kalpakjian 18

CHAPTER

Fusion-Welding

Processes

_ _ _ _ _ _ _ _ 30.I lntroduction 865

° This chapterdescribesfusion-welding processes, in whichtwopieces arejoined 39,2 Qxyf,,e|_g,s

together by the application of heat, which melts and fuses the interface; the Welding 866

operation issometimes assisted witha filler metal. 3°-3 A'°'W°|di“8P'°°°SS°S=

H f , ld, ____ _ h, _ _ _ Nonconsumable

° A usion-we ing processes are iscussed in t is chapter, beginning with Hectrode 86,

oxyfuel-gas welding, in which acetylene and oxygen provide the energy for 36,4 An;-weldingpmesses;

Welding, Consumable

'

V ld. h d .b d _ h. h I . 1 Electrode ₈₇₃

arious arc-we ing processes are t en escri e , in W ic eectrica energy 305 E|ect_°desf°rA_c

and consumable or nonconsumable electrodes are used to produce the weld; Wdding 879

specific processesexamined include shielded metal arc welding, flux-cored arc 30.6 Electron-beam

Welding, gas tungsten-arc welding, submerged arc welding, and gas metal-arc Welding 88°

Weldin 30.7 Laser-beamWelding 880

g- _

_ _ 30.8 Cutting 882

U Weldingwith high-energy beams is then discussed, in which electron beamsor _30.9 The_Weldjoint, Quality,

lasers provide highly focused heat sources. and Testing 884

_ _ _ _ _ . 3o.|o

'ro'

dP

6 The chapter _{ends witha discussionof the weldjoint, including quality, inspec- glllilctiglgnagg mess}

tion, and testing procedures, alongwith adiscussionofgood weld design prac- EXAMPLE*

tices and process selection. `

30.| WeldingSpeed for DifferentMaterials 87| 30.2 LaserWeldingof

Razor Blades 88|

30.1

Introduction

=°-= WeldDesig"

The welding processes described in this chapter involve the partial melting and

fusion between two members to be joined. Here, fusion welding is defined as

melting togetherandcoalescing materials bymeans ofheat. Fillermetals, which are

metals added to the weld area duringwelding, also may beused. Fusionwelds made withoutthe use of fillermetals are known asautogenous welds.

The chapter describes the major classes of fusion-welding processes. It covers the basic principles of each process; the equipment used; the relative advantages, limitations, and capabilities of the process; and the economic considerations affect-ing process selection (Table

30.l).

These processes includethe oxyfuel-gas, arc, and high-energy-beam (laser-beam and electron-beam) welding processes, which have important and unique applicationsin modern manufacturing.

Thechapter continues with a descriptionofweld-zone features andthe variety

of discontinuities and defects that can exist in welded joints. The weldability of

Selection 896

866 Chapter30 Fusion-WeldingProcesses

TABLE 30.|

GeneralCharacteristics ufFusion-weldingProcesses

Typical

joining Skill level Welding Current costof

process Operation Advantage required position type Distortion* equipment (S)

Shielded Manual Portable High All AC,DC 1 to2 Low (1500-I-)

metal arc and flexible

Submerged Automatic High Low to Flat and AC,DC 1 to2 Medium (50004-)

arc deposition medium horizontal

Gas metal arc Semiautomatic Mostmetals Low to All DC 2to3 Medium (3000+)

or automatic high

Gas tungsten Manual or Mostmetals Low to All AC,DC 2to3 Medium (50004-)

arc automatic high

Flux-coredarc Semiautomatic High Low to All DC 1 to3 Medium (2000+)

or automatic deposition high

Oxyfuel Manual Portable High All

-

2to4 Low(500-+-)

and flexible

Electron beam, Semiautomatic Mostmetals Medium All

-

3 to5 High

(100,000-laser beam or automatic to high 1million)

°*1= highest;5 lowest.

various ferrous and nonferrous metals and alloys are then reviewed. The chapter

ends with a discussion of design guidelines for welding, giving several examples of

good weld-design practices. As in all manufacturing processes, the economics of

welding is a significant aspect of the overall operation. Welding processes, equip-ment, andlabor costs are discussed inSection 31.8.

30.2

Oxyfuel-gas Welding

Oxyfuel-gas welding (OFW) is ageneral term used to describe any welding process

that uses a fuel gas combined with oxygen to produce a flame. The flame is the

source ofthe heat thatis usedto meltthe metals atthe joint. The most common

gas-welding process uses acetylene; the process is known as ox;/acetylene-gas welding

(OAW) and is typicallyused for structural metal fabrication and repair work. Developed inthe early 1900s, OAW utilizesthe heat generated bythe combus-tion of acetylene gas (CZHZ) in a mixturewith oxygen. The heat is generated in

ac-cordance with a pair ofchemical reactions. Theprimary combustionprocess, which

occurs inthe inner core of theflame (Fig. 30.1), involvesthe following reaction: CZHZ +

oz

->

zco

+ H2 + Heat. <30.1>

This reaction dissociates the acetylene intocarbon monoxide and hydrogen and

pro-duces about one-third of the total heat generated in the flame. The secondary

com-bustionprocess is

zco

+ H2 + 1.502

->

zcoz

+ H20 + Heat. (30.2)

This reaction consists of the further burning of both the hydrogen and the carbon monoxide and produces about two-thirds ofthetotalheat. Note thatthereactionalso

Section30.2 Oxyfuel gas Welding 867

0 O Outerenvelope Acetylene

21030

li()

C (small and narrow) feather

Innercone Outer Innercone Bright lumlnous Blue

3040to 3300°C envelope (pointed) innercone envelope

(a) Neutralflame (b) Oxidizing flame (C) Carburizing (reduclng)flame

~» Gas mixture

/If;

I `

Filler rod ` _gg If ," We|din9 torch

Molten t~,a »

weldmetal 'teff

il*

Flaw?

(d)

FIGURE 30.I Three basic types of _{oxyacetylene flames used} in oxyfuel-gas welding and

cutting operations: (a)neutral flame; (b) oxidizing flame; (c) carburizing, or reducing,flame. Thegas mixturein (a) is basicallyequal volumes ofoxygen and acetylene. (d)Theprinciple of

the oxyfuel-gas weldingprocess.

Flame Types. The proportion of acetylene and oxygen in the gas mixture is an

important factor in oxyfuel-gas welding. At a ratio of 1:1 (i.e., when there is no excess oxygen), the flame is considered to be neutral (Fig. 30.1a). With a greater oxygen supply, the flame can be harmful (especially for steels), because itoxidizes

the metal. For this reason, a flame with excess oxygen is known as an oxidizing flame (Fig. 3O.1b). Only in the welding of copper and copper-based alloys is an

oxidizing flame desirable, because in those cases, a thin protective layer of slag

(compounds of oxides) forms over the molten metal. If the oxygen is insufficient for full combustion, the flameis known asareducing,or carburizing, flame (a flame

having excess acetylene; Fig. 30.1c). The temperature of areducing flame is lower;

hence, such aflame is suitable for applications requiring lowheat, such as brazing, soldering, and flame-hardening operations.

Other fuel gases (such as hydrogen and methylacetylene propadiene) also can be used inoxyfuel-gas welding. However, thetemperaturesdeveloped by thesegases are lower than those produced by acetylene. Hence, they are used for welding (a) metals with low melting points (such as lead) and (b) parts that are thin and

small. The flame with pure hydrogen gas is colorless; therefore, it is difficult to adjust the flame by eyesight.

Filler Metals. Filler metals are used to supply additional metal to the weld zone during welding. They are available as filler rods or wire (Fig. 3O.1d) and may be bare or coated with flux. The purpose of the flux is to retard oxidation of the sur-faces ofthe _partsbeing weldedby generating agaseous shield aroundthe weldzone.

The flux also helps to dissolve and remove oxides and other substances from the weld zone, thus contributing to the formation of a stronger joint. The slag

devel-oped (compounds of oxides, fluxes, and electrode-coating materials) protects the

8 8 Chapter30 Fusion-Welding Processes

Welding Practice and Equipment. Oxyfuel-gas welding can be used with most

ferrous and nonferrous metals for almostany workpiece thickness, butthe relatively

lowheat inputlimits the process to thicknessesof lessthan 6 mm _

Small joints made by thisprocess may consist of a single-weld bead. Deep-V groove joints are made inmultiplepasses. Cleaning the surface of each weld bead prior to depositinga second layer is important for joint strength and in avoiding defects (see Section 30.9). Wire brushes (hand or power) may be used for this purpose.

