kalpakjian 19

CHAPTER

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

' This last chapter of Part VI describes various joining, bonding, and fasteningprocesses that involve mechanisms unlike those in the preceding two chapters.

° Brazing and soldering are different from welding in that no diffusion takesplace at the interface, thus bond strength depends on adhesive forces.

° Brazing and soldering are differentiated by the temperature at which filler met-als melt: Brazing takes place above 450°C and produces stronger joints thansoldering, while soldering involves lower temperatures and is widely applied inthe electronics industry.

' Adhesive bonding is versatile, and a Wide variety of adhesives is available fornumerous 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 joiningoperations.

32.l Introduction

In most of the joining processes described in Chapters 30 and 31, the mating surfacesof the components are heated to elevated temperatures by various external or internalmeans, to cause fusion and bonding at the joint. But what if we want to join a pair ofmaterials 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 materialswith very different characteristics, properties, sizes, thicknesses, and cross sections?

This chapter first describes two joining processes-brazing and soldering-that require lower temperatures than those used for fusion welding. Filler metals areplaced in or supplied to the joint and are melted by an external source of heat; uponsolidification, a strong joint is obtained. Brazing and soldering are distinguishedarbitrarily by temperature. Temperatures for soldering are lower than those forbrazing, and the strength of a soldered joint is much lower.

The chapter also describes the principles and types of adhesive bonding. Theancient method of joining parts with animal-derived glues (typically employed inbookbinding, labeling, and packaging) has been developed into an important joiningtechnology for metallic and nonmetallic materials. The process has wide application

32.I Introduction 92|32.2 Brazing 92232.3 Soldering 92632.4 Adhesive Bonding 93|32.5 Mechanical Fastening 93932.6 joining Plastics, Ceramics,

and Glasses 94232.1 Economics of joining

Operations 945

EXAMPLE:

32.l Soldering of Componentsonto a Printed CircuitBoard 929

CASE STU DY:

32.l Light Curing Acrylic

Adhesives for MedicalProducts 937

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, andglasses, either to each other or to other materials, present various challenges.

Although all of the joints described thus far are of a permanent nature, inmany applications joined components have to be taken apart for replacement, main-tenance, repair, or adjustment. But how, for example, do we take apart a productwithout destroying the joint? If joints are required that are not permanent, but stillmust be as strong as welded joints, the obvious solution is to use mechanical fasten-ing, 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 sur-faces to be joined (or at their periphery) and the temperature is raised sufficiently tomelt the filler metal, but not the components (the base metal)-as would be the casein fusion welding. Thus, brazing is a liquid-solid-state bonding process. Upon cool-ing and solidification of the filler metal, a strong joint is obtained (Fig. 32.1). Fillermetals 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). Brazingis derived from the word brass, an archaic word meaning “to harden,” and theprocess 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 theperiphery 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-bulbfilament; (b) brazed radiator heat exchanger; (c) soldered circuit board; (d) brazed ringhousing; and (e) brazed heat exchanger. Source: Courtesy of Edison Welding Institute.

Section 32 2 Brazing 2

|='|| i Fillefmeral .... viiirgrmea (thickness exaggerated)

if ';'l" ":` I Tiff'le' <a> <b>

FIGURE 32.2 An example of furnace brazing (a) before and (b) after brazing. The filler metalis a 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 betweenthe two parts being brazed is an important factor in joint strength. lf the clearance is toosmall, the molten braze metal will not penetrate the interface fully. lf it is too large, there willbe insufficient capillary action for the molten metal to fill the interface.

by various external means, melting the braze metal and, by capillary action, fillingthe closely fitting space (joint clearance) at the interfaces (Fig. 32.2b).

In braze welding, filler metal (typically brass) is deposited at the joint by atechnique 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 repairwork, typically on parts made of cast steels and irons. Because of the wider gaps be-tween the components being welded (as in oxyfuel-gas welding), more braze metalis 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 bejoined rapidly and with little distortion.

Filler Metals. Several filler metals (braze metals) are available with a range ofbrazing temperatures (Table 32.1). Note that, unlike those for other welding opera-tions, filler metals for brazing generally have a composition that is significantly dif-ferent from those of the metals to be joined. They are available in a variety ofshapes, such as wire, rod, ring, shim stock, and filings. The selection of the type offiller metal and its composition are important in order to avoid enibrittlement of thejoint by (a) grain-boundary penetration of liquid metal (Section 1.5.2); (b) the for-mation of brittle interinetallic compounds at the joint (Section 4.2.2); and (c) galvaniccorrosion in the joint (Section 3.8).

Because of diffusion between the filler metal and the base metal, the mechanicaland metallurgical properties of a joint can change as a result of subsequent process-ing or during the service life of a brazed part. For example, when titanium is brazedwith pure tin as the filler metal, it is possible for the tin to diffuse completely into the

Chapter 32 Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

3 “hsC 'D

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Joint clearance ->

TABLE 32.l

Typical Fiiler Metals for Erasing Various Metals andhiioysBrazing temperature

Base metal Filler metal (°C)

Aluminum and its alloys Aluminum-silicon 570-620Magnesium alloys Magnesium-aluminum 580-625Copper and its alloys Copper-phosphorus 700-925Ferrous and nonferrous Silver and copper alloys, 620-1150

(except aluminum copper-phosphorusand magnesium)

Iron~, nickel-, and Gold 900-1100cobalt-based alloys

Stainless steels, nickel- and Nickel-silver 925-1200cobalt-based alloys

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

Fluxes. The use of a flux is essential in brazing; a flux preventsoxidation 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 spreadingof the molten filler metal in the joint and (b) to develop maximum bondstrength. Sand blasting also may be used to improve the surface finish ofthe faying surfaces for brazing. Because they are corrosive, fluxes must beremoved after brazing, usually by washing with hot water.

FIGURE 32 4 The effect of joint clear-ance on the tensile and shear strength of

brazed joints. Note that, unlike tensilestrength shear strength continually de-creases as the clearance increases.

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 theinterfaces between the components and the filler metal. joint clearancestypically range from 0.025 to 0.2 mm. As shown in Fig. 32.4, the small-er 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 (silversolder). Note that there is an optimum gap for achieving maximum tensile strengthof 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 toensure 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 carburiz-ing flame (see Fig. 30.1c). Brazing is performed by first heating the joint with the torchand then depositing the brazing rod or wire in the joint. Suitable part thicknesses aretypically in the range from 0.25 to 6 mm. Torch brazing is difficult to control and re-

quires skilled labor; however, it can be automated as a production process by usingmultiple torches. Torch brazing can also be used for repair work.

Section 32.2 Brazing

Furnace Brazing. The parts in furnace brazing (PB) arefirst cleaned and preloaded with brazing metal in appro- Guide _ _ 5%priate configurations; then the assembly is placed in a _ Q 5' :ii furnace, where it is heated uniformly. Furnaces may be either batch type, for complex shapes, or continuous f;,j j

type, for high production runs-especially for small Eirfééo be \ _,;, 5* parts with simple joint designs. Vacuum furnaces or neu- \ tral atmospheres are used for metals that react with theenvironment. Skilled labor is not required, and complexshapes can be brazed because the Whole assembly is

heated uniformly in the furnace.

Induction Brazing. The source of heat in inductionbrazing (IB) is induction heating by high-frequency AC current. Parts are pre-loaded 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 generallyare used. Part thicknesses usually are less than 3 mm. Induction brazing is particu-larly suitable for brazing parts continuously (Fig. 32.5).

Resistance Brazing. In resistance brazing (RB), the source of heat is the electricalresistance of the components to be brazed. Electrodes are utilized in this method, asthey are in resistance Welding. Parts typically with thicknesses of 0.1 to 12 mmeither are preloaded with filler metal or supplied externally with the metal duringbrazing. As in induction brazing, the process is rapid, heating zones can be confinedto very small areas, and the process can be automated to produce reliable and uni-form quality.

Dip Brazing. Dip brazing (DB) is carried out by dipping the assemblies to be brazedinto either a molten filler-metal bath or a molten salt bath (Section 4.12) at a temper-ature just above the melting point of the filler metal. Thus, all component surfacesare coated with the filler metal. Consequently, dip brazing in metal baths is typicallyused for small parts (such as sheet, wire, and fittings), usually less than 5 mm inthickness or diameter. Molten salt baths, which also act as fluxes, are used for com-plex assemblies of various thicknesses. Depending on the size of the parts and thebath 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 quartzlamp. The process is particularly suitable for brazing very thin components, usuallyless than 1 mm thick, including honeycomb structures (Section 16.12). The radiant en-ergy is focused on the joint, and brazing can be carried out in a vacuum. Microwaveheating also can be used.

Diffusion Brazing. Diffusion brazing (DFB) is carried out in a furnace where, withproper control of temperature and time, the filler metal diffuses into the fayingsurfaces of the components to be joined. The brazing time required may range from30 minutes to 24 hours. This process is used for strong lap or butt joints and for dif-ficult joining operations. Because the rate of diffusion at the interface does not de-pend 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 withhigh-temperature metals and alloys, electron-beam or laser-beam heating may beused (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

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

925

Inductioncoil

Insulatingboard

Ejector


Good Poor Comments

ff ti' Too little joint '====;==f¢' Improved design when fatigue loading is a factorto be considered

Insufficientbonding area

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

used, filler metal is deposited at the joint (hence the termwelding) rather than drawn in by capillary action. As a re-sult, considerably more filler metal is used than in brazing.However, temperatures in braze welding generally arelower than in fusion welding, and thus part distortion is

minimal. The use of a flux is essential in this process. Theprincipal use of braze welding is for maintenance andrepair work, such as work on ferrous castings and steelcomponents, although the process can be automated formass production.

32.2.2 Design for Brazing

As in all joining processes, joint design is important in braz-ing. Some design guidelines are given in Fig. 32.6. Strongjoints require a larger contact area for brazing than for weld-ing. A variety of special fixtures and work-holding devicesmay be required to hold the parts together during brazing;some will allow for thermal expansion and contraction dur-ing the brazing operation.

32.3 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 surfacetension and high wetting capability. Heat sources for soldering are usually solderingirons, 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 practicedas 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 exam-ple, Fig. 4.7). Solders traditionally have been tin-lead alloys in various proportions. Forexample, a solder of 61.9% Sn-38.1% Pb composition melts at 188°C, whereas tinmelts 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-sil-ver, cadmium-silver, and zinc-aluminum alloys (Table 322).

Because of the toxicity of lead and its adverse effects on the environment, lead-

free solders are being developed continuously and are coming into wider use. Amongthe various candidate materials are silver, indium, and bismuth eutectic alloys in

TABLE 32.2

Types of Solders and Their Applications

Tin-lead General purposeTin-zinc AluminumLead-silver Strength at higher than room temperatureCadmium-silver Strength at high temperaturesZinc-aluminum Aluminum, corrosion resistanceTin-silver ElectronicsTin-bismuth Electronics

Section 32 3 Soldering 2

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 suit-able for every soldering application.

Fluxes are used in soldering and for the same purposes as they are in welding andbrazing, as described in Section 32.2. Fluxes for soldering are generally of two types:

I. Inorganic acids or salts, such as zinc-ammonium-chloride solutions, whichclean the surface rapidly. To avoid corrosion, the flux residues should be re-moved after soldering by washing the joint thoroughly with water.

2. Noncorrosive resin-based fluxes, used typically in electrical applications.

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 induceinterfacial bonding. This method is similar to that used for joining carbidesand 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 TechniquesThe following soldering techniques are somewhat similar to brazing methods:

a. Torch soldering (TS).

b. Furnace soldering (FS).

c. Iron soldering (INS) (with the use of a soldering iron).

d. Induction soldering (IS).

e. Resistance soldering (RS).

f. Dip soldering (DS).

g. Infrared soldering (IRS).

Other soldering techniques, for special applications, are:

h. Ultrasonic solderin in which a transducer sub'ects the molten solder tog 1ultrasonic 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).

The last two techniques are widely used for bonding and packaging in surface-mount technology, as described in Section 28.11. Because they are significantly dif-ferent 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 vis-cosity, and thus are capable of maintaining their shape for relatively long periods.


Squeegee

Tensioned screen

_ _ _ _ _ _ §<=L@<a1m2§_@iaL 'ia§e_ _e___ S é . .y , E 7 _ __

. if if iiii '”"C5}§i;f¢fgr¢‘g"`i

Paste deposited EmU|S|0non contact area

(8)

Copper |and Gull wilpg lead

Cgpper land W6-fied SOldel'

'Ag Plating or Coat *it \ coating JT Oil orair

_ ¢¢ Circuit board

"AV"; ArA '5 il” Turbulent zoneIC Ieads (oil prevents dross) a e Solder ,_“__ ‘,','

Turbulent zone(dross formedin air)

(D) (C)

FIGURE 32.1 (a) Screening solder paste onto a printed circuit board in reflow soldering.

(b) Schematic illustration of the Wave-soldering process. (c) SEM image of wave-solderedjoint on surface-mount device. Source: (a) After V Solberg.

The paste is placed directly onto the joint, or on flat objects for finer detail, and it canbe applied via a screening or stenciling process, as shown in Fig. 32.7a. Stenciling is

commonly used during the attachment of electrical components to printed circuit

boards. An additional benefit of reflow soldering is that the surface tension of the

paste helps keep surface-mount packages aligned on their pads; this feature improvesthe reliability of the solder joints. (See also Section 28.ll.)

Once the paste has been placed and the joint assembled, it is heated in a fur-

nace and soldering takes place. In reflovv soldering, the product is heated in a con-trolled manner, so that the following sequence of events occurs:

I. Solvents present in the paste are evaporated.

2. The flux in the paste is activated, and fluxing action occurs.

3. The components are preheated carefully.

4. The solder particles are melted, and they Wet the joint.

5. The assembly is cooled at a lovv rate to prevent thermal shock and fracture of

the solder joint.

Section 32.3 Soldering 92

Although this process appears to be straightforward, there are several process vari-ables for each stage, and good control over temperatures and exposures must bemaintained at each stage in order to ensure proper joint strength.

Wave Soldering. Wai/e soldering is a common technique for attaching circuit com-ponents to their boards (see Section 28.11). To understand the principle of wave sol-dering, it is important to note that molten solder does not wet all surfaces. Thesolder will not stick to most polymer surfaces, and it is easy to remove while molten.Also, as can be observed with a simple handheld soldering iron, the solder wetsmetal surfaces and forms a good bond only when the metal is preheated to a certaintemperature. Thus, wave soldering requires separate fluxing and preheating opera-tions before it can be completed.

A typical wave-soldering operation is illustrated in Fig. 32.7b. A standing lam-inar wave of molten solder is generated by a pump. Preheated and prefluxed circuitboards are then conveyed over the wave. The solder wets the exposed metal surfaces,but it does not remain attached to the polymer package for the integrated circuits,and it does not adhere to the polymer-coated circuit boards. An air knife (basically a

high-velocity jet of hot air) blows excess solder away from the joint to prevent bridg-ing between adjacent leads.

