kalpakjian 19

Brazing, Soldering,

Adhesive-Bonding, and MechanicalFastening Processes ' This last chapter of Part VI describes various joining, bonding, and fastening

processes that involve mechanisms unlike those in the preceding two chapters. ° Brazing and soldering are different from welding in that no diffusion takes place at the interface, thus bond strength depends on adhesive forces. °

' ' °

Brazing and soldering are differentiated by the temperature at which filler metals melt: Brazing takes place above 450°C and produces stronger joints than soldering, while soldering involves lower temperatures and is widely applied in the electronics industry. Adhesive bonding is versatile, and a Wide variety of adhesives is available for numerous applications. Mechanical joining processes are then described, such as using bolts, nuts, rivets, snap fasteners, or shrink fits in assembly. The chapter ends with a discussion of economic considerations in joining

operations.

32.l

32.I 32.2 32.3 32.4 32.5 32.6

32.1

Introduction 92| Brazing 922 Soldering 926 Adhesive Bonding 93| Mechanical Fastening 939 joining Plastics, Ceramics, and Glasses 942 Economics of joining Operations 945

EXAMPLE:

32.l

Soldering of Components onto a Printed Circuit Board

929

CASE STU DY:

32.l

Light Curing Acrylic Adhesives for Medical

Products

937

Introduction

In most of the joining processes described in Chapters 30 and 31, the mating surfaces of the components are heated to elevated temperatures by various external or internal means, to cause fusion and bonding at the joint. But what if we want to join a pair of materials that cannot withstand high temperatures, such as electronic components? What if the parts to be joined are fragile, intricate, or made of two or more materials with very different characteristics, properties, sizes, thicknesses, and cross sections? This chapter first describes two joining processes-brazing and solderingthat require lower temperatures than those used for fusion welding. Filler metals are placed in or supplied to the joint and are melted by an external source of heat; upon solidification, a strong joint is obtained. Brazing and soldering are distinguished arbitrarily by temperature. Temperatures for soldering are lower than those for brazing, and the strength of a soldered joint is much lower. The chapter also describes the principles and types of adhesive bonding. The ancient method of joining parts with animal-derived glues (typically employed in bookbinding, labeling, and packaging) has been developed into an important joining technology for metallic and nonmetallic materials. The process has wide application

92|

2

Chapter 32

Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

in numerous consumer and industrial products, as well as in the aircraft and aero-

space industries. Bonding materials such as thermoplastics, thermosets, ceramics, and glasses, either to each other or to other materials, present various challenges. Although all of the joints described thus far are of a permanent nature, in many applications joined components have to be taken apart for replacement, maintenance, repair, or adjustment. But how, for example, do we take apart a product without destroying the joint? If joints are required that are not permanent, but still must be as strong as welded joints, the obvious solution is to use mechanical fastening, such as fastening with bolts, screws, nuts, or a variety of other fasteners.

32.2

Brazing

Brazing is a joining process in which a #Her metal is placed between the faying surfaces to be joined (or at their periphery) and the temperature is raised sufficiently to melt the filler metal, but not the components (the base metal)-as would be the case in fusion welding. Thus, brazing is a liquid-solid-state bonding process. Upon cooling and solidification of the filler metal, a strong joint is obtained (Fig. 32.1). Filler metals used for brazing typically melt above 45 0°C, which is below the melting point (solidus temperature) of the metals to be joined (see, for example, Fig. 4.5). Brazing is derived from the word brass, an archaic word meaning “to harden,” and the process was first used as far back as 3000 to 2000 B.C. In a typical brazing operation, a filler (braze) metal wire is placed along the periphery of the components to be joined, as shown in Fig. 32.2a. Heat is then applied

(H)

(D)

(C)

(Ol)

(G)

FIGURE 32.I Examples of brazed and soldered parts. (a) Resistance-brazed light-bulb filament; (b) brazed radiator heat exchanger; (c) soldered circuit board; (d) brazed ring housing; and (e) brazed heat exchanger. Source: Courtesy of Edison Welding Institute.

Section 32 2

|='||

Fillefmeral (thickness exaggerated)

i

viiirgrmea

....

if

';'l"

":`

Tiff

'le'

I





FIGURE 32.2 is a

An example of furnace brazing (a) before and (b) after brazing. The filler metal shaped wire and moves into the interfaces by capillary action with the application of heat.

