Eagar 138

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Welding Brazing and

Soldering

Prepared under the direction of the

ASM International Handbook Committee

Volume Chairmen

David LeRoy Olson

Thomas A. Siewerl

Stephen Liu

Glen R. Edwards

Scoti D. Henry; Manager of Handbook Development

Suzanne E Hampson, Production Project Manager

Theodore B Zorc, Technical Editor

Faith Reidenbach, Chief Copy Editor

Laurie A. Harrison, Editorial Assistant

Nancy M. Sobie, Production Assistant

Wil liam W. Scott, Jr., Director of Technical Publications

Editorial Assistance

JosephR Davis

Kelly Ferjutz

Nikki D Wheaton

Mara S Woods

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undamentals ofWeldinghairperson: Thomas W Eagar Massachusetts Institute of Technology

nergy Sources Used for Fusion Welding 3

eat Flow in Fnsion Weldingluid Flow Phenomena During Welding 9

Transfer of Heat and Mass to the ase Metal in Gas-Metal AICWelding 25

rc Physics of Gas-Tungsten A x Welding 3

Power Sources 6

undamentals of Weld S ol~d ~fica tion 45

Nature and Behavior of Fluxes Used fo r Welding 5

Shielding Gases 64

Solid-State Transfo nnat~on sn Weidmenis 7

Cracking Phenomena Associated With Welding

haracte zation of Weld s 97

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nerm Sources Use

Thomas W Eagar, Massachusetts Institute of Technology

WELD ING AND JOINING processes ar e es-sential for the development of virtually everymanufactarcd pmduct. However, these proc-

esses often appear to consume greater fractionsof the product cost and to create more of theproduction difficulties than might be expected.There are a number of reasons that explain this

situation.First, w elding and joining ar e nlultifaceted ,

h t h in t e r ns of precess variations (such as fas-tening, adhesive bonding, soldering, brazing,~IC weldinz. diffusion bond in^. and resistancewe ld ing ) a id in the d i~ c i~ l in e c ne e de dor prob-lem so1vin.e (such as mechanics, materials sci-ence, physics, chemistry, and electronics). Anengineer w ith unusually broad and deep trainingis required to bring these disciplines togetherand to apply them effectively to a variety of

processes.~ e c o n b elding or joining difficulties usu-

ally occur far into the manufacturing process,where the relative value of scrapped parts ishigh.

'Third. a V ~ I V a m mrcentaee of nroduct fail-. u

we s occu r at joints because they are usually lo-cated at the h i~ he st tress mints of an assemblyand are the rec re the weikest pm s of that -sem bly. Care ful attention to the joining proc-esses can produce great rewards in manufactur-ing economy and product reliability.

The Section ''Fusion Welding Processes" inthis Volume provides details about equipmentand systems for the maior fusion welding proc-esses: The purpose of tii s Section of the co iu m eis to discuss the fundamentals of fusion w eldingprocesses, with an emphasis on the underlyiniscientific principles.

Because Chzre are many fusion welding proc-esses, one of the greatest difficulties for the

manufacturing engineer is to determine which

preferable to welding, whereas for long produc-tion runs, welds can be stronger and less expen-sive.

The perfect joint is indistinguishable fro m thematerial surtounding it. Although some proc-esses, such as difrusion bonding, can achieveresults that are very close to this ideal, they areeither expensive or restricted to use with just afew materials. T here is no universal process thatperforms adequately on all materials in all ge-ometries. Nevertheless, virtually any materialcan bejoined in some way , although joint prop-erties equal to those of the bulk material cannotalways be achieved.

Th2 economics ofjoin ing a material may limitits usefulness. For exam ple, aluminum is usedextensively in aircraft manufacturing and can bejoined by using adhesives or fasteners, or by

welding. However, none of these processes hasproven economical enough to allow the exten-

sive replacement of steel by aluminum in theframes of automobiles. An increased use ofcomposites in aircrafts is limited by an inabilityto achieve adequa te joint strength.