When surface-mount packages are to be wave soldered, they must be bondedadhesively to the circuit board before soldering can commence. Bonding usually is

accomplished by (1) screening or stenciling epoxy onto the boards, (2) placing thecomponents in their proper locations, (3) curing the epoxy, (4) inverting the board,and (5) performing wave soldering. A scanning-electron-microscope (SEM) photo-graph of a typical surface-mount joint is shown in Fig. 32.7c.

EXAMPLE 32.1 Soldering of Components onto a Printed Circuit Board

The computer and consumer electronics industriesplace extremely high demands on electronic compo-nents. Integrated circuits and other electronic devicesare expected to function reliably for extended peri-ods, during which they may be subjected to signifi-cant temperature variations and to vibration. Inrecognition of this requirement, it is essential that thesolder joints used to attach such devices to circuitboards be sufficiently strong and reliable and alsothat the solder joints be applied extremely rapidlywith automated equipment.

A continuing trend in the computer and theconsumer electronics industries is toward the reduc-tion of chip sizes and increasing compactness ofcircuit boards. Further space savings are achieved bymounting integrated circuits into surface-mountpackages, which allow tighter packing on a circuitboard. More importantly, the technique allows com-ponents to be mounted on both sides of the board.

A challenging problem arises when a printed cir-cuit board has both surface-mount and in-line circuitson the same board and it is desired to solder all of thejoints via a reliable automated process. It is important

to recognize that, for efficiency of assembly, all of thein-line circuits should be restricted to insertion fromone side of the board. Indeed, there is no performancerequirement that would dictate otherwise, and this re-striction greatly simplifies manufacturing.

The basic steps in soldering the connections onsuch a board are as follows (see Figs. 32.7b and c):

I. Apply solder paste to one side.

2. Place the surface-mount packages onto theboard, and insert in-line packages through theprimary side of the board.

3. Reflow the solder.

4. Apply adhesive to the secondary side of theboard.

5. Using the adhesive, attach the surface-mountdevices onto the secondary side.

6. Cure the adhesive.

7. Perform a wave-soldering operation on the sec-ondary side to produce an electrical attachmentofthe surface mounts and the in-line circuits tothe board.


Applying solder paste is done with chemicallyetched stencils or screens so that the paste is placedonly onto the designated areas of a circuit board.(Stencils are used more widely for fine-pitched de-vices and produce a more uniform paste thickness.)Surface-mount circuit components are then placed onthe board, and the board is heated in a furnace toaround 200°C to reflow the solder and form strongconnections between the surface mount and the cir-cuit board.

At this point, the components with leads are in-serted into the primary side of the board, their leadsare crimped, and the board is flipped over. A dot of

epoxy at the center of a surface mount component lo-cation is printed onto the board. The surface-mountpackages are then placed onto the adhesive by high-speed automated, computer-controlled systems, Theadhesive is then cured, the board is flipped, and wavesoldering is done.

The wave-soldering operation simultaneouslyjoins the surface-mount components to the second-ary side and solders the leads of the in-line compo-nents from the board’s primary side. The board is

then cleaned and inspected prior to the performanceof electronic quality checks.

32.3.4 Soldering Applications and Design Guidelines

Soldering is used extensively in the electronics industry. Note, however, that becausesoldering temperatures are relatively low, a soldered joint has very limited utility atelevated temperatures. Moreover, since solders generally do not have much strength,the process cannot be used for load-bearing (structural) members. ]oint strength canbe improved significantly by mechanical interlocking of the joint, as illustrated in

Fig. 32.8.Soldering can be used to join various metals of different thicknesses. Copper

and precious metals such as silver and gold are easy to solder. Aluminum and stainless steels are difficult to solder because of their strong, thin oxide film. However,these and other metals can be soldered with the aid of special fluxes that modify surfaces. Although manual operations require skill and are time consuming, solderingspeeds can be high with the use of automated equipment./ / //

ML’ 1(a) Flanged T (b) Flush lap (0) Flanged corner (d) Line contact

Q-fe~~~i». BO" 'l"1l'l »»..,,,, _M (e) Flat lock seam (f) Flanged bottom (g) Combination joint

Crimp

-\e=~<

PC board Wife -/(h) Through (i) Crimped (j) Twisted

hole connection combination joint wire joint

FIGURE 32.8 ]oint designs commonly used for soldering.

Section 32.4 Adhesive Bonding 93|

Design guidelines for soldering are similar to those for brazing. Some frequentlyused joint designs are shown in Fig. 32.8. Note the importance of large Contact sur-faces (because of the low strength of solders) for developing sufficient joint strengthin soldered products. Since the faying surfaces generally would be small, solders arerarely used to make butt joints.

32.4 Adhesive Bonding

Numerous parts and components can be joined and assembled by adhesives ratherthan by one or more of the joining methods described thus far. A common exampleof adhesive bonding is plywood, where several layers of wood are bonded withwood glue. Modern plywood was developed in 1905, but the practice of adhesivebonding wood layers dates back to 3500 B.C.

Adhesive bonding has gained increased acceptance in manufacturing ever sinceits first use on a large scale: the assembly of load-bearing components in aircraftduring World War II (1939-1945). Adhesives are available in various forms: liquid,paste, solution, emulsion, powder, tape, and film. When applied, adhesives typicallyare about 0.1 mm thick.

To meet the requirements of a particular application, an adhesive may requireone or more of the following properties (Table 32.3):

' Strength: shear and peel° Toughness° Resistance to various fluids and chemicals° Resistance to environmental degradation, including heat and moisture° Capability to wet the surfaces to be bonded.

TABLE 32.3

Typical Properties and Characteristics of Chemically Reactive Structural Adhesives

Epoxy Polyurethane Modified acrylic Cyanoacrylate AnaerobicImpact resistance Poor Excellent Good Poor FairTension-shear 15-22 12-20 20-30 18.9 17.5

strength, MPaPeel streI1gth”‘, N/m <523 14,000 5250 <525 1750Substrates bonded Most Most smooth, Most smooth, Most non- Metals, glass,

nonporous nonporous porous metals thermosetsor plastics

Service temperaturerange, °C -55 to 120 -40 to 90 -70 to 120 f55 t0 80 -55 to 150

Heat cure or mixing Yes Yes No No Norequired

Solvent resistance Excellent Good Good Good ExcellentMoisture resistance Good- Fair Good Poor Good

ExcellentGap limitation, mm None None 0.5 0.25 0.60Odor Mild Mild Strong Moderate MildToxicity Moderate Moderate Moderate Low LowFlammability Low Low High Low Low

‘F Peel strength varies widely depending on surface preparation and quality.


32.4.l Types of Adhesives and Adhesive Systems

Several types of adhesives are available, and more continue to be developed thatprovide adequate joint strength-including fatigue strength (Table 32.4). The three

basic types of adhesives are the following:

a. Natural adhesives-such as starch, dextrin (a gummy substance obtained fromstarch), soya flour, and animal products.

b. Inorganic adhesives--such as sodium silicate and magnesium oxychloride.

c. Synthetic organic adhesives-which may be thermoplastics (used for nonstruc-tural and some structural bonding) or thermosetting polymers (used primarilyfor structural bonding).

TABLE 32.4

General Characteristics of Adhesives

Type Comments Applications

Acrylic Thermoplastic; quick setting; tough bond at room temper- Fiberglass and steel sandwich bonds,ature; two components; good solvent chemical and impact tennis racquets, metal parts, and plasticsresistance; short work life; odorous; ventilation required

Anaerobic Thermoset; easy to use; slow curing; bonds at room tem- Close-fitting machine parts, such as

perature; curing occurs in absence of air; will not cure shafts and pulleys, nuts and bolts, and

where air contacts adherents; one component; not good bushings and pins

on permeable surfacesEpoxy Thermoset; one or two components; tough bond; Metal, ceramic, and rigid plastic parts

strongest of engineering adhesives; high tensile and lowpeel strengths; resists moisture and high temperature;difficult to use

Cyanoacrylate Thermoplastic; quick setting; tough bond at room temper- “Crazy glueTM”

ature; easy to use; colorlessHot melt Thermoplastic; quick setting; rigid or flexible bonds; easy Bonds most materials; packaging,

to apply; brittle at low temperatures; based on ethylene book binding, and metal can jointsvinyl acetate, polyolefins, polyamides, and polyesters

Pressure sensitive Thermoplastic variable strength bonds; primer anchors Tapes, labels, and stickers

adhesive to roll tape backing material-a release agent onthe back of web permits unwinding; made of polyacrylateesters and various natural and synthetic rubbers

Phenolic Thermoset; oven cured; strong bond; high tensile and Acoustical padding, brake lining and

low impact strength; brittle; easy to use; cures by solvent clutch pads, abrasive grain bonding, and

evaporation honeycomb structures

Silicone Thermoset; slow curing; flexible; bonds at room Gaskets and sealants

temperature; high impact and peel strength; rubber-likeFormaldehyde Thermoset; strong with wood bonds; urea is inexpensive, Wood joints, plywood, and bonding

Urea is available as powder or liquid, and requires a catalyst;Melamine melamine is more expensive, cures with heat, and thePhenol bond is waterproof; resorcinol forms a waterproof bondResorcinol at room temperature. Types can be combined

Urethane Thermoset; bonds at room temperature or oven cure; Fiberglass body parts, rubber, and

good gap-filling qualities fabric

Water-based Inexpensive, nontoxic, nonflammable Wood, paper, fabric, leather, and dry-

Animal seal envelopes

VegetableRubbers

Section 32.4 Adhesive Bonding

Because of their strength, synthetic organic adhesives are the most importantadhesives in manufacturing processes, particularly for load-bearing applications.They are classified as follows:

° Chemically reactive: polyurethanes, silicones, epoxies, cyanoacrylates, modi-fied acrylics, phenolics, and polyimides. Also included are anaerobics, whichcure in the absence of oxygen, such as Loctite® for threaded fasteners (see alsoCase Study 32.1).

° Pressure sensitive: natural rubber, styrene-butadiene rubber, butyl rubber, ni-trile rubber, and polyacrylates.

° Hot melt: thermoplastics (such as ethylene-vinyl acetate copolymers, poly-olefins, polyamides, and polyester) and thermoplastic elastomers.

° Reactive hot melt: a thermoset portion (based on urethane’s chemistry) withimproved properties.

° Evaporative or diffusion: vinyls, acrylics, phenolics, polyurethanes, syntheticrubbers, and natural rubbers.

° Film and tape: nylon epoxies, elastomer epoxies, nitrile phenolics, vinyl pheno-lics, and polyimides.

° Delayed tack: styrene-butadiene copolymers, polyvinyl acetates, polystyrenes,and polyamides.

° Electrically and thermally conductive: epoxies, polyurethanes, silicones, andpolyimides. Electrical conductivity is obtained by the addition of fillers, such assilver (used most commonly), copper, aluminum, and gold. Fillers that improvethe electrical conductivity of adhesives generally also improve their thermalconductivity.

Adhesive Systems. These may be classified on the basis of their specific chemistries:

° Epoxy-based systems: These systems have high strength and high-temperatureproperties to as high as 200°C. Typical applications include automotive brakelinings and bonding agents for sand molds for casting.

° Acrylics: These adhesives are suitable for applications with substrates that arenot clean.

° Anaerobic systems: The curing of these adhesives is done under oxygen depri-vation, and the bond is usually hard and brittle. Curing times can be reducedby external heat or by ultraviolet (UV) radiation.

° Cyanoacrylate: The bond lines are thin and the bond sets within 5 to 40s.° Urethanes: These adhesives have high toughness and flexibility at room tem-

perature, and they are used widely as sealants.° Silicones: Highly resistant to moisture and solvents, these adhesives have high

impact and peel strength; however, curing times are typically in the range from1 to 5 days.

Many of these adhesives can be combined to optimize their properties, such asthe combinations of epoxy-silicon, nitrile-phenolic, and epoxy-phenolic. The least ex-pensive adhesives are epoxies and phenolics, which are followed in affordability bypolyurethanes, acrylics, silicones, and cyanoacrylates. Adhesives for high-temperatureapplications in a range up to about 26O°C (such as polyimides and polyben-zimidazoles) are generally the most expensive. Most adhesives have an optimumtemperature (ranging from about room temperature to about 200°C) for maximumshear strength.

32.4.2 Electrically Conducting Adhesives

Although the majority of adhesive bonding applications require mechanical strength, arelatively recent advance is the development and application of electrically conducting


adhesives to replace lead-based solder alloys, particularly in the electronics industry.

These adhesives require curing or setting temperatures that are lower than those re-

quired for soldering.In electrically conducting adhesives, the polymer is the matrix and contains

conducting metals (fillers) in forms such as flakes and particles (see also Section 7.3

on electrically conducting polymers). There is a minimum proportion of fillers

necessary to make the adhesive electrically conducting; typically, it is in the range

of 40 to 70% by volume.The size, shape, and distribution of the metallic particles, the method of heat

and pressure application, and the individual conducting particle contact geometrycan be controlled to impart isotropic or anisotropic electrical conductivity to the ad-hesive. Metals used are typically silver, nickel, copper, and gold, as well as carbon.More recent developments include polymeric particles (such as polystyrene) coatedwith thin metallic films of silver or gold. Matrix materials are generally epoxies, al-

though thermoplastics also are used and are available as film or as paste. Applicationsof electrically conducting adhesives include calculators, remote controls, and control

panels. In addition, there are high-density uses in electronic assemblies, liquid-crystaldisplays, pocket TVs, and electronic games.

32.4.3 Surface Preparation, Process Capabilities, and Applications

Surface preparation is very important in adhesive bonding. ]oint strength depends

greatly on the absence of dirt, dust, oil, and various other contaminants. This de-

pendence can be observed when one is attempting to apply an adhesive tape over a

dusty or oily surface. Contaminants also affect the wetting ability of the adhesive

and prevent even spreading of the adhesive over the interface. Thick, weak, or loose

oxide films on workpiece surfaces are detrimental to adhesive bonding. On the otherhand, a porous or a thin and strong oxide film may be desirable-particularly one

with some surface roughness to improve adhesion or to introduce mechanical lock-

ing. However, the roughness must not be too high, because air may be trapped, in

which case the joint strength is reduced. Various compounds and primers are avail-able that modify surfaces to improve adhesive-bond strength. Liquid adhesives may

be applied by brushes, sprayers, or rollers.

Process Capabilities. Adhesives can be used for bonding a wide variety of similarand dissimilar metallic and nonmetallic materials and components with differentshapes, sizes, and thicknesses. Adhesive bonding can also be combined with me-

chanical joining methods (Section 32.5 ) to further improve the strength of the bond.joint design and bonding methods require care and skill. Special equipment is usuallyrequired, such as fixtures, presses, tooling, and autoclaves and ovens for curing.

