ISF Aachen Welding Technology Part I

0. Introduction

0. Introduction

4

Welding fabrication processes are classified in accordance with the German Standards DIN 8580 and DIN 8595 in main group 4 “Joining”, group 4.6 “Joining by Welding”, Figure 0.1.

2 Forming

1 Casting

4.1 Joining by composition

4.2 Joining by filling

3 Cutting

4.3 Joining by pressing

4.4 Joining by casting

4 Joining

4.5 Joining by forming

5 Coating

4.6 Joining by welding

4.6.1 Pressure welding

4.7 Joining by soldering

6 Changing of materials properties

4.8 Joining by adhesive bonding

4.6.2 Fusion welding

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Production Processes acc. to DIN 8580

Figure 0.1

Welding: permanent, positive joining method. The course of the strain lines is almost ideal. Welded joints show therefore higher strength

Screwing

properties than the joint types depicted in Figure 0.2. This is of advantage, especially in Riveting

the case of dynamic stress, as the notch effects are lower.

Adhesive bonding

Soldering

Welding

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© ISF 2002

Connection Types

Figure 0.2 2005

0. Introduction

5

Figures 0.3 and 0.4 show the further subdivision of the different welding methods according to DIN 1910.

Production processes 4 Joining 4.6 Joining by welding

4.6.1 Pressure welding

4.6.2 Fusion welding

4.6.1.1 Welding by solid bodies

4.6.1.2 Welding by liquids

4.6.1.3 Welding by gas

4.6.1.4 Welding by electrical gas discharge

4.6.1.6 Welding by motion

4.6.1.7 Welding by electric current

Heated tool welding

Flow welding

Gas pressure-/ roll-/ forge-/ diffusion welding

Arc pressure welding

Cold pressure-/ shock-/ friction-/ ultrasonic welding

Resistance pressure welding © ISF 2002

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Joining by Welding acc. to DIN 1910 Pressure Welding

Figure 0.3

Production processes 4 Joining 4.6 Joining by welding

4.6.1 Pressure welding

4.6.2 Fusion welding

4.6.2.2 Welding by liquids

4.6.2.3 Welding by gas

4.6.2.4 Welding by electrical gas discharge

4.6.2.5 Welding by beam

4.6.2.7 Welding by electric current

Cast welding

Gas welding

Arc welding

Beam welding

Resistance welding

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Joining by Welding acc. to DIN 1910 Fusion Welding

Figure 0.4

2005

10. Laser Beam Welding

10. Laser Beam Welding

134

The term laser is the abbreviation for ,,Light Amplification by Stimulated Emission of Radiation”. The laser is the further development of the maser (m=microwave), Figure 10.1. Although the principle of the stimulated emis1917 postulate of stimulated emission by Einstein

sion and the quantum-mechanical fundamen-

1950 work out of physical basics and realisation of a maser (Microwave Amplification by Stimulated Emission of Radiation) by Towens, Prokhorov, Basov

tals have already been postulated by Einstein

1954 construction of the first maser 1960 construction of the first ruby laser (Light Amplification by Stimulated Emission of Radiation)

in the beginning of the 20th century, the first laser - a ruby laser - was not implemented

1961 manufacturing of the first HeNe lasers and Nd: glass lasers

until 1960 in the Hughes Research Laborato-

1962 development of the first semiconductor lasers

ries. Until then numerous tests on materials

1964 nobel price for Towens, Prokhorov and Basov for their works in the field of masers construction of the first Nd:YAG solid state lasers and CO2 gas lasers

had to be carried out in order to gain a more

1966 established laser emission on organic dyes

The following years had been characterised

precise knowledge about the atomic structure.

since increased application of CO2 and solid state laser 1970 technologies in industry

by a fast development of the laser technology.

1975 first applications of laser beam cutting in sheet fabrication industry

Already since the beginning of the Seventies

1983 introduction into the market of 1-kW-CO2 lasers

and, increasingly since the Eighties when the

1984 first applications of laser beam welding in industrial serial production

first high-performance lasers were available, CO2 and solid state lasers have been used

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History of the Laser

for production metal working.

Figure 10.1 The number of the annual 3

sales of laser beam sources 9

has constantly increased in the course of the last few years, Figure 10.2.

10 €

2

1.5

1

The application areas for the

0.5

laser beam sources sold in

0

1986

1988

1990

1992

1994

1996

1998

2000

1994 are shown in Figure 10.3. The main application

Japan and South East Asia North America West Europe br-er10-02e.cdr

areas of the laser in the field of production metal working are joining and cutting jobs.

Figure 10.2

2005

10. Laser Beam Welding

135

The availability of more efficient laser beam sources opens up new application possibilities and - guided by financial considerations - makes the use of the laser also more attractive, Figure 10.4. Figure

10.5

shows

the

characteristic properties drilling 1,8%

welding 18,7%

of the laser beam. By reason

inscribe 20,5%

others 9,3%

of

the

stimulated

induced

emission

or the

radiation is coherent and monochromatic.

As

the

divergence is only 1/10 cutting 44,3%

micro electronics 5,4%

br-er10-03e.cdr

mrad,

long

paths

without

beam

transmission significant

divergences

are

possible. Figure 10.3 Inside the resonator, Figure 10.6, the laser-active medium (gas molecules, ions) is excited to a higher energy level (“pumping”) by energy input (electrical gas discharge, flash lamps). 40

During retreat to a lower

kW

laser power

level, the energy is re-

CO2

20 10

leased in the form of a light

5

quantum

4

wave length depends on

3

tween both excited states

1 2000

1995

1990

1985

1980

1975

diode laser 1970

0

The

the energy difference be-

Nd:YAG

2

(photon).

and is thus a characteristic for the respective la-

br-er10-04e.cdr

ser-active medium.

Figure 10.4

2005

10. Laser Beam Welding

136 A distinction is made be-

light bulb

Laser

tween

induced emission

spontaneous

and

induced transition. While E2

0,46"

E1

the spontaneous emission is non-directional and in exited state

ground state

coherent (e.g. in fluores-

0,9 4"

cent tubes) is a laser beam monochromatic

polychromatic

generated

(multiple wave length)

coherent

incoherent

(in phase)

induced

emission when a particle

(not in phase)

small divergence

large divergence

by

with a higher energy level © ISF 2002

br-er10-05e.cdr

is hit by a photon. The reCharacteristics of Laser Beams

Figure 10.5

sulting

photon

has

same

properties

the (fre-

quency, direction, phase) as the exciting photon (“coherence”). In order to maintain the ratio of the desired induced emission I spontaneous emission as high as possible, the upper energy level must be constantly overcrowded, in comparison with the lower one, the so-called “laser-inversion”. As result, a stationary light wave is formed between the mirrors of the resonator (one of which is semi-reflecting) causing parts of the excited laser-active medium to emit light. In the field of production metal working, and particularly in welding, especially CO2 and Nd:YAG lasers are applied for their high power outputs. At present, the development of diode lasers is so far advanced resonator

that their sporadic use in the

energy source

field of material processing is also possible. The indusCO2 lasers are, nowadays,

laser beam

trial standard powers for active laser medium

approximately 5 - 20 kW, lasers with powers of up to 40 kW are available. In the field of solid state lasers

fully reflecting mirror R = 100%

energy source

© ISF 2002

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average output powers of

partially reflecting mirror R < 100%

Laser Principle

up to 4 kW are nowadays obtainable.

Figure 10.6

2005

10. Laser Beam Welding

137

In the case of the CO2 laser, Figure 10.7, where the resonator is filled with a N2-C02-He gas mixture, pumping is carried out over the vibrational excitation of nitrogen molecules which again, with thrusts of the second type, transfer their 0,6

002

eV

energy

vibrational energy to the

thrust of second type 2

transmission of vibration energy

0,4 0,3 0,288 eV

carbon dioxide. During the transition without emission

transition to the lower energy level, CO2 molecules

thrust of first type 1

001 ∆E = 0,002 eV

0,290 eV

0,2

100

0,1

discharge through thrust with helium

0

emit a radiation with a

LASER λ = 10,6 µm

0

wavelength of 10.6 µm. The helium atoms, finally,

000

N2

lead the CO2 molecules

CO2 © ISF 2002

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back to their energy level.

Energy Diagram of CO2 Laser

Figure 10.7

The efficiency of up to

15%, which is achievable with CO2 high performance lasers, is, in comparison with other laser systems, relatively high. The high dissipation component is the heat which must be discharged from the resonator. This is achieved by means of the constant gas mixture circulation and cooling by heat exchangers. In dependence of the type of gas transport, laser sys-

radio frequency high voltage exitaion

tems are classified into laser beam

longitudinal-flow and transverse-flow laser systems, cooling water

cooling water

Figures 10.8 and 10.9. laser gas

laser gas: CO2: 5 l/h He: 100 l/h N2 : 45 l/h

vakuum pump

gas circulation pump br-er10-08e.cdr

Figure 10.8

2005

10. Laser Beam Welding

138

With transverse-flow laser systems of a compact design can the multiple folding ability of the beam reach higher output powers than those achievable with longitudinal-flow systems, the beam quality, however, is worse. In d.c.-excited systems (high voltage), the electrodes are positioned inside the resonator. The interaction between the electrode material and the gas molecules causes electrode burn-off. In addition to the wear of the electrodes, the burn-off also entails a contamination of the laser gas. Parts of the gas mixture must be therefore exchanged permanently. In high-frequency a.c.-excited systems the electrodes are positioned outside the gas discharge tube where the electrical energy is capacitively

coupled.

Cooling water

High

electrode lives and high achievable pulse frequen-

gas circulation pump

laser gas: CO2: 11 l/h He: 142 l/h N2: 130 l/h

turning mirrors

laser beam

cies characterise this kind mirror

of excitation principle. In

(partially reflecting)

diffusion-cooled CO2 sys-

gas discharge

tems beams of a high quality are generated in a mini-

laser gas

end mirror

mum of space. Moreover, br-er10-09e.cdr

cooling water

gas exchange is hardly ever necessary. Figure 10.9 The intensity distribution is not constant across the la-

f2,57" d0

ser beam. The intensity distribution in the case of the

unfocussed beam

ΘF

focussed beam

dF

ideal beam is described by TEM modes (transversal electronic-magnetic). In the Gaussian or basic mode K = 2λ .1 π dσΘσ

TEM00 is the peak energy in

0 90% - well suitable to automatic function

installation

ductivity), the design of the resonator (beam quality), the focal position and

- minimum thermal stress - little distortion - completely processed components - welding at positions difficult to access - different materials weldable

work piece

the material (thermal con-

the applied optics (focal length; focus diameter). Figure 10.26 shows several joint shapes which are typi-

© ISF 2002

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Advantages and Disadvantages of Laser Beam Welding

Figure 10.24

and which can be welded by laser beam application.

The high cooling rate during laser beam

28 0,2% C-steel CO2-laser

mm

penetration depth

cal for car body production

welding leads, when transforming steel mate-

(cross flow)

rials are used, to significantly increased

20 laser power:

16

15 kW

hardness values in comparison with other

10 kW

welding methods, Figure 10.27. These are a

12 8 kW

8

6 kW 4 kW

sign for the increased strength at a lower

4 1,5 kW

toughness and they are particularly critical in

0 0

0,6

1,2

1,8

m/min

3,0

circumstances of dynamic loads.

welding speed

penetration depth

15 X 5 CrNi 18 10 CO2-laser

The small beam diameter demands the very

(axial flow)

mm

precise manipulation and positioning of the laser power:

5 4 kW

2 kW

0

preparation, Figure 10.28. Otherwise, as re-

6 kW

1 kW

0

1

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2

3

4

5

6

7

m/min

workpiece or of the beam and an exact weld

9

sult, lack of fusion, sagged welds or concave

welding speed

root surfaces are possible weld defects. Penetration Depths

Figure 10.25

2005

10. Laser Beam Welding

146

butt weld

fillet weld at overlap joint

lap weld at overlap joint

flanged weld at overlap joint

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Figure 10.26

500 HV 0,4

MAZ

hardness

laser beam weld

WMA MAZ

MAZ

MAZ

0

12

distance from the weld centre

submerged arc weld

weld

submerged arc weld br-er10-27e_f.cdr

Figure 10.27

misalignment

edge preparation

(e ≤ 0,1 x plate thickness)

gap

beam

mispositioning

(a ≤ 0,1 x plate thickness)

© ISF 2002

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Welding Defects

Figure 10.28 2005

10. Laser Beam Welding

147

Caused by the high cooling rate and, in connection with this, the insufficient degassing of the molten metal, pore formation may occur during laser beam welding of, in particular, thick plates (very deep welds) or while carrying out welding-in works (insufficient degassing over the root), Figure 10.29. However, too low a weld speed may also cause pore formation when the molten metal picks up gases from the root side. The materials that may be welded with the laser reach from unalloyed and lowalloy steels up to high quality

titanium

based

and

alloys.

carbon

nickel

The

content

high

of

the

transforming steel materials is, due to the high cooling vw = 0,7 m/min

vw = 0,9 m/min

vw = 1,5 m/min

rate, to be considered a

material: P460N (StE460), s = 20 mm, P = 15 kw © ISF 2002

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critical

Porosity

influential

factor

where contents of C > Figure 10.29

0.22% may be stipulated as the limiting reference value. Aluminium

welding direction

and

copper

properties cause problems filler wire

laser beam

during energy input and

filler wire

laser beam

gas

gas plasma

process

stability.

Highly

plasma

reactive materials demand, weld metal

work piece

weld metal

work piece

also during laser beam welding,

molten pool

keyhole

molten pool

keyhole

sufficient

gas

shielding beyond the solidification of the weld seam.

forward wire feeding br-er10-30e.cdr

backward wire feeding

The

sole

application

of

working gases is, as a rule, not adequate. Figure 10.30

2005

10. Laser Beam Welding

148

The application of laser beam welding may be extended by process variants. One is laser beam welding with filler wire, Figures 10.30 and 10.31 which offers the following advantages: - influence on the mechanic-technological properties of the weld and fusion zone (e.g. strength, toughness, corrosion, wear resistance) over the metallurgical composition of the filler wire - reduction of the demands

without filler wire

with filler wire

increase of gap bridging ability material: S380N (StE 380) gap: 0,5 mm PL = 8,3 kW VW = 3 m/min ES = 166 J/min s = 4 mm

filler wire: Sg2 dw = 0,8 mm

weld zone

Possibility of metallurgical influence

weld zone

material combination:

10CrMo9-10/ X6CrNiTi18-10 PL = 5,0 kW

on the accuracy of the weld

gap: 0 mm vw = 1,6 m/min

vw = 1,0 m/min dw = 1,2 mm

gap: 0,5 mm wire: SG-Ni Cr21 Fe18 Mo

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preparation in regard to edge misalignment, edge preparation and beam mis-

Figure 10.31

alignment, due to larger molten pools - “filling” of non-ideal, for example, V-shaped groove geometries - a realisation of a defined weld reinforcement on the beam entry and beam exit side. The exact positioning of the filler wire is a prerequisite with sensing device; fill factor 120 %

for a high weld quality and a sufficient dilution of the mol-

KB 4620/9 20:1 10/92

KB 4620/6 20:1 10/92

KB 4620/4 20:1 10/92

KB 4620/0 20:1 10/92

KB 4620/41 20:1 10/92

KB 4620/38 20:1 10/92

Probe MS1-6C Probe MS1-5A Probe MS1-4C Probe MS1-3A Probe MS1-2B Probe MS1-1C

ten pool through which filler

0.1 mm

0.2 mm

0.3 mm

0.4 mm

0.5 mm

0.6 mm

KB 4620/12 20:1 10/92

KB 4620/17 20:1 10/92

KB 4621/15 20:1 10/92

KB 4621/12 20:1 10/92

KB 4621/9 20:1 10/92

KB 4621/7 20:1 10/92

wire of different composition as the base can reach right to the root. Therefore, the use of sensor systems is indispensable for industrial

Probe OS1-6A Probe OS1-5C Probe OS1-4C Probe OS1-3B Probe OS1-2B Probe OS1-1B without sensing device; wire speed vD = 4 m/min constant

1 mm

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application,

Figure 10.32.

The sensor systems are to take over the tasks of

Figure 10.32

- process control,

2005

10. Laser Beam Welding

149

- weld quality as surance - beam positioning and joint tracking, respectively. The present state-of-the-art is the further development of systems for industrial applications which until now have been tested in the laboratory. Welding by means of solid state lasers has, in the past, mainly been applied by manufacturers from the fields of precision mechanics and microelectronics. Ever since solid state lasers with higher powers are available on the market, they are applied in the car industry to an ever increasing degree. This is due to their more variable beam manipulation possibilities when comparing with CO2 lasers. The CO2 laser is

aerospace industry automotive industry

- engine components - instrument cases

- gear parts

mostly used by the car in-

steel industry - pipe production - vehicle superstructures - continuous metal strips - tins

(cog-wheels, planet gears)

- body-making (bottom plates, skins)

- engine components (tappet housings, diesel engine precombustion chambers)

dustry and by their ancillary industry for welding rotation-symmetrical mass-

electronic industry medical industry - heart pacemaker cases - artificial hip joints

plant and apparatus engineering

- PCBs - accumulator cases - transformer plates - CRTs

- seal welds at housings - measurement probes

Figure 10.33 shows some typical application examples for laser beam weld-

© ISF 2002

br-er10-33e.cdr

produced parts or sheets.

ing.

Practical Application Fields

Figure 10.33

2005

11. Surfacing and Shape Welding

11. Surfacing and Shape Welding

151 DIN 1910 (“Welding”) classifies the welding process according to its applica-

base metal/ surfacing metal

tions: welding of joints and surfacing. According

similar

dissimilar


for repair welding


hardfacing (wear protection)

to DIN 1910 surfacing is the coating of a workpiece


cladding (corrosion prevention)

by means of welding.


buffering (production of an appropriate-to-the-type-of-duty joint of dissimilar materials)

Dependent on the applied filler

material

a

further

br-er11-01e.cdr

classification made: Figure 11.1

may

deposition

be repair

welding and surfacing for

the production of a composite material with certain functions. Surfacing carried out with wearresistant materials in preference to the base metal material is called hardfacing; but when mainly chemically stable filler materials are used, the method is called cladding. In the case of buffering, surfacing layers are pro

wear caused by very high impact and compressive stress



wear by friction (metal against metal) during high impact and compression stress



strong sanding or grinding wear



very strong wear caused by grinding during low impact stress



cold forming tools



hot forming tools



cavitation

between two layers with strongly differing



wear parts (plastics industry)

thermal expansion coefficients.



corrosion

Figure 11.2 shows different kinds of stresses



temperature stresses

duced which allow the appropriate-to-thetype-of-duty joining of dissimilar materials and/or of materials with differing properties, Figure 11.1. A buffering, for instance, is an intermediate layer made from a relatively tough material

which demand the surfacing of components. Furthermore surfacing may be used for pri-

br-er11-02e.cdr

Components Kinds of Stress

mary forming as well as for joining by primary forming. Figure 11.2

2005

11. Surfacing and Shape Welding

152

In case of surfacing - as for all fabrication processes - certain limiting conditions have to be observed. For example, hard and wear-resistant weld filler metals cannot be drawn into solid wires. Here, another form has to be selected (filler wire, continuously cast rods, powder). Process materials, as for example SA welding flux demand a certain welding position which in terms limits the method of welding. The coating material must be selected with view to the type of duty and, moreover, must be compatible with the base metal, Figure 11.3. For all surfacing tasks a large product line of welding filler metals is available. In dependence on the welding method as well as on the selected materials, filler metals in the form of wires, filler wires, strips, cored strips, rods or powder are applied, Figure 11.4. The filler/base metal dilution is rather important, as the desired high-quality properties of the surfacing layer deteriorate with the increasing degree of dilution.

component (material)

wearing protection (armouring) hard facing on  cobalt base  nickel base

coating

stress compatibility

manufacturing conditions availability

 iron base

corrosion prevention  ferritic to martensitic chromium steel alloys  soft martensitic chromium-nickel steel alloys

coating material (filler)

consumable

surfacing method

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 austenitic-ferritic chromium-nickel steel alloys  austenitic chromium-nickel steel alloys

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Materials for Surfacing

Boundary Conditions in Surfacing

Figure 11.3

Figure 11.4

2005

11. Surfacing and Shape Welding

153

A weld parameter optimisation has the objective to optimise the degree of dilution in order to guarantee a sufficient adherence of the layer with the minimum metal dissimilation. A planimetric

determination

of the surfacing and penesurface built up by welding FB

tration areas will roughly assess the proportion of

penetration area FP

filler to base metal. When base metal FP FP + FB

AD= AD =

the analysis of base and filler metal is known, a

x 100%

(X-contentsurfacing layer - X-contentFM) [% in weight] (X-contentbase metal - X-contentFM) [% in weight]

FM: weld filler metal

more precise calculation is x 100%

possible by the determination of the content of a cer-

AD: dilution © ISF 2002

br-er11-05e.cdr

tain element in the surfac-

Definition of Dilution

ing layer as well as in the base metal, Figure 11.5.

Figure 11.5 Figure 11.6 shows record charts of an electron nickel and chromium. It is evident that - after passing a narrow transition zone between base metal and layer — the analysis inside the layer is quasi constant.

Cr percentages by mass

beam microprobe analysis for the elements

30 % 20

10

0 0

300

µm

500

300

µm

500

30 Ni percentages by mass

but also for surfacing.

200 distance

As depicted in Figure 11.7 almost all arc welding methods are not only suitable for joining

100

% 20

10

0

0

100

br-er11-06e.cdr

200 distance

Microprobe Analyses

Figure 11.6

2005

11. Surfacing and Shape Welding

154

In the case of the strip-electrode submerged-arc surfacing process normally strips (widths: 20 - 120mm) are used. These strips allow high cladding rates. Solid wire electrodes as well as flux-cored strip electrodes are used. The flux-cored strip electrodes contain certain alloying elements. The strip is continuously fed into the process via feed rollers. Current contact is normally carried out metal-arc welding - stick electrode - filler wire

inert gas-shielded arc welding

via copper contact jaws

- wire electrode - strip electrode

which in some cases are

- MIG / MAG - MIG cold wire - filler wire

arc welding with self-shielded cored wire electrode - filler wire

submerged arc welding

TIG welding

protected against wear by electroslag welding

hard metal inserts. The

- wire electrode

slag-forming flux is sup-

- TIG cold wire

plied onto the workpiece in arc spraying

plasma welding

front of the strip electrode - powder - wire

- plasma powder - plasma hot wire

by means of a flux support. The non-molten flux can be

plasma spraying br-er11-07e.cdr

extracted and returned to the flux circuit. Figure 11.7 Should the slag developed on top of the welding bead power source drive rolls

+

-

not detach itself, it will have to be removed me-

filler metal

chanically in order to avoid slag

flux support

inclusions

during

overwelding. The arc wanflux application slag

ders along the lower edge

surfacing bead base metal

of the strip. Thus the strip is melted consistently, Fig-

br-er11-08e.cdr

ure 11.8.

Figure 11.8

2005

11. Surfacing and Shape Welding

155 Figure

11.9

shows

the

cladding of a roll barrel. The coating is deposited helically while the workpiece is rotating. The weld head is moved axially over the workpiece. The

macro-section

and

possible weld defects of a br-er11-09e.cdr

strip-electrode submergedarc surfacing process are Figure 11.9

depicted in Figure 11.10.

Electroslag surfacing using a strip electrode is

coarse grain zone

lack of fusion

mirco slag inclusions

sagged weld

similar to strip-electrode SA surfacing,

Figure

11.11.

base metal

The difference is that the weld

filler metal is not

melted in the arc but in liq-

crack formation in these areas of the coarse grain zone

gusset

undercuts

uefied welding flux — the liquid slag – as a result of Joule resistance heating. br-er11-10e.cdr

The slag is held by a slight inclination of the plate and the flux mound to prevent it

Figure 11.10

from running off.

2005

11. Surfacing and Shape Welding

156

TIG weld surfacing is a suitable surfacing method for small and complicated contours and/or low quantities (e.g. repair work) with normally relatively low deposition rates. The process principle has already been shown when the TIG joint welding process was explained, Figure 11.12. The arc is burning between a gas-backed nonconsumable tungsten electrode and the workpiece. The arc melts the base metal and the wire or rodmolten pool

shaped weld filler metal which is fed either continuously or intermittently. Thus a fusion welded joint devel-

br-er11-11e.cdr

ops between base metal and surfacing bead. Figure 11.11 In the case of MIG/MAG surfacing processes the arc burns between a consumable shielding gas nozzle

rod/ wire-shaped filler metal

wire electrode and the workpiece. This method allows higher deposition rates. Filler as well as solid wires are used. The wire elec-

arc

trode has a positive, while the workpiece to be surfaced has a negative polarity, Figure 11.13.

base metal (+ / ~)

tungsten electrode (- / ~) surfacing bead

br-er11-12e.cdr

© ISF 2002

Process Principle of TIG Weld Surfacing

Figure 11.12

2005

11. Surfacing and Shape Welding

157 A further development of the TIG welding process is plasma welding. While the

contact tube wire feed device

shielding gas shielding gas nozzle

+ -

weld filler metal

TIG arc develops freely, the plasma welding arc is me-

power source

arc shielding gas surfacing bead

chanically

and

constricted

by

thermally a

water-

cooled copper nozzle. Thus workpiece

the arc obtains a higher

oscillation

energy density.

feed direction br-er11-13e.cdr

In the case of plasma arc Figure 11.13

powder

surfacing

this

constricting nozzle has a positive, the tungsten electrode has a negative polarity, Figure 11.14. Through a pilot arc power supply a non-transferred arc (pilot arc) develops inside the torch. A second, separate power source feeds the transferred arc between electrode and workpiece. The nontransferred arc ionises the centrally fed plasma gas (inert gases, as, e.g., Ar or He) thus causing a plasma jet of high energy to emerge from the nozzle. This plasma jet serves to produce and to stabilise the arc striking ability of the transferred arc gap. The surfacing filler metal powder added by a feeding gas flow is melted in the plasma jet. The partly liquefied weld filler metal meets the by transferred arc molten

tungsten electrode

filler metal

base metal and forms the plasma gas HIG

surfacing bead. A third gas flow, the shielding gas, protects the surfacing bead and

the

adjacent

high-

UNTA

conveying gas power sources shielding gas pilot arc welding arc

UTA

surfacing bead

temperature zone from the surrounding influence. The workpiece

applied gases are mainly

oscillation

br-er11-14e.cdr

inert gases, as, for example, Ar and He and/or Ar/He mixtures.

Figure 11.14

2005

11. Surfacing and Shape Welding

158

The method is applied for surfacing small and medium-sized parts (car exhaust valves, extruder spirals). Figure 11.15 shows a cross-section of armour plating of a car exhaust valve seat. The fusion line, i.e., the region between surfacing and base metal, is section A

shown ZW

enlarged

on

the

right side of Figure 11.15

GW

(blow-up).

It

shows

hardfacing

with

cobalt

which is high-temperature and

hot

gas

corrosion

resistant. br-er11-15e.cdr

In plasma arc hot wire surfacing the base metal Figure 11.15

is melted by an oscillating plasma torch, Figure 11.16.

The weld filler metal in the form of two parallel wires is added to the base metal quite independently. The arc between the tips of the two parallel wires is generated through the application of a separate power shielding gas

plasma power source

plasma gas

=

tungsten electrode arc

wires from spool surfacing bead

source. The plasma arc with a length of approx. 20 mm is oscillating (oscillation width between 20 to 50 mm). The two wires are fed in a V-formation at an an-

~ workpiece

weld pool

hot wire power source

gle of approx. 30° and melt in

the

high-temperature

region in the trailing zone

br-er11-16e.cdr

of the plasma torch.

Figure 11.16

2005

11. Surfacing and Shape Welding For

surfacing

besides

the

159

purposes, arc-welding

methods, the beam welding methods laser beam and electron beam welding may also be applied. Figure 11.17 shows the process principle of laser surfacing. The powder filler metal is added to the laser beam via a powder nozzle and the powder gas flow is, in addition, constricted by shielding gas flow.

Figure 11.17

Friction surfacing is, in principle, similar to friction welding for the production of joints which due to the different materials are difficult to produce with fusion welding, Figure 11.18. The filler metal is “advanced” over the workpiece with high pressure and rotation. By the pressure and the relative movement frictional heat develops and puts the weld filler end into a pasty condition. The advance motion causes an adherent, “spreaded” layer on the base metal. This method is not applied frequently and is mainly used for materials which show strong differences in their melting and oxidation behaviours. A comparison of the different

surfacing

force

methods

filler metal

shows that the application fields are limited - de-

rotation advance

pendent on the welding method.

A

specific

method, for example, is the low filler/base metal

surfacing layer bulge base metal

dilution. These methods br-er11-18e.cdr

are applied where highquality filler metals are Figure 11.18 2005

11. Surfacing and Shape Welding

160

welded. Another criterion for the selection of a surfacing method is the deposition rate. In the case of cladding large surfaces a method with a high deposition rate is chosen,

compressed air

spraying material

workpiece

this with regard to profitability. In thermal spraying the filler metal is melted inside the torch and then, with a high kinetic energy, dis-

fuel gas-oxygen mixture

flame cone

spray deposit

br-er11-19e.cdr

charged onto the unfused but preheated workpiece surface.

Figure 11.19

There is no fusion of base and filler metal but rather adhesive binding and mechanical interlocking of the spray deposit with the base material. These mechanisms are effective only when the workpiece surface is coarse (pre-treatment by sandblasting) and free of oxides. The filler and base materials are metallic and non-metallic. Plastics may be sprayed as well. The utilisation of filler metals in thermal spraying is relatively low. The most important methods of thermal spraying are: plasma arc spraying, flame spraying and arc spraying. compressed air

spraying jet

In powder flame spraying an oxyacetylene flame pro-

non-binding sprayed particles (loss in spraying)

gas mixture

vides the heating source where the centrally fed filler metal

is

melted,

Figure

11.19. The kinetic energy for

adjustable wire feed device

the acceleration and atomispraying wire

sation of the filler metal is

fusing wire tip

spray deposit

br-er11-20e.cdr

produced

by

compressed

gas (air). Figure 11.20 2005

11. Surfacing and Shape Welding

161 In contrast to powder flame spraying,

is

for

flame

powder injector back frame

isolation ring

gas middle distributor frame

spraying a wire filler metal

anode carrier

fed mechanically into the copper anode

centre cone, melted, atomised and accelerated in direction of the substrate,

jet of particles cooling water

plasma gas

cooling water tungsten cathode

Figure 11.20.

arc

In plasma arc spraying an © ISF 2002

br-er11-21e.cdr

internal, high-energy arc is

Plasma Powder Spraying Unit

ignited between the tungFigure 11.21

sten cathode and the anode, Figure 11.21. This arc

ionises the plasma gas (argon, 50 - 100 l/min). The plasma emerges from the torch with a high kinetic and thermal energy and carries the side-fed powder along with it which then meets the workpiece surface in a semi-fluid state with the necessary kinetic energy. In the case of shape welding, steel shapes with larger dimensions and higher weights are produced from molten weld metal only. In comparison to cast parts this method brings about essentially more favourable mechano-technological material properties, especially a better toughness characteristic. The reason for this lies mainly in the high purity and the homogeneity of the steel which is helped by the repeated melting process and the resulting slag reactions. These properties are also put down to the favourable fine-grained struc-

primary forming (casting)

ture

formation

which

is

achieved by the repeated shape welding

subsequent thermal treatment with the multi-pass

forming (forging)

technique. Also in contrast

joining (welding)

with the shapes produced by forging, the workpieces © ISF 2002

br-er11-22e.cdr

produced by shape welding Shape Welding - Integration

show quality advantages,

Figure 11.22 2005

11. Surfacing and Shape Welding

162

especially in the isotropy and the regularity of their toughness and strength properties as far as larger workpiece thicknesses are concerned. In Europe, due to the lack of expensive forging equipment, very high individual weights may not be produced as forged parts. Therefore, shape welding is, for certain applications, a sensible technological and economical alternative to the methods of primary forming, forming or joining, Figure 11.22. Figure 11.23 shows an early application which is related to the field of arts. The higher tooling costs in forging make the shape welding method less expensive; this applies to parts with certain increasing complexity. This comparison is, however, related to relatively low numbers of pieces, where the

br-er11-23e.cdr

Shape Welded Goblet (1936)

tooling costs per part are accordingly higher, Figure 11.24.

Figure 11.23

forged products

€/kg

pipe bends

spherical caps

braces

boiler shell rings

shafts

shape-welded products

complexity of the parts br-er11-24e.cdr

Figure 11.24

2005

11. Surfacing and Shape Welding

163

Figure 11.25 shows the principal procedure for the production of typical shape-welded parts.

Baumkuchenmethode

Cylindrical containers are produced with the

- core made of foreign material necessary

“Baumkuchenmethode” method: the filler metal

applications: shafts, large boiler shell rings, flanges

is welded by submerged-arc with helical movement in multiple passes into a tube which

+ several weld heads possible + no interruption during weld head failure

Töpfermethode

has the function of a traction mechanism (for the most part mechanically removed later).

+ free rotationally-symmetrical shapes + several weld heads possible + weld head manipulation not necessary + each head capable to weld a specific layer + small diameters possible - component movement must correspond with the contour - number of weld heads limited when smaller diameters are welded

This brings about the possibility to produce seamless containers with bottom and flange in

applications: spherical caps, pipe bends, braces

one working cycle. Klammeraffe

- limited welding efficiency

Elbows are mainly manufactured with the

applications: welding-on of connection pieces

Töpfer method. On the traction mechanism a rotationally symmetrical part with a semicircle

+ transportable unit

br-er11-25e.cdr

© ISF 2002

Shape Welding Procedures

cross-section is produced which is later separated and welded to an elbow, Figures 11.26 and 11.27. The Klammeraffe method serves

Figure 11.25 the purpose to weld external connection pieces onto pipes.

A

portable

unit

which is connected with the phase 7

phase 5

phase 6 phase 4 phase 2

joist

phase 3 traction mechanism

phase 1

pipe welds the connection pipe in a similar manner to the Töpfer method.

turntable

br-er11-26e.cdr

Figure 11.26

2005

11. Surfacing and Shape Welding

164 In the case of electron beam surfacing the filler metal is added to the process in the form

ti tes ng

1. welding of the half-torus 2. stress relief annealing 3. mechanical treatment 4. seperating/ halving 5. folding 6. welding togehter 7. stress relief annealing 8. testing

of a film, Figure 11.28.

br-er11-27e.cdr

Production of a Pipe Bend by Shape Welding

Figure 11.27

electron beam surface layer base material metal foil metal foil feeding workpiece

feed direction ka11-18.cdr

© ISF 1998

Process Principle Electron Beam Surface Welding

Figure 11.28

2005

12. Thermal Cutting

12. Thermal Cutting

166

Thermal cutting processes are

applied

in

different

fields of mechanical engineering, shipbuilding and

Classification of thermal cutting processes

process technology for the

- physics of the cutting process

production of components

- degree of mechanisation

and for the preparation of - type of energy source

welding edges. The ther-

- arrangement of water bath

mal cutting processes are classified

into

different

categories according to DIN

br-er12-01e.cdr

Classification of Thermal Cutting Processes acc. to DIN 2310-6

2310, Figure 12.1. Figure 12.1 Figure 12.2 shows the clas-

sification according to the physics of the cutting process: - flame cutting – the material is mainly oxidised (burnt) - fusion cutting – the material is mainly fused - sublimation cutting – the material is mainly evaporat The gas jet and/or evaporation expansion is in all processes responsible for the ejection of molten material or emerging reaction products such as slag. The different energy carriers for the thermal cutting are depicted in Figure 12.3: - gas, Flame cutting The material is mainly oxidised;the products are blown out by an oxygen jet.

- electrical gas discharge and - beams.

Fusion cutting The material is mainly fused and blown out by a high-speed gas jet. Sublimation cutting The material is mainly evaporated. It is transported out of the cutting groove by the created expansion or by additional gas.