The equipmentfor oxyfuel-gas welding consists basically of a welding torch connected byhoses to high-pressure gas cylinders and equipped withpressuregages

andregulators (Fig.

302).

Theuse ofsafetyequipment (such as goggles withshaded

lenses, face shields, gloves, andprotective clothing) is essential. Proper connection of the hoses to the cylinders is an important factor in safety. Oxygen and acetylene

cylinders have different threads, so the hoses cannot be connected to the wrong cylinders. The low equipment costis an attractive feature of oxyfuel-gas welding. Although it can be mechanized, this operation is essentially manual and, hence,

Valves Mixer Tip

... ...

I

£3 i1¢'1§'§'if1z ~~¢1=ff;

Enlargedview 1

(H)

Oxygen

:§

i

Mixing chamber

HN ,i~ .n _.¢v._ W1 wi

nn V T* ,, f J‘v‘¢fy¢v%-%-‘~‘v5v‘vfv5~‘v‘,T T

Torch head

“

`

it,

Oxygen Unionnut Mixer Tip

(D)

Gas regulators

as

V

Hoses

Gas controlvalves

Van:

"7

_ Weldingtorch

Oxygen cylinder

/

Weldingtip

Combustible-gas cylinder

`

Flame

(C)

FIGURE 30.2 (a) General view of, and (b) cross section of, a torch used in oxyacetylene

welding. The acetylene valve isopenedfirst. Thegas is litwitha spark lighteror a pilot light.

Then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in

oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left

handed, whereas those foroxygen are right handed. Oxygen regulators usually are painted

Section 30.3 Arc-welding Processes: Nonconsumable Electrode C2H2 + O2mixture Torch Flameheating = _Ofsurfaces if Clam 'si p ’=<ff2@1'

4.

¥t..~ ; g»»=g;k§=,,em:; (H) (D)

FIGURE 30.3 Schematic illustration of the pressure-gas welding process: (a) before and (b) after. Notethe formation of a flashat the joint; later theflash can betrimmed off.

slow. However, it has the advantages of being portable, versatile, and economical for simple and low-quantity work.

Pressure-gas Welding. Inthis method, the welding oftwo components starts with

the heating of the interface by means ofatorch using(typically) anoxyacetylene-gas mixture (Fig. 30.3a). After the interface begins to melt, the torch is withdrawn. A

force is applied to pressthe two components together (Fig. 30.3b) and is maintained

untilthe interface solidifies.Note the formationof aflash dueto the upsetting ofthe joined ends of the two components.

30.3

Arc-welding Processes:

Nonconsumable

Electrode

In arc welding, developed in the mid-18005, the heat required is obtained from

electrical energy. The process involves either a consumable or a nonconsumable electrode. An AC or a DC power supply produces an arc between the tip of the elec-trode and the workpiece to be welded. The arc generates temperatures of about 30,000°C, which are much higher thanthose developed in oxyfuel-gas welding.

In nonconsurnable-electrode welding processes, the electrode is typically a

tungsten electrode (Fig. 30.4). Because of the high temperatures involved, an exter-nally supplied shielding gas is necessary to prevent oxidation of the weld zone. Typically, directcurrent is used, andits polarity (the direction of current flow) is

im-portant. The selection of current levels depends on suchfactors as the type of

elec-trode, metals tobe welded, and depth and widthofthe weld zone.

In straight

polarity-also

knownas direct-current electrode negative

(DCEN)-the workpiece is positive (anode), andthe electrode is negative (cathode). DCEN gen-erally produces welds thatare narrow anddeep (Fig. 30.5a). Inreverse

polarity-also

known as direct-current electrode positive (DCEP)-theworkpiece isnegative andthe

electrode is positive. Weld penetration is less, and the weld zone is shallower and

wider (Fig. 30.5b). Hence, DCEP is preferred forsheet metals andfor joints with very wide gaps. ln theAC current method, the arc pulsatesrapidly. This method is suitable for welding thick sections and for using large-diameter electrodes at maximum currents (Fig. 30.5c).

Torch withdrawn

Upsetting force

870 Chapter30 Fusion-WeldingProcesses

Foot pedal (optional)

DC(+) (H) DC

(-)

(D) AC (C)

FIGURE 30.5 The effect of

polarity and current type on

weld beads: (a) DC current

with straight polarity; (b) DC

currentwithreverse polarity;

(c) ACcurrent. Travel if If 5 Electrical conductor Tungsten electrode Gas passage

f l

V

if;

Shielding gas

Filler wire

\ §¥&`

Arc

rn..

_~»

“`

_Sldfd

._

ld

tl

Molten weld metal

_

“'°'°‘

“""'“'

0|

He We

mea

(H) inert-gas AC or DC su ppV Weider Cooling-water sup Torch Filler rod Workpiece (bl

FIGURE 30.4 (a) The gas tungsten-arc welding process, formerly known as TIG (for

tungsten-inert-gas) welding. (b) Equipment forgastungsten-arc welding operations.

Heat Transferin ArcWelding. The heatinputinarcWeldingis givenbythe equation

H VI

-

=

e-,

(30.3)

I 1/

Where His the heat input (] or BTU), Iis theWeld length, V is the voltage applied, I is

the current (amperes), and 1/ is the welding speed. The term eis the efficiency of the process and varies from around 75% forshielded metal-arc welding to 90% for gas

metal-arc welding and submerged-arc Welding. The efficiencyis an indication thatnot

all of the available energy is _{beneficially used to melt material,} because the heat is

conducted through the workpiece, someis lost by radiation, and still more is lost by

convectionto thesurrounding environment.

The heat inputgiven by Eq. (30.3) melts acertainvolume ofmaterial, usually

the electrodeor fillermetal, and can also beexpressedas

H = uV,,, = uAI, (30.4)

where u is the specific energy required for melting, V," is the volume of material melted, and A is the cross section of the Weld. Some typical values of u are given in

Table 30.2. Equations (30.3) and (30.4) allow an expressionof the welding speed: VI

1/ =

eg.

(30.5)

Although these equations have been developed forarc Welding, similar ones can be

obtained for other fusion-welding operations asWell, taking into accountdifferences

Section30.3 Arc-welding Processes: NonconsumableElectrode

TABLE 30.2

Approximate SpecificEnergiesRequiredto Melta

UnitVolumeufCommonlyWelded Metals

Specific energy,u

Material ]/mm3

Aluminum and itsalloys 2.9

Cast irons 7.8 Copper 6.1 Bronze (90Cu-10511) 4.2 Magnesium 2.9 Nickel 9.8 Steels 9.1-10.3 Stainless steels 9.3-9.6 Titanium 14.3

EXAMPLE 30.l Welding Speed forDifferentMaterials

Consider a situation inwhich a welding operation is Therefore, from _Eq. (30.5) being performed with V == 20 volts,

I

= 200 A, and

the _{cross-sectional area} of the weld bead is 30 mrnz. _ _Yi __ 0 75 (20l(200l 34 5 mf

Estimate the welding speed if theworkpiece andelec- V T euA T l ' ) (2.9)(30) m S

trode aremade of (a) aluminum, (b) carbonsteel,and

(c) titanium.Use anefficiencyof75%. Similarly, for carbon steel, u is estimated as

9.7 ]/mm3 (average of extreme values in the table)

Snlution For aluminum, we note from Table 30.2 leading to 1/ = 10.3 mm/s For titanium u 14₃

that the specific energy required is u = 2.9 ]/mm3. ]/mrn3,so that 1/ = 7.0mm/s

Gas Tungsten-arc Welding. In gas tungsten-are welding (GTAW), formerly

knownas TIG (for “tungsten inert gas”) welding, the filler metal is supplied from

a filler wire (Fig. 30.4a). Because the tungsten electrodeis not consumed in this operation, a constant and stable arcgap is maintained at a constant current level.

The filler metals are similar tothe metals to be welded, and flux is not used. The

shielding gas is usually argon _{or helium (or} a mixture of the two). Welding with

GTAWmay be done withoutfiller

metals-for

example, inthe weldingof close-fit joints.

