Part VIII Joining anddantn/kalpakjian/pdf/part-VIII.pdf · OF WELDING Chapter Contents 29.1 Overview of Welding Technology 29.1.1 Types of Welding Processes 29.1.2 Welding as a Commercial

E1C29 11/11/2009 16:9:57 Page 693

Part VIII Joining andAssembly Processes

29FUNDAMENTALSOF WELDING

Chapter Contents

29.1 Overview of Welding Technology29.1.1 Types of Welding Processes29.1.2 Welding as a Commercial Operation

29.2 The Weld Joint29.2.1 Types of Joints29.2.2 Types of Welds

29.3 Physics of Welding29.3.1 Power Density29.3.2 Heat Balance in Fusion Welding

29.4 Features of a Fusion-Welded Joint

In this part of the book, we consider the processes that areused to join two or more parts into an assembled entity.These processes are labeled in the lower stem of Figure 1.4.The term joining is generally used for welding, brazing,soldering, and adhesive bonding, which form a permanentjoint between the parts—a joint that cannot easily be sepa-rated. The term assembly usually refers tomechanicalmeth-ods of fastening parts together. Someof thesemethods allowfor easy disassembly, while others do not. Mechanical as-sembly is covered in Chapter 32. Brazing, soldering, andadhesive bonding are discussed in Chapter 31.We begin ourcoverageof the joining andassembly processeswithwelding,covered in this chapter and the following.

Welding is amaterials joining process inwhich twoormore parts are coalesced at their contacting surfaces by asuitable application of heat and/or pressure.Manyweldingprocesses are accomplished by heat alone,with no pressureapplied; others by a combination of heat and pressure; andstill others by pressure alone, with no external heat sup-plied. In some welding processes a fillermaterial is addedto facilitate coalescence. The assemblage of parts that arejoined by welding is called a weldment. Welding is mostcommonly associated with metal parts, but the process isalso used for joining plastics.Our discussion ofweldingwillfocus on metals.

Welding is a relatively new process (Historical Note29.1). Its commercial and technological importance derivesfrom the following:

693

E1C29 11/11/2009 16:9:58 Page 694

� Welding provides a permanent joint. The welded parts become a single entity.

� The welded joint can be stronger than the parent materials if a filler metal is used thathas strength properties superior to those of the parents, and if proper weldingtechniques are used.

� Welding is usually the most economical way to join components in terms of materialusage and fabrication costs. Alternative mechanical methods of assembly requiremore complex shape alterations (e.g., drilling of holes) and addition of fasteners (e.g.,rivets or bolts). The resulting mechanical assembly is usually heavier than a corre-sponding weldment.

� Welding is not restricted to the factory environment. It can be accomplished ‘‘in thefield.’’

Although welding has the advantages indicated above, it also has certain limita-tions and drawbacks (or potential drawbacks):

� Most welding operations are performedmanually and are expensive in terms of laborcost. Many welding operations are considered ‘‘skilled trades,’’ and the labor toperform these operations may be scarce.

� Most welding processes are inherently dangerous because they involve the use ofhigh energy.

� Since welding accomplishes a permanent bond between the components, it does notallow for convenient disassembly. If the product must occasionally be disassembled(e.g., for repair or maintenance), then welding should not be used as the assemblymethod.

� The welded joint can suffer from certain quality defects that are difficult to detect.The defects can reduce the strength of the joint.

Historical Note 29.1 Origins of welding

A lthough welding is considered a relatively newprocess as practiced today, its origins can be traced toancient times. Around 1000 BCE, the Egyptians and othersin the eastern Mediterranean area learned to accomplishforge welding (Section 30.5.2). It was a natural extensionof hot forging, which they used to make weapons, tools,and other implements. Forge-welded articles of bronzehave been recovered by archeologists from the pyramidsof Egypt. From these early beginnings through the MiddleAges, the blacksmith trade developed the art of weldingby hammering to a high level of maturity. Weldedobjects of iron and other metals dating from these timeshave been found in India and Europe.

It was not until the 1800s that the technologicalfoundations of modern welding were established. Twoimportant discoveries were made, both attributed toEnglish scientist Sir Humphrey Davy: (1) the electric arc,and (2) acetylene gas.

Around 1801, Davy observed that an electric arccould be struck between two carbon electrodes.However, not until the mid-1800s, when the electric

generator was invented, did electrical power becomeavailable in amounts sufficient to sustain arc welding. Itwas a Russian, Nikolai Benardos, working out of alaboratory in France, who was granted a series of patentsfor the carbon arc–welding process (one in England in1885, and another in the United States in 1887). By theturn of the century, carbon arc welding had become apopular commercial process for joining metals.

Benardos’ inventions seem to have been limited tocarbon arc welding. In 1892, an American namedCharles Coffin was awarded a U.S. patent for developingan arc–welding process utilizing a metal electrode. Theunique feature was that the electrode added filler metalto the weld joint (the carbon arc process does not depositfiller). The idea of coating the metal electrode (to shieldthe welding process from the atmosphere) wasdeveloped later, with enhancements to the metal arc–welding process being made in England and Swedenstarting around 1900.

Between 1885 and 1900, several forms of resistancewelding were developed by Elihu Thompson. These

694 Chapter 29/Fundamentals of Welding

E1C29 11/11/2009 16:9:58 Page 695

29.1 OVERVIEW OF WELDING TECHNOLOGY

Welding involves localized coalescence or joining together of two metallic parts at theirfaying surfaces. The faying surfaces are the part surfaces in contact or close proximity thatare to be joined.Welding is usually performed on parts made of the samemetal, but somewelding operations can be used to join dissimilar metals.

29.1.1 TYPES OF WELDING PROCESSES

Some 50 different types of welding operations have been cataloged by the AmericanWelding Society. They use various types or combinations of energy to provide therequired power. We can divide the welding processes into two major groups: (1) fusionwelding and (2) solid-state welding.

Fusion Welding Fusion-welding processes use heat to melt the base metals. In manyfusionwelding operations, a fillermetal is added to themolten pool to facilitate the processand provide bulk and strength to the welded joint. A fusion-welding operation in which nofiller metal is added is referred to as an autogenousweld. The fusion category includes themost widely used welding processes, which can be organized into the following generalgroups (initials in parentheses are designations of the American Welding Society):

� Arcwelding (AW).Arcwelding refers to a group of welding processes inwhich heatingof the metals is accomplished by an electric arc, as shown in Figure 29.1. Some arc-welding operations also applypressureduring the process andmost utilize a fillermetal.

� Resistance welding (RW). Resistance welding achieves coalescence using heat fromelectrical resistance to the flow of a current passing between the faying surfaces oftwo parts held together under pressure.

� Oxyfuel gas welding (OFW). These joining processes use an oxyfuel gas, such as amixture of oxygen and acetylene, to produce a hot flame for melting the base metaland filler metal, if one is used.

included spot welding and seam welding, two joiningmethods widely used today in sheet metalworking.

Although Davy discovered acetylene gas early in the1800s, oxyfuel gas welding required the subsequentdevelopment of torches for combining acetylene andoxygen around 1900. During the 1890s, hydrogen and

natural gas were mixed with oxygen for welding, but theoxyacetylene flame achieved significantly highertemperatures.

These three welding processes—arc welding, resistancewelding, and oxyfuel gas welding—constitute by far themajority of welding operations performed today.

FIGURE 29.1 Basics ofarc welding: (1) before the

weld; (2) during the weld(the base metal is meltedand fillermetal is added to

the molten pool); and (3)the completed weldment.There are many variations

of the arc-weldingprocess.

Section 29.1/Overview of Welding Technology 695

E1C29 11/11/2009 16:9:58 Page 696

� Other fusion-welding processes. Other welding processes that produce fusion of themetals joined include electron beam welding and laser beam welding.

Certain arc and oxyfuel processes are also used for cutting metals (Sections 26.3.4and 26.3.5).

Solid-State Welding Solid-state welding refers to joining processes in which coales-cence results from application of pressure alone or a combination of heat and pressure. Ifheat is used, the temperature in the process is below the melting point of the metals beingwelded. No filler metal is utilized. Representative welding processes in this groupinclude:

� Diffusion welding (DFW). Two surfaces are held together under pressure at anelevated temperature and the parts coalesce by solid-state diffusion.

� Friction welding (FRW). Coalescence is achieved by the heat of friction between twosurfaces.

� Ultrasonic welding (USW). Moderate pressure is applied between the two parts andan oscillating motion at ultrasonic frequencies is used in a direction parallel to thecontacting surfaces. The combination of normal and vibratory forces results in shearstresses that remove surface films and achieve atomic bonding of the surfaces.

In Chapter 30, we describe the various welding processes in greater detail. Thepreceding survey should provide a sufficient framework for our discussion of weldingterminology and principles in the present chapter.

29.1.2 WELDING AS A COMMERCIAL OPERATION

The principal applications of welding are (1) construction, such as buildings and bridges;(2) piping, pressure vessels, boilers, and storage tanks; (3) shipbuilding; (4) aircraft andaerospace; and (5) automotive and railroad [1]. Welding is performed in a variety oflocations and in a variety of industries.Owing to its versatility as an assembly technique forcommercial products, many welding operations are performed in factories. However,several of the traditional processes, such as arc welding and oxyfuel gas welding, useequipment that can be readily moved, so these operations are not limited to the factory.They can be performed at construction sites, in shipyards, at customers’ plants, and inautomotive repair shops.

Most welding operations are labor intensive. For example, arc welding is usuallyperformedbya skilledworker, calledawelder,whomanually controls thepathorplacementof the weld to join individual parts into a larger unit. In factory operations in which arcwelding ismanually performed, thewelder oftenworkswith a secondworker, called a fitter.It is the fitter’s job to arrange the individual components for the welder prior tomaking theweld.Welding fixtures andpositionersareused for thispurpose.Awelding fixture is adevicefor clamping and holding the components in fixed position for welding. It is custom-fabricated for the particular geometry of the weldment and thereforemust be economicallyjustifiedon thebasis of thequantitiesof assemblies tobeproduced.Aweldingpositioner is adevice that holds the parts and also moves the assemblage to the desired position forwelding.Thisdiffers fromawelding fixture that onlyholds theparts ina single fixedposition.The desired position is usually one in which the weld path is flat and horizontal.