,

to

_

a

FIGURE 32.3 joint designs commonly used in brazing operations. The clearance between the two parts being brazed is an important factor in joint strength. lf the clearance is too small, the molten braze metal will not penetrate the interface fully. lf it is too large, there will be insufficient capillary action for the molten metal to fill the interface.

by various external means, melting the braze metal and, by capillary action, filling the closely fitting space (joint clearance) at the interfaces (Fig. 32.2b). In braze welding, filler metal (typically brass) is deposited at the joint by a technique similar to oxyfuel-gas welding (see Fig. 3O.1d); the major difference is that the base metal does not melt. The main application of braze welding is in repair work, typically on parts made of cast steels and irons. Because of the wider gaps between the components being welded (as in oxyfuel-gas welding), more braze metal is used than in conventional brazing.

In general, dissimilar metals can be assembled with good joint strength. Examples of joints made are shown in Fig. 32.3. Intricate, lightweight shapes can be joined rapidly and with little distortion.

Filler Metals.

Several filler metals (braze metals) are available with a range of brazing temperatures (Table 32.1). Note that, unlike those for other welding operations, filler metals for brazing generally have a composition that is significantly different from those of the metals to be joined. They are available in a variety of shapes, such as wire, rod, ring, shim stock, and filings. The selection of the type of filler metal and its composition are important in order to avoid enibrittlement of the joint by (a) grain-boundary penetration of liquid metal (Section 1.5.2); (b) the formation of brittle interinetallic compounds at the joint (Section 4.2.2); and (c) galvanic corrosion in the joint (Section 3.8). Because of diffusion between the filler metal and the base metal, the mechanical and metallurgical properties of a joint can change as a result of subsequent processing or during the service life of a brazed part. For example, when titanium is brazed with pure tin as the filler metal, it is possible for the tin to diffuse completely into the

Brazing

2

Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

Chapter 32

TABLE 32.l

Typical Fiiler Metals for Erasing Various Metals andhiioys Brazing temperature Filler metal

Base metal

(°C)

Aluminum and its alloys Magnesium alloys Copper and its alloys Ferrous and nonferrous (except aluminum and magnesium)

Aluminum-silicon Magnesium-aluminum Copper-phosphorus Silver and copper alloys, copper-phosphorus

570-620 580-625 700-925 620-1150

Iron~, nickel-, and cobalt-based alloys Stainless steels, nickel- and cobalt-based alloys

Gold

900-1100

Nickel-silver

925-1200

titanium base metal when it is subjected to subsequent aging or to heat treatment. Consequently, the joint no longer exists. Q

Fluxes. The use of a flux is essential in brazing; a flux prevents oxidation and removes oxide films. Brazing fluxes generally are made of borax, boric acid, borates, fluorides, and chlorides. Wetting agents may be added to improve both the wetting characteristics of the molten filler metal and the capillary action. It is essential that the surfaces to be brazed be clean and free from rust, oil, and other contaminants in order (a) for proper wetting and spreading of the molten filler metal in the joint and (b) to develop maximum bond strength. Sand blasting also may be used to improve the surface finish of the faying surfaces for brazing. Because they are corrosive, fluxes must be removed after brazing, usually by washing with hot water.

zeQ

3C

“hs

'D

“’

‘%

S6

rn

@~9r

S"""Qrh

Joint clearance

->

The effect of joint clearance on the tensile and shear strength of brazed joints. Note that, unlike tensile strength shear strength continually decreases as the clearance increases. FIGURE 32 4

Brazed joint Strength. The strength of the brazed joint depends on (a) joint clearance, (b) joint area, and (c) the nature of the bond at the interfaces between the components and the filler metal. joint clearances typically range from 0.025 to 0.2 mm. As shown in Fig. 32.4, the smaller the gap, the higher is the shear strength of the joint. The shear strength of brazed joints can reach 800 MPa by using brazing alloys containing silver (silver solder). Note that there is an optimum gap for achieving maximum tensile strength of the joint. Because clearances are very small, roughness of the mating surfaces becomes important. The surfaces to be brazed must be cleaned chemically or mechanically to ensure full capillary action; thus, the use of a flux is also important.