Adhesive joints are designed to withstand shear, compressive, and tensile

forces, but they should not be subjected to peeling (Fig. 32.9). Note, for example,

how easily you can peel adhesive tape from a surface, yet be unable to slide it along

Peelingforce1; 5 r ........ _”({'~~ ,,,, ,,

\L.._;,__ li .._. .rrr. . <a> <b>

FIGURE 32.9 Characteristic behavior of (a) brittle and (b) tough adhesives in a peeling test.

This test is similar to the peeling of adhesive tape from a solid surface.

Section 32.4 Adhesive Bonding

the surface. During peeling, the behavior of an adhesive may be either brittle or duc-tile and tough-requiring high forces to peel the adhesive.

Applications. Major industries that use adhesive bonding extensively are the aero-space, automotive, appliances, and building products industries. Applications includeautomotive brake-lining assemblies, laminated windshield glass, appliances, helicop-ter blades, honeycomb structures, and aircraft bodies and control surfaces.

An important consideration in the use of adhesives in production is curing time,which can range from a few seconds (at high temperatures) to several hours (at roomtemperature), particularly for thermosetting adhesives. Thus, production rates can below compared with those of other joining processes. Furthermore, adhesive bondsfor structural applications rarely are suitable for service above 25O°C.

Nondestructive inspection of the quality and strength of adhesively bondedcomponents can be difficult. Some of the techniques described in Section 36.10(such as acoustic impact (tapping), holography, infrared detection, and ultrasonictesting) are effective nondestructive-testing methods for adhesives.

The major advantages of adhesive bonding are as follows:

° The interfacial bond has sufficient strength for structural applications, but is

also used for nonstructural purposes, such as sealing, insulation, the preven-tion of electrochemical corrosion between dissimilar metals, and the reductionof vibration and of noise (by means of internal damping at the joints).

° Adhesive bonding distributes the load at an interface and thereby eliminateslocalized stresses that usually result from joining the components with mechan-ical fasteners, such as bolts and screws. Moreover, structural integrity of thesections is maintained (because no holes are required).

° The external appearance of the bonded components is unaffacted.° Very thin and fragile components can be bonded without significant increase

in their weight.° Porous materials and materials of very different properties and sizes can be

joined.° Because adhesive bonding usually is carried out at a temperature between

room temperature and about 200°C, there is no significant distortion of thecomponents or change in their original properties. Avoiding distortion is im-portant, particularly for materials that are heat sensitive.

The major limitations of adhesive bonding are the following:

° There is a limited range of service temperatures.° Bonding time can be long.° There is a need for great care in surface preparation.° Bonded joints are difficult to test nondestructively, particularly for large

structures.° The limited reliability of adhesively bonded structures during their service life and

under hostile environmental conditions (such as degradation by temperature, ox-idation, stress corrosion, radiation, or dissolution) may be a significant concern.

The cost of adhesive bonding depends on the particular operation. In manycases, however, the overall economics of the process make adhesive bonding anattractive joining process. Sometimes it is the only one that is feasible or practical.The cost of equipment varies greatly, depending on the size and type of operation.

32.4.4 Design for Adhesive Bonding

° Designs for adhesive bonding should ensure that joints are subjected only tocompressive, tensile, and shear forces and not to peeling or cleavage.

Chapter 32 Brazi ng, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

° Several joint designs for adhesive bonding are shown in Figs. 32.10 through32.12. They vary considerably in strength; hence, selection of the appropriatedesign is important and should include considerations such as the type of load-ing and the environment.

Poor Good

Adhesive

\/(H) (bl

FIGURE 32.10 Various joint designs in adhesive bolarge contact areas between the members to be joined.

Simple Simple

Beveled Beveled

Badiused Ftadiused

(H) (D)

Single taper Single

Double taper Double

Increased thickness Beveled

(C) (Ci)

FIGURE 32.lI Desirable configurations for adhesivelybonded joints: (a) single lap, (b) double lap, (c) scarf, and(d) strap.

Very good*`

(C)

nding. Note that good designs require

hesiveitque, ysyyy i%

Rivet

(H)

"`“*‘eS'“eA ‘fféea

Spot weldbead

(bl

FIGURE 32.|2 Two exam-ples of combination joints,for purposes of improvedstrength, air tightness, and re-sistance to crevice corrosion.

Section 32.4 Adhesive Bonding 937

° Butt joints require large bonding surfaces. Simple lap joints tend to distortunder tension because of the force couple at the joint. (See Fig. 31.9a.)

° The coefficients of thermal expansion of the components to be bonded shouldpreferably be close to each other in order to avoid internal stresses during ad-hesive bonding. Also, situations in which thermal cycling can cause differentialmovement across the joint should be avoided.

CASE STUDY 32.I Light Curing Acrylic Adhesives for Medical Products

Cobe Cardiovascular, Inc., is a leading manufacturerof blood collection and processing systems, as Well as

extracorporeal systems for cardiovascular surgery.The company (like many other device manufacturers)traditionally used solvents for bonding device compo-nents and subassemblies. However, several federalagencies began to encourage industries to avoid theuse of solvents, and Cobe particularly wanted to elim-inate its use of methylene chloride for environmentaland occupational safety reasons. Towards this goal,the company began to redesign most of its assembliesto accept light-curing (ultraviolet or visible) adhesives.Most of the company’s devices were made of trans-parent plastics. Consequently, its engineers required

clear adhesive bonds for aesthetic purposes and withno tendency for stress cracking or crazing.

As an example of a typical product, Cobe’sblood salvage or collection reservoir is an oval poly-carbonate device approximately 300 mm tall,200 mm in major diameter, and 100 mm deep (Fig.32.13). The reservoir is a one-time use, disposabledevice; its purpose is to collect and hold the bloodduring open-heart and chest surgery or for arthro-scopic and emergency room procedures. Up to 3000cc of blood may be stored in the reservoir While theblood awaits passage into a 250-cc centrifuge, whichcleans the blood and returns it to the patient after thesurgical procedure is completed. The collection reser-

FIGURE 32.l3 The Cobe Laboratories blood reservoir. The lid is bondedto the bowl with an airtight adhesive joint and tongue-in-groove joint.Source: Courtesy of Cobe Laboratories.


voir consists of a clear, polycarbonate lid joined to apolycarbonate bucket. The joint is a tongue-and-groove configuration; the goal was to create a strong,elastic joint that could withstand repeated stresseswith no chance of leakage.

Light-cured acrylic adhesives offer a range of per-formance properties that make them well suited for thisapplication. First and foremost, they achieve high bondstrength to the thermoplastics typically used to formmedical-device housings. For example, Loctite® 3211(see anaerobic adhesives, Section 32.4.1 ) achieves shearstrengths of 1 1 MPa on polycarbonate. As important asthe initial shear strength may be, it is even more impor-tant that the adhesive be able to maintain the high bondstrength after sterilization. Fortunately, disposablemedical devices are typically subjected to very few ster-ilization cycles during manufacturing. Also, these adhe-sives can endure a limited number of cycles of gammairradiation, electron beam irradiation, autoclaving,ethylene oxide, or chemical immersion.

Another consideration that makes light-curedadhesives well suited for this application is their avail-ability in formulations that allow them to withstandlarge strains prior to yielding; for example, Loctite®3211 yields at elongations in excess of 2()0%. Thisflexibility is critical, because the bonded joints aretypically subjected to large amounts of bending andflexing when the devices are pressurized during qualifi-cation testing and use. If an adhesive is too rigid, it willfail this type of testing, even if it offers higher shearstrength than a comparable and more flexible adhe-sive. Finally, light-cured acrylics are widely available informulations that meet international quality standardcertification (ISO, see Section 36.6 ), which means that,when processed properly, they will not cause biocom-patibility problems in the final assembly.

While these performance features are attractive,the adhesive also must meet certain processing charac-teristics during manufacturing. Light-cured acrylic ad-hesives have found wide use in medical-device assembly/joining operations, because their processing character-istics are compatible with the high-speed automatedmanufacturing processes employed. These adhesivesare available in a wide variety of viscosities and aredispensed easily through either pressure-time or posi-tive-displacement dispensing systems. Gnce dispensedon the part, they can remain in contact with evenhighly stressed plastic parts for several minutes orlonger without causing stress cracking or degradationof the plastic. For example, Liquid Loctite® 3211 canremain in contact with polycarbonate that has been

bent to induce stresses up to 17 MPa for more than 15_

minutes without stress cracking. Finally, the adhesivecan be converted completely from a liquid to a solidstate in seconds when exposed to light of the properintensity and wavelength.

Since Loctite® 3211 absorbs light in the visibleas well as the ultraviolet range, it can be used success-fully on plastics that contain UV blockers, such as

many grades of polycarbonate. The ability to have along open time when parts can be positioned, yet curethe adhesive on demand, is a unique benefit to light-curing adhesives, dramatically reducing scrap costs.The equipment used to irradiate the part with high-intensity light typically requires a space of 1 X 2 m2 ona production line, which generally is much less thanthat required for the ovens used by heat~cured adhe-sives or the racking shelves required for slower curingadhesives. Since floor space carries a cost premium inclean~room environments, this is a significant benefit.

It also is important that the joint be designedproperly to maximize performance. If the enclosure is

bonded with a joint consisting of two flat faces in inti-mate contact, peel stresses (see Fig. 32.9) will be actingon the bond when the vessel is pressurized. Peel stressesare the most difficult type for an adhesive joint towithstand, due to the fact that the entire load is con-centrated on the leading edge of the joint. The tongue-and-groove design that the company used addressedthis concern, with the groove acting as a reservoir forholding the adhesive during the dispensing operation.When the parts are mated and the adhesive is cured,this design allows much of the load on the joint (whenthe device is pressurized) to be translated into shearand tensile forces, which the adhesive is much bettersuited to withstand. The gap between the tongue andthe groove can vary widely, because most light-curedadhesives quickly can be cured to depths in excess of5 mm. This feature allows the manufacturer to have a

robust joining process (meaning that wide dimension-al tolerances can be accommodated).

With the new design and with the use of this ad-hesive, the environmental concerns and the issues as-sociated with solvent bonding were eliminated, withthe accompanying benefit of a safer, faster, and moreconsistent bond. The light-curing adhesive providedthe aesthetic-bond line the company wanted-one thatwas clear and barely perceptible. It also provided thestructural strength needed and thus maintained a com-petitive edge for the company in the marketplace.

Source: Courtesy of P.]. Courtney, Loctite Corporation.

Section 32.5

32.5 Mechanical Fastening

Two or more components may have to be joined or fastened in such a way that theycan be taken apart sometime during the product’s service life or life cycle. Numerousproducts, such as mechanical pencils, watches, computers, appliances, engines, andbicycles, have components that are fastened mechanically. Mechanical fasteningmay be preferred over other methods for the following reasons:

° Ease of manufacturing.° Ease of assembly and transportation.° Ease of disassembly, maintenance, parts replacement, or repair.

in creating designs that require movable joints such as hinges, sliding° Easemechanisms, and adjustable components and fixtures.

° Lower overall cost of manufacturing the product.

The most common method of mechanical fastening is by the use of bolts, nuts,screws, pins, and a variety of other fasteners. Also known as mechanical assembly,mechanical fastening generally requires that the components have holes throughwhich the fasteners are inserted. These joints may be subjected to both shear andtensile stresses and should be designed to resist such forces.

Hole Preparation. An important aspect of mechanical fastening is hole prepara-tion. As described in Chapters 16, 23, and 27, a hole in a solid body can be pro-duced by several processes, such as punching, drilling, chemical and electricalmeans, and high-energy beams. Recall from Parts II and III that holes also may beproduced integrally in the product during casting, forging, extrusion, and powdermetallurgy. For improved accuracy and surface finish, many of these hole-makingoperations may be followed by finishing operations, such as shaving, deburring,reaming, and honing, as described in various sections of Part IV.

Because of the fundamental differences in their characteristics, each of thehole-making processes produces holes with different surface finishes, surfaceproperties, and dimensional accuracy and characteristics. The most significant in-fluence of a hole in a solid body is its tendency to reduce the component’s fatiguelife by stress concentration. For holes, fatigue life can be improved best by induc-ing compressive residual stresses on the cylindrical surface of the hole. These stress-es usually are developed by pushing a round rod (drift pin) through the hole andexpanding it by a very small amount. This operation plastically deforms the sur-face layers of the hole in a manner similar to that seen in shot peening or in rollerburnishing (Section 342).

Threaded Fasteners. Bolts, screws, and nuts are among the most commonly usedthreaded fasteners. Numerous standards and specifications (including thread dimen-sions, dimensional tolerances, pitch, strength, and the quality of the materials usedto make these fasteners) are described in the references at the end of this chapter.Bolts and screws may be secured with nuts, or they may be self-tapping-wherebythe screw either cuts or forms the thread into the part to be fastened. The self-tappingmethod is particularly effective and economical in plastic products in which fasteningdoes not require a tapped hole or a nut.

If the joint is to be subjected to vibration (such as in aircraft, engines, andmachinery), several specially designed nuts and lock washers are available. Theyincrease the frictional resistance in the torsional direction and so inhibit any vibra-tional loosening of the fasteners.

Mechanical Fastening


i(3) (bl (Cl (d)

FIGURE 32.|4 Examples of rivets: (a) solid, (b) tubular, (c) split or bifurcated, and(d) compression.

L1Poor alll s,e, Good

<a> im <C> <d>

FIGURE 32.l5 Design guidelines for riveting. (a) Exposed shank is too long; the result is

buckling instead of upsetting. (b) Rivets should be placed sufficiently far from edges to avoidstress concentrations. (c) joined sections should allow ample clearance for the riveting tools.(d) Section curvature should not interfere with the riveting process. Source: After ].G. Bralla.

Rivets. The most common method of permanent or semipermanent mechanicaljoining is by riveting (Fig. 32.14). Hundreds of thousands of rivets may be used in

the construction and assembly of one large commercial aircraft. Riveting may bedone either at room temperature or at elevated temperatures. Rivets may be solid ortubular. Installing a solid rivet takes two steps: placing the rivet in the hole (usuallypunched or drilled) and deforming the end of its shank by upsetting it Uveading; see

Fig. 14.11). A hollow rivet is installed by flaring its smaller end (see Section 16.6).Explosives can be placed within the rivet cavity and detonated to expand the end ofthe rivet. The riveting operation also may be performed by hand or by mechanizedmeans, including the use of programmable robots. Some design guidelines for rivet-ing are illustrated in Fig. 32.15.

32.5.1 Other Fastening Methods

Numerous other techniques are used in joining and assembly applications. The mostcommon types are described here.

Metal Stitching and Stapling. The process of metal stitching and stapling(Fig. 32.16) is much like that of the ordinary stapling of papers. The operation is fast,and it is particularly suitable for joining thin metallic and nonmetallic materials,

Section 32.5 Mechanical Fastening 94|

including wood. A common example is the stapling of_; i

cardboard containers. In clinc/Qing, the fastener mate-rial must be sufficiently thin and ductile to withstandthe large localized deformation during sharp bending. (3)

Searning. Seanfiing is based on the simple principleof folding two thin pieces of material together, muchlike the joining of two pieces of paper by folding themat the corners (Fig. 32.17). Common examples ofseaming are seen at the tops of beverage cans (see last (C)

illustration in Fig. 16.3O), in containers for food andhousehold products, and in heating and air-conditioningducts. In seaming, the materials should be capable ofundergoing bending and folding at very small radii;otherwise, they will crack. The performance and reliability of seams may be im-proved as well as making them impermeable by the addition of adhesives or poly-meric coatings and seals or by soldering.