The Safety Issue Welding is inherently dangerous to human workers. Strict safetyprecautions must be practiced by those who perform these operations. The high tempera-tures of the molten metals in welding are an obvious danger. In gas welding, the fuels


E1C29 11/11/2009 16:9:58 Page 697

(e.g., acetylene) are a fire hazard.Most of the processes use high energy to causemelting ofthe part surfaces to be joined. In many welding processes, electrical power is the source ofthermal energy, so there is the hazard of electrical shock to the worker. Certain weldingprocesses have their own particular perils. In arcwelding, for example, ultraviolet radiationis emitted that is injurious to human vision. A special helmet that includes a dark viewingwindowmust be worn by thewelder. This window filters out the dangerous radiation but isso dark that it renders the welder virtually blind, except when the arc is struck. Sparks,spatters of molten metal, smoke, and fumes add to the risks associated with weldingoperations.Ventilation facilitiesmust beused to exhaust thedangerous fumesgeneratedbysome of the fluxes and molten metals used in welding. If the operation is performed in anenclosed area, special ventilation suits or hoods are required.

Automation in Welding Because of the hazards of manual welding, and in efforts toincrease productivity and improve product quality, various forms of mechanization andautomation have been developed. The categories include machine welding, automaticwelding, and robotic welding.

Machine welding can be defined as mechanized welding with equipment thatperforms the operation under the continuous supervision of an operator. It is normallyaccomplished by awelding head that ismovedbymechanicalmeans relative to a stationarywork, or bymoving thework relative to a stationarywelding head.Thehumanworkermustcontinually observe and interact with the equipment to control the operation.

If the equipment is capable of performing the operation without control by a humanoperator, it is referred to as automatic welding. A human worker is usually present tooversee the process and detect variations from normal conditions. What distinguishesautomatic welding from machine welding is a weld cycle controller to regulate the arcmovement and workpiece positioning without continuous human attention. Automaticwelding requires a welding fixture and/or positioner to position the work relative to theweldinghead. It also requires a higherdegreeof consistency andaccuracy in thecomponentparts used in the weldment. For these reasons, automatic welding can be justified only forlarge quantity production.

In robotic welding, an industrial robot or programmable manipulator is used toautomatically control the movement of the welding head relative to the work (Section38.4.3). The versatile reachof the robot armpermits the use of relatively simple fixtures, andthe robot’s capacity to be reprogrammed for new part configurations allows this form ofautomation to be justified for relatively low production quantities. A typical robotic arc-welding cell consists of two welding fixtures and a human fitter to load and unload partswhile the robot welds. In addition to arc welding, industrial robots are also used inautomobile final assemblyplants toperformresistanceweldingon carbodies (Figure39.11).

29.2 THE WELD JOINT

Welding produces a solid connection between two pieces, called a weld joint. Aweld jointis the junction of the edges or surfaces of parts that have been joined by welding. Thissection covers two classifications related to weld joints: (1) types of joints and (2) thetypes of welds used to join the pieces that form the joints.

29.2.1 TYPES OF JOINTS

There are five basic types of joints for bringing two parts together for joining. The fivejoint types are not limited to welding; they apply to other joining and fastening

Section 29.2/The Weld Joint 697

E1C29 11/11/2009 16:9:58 Page 698

techniques as well. With reference to Figure 29.2, the five joint types can be defined asfollows:

(a) Butt joint. In this joint type, the parts lie in the same plane and are joined at theiredges.

(b) Corner joint. The parts in a corner joint form a right angle and are joined at the cornerof the angle.

(c) Lap joint. This joint consists of two overlapping parts.

(d) Tee joint. In a tee joint, one part is perpendicular to the other in the approximateshape of the letter ‘‘T.’’

(e) Edge joint. The parts in an edge joint are parallel with at least one of their edges incommon, and the joint is made at the common edge(s).

29.2.2 TYPES OF WELDS

Each of the preceding joints can be made by welding. It is appropriate to distinguishbetween the joint type and the way in which it is welded—the weld type. Differencesamong weld types are in geometry (joint type) and welding process.

A fillet weld is used to fill in the edges of plates created by corner, lap, and teejoints, as in Figure 29.3. Filler metal is used to provide a cross section approximately theshape of a right triangle. It is the most common weld type in arc and oxyfuel weldingbecause it requires minimum edge preparation—the basic square edges of the parts areused. Fillet welds can be single or double (i.e., welded on one side or both) and can becontinuous or intermittent (i.e., welded along the entire length of the joint or withunwelded spaces along the length).

Groove welds usually require that the edges of the parts be shaped into a groove tofacilitate weld penetration. The grooved shapes include square, bevel, V, U, and J, in

FIGURE 29.2 Five basic types of joints: (a) butt, (b) corner, (c) lap, (d) tee, and (e) edge.

FIGURE 29.3 Variousforms of fillet welds:(a) inside single fillet

corner joint; (b) outsidesingle fillet corner joint;(c) double fillet lap joint;

and (d) double fillet teejoint. Dashed lines showthe original part edges.


E1C29 11/11/2009 16:9:58 Page 699

single or double sides, as shown in Figure 29.4. Filler metal is used to fill in the joint,usually by arc or oxyfuel welding. Preparation of the part edges beyond the basic squareedge, although requiring additional processing, is often done to increase the strength ofthe welded joint or where thicker parts are to be welded. Although most closelyassociated with a butt joint, groove welds are used on all joint types except lap.

Plug welds and slot welds are used for attaching flat plates, as shown in Figure 29.5,using one or more holes or slots in the top part and then filling with filler metal to fuse thetwo parts together.

Spot welds and seam welds, used for lap joints, are diagrammed in Figure 29.6. Aspot weld is a small fused section between the surfaces of two sheets or plates. Multiplespot welds are typically required to join the parts. It is most closely associated withresistance welding. A seam weld is similar to a spot weld except it consists of a more orless continuously fused section between the two sheets or plates.

FIGURE 29.4 Sometypical groove welds:

(a) square groove weld,one side; (b) single bevelgroove weld; (c) singleV-groove weld; (d) single

U-groove weld; (e) singleJ-groove weld; (f) doubleV-groove weld for thicker

sections. Dashed linesshow the original partedges.

FIGURE 29.5 (a) Plugweld; and (b) slot weld.

FIGURE 29.6 (a) Spot weld; and (b) seam weld.

Section 29.2/The Weld Joint 699

E1C29 11/11/2009 16:9:58 Page 700

Flange welds and surfacing welds are shown in Figure 29.7. A flange weld is madeon the edges of two (or more) parts, usually sheet metal or thin plate, at least one of theparts being flanged as in Figure 29.7(a). A surfacing weld is not used to join parts, butrather to deposit filler metal onto the surface of a base part in one or more weld beads.The weld beads can be made in a series of overlapping parallel passes, thereby coveringlarge areas of the base part. The purpose is to increase the thickness of the plate or toprovide a protective coating on the surface.

29.3 PHYSICS OF WELDING

Although several coalescingmechanisms are available forwelding, fusion is by far themostcommon means. In this section, we consider the physical relationships that allow fusionwelding to be performed. We first examine the issue of power density and its importance,and then we define the heat and power equations that describe a welding process.

29.3.1 POWER DENSITY

Toaccomplish fusion, a sourceofhigh-density heat energy is supplied to the faying surfaces,and the resulting temperatures are sufficient to cause localizedmeltingof thebasemetals. Ifa fillermetal is added, theheat densitymust behigh enough tomelt it also.Heat density canbedefinedas the power transferred to theworkperunit surface area,W/mm2 (Btu/sec-in2).The time to melt the metal is inversely proportional to the power density. At low powerdensities, a significant amount of time is required to cause melting. If power density is toolow, the heat is conducted into the work as rapidly as it is added at the surface, andmeltingnever occurs. It has been found that the minimum power density required to melt mostmetals in welding is about 10 W/mm2 (6 Btu/sec-in2). As heat density increases, meltingtime is reduced. If power density is too high—above around 105 W/mm2 (60,000 Btu/sec-in2)—the localized temperatures vaporize themetal in the affected region. Thus, there is apractical range of values for power density within which welding can be performed.Differences among welding processes in this range are (1) the rate at which weldingcanbeperformedand/or (2) the size of the region that can bewelded.Table 29.1 provides acomparison of power densities for the major fusion welding processes. Oxyfuel gaswelding is capable of developing large amounts of heat, but the heat density is relativelylow because it is spread over a large area. Oxyacetylene gas, the hottest of the OFW fuels,burns at a top temperature of around 3500�C (6300�F). By comparison, arc weldingproduces high energy over a smaller area, resulting in local temperatures of 5500�C to6600�C (10,000�F–12,000�F). For metallurgical reasons, it is desirable to melt the metalwith minimum energy, and high power densities are generally preferable.

FIGURE 29.7 (a) Flangeweld; and (b) surfacing

weld.


E1C29 11/11/2009 16:9:58 Page 701

Power density can be computed as the power entering the surface divided by thecorresponding surface area:

PD ¼ PA

ð29:1Þ

where PD ¼ power density, W/mm2 (Btu/sec-in2); P ¼ power entering the surface,W (Btu/sec); andA¼ surface area over which the energy is entering, mm2 (in2). The issueis more complicated than indicated by Eq. (29.1). One complication is that the powersource (e.g., the arc) is moving in many welding processes, which results in preheatingahead of the operation and postheating behind it. Another complication is that powerdensity is not uniform throughout the affected surface; it is distributed as a function ofarea, as demonstrated by the following example.