It is essential that the manufacturing engineerwork with the designer from the p i n t of product

conception to en sure that com patible materials,processes, and properties selected for the

final assembly. Often, the designer leaves theproblem of joining the parts to the manufactur-ing engineer. This can cause an escalation incost and a decrease in reliability. If the dezignhas. been planned carefully and the p ans h avebeen produced accurately, the joining processbecomes much easier and cheape r, and both thequality and reliability of the prod~dct are e n-hanced.

Generally, any two solids will bond if theirsurfaccs are brought into intimate confact. One

factor that ~ en er al ly nhibits this contact is sur-

0.001 Pa atm . ). For example, apressure of 1 Pa atm ), the contam inattime is s, whereas at 0.1 M Pa (1 atm), ion ly I0 X s ,

In fusion welding, intimate interfacial cotact is achieved by interposing a liquid substantially similar composition as the bmetal. If thc surt3ce contaminat~on S su:ubthen i t is d~ssolvcdn the liquid. If it IS i n~n lut l~ent will fluat away (rum thc l~q uid -so lid face.

Energy-Source Intensity

One distinguishing feature of all fusi on weing processes is the intensity of the heat souused to melt the liquid. Virtually every c

centrated heat source has been applied to welding process. However, many of the charteristics of each type of heat source are demined by its intensity. For example, when csidering a planar heat source diffusing intov e q thick slab, the surface temperature will bfunction of both the surface power density athe time.

Figure shows how this temperature will von steel with power densities that range fr400 to SOW W/cm 2, At the lower valuetakes 2 min to melt the surface. If that hsource were a point on the flat surface, ththe heat flow would be divergent and mi

not melt the steel. Rather, the solid mwould be able to conduct away the beat as fas

it was being introduced. It is generally fothat heat-source power densities of appro

mately I000 W/cm 2 are necessary to m elt mmetals.

t the other end of the power-density sptrum , heat intensities of lo 6 or 10' W /cm 2--

process % I 1 produce ~.'ccptahlc pro vrtlc 5 3 the I ~.ontam nmc~nAn> ircihly prnduccd hur- \ 2 p o r i ~ emu5t met.11- tbith~c f u nl:rtl\el o w s t c x t . T he re ire no vn ~ pl c n\\vcrh. Any Ficc cxposcd tu ihe ainlwpncie abwrh A V - %md<.At level> . ~ h o \ c hcx \2 lues , 2 oi

change in the p& g eo me tq, material, value of gen, water vapor, c&bon dioxide, and solid that interacts with the heat source wilthe end product, or size of the prcduction run , as hydrocarbons very rapidly. If it is assumed that vaporized , and n o fusion welding can o cc

well as the availability ofjoining equipment, can each molec ule that hits the surfa ce will be ab- Th us, the heat sources for all fusion weldinfluence the choice of joining methcd. For sorbed, then the time-pressure value to produce processes should have power densities betwsmall lots of complex paris, fastening may be a monolayer of contamination is approximately approximately 0.001 and I MW /cm2. T

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4 1 Fundamentalsof Welding

Dista nce from joint, in.

0 0.8 1 6 2 4 3.2 4 .0 4 .8

At W w/cm2 s heat

1800A t m l c m ~ , 4 seat

A ~ W Q 1cm2,1s heat-?

1200

600

0

Distance froni joint, crn

. Temperature d is t r ibu t ion af ter a specif icF g eating tim e in a thick s'eel plate leaTed uni-f o r m ly o n o n e su r f a c e as a funct ion of app l ied heatintensity; initial temperature of plate i s 25 C (77 OF

Electroslag, Resistance welding

oxyacetylene flame, (oxygen cutting]thermite I

Airlfuel ga s Electron bea m,flam e Arc welding l s e r beam

Spectru m of p rac t ica l hea t in tensi t ies us edF g fo r fusion weld ing

power-density spectruni is shown in Fig. 2,along with the points at which common joiningprocesses are employed.

The fact that power density is inversely re-lated to the interaction time of the heat source onthe material is evident in Fig. 1. Because thisrepresents a transient heat conduction problem,one can expect the heat to diffuse into the steel toa depth that increases as the square root of time,that is, from the Einstein equation:

where is the distance that the heat diffuses intothe solid, in centimeters; s the thermal diffu-siviiy of the solid, in cm2/s; and is the time inseconds. Tables 1 and 2 give the thermal diffu-sivities of common elements and common al-

loys, respectively.

