Welding Technology Part 1

Haward Technology Middle E

Applied Welding Technology: Welding Fabrication and Inspection to Meet AWS, ASME and API Codes

WHY WELD? Metal Structures can generally be made in 2 ways Cast

Fabricate

Castings can weigh 3 to 4 times as much as a fabrication economics and limited by scale

Fabrication We can fabricate using rivetting, bolting or welding Compare: Rivetting

Bolting

Or we can WELD

What is a WELD? Definition:

A localised coalescence of metals or non-metals produced either by heating the material to the welding temperature with our without the application of pressure or by the application of pressure alone and with or without the use of a filler metal

WELDING is therefore a special process It brings together the disciplines of design, metallurgy, process and inspection It is probably one of the most governed activities in the world Governed by National and International Codes and Client/Customer Specifications

Benefits of this Course to Engineers and Inspectors This course is designed to improve participants general knowledge of welding and: An aid to identify potential problems To enable a better understanding of inspection And to gain respect in this field and assist in a better cooperation and understanding within the engineering community

Chapter 1

Metal Joining and Cutting Processes Three Basic Groups: Welding

Cutting

Brazing

Well over 100 different processes with new process being developed - especially hybrids

Coverage of Processes For the more common processes we will discuss: Advantages Limitations Techniques and filler metals will be discussed later

Welding Processes to be discussed Shielded Metal Arc Welding -SMAW (or MMAW) Gas Tungsten Arc Welding - GTAW (or TIG) Gas Metal Arc Welding - GMAW (or MIG/MAG) Flux Cored Arc Welding -FCAW Submerged Arc Welding -SAW Oxy-fuel Gas Welding Stud Welding Special Welding Processes

Shielded Metal Arc Welding

SMAW Equipment

Simple Circuit Diagram for SMAW

SMAW Equipment

SMAW Advantages Simple equipment Inexpensive Very portable Large range of alloys High quality welds

SMAW Limitations Requires a degree of welder skill esp for position Relatively slow i.e. low deposit rate Can requires strict control on electrode storage and preparation for welding Large range of alloys A degree of fume is produced The slag must be removed Large range of alloys

SMAW - Effect of Current Can use: AC current, DC electrode +ve and DC electrode -ve

DC Straight or DC Reversed Polarity are not recommended can mean different things and very confusing Welding with AC transformers : - Cheapest for equipment and is very robust - Suffers less from arc blow - Good heat balance between electrode and workpiece - Generally operates on higher open circuit voltage

SMAW - Effect of Current Welding with DC transformer-rectifier or converters (engine driven): - Unbalanced heat between electrode and workpiece - Suffers more from arc blow - Generally operates on lower open circuit voltage For DC Electrode -ve : - More heat developed at electrode, increases melt-off rate - Less arc penetration into workpiece - Less dilution of base metal - Good for surfacing

SMAW - Effect of Current For DC Electrode +ve: - More heat developed at workpiece, decreased melt-off rate - More arc penetration into workpiece - More dilution of base metal DC Electrode +ve is probably the most common method of welding with covered electrodes

Gas Tungsten Arc Welding

GTAW Equipment

GTAW Equipment

Equipment can be simple and cost effective or expensive, it depends upon the application!

GTAW - DC Heat Balance

DC electrode -ve is common method for ferrous alloys

GTAW Electrodes Class

Alloy

Colour

EWP

Pure Tungsten

Green

EWCe – 2

1.8 - 2.2% Ceria

Orange

EWLa – 1

1% Lanthanum oxide

Black

EWTh – 1

0.8 - 1.2% Thoria

Yellow

EWTh – 2

1.7 - 2.2% Thoria

Red

EWZr

0.15 - 0.4% Zirconia

Brown

GTAW Electrodes Pure Tungsten - General Purpose AC electrode primarily used for Al, Mg, Ni and their alloys.