Example 29.1Power Density inWelding

Aheat source transfers 3000W to the surface of ametal part. The heat impinges the surfaceina circular area,with intensities varying inside the circle. Thedistribution is as follows: 70%of the power is transferredwithin a circle of diameter¼ 5mm, and90%is transferredwithina concentric circle of diameter ¼ 12 mm. What are the power densities in (a) the 5-mmdiameter inner circle and (b) the 12-mm-diameter ring that lies around the inner circle?

Solution: (a) The inner circle has an area A ¼ p 5ð Þ2

4¼ 19:63mm2.

The power inside this area P ¼ 0.70 � 3000 ¼ 2100 W.Thus the power density PD ¼ 2100

19:63¼ 107W /mm2.

(b) The area of the ring outside the inner circle is A ¼ p 122 � 52� �

4¼ 93:4 mm2.

The power in this region P ¼ 0.9 (3000) � 2100 ¼ 600 W.The power density is therefore PD

600

93:4¼ 6:4W/mm2.

Observation: The power density seems high enough for melting in the inner circle, butprobably not sufficient in the ring that lies outside this inner circle. n

29.3.2 HEAT BALANCE IN FUSION WELDING

The quantity of heat required to melt a given volume of metal depends on (1) the heat toraise the temperature of the solid metal to its melting point, which depends on themetal’svolumetric specific heat, (2) the melting point of the metal, and (3) the heat to transformthe metal from solid to liquid phase at the melting point, which depends on the metal’sheat of fusion. To a reasonable approximation, this quantity of heat can be estimated by

TABLE 29.1 Comparison of several fusion weldingprocesses on the basis of their power densities.

Approximate Power Density

Welding Process W/mm2 Btu/sec-in2

Oxyfuel welding 10 6Arc welding 50 30Resistanc welding 1000 600Laser beam welding 9000 5000Electron beam welding 10,000 6000

Section 29.3/Physics of Welding 701

E1C29 11/11/2009 16:9:59 Page 702

the following equation [5]:

Um ¼ KT2m ð29:2Þ

where Um ¼ the unit energy for melting (i.e., the quantity of heat required to melt a unitvolume of metal starting from room temperature), J/mm3 (Btu/in3);Tm¼melting point ofthemetal onanabsolute temperature scale, �K(�R); andK¼ constantwhosevalue is 3.33�10�6 when the Kelvin scale is used (and K ¼ 1.467 � 10�5 for the Rankine temperaturescale). Absolute melting temperatures for selected metals are presented in Table 29.2.

Not all of the energy generated at the heat source is used to melt the weld metal.There are two heat transfer mechanisms at work, both of which reduce the amount ofgenerated heat that is used by the welding process. The situation is depicted in Figure 29.8.The firstmechanism involves the transferof heatbetween theheat source and the surfaceofthework. This process has a certain heat transfer factor f1, defined as the ratio of the actualheat received by the workpiece divided by the total heat generated at the source. Thesecond mechanism involves the conduction of heat away from the weld area to bedissipated throughout the work metal, so that only a portion of the heat transferred tothe surface is available for melting. Thismelting factor f2 is the proportion of heat receivedat thework surface that can be used formelting. The combined effect of these two factors is

TABLE 29.2 Melting temperatures on the absolute temperature scale forselected metals.

MeltingTemperature

MeltingTemperature

Metal �Ka �Rb Metal �Ka �Rb

Aluminum alloys 930 1680 SteelsCast iron 1530 2760 Low carbon 1760 3160Copper and alloys Medium carbon 1700 3060Pure 1350 2440 High carbon 1650 2960Brass, navy 1160 2090 Low alloy 1700 3060Bronze (90 Cu–10 Sn) 1120 2010 Stainless steels

Inconel 1660 3000 Austenitic 1670 3010Magnesium 940 1700 Martensitic 1700 3060Nickel 1720 3110 Titanium 2070 3730

Based on values in [2].aKelvin scale ¼ Centigrade (Celsius) temperature þ 273.bRankine scale ¼ Fahrenheit temperatureþ 460.

FIGURE 29.8 Heattransfer mechanisms infusion welding.

Heat source for welding

Heat used for melting

(1-f1) Heat losses

Heat transferred to work surface

Worksurface

(1-f2) Heat dissipatedinto work

f1

f2


E1C29 11/11/2009 16:9:59 Page 703

to reduce the heat energy available for welding as follows:

Hw ¼ f 1f 2H ð29:3Þwhere Hw ¼ net heat available for welding, J (Btu), f1 ¼ heat transfer factor, f2 ¼ themelting factor, and H ¼ the total heat generated by the welding process, J (Btu).

The factors f1 and f2 range in value between zero and one. It is appropriate toseparate f1 and f2 in concept, even though they act in concert during the welding process.The heat transfer factor f1 is determined largely by the welding process and the capacityto convert the power source (e.g., electrical energy) into usable heat at the work surface.Arc-welding processes are relatively efficient in this regard, while oxyfuel gas-weldingprocesses are relatively inefficient.

The melting factor f2 depends on the welding process, but it is also influenced by thethermal properties of the metal, joint configuration, and work thickness. Metals with highthermal conductivity, such as aluminumand copper, present a problem inwelding because ofthe rapid dissipation of heat away from the heat contact area. The problem is exacerbated byweldingheat sourceswith lowenergydensities (e.g.,oxyfuelwelding)because theheat input isspread over a larger area, thus facilitating conduction into thework. In general, a high powerdensity combined with a low conductivity work material results in a high melting factor.

We can now write a balance equation between the energy input and the energyneeded for welding:

Hw ¼ UmV ð29:4Þwhere Hw ¼ net heat energy used by the welding operation, J (Btu); Um ¼ unit energyrequired tomelt themetal, J/mm3 (Btu/in3); andV¼ the volume ofmetalmelted,mm3 (in3).

Most welding operations are rate processes; that is, the net heat energy Hw isdelivered at a given rate, and the weld bead is made at a certain travel velocity. This ischaracteristic for example of most arc-welding, many oxyfuel gas-welding operations,and even some resistance welding operations. It is therefore appropriate to expressEq. (30) as a rate balance equation:

RHw ¼ UmRWV ð29:5ÞwhereRHw¼ rateofheatenergydeliveredtotheoperationforwelding,J/s¼W(Btu/min);andRWV¼volumerateofmetalwelded,mm3/s (in3/min). In theweldingofacontinuousbead, thevolume rate ofmetal welded is the product ofweld areaAw and travel velocity v. Substitutingthese terms into the above equation, the rate balance equation can now be expressed as

RHw ¼ f 1f 2RH ¼ UmAwv ð29:6Þwhere f1 and f2 are the heat transfer and melting factors; RH ¼ rate of input energygenerated by the welding power source, W (Btu/min); Aw ¼ weld cross-sectional area,mm2 (in2); and v ¼ the travel velocity of the welding operation, mm/s (in/min). InChapter 30,we examine how thepower density inEq. (29.1) and the input energy rate forEq. (29.6) are generated for some of the individual welding processes.

Example 29.2Welding TravelSpeed

The power source in a particular welding setup generates 3500W that can be transferred tothe work surface with a heat transfer factor ¼ 0.7. The metal to be welded is low carbonsteel, whose melting temperature, from Table 29.2, is 1760�K. The melting factor in theoperation is 0.5.Acontinuous filletweld is tobemadewith a cross-sectional area¼ 20mm2.Determine the travel speed at which the welding operation can be accomplished.

Solution: Let us first find the unit energy required tomelt themetalUm fromEq. (29.2).

Um ¼ 3:33 10�6� �� 17602 ¼ 10:3 J/mm3

Section 29.3/Physics of Welding 703

E1C29 11/11/2009 16:9:59 Page 704

Rearranging Eq. (29.6) to solve for travel velocity, we have v ¼ f 1f 2RHUmAw

; and solving for

the conditions of the problem, v ¼ 0:7 (0:5) (3500)10:3 (20)

¼ 5:95mm/s:n

29.4 FEATURES OF A FUSION-WELDED JOINT

Most weld joints are fusion welded. As illustrated in the cross-sectional view of Figure 29.9(a), a typical fusion-weld joint inwhich fillermetal has been addedconsists of several zones:(1) fusion zone, (2) weld interface, (3) heat-affected zone, and (4) unaffected base metalzone.

The fusion zone consists of a mixture of filler metal and base metal that havecompletely melted. This zone is characterized by a high degree of homogeneity amongthe component metals that have been melted during welding. The mixing of these compo-nents is motivated largely by convection in themolten weld pool. Solidification in the fusionzonehas similarities toacastingprocess. Inwelding themold is formedby theunmeltededgesor surfacesof the componentsbeingwelded.Thesignificantdifferencebetween solidificationin casting and in welding is that epitaxial grain growth occurs in welding. The reader mayrecall that in casting, the metallic grains are formed from the melt by nucleation of solidparticles at the mold wall, followed by grain growth. In welding, by contrast, the nucleationstage of solidification is avoided by themechanismof epitaxial grain growth, inwhich atomsfrom the molten pool solidify on preexisting lattice sites of the adjacent solid base metal.Consequently, the grain structure in the fusion zone near the heat-affected zone tends tomimic the crystallographic orientation of the surrounding heat-affected zone. Further intothe fusion zone, a preferred orientation develops in which the grains are roughly perpendic-ular to the boundaries of the weld interface. The resulting structure in the solidified fusionzone tends to feature coarse columnar grains, as depicted in Figure 29.9(b). The grainstructure depends on various factors, including welding process, metals being welded (e.g.,identicalmetals vs. dissimilarmetals welded), whether a fillermetal is used, and the feed rateat which welding is accomplished. A detailed discussion of weldingmetallurgy is beyond thescope of this text, and interested readers can consult any of several references [1], [4], [5].