For the planar heat source on a steel surfice,as represented. by Fig. 1, the time in seconds toproduce melting on the surface, tm s given by:

where H.I. is the net heat intensity in W/cm2)transferred to the workpiece.

Equation 2 provides a rough estimate of thetime required to produce melting, and is basedupon the thermal diffusivity of steel. Materialswith higher thermal diffusivities-or the use of alocal point heat source rather than a planar heatsource-will increase the time to produce melt-

Table 1 Thermal diffusivities of common elements from 20 to 100 OC (68 to 212 OF

AluminumAntimonyBerylliumBismuthCadmiuni

CarbonCobattCopperGalliumGermaniumGoldHafniumIndiumIridiumIronLeadMagnesiumMolybdenumNickelNiobiumPalladiumPlatinumPlutoniumRhodium

SiliconSilverSodium

TinTitaniumTungstenUraniumVanadiumZincZironium

Density Heat capacity T h e m 1 condmctivity.Tbermaf d

Element dcm3 lblin.' Jk g K caI C Wlm K cal~lcm s T mm21s cm2

2.699 0.098 900 0.215 22 1

Table 2 Thermal diffusivities of common alloys from 20 to 100 'C (68 to 212 OF)--

Qemsily Heat capacity Thermal c~nducliv ity 'Thema1 di--

lloys glcm3 1b1in.~ Jk g K calmlg C Wlm K ~ l IT l c m C m2 1s

Alumiuum alloys

I I002014505264x17075

Copper alloys

Commercial bronzeCartridge brassNaval brassBeryllium copper9 aIumin11rn bronze

Maguesium alloys

AZ 31AZ 91zw IR7 5

Stainless steeIsType 301Type 304Type 316Type 410Type 430Type 501

Nickel-base alloys

Nimonic 8OAInconel 600Moue1400

Titanium alloys

Ti-6A I4 VTi-5Al-2.5Sn

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tieat intensity, w1cm2

Typical weld pool-heat source interactionFig. times as function of heat-source intensity.

Materials with a high thermal diffusivity, such as cop-

per or aluminum, would lie near the top of this band,

whereas steels, nickel alloys, or titanium would lie inthe middle. Uranium and ceramics, with very low ther-

mal diffusivities, would lie near the bottom of theband.

Heal inlensizywlcm

. th~ xim um eld travel velocityas afunction ofF gq heat-source intensity based on typical heat-

source spot diameters

ing by a factor of up to two to tjve times. O n theother hand, thin materials tend to heat morequickly.

If the time to meXing is considered to be acharacteris,tic nteraction time, t I hen the graphshown in Fig. 3 can be generated. Heat sourceswith powe r densities that are of the order of 1000W crn2, such as oxya cety lene flames or electro-

slag welding, require interaction times of 5 swith steel, w hereas laser and electron beams , atSW "cm 2, need interaction times on the order ofonly 5 k s . If this interaction time is dividedinto the heat-source diameter, dH hen a maxi-mum travel speed, Vmm, is obtained for thewelding process (Fig. 4).

The reBon why welders begin their trainingwith ?he oxyacetylen e process should be c lear: itis inherently slow and does not require rapidresponse time in order to control the size of theweld puddle. Greater skill is needed to controlthe more-rapid fluctuations in arc processes. T he

ProducLivity, n. of weldls

0.004 0.04 0.4 4 40 4Oi3

i 07

z3 lo6

a.