Dependingon the metals tobe welded, thepower supplyis eitherDC at200 A or

ACat500 A (Fig.30.4b). Ingeneral,AC ispreferred for aluminum and magnesium,

be-cause the cleaningactionofACremovesoxides andimproves weld quality. Thorium or

zirconium may be used inthe tungsten electrodes to improve their electron emission

characteristics. The powersupply rangesfrom 8to20_{kW Contamination}ofthe tung-sten electrode by themolten metal can be a significantproblem, particularly incritical

applications, because itcancause discontinuities inthe weld.Therefore, contactofthe

electrode withthe _{molten-metal pool should} be avoided.

The GTAWprocess is used for a wide variety of applications and metals, par-ticularly aluminum, magnesium, titanium, and the refractory metals. It is especially

suitable for thin metals. The cost of the inert gas makes this process more expensive

2 Chapter 30 Fuslon-Welding Processes

used in avariety ofcritical applications witha wide range ofworkpiece thicknesses and shapes. Theequipment is portable.

Plasma-arc Welding. In plasma-arc welding (PAW), developed in the 1960s, a

concentrated plasma arc is producedand directed towards the weld area.The arc is

stable andreaches temperaturesas high as 33,000°C. A plasmais an ionized hot gas

composed of nearly equal numbers of electrons and ions.The plasma is initiated

be-tween the tungsten electrode and the orifice by a low-currentpilot arc. What makes plasma-arc welding unlike otherprocesses is that the plasma arc is concentrated be-cause it is forced through a relatively small orifice. Operating currents usually are

below 100 A, butthey can be higher for special applications. When a filler metal is

used, it is fed into the arc, as is done in GTAW. Arcand weld-zone shielding is sup-plied bymeans of an outer-shieldingring and the use of gases suchas argon,helium,

or mixtures.

There are two methods of plasma-arc welding:

° In the transferred-arc method(Fig. 30.6a), the workpiece being welded is part oftheelectrical circuit. The arctransfers fromthe electrode tothe

workpiece-hence the term transferred.

° In the nontransferred method (Fig. 30.6b), the arc occurs between the

elec-trode and the nozzle, and the heat is carried to the workpiece by the plasma

gas. This thermal-transfer mechanism is similar to that for an oxyfuel flame (see Section 30.2).

Compared with other arc-welding processes, plasma-arc welding has better

arc stability,less thermal distortion, and higher energy concentration, thus permit-ting deeper and narrower welds. In addition, higher welding speeds, from 120 to

1000 mm/min, can be achieved. A variety of metals can be weldedwith part

thick-nesses generally less than6 mm.

The high heat concentration can penetrate completely through the joint

(known as the keyholetechnique),with thicknessesas much as 20 mm for some

ti-tanium and aluminum alloys. In the keyhole technique, the force of the plasma arc

displaces the molten metal and produces a hole at the leadingedge of the weld pool. Plasma-arc welding (rather than the GTAW process) often is used for butt and lap

joints because of its higher energy concentration, better arc stability, and higher welding speeds. Proper training and skill are essential for operators who use this

Tungsten electrode 9 ig Plasmagas 1° ... Shielding gas gg ,. l=¢wa .;::v=‘I!§ V.:fel I

(3) (U)

FIGURE 30.6 Two types ofplasma-arc welding processes: (a) transferredand (b)

Section30.4 Arc-welding Processes:

equipment. Safety considerations includeprotectionagainst glare, spatter, andnoise

from the plasma arc.

Atomic-hydrogen Welding. In atomic-hydrogen welding (AHW), an arc is

gener-atedbetweentwo tungsten electrodesin ashielding atmosphereofhydrogengas.The

arc is maintained independentlyofthe workpiece or partsbeing welded. The

hydro-gen gasnormallyis diatomic (HZ),but wherethe temperaturesare over6,000°C near the arc, the hydrogen breaks down into its atomic form, simultaneously absorbing a

large amount ofheat fromthe arc.Whenthehydrogen strikesthe cold_surface ofthe

workpieces tobejoined, it recombines intoits diatomic form and rapidly releases the

stored heat. The energyin AHW can be varied easily by changing the distance

be-tween the arc stream and the workpiece surface. This process is being replaced by

shielded metal-arc welding, mainly because of the availability of inexpensive inert

gases.

30.4

_{Arc-welding Processes:}

_Consumable

_Electrode

Thereare severalconsumable-electrode arc-welding processes.

30.4.l

Shielded Metal-arc Welding

Shielded metal-arc welding(SMAW) is one ofthe oldest, simplest, and most versatile joining processes. About 50% ofall _{industrial and maintenance welding currently} is

performed by this process. The electric arc is generated by touching the tip of a

coatedelectrode against theworkpiece andwithdrawing it quickly toa distance

suf-ficientto maintain the arc (Fig. 30.7a). The electrodes are inthe shapes ofthin, long rods (hence,this process also is known as stick welding) that are held manually.

The heat generated melts a portion of the electrode tip, its coating, and the basemetal inthe immediatearc area. The molten metal consists of amixture of the basemetal (the workpiece), the electrode metal, and substances fromthe coatingon the electrode; this mixture forms the weld when it solidifies. The electrode coating deoxidizes the weld area and provides a shielding gas to protect it from oxygen in

the environment.

A bare section at the end of the electrode is clamped to one terminal of the

power source, while the other terminal is connected to the workpiece being welded

(Fig.30.7b). The current, whichmay be DC or AC, usually ranges from 50 to 300 A.

Welding machineAC or DC

Consumable Electrode

power source and controls SolidifiedS189

Work AVC Elect d

pg cable |q0|der;D e x

\\\

lp;/l /

Coating

-

E|e<;trode

y,

_/.

_Electrode

iff /

/

V

S workpiece iiii‘\ er .. Sgéeldmg

12; ._ .. ~1~11if1f

L

Electrode

Cable Weld metal Arc

FIGURE 30.7 Schematic illustration ofthe shielded metal-arc weldingprocess. About50%

874 Chapter30 Fusion-Welding Processes

FIGURE 30.8 A deep weld

showing thebuildup sequence

ofeight individual weld beads.

For sheet-metal welding,DC is preferred because ofthe steadyarc itproduces. Power requirements generally are less than10 kW.

The SMAW process has the advantages of being relatively simple, versatile, and requiring a smaller variety of electrodes. The equipmentconsists of a power

supply,cables, and an electrode holder. The SMAW process commonly is used in

general construction, shipbuilding, pipelines, and maintenance work. Itis especially useful for work in remote areas where a portable fuel-powered generator can be used as the power supply. SMAW is best suited for workpiece thicknesses of 3 to

19mm, although this range can beextended easily by skilled operatorsusing multi-ple-pass techniques (Fig. 30.8).

The multiple-pass approach requires that the slag be removed after each weld

bead. Unless removed completely, the solidified slag can cause severe corrosion of the weld area and lead to failure ofthe weld, but italso prevents the fusion of weld

layers and, therefore, compromises the weld strength. Before another weld is

applied, the slag should be removed

completely-for

example, by wire brushing or weld chipping. Consequently, both labor costs and material costs are high.

30.4.2

Submerged-arc Welding

Insubmerged-arc welding (SAW), the weld arc is shielded by agranular flux consist-ingof lime, silica, manganese oxide, calciumfluoride, and other compounds.The flux is fed into the weld zone from ahopper bygravityflow through a nozzle (Fig. 30.9). The thick layer of flux completely covers themolten metal. It prevents spatter and sparks and suppressesthe intense ultraviolet radiationandfumes characteristic ofthe SMAWprocess. The flux also acts as a thermalinsulator by promotingdeep penetra-tion of heat into the workpiece.The unused flux can be recovered (using arecovery tube), treated, andreused.

The consumable electrode is acoil ofbare round wire 1.5 to 10 mmin

diame-ter; itis fed automatically through a tube (welding gun). Electric currents typically range from 300 to 2000 A. The power supplies usually are connected to standard

single- or three-phase power lines with a primaryrating up to 440V

Because theflux is gravity fed, the SAW processis limited largely towelds in a

flat or horizontal position having a backup piece. Circular welds can be made on pipes and

cylinders-provided

that they are rotated during welding. As Fig. 30.9 shows, the unfused flux can be recovered, treated, and reused. SAW is automated

Electrode-wire reel

____.,T

Voltage and

i current control

Ul1fUSeCl-f|UX 2

recoverytube _ yl..