Nonmetal

Metal channel

Crimping. The crimping process is a method of joining Without using fasteners. Itcan be done with beads or dimples (Fig. 32.18), which can be produced by shrink-ing or swaging operations. Crimping can be done on both tubular and flat parts,provided that the materials are sufficiently thin and ductile to withstand the large lo-calized deformations. Metal caps are fastened to glass bottles by crimping just assome connectors are crimped to electrical wiring.

Spring and Snap-in Fasteners. Several types of spring and snap-in fasteners areshown in Fig. 32.19. Such fasteners are used widely in automotive bodies andhousehold appliances. They are economical, and they permit easy and rapid compo-nent assembly.

Shrink and Press Fits. Components also may be assembled by shrink fitting andpress fitting. Shrink fitting is based on the thermal contractions of two components.Typical applications are assembling die components and mounting gears and camsonto shafts. In press fitting, one component is forced over another; when the com-ponents are designed properly, this process results in high joint strength.

Shape-memory Alloys. The characteristics of these materials were described inSection 6.13. Recall their use as fasteners because of their unique capability to re-cover their shape upon heating. Several advanced applications include their use ascoupling in the assembly of titanium-alloy tubing for aircraft.

@i:~w . .. (H) (D)

FIGURE 32.I8 Two examples of mechanical joining by crimping.

Standard loop

FIGURE 32.|6 Typical example

Flat clinch

(bi

ld)

s of metal stitching.

1

2.

3.

4.

FIGURE 32.I1 Stages in forming a double-lock seam.

Chapter 32 Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes Spring clip

.:...:

Rod-end attachment pUSh`°nto sheet-metal part fastener

(H) (D) (C)

‘lik /' f-+...Ws ‘ssr‘r rrrrr '35 if rrss ~ ff . ,i ssrr . . Deflected Rigid

Sheet metal cover Sheet-metal cover Integrated snap fasteners

(dl (9) (f) (Q)

FIGURE 32.l9 Examples of spring and snap-in fasteners used to facilitate assembly.

32.5.2 Design for Mechanical Fastening

The design of mechanical joints requires a consideration of the type of loading towhich the structure will be subjected and of the size and spacing of holes.

Compatibility of the fastener material with that of the components to be joined is

important. Incompatibility may lead to galvanic corrosion, also known as crevicecorrosion (Section 3.8). For example, in a system in which a steel bolt or rivet is usedto fasten copper sheets, the bolt is anodic and the copper plate is cathodic; this com-bination causes rapid corrosion and loss of joint strength. Aluminum or zinc fasten-ers on copper products will react in a similar manner.

Other general design guidelines for mechanical joining include the following(see also Section 37.10):

° It is generally less costly to use fewer, but larger, fasteners than to use a largenumber of small ones.

° Part assembly should be accomplished with a minimum number of fasteners.° The fit between parts to be joined should be as loose as possible to reduce costs

and to facilitate the assembly process.° Fasteners of standard size should be used whenever possible.' Holes should not be too close to edges or corners, to avoid the possibility of

tearing the material when it is subjected to external forces.

32.6 joining Plastics, Ceramics, and Glasses

Plastics can be joined by many of the methods already described for joining metalsand nonmetallic materials, especially adhesive bonding and mechanical fastening.

32.6.l joining Thermoplastics

Thermoplastics can be joined by thermal means, adhesive bonding, solvent bonding,and mechanical fastening.

Section 32.6 joining Plastics,

Thermal Methods. Thermoplastics soften and melt as the temperature is in-creased. Consequently, they can be joined when heat is generated at the interface(from either an external or internal source), allowing fusion to take place. The heatsoftens the thermoplastic at the interface to a viscous or molten state and ensures agood bond with the application of pressure.

Because of the low thermal conductivity of thermoplastics, the heat sourcemay burn or char the surfaces of the components if applied at too high a rate. Suchburning or charting can cause difficulties in obtaining sufficiently deep fusion forproper joint strength. Oxidation also can be a problem in joining some polymers(such as polyethylene), because it causes degradation. Typically, an inert shieldinggas (such as nitrogen) is used to prevent oxidation.

External heat sources may be chosen from among the following (the choice de-pends on the compatibility of the polymers to be joined):

° Hot air; inert gases, or a filler material of the same type is also used.° In a process known as loot-tool welding or loot-plate welding, heated tools and

dies are pressed against the surfaces to be joined and heat them by the interdif-fusion of molecular chains. This process commonly is used in butt-weldedpipes and (end-to-end) tubing.

° Infrared radiation (from high-intensity quartz heat lamps) is focused into anarrow beam onto the surface to be joined.

° Radio waves are particularly useful for thin films; frequencies are in the rangefrom 100 to 500 Hz.

° Dielectric heating at frequencies of up to 100 MHZ are effecive for the throughheating of polymers such as nylon, polyvinyl chloride, polyurethane, and rubber.

° Electrical-resistance elements (such as wires or braids, or carbon-based tapes,sheets, and ropes) are placed at the interface to create heat by the passing of elec-trical current-a process known as resistive-implant welding. Alternatively, ininduction welding, these elements at the interface may be subjected to radio-frequency exposure. In both cases, because they are left in the weld zone, the el-ements at the interface must be compatible with the use of the joined product.

° Lasers emitting defocused beams at low power prevent degradation of thepolymer.

Internal heat sources are developed by the following means:

° Ultrasonic welding is the most commonly used process for thermoplastics,particularly amorphous polymers such as acrylonitrile-butadiene-styrene(ABS) and high-impact polystyrene; frequencies are in the range from 20 to40 kHz. The ultrasonic welding process illustrated in Fig. 31.2 is still appli-cable, but note that the tool can apply vertical motion, causing a released-compression loading. Due to the high hysteresis of polymers in a loading cycle,the heat for welding is developed in the polymer and not at the interface.

° Friction welding (also called spin welding for polymers) and linear frictionwelding (also called vibration welding) are particularly useful for joining poly-mers with a high degree of crystallinity, such as acetal, polyethylene, nylons,and polypropylene.

° Orbital welding is similar to friction welding, except that the rotary motion ofone component is in an orbital path.

The fusion method is particularly effective with plastics that cannot be bondedeasily by means of adhesives. Plastics (such as PVC, polyethylene, polypropylene,acrylics, and ABS) can be joined in this manner. For example, specially designedportable fusion-sealing systems are used to allow in-field joining of plastic pipe (usu-ally made of polyethylene and used for natural-gas delivery).

Ceramics, and Glasses


Coextruded multiple food wrappings consist of different types of films, which arebonded by heat during extrusion (Section 19.2.1). Each film has a different function-for example, one film may keep out moisture, another may keep out oxygen, and a

third film may facilitate heat sealing during the packaging process. Some wrappingshave as many as seven layers-all bonded together during production of the film.

Adhesive Bonding. This method is best illustrated in the joining of sections ofPVC pipe (used extensively in plumbing systems) and ABS pipe (used in drain,waste, and vent systems). A primer that improves adhesion is used to apply the ad-hesive to the connecting sleeve and pipe surfaces (a step much like that using primersin painting), and then the pieces are pushed together.

Adhesive bonding of polyethylene, polypropylene, and polytetrafluoroethyl-ene (Teflon) can be difficult, because adhesives do not bond readily to them. Thesurfaces of parts made of these materials usually have to be treated chemically toimprove bonding. The use of adhesive primers or double-sided adhesive tapes also is

effective.

Mechanical Fastening. This method is particularly effective for most thermoplas-tics (because of their inherent toughness and resilience) and for joining plastics tometals. Plastic or metal screws may be used. The use of self-tapping metal screws is

a common practice. Integrated snap fasteners have gained wide acceptance for sim-

plifying assembly operations; fastener geometries are shown in Figs. 32.19f and g.

Because the fastener can be molded directly at the same time as the plastic, it addsvery little to the cost of the assembly. This technique is very cost effective, because it

reduces assembly time and minimizes the number of parts required.

Solvent Bonding. This method consists of the following sequence of steps:

I. Roughening the surfaces with an abrasive;

2. Wiping and cleaning the surfaces with a solvent appropriate for the particularpolymer;

3. Pressing the surfaces together and holding them together until sufficient jointstrength is developed.

Electromagnetic Bonding. Thermoplastics also may be joined by magnetic meansby embedding tiny particles on the order of 1 um in the polymer. A high-frequencyfield then causes induction heating of the polymer and melts it at the interfaces to be

joined.

32.6.2 joining Thermosets

Thermosetting plastics (such as epoxy and phenolics) can be joined by the followingtechniques:

0 Threaded or other molded-in inserts.° Mechanical fasteners, particularly those using self-tapping screws and inte-

grated snap fasteners.° Solvent honding.0 Co-curing, in which the two components to be joined are placed together and

cured simultaneously.° Adhesive honding.

32.6.3 joining Ceramics and Glasses

A wide variety and numerous types of ceramics and glasses are now available withunique and important properties. Ceramics and glasses are used as products, as

Section 32.7 Economics ofjoming Operations

components of products, or as tools, molds, and dies. These materials often areassembled into components or subassemblies and are joined either with the sametype of material or with different metallic or nonmetallic materials. Generally, ce-ramics, glasses, and many materials can be joined by adhesive bonding. A typicalexample is assembling broken ceramic pieces with a two-component epoxy, whichis dispensed from two separate tubes and is mixed just prior to application. Otherjoining methods include mechanical means, such as fasteners and spring or pressfittings.

Ceramics. As described in Chapter 8, ceramics have properties that are very dif-ferent from metallic and nonmetallic materials, especially when it comes to stiff-ness, hardness, brittleness, resistance to high temperatures, and chemical inertness.Thus, joining them to each other or to other metallic or nonmetallic materials re-quires special considerations, and several highly specialized joining processes havebeen developed.

A common technique that is effective in joining difficult-to-bond combinationsof materials consists of first applying a coating of a material that bonds itself well toone or both components-thus acting as a bonding agent. For example, the surfaceof alumina ceramics can be metallized, as described in Section 34.5 _ In this tech-nique, known as the Mo-Mn process, first the ceramic part is coated with a slurry ofoxides of molybdenum and manganese. Next, the part is fired, forming a glassylayer on its surface. Then this layer is plated with nickel, and since the part now hasa metallic surface, it can be brazed to a metal surface by means of an appropriatefiller metal.

Tungsten carbide and titanium carbide can be brazed easily to other metalsbecause they both have a metallic matrix: WC has a matrix of cobalt, and TiC hasnickel-molybdenum alloy as a matrix. Common applications include brazing cubicboron nitride or diamond tips to carbide inserts (Figs. 22.10 and 22.11) and carbidetips to masonry drills (Fig. 23.21). Depending on their particular structure, ceramicsand metals also can be joined by diffusion bonding. It may be necessary to place ametallic layer at the joint to make it stronger.

In addition, ceramic components can be joined or assembled together duringtheir primary shaping process; a common example is attaching handles to coffeemugs prior to firing them. Thus, the shaping of the whole product is done integrallyrather than as an additional operation after the part is already made.

Glasses. As evidenced by the availability of numerous glass objects, glasses can bebonded easily to each other. This is commonly done by first heating and softeningthe surface to be joined, then pressing the two pieces together, and finally coolingthem. Bonding glass to metals is also possible, because of the diffusion of metal ionsinto the amorphous surface structure of glass. However, the differences in the coef-ficients of thermal expansion of the two materials must be taken into account.

32.1 Economics of joining Operations

As in the economics of welding operations (described in Section 31.8), the joiningprocesses discussed in this chapter depend greatly on several considerations. FromTable VI.1, it can be seen that, in relative terms, the cost distribution for some ofthese processes is as follows:

° Highest: brazing, bolts, nuts, and other fasteners.0 Intermediate: riveting and adhesive bonding.° Lou/est: seaming and crimping.

Chapter 32 Brazing Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes

The variety of processes involved and the general costs are as follows:

Brazing

° Manual brazing: The basic equipment costs about $300, but it can run over$5 0,000 for automated systems.

° Furnace brazing: Costs vary Widely, ranging from about $2000 for simplebatch furnaces to $300,ooo+ for continuous-vacuum furnaces.

0 Induction brazing: For small units, the cost is about $10,000.° Resistance brazing: Equipment costs range from $1000 for simple units to

more than $10,000 for larger, more complex units.° Dip brazing: The cost of equipment varies Widely, from $2000 to more than

$200,000; the more expensive equipment includes various computer-controlfeatures.

° Infrared brazing: Equipment cost ranges from $500 to $30,000.° Diffusion brazing: The cost of equipment ranges from $50,000 to $300,000.

Soldering. The cost of soldering equipment depends on its complexity and on thelevel of automation. The cost ranges from less than $20 for manual soldering irons

to more than $50,000 for automated equipment.

SUMMARY

]oining processes that do not rely on fusion or pressure at the interfaces includebrazing and soldering. These processes instead utilize filler material that requiressome temperature rise in the joint. They can be used to join dissimilar metals ofintricate shapes and various thicknesses.

Adhesive bonding has gained increased acceptance in major industries, such asthe aerospace and automotive industries. In addition to possessing good bondstrength, adhesives have other favorable characteristics, such as the ability toseal, to insulate, to prevent electrochemical corrosion between dissimilar met-als, and to reduce vibration and noise by means of internal damping in thebond.

Surface preparation and joint design are important factors in adhesive bonding.

Mechanical fastening is one of the oldest and most common joining methods.Bolts, screws, and nuts are common fasteners for machine components and struc-tures that are likely to be taken apart for maintenance, for ease of transportation,or for various other reasons.

Rivets are semipermanent or permanent fasteners used in buildings, bridges,and transportation equipment. A wide variety of other fasteners andfastening techniques is available for numerous permanent or semipermanentapplications.

Thermoplastics can be joined by fusion-Welding techniques, by adhesive bonding,or by mechanical fastening. Thermosets usually are joined by mechanical means(such as molded-in inserts and fasteners) or by solvent bonding. Ceramics can bejoined by adhesive bonding and metallizing techniques. Glasses are joined by

heating the interface and by using adhesives.

KEY TERMS

Adhesive bonding Filler metalBraze welding FluxBrazing Hole preparationCrimping Integrated snap fastenerElectrically conducting Lead-free solders

adhesives Mechanical fasteningFasteners Press fitting

BIBLIOGRAPHY

Review Questions 947

Reflow soldering StaplingRivet StitchingSeaming Threaded fastenersShrink fitting Wave solderingSnap-in fastener

SolderingSolvent bonding

Adams, R.D. (ed.), Adhesive Bonding: Science, Technologyand Applications, CRC Press, 2005.