32.2.l Brazing Methods The heating methods used in brazing identify the various processes. Torch Brazing. The heat source in torch brazing (TB) is oxyfuel gas with a carburizing flame (see Fig. 30.1c). Brazing is performed by first heating the joint with the torch and then depositing the brazing rod or wire in the joint. Suitable part thicknesses are typically in the range from 0.25 to 6 mm. Torch brazing is difficult to control and requires skilled labor; however, it can be automated as a production process by using multiple torches. Torch brazing can also be used for repair work.

Section 32.2

Furnace Brazing. The parts in furnace brazing (PB) are first cleaned and preloaded with brazing metal in appropriate configurations; then the assembly is placed in a furnace, where it is heated uniformly. Furnaces may be either batch type, for complex shapes, or continuous type, for high production runs-especially for small parts with simple joint designs. Vacuum furnaces or neutral atmospheres are used for metals that react with the environment. Skilled labor is not required, and complex shapes can be brazed because the Whole assembly is heated uniformly in the furnace.

Guide

\ _

Eirfééo be

\

_

_

Q 5' :ii

5%

f;,j

925

Brazing

Induction coil

j

_,;, 5*

Insulating board

Ejector

Schematic illustration of a continuous induction-brazing setup for increased productivity. FIGURE 32.5

Induction Brazing. The source of heat in induction brazing (IB) is induction heating by high-frequency AC current. Parts are preloaded with filler metal and are placed near the induction coils for rapid heating (see Fig. 4.26). Unless a protective (neutral) atmosphere is utilized, fluxes generally are used. Part thicknesses usually are less than 3 mm. Induction brazing is particularly suitable for brazing parts continuously (Fig. 32.5). Resistance Brazing. In resistance brazing (RB), the source of heat is the electrical resistance of the components to be brazed. Electrodes are utilized in this method, as they are in resistance Welding. Parts typically with thicknesses of 0.1 to 12 mm either are preloaded with filler metal or supplied externally with the metal during brazing. As in induction brazing, the process is rapid, heating zones can be confined to very small areas, and the process can be automated to produce reliable and uniform quality. Dip Brazing. Dip brazing (DB) is carried out by dipping the assemblies to be brazed into either a molten filler-metal bath or a molten salt bath (Section 4.12) at a temperature just above the melting point of the filler metal. Thus, all component surfaces are coated with the filler metal. Consequently, dip brazing in metal baths is typically used for small parts (such as sheet, wire, and fittings), usually less than 5 mm in thickness or diameter. Molten salt baths, which also act as fluxes, are used for complex assemblies of various thicknesses. Depending on the size of the parts and the bath size, as many as 1000 joints can be made at one time by dip brazing.

Infrared Brazing. The heat source in infrared brazing (IRB) is a high-intensity quartz lamp. The process is particularly suitable for brazing very thin components, usually less than 1 mm thick, including honeycomb structures (Section 16.12). The radiant energy is focused on the joint, and brazing can be carried out in a vacuum. Microwave heating also can be used. Diffusion Brazing. Diffusion brazing (DFB) is carried out in a furnace where, with proper control of temperature and time, the filler metal diffuses into the faying surfaces of the components to be joined. The brazing time required may range from 30 minutes to 24 hours. This process is used for strong lap or butt joints and for difficult joining operations. Because the rate of diffusion at the interface does not depend on the thickness of the components, part thicknesses may range from foil to as much as 5 0 mm.

High-energy Beams. For specialized and high-precision applications and with high-temperature metals and alloys, electron-beam or laser-beam heating may be used (see also Sections 27.6 and 27.7).

Braze Welding. The joint in braze welding is prepared as it is in fusion welding, described in Chapter 30. While an oxyacetylene torch with an oxidizing flame is

Chapter 32

926

Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

Comments

Poor

Good

ti'

ff

'====;==f¢'

Too little joint

Improved design when fatigue loading is a factor to be considered

used, filler metal is deposited at the joint (hence the term welding) rather than drawn in by capillary action. As a result, considerably more filler metal is used than in brazing. However, temperatures in braze welding generally are lower than in fusion welding, and thus part distortion is minimal. The use of a flux is essential in this process. The principal use of braze welding is for maintenance and repair work, such as work on ferrous castings and steel components, although the process can be automated for mass production.

32.2.2 Design for Brazing Insufficient bonding area

Examples of good and poor design for brazing. Source: American Welding Society. FIGURE 32.6

32.3

As in all joining processes, joint design is important in brazing. Some design guidelines are given in Fig. 32.6. Strong joints require a larger contact area for brazing than for welding. A variety of special fixtures and work-holding devices

may be required to hold the parts together during brazing; some will allow for thermal expansion and contraction during the brazing operation.