Wire-feed motor

Electrode cable

Wolkplece ipgp s/V

`

Voltage-pickup

Weld backing leads (optional)

Grciind

FIGURE 30.9 Schematic illustrationof the submerged-arc welding process and equipment.

Section30.4 Arc-welding Processes: Consumable Electrode 8

and is used to weld a variety of carbon and alloy steel and stainless-steel sheets or plates at speeds ashigh as 5 m/min. Thequality ofthe Weld is very

high-With

good

toughness, ductility, and uniformity of properties. The SAW process provides very high welding productivity, depositing 4 to 10 times the amount of Weld metal per houras the SMAWprocess. Typical applications includethick-plateWelding for

ship-buildingand for pressure vessels.

30.4.3

Gas

Metal-arc

Welding

Ingas metal-arc welding (GMAW), developedin the 19505 and formerly called metal

inert-gas (MIG) welding, the Weld area is shielded byan effectivelyinert atmosphere

of argon, helium, carbon dioxide, or various other gas mixtures (Fig. 30.10a). The consumable bare Wire is fed automatically through a nozzle into the Weld arc by a

Wire-feeddrive motor (Fig.30.10b). In additionto usinginert shielding gases,

deoxi-dizers usually arepresent inthe electrode metal itselfin order to prevent oxidationof themolten-weld puddle. Multiple-weld layers can be depositedat the joint.

Metal can betransferred by three methods inthe GMAW process:

l. In spray transfer, small, molten metal droplets from the electrode are trans-ferred to the Weld area at a rate of several hundred droplets per second. The

Solid wireelectrode 4 I Shieminggas

Current conductor

wif: Travel

Nozzle

Wire guideand

Shieldinggas

i

contact tube

Arc g; solidified weld meiai

i. Molten weld metal

fl=’ = 5 rr

,.

5 I

in

,lf

“~

I\ pryr N (H) Feed control

Control system A Wife

Gas our

Gun control _{Gas in §}

Gun Shielding-gas source

Workpiece I'

ii ,..,__ ,I..

‘N

E

U

Voltage control

Wire-feed

1-::

drive motor

f

‘~»iir1i»lr Welding machine

Contactorcontrol Power supply

(bl

FIGURE 30.I0 (a) Schematic illustration of the gas metal-arc Welding process, formerly

known as MIG (for metal inert-gas) Welding. (b) Basic equipment used in gas metal-arc

876 Chapter30 Fusion-Welding Processes

Arc shield composed of

vaporizedand slag-forming

compounds protectsmetal

transfer through arc

Solidifiedslag

Molten slag

Solidified weld metal

transfer is spatter free and very stable. High DC currents and voltages and large-diameter electrodes are used with argon or an argon-rich gas mixture as

the shielding gas. The average current required inthis process can be reduced withthe use ofa pulsed arc, which superimposes high-amplitude pulses onto a

low, steady current. The process can be used in all welding positions.

In globular transfer, carbon-dioxide-rich gases are utilized, and globules are

propelled by the forces of the electric-arc transfer of the metal, resulting in

considerable spatter. High Welding currents are used, making it possible for greaterWeld penetration and higher welding speed thanare achieved inspray transfer. Heavier sections commonly arejoined by this method.

2.

In short circuiting, the metal is transferred in individual droplets (more than

50 per second) as the electrode tip touches the molten Weld metal and

short-circuits. Lowcurrents and voltages are utilized with carbon-dioxide-rich gases

and electrodes madeofsmall-diameterWire. The power requiredis about2 kW 3.

The temperatures generated in GMAW are relatively low; consequently, this

method is suitable only for thin sheets and sections of less than 6 mm; otherwise

in-complete fusion may occur. The operation, which is easy to perform, is commonly used for welding ferrous metals inthin sections. Pulsed-arc systemsare used for thin ferrous and nonferrous metals.

The GMAWprocess is suitableforWeldingmost ferrous and nonferrous metals and is used extensively in the metal-fabrication industry. Because of the relatively

simplenature ofthe process, thetraining ofoperatorsis easy. Theprocess is versatile,

rapid, and economical, and Welding productivity is double that of the SMAW

process. The GMAW process can be automated easily and lends itself readily to robotics and toflexible manufacturing systems (see Chapters37 and 39).

30.4.4

Flux-cored

Arc

Welding

The flux-cored arc welding (FCAW) process, illustrated in Fig. 30.11, is similar to

gasmetal-arc Welding, exceptthat the electrode is tubular inshape and is filledwith

Current-carrying guide tube insulated extension tip

YE?-,_

Powdered metal, vapor- or

gas-forming materials, deoxidizers and scavengers

$

Arc

Base metal

Metal dropletscoveredwith thin slag coating forming

Molten molten puddle

weld metal

FIGURE 30.1I Schematic illustration ofthe flux-cored arc-welding process. Thisoperation

Section 30.4 Arc-welding Processes:

flux (hence the term flux-cored). Cored electrodes produce a more stable arc,

im-proveweldcontour, and produce better mechanical properties oftheweld metal. The

flux inthese electrodes is much more flexible than the brittlecoatingused on SMAW

electrodes, so the tubularelectrode can beprovided inlongcoiled lengths.

The electrodes are usually 0.5 to 4 mm in diameter, and the power required is

about 20 kW. Self~shielded cored electrodes also are available. They do _{not require}

anyexternal shielding gas, because they contain emissive fluxes thatshield the weld area against the _surroundingatmosphere. Small-diameter electrodes have made the

welding of thinner materials not only possible, but often preferable. Also,

small-di-ameter electrodes make itrelativelyeasy to weldparts in different positions, and the fluxchemistry permits thewelding ofmany metals.

TheFCAWprocess combines the versatility of SMAWwith the continuous and automaticelectrode-feeding featureof GMAWThe process is economicaland versa-tile,so itis used for welding a varietyof joints, mainlyon steels, stainless steels, and

nickel alloys. The higher weld-metal deposition rateofthe FCAWprocess (compared with that ofGMAW) has led toitsuse in thejoiningofsections of allthicknesses.The

use of tubular electrodes with very small diameters has extended the use of this process to workpieces ofsmaller sectionsize.

A major advantage of FCAW is the ease with which specific weld-metal

chemistries can bedeveloped. By adding alloying elements tothe flux core, virtually

any alloy composition canbeproduced. The process is easyto automateandis

read-ily adaptable toflexible manufacturing systems and robotics.

30.4.5

Electrogas

Welding

Electrogas welding (EGW) is used primarily for welding the edges of sections verti-callyand in onepass with the pieces placed edgeto edge (butt joint). It is classified as

a machine-welding process, because it requires special equipment (Fig. 3012). The weld metal is deposited into a weldcavity between the two pieces to be joined. The

spaceis enclosed by twowater-cooled copper dams (shoes)to preventthe moltenslag

from running off; mechanical drives move the shoes upward. Circumferential welds (such as thoseon pipes) also are possible, with the workpiece rotating.

Drive rolls Electrode conduit Welding wire Gas

4->

Oscillator Welding gun

gas-

Gas box

Gas Water out

->

Waterout

Welding wire 3#P=?g€m€maVY

Waterin Waterin S le mggas

Fixedshoe Moveable shoe

Weld metal

il'

Primaryshielding gas

FIGURE 30.l2 Schematic illustration ofthe electrogas-welding process.

Chapter30 Fusion-Welding Processes

Single or multiple electrodes are fed througha conduit,and acontinuousarc is

maintained by flux-cored electrodes at up to 750 A or solid electrodes at 400 A.

Power requirements are about 20 kW Shielding is done by means of an inert gas, such ascarbon dioxide, argon, or

helium-depending

on the type of material being Welded. The gas may be providedeither from an external source, from a flux-cored electrode, or from both.

The equipment for electrogas Welding is reliable and training for operators is

relatively simple. Weld thickness ranges from 12to 75 mm on steels, titanium, and aluminum alloys. Typical applications are in the construction of bridges, pressure

vessels, thick-Walled andlarge-diameter pipes, storage tanks, and ships.

30.4.6

Electroslag

Welding

Electroslag welding (ESW) and its applications are similar to electrogas welding

(Fig. 30.13). The main difference is that the arc is started betweenthe electrode tip

andthe bottom of the parttobe vvelded. Fluxis added, which then meltsbythe heat

of the arc. After the molten slag reaches the tip of the electrode, the arc is

extin-guished. Heat is produced continuously by the electrical resistance of the molten

slag. Because the arc is extinguished, ESW is not strictly an arc-welding process. Single or multiple solid asWellas flux-cored electrodesmay be used. Theguide may

be nonconsumable(conventional method) or consumable.