Adams, R.D., Comyn, ]., and Wake, WC., Structural Adhesive]oints in Engineering, Chapman 86 Hall, 1997.

Bath, ]. (ed.), Lead-Free Soldering, Springer 2007.Bickford, ].H., Introduction to the Design and Behavior of

Bolted ]oints, 4th ed., Marcel Dekker, 2007.Brockmann, W, Geiss, P.I.., Klingen, ]., and Schreoeder,

K.B., Adhesive Bonding: Materials, Applications andTechnology, Wiley, 2009.

Hamrock, BJ., jacobson, B., and Schmid, S.R., Fundamentalsof Machine Elements, 2nd ed., McGraw-Hill, 2005.

Handbook of Plastics ]oining: A Practical Guide, WilliamAndrew, Inc., 1996.

Humpston, G., and Jacobson, D.M., Principles of Soldering,ASM International, 2004.

Hwang, ].S., Modern Solder Technology for CompetitiveElectronics Manufacturing, McGraw-Hill, 1996.

jacobson, D.M., and Humpston, G., Principles of Brazing,ASM International, 2005.

Manko, H.H., Soldering Handbook for Printed Circuits andSurface Mounting, Van Nostrand Reinhold, 1995.

REVIEW QUESTIONS

32.l. What is the difference between brazing and brazewelding?

32.2. Are fluxes necessary in brazing? If so, why?

32.3. Why is surface preparation important in adhesivebonding?

32.4. Soldering is generally applied to thinner components.Explain Why.

32.5. Explain the reasons that a variety of mechanical join-ing methods have been developed over the years.

-, Solders and Soldering-Materials, Design, Production,and Analysis for Reliable Bonding, McGraw-Hill, 1992.

Nicholas, M.G., _loining Processes: Introduction to Brazingand Diffusion Bonding, Chapman 86 Hall, 1998.

Parmley, R.O. (ed.), Standard Handbook of Fastening andjoining, 3rd ed., McGraw-Hill, 1997.

Peaslee, R.L., Brazing Footprints: Case Studies in High-Temperature Brazing, Wall Colmonoy Corp., 2003.

Petrie, E.M., Handbook of Adhesives and Sealants, McGraw-Hill, 2nd ed., 2006.

Roberts, P., Industrial Brazing Practice, CRC Press, 2004.Rotheiser, ]., _loining of Plastics: Handbook for Designers and

Engineers, Hanser Gardner, 2004.Satas, D. (ed.), Handbook of Pressure Sensitive Adhesive

Technology, 3rd ecl., Satas 85 Associates, 1999.Schwartz, M.M., Brazing, 2nd ed., ASM International, 2003.l, joining of Composite-Matrix Materials, ASM

International, 1994.Speck, ].A., Mechanical Fastening, joining, and Assembly,

Marcel Dekker, 1997.Woodgate, R.W, Handbook of Machine Soldering, Wiley,

1996 .

32.6. Describe the similarities and differences between thefunctions of a bolt and those of a rivet.

32.7. What precautions should be taken in the mechanicaljoining of dissimilar metals?

32.8. What difficulties are involved in joining plastics?Why?

32.9. What are the principles of (a) wave soldering and(b) reflow soldering?

32.l0. What is a peel test? Why is it useful?

948 Chapter 32 Brazing, Soldering, Adhesive-Bonding,

QUALITATIVE PROBLEMS

and Mechanical-Fastening Processes

32.| I. Describe some applications in manufacturing forsingle-sided and some for double-sided adhesive tapes.

32.|2. Comment on your observations concerning the jointsshown in Figs. 32.3, 32.6, 32.10, and 32.11.

32.I3. Give examples of combination joints other thanthose shown in Fig. 32.12.

32.I4. Discuss the need for fixtures for holding workpiecesin the joining processes described in this chapter.

32.|5. Explain why adhesively bonded joints tend to be

weak in peeling.

QUANTITATIVE PROBLEMS

u32.I9. Refer to the simple butt and lap joints shown at

the top row of Fig. 32.10a. (a) Assuming that the area of thebutt joint is 5 >< 20 mm, and referring to the adhesive prop-erties given in Table 32.3, estimate the minimum and maxi-mum tensile force that this joint can withstand. (b) Estimatethese forces for the lap joint, assuming that its area is 15 ><

15 mm.

|’32.20. In Fig. 32.12a, assume that the cross section of

the lap joint is 20 >< 20 mm, that the diameter of the solidrivet is 4 mm, and that the rivet is made of copper. Using the

32.|6. It is common practice to tin-plate electrical terminalsto facilitate soldering. Why is it tin that is used?

32.| 7. How important is a close fit for two parts that are to

be brazed? Explain.

32.18. If you are designing a joint that must be strongand also needs to be disassembled several times during theproduct’s life, what kind of joint would you recommend?Explain.

strongest adhesive shown in Table 32.3, estimate the maxi-mum tensile force that this joint can withstand.

32.2|. As shown in Fig. 32.1Sa, a rivet can buckle if it is toolong. Referring to Chapter 14 on forging, determine the max-imum length-to-diameter ratio of a rivet so that it would notbuckle during riveting.

32.22. Figure 32.4 shows qualitatively the tensile and shearstrength in brazing as a function of joint clearance. Search thetechnical literature, obtain data, and plot these curves quanti-tatively. Comment on your observations.

SYNTHESIS, DESIGN, AND PROIECTS

32.23. Examine various household products and describehow their components are joined and assembled. Explainwhy those particular processes were used and not others.

32.24. Name several products that have been assembled by

(a) seaming, (b) stitching, and (c) soldering.

32.25. Suggest methods of attaching a round bar (made of a

thermosetting plastic) perpendicularly to a flat metal plate.Discuss their advantages and limitations.

32.26. Describe the tooling and equipment that would be

necessary to perform the double-lock seaming operationshown in Fig. 32.17, starting with a thin, flat sheet.

32.27. Prepare a list of design guidelines for joining by theprocesses described in this chapter. Would these guidelines becommon to most processes? Explain.

32.28. What joining methods would be suitable for assem-bling a thermoplastic cover over a metal frame? Assume thatthe cover is removed periodically, as is the top of a coffee can.

32.29. Answer Problem 32.28, but for a cover made of (a) a

thermoset, (b) a metal, and (c) a ceramic. Describe the factorsinvolved in your selection of methods.

32.30. Comment on workpiece size and shape limitations, if

any, for each of the processes described in this chapter.

32.3I. Describe part shapes that cannot be joined by theprocesses covered in this chapter. Give specific examples.

32.32. Give examples of products in which rivets in a struc-ture or in an assembly may have to be removed and later re-placed by new rivets.

32.33. Visit a hardware store and investigate the geometryof the heads of screws that are permanent fasteners-that is,

fasteners that can be screwed in, but not out.

32.34. Obtain a soldering iron and attempt to solder twowires together. First, try to apply the solder at the same timeas you first put the soldering iron tip to the wires. Second,preheat the wires before applying the solder. Repeat the sameprocedure for a cool surface and a heated surface. Recordyour results and explain your findings.

32.35. Perform a literature search to determine the proper-ties and types of adhesives used to affix artificial hips ontothe human femur.

RTPA

SurfaceTechnology

Our first visual or tactile contact with the objects around us is through theirsurfaces. We can see or feel surface roughness, vvaviness, reflectivity, and other fea-tures, such as scratches, nicks, cracks, and discoloration. The preceding chaptersdescribed the properties of materials and manufactured components basically interms of their bulk characteristics, such as strength, ductility, hardness, and tough-ness. Also included vvere some descriptions of the influences of surfaces on theseproperties-influences such as the effect of surface preparation on fatigue life andthe sensitivity of brittle materials to surface scratches and defects.

Machinery and accessories have numerous members that slide against eachother: slideways, bearings, tools and dies for cutting and forming, and pistons andcylinders. Close examination will reveal that (a) some of these surfaces are smoothwhile others are rough, (bl some are lubricated While others are dry, (c) some aresubjected to heavy loads While others support light loads, (d) some are subjected toelevated temperatures while others are at room temperature, and (e) some surfacesslide against each other at high relative speeds While others move slowly.

In addition to possessing geometric features, a surface constitutes a thin layer onthe bulk material. A surface’s physical, chemical, metallurgical, and mechanical prop-erties depend not only on the material and its processing history, but also on the envi-ronment to which the surface is exposed. The term surface integrity is used to describethe chemical, mechanical, and metallurgical characteristics of a surface.

Because of the various mechanical, physical, thermal, and chemical effects thatresult from its processing history, the surface of a manufactured part usually possessesproperties and behavior that are significantly different from those of its bulk.Although the bulk material generally determines the component’s overall mechanicalproperties, the component’s surfaces directly influence the part’s performance in thefollowing areas (Fig. VH.1):

° Friction and wear of tools, molds, dies, and of the products made.° Effectiveness of lubricants during the manufacturing process and throughout the

part’s service life.° Appearance and geometric features of the part and their role in subsequent

operations, such as welding, soldering, adhesive bonding, painting, andcoating.

° Resistance to corrosion.

0 Part VII Surface Technology

Coating

Valves, seals,cylinders,piston rings

Bearings

PlatingGrease-Galvanized steel

Wheel bearings

Paint

Corrosionprotection

Brake drums,rotors

FIGURE Vll.l Components in a typical automobile that are related to the topics describedin Part VII.

l or ,rttta tttl_ Surfaces Tribology ESurface treatments

,t;> it,,tt _rttt r tttl - ll,tt _ttl ,T ,crt N tt,t; N t.,,ttrii,r ,tclzL, _,vt.t,,tttrttair-r,r~. I I I

Integrity Friction BurnishingStructure Wear HardeningTexture Lubrication Deposition

Roughness implantationCoatingsCleaning

FIGURE Vll.2 An outline of topics covered in Part VII.

° Crack initiation as a result of surface defects such as roughness, scratches,seams, and heat-affected zones, which can lead to weakening and prematurefailure of the part, through fatigue, for instance.

° Thermal and electrical conductivity of contacting bodies. For example, rough sur-faces have higher thermal and electrical resistances than smooth surfaces.

Following the outline shovvn in Fig. VIL2, this part of the book will presentsurface characteristics in terms of their structure and topography. The material andprocess variables that influence the friction and wear of materials will then be described.Several mechanical, thermal, electrical, and chemical methods can be used to modifysurfaces for improved frictional behavior, effectiveness of lubricants, resistance to Wear

and corrosion, and surface finish and appearance.

CHAPTER

Surface Roughnessand Measurement;Friction, Wear, andLubrication

¢ This chapter describes the features of surfaces that have a direct effect on boththe selection of manufacturing processes and the service life of the partsproduced.

° Surface features such as roughness, texture, and lay are discussed, as well asapproaches used to quantitatively describe and measure surfaces.

° The chapter also examines the nature of friction, its role in manufacturing, andthe factors involved in its magnitude.

° Wear and lubrication are then discussed, along with various approaches tominimizing wear.

° The chapter ends with a summary of commonly used lubricants, their addi-tives, and their selection for a particular manufacturing process.

33.1 Introduction

This chapter begins with a description of the nature of surfaces, which are distinctentities and have properties significantly different from those of the bulk; this is

particularly true for metals, because of various surface-oxide layers. Several defectscan exist on a surface, depending on the manner in which the surface was generated.These defects (as well as various surface textures) can have a major influence on thesurface integrity of workpieces, tools, and dies.

Two common methods of surface-roughness measurement in engineering prac-tice are described in this chapter, including the instrumentation involved and a briefdescription of surface-roughness requirements in engineering design. Because oftheir increasing importance in precision manufacturing and nanofabrication, three-dimensional surface measurements are discussed and illustrated as well.

The chapter also describes those aspects of friction, wear, and lubrication-collectively known as tribology-which are relevant to manufacturing processes andoperations and to the service life of products. We first describe friction and wear formetallic and nonmetallic materials and how they are influenced by various material andprocess variables. An understanding of these relationships is necessary for the properselection of tool and die materials and of appropriate metalworking fluids for a partic-ular operation. Wear has a major economic impact, as it is estimated that in the UnitedStates alone the total cost of replacing worn parts is more than $100 billion per year.

33.| Introduction 95|33.2 Surface Structure and

Integrity 95233.3 Surface Texture and

Roughness 953

33.4 Friction 95733.5 Wear 96|33.6 Lubrication 96433.7 Metalworking Fluids and

Their Selection 966

EXAMPLE:

33.I Determination ofCoefficient ofFriction 960

`95l

2 Chapter 33 Surface Roughness and Measurement; Friction, Wear, and Lubrication

The chapter then describes the fundamentals of metalworking fluids, includingthe types, characteristics, and application of commonly used liquid and solid lubri-cants and the lubrication practices employed. The importance of biological andenvironmental considerations in the use, application, recycling, and ultimate disposalof metalworking fluids is also discussed.

33.2 Surface Structure and Integrity

Upon close examination of the surface of a piece of metal, it is found that it generallyconsists of several layers (Fig. 33.1):

I.

2.

3.

1-100 nm

The bulk metal (also known as the metal substrate) has a structure that dependson the composition and processing history of the metal.

Above the bulk metal is a layer that usually has been deformed plastically andwork hardened to a greater extent than the bulk during the manufacturingprocess. The depth and properties of the work-hardened layer (surface struc-ture) depend on such factors as the processing method used and how muchfrictional sliding the surface encountered. For example, if the surface is pro-duced by machining with a dull and worn tool or the surface is ground by a

dull grinding wheel, the work~hardened layer will be relatively thick and usu-ally will also have residual stresses.

Unless the metal is processed and kept in an inert (oxygen-free) environmentor is a noble metal (such as gold or platinum), an oxide layer forms over thework-hardened layer. The oxide on a metal surface is generally much harderthan the base metal; hence, it is more abrasive. As a result, it has importanteffects on friction, wear, and lubrication. For example,

° Iron has an oxide structure with Fe() adjacent to the bulk metal, followedby a layer of Fe3O4, and then a layer of Fe2O3 (which is exposed to theenvironment).

° Aluminum has a dense, amorphous (without any crystalline structure) layerof A1203 with a thick, porous, and hydrated aluminum-oxide layer over it.

° Copper has a bright, shiny surface when freshly scratched or machined.Soon after, however, it develops a Cuz() layer, which then is covered with alayer of CuO. The latter layer gives copper its somewhat dull color.

° Stainless steels are “stainless” because they develop a protective layer ofchromium oxide (by passivation, as described in Section 3.8).

Contaminant

Adsorbed gas

1 nm 10-100 nm Oxide 'aw1-100 nm

1-100 m Be"bV (a'“°'P“°“S) 'ayefgigs * '°"'”"*W1»@v Work-hardened layer Metal substrate

FIGURE 33.I Schematic illustration of a cross section of the surface structure of a metal. Thethickness of the individual layers depends on both processing conditions and the processingenvironment. Source: After E. Rabinowicz and B. Bhushan.