Electron beams for thermal cutting are listed in the DIN-Standard,

they

pro-

duce, however, only very small boreholes. Cutting is impossible.

br-er12-02e.cdr

Classification of Processes by the Physics of Cutting

Figure 12.2 2005

12. Thermal Cutting

167

Figure 12.4 depicts the different methods of thermal cutting with gas according to DIN 8580. These are: - flame cutting - metal powder flame cutting

thermal cutting by:

- gas

- metal powder

- electrical gas discharge - sparks - arc - plasma

fusion cutting - flame planing

- beams - laser beam (light) - electron beam - ion beam

-oxygen-lance cutting - flame gouging or scarfing

br-er12-03e.cdr

-flame cleaning

Classification of Thermal Cutting Processes acc. to DIN 2310-6

In flame cutting (principle

Figure 1.3

is depicted in Figure 12.5) the material is brought to thermal cutting processes using gas:

the ignition temperature by a heating flame and is then


burnt in the oxygen stream.

oxygen cutting




During the process the igni-

metal powder




metal powder

flame cutting

fusion cutting

tion temperature is main


tained on the plate top side

flame planing

by the heating flame and conduction

oxygen-lance cutting




flame gouging




below the plate top side by thermal







flame cleaning

scarfing

br-er12-04e.cdr

and

Thermal Cutting Processes Using Gas

convection. Figure 1.4 However, this process is suited for automation and is, also easy to apply on site. Figure 12.6. shows a commercial torch which combines a welding with a cutting torch. By means of different nozzle shapes the process may be adapted to varying materials and plate thicknesses. Hand-held torches or machine-type torches are equipped with different cutting nozzles: Standard or blocktype nozzles (cutting-oxygen pressure 5 bar) are used for hand-held torches and for torches which are fixed to guide carriages.

2005

12. Thermal Cutting

168

The high-speed cutting nozzle (cutting-oxygen pressure 8 bar)

allows higher cutting

speeds with increased cutting-oxygen pressure. The heavy-duty cutting nozzle (cuttingoxygen pressure 11 bar) is cutting oxygen heating oxygen gas fuel

mainly applied for economic cutting with flamecutting machines. A further

heating flame

development of the heavyduty nozzle is the oxygenshrouded allows

cutting jet

which

nozzle

even

faster

and

more economic cutting of

workpiece

plates within certain thickbr-er12-05e.cdr

ness ranges. Gas mixing is Principle of Oxygen Cutting

either carried out in the torch handle, the cutting

Figure 12.5

attachment, the torch head or in the nozzle (gas mixing

cutting oxygen heating oxygen

cases

gas fuel

in

nozzle); also

special

outside

the

torch – in front of the noz-

mixing chamber

zle. As the design of cutting torches is not yet subject to manual cutting equipment as a cutting and welding torch combination

standardisation,

many

types and systems exist on gas mixing nozzle

block-type nozze

the market.

br-er12-06e.cdr

Cutting Torch and Nozzle Shapes

Figure 12.6

2005

12. Thermal Cutting

169 The selection of a torch or

heating and cutting nozzle

nozzles important and de-

nozzle-to-work distance

torch kerf width

cutting jet

kerf

pends mainly on the cutting thickness, the desired cut-

start

ting quality, and/or the ge-

cut thickness

ometry of the cutting edge.

cutting le

cut lengt h ngth

Figure 12.7 gives a survey of the definitions of flamecutting.

end of the cut

br-er12-07e.cdr

Flame Cutting Terms

In flame cutting, the thermal conductivity of the ma-

Figure 12.7

terial must be low enough to

constantly

maintain

the ignition temperature, The heating flame has to perform the following tasks:

Figure 12.8. Moreover, the

- rapid heating of the material (about 1200°C)

material

- substitution of losses due to heat conduction

melt during the oxidation

in order to maintain a positive heat balance

must

- preheating of cutting oxygen

nor

- stabilisation of the cutting oxygen jet; formation

oxides, as these would

of a cylindrical geometry over a extensive length and protection against nitrogen of the surrounding air

form

neither

produce

high-melting

difficult

cutting

surfaces. In accordance, only steel or titanium mate-

br-er12-08e.cdr

Function of the Flame During Flame Cutting

rials fulfill the conditions for oxygen

Figure 12.8

cutting.,

Figure

12.9

2005

12. Thermal Cutting

170

Steel materials with a C-content of up to approx. 0.45% may be flame-cut without preheating, with a C-content of approx. 1.6% flame-cutting is carried out with preheating, because an increased C-content demands more heat. Carbon accumulates at the cutting surface, so a very high degree of hardness is to be expected. Should the carbon content exceed 0.45% and should the material not have been subject to prior heat treatment, hardening cracks on the cutting surface are regarded as likely. Some alloying elements form high-melting oxides which impair the slag expulsion and influence the

The material has to fulfill the following requirements: - the ignition temperature has to be lower than the melting temperature - the melting temperature of the oxides has to be lower than the melting temperature of the material itself

thermal conductivity.

- the ignition temperature has to be permanently maintained; i. e. the sum of the supplied energy and heat losses due to

The iron-carbon equilibrium diagram illustrates the carbon

heat conduction has to result in a positive heat balance

br-er12-09e.cdr

content-temperature

Conditions of Flame Cutting

interrelation, Figure 12.10. As the carbon content increases,

the

Figure 12.9

melting steel

temperature is lowered.

liquid

temperature [°C]

That means: from a certain carbon content upwards, the ignition temperature is higher than the melting temperature,

i.e.,

cast iron

1500

this

1000

pasty

solid

Liquidus

rve n cu o i t i ign

Solidus solid

would be the first violation to the basic requirement in flame cutting.

2,0

carbon content [%]

br-er12-10e.cdr

Ignition Temperature in the Iron-Carbon-Equilibrium Diagram

Figure 12.10

2005

12. Thermal Cutting Steel

171

compositions

may

influence flame cuttability substantially - the individual alloying show

elements

reciprocate

may effects

(reinforcing/weakening), Figure 12.11. The content limits of the alloying constituents are therefore only

Maximum allowable contents of alloy-elements: carbon:

up to 1,6 %

silicon:

up to 2,5 % with max. 0,2 %C

manganese:

up to 13 % and 1,3 % C

chromium:

up to 1,5 %

tungsten:

up to 10 % and 5 % Cr, 0,2 % Ni, 0,8 % C

nickel:

up to 7,0 % and/or up to 35 % with min. 0,3 % C

copper:

up to 0,7 %

molybdenum: up to 0,8 %, with higher proportions of W, Cr and C not suitable for cutting

reference values for the br-er12-11e.cdr

evaluation of the flame cut-

Flame Cutting Suitability in Dependance of Alloy-Elements

tability of steels, as the cutting quality is substantially

Figure 12.11

deteriorating, as a rule already with lower alloy contents. square butt weld

single-V butt weld

By an arrangement of one

single-V butt weld with rootface

or several nozzles already during the cutting phase a weld preparation may be carried

out

and

certain

welding grooves be pro-

double-V butt weld

double-V butt weld with root face

br-er12-12e.cdr

duced. Figure 12.12 shows

Weld-Groove Preparation by Oxygen Cutting

torch arrangements for - the square butt weld,

Figure 12.12

- the single V butt weld, - the single V butt weld with root face, - the double V butt weld and - the double V butt weld with root face.

2005

12. Thermal Cutting

172

It has to be considered that, particularly in cases where flame cutting is applied for weld preparations, flame cutting-related defects may lead to increased weld dressing work. Slag adhesion or chains of molten globules have to be removed in order to guarantee process safety and part accuracy for the subsequent processes. Figure 12.13 gives a survey of possible defects in flame cutting. In order to improve the edge defect: edge rounding chain of fused globules edge overhang

cratering: sporadic craterings connected craterings cratering areas adherent slag: slag adhearing to bottom cut edge

cut face defects: kerf constriction or extension angular deviation step at lower edge of the cut excessive depth of cutting grooves

flame-cutting

capacity

and/or cutting of materials which are normally not to be flame-cut the powder flame cutting process may

cracks: face cracks cracks below the cut face

be applied. Here, in addition to the cutting oxygen, iron powder is

br-er12-13e.cdr

Possible Flame Cutting Defects

blown into the cutting gap. In the flame, the iron powder oxidises very fast and

Figure 12.13

adds further energy to the process. Through the additional oxygen water seperator

compressed air

acetylene

energy

high-melting

input

the

oxides

of

the high-alloy materials are molten. Figure 12.14

powder dispenser

shows a diagrammatic representation

of

a

metal

powder cutting arrangement.

br-er12-14e.cdr

Metal Powder Flame Cutting

Figure 12.14

2005

12. Thermal Cutting

173

Figure 12.15 shows the principle of flame gouging

flame gouging

scarfing

and scarfing. Both methgas-heat oxygen mixture

ods are suited for the weld

gouging oxygen

preparation; material is removed but not cut. This

gas-heat oxygen mixture scarfing oxygen

way, root passes may be grooved out or fillets for welding may be produced later.

br-er12-15e.cdr

Flame Gouging and Scarfing

Figure 12.16 shows the methods of thermal cut-

Figure 12.15

ting processes by electrical gas discharge: -

plasma cutting with non-transferred arc

-

plasma cutting with transferred arc

-

plasma cutting with transferred arc and secondary gas flow

-

plasma cutting with transferred arc and water injection

-

arc air gouging (represented diagrammatically)

-

arc oxygen cutting (represented diagrammatically) Thermal cutting processes by electrical gas discharge:

plasma cutting

- with non-transferred arc - with transferred arc -with secondary gas flow -with water injection

arc air gouging

carbon electrode

compressed air

arc oxygen cutting

cutting oxygen

=

electrode coating tube arc

br-er2-16e.cdr

Thermal Cutting Processes by Electrical Gas Discharge

Figure 12.16

2005

12. Thermal Cutting

174 In plasma cutting the entire

electrode

plasma gas

-

cooling water

must

be

heated to the melting tempower source

HF R

workpiece

+

perature by the plasma jet. The

nozzle

forms

the

plasma jet only in a restricted way and limits thus

nozzle

the cutting ability of plate to a

thickness

of

approx.

Figure

12.17.

workpiece

150 mm,

br-er12-17e.cdr

Characteristic

Plasma Cutting

for

the

plasma cut are the coneFigure 12.17

shaped formation of the kerf

and

the

rounded

edges in the plasma jet entry zone which were caused by the hot gas shield that envelops the plasma jet. These process-specific disadvantages may be significantly reduced or limited to just one side of the plate (high quality or scrap side), respectively, by the inclination of the torch and/or water addition. With the plasma cutting process, all electrically conductive materials may be separated. Nonconductive materials, or similar materials, may be separated by the emerging plasma flame, but only with limited ability. In order to cool and to replasma gas

electrode

duce

the

emissions,

plasma torches may be surrounded by additional water curtain

gas cutting water swirl chamber

nozzle

cone of water

workpiece

water

curtains

which also serve as arc constriction, Figure 12.18. In

water bath

or

dry

plasma

cutting

where Ar/H2, N2, or air are used, harmful substances

br-er12-18e.cdr

Water Injection Plasma Cutting

always develop which not only have to be sucked off

Figure 12.18

very carefully but which

2005

12. Thermal Cutting

175

also must be disposed of. In water-induced plasma cutting (plasma arc cutting in water or under water) gases, dust, also the noise,

cutting with water bath

water injection plasma cutting with water curtain

plasma cutting with workpiece on water surface

underwater plasma cutting

and the UV radiation are, for the most part, held back by the water. A further, positive effect is the cooling of the cutting surface, Fig-

br-er12-19e.cdr

ure 12.18. Careful disposal

Types of Water Bath Plasma Cutting

of the residues is here inFigure 12.19

evitable. Figure 12.19 gives a survey of the different cutting meth-

plasma gas

electrode

ods using a water bath.

secondary gas

Figure 12.20 shows a torch

nozzle

which is equipped with an additional gas supply, the so-called secondary gas. The secondary gas shields the

plasma

jet

and

workpiece br-er12-20e.cdr

in-

Plasma Cutting With Secondary Gas Flow

creases the transition resistance at the nozzle front.

Figure 12.20

The so-called “double and/or parasite arcs” are avoided and nozzle life is increased.

2005

12. Thermal Cutting

176

Thanks to new electrode materials, compressed air may be applied as plasma gas – therefore, in flame cutting, the burning of unalloyed steel may be used for increased

capacity

cutting speed [m/min]

and even pure oxygen

1 6 2 4 2

and

forming

3 4 5

quality. The selection of the plasma

machine type and plasma medium 1 WIPC, 400 A, O2 2 WIPC, 400 A, N2 3 200 A, s < 8 mm: N2 s > 8 mm: Ar/H2 4 40 A, compressed air

8

10

15

20

plate thickness [mm]

gases br-er12-21e.cdr

depends on the require-

Cutting Speeds of Different Plasma Cutting Equipment for Steel Plates

ments of the cutting process. Plasma forming media

Figure 12.21

are argon, helium, hydrogen, nitrogen, air, oxygen or water. The advantage of the use of oxygen as plasma gas is in the achievable cutting speeds within the plate thickness range of approx. 3 – 12 mm (400 A, WIPC). In the steel plate thickness range of approx. 1 – 10 mm the application of 40 A-compressed air units is recommended. In comparison with 400 A WIPC systems, these allow vertical and significantly narrower cutting kerfs, but with lower cutting speeds. Figure 12.21 shows different

Thermal cutting processes by laser beam

cutting speeds for different units and plasma gases.

- laser beam combustion cutting

In

the

thermal

processes

with

cutting

- laser beam fusion cutting

beams

- laser beam sublimation cutting

only the laser is used as the jet generator for cutting,

br-er12-22e.cdr

Figure 12.22.

Thermal Cutting With Beams

Figure 12.22

2005

12. Thermal Cutting

177

Variations of the laser beam cutting process: -

laser beam combustion cutting, Figure 12.25

-

laser beam fusion cutting, Figure 12.26

-

laser beam sublimation cutting, Figure 12.27.

The process sequence in laser beam combustion cutting is comparable to oxygen cutting. The material is heated to the ignition temperature and subsequently burnt in the oxygen stream, Figure 12.23. Due to the concentrated energy input almost all metals in the plate thickness range of up to approx. 2 mm may be cut. In addition, it is possible to achieve very good bur-free cutting qualities for stainless steels lens

(thickness of up to approx. 8 mm) and for structural

cutting oxygen

steels (thickness of up to 12 mm). Very narrow and

laser focus thin layer of cristallised molten metal

workpiece

parallel cutting kerfs are characteristic

for

laser

beam cutting of structural slag jet

steels.

br-er12-23e.cdr

Laser Beam Cutting

In laser beam cutting, eiFigure 12.23

ther oxygen (additional energy contribution for oxidising materials) or an inactive 80

20

evaporating

40

melting

60

heating-up

absorption factor

cutting gas may be applied depending on the cutting job. Besides, the very high beam

powers

(pulsed/superpulsed mode

-laser) d:YAG 6 µm (N aser) λ = 1,0 O 2 (C -l ,06 µm 0 1 = λ

of operation) allow a direct

melting point Tm

boiling point Tb

temperature br-er12-24e.cdr

Qualitative Temperature Dependency on Absorption Ability

evaporation of the material (sublimation).

In

laser

beam combustion cutting and laser beam sublima-

Figure 12.24 2005

12. Thermal Cutting

178

tion cutting the reflexion of the laser beam of more than 90 % on the workpiece surface decreases unevenly when the process starts. In laser beam fusion cutting remains the reflexion on the molten material, however, at more than 90%! Figure 12.24 shows the absorption factor of the laser light in dependence on the temlaser cutting (with oxygen jet)

perature. This factor mainly depends

on

the

- the laser beam is focused on the workpiece surface and the material burns in the oxygen jet starting from the heated surface

wave

length of the used laser

materials: - steel aluminium alloys, titanium alloys

light. When the melting point of the material has

cutting gas: - O2, N2, Ar

been reached, the absorp-

criteria: - high cutting speed, cut faces with oxide skin

tion factor increases unevenly and reaches values

br-er12-25e.cdr

of more than 80%.

Characteristics of the Laser Beam Cutting Processes I

During laser beam combus-

Figure 12.25

tion cutting of structural steel high cutting speeds are achieved due to the exothermal energy input and the low laser beam powers, Figure 12.25. In the above-mentioned case (dependent on beam quality, focussing, laser fusion cutting: - the laser beam melts the entire plate thickness (optimum focus point 1/3 below plate surface) - high reflection losses (>90%) materials: - metals, glasses, polymers cutting gas: - N2, Ar, He criterions: - cutting speed is only 10-15% in comparison to cutting with oxygen jet, characteristics melting drag lines

etc.), above

a

beam power of approx. 3,3 kW,

spontaneous

evaporation of the material takes place and allows sublimation cutting. Significantly higher laser powers are necessary to fuse the material and blow it out with an inert gas, as the

br-er12-26e.cdr

Characteristics of the Laser Beam Cutting Processes II

reflexion loss remains constant.

Figure 12.26

2005

12. Thermal Cutting

179

Important influence quantities for the cutting speed and quality in laser beam cutting are the focus intensity, the position of the focus point in relation to the plate surface and the formation of the cutting gas flow. A prerequisite for a high intensity in the focus is the high beam quality (Gaussian intensity distribution in the beam) with a high beam power and suitable focussing optics. Laser beam cutting of contours, especially of pointed corners and narrow root faces, requires adaptation of the beam power in order to avoid heat accumulation and burning of the material. In such a case the beam power might be reduced in the continuous wave

(CW)

operating

mode. With a decreasing beam efficiency decreases the cuttable plate thickness as well. Better suited is the switching of the laser to pulse

mode

equipment

of

(standard

laser evaporation cutting:

- spontaneous evaporation of the material starting from 105 W/cm2 with high absorption rate and deep-penetration effect - metallic vapour is pressed from the cavity by own vapour pressure and by a supporting gas flow materials: - metals, wood, paper, ceramic, polymer cutting gas: - N2, Ar, He (lens protection) criteria: - low cutting speed, smooth cut edges, minimum heat input br-er12-27e.cdr

HF-excited

Characteristics of the Laser Beam Cutting Processes III

lasers) where pulse height can be selected right up to

Figure 12.27

the height of the continuous wave.

A

super

pulse

equipment (increased excitation) allows significantly higher pulse efficiencies to be

selected

than

those

laser 600 W 1500 W 600 W 1500 W 1500 W

steel Cr-Ni-steel aluminium

plasma 50 A 5 kW 250 A 25 kW 500 A 150 kW

steel Cr-Ni-steel aluminium

achieved with CW. Further fields of application for the

Stahl Cr-NiStahl

oxy-flame

pulse and super pulse op-

1

eration mode are punching

10

100 plate thickness [mm]

1000

br-er12-28e.cdr

and laser beam sublimation Fields of Application of Cutting Processes

cutting. Figure 12.28

2005

12. Thermal Cutting

180

Laser beam cutting of aluminium plates thicker than appx. 2 mm does not produce bur-free results due to a high reflexion property, high heat and

temperature

differences

10

large cuttig speeds [m/min]

conductivity

CO2-laser (1500 W)

between Al and Al2O3. The addition of iron powder allows the flame cutting of stainless

steels

plasma cutting (WIPC, 300-600 A)

1 oxygen cutting (Vadura 1210-A)

(energy

0,1

input and improvement of

10

1

100

plate thickness [mm]

the molten-metal viscosity). br-er12-29e.cdr

The cutting quality, how-

Cutting Speeds of Thermal Cutting Processes

ever, does not meet high Figure 12.29

standards.

Figure 12.28 shows a comparison of the different plate thicknesses which were cut using different processes. For the plate thickness range of up to 12 mm (steel plate), laser beam cutting is the approved precision cutting process. Plasma cutting of plates > 3 mm allows higher cutting speeds, in comparison to laser beam cutting, the cutting quality, however, is significantly lower. Flame cutting is used for cutting plates > 3 mm, the cutting speeds are, in comparison to plasma cutting, significantly lower. With an increasing plate thickness the difference costs [DM/m cut length]

machine costs

5

the

cutting

speed is reduced. Plates

total costs

6

in

with a thickness of more than 40 mm may be cut

4 laser

even faster using the flame

3

flame cutting with 3 torches

plasma

2

cutting process.

1

Figure 12.29 shows the 5

10

15

20

25

plate thickness [mm]

30

35

40

cutting speeds of some thermal cutting processes.

br-er12-30e.cdr

Thermal Cutting Costs - Steal

Figure 12.30

2005

12. Thermal Cutting

181

Apart from technological aspects, financial considerations as well determine the application of a certain cutting method. Figures 12.30 and 12.31 show a comparison of the costs of flame cutting, plasma arc

extract from a costing acc. to VDI 3258

and laser beam cutting – the costs per m/cutting

flame cutting (6-8 torches)

plasma cutting (plasma 300A)

laser beam cutting (laser 1500W)

170,000.00

220,000.00

500,000.00

investment total (replacement value)

€

operating hour. The high

calculation for a 6-yearaccounting depreciation

€/h

23.50

29.00

65.00

investment costs for a laser

maintenance costs

€/h

3.50

4.00

10.00

beam

energy costs

€/h

1.00

2.50

2.50

production cost unit rate costs/1 operating hour

€/h

65.00

75.00

130.00

length and the costs per

cutting

equipment

might be a deterrent to exploit the high cutting qualities obtainable with this

1 shift, 1600h/year, 80% availability, utilisation time 1280h/year br-er12-31e.cdr

process.

Cost Comparison of Cutting Processes

Figure 12.31

2005

13. Special Processes

13. Special Processes

183

Apart from the welding processes explained earlier there is also a multitude of special welding processes. One of them is stud welding. Figure 13.1 depicts different stud shapes. Depending on the application, the studs are equipped with either internal or external screw threads; also studs with pointed tips or with corrugated shanks are used. In arc stud welding, a distinction is basically made between

three

variations.

process

Figure 13.2.

depicts the three variations – the differences lie in the kind of arc ignition and in the cycle of motions during rammed flange

the welding process.

br-er13-01e.cdr

The switching arrangement of an arc stud weldFigure 13.1

ing unit is shown in Figure 13.3. Besides a power

drawn-arc stud welding

capacitordischarge stud welding with tip ignition

drawn-arc stud welding with ferrule ignition

source

which

produces

high currents for a shorttime, a control as well as a lifting device are necessary.

ceramic ferrule

cold-upset tip ignition

ignition ring

br-er13-02e.cdr

Figure 13.2

2005

13. Special Processes

184

In drawn-arc stud welding the stud is first mounted onto the plate, Figure 13.4. The arc is ignited by lifting the stud and melts the entire stud diameter in a short time. When stud and base plate are fused, the stud is dipped into the molten weld pool while the ceramic ferrule is forming the weld. After the solidification of the liquid weld pool the ceramic ferrule is knocked off. Figure 13.5 illustrates tip ignition

stud

welding.

lifting device

The tip melts away imme-

control device

diately after touching the plate and allows the arc to

stud holding device

welding time adjustment

be ignited. The lifting of the

stud

stud is dispensed with. When the stud base is molpower source

ten, the stud is positioned onto

the

partly

ceramic ferrule

V

workpiece

A

molten br-er13-03e.cdr

workpiece. Studs with diameters of up

Figure 13.3

to 22 mm can be used. Welding currents of more

start

3

lifting L

dipping > (L + P)

4

L

projection P

2

L

end

current

materials, see Figure 13.6. Problematic are the differ-

1

time P

ess allows to join different

0

P

The arc stud welding proc-

stud movement

than 1000 A are necessary.

ent melting points and the heat dissipation of the individual materials. Aluminium

time br-er13-04e.cdr

studs, for example, may not be welded onto steel. Figure 13.4

2005

13. Special Processes

185

The relatively high welding currents in the arc stud welding process cause the somewhat troublesome side-effects of the arc blow. Figure 13.7 depicts different arrangements of current contact points and cable runs and illustrates the developing arc deflection (B,C,E). A, D and F show possible countermeasures. a

b

c

d

In high-frequency welding of pipes the energy input into the workpiece may be carried out via sliding contacts, as shown in Figure 13.8, or via rollers, as shown in Figure 13.9. Only the high-frequency technique allows a safe current transfer in spite of the scale or oxide layers. Through the skin effect

br-er13-05e.cdr

© ISF 2002

Phases of Capacitor-Discharge Stud Welding With Tip Ignition

the current flows only conditionally at the surface. Therefore no thorough fusion of thickFigure 13.5

wall pipes may be achieved.

unalloyed sructural steel S235J0 and/or comparable steels

other unalloyed steels

stainless steels acc. DIN EN 17440

heat resisting steels acc. SEW 470

aluminium and aluminium alloys

unalloyed structural steel S235J0, S355J0 and/or comparable steels (acc. DIN EN 10 025)

1

2

3

2

0

other unalloyed steels

2

2

3

2

0

stainless steels acc. DIN EN 17440

3

3

1

3

0

heat resisting steels acc. SEW 470

2

2

2

2

0

aluminium and aluminium alloys

0

0

0

0

2

stud material base meatl

explanation of the weldability classification numbers: 1 = well suitable (transmission of energy) 2 = suitable (transmission of energy possible with restriction)

3 = suitable only up to a point (not for transmission of energy 0 = not possible

br-er13-06e.cdr

Figure 13.6

2005

13. Special Processes

186 Only welding of small wall thicknesses is profitable – as the weld speed must be greatly reduced with in-

A

creasing wall thicknesses,

B

Figure 13.10.

C

D

br-er13-07e.cdr

Figure 13.7

rotary transformer

moving direction of the pipe

~

pressure rollers

Isolation copper electrode wheel (water-cooled)

sliding contacts (fixed)

slot pipe

∼ interstage transformer

pressure rollers

HF-valve generator

br-er13-08e.cdr

counterpressure rollers © ISF 2002

br-er13-09e.cdr

Rotary Transformer Resistance Welding

High-Frequency Welding of Pipes

Figure 13.8

© ISF 2002

Figure 13.9

2005

13. Special Processes

187 In induction welding – a process which is

80 4

5

used frequently nowadays – the energy input

6

is received contactless, Figure 13.11. Varying

3

m/min

magnetic fields produce eddy currents inside

welding speed

2

the workpiece, which again cause resistance heating in the slotted tube. A distinction is

40

made between coil inductors (left) and line 1

inductors (right).

20

Also in case of induction welding flows the 0 0

2

4

6

8 10 wall thickness

12

mm

16

current flows only close to the surface areas of the pipe. Only the current part which

1: 36 kA; 100 kVA; 60 Hz 2: 57 kA; 200 kVA; 60 Hz 3: 75 kA; 300 kVA; 60 Hz 4: 125 kA; 500 kVA; 60 Hz 5: 150 kA; 1200 kVA; 120 Hz 6: 200 kA; 1850 kVA; 120 Hz

reaches the joining zone and causes to fill the gap may be utilised. Figure 13.12 illustrates

Br-er13-10e.cdr

© ISF 2002

Welding Speeds in HF-Resistance Welding

two current paths. On the left side: the useful current path, on the right side: the useless current path which does not contribute to the

Figure 13.10

Figure 13.13

fusion of the edges. shows

the moving direction of the pipe

effective depth during the

moving direction of the pipe

inductive heating for different materials, in dependence on the frequency. As pressure rollers

soon as the Curie tempera-

pressure rollers

ture point is reached, the effective depth for ferritic coil inductor

steels increases.

line inductor

br-er13-11e.cdr

Figure 13.11

2005

13. Special Processes

188 The application of the induction

l

δ2

welding

method

of more than 100m/min,

b

b

allows high welding speeds

δ1

Figure 13.14.

δ1

s

d

δ2

b width of the heating inductor s wall thickness of the pipe δ1 current penetration depth on pipe backside

d l

current penetration depth at the strip edges outside diameter of the pipe distance inductor- welding point

br-er13-12e.cdr

Figure 13.12

160

100

corrective factor

effective depth δ

20 mm 10 8 6 4

1

m/min 2 3

2 1,0 0,8 0,6 0,4

4

5

120

%

0

6

0

7

50 100 mm 200 pipe diameter

100 high frequency 200 - 450 kW

0,2

weld speed

0,10 0,08 0,06 0,04 0,02 1

4

1 2 3 4 5 6 br-er13-13e.cdr

7

10

frequency f

steel (ferritic steel (austenitic) brass aluminium copper brass copper aluminium steel (ferritic)

100

200

kHz 1000

800°C 20....1400°C 800°C 600°C 850°C 20°C 20°C 20°C 20°C

80

60

600 kW

40

450 kW 300 kW

20 200 kW 60 kW 100 kW 150 kW

0 0 © ISF 2002

2

4

8 10 12 14 wall thickness

br-er13-14e.cdr

Standard Values of the Effective Depths During Inductive Heating

Figure 13.13

6

16

mm 20 © ISF 2002

Welding Speeds in Induction Welding

Figure 13.14

2005

13. Special Processes

189 Aluminothermic

fusion

welding or cast welding is mainly used for joining

3FeO

+ 2Al

Al2O3 + 3Fe - 783 kJ

railway tracks on site. A crucible is filled with a mix-

Fe2O3

+ 2Al

ture consisting of alumin-

Al2O3 + 2Fe - 758 kJ

ium powder and iron ox-

3Fe3O4 + 8Al

ide. An exothermal reac-

4Al2O3 + 9Fe - 3012 kJ

tion is initiated by an igniter – the aluminium oxidises

br-er13-15e.cdr

and the iron oxide is reduced

to

iron,

Fig-

ure 13.15. The molten iron

Figure 13.15

flows into a ceramic mould which matches the contour of the track. After the melt has cooled, the mould is knocked off. Figure 13.16 shows the process assembly. Explosion welding or explosion

cladding

is

fre-

quently used for joining

mould

runner gate riser

slag mould

preheating gas fuel

workpiece

air

riser c b

dissimilar materials, as, for

example,

steel/alloyed

cut A-B

unalloyed steel,

cop-

thermit steel

steel/aluminium. The materials which are to be joined are pressed together by a

A

riser thermit bulge weld cross-section

b

or

blow-hole orifice

thermit slag thermit crucible slag mould

per/aluminium

workpiece

thickness of the cast b

channel between riser and runner gate runner gate blow-hole orifice iron or sand plug

B

foundry sand

riser runner gate workpiece cast-around bulge

br-er13-16e.cdr

shock wave. Wavy transitions develop in the joining area,

Figures 13.17

and

Figure 13.16

13.18.

2005

13. Special Processes

190

The determined cladding adhered

to

during

a)

explosive charge buffer flyer plate

igniter

the

parent plate

anvil

vd t

welding speed is too low,

vd

vP B

α A' A

vF

K

If the welding speed is

A'

β

B'

K

vK

t

β B vP B'

vF

A

vK

B 90 - β + α /2

exceeded, the develop-

v

ment of the waves in the joining zone is erratic.

explosive charge buffer flyer plate

igniter

parent plate

anvil Amboß

welding process. If the lack of fusion is the result.

b) d

speed must be strictly

F

vP

β K

vK

vF 90 - α /2 B'

K

β vK = vD

B vP B'

br-er13-17e.cdr

Figure 13.19 shows the critical cladding speeds for different material com-

Figure 13.17

binations. Figure 13.20 shows a diagrammatic representation of a diffusion welding unit. Diffusion welding, like ultrasonic welding, is welding in the solid state. The surfaces which are to be joined are cleaned, polished and then joined in a vacuum with pressure and temperature. After a certain time (minutes, right up to several days) joining is achieved by diffusion processes. The advantage of this costly welding method lies in the possibility of joining dissimilar materials without taking the risk of structural transformation due to the heat input. Figure 13.21 shows several possible material combinations. The joining of two extremely different materials, as, e.g. austenite and a zirconium alloy, may be obtained by several interBr-er13-18e.cdr

mediate layers. Figure 13.18 2005

13. Special Processes

191

measuring amplifier

-1

critical speed [m s ]

materials flyer plate/ parent plate

working pressure 1,33 mPa

vk1

vk2

vk3

600

1000

>4000

copper/ copper

1200

1600

>3600

steel/ steel

2100

2700

>3900

copper/ aluminium

1000

1400

aluminium/ steel

1200

1600

cooper/ steel

1400

2400

aluminium/ zinc

500

1000

3000

copper/ zinc

800

1400

3300

hydraulic aggregate unit

P

aluminium/ aluminium

workpieces HFgenerator

pumping station recorder p,T = f(t)

br-er13-19e.cdr

loading device

© ISF 2002

br-er13-20e.cdr

Schematic Representation of a Diffusion Welding Unit

Critical Cladding Speeds in Explosive Cladding

tantalum

niobium

zirconium

Figure 13.22 shows the structure of a joint tungsten

molybdenum

titanium

nickel

copper

aluminium

stainless steel

tool steel

Figure 13.20 structural steel

cast iron

Figure 13.19

material

© ISF 2002

tantalum

where nickel, copper and vanadium had been used as intermediate layers. As the diffusion of the individual components takes place only

niobium zirconium

in the region close to the surface, very thin

tungsten molybdenum

layers may be realised.

titanium nickel copper aluminium stainless steel tool steel very good weld quality

structural steel cast iron

good weld quality bad weld quality not tested/ results not reported

br-er13-21e.cdr

© ISF 2002

Possible Material Combinations for Diffusion Welding

Figure 13.21 2005

13. Special Processes

192

In cold pressure welding in contrast to diffusion welding - a deformation is produced by the high contact pressure in the bonding

X10CrNiTi18 9

Ni

Cu

V

Zr2Sn

plane, Figure 13.23. The joint surfaces are moved very close towards each other, i.e., to the atomic distance. Through transpobr-er13-22e.cdr

sition processes as well as through

adhesion

forces

can joining of similar and

Figure 13.22

dissimilar materials be realised. Ultrasonic welding is used as a microwelding method. The process principle is shown in Figure 13.24. The surface layers of overlap arranged plates are destroyed by applying mechanical vibrator energy. At this instance are joining surfaces deformed by very short localised warming up and point-interspersed connected. The joining members are welded under pressure, where one part small amplitudes (up to 50 µm) relative to the other is moved with with ultrasonic frequency. dies

d1

As far as metals are concerned, the vibratory vector is in the joining zone, in

specimen A

contrast to ultrasonic weld-

guide and buffer

ing of plastics. The ultra-

specimen B d2

sonics which have been produced

by

a

magne-

tostrictive transducer and br-er13-23e.cdr

transmitted by a sonotrode lie in the frequency range of 20 up to 60 Hz.

Figure 13.23

2005

13. Special Processes

193

Figure 13.25 shows possible material combinations for ultrasonic welding. Further microwelding processes are methods which HFgenerator

are also called heated ele-

process observation optics

ment welding methods, as,

pressure force

for

sonotrode

example,

nailhead

bonding and wedge bond-

sonotrode tip

ing. These methods are

workpiece

applied in the electronics anvil

industry for joining very fine

ultrasonic vibrator

wires, as, for example, gold br-er13-24e.cdr

wires from microchips with aluminium strip conductors. Figure 13.24 In wedge bonding a wire is positioned onto the contact point via a feeding nozzle. The welding wedge is lowered and the wire is welded with the aluminium thin foil, Figure 13.26. The wire is cut with a cutting aluminium+alloy beryllium+alloy copper, Cu-Zn-alloy germanium gold iron magnesium+alloy molybdenum+alloy nickel+alloy palladium+alloy platin+alloy silicon silver+alloy tantalium+alloy tin titanium+alloy tungsten+alloy zirconium+alloy

tool. In nailhead bonding, the wire which emerges from the feeding nozzle may have diameters from 12 to 100 µm. By a reducing hydrogen flame its end is molten to a globule, Figure 13.27. The nozzle then presses this globule onto the part aimed at and shapes it into a nail head. Figure 13.28 depicts this type of weld.

aluminium+alloy beryllium+alloy copper, Cu-Zn-alloy germanium gold iron magnesium+alloy molybdenum+alloy nickel+alloy palladium+alloy platin+alloy silicon silver+alloy tantalium+alloy tin titanium+alloy tungsten+alloy zirconium+alloy

A further method related to welding is soldering. The process principle of soldering is briefly explained in Figure 13.29.

br-er13-25e.cdr

© ISF 2002

Possible Material Combinations for Ultrasonic Welding

Figure 13.25

2005

13. Special Processes

194

heated wedge (tungsten-carbide)

5-50 µm gold wire

wedge bonding

Al-strip conductor

cutting tool

br-er13-26e.cdr

Figure 13.26 heated wedge (tungsten-carbide)

H2-flame

5-50 mm gold wire

wedge bonding

Al-strip conductor

nailhead

br-er13-27e.cdr

Figure 13.27

br-er13-28e.cdr

Figure 13.28 2005

13. Special Processes

195

The individual soldering methods are classified into different mechanisms depending on the type of heating, Figure 13.30. There are two basic distinctions: soft soldering (melting temperature of the solder is approx. up to 450°C) and brazing (melting temperature of the brazing solder is approx. up to 1100°C. For high-temperature soldering solders with high melting points (melting temperature is approx. up to 1200°C) are used. This process is frequently subject to automation.

In soldering, atomar forces of attraction are effective. Similar and dissimilar metals are joined by addition of a solder with a low melting point. In the boundary area

classification according to the type of heating:

transposition processes occur between solder and base metal. This is called a “two-dimensional”diffusion. In the subsequent diffusion glowing phase

- flame brazing

(high-temperature soldering) the solder may be

- iron soldering

completely absorbed by the base metal.