Thoriated - Standard DC electrode for TiG and plasma welding. ThO2 is a radioactive element. Ceriated - Similar to thoriated in performance but thoria-free. For DC TiG or plasma welding Ni, Mo, Ti, Cu, Ta and their alloys. A good replacement for Thoriated Lanthanated - An AC/DC electrode for plasma cutting, welding and spray applications. Zirconiated - Very similar to the pure tungsten electrode but with better overall performance.

GTAW Electrodes

Point shape - Usually ground to a point but in AC and DC+ve the electrode will ball up Point shape - For thoriated use a purpose built grinder to collect dust particles

GTAW Advantages High quality welds in almost all metals and alloys Good control of current - good for thin materials Very little, if any, post weld cleaning is required Arc and weld pool clearly visible to the welder

No filler metal carried across the arc therefore little or no spatter No slag produced that might be trapped in the weld All positional

GTAW Limitations Very low deposition rate - slow process Two handed operation Low tolerance for contamination Equipment is similar but more expensive than SMAW

GTAW Be careful to avoid tungsten inclusions in weld

Avoid using scratch start power sources esp for aluminium

Consider using lift start or HF start

Gas Metal Arc Welding

GMAW Equipment Equipment becomes more complex

GMAW Transfer Modes

Factors affecting transfer mode includes shielding gas, current, voltage and power supply

GMAW - Spray Transfer High current - good penetration and high deposition Droplet size than wire diameter Wire contacts workpiece and resistance heating at A to D Droplet transferred across arc at D to F Arc extinguished at H and wire moves to re-contact workpiece at I All positional but metal must have high electrical resistance

GMAW - Pulsed Transfer 1 droplet transferred at each pulse Spray transfer achieved at low currents - thin materials Low heat input Suitable for most metals

GMAW Transfer Modes Typical operating ranges for Spray, Dip and Pulse transfer with 1.2 mm diameter wire

GMAW Advantages High productivity No slag to remove Clean process Welds most alloys Lowest hydrogen potential of all processes Continuous wire feed - semi automatic

GMAW Limitations Unsuitable for windy conditions Usually limited to shop welding Little tolerance for contamination Equipment more complex Suffers from inherent weld defects (discussed later)

GMAW Wire Feed Units Many types of feed systems, chose one that is appropriate to the work intended

Spool mounted on gun or “push-pull’ system

2 or 4 roll drives

Flux Cored Arc Welding 2 main types of FCAW Self shielded or gas shielded Equipment similar/same as GMAW

FCAW Advantages High Productivity Tolerates contamination High current density ∴ deep penetration esp with DC electrode +ve Suitable for field work, esp self shielded Sometimes the only method of obtaining semi-automatic welding with high alloys esp surfacing materials

Combines advatages of GMAW and SMAW without inherent defects

FCAW Limitations Slag to be removed Fumes are produced Higher cost of consumables Equipment can be more complex

Metal Cored Arc Welding A derivative of Flux Cored Arc welding No flux used - core filled with metal powders Does not suffer from GMAW inherent defects with same penetration proerties as FCAW

Submerged Arc Welding

Sub-arc Equipment

Sub-arc Advantages High deposition rate Deep penetraion Automatic process Good for surfacing Can be hand held Can use alloy fluxes - but critical voltage dependency

Sub-arc Deposition Rates

Sub-arc Limitations Flat or horizontal - but overhead has been developed High set-up time Needs positioning equipment Arc is not visible Needs slag removal and flux recovery High heat input - not suitable for thin materials, aluminium, single phase and quenched and tempered materials/steels Defects can be large i.e. lack of fusion or slag lines

Stud Welding Post Stud holder Ferrule* Ferrule holder* Workpiece Force applied Arc struck and stud withdrawn Arc maintained Stud plunged into workpiece

Stud Welding Advantages Simple, fats and repetitive esp automatic feed Limitations Needs clean surface to ignite arc and high maintenance Discontinuities Lack of 3600 and lack of fusion

Oxy-Fuel Gas Welding Principles

Hand held torch

Gas bottles in trolley

OAW Advantages Very simple equipment Suitable for thin materials Suitable for pipe welds Extremely portable No electricity required