Section 33.3 Surface Texture and Roughness

4. Under normal environmental conditions, surface oxide layers are generallycovered with adsorbed layers of gas and moisture.

5. Finally, the outermost surface of the metal may be covered with contaminantssuch as dirt, dust, grease, lubricant residues, cleaning-compound residues, andpollutants from the environment.

Surfaces have properties that generally are very different from those of the sub-strate material. The factors that pertain to the surface structures of the metals justdescribed are also factors in the surface structure of plastics and ceramics. The surfacetexture of these materials depends (as with metals) on the method of production.

Surface Integrity. Surface integrity describes not only the topological (geometric)features of surfaces and their physical and chemical properties, but also their mechan-ical and metallurgical properties and characteristics. Surface integrity is an importantconsideration in manufacturing operations, because it influences such properties as

fatigue strength, resistance to corrosion, and service life (see, for example, Fig. 2.29).Several surface defects caused by and produced during component manufactur-

ing can be responsible for inadequate surface integrity. These defects usually are causedby a combination of factors, such as (a) defects in the original material, (b) the methodby which the surface is produced, and (c) improper control of the process parameters(which can result in excessive stresses, temperatures, or surface deformation).

The following are general definitions of the major surface defects (listed inalphabetical order) found in practice:

° Cracks may be external or internal; cracks that require a magnification of 1O><

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 liquid-

metal 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 tem-

perature cycling of the material; these changes may consist of phase transfor-mations, recrystallization, alloy depletion, decarburization, and molten andthen recast, resolidified, or redeposited material.

° Pits are shallow surface depressions, usually the result of chemical or physicalattack.

° Residual stresses (either tension or compression) on the surface are caused bynonuniform 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, andprocessing methods.

33.3 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 sur-face 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 measur-able 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, usuallyvisible to the naked eye.

Flaw

e'9 Lay directionRoughness y Q 1

_- t Jf. er 51. if LQ Q Roughness spacing

Roughness-width cutoffWaviness width

/--...,/'M = + "\./" + -'------~Surface profile Error of form Waviness Roughness

Maximum waviness height WOOOOOS-0.05-Maximum waviness widthMaximum Ra --/1 fa 0.00025 -_-Roughness-width cutoffMinimum Ffa -f 10.00013-Maximum roughness width

Lay l/la)

w¢¢X¢f§§:;-.s~»» w~aa,a.,»f»»»7§ss»a“.s_~a,`a;sa,. saas.sa-aa Lay _

Symbm lnterpretatlon Examples

_ 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 MJ.

Lay angular in both directions to line 3

X representing the surface to which symbol is applied AL

P Pitted, protuberant, porous, or particulatenondirectlonal lay 3 fp

lb)

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

Section 33.3 Surface Texture and Roughness

° Roughness is defined as closely spaced, irregular devia-tions on a small scale; it is expressed in terms of its V

height, width, and distance along the surface.° Waviness is a recurrent deviation from a flat surface; it is A

measured and described in terms of the space betweenadjacent crests of the waves (waz/iness width) and heightbetween the crests and valleys of the waves (u/ai/inessheight).

abode

Digitized data

fg hi j k/

Surface profile Center (datum) line

FIGURE 33.3 Coordinates used for surface roughnessSurface roughness is generally characterized by two

methods. The arithmetic mean value (Ra) is based on theschematic illustration of a rough surface, as shown in Fig. 33.3,and is defined as

[9 Ra = ;1} , (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 £72 2 dl _ __

RL] = (33.2)

The datum line AB in Fig. 33.3 is located so that the sum of the areas abovethe 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 theheight from the deepest trough to the highest peak. It indicates how much materialhas 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 interna-

tionally in the mid-19505 and is used widely in engineering practice. DividingEq. (33.2) by Eq. (33.1) yields the ratio Rq/Ru, which, for typical surfaces producedby machining and finishing processes is 1.1 for cutting, 1.2 for grinding, and 1.4 forlapping and honing.

In general, a surface cannot be described by its Ra or Rq value alone, since thesevalues are averages. Two surfaces may have the same roughness value, but haveactual topographies that are very different. For example, a few deep troughs on anotherwise smooth surface will not affect the roughness values significantly. However,this type of surface profile can be significant in terms of friction, wear, and fatiguecharacteristics of a manufactured product. Consequently, it is important to analyze asurface in great detail, particularly for parts to be used in critical applications.

Symbols for Surface Roughness. Acceptable limits for surface roughness are spec-ified on technical drawings by symbols, typically shown around the check mark inthe lower portion of Fig. 33.2a, and the values of these limits are placed to the leftof the check mark. The symbols and their meanings concerning the lay are given inFig. 33.2b. Note that the symbol for the lay is placed at the lower right of the checkmark. Symbols used to describe a surface specify only its roughness, waviness, andlay; they do not include flaws. Therefore, whenever necessary, a special note is in-cluded in technical drawings to describe the method that should be used to inspectfor surface flaws.

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

measurement defined by Eqs. (33.1) and (33 2)


Stylus

Head If Stylus path ,,,,_, ’ \ f ‘

Rldef / \ /'_ \ ,’ ‘ Actual surface Stylus \\ If LR \\

Eff. "V, ily, ' ‘f p \ I ylli (8) (D)

o 5 ,rm 0.6 ,rm

_L-""" ""‘f\~/Y-=-v~f\-"-‘- =“'=‘v-'vyY~'\i=r=

l<->l T0.4 mm

(c) Lapping (d) Finish grinding

3 8 ,um_L 5 ;.Lm

_l_

T T

(e) Fiough grinding (f) Turning

FIGURE 33.4 la) Measuring surface roughness with a stylus. The rider supports the stylusand guards against damage. (b) Path of the stylus in surface-roughness measurements (brokenline), 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 verticaland horizontal scales.

stylus that travels along a straight line over the surface (Fig. 33.4a). The distancethat the stylus travels is called the cutoff, which generally ranges from 0.08 to25 mm. A cutoff of 0.8 mm is typical for most engineering applications. The rule ofthumb is that the cutoff must be large enough to include 10 to 15 roughness irregu-larities, as well as all surface waviness.

In order to highlight roughness, profilometer traces are recorded on an exag-gerated 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 recordinginstrument. Thus, the recorded profile is distorted significantly, and the surfaceappears to be much rougher than it actually is. The recording instrument compen-sates 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 diameteris 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 Friction

Surface roughness can be observed directly through an optical or scanning-electron microscope. Stereoscopic photographs are particularly useful for three-dimensional views of surfaces and also can be used to measure surface roughness.

Three-dimensional Surface Measurement. Because surface properties can varysignificantly with the direction in which a profilometer trace is taken, there is oftena need to measure three-dimensional surface profiles. In the simplest case, this canbe done with a surface profilometer that has the capability of indexing a short dis-tance between traces. A number of other alternatives have been developed, two ofwhich are optical interferometers and atomic-force microscopes.

Optical-interference microscopes shine a light against a reflective surface andrecord 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 areaof interest. As the vertical distance between the sample and the interference objec-tive is changed, the fringe patterns also change, thus allowing for a surface heightmeasurement.

Atomic-force microscopes (AFMS) are used to measure extremely smooth sur-faces and even have the capability of distinguishing atoms on atomically smoothsurfaces. ln principle, an AFM is merely a very fine surface profilometer with a laserthat is used to measure probe position. The surface profile can be measured withhigh accuracy and with vertical resolution on the atomic scale, and scan areas can beon the order of 100 /im square, although smaller areas are more common.

Surface Roughness in Engineering Practice. Requirements for surface-roughnessdesign in typical engineering applications vary by as much as two orders of magni-tude. Some examples are as follows:

° Bearing balls 0.025 /.im° Crankshaft bearings 0.32 ,um0 Brake drums 1.6 /im° Clutch-disk faces 3.2 um.

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

33.4 Friction

Friction plays an important role in manufacturing processes because of the relativemotion 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 anoperation, and (b) impedes free movement at interfaces, thus significantly affectingthe flow and deformation of materials in metalworking processes. However, frictionis not always undesirable; for example, without friction, it would be impossible toroll metals, clamp workpieces on machines, or hold drill bits in chucks.

There are a number of explanations for the phenomenon of friction. A com-monly accepted theory of friction is the adhesion theory, based on the observationthat two clean and dry surfaces, regardless of how smooth they are, contact eachother at only a fraction of their apparent contact area (Fig. 33.5 ). The maximumslope 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 thesurface) that are in contact with each other. Therefore, the normal stresses at theseasperities are high; this causes plastic deformation at the junctions. Their contact


N

F-» JH g* _ V _ Microweld

i i

H it ii ijj it si rl jj ' Q PlasticProjected u jj rr deformation

| I

Contact ,L 4/\‘ _H Elasticareas T distortion

FIGURE 33.5 Schematic illustration of the interface of two bodies in contact showing realareas of contact at the asperities. In engineering surfaces, the ratio of the apparent-to-realareas 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 thatan asperity from a hard surface (such as a tool or a die) penetrates and plows througha softer surface (the workpiece). Plowing may (a) cause displacement of the materialand/or (b) produce small chips or slivers, as in cutting and abrasive processes. Otherexplanations 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 processesinvolved, 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 hotworking and from 0.5 to as much as 2 for machining.

Almost all of the energy dissipated in overcoming friction is converted intoheat, which raises surface temperature. A small fraction of the energy becomes storedenergy (Section 1.6) in the plastically deformed surfaces. The temperature increaseswith increasing friction and sliding speed, decreasing thermal conductivity, and de-creasing specific heat of the sliding materials (see also Section 21.4). The interfacetemperature may be high enough to soften and even melt the surfaces and, some-times, to cause microstructural changes in the materials involved. Note that tempera-ture also affects the viscosity and other properties of lubricants, with a sufficientlyhigh temperature causing their breakdown.

Friction in Plastics and Ceramics. Because their strength is low compared withthat of metals (Tables 2.2 and 7.1), plastics generally possess low frictional charac-teristics. This property makes plastics better than metals for bearings, gears, seals,

prosthetic joints, and general friction-reducing applications, provided that the loadsare not high. Because of this characteristic, polymers sometimes are described as selflubricating.

The factors involved in the friction and wear of metals are generally applica-ble to polymers as well. In sliding, the plowing component of friction in thermo-plastics 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 Friction

the surface of a piece of rubber and observing how the rubber quickly recovers itsshape.

An important factor in plastics applications is the effect of temperature rise atthe sliding interfaces caused by friction. As described in Section 7.3, thermoplasticsrapidly lose their strength and become soft as temperature increases. If the tempera-ture rise is not controlled, sliding surfaces can undergo permanent deformation andthermal degradation.

The frictional behavior of various polymers on metals is similar to that of met-als on metals. The well-known low friction of PTFE (Teflon) is attributed to itsmolecular structure, which has no reactivity with metals. Consequently, its adhesionis poor and thus its friction is low.

The frictional behavior of ceramics is similar to that of metals; hence, adhesionand 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 thathave low adhesion (such as carbides and ceramics) and through the use of surfacefilms and coatings. Lubricants (such as oils) or solid films (such as graphite) inter-pose an adherent film between the tool, die, and workpiece. This film minimizesadhesion and interactions of one surface with the other, thus reducing friction.Friction also can be reduced significantly by subjecting the tool- or die-workpieceinterface to ultrasonic vibrations, typically at 20 kHz. The amplitude of the vibra-tions periodically separates the two surfaces and allows the lubricant to flow morefreely into the interface during these separations.

Friction Measurement. Although the coefficient of friction can be estimated theo-retically, it usually is determined experimentally, either during actual manufac-turing processes or in simulated laboratory tests using small-scale specimens ofvarious shapes. A test that has gained wide acceptance-particularly for bulk-deformation processes-is the ring-compression test. A flat ring is upset plasticallybetween two flat platens (Fig. 33.6a). As its height is reduced, the ring expands

Good lubrication Poor lubrication

i

i i ( i (

(2)

(D)

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


U

/\ 0.40/\

80 - ,, 0.30

70 - Q

60 -A 0.20

§ 50 -§ 0.15

40 -E 0.12

2 3° ° 0.105 0.099 2° ' 0.08-‘E 0.07 1° ` 0.062 0 _fi 0.055U i0058 -10 -U: 0-04 Original dimensions of specimen:

_20` O03 OD=19mmID = 9.5 mm_30 _

Height = 0.64 mm

-40- .002 0 0 10 20 30 40 50 60 70

Reduction in height (%)

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

radially outward. If friction at the interfaces is zero, both the inner and outerdiameters 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 partic-ular reduction in height, there is a critical friction value at which the internaldiameter 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 thecurves shown in Fig. 33.7 (which are obtained through theoretical analyses), thecoefficient of friction can be determined. Note that each ring geometry and materialhas its own specific set of curves. The most common geometry of a specimen has anouter diameter, an inner diameter, and height proportions of 6:3:2, respectively. Theactual size of the specimen is usually not relevant in these tests. Thus, once the per-centage of reduction in internal diameter and height is known, the magnitude ofit 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 thickness by 50%. Determine the coefficient of fric-height and with an outside diameter (OD) of 30 mm tion, /JL, if the GD is 38 mm after deformation.and an inside diameter (ID) of 15 mm is reduced in

Solution First it is necessary to determine the newID (which is obtained from volume constancy) asfollows:

volume = -1-i<so2 - 152)1O = $382 - ID2)5.

Section 33.5 Wear

Thus, the change in internal diameter is

9.7 - 15AID == L-T5--l = ~O.35 or 35% (decrease)

With a 50% reduction in height and a 35% reductionin internal diameter, the friction coefficient can beobtained from Fig. 33.7 as

From this equation, the new ID is calculated as 9.7 mm. ;.t = 0.21.

33.5 Wear

Wear has important technological and economic significance because it changesthe shapes of tools and dies and, consequently, affects the tool life, tool and diedimensions, and thus the quality of the parts produced. The importance of wearis evident in the number of parts and components that continually have to bereplaced or repaired. Examples of Wear in manufacturing processes include dulldrill bits that have to be reground, Worn cutting tools that have to be indexed orchanged, tools and dies that have to be repaired or replaced, and countless othersituations. Wear plates, placed in dies and sliding mechanisms where the loadsare high, are an important component in some metalworking machinery. Theseplates, also known as wear parts, are expected to Wear, but they can easily bereplaced.

Although wear generally alters a part’s surface topography and may resultin severe surface damage, it can also have a beneficial effect. The running-inperiod for various machines and engines produces wear in order to remove thepeaks from asperities (Fig. 33.8). Thus, under controlled conditions, wear may

Scale: 250 am

25 ,u.|'T1

15 .~<;i3§1;;y;\a§,~ ;,;,e,@t11', -‘~~' >'~1=~~:fcir¢-f.,f.':§1 <'4»»-»/'~',;~ ,Lyra ;>.,!'5;_;;,1;<.,_,: 3, Ny.; 3 <_ ~;,,__;

- L - v _ \;~ »;- (H)

Unwom W0 V fl , ' 9 *~"` fi “céllif *QQ »l<3£,¢;z 5

<b> T-TFIGURE 33.8 Changes in original (a) Wire-brushed and (b) ground-surface profiles after Wear.Note the difference in the vertical and horizontal scales. Source: After E. Wild and KJ. Mack.