- block brazing A distinction is made between soft soldering (melting

- furnace soldering

temperature of the solder is below 450°C) and brazing

- salt bath brazing

(melting temperature of the solder is 450°C up to 1100°C) as well as high-temperature soldering (melting

- dip soldering

temperature of the solder is up to 1200°C). Heating of

- wave soldering

the component for melting the solder may be effected in

- resistance soldering

various ways.

- induction brazing br-er13-29e.cdr

© ISF 2002

br-er13-30e.cdr

Classification of Soldering Methods

Soldering - Definition and Process Principle

Figure 13.29

© ISF 2002

Figure 13.30

2005

14. Mechanisation and Welding Fixture

14. Mechanisation and Welding Fixtures

197

As the production costs of the metal-working industry are nowadays mainly determined by the costs of labour, many factories are compelled to

Designation

rationalise their manufac-

manual welding

turing methods by partially

m

and fully mechanised pro-

partially mechanised welding t

duction processes. In the field of welding engineering

examples gas-shielded arc welding TIG GMAW

fully mechanised welding

movement/ working cycles torch-/ workpiece control

filler wire feeding

workpiece handling

manually

manually

manually

manually

mechanically

manually

mechanically mechanically

manually

v

where a consistently good

automatic welding

quality with a maximum

mechanically mechanically mechanically

a

productivity

is

a

automation

aspects

must,

br-er14-01e.cdr

are

conse-quently taken into account.

Figure 14.1

The levels of mechanisation in welding are stipulated in DIN 1910, part 1. Distinctions are made with regard to the type of torch control and to filler addition and to the type of process sequence, as, e.g., the transport of parts to the welding point. Figure 14.1 explains the four levels of mechanisation. Figure 14.2. shows manual welding, in this case: manual electrode welding. The control of the electrode and/or the arc is carried out manually. The filler metal (the consumable electrode) is also fed manually to the welding point. br-er14-02e.cdr

© ISF 2002

Manual Welding (Manual Electrode Welding)

Figure 14.2

2005

14. Mechanisation and Welding Fixtures

198 In partially mechanised welding, e.g. gas-shielded metal-arc welding, the arc manipulation is carried out manually, the filler metal addition, however, is executed

mechanically

by

means of a wire feed motor, Figure 14.3. br-er14-03e.cdr

In fully mechanised weld-

Partially Mechanised Welding (Gas-Shielded Metal-Arc Welding)

ing, Figure 14.4, an auto-

Figure 14.3

matic

equipment

mechanism carries out the welding advance and thus the torch control. Wire feeding is realised by means of wire feed units. The workpieces must be positioned manually in accordance with the direction of the moving machine support. In automatic welding, besides

the

process

se-

quences described above, the work-pieces are mechanically positioned at the welding point and, after matically

welding,

auto-

trans-ported

to

the next working station. Figure14. 5 shows an exbr-er14-04e.cdr

ample of automatic welding

Fully Mechanised Welding (Gas-Shielded Metal-Arc Welding)

(assembly line in the car industry).

Figure 14.4

2005

14. Mechanisation and Welding Fixtures

199 Apart from the actual welding device, that is, the welding power source, the filler metal feeding unit and the simple torch control units, there is a variety of auxiliary devices available which facilitate or make the welding process at all possible. Figure 14.6 shows a

br-er14-05e.cdr

Automatic Welding (Assembly Line)

survey of the most important assisting devices.

Figure 14.5 Before welding, the parts are normally aligned and assembly line

then

welding robot

tack-welded.

Fig-

ure 14.7 depicts a simple

machine carrier

tack-welding jig for pipe

linear travelling mechanism

clamping. The lower part of

track-mounted welding robots spindle / sliding head turntable

the device has the shape

turn-/ tilt table

of a prism. This allows to

dollies

clamp pipes with different

assembly devices

diameters. br-er14-06e.cdr

Devices, however, may be Figure 14.6

significantly more complex.

Figure 14.8 shows an example of an assembly equipment used in car body manufacturing. This type of device allows to fix complex parts at several points. Thus a defined position of any weld seam is reproducible.

2005

14. Mechanisation and Welding Fixtures

200

In apparatus engineering and tank construction it is often necessary to rotate the

components,

e.g.,

when welding circumferential seams. The equipment should be as versatile as possible and suit several tank diameters. Figure 14.9 shows three types of turnbr-er14-07e.cdr

ing rolls which fulfil the

Simple Tack Welding Jig for Welding Circumferential Welds

demands. Figure bottom: the rollers are adjustable;

Figure 14.7

Figure middle: the rollers automatically adapt to the tank diameter; Figure top: the roller spacing may be

1 portal with 2 industrial robots IR 400, equipped with tool change system 2 resting transformer welding tongs 3 depot of welding tongs 4 clamping tool 5 copper back-up bar for car roof welding 6 transformer welding tongs for car roof welding 7 driverless transport system 8 component support frame 9 swivelled support for component support frames 10 resting transformer welding tongs for car boot

varied by a scissor-like arrangement. In general, dollies are motor-driven.

This provides

also an effortless movebr-er14-08e.cdr

ment

of

heavy

compo-

nents, Figure 14.10. Figure 14.8 A work piece positioner, e.g. a turn-tilt-table, is part of the standard equipment of a robot working station. Figure 14.11 shows a diagrammatic representation of a turn-tilt-table. Rotations around the tilting axis of approx. 135° are possible while the turn-table can be turned by 365°. Those types of turn-tables are designed for working parts with weights of just a few kilograms right up to several hundred tons.

2005

14. Mechanisation and Welding Fixtures

201

set of rollers 2

set of rollers 1 br-er14-09e.cdr

br-er14-10e.cdr

Turning Rolls

Turning Rolls

Figure 14.9

Figure 14.10 A turn-tilt-table with hydrautable top

rotational axis

gear segment

lic adjustment of the tilting

table support tilting axis

and vertical motion as well

support

as chucking grooves for the part fixture is depicted in Figure 14.12.

br-er14-11e.cdr

Figure 14.11

2005

14. Mechanisation and Welding Fixtures

202

In robot technology the types of turn-tilt-tables - as shown in Figure 14.13 - are gaining importance. Positioners with orbital design

have

advantage

a

decisive

because

the

component, when turning around the tilting axis, remains approx. equally disbr-er14-12e.cdr

tant to the welding robot. Other types of workpiece

Turn-Tilt-Table With Hydraulic Adjustment

Figure 14.12

positioners are shown in Figure 14.14 – the double

single-column turn-tilt-table

column turn-tilt-table and

table top

turn-tilt-table.

table top

tilting axis support

the spindle and sliding holder

table support

orbital turn-tilt-table table support tilting axis support rotational axis

rotational axis

Those types of positioners are used for special component

geometries

and

allow welding of any seam in the flat and in the horizontal position. In the field of welding, spe-

© ISF 2002

br-er14-13e.cdr

Turn-Tilt-Tables

Figure 14.13

cial units are designed for special tasks. Figure 14.16 shows a pipe-flange-welding machine. This machine allows the welding of flanges to a pipe. The weld head has to be guided to follow the seam contour.

2005

14. Mechanisation and Welding Fixtures tilting axis

203

rotational axis table top table support

support

© ISF 2002

br-er14-14e.cdr

Double-Column Turn-Tilt-Table

Figure 14.14 table tops

spindle holder sliding holder

bed way

© ISF 2002

br-er14-15e.cdr

Spindle / Sliding Holder Turntable

Figure 14.15

br-er14-16e.cdr

Figure 14.16 2005

14. Mechanisation and Welding Fixtures

204 Plain plates or rounded tanks

are

clamped

by

means of longitudinal jigs for the welding of a longitudinal seam, Figure 14.17. The design and the gripping power are very dependent of the thickness of the plates to be welded. br-er14-17e.cdr

A simple example of a special welding machine is Figure 14.17

the tractor travelling carriage for submerged-arc

welding, Figure 14.18. This device is designed for the application on-site and provides, besides the supply of the filler metal, also the welding speed as well as the feeding and suction of the welding flux. For the guidance of a welding head and/or welding device, machine supports may be used. Figure 14.19 shows different types of machine supports for welding and cutting. Apart from the translatory and rotary principal axes they are often also equipped with additional axes to allow precise positioning.

br-er14-18e.cdr

Tractor for Submerged-Arc Welding

Figure 14.18

2005

14. Mechanisation and Welding Fixtures To

increase

levels

205

of

mechanisation of welding

c

b

a boom

processes robots are fre-

main piloting system case

pillar

quently applied. Robots are

cross piloting system case

travelling mechanism

handling devices which are equipped with more than three

user-programmable

axes.

Figure 14.20

e

d

auxiliary piloting system case

de-

auxiliary piloting system case

scribes kinematic chains which can be realised by br-er14-19e.cdr

different

combinations

of

translatory and rotary axes. Figure 14.19 The most common design of a track-mounted welding robot is shown in Figure 14.21.

The

designation

cartesian robot

cylinder coordinated robot

spherical coordinated robot

horizontal knuckle arm robot

vertical knuckle arm robot

robot

depicted here is a hinged-

arrangement

arm robot with six axes. The axes are divided into three

A

R

x

kinematic schedule

z y

z C

R B C

D

B

C z

C

principal and three additional axes or hand axes. The wire feed unit and the spool carriers for the wire

operating space

Kinematic Chains

electrodes are often fixed on the robot. This allows a

© ISF 2002

br-er14-20e.cdr

Figure 14.20

compact welding design.

2005

14. Mechanisation and Welding Fixtures

206

Varying lever lengths permit the design of robots

with different operating ranges. Fig-

ure 14.22 shows the operating range of a robot. In the unrestricted operating range the component may be reached with the torch in any position. The restricted operating range allows the torch to reach the component only certain positions. In the case of a suspended arrangement the robot fixing device is shortened thus allowing a compact design. For the completion of a robot welding station workpiece positioners are necessary. Figure 14.23 shows positioner devices where also several axes may be combined. These axes may either turn to certain defined positions or be guided by the robot control and moved synchronically with the internal axes. The complexity and versatility of the axis posi-

br-er14-21e.cdr

tions increases with the number of axes which

Robot Motions

participate in the movement. Figure 14.21

br-er14-22e.cdr

Figure 14.22

2005

14. Mechanisation and Welding Fixtures

207 Movement by means of a linear travelling mechanism increases the operating range of the robot, Figure 14.24. This may be done in ease of stationary as well as suspended arrangement, where there is a possibility to move to fixed end positions or to

br-er14-23e.cdr

stay in a synchronised motion with the other moveFigure 14.23

ment axes.

br-er14-24e.cdr

Figure 14.24

2005

15. Welding Robots

15. Welding Robots

209

Increased quality requirements for products and the trend to automate production processes along with increased profitability result in the use of industrial robots in modern manufacturing, Figures 15.1 – 15.2. Since robots have been introduced in industry in the 70s, their most frequently fields of application ranged from installation jobs up to spot welding, and seam welding. The definition says that an 8000

industrial

Europa Europe

7000

Amerika America

6000

for

gas

welding is an universal

Japan Japan

5000

robot

4000

movement automaton with

3000

more

2000

which

three are

axes user-

2002

2000

1998

programmable and may be 1996

1994

1992

1990

1000 0

than

sensor-controlled.

It

is

equipped with a welding

br-er15-01e.cdr

torch and carries out weld-

Inernational Distribution of Installed Welding Robots (1990 -2002)

ing jobs. Figure 15.1 Core of a modern robot welding cell are one or more seam welding robots of swan neck type. Normally, they have six user-programmable axes; so they can access any point within the working range at any orientation of the welding torch. To extend their working range, robots may be installed in overassembly 10.229

others 1.562

machining 1.767 seam welding 8.749

measurement 1.251 commissioning

head position. A further

and palletising 3.234 pressing and forging 2.064 diecasting and injection moulding 4.681

metal cutting machine tools 6022

spot welding 12.349

br-er15-02e.cdr

applying bonding and sealing agents 1.485

surfacing 2.337

research and training 2.656

other workpiece manipulators 8.214

extension of the working range can be achieved by installation

of

the robot

onto a linear carriage with Cartesian axes. Such 'external' axes are also userprogrammable,

Figure

15.3.

Figure 15.2 2005

15. Welding Robots

210

To turn the workpiece in the welding-favourable downhand position and to ensure accessibility to any joints, workpiece positioners are used as external axes which are steered by the robot control. Multi-station cycle tables are often used to increase profitability of the complete system installation. The operator feeds and removes the welded workpiece

on

one

side,

while the robot is welding on the other side. br-er15-03e.cdr

The robot control is the

Examples for Robot Arrangements

centre of an industrial robot system

for

arc

welding,

Figure 15.3

Figure 15.4. It provides and processes all information for robot mechanics, positioner, welding unit, safety equipment, and external sensors. The robot program transforms information into signals for control of robot- and positioner-mechanics as well as welding power source. Communication with external systems is possible by a host or master computer. Modern industrial robot controls are build as multi-processor controls due to the multitude of parallel calculations and control functions. Figure 15.5 shows the internal structure of such a control. Individual assemblies which are designed

robot mechanics power source safety device

welding installation

link to SPS hostcomputer

special

jobs

and

equipped with an own miwelding gun

industrial robot control

for

cro-processor are linked with the host computer via the system bus. The host

positioner

offline CAD expert

tools sensors

controls and coordinates the actions of the compo-

MDR

nents based on the operat-

br-er15-04e.cdr

Industrial Robot System for Arc Welding

ing system and the robot program.

Examples

of

Figure 15.4 2005

15. Welding Robots

211

such assemblies, which are mostly installed on individual printed boards, are e.g. the axes computers. They are responsible for calculation of movement and for control of power units of the individual axes. To control the drive motors, two interconnected control loops per axis are available which control speed and position of each axis. Further assemblies control the display screen, the manual programming unit (PHG); these assemblies are responsible for communication with the welding power source, external sensors, and peripheral units via digital and analogue in- and outputs and field bus systems. Or they complete the data transmission with external control systems. To reduce downtimes in the case of malfunction, some robot controls can telediagnosis

sys-

tems of the robot manu-

expert system offline programm

printer

telediagnosis

brake motor

bulk memory

sensors

chaining

keyboard

welding unit

host Computer

screen

tools

welding unit

welding unit

positioner

sensors

facturer to support service

speed control loop

with

encoder tacho

MDR PHG

position control loop

be connected via internet

personnel during troubleshooting and commission-

Internet

memory assembly

master programm computer interface

fieldbus

analog I/0

digital I/0

compiler computer

axsescomputer

ing. br-er15-05e.cdr

Programming of welding

Industrial Robot Control

robots can be carried out in different ways which are

Figure 15.5

distinguished in On-Line

+

B

+

+

- Z

+

- Y

A

C

-

+

+

- X

-

bot) and Off-Line (pro-

play-back programming direct programming (online)

-

(programming at the ro-

NOT-AUS

sensor supported programming

gramming out of the robot

movement oriented

textual programming, teach-in points

mixed proced. (online/ offline) + +

A B

+

+

+

Z

+

Y

-

C

-

X

-

-

-

-

cell), Figure 15.6.

teach-in programming

NOT-AUS

+

macropro-gramming, teach-in points chains

function oriented

The robot is manually textual programming with point coordinates

guided along the later track

with

decoupled

indirect programming (off-line)

knowledge-based programming with expert systems

drives during Play-Back programming. The path of

function oriented

br-er15-06e.cdr

Programming Procedures for Welding Robots

the track is recorded and transformed into a corre-

movement oriented

graphical programming with CAD data

Figure 15.6 2005

15. Welding Robots

212

sponding robot control pro-

TEA 1PTP

gram. This procedure is

8 2 3

4

$1

preferably used for painting

7

$2

jobs.

5$ 16

point file point no.

CP/ OV/ AUSG X Y Z ALPHA BETA GAMMA PTP SPD 1 2 3 4 EXT1 EXT2 EXT3 EXT4 EXT5 EXT6

0 LIST1=(20,0,0,50,60,75,15,12,0,0) definition of

1

PTP 100 0000 10560 1 17204

0

1317

0

1 LIST2=(30,0,0,55,70,0,0,0,0,0)

2

PTP 100 0000 10700 1128 15164

0

1344

0

2 MAIN

3

PTP 100 1100 10700 1513 14220

0

1344

0

3 $(1)

4

PTP 100 1100 10700 2420 14229

0

1344

0

4 GP(1-3)

5

PTP 100 1100 10700 3190 13294

0

1344

0

5 GC (4,$2,5,$1,6)

6

PTP 100 1100 10700 3852 14448

0

1344

0

7

PTP 100 0000 10700 4510 15520

0

1344

0

6 GP(7,8,1)

8

PTP 100 0000 10700 4510 15520

0

1344

0

7 END

welding parameters

A common technique to program a robot is the

selection of welding parameter set1 move to point 1-3 in PTP operation welding with change welding parameters

Teach-In procedure. During Teach-In

programming,

with the help of the manual

br-er15-07e.cdr

programming

unit,

the

welding torch is moved to Figure 15.7

notable

points

of

the

groove to be welded which are stored with information about position and orientation. In addition, track parameters must be entered, like e.g. type of movement and speed or welding parameter sets. During sensor supported Teach-In programming, the path progress through some typical points is only roughly indicated. Then the accurate path is picked-up by sensors and automatically calculated in the robot steering control. Afterwards the movement program is supplemented by additional information about e.g. welding parameter sets. Textual programming begeometric macro

welding macro

longs to mixed procedures. The sequence program in

Length macro

form of a text file is created = TCP = torch angel - welding parameters

on an external computer and is then transmitted to the robot steering control,

Profile macro = TCP - torch angel - welding parameters - welding programme (ignition, welding, crater filling)

Figure 15.7. The recording of the position of points is carried out in the same way

br-er15-08e.cdr

as with Teach-In programming: moving into position Figure 15.8

and recording. 2005

15. Welding Robots

213

Macro-programming is also regarded as a mixed method which shortens programming time at the robot, Figure 15.8. Macros are structured processing sequences which are created online to fulfil working functions and which can be repeated for further similar working functions.

Geometry

macros

contain information about torch guidance to produce certain joints or joint sections. Welding technology parameters welding

for individual

situations

summarised

in

are

welding

macros. This applies for torch positioning, torch in-

Quelle:Cloos, Kuka 2002

br-er15-09e.cdr

clination, relative position of

Graphical Simulation of Robot Movement

beads to root and welding parameters.

Figure 15.9

Using a collection (can be created online or offline) of such macros, the programming time can be shortened for workpieces with often repeated welding jobs, e.g. steel construction when welding stiffeners and head plates Using offline programming practice, the programming work is shifted out from the producing robot cell. This avoids unproductive stoppages and allows for economic-viable, limited number of pieces to be reduced. During ming,

textual the

program-

3-dimensional

point coordinates and torch orientations are entered into an external computer in a manufacturer-specific

pro-

movement instructions

program sequence instructions

arithmetical and logical functions

− synchronical PTP procedure (point to point)

− sub program technique

− +, -, *, : − boolean operations

− linear interpolation, CP (continious path)

− jump instruction

− etc.

− cicule and graduated cicule interpolation − continuously programmable tool speed

− conditional instructions − repeated loops − inquiry of entries − programmed stop

special functions − 3D online and offline transformation of program parts − mirroring of program parts − processing variables − communication with sensors − communication with external computers

gram language. To achieve a complete program sequence,

each

instruction

br-er15-10e.cdr

must be entered individually. Figure 15.10 2005

15. Welding Robots

214

The graphical offline programming uses CAD data for modelling the complete robot working cell and parts to be welded. Planning of the path is carried out with CAD functions directly at the workpiece which is displayed on a screen. In

TEA

EDI

most cases, the program-

5

1

0 LIST1=(20,0,0,50,60,75,15,12,0,0)

ming systems provide a

definition of welding parameters

1 MAIN 2 $(1)

graphical simulation of the

selection of welding parameters set 1

3 GP(1-3)

movement, e.g. to check

move to dots 1-3 in PTP mode

4

4 GC(4)

for collisions between torch and

workpiece,

Figure

move to dot 4 in CP mode

5 GP(5,1)

2

move to dot 5 and 1 in PTP mode 6 END

3

15.9.

For

the

following

transformation of the probr-er15-11e.cdr

gram into the robot control, a

calibration

between

model and physical robot

Figure 15.11

working cell is required. In the case of knowledgebased

offline

TEA

program-

EDI 0 LIST1=(20,0,0,50,60,75,15,12,0,0)

ming, the operator is supported

by

integrated

definition of welding parameters

1 7

6

job-

9 2 5

with rotating the 6th axis

6 Cir(3,4,5,50) circle instruction

7 GP(6-8) 8 $(2) selection of welding parameter set 2

3

4 120°

specific welding parameters. However, checking

5 CIRO(1)

8

° 10

of

move to dot 1-3 in PTP mode

10

comes to creation of robot determination

selection of welding parameter set 1

4 GP(1-3)

expert systems when it welding programs, e.g. for

1 LIST2=(30,0,0,55,70,0,0,0,0,0) 2 MAIN 3 $(1)

11

9 CIRO(0) lock 6th axis

10 CIR(8,9,10,0) 11 GP(11,1) 12 END

br-er15-12e.cdr

and adapting the program must be carried out by the operator.

Figure 15.12

Modern robot controls provide the programmer with some functions for movement control and for modification of program sequence, Figure 15.10. PTP movement (point to point) serves to move the robot in the space. All axes are controlled in such a way that they reach 2005

15. Welding Robots

215

their set-point at the same time. Thereby the actual path of the torch depends on kinematics of the robot and on current position of the axes. A linear interpolation (CP procedure, continuous Path), Figure 15.11, is used for accurate movement along a straight line, e.g. movement to weld start point or welding. The active point of the tool 'arc' (Tool-Centre-Point, TCP) is moved along a straight line between two programmed points, adapting torch angle and torch inclination between the two points. Circles and graduated circles are entered by means of circle interpolation programs, Figure 15.12. Then the orientation of the torch can be adapted through turning the knuckle axis or 6th axis of the robot and the value of spill-weld at the end of the seam can be indicated. Speed of the torch is user-programmable and, if required, can be superimposed by an oscillation. To control the program run, commands are available for: repeated loops, conditional and unconditional program jumps, waiting periods, waiting for inputs, and working with sub-programs. The software of modern seam welding robots contains – as special functions – 3-dimansional transfor-mations and mirroring of programs and y

partial programs, palletisx

offset

ing functions, processing

y

z y

x

z

sensor data and commands for communication

y

with other robot controls

x x

z z

(Master/Slave

operation)

as well as with external computers, Figure 15.13.

br-er15-13e.cdr

Figure 15.13

2005

16. Sensors

16. Sensors

217

The welding process is exstrategy

posed to disturbances like misalignment of workpiece,

brain

inaccurate preparation, machine

and

device

sensor

eye

hand

control

tolerances, and proess disturbances, Figure 16.1. welding process

The manual welder notices them by eyesight and cor-

process disturbances br-er16-01e.cdr

rects

them

according

to

manually

Adaptive Process Control Manually - Fully Mechanised

strategies

learned and gained by ex-

© ISF 2002

Figure 16.1

perience. To record process irregularities and path deviations, a fully mechanised welding plant requires sensors providing control signals which are then used in accordance with implemented rules. Using corresponding control elements, the control loop is closed for the welding process. Scopes of duty of the sensors is finding the weld start point and seam tracking. In addition, with the help of information about joint geometry, process parameters can be adapted online and offline. The ideal sensor for a robot application should measure the welding point (avoidance of tracking misalignment), detect in advance (finding the start point of the seam, recognising corners, avoiding collisions) and should be as small as possible (no restriction in accessibility). The ideal sensor which combines

all

three

re-

quirements, does not yet exist, therefore one must select a sensor which is suitable for the individual br-er16-02e.cdr

Sensors for Arc Welding Systems Survey

welding

job.

Figure 16.2

shows

different

sensor

Figure 16.2 2005

16. Sensors

218

principles used in welding engineering. The most frequently used systems in practice are tactile, optical, and arc based V, X Y seam with ball-probe

sensor systems with me-

overlapp seam with ball-probe

chanical arc adjustment. fillet seam with ball-probe

With tactile scanning systems, the simplest type of

edge seam with edge-probe

I seam with blanc-probe

scanning is a mechanical sensor. Pins, rollers, balls, or similar devices may be

multilayer seam with ball-probe

used as sensors. br-er16-03e.cdr

Scanning Principles With Tactile Sensors

Such

scanning

systems

show a long distance be-

Figure 16.3

tween sensor and torch, the application range is limited. Only grooves with large dimensions and relatively straight seam path can be scanned with these systems. Figure 16.3 shows some examples of different groove geometries. Tactile sensors can recognise 3-dimensional offsets of the workpiece. Through scanning of three levels the 3-dimensional point of intersection can be calculated and the robot program for correcting the deviation can be shifted accordingly thus finding the start point A

of the weld. In this case, the gas nozzle of the

A'

torch serves as a sensor, Figure 16.4, which is charged with electrical tension. As soon as the torch touches the workpiece, a current C'

flows, which is then taken by the robot control

B'

as a signal for obtaining the level to be scanned.

B br-er16-04e.cdr

© ISF 2002

Inductive sensors are graded as non-contact measurement systems. Due to their function Figure 16.4 2005

16. Sensors

219

principle, they can be applied for metallic and electrically conductive materials. The simplest type is a ring coil. If alternating current flows though the coil, ,a magnetic field is generated close to the workpiece. When the coil approaches

coil arrangement for distance measurement

the workpiece surface, the magnetic coil arrangement for groove position

weakens.

Figure 16.5 shows the dis-

sensor signals

tance-dependent electrical

groove position

transmitter coil reception coil

field

signal. Such simple sen-

distance A

B

sors are used to recognise the Using

br-er16-05e.cdr

workpiece

position.

several

distance

© ISF 2002

sensors, also a welding

Principle of an Inductive Sensor (Single Coil and Multicoil Arrangement)

groove can be scanned.

Figure 16.5 With multi-coil arrangements in one sensor, the position of the welding groove, the angle between sensor and workpiece surface and the distance can be recorded. Figure 16.6 shows a principle arrangement. A transmitter coil generates an magnetically alternating field which induces alternating currents in the two receiver coils. In the undisturbed case, these currents are phase-shifted by 180° and neutralise each other. If the sensor is moved crosswise to the groove, magnetical asymmetries will occur in the scanning area, which will show in the presented signal shape. The output signal will be zero, if the coils are positioned ex-

functional principle of continuous wave doppler’s radar

transmitting wave receiving wave

work piece

radar sensor

actly above the centre of phase difference

the groove. The radar sensor in Figure 16.6

uses

radar sensor

Doppler's

signal path

oscillation

effect to generate a signal. Here the phase difference between transmitter signal and

receiving

signal

workpiece br-er16-06e.cdr

is

© ISF 2002

Functional Principle of a Radar Sensor

evaluated. A

mathematical

process

Figure 16.6

2005

16. Sensors

220

transforms such signals into distance values. To record the position and the depth of the groove, the sensor must be continuously moved along the seam. Radar sensors form a so called radar baton, which is focussed onto a measurement spot of about 0,7 mm diameter for this application. Figure 16.6 shows the sensor signal, which represents the relative movement along the workpiece. At the moment, the characteristic values of the weld groove can be determined with a resolution in the range of 1/10 mm. Arc sensors evaluate the continuous change of the welding current with a change of the contact

tip-to-work

distance,

Figure 16.7. A signal for side control of the torch is determined

measure-

l2

l1

ment and subtraction of the l1,

2

IO

∆l

by

U

groove. A comparison be-

l2 l0 I1 side correction ∆l = 0

∆I br-er16-07e.cdr

currents on the flanks of a

height correction ΣI = 2 x ISoll

tween

actual

welding

current and programmed rated current provides a

I

© ISF 2002

signal for distance control Arc Sensor

of the welding torch. To let this sensor method

Figure 16.7

work, a divergence of the arc or the use of a second arc is required. To realise this principle, mechanical oscillation

twin wire welding

there are numerous possibili-ties. Figure 16.8 shows some variants of signal recording.

The

most

frequently used method is a magnetical oscillation br-er16-08e.cdr

rotating wire © ISF 2002

Arc Sensor - Signal Detection -

mechanical

oscillation

of

the welding torch, which is carried

out

by

a

rotor

Figure 16.8 2005

16. Sensors

221

movement with an oscillation frequency up to 5 Hz. The second method is mainly used with submerged arc welding. Both wires are aligned crossways to welding direction and the difference of the two currents is evaluated. Magnetic fields can diverge only the arc itself. The advantage of this method is a high divergence frequency of about 15 Hz. A disadvantage is the size of the electromagnets and the limited accessibility to the workpiece. The last variant of an arc sensor incorporates a mechanical rotation of the welding wire. In this case, the divergence frequency of the arc can reach up to 30 Hz. The signal recording is continuous during the movement. In this way, information about orientation of the torch and groove width is also provided. The arc sensor principle is limited to groove shapes with clear flanks. Together with the tactile torch gas nozzle sensor, it provides a frequently used combination for seam finding and seam tracking during robot welding. Optical sensors can be used for a great number of jobs. The easiest method is the recognition of the radiation intensity, which is reflected during welding. E.g. with laser beam welding, this is carried out through recording the reflected laser radiation with simple sensors for control of penetration depth, Figure 16.9. The procedure is based on the line-up between the degree of reflection and shaft relation (penetration depth/focus position) of the capillary. The amount of back-reflection of the laser beam power is measured, which NdYAG-Laser

due

to

multi-

CO2-Laser

reflection is not absorbed by the workpiece. Changes of penetration depth due to modified laser power or a shifted focus position can be identified by the signal of reflected laser power br-er16-09e.cdr

© ISF 2002

Back Reflection Procedure for Laser Beam Welding

and can be used for control of the penetration depth.

Figure 16.9 2005

16. Sensors

222

However, optical sensors can also be used for measuring geometrical values. Such information may be used for finding the start point of a seam, for seam tracking, and for identification of groove profile. The two last mentioned functions provide the possibility to use the information for filling rate control and/or quality control. Geometry-measuring optical sensors are normally external systems, which are positioned in front of the torch as a leading element. It is practical to equip the sensor with additional axes, because both, torch and sensor, must be moved along the groove. Without additional axes, a robot would be limited in its accessibility to the workpiece and in its working range. Another problem is the tremendous effort to introduce the control-technical integration into the robot control. Among other things, information must be exchanged in real time. Most of geometry-measuring sensors use the triangulation principle or a variant of this measurement procedure. The triangulation measurement procedure provides information about the distance to the workpiece surface. A light spot is projected onto the workpiece surface and displayed to a line-type receiver element under a certain angle. With distance changes laser

emerge corresponding positions on the reCCD camera

ceiver element, Figure 16.10. Sensors which use this triangulation principle are applied for

lens

recognition of workpiece position and for off-

laser beam

line seam finding. Both, the laser scanner and the light-section procedure are based on the triangulation measurement principle. With the laser scan-

depth measurement range

ner,

Figure 16.11,

this

principle

is

complemen-ted by an oscillating axis in parallel to the groove axis. The measurement of a br-er16-10e.cdr

© ISF 2002

Triangulation Principle

sequence of distances along a line becomes possible and provides a 2-dimensional record and evaluation of the groove contours.

Figure 16.10

2005

16. Sensors

223

Sensors as part of the light-section procedure, also provide information about the 2dimensional position of the groove. As a function of this system, one or more light lines are projected onto the workpiece surface and displayed to a CCD matrix under a certain angle, Figure 16.12. In contrast to scanning, information about the groove profile is provided by taking a picture scene. Using sensors, it is pssible to obtain additional 3-dimensional information through evaluation of more, in succession taken, while the camera moves over the grooves. Systems, which generate their information through a projection of several light lines, provide additional information about the path of the seam and the orientation of the sensor related to the workpiece surface. Both,

scanning

systems

and sensors based on the

laser motor

focusing lens

light section procedure, can be used for recognition of

CCD camera angle transmitter

the welded seam to make

beam deflection mirror

lens mirror

oscillation

an automised quality control of the outer weld character-

workpiece

istics possible. Another optical measurebr-er15-20e.cdr

© ISF 2002

ment principle uses, similar Laser Scanner Principle

to human sight, the stereo procedure to record ge-

Figure 16.11

ometry information across the weld groove. Two inde-

laser ligthing

CCD camera Diodendiode Laser laser

pendent optics photograph

2-D 2-D detector Detektor

the interesting groove area and displays them onto two image converter elements line projection

(CCD-lines or CCD-matrix). Based on the correspondworkpiece

ing image points in both picture

scenes,

the

3-

br-er16-12e.cdr

dimensional position of ob-

© ISF 2002

Principle of Light-Section Procedure

ject points is evaluated. Figure 16.12 2005

16. Sensors

224

Figure 16.13 shows the measurement principle, which uses CCD lines as image converter elements, and idealised signals for generating information. The grey scale drop in the signal is ideally used as corresponding image area, which occurs with butt welds due to different reflection intensity between stereo measurement principle

stereo measurement principle

workpiece surface and gap. Both, the lateral position of

CCD lines

left signal

right signal

the groove and the dis-

laser display levels

side position :

signal drop

tance to the sensor can be determined by evaluating

light line projection workpiece

distance :

the centre positions of both signal drops. The width of groove width :

the groove is taken from br-er16-13e.cdr

© ISF 2002

the width of the signal drop.

Principle of Stereo Measurement Procedure

Optical sensors may also Figure 16.13

be used for geometrical

recognition of the weld pool, to adapt process parame-ters in the case of possible deviations. Figure 16.14 depicts such a system for use with laser beam welding. The welding process is monitored by a CCD camera through a filter system. An optical filter allows to observe the weld pool surface without disturbing effects of the plasma in the near infrared spectrum. Picture data are transferred to an image processing computer which measures the geometry of the weld pool. Geometry data

contain

information

which is used online for control of the welding process.

Among

others,

penetration depth and focus

position

can

be

controlled. The system also provides the recognition of br-er16-14e.cdr

© ISF 2002

Weld Pool Recognition

protrusion-welded

joints

and welding defects like e.g. molten pool ejections.

Figure 16.14

2005

16. Sensors

225

During electron beam welding, the beam is in combination with a detector used for both, to carry out a seam tracking and a monitoring of the welded seam. For this, the beam can be diverged as well as bent, Figure 16.15. Backscattered electrons are recognised by a special detector and converted into grey values. The line or area surface scanning by the spotted functional principle

seam tracking

(evaluation of back-scattered electrons)

electron beam provides a progressive series of greys across the scanned line or area. During electron beam welding, these signals can

monitoring

be used for seam tracking by scanning an edge which is parallel to the groove. The

area-type

scanning

provides the possibility for br-er16-15e.cdr

observing the welded seam Sensor Principle of Electron Beam Welding

or the focus position.

Figure 16.15

2005

3. Submerged Arc Welding

3. Submerged Arc Welding

32

In submerged arc welding a mineral weld flux layer protects the welding point and the freezing weld from the inelectrode

flux hopper

fluence of the surrounding atmosphere,

contact piece

flux solid slag

arc

Figure

3.1.

The arc burns in a cavity filled with ionised gases and vapours where the

weld metal

droplets from the continuously-fed

base metal

liquid slag

molten pool

weld cavity

wire

electrode

are transferred into the weld pool. Unfused flux can

br-er3-01e.cdr

© ISF 2002

be extracted from behind

Process Principle of Submerged Arc Welding

the welding head and subsequently recycled.

Figure 3.1 Main components of a submerged arc welding unit are:

the wire electrode reel, the wire feed motor equipped with grooved wire feed rolls which are suitable for the demanded wire diameters, a wire straigthener as well as a torch head for current transmission, Figure 3.2. Flux supply is carried out via a hose from the flux container to the feeding hopper which is mounted on the torch head. Depending on the degree of automation it is possible to install a flux excess pickup behind the torch. Submerged arc AC or DC current supply wire straightener wire feed rolls flux supply indicators power source wire reel welding machine holder

welding can be operated using either an a.c. power source or a d.c. power source where the electrode is normally connected to the positive terminal. Welding advance is provided by the welding ma-

br-er3-02e.cdr

© ISF 2002

chine Assembly of a SA Welding Equipment

or

by

workpiece

movement.

