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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:
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� 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
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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.
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� 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
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(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
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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.
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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.
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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.
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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
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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
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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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
Section 30.1/Arc Welding 713
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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
Molten weld metalBase metal
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.)
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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
Solidified weld metal
Molten weld metalBase metal
Section 30.1/Arc Welding 715
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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.
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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
Solidified weld metal
Direction of travel
Molten weld metalBase metal
Shielding gas
Tungsten electrode(nonconsumable)
FIGURE 30.8Submerged arc welding(SAW).
Consumableelectrode
Blanket ofgranular flux
Vacuum system forrecovery of granular flux
Slag (solidified flux)
Solidified weld metal
Molten weld metalMolten flux
Base metal
Direction of travel
Granular fluxfrom hopper
Section 30.1/Arc Welding 717
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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
Solidified weld metal
Molten weld metal
Base metal
Plasma stream
Tungsten electrode
Direction of travel
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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
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(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:
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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
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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.
722 Chapter 30/Welding Processes
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various sizes and configurations. These devices consist of two
opposing electrodescontained in a pincer mechanism. Each unit is
lightw