OAW Limitations Very slow process 2 handed manual operation Suitable for pipe welds Less concentrated heat -but high heat input Low productivity Limited to carbon and low alloy steels

Special Welding Process There are variations of the processes discussed Many hybrid process being developed Many other special welding process There are too many to cover in this course but some can be mentioned

Plasma Arc Welding

Equipment can be complex but low cost units available which run on compressed air

Electroslag Welding

Electrogas Welding

Resistance Welding

High Energy Processes Electron Beam

Laser - CO2 shown also Nd-YAG

Request for Day 2 Discussion On day 2, we will discuss welding dissimilar metals If you have any specific topics which you wish to be considered, please let me know at the end of today’s session Thank you

Power Sources for Arc Welding Will limit discussion to three most common types Transformer Transformer /rectifier Inverter - solid state

Main Components of a Power Source ▪Transformer ▪Rectifier ▪Inductor ▪High Frequency unit or Capcitor Discharge (GTAW) ▪Protection System for overheating and over current ▪Burn-back ▪Cooling fan or oil ▪Enclosure

Transformer Simple operation - V1/V2 = A2/A1 Many types including movable shunt, cntre tap, single phase and 3 phase etc Cost increase with complexity AC current only

Transformer Rectifier

Inductance An inductive circuit is where the current lags voltage With zero inductance burn-back of the consumable electrode is instantaneous Adding inductance allows soft start and slow build up of current in the electrode

Arc Characteristics Constant current Constant voltage CC/CV inverters

Constant Current

Constant Current Manual welding has poor control of arc length When arc length is increased or decreased, the arc energy remains fairly constant and therefore electrode melt-off rate remains constant as does the heat of the arc Especially true when using equipment with high open circuit voltage SMAW and GTAW trypically use constant current arc control

Constant Voltage

Constant Voltage Typical arc control for semi automatic and automatic processes These process could use CC but would need voltage sensing device to control wire feed rate to keep arc length constant Prior to electroonic control, sensing was limited to supply frequency and feed motor could not react fast enough Controls arc length by adjusting melt-off rate Current and ∴ heat is not constant

Inverters

Inverters Solid state units Compact, portable and large weight saving High frequency of operation Combines best features of CC and CV unots

Duty Cycle The permissible temperature rise in a transformer without harming the insulation

Arc Blow The result of magnetic disturbances (including residual) which forcibly directs the arc away from the point of welding

Ends of ferromagnetic workpiece Location of lead

Edge of steel plate

Arc Blow Cannot always be elimimated, but may be controlled or reduced Change from DC to AC Use short arc technique Reduce welding current or voltage Use heavy tack welds at either end and intermittent tacks along length Angle electrode in the direction opposite to the arc blow Weld towards heavy tack or completed weld

Arc Blow Use backstep technique Attach work cable to both ends of the joint to be welded Extend end of joint by attaching run-off plates Bridge joint in piping Wrap cable round workpiece and pass current throught it try winding both ways - aim to neutralise the residual magnetism Can try magnetic particle yoke

Cutting Processes Oxy-Fuel Gas Cutting Plasma Cutting Carbon Arc Cutting (more often for gouging) Laser Beam Cutting Water Jet Cutting Mechanical

Oxy-Fuel Gas Cutting Uses same basic set-up as Oxyfuel welding Can be used for gouging Generally limited to carbon steels in a wide range of thicknesses Can be mechanised Needs fluxing or abrasive powder additions to cut other metals DO NOT USE CUTTING HEAD TO PREHEAT WELD JOINTS

Plasma Arc Cutting More expensive than Oxy-Fuel Small systems using compressed air as plasma gas are available Cuts most metals Can inject water to supplement superheat or shroud to minimise noise, fume etc. Can submerge workpiece in water to minimise HAZ and distortion

Carbon Arc Cutting Uses same equipment as SMAW but needs higher power i.e. 600 800A power sources are better Can cut most metals Can be automated High fume and noise Carburises cut surface *Fire Hazard*