Figure 3.2 2005

3. Submerged Arc Welding

33

Identification of wire electrodes for submerged arc welding is based on the average Mncontent and is carried out in steps of 0.5%, Figure 3.3. Standardisation for welding filler materials for unalloyed steels as well as for fine-grain structural steels is contained in DIN EN 756, for creep resistant steels in DIN pr EN 12070 (previously DIN 8575) and for stainless and heat resistant steels in DIN pr EN 12072 (previously DIN 8556-10). The proportions of additional alloying elements are dependent on the materials to be welded and on the mechanical-technological demands which emerge from the prevailing operating conditions, Figure 3.4. Connected to this, most important alloying elements are manganese for strength, molybdenum for high-temperature strength and nickel for toughness.

alloy type

commercial wire electrodes

main alloying elements Mn Ni Mo Cr V

Mn

S1 S2 S3 S4

0,5 1,0 1,5 2,0

S2Mo S3Mo S4Mo

1,0 1,5 2,0

Ni

S2Ni1 S2Ni2

1,0 1,0

1,0 2,0

NiMo

S2NiMo1 S3NiMo1

1,0 1,5

1,0 1,0

NiV

S3NiV1

1,5

1,0

NiCrMo

S1NiCrMo2,5 S2NiCrMo1 S3NiCrMo2,5

0,5 1,0 1,5

2,5 1,0 2,5

MnMo

0,5 0,5 0,5

DIN EN 756 Reference analysis mat.-no. approx. weight % S1 C = 0,08 1.0351 Si = 0,09 Mn = 0,50 C = 0,10 Si = 0,10 Mn = 1,00

For higher welding joint quality requirements; in: pipe production, boiler and tank construction, sructural steel engineering, shipbuilding. Fine-grain structural steels up to StE 380.

S3 1.5064

C = 0,11 Si = 0,15 Mn = 1,50 C = 0,10 Si = 0,30 Mn = 1,00

For high-quality welds with medium wall-thicknesses. Fine-grain structural steels up to StE 420. Especially suitable for welding of pipe steels, no tendency to porosity of unkilled steels. Fine-grain structural steels up to StE 420.

S2Mo 1.5425

C = 0,10 Si = 0,15 Mn = 1,00 Mo = 0,50

For welding in boiler and tank construction and pipeline production with creep-resistant steels. Working temperatures of up 500 °C. Suitable for higher-strength fine-grain structural steels.

S2Ni1

C = 0,09 Si = 0,12 Mn = 1,00 Ni = 1,20 C = 0,10 Si = 0,12 Mn = 1,00 Ni = 2,20

For welding low-temperature fine-grain structural steels. Non-ageing.

C = 0,12 Si = 0,15 Mn = 1,00 Mo = 0,50 Ni = 1,00

For quenched and tempered fine-grain structural steels. Suitable for normalising and/or re-quenching and tempering.

0,15 0,6 0,8 0,6 0,5 0,6 0,8

S2Ni2

From a diameter of 3 mm upwards all wire electrodes have to be marked with the following symbols: S1 Si Mo

S6: I : _ :

IIIIII

S3NiMo1

Example: S2Si: II _ S3Mo: III

br-er3-03e.cdr

© ISF 2002

Wire Electrodes for Submerged Arc Welding

Figure 3.3

For lower welding joint quality requirements;in: boiler and tank construction, pipe production, structural steel engineering, shipbuilding

S2 1.5035

S2Si 1.5034

0,5 0,5

Properties and application

br-er3-04e.cdr

Especially suitable for low-temperature welds. Non-ageing.

© ISF 2002

Properties and Application Areas for Wire Electrodes in Submerged Arc Welding

Figure 3.4

The identification of wire electrodes for submerged arc welding is standardised in DIN EN 756, Figure 3.5. During manufacture of fused welding fluxes the individual mineral constituents are, with regard of their future composition, weighed and subsequently fused in a cupola or electric furnace, Figure 3.6. In the dry granulation process, the melt is poured stresses break the 2005

3. Submerged Arc Welding

34

crust into large fragments. During water granulation the melt hardens to form small grains with a diameter of approximately 5 mm. Wire electrode DIN EN 756 - S2Mo

As a third variant, compressed air is additionally DIN main no.

blown into the water tank resulting in finely blistered

Symbols of the chemical composition: S0, S1...S4, S1Si, S2Si, S2Si2, S3Si, S4Si, S1Mo,..., S4Mo, S2Ni1, S2Ni1.5, S2Ni2, S2Ni3, S2Ni1Mo, S3Ni1.5, S3Ni1Mo, S3Ni1.5Mo

grains with low bulk weight. The fragments or grains are subsequently ground

br-er3-05e.cdr

and screened – thus bring-

Identification of a Wire Electrode in Accordance with DIN EN 756

ing about the desired grain size.

Figure 3.5

lime

quarz

rutile

bauxite

magnesite

roasting kiln

rutile

Mn - ore

fluorspar

magnesite

alloys

sintering furnace

silos

silos ball mill

balance mixer

coke

balance coke

raw material

dish granulator molten metal air

gas

tapping

drying oven coal-burning stove

electrical furnace granulation

tub foaming

air

heat treatment furnace

cylindrical crusher screen

screen cooling pipe

drying oven

balance

balance br-er3-06e.cdr

© ISF 2002

br-er3-07e.cdr

Production of Fused Welding Fluxes

Figure 3.6

© ISF 2002

Production of Agglomerated Welding Fluxes

Figure 3.7

2005

3. Submerged Arc Welding

35 During manufacture of agglomerated weld

Fused fluxes1)

Properties

Agglomerated fluxes1)

ground, Figure 3.7. After weighing and with

uniformity of grain size distribution

+/++

-/++

the aid of a suitable binding agent (water-

grain strength

+/++

-/++

glass) a pre-stage granulate is produced in the

homogeneity

+/++

--2)/++

susceptibility to moisture

+/++

-/+

storing properties

+/++

-/++

resistance to dirt

--/+

-/++

rotary dish granulator where the individual

current carrying capacity

+/++

+/++

grains are rolled up to their desired size and

-/+

+/++

high-speed welding properties

+/++

+/++

multiple-wire weldability

-/++

+/++

flux consumption

-/+

+/++

slag removability

mixer. Manufacture of the granulate is finished on a

consolidate. Water evaporation in the drying oven hardens the grains. In the annealing furnace the remaining water is subsequently removed at temperatures of between 500°C and

1) 2)

fluxes the raw materials are very finely

assessment : -- bad, - moderate, + good, ++ very good core agglomerated flux

br-er3-08e.cdr

900°C, depending on the type of flux.

© ISF 2002

Properties of Fused and Agglomerated Welding Fluxes

The fused welding fluxes are characterised by high homogeneity, low sensitivity to moisture,

Figure 3.8

good storing properties and high abrasion re-

sistance. An important advantage of the agglomerated fluxes is the relatively low manufacturing temperature, Figure 3.8. The technological properties of the welded joint can be improved by the addition of temperature-sensitive deoxidation and alloying constituents to the flux. Agglomerated fluxes have, in general, a lower bulk weight

MS

(lower consumption) which

CS

allows the use of compo-

ZS RS

nents which are reacting

AR

among themselves during

AB

the melting process. However, the higher susceptibil-

AS AF

ity to moisture during stor-

FB

age andprocessing has to

Z

be taken intoconsideration.

MnO + SiO2 CaO CaO + MgO + SiO2 CaO + MgO ZrO2 + SiO2 + MnO ZrO2 TiO2 + SiO2 TiO2 Al2O3 + TiO2 Al2O3 + CaO + MgO Al2O3 CaF2 Al2O3 + SiO2 + ZrO2 CaF2 + MgO ZrO2 Al2O3 + CaF2 CaO + MgO + CaF2 + Mo SiO2 CaF2 other compositions

min. 50% max. 15% min. 55% min.15% min. 45% min. 15% min. 50% min. 20% min. 40% min. 40% min. 20% max. 22% min. 40% min. 30% min. 5% min. 70% min. 50% max. 20% min. 15%

manganese-silicate calcium-silicate zirconium-silicate rutile-silicate aluminate-rutilel aluminate-basic

aluminate-silicate aluminate-fluoride-basic fluoride-basic

br-er3-09e.cdr

Different Welding Flux Types According to DIN EN 760

Figure 3.9 2005

3. Submerged Arc Welding

36

The SA welding fluxes are, in accordance with their mineralogical constituents, classified into nine groups, Figure 3.9. The composition of the

MS - high manganese and silicon pickup - restricted toughness values - high current carrying capacity/ high weld speed - unsusceptible to pores and undercuts - unsuitable for thick parts - suitable for high-speed welding and fillet welds

individual flux groups is to be considered as in CS

acidic types - highest current carrying capacity of all fluxes - high silicon pickup - suitable for welding by the pass/ capping method of thick parts with low requirements basic types - low silicon pickup - suitable for multiple pass welding - current carrying capacity decreases with increaseing basicity

there is also the Z-group which allows free

ZS

- high-speed welding of single-pass welds

compositions of the flux.

RS

- high manganese pickup/ high silicon pickup - restricted toughness values of the weld metal - suitable for single and multi wire welding - typical: welding by the pass/ capping pass method

AR

- average manganese and silicon pickup - suitable for a.c. and d.c. - single and multi wire welding - application fields: thin-walled tanks, fillet welds for structural steel construction and shipbuilding

principle, as fluxes which belong to the same group may differ substantially with regards to their welding or weld metal properties. In addition to the groups mentioned above

The calcium silicate fluxes are recognized by their effective silicon pickup. A low Si pickup has low cracking tendency and liability to rust,

br-er3-10ae.cdr

on the other hand the lower current carrying AB

AS

AF

- medium manganese pickup - good weldability - good toughness values in welding by the pass/ capping pass method - application field:unalloyed and low alloyed structural steels - suitable for a.c. and d.c. - applicable for multilayer welding or welding by the pass/ capping pass method - mainly neutral metallurgical behavior - manganese burnoff possible - good weld appearance and slag removability - to some degree suitable for d.c. - recommended for multi layer welds for high toughness requirements - application field: high-tensile fine grain structural steels, pressure vessels, nuclear- and offshore components - suitable for welding stainless steels and nickel-base alloys - neutral behaviour as regards Mn, Si and other constituents

FB - mainly neutral metallurgical behaviour - however, manganese burnoff possible - highest toughness values right down to very low temperatures - limited current carrying capacity and welding speed - recommended for multi layer welds - application field: high-tensile fine-grain structural steeels Z

- all other compositions

br-er3-10be.cdr

© ISF 2002

Classification of Fluxes for SA Welding According to DIN EN 760 (II)

© ISF 2002

Classification of Fluxes for SA Welding According to DIN EN 760 (I)

Figure 3.10a capacity of these fluxes has to be accepted. A high Si pickup leads to a high current currying capacity up to 2500 A and a deep penetration. Aluminate-basic fluxes have, due to the higher Mn pickup, good mechanical properties. With the application of wire electrodes, as S1, S2 or S2Mo, a low cracking tendency can be obtained. Fluoride-basic fluxes are characterised by good weld metal impact values and high cracking insensitivity. Figures 3.10a and 3.10b show typical properties and application areas for the different flux types.

Figure 3.10b

2005

3. Submerged Arc Welding

37

Figure 3.11 shows the identification of a welding flux according to DIN EN 760 by the example of a fused calcium silicate flux. This type of flux is suitable for the welding of joints as well as for overlap welds. The flux can be used for SA welding of unalloyed and low-alloy steels, as, e.g. general structural steels, as well as for welding high-tensile and creep resistant

steels.

The

silicon

pickup is 0.1 – 0.3% (6), welding flux DIN EN 760-SF CS 1 67 AC H10

while

the

manganese

pickup is expected to be DIN main no.

hydrogen content (table 4)

flux/SA welding

type of current

method of manufacture

metallurgical behaviour

F fused A agglomerated M mechanically mixed flux

flux type

or a.c. can be used, as, in principle, a.c. weldability

(table 2)

allows also for d.c. power

flux class 1-3

source. The hydrogen con-

(table 1)

(figure 3.9)

0.3 – 0.5% (7). Either d.c.

tent in the clean weld br-er3-11e.cdr

metal is lower than the

Identification of a Welding Flux According to DIN EN 760

10 ml/100 g weld metal.

Figure 3.11 The flux classes 1-3 (table 1) explain the suitability of a flux for welding certain material groups, for welding of joints and for overlap welding. The flux classes also characterise the metallurgical material be-

table 2 table 1 unalloyed and low-alloyed steel general structural steel high-tensile & creep resistant steels stainless and heat resistant steels Cr- & CrNi steels welding of joints hardfacing pickup of elements as C, Cr, Mo

identification proportion flux in all-weld metal figure %

metallurgial behaviour

flux class 1 2 3

burnoff

1 2 3 4

over 0,7 0,5 up to 0,7 0,3 up to 0,5 0,1 up to 0,3

pickup or burnoff

5

0 up to 0,1

pickup

6 7 8 9

0,1 up to 0,3 0,3 up to 0,5 0,5 up to 0,7 over 0,7

table 4 identification

hydrogen content ml/100g all-weld metal max.

H5

5

H10

10

H15

15

br-er3-12e.cdr

haviour. In table 2 defines the identification figure for the pickup or burn-off behaviour of the respective element. Table 4 shows the gradation of the diffusible hydrogen content in the weld metal, Figure 3.12.

Parameters for Flux Identification According to DIN EN 760

Figure 3.12

2005

3. Submerged Arc Welding

38

Figure 3.13 shows the identification of a wire-flux combination and the resultant weld metal. It is a case of a combination for multipass SA welding where the weld metal shows a minimum yield point of 460 N/mm² (46) and a minimum metal impact value of 47 J at

wire-flux combination DIN EN 756 - S 46 3 AB S2

–30°C (3). The flux type is aluminate-basic (AB) and is

standard no.

used with a wire of the qual-

wire electrode and/or wire-flux combination for submerged arc welding

ity S2. The tables for the identifica-

strength and fracture strain

tion of the tensile properties

chemical composition of the wire electrode type of flux (figure 3.10)

impact energy (table 3)

(table1 and 2)

as well as of the impact en-

br-er 3-13e.cdr

Identification of a Wire-Flux Combination According to DIN EN 756

ergy are combined in Figure 3.14.

Figure 3.13

The chemical composition of the weld metal and the structural constitution are dependent on the different metallurgical reactions during the welding process as well as on the used

Identification for strength properties of multipass weld joints

table 1 identification

minimum yield point n/mm2

35

355

440 up to 570

22

38

380

470 up to 600

20

42

420

500 up to 640

20

46

460

530 up to 680

20

50

500

560 up to 720

18

tensile strength minimum fracture strain % N/mm2

materials, Figure 3.15. The welding flux influences the slag viscosity, the pool motion and the bead surface. The different combinations of filler material and welding flux cause, in di-

table 2

Identification for strength properties of welding by the pass/ capping pass method welded joints

rect dependence on the weld parameters (cur-

identification

minimum base metal yield strength N/mm2

minimum tensile strength N/mm2

rent, voltage), a different melting behaviour

2T

275

370

3T

355

470

4T

420

520

5T

500

600

and also different chemical reactions. The dilution with the base metal leads to various strong weld pool reactions, this being depend-

table 3

Identification for the impact energy of clean all-weld metal or of welding by the pass/ capping pass method welded joints

identification

Z

A

0

temp. for minimum impact energy 47J °C

no demands

+20

0

br-er3-14e.cdr

2

3

4

5

6

7

ent on the weld parameters.

8

-20 -30 -40 -50 -60 -70 -80

The diagram of the characteristics for 3 dif© ISF 2002

Parameter for Weld Metal Identification According to DIN EN 756

ferent welding fluxes assists, in dependence of the used wire electrodes, to determine the pickup and burn-off behaviour of the element

Figure 3.14 2005

3. Submerged Arc Welding

39 manganese, Figure 3.16.

welding flux

welding filler metal droplet reaction

welding data

For example: A welding flux with the mean characteristic and when a wire

base metal slag

welding data

dilution

electrode S3 is used, has a neutral point where neither pickup nor burn-off occur.

welding data

weld pool reaction

The pickup and burn-off weld metal

behaviour is, besides the

br-er 3-15e.cdr

filler material and the weld-

Metallurgical Reactions During Submerged Arc Welding

ing flux, also directly de-

Figure 3.15

pendent on the welding amperage

and

welding

voltage, Figure 3.17. By the example of the selected flux a higher welding voltage causes a more steeply descending manganese characteristic at a constant neutral point. Silicon pickup increases with the increased voltage. The influence of current and voltage on the carbon content is, as a rule, negligible. Inversely proportional to the voltage is the rising characteristic as regards manganese in dependence on the welding Mn-pickup

current,

Figure

3.18.

Higher currents cause the characteristic curve to flat1,0% S1

S2

2,0% S3

S4

S5

3,0% Mn in wire

ten. As the welding volt-

S6

age, the welding current also has practically no influence on the location of the neutral point. Silicon

Mn-burnoff

pickup decreases with in-

br-er 3-16e.cdr

Manganese-Pickup and Manganese-Burnoff During Submerged Arc Welding

creasing current intensity.

Figure 3.16

2005

3. Submerged Arc Welding

40

weld flux LW 280 (DIN EN 760 SF CS 1 76 AC H 10) current intensity 580 A welding speed 55 cm/min 0,6

0,6

36 V

33 V

0,2

0,2

25 V

0

0,5

-0,2

pickup/ burnoff
X in weight %

% Mn

neutral point 1,0

2,0

% Mn wire

-0,4

27 V 29 V

-0,6 0,6

36 V

% Si 0,2 0 -0,2

25 V 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 % Si wire

-0,4 0,05 0

0,05

-0,05

0,15

25 - 36 V

-0,10

0

2,5

0,20 0,25 % C wire

450 A 650 A

neutral point

580 A

700 A 0,5

-0,2

pickup/ burnoff
X in weight %

% Mn

weld flux LW 280 (DIN EN 760 SF CS 1 76 AC H 10) arc voltage 29 V welding speed 55 cm/min

-0,4

1,0 % Mn wire

2,0

2,5 800 A

-0,6 0,6

450 A

% Si 0,2 0 -0,2

800 A 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 % Si wire

-0,4 0,05

800 A

0

0,15

-0,05

450 A

-0,10

0,20 0,25 % C wire

-0,15

-0,15

% XSZ

br-er3-17e.cdr

© ISF 2006

Pickup and Burnoff Behaviour in Dependence on Welding Voltage and Wire Electrode

Figure 3.17

br-er3-18e.cdr

© ISF 2006

Pickup and Burnoff Behaviour in Dependence on Welding Current and Wire Electrode

Figure 3.18

The Mn-content of the weld metal can be deflux diagramm LW 280 (DIN EN 760 SF CS 1 76 AC H 10),

termined by means of a welding flux dia-

manganese

gram, Figure 3.19.

wire electrode 4 mm Ø acc. to Prof. Thier example:

I = 580 A U = 29 V MnSZ1 = 0.48 % Mn MnSZ2 = 1.69 % Mn

In this example, the two points on the axis which determine the flux characteristic are defined for the parameters 580A welding current and 29V welding voltage, with the aid of the auxiliary straight line and the neutral point curve (MnNP). In this case, the two points are positioned at 0.6% ∆Mn and 1.25% MnSZ. Dependent on the manganese content of the used filler material, the pickup or burn-off

br-er3-19e.cdr

© ISF 2002

Welding Flux Diagramm for Determination of the Mn Content in the Weld Metal

Figure 3.19

contents can be recognized by the reflection with respect to the characteristic line (0.38%

2005

3. Submerged Arc Welding

41

Mn-pickup with a wire containing 0.5%Mn, 0.2% Mn-burnoff with a wire containing 1.75%Mn). The structure of the characteristic line for the

flux diagramm LW 280 (DIN EN 760 SF CS 1 76 AC H 10),

determination of the silicon pickup con-

silicon wire electrode 4 mm Ø acc. to Prof. Thier example:

tent, is, in principle, exactly the same as de-

I = 580 A U = 29 V SiSZ = 0.16 % Si

scribed above, Figure 3.20. As silicon has only pickup properties and therefore no neu-

auxiliary straight line

tral point exists, the second auxiliary straight line must be considered for the determination of the second characteristic line point. Weld preparations for multipass fabrication are dependent on the thickness of the plates to be welded, Figure 3.21. If no root is

auxiliary straight line

planned during weld preparation and also no

br-er3-20e.cdr

© ISF 2006

support of the weld pool is made, the root

Welding Flux Diagram for Determination of the Si Content in the Weld Metal

Figure 3.20

pass must be welded using low energy input.

When welding very thick plates which are accessible

from both sides, the double-U butt weld may be

preparation geometry

weld buildup

applied, Figure 3.22. Beand

welded, the root must be milled

out

manual metal arc welding SA SA

fore the opposite side is

SA SA SA SA

(goug-

ing/sanding). This type of weld cannot be produced

manual metal arc welding manual metal arc welding

by flame cutting and is, as

SA SA SA SA

milling is necessary, more expensive, although exact

br-er 3-21e.cdr

Welding Procedure Sheets for Single-V Butt Welds, Single-Y Butt Welds with Broad Root Faces and Double-V Butt Welds

weld preparation and correct selection of the welding parameters lead to a

Figure 3.21

high weld quality.

2005

3. Submerged Arc Welding

42 Another variation of heavy-

preparation geometry

weld buildup

plate welded joints is the so-called „steep single-V manual metal arc welding turning and sanding manual metal arc welding

butt weld”, Figure 3.23.

SA SA

The very steep edges keep

side 1

turn SA SA

the welding volume at a

SA SA

very low level. This tech-

turn

side 2

turn

nique, however, requires

SA SA

the application of special © ISF 2002

br-er3-22e.cdr

Welding Procedure Sheet for Double-U Butt Welds

narrow-gap torches. The geometry during slag detachment and also during

Figure 3.22

reworking weld-related defects may cause problems. Here, high demands are made on torch manipulation and process control. Special narrow-gap welding fluxes facilitate slag removal. The most important welding parameters as regards GMA welding

weld bead formation are welding

GMA welding

voltage

and speed, Figure 3.24. A higher

SA welding

current, welding

current

causes higher deposition SA welding

rates and energy input,

oscillated

which leads to reinforced beads and a deeper pene© ISF 2002

br-er3-23e.cdr

Welding Procedure Sheet for Square-Edge Welds

Figure 3.23

tration. The weld width remains

roughly

constant.

The increased welding voltage leads to a longer arc

which also causes the bead to be wider. The change in welding speed causes - on both sides of an optimum - a decrease of the penetration depth. At lower weld speeds, the weld pool running ahead of the welding arc acts as a buffer between arc and base metal. At high speeds, the energy per unit length decreases which leads, besides lower penetration, also to narrower beads.

2005

3. Submerged Arc Welding

plate thickness: 25 mm 10 wire electrode: 4 mm flux: MS-Typ

tp 400

500 600 welding current (I)

700

800 Amp.

constant: II= 600 Amp.

v = 60 cm/min

w tp

28

consumption kg flux / kg wire

20

w

30

32

34

36

38

40 Volt

arc voltage (U) 12 10 w 8 6 te 4 constant: II = 600 Amp. 2 U = 32 Volt 0 30 40 50 60 70

30 20 10

80

90

100

A) flat weld - I square butt joint

2,4 2,2 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0

fused composition fluxes

agglomerated fluxes

400

consumption kg flux/ kg wire

12 10 8 6 4 2 0

30

U = 32 Volt v = 60 cm/min

constant:

weld width w in mm

penetration depth tp in mm

12 10 8 6 4 2 0

43

110 cm/min

500

600

700

B) fillet weld

1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2

fused composition fluxes

agglomerated fluxes

0 400

500

600

700

welding speed (v) br-er3-24e.cdr

© ISF 2002

800 900 1000 1100 current intensity (A)

br-er3-25e.cdr

© ISF 2002

Influence of the Weld Parameters on Penetration Depth and Weld Width

Figure 3.24

800 900 1000 1100 current intensity (A)

Welding Flux Consumption in Dependence on Current Intensity and Seam Shape

Figure 3.25

Weld flux consumption is dependent on the selected weld type, Figure 3.25. Due to geometrical shape, the flux consumption of a fillet weld is significantly lower than that of a butt weld. Because of their lower bulk weight, the specific consumption of agglomerated fluxes is lower than that of fused fluxes.

direction of welding

1

2

Two different control con-

3

cepts allow the regulation

L1

of the arc length (the prin-

L3

L2

ciple is shown in Figure 3.26). The application of the appropriate control system is dependent on the © ISF 2002

br-er3-26e.cdr

Control of the Arc Length

available

power

source

characteristics.

Figure 3.26 2005

3. Submerged Arc Welding

44

The external regulation of the arc length by the control of the wire feed speed requires a

U U0

power source with a steeply descending characteristic, Figure 3.27. In this case, the short-

US

A

I

A´

I´

IS IK

I

DU

ening of the arc caused by some process disturbance, entails a strong voltage

DI

drop at a low current rise. As a regulated quantity, this voltage drop reduces the wire feed

external regulation ( D U-regulation)

U

speed. Thus, the initial arc length can be regulated at an almost constant deposition rate. In contrast, the internal regulation effects, when

I

U0 US

the arc is reduced, a strong current rise at a

A

A´

I´

DI

DU

low voltage drop (slightly descending characIS

teristic). At a constant wire feed speed the ini-

I

internal self regulation ( D I-regulation) br-er3-27e.cdr

tial arc length is independently regulated by the

© ISF 2002

Control System for Constant Length of Arc

increased burn-off rate which again is a consequence of the high current. Figure 3.27

backing flux

The reaction of the internal regulation to process disturbance is very fast. This process is self regulating and does not require any machine expenditure.

ceramic backing bar

In submerged arc welding of butt joints, it is, depending on the weld preparation, necessary to support the liquid weld pool with a backing, Figure 3.28. This is normally done

flux copper backing

with either a ceramic or copper backing with a flux layer or by a backing flux. Dependent on the shape of the backing bar, direct formation

br-er3-28e.cdr

of the underside seam can be achieved. Examples of Weld Pool Backups

When welding circumferential tubes, the inclination angle of the electrode has a direct

Figure 3.28 2005

3. Submerged Arc Welding

45 influence onto the formation of the weld bead,

3 0° -

0°

Figure 3.29. For external as well as for internal tube welds, the best weld shapes may be obtained with an adjusted angular position of the torch. If the advance is too low, the molten bath runs ahead and produces a narrow weld with a medium-sized ridge, too high an ad-

a1 = 0°

a2

a3

vance causes the flowback of the molten bath and a wide seam with a formed trough in the centre. The processes described here for external tube welds are, the other way round, b1

b2

b3

t3

t2

t1

also applicable to internal tube welds. To increase the efficiency of submerged

inclusion br-er3-29e.cdr

© ISF 2002

arc welding, different process variations are

Wire Position in SA-Welding for Circumferential Tube Welds

applied, Figure 3.30. In multiwire welding, where up to 6 wires are used, each welding

Figure 3.29

torch is operated from a separate power

source. In twin wire welding, two wire electrodes are connected in one torch and supplied from one power source. Dependent on the application, the wires can be arranged in a parallel or in a tandem. In submerged arc welding with iron powder addition can the deposition rate be

single wire

tandem

parallel twin wire

tandem, twin wire

substantially increased at constant electrical parameters, Figure 3.31. The increased deposition rate is realised by either the addition of a currentless wire (cold wire) or of a pre© ISF 2002

br-er3-30e.cdr

heated filler wire (hot wire).

Process Variations of Submerged-Arc Welding

The use of a rectangular Figure 3.30

2005

3. Submerged Arc Welding

46 strip instead of a wire electrode allows a higher current carrying capacity and opens the SA method also cold wire

iron powder/ chopped wire

for the wide application range of surfacing. However, the mentioned

hot wire

process variations can be

strip

over

combined

wide

© ISF 2002

br-er3-31e.cdr

ranges, where the elec-

Process Variations of Submerged-Arc Welding

trode distances and posi-

Figure 3.31

tions have to be appropriately

optimised,

Figure

3.32. Current type, polarity, geometrical co-ordination of the individual weld heads and the selected weld parameters also have substantial influence on the weld result.

1. WH

2. WH

65°

tandem welding

12..16

2. WH

1. WH

3. WH

~

=

~

deposition rate

100 kg/h

~

=

65°

three-wire welding

35

12..16

80 70 60

0

HW

2. WH 3. WH ~

=

three-wire, hot wire welding

15

~ 80°

four-wire welding br-er3-32e.cdr

~

~

~ 75°

15

18

three-wire tandem single wire

0

500

1000

1500

2000

2500

A

3500

9

Æ5,0 mm

speed = 40 cm/min

Æ 4,0 mm

6

Æ3,0 mm 3~ ~ 0 300 400 500 600

A

voltage = 30 V wire protrusion = 10d length

800

current intensity

12 © ISF 2002

Position of Wire Electrode in Submerged-Arc Multi-Wire Welding

Figure 3.32

four-wire

double wire

12 kg/h

10 10 35 12..16

~

single wire+ hot wire

current intensity

weld metal

1. WH

single wire+ metal powder

50 40 30 20 10

br-er3-33e.cdr

© ISF 2002

Application Fields for Submerged-Arc Process Variants

Figure 3.33 2005

3. Submerged Arc Welding

47

The description of these individual process variations of submerged arc welding shows that this method can be applied sensibly and economically over a very wide operating range, Figure 3.33. It is a high-efficiency welding process with a deposition rate of up to 100 kg/h. Due to large molten pools and flux application positional welding is not possible. When more than one wire is used in order to obtain a elektrode

high deposition rate, arc

(_)

(_)

interactions occur due to

+

+

+

_

+

_( ) +

_

+

_

~

magnetic arc blow, Figure 3.34. Therefore, the selection of the current

(+) _

type (d.c. or a.c.) and also arc

sensible phase displaceworkpiece

ments between the indi-

© ISF 2002

br-er3-34e.cdr

vidual welding torches are

Magnetic Interaction of Arcs at SA Tandem Welding

very important. Figure 3.34

2005

4. TIG Welding and Plasma Arc Welding

4. TIG Welding and Plasma Arc Welding

49

TIG welding and plasma welding belong to the group of the gas-shielded tungsten arc welding processes, Figure 4.1. In the gas-shielded tungsten arc welding processes mentioned in Figure 4.1, the arc burns between a non- consumable

tungsten electrode and the work-

piece or, in plasma arc weldGas-shielded arc welding

ing, between the tungsten electrode and a live copper electrode inside the torch.

Gas-shielded metal arc welding GMAW

Gas-shielded tungsten arc welding

Exclusively inert gases (Ar, He) are used as shielding gases.

Metal inert-gas welding MIG

narrow-gap gas-shielded arc welding

Metal active gas welding MAG

electrogas welding

CO2 welding

The potential curve of the

plasma metal arc welding

Mixed gas welding

Tungsten inert- Tungsten plasma gas welding welding with TIG electrode

Tungsten hydrogen welding

Plasma arc Plasma arc Plasma welding with welding with arc welding non-transferred with transferred semi-transferred arc arc arc

ideal arc, as shown in Figure © ISF 2002

br-er4-01e.cdr

4.2, can be divided into

Classification of Gas-Shielded Arc Welding acc. to DIN ISO 857

three characteristic sectors: 1.cathode- drop region

Figure 4.1

2.arc

l

3. anode-drop region In the cathode-drop region almost 50%

of the total

-

+

K

A L

voltage drop occurs over a

U

-4

length of 10 mm. US

A similarly high voltage drop

A: anode spot (up to 4000°C) K: cathode spot (approx. 3600°C) L: arc column (4500-20000°C) l: arc length

20 V

occurs in the anode-drop

10 0

region, here, however, over

10

-4

1

2

3

4 mm 5

l

arc potential curve (example)

0,5

a length of 0.5 mm. The © ISF 2002

br-er4-02e.cdr

voltage drop on the remain-

Arc Potential Curve

ing arc length is comparatively low. Main energy con-

Figure 4.2

version occurs accordingly in the anode-drop and cathode-drop region. Figure 4.3 shows the potential distribution by the example of a real TIG arc under the influence of different shielding gases. UA and UK have different values, the potential curve in the

2005

4. TIG Welding and Plasma Arc Welding

50

arc is not exactly linear. There is no discernible expansion of the cathode-drop and anodedrop region. 20 Argon 60 A V

UK = 6,5 V

25

10

UARC

4 mm 2

V

anode

5

cathode 20

1

3

2

4

6

mm

X ARC 40

he

15

4

Helium 60 A

V

2 n argo

UK = 6,5 V

UARC

lium

arc length

0

arc voltage

UA = 3,5 V

10

20 anode

10

cathode

0

50

100

150

200

250

350

A

weld current

UA = 6,1 V 0

1

2

3

4

mm

6

XARC br-er4--03e.cdr

© ISF 2002

br-er4-04e.cdr

Figure 4.3

© ISF 2002

Figure 4.4

The electrical characteris9 000 K

10 000 K x

x

x x

x

x

x

x

x

2

x

x

shielding gas, Figure 4.4. As

2

x

pending on the selected

8 000 K

TIG cathode

tics of the arc differ, de-

mm

mm x

x

4

x

4

x

the ionisation potential of helium in comparison with

x

x

x

8

4

3

2

6

x

must necessarily be higher.

x

6

x

argon is higher, arc voltage

1

0

1

2

mm

4

8

anode spot weld pool

The temperature distribubr-er4-05e.cdr

tion of a TIG arc is shown in

© ISF 2002

Temperature Distribution in a TIG Arc (at I=100 A)

Figure 4.5. Figure 4.5

2005

4. TIG Welding and Plasma Arc Welding

51

In TIG welding just approximately 30% of the input electrical energy may be used for

P = U.I

melting the base metal, Figure 4.6. Losses

welding direction

result from the arc radiation and heat dissipation in the workpiece and also from the heat

radiation

conversion in the tungsten electrode. R.I2 melting of wire

Figure 4.7 describes the process principle of TIG welding. Figure 4.8 explains by an example the code for a TIG welding wire, as stipulated in the

thermal conductivity [W/m K]

drafts of the European Standardisations. A table with the chemical compositions of the

fusion heat [kJ/kg] specific heat [kJ/kg K]

br-er4-06e.cdr

© ISF 2002

filler materials is shown in Figure 4.9. According to Figure 4.10, a conventional

Figure 4.6 TIG welding installation

tungsten electrode

consists of a transformer, a

electric contact

set of rectifiers and a torch.

shielding gas shielding gas nozzle

For most applications an

welding power source

filler metal

electrode with a negative polarity is used. However,

weld

for welding of aluminium, alternating current must be workpiece arc

used. For arc ignition a high-frequency high volt-

© isf 2002

br-er4-07e.cdr

Tungsten Inert Gas Welding (TIG)

age is superimposed and causes ionisation between

Figure 4.7

electrode and workpiece.

2005

4. TIG Welding and Plasma Arc Welding

52 The central part of the torch for TIG welding is

W 46 3 W2

the

chemical composition table

tungsten

electrode

which is held in a collet

rods and wires for tig-welding minimum impact energy value 47 J at -30°C

inside the torch body, Figure 4.11. The hose pack-

minimum weld metal yield point: 460 N/mm2 identification letter for TIG-welding

age contains the supply lines for shielding gas and

identification of filler rod as an individual product: W2

welding

current.

shielding

gas

The

nozzle

is

© ISF 2002

br-er4-08e.cdr

mostly made of ceramic.

Designation of a Tungsten Innert Gas Welding Wire to EN 1668

Manually operated torches

Figure 4.8

for TIG welding which are used for high amperages

as well as machine torches for long duty cycles are water-cooled. In order to keep the influence of torch distance variations on the current intensity and thus on the penetration depth as low as possible, power sources used for TIG welding always have a steeply dropping characteristic, Figure 4.12. The non-contact reignition of the A.C. TIG arc after a voltage zero crossover requires ionisation of the

electrode-workpiece

gap

by

high-frequent

high voltage pulses, Figbr-er4-09e.cdr

© ISF 2002

Chemical composition of filler rods and wires for TIG-welding

ure 4.13.

Figure 4.9

2005

4. TIG Welding and Plasma Arc Welding

53 When argon is used as a

mains

L1 L2 L3 N PE

shielding gas, metals as, for example, aluminium and

magnesium,

which

filter capacitor

high-frequency choke coil

_ O

have low melting points and

high voltage impulse generator

~

simultaneously

forming tight and high melt-

St

rectifier transformer SC: scattering core for adjusting the characteristic curve

also

ing oxide skins, cannot be + O

= ~

welded with a negative po-

selector switch

larity

electrode.

With

a

© ISF 2002

br-er4-10e.cdr

positive polarity, however,

Principle Structure of a TIG Welding Installation

a “cleaning effect” takes

Figure 4.10

place which is caused by the impact of the positive

charged ions from the shielding gas atmosphere on the negative charged work surface, thus destroying the oxide skin due to their large cross-section. However, as a positive polarity longer arc

torch cap with seal

shorter arc

R and U rise

R and U drop

I drops

I rises

handle of the torch control switch control cable

long

short

arc length

electrode collet collet case tungsten electrode gas nozzle

shielding gas supply

voltage

torch body with cooling device

cooling water supply

increasing

U

cooling water return with welding current cable

decreasing

current intensity br-er4-11e.cdr

© ISF 2002

br-er4-12e.cdr

decreasing

increasing

i © isf 2002

Construction of a Water-Cooled Torch for TIG Welding

Figure 4.11

Figure 4.12

2005

4. TIG Welding and Plasma Arc Welding

54

would cause thermal overload of the electrode, these materials are welded with alternating current. However, this has a disturbing side-effect. The electron emission and, consequently, the current flow are dependent on the temperature of the cathode. During the negative phase on the work surface the emission is, due to the lower melting temperature substantially lower than during the negative phase on the tungsten electrode. As a consequence, a positively connected electrode leads to lower welding current flows than this would be the case with a negatively connected electrode, Figure 4.14. A filter capacitor in the welding current circuit filters out the D.C. compovoltage

reignition of the arc by voltage impulses

nent which results in equal A.C.

half-wave

components.