Soldering

In soldering, the filler metal (called solder) melts at a relatively low temperature. As in brazing, the solder fills the joint by capillary action between closely fitting or closely placed components. Two important characteristics of solders are low surface tension and high wetting capability. Heat sources for soldering are usually soldering irons, torches, or ovens. The word “solder” is derived from the Latin solidare, meaning “to make solid.” Soldering with copper-gold and tin-lead alloys was first practiced as far back as 4000 to 3000 B.C.

32.3.l Types of Solders and Fluxes Solders melt at a temperature that is the eutectic point of the solder alloy (see, for example, Fig. 4.7). Solders traditionally have been tin-lead alloys in various proportions. For example, a solder of 61.9% Sn-38.1% Pb composition melts at 188°C, whereas tin melts at 232°C and lead at 327°C. For special applications and higher joint strength (especially at elevated temperatures), other solder compositions are tin-zinc, lead-silver, cadmium-silver, and zinc-aluminum alloys (Table 322). Because of the toxicity of lead and its adverse effects on the environment, leadfree solders are being developed continuously and are coming into wider use. Among the various candidate materials are silver, indium, and bismuth eutectic alloys in TABLE 32.2

Types of Solders and Their Applications Tin-lead Tin-zinc Lead-silver Cadmium-silver Zinc-aluminum Tin-silver Tin-bismuth

General purpose Aluminum Strength at higher than room temperature Strength at high temperatures Aluminum, corrosion resistance Electronics Electronics

Section 32

combination with tin. Three typical compositions are 96.5% Sn-3.5% Ag, 42% Sn-58% Bi, and 48% Sn-52% In. However, none of these combinations are suitable for every soldering application. Fluxes are used in soldering and for the same purposes as they are in welding and brazing, as described in Section 32.2. Fluxes for soldering are generally of two types:

Inorganic acids or salts, such as zinc-ammonium-chloride solutions, which clean the surface rapidly. To avoid corrosion, the flux residues should be removed after soldering by washing the joint thoroughly with water. 2. Noncorrosive resin-based fluxes, used typically in electrical applications. I.

32.3.2 Solderability Solderability may be defined in a manner similar to weldability (Section 30.9.2). Special fluxes have been developed to improve the solderability of metals and alloys. As a general guide, ° ° ° °

Copper, silver, and gold are easy to solder Iron and nickel are more difficult to solder Aluminum and stainless steels are difficult to solder because of their thin, strong oxide films Steels, cast irons, titanium, and magnesium, as well as ceramics and graphite, can be soldered by first plating them with suitable metallic elements to induce interfacial bonding. This method is similar to that used for joining carbides and ceramics (see Section 32.6.3). A common example of the method is tinplate, which is steel sheet coated with tin, thus making it very easy to solder. Tinplate is a common material used in making cans for food.

32.3.3 Soldering Techniques The following soldering techniques are somewhat similar to brazing methods: a.

b. c.

d. e. f. g.

Torch soldering (TS). Furnace soldering (FS). Iron soldering (INS) (with the use of a soldering iron). Induction soldering (IS). Resistance soldering (RS). Dip soldering (DS). Infrared soldering (IRS).

Other soldering techniques, for special applications, are: in which a transducer sub'ects the molten solder to g ultrasonic cavitation. This action removes the oxide films from the surfaces to be joined and thus eliminates the need for a flux-hence the term fluxless soldering). i. Reflow (paste) soldering (RS). j. Wave soldering (WS).

h. Ultrasonic solderin

1

The last two techniques are widely used for bonding and packaging in surfacemount technology, as described in Section 28.11. Because they are significantly different from other soldering methods, they are described next in greater detail.

Reflow Soldering. Solder pastes are solder-metal particles held together by flux, binding, and wetting agents. The pastes are semisolid in consistency, have high viscosity, and thus are capable of maintaining their shape for relatively long periods.