Electroslag Welding is capable of Welding plates with thicknesses ranging from 50mm to morethan 900 mm, andWeldingis doneinone pass. Thecurrent requiredis

about600 Aat40to50\L although higher currentsare usedforthick plates. The trav-el speed ofthe weld is intherange from12to 36mm/min.Weld qualityis good. This

process is used for largestructural-steel sections,such as heavy machinery,bridges, oil rigs,ships,and nuclear-reactorvessels.

Power source Control panel

r §3i "' S agar?

~

Electrode lead X.; *Oscillation (optional) Consumable guide tube work

5*

Workpiece

(QVOUHG) lead

,

if

4-

Water_in

\Water

out FIGURE 30.|3 Equipmentusedfor electroslag-Welding operations.

Wirereel

Wire-feeddrive

Molten slag

Molten weldpool

Section30.5 Electrodes forArcWeld|ng 87

30.5

Electrodes

for Arc

Welding

Electrodes for consumable arc-welding processes are classified according to the

following properties:

° Strength ofthe deposited weld metal

° Current (Acor Dc)

° Type of coating.

Electrodes are identified by numbers and letters (Table

_30.3)-or

by color code if the numbers and letters are too small to imprint. Typical coated-electrode

dimensions are in the range from 150 to 460 mm in length and 1.5 to 8 mm in

diameter.

Specifications for electrodes and filler metals (including dimensional toler-ances, quality control procedures, and processes) are published by the _American

Welding Society (AWS) and the American National Standards Institute (ANSI).

Some specifications appear in the AerospaceMaterials Specifications (AMS) by the Society of Automotive Engineers (SAE). Electrodes are soldbyweight and are

avail-able in a wide variety of sizes and specifications. Criteria for selection and recom-mendations for electrodes for a particular metal and its application can be found in

supplier literature and in the _{various handbooks and references listed} atthe end of this chapter.

TABLE 30.3

Designations forMild-steel Coated Electrodes

The prefix “E” designates arc welding electrode.

The first twodigits of four-digitnumbersand the first three digits offive-digitnumbers

indicate minimum tensile strength:

E60XX 60,000 psi

E70XX 70,000

E110XX 110,000

Thenext-to-last digit indicatesposition:

EXX1X Allpositions

EXXZX Flatposition and horizontalfillets

Thelast two digitstogetherindicate thetype ofcovering and the current tobe used.

The suffix (Example: EXXXX-A1) indicates theapproximate alloy in theweld deposit:

-A1

0.5% Mo

-B1

0.5% Cr, 0.5% Mo

-B2

1.25% Cr, 0.5% Mo

-B3

2.25% Cr, 1% M0

-B4

2% Cr, 0.5% M0

-B5

0.5%Cr, 1% Mo

-C1

2.5% Ni

-C2

3.25%Ni

-C3

1% Ni, 0.35%Mo, 0.15%Cr

-D1

and D2 0.25-0.45%Mo, 1.75% Mn

-G

0.5% min. Ni, 0.3% min. Cr, 0.2% min. Mo,

0.1% min.\L 1% min.Mn (only oneelementrequired)

8 0 Chapter30 Fus|on-Welding Processes

Electrode Coatings. Electrodesare coated withclaylike materials that include

sili-cate binders andpowdered materials,such as oxides,carbonates, fluorides, metal al-loys, cottoncellulose, and wood flour.The coating, which is brittle and takes part in

complex interactions during welding, has the following basic functions:

° Stabilize the arc.

° Generate gasesto actas a shield against the surrounding atmosphere; the gases

produced are carbon dioxide, water vapor, and small amounts of carbon

monoxide and hydrogen.

° Controlthe rate atwhich the electrode melts.

° Act asa flux to protect the weld againstthe formation of oxides, nitrides, and other inclusions and, with the resulting slag, to protect the molten-weld pool.

° Add alloying elements tothe weld zoneto enhancethe propertiesof the

joint-among these elements are deoxidizers to prevent the weld from becoming

brittle.

The deposited electrode coating or slag must be removed after each pass in

order to ensure a goodweld; a wire brush (manual or power) can be used for this

purpose. Bare electrodes and wires madeofstainlesssteelsand aluminum alloys also are available. They are used as filler metalsinvarious welding operations.

30.6

Electron-beam

Welding

In electron-beam welding (EBW), developed inthe 1960s, heat is generated by high-velocity narrow-beam electrons. The kineticenergy of the electrons is converted into

heat as they strike the workpiece. The process requires special equipment to focus

the beam on the workpiece, typicallyin avacuum. The higher the vacuum, the more

the beam penetrates, and the greater is the depth-to-width ratio; thus, the methods

are called EBW-HV (for “high vacuum”) and EBW-MV (for “medium vacuum”);

some materials also may also be welded byEBW-NV (for “no vacuum”).

Almostany metal canbewelded byEBW, and workpiece thicknesses can range from foil to plate. Capacities of electron guns range up to 100 kW The intense energy also is capable of producing holes in the workpiece (see keyhole technique,

Section 30.3). Generally, no shielding gas, flux, or fillermetal is required.

The EBW process has the capability of making high-quality welds that are

almost parallel sided, are deep and narrow, and have small heat-affected zones (see Section 30.9). Depth-to-width ratios range between 10 and 30. The sizes of welds made by EBW are much smaller than those of welds made by conventional processes. With the use of automation and servo controls, parameters can be con-trolled accuratelyat welding speeds ashigh as 12 m/min.

Almost any metal can be buttor lap welded with this process atthicknessesup

to 150 mm. Distortion and shrinkage in the weld area are minimal. The weld qualityis good and of very high purity. Typical applications includethe welding of

aircraft,missile, nuclear, and electronic components, as well as gears and shafts for

the automotive industry. Electron-beam weldingequipmentgenerates X-rays; hence, propermonitoring and periodic maintenance are essential.

30.7

Laser-beam

Welding

Laser-beam welding (LBW) utilizes a high-power laser beamas the source of heat, to produce a fusion weld. Because the beam can be focused onto a very small area,

it has highenergy densityanddeep-penetrating capabil-ity. The beam can bedirected, shaped, and focused pre-cisely on the workpiece. Consequently, this process is

suitable particularly for welding deep andnarrowjoints

(Fig. 30.14) with depth-to-width ratios typically

rang-ing from4 to 10.

Laser-beam weldinghas become extremely popular andis usedinmost industries. Inthe automotiveindustry,

welding transmission components are the most

wide-spread application. Among numerous other applications

is the welding of thin parts for electronic components.

The laser beam may be pulsed (in milliseconds) with power levels up to 100 kW for applications such as the

spot welding of thin materials. Continuous multi-kW

laser systems are used for deep weldson thick sections.

Laser-beam weldingproduces welds ofgood

qual-ity with minimum shrinkage or distortion. Laser welds

Section30.7 Laser-beam Welding 88|

(H) (D)

FIGURE 30.I4 Comparison of the sizes of weld beads:

(a) laser-beam or electron-beam welding and (b)

tungsten-arcwelding. Source:CourtesyofAmerican WeldingSociety.

have goodstrength and generally are ductile andfree of

porosity. The process can be automatedto be used on a

varietyof materials withthicknesses upto 25 mm; it is particularly effective onthin workpieces. As described in Section 16.2.2, tailor-welded sheet-metal blanks are

joined principally by laser-beam welding using robotics for precise control of the

beam path.

Typical metals and alloys welded include aluminum, titanium, ferrous metals,

copper, superalloys,and the refractory metals. Weldingspeeds range from2.5 m/min

toas highas 80 m/min for thin metals. Because of the natureof the process, welding can be done in otherwise inaccessible locations. As in other and similar automated welding systems, the _operatorskill required is minimal. Safety is particularly

impor-tantin laser-beam welding due to the extreme hazards to the eye as well as the skin;

solid-state (YAG) lasers also aredangerous. (See Table 27.2 on types of lasers.)

The major advantages of LBW overEBW are the following:

° A vacuumis not required, andthe beam can betransmitted throughair.

° Laser beams can be shaped, manipulated, and focused optically (by means of

fiber optics), so the process can beautomated easily.

° The beams do notgenerate X-rays.