With modern transistorised power sources which use

+

+

time

-

-

alternating current (square wave) for a faster zero cross-over, is duration and height of the phase com-

© ISF 2002

br-er4-13e.cdr

Reignition of the Tungsten A.C. Arc Through Voltage Impulses

Figure 4.13

ponents

adjustable.

The

electrode

thermal

and the

cleaning effect

stress

-

0

+

+

time

+

+

0

-

+

-

time

-

-

lower

+

-

+

+

smaller

current a

-

time

-

-

- time +

+

time

+

+

+

-

- time +

cleaning effect

stronger

heat load of the electrode

increasing

balanced half-wave components

0

with filter capacitor

without filter capacitor electronic controled power source

electrode polarity

may be freely influenced. current a

Figure 4.15 shows that the thermal

electrode

load

can be recognized from the shape of the electrode tip. While

the

normal-load

negative connected electrode end has the shape of a pointed cone (point angle

weld seam width © isf 2002

br-er4-14e.cdr

Influence of the Half-Wave Components during A.C. TIG Welding

approx. 10°), a flattened electrode tip is the result

Figure 4.14 2005

4. TIG Welding and Plasma Arc Welding

55

from a.c. welding (higher thermal load by positive half-waves).The tip of a thermally overloaded electrode is hemispherical and leads to a stronger spread of the arc and thus to wider welds with lower penetration.

materials: - steels, especially high-alloy steel - aluminium and aluminium alloys - copper and copper alloys - nickel and nickel alloys - titanium - circonium - tantalum

electrode for A.C. welding (alternating current)

electrode for D.C. welding (direct current)

workpiece thickness: - 0,5 - 5,0 mm weld types: - plain butt weld, V-type welds, flanged weld, fillet weld - all positions - surfacing

overloaded electrode

application examples: - tube to tube sheet welding - orbital welding - root welding

influence of the electrode shape on penetration profile

© ISF 2002

br-er4-15e.cdr

© ISF 2002

br-er4-16e.cdr

Electrode Shapes for TIG Welding

Applications of TIG Welding

Figure 4.15

Figure 4.16

preflow of the shielding gas

postflow of the shielding gas

movement in switch-on position

All fusion weldable materials can be joined using the TIG process; from the eco-

shielding gas

nomical point of view this orbital movement

applies especially to plate 0

360

0

welding current

thickness of less than 5 mm.

The

method

is,

moreover, predestined for rise of current

preheating

pulsing

current decay

br-er4-17e.cdr

overlap © ISF 2002

Flow Chart of TIG Orbital Welding

welding

root

passes

without backing support, Figure 4.16.

Figure 4.17 2005

4. TIG Welding and Plasma Arc Welding

56

For circumferential welding of fixed pipes, the TIG orbital welding method is applied. The welding torch moves orbitrally around the pipe, i.e., the pipe is welded in the positions flat, vertical down, overhead, contact tube

vertical-up and also inter-

tungsten electrode

pass welding is applied.

shielding gas nozzle

shielding gas

Moreover,

Ignition device

plasma gas nozzle

defect-free

weld bead overlap must be

plasma gas welding power source

filler material

a

surface weld

achieved. Orbital welding installations are equipped with process operational

workpiece

non-transferred arc

controls which determine the

appropriate

process

© isf 2002

br-er4-18e.cdr

parameters, Figure 4.17.

Plasma Arc Welding with Non-Transferred Arc

In plasma arc welding

Figure 4.18

burns the arc between the tungsten electrode (- pole) and the plasma gas nozzle (+ pole) and is called the “nontransferred” arc, Figure 4.18. The non-transferred arc is mainly used for metal-spraying and for the welding of metal-foil strips. In plasma arc welding with transferred arc burns the arc between the tungsten electrode (pole) and the workpiece (+ pole)

and

“transferred

is

called

arc”,

the

Figure

4.19. The plasma gas constricts the arc and leads to a more concentrated heat input than in TIG welding and allows thus the exploitation

contact tube

tungsten electrode

shielding gas nozzle shielding gas Ignition device

plasma gas nozzle plasma gas

welding power source

filler material seam

of the “keyhole effect”. Plasma arc welding with transferred arc is mainly

work piece transferred arc br-er4-19e.cdr

© isf 2002

Plasma Arc Welding with Transferred Arc

used for welding of joints. Figure 4.19

2005

4. TIG Welding and Plasma Arc Welding

57 Plasma arc welding with

contact tube

tungsten electrode

semi-transferred arc is a

shielding gas nozzle

combination ignition device

shielding gas

the

two

methods mentioned above.

conveying gas and welding filler (powder) welding power source

plasma gas

of

plasma gas nozzle

This

process

used

for

variant

is

microplasma

welding, plasma-arc pow-

surface weld non-transferred arc workpiece

transferred arc

der surfacing and weldjoining of aluminium, Figure 4.20

© ISF 2002

br-er4-20e.cdr

Plasma Arc Welding with Semi-Transferred Arc

The

Figure 4.20

plasma

welding

equipment includes, be-

sides the water-cooled welding torch, a gas supply for plasma gas (Ar) and shielding gas (ArH2-mixture, Ar/He mixture or Ar); the gas supply is, in most cases, separated, Figure 4.21. The copper anode and the additional focusing gas flow constrict the plasma arc which leads,

br-er4-21e.cdr

Figure 4.21

© ISF 2002

br-er4-22e.cdr

© ISF 2002

Figure 4.22 2005

4. TIG Welding and Plasma Arc Welding

58

in comparison with TIG welding, to a more concentrated heat input and thus to deeper penetration. An arc

Arc shapes of shielding gases:

that has been generated in

argon with 6,5% hydrogen helium 50% argon, 50% helium argon

and is not easy to deflect, as, for example, at work-

arc length

this way burns more stable

plasma gas: argon

piece edges, Figure 4.21. br-er4-23e.cdr

The TIG arc is cone shaped or

bell

shaped,

Arc Shapes in Microplasma Welding with Different Shielding Gases

respec-

tively, and has an aperture

© ISF 2002

Figure 4.23

angle of 45°. The plasma arc, in comparison, burns highly concentrated with almost parallel flanks, Figure 4.22. The shielding gas used in plasma arc welding exerts, due to its thermal conductivity, a deci-

plasma torch

sive influence onto the arc configuration. The use of a mixture of argon with hydrogen welding direction

results in the often desired cylindrical arc shape, Figure 4.23.

weld (seam)

In plasma arc welding of plates thicker than 2.5mm the so-called “keyhole effect” is utilweld surface

ised, Figure 4.24. The plasma jet penetrates the material, forming a weld keyhole. During

keyhole

welding the plasma jet with the keyhole moves along the joint edges. Behind the root

plasma jet as result of the surface tension and br-er4-24e.cdr

© ISF 2002

the vapour pressure in the keyhole, the liquid metal flows back together and the weld bead is created.

Figure 4.24

2005

4. TIG Welding and Plasma Arc Welding

59

Very thin sheets and metal-foils can be welded using microplasma welding with amperages between 0.05 and 50 A. Figures 4.25 and 4.26 show these

application

ples:

The

exam-

circumferential

weld in a diaphragm disk with a wall thickness of 0.15mm and the joining of fine metal grids made of CrNi steel.

© ISF 2002

br-er4-25e.cdr

Microplasma Welding of a Diaphragm Disk Made of CrNi

Figure 4.25

br-er4-26e.cdr

© ISF 2002

Figure 4.26 2005

5. Gas– Shielded Metal Arc Welding

5. Gas-Shielded Metal Arc Welding

61

The difference between gas-shielded metal arc welding (GMA) and the gas tungsten arc welding process is the consumable electrode. Essentially the process is classified as metal inert gas welding (MIG) gas-shielded arc welding (SG)

and

gas-shielded metal-arc welding (GMAW) metal inert gas welding (MIG) electrogas welding (MSGG) Narrow-gap gasshielded arc welding (MSGE)

tungsten gasshielded welding

metal active gas welding

plasma gas metal arc welding

tungsten inert-gas welding

tungsten plasma welding

hydrogen tungsten arc welding

(MAG)

(MSGP)

(TIG)

(WP)

(WHG)

plasma jet welding

plasma arc welding

(WPS)

(WPL)

plasma jet plasma arc welding (WPSL)

gas mixture gas metalarc CO2 metal-arc welding welding (GMMA)

(MAGC)

consumable electrode

non consumable electrode

br-er5-01e.cdr

metal

active

gas

welding (MAG). Besides, there are two more process variants,

the

electrogas

and the narrow gap welding

and

also

the

gas-

shielded plasma metal arc welding, a combination of both plasma welding and

© ISF 2002

MIG welding, Figure 5.1.

Classification of Gas-Shielded Arc Welding Processes

Figure 5.1

In contrast to TIG welding, where

the

electrode

is

normally negative in order to avoid the melting of the tungsten electrode, this effect is exploited in MIG welding, as the positive pole is

wire feed unit

strongly heated than the negative pole, thus improving the melting characteristics of the water cooling

feed wire.

shielding gas control device

Figure 5.2 shows the principle of a GMA weld-

control switch

ing installation. The welding power source is assembled

using

the

following

cooling water control

assembly

rectifier transformer

groups: The transformer converts the mains voltage to low voltage which is subsequently

welding power source

rectified. Apart from the torch cooling and the shielding br-er5-02e.cdr

© ISF 2002

gas control, the process control is the most GMA Welding Installation

important installation component. The process control ensures that once set welding data are adhered to.

Figure 5.2

2005

5. Gas-Shielded Metal Arc Welding

62

A selection of common welding installation variants is depicted in Figure 5.3, where the universal device with a separate wire feed housing is the most frequently used variant in the industry. compact device

3 to 5m

universal device

5, 10 or 20m 3 to 5m

mini-spool device

10, 20 or 30m

push-pull device 1 torch handle 2 torch neck 3 torch trigger 4 hose package 5 shielding gas nozzle 6 contact tube 7 contact tube fixture 8 insulator 9 wire core 10 wire guide tube 11 wire electrode 12 shielding gas supply 13 welding current supply

5 to 10m

© ISF 2002

br-er5-03e.cdr

Manual Gas-Shielded Arc Welding Torch

Types of Welding Installations

Figure 5.3

© ISF 2002

br-er5-04e.cdr

Figure 5.4

Figure 5.4 shows in detail a manually operated inert-gas shielded torch with the common swan-neck shape. A machine torch has no handle and its shape is straight or swan-necked. The hose package contains the wire core and also supply lines for shielding gas, current and cooling water, the latter for contact tube cooling. The current is transferred to the wire electrode over the contact tube. The shielding gas nozzle is shaped to ensure a steady gas flow in the arc space, thus protecting arc and molten pool against the atmosphere. A so-called “Two-Wire-Drive” wire feed system is of the most simple design, as shown in Figure 5.5. The wire is pulled off a wire reel and fed into the hose package. The wire transport roller, which shows different grooves depending on the used material, is driven by an electric motor. The counterpressure roller generates the frictional force which is needed for wire feeding.

2005

5. Gas-Shielded Metal Arc Welding

63

1

4-roller drive

2

4

4

3

1

3

2

F

4

4

3 1

2

1 wire guide tube 2 drive rollers 3 counter pressure rollers 4 wire guide tube

2

planetary drive 3

direction of rotation

5

6

1 wire reel

3 wire transport roll

2 wire guide tube

4 counter pressure roll

3

5 wire feed roll with a V-groove for steel electrodes 6 wire feed roll with a rounded groove for aluminium br-er5-05e.cdr

1 © ISF 2002

br-er5-06e.cdr

© ISF 2002

Wire Feed System

Figure 5.5

1 wire guide tube 2 roller holding device 3 drive rollers

2

Wire Drives

Figure 5.6

More complicated but following the same operation principle is the “Four-Wire-Drive”, Figure 5.6. Here, the second pair of rollers guarantees higher feeding reliability by reducing the risk of wheel slip. Another design among the wire feed drive systems is the planetary drive, where the wire is fed in axial direction by the motor. A rectilinear rotation-free wire feed motion is the outcome of the welding voltage

motor rotation and the angular offset of the drive rollers

time

which

are

firmly

welding current

connected to the motor shaft. time

1 ms 1 mm

Figure

5.7

depicts

the

metal transfer in the short arc © ISF 2002

br-er5-07e.cdr

Short-Circuiting Arc Metal Transfer

Figure 5.7

range.

During

the

burning phase of the arc, material is molten and ac2005

5. Gas-Shielded Metal Arc Welding

64

cumulates at the electrode end. The voltage drops slowly while the arc shortens. Electrode and workpiece make contact and a short-circuit occurs. In the short-circuit phase is the liquid

the molten pool. The narrowing liquid root and the

welding current

result of surface tension into

welding current

electrode material drawn as

rising current lead to a very high current density that causes a sudden evapora-

time

time

tion of the remaining root. The arc is reignited. The choke effect

low

short-arc technique is par-

medium

br-er5-08e.cdr

© ISF 2002

ticularly suitable for out-ofChoke Effect

position and root passes welding.

welding current

welding current

Figure 5.8

time

welding voltage

welding voltage

time

time

time br-er5-09e.cdr

© ISF 2002

br-er5-10e.cdr

Long Arc

Figure 5.9

© ISF 2002

Spray Arc

Figure 5.10

2005

5. Gas-Shielded Metal Arc Welding

65

The limitation of the rate of the current rise during the short-circuit phase with a choke leads to a pointed burn-off process which is smoother and clearly shows less spatter formation, Figures 5.8 In shielding gases with a 35

C1 shielding gas composition: C1: CO2 M21: 82% Ar, 18% CO2 M23: 92% Ar, 8% O2

welding voltage

V

long arc

high CO2 proportion a

M21 M23

long arc is formed in the upper power range, Figure

25

5.9. Material transfer is 20

undefined and occurs as mixed circuiting arc

15

short arc contact tube distance: approx. 15 mm 150 3,5 br-er5-11e.cdr

4,5

illustrated in Figures 5.13

spray arc

and

contact tube distance: approx. 19 mm

5.14.

Short-circuits

with very strong spatter

200 welding current

250

A

300

5,5 7,0 wire feed

8,0

m/min

10,5

formation are caused by

© ISF 2002

the formation of very large

Welding Parameters in Dependence on the Shielding Gas Mixture (SG 2, Ø1,2 mm)

droplets at the electrode

Figure 5.11

end.

If the inert gas content of the shielding gas exceeds 80%, a spray arc forms in the upper characterised by a non-short-circuiting and spray-like material transfer. For its high deposition rate the spray arc is used for welding filler

thermal conductivity

power range, Figure 5.10. The spray arc is

helium

hydrogen

CO2 nitrogen

and cover passes in the flat position. argon

Connections between welding parameters,

temperature

shielding gas and arc type are shown in Figure 5.11. When the shielding gas M23 is used,

argon 82%Ar+18%CO2

CO2

helium

the spray arc may already be produced with an amperage of 260 A. With the decreasing argon proportion the amperage has to be increased

br-er2-12e.cdr

© ISF 2002

in order to remain in the spray arc range. When pure carbon dioxide is applied, the spray arc Figure 5.12 2005

5. Gas-Shielded Metal Arc Welding

66

cannot be produced. Figure 5.11 shows, moreover, that with the increasing CO2 content the welding voltage must also be increased in order to achieve the same deposition rate. current-carrying arc core

The different thermal conductivity of the shielding gases has a considerable influence

temperature

on the arc configuration and weld geometry, Figure 5.12. Caused by the low thermal conductivity of the argon the arc core becomes r

r

argon

carbon dioxide

Fa

F Fr

wire elektrodes

Fr F

argon

current-carrying arc core

Fa carbon dioxide

br-er5-13e.cdr

© ISF 2006

argon

Influence of Shielding Gas on Forces in the Arc Space

carbon dioxide

Figure 5.13 very hot – this results in a deep penetration in the weld centre, the so-called “argon fingertype

penetration”.

Weld

reinforcement

is br-er5-14e.cdr

© ISF 2002

strongly pronounced. Application of CO2 and helium leads, due to the better thermal conductivity of these shielding gases, to a wide and

Figure 5.14

deep penetration. A recombination (endothermic break of the linkage in the arc space – exothermal reaction 2CO + O2 ->2CO2 in the workpiece proximity) intensifies this effect when CO2 is used. In argon, the current-carrying arc core is wider and envelops the wire electrode end, Figure 5.13. This generates electromagnetic forces which bring about the detachment of the liquid electrode material. This so-called “pinch effect” causes a metal transfer in small drops, Figure 5.14.

2005

5. Gas-Shielded Metal Arc Welding

67 The pointed shape of the arc attachment in carbon dioxide produces a reverse-direction

acceleration due to gravity

force component, i.e., the molten metal is wire electrode

electromagnetic force FL (pinch effect)

pushed up until gravity has overcome that force component and material transfer in the form of very coarse drops appear.

viscosity surface tension S

droplets necking down

backlash forces fr of the evaporating material

inertia electrostatic forces

suction forces, plasma flow induced

Besides the pinch effect, the inertia and the gravitational force, other forces, shown in Figure 5.15, are active inside the arc space; however these forces are of less importance. If the welding voltage and the wire feed speed are further increased, a rotating arc occurs

work piece br-er5-15e.cdr

© ISF 2002

Forces in Arc Space

after an undefined transition zone, Figure 5.16. High-efficiency MAG welding has been applied since the beginning of the nine-

Figure 5.15

ties; the deposition rate, when this process is

used, is twice the size as, in comparison, to spray arc welding. Apart from a multicomponent gas with a helium proportion, also a high-rating power source and a precisely controlled wire feed system for high wire feed speeds are necessary. Figure

5.17

depicts

the

deposition rates over the wire feed speed, as achievable

with

efficiency

modern MAG

high-

welding

processes. During the transition from the short to the spray arc the drop frequency rate inbr-er5-16e.cdr

creases erratically while the

© ISF 2002

Rotating Arc

drop volume decreases at Figure 5.16 2005

5. Gas-Shielded Metal Arc Welding

68 the same degree. With an

25 deposition rate

increasing

Ø 1,2 mm

kg/h

high performance GMA welding

20

this

“critical

current

range” moves up to higher

Ø 1,0 mm

15

power ranges and is, with

10

Ø 0,8 mm

conventional GMA

inert gas constituents of lower than 80%, hardly

5 0

CO2-content,

achievable thereafter. This 5

0

10

15

20

25

30

35

40

45 m/min

effect

wire feed speed br-er5-17e.cdr

facilitates

the

pulsed-arc welding tech-

© ISF 2002

nique, Figure 5.18.

Deposition Rate

Figure 5.17

In pulsed-arc welding, a change-over

occurs

be-

tween a low, subcritical background current and a high, supercritical pulsed current. During the background phase which corresponds with the short arc range, the arc length is ionised 300

300

200

200 critical current range

100

100

UEff

3

V arc voltage

10 cm

drop volume

number of droplets

35 -4

1/s

25 20 Um

15 10 5

500

0

0

200

A

400

tP

0

600

A 400 welding current

Ikrit

Im

- background current IG - pulse voltage UP - impulse time tP - background time tG or frequency f with f = 1 / ( tG + tP), resp. - wire feed speed vD

350 300 IEff

250 200

Im

150 100 50 0

time

IG

tG

Setting parameters:

0

br-er5-18e.cdr

© ISF 2002

5

br-er5-19e.cdr

10

15 time

20

ms

30 © ISF 2002

Pulsed Arc

Figure 5.18

Figure 5.19

2005

5. Gas-Shielded Metal Arc Welding

69

welding current

and wire electrode and work surface are preheated. During the pulsed phase the material is molten and, as in spray arc welding, superseded

by

the

pulsed current intensity Non-short-circuiting metal tranfer range

backround current intensity

magnetic

forces. Figure 5.20.

time

Figure 5.19 shows an example of pulsed arc real

© isf 2002

br-er5-20e.cdr

current path and voltage

Pulsed Metal Transfer

time curve. The formula for Figure 5.20

mean current is:

Im =

1T idt T ∫0

for energy per unit length of weld is:

Ieff =

1T 2 i dt T ∫0

By a sensible selection of welding parameters, the GMA welding technique allows a selection of different arc types which 50

are distinguished by their

working range welding current / arc voltage 45

metal transfer way. Figure shows

the

40

setting

range for a good welding process in the field of conventional GMA welding.

spray arc

optimal setting lower limit upper limit

35 voltage [v]

5.21

30 transition arc 25 short arc shielding gas: 82%Ar, 18%CO2 wire diameter: 1,2 mm wire type: SG 2

20 15

Figure 5.22 shows the extended setting range for the

10 50

75

100

125

150

175 200 225 250 welding current

275

350

375

400

Parameter Setting Range in GMA Welding

ing process with a rotating arc.

325

© ISF 2002

br-er5-21e.cdr

high-efficiency MAGM weld-

300

Figure 5.21

2005

5. Gas-Shielded Metal Arc Welding

70

Some typical applications of the different arc types are depicted in Figure 5.23. The rotating arc, (not mentioned in the figure), is applied in just the same way as the spray arc, however, it is not used for the welding of copper and aluminium. The arc length within the

filler metal: SG2 -1,2 mm shielding gas: Ar/He/CO2/O2-65/26,5/8/0,5

working range is linearly dependent on the set weld-

V

The weld seam shape is

30

voltage

ing voltage, Figure 5.24. considerably influenced by

rotating arc

50 transition zones spray arc

high-efficiency spray arc

20

the arc length. A long arc

high-efficiency short arc

10

produces a wide flat weld

short arc

seam and, in the case of 100

fillet welds, generally under-

200

br-er5-22e.cdr

cuts. A short arc produces a

300 welding current

400

A

600

Quelle: Linde, ISF2002

Setting Range or Welding Parameters in Dependence on Arc Type

narrow, banked weld bead. Figure 5.22

On the other hand, the arc length is inversely proportional to the wire feed speed, Figure 5.25. This has influence on the current over the internal adjustment with a slightly dropping power source characteristic. This again is of considerable importance for the deposition rate, i.e., a low wire feed speed leads to a low deposition rate, the result is flat penetration and low base metal fusion. At a constant weld speed and a high wire feed speed a deep penetration can be obtained. arc types

intensity is

pendent

on

the

de-

contact

tube distance, Figure 5.26. With a large contact tube distance, the wire stickout is longer

and

is

therefore

applications

current

seam type, positions workpiece thickness

At equal arc lengths, the

welding methods MAGC MAGM MIG

spray arc

long arc

-

aluminium copper steel unalloyed, lowalloy, high-alloy

fillet welds or inner passes and cover passes of butt welds at medium-thick or thick components in position PA, PB welding of root layers in position PA

characterised by a higher

short arc aluminium (s < 1,5 mm)

steel unalloyed, low-alloy

steel unalloyed, low-alloy, steel low-alloy, high-alloy high-alloy

steel unalloyed, low-alloy

steel unalloyed, low-alloy

fillet welds or inner passes and cover passes of butt welds at medium-thick or thick components in position PA, PB

fillet welds or butt welds fillet welds or inner at thin sheets, all positions passes and cover passes of thin and root layers of butt welds medium-thick at medium-thick or thick components, all components, all positions positions inner passes and cover passes of fillet or butt welds in position PC, PD, PE, PF, PG (out-of-position)

br-er5-23e.cdr

ohmic

resistance

pulsed arc aluminium copper

-

root layer welds only conditionally possible

© ISF 2002

which Applications of Different Arc Types

leads to a decreased current Figure 5.23

2005

5. Gas-Shielded Metal Arc Welding

71

arc length: long medium short

U AL AM AK

U

AL

AM

AK

arc length: long medium short

vD, I vD, I operating point: welding voltage: arc length:

AL

AM

AK

high long

medium medium

low short

operating point: wire feed speed: arc length: welding current: deposition efficiency:

weld appearance butt weld

AL

AM

AK

low long low low

medium medium medium medium

high short high high

weld appearance: weld appearance fillet weld

br-er5-25e.cdr

© ISF 2002

br-er5-24e.cdr

© ISF 2002

Wire Feed Speed

Welding Voltage

Figure 5.24

Figure 5.25 intensity. For the adjustment of the contact tube distance, as a thumb rule, ten to twelve times the size of the wire diameter should be

contact tube-to-work distance lk

lk1

lk2

lk3

The torch position has considerable influ-

3

30

considered.

ence on weld formation and welding proc-

mm 2

20

operating rule: lk = 10 to 12 dD

pointed in forward direction of the weld, a part

1

10

ess, Figure 5.27. When welding with the torch of the weld pool is moved in front of the arc.

0 200

250

300 A

This results in process instability. However, it

350

current wire electrode:

1,2 mm diameter

shielding gas:

82% Ar + 18% CO2

arc voltage:

29 V

wire feed speed:

8,8 m/min

welding speed:

58 cm/min

br-er5-26e.cdr

ha s the advantage of a flat smooth weld surface with good gap bridging. When welding with the torch pointed in reversing direction of

© ISF 2002

the weld, the weld process is more stable and Contact Tube-to-Work Distance

Figure 5.26

the penetration deeper, as base metal fusion 2005

5. Gas-Shielded Metal Arc Welding

72 by the arc is better, although the weld bead

advance direction

surface is irregular and banked. Figure 5.28 shows a selection of different application areas for the GMA technique and the appropriate shielding gases.

penetration:

shallow

average

deep

gap bridging:

good

average

bad

arc stability:

bad

average

good

spatter formation: strong

average

low

weld width:

average

narrow

average

rippled

The welding current may be produced by different welding power sources. In d.c. welding the transformer must be equipped with downstream rectifier assemblies, Figure 5.29. An additional ripple-filter choke suppresses the

wide

residual ripple of the rectified current and has weld appearance: smooth

br-er5-27e.cdr

also a process-stabilising effect. With the development of efficient transistors

© ISF 2002

the design of transistor analogue power

Torch Position

sources became possible, Figure 5.29. The Figure 5.27

operating principle of a transistor analogue

power source follows the principle of an audio frequency amplifier which amplifies a low-level to a high level input signal, possibly distortion-free. The transistor power source is, as conventional power sources, also equipped with a three-phase transformer, with generally only one secondary tap. The secondary voltage is rectified by silicon diodes into full wave opera-

transistor

cascade.

The

welding voltage is steplessly industrial sections

adjustable until no-load voltage is reached. The difference between source voltage and welding voltage reduces at the transistor cascade and produces a

shielding gases

and fed to the arc through a

chemical-apparatus engineering shopwindow construction pipe production aluminium-working industry nuclear engineering aerospace engineering fittings production electrical engineering industry automotive industry motor car accessories materials-handling technology sheet metal working crafts motor car repair steel production boiler and tank construction machine engineering structural steel engineering agricultural machine industry rail car production

Argon 4.6 Argon 4.8 Helium 4.6 Ar/He-mixture Ar + 5% H2 or 7,5% H2 99% Ar + 1% O2 or 97% Ar + 3% O2 97,5% Ar + 2,5% CO2 83% Ar + 15% He + 2% CO2 90% Ar + 5% O2 + 5% CO2 80% Ar + 5% O2 + 15% CO2 92% Ar + 8% O2 88% Ar + 12% O2 82% Ar + 18% CO2 92% Ar + 8% CO2 forming gas (N2-H2-mixture)

tion, smoothed by capacitors

application examples autoclaves, vessels, mixers, cylinders panelling, window frames, gates, grids stainless steel pipes, flanges, bends spherical holders, bridges, vehicles, dump bodies reactors, fuel rods, control devices rocket, launch platforms, satellites valves, sliders, control systems stator packages, transformer boxes passenger cars, trucks radiators, shock absorbers, exhausts cranes, conveyor roads, excavators (crawlers) shelves (chains), switch boxes braces, railings, stock boxes mud guards, side parts, tops, engine bonnets attachments to flame nozzles, blast pipes, rollers vessels, tanks, containers, pipe lines stanchions, stands, frames, cages beams, bracings, craneways harvester-threshers, tractors, narrows, ploughs waggons, locomotives, lorries

br-er5-28e.cdr

comparatively

high

stray

© ISF 2002

Fields of Application of Different Shielding Gases

power which, in general, Figure 5.28

2005

5. Gas-Shielded Metal Arc Welding

73

makes water-cooling necessary. The efficiency factor is between 50 and 75%. This disadvantage is, however, accepted as those power sources are characterised by very short reaction times (30 to 50 µs). Along with the development of transistor analogue power sources, the consequent separation of the power section (transthree-phase transformer

fully-controlled three-phase bridge rectifier

energy store

former and rectifier) and

transistor power section

mains supply

electronic

welding current

control

took

place. The analogue or digital control sets the refuist u1 . . un

erence values and also

iist

controls the welding procreference input values

signal processor (analog-to-digital)

current pickup

ess. The power section operates exclusively as an

© isf 2002

br-er5-29e.cdr

amplifier for the signals

GMA Welding Power Source, Electronically Controlled, Analogue

coming from the control.

Figure 5.29 The output stage may also be carried out by clocked cycle. A secondary clocked transistor power source features just as the analogue power sources, a transformer and a rectifier, Figure 5.30. The transistor unit functions as an on-off switch. By varying the on-off period, i.e., of the pulse duty factor, the average voltage at the output of the transistor stage may be varied. The arc voltage achieves small ripples, which are of a limited amplitude, in the switching frequency of, in general, 20 kHz; whereas the welding current shows to be strongly smoothed during the high pulse frequencies caused by

3-phase transformer

3-phase bridge rectifier

energy store

transistor switch

protective reactor welding current

mains supply

inductivities. As the transistor unit has only a switching function, the stray power is

Uist U1 . . Un

lower than that of analogue sources.

The

reference input values

efficiency

Iist

signal processor (analog-to-digital)

current pickup

factor is approx. 75 – 95%. br-er5-30e.cdr

The reaction times of these

© ISF 2002

GMA Welding Power Source, Electronically Controlled, Secondary Chopped

clocked units are within of Figure 5.30

2005

5. Gas-Shielded Metal Arc Welding

74 300 – 500 µs clearly longer than those of analogue

3-phase bridge rectifier

filter

energy storage

transistor inverter

medium frequency transformer

power sources.

rectifier welding current

mains supply

Series

regulator

power

sources, the so-called “inverter power sources”, dif-

Uist U1 . . Un

Iist

reference input values

fer widely from the afore-

signal processor (analog-to-digital)

current pickup

mentioned

welding

ma-

chines, Figure 5.31. The © ISF 2002

br-er5-31e.cdr

GMA Welding Power Source, Electronically Controlled, Primary Chopped, Inverter

Figure 5.31

alternating voltage coming from the mains (50 Hz) is initially rectified, smoothed and converted into a me-

dium frequency alternating voltage (approx. 25-50 kHz) with the help of controllable transistor and thyristor switches. The alternating voltage is then transformer reduced to welding voltage levels and fed into the welding process through a secondary rectifier, where the alternating voltage also shows switching frequency related ripples. The advantage of inverter power sources is their low weight. A transformer that transforms voltage with frequency of 20 kHz, has, compared with a 50 Hz transformer, considerably lower magnetic losses, that is to say, its size may accordingly be smaller and its weight is just 10% of that of a 50 Hz transformer. Reaction time and efficiency factor are compa-

manufacturer insulations class

rotary current welding rectifier

~ type

_

protective IP21 system

VDE 0542 production number

welding MIG/MAG U0 15 - 38 V input 3~50Hz 6,6 kVA (DB) cos
0,72

F

cooling type

F

rable to the corresponding

DIN 40 050

values

switchgear number

S

35A/13V - 220A/25V

power range

X 60% ED 100% ED 170 A I2 220 A

power capacity in dependence of current flow

U2 25 V

23 V

U1 220 V

I1 26 A

U1 380 V

I1 15 A

17 A 10 A

U1

V

I1

A

A

U1

V

I1

A

A

power supply

power sources. All welding power sources plate, Figure 5.32. Here the performance capability

© ISF 2002

Rating Plate

switching-type

are fitted with a rating

min. and max. no-load voltage br-er5-32e.cdr

of

and the properties of the power source are listed.

Figure 5.32 2005

5. Gas-Shielded Metal Arc Welding

75 The S in capital letter (former K) in the middle shows that the power source is suitable for welding operations

under

hazardous

situations, i.e., the secona

seamless flux-cored wire electrode

b

c

dary no-load voltage is lower than 48 Volt and

form-enclosed flux-cored wire electrode

therefore not dangerous to the welder. br-er5-33e.cdr

© ISF 2002

Cross-Sections of Flux-Cored Wire Electrodes

Besides the familiar solid

Figure 5.33

wires also filler wires are used

for

gas-shielded

metal arc welding. They consist of a metallic tube and a flux core filling. Figure 5.33 depicts common cross-sectional shapes. Filler wires contain arc stabilisators, slag-forming and also alloying elements which support a stable welding process, help to protect the solidifying weld from the atmosphere and, more often than not, guarantee symbol R

slag characteristics rutile base, slowly soldifying slag rutile base, rapidly soldifying slag basic filling: metal powder

P B M V W

rutile- or fluoride-basic fluoride basic, slowly soldifying slag fluoride basic, slowly soldifying slag other types

Y S

customary application* S and M

shielding gas **

very

good

mechanical

C and M2

S and M

C and M2

S and M S and M S S and M

C and M2 C and M2 without without

S and M

without

properties. An

important

distinctive

criteria is the type of the filling. The influence of the filling is very similar to that of the electrode covering in

*) S: single pass welding - M: multi pass welding **) C: CO2 - M2: mixed gas M2 according to DIN EN 439

manual electrode welding (see chapter 2). Figure

br-er5-34e.cdr

© ISF 2002

Type Symbols of Flux-Cored Wire Electrodes According to DIN EN 12535

5.34 shows a list of the different types of filler wire.

Figure 5.34

2005

6. Narrow Gap Welding, Electrogas - and Electroslag Welding

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

77

Up to this day, there is no universal agreement about the definition of the term “Narrow Gap Welding” although the term is actually self-explanatory. In the international technical literature,

the

characteristics Process characteristics: - narrow, almost parallel weld edges. The small preparation angle has the function to compensate the distortion of the joining members - multipass technique where the weld build-up is a constant 1 or 2 beads per pass - usually very small heat affected zone (HAZ) caused by low energy input

process mentioned

in the upper part of Figure 6.1

are frequently

con-

nected with the definition Advantages: - profitable through low consumption quantities of filler material, gas and/ or powder due to the narrow gaps - excellent quality values of the weld metal and the HAZ due to low heat input - decreased tendency to shrink

for narrow gap welding. In

Disadvantages - higher apparatus expenditure, espacially for the control of the weld head and the wire feed device - increased risk of imperfections at large wall thicknesses due to more difficult accessibility during process control - repair possibilities more difficult

spite of these “definition difficulties”

all

questions

about the universally valid advantages and disadvan-

© ISF 2002

br-er6-01e.cdr

tages of the narrow gap

Narrow Gap Welding

welding method can be clearly answered.

Figure 6.1

The numerous variations of narrow gap welding are, in general, a further development of the conventional welding technologies. Figure 6.2 shows a classification with emphasis on several important process characteristics. Narrow gap TIG welding with cold or hot wire addition is mainly applied as an orbital process method or for the joining of highalloy

as

ferrous

well

as

metals.

nonThis

method is, however, hardly applied in the practice. The other processes are more

widely spread and

are explained in detail in

submerged arc electroslag narrow narrow gap welding gap welding process with straightened wire electrode (1P/L, 2P/L, 3P/L) process with oscillating wire electrode (1P/L) process with twin electrode (1P/L, 2P/L) process with lengthwise positioned strip electrode (2P/L) flat position br-er6-02e.cdr

the following.

process with linearly oscillating filler wire

process with stripshaped filler and fusing feed

gas-shielded metal arc narrow gap welding

electrogas process with linearly oscillating wire electrode electrogas process with bent, longitudinally positioned strip electrode

vertical up position

tungsten innert gas-shielded narrow gap welding

process with hot wire addition (1P/L, 2P/L) MIG/MAGprocesses (1P/L,2P/L,3P/L) process with cold wire addition (1P/L, 2P/L)

all welding positions © ISF 2006

Survey of Narrow Gap Welding Techniques Based on Conventional Technologies

Figure 6.2

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

78

In Figure 6.3, a systematic subdivision of the GMA narrow gap welding no wire-deformation

various GMA narrow gap technologies is shown. In accordance with this, the fundamen-

long-wire method (1 P/L, 2 P/L) thick-wire method (1 P/L, 2 P/L)

tal distinguishing feature of the methods is whether the process is carried out with or without wire deformation. Overlaps in the structure

GMA narrow gap welding wire-deformation

D

result from the application of methods where a

A

twin-wire method (1 P/L)

tandem-wire method (1 P/L, 2 P/L, 3 P/L) twisted wire method (1 P/L)

rotation method (1 P/L)

single or several additional wires are used.

corrugated wire method with mechanical oscillator (1 P/L) corrugated wire method with oscillating rollers (1 P/L) corrugated wire method with contour roll (1 P/L) zigzag wire method (1 P/L) wire loop method (1 P/L)

While most methods are suitable for single pass per layer welding, other methods require a weld build-up with at least two passes per layer. A further subdivision is made in accordance with the different types of arc moveexplanation: P/L: Pass/Layer

ment.