o5

lo4.2

lo3.kl

1000.01 0.1 1 lo 100 lo3

Energy Sources Used for Fusion Wdding

Productivity, cm of weldls

Fig. Approximate relationship between capitalcost of welding equipment and speed at

which sheet metal joints can be produced

weld pool creatcd by the high-heat-intensitypraceszes, such s laser-beam and electron-beam welding, cannot be humanly controlledand m ust therefore be automated. This need toautomate leads to increased capital costs. O n anapproximate basis, the W /cm2 of a process canbe substituted w ith the dollar cost of the capitaleq~tipment.With reference to Fig. 2, the cost ofoxyacetylene .welding equipment is nearly$1000, whel-eas a fully automated laser-beam orelectron-bean system can cost 1 million. Notethat the capital cost includes only the energysource, control system, fixturing, and materialshandling equipment. It does cot include operat-ing m aintenance or inspection costs, which canvary widely depending on the specific applica-

tion.For constant total power, a decrease in the

spot size will produce a squared increase in theheat intensity. This is one of the reasons whythe spot size decreases with increasing heat in-tensity (Fig. 4). I? is easier to make the spotsmaller than it is to increase the power ratingof the equipment. In addition, only a smallvolume of material usually needs to be melted.If the spot size were kept constant and th.e in-put pow er were sq uared in order to obtain higherdensities, then the volume of fused metalw o ~ ~ l dncrease dramatically. with no beneficialeffect..

However, a decreasing spot size, coupled

with a decreased interaction time at higherpower densities, compounds the problem ofcontrolling the higher-heat-intensity process.A shorter interaction time means that the sen-sors and controllers necessary for automationmust operate at higher frequencies. The smallerspot size m eans that the positioning of .theheat source must be ~ v e amore precise, thatis, on the order of the heat,-source diame ter,dH. The control frequency must be greaterthan the travel velocity divided by the diamete rof the heat source. For processes that operatenear the maximum travel velocity, this is

103 lo4 lo5 lo6 lo7

Heat intermity, ~ l c t n ~

Range of weld HA widths as function

Fig* heat-source ntensity

the inverse of the process interaction time,(Fig. 3 .

Thus, not only must the high-heat-intensprocesses be automated because of an inherenhigh travel speed, but the fixturing requireme

become greater, and the control systems asensors must have ever-higher frequency sponses. These factors lead to increased coswhich is one reason that the very productlaser-beam and electron-beam welding presses have not found wider use. The appromate producti.vity of selec ted welding proc essexpressed as length of weld produced per sond, to the relative capital cost of equipmenshown in Fig. 5.

Another important welding process paramethat is related to the power density of the hsource is the width of the heat-affected zo(HAZ). This zone is adjacent to the weld meand is not melted itself but is structurachanged because of the heat of welding; Us

the Einstein equation, the H A 2 width canestimated from the process interaction .tand the thermal diffusivity of the material. Tis shown in Fig. 6, with one slight modifition. At levels above approximately lo4W/cmthe HAZ width 6icomes roughly constant. Tis due to the fact that the HA2 grows durthe heating stage at power densities that below lo4 W/cm2, but at higher power denties it grows during the cooling cycle. Thuslow power densities, the HAZ width is ctrolled by the i~t eract ion.tim e, whereas at hpower densities, it is independent of the hesource interaction time. In the latter case, HAZ width grows during the cooling cycle

the heat of fusion is removed from the wmetal, and is proportional to the fusion zwidth.

The change of slope in Fig. 6 also represethe heat intensity at which the heat utilizatefficiency of the process changes. At high hintensities, nearly all of the heat is usedmelt the matenal and little is wasted in preheing the surroundings. As heat intensity creases, this efficiency is reduced. Fo r arc weing, as little as half of the heat generamay enter the plate, and only 40% of this heaused to fuse the metal. For oxyacetylene we

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undamentalsof Welding

ing, theheatenteringthe metal may e 10% orless intensity processes to more than 10 in high-heat- figures in this article. Nonetheless, it is useful

of the total heat, and the heat necessary to fuse intensity processes. more fully understand each of the comm

the metal may be less than 2 of the total heat. It should now e evident that all fusion weld- welding heat sources, such as flames, arcs, las

A final point is that the heat intensity also ing processes can be characterized generally by beams,electron beams, and electrical resistan

controls the depth-to-width ratio of the molten heat-source intensity. The properties of any new These arc described in separate articles in the Se

pool. This value can vary from 0.1 in low-heat- heat source can be estimated readily from the tion Fusion Welding Processes in this Volum

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