° Thequalityofthe weld is better thanin EBW;the weld has lesstendencytoward incomplete fusion, spatter,andporosity; andthere is lessdistortion.

EXAMPLE 30.2 Laser WeldingofRazorBlades

A close-up of the Gillette Sensorm razor cartridge is

shown in Fig. 30.15. Each of the two narrow,

high-strength blades has 13 pinpoint

welds-11

ofwhich can be seen (as darker spots,about 0.5 mm in diame-ter) oneach bladeinthe photograph. Youcaninspect

the welds on actual bladeswitha magnifyingglass or

amicroscope.

The welds are made with an Nd:YAG laser

equipped with fiber-optic delivery. This equipment

provides very flexible beam manipulation and can

target exact locations along the length ofthe blade.

With a set ofthese machines, production is at a rate

of 3millionwelds per hour, withaccurate and consis-tentweld quality.

Source: Courtesyof LumonicsCorporation, Industrial Products Division.

2 Chapter30 Fusion-Welding Processes

FIGURE 30.15 Detail of Gillette Sensor”

razor cartridge, showinglaserspotwelds.

30.8

Cutting

In additionto being cut bymechanical means, material can be cut into various con-tours with the use of a heat source that melts and removes a narrow zone in the

workpiece. The sources of heat can betorches,electric arcs, or lasers.

Oxyfuel-gas Cutting. Ox;/fuel-gas cutting (OFC) is similar to oxyfuel welding,

but the heat source is nowused to remove anarrow zonefrom ametal plate or sheet

(Fig.30.16a). Thisprocessis suitable particularly for steels.The basic reactionswith

steel are

Fe + O

l>

FeO + Heat, (30.6) 3Fe -1- 202

->

Fe3O4 + Heat, (30.7)

and

4Fe + 302

->

2 Fe2O3 + Heat. (30.8)

The greatest heat isgenerated by the second reaction,anditcan producea

tem-perature rise to about 870°C. However, this temperature is not sufficiently high to cut steels; therefore, the workpiece is preheated with fuel gas, and oxygen is

intro-duced later (see the nozzle cross section in Fig.30.16a). The higher the carbon con-tent of the steel, the higher is the preheating temperature required. Cutting takes place mainlyby the oxidation (burning) of the steel; some melting also takes place.

Cast irons and steel castings alsocan be cut by this method. The process generates a

kerf similar to that produced by sawing with a saw blade or by wire

Section 30 8 Cuttlng 883 fg Torch Oxygen Preheat flames (oxyacetylene) Torch

l-Workpiece

Plate Drag lines

Kerr_,| Thlckness

i

Slag(ironand /ll _>| (__

rroh oxrde) Drag

_//(|_l»(

(3) (b)

FIGURE 30.l6 (a) Flame cutting of a steel plate withan oxyacetylene torch, and a cross

section ofthe torch nozzle.(b) Cross sectionof a flame-cut plate, showing draglines.

The maximum thickness that can be cut byOFC depends mainly on the gases

used. With oxyacetylene gas,the maximum thickness is about 300 mm; with oxyhy-drogen, it is about 600 mm. Kerf widths range from about 1.5 to 10 mm, with

rea-sonably good control of tolerances. The flame leaves drag lines on the cut surface

(Fig. 30.l6b), resulting in arougher surface than that producedby processessuch as

sawing, blanking, or other operations that use mechanical cutting tools. Distortion caused by uneventemperature distribution can be aproblem in OFC.

Although longused for salvage and repair work, OFC can beused in manufac-turingas _{well. Torches may be guided along}specified paths either manually,

mechan-ically, or automatically by machines using programmable controllers and robots.

Underwater cuttingis done with specially designedtorchesthat producea blanketof

compressedair between the flameand the surroundingwater.

ArcCutting. Arc-cutting processesare basedon the same principles as arc-welding processes.A variety of materials can becut athigh speeds by arc cutting. As in

weld-ing,these processesalso leaveaheat-affected zonethatneedstobetaken into account, particularlyincriticalapplications.

In aircarbon-arc cutting (CAC-A), a carbon electrode is used and the molten

metal is blown awayby a high-velocityair jet; thus,the metal being cut doesn’t have to oxidize. Thisprocessis used especially for gouging and scarfing (removal ofmetal from a surface). However, the process is noisy, and the molten metal can be blown substantial distances andcause safetyhazards.

Plasma-arc cutting (PAC) produces the highest temperatures. It is used forthe

rapid cutting of nonferrous and stainless-steel plates. The cutting productivity of this process is higher than that of oxyfuel-gas methods. PAC produces a good sur-face finish and narrow kerfs, and is the most _popular cutting process utilizing pro-grammable controllers employed in manufacturing today.

Electron beams andlasersalso are usedforveryaccuratelycuttingawidevariety

ofmetals,as was described inSections 27.6 and27.7. The surface finishis better than thatofotherthermal cutting processes, andthe kerfisnarrower.

® Q wh G D' so 'U Ph ra "1 ua C 71 C 2. o

?

5

_E: :l on 'o -\ o Q N U) U1 (U |-/I

30.9

The

Weld

joint,

Quality,

and

Testing

Three distinct zones can beidentified in atypical weldjoint, asshown in Fig. 30.171

I. Basemetal

2. Heat-affectedzone 3. Weldmetal.

The metallurgy and properties of the second andthird zones depend strongly on the type of metals joined, the particular joining process, the filler metals used (if any),

and welding process variables. A joint produced without a filler metal is called autogenous, and its weld zone is composed of the resolidified base metal. A joint made witha fillermetal has a centralzone called the weld metal and is composed of

a mixture ofthe base and the filler metals.

Solidification of

the

WeldMetal. After the application of heat and the introduc-tion ofthe filler metal (ifany)into the weld zone,the weld jointis allowed tocool to ambient temperature. The solidijqcation process is similar to that in casting and be-gins with the formation ofcolumnar (dendritic) grains. (See Fig. 10.3.) These grains

are relatively long and form parallel tothe heat flow. Becausemetals are much

bet-ter heat conductors than the surrounding air, the grains lieparallel to the plane of the twocomponentsbeing welded (Fig. 30.18a). Incontrast, the grains in a shallow weld are shown in Figs. 30.18b andc.

Grain structure and grain size depend on the specific metal alloy, the particu-lar welding process employed, and the type of filler metal. Because it began in a

molten state, the weld metal basically has a cast structure, and since it has cooled

slowly, ithas coarse grains. Consequently, this structure generally has low strength, toughness, and ductility. However, the proper selection of filler-metal composition

or of heat treatments following welding can improve the mechanical properties of the joint.

The resulting structure depends on the particular alloy, its composition, and

the thermalcyclingto which the jointis subjected. For example, cooling rates may be

controlled and reduced by pre/venting the general weld area prior to welding.

Original Fusion zone Heat-affected

structure (weld metal) zone

fa G o ><C 13| gm 'Lu lo (QL. c/:N sn 2_0 ma-39.

és

1% gs: QQQ o *'h N FY

E

E. ‘L *'h c 2. o

F

2

‘L Q. N o :s FD Tmpe ture O 2. Q. IJ Q I ___`

|

s

mga;

§

~" §Dcn§ Q

2

:"¢.(D.U :°Z

6

EéaéSgg’= mage

2

Em Q35

9

2

Qw 3‘°3 Q §'22.

2

OEO' 5; 9.23 Egfn 9.._.3 ESQ <1>‘°gi temperature of base metal

Preheating is important, particularly for metals hav-ing high thermal conductivity, such as aluminum and copper.Withoutpreheating, theheat produced during welding dissipatesrapidlythroughthe restoftheparts

being joined.

Heat-affected Zone. Theheat-affected zone (HAZ)

is within the base metal itself. It has a microstructure different from that ofthe base metal priorto welding, because ithas been temporarily subjected to elevated

temperatures during welding. The portions of the

base metal that are far enough away from the heat source do not undergo any microstructural changes during welding because of the far lower temperature to which they aresubjected.

The properties and microstructure of the HAZ depend on (a) the rate of heat input and cooling and (b) the temperature to which this zone was raised. In

addition to metallurgical factors (such as the original grain size, grain orientation, and degree ofprior cold

Section30.9 TheWeldjoint Quality and Testmg 88 (H) (b) (C) '< 1 mm >1 0.1 mm

-

Melt zone 0.43mm Heat-affected zone Z T (d)

FIGURE 30.18 Grain structure in (a) a deep weld and (b) a shallow weld. Note that the

grains in the solidified weld metal are perpendicular totheir interface with the base metal.