The second zone in the weld joint is the weld interface, a narrow boundary thatseparates the fusion zone from the heat-affected zone. The interface consists of a thin bandofbasemetal thatwasmeltedorpartiallymelted (localizedmeltingwithin thegrains)duringthe welding process but then immediately solidified before anymixing with themetal in thefusion zone. Its chemical composition is therefore identical to that of the base metal.

The third zone in the typical fusion weld is the heat-affected zone (HAZ). Themetal in this zone has experienced temperatures that are below its melting point, yet high

FIGURE 29.9 Cross section of a typical fusion-welded joint: (a) principal zones in the joint and (b) typical grain structure.


E1C29 11/11/2009 16:10:0 Page 705

enough to cause microstructural changes in the solid metal. The chemical composition inthe heat-affected zone is the same as the base metal, but this region has been heat treateddue to the welding temperatures so that its properties and structure have been altered.The amount of metallurgical damage in the HAZ depends on factors such as the amountof heat input and peak temperatures reached, distance from the fusion zone, length oftime the metal has been subjected to the high temperatures, cooling rate, and the metal’sthermal properties. The effect on mechanical properties in the heat-affected zone isusually negative, and it is in this region of the weld joint that welding failures often occur.

As the distance from the fusion zone increases, the unaffected base metal zone isfinally reached, in which no metallurgical change has occurred. Nevertheless, the basemetal surrounding the HAZ is likely to be in a state of high residual stress, the result ofshrinkage in the fusion zone.

REFERENCES

[1] ASM Handbook, Vol. 6, Welding, Brazing, andSoldering. ASM International, Materials Park,Ohio, 1993.

[2] Cary, H. B., and Helzer, S. C. Modern WeldingTechnology, 6th ed. Pearson/Prentice-Hall, UpperSaddle River, New Jersey, 2005.

[3] Datsko, J. Material Properties and ManufacturingProcesses. John Wiley & Sons, Inc., New York,1966.

[4] Messler, R. W., Jr. Principles of Welding: Processes,Physics, Chemistry, and Metallurgy. John Wiley &Sons, Inc., New York, 1999.

[5] Welding Handbook, 9th ed., Vol. 1,Welding Scienceand Technology.American Welding Society, Miami,Florida, 2007.

[6] Wick, C., and Veilleux, R. F. Tool and Manufactur-ing Engineers Handbook, 4th ed., Vol. IV, QualityControl and Assembly. Society of ManufacturingEngineers, Dearborn, Michigan, 1987.

REVIEW QUESTIONS

29.1. What are the advantages and disadvantages ofwelding compared to other types of assemblyoperations?

29.2. What were the two discoveries of Sir HumphreyDavy that led to the development of modern weld-ing technology?

29.3. What is meant by the term faying surface?29.4. Define the term fusion weld.29.5. What is the fundamental difference between a

fusion weld and a solid state weld?29.6. What is an autogenous weld?29.7. Discuss the reasons why most welding operations

are inherently dangerous.

29.8. What is the difference between machine weldingand automatic welding?

29.9. Name and sketch the five joint types.29.10. Define and sketch a fillet weld.29.11. Define and sketch a groove weld.29.12. Why is a surfacing weld different from the other

weld types?29.13. Why is it desirable to use energy sources for weld-

ing that have high heat densities?29.14. What is the unit melting energy in welding, and

what are the factors on which it depends?29.15. Define and distinguish the two terms heat transfer

factor and melting factor in welding.29.16. What is the heat-affected zone in a fusion weld?

MULTIPLE CHOICE QUIZ

There are 14 correct answers in the following multiple choice questions (some questions have multiple answers that arecorrect). To attain a perfect score on the quiz, all correct answers must be given. Each correct answer is worth 1 point. Each

Multiple Choice Quiz 705

E1C29 11/11/2009 16:10:0 Page 706

omitted answer or wrong answer reduces the score by 1 point, and each additional answer beyond the correct number ofanswers reduces the score by 1 point. Percentage score on the quiz is based on the total number of correct answers.

29.1. Welding can only be performed on metals thathave the same melting point; otherwise, the metalwith the lower melting temperature always meltswhile the other metal remains solid: (a) true,(b) false?

29.2. A fillet weld can be used to join which of thefollowing joint types (three correct answers):(a) butt, (b) corner, (c) edge, (d) lap, and (e) tee?

29.3. A fillet weld has a cross-sectional shape that isapproximately which one of the following: (a) rect-angular, (b) round, (c) square, or (d) triangular?

29.4. Groove welds are most closely associated withwhich one of the following joint types: (a) butt,(b) corner, (c) edge, (d) lap, or (e) tee?

29.5. A flange weld is most closely associated with whichone of the following joint types: (a) butt, (b) corner,(c) edge, (d) lap, or (e) tee?

29.6. For metallurgical reasons, it is desirable to melt theweld metal with minimum energy input. Which oneof the following heat sources is most consistentwith this objective: (a) high power, (b) high powerdensity, (c) low power, or (d) low power density?

29.7. The amount of heat required to melt a givenvolume of metal depends strongly on which ofthe following properties (three best answers):

(a) coefficient of thermal expansion, (b) heat offusion, (c) melting temperature, (d) modulus ofelasticity, (e) specific heat, (f) thermal conductivity,and (g) thermal diffusivity?

29.8. The heat transfer factor in welding is correctlydefined by which one of the following descriptions:(a) the proportion of the heat received at the worksurface that is used for melting, (b) the proportionof the total heat generated at the source that isreceived at the work surface, (c) the proportion ofthe total heat generated at the source that is usedfor melting, or (d) the proportion of the total heatgenerated at the source that is used for welding?

29.9. The melting factor in welding is correctly definedby which one of the following descriptions: (a) theproportion of the heat received at the work surfacethat is used for melting, (b) the proportion of thetotal heat generated at the source that is received atthe work surface, (c) the proportion of the totalheat generated at the source that is used for melt-ing, or (d) the proportion of the total heat gener-ated at the source that is used for welding?

29.10. Weld failures always occur in the fusion zone of theweld joint, since this is the part of the joint that hasbeen melted: (a) true, (b) false?

PROBLEMS

Power Density

29.1. A heat source can transfer 3500 J/sec to a metalpart surface. The heated area is circular, and theheat intensity decreases as the radius increases, asfollows: 70% of the heat is concentrated in acircular area that is 3.75 mm in diameter. Is theresulting power density enough to melt metal?

29.2. In a laser beam welding process, what is the quan-tity of heat per unit time (J/sec) that is transferredto the material if the heat is concentrated in circlewith a diameter of 0.2 mm? Assume the powerdensity provided in Table 29.1.

29.3. A welding heat source is capable of transferring150 Btu/min to the surface of a metal part. Theheated area is approximately circular, and the heatintensity decreases with increasing radius as fol-lows: 50% of the power is transferred within acircle of diameter ¼ 0.1 in and 75% is transferredwithin a concentric circle of diameter ¼ 0.25 in.What are the power densities in (a) the 0.1-indiameter inner circle and (b) the 0.25-in diameterring that lies around the inner circle? (c) Are thesepower densities sufficient for melting metal?

Unit Melting Energy

29.4. Compute the unit energy for melting for the fol-lowing metals: (a) aluminum and (b) plain lowcarbon steel.

29.5. Compute the unit energy for melting for the fol-lowing metals: (a) copper and (b) titanium.

29.6. Make the calculations and plot on linearly scaledaxes the relationship for unit melting energy as a

function of temperature. Use temperatures as fol-lows to construct the plot: 200�C, 400�C, 600�C,800�C, 1000�C, 1200�C, 1400�C, 1600�C, 1800�C,and 2000�C. On the plot, mark the positions ofsome of the welding metals in Table 29.2. Use of aspreadsheet program is recommended for thecalculations.


E1C29 11/11/2009 16:10:0 Page 707

29.7. Make the calculations and plot on linearly scaledaxes the relationship for unit melting energy as afunction of temperature. Use temperatures as fol-lows to construct the plot: 500�F, 1000�F, 1500�F,2000�F, 2500�F, 3000�F, and 3500�F. On the plot,mark the positions of some of the welding metals inTable 29.2. Use of a spreadsheet program is rec-ommended for the calculations.

29.8. A fillet weld has a cross-sectional area of 25.0 mm2

and is 300 mm long. (a)What quantity of heat (in J)is required to accomplish the weld, if the metal tobe welded is low carbon steel? (b) How much heatmust be generated at the welding source, if the heattransfer factor is 0.75 and the melting factor ¼0.63?

29.9. AU-grooveweld is used to butt weld 2 pieces of 7.0-mm-thick titanium plate. The U-groove is preparedusing a milling cutter so the radius of the groove is3.0mm.Duringwelding, the penetration of theweldcauses an additional 1.5 mm of material to bemelted. The final cross-sectional area of the weldcan be approximated by a semicircle with a radius of4.5 mm. The length of the weld is 200 mm. Themelting factor of the setup is 0.57 and the heattransfer factor is 0.86. (a) What is the quantity ofheat (in J) required to melt the volume of metal inthis weld (fillermetal plus basemetal)?Assume theresulting top surface of the weld bead is flush withthe top surface of the plates. (b) What is the re-quired heat generated at the welding source?

29.10. A groove weld has a cross-sectional area¼ 0.045 in2and is 10 in long. (a)What quantity of heat (inBtu) isrequired to accomplish the weld, if the metal to bewelded ismedium carbon steel? (b)Howmuchheatmust be generated at the welding source, if the heattransfer factor ¼ 0.9 and the melting factor ¼ 0.7?

29.11. Solve the previous problem, except that the metalto be welded is aluminum, and the correspondingmelting factor is half the value for steel.

29.12. In a controlled experiment, it takes 3700 J to meltthe amount of metal that is in a weld bead with across-sectional area of 6.0 mm2 that is 150.0 mmlong. (a) Using Table 29.2, what is the most likelymetal? (b) If the heat transfer factor is 0.85 and themelting factor is 0.55 for a welding process, howmuch heat must be generated at the welding sourceto accomplish the weld?