T'""* ** Hard

Metal transfer(possible wearparticle)

Plastic zone so i n (microweld) '

lb) (C)

FIGURE 33.9 Schematic illustration of (a) two contacting asperities, (b) adhesion betweentwo asperities, and (c) the formation of a wear particle.

be regarded as a type of smoothing or polishing process. Note that writing with anordinary pencil or chalk is a wear process, and the words written actually areformed from wear particles.

Adhesive Wear. If a tangential force is applied to the model shown in Fig. 33.9,shearing can take place either (a) at the original interface or (b) along a path belowor above the interface, in either case causing ad/aesiz/e u/ear, also called sliding wear.Because of factors such as strain hardening at the asperity contact, diffusion, andmutual solid solubility, the adhesive bonds often are stronger than the base metals.Thus, during sliding, fracture usually follows a path in the weaker or softer compo-nent; that is how a wear fragment is generated. Although this fragment is typicallyattached to the harder component (the upper surface in Fig. 33.9c), it eventuallybecomes detached during further rubbing at the interface and develops into a looseWear particle.

In more severe cases, such as ones with high loads and strongly bonded asper-ities, adhesive wear is described as scufyqng, smearing, tearing, galling, or seizure(severe wear). Oxide layers on surfaces have a large influence on adhesive wear,sometimes acting as a protective film, resulting in mild wear-which consists ofsmall wear particles.

Adhesive wear can be reduced by one or more of the following methods:

a. Selecting materials that do not form strong adhesive bonds.

b. Using a harder material as one member of the pair.

c. Using materials that oxidize more easily.

d. Applying hard coatings that serve methods a to c. Coating one surface with a

soft material (such as tin, silver, lead, or cadmium) also is effective in reducingwear.

e. Using an appropriate lubricant.

Abrasive Wear. This type of wear is caused by a hard, rough surface or a surfacecontaining hard, protruding particles sliding across another surface. As a result,rnicroclaips or slit/ers are produced as wear particles, thereby leaving grooves orscratches on the softer surface (Fig. 33.1O). In fact, processes such as filing, grinding,ultrasonic machining, and abrasive-jet and abrasive water-jet machining, describedin the preceding chapters, act in this manner. The difference in these operations is

that, unlike wear that is generally not intended or wanted, the process parametersare controlled to produce the desired shapes and surfaces through wear.

There are two basic types of abrasive wear. In two-body wear, abrasive actiontakes place between two sliding surfaces or between a hard, abrasive particle in con-tact with a solid body. This type is the basis of erosive wear. In three-body wear, anabrasive particle is present between two sliding solid bodies, such as a wear particlecarried by a lubricant. Such a situation indicates the importance of properly filteringlubricants in metalworking operations to remove any contaminants.

The abrasive-wear resistance of pure metals and ceram-ics has been found to be directly proportional to their hard-ness. Thus, abrasive wear can be reduced by increasing thehardness of materials (usually by heat treating) or by reducingthe normal load. Elastomers and rubbers resist abrasivewear as well, because they deform elastically and then recoverwhen abrasive particles cross over their surfaces. The bestexamples are automobile tires, which last a long time eventhough they are operated on road surfaces that generally arerough and abrasive. Even hardened steels would not last longunder such conditions.

Section 33.5 Wear 963

*PChip

Hard particle

FIGURE 33.I0 Schematic illustration of abrasivewear in sliding. Longitudinal scratches on a surfaceusually indicate abrasive wear.

Corrosive Wear. Also known as oxidation or chemical wear, this type of wear is

caused by chemical and electrochemical reactions between the surface and the envi-ronment. The fine corrosive products on the surface constitute the wear particles in

corrosive wear. When the corrosive layer is destroyed or removed through sliding orabrasion, another layer begins to form, and the process of removal and corrosive-layer formation is repeated. Among corrosive media are water, seawater, oxygen,acids, chemicals, and atmospheric hydrogen sulfide and sulfur dioxide.

Corrosive wear can be reduced by

° Selecting materials that will resist environmental attack.° Applying a coating (Chapter 34).° Controlling the environment.° Reducing operating temperatures in order to lower the rate of chemical

reaction.

Fatigue Wear. Fatigue wear, also called surface fatigue or surface-fracture wear, is

caused when the surface of a material is subjected to cyclic loading, such as rollingcontact in bearings. The wear particles usually are formed through spalling orpitting. Thermal fatigue is another type of fatigue wear, whereby surface cracks aregenerated by thermal stresses from thermal cycling, as when a cool die repeatedlycontacts hot workpieces. The cracks then join, and the surface begins to spall,producing fatigue wear in a phenomenon known as heat checking.

In manufacturing operations, fatigue wear usually occurs on dies in hot-workingand die-casting operations. This type of wear can be reduced by

° Lowering contact stresses.° Reducing thermal cycling.° Improving the quality of materials by removing impurities, inclusions, and

various other flaws that may act as local points for crack initiation.

Other Types of Wear. Several other types of wear can be seen in manufacturingoperations.

° Erosion is caused by loose particles abrading a surface.° Fretting corrosion occurs at interfaces that are subjected to very small recipro-

cal movements.° Impact Wear is the removal (by impacting particles) of small amounts of mate-

rial from a surface.

In many cases, component wear is the result of a combination of differenttypes of wear. Note, for example, in Fig. 33.11, that even in the same forging die,various types of wear take place in different locations; a similar situation can exist incutting tools, as shown in Fig. 21.18.


;TTopdfeT TWA?

' 9 9 06999 0 ® Erosion_ ® Pitting (lubricated dies only)

Q ag’ 99 “ © Thermal fatigueQD Mechanical fatigue © Plastic deformation

Q

FIGURE 33.Il Types of wear observed in a single die used for hot forging. Source: AfterT.A. Dean.

Wear of Thermoplastics. The wear behavior of thermoplastics is similar to that ofmetals. Their abrasive-wear behavior depends partly on the ability of the polymer todeform and recover elastically, as in elastomers. Typical polymers with good wearresistance are polyimides, nylons, polycarbonate, polypropylene, acetals, and high-

density polyethylene. These polymers are molded or machined to make gears, pul-

leys, sprockets, and similar mechanical components. Because thermoplastics can bemade with a wide variety of compositions, they also can be blended with internallubricants, such as polytetrafluoroethylene, silicon, graphite, molybdenum disulfide,and rubber particles, that are interspersed within the polymer matrix.

Wear of Reinforced Plastics. The wear resistance of reinforced plastics depends onthe type, amount, and direction of reinforcement in the polymer matrix. Carbon, glass,and aramid fibers all improve wear resistance. Wear takes place when fibers are pulledout of the matrix (Hber pullout). Wear is highest when the sliding direction is parallelto the fibers, because they can be pulled out more easily in this case. Long fibersincrease the wear resistance of composites, because they are more difficult to pull outand they prevent cracks in the matrix from propagating to the surface as easily.

Wear of Ceramics. When ceramics slide against metals, wear is caused by (a) small-scale plastic deformation and brittle surface fracture, (b) surface chemical reactions,(c) plowing, and (d) fatigue. Metals can be transferred to the oxide-type ceramic sur-faces, forming metal oxides. Thus, sliding actually takes place between the metaland the metal-oxide surface.

33.6 Lubrication

There is evidence that the lubrication of surfaces to reduce friction and wear datesback about four millenia to lubricate various linear and rotary moving components.For example, chariot wheels were lubricated with beef tallow in 1400 B.C. Variousoils also were used for lubrication in metalworking processes (see Table l.2), begin-ning in about 600 A.D.

In manufacturing processes, as noted in various chapters, the surfaces of tools,dies, and workpieces are subjected to (a) force and contact pressure, which rangesfrom very low values to multiples of the yield stress of the workpiece material;

Section 33.6 Lubrication

(b) relative speed, from very low to very high; and lc) temperature, which rangesfrom ambient to melting. In addition to selecting appropriate materials and control-ling process parameters to reduce friction and wear, lubricants, or, more generally,metalworking fluids, are applied widely.

Regimes of Lubrication. There are four regimes of lubrication that are generally ofinterest in manufacturing operations (Fig. 3312):

I. In thick-film lubrication, the surfaces are separated completely by a film oflubricant and lubricant viscosity is an important factor. Such films can developin some regions of the workpiece in high-speed operations and also can developfrom high-viscosity lubricants that become trapped at die-workpiece inter-faces. A thick lubricant film results in a dull, grainy surface appearance on theworkpiece after forming operations, with the degree of roughness varying withgrain size. In operations such as coining and precision forging, trapped lubri-cants are undesirable, because they prevent accurate shape generation.

2. As the load between the die and the workpiece increases or as the speed andviscosity of the metalworking fluid decrease, the lubricant film becomes thin-ner and the process is known as thin-film lubrication. This condition raises thefriction at the sliding interfaces and results in slight wear.

3. In mixed lubrication, a significant portion of the load is carried by the physicalcontact between the asperities of the two contacting surfaces. The rest of theload is carried by the fluid film trapped in pockets, such as the valleys betweenasperities.

4. In boundary lubrication, the load is supported by contacting surfaces that arecovered with a boundary film of lubricant (Fig. 33.12d)-a thin molecularlubricant layer. The film is attracted to the metal surfaces and prevents directmetal-to-metal contact of the two bodies, thus reducing wear. Boundary lubri-cants typically are natural oils, fats, fatty acids, esters, or soaps. However,boundary films can break down (a) as a result of desorption caused by hightemperatures developed at the sliding interfaces or (b) by being rubbed off dur-ing sliding. Deprived of this protective film, the sliding metal surfaces thenbegin to wear and may also score severely.

Lubricant

(a) Thick film (b) Thin film

Boundary film

(C) Mixed (d) Boundary

FIGURE 33.I2 Regimes of lubrication generally occurring in metalworking operations.Source: After WR.D. Wilson.


Other Considerations. Note that the valleys in the surface of the contacting bodies(see Figs. 33.2a, 33.4, and 33.5) can serve as local reservoirs or pockets for lubricants,thereby supporting a substantial portion of the load. The workpiece, but not the die,should have the rougher surface; otherwise, the rougher and harder die surface, actinglike a file, may damage the workpiece surface. The recommended surface roughnesson most dies is about 0.4 ,u.m. The overall geometry of the interacting bodies is anoth-er important consideration in ensuring proper lubrication. The movement of theworkpiece into the deformation zone, as occurs during wire drawing, extrusion, androlling, should allow a supply of lubricant to be carried into the die-workpieceinterface. With proper selection of process parameters, a relatively thick lubricantfilm can be entrained and maintained.

33.1 Metalworking Fluids and Their Selection

The functions of a metalworking fluid are to

° Reduce friction, thus reducing force and energy requirements and any rise in

temperature.° Reduce u/ear, thus reducing seizure and galling.° Improve material flow in tools, dies, and molds.° Act as a thermal barrier between the workpiece and its tool and die surfaces,

thus preventing workpiece cooling in hot-working processes.° Act as a release or parting agent-a substance that helps in the removal or

ejection of parts from dies and molds.

Several types of metalworking fluids are now available with diverse chemistries,properties, and characteristics that fulfill these requirements. (See also Section 22.12on cutting fluids.)

33.1.I Oils

Oils maintain high film strength on the surface of a metal, as it readily can beobserved when trying to clean an oily surface. Although they are very effective in

reducing friction and wear, oils have low thermal conductivity and low specific heat.Consequently, they do not effectively conduct away the heat generated by frictionand plastic deformation. Moreover, it is difficult and costly to remove oils fromcomponent surfaces that are to be painted or welded, and it is difficult to dispose ofthem. (See also Section 34.l6.)

The sources of oils may be mineral (petroleum or hydrocarbon), animal, orvegetable. Oils may be compounded with any number of additives or with other oils.Compounding is used to change such properties as viscosity-temperature behavior,surface tension, heat resistance, and boundary-layer characteristics. Undiluted min-eral oils, with or without additives, are known as neat oils. Oils can be contaminatedby the lubricants used for the slideways and various components of the machinetools and metalworking machinery. Because these oils have characteristics differentfrom those used for the process itself, they can have adverse effects as a metalwork-ing lubricant. When present in the metalworking fluid itself, such oils are known astramp oil.

33.1.2 Emulsions

An emulsion is a mixture of two immiscible liquids (usually oil and water in variousproportions), along with additives. Emulsiners are substances that prevent the dis-persed droplets in a mixture from joining together-hence the term immiscible. Milky

Section 33.7 Metalworking Fluids and Their Selection

in appearance, emulsions also are known as water-soluble oils or water-based coolantsand are of two types. In indirect emulsion, water droplets are dispersed in the oil. Indirect emulsion, mineral oil is dispersed in water in the form of very small droplets.Direct emulsions are important metalworking fluids, because the presence of watergives them high cooling capacity. They are effective particularly in high-speed machin-ing (Section 25.5), where a severe temperature rise has detrimental effects on tool life,the surface integrity of workpieces, and the dimensional accuracy of parts.

33.7.3 Synthetic and Semisynthetic Solutions

Synthetic solutions are chemical fluids that contain inorganic and other chemicalsdissolved in water; they do not contain any mineral oils. Chemical agents are addedto impart various properties. Semisynthetic solutions are basically synthetic solu-tions to which small amounts of emulsifiable oils have been added.

33.1.4 Soaps, Greases, and Waxes

Soaps are typically reaction products of sodium or potassium salts with fatty acids.Alkali soaps are soluble in water, but other metal soaps generally are insoluble. Soapsare effective boundary luhricants and can form thick film layers at die-workpieceinterfaces, particularly when applied on conversion coatings for cold metalworkingapplications (Section 34.1O).

Greases are solid or semisolid lubricants and generally consist of soaps, mineraloil, and various additives. They are highly viscous and adhere well to metal surfaces.Although used extensively in machinery, greases are of limited use in manufacturingprocesses.

Waxes may be of animal or plant (parafjqn) origin. Compared with greases,they are less “greasy” and are more brittle. Waxes are of limited use in metalwork-ing operations-except as lubricants for copper and, in the form of a chlorinatedparaffin, as lubricants for stainless steels and high-temperature alloys.

33.1.5 Additives

Metalworking fluids usually are blended with various additives, including

0 Oxidation inhibitors° Rust-preventing agents° Foam inhibitors° Wetting agents° Odor-controlling agents° Antiseptics.

Sulfun chlorine, and phosphorus are important additives to oils. Known asextreme-pressure (EP) additives, and used either singly or in combination, they reactchemically with metal surfaces and form adherent surface films of metallic sulfidesand chlorides. These films have low shear strength and good antiweld properties andthus can reduce friction and wear effectively. However, they may preferentially attackthe cobalt binder in tungsten-carbide tools and dies (through selective leaching), caus-ing changes in the surface roughness and integrity of those tools (Section 22.4).