3

Soldering

2

28

Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

Chapter 32

Squeegee Tensioned screen

_ _

__ _

_

'ia§e_

§< or higher to be seen by the naked eye are called microcracks. Craters are shallow depressions. The heat-affected zone is the portion of a metal which is subjected to thermal cycling without melting, such as that shown in Fig. 30.17. Inclusions are small, nonmetallic elements or compounds in the material. Intergranular attack is the weakening of grain boundaries through liquidmetal embrittlement and corrosion. Laps, folds, and seams are surface defects resulting from the overlapping of material during processing. Metallurgical transformations involve microstructural changes caused by temperature cycling of the material; these changes may consist of phase transformations, recrystallization, alloy depletion, decarburization, and molten and then recast, resolidified, or redeposited material. Pits are shallow surface depressions, usually the result of chemical or physical attack. Residual stresses (either tension or compression) on the surface are caused by nonuniform deformation and a nonuniform temperature distribution. Splatter is small resolidified molten metal particles deposited on a surface, as during welding. Surface plastic deformation is a severe surface deformation caused by high stresses due to factors such as friction, tool and die geometry, worn tools, and processing methods.

Surface Texture and Roughness

Regardless of the method of production, all surfaces have their own characteristics, which collectively are referred to as surface texture. Although the description of surface texture as a geometrical property is complex, the following guidelines have

Chapter 33

Surface Roughness and Measurement; Friction, Wear, and Lubrication

been established for identifying surface texture in terms of Well-defined and measurable quantities (Fig. 33.2): ° °

Flaws or defects are random irregularities, such as scratches, cracks, holes, depressions, seams, tears, or inclusions. Lay (directionality) is the direction of the predominant surface pattern, usually visible to the naked eye.

1

Flaw

Roughness

e'9 Q

y

_-

t

51. if

Jf. er

Roughness spacing

LQ

Lay direction

Q

Roughness-width cutoff Waviness width

/--...,/'M Surface profile

=

Maximum waviness height Maximum Ra Minimum Ffa Lay

w¢¢X¢f§§:;-.s~»»

Lay Symbm

_--

"\./"

+

+

Waviness

Error of form

-'------~ Roughness

WOOOOOS-0.05-Maximum waviness

l/ -f

--/1

fa

width cutoff width

-_- Roughness-width 10.00013-Maximum roughness 0.00025

.sa-aa

la)

w~aa,a.,»f»»»7§ss»a“.s_~a,`a;sa,. saas

_

Examples

lnterpretatlon

Lay parallel to the line representing the

surface to which the symbol

is

applied

3

£3

Lay perpendicular to the llne representing the _l_

surface to which the symbol

is

applied

Lay angular in both directions to line

X

P

representing the surface to which symbol is applied

Pitted, protuberant, porous, or particulate nondirectlonal lay

MJ.

AL 3

3

fp

lb)

FIGURE 33.2 (a) Standard terminology and symbols to describe surface finish. The quantities are given in microinches. (b) Common surface lay symbols.

Section 33.3 °

°

Roughness is defined as closely spaced, irregular deviations on a small scale; it is expressed in terms of its height, width, and distance along the surface. Waviness is a recurrent deviation from a flat surface; it is measured and described in terms of the space between adjacent crests of the waves (waz/iness width) and height between the crests and valleys of the waves (u/ai/iness height).

Surface Texture and Roughness

Digitized data V

A

abode

fg hi j k/

Surface profile Center (datum) line FIGURE 33.3

Surface roughness is generally characterized by two methods. The arithmetic mean value (Ra) is based on the schematic illustration of a rough surface, as shown in Fig. 33.3, and is defined as Ra =

;1} , [9

Coordinates used for surface roughness measurement defined by Eqs. (33.1) and (33 2)

(33.1)

where all ordinates a, h, c, are absolute values and n is the number of readings. The root-mean-square roughness (Rq, formerly identified as RMS) is defined as .

.

,

2

RL]

=

£72

2

dl

_

__

(33.2)