29.13. Compute the unit melting energy for (a) aluminumand (b) steel as the sum of: (1) the heat required toraise the temperature of the metal from roomtemperature to its melting point, which is thevolumetric specific heat multiplied by the temper-ature rise; and (2) the heat of fusion, so that thisvalue can be compared to the unit melting energycalculated by Eq. (29.2). Use either the SI units orU.S. customary units. Find the values of the prop-erties needed in these calculations either in thistext or in other references. Are the values closeenough to validate Eq. (29.2)?

Energy Balance in Welding

29.14. The welding power generated in a particular arc-welding operation ¼ 3000 W. This is transferred tothe work surface with a heat transfer factor ¼ 0.9.The metal to be welded is copper whose meltingpoint is given in Table 29.2. Assume that themelting factor ¼ 0.25. A continuous fillet weld isto be made with a cross-sectional area ¼ 15.0 mm2.Determine the travel speed at which the weldingoperation can be accomplished.

29.15. Solve the previous problem except that the metalto be welded is high carbon steel, the cross-sectional area of the weld ¼ 25.0 mm2, and themelting factor ¼ 0.6.

29.16. Awelding operation on an aluminum alloymakes agroove weld. The cross-sectional area of the weld is30.0 mm2. The welding velocity is 4.0 mm/sec. Theheat transfer factor is 0.92 and the melting factor is0.48. The melting temperature of the aluminumalloy is 650�C. Determine the rate of heat genera-tion required at the welding source to accomplishthis weld.

29.17. The power source in a particular welding operationgenerates 125 Btu/min, which is transferred to thework surface with heat transfer factor ¼ 0.8. Themelting point for the metal to be welded ¼ 1800�Fand its melting factor ¼ 0.5. A continuous filletweld is to be made with a cross-sectional area ¼0.04 in2. Determine the travel speed at which thewelding operation can be accomplished.

29.18. In a certain welding operation tomake a fillet weld,the cross-sectional area ¼ 0.025 in2 and the travelspeed¼ 15 in/min. If the heat transfer factor¼ 0.95and melting factor ¼ 0.5, and the melting point ¼2000�F for the metal to be welded, determine therate of heat generation required at the heat sourceto accomplish this weld.

29.19. A fillet weld is used to join 2 medium carbon steelplates each having a thickness of 5.0 mm. Theplates are joined at a 90� angle using an insidefillet corner joint. The velocity of the welding headis 6 mm/sec. Assume the cross section of the weldbead approximates a right isosceles triangle with a

Problems 707

E1C29 11/11/2009 16:10:0 Page 708

leg length of 4.5mm, the heat transfer factor is 0.80,and the melting factor is 0.58. Determine the rateof heat generation required at the welding sourceto accomplish the weld.

29.20. A spot weld was made using an arc-welding pro-cess. In a spot-welding operation, two 1/16-in thickaluminum plates were joined. The melted metalformed a nugget that had a diameter of 1/4 in. Theoperation required the power to be on for 4 sec.Assume the final nugget had the same thickness asthe two aluminum plates (1/8 in thick), the heattransfer factor was 0.80 and the melting factor was0.50. Determine the rate of heat generation thatwas required at the source to accomplish this weld.

29.21. A surfacingweld is to be applied to a rectangular lowcarbon steel plate that is 200 mm by 350 mm. Thefiller metal to be added is a harder (alloy) grade ofsteel, whosemelting point is assumed to be the same.A thickness of 2.0mmwill be added to the plate, butwith penetration into the basemetal, the total thick-ness melted during welding ¼ 6.0 mm, on average.The surface will be applied by making a series ofparallel, overlapped welding beads running length-wise on the plate. The operation will be carried outautomatically with the beads laid down in one longcontinuous operation at a travel speed ¼ 7.0 mm/s,usingweldingpasses separatedby5mm.Assume thewelding bead is rectangular in cross section: 5mmby6 mm. Ignore the minor complications of the turn-arounds at the ends of the plate. Assuming the heat

transfer factor ¼ 0.8 and the melting factor ¼ 0.6,determine (a) the rate of heat that must be gener-ated at the welding source, and (b) how long will ittake to complete the surfacing operation.

29.22. An axle-bearing surface made of high carbon steelhas worn beyond its useful life. When it was new,the diameter was 4.00 in. In order to restore it, thediameter was turned to 3.90 in to provide a uniformsurface. Next the axle was built up so that it wasoversized by the deposition of a surface weld bead,which was deposited in a spiral pattern using asingle pass on a lathe. After the weld buildup, theaxle was turned again to achieve the original diam-eter of 4.00 in. The weld metal deposited was asimilar composition to the steel in the axle. Thelength of the bearing surface was 7.0 in. During thewelding operation, the welding apparatus was at-tached to the tool holder, which was fed toward thehead of the lathe as the axle rotated. The axlerotated at a speed of 4.0 rev/min. The weld beadheight was 3/32 in above the original surface. Inaddition, the weld bead penetrated 1/16 in into thesurface of the axle. The width of the weld bead was0.25 in, thus the feed on the lathe was set to 0.25 in/rev. Assuming the heat transfer factor was 0.80 andthe melting factor was 0.65, determine (a) therelative velocity between the workpiece and thewelding head, (b) the rate of heat generated atthe welding source, and (c) how long it took tocomplete the welding portion of this operation.


E1C30 11/11/2009 16:12:29 Page 709

30WELDINGPROCESSES

Chapter Contents

30.1 Arc Welding30.1.1 General Technology of Arc Welding30.1.2 AW Processes—Consumable

Electrodes30.1.3 AW Processes—Nonconsumable

Electrodes

30.2 Resistance Welding30.2.1 Power Source in Resistance Welding30.2.2 Resistance-Welding Processes

30.3 Oxyfuel Gas Welding30.3.1 Oxyacetylene Welding30.3.2 Alternative Gases for Oxyfuel

Welding

30.4 Other Fusion-Welding Processes

30.5 Solid-State Welding30.5.1 General Considerations in Solid-State

Welding30.5.2 Solid State-Welding Processes

30.6 Weld Quality

30.7 Weldability

30.8 Design Considerations in Welding

Welding processes divide into two major categories:(1) fusion welding, in which coalescence is accomplishedby melting the two parts to be joined, in some cases addingfiller metal to the joint; and (2) solid-state welding, inwhich heat and/or pressure are used to achieve coalescence,but no melting of the base metals occurs and no filler metalis added.

Fusion welding is by far the more important category.It includes (1) arc welding, (2) resistance welding, (3) oxy-fuel gas welding, and (4) other fusion welding processes—ones that cannot be classified as any of the first three types.Fusion welding processes are discussed in the first foursections of this chapter. Section 30.5 covers solid-statewelding. And in the final three sections of the chapter,we examine issues common to all welding operations:weld quality, weldability, and design for welding.

30.1 ARC WELDING

Arc welding (AW) is a fusion-welding process in whichcoalescence of the metals is achieved by the heat of anelectric arc between an electrode and the work. The samebasic process is also used in arc cutting (Section 26.3.4). AgenericAWprocess is shown inFigure 30.1.Anelectric arc isa discharge of electric current across a gap in a circuit. It issustained by the presence of a thermally ionized column ofgas (called a plasma) throughwhich current flows. To initiatethe arc in an AW process, the electrode is brought intocontact with the work and then quickly separated from itby a short distance. The electric energy from the arc thusformed produces temperatures of 5500�C (10,000�F) orhigher, sufficiently hot to melt any metal. A pool of moltenmetal, consisting of base metal(s) and filler metal (if one isused) is formed near the tip of the electrode. In most arc-welding processes, filler metal is added during the operationto increase the volume and strength of the weld joint. As the

709

E1C30 11/11/2009 16:12:29 Page 710

electrode is moved along the joint, the molten weld pool solidifies in its wake. Our VideoClip on welding illustrates the various forms of arc welding described in this section.

VIDEO CLIP

Welding: View the segment on arc welding.

Movement of the electrode relative to the work is accomplished by either a humanwelder (manual welding) or by mechanical means (i.e., machine welding, automaticwelding, or robotic welding). One of the troublesome aspects of manual arc welding isthat the quality of the weld joint depends on the skill and work ethic of the humanwelder.Productivity is also an issue. It is often measured as arc time (also called arc-on time)—the proportion of hours worked that arc welding is being accomplished:

Arc time ¼ time arc is onð Þ= hours workedð Þ ð30:1ÞThis definition can be applied to an individual welder or to a mechanized work-

station. For manual welding, arc time is usually around 20%. Frequent rest periods areneeded by the welder to overcome fatigue in manual arc welding, which requires hand-eye coordination under stressful conditions. Arc time increases to about 50% (more orless, depending on the operation) for machine, automatic, and robotic welding.

30.1.1 GENERAL TECHNOLOGY OF ARC WELDING

Before describing the individual AW processes, it is instructional to examine some of thegeneral technical issues that apply to these processes.

Electrodes Electrodes used in AW processes are classified as consumable or non-consumable. Consumable electrodes provide the source of the filler metal in arc welding.These electrodes are available in two principal forms: rods (also called sticks) and wire.Welding rods are typically 225 to 450 mm (9–18 in) long and 9.5 mm (3/8 in) or less indiameter. The problem with consumable welding rods, at least in production weldingoperations, is that they must be changed periodically, reducing arc time of the welder.Consumable weldwire has the advantage that it can be continuously fed into the weld poolfrom spools containing long lengths of wire, thus avoiding the frequent interruptions thatoccur when using welding sticks. In both rod and wire forms, the electrode is consumed bythe arc during the welding process and added to the weld joint as filler metal.

Nonconsumable electrodes are made of tungsten (or carbon, rarely), which resistsmelting by the arc. Despite its name, a nonconsumable electrode is gradually depleted

FIGURE 30.1 The basicconfiguration andelectrical circuit of an arc-

welding process.