C

A: method without forced arc movement B: method with rotating arc movement C: method with oscillating arc movement D: method with two or more filler wires

br-er6-03e.cdr

In the following, some of the GMA narrow gap

B

coiled-wire method (1 P/L)

© ISF 2006

Survey and Structure of the Variations of Gas-Shielded Metal Arc Narrow Gap Welding

technologies are explained: Using the turning tube method, Figure 6.4, side

Figure 6.3

wall fusion is achieved by the turning of the contact tube; the contact tip angles are set in degrees of between 3° and 15° towards the torch axis. With an electronic stepper motor control, arbitrary transverse-arc oscillating motions with defined dwell periods of oscillation and oscillation frequencies can be realised - independent of the filler wire properties. In contrast, when the

corrugated

wire

corrugated wire method with mech. oscillator 1

method

1

oscillation is produced by

3

4

4

5

5

6

6

the plastic, wavy deformation of the wire electrode.

1 - wire reel 2 - mechanical oscillator for wire deformation 3 - drive rollers 4 - inert gas shroud 5 - wire feed nozzle and shielding gas tube 6 - contact tip

8 - 10

12 - 14

The 1 - wire reel 2 - drive rollers 3 - wire mechanism for wire guidance 4 - inert gas shroud 5 - wire guide tube and shielding gas tube 6 - contact tip

mechanical

oscillator is applied, arc

2

2 3

with

deformation

is

ob-

tained by a continuously swinging oscillator which is fixed above the wire feed

br-er 6-04e.cdr

Principle of GMA Narrow Gap Welding

rollers. Amplitude and frequency of the wave motion

Figure 6.4 2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

79

can be varied over the total amplitude of oscillation and the speed of the mechanical oscillator or, also, over the wire feed speed. As the

plate thickness: gap preparation:

300 mm square-butt joint, 9 mm flame cut elctrode diameter: 1.2 mm amperage: 260 A pulse frequency: 120 HZ arc voltage: 30 V welding speed: 22 cm/min -1 wire oscillation: 80 min oscillation width: 4 mm shielding gas: 80% Ar/ 20% Co2 primery gas flow: 25 l/min secondary gas flow: 50 l/min number of passes: approx. 70

contact tube remains stationary, very narrow gaps with widths from 9 to 12 mm with plate thicknesses of up to 300 mm can be welded. Figure 6.5 shows the macro section of a GMA narrow gap welded joint with plates (thickness: 300 mm) which has been produced by the mechanical oscillator method in approx. 70 passes. A highly regular weld build-up and an almost straight fusion line with an extremely narrow heat affected zone

br-er6-05e.cdr

© ISF 2002

can be noticed. Thanks to the correct setting of the oscillation parameters and the precise, centred torch manipulation no sidewall fusion

Figure 6.5

defects occurred, in spite of the low sidewall

penetration depth. A further advantage of the weave-bead technique is the high crystal restructuring rate in the weld metal and in the basemetal adjacent to the fusion line – an advantage that gains good toughness properties. Two

narrow-gap

welding

variations with a rotating

rotation method 1

spiral wire method 1

arc movement are shown in

2 3

2 3

Figure 6.6. When the rota-

4

tion method is applied, the

4

5

arc movement is produced

6

5

from a contact tube nozzle which is rotating with fre-

1 - wire reel 2 - drive rollers 3 - mechanism for nozzle rotation 4 - inert gas shroud 5 - shielding gas nozzle 6 - wire guiding tube

1 - wire reel 2 - wire mechanism for wire deformation 3 - drive rollers 4 - wire feed nozzle and shielding gas supply 5 - contact piece

9 - 12

ing wire electrode (1.2 mm)

13 - 14

by an eccentrically protrud-

br-er 6-06e.cdr

quencies between 100 and

Principle of GMA Narrow Gap Welding

150 Hz. When the wave Figure 6.6

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

80

wire method is used, the 1.2 mm solid wire is being spiralwise deformed. This happens before it enters the rotating 3 roll wire feed device. With a turning speed of 120 to 150 revs per minute the welding wire is deformed. The effect of this is such that after leaving the contact piece the deformed wire creates a spiral diameter of 2.5 to 3.0 mm in the gap – adequate enough to weld plates with thicknesses of up to 200 mm at gap widths between 9 and 12 mm with a good sidewall fusion. Figure 6.7 explains two tandem method

twin-wire method

GMA narrow gap welding

1

1

350

methods which are oper-

2 3

2

4

3

5

4

6

ated without forced arc movement, where a reliable sidewall fusion is ob-

5

9 - 12

1 - wire reel 2 - deflection rollers 3 - drive rollers 4 - inert gas shroud 5 - shielding gas nozzle 6 - wire feed nozzle and contact tip

1 - wire reel 2 - drive rollers 3 - inert gas shroud 4 - wire feed nozzle and shielding gas supply 5 - contact tips

15 - 18

tained either by the wire deflection through constant deformation (tandem wire method) or by forced wire

br-er 6-07e.cdr

Principle of GMA Narrow Gap Welding

Figure 6.7

deflection with the contact tip (twin-wire method). In both cases, two wire electrodes

with

thicknesses

between 0.8 and 1.2 mm are used. When the tandem technique is applied, these electrodes are transported to the two weld heads which are arranged inside the gap in tandem and which are indeFigure pendently selectable. When the twin-wire method is applied, two parallel switched electrodes are transported by a common wire feed unit, and, subsequently, adjusted in a common sword-type torch at an incline towards the weld edges at small spaces behind each other (approx. 8 mm) and molten. In place of the SA narrow gap welding methods, mentioned in Figure 6.2, the method with a lengthwise positioned strip electrode as well as the twin-wire method are explained in more detail, Figure 6.8. SA narrow gap welding with strip electrode is carried out in the multipass layer technique, where the strip electrode is deflected at an angle of approx. 5° towards the edge in order to avoid collisions. After completing the first fillet weld and slag removal the opposite fillet is made. Solid wire as well as cored-strip electrodes with widths between 10

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

strip electrode

81

and 25 mm are used. The gap width is, depending on the number of passes per layer,

SO stick out s

α

s a x α

gap width electrode deflection distance of strip tip to flank twisting angle

h w

bead hight bead width

so x

a

h

f

between 20 and 25 mm. SA twin-wire welding is, in general, carried out using two electrodes (1.2 to 1.6 mm) where one electrode is deflected towards one weld edge and the other towards the bottom of the groove or to-

w

twin-wire electrode

wards the opposite weld edge. Either a single pass layer or a two pass layer technique are

vw

s

H z

a h

vw a H z

weld speed electrode deflection stick out distance torch - flank

s h w p

gap width bead height bead width penetration depth

applied. Dependent on the electrode diameter and also on the type of welding powder, is the gap width between 12 and 13 mm. Figure 6.9 shows a comparison of groove

p

w

shapes in relation to plate thickness for SA

br-er6-08e.cdr

Submerged Arc Narrow Gap Welding

welding (DIN 8551 part 4) with those for GMA welding (EN 29692) and the unstandardised,

Figure 6.8 10°

7°

mainly used, narrow gap welding. Depending on the plate thickness, significant differences in

s

8

the weld cross-sectional dimensions occur

s

8

16

which may lead to substantial saving of material and energy during welding. For example, when welding thicknesses of 120 mm to 200 mm with the narrow gap welding technique,

double-U butt weld SA-DU weld preparation (8UP DIN 8551) 8°

square-edge butt weld SA-SE weld preparation (3UP DIN 8551) 10

66% up to 75% of the weld metal weight are saved, compared to the SA square edge weld.

s

s

3

6

3

The practical application of SA narrow gap welding for the production of a flanged calotte joint for a reactor pressure vessel cover is de-

double-U butt weld GMA-DU weld preparation (Indexno. 2.7.7 DIN EN 29692)

narrow gap weld GMA-NG weld preparation (not standardised)

br-er6-09e.cdr

picted in Figure 6.10. The inner diameter of the

Comparison of the Weld Groove Shape

pressure vessel is more than 5,000 mm with Figure 6.9

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

82

wall thicknesses of 400 mm and with a height of 40,000 mm. The total weight is 3,000 tons. The weld depth at the joint was 670 mm, so it had been necessary to select a gap width of at least 35 mm and to work in the three pass layer technique.

workpiece

wire guide

electrode shielding gas +

arc weld pool Cu-shoe

weld advance weld metal

water

designation: gas-shielded metal arc welding (GMAW acc. DIN 1910 T.4) position: vertical (width deviations of up to 45°) plate thickness: 6 - 30 mm square-butt joint or V weld seam 30 mm double-V weld seam materials: unalloyed, lowalloy and highalloy steels gap width: 8 - 20 mm electrodes: only 1 (flux-cored wire, for slag formation between copper shoe and weld surface) Ø 1.6 - 3.2 mm 350 - 650 A amperage: 28 - 45 V voltage: weld speed: 2 - 12 m/h shielding gas: unalloyed and lowalloy steels CO2 or mixed gas (Ar 60% and 40% Co2) highalloy steels: argon or helium br-er6-10e_sw.cdr

© ISF 2002

br-er6-11e.cdr

Electrogas Welding

Figure 6.10

Figure 6.11

Electrogas welding (EG) is characterised by a vertical groove which is bound by two watercooled copper shoes. In the groove, a filler wire electrode which is fed through a copper nozzle, is melted by a shielded arc, Figure 6.11. During this process, are groove edges fused. In relation with the ascending rate of the weld pool volume, the welding nozzle and the copper shoes are pulled upwards. In order to avoid poor fusion at the beginning of the welding, the process has to be started on a run-up plate which closes the bottom end of the groove. The shrinkholes forming at the weld end are transferred onto the run-off plate. If possible, any interruptions of the welding process should be avoided. Suitable power sources are rectifiers with a slightly dropping static characteristic. The electrode has a positive polarity.

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

83

The application of electrogas welding for lowalloyed steels is – more often than not - limited, as the toughness of the heat affected zone with

1 2 3 4 5 6 7 8 9

the complex coarse grain formation does not meet sophisticated demands. Long-time exposure to temperatures of more than 1500°C and

1. base metal 2. welding boom 3. filler metal 4. slag pool 5. metal pool

low crystallisation rates are responsible for this. 6. copper shoe 7. water cooling 8. weld seam 9. Run-up plate

The same applies to the weld metal. For a more wide-spread application of electrogas welding, the High-Speed Electrogas Welding Method has been developed in the ISF. In this process, the gap cross-section is reduced and additional metal powder is added to increase the deposition rate. By the increase of the welding speed, the dwell times of weld-

designation: position: plate thickness: gap width: materials: electrodes:

resistance fusion welding vertical (and deviation of up to 45°) 30 mm (up to 2,000 mm) 24 - 28 mm unalloyed, lowalloy and highalloy steels 1 or more solid or cored wires Ø 2.0 - 3.2 mm plate thickness range per electrode: fixed 30 - 50 mm oscillated: up to 150 mm amperage: 550 - 800 A per electrode voltage: 35 - 52 V welding speed: 0.5 - 2 m/h slag hight: 30 - 50 mm br-er6-12e.cdr

adjacent regions above critical temperatures

Electroslag Welding

and thus the brittleness effects are significantly Figure 6.12

reduced.

Figure 6.12 shows the process principle of Electroslag Welding. Heating and melting of the groove faces occurs in a slag bath. The temperature of the slag bath must always exceed the melting temperature of the metal. The Joule effect, produced when the current is transferred through

the

conducting

bath, keeps the slag bath ~

temperature constant. The powder

welding current is fed over

slag

one or more endless wire ignition with arc

powder fusion

the

slag

highly

heated

slag

bath. Molten pool and slag

molten pool weld metal

start of welding

electrodes which melt in

bath which both form the welding

end of welding © ISF 2002

br-er6-13e.cdr

Process Phases During ES Welding

weld pool are, sideways retained

by

the

groove

faces and, in general, by

Figure 6.13 2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

84

water-cooled copper shoes which are, with the complete welding unit, and in relation with the welding speed, moved progressively upwards. To avoid the inevitable welding defects at the beginning of the welding process (insufficient penetration, inclusion of unmolten powder) and at the end of the welding (shrinkholes, slag inclusions), run-up and run-off plates are used. The electroslag welding process can be divided into four process phases, Figure 6.13. At the beginning of the welding process, in the so-called “ignition phase”, the arc is ignited for a short period and liquefies the non-conductive welding flux powder into conductive slag. The arc is extinguished as the electrical conductivity of the arc length exceeds that of the conductive slag. When the desired slag bath level is reached, the lower ignition parameters (current and voltage) are, during the so-called “Data-Increase-Phase”, raised to the values of the stationary welding process. This occurs on the run-up plate. The subsequent actual welding process starts, the process phase. At the end of the weld, the switch-off phase is initiated in the run-off plate. The solidifying slag bath is located on the run-off plate which is subsequently removed. The electroslag welding with consumable feed wire (channel-slot welding) proved to be very useful for shorter welds. The copper sliding shoes are replaced by fixed Cu cooling bars and the welding nozzle by a steel tube, Figure 6.14. The length of the sheathed steel tube, in general, corresponds with the weld seam length (mainly shorter than 2.500 mm) and the steel tube melts during welding in the ascending slag bath. Dependent on the plate thickness, welding can be carried out with one single or with several wire and strip electrodes. A feature

of

this

process

drive motor

run-off plate workpiece

workpiece

= ~

fusing feed nozzle

Also curved

seams can be welded with a bent consumable elec-

position: vertical plate thickness: 15 mm materials: unalloyed, lowalloy and highalloy steels welding consumables:

the easier welding area preparation.

Electroslag fusing nozzle process (channel welding)

welding cable

variation is the handiness of the welding device and

wire or strip electrode

workpiece cable workpiece

trode. As the groove width

run-up plate copper shoes workpiece

wire electrodes: Ø 2.5 - 4 mm strip electrodes: 60 x 0.5 mm plate electrodes: 80 x60 up to 10 x 120 mm fusing feed nozzle: Ø 10 - 15 mm welding powder: slag formation with high electrical conductivity

copper shoes br-er 6-14e.cdr

can

be

duced

significantly rewhen

Electroslag Welding with Fusing Wire Feed Nozzle

comparing Figure 6.14

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

85

with conventional processes, and a strip shaped filler material with a consumable guide piece is used, this welding process is rightly placed under the group of narrow gap welding techniques. Likewise in electrogas welding, the electroslag welding process is also characterised by a large molten pool with a – simultaneously - low heating and cooling rate. Due to the low cooling rate good degassing and thus almost porefree hardening of the slag bath is possible. Disadvantageous, however, is the formation of a coarse-grain structure. There are, however, possibilities to improve the weld properties, Figtechnological measures post weld heat treatment

ure 6.15.

metallurgical measures increase of purity

decrease of peak temperature and dwell times at high temperatures

application of suitable base and filler metals addition of suitable alloy and micro-alloy elements (nucleus formation)

increase of welding speed reduction of energy per unit length

To avoid postweld heat treatment the electroslag welding process with lo-

continuous normalisation furnace normalisation

increase of deposit rate

decrease of groove volume

application of several wire electrodes, metal powder addition

V, double-V butt joints, multi-pass technique

reduction of S-, P-, H2-, N2and O2 - contents and other unfavourable trace elements

C-content limits Mn, Si, Mo, Cr, Ni, Cu, Nb, V, Zr, Ti

cal continuous normalisation has been developed for plate thicknesses

br-er 6-15e.cdr

of up to approx. 60 mm,

Possibilities to Improve Weld Seam Properties

Figure 6.16. The welding temperature in the weld

Figure 6.15

region drops below the Ar1temperature and is subsequently heated to the nor-

temperature °C 1. filler wire 2. copper shoes 3. slag pool 4. metal pool 5. water cooling 6. slag layer 7. weld seam 8. distance plate 9. postheating torch 10. side plate 11. heat treated zone

malising

2 2000 1500 900

7 8 9

500 10 950

11

1 2 3 4 5 6 7 8 9 10

temperature

(>Ac3). The specially designed torches follow the copper shoes along the weld seam. By reason of the residual heat in the workpiece the necessary

br-er 6-16e.cdr

temperature ES Welding with Local Continuous Normalisation

can

be

reached in a short time.

Figure 6.16 2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

86

In order to circumvent an expensive postheat weld treatment which is often unrealistic for use on-site, the electroslag high-speed welding process with multilayer technique has been developed. Similar to electrogas welding, the weld cross-section is reduced and, by application of a twin-wire electrode in tandem arrangement and addition of metal powder, the weld speed is increased, as in contrast to the conventional technique. In the heat affected zones toughness values are determined which correspond with those of the unaffected base metal. The slag bath and the molten pool of the first layer are retained by a sliding shoe, Figure 6.17. The weld preparation is a double-V butt weld with a gap of approx. 15 mm, so the carried along sliding shoe seals the slag and the metal bath. Plate preparation is, as in conventional electroslag welding, exclusively done by flame cutting. Thus, the advantage of easier weld preparation can be maintained.

12 11 1

1

2

2

3

3

4

4

9

9

5

5

6

6

7

7

8

8

4

10

br-er6-17e.cdr

4 1 magnetic screening 2 metal powder addition 3 tandem electrode 4 water cooling 5 copper shoe (water cooled) 6 slag pool 7 molten pool 8 solidified slag 9 welding powder addition 10 weld seam

ES-welding in 2 passes with sliding shoe

Figure 6.17

© ISF 2002

10

br-er6-18e.cdr

1 magnetic screening 2 metal powder supply 3 three-wire electrode 4 water cooling 5 copper shoe (water cooled) 6 slag pool 7 molten pool 8 solidified slag 9 welding powder supply 10 weld seam 11 first pass 12 second pass

ES-welding of the outer passes

© ISF 2002

Figure 6.18

For larger plate thicknesses (70 to 100 mm), the three passes layer technique has been developed. When welding the first pass with a double-V-groove preparation (root width: 20 to 30 mm; gap width: approx. 15 mm) two sliding shoes which are adjusted to the weld

2005

6. Narrow Gap Welding, Electrogas- and Electroslag Welding

87

groove are used. The first layer is welded using the conventional technique, with one wire electrode without metal powder addition. When welding the outer passes flat Cu shoes are again used, Figure 6.18. Three wire electrodes, arranged in a triangular formation, are used. Thus, one electrode is positioned close to the root and on the plate outer sides two electrodes in parallel arrangement are fed into the bath. The single as well as the parallel wire electrodes are fed with different metal powder quantities which as outcome permit a welding speed 5 times higher than the speed of the single layer conventional technique and also leads to strong increase of toughness in all zones of the welded joint. If wall thicknesses of more than 100 mm are to be welded, several twin-wire electrodes with metal powder addition have to be used to reach deposition rates of approx. 200 kg/h. The electroslag welding process is limited by the possible crack formation in the centre of the weld metal. Reasons for this are the concentration of elements such as sulphur and phosphor in the weld centre as well as too fast a cooling of the molten pool in the proximity of the weld seam edges.

2005

7. Pressure Welding

7. Pressure Welding

89

Figure 7.1 shows a survey of the pressure welding processes for joining of metals in accordance with DIN 1910. In gas pressure welding a distinction is made between open square and closed square gas pressure welding, Figure 7.2. Both methods use efficient gas torches to bring the workpiece ends up to the welding temperature. When the welding temperature is reached, both joining members are butt-welded by the application of axial force when a flash edge forms. The long preheating time leads to the formation of a coarsegrained structure in the

welding

joining area, therefore the pressure welding

fusion welding

gas pressure welding

resistance pressure welding

induction pressure welding

conductive pressure welding

resistance spot welding

welds are of low toughness values. As the process is friction welding

operated independently

mainsand

the

process equipment is low in

projection welding

roll seam welding

pressure butt welding

flash butt welding

weight and also easy to handle, the main applica-

br-er7-01e.cdr

tion areas of gas pressure

Classification of Welding Processes acc. to DIN 1910

welding are the welding of

Figure 7.1

reinforcement steels and of pipes in the building trade.

initial state: gap closed

initial state: gap opened (for special cases)

gas flame torch in the open gap stationary

In pressure butt welding, the input of the necessary

mobile

welding heat is produced workpiece closed gap

ring-shaped burner (sectional view) pressure

1. heating

by resistance heating. The necessary axial force is

2. torch positioning 3. welding by rapid pressing

completed weld seam working cycles: 1. heating 2. welding by pressing

applied by copper clamping jaws which also receive the current supply, Figure 7.3.

br-er7-02e.cdr

Open Square and Closed Square Gas Pressure Welding

The current circuit is closed over the abutting surfaces

Figure 7.2 2005

7. Pressure Welding

90

of the two joining members where, by the increased interface resistance, the highest heat generation is obtained. After the welding temperature - which is lower than the melting temperature of the weld metal – is reached, upset

before upset force has been applied

upset force

pressure is applied and the current circuit is opened. This produces a thick flash-

water-cooled clamping chucks (Cu electrodes)

free upset seam which is typical for this method. In order to guarantee the uni-

bulging at the end of the weld

_ ~

form heating of the abutting faces, they must be conbr-er7-03e.cdr

formable in their cross-

Process Principle of Pressure Butt Welding

sectional sizes and shapes with each other and they

Figure 7.3

must have parallel faces. As no molten metal develops during pressure upset butt welding, the joining surfaces must be free from contaminations and from oxides. Suitable for welding are unalloyed and lowalloy steels. The welding of aluminium and copper ma-

fixed clamping chuck

mobile clamping chuck

a+b b 2

terial is, because of the

clamping force

a

tendency towards oxidation and

good

conductivity,

possible only up to a point. For the most part, smaller cross-sections

steel chuck

with

copper shoe secondary side

sur-

faces of up to 100 mm² are

primary side welding transformer

welded. Areas of applica-

a = flashing length b = upset loss

br-er7-04e.cdr

tions are chain manufactur-

Schematic Structure of a Flash Butt Welding Equipment

ing and also extensions of wires in a wire drawing

Figure 7.4

shop.

2005

7. Pressure Welding

91

A flash butt welding equipment is, in its principal structure, similar to the pressure butt welding device, Figure 7.4. While in pressure upset butt welding the joining members are always strongly pressed together, in flash butt welding only “fusing contact” is made during the heating phase. During the welding process, the workpiece ends are progressively advanced towards each other until they make contact at several points and the current circuit is over these contact bridges closed. As the local current density at these points is high, the heating also develops very fast. The metal is liquified and, partly, evaporated. The metal vapour pressure causes the liquified metal to be thrown out of the gap. At the same time, the metal vapour is generating a shielding gas atmosphere; that is to say, with the exception of pipe welds, welding can be carried out without the use of shielding gas. The constant creation and destruction of the contact bridges causes the abutting faces to “burn”, starting from the contact points, with heavy spray-type ejection. Along with the occurrence of material loss, the parts are progressively advanced towards each other again. New contact bridges are created again and again. When the entire abutting face is uniformly fused, the two workpiece ends are, through a high axial force, abruptly pressed together and the welding current is switched off. This way, a narrow, sharp and, in contrast to friction welding, irregular weld edge is produced during the upsetting pro-

br-er7-05e.cdr

© ISF 2002

gress, which, if necessary, can be easy mechanically removed while the weld is still warm, Figure 7.5.

Figure 7.5

In flash butt welding, a fundamental distinction is made between two different working techniques. During hot flash butt welding a preheating operation precedes the actual flashing process, Figure 7.6. The preceding resistance heating is carried out by “reversing”, i.e., by the changing short-circuiting and pressing of the joining surfaces and by the mechani-

2005

7. Pressure Welding

92

cal separation in the reversed motion. When the joint ends are sufficiently heated, is the flashing process is initialised automatically and the following process is similar to cold flash butt welding. In contrast to cold flash butt welding, the advantage of hot flash butt welding is that, on one hand, sections of 20 times the size can be welded with the same machine efficiency and, on the other hand, a smaller temperature drop and with that a lower cooling rate in the workpiece can be obtained. This is of importance, especially with steels which because of their chemical composition have a tendency to harden. The cooling rate may also be reduced by conductive reheating inside the machine.

upset travel

A smooth and clean sur-

flashing travel

face is not necessary with hot flash butt welding. If the abutting faces differ greatly

upset force

from the desired planeparallelism, an additional

preheating

flashing

flashing

amperage

flashing process (a short

time

flashing period with low

hot flash welding

time

cold flash welding

br-er7-06e.cdr

speed and high energy)

Flashing Travel, Upset Travel, Upset Force and Welding Current in Timely Order

may be carried out first. Figure 7.6

The welding area of the structure of a flash butt heat affected zone

weld shows a zone which

10 mm

is reduced in carbon and other

alloying

elements,

Figure 7.7.

Moreover,

flash

welded

butt

material: C60 E

all

0,1 mm

joints

have a pronounced coarse grain zone, whereby the weld

coarse grain zone

fine grain zone

soft-annealing zone

base metal

toughness properties of the br-er7-07e.cdr

welded joint are lower than

Secondary Structure Along a Flash Butt Weld

of the base metal. By the impact normalizing effect in

Figure 7.7

2005

7. Pressure Welding

93

the machine successive to the actual welding process, can the toughness properties be considerably increased. By one or several current impulses the weld temperatures are increased by up to approximately 50° over the austeniting temperature of the metal. Steels, aluminium, nickel and copper alloys can be welded economically with the flash butt welding process. Supported by the axial force, shrinkage in flash butt welding is so insignificant that only very low residual stresses occur. It is, therefore, possible to weld also steels with a higher carbon content.

n

n F1 friction force

F2 upset force

br-er7-08e.cdr

© ISF 2002

br-er7-09e_sw.cdr

Phases of Friction Welding Process

Figure 7.8

Figure 7.9

Profiles of all kind are butt welded with this method. The method is used for large-scale manufacture and with components of equal dimensions. The weldable cross-sections may reach dimensions of up to 120,000 mm². Areas of application are the welding of rails, the manufacture of car axles, wheel rims and shafts, the welding of chain links and also the manufacture of tools and endless strips for pipe production. Friction welding is a pressure welding method where the necessary heat for joining is produced by mechanical friction. The friction is, as a rule, generated by a relative motion be-

2005

7. Pressure Welding

94 tween a rotating and a stationary workpiece

clamping tool

brake

clamping tool

workpiece

clutch

while axial force is being applied at the same

pressure element for axial pressure

time, Figure 7.8. After the joint surfaces are adequately heated, the relative motion is discontinued and the friction force is increased to upsetting force.

conventional friction welding driving motor

flywheel

An even, lip-shaped bead is produced which may be removed in the welding machine by

clamping tool

clamping tool workpiece

pressure element for axial pressure

an additional accessory unit. The bead is often considered as the first quality criterion. Figure 7.9 shows all phases of the friction welding process. In most cases this method

flywheel friction welding

is used for rotation-symmetrical parts. It is, br-er7-10e.cdr

© ISF 2002

nowadays, also possible to accurately join rectangular and polygonal cross-sections.

Figure 7.10

The most common variant of friction weld-

ing is friction welding with a continuous drive and friction welding with a flywheel drive, Figure 7.10. In friction welding with continuous drive, the clamped-on part to be joined is kept at a constant nominal speed by a drive, while the workpiece in the sliding chuck is pressed with a defined friction force. The nominal speed is maintained until the demanded temperature

profile

has friction welding time 1...100s

been achieved. Then the motor is declutched and the relative motion is neu-

braking 0,1...0,5s

1800...

number of revolutions

5400 min

friction welding time 0,125...2s

900...

-1

-1

5400min

time

tralised by external braking. In general, the friction force

axial pressure

40...280 -2

20...100 Nmm

is raised to upsetting force

40...280

-2

-2

Nmm

Nmm

after the rotation movement has

been

discontinued.

During flywheel friction

torque

conventional friction welding

flywheel friction welding

br-er7-11e.cdr

welding, the inertia mass

Comparison of the Welding Processes for Conventional and Flywheel Friction Welding

is raised to nominal speed, Figure 7.11

2005

7. Pressure Welding

95

the drive motor is declutched and the stationary workpiece is, with a defined axial force, pressed against the rotating workpiece. Welding is finished when the total kinetic energy stored in the flywheel – has been consumed by the friction processes. This is the so-called self-breaking effect of the system. The method is used when, based on metallurgical processes, extremely short welding times may be realised. Further process variants are radial friction welding, orbital friction welding, oscillation friction welding and friction surfacing. However, these process variants are until today still in the experimental stage. Recently, new developments in the field of friction stud welding – studs on plates – have been introduced. Figure 7.11 depicts the variation in time of the most important process parameters in friction welding with continuous drive and flywheel friction welding. The occuring moments’ maxima may be interpreted as follows: The first maximum, at the start of the frictional contact, is explained by the formation of local fusion zones and their shearing off in the lower temperature range. The torque decreases as a result of the risen temperature - which again is a consequence of the increased plasticity - and of the lowered deformation resistance. The second maximum is generated during the braking phase which precedes the spindle standstill. The second maximum is explained by number of revolutions

the increased deformation resistance

at

temperatures.

upset force

dropping The

tem-

perature drop in the joining friction force

zone is explained by the lowered energy input – due to the rotation-speed decrease – and also by the augmented

radial

reduction

dis-

time © ISF 2002

br-er7-12e.cdr

placement of the highly

Combined Friction Welding

heated material into the weld upset.

Figure 7.12

In friction welding with a continuous drive, the process variation “combined friction welding” allows the free and independent from each other selection of the braking and upsetting moments, Fig. 7.12.

2005

7. Pressure Welding

96

In this case, the rotation-energy which has been stored in the drive motor, the spindle and also in the clamping chuck, may be totally or partially converted by self-breaking. Here, the breaking phase matches the breaking phase in flywheel welding. The use of this process variant allows the welding structures to influence each other in a positive way when many welding tasks are to be carried out. Moreover, the torque range may be accurately predetermined by the microcontroller of the braking initiator which prevents the slip-through of the workpieces in the clamping chuck. The universal friction welding machine is in its struc-

a)

b)

P

ture similar to a turning lathe,

however,

for

P

the

transmission of the high

c)

axial forces, the machine

P

d)

P

P

P

structure must be considerably more rigid.

e)

Basically, there are three

f)

P

P

P

types of friction welding: a) Types of Friction Welding Processes

ing workpiece and a translational motion of the other

© ISF 2002

br-er7-13e.cdr

friction welding with a rotat-

Figure 7.13

workpiece; b) friction weldbefore welding

ing with rotation and transone

1. a)round stock with round stock

workpiece facing a station-

b) round stock with round stock, chamfered

ary other workpiece, c)

before welding 4. pipe with pipe

1..2°

5. round material with plate g/d » 0,25...0,3

d » 0,6D

stationary

intermediate

piece. The remaining variations,

b) round stock with round stock (different cross-sections, bevelled)

shown

in

3.

d 0,75d

7. round material with plate, without preparation

d

two workpieces against a

d=0,75D

8. pipe with plate, without preparation

round stock with pipe

Fig-

(1/6)d © ISF 2002

br-er7-14e.cdr

ure 7.13, also find applications

when

both

Joint Types Obtained by Friction Welding

work-

pieces have to rotate in

d

6. pipe with plate

2. a)round stock with round stock (different cross-sections, partially machined) D

rotation and translation of

after welding

g

of

D

motion

d

lational

after welding

Figure 7.14 2005

7. Pressure Welding

97

opposite direction to each other. For example, when a low diameter and, in connection with this, the low relative speeds demand the necessary heat quantity. A survey of possible joint shapes achievable with friction welding is given in Figure 7.14. The specimen preparation of the joining members should, if possible, be carried out in a way that the heat input and the heat dissipation is equal for both members. Depending on the combination of materials can this provision facilitate the joining task considerably. The abutting surfaces should be smooth, angular and of equal dimensions. A simple saw cut is, for

cirkon tungsten vanadium titanium tantalum stellite free cutting steel cast steel steel, austentic steel, high alloyed steel, alloyed steel, unalloyed silver niobium nickel alloys nickel molybdenum brass magnesium copper cobalt hard metal, sintered cast iron (GGG, GT) lead aluminium, sintered aluminium alloys aluminium

many applications, sufficient.

aluminium aluminium alloys aluminium, sintered lead cast iron (GGG, GT) hard metal, sintered cobalt copper magnesium brass molybdenum nickel nickel alloys niobium silver steel, unalloyed steel, alloyed steel, high alloyed steel, austentic cast steel free cutting steel stellite tantalum titanium vanadium tungsten cirkon

The method of heat generation causes a comparatively low joining temperature – lower than the melting temperature of the metals. This is the main reason why friction welding is the suitable method for metals and material combinations which are difficult to weld. It is also possible to weld material combinations (e.g. Cu/Al or Al/steel) which cannot be joined using other welding processes otherwise only

friction weldable restricted friction weldable not friction weldable not tested

with

increased

expenditure.

Figure 7.15

shows a survey of possible material combinations. Many combinations have, however,

br-er7-15e.cdr

© ISF 2002

not yet been tested on their suitability to friction welding. Metallurgical reasons which may reduce the friction weldability are:

Figure 7.15 1. the quantity and distribution of non-metal inclusions, 2. formation of low-melting or intermetallic phases, 3. embrittlement by gas absorption (as a rule, the costly, inert gas shielding can be dispensed with, even when welding titanium), 4. softening of hardened or precipitataly-hardened materials and 5. hardening caused by too high a cooling rate.

2005

7. Pressure Welding

98

By the adjustment of the welding parameters in respect toweld joints, can in most cases joints with good mechano-technological properties be obtained. The secondary structure along the friction-welded metal: S235JR 10 mm

joint is depicted in Fig-

p = 30 N/mm2 t =6s 2 tSt = 250 N/mm n = 1500 U/min

ure 7.16.

An

fine-grained

extremely structure

(forge structure) develops

1 mm

in the joining zone region.

structures on parallels with a 5 mm distance from the sample axis

This

structure

which

is

typical of a friction-welded base metal

heat affected zone

transition heat affected zone - weld metal

weld metal

10 µm

joint is characterised by

br-er7-16e.cdr

high strength and tough-

Secondary Structure Along a Friction Weld

ness properties.

Figure 7.16 Figure 7.17 shows a comparison between a flash butt-welded and a friction-welded cardan shaft. The two welds are distinguished by the size of their heat affected zone and the development of the weld upset. While in friction welding a regular, lip-shaped upset is produced, the weld flash formation in flash butt welding is narrower and sharper and also considerably more irregular. Besides, the heat affected zone during friction welding is substantially smaller than during flash butt weld-

flash butt welding

ing. Friction

welding

machines

are

fully

mechanized and may well be integrated into production

lines.

Loading

and

unloading

equipment, turning attachments for the preparation of the abutting surfaces and for upset friction welding

br-er7-17e.cdr

removal and also storage units for © ISF 2002

complete welding programs make these machines well adaptable to automation. The machines may furthermore be equipped with

Figure 7.17 2005

7. Pressure Welding

99

1

2

3

4

1 cardan shaft, AIZn 4,5 Mg 1 2 cardan shaft, retracted tube

1,2 joint ring

3 loading device

material combination: Cf53/ Ck45

4 unloading grippers

br-er7-18e_sw.cdr

3 cardan shaft, flattening test specimen 4 crown wheel, 16MnCr5/ 42Cr4 5 bimetal valve, X45CrSi9-3/ NiCr20 TiAl © ISF 2002

Figure 7.18

br-er7-19e_sw.cdr

© ISF 2002

Figure 7.19 parameter supervisory systems. During welding are parameters: welding path, pressure, rotational speed, and time are governed by the desired value/actual value comparison. This allows an indirect quality control. A further complement to the retension of parameters is the torque control, however this method is costly and it cannot be used for all applications because of its structural dimensions.