The datum line AB in Fig. 33.3 is located so that the sum of the areas above the line is equal to the sum of the areas below the line. The maximum roughness height (R,) also can be used and is defined as the height from the deepest trough to the highest peak. It indicates how much material has to be removed in order to obtain a smooth surface, such as by polishing. The units generally used for surface roughness are /.tm (microns). Because of its simplicity, the arithmetic mean value (Ra) was adopted internationally in the mid-19505 and is used widely in engineering practice. Dividing Eq. (33.2) by Eq. (33.1) yields the ratio Rq/Ru, which, for typical surfaces produced by machining and finishing processes is 1.1 for cutting, 1.2 for grinding, and 1.4 for lapping and honing. In general, a surface cannot be described by its Ra or Rq value alone, since these values are averages. Two surfaces may have the same roughness value, but have actual topographies that are very different. For example, a few deep troughs on an otherwise smooth surface will not affect the roughness values significantly. However, this type of surface profile can be significant in terms of friction, wear, and fatigue characteristics of a manufactured product. Consequently, it is important to analyze a surface in great detail, particularly for parts to be used in critical applications. Symbols for Surface Roughness. Acceptable limits for surface roughness are specified on technical drawings by symbols, typically shown around the check mark in the lower portion of Fig. 33.2a, and the values of these limits are placed to the left of the check mark. The symbols and their meanings concerning the lay are given in Fig. 33.2b. Note that the symbol for the lay is placed at the lower right of the check mark. Symbols used to describe a surface specify only its roughness, waviness, and lay; they do not include flaws. Therefore, whenever necessary, a special note is included in technical drawings to describe the method that should be used to inspect for surface flaws.

Measuring Surface Roughness. Typically, instruments called surface profilometers are used to measure and record surface roughness. A profilometer has a diamond

Chapter 33

Surface Roughness and Measurement; Friction, Wear, and Lubrication

Stylus

Head

Stylus path

If ,,,,_,

/

Rldef

’

\

Stylus

\

/'_ If

\\ Eff.

I

"V, ily,

'

f

LR ,’

‘f p

‘

Actual surface

‘

\\

\

ylli

(8)

(D)

o 5 ,rm

-""" ""‘f\~/Y-=-v~f\-"-‘-

\

0.6 ,rm =“'=‘v-'vyY~'\i=r=

ll 0.4 mm

(c) Lapping

3 8 ,um _L

_L

T (d) Finish grinding

5

;.Lm

_l_

T (e) Fiough grinding

T (f)

Turning

la) Measuring surface roughness with a stylus. The rider supports the stylus and guards against damage. (b) Path of the stylus in surface-roughness measurements (broken line), compared with the actual roughness profile. Note that the profile of the stylus path is smoother than that of the actual surface. (c) through (f) Typical surface profiles produced by various machining and surface-finishing processes. Note the difference between the vertical and horizontal scales. FIGURE 33.4

stylus that travels along a straight line over the surface (Fig. 33.4a). The distance that the stylus travels is called the cutoff, which generally ranges from 0.08 to 25 mm. A cutoff of 0.8 mm is typical for most engineering applications. The rule of thumb is that the cutoff must be large enough to include 10 to 15 roughness irregularities, as well as all surface waviness. In order to highlight roughness, profilometer traces are recorded on an exaggerated vertical scale (a few orders of magnitude larger than the horizontal scale; see Fig. 33.4c through f); the magnitude of the scale is called gain on the recording instrument. Thus, the recorded profile is distorted significantly, and the surface appears to be much rougher than it actually is. The recording instrument compensates for any surface waviness; it indicates only roughness. Because of the finite radius of the diamond stylus tip, the path of the stylus is different from the actual surface (note the path with the broken line in Fig. 33.4b), and the measured roughness is lower. The most commonly used stylus-tip diameter is 10 /sum. The smaller the stylus diameter and the smoother the surface, the closer is the path of the stylus to the actual surface profile.

Section 33 4

Surface roughness can be observed directly through an optical or scanningelectron microscope. Stereoscopic photographs are particularly useful for threedimensional views of surfaces and also can be used to measure surface roughness.

Three-dimensional Surface Measurement. Because surface properties can vary significantly with the direction in which a profilometer trace is taken, there is often a need to measure three-dimensional surface profiles. In the simplest case, this can be done with a surface profilometer that has the capability of indexing a short distance between traces. A number of other alternatives have been developed, two of which are optical interferometers and atomic-force microscopes. Optical-interference microscopes shine a light against a reflective surface and record the interference fringes that result from the incident and its reflected waves. This technique allows for a direct measurement of the surface slope over the area of interest. As the vertical distance between the sample and the interference objective is changed, the fringe patterns also change, thus allowing for a surface height measurement. Atomic-force microscopes (AFMS) are used to measure extremely smooth surfaces and even have the capability of distinguishing atoms on atomically smooth surfaces. ln principle, an AFM is merely a very fine surface profilometer with a laser that is used to measure probe position. The surface profile can be measured with high accuracy and with vertical resolution on the atomic scale, and scan areas can be on the order of 100 /im square, although smaller areas are more common. Surface Roughness in Engineering Practice. Requirements for surface-roughness design in typical engineering applications vary by as much as two orders of magnitude. Some examples are as follows: ° ° 0

°

Bearing balls Crankshaft bearings Brake drums Clutch-disk faces

0.025 /.im 0.32 ,um 1.6 /im 3.2 um.