(c)Weld bead onacold-rolled nickel stripproducedby alaser beam. (d)Microhardness (HV)

profile across a weld bead.

work), physical properties (such as the specific heat and thermal conductivity of the metals) influence the size and characteristics of the HAZ.

The strength and hardness ofthe HAZ (Fig. 3O.18d) dependpartly onhow the

original strength and hardness ofthe base metal was developed priortothe welding.

As was described in Chapters 2 and 4, they may have beendeveloped by (a) cold

working, (b) solid-solution strengthening, (c) precipitation hardening, or (d) various heat treatments. The effects of these strengthening methods are complex, and the

simplest to analyze are those in a base metalthat has beencold worked,such as by cold rolling or cold forging.

The heat applied during welding recrystallizesthe elongated grains ofthe cold-worked base metal. Onthe onehand, grains that are away from the weld metal will

recrystallize into fine, equiaxed grains. Onthe other hand, grains close to the weld metal have been subjected to elevated temperatures for alonger time. Consequently,

the grains will grow in size (grain growth), and this region will be softer and have lower strength. Such ajoint will be weakestat itsHAZ.

The effects ofheat on theHAZ for joints made from dissimilar metals and for alloys strengthened by other methods are so complex as to be beyond the scope of this book. Details can befound in the moreadvanced references listedinthe bibliog-raphy at the end ofthis chapter.

30.9.l

Weld

Quality

As a resultof ahistoryofthermal cycling and itsattendant microstructuralchanges, a welded joint may develop various discontinuities. Welding discontinuities also can

886 Chapter30 Fusion-WeldingProcesses

or bypoor operator training. The majordiscontinuities that affect weld qualityare

described here.

Porosity. Porosity inwelds may becaused by

° Gases released during melting of the weld area, buttrappedduring solidification. ° Chemical reactions during welding.

° Contaminants.

Most welded joints contain some porosity, which is generally in the shape of spheres or ofelongated pockets. (See also Section 10.6.1.) The distribution of

poros-ityinthe weld zonemay be random, orthe porositymay beconcentrated in acertain region in the zone.

Porosity inwelds can be reduced bythe following practices:

° Proper selection of electrodes and fillermetals.

'

Improved welding techniques, such aspreheatingthe weld area or increasing

the rateof heat input.

° Proper cleaning and the prevention of contaminants from entering the weld zone.

° Reduced weldingspeeds to allow time for gasto escape.

Slaglnclusions. Sluginclusionsarecompoundssuchas oxides,fluxes,and

electrode-coating materials thatare trappedin the weld zone.Ifshieldinggasesarenoteffective

during welding, contamination fromthe environmentalso maycontribute tosuch

in-clusions. Welding conditions are important as well: Withcontrol of welding process

parameters, themoltenslag willfloat tothe surfaceofthemoltenweld metal and thus

willnot become entrapped.

Slaginclusions can beprevented bythe following practices:

° Cleaningthe weld-bead surfaceby means ofawire brush (hand or power) ora

chipper before the next layer is deposited. ° Providing sufficient shielding gas.

° Redesigning thejoint topermitsufficient space forproper manipulation ofthe

puddle of molten weld metal.

IncompleteFusionand Penetration. Incomplete fusionproduces poor weldbeads, such as those shown in Fig. 30.19. A better weld can be obtained bythe use of the

following practices:

° Raising the temperature of the base metal.

° Cleaning the weld area before welding.

let

Weld Incomplete Weld nigggn e

I

/I

We|d Bndgmg fUSIOI'\

Base ` ` " meta' Incompletefusion

(3) (D) (C)

Section 30.9 TheWeld joint. Quality, and Testing Underfilt Crack Inclusions BHS9 meta' Incomplete penetration (8) Good weld Overlap Undercut Porosity Lackof penetration (D) (C)

FIGURE 30.20 Examples of various defects in fusionWelds.

'

Modifyingthe joint design and changing the type ofelectrode used.

° Providing sufficient shielding gas.

Incompletepenetrationoccurswhen thedepth ofthewelded jointis insufficient.

Penetration can beimprovedbythe following practices:

° Increasing the heat input.

° Reducingthe travel speed during the Welding. ° Modifyingthe joint design.

° Ensuring that the surfaces to bejoined fiteach other properly.

Weld Profile. Weld profile is important not only because of its effects on the

strength and appearance of the Weld, but also because it can indicate incomplete

fusion or the presence of slaginclusionsinmultiple-layer welds.

° Underfilling resultsWhenthe jointis notfilled with theproper amount ofweld

metal (Fig. 30.20a).

° Undercuttingresults fromthemeltingawayofthe basemetal andtheconsequent generation ofagroove inthe shape ofasharp recess ornotch (Fig.3O.20b).If it

is deepor sharp,an undercut can actasastressraiser and can reducethe fatigue strengthofthejoint; insuch cases,itmay leadtopremature failure.

° Overlap is a surface discontinuity (Fig. 30.20b) usually causedby poor

Weld-ing practiceor by theselection ofimpropermaterials.A goodWeldis shownin

Fig.3o.20¢_

Cracks. Cracks may occur in various locations and directions in the Weld area. Typical types ofcracks are longitudinal, transverse, crater, underbead, and toecracks

(Fig. 3021).

Cracks generally result froma combination of the follovving factors:

° Temperature gradients that cause thermal stresses in the Weldzone.

° Variations in the composition of the Weld zone that cause different rates of

contractionduring cooling.

° Embrittlement of grain boundaries (Section 1.5.2), caused by the segregation

of such elements as sulfur to the grain boundaries and occurring when the

888 Chapter30 Fusion-WeldingProcesses W Id Toe Crack e

¢,

Transverse ` Crack Longitudinal ssssf Cracks Underbead "”" Cravk Weld Weld Transverse crack » >;§ Longitudinal Crack Base metal Base

meta'

l-

Toecrack

(H)

FIGURE30.21 Types ofcracks developedinweldedjoints. The cracks are caused bythermal

stresses, similar to thedevelopmentof hottears incastings, as shown in Fig. 10.12.

solidified.

cooling.

° Hydrogen _{embrittlement} (Section2.10.2).

° Inability of the weld metal to contract during cooling (Fig. 3022). This is a

situationsimilar to hottearsthat develop in castings (Fig 10.12) andis related

to excessive restraint ofthe workpiece during the welding operation.

Cracks also are classified as hot cracks, which occur while the joint is still at

elevated temperatures, and cold cracks, which develop after the weld metal has

The basic crack-prevention measures inwelding are the following:

° Modify the joint design to minimize stresses developed from shrinkage during ° Change the parameters, procedures, and sequence of the _welding operation.

° Preheat the components to be welded.

° Avoid rapid coolingof the weldedcomponents.

"GURE 3°-22 Crack in H Lamellar Tears. In describing the anisotropy of plastically deformed metals in

Weld bead- The “Ve Welded Section 1.5,itwas statedthat theworkpiece is weaker when testedinitsthickness di~ components were not

al-lowed to contract freelyafter

the weld was completed.

Source: Courtesy of Packer

Engineering.

rection because ofthe alignment ofnonmetallic impurities and inclusions (stringers).

This condition is particularly evident in rolled plates and in structural shapes. In

welding such _{components, lamellar tears} may develop because of shrinkage of the

restrainedcomponents ofthe structure during cooling. Such tears can beavoided by

providing for shrinkage ofthe members or by modifyingthe joint designto makethe weld bead penetratethe weakercomponent more deeply.

Surface Damage. Some ofthe metal may spatterduring welding and be deposited

as small droplets on adjacent surfaces. In arc-welding processes, the electrode inad-vertentlymay touch the partsbeing welded atplaces otherthanthe weld zone. (Such

encounters are called arc strikes.) The surface discontinuities thereby produced may be objectionable for reasons of appearance or of subsequent use of the weldedpart.

If severe, these discontinuities adversely may affect the properties of the welded structure, particularly for notch-sensitive metals. Using proper welding techniques and procedures is important inavoiding surface damage.