Laser Cutting High speed cutting

Can cut most materials Equipment is expensive

Applicability of Thermal Cutting Process to Materials Material

Cutting Processes OFC

PAC

CAC-A

LBC

Carbon Steel

X

X

X

X

Stainless Steel

X*

X

X

X

Cast Iron

X*

X

X

X

X

X

X

X

X

X

Copper

X

X

X

Refractory Materials

X

X

X

Aluminium Titanium

X* = Process applicable with special techniques This table should be used only as a very general guide

X*

Water Jet Cutting High velocity water jet large expensive installations Can cut range of non metallics up to about 9 mm Can cut metals but limited No thermal distortion of cut material Relatively slow

Mechanical Cutting Grinding Sawing As with Water Jet cutting there is no heat from an arc Ideal when there is a need to preserve the metallurgical structure or for cutting non-metals

Brazing Achieves a bond between materials by heating them in the presence of a filler metal that has a liquidus above 4500C and below the solidus of the base metal Brazing process include - Torch Brazing - Furnace Brazing - Induction Brazing - Resistance Brazing - Dip Brazing - Infrared Brazing

Brazing Depends on surface diffusion to create joint ∴ needs large surface contact area for high strength joints Must have capillary action to wet surfaces to be joined this requires close fit up or high tolerance Surfaces must be metallurgically clean ∴ fluxes needed to assist cleaning action and to protect cleaned surface through heating cycle

Brazing Consumable Forms Brazing consumables are available in a variety of forms including: Rod, Wire, Strip, Foil, Paste and Preforms The above forms can come “prefluxed” or require the addition of additional flux

Braze Joint Configurations Lap Flare Splice Socket etc.

Brazing Consumables AWS Brazing Filler Metals Designation

Primary Element

BAlSi

Aluminium - Silicon

BCuP

Copper - Phosphorus

BAg

Silver

BAu

Gold

BCu

Copper

BCuZn

Copper - Zinc

BMg

Magnesium

BNi

Nickel

Brazing Advantages Strong joints (3D stress fields) Joins dissimilar metals Joins metals to non metals Joins “unweldable” metals Less heat and less distortion

Brazing Limitations Cleanliness is critical Joint design is critical Some fluxes can be corrosive and need to be completely removed form workpiece Voids and unbonded areas

Arc Welding Consumables Arc welding consumables are manufactured by many companies all over the world Implies that there are hundreds of consumables available that will perform in the same manner when welded to similar metals It was recognised many years ago that all this could lead to considerable confusion in distinguishing between consumables - not good for the industry!

Arc Welding Consumables As we will discuss later, most, if not all, common base metals have been classified - so therefore have consumables The AWS method is by far the most common method of classification Even if a country has their own national method, it is common for consumables to carry both methods when marketed in that country All classifications use similar systems even if their specifics are different The AWS method is probably the simplest

Arc Welding Consumables In this section we will discuss both the consumable identification system and their properties, for the following: Coated electrodes Welding Wires Shielding gases Welding Fluxes The AWS method is probably the simplest

Coated Electrodes A coated electrode is one in which a clay type materials is extruded over the core wire during production

This coating is called a “FLUX” and is a mixture of different substances which have a decisive influence on welding characteristics and mechanical properties of the joint

Electrode Fluxes The mixture of materials which make up this flux generally contain the following five main components: ▪Slag-forming materials ▪Deoxidants ▪Shielding gas-forming agents ▪Binders, and, if necessary ▪Alloying elements (including iron powder

Electrode Flux Purpose: 1. To produce shielding gas vapours to displace air 2. De-oxidisers and other scavengers to purify the weld 3. Produce a slag which shapes the final weld 4. Protects the weld when solidifying and subsequent cooling 5. Adjust depsoition rates 6. Provide easy arc initiation and stability

General SMAW Flux Descriptions There are about 8 general types of fluxing systems and each perform a specific role during welding. NB: Not all are recognised by the AWS classification system ▪Acid ▪Cellulosic ▪Rutile ▪Rutile-thick