1 pump shaft 2 shaft C22E/ E295 3 press cylinder S185/9S 20K 4 hydraulic cylinder S235J3G2/ C60E or S235JR/ C15 5 cylinder case S235JR/ S355J2G3 6 piston rod 42Cr4 7 connecting rod 100Cr6/ S235JR 8 stud S235J2G3/ X5CrNi18-10 9 knotter hook 15CrNi6 br-er7-20e_sw.cdr

Friction welding machines are mainly used in the series production and industrial mass production. © ISF 2002

Nevertheless, these machines are also always applied when metals and material com-

Figure 7.20

2005

7. Pressure Welding

100

binations which are difficult to weld have to be joined in a reliable and cost-effective way. With the machines that are presently used in Germany, it is possible to weld massive workpieces

in

the

diameter

range of 0.6 up to 250 mm

friction welds

160 mm

Ø40 mm

Ø30 mm

For steel pipes, the maximum weldable diameter is at present approximately

940 mm forged piece motor shaft

900 mm, the wall thick-

friction-welded piece € 20,-

flange,forged material costs shaft Ø30 und 40 mm 2x friction welds incl. upset removal

€ 20,-

€

7,50

€

4,25

€

3,-

€

14,75

nesses are approx. 6 mm. Figures 7.18 to 7.20 show a selection of examples for

br-er7-21e.cdr

the application of friction

Cost Comparison of Forging/ Friction Welding in a Case of a Cardan Shaft

welding.

Figure 7.21 Figure 7.21 shows a comparison of the cost expenditure for the manufacture of a cardan shaft, carried out by forging and by friction welding, respectively. It shows that the application of the friction welding method may save approx. 20% of the production costs. This comparison is, however, not an universally valid statement as for each component a profitability evaluation must be carried out separately. The comparison is just to show that, in many applications, considerable savings can be made if the matter of the joining technology by “friction welding” could be circulated to a wider audience of design and production engineers. Figure 7.22 shows in brief the important advantages and disadvantages of friction

Advantages and disadvantages of friction welding in comparison with the competitive flash butt welding advantages: - clean and well controllable bulging - low heat influence on joining members - better control of heat input - low phase seperation phenomena in the bond zone - hot forming causes permanent recovery and recrystallisation processes in the welding area thus forming a very fine-grained structure with good toughness and strength properties (forged structure) - low susceptibility to defects, extremely good reproducibility within a wide parameter range - frequently shorter welding times - more choice in the selection of weldable materials and material combinations disadvantages: - torque-safe clamping necessary - machine-determined smaller maximum weldable cross-sections - susceptibility to non-metal inclusions - high expenditure requested because of high manufacturing tolerances - high capital investment for the machine br-er7-22e.cdr

© ISF 2002

welding in comparison with the competitive method of flash butt welding. Figure 7.22 2005

7. Pressure Welding

101

Pressure welding with magnetically impelled arc, “Magnetarc Welding”, is an arc pressure welding method for the joining of closed structural tubular shapes, Figure 7.23. The weldable 1. starting position

wall

thickness

range is between 0.7 and

a) both workpieces are brought into contact b) welding current and magnetic field are switched on

5 mm, the weldable diame-

2. starting of welding

ter range between 5 and

a) both workpieces are seperated until a defined gap width is reached (retracting movement) - the arc ignites

300 mm.

In

“Magnetarc

Welding” an arc burns be-

3. heating a) the arc rotates b) the joint surfaces are melting

tween the joining surfaces and is rotated by external

4. completion of welding a) both workpieces are broght into contact again and upset b) welding current and magnetic field are switched off

magnetic forces. This is achieved by a magnet coil

br-er7-23e.cdr

system that produces a

Diagrammatic Representation of Magnetic Arc Welding

magnetic field.

Figure 7.23 The combined action of this magnetic field and the arc’s own magnetic field effects a tangential force to act upon the arc. The rotation of the arc heats and melts the joint surfaces. After

malleable

normally not removed. The welding operation

cast steel

materials

free cutting steel

gether. A regular weld upset develops which is

steel, unalloyed

piece members are pressed and fused to-

steel, lowalloyed

an adequate heating operation, the two work-

takes place under shielding gas (mainly CO2). steel, unalloyed

The shielding gas’ function is not the protection of the weld from the surrounding atmos-

steel, lowalloyed free cutting steel

phere but rather a contribution towards the stabilisation of the arc. The reproducibility of

cast steel

the arc ignition and motion behaviour and the

malleable

regularity of the weld bead are therefore imsuitable for magnetic arc welding

proved. The prerequisite for the application of a mate-

br-er7-24e.cdr

not tested

© ISF 2002

rial in “Magnetarc Welding” is its electrical conductivity and melting behaviour. FigFigure 7.24 2005

7. Pressure Welding

102

ure 7.24 gives a survey of the material combinations which are nowadays already weldable under industrial conditions. As reason is the symmetric heat input, the subsequent upsetting

of

the

liquid

phase and the cooling off under

pressure.

The © ISF 2002

br-er7-25e_sw.cdr

cracking sensitivity of the Applications for Magnetic Arc Welding

welds is, in general, relatively low. This has a posi-

Figure 7.25

tive effect, particulary when steels with a high carbon content or machining steels are welded. The joining faces of the workpieces must be free from contamination, such as rust or scale. To obtain a defect-free weld, normally a simple saw cut is a sufficient preparation of the abutting surfaces. If special demands are put on the dimensional accuracy of the joints, the prefabrication tolerances have to be adjusted accordingly. This applies also to friction welding. Figures 7.25 and 7.26 show several application examples of pressure welding with magnetically impelled arc. Figure 7.27 shows a summary of the most important advantages and disadvantages of br-er7-26e_sw.cdr

© ISF 2002

this method in comparison with the competitive methods of friction welding and flash butt welding.

Figure 7.26

2005

7. Pressure Welding

103 In friction-stir welding a cylindrical, mandrellike tool carries out rotating self-movements

Advantages and disadvantages of magnetic arc welding in comparison with flash butt welding and/ or friction welding

between two plates which are knocked and clamped onto a fixed backing. The resulting friction heat softens the base metal, although

advantages: - lower energy demands

the melting point is not reached. The plastified

- material savings through lower loss of length - better dimensional accuracy in joining especially for small wall thicknesses

material is displaced by the mandrel and transported behind the tool where a longitudi-

- in comparison with friction welding less moving parts (only axial movement of one joining member during upsetting)

nal seam develops.

- no restrictions to the free clamping length - smaller and more regular welding edge - no spatter formation

The advantages of this method which is

disadvantages:

mainly used for welding of aluminium alloys is

- suitable for small wall thicknesses only (maximum wall thickness: 4 - 5 mm)

the low thermal stress of the component

- welding parameters must be kept within narrow limits - only magnetizing steels are weldable without any difficulties

br-er7-27e.cdr

© ISF 2002

which allows joining with a minimum of distortion and shrinkage. Welding fumes do not develop and the addition of filler metal or shielding gases is not required.

Figure 7.27

workpiece tool collar

fixed backing

contoured pin

br-er7-28e.cdr

Friction-Stir Welding

Figure 7.28

2005

8. Resistance Spot Welding, Resistance Projection Welding and Resistance Seam Welding

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

105

Figure 8.1 shows an extract from the classification of the welding methods according to DIN 1910 with a detailed account of the conductive resistance pressure welding. In the case of resistance pressure welding, the heating occurs at the welding point as a consequence of Joule resistance heating caused by current flow through an electrical conductor, Figure 8.2. In spot and projection welding, the plates

welding

to be welded in overlap. pressure welding

fusion welding

cold pressure welding

resistance pressure welding

induction pressure welding

Conduction pressure welding

Current supply is carried out through spherical or

friction welding

flat

electrodes,

respec-

tively. In roller seam welding, two driven roller elec-

resistance spot welding

projection welding

roller seam welding

resistance butt welding

flash butt welding

© ISF 2002

br-er8-01e.cdr

trodes are applied. The plates to be welded are mainly

Classification of Welding According to DIN 1910

overlapped.

The

heat input rate Qinput is generated by resistance

Figure 8.1

heating

in

a

current-

carrying conductor, Figure spot welding

roller seam welding

projection welding


workpieces overlap
electrode
weld nugget


workpiece usually in general overlap
driven roller electrode
spot rows (stitch weld, roller spots)


workpieces with elevations (concentration of electicity)
workpieces overlap
pad electrode
several joints in a single weld
weld nugget joint

1

3

2

2

1

3

3

5

1 1

1 electrode force 2 elektrodes 3 production part

4 loaded area

instrumental in the formaQeff is composed of the input heat minus the dissi-

4 1

fective heat quantity Qeff is tion of the weld nugget.

1

2

8.3. However, only the ef-

pation heat. The heat loss arises from the heat dissi-

5 projection

pation into the electrodes

br-er8-02e.cdr

and the plates and also from thermal radiation. Figure 8.2

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

106

The resistance during resistance heating is composed of the contact resistances on the two plates and of their material resistance. The reduction of the electrode force down to 90% increases the heat input rate by 105%, the reduction of the welding current down to 90% decreases the heat rate to 80% and a welding time reduction to 90% decreases the heat rate to 92%. The time progression of the resistance is shown in Figure 8.4. The contact resistance is composed of the interface resistances between the electrode and the plate (electrode/plate) and between the plates (plate/plate).

The

resis-

Fel: Qeff: Qinput: I: Q1: Q2 : Q3 : Q4: R(t): Rmaterial(t): Rc(t):

tance height is greatly dependent on the applied electrode force. The higher this force is set, the larger are the conductive cross-

electrode force effective heat total heat input current (time dependence) heat losses losses into the electrodes losses into the sheet metal losses by heat radiation total resistance material resistance contact resistance

Fel

Q4

Q3

Qeff = Qinput - Q1l

Q3

Q4 Q2

2

Qinput = C

I (t) R(t) dt

t=0

points and smaller the re-

Qeff

Q4

t=tS

sections at the contact

Q4

Q2

Fel

Q1 = Q2 + Q3 + Q4

sistances. The contact sur-

R(t) = Rmaterial(t) + Rc(t) © ISF 2002

br-er8-03e.cdr

faces, which are rapidly Heat Balance in Spot Welding

increasing at the start of welding, effect a rapid re-

Figure 8.3

duction of interface resistances.

theoretical contact area 100% metallic conduction contact mOhm

proportion at room temperature

With the formation of the resistances

between

the

resistance

weld nugget the interface

total resistance

low electrode force high resistance

sum of material resistance

high electrode force low resistance

plates disappear. During

proportion after first milliseconds welding time

sum of contact resistances

the progress of the weld

5

10

welding time

the material resistance in-

periods

surface resistance is collapsed, a3 is highly extended A1: area out-of-contact A2: contact area with high resistance A3: contact area completely conductive

creases from a low value br-er8-04e.cdr

(surrounding temperatures) to a maximum value above the melting temperature.

Figure 8.4 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding Figure

8.5

shows

diaelectrode force

grammatically the different

resistance rate

resistances during the spot

R1

welding process with acting electrode force, but

R3

R3

R6 R6

_ ~

R7

without Weld

welding nugget

107

current.

R4

formation

R5

R5 R7

R2

R4

must therefore start in the

0

100

200

joining zone because of

R [µOhm]

the existing high contact br-er8-05e.cdr

resistance there. Figure 8.6 shows directly

Figure 8.5

cooled electrodes for resistance welding. The coolant

cooling tube cooling drill-hole 6-8

is normally water. In the cooling tube, the cooling water is transported to the

2-5

10 - 20

slope

electrode base. The diagram shows the temperature distribution in the elec°C

trodes and in the plates. The maximum temperature Electrode Cooling

the weld nugget and decreases

strongly in

the

© ISF 2002

br-er8-06e.cdr

is reached in the centre of

Figure 8.6

electrode direction. Sequence of a resistance spot welding process, Figure 8.7: 1->2 Lowering of the top electrode 2->3 Application of the adjusted electrode force Set-up time tpre, sequence 3->4 Switching-on of the adjusted welding current for the period of the welding time tw. Formation of the weld nugget in the joining zone of both workpieces.

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

108

An example shows the macrosection of a weld nugget after the welding time has ended. 4->5 Maintaining the electrode force for the period of the set post-weld holding

time th.

5->6 Switching-off the force generating system and lifting the electrodes off the workpiece. The functions of the set-up time and the post-weld holding time are listed in Figure 8.8. Dependent on the welding task different force and current programs can be set in the welding machines, Figure 8.9. In the top the simplest possible welding program sequence is shown: The application of the electrode force, the set-up time sequence tpre, the switching-on of the welding current and the sequence of the welding time tw, the sequence of the postweld holding time th and the switching-off of the force generating system. The diagram in the centre is almost identical to the one just described. Merely in the welding current range, welding is carried out using an adjustable current rise (7) and current decay (8). The diagram below depicts a more sophisticated current program. In addition, welding is carried out with a variable electrode force (2) and with preheating (4) and post-heating current (6). Dependent on the control system, the process can be influenced by adjustment. Fel

Iw

set-up time

electrode force Fel

- compressing the workpiece - build-up of electrode force to preset value - setting-up of reproducible resistance before welding - electrode resting after bounce - preventing resting of electrode on workpiece under electricle voltage

welding current Iw

time t

tpre

tw

th

top electrode

postweld-holding time - holding time of workpiece during cooling of molten metal - prevention of pore formation in the welding nugget - prevention of lifting the electrode under voltage

workpiece lower electrode

insufficiently melted weld nugget

weld nugget

The postweld-holding time has influence on the weld point hardening within certain limits.

totally melted weld nugget

br-er8-07e.cdr

© ISF 2002

br-er8-08e.cdr

Time Sequence of Resistance Spot Welding

Figure 8.7

© ISF 2002

Functions of Pre- and Postwelding

Figure 8.8 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

109

A controlled variable may be, for instance, the electrode path, the resistance progress, the welding current or the welding voltage. Figure 8.10 shows the principle structure of a resistance spot welding machine. The main components are: the machine frame, the welding transformer with secondary lines, the electrode pressure system and the control system. This principle design applies to spot, projection and roller seam welding machines. Differences are to be found merely in the type of

Fel

welding current

electrode force

electrode fittings and in the electrode shapes.

5

Iw

tw

th

tpre = pre-weld time tw = welding time th = holding time tpres = pressure time

1

9 tpre

time

10

tp res

2 3

11

Fel

welding current

electrode force

6 12 4

Iw 8

5 7

7 5 8

electrode force

welding current

time

1 2 5 7

4

Fel

3 Iw 6

8

1 - initial force 2 - welding pressure force 3 - post pressure force 4 - preheating current 5 - welding current 6 - postheating current 7 - ascending current 8 - descending current

1 electrode force cylinder 2 pneumatic equipment 3 machine tool frame 4 welding transformer 5 power control unit 6 current conductor 7 lower arm 8 foot switch 9 top arm 10 electrical power supply cable 11 water cooled electrode holder 12 electrode

time © ISF 2006

br-er8-09e. cdr

br-er8-10e.cdr

Schematic Assembly of Spot Welding Machine

Course of Force and Current

Figure 8.9

© ISF 2002

Figure 8.10

Figure 8.11 depicts the possible process variations of resistance spot welding. These are distinguished by the number of plates to be welded and by the arrangement of the electrodes or, respectively, of the transformers. It has to be noted that with a corresponding arrangement also plates can be welded where one of the two plates has a non-conductive surface (as, for example, plastics).

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

110

Figure 8.12 shows the current types which are normally used for resistance welding. Alternating current has the ~

simplest structure (Figure 8.13) and is most price ef-

~

~

fective, unavoidable are, however,

the

disadvan-

tages of current zeros and

two-sided single-shear single-spot welding

two-sided two-shear spot welding (stack welding)

one-sided single-spot welding with contact electrode

~

~

~

weld nugget cooling. In

+

+

+

relation to the average cur-

+

+

rent values, peak loads ~

two-sided duplex spot welding

occur and, with that, increased

electrode

one-sided multi-spot welding with conductive base © ISF 2002

br-er8-11e.cdr

wear.

Variants of Spot Welding

These extreme peak loads do not occur with direct

one-sided duplex spot welding with conductive base

Figur 8.11

current. The structural design of a d.c. supply unit is, however, more complicated and, therefore, more expensive than an a.c. supply unit. As conventional welding machines operate with a 50 Hz primary current supply, the welding current can be controlled only in 20 ms units (1 period). When the inverter-direct current technique or, respectively, the medium-frequency technique is used, a finer setting of the current-on period and a more alternating current

medium frequency direct current 12

15

[kA]

[kA]

20 10 0 0.00 0.02 0.04 0.07 0.09 0.11 0.13 0.16 -5

current

current

5

-10 -15 -20

ing current is possible.

8 6 4 2 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

welding time [s]

welding time

[kA] 0.00

0.02 0.04

0.06 0.08 0.10 0.12

welding time

0.14 0.16

45 40 35 30 25 20 15 10 5 0 0.00

currents and shorter weldpacitor resistance welding technique is applied. The

0.02 0.04

0.06 0.08 0.10

0.12 0.14 0.16

[s] © ISF 2002

Current Types

In order to realise higher ing times, the impulse ca-

welding time

[s]

br-er8-12e.cdr

Figur 8.12

[s]

impulse capacitor current

current

current

[kA]

"conventional" direct current 18 16 14 12 10 8 6 4 2 0

precise control of the weld-

10

rectified primary current is stored in capacitors and, through

a

high-voltage

transformer, converted to

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

111

high welding currents. The single-phase alternating current

advantages of this tech-

static-inverter direct current

nique are low heat input and

high

reproducibility.

Because of the high energy density,

materials

with

capacitor impulse discharge

3-phase direct current

good conductivity can be welded and also multipleprojection welds can be carried out. A disadvantage br-er8-13e.cdr

of this method is, apart from the high equipment costs, the difficult regula-

Figure 8.13

tion of the welding current. Electrodes for spot resistance welding have the property of transferring the electrode force and the welding current. They are wearing parts and, therefore, easily replaceable.

requirements

electrodes form A

form B

form C

form E

form F

form G

form D

- good electrical conductivity - good thermal conductivity - high high-temperature strength - high temperature stability - high softening temperature - little tendency to alloying with workpiece material - easy options in machining ISO 5182 Group

Type 1

electrode caps 2 A

3

4

ISO 5182

Key

Group Type

No. 1

Cu - ETP

2

Cu Cdl

1

Cu Crl

2

Cu Crl Zr

1

Cu CO2 Be

2

Cu Ni2 Si

1

Cu Ni1 P

2

Cu Be2 Co Ni

3

Cu Ag6

4

CuAl10NiFe5Ni5

Key No.

10

W75 Cu

11

W78 Cu

12

WC70 Cu

13

Mo

14

W

15

W65 Ag

B

electrode holders br-er8-14e.cdr

© ISF 2002

br-er8-15e.cdr

Electrodes, Electrode Caps and Holders

Figure 8.14

Electrode Materials

Figure 8.15 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

112

Depending on the shape and type of electrode, solid electrodes or electrode caps, must be either remachined or recycled. Figure 8.14 depicts various types of electrodes, electrode caps and holders. Dependent upon the electrode application, different alloyed electrode materials are used, Figure 8.15. The added alloying elements influence the red hardness, the tempering resistance, the conductivity, the fusion temperature, the electrode alloying tendency, and, finally, the machinability of the electrode material. When beryllium is used as an alloying element, the admissible MAC values must be strictly adhered to during remachining or dressing of the electrodes. Already during the design phase of the components to be welded, importance must be attached to a good accessibility of the welding point. Moreover, the electrode force which is imperative to the process must be applied in a way that no damage is done to the workpiece. In the ideal case, the welding point is accessible from the top and from below, Figure 8.16. poor

good

br-er8-16e.cdr

poor

© ISF 2002

br-er8-17e.cdr

Accessibility for Spot Welding Electrode

Figure 8.16

good

© ISF 2002

Contact Area for Spot Welding Electrodes

Figure 8.17

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

113

In order to avoid the displacement of the electrodes, the electrode working surface must be flat. Also during the design phase space must be provided for an adequately large clearing zone around the working point, in

spot welding

order to guarantee the unimpeded electrode

A

approach to the working point, Figure 8.17. shunt connection current

Dependent on the joining job, the process copper

current path

indirect welding one side

variation, or the resistance welding method, a so-called “shunt current/effect” may be noticed. This current component, as a rule, does not contribute to the formation of the weld nugget; under certain circumstances it might

roller seam welding br-er8-18e.cdr

even prevent a reliable welding process. In

Shunting

the example, shown in Figure 8.18, the shunt

current leads to undesired fusing contacts

Welding spatter: Discharge of molten material between two steel sheets or from the surface of steel sheets.

Figure 8.18

and, because of the lacking electrode force at this point, also to damages to the plate surface. If unsuitable welding parameters have been fig. 1

set, weld spatter formation may occur, Fig-

fig. 2

Reason here is high welding current, (fig. 1) or too-small edge distance (fig. 2)

ure 8.19. Liquid molten metal forms on the plate surface or in the joining zone. The reasons for this kind of process disturbance are, for example, too low an electrode force with regard to the set welding current or welding time, too high an energy input with regard to

porosity in the joint caused by welding spatter

discharge of molten material at the joint plane

br-er8-19e.cdr

the plate thickness or too small an edge dis-

Welding Spatter

tance of the welding point. Figure 8.19 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

114

Figure 8.20 shows a list of welding current changes shunt connection

wear of electrodes

wear of cable

mains voltage fluctuation

a large number of possible secondary electrical impedance

disturbances in resistance

Qeff = Qinput - Qlosses

wear Qeff diversion heat

plate

plate thickness

quality of plates

number of plates

welding equipment

alteration to force

spot welding. Welding curalteration of pressure

rent changes are caused by: shunt, electrode wear, cable wear, mains voltage variations, secondary im-

plate surface

edge distance

pedance.

modification of the unit br-er8-20e.cdr

Different welding conditions are caused by weldFigure 8.20

ing machine wear, different heat

dissipation.

Non-

uniform conditions by alterations to components are: different plate thicknesses, plate quality, number of plates, plate surfaces, edge distances. Electrode force changes are caused by: pressure fluctuations and -changes, plate bouncing. The resistance spot welding method allows welding of a large number of materials. A list of the various materials is shown in Figure

materials

aluminium

weldability

alloying elements

good weldability

sufficient weldability

satisfactory maximum content [%]

8.21. The alloying elements which are used for steels have a varying influence on the suitability for resistance spot welding. The values which are indicated in the table are valid only when the stated element is the sole alloying constituent of the steel material.

iron

very good

gold

satisfactory

C

0,25

0,40

C + Cr

0,35

1,60

C + Mo

0,50

0,70

C+V

0,40

0,60

C + Mn

1,40

1,60

molybdenum satisfactory

C + Ni

3,00

4,00

nickel

very good

Si

0,40

1,00

platinum

very good

Cu

0,60

0,60

P+S

0,10

0,10

C+Cr+Mo+V

0,60

1,60

cobalt

very good

copper

poor

magnesium

silver tantalum

Figure 8.22 shows a comparison between

good

very good very good

titanium

very good

tungsten

satisfactory

resistance spot and resistance projection welding. The fundamental difference between the two methods lies in the definition of the

weldable materials

influence of alloying elements (steel materials)

br-er8-21e.cdr

current transition point.

Weldable Materials

Figure 8.21 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

115

The differences between both methods are illustrated in Figure 8.23. The short life of the electrodes used for resistance spot welding is explained by the higher thermal load and the larger pressing area caused by the smaller electrode contact areas. The term “electrode life” stands for the number of welds that can be carried out with one pair of electrodes without further rework and without exceeding the tolerances

after welding

before welding

for quality criteria of the weld. Depending

on

the

defollow-up distance

mands on the joint strength or on the projection rigidity,

elektrode

different projection shapes are applied. These are an-

projection br-er8-22e.cdr

nular, circular or longitudinal projections. The welding projections are, accord-

Figure 8.22

spot welding

projection welding

up to 20 mm

> 20 mm

embossed projection shape

elektrodes: diameter tip face

pressed mould pressed

convex

flat

electrode life

less

longer

place where the nugget originates

elektrodes

projections

one

several

small

big

current distribution

no

yes

force distribution

no

yes

number of welding nuggets

circular longitudinal annular

solid projection shape

natural projection shape

struck machined cut pushed

circular longitudinal annular interrupted annular

spot contact line contact

Circular

follow-up distance

weld nut

problems:

br-er8-23e.cdr

© ISF 2002

Longitudinal

cut

Annular

pushed

wire-plate

bolt-pipe

br-er8-24e.cdr

Differences Between Resistance Spot and Projection Welding

Figure 8.23

crossed wires

Customary Projection Shapes

Figure 8.24 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

116

ing to their size, adapted to the used plate thickness and may, therefore, appear as difdie

die plate

ferent types in the workpiece: embossed proplate

jections, solid projections and natural projections. The survey is shown in Figure 8.24. d1

mould plate

mould plate

counter-die

d1

In Figure 8.25 the production of embossed projections in different shapes is shown. The

ring projection

embossed projection

shape is embossed onto the plate surface by appropriate die plates, dies and, if necessary,

die

b

l mould plate

counter dies. plate

Various problems occur in projection welding caused by the welding of several joints in a single working cycle. Due to different current

longitudinal projection br-er8-25e.cdr

© ISF 2002

Production of Embossed Projection Shapes

paths - when using direct current - and a current displacement - when using alternating current -, welding nuggets with differing quali-

Figure 8.25

alternating current distribution intensity of current increases from the center to the outer area caused by current displacement

force distribution of a C-frame projection press welder during bending of machine tool frame

direct current distribution intensity of current decreases from the center to the outer area caused by the longer current path br-er8-26e.cdr

force distribution of a C-frame projection press welder with non-parallel positioning tables © ISF 2002

br-er8-27e.cdr

Problem of Current Distribution During Projection Welding

Figure 8.26

© ISF 2002

Problems of Force Distribution During Projection Welding

Figure 8.27 2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

117

ties are produced when no preventive remedies are taken, Figure 8.26. A varying force distribution, as shown by the example in Figure 8.27, also leads to differing qualities of the produced weld nuggets. In Figure 8.28 several examples of application using projection welding are de© ISF 2002

br-er8-30e_sw.cdr

Application Examples of Projection Welding

picted. In this example, the shapes are of the embossed type.

Figure 8.28 Figures 8.29 and 8.30 show several process variations of roller seam welding. Seam welding is actually a continuous spot welding process, but with the application of roller electrodes. In contrast to resistance spot welding the electrodes remain in contact and turn continuously after the first weld spot has been produced. At the points where a welding spot is to be produced again current flow is lap joint

lap joint with wire electrode

lap joint with foil

squash seam weld

butt weld with foil

initiated. Dependent on the electrode feed rate and on the welding current frequency, spot welds or seal

before welding

welds

with

weld

nuggets

overlapping are

pro-

duced. The application of d.c. current also produces

after welding

seal welds. © ISF 2002

br-er8-28e.cdr

Roller Seam Welding

Figure 8.29

2005

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding

118

interrupted-current roller seam weld

overlap seal weld

continuous D.C. seal weld br-er8-29e.cdr

© ISF 2002

Weld Types for Roller Seam Welding

Figure 8.30

2005

9. Electron Beam Welding

9. Electron Beam Welding

120 The application of highly accelerated elechigh voltage supply

beam generation beam forming and guidance

fusion, drilling and welding process and also

cathode control elektrode

for surface treatment has been known since

anode

the Fifties. Ever since, the electron beam

adjustment coil to vacuum pump

valve

trons as a tool for material processing in the

welding process has been developed from the laboratory stage for particular applications. In

viewing optics

stigmator

focussing coil

this cases, this materials could not have been

defelction coil

joined

by any industrially applied

high-

working chamber

production joining method. The electron beam welding machine is made

workpiece workpiece handling

to vacuum pump

beam generation, beam manipulation and forming and working chamber. These compo-

chamber door br-er9-01e.cdr

up of three main components:

© ISF 2002

Schematic Representation of an Electron Beam Welding Machine

nents may also have separate vacuum systems, Figure 9.1.

Figure 9.1 A tungsten cathode which has been heated under vacuum emits electrons by thermal emission. The heating of the tungsten cathode may be carried out directly - by filament cur-

power supply

chamber evacuation system valve

evacuation system for gun

control cabinet

EB-gun

rent - or indirectly - as, for example, by coiled filaments. The electrons are accelerated by high voltage between the cathode and the pierced anode. A modulating electrode, the socalled “Wehnelt cylinder”, which is positioned between anode and cathode, regulates the electron flow. Dependent on the height of the cut-off voltage between the cathode and the

working chamber workpiece receiving platform workpiece handling

modulating electrode, is a barrier field which may pass only a certain quantity of electrons.

control panel control desk

br-er9-02e_f.cdr

© ISF 2002

All-Purpose EBW Machine and Equipment

This happens during an electron excess in front of the cathode where it culminates in Figure 9.2

2005

9. Electron Beam Welding

121

form of an electron cloud. Due to its particular shape which can be compared to a concave mirror as used in light optic, the Wehnelt cylinder also effects, besides the beam current adjustment, the electrostatic focussing of the electron beam. The electron beam which diverges after having passed the pierced anode, however, obtains the power density which is necessary for welding only after having passed the adjacent alignment and focussing system. One or several electromagnetic focussing lenses bundle the beam onto the workpiece inside the vacuum chamber. A deflection coil assists in maintaining the electron beam oscillating motion. An additional stigmator coil may help to correct aberrations of the lenses. A viewing optic or a video system allows the exact positioning of the electron beam onto the weld groove. The core piece of the electron beam welding machine is the electron beam gun where the electron beam is generated under high vacuum. The tightly focussed electron beam diverges rapidly under atmospheric pressure caused by scattering and ionisation development with air. As it would, here, loose power density and efficiency, the welding process is, as a rule, carried out under medium or high vacuum. The necessary vacuum is generated in separate vacuum pumps for working chamber and beam gun. A shut-off valve which is positioned between electron gun and working chamber serves to maintain the gun vacuum while the working chamber is flooded. In universal machines, Figure 9.2, the workpiece manipulator assembly inside the vacuum chamber is a slide with working table positioned over NCcontrolled stepper motors. For workpiece removal, the slide is moved from the vacuum chamber onto the workpiece platform. A distinction is made between electron beam machines with vertical and horizontal beam manipulation systems.

back-scattered electrons

The energy conversion in

x-ray

the workpiece, which is thermal radiation secondary electrons

schematically

shown

in

Figure 9.3, indicates that the kinetic energy of the

x

convection

highly

accelerated

elec-

trons is, at the operational y

point, not only converted

heat conduction

into the heat necessary for

z © ISF 2002

br-er9-03e.cdr

Energy Transformation Inside Workpiece

welding, but is also released by heat radiation

Figure 9.3 2005

9. Electron Beam Welding

122

and heat dissipation. Furthermore, a part of the incident electrons (primary electrons) is subject to backscatter and by secondary processes the secondary electrons are emitted from the workpiece thus generating X-rays. The impact of the electrons, which are tightly focussed into a corpuscular beam, onto the workpiece surface stops the electrons; their penetration depth into the workpiece is very low, just a few µm. Most of the kinetic energy is released in the form of heat. The high energy density at the impact point causes the metal to evaporate thus allowing the following electrons a deeper penetration. This finally leads to a metal vapour cavity which is surrounded by a shell of fluid metal, covering the entire weld depth, Figure 9.4. This deep-weld nowadays a)

b)

c)

d)

effect

allows

penetration

depths into steel materials of up to 300 mm, when

© ISF 2002

br-er9-04e.cdr

modern high vacuum-high Principle of Deep Penetration Welding

voltage machines are used.

Figure 9.4 The diameter of the cavity corresponds approximately electron beam

motion of the molten metal groove melting pool welding direction

with the beam diameter. By groove front side

a relative motion in the di-

keyhole molten zone

vapour capillary

rection of the weld groove between F1

solidified zone

F1 : force resulting from vapour pressure F2 : force resulting from surface tension F3 : force resulting from hydrostatic pressure

workpiece

and

electron beam the cavity

F2

penetrates through the ma-

F3

terial, Figure 9.5. At the

F1

front side of the cavity new material is molten which, to © ISF 2002

br-er9-05e.cdr

Condition in Capillary

some extent, evaporates, but for the most part flows

Figure 9.5 2005

9. Electron Beam Welding

123

around the cavity and rapidly solidifies at the backside. In order to maintain the welding cavity open, the vapour pressure must press the molten metal round the vapour column against the cavity walls, by counteracting its hydrostatic pressure and the surface tension. However, this equilibrium of forces is unstable. The transient pressure and temperature conditions inside the cavity as well as their respective, momentary diame-

β

ters are subject to dynamic

I

II

III

changes. Under the influence of the resulting, dy-

workpiece movement

namically changing geome© ISF 2002

br-er9-06e.cdr

try of the vapour cavity and

Model of Shrinkage Cavity Formation

with an unfavourable selecFigure 9.6 tion of the welding parameters, metal fume bubbles may be included which on cooling turn into shrinkholes, Figure 9.6.

150

The unstable pressure exposes the molten backside of the vapour cavity to strong and irregular changes in shape (case II). Pressure EBW MSG UP (narrow gap)(narrow gap) EBW

variations interfere with the regular flow at the

UP (conventional)

MSG (narrow gap)

UP (narrow gap)

UP (conventional)

welding current

0,27 A

260 A

650 A

510 A

welding voltage groove area

150.000 V

30 V

30 V

28 V

cavity backside, act upon the molten metal and, in the most unfavourable case, press the

number of passes

896 mm 1

2098 mm 35

4905 mm 81

5966 mm 143

unevenly distributed molten metal into differ-

filler metal

0

23 kg

54 kg

66 kg

melting efficiency

7,7 kg/h

ent zones of the molten cavity backside, thus

energy input

64·10 kJ

128·10 kJ

293·10 kJ

377·10 kJ

welding time

27 min

4 h 35 min

4 h 11 min

7 h 20 min

2

3

2

5 kg/h

2

13 kg/h 3

3

2

9 kg/h 3

forming the so-called vapour pockets. The cavities are not always filling with molten

br-er9-19e.cdr

© ISF 2002

Comparison of EB, GMAW and SAWNarrow Gap and Conventional SAW

metal, they collapse sporadically and remain as hollow spaces after solidification (case Ill). The angle ß (case I) increases with the rising

Figure 9.7 2005

9. Electron Beam Welding

124

weld speed and this is defined as a turbulent process. Flaws such as a constantly open vapour cavity and subsequent continuous weld solidification could be avoided by selection of job-suitable welding parameter combination and in particular of beam oscillation characteristics, it has to be seen to a constantly of the molten metal, in order to avoid the abovementioned defects. Customary beam oscillation types are: circular, sine, double parabola or triangular functions. Thick plate welding accentuates the process-specific advantage of the deep-weld effect and, with that, the possibility to join in a single working cycle with high weld speed and low heat input quantity. A comparison with the submerged-arc and the gas metal-arc welding processes illustrates the depth-to-width ratio which is obtainable with the electron beam technology, Figure 9.7. Electron beam welding of thick plates offers thereby decisive advantages. With modern equipment, wall thicknesses of up to 300 mm with length-to-width ratios of up to 50 : 1 and consisting of low and high-alloy materials can be welded fast and precisely in one pass and without adding any filler metal. A corresponding quantification shows the advantage in regard of the applied filler metal and of the primary energy demand. Compared with the gas-shielded narrow gap welding process, the production time can be

in vacuum

reduced by the factor of approx. 20 to 50.




thin and thick plate welding (0,1 mm bis 300 mm)




extremely narrow seams (t:b = 50:1)




low overall heat input => low distortion => welding of completely processed components




high welding speed possible




no shielding gas required

practice, Figure 9.8. Pointing to series produc-




high process and plant efficiency

tion, the high profitability of this process is




material dependence, often the only welding method

dominant. This process depends on highly

Numerous specific advantages speak in favour of the increased application of this high productivity process in the manufacturing

energetic efficiency together with a sparing

at atmosphere


very high welding velocity




good gap bridging




no problems with reflection during energy entry into workpiece

br-er9-12e.cdr

use of resources during fabrication.

© ISF 2002

Advantages of EBW

Figure 9.8 2005

9. Electron Beam Welding

125 Considering the

above-mentioned

advan-

tages, there are also disadvantages which in vacuum

emerge from the process. These are, in par-




electrical conductivity of materials is required




high cooling rates => hardening => cracks

ticular, the high cooling rate, the high equip-




high precision of seam preparation

ment costs and the size of the chamber, Fig-




beam may be deflected by magnetism

ure 9.9.