Section30.9 TheWeldjoint, Quality, and Testing 889

Weld

Weld

I

t 1 :

-

;

-Transverse shrinkage

:

E

_{Weld Neutramxis Wejd}

l

I

j

Angular distortion Lgagmjgglal

(H) (D) (C) ld)

FIGURE 30.23 Distortion ofparts after welding.Distortion iscausedby differentialthermal expansionand contractionof different regions ofthe weldedassembly.

ResidualStresses. Because of localized heating and cooling during

weld-ing, the expansion andcontractionof theweld areacauses residual stresses

inthe workpiece. (See also Section2.11.) Residualstresses can lead to the

following defects:

° Distortion, warping, and buckling of the welded parts (Fig. 30.23).

° Stress-corrosion cracking (Section 2.10.2).

° Furtherdistortionif a portionof the weldedstructureis subsequently

removed, such as bymachining or sawing.

° Reduced fatigue life ofthe welded structure.

The type and distribution of residual stresses in welds is described

best byreference toFig. 30.24a. Whentwoplates are being welded, along,

narrow zone is subjected to elevated temperatures, while the plates, as a

whole, are essentiallyat ambient temperature. After the weld is completed

and as time elapses, the heat from the weld zone dissipates laterally into

the plates,while the weld area cools. Thus, the plates begin to expand

lon-gitudinally, while thewelded length begins tocontract (Fig. 30.23).

Ifthe plate is not constrained, it will warp, as shown in Fig.30.23a. However, if the plate is not free to warp, it will develop residual stresses

that typically are distributed throughout the material, such as the stresses

shown in Fig. 30.24. Note thatthe magnitude of the compressive residual

stresses inthe plates diminishes tozero at apoint far away from the weld

area. Because noexternal forces are acting on thewelded plates,the tensile

and compressive forces representedby these residualstresses must balance

each other.

Events leading to the distortion of a welded structure are shown in Fig. 30.25_ Before welding, the structure is stress free, as shown in

Fig. 30.25a. The shape may be fairly rigid,and fixturing also may be pres-ent to support the structure. When the weld bead is placed, the molten

Base .

I

eld (H) Residual stress Compressive

_<--

Tensile (bl

FIGURE 30.24 Residual stresses

de-veloped in (a) a straight-butt joint.

Note that the residual stresses shown

in (b)must bebalanced internally. (See

also Fig. 2.29.)

metal fills the gap between the surfaces to be joined, and flows outward to form

the weld bead. At this point, the weld is not under any stress. Afterward, the weld

bead solidifies, and both the weld beadand the surrounding material cool to room temperature. As these materials cool, they tend to contract, but are constrained by

the bulk of the weldment. The result is that the weldmentdistorts (Fig. 30.25c) and residual stressesdevelop.

The residual-stress distribution shown in Fig. 30.25 places the weld and the

0 Chapter30 Fusron-Welding Processes

Hotzone

Rgid (expanded) No Snape C t t_

on rac |on

frame Men change gg

It

(pushed Out) V V' ema

{

ssss (residual)

1-~.

1

sss s tensile

, sss . ssss

,

.

stress

i if T

@

Distortion

(D) (C)

FIGURE 30.25 Distortion ofa welded structure. Source:After ].A. Schey.

Many welded structures will use cold-worked materials (such as extruded or

roll-formed shapes), and these are relatively strong and fatigue resistant. The weld itself may haveporosity (see Fig. 3O.20b), which can act as a stress riser and aid

fatigue-crack growth,or there could beothercracks that cangrow in fatigue. Ingeneral,the

HAZ is less fatigue resistant than the base metal. Thus, the residualstresses

devel-oped can be very harmful, and it is not unusual to further treat welds in highly stressed or fatigue-susceptible applications.

Incomplex weldedstructures,residual-stressdistributionsarethreedimensional and, consequently, difficult to analyze. The previous discussioninvolved twoplates thatwere not restrainedfrom movement. Inotherwords,the plates were notan

inte-gralpartofalargerstructure.If, however, they arerestrained, reaction stresses will be

generated, becausethe plates arenotfree to expand or contract. Thissituation arises particularly instructures with highstiffness.

Stress Relieving of Welds. The problems caused by residualstresses (such as

dis-tortion, buckling, and cracking) can be reducedby preheatingthe base metal or the

parts to be welded. Preheating reduces distortion by reducing the cooling rate and

the level of thermal stresses developed (by lowering the elastic modulus). This

tech-nique also reduces shrinkage and possiblecracking ofthe joint.

For optimum results, preheating temperaturesand cooling rates must be

con-trolled carefullyinorder tomaintain acceptable strength and toughness inthe welded structure. The workpieces may be heated inseveral ways, including (a) in a_furnace,

(b) electrically(resistively or inductively),or (c) byradiantlampsor ahot-air blastfor

thin sections. The temperature and time required for stress relieving depend on the type of material and on the magnitude ofthe residual stressesdeveloped.

Other methods of stress relieving include peening, hammering, or surface rolling ofthe weld-beadarea. These techniques induce compressive residualstresses,

which, inturn, lower or eliminate tensile residualstresses in the weld. Formultilayer welds, the first and last layersshould not bepeened,in order toprotectthem against possible peening damage.

Residual stresses can also be relieved or reducedby plastically deforming the

structure by a small amount. For instance, this technique can be used in welded pressurevessels by pressurizingthe vessels internally (proofstressing).Inorder to

re-ducethe possibilityof sudden fracture under high internal pressure, the weld must be made properly and must be free of notches and discontinuities, which could act

as pointsof stress concentration.

In addition to being preheated for stress relieving, welds may be heat treated

Section30.9 TheWeldjoint Quality and Testing

includethe annealing, normalizing, quenching, and tempering of steelsandthe

solu-tion treatment andaging ofvarious alloys asdescribed in Chapter4.

30.9.2

Weldability

The u/eldability of a metal is usually defined as its capacity to be Welded into a spe-cificstructure that has certainproperties and characteristics and will satisfactorily meet servicerequirements. Weldability involves a large number of variables; hence, generalizations are difficult. As noted previously, the material characteristics (such

as alloying elements, impurities, inclusions, grain structure, and processing history)

ofboth the basemetal andthe filler metalare important. Forexample,the Weldabil-ity of steels decreases with increasing carbon content because of martensite

forma-tion (vvhich is hard and brittle) and thus reducesthe strength of the Weld. Coated

steel sheets present various challenges inwelding, depending on the type and

thick-ness of the coating.

Because of the effects of melting and solidification and of the associated

mi-crostructuralchanges, a thoroughknowledge ofthe phase diagramandthe response

ofthe metal or alloyto sustained elevatedtemperatures is essential. Also influencing Weldability are mechanical and physical properties: strength, toughness, ductility, notch sensitivity, elastic modulus, specific heat, melting point, thermal expansion, surface-tension characteristics ofthe molten metal, and corrosion resistance.

The preparation of surfaces for Welding is important, as are the nature and properties of surface-oxide films and of adsorbed gases. The particular Welding process employed significantly affects the temperatures developed and their distri-bution in the Weld zone. Other factors that affect Weldability are shielding gases, fluxes, moisture content of the coatings onelectrodes, Weldingspeed, Welding

posi-tion, cooling rate, and level ofpreheating, asWellassuch postvvelding techniques as stress relieving and heat treating.

Weldability ofFerrous Materials:

° Plain-carbonsteels: Weldabilityis excellent for lovv-carbon steels, fair to good formedium-carbon steels, poor forhigh-carbonsteels.

° Lou/-alloysteels: Weldability is similar to that of medium-carbon steels. ° High-alloysteels:\X/eldability generallyis good under well-controlledconditions.

° Stainlesssteels: Thesegenerally areweldable by various processes.

° Cast irons: These generally are weldable, although their vveldability varies

greatly.

Weldability of Nonferrous Materials:

° Aluminum alloys: These are weldable at a high rate of heat input. An inert shielding gas and lackof moistureare important. Aluminum alloyscontaining

zinc or coppergenerally are considered unvveldable.

° Copper alloys: Dependingoncomposition, these generallyareweldableatahigh

rateofheatinput. Aninert shieldinggas and lackofmoisture areimportant.

° Magnesium alloys: These are weldable With the use of a protective shielding

gas andfluxes.

° Nickel alloys: Weldability is similar to that of stainless steels. The lack of

sul-fur is undesirable.

° Titanium alloys: Theseare weldable with the proper use of shielding gases. ° Tantalum: Weldabilityis similar tothat oftitanium.