▪Rutile-cellulosic ▪Rutile-acid ▪Rutile-basic ▪Basic

Acid Flux Contain large proportions of iron oxides Due to high oxygen potential - they also contain deoxidents such as ferro-manganese The acid slag causes a very fine droplet transfer across the arc and produces a flat smooth weld They have limited application for positional welding and more susceptible to solidification cracking

Cellulosic Flux Contains large quantities of combustible organic substances, particularly cellulose and produces less slag The arc is very intense and deeply penetrating Especially suitable for vertical down welding

Rutile Flux Contains rutile (titanium oxide) and produces a coarse droplet transfer suitable for welding sheet metal Suitable for welding in all positions except vertical down

Rutile-thick Flux Have a diameter ration of covering to core wire greater than 1.6 High rutile content exhibits good restriking characteristics of the electrode and produces a finely rippled weld with good slag detachability

Rutile-acid Flux An improvement on acid covered electrodes Obviously contains more rutile A thick covering and suitable for all positions except vertical down

Rutile-basic Flux Contain basic minerals such as calcium carbonate and flourspar Improves mechanical properties Suitable for all position except vertical down

Basic Flux Contains higher quantities of basic minerals Produces stronger and tougher weld metal More resistant to solidification cracking due to higher weld metal purity Low risk of hydrogen cracking when “dry” electrodes are used Suitable for all positions including some designed for vertical down Generally requires a higher degree of welder skill

AWS System for Coated SMAW Fusion Welding Electrodes This system is described in the appropriate section of the AWS A5.XX series of Standards It is recognised by the ASME IX standard and enables grouping of electrodes within their classifications The system for SMAW electrodes comprises 5 sections, each describing a particular and property of the weld metal

AWS A5.5 for SMAW Electrodes E XX X X - XX Electrode Tensile Strength in kpsi Position capability

Alloy content when required Coating characteristics

AWS A5.5 for SMAW Electrodes Tensile strength e.g. 70 represents an electrode having an ultimate tensile strength of 70,000psi Usual to have 60, 70, 80, 90, 100, 110 or 120. Position capability: ▪1 = All positions ▪2 = Flat and Horizontal ▪4 = Downhill

AWS A5.4 for Stainless Steels E XXX X - X X Arc characteristics

Electrode Alloy e.g 308, 316 type stainless etc.

Position capability Carbon Content

Last 2 digits together denote flux coating

AWS A5.5 for SMAW Electrodes Coating and Arc Characteristics e.g. 10 11 12 13 14 15 16

High cellulose sodium High cellulose potassium High titania sodium High titania potassium Iron powder titania Low hydrogen sodium Low hydrogen potassium

DCEP AC or DCEP AC or DCEN AC or DCEN AC or DC DCEP AC or DCEP

Cellulosic Cellulosic Rutile Rutile Rutile-thick Basic Basic

18

Iron powder low hydrogen

AC or DC

Basic

Note that the flux covering in yellow is an approximation of flux type in accordance with AWS

AWS A5.5 for SMAW Electrodes Coating and Arc Characteristics (Contd) 20 24 27 28 30

High iron oxide sodium Iron powder titania Iron powder iron oxide High iron oxide sodium Iron powder titania

AC or DC Acid? AC or DC Rutile AC or DC Acid? AC or DC Acid? AC or DC Acid? (Less fluid slag than Exx20)

Note that the flux covering in yellow is an approximation of flux type in accordance with AWS

AWS A5.5 for SMAW Electrodes Alloy Content

A1 0.5% Molybdenum B1 1/2% Chromium, 1/2% Molybdenum B2 1- 1/4% Chromium, 1/2% Molybdenum B2L Low Carbon version of B2 type. Carbon content is 0.05% or less B3 2- 1/4% Chromium, 1% Molybdenum B3L Low Carbon version of B3 type. Carbon content is 0.05% or less B4L 2% Chromium, 1/2% Molybdenum, low carbon (0.05% or less) B5 1/2% Chromium, 1.1% Molybdenum