Because of the many material and process variables involved, the range of roughness produced even within the same manufacturing process can be significant.

33.4

Friction

Friction plays an important role in manufacturing processes because of the relative motion and the forces that always are present on tools, clies, and workpieces. Friction (a) dissipates energy, thus generating heat, which can have detrimental effects on an operation, and (b) impedes free movement at interfaces, thus significantly affecting the flow and deformation of materials in metalworking processes. However, friction is not always undesirable; for example, without friction, it would be impossible to roll metals, clamp workpieces on machines, or hold drill bits in chucks. There are a number of explanations for the phenomenon of friction. A commonly accepted theory of friction is the adhesion theory, based on the observation that two clean and dry surfaces, regardless of how smooth they are, contact each other at only a fraction of their apparent contact area (Fig. 33.5 ). The maximum slope of the surface ranges typically from 5° to 15 °. In such a situation, the normal (contact) load, N, is supported by minute asperities (small projections from the surface) that are in contact with each other. Therefore, the normal stresses at these asperities are high; this causes plastic deformation at the junctions. Their contact

Friction

8

Surface Roughness and Measurement; Friction, Wear, and Lubrication

Chapter 33

N

F-»

H g* _ H

jj Projected Contact

areas

u

V

_

i

i

it it

ii

i

si

rl

jj rr

jj

,L 4/\‘ |

I

_H T

' J

Microweld

Q

Plastic deformation Elastic distortion

FIGURE 33.5 Schematic illustration of the interface of two bodies in contact showing real areas of contact at the asperities. In engineering surfaces, the ratio of the apparent-to-real areas of contact can be as high as 4 to 5 orders of magnitude.

creates an adhesive bond-the asperities form inicrou/elds. Cold-pressure welding (see Section 31.2) is based on this principle. Another theory of friction is the abrasion theory, which is based on the notion that an asperity from a hard surface (such as a tool or a die) penetrates and plows through a softer surface (the workpiece). Plowing may (a) cause displacement of the material and/or (b) produce small chips or slivers, as in cutting and abrasive processes. Other explanations for frictional behavior have been suggested, but for most applications in manufacturing, adhesion and abrasion mechanisms are the most relevant. The sliding motion between two bodies with an interface as just described is possible only if a tangential force is applied. This force, called the friction force, F, is required to shear the junctions or plow through the softer material. The ratio F/N (Fig. 33.5a) is the coefficient of friction, /J.. Depending on the materials and processes involved, coefficients of friction in manufacturing vary significantly. For example, in metal-forming processes, ,us ranges from about 0.03 for cold working to 0.7 for hot working and from 0.5 to as much as 2 for machining. Almost all of the energy dissipated in overcoming friction is converted into heat, which raises surface temperature. A small fraction of the energy becomes stored energy (Section 1.6) in the plastically deformed surfaces. The temperature increases with increasing friction and sliding speed, decreasing thermal conductivity, and decreasing specific heat of the sliding materials (see also Section 21.4). The interface temperature may be high enough to soften and even melt the surfaces and, sometimes, to cause microstructural changes in the materials involved. Note that temperature also affects the viscosity and other properties of lubricants, with a sufficiently high temperature causing their breakdown.

Friction in Plastics and Ceramics. Because their strength is low compared with that of metals (Tables 2.2 and 7.1), plastics generally possess low frictional characteristics. This property makes plastics better than metals for bearings, gears, seals, prosthetic joints, and general friction-reducing applications, provided that the loads are not high. Because of this characteristic, polymers sometimes are described as self lubricating. The factors involved in the friction and wear of metals are generally applicable to polymers as well. In sliding, the plowing component of friction in thermoplastics and elastomers is a significant factor because of their viscoelastic behavior (i.e., they exhibit both viscous and elastic behavior) and subsequent hysteresis loss (see Fig. 7.14). This condition can easily be simulated by dragging a dull nail across