710 Chapter 30/Welding Processes

E1C30 11/11/2009 16:12:29 Page 711

during the welding process (vaporization is the principal mechanism), analogous to thegradual wearing of a cutting tool in a machining operation. For AW processes that utilizenonconsumable electrodes, any filler metal used in the operation must be supplied bymeans of a separate wire that is fed into the weld pool.

Arc Shielding At the high temperatures in arc welding, the metals being joined arechemically reactive tooxygen,nitrogen,andhydrogen in theair.Themechanicalpropertiesoftheweld joint canbe seriouslydegradedby these reactions.Thus, somemeans to shield thearcfromthesurroundingair isprovided innearlyallAWprocesses.Arc shielding isaccomplishedby covering the electrode tip, arc, andmoltenweld pool with a blanket of gas or flux, or both,which inhibit exposure of the weld metal to air.

Common shielding gases include argon and helium, both of which are inert. In thewelding of ferrous metals with certain AWprocesses, oxygen and carbon dioxide are used,usually in combinationwithArand/orHe, toproduceanoxidizing atmosphereor to controlweld shape.

A flux is a substance used to prevent the formation of oxides and other unwantedcontaminants, or to dissolve them and facilitate removal. During welding, the flux meltsand becomes a liquid slag, covering the operation and protecting the molten weld metal.The slag hardens upon cooling andmust be removed later by chipping or brushing. Flux isusually formulated to serve several additional functions: (1) provide a protectiveatmosphere for welding, (2) stabilize the arc, and (3) reduce spattering.

The method of flux application differs for each process. The delivery techniquesinclude (1) pouring granular flux onto the welding operation, (2) using a stick electrodecoatedwith fluxmaterial inwhich the coatingmelts duringwelding to cover the operation,and (3) using tubular electrodes in which flux is contained in the core and released as theelectrode is consumed. These techniques are discussed further in our descriptions of theindividual AW processes.

Power Source in Arc Welding Both direct current (DC) and alternating current (AC)are used in arc welding. ACmachines are less expensive to purchase and operate, but aregenerally restricted to welding of ferrous metals. DC equipment can be used on all metalswith good results and is generally noted for better arc control.

In all arc-weldingprocesses, power todrive theoperation is theproduct of the currentI passing through the arc and the voltageE across it. This power is converted into heat, butnot all of the heat is transferred to the surface of the work. Convection, conduction,radiation, and spatter account for losses that reduce theamountofusableheat.Theeffect ofthe losses is expressed by the heat transfer factor f1 (Section 29.3). Some representativevalues of f1 for several AW processes are given in Table 30.1. Heat transfer factors are

TABLE 30.1 Heat transfer factors for severalarc-welding processes.

Arc-Welding ProcessaTypical Heat

Transfer Factor f1

Shielded metal arc welding 0.9Gas metal arc welding 0.9Flux-cored arc welding 0.9Submerged arc welding 0.95Gas tungsten arc welding 0.7

Compiled from [5].aThe arc-welding processes are described in Sections 30.1.2 and30.1.3.

Section 30.1/Arc Welding 711

E1C30 11/11/2009 16:12:29 Page 712

greater for AW processes that use consumable electrodes because most of the heatconsumed in melting the electrode is subsequently transferred to the work as moltenmetal. The process with the lowest f1 value in Table 30.1 is gas tungsten arc welding, whichuses a nonconsumable electrode. Melting factor f2 (Section 29.3) further reduces theavailable heat for welding. The resulting power balance in arc welding is defined by

RHw ¼ f 1f 2IE ¼ UmAwv ð30:2Þ

whereE¼ voltage, V; I¼ current,A; and the other terms were defined in Section 29.3. Theunits of RHw are watts (current multiplied by voltage), which equal J/sec. This can beconverted to Btu/sec by recalling that 1 Btu ¼ 1055 J, and thus 1 Btu/sec ¼ 1055 watts.

Example 30.1Power in ArcWelding

Agas tungsten arc-welding operation is performed at a current of 300A and voltage of 20V.The melting factor f2 ¼ 0.5, and the unit melting energy for the metal Um ¼ 10 J/mm3.Determine (a) power in the operation, (b) rate of heat generation at theweld, and (c) volumerate of metal welded.

Solution: (a) The power in this arc-welding operation is

P ¼ IE ¼ 300Að Þ 20 Vð Þ ¼ 6000W(b) FromTable 30.1, the heat transfer factor f1¼ 0.7. The rate of heat used for welding isgiven by

RHw ¼ f 1f 2IE ¼ 0:7ð Þ 0:5ð Þ 6000ð Þ ¼ 2100W ¼ 2100 J/s(c) The volume rate of metal welded is

RVW ¼ 2100 J/sð Þ= 10 J/mm3� � ¼ 210mm3/s n

30.1.2 AW PROCESSES—CONSUMABLE ELECTRODES

A number of important arc-welding processes use consumable electrodes. These arediscussed in this section. Symbols for the welding processes are those used by theAmerican Welding Society.

Shielded Metal Arc Welding Shielded metal arc welding (SMAW) is an AW processthat uses a consumable electrode consisting of a filler metal rod coated with chemicals thatprovide flux and shielding. The process is illustrated in Figures 30.2 and 30.3. The weldingstick (SMAWis sometimes called stickwelding) is typically 225 to450mm(9–18 in) longand2.5 to 9.5 mm (3/32–3/8 in) in diameter. The filler metal used in the rodmust be compatiblewith the metal to be welded, the composition usually being very close to that of the basemetal. The coating consists of powdered cellulose (i.e., cotton and wood powders) mixedwith oxides, carbonates, and other ingredients, held together by a silicate binder. Metalpowders are also sometimes included in the coating to increase the amount of filler metaland to add alloying elements. The heat of the welding processmelts the coating to provide aprotective atmosphere and slag for the welding operation. It also helps to stabilize the arcand regulate the rate at which the electrode melts.

During operation the bare metal end of the welding stick (opposite the welding tip)is clamped in an electrode holder that is connected to the power source. The holder has aninsulated handle so that it can be held and manipulated by a human welder. Currentstypically used in SMAW range between 30 and 300 A at voltages from 15 to 45 V.Selection of the proper power parameters depends on the metals being welded, electrodetype and length, and depth of weld penetration required. Power supply, connectingcables, and electrode holder can be bought for a few thousand dollars.


E1C30 11/11/2009 16:12:29 Page 713

Shielded metal arc welding is usually performed manually. Common applicationsinclude construction, pipelines, machinery structures, shipbuilding, job shop fabrication,and repair work. It is preferred over oxyfuel welding for thicker sections—above 5 mm(3/16 in)—because of its higher power density. The equipment is portable and low cost,making SMAWhighly versatile and probably the most widely used of the AW processes.Base metals include steels, stainless steels, cast irons, and certain nonferrous alloys. It isnot used or seldom used for aluminum and its alloys, copper alloys, and titanium.

A disadvantage of shielded metal arc welding as a production operation is the useof the consumable electrode stick. As the sticks are used up, they must periodically bechanged. This reduces the arc time with this welding process. Another limitation is thecurrent level that can be used. Because the electrode length varies during the operationand this length affects the resistance heating of the electrode, current levels must bemaintained within a safe range or the coating will overheat and melt prematurely whenstarting a newwelding stick. Some of the other AWprocesses overcome the limitations ofwelding stick length in SMAW by using a continuously fed wire electrode.

GasMetal ArcWelding Gasmetal arc welding (GMAW) is anAWprocess inwhich theelectrode is a consumablebaremetalwire, and shielding is accomplishedby flooding thearc

FIGURE 30.2 Shieldedmetal arc welding (stick

welding) performed by a(human) welder. (Photocourtesy of Hobart

Brothers, Troy, Ohio.)

FIGURE 30.3 Shieldedmetal arc welding(SMAW).

Consumable electrode

Electrode coating

Molten weld metalBase metal

Protective gasfrom electrode

coatingSolidifiedweld metal

Slag

Direction of travel


E1C30 11/11/2009 16:12:29 Page 714

with a gas. The bare wire is fed continuously and automatically from a spool through thewelding gun, as illustrated in Figure 30.4. A welding gun is shown in Figure 30.5. Wirediameters ranging from 0.8 to 6.5 mm (1/32–1/4 in) are used inGMAW, the size dependingon the thickness of the parts being joined and the desired deposition rate. Gases used forshielding include inert gases such as argon and helium, and active gases such as carbondioxide. Selection of gases (and mixtures of gases) depends on the metal being welded, aswell as other factors. Inert gases are used for welding aluminum alloys and stainless steels,

FIGURE 30.4 Gas metalarc welding (GMAW).

Shielding gas

Solidified weld metal

Direction of travel


Shielding gas

Nozzle

Electrode wire

Feed from spool

FIGURE 30.5 Welding gun for gas metal arc welding. (Courtesy of Lincoln Electric Company,Cleveland, Ohio.)


E1C30 11/11/2009 16:12:29 Page 715

while CO2 is commonly used for welding low and medium carbon steels. The combinationofbare electrodewire and shielding gases eliminates the slag coveringon theweldbeadandthusprecludes theneed formanual grindingandcleaningof the slag.TheGMAWprocess istherefore ideal for making multiple welding passes on the same joint.

The various metals on which GMAW is used and the variations of the process itselfhave given rise to a variety of names for gas metal arc welding. When the process was firstintroduced in the late 1940s, it was applied to the welding of aluminum using inert gas(argon) for arc shielding. The name applied to this process was MIG welding (for metalinert gas welding). When the same welding process was applied to steel, it was found thatinert gases were expensive and CO2 was used as a substitute. Hence the termCO2weldingwas applied. Refinements in GMAW for steel welding have led to the use of gas mixtures,including CO2 and argon, and even oxygen and argon.