AWS A5.18/28 System for GMAW & GTAW Wires E R XX S - X Electrode

Alloy

R = Rod/Wire Tensile Strength in kpsi

S = Solid

AWS A5.18/28 GMAW Alloys 1 - Lowest silicon content of all classifications, tested with argon/ oxygen and may be used with CO2 when quality requirements are not critical and lower cost desired. 2 -Multiple deoxidised with combined total of 0.2% of Zr, Ti, and Al in addition to silicon and manganese. Used with argon/ oxygen, CO2 and argon/CO2. Preferred for out of position welding. 3 - Simliar to 1 but with higher silicon. Can be used out of position with argon/CO2 mixtures or CO2. Be careful with high heat inputs and strength

AWS A5.18/28 GMAW Alloys 4-

Higher silicon content than 3 and produce higher tensile strength. Primarily for CO2 requiring more deoxidation than 1 or 3.

5 - Contain aluminium in addition to manganese and silicon as deoxidisers. Used for steels with rusty or contaminated surface sacrifice in weld quality. 6-

with a

Produce welds with highest impact properties when used with CO2 Other classifications exist but the user should refer to manufacturers recommendations and the apprioriate AWS specification

AWS A5.18/28 GMAW Alloys

AWS A5.9 Stainless GMAW

AWS A5.10 Alumium GMAW

AWS 5.20/29 System for FCAW Wires ER XX T-XX Alloy

Electrode R = Rod/Wire Tensile Strength in 10 x kpsi Position Capability 0 = Flat and Horizontal 1= all position

When Operating necessary! Characteristics T= Tubular

AWS A5.20/29 System for FCAW Wires Operating Characteristics 1, 2, 5, 9 & 12 require shielding gas 3, 4, 6, 7, 8, 10, & 11 do not require shielding gas Sometimes termed “Open Arc” wires Alloy System As for AWS A5.5 when used

AWS A5.17/23 System for Sub Arc Wire

Sub Arc Wire Chemistry

AWS A5.17/23 System for Sub Arc Flux

Sub Arc Flux and Wire In submerged arc welding, the wire and flux are classified together E.g. F7A6-EM12K; indicates a combination that will produce weld metal in the welded condition with a tensile strength not less than 70kpsi and a Charpy V notch strength of at least 20ft lb at - 600F when depsoited under the conditions called for in the specification Each wire flux combination is unique therefore you CANNOT substitute either without the need for further testing to ensure that weld metal properties are maintained

Submerged-Arc Fluxes As with SMAW, Sub-arc fluxes can be classified into different groups i.e. Acid or Basic Within these groups, there are different types of fluxes i.e. Neutral or Active The manner in which they are made is also grouped i.e. Fused or Agglomerated

Acid or Basic

The degree of basicity or the basicity index ( BI ) of a SAW–flux is used to describe the chemical characteristic of a SAW–flux slag and its metallurgical behavior BI or basicity of a flux according to Boniczewsky [ in weight % ]: CaO + MgO + BaO + CaF2 + Na2O + K2O + 0,5 ( MnO + FeO) [wt.–%] SiO2 + 0,5 ( AL2O3 + Ti O2 + ZrO2) [wt.–%]

Acid or Basic The BI defines the ratio between the basic and the acid oxides of a flux and can be an indication of the achievable O2–content in the weld metal A high BI gives low O2–content in the weld metal. Higher BI's normally increase impact toughness, but with less arc constancy and more difficult weld bead shaping. One common method of compensation is to use higher silicon–bearing wire

Acid or Basic

Neutral Fluxes Will not cause a significant change in the weld metal chemistry even with variations of welding voltages. Will not significantly affect the strength of the weld metal regardless of the welding voltage used or the number of passes. As a rule neutral fluxes should be specified for multipass welding.