X-ray formation




size of workpiece limited by chamber size




high investment

In accordance with DIN 32511 (terms for methods and equipment applied in electron

at atmosphere

and laser beam welding), the specific desig-




X-ray formation




limited sheet thickness (max. 10 mm)




high investment

nations, shown in Figure 9.10, have been standardised for electron beam welding.


small working distance

Electron beam units are not only distinguished br-er9-13e.cdr

© ISF 2002

Disadvantages of EBW

by their working vacuum quality or the unit concept but also by the acceleration voltage level, Figure 9.11. The latter exerts a consid-

Figure 9.9

erable influence onto the obtainable welding

results. With the increasing acceleration voltage, the achievable weld depth and the depth-towidth ratio of the weld geometry are also increasing. A disadvantage of the increasing accelerating voltage is, however, the exponential increase of X-rays and, also, the likewise increased sensitivity to flash-over voltages. In correspondence with the size of the workpiece to be welded and the size of groove

the chamber volume, high-

in

industrial

production,

while the low-voltage technology (max. 60 kV) is a good alternative for smaller units and weld thicknesses. The design of the unit for the low-voltage

technique

gt

a se

m

width of seam

g len

t

fs ho

m ea

weld thickness

of up to 200 kW are applied

len

f ho

weld penetration depth

(150 - 200 kV) with powers

Nahtdicke

generators

weld reinforcement

beam

root reinforcement

voltage

end crater

upper bead

blind bead molten area

unapproachable gap lower bead

root weld © ISF 2002

br-er9-07e.cdr

Basic Definitions

is Figure 9.10

2005

9. Electron Beam Welding

126

simpler as, due to the lower acceleration voltage, a separate complete lead covering of the unit is not necessary.

by accelerating voltage:  high voltage machine (UB=150 kV)  low voltage machine (UB=60 kV)

-6

< 1 x 10 mbar

< 5 x 10-4 mbar

by pressure:  high vacuum machine  fine vacuum machine  atmospheric machine (NV-EB welding)

by machine concept:  conveyor machine  clock system  all-purpose EBW machine  local vacuum machine  mobile vacuum machine  micro and fine welding machine br-er9-20e.cdr

br-er9-09e_f.cdr

Classification of EBW Machines

Figure 9.11

EB-Welding in High Vacuum

Figure 9.12

While during the beam generation, the vacuum (p = 10-5 mbar) for the insulation of the beam generation compartment and the prevention of cathode oxidation is imperative, the possible working pressures inside the vacuum chamber vary between a high vacuum (p = 10-4 mbar) and atmospheric pressure. A collision of the electrodes with the residual gas molecules and the scattering of the electron beam which is connected to this is, naturally, lowest in high vacuum. The beam diameter is minimal in high vacuum and the beam power density is maximum in high vacuum, Figure 9.12. The reasons for the application of a high vacuum unit are, among others, special demands on the weld (narrow, deep welds with a minimum energy input) or the choice of the materials to be welded (materials with a high oxygen affinity). The application of the electron beam welding process also entails advantages as far as the structural design of the components is concerned.

2005

9. Electron Beam Welding

127

With a low risk of oxidation and reduced demands on the welds, the so-called “mediumvacuum units” (p = 10-2 mbar) are applied. This is mainly because of economic considerations, as, for instance, the reduction of cycle times, Figure 9.13. Areas of application are in the automotive industry (pistons, valves, torque converters, gear parts) and also in the metalworking industry (fittings, gauge heads, accumulators). Under extreme demands on the welding time, reduced requirements to the weld geometry, distortion and in case of full material compatibility with air or shielding gas, out-of-vacuum welding units are applied, Figure 9.14. Their advantages are the continuous welding time and/or short cycle times. Areas of application are in the metal-working industry (precision tubes, bimetal strips) and in the automotive industry (converters, pinion cages, socket joints and module holders).

< 1 x 10-6 mbar

-4

< 1x 10 mbar

< 5 x 10-2 mbar ~ 10

-1

mbar

~ 1 mbar

br-er9-10e_f.cdr

br-er9-11e_f.cdr

EB-Welding in Fine Vacuum

Figure 9.13

Atmospheric Welding (NV-EBW)

Figure 9.14

2005

9. Electron Beam Welding

128

A further distinction criterion is the adjustment of the vacuum chambers to the different joining tasks. Universal machines are characterised by their simply designed working chamber, Figure 9.15. They are equipped with vertically or horizontally positioned and, in most cases, travelling beam generators. Here, several workpieces can be welded in subsequence during an evacuation cycle. The largest, presently existing working chamber has a volume of 265 m³.

br-er9-14e_f.cdr

br-er9-15e_f.cdr

EBW Clock System Machine

Machine Concept - Conventional Plant

Figure 9.15

Figure 9.16

Clock system machines, in contrast, are equipped with several small vacuum chambers which are adapted to the workpiece shape and they are, therefore, characterised by short evacuation times, Figure 9.16. Just immediately before the welding starts, is the beam gun coupled to the vacuum chamber which has been evacuated during the preceding evacuation cycle, while, at the same time, the next vacuum chamber may be flooded and charged/loaded.

2005

9. Electron Beam Welding

129

Conveyor machines allow the continuous production of welded joints, as, for example, bimetal semi finished products such as saw blades or thermostatic bimetals, Figure 9.17. In the main chamber of these units is a gradually raising pressure system as partial vacuum pre and post activated, to serve as a vacuum lock.

butt weld

T-joint/ fillet weld

a)

T-joint butt welded

b)

lap weld

semi-finished material

endproduct

br-er9-17e.cdr

br-er9-16e_f.cdr

Seam Appearance for EB-Welding in Vacuum

EBW Conveyor Machine

Figure 9.17

© ISF 2002

Figure 9.18

Systems which are operating with a mobile and local vacuum are characterised by shorter evacuation times with a simultaneous maintenance of the vacuum by decreasing the pumping volume. In the “local vacuum systems”, with the use of suitable sealing, is the vacuum produced only in the welding area. In “mobile vacuum systems” welding is carried out in a small vacuum chamber which is restricted to the welding area but is travelling along the welded seam. In this case, a sufficient sealing between workpiece and vacuum chamber is more difficult. With these types of machine design, electron beam welding may be carried out with components which, due to their sizes, can not be loaded into a stationary vacuum chamber (e.g. vessel skins, components for particle accelerators and nuclear fusion plants). 2005

9. Electron Beam Welding

130

In general the workpiece is moved during electron beam welding, while the beam remains stationary and is directed onto the workpiece in the horizontal or the vertical position. Depending on the control systems of the working table and similar to conventional welding are different welding positions possible. The weld type preferred in electron beam welding is the plain butt weld. Frequently, also centring allowance for centralising tasks and machining is made. For the execution of axial welds, slightly oversized parts (press fit) should be selected during weld preparation, as a transverse shrinkage sets in at the beginning of the weld and may lead to a considerable increase of the gap width in the opposite groove area. In some cases also T-welds may be carried out; the T-joint with a plain butt weld should, however, be chosen only when the demands on the strength of the joints are low, Figure 9.18. As the beam spread is large under atmosphere, odd seam formations have to be considered during Non-Vacuum Electron Beam Welding, Figure 9.19. In order to receive uniform and reproducible results with electron beam welding, an exact knowledge about the beam geometry is necessary and a prerequisite for: - tests on the interactions between beam and substance - applicability of welding parameters to br-er9-18e.cdr

© ISF 2002

Seam Appearence at Atmospheric Welding (NV-EBW)

other welding machines - development of beam generation systems.

Figure 9.19 The objective of many tests is therefore the exact measurement of the beam and the investigation of the effects of different beam geometries on the welding result. For the exact measurement of the electron beam, a microprocessor-controlled measuring system has been developed in the ISF. The electron beam is linearly scanned at a high speed by means of a point probe, which, with a diameter of 20 µm is much smaller than the beam diameter in the focus, Figure 9.20. When the electron beam is deflected through the aperture diaphragm located inside the sensor, the electrons flowing through the diaphragm

2005

9. Electron Beam Welding

131

are picked up by a Faraday shield and diverted over a precision resistor. The time progression of the signal, intercepted at the resistor, corresponds with the intensity distribution of the electron beam in the scanning path. In order to receive an overall picture of the power density distribution inside the electron beam, the beam is line scanned over the slit sensor (60 lines). An evaluation program creates a perspective view of the power density distribution in the beam and also a two-dimensional representation of lines with the same power density.

hole sensor hole with aperture diaphragm Faraday cup (20 µm)

track of the beam

cross section of the beam

measurement field

slit sensor

slit with Faraday cup

FILENAME: R I N G S T R Accel. voltage: 150 kV Beam current: 600 mA Prefocus current: 700 mA Main focus current: 1500 mA Cath. heat current: 500 mm Max. Density: 26,456 kW/mm2 2 Ref. Density: 26,456 kW/mm

voltage

cross section of the beam

beam deflection br-er9-21e.cdr

br-er9-22e_f.cdr

Two Principles of Electron Beam Measuring

Figure 9.20

© ISF 2002

Energy Concentration and Development in Electron Beam

Figure 9.21

An example for a measured electron beam is shown in Figure 9.21. It can be seen clearly that the cathode had not been heated up sufficiently. Therefore, the electrons are sucked off directly from the cathode surface during saturation and unsaturated beams, which may lead to impaired welding results, develop. During the space charge mode of a generator, the electron cloud is sufficiently large, i.e., there are always enough electrons which can be sucked off. In the ideal case, the developed power density is rotationally symmetrical and in accordance with the Gaussian distribution curve. The electron signals are used for the automatic seam tracking. These may be either primary or secondary electrons or passing-through current or the developing X-rays. When backscattered primary electrons are used, the electron beam is scanned transversely to the groove. A 2005

9. Electron Beam Welding

132

computer may determine the position of the groove relative to the beam by the signals from the reflected electrons. In correspondence with the deflection the beam is guided by electromagnetic deflection coils or by moving the working table. This kind of seam tracking system may be used either on-line or off-line. The broad variation range of the weldable maindustrial areas:  automotive industries  aircraft and space industries  mechanical engineering  tool construction  nuclear power industries  power plants  fine mechanics and electrical industries  job shop material:  almost all steels  aluminium and its alloys  magnesium alloys  copper and its alloys  titanium  tungsten  gold  material combinations (e.g. Cu-steel, bronze-steel)  ceramics (electrically conductive)

terials and also material thicknesses offer this joining method a large range of application, Figure 9.22. Besides the fine and micro welding carried out by the electronics industry where in particular the low heat input and the precisely programmable control is of importance, electron beam welding is also particularly suited for the joining of large crosssections.

br-er9-20e.cdr

EBW Fields of Application

Figure 9.22

2005

1. Gas Welding

1. Gas Welding

7

Although the oxy-acetylene process has been introduced long time ago it is still applied for its flexibility and mobility. Equipment for oxyacetylene welding consists of just a few elements, the energy necessary for welding can be transported in cylinders, Figure 1.1. density in normal state [kg/m3]

7

br-er1-01.cdr

© ISF 2002

0

0

°C

10.3

370

8.5

330

KW k

/cm2

natural gas natural gas

2850 2770

propane

flame temperature with O2 flame efficiency with O2 flame velocity with O2 43 1350 acetylene

3200

510 propane

air

300

490 335 acetylene

200

oxygen

400

oxygen cylinder with pressure reducer acetylene cylinder with pressure reducer oxygen hose acetylene hose welding torch welding rod workpiece welding nozzle welding flame

cm

/s

br-er1-02.cdr

© ISF 2002

Properties of Fuel Gas in Combination with Oxygen

Equipment Components for Gas Welding

Figure 1.1

oxygen

645

645

600

2

1 2 3 4 5 6 7 8 9

1.43 0.9

ignition temperature [OC]

9

1

1.17

natural gas

8

1.29

propane

6

5

2.0

acetylene

4

2.5 2.0 1.5 1.0 0.5 0

air

3

Figure 1.2

Process energy is obtained from the exothermal chemical reaction between oxygen and a combustible gas, Figure 1.2. Suitable combustible gases are C2H2, lighting gas, H2, C3H8 and natural gas; here C3H8 has the highest calorific value. The highest flame intensity from point of view of calorific value and flame propagation speed is, however, obtained with C2H2.

2005

1. Gas Welding

8 C2H2 is produced in acetylene gas genera-

loading funnel

tors by the exothermal transformation of calcium carbide with water, Figure 1.3. Carbide

material lock

is obtained from the reaction of lime and carbon in the arc furnace. gas exit

C2H2 tends to decompose already at a pres-

feed wheel

sure of 0.2 MPa. Nonetheless, commercial quantities can be stored when C2H2 is disgrille

solved in acetone (1 l of acetone dissolves

sludge

approx. 24 l of C2H2 at 0.1 MPa), Figure 1.4.

to sludge pit br-er1-03.cdr

© ISF 2002

Acetylene Generator

Figure 1.3

Acetone disintegrates at a pressure of more

acetone

acetylene

than 1.8 MPa, i.e., with a filling pressure of 1.5 MPa the storage of 6m³ of C2H2 is possible in a standard cylinder (40 l). For gas ex-

porous mass

change (storage and drawing of quantities up to 700 l/h) a larger surface is necessary,

N

acetylene cylinder

therefore the gas cylinders are filled with a

acetone quantity :

porous mass (diatomite). Gas consumption

acetylene quantity :

6000 l

during welding can be observed from the

cylinder pressure :

15 bar

~13 l

weight reduction of the gas cylinder. filling quantity : up to 700 l/h

br-er1-04.cdr

© ISF 2002

Storage of Acetylene

Figure 1.4 2005

1. Gas Welding

9 Oxygen is produced by fractional distillation of

gaseous cooling

liquid air and stored in cyl-

cylinder

nitrogen air

inders with a filling presbundle

sure of up to 20 MPa, Fig-

oxygen

liquid air

oxygen

ure 1.5. For higher oxygen

pipeline liquid

consumption, storage in a liquid state and cold gasifi-

tank car nitrogen vaporized cleaning

compressor

separation

cation is more profitable. supply

br-er1-05.cdr

© ISF 2002

Principle of Oxygen Extraction

Figure 1.5

The standard cylinder (40 l) contains, at a

50 l oxygen cylinder

filling pressure of 15 MPa, 6m³ of O2 (pres-

protective cap cylinder valve

sureless state), Figure 1.6. Moreover, cylin-

gaseous

take-off connection

N

ders with contents of 10 or 20 l (15 MPa) as

p = cylinder pressure : 200 bar V = volume of cylinder : 50 l Q = volume of oxygen : 10 000 l

well as 50 l at 20 MPa are common. Gas consumption can be calculated from the pres-

content control Q=pV

sure difference by means of the general gas foot ring

equation. manometer

liquid

safety valve

vaporizer

filling connection user

still liquid

gaseous

br-er1-06.cdr

Storage of Oxygen

Figure 1.6

2005

1. Gas Welding

10

In order to prevent mistakes, the gas cylinders are colour-coded. Figure 1.7 shows a survey of the present colour code and the future colour code which is in accordance with DIN EN 1089. old condition

DIN EN 1089

blue

white

old condition

DIN EN 1089

grey

brown grey

blue (grey)

also of

are

different designs.

Oxygen cylinder connec-

helium

oxygen techn. yellow

The cylinder valves

brown

red

red

tions show a

right-hand

thread union nut. Acetylene acetylene grey

hydrogen dark green

grey

vivid green grey

grey

argon-carbon-dioxide mixture

argon darkgreen

black

grey

grey

carbon-dioxide

br-er1-07.cdr

© ISF 2006

valves

are

equipped with screw clamp retentions. Cylinder valves for

darkgreen

nitrogen

cylinder

other

combustible

gases have a left-hand thread-connection with a

Gas Cylinder-Identification according to DIN EN 1089

circumferential groove.

Figure 1.7

cylinder pressure

working pressure

Pressure

regulators

re-

duce the cylinder pressure to the requested working pressure, Figures 1.8 and 1.9.

br-er1-08.cdr

© ISF 2002

Single Pressure Reducing Valve during Gas Discharge Operation

Figure 1.8

2005

1. Gas Welding

11

At a low cylinder pressure (e.g. acetylene cylinder) and low pressure fluctuations, singlestage regulators discharge pressure

locking pressure

are applied; at higher cylinder pressures normally two-stage pressure regulators are used. The requested pressure is set by the adjusting screw. If the pressure increases on the low pressure side, the throttle valve closes the

br-er1-09.cdr

© ISF 2002

increased

Single Pressure Reducing Valve, Shut Down

pressure

onto

the membrane.

Figure 1.9 The injector-type torch consists of a body with valves and welding chamber with welding nozzle, Figure 1.10. By the selection of suitable welding chambers, the flame intensity can be adjusted for welding different plate thicknesses. The special form of the mixing chamber guarantees highest possible safety against flashback, Figure 1.11.

welding torch injector or blowpipe

The high outlet speed of the escaping O2 generates a negative pressure

mixer tube

coupling nut mixer nozzle oxygen valve

hose connection for oxygen A6x1/4" right

in the acetylene gas line, in consequence C2H2 is sucked

and

injector pressure nozzle suction nozzle

drawn-in.

C2H2 is therefore avail-

fuel gas valve

welding nozzle

hose connection for fuel gas A9 x R3/8” left

able with a very low pressure of 0.02 up to 0.05 MPa with

O2

0.3 MPa).

welding torch head br-er1-10.cdr

-compared (0.2

up

torch body © ISF 2002

Welding Torch

to Figure 1.10

2005

1. Gas Welding

12

A neutral flame adjustment allows the differentiation of three zones of a chemical reaction, Figure 1.12: 0. dark core:

escaping gas mixture

1. brightly shining centre cone:

acetylene decomposition C2H2 -> 2C+H2 1st stage of combustion

2. welding zone:

2C + H2 + O2 (cylinder) -> 2CO + H2 2nd stage of combustion

3. outer flame:

4CO + 2H2 + 3O2 (air) -> 4CO2 + 2H2O 2C2H2 + 5O2 ->

complete reaction:

4CO2 + 2H2O

acetylene oxygen acetylene

welding torch head injector nozzle coupling nut

pressure nozzle

torch body

br-er1-11.cdr

© ISF 2002

Injector-Area of Torch

Figure 1.11

2005

1. Gas Welding

13

welding flame ratio of mixture

welding flame combustion welding nozzle centre cone welding zone 2-5

excess of oxygen

normal (neutral)

excess of acetylene

outer flame

3200°C

2500°C

1800°C

1100°C

effects in welding of steel foaming spattering

sparking 400°C

consequences: carburizing hardening

br-er1-12.cdr

© ISF 2002

Temperature Distribution in the Welding Flame

reducing

br-er1-13.cdr

oxidizing © ISF 2002

Effects of the Welding Flame Depending on the Ratio of Mixture

Figure 1.12

Figure 1.13 welding flame

By changing the mixture ratio of the volumes

balanced (neutral) flame nozzle size: for plate thickness of 2-4 mm

O2:C2H2 the weld pool can greatly be influ-

discharging velocity and weld heat-input rate: low 2

enced, Figure 1.13. At a neutral flame adjustment the mixture ratio is O2:C2H2 = 1:1. By reason of the higher flame temperature, an

soft flame

excess oxygen flame might allow faster

discharging velocity and weld heat-input rate: middle 3

welding of steel, however, there is the risk of oxidizing (flame cutting). Area of application: brass

moderate flame discharging velocity and weld head-input rate: high 4

The excess acetylene causes the carburising of steel materials. Area of application: cast iron

hard flame br-er1-14.cdr

© ISF 2002

Effects of the Welding Flame Depending on the Discharge Velocity

Figure 1.14

2005

1. Gas Welding

14

By changing the gas mixture outlet speed the flame can be adjusted to the heat requirements of the welding job, for example when welding plates (thickness: 2 to 4 mm) with the welding chamber size 3: “2 to 4 mm”, Figure 1.14. The gas mixture outlet speed is 100 to 130 m/s when using a medium or normal flame, applied to at, for example, a 3 mm plate. Using a soft flame, the gas outlet speed is lower (80 to 100 m/s) for the 2 mm plate, with a hard flame it is higher (130 to 160 m/s) for the 4 mm plate.

Depending on the plate thickness are the working methods “leftward welding” and “rightward welding” applied, Figure 1.15. A decisive factor for the designation of the working method is the sequence of flame and welding rod as well as the manipulation of flame and welding rod. The welding direction itself is of no importance. In leftward welding the flame is pointed at the open gap and “wets” the molten pool; the heat input to the molten pool can be well controlled by a slight movement of the torch (s ≤ 3 mm). Leftward welding is applied to a plate thickness of up to 3 mm. The weld-rod dips into the molten pool from time to time, but remains calm otherwise. The torch swings a little.

1,5 welding-rod

flame

welding bead

s

symbol

flange weld

plain butt weld

1,0

4,0

3,0

12,0

1,0

8,0

1,0

8,0

lap seam

1,0

8,0

fillet weld

V - weld 1-2 1-2

corner weld

flame

br-er1-15e.cdr

© ISF 2002

Flame Welding

Figure 1.15

denotation

1,0

Rightward welding ist applied to a plate thickness of 3mm upwards. The wire circles, the torch remains calm. Advantages: - the molten pool and the weld keyhole are easy to observe - good root fusion - the bath and the melting weld-rod are permanently protected from the air - narrow welding seam - low gas consumption

weld-rod

gap preparations ~ s+1 ~

plate thickness range s [mm] from to

r=

Advantages: easy to handle on thin plates

br-er1-16.cdr

© ISF 2002

Gap Shapes for Gas Welding

Figure 1.16

2005

1. Gas Welding

15 In rightward welding the flame is directed PA

butt-welded seams in gravity position

onto the molten pool; a weld keyhole is

gravity fillet welds

formed (s ≥ 3 mm). Flanged welds and plain butt welds can be

PB s

f

horizontal fillet welds

applied to a plate thickness of approx.

vertical fillet and butt welds

1.5 mm without filler material, but this does

PF PG

vertical-upwelding position vertical-down position

PC

horizontal on vertical wall

not apply to any other plate thickness and weld shape, Figure 1.16. By the specific heat input of the different

PE

welding methods all welding positions can be

overhead position

carried out using the oxyacetylene welding PD

method, Figures 1.17 and 1.18 horizontal overhead position

© ISF 2002

br-er1-17.cdr

Welding Positions I

Figure 1.17

When working in tanks and confined spaces, the welder (and all other persons present!) have to be protected against the

welding heat, the gases produced during welding and lack of oxygen ((1.5 % (vol.) O2 per 2 % (vol.) C2H2 are taken out from the PA

ambient atmosphere)), Figure 1.19. The addition of pure oxygen is unsuitable (explosion hazard!).

PB PF

A special type of autogene method is flame-

PC

straightening, where specific locally applied flame heating allows for shape correction of

PG

workpieces, Figure 1.20. Much experience is

PD

needed to carry out flame straightening proc-

PE

esses. The basic principle of flame straightening de© ISF 2002

br-er1-18.cdr

pends on locally applied heating in connecWelding Positions II

tion with prevention of expansion. This procFigure 1.18

2005

1. Gas Welding

16

ess causes the appearance of a heated zone. During cooling, shrinking forces are generated in the heated zone and lead to the desired shape correction.

Safety in welding and cutting inside of tanks and narrow rooms

Flame straightening

welded parts

first warm up both lateral plates, then belt

Hazards through gas, fumes, explosive mixtures, electric current protective measures / safety precautions 1. requirement for a permission to enter 2. extraction unit, ventilation

butt weld 3 to 5 heat sources close to the weld-seam

3. second person for safety reasons 4. illumination and electric machines: max 42volt

double fillet weld 1,3 or 5 heat sources

5. after welding: Removing the equipment from the tank

br-er1-19e.cdr

© ISF 2002

br-er1-20.cdr

Gas Welding in Tanks and Narrow Rooms

Figure 1.19

© ISF 2002

Flame Straightening

Figure 1.20

2005

2. Manual Metal Arc Welding

2. Manual Metal Arc Welding

18 Figure 2.1 describes the burn-off of a covered stick electrode. The stick electrode

air (O2, N2, etc.)

electrode core

consists of a core wire with a mineral cover-

electrode coating

ing. The welding arc between the electrode and the workpiece melts core wire and covering. Droplets of the liquefied core wire mix

Smoke and gas

with the molten base material forming weld metal while the molten covering is forming slag which, due to its lower density, solidifies on the weld pool. The slag layer and gases which are generated inside the arc protect the metal during transfer and also the weld pool

liquid slag solid slag

from the detrimental influences of the surrounding atmosphere. c ISF 2002

br-er2-01.cdr

Weld Point

Figure 2.1

Covered stick electrodes have replaced the initially applied metal arc and carbon arc electrodes. The covering has taken on the functions which are described in Figure 2.2.

Figure 2.2

2005

2. Manual Metal Arc Welding

19

The covering of the stick electrode consists of a multitude of components which are mainly mineral, Figure 2.3. coating raw material

effect on the welding characteristics

quartz - SiO2

to raise current-carrying capacity

rutile -TiO2

fluorspar - CaF2

to increase slag viscosity, good re-striking to refine transfer of droplets through the arc to reduce arc voltage, shielding gas emitter and slag formation to increase slag viscosity of basic electrodes, decrease ionization

calcareous- fluorspar K2O Al2O3 6SiO2

easy to ionize, to improve arc stability

ferro-manganese / ferro-silicon

deoxidant shielding gas emitter

magnetite - Fe3O4 calcareous spar -CaCO3

cellulose kaolin Al2O3 2SiO2 2H2O potassium water glass K2SiO3 / Na2SiO3

lubricant bonding agent

br-er2-03.cdr

© ISF 2002

Influence of the Coating Constituents on Welding Characteristics

Figure 2.3 For the stick electrode manufacturing mixed ground and screened covering materials are used as protection for the core wire which has been drawn to finished diameter and subsequently cut to size, Figure 2.4. raw material storage for flux production raw wire storage jaw crusher

1

magnetic separation

wire drawing machine and cutting system 2

3

descaling

inspection

example of a three-stage wire drawing machine drawing plate

cone crusher for pulverisation

Ø 6 mm

sieving to further treatment like milling, sieving, cleaning and weighing

sieving system

Ø 5,5 mm

Ø 4 mm

weighing and mixing inspection

br-er2-04.cdr

to the pressing plant electrode compound

3,25 mm

wet mixer

inspection © ISF 2002

Stick Electrode Fabrication 1

Figure 2.4

2005

2. Manual Metal Arc Welding

20 The core wires are coated

the pressing plant

with the covering material which inspection electrodepress

electrode compound

sion presses. The defect-

packing inspection

core wire magazine

nozzleconveying wire wire pressing belt feeder magazine head

binding

agents in electrode extru-

inspection compound

contains

free electrodes then pass

TO DELIVERY

through a drying oven and are, after a final inspection,

drying stove

automatically packed, Fig-

inspection inspection

ure 2.5.

inspection © ISF 2002

br-er10-33e.cdr

Stick Electrode Fabrication 2

Figure 2.5

Figure 2.6 shows how the moist extruded covering is deposited onto the core wire inside an electrode extrusion press.

pressing cylinder

core rod coating pressing nozzle pressing cylinder

pressing mass

core rod guide

br-er2-06.cdr

Production of Stick Electrodes

Figure 2.6

2005

2. Manual Metal Arc Welding

21

Stick electrodes are, according to their covering compositions, categorized into four different types, Figure 2.7. with concern to burn-off characteristics and achievable weld metal toughness these types show fundamental differences.

cellulosic type

acid type

cellulose 40 rutile TiO2 20 quartz SiO2 25 Fe - Mn 15 potassium water glass almost no slag droplet transfer : medium- sized droplets toughness value: good

slag solidification time: long droplet transfer : fine droplets to sprinkle toughness value:

basic typ

rutile type

magnetite Fe3O4 50 SiO2 20 quartz CaCO3 10 calcite Fe - Mn 20 potassium water glass

rutile TiO2 45 magnetite Fe3O4 10 SiO2 quartz 20 CaCO3 10 calcite Fe - Mn 15 potassium water glass

fluorspar CaF2 45 CaCO3 40 calcite SiO2 10 quartz 5 Fe - Mn potassium water glass

slag solidification time: medium

slag solidification time: short

droplet transfer : medium- sized to fine droplets toughness value:

droplet transfer : medium- sized to big droplets toughness value:

good

very good

normal

© ISF 2002

br-er2-07.cdr

Characteristic Features of Different Coating Types

Figure 2.7

The melting characteristics of the different coverings and the slag properties result in further properties; these determine the areas of application, Figure 2.8.

coating type symbol

cellulosic type C

acid type A

rutile type R

basic type B

~/+

~/+

~/+

=/+

very good

moderate

good

good

PG,(PA,PB, PC,PE,PF)

PA,PB,PC, PE,PF,PG

PA,PB,PC, PE,PF,(PG)

PA,PB,PC, PE,PF,PG

low

high

low

very low

moderate

good

good

moderate

slag detachability

good

very good

very good

moderate

characteristic features

spatter, little slag, intensive fume formation

high burn-out losses

universal application

low burn-out losses hygroscopic predrying!!

current type/polarity gap bridging ability welding positions sensitivity of cold cracking weld appearance

br-er2-08.cdr

© ISF 2002

Characteristics of Different Coating Types

Figure 2.8

2005

2. Manual Metal Arc Welding The

dependence

22

on

temperature of the slag’s determines the reignition behaviour of a stick electrode,

Figure

2.9.

ing slag -contain le ti u r high r nducto semico

reignition threshold

h ac co igh id s n d - te l a uc mp g to e r r at ur e hig bas i c hs co tem lag nd pe uc rat to ur r e

conductivity

conductivity

electrical

The

electrical conductivity for a rutile stick electrode lies, also at room temperature,

temperature

above the threshold value which

is

© ISF 2002

br-er2-09.cdr

necessary for

Conductivity of Slags

reignition. Therefore, rutile electrodes are given pref-

Figure 2.9

erence in the production of tack welds where reignition occurs frequently.

DIN EN 499 - E 46 3 1Ni B 5 4 H5 3

hydrogen content < 5 cm /100 g welding deposit butt weld: gravity position fillet weld: gravity position suitable for direct and alternating current recovery between 125% and 160% basic thick-coated electrode chemical composition 1,4% Mn and approx. 1% Ni o minimum impact 47 J in -30 C 2 minimum weld metal deposit yield strength: 460 N/mm distinguishing letter for manual electrode stick welding

The complete designation for filler materials, following European Standardisation, includes details– partly as encoded abbre-

The mandatory part of the standard designation is: EN 499 - E 46 3 1Ni B

viation – which are rele-

© ISF 2002

br-er2-10.cdr

vant for welding, Figure 2.10.

The

Designation Example for Stick Electrodes

identification

letter for the welding proc-

Figure 2.10

ess is first: E

-

manual electrode welding

G

-

gas metal arc welding

T

-

flux cored arc welding

W

- tungsten inert gas welding

S

-

submerged arc welding

2005

2. Manual Metal Arc Welding

23

The identification numbers give information about yield point, tensile strength and elongation of the weld metal where the tenfold of the identification number is the minimum yield point in N/mm², Figure 2.11.

key number

minimum yield strength N/mm2

tensile strength N/mm2

minimum elongation*) %

35

355

440-570

22

38

380

470-600

20

42

420

500-640

20

46

460

530-680

20

50

500

560-720

18

*) L0 = 5 D0

br-er2-11.cdr

© ISF 2002

Characteristic Key Numbers of Yield Strength, Tensile Strength and Elongation

Figure 2.11

The identification figures for the minimum impact energy value of 47 J – a parameter for the weld metal toughness – are shown in Figure 2.12.

characteristic figure Z A 0 2 3 4 5 6 7 8

0

minimum impact energy 47 J [ C] no demands +20 0 -20 -30 -40 -50 -60 -70 -80

The minimum value of the impact energy allocated to the characteristic figures is the average value of three ISO-V-Specimen, the lowest value of whitch amounts to 32 Joule. br-er2-12.cdr

Characteristic Key Numbers for Impact Energy

Figure 2.12

2005

2. Manual Metal Arc Welding

24

The chemical composition alloy symbol

of the weld metal is shown by the alloy symbol, Figure

The properties of a stick electrode are characterised by the covering thickness

Ni -

0,3 - 0,6 0,3 - 0,6 0,3 - 0,6

0,6 - 1.2 1,8 - 2,6 2,6 - 3,8 0,6 - 1,2 0,6 - 1,2

£ 2,0

Mo MnMo 1 Ni 2 Ni 3 Ni Mn 1 Ni 1 Ni Mo

1,4 >1,4 - 2,0 1,4 1,4 1,4 >1,4 - 2,0 1,4

Z

other specified compositions

*) companion elements: Mo 0,2; Ni 0,5; Cr 0,2; V 0,08; Nb 0,05; Cu 0,3; Al 2,0 (applies only to self-shielded flux-cored electrodes). single values are maxima

and the covering type. Both details are determined by

br-er2-13.cdr

the identification letter for electrode

Mo _

Mn without

2.13.

the

chemical composition*) %

© ISF 2002

Alloy Symbols for Weld Metals Minimum Yield Strength up to 500 N/mm2

covering, Figure 2.13

Figure 2.14.

Figure 2.15 explains the additional identificakey letter

tion figure for electrode recovery and applica-

type of coating

ble type of current. The subsequent identifiA

acid coating

cation figure determines the application possi-

B

basic coating

bilities for different welding positions:

C

cellulose coating

R

rutile coated (medium thick)

RR

rutile coated (thick)

RA

rutile acid coating

RB

rutile basic coating

RC

rutile cellulose coating

br-er2-14.cdr

1-

all positions

2-

all positions, except vertical down postion

3-

© ISF 2002

flat position butt weld, flat position fillet weld, horizontal-, vertical up position

4-

flat position butt and fillet weld

5-

as 3; and recommended for vertical down position

Key Letters for Electrode Coatings

Figure 2.14

2005

2. Manual Metal Arc Welding

25 The last detail of the Euro-

additional characteristic number

deposition efficiency %

current type*)

1 2

105

125 125

alternating and direct current direct current

5 6

>125 >125

160 160

alternating and direct current direct current

7 8

>160 >160

alternating and direct current direct current

*) To prove the suitability for direct current, the tests have to be run with a no-load voltage of max. 65 V.

determines the maximum hydrogen content of the weld metal in cm³ per 100 g weld metal. Welding current amperage and core wire diameter of the stick electrode are

© ISF 2002

br-er2-15.cdr

determined

by

the

thickness of the workpiece

Additional Characteristic Numbers for Deposition Efficiency and Current Type

to be welded. Fixed stick

Figure 2.15

electrode lengths are assigned to each diameter, Figure 2.16.

diameter

d mm l

length

2,0

2,5

250/300

350

3,25

4,0

350/450 350/450

5,0

6,0

450

450

mm

current

I A

rule-of -thumb min. for current[A] max.

Figure

2.17

shows

the

process principle of manual metal arc welding.

40-80

50-100

20 x d 40 x d

90-150 120-200 180-270 220-360 30 x d 50 x d

35 x d 60 x d

Polarity and type of current depend

on

the

applied

electrode types. All known power sources with a de-

br-er2-16.cdr

© ISF 2002

Size and Welding Current of Stick Electrodes

scending

characteristic

curve can be used.

Figure 2.16 Since in manual metal arc welding the arc length cannot always be kept constant, a steeply descending power source is used. Different arc lengths lead therefore to just minimally altered weld current intensities, Figure 2.18. Penetration remains basically unaltered.

2005

2. Manual Metal Arc Welding

26

Simple welding transformers are used for a.c. welding. For d.c. welding mainly converters, rectifiers and series regulator transistorised power sources (inverters) are applied. Converters are specifically suitable electrode holder

for site welding and are mains-independent

stick electrode

- (+)

when

an internal combustion engine is used. The advan-

power source = or ~

tages of inverters are their

+ (-)

small size and low weight, arc

however, a more complicated electronic design is necessary, Figure 2.19.

work piece © ISF 2002

br-er2-17.cdr

Principle Set-up of MMAW Process

Figure 2.17

arc welding converter

power source characteristic

A2

U

A1

transformer

A2

2

rectifier

1

A1

21 characteristic of the arc br-er2-18e.cdr

inverter type

I

© ISF 2002

br-er2-19.cdr

Operating Point at Different Arc Lengths

Figure 2.18

© ISF 2002

Power Sources for MMAW

Figure 2.19

2005

2. Manual Metal Arc Welding

27

45

cy

7

RA73 V

eff ici en

kg/h

sit ion

6

30

B53

de

cy

0%

ie n

22

ef fic io n

po

sit

b

3

oa

ed

t

-c ick

a

th

2

oa

d te

c inth 1

B15

25

de

RR12 RA12

X

4

0%

35

5

16

burn-off rate at 100% duty cycle

medium weld voltage

RR73

= RR12 = = = =

20

c

po

40

100

200

300

3,25 4 5 6

A

X = RR73 -

0

400

medium weld current br-er2-20.cdr

© ISF 2002

0

100

200 300 welding amperage

5 mm 5 mm

400 A 500

a = A- and R- coated electrodes, recovery 105% b = basic-coated electrodes, recovery