GMAW is widely used in fabrication operations in factories for welding a variety offerrous and nonferrous metals. Because it uses continuous weld wire rather than weldingsticks, it has a significant advantage over SMAW in terms of arc time when performedmanually. For the same reason, it also lends itself to automation of arcwelding. The electrodestubs remaining after stick welding also wastes filler metal, so the utilization of electrodematerial is higher with GMAW. Other features of GMAW include elimination of slagremoval (since no flux is used), higher deposition rates than SMAW, and good versatility.

Flux-Cored Arc Welding This arc-welding process was developed in the early 1950s asan adaptationof shieldedmetal arcwelding to overcome the limitations imposed by the useof stick electrodes. Flux-cored arc welding (FCAW) is an arc-welding process in which theelectrode is a continuous consumable tubing that contains flux and other ingredients in itscore. Other ingredients may include deoxidizers and alloying elements. The tubular flux-cored ‘‘wire’’ is flexible andcan thereforebe supplied in the formof coils tobe continuouslyfed through the arc-welding gun. There are two versions of FCAW: (1) self-shielded and(2) gas shielded. In the first version of FCAW to be developed, arc shielding was providedby a flux core, thus leading to the name self-shielded flux-cored arc welding. The core inthis form of FCAW includes not only fluxes but also ingredients that generate shieldinggases for protecting thearc.The second versionofFCAW,developedprimarily forweldingsteels, obtains arc shielding from externally supplied gases, similar to gas metal arcwelding. This version is called gas-shielded flux-cored arc welding. Because it utilizes anelectrode containing its own flux together with separate shielding gases, it might beconsidered a hybrid of SMAW and GMAW. Shielding gases typically employed arecarbon dioxide for mild steels or mixtures of argon and carbon dioxide for stainlesssteels. Figure 30.6 illustrates the FCAW process, with the gas (optional) distinguishingbetween the two types.

FIGURE 30.6 Flux-cored arc welding. The

presence or absence ofexternally suppliedshielding gas

distinguishes the twotypes: (1) self-shielded, inwhich the core provides

the ingredients for shield-ing;and (2)gasshielded, inwhich external shielding

gases are supplied.

Shielding gas

Direction of travel

Shielding gas (optional)

Arc

Nozzle (optional)

Guide tube

Slag

Tubular electrode wire

Flux core

Feed from spool




E1C30 11/11/2009 16:12:29 Page 716

FCAWhas advantages similar toGMAW,due to continuous feeding of the electrode.It is used primarily for welding steels and stainless steels over a wide stock thickness range.It is noted for its capability to produce very-high-quality weld joints that are smooth anduniform.

Electrogas Welding Electrogas welding (EGW) is anAWprocess that uses a continuousconsumable electrode (either flux-cored wire or bare wire with externally supplied shieldinggases) and molding shoes to contain the molten metal. The process is primarily applied tovertical butt welding, as pictured in Figure 30.7. When the flux-cored electrode wire isemployed, no external gases are supplied, and the process can be considered a specialapplication of self-shielded FCAW. When a bare electrode wire is used with shielding gasesfromanexternal source, it is consideredaspecial caseofGMAW.Themoldingshoesarewatercooledtoprevent theirbeingaddedtotheweldpool.Togetherwiththeedgesof thepartsbeingwelded, theshoes formacontainer,almost likeamoldcavity, intowhichthemoltenmetal fromtheelectrodeandbaseparts is graduallyadded.Theprocess isperformedautomatically,withamoving weld head traveling vertically upward to fill the cavity in a single pass.

Principal applications of electrogas welding are steels (low- and medium-carbon,low-alloy, and certain stainless steels) in the construction of large storage tanks and inshipbuilding. Stock thicknesses from 12 to 75 mm (0.5–3.0 in) are within the capacity ofEGW. In addition to butt welding, it can also be used for fillet and groove welds, always ina vertical orientation. Specially designedmolding shoesmust sometimes be fabricated forthe joint shapes involved.

Submerged Arc Welding This process, developed during the 1930s, was one of the firstAW processes to be automated. Submerged arc welding (SAW) is an arc-welding processthat uses a continuous, consumable bare wire electrode, and arc shielding is provided by acover of granular flux. The electrode wire is fed automatically from a coil into the arc. Theflux is introduced into the joint slightly ahead of the weld arc by gravity from a hopper, asshown in Figure 30.8. The blanket of granular flux completely submerges the weldingoperation, preventing sparks, spatter, and radiation that are so hazardous in other AWprocesses. Thus, the welding operator in SAW need not wear the somewhat cumbersomeface shield required in the other operations (safety glasses and protective gloves, of course,are required). The portion of the flux closest to the arc is melted, mixing with the moltenweldmetal to remove impurities and then solidifyingon topof theweld joint to formaglass-like slag. The slag and unfused flux granules on top provide good protection from theatmosphere and good thermal insulation for the weld area, resulting in relatively slowcooling and a high-quality weld joint, noted for toughness and ductility. As depicted in our

FIGURE 30.7Electrogas welding using

flux-cored electrode wire:(a) frontviewwithmoldingshoe removed for clarity,

and (b) side view showingmolding shoes on bothsides.


E1C30 11/11/2009 16:12:30 Page 717

sketch, the unfused flux remaining after welding can be recovered and reused. The solidslag covering the weld must be chipped away, usually by manual means.

Submerged arc welding is widely used in steel fabrication for structural shapes (e.g.,welded I-beams); longitudinal and circumferential seams for large diameter pipes, tanks,and pressure vessels; and welded components for heavy machinery. In these kinds ofapplications, steel plates of 25-mm (1.0-in) thickness and heavier are routinely welded bythis process. Low-carbon, low-alloy, and stainless steels can be readily welded by SAW;but not high-carbon steels, tool steels, andmost nonferrous metals. Because of the gravityfeed of the granular flux, the parts must always be in a horizontal orientation, and abackup plate is often required beneath the joint during the welding operation.

30.1.3 AW PROCESSES—NONCONSUMABLE ELECTRODES

TheAWprocesses discussed above use consumable electrodes. Gas tungsten arc welding,plasma arc welding, and several other processes use nonconsumable electrodes.

Gas Tungsten Arc Welding Gas tungsten arc welding (GTAW) is an AW process thatuses a nonconsumable tungsten electrode and an inert gas for arc shielding. The term TIGwelding (tungsten inert gas welding) is often applied to this process (in Europe, WIGwelding is the term—the chemical symbol for tungsten is W, for Wolfram). GTAW can beimplemented with or without a filler metal. Figure 30.9 illustrates the latter case. When afiller metal is used, it is added to theweld pool from a separate rod or wire, beingmelted bytheheat of the arc rather than transferred across the arc as in the consumable electrodeAWprocesses. Tungsten is a good electrode material due to its high melting point of 3410�C(6170�F). Typical shielding gases include argon, helium, or amixture of these gas elements.

GTAWisapplicable tonearly allmetals inawide rangeof stock thicknesses. It canalsobe used for joining various combinations of dissimilarmetals. Itsmost common applications

FIGURE 30.9 Gastungsten arc welding

(GTAW).

Shielding gas

Gas nozzle

Electrode tip


Direction of travel


Shielding gas

Tungsten electrode(nonconsumable)

FIGURE 30.8Submerged arc welding(SAW).

Consumableelectrode

Blanket ofgranular flux

Vacuum system forrecovery of granular flux

Slag (solidified flux)


Molten weld metalMolten flux

Base metal

Direction of travel

Granular fluxfrom hopper


E1C30 11/11/2009 16:12:30 Page 718

are for aluminum and stainless steel. Cast irons, wrought irons, and of course tungsten aredifficult to weld by GTAW. In steel welding applications, GTAW is generally slower andmore costly than the consumable electrode AW processes, except when thin sections areinvolved and very-high-quality welds are required. When thin sheets are TIG welded toclose tolerances, fillermetal is usually not added. Theprocess can beperformedmanually orby machine and automated methods for all joint types. Advantages of GTAW in theapplications towhich it is suited includehigh-qualitywelds, noweld spatter because no fillermetal is transferred across the arc, and little or no postweld cleaning because no flux is used.

Plasma Arc Welding Plasma arc welding (PAW) is a special form of gas tungsten arcwelding in which a constricted plasma arc is directed at the weld area. In PAW, a tungstenelectrode is contained in a specially designed nozzle that focuses a high-velocity stream ofinert gas (e.g., argon or argon–hydrogenmixtures) into the region of the arc to form a high-velocity, intensely hot plasma arc stream, as in Figure 30.10. Argon, argon–hydrogen, andhelium are also used as the arc-shielding gases.

Temperatures in plasma arc welding reach 17,000�C (30,000�F) or greater, hot enoughto melt any known metal. The reason why temperatures are so high in PAW (significantlyhigher than those in GTAW) derives from the constriction of the arc. Although the typicalpower levels used inPAWarebelow thoseused inGTAW, thepower is highly concentrated toproduce a plasma jet of small diameter and very high power density.

Plasma arc welding was introduced around 1960 but was slow to catch on. In recentyears its use is increasing as a substitute for GTAW in applications such as automobilesubassemblies, metal cabinets, door and window frames, and home appliances. Owing tothe special features of PAW, its advantages in these applications include good arc stability,better penetration control thanmost otherAWprocesses, high travel speeds, and excellentweld quality. The process can be used to weld almost any metal, including tungsten.Difficult-to-weld metals with PAWinclude bronze, cast irons, lead, andmagnesium. Otherlimitations include high equipment cost and larger torch size than other AW operations,which tends to restrict access in some joint configurations.

Other Arc-Welding and Related Processes The precedingAWprocesses are themostimportant commercially. There are several others that should be mentioned, which arespecial cases or variations of the principal AW processes.