Section 33 4

the surface of a piece of rubber and observing how the rubber quickly recovers its shape. An important factor in plastics applications is the effect of temperature rise at the sliding interfaces caused by friction. As described in Section 7.3, thermoplastics rapidly lose their strength and become soft as temperature increases. If the temperature rise is not controlled, sliding surfaces can undergo permanent deformation and

thermal degradation. The frictional behavior of various polymers on metals is similar to that of metals on metals. The well-known low friction of PTFE (Teflon) is attributed to its molecular structure, which has no reactivity with metals. Consequently, its adhesion is poor and thus its friction is low. The frictional behavior of ceramics is similar to that of metals; hence, adhesion and plowing at interfaces contribute to the friction force in ceramics as well. Usually, adhesion is less important with ceramics because of their high hardness, whereby the real area of Contact at sliding interfaces is small. Reducing Friction. Friction can be reduced through the selection of materials that have low adhesion (such as carbides and ceramics) and through the use of surface films and coatings. Lubricants (such as oils) or solid films (such as graphite) interpose an adherent film between the tool, die, and workpiece. This film minimizes adhesion and interactions of one surface with the other, thus reducing friction. Friction also can be reduced significantly by subjecting the tool- or die-workpiece interface to ultrasonic vibrations, typically at 20 kHz. The amplitude of the vibrations periodically separates the two surfaces and allows the lubricant to flow more freely into the interface during these separations. Friction Measurement. Although the coefficient of friction can be estimated theoretically, it usually is determined experimentally, either during actual manufacturing processes or in simulated laboratory tests using small-scale specimens of various shapes. A test that has gained wide acceptance-particularly for bulkdeformation processes-is the ring-compression test. A flat ring is upset plastically between two flat platens (Fig. 33.6a). As its height is reduced, the ring expands

Good lubrication

Poor lubrication

i

i

i

(

i

(

(2)

(D)

Ring-compression test between flat dies. (a) Effect of lubrication on type of ring-specimen barreling. (b) Test results: (1) original specimen and (2) to (4) increasing friction. Source: After A.T. Male and M.G. Cockcroft. FIGURE 33.6

Friction

0

Chapter 33

Surface Roughness and Measurement; Friction, Wear, and Lubrication

/\ /\

80 -

0.30

,,

A

§ § E

70

-

60

-

0.40

Q

0.20

50 40

0.15

-

0.12

2

3° °

0.10

5

'

0.09

9 2°

0.08 0.07

-‘E

0.06 0.055

1° `

2

0

i005

_fi

U

8 -10 -

0-04

U:

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O03

Original dimensions of specimen:

OD=19mm ID = 9.5 mm Height = 0.64 mm

_30 _

-40-

.002 0

0

10

20

30

Reduction

40 50 60 height (%)

70

in

Chart to determine friction coefficient from a ring-compression test. Reduction height and change in internal diameter of the ring are measured; then /.L is read directly from this chart. For example, if the ring specimen is reduced in height by 40% and its internal diameter decreases by 10%, the coefficient of friction is 0.10. FIGURE 33.1 in

radially outward. If friction at the interfaces is zero, both the inner and outer diameters of the ring expand as if it were a solid disk. With increasing friction, however, the internal diameter becomes smaller and barreling occurs. For a particular reduction in height, there is a critical friction value at which the internal diameter increases from its original value if ,rr is lower and decreases if /,L is higher (Fig. 33.6b>. By measuring the change in the specimen’s internal diameter and using the curves shown in Fig. 33.7 (which are obtained through theoretical analyses), the

coefficient of friction can be determined. Note that each ring geometry and material has its own specific set of curves. The most common geometry of a specimen has an outer diameter, an inner diameter, and height proportions of 6:3:2, respectively. The actual size of the specimen is usually not relevant in these tests. Thus, once the percentage of reduction in internal diameter and height is known, the magnitude of it can be determined from the appropriate chart.

EXAMPLE 33

I

Determination uf Coefficient of Friction

In a ring compression test, a specimen 10 mm in height and with an outside diameter (OD) of 30 mm and an inside diameter (ID) of 15 mm is reduced in

thickness by 50%. Determine the coefficient of friction, /JL, if the GD is 38 mm after deformation.

Section 33.5

Solution First it is necessary to determine the new ID (which is obtained from volume constancy) as

Thus, the change in internal diameter is

follows:

AID

volume =

-1-i