33.7.6 Solid Lubricants

Because of their unique properties and characteristics, several solid materials areused as lubricants in manufacturing operations. Described here are four of the mostcommonly used solid luhricants.


Graphite. The general properties of graphite were described in Section 8.6.

Graphite is weak in shear along its basal planes (see Fig. 1.4); thus, it has a lowcoefficient of friction in that direction. It can be an effective solid lubricant, particu-larly at elevated temperatures. However, friction is low only in the presence of air or

moisture. ln a vacuum or an inert-gas atmosphere, friction is very high; in fact,

graphite can be abrasive in these environments. Graphite can be applied either by

rubbing it on surfaces or by making it part of a colloidal (dispersion of small parti-cles) suspension in a liquid carrier such as water, oil, or an alcohol.

Molybdenum Disulfide. A widely used lamellar solid lubricant, molybdenumdisulfide is somewhat similar in appearance to graphite. However, unlike graphite,it has a high friction coefficient in an ambient environment. Oils commonly are

used as carriers for molybdenum disulfide (MoS2) and as a lubricant at roomtemperature. Molybdenum disulfide is applied by rubbing it on the workpiecesurface.

Metallic and Polymeric Films. Because of their low strength, thin layers of soft

metals and polymer coatings also are used as solid lubricants. Suitable metalsinclude lead, indium, cadmium, tin, and silver; polymers such as polytetrafluoroeth-ylene, polyethylene, and methacrylates are also used. However, these coatings havelimited applications because of their lack of strength under high contact stresses,especially at elevated temperatures.

Soft metals also are used to coat high-strength metals, such as steels, stainlesssteels, and high-temperature alloys. For example, copper or tin is chemically de-

posited on the surface of a metal before it is processed. If the oxide of a particularmetal has low friction and is sufficiently thin, the oxide layer can serve as a solid

lubricant, particularly at elevated temperatures.

Glasses. Although it is a solid material, glass becomes viscous at elevated tempera-tures and hence can serve as a liquid lubricant. Viscosity is a function of temperature(but not of pressure) and depends on the type of glass. Poor thermal conductivity also

makes glass attractive, since it acts as a thermal barrier between hot workpieces andrelatively cool dies. Glass lubrication is typically used in such applications as hot ex-trusion and hot forging.

Conversion Coatings. Lubricants may not always adhere properly to workpiecesurfaces, particularly under high normal and shearing stresses. Failure to adhere has

the greatest effects in forging, extrusion, and the wire drawing of steels, stainlesssteels, and high-temperature alloys. For these applications, the workpiece surfaces are

first transformed through a chemical reaction with acids-hence the term conversion.(See also Section 34.10.) This reaction leaves a somewhat rough and spongy surface,which acts as a carrier for the lubricant. After treatment, any excess acid from the

surface is removed with the use of borax or lime. A liquid lubricant, such as a soap, is

then applied to the surface. The lubricant film adheres to the surface and cannot be

scraped off easily. Zinc-phosphate conversion coatings often are used on carbon andlow-alloy steels. Oxalate coatings are used for stainless steels and high-temperaturealloys.

Fullerenes (Buckyballs). As described in Section 8.6.1, these are carbon moleculesin the shape of soccer balls. When placed between sliding surfaces, buckyball mole-cules act like tiny ball bearings. They can perform well as solid lubricants and arebeing investigated as bearings in aerospace applications.

Section 33.7 Metalworking Fluids and Their Selection

33.7.7 Selection of Metalworking Fluids

Selecting a metalworking fluid for a particular application and workpiece materialinvolves a consideration of several factors:

l. Specific manufacturing process.

2. Workpiece material.

3. Tool or die material.

4. Processing parameters.

5. Compatibility of the fluid with the tool and die materials and the workpiece.6. Surface preparation required.

7. Method of applying the fluid.

8. Removal of the fluid and cleaning of the workpiece after processing.

9. Contamination of the fluid by other lubricants, such as those used to lubricatemachinery.

|0. Storage and maintenance of fluids.

I l. Treatment of waste lubricant.

l2. Biological and environmental considerations.

I3. Costs involved in all of the factors listed here.

In selecting an oil as a lubricant, it is important to investigate its viscosity, tem-perature, and pressure characteristics. Low viscosity can have significant detrimen-tal effects and cause high friction and wear. The specific function of a metalworkingfluid-whether it is primarily a lubricant or a coolant--also must be taken intoaccount. Water-based fluids are very effective coolants, but as lubricants, they arenot as effective as oils. It is estimated that water-based fluids are used in 80 to 90%of all machining operations.

Specific requirements for metalworking fluids are as follows:

° They should not leave any harmful residues that could interfere with operations.° They should not stain or corrode the workpiece or the equipment.° Periodic inspection is necessary to detect deterioration caused by bacterial

growth, accumulation of oxides, chips, wear debris and general degradationand breakdown due to temperature and time. The presence of wear particles is

particularly important, because they cause damage to the system; properinspection and filtering are thus essential.

After the completion of manufacturing operations, workpiece surfaces usuallyhave lubricant residues; these should be removed prior to further processing, such aswelding or painting. Oil-based lubricants are more difficult and expensive to removethan water-based fluids. Various cleaning solutions and techniques used for this pur-pose are described in Section 34.16.

Biological and Environmental Considerations. Biological and environmental con-siderations are important factors in the selection of a metalworking fluid. Hazardsmay result if one contacts or inhales some of these fluids, such as inflammation ofthe skin (dermatitis) and long-term exposure to carcinogens. The improper disposalof metalworking fluids may cause adverse effects on the environment as well. Toprevent or restrict the growth of microorganisms such as bacteria, yeasts, molds,algae, and viruses, chemicals (biocides) are added to metalworking fluids.

Much progress has been made in developing environmentally safe (green) fluidsand the technology and equipment for their proper treatment, recycling, and disposal.In the United States, laws and regulations concerning the manufacture, transportation,


KEY TERMS

Abrasive Wear

AdditivesAdhesionAdhesive wearArithmetic mean valueAsperitiesBoundary lubricationCoefficient of frictionCompounded oils

Conversion coatingsCoolantEmulsionExtreme-pressure additivesFatigue wear

use, and disposal of metalworking fluids are promulgated by the U.S. OccupationalSafety and Health Administration (OSHA), the National Institute for OccupationalSafety and Health (NIOSH), and the Environmental Protection Agency (EPA).

SUMMARY __

° Surfaces and their properties are as important as the bulk properties of materials.A surface not only has a particular shape, roughness, and appearance, but also

has properties that differ significantly from those of the bulk material.

° Surfaces are exposed to the environment and thus are subject to environmentalattack. They also may come into contact with tools and dies (during processing)or with other components (during their service life).

° The geometric and material properties of surfaces can affect their friction, wear,fatigue, corrosion, and electrical and thermal conductivity properties significantly.

° The measurement and description of surface features (including their characteris-tics) are important aspects of manufacturing. The most common surface-rough-ness measurement is the arithmetic mean value. The instrument usually used tomeasure surface roughness is a profilometer.

° Friction and wear are among the most significant factors in processing materials.Much progress has been made in understanding these phenomena and identifyingthe factors that govern them.

° Other important factors are the affinity and solid solubility of the two materialsin contact, the nature of surface films, the presence of contaminants, and processparameters such as load, speed, and temperature.

° A wide variety of metalworking fluids, including oils, emulsions, synthetic solu-tions, and solid lubricants, is available for specific applications.

° The selection and use of lubricants requires a careful consideration of many fac-tors regarding the workpiece and die materials and the particular manufacturingprocess.

° Metalworking fluids have various lubricating and cooling characteristics.Biological and environmental considerations also are important factors in select-ing a metalworking fluid.

Flaw

Eretting corrosionFriction forceGreasesImpact wearLayLubricantLubricationMaximum roughness

heightMetalworking fluids

MicroweldsMixed lubricationOils

Oxide layerPitPlowingRing-compression testRoot-mean-square

averageRunning-inSelective leaching

Self lubricatingSevere wearSoapsSolid lubricantsSubstrateSurface defects

Surface finishSurface integritySurface profilometerSurface roughnessSurface structureSurface textureThick-film lubricationThin-film lubricationTribology

Ultrasonic vibrationsWater-soluble oils

WavinessWaxesWear parts

BIBLIOGRAPHY

Astakhov, V, Tribology of Metal Cutting, Elsevier, 2007.Bhushan, B., Introduction to Tribology, Wiley, 2002.1- (ed.), Handbook of Micro/Nanotribology, 2nd ed.,

CRC Press, 1998.--, Modern Tribology Handbook, CRC Press, 2001.Blau, P.]., Friction Science and Technology, Marcel Dekker,

1995.Booser, E.R. (ed.), Tribology Data Handbook, CRC Press,

1998.Burakowski, T., and Wiershon, T., Surface Engineering of

Metals: Principles, Equipment, Technologies, CRCPress, 1998.

Byers, ].P. (ed.), Metalworking Fluids, Marcel Dekker, 1994.Chattopadhyay, R., Surface Wear: Analysis, Treatment, and

Prevention, ASM International, 2001.Gohar, R., Fundamentals of Tribology, Imperial College

Press, 2008.Ludema, K.C., Friction, Wear, Lubrication: A Textbook in

Tribology, CRC Press, 1996.

REVIEW QUESTIONS

Qualitative Problems 97|

Nachtman, E.S., and Kalpakjian, S., Lubricants andLubrication in Metalworking Operations, MarcelDekker, 1985.

Neele, M.]. (ed.), The Tribology Handbook, 2nd ed.,Butterworth-Heinemann, 1995.

Rabinowicz, E., Friction and Wear of Materials, 2nd ed.,Wiley, 1995.

Schey, ].A., Tribology in Metalworking-Friction,Lubrication and Wear, ASM International, 1983.

Stachowiak, G.W, Wear: Materials, Mechanisms andPractice, Wiley, 2006.

Stachowiak, G.W, and Batchelor, A.W, EngineeringTribology, 3rd ed., Butterworth-Heinemann, 2005.

Totten, G.E., and Liang, I-I., Mechanical Tribology: MaterialsCharacterization and Applications, CRC Press, 2004.

Williams, ].A., Engineering Tribology, Oxford UniversityPress, 2005 _

33.|. Explain what is meant by (a) surface texture and(b) surface integrity.

33.2. List and explain the types of defects typically foundon surfaces.

33.3. Define the terms (a) roughness and (b) waviness.33.4. Explain why the results from a profilometer are not a

true depiction of the actual surface.

33.5. Describe the features of the ring-compression test.Does it require the measurement of forces?

QUALITATIVE PROBLEMS

33.l I. Give several examples that show the importance offriction in manufacturing processes as described in Parts IIIand IV

33.12. Explain the significance of the fact that the hardnessof metal oxides is generally much higher than that of the basemetals themselves. Give some examples.

33.|3. What factors would you consider in specifying thelay of a surface for a part? Explain.

33.|4. Explain why identical surface-roughness values donot necessarily represent the same type of surface.33.l5. Why are the requirements for surface-roughnessdesign in engineering applications so broad? Explain withspecific examples.

33.| 6. What is the significance of a surface-temperature riseresulting from friction? Give some examples based on topicscovered in the preceding chapters.

33.6. List the types of wear generally observed in engineer-ing practice.

33.7. How can adhesive wear be reduced? Abrasive wear?33.8. Explain the functions of a lubricant in manufacturingprocesses?

33.9. What is the role of additives in metalworkingfluids?

33.10. Describe the factors involved in lubricant selection.

33.|7. Explain the causes of lay on surfaces.

33.|8. Give several examples of how wear on molds, tools,and dies affects a manufacturing operation.33.l9. Comment on the surface roughness of various partsand components with which you are familiar. What types ofparts exhibit the coarsest surface? What types exhibit thefinest? Explain.

33.20. Give two examples in which waviness on a surfacewould be desirable. (b) Give two examples in which it wouldbe undesirable.

33.2I. Do the same as for Problem 33.20, but for surfaceroughness.

33.22. Describe your observations regarding Fig. 33.7.33.23. Give the reasons that an originally round specimenin a ring-compression test may become oval after it is upset.

972 Chapter 33 Surface Roughness and Measurement;

33.24. Explain the reason that the abrasive-wear resistanceof a material is a function of its hardness.

33.25. On the basis of your own experience, make a list of

parts and components that have to be replaced because of

wear.

QUANTITATIVE PROBLEMS

Friction, Wear, and Lubrication

33.26. Explain why the types of wear shown in Fig. 33.11occur in those particular locations in the forging die.

33.27. List manufacturing operations in which high frictionis desirable and those in which low friction is desirable.

|l33.28. Refer to the profile shown in Fig. 33.3, and offer

some reasonable numerical values for the vertical distancesfrom the centerline. Calculate the R, and R, values. Then give

another set of values for the same general profile and calculatethe same two quantities. Comment on your observations.

33.29. Obtain several different parts made of various mate-rials, inspect their surfaces under an optical microscope at

different magnifications, and make an educated guess as towhat manufacturing process or finishing process was likelyused to produce each of these parts. Explain your reasoning.

|]33.30. Refer to Fig. 33.6b, and make measurements of

the external and internal diameters (in the horizontal direction

in the photograph) of the four specimens shown. Rememberingthat in plastic deformation the volume of the rings remainsconstant, estimate (a) the reduction in height and (b) thecoefficient of friction for each of the three compressedspecimens.

|l33.3l. Using Fig. 33.7, make a plot of the coefficient offriction versus the change in internal diameter for a constantreduction in height of 35%.

33.32. Assume that in Example 33.1 the coefficient of

friction is 0.16. If all other parameters remain the same, whatis the new internal diameter of the specimen?

SYNTHESIS, DESIGN, AND PROIECTS

33.33. Discuss the tribological differences between ordinarymachine elements (such as gears, cams, and bearings) and met-alworking processes using tools, molds, and dies. Considersuch factors as load, speed, and temperature.

33.34. Section 33.2 listed major surface defects. Howwould you go about determining whether or not each of thesedefects is a significant factor in a particular application?

33.35. Describe your own thoughts regarding biological andenvronmental considerations in the use of metalworking fluids.

33.36. Wear can have detrimental effects in manufacturingoperations. Can you visualize situations in which wear couldbe beneficial? Explain, and give some examples.

33.37. Many parts in various appliances and automobileshave to be replaced because they were worn. Describe the

methodology you would follow in determining the type(s) of

wear these components have undergone.

33.38. In the second paragraph of the introduction toPart VH, five different sets of interfacial conditions were out-lined, from (a) to (e). For each of these, give several examplesfrom the manufacturing processes described in this book.

33.39. Describe your thoughts on the desirability of inte-grating surface-roughness measuring instruments into themachine tools described in Parts III and IV? How would yougo about doing so, giving special consideration to the factoryenvironment in which they are to be used? Make some pre-liminary sketches of such a system.

kalpakjian 19

Documents

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soldering of componentsonto

types of adhesive bonding

mechanical joining processes

lower temperatures

various joining

adhesive forces

mechanical fastening