Carbon arc welding (CAW) is an arc-welding process in which a nonconsumablecarbon (graphite) electrode is used. It has historical importance because it was the firstarc-welding process to be developed, but its commercial importance today is practicallynil. The carbon arc process is used as a heat source for brazing and for repairing ironcastings. It can also be used in some applications for depositing wear-resistant materialson surfaces. Graphite electrodes for welding have been largely superseded by tungsten(in GTAW and PAW).

FIGURE 30.10 Plasmaarc welding (PAW).

Plasma gas

Shielding gas

Shielding gas


Molten weld metal

Base metal

Plasma stream

Tungsten electrode

Direction of travel


E1C30 11/11/2009 16:12:30 Page 719

Stud welding (SW) is a specialized AW process for joining studs or similar compo-nents to base parts. A typical SWoperation is illustrated in Figure 30.11, in which shieldingis obtained by the use of a ceramic ferrule. To begin with, the stud is chucked in a specialweld gun that automatically controls the timing andpower parameters of the steps shown inthe sequence. Theworkermust only position the gun at theproper location against the baseworkpart to which the stud will be attached and pull the trigger. SW applications includethreaded fasteners for attachinghandles to cookware, heat radiation fins onmachinery, andsimilar assembly situations. In high-production operations, stud welding usually hasadvantages over rivets, manually arc-welded attachments, and drilled and tapped holes.

30.2 RESISTANCE WELDING

Resistance welding (RW) is a group of fusion-welding processes that uses a combination ofheat and pressure to accomplish coalescence, the heat being generated by electricalresistance to current flow at the junction to be welded. The principal components inresistance welding are shown in Figure 30.12 for a resistance spot-welding operation, themost widely used process in the group. The components include workparts to be welded

FIGURE 30.11 Stud arc welding (SW): (1) stud is positioned; (2) current flows from the gun, and stud is pulledfrom base to establish arc and create a molten pool; (3) stud is plunged into molten pool; and (4) ceramic ferrule is

removed after solidification.

FIGURE 30.12Resistance welding (RW),showing the componentsin spot welding, thepredominant process in

the RW group.

Section 30.2/Resistance Welding 719

E1C30 11/11/2009 16:12:30 Page 720

(usually sheet metal parts), two opposing electrodes, a means of applying pressure tosqueeze the parts between the electrodes, and anACpower supply fromwhich a controlledcurrent can be applied. The operation results in a fused zone between the two parts, called aweld nugget in spot welding.

By comparison to arc welding, resistance welding uses no shielding gases, flux, orfiller metal; and the electrodes that conduct electrical power to the process are non-consumable. RW is classified as fusion welding because the applied heat almost alwayscauses melting of the faying surfaces. However, there are exceptions. Some weldingoperations based on resistance heating use temperatures below the melting points of thebase metals, so fusion does not occur.

30.2.1 POWER SOURCE IN RESISTANCE WELDING

The heat energy supplied to the welding operation depends on current flow, resistance ofthe circuit, and length of time the current is applied. This can be expressed by the equation

H ¼ I2Rt ð30:3Þ

where H ¼ heat generated, J (to convert to Btu divide by 1055); I ¼ current, A; R ¼electrical resistance, V; and t ¼ time, s.

The current used in resistance welding operations is very high (5000 to 20,000 A,typically), althoughvoltage is relatively low(usuallybelow10V).Theduration tof the currentis short in most processes, perhaps lasting 0.1 to 0.4 s in a typical spot-welding operation.

The reason why such a high current is used in RW is because (1) the squared termin Eq. (30.3) amplifies the effect of current, and (2) the resistance is very low (around0.0001 V). Resistance in the welding circuit is the sum of (1) resistance of the electrodes,(2) resistances of theworkparts, (3) contact resistances between electrodes andworkparts,and (4) contact resistance of the faying surfaces. Thus, heat is generated in all of theseregions of electrical resistance. The ideal situation is for the faying surfaces to be the largestresistance in the sum, since this is the desired location of the weld. The resistance of theelectrodes is minimized by using metals with very low resistivities, such as copper. Also, theelectrodes are oftenwater cooled to dissipate the heat that is generated there. Theworkpartresistances are a function of the resistivities of the basemetals and the part thicknesses. Thecontact resistances between the electrodes and the parts are determinedby the contact areas(i.e., size and shape of the electrode) and the condition of the surfaces (e.g., cleanliness of thework surfaces and scale on the electrode). Finally, the resistance at the faying surfacesdepends on surface finish, cleanliness, contact area, and pressure. Nopaint, oil, dirt, or othercontaminants should be present to separate the contacting surfaces.

Example 30.2ResistanceWelding

A resistance spot-welding operation is performed on two pieces of 1.5-mm-thick sheetsteel using 12,000 A for a 0.20 s duration. The electrodes are 6 mm in diameter at thecontacting surfaces. Resistance is assumed to be 0.0001 V, and the resulting weld nuggetis 6 mm in diameter and 2.5 mm thick. The unit melting energy for the metalUm¼ 12.0 J/mm3. What portion of the heat generated was used to form the weld nugget, and whatportion was dissipated into the work metal, electrodes, and surrounding air?

Solution: The heat generated in the operation is given by Eq. (30.3) as.

H ¼ 12; 000ð Þ2 0:0001ð Þ 0:2ð Þ ¼ 2880 JThe volume of the weld nugget (assumed disc-shaped) is

v ¼ 2:5p 6ð Þ2

470:7 mm3:


E1C30 11/11/2009 16:12:30 Page 721

The heat required to melt this volume of metal isHw¼ 70.7(12.0)¼ 848 J. The remainingheat, 2880� 848¼ 2032 J (70.6% of the total), is lost into the work metal, electrodes, andsurrounding air. In effect, this loss represents the combined effect of the heat transferfactor f1 and the melting factor f2 (Section 29.3). n

Success in resistance welding depends on pressure as well as heat. The principalfunctions of pressure in RW are to (1) force contact between the electrodes and theworkparts and between the two work surfaces prior to applying current, and (2) press thefaying surfaces together to accomplish coalescence when the proper welding temperaturehas been reached.

General advantages of resistance welding include (1) no filler metal is required,(2) high production rates are possible, (3) lends itself to mechanization and automation,(4) operator skill level is lower than that required for arc welding, and (5) goodrepeatability and reliability. Drawbacks are (1) equipment cost is high—usuallymuch higher than most arc-welding operations, and (2) types of joints that can bewelded are limited to lap joints for most RW processes.

30.2.2 RESISTANCE-WELDING PROCESSES

The resistance-welding processes of most commercial importance are spot, seam, andprojection welding. These processes are illustrated in our Video Clip on welding.

VIDEO CLIP

Welding: View the segment titled Resistance Welding.

Resistance SpotWelding Resistance spot welding is by far the predominant process inthis group. It is widely used in mass production of automobiles, appliances, metalfurniture, and other products made of sheet metal. If one considers that a typical carbody has approximately 10,000 individual spot welds, and that the annual production ofautomobiles throughout the world is measured in tens of millions of units, the economicimportance of resistance spot welding can be appreciated.

Resistance spot welding (RSW) is an RW process in which fusion of the fayingsurfaces of a lap joint is achieved at one location by opposing electrodes. The process isused to join sheet-metal parts of thickness 3 mm (0.125 in) or less, using a series of spotwelds, in situations where an airtight assembly is not required. The size and shape of theweld spot is determined by the electrode tip, the most common electrode shape beinground, but hexagonal, square, and other shapes are also used. The resulting weld nugget istypically 5 to 10 mm (0.2–0.4 in) in diameter, with a heat-affected zone extending slightlybeyond the nugget into the base metals. If the weld is made properly, its strength will becomparable to that of the surroundingmetal. The steps in a spotwelding cycle are depictedin Figure 30.13.

Materials used forRSWelectrodes consist of twomaingroups: (1) copper-based alloysand (2) refractory metal compositions such as copper and tungsten combinations. Thesecond group is noted for superior wear resistance. As in most manufacturing processes,the tooling in spot welding gradually wears out as it is used. Whenever practical, theelectrodes are designed with internal passageways for water cooling.

Because of itswidespread industrial use, variousmachines andmethods are availableto perform spot-welding operations. The equipment includes rocker-arm and press-typespot-welding machines, and portable spot-welding guns. Rocker-arm spot welders, shownin Figure 30.14, have a stationary lower electrode and a movable upper electrode that canbe raised and lowered for loading and unloading thework. The upper electrode ismounted

Section 30.2/Resistance Welding 721

E1C30 11/11/2009 16:12:30 Page 722

on a rocker arm (hence the name) whosemovement is controlled by a foot pedal operatedby the worker. Modern machines can be programmed to control force and current duringthe weld cycle.

Press-type spot welders are intended for larger work. The upper electrode has astraight-line motion provided by a vertical press that is pneumatically or hydraulicallypowered. The press action permits larger forces to be applied, and the controls usuallypermit programming of complex weld cycles.

The previous twomachine types are both stationary spot welders, in which the workis brought to the machine. For large, heavy work it is difficult to move and position thepart into stationarymachines. For these cases, portable spot-welding guns are available in

FIGURE 30.13 (a) Stepsin a spot-welding cycle,and (b) plot of squeezingforce and current during

cycle. The sequence is:(1) parts insertedbetweenopen electrodes, (2) elec-

trodes close and force isapplied, (3) weld time—current is switched on,

(4) current is turnedoffbutforce is maintained or in-creased (a reduced cur-rent is sometimes applied

near the end of this stepfor stress relief in theweldregion), and (5) electrodes

are opened, and thewelded assembly isremoved.

FIGURE 30.14 Rocker-arm spot-weldingmachine.


E1C30 11/11/2009 16:12:30 Page 723

various sizes and configurations. These devices consist of two opposing electrodescontained in a pincer mechanism. Each unit is lightw

Documents

Part VIII Joining anddantn/kalpakjian/pdf/part-VIII.pdf · OF WELDING Chapter Contents 29.1 Overview of Welding Technology 29.1.1 Types of Welding Processes 29.1.2 Welding as a Commercial