Active Fluxes Cause a substantial change in weld metal chemistry when voltage (and ∴ the amount of flux fused) is changed. Can add large amounts of Mn and Si to the weld metal and the weld strength increases. If is used for multipass welding, excessive Mn and Si buildup, the weld becomes brittle and crack prone. Must limited in the number of passes, esp. over rust and mill scale. In multipass welding, carefully control voltage to avoid excessive build-up of Mn and Si

Fused or Agglomerated Flux This refers to the manufacturing method used to produce the flux Fused flux is produced by melting the ingredients in a furnace, cooling the mixture and crushing to desired particle size They have a characteristic glass appearance Agglomerated flux is simpler to produce Ingredients are crushed, sieved and mixed with silica gel to a paste form and baked in a rotating oven, on cooling flux is sieved to desired particle

Fused Flux Because the method of manufacture, fused fluxes are limited in chemistry and tend to be neutral Can be basic but acid is more common Benefit of fused neutral flux is their ability to be recycled and they are non-hydroscopic

Agglomerated Flux As agglomerated fluxes are simply a physical mixing of ingredients no chemical reactions occur between ingredients and a wide range of fluxes can be produced Can be acid but basic is more common They are hygroscopic! And great care is taken by manufactrurer to ensure they are dry when packed

AWS Consumable Classification System It is important to note that these classifications are critical in their meaning and their control in producing and testing these consumables They refer to 100% weld metal and do not take any account of effects such as weld metal dilution or variations in welding technique These plain facts form some of the basic reasons for carrying out welding procedure testing and will be discussed later

Care of Consumables Discuss what precautions and you would take when storing and distributing welding consumables from a site store Assume there is a range of alloys for SMAW, GTAW and GMAW welding processes

Storage Considerations ▪Clean and dry ▪Baking and holding equipment and conditioning electrodes esp Low hydrogen ▪Hermetically sealed electrodes ▪Consumable identification ▪Maximum amounts issued from store ▪Return and scrap or recondition

Shielding Gases Shielding gases influence the following: ▪Metal transfer modes - Dip, Globular or Spray ▪Welding parameters - Current & Voltage ▪Arc stability ▪Welding speed ▪Wetting ▪Weld bead shape ▪Deposited weld metal chemistry ▪Welding fumes and gas emissions

Shielding Gases There are volt and amp ranges which will be optimum for each wire and gas combination. This range is dependent on the physical properties of the gas particularly ionisation potential and thermal conductivity Other effects will be discussed in relation to particular gases

Argon Lower ionisation potential, less energy and shorter arc length Tends to promote small droplet spray transfer in GMAW -droplets propelled at high energy to weld pool and promotes penetration Argon is heavier than air and advantageous when welding in the flat position Argon is inert and no chemical reactions in weld pool Used in GMAW and GTAW on Al, Mg copper and alloys GTAW on carbon steels and high purity for GTAW on Ti

Helium Higher ionisation potential than argon, greater energy and longer arc length Promotes large droplet globular transfer in GMAW Lighter than air so need larger flow rates Inert and used particularly in Argon mixtures on heavy heat sink materials such as aluminium Helium is much more expensive than argon

Carbon Dioxide Ionisation potential between argon and helium On disassociation into carbon monoxide and oxygen higher thermal efficiency Transfers greatest amount of heat to weld pool than any other shielding gas Supports globular and dip transfer but with spatter Increases wetting and arc stability - reacts with weld metal Used on its own or as a mixture with argon in GMAW do not use with GTAW

Oxygen Low ionisation potential, but O2 molecule splits and recombines to produce higher heat input than argon alone Generally only used in mixtures with argon up to around 5% Better arc stability than with pure argon Used in certain applications to enhance wetting of the weld pool Mixtures only used in GMAW - not for use with GTAW or with metals having high affinity for oxygen - Ti, etc

25 - 29 March 2006

Hydrogen High ionisation potential and high heat transfer Strong reducing agent and produces cleaner less oxidised weld bead Usually limited to about 5% in argon but more for certain purging applications Usually only for austenitic stainless steels Can be used in other applications - but must beware of solubility of hydrogen in weld metal its consequences!!

25 - 29 March 2006