nucleation mechanisms

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W E L D I N G R E S E A R C HSUPPLEMENT TO THE WELDING JOURNAL. DECEMBER 1986Sponsored by the American Welding Society and the Welding Research Council

mReaders are advised that all papers published in the Welding Journal's Research Supplement undergoPeer Review before publicat ion for: 1) or iginality of the contr ibut ion; 2) technical value to the weldingcom mu nity; 3) pr ior publicat ion of the mater ial being reviewed; 4) proper cred it to others working in thesame area; and 5) justif ication of the conclusions based on the results.The names of the more than 160 individuals serving on the AWS Peer Review Panel are publishedperiodically. All are experts in their respective technical areas, and all are volunteers in the program.

Nucleation Mechanisms and Grain Refiningof Weld Metal

For the first time, heterogeneous nuclei are observed in theweld pool and a method for identifying the nucleationmechanism is demonstrated

BY S. KOU AND Y. LE

i.e., nucleat ion and grainThe microstructure in the w e l d

. Heterogeneous nucle i in the w eldas well as partially melting grains

weld p o o l

i.e., dendr i te f ragm entat ion, gra inThe in te rac t ion be twe en we ld poo l

KOU is a Professor an d Y. LE is a GraduateDepartment of Metallurgical andEngineering, University of Wisconsin,

nucleat ion and grain ref ining was discussed. A method was presented forident ifying the nucleat ion and grain refining mechanism of the weld meta l , bychanging the microst ructure around thewe ld po ol boun dary and d issolv ing heter ogeneous nucle i through over lap w e l d ing, and by examining the grains of theweld meta l wi th e lect ron microscopy.Welds of commerc ia l 6061 a luminumalloy were used as an example for demonstrat ing this mechanism ident if icat ionm e t h o d .

I n t r o d u c t i o nThe grain structure of the weld metalcan af fect signif icant ly the soundness an dpropert ies of the resultant w e l d . Fineequiaxed grains tend to reduce solidif icat ion cracking (Refs. 1-3) and improvemechanical proper t ies of the weld meta l ,such as toughness, duct i l i ty, strength andfat igue life (Refs. 4-6).The grain structure near the fusionboundary of the weld meta l is dominatedby epitaxial gr ow th , as f irst repo rted by

Savage, ef al . (Refs. 7, 8). The grainst ructure in the bulk weld meta l , however , is dom inated by comp et i t ive gro wt h.In the absence of nucleat ion in the bulkwe ld m eta l , the columnar gra ins grow ingepitaxially f rom the fusion boundarycompete wi th each other for space. Thecolumnar grains which have their easygrowth d irect ion l in ing up favorably wi ththe d irect ion of maximum temperaturegradient tend to grow faster and crowdout other co lumnar gra ins. When nucleat ion is operat ing, ho we ver , ne w gra inswi l l nuc leate and grow in the bulk weldmetal, blocking of f the columnar grainsgrowing epitaxially f rom the fusionboundary. The greater the extent ofnucleat ion, the more numerous and f inerthe new grains are, i.e., the more grainref ining of the weld metal.

The nucleat ion mechanisms and grainref ining of the weld metal have beenstudied by a number of invest igators.However , an in tegrated p ic ture of themicrost ructure of the weld ing zone,wh ich is an impor tant key to the under standing of weld metal nucleat ion andgrain ref ining, has not been provided. As

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Region ofpart ial ly - melt ing grains

WeldingD i r e c t i o n

0

Region ofgrowing dendr i tes

F u s i o n Z o n e

B a s e M e t a lW e l d i n g D i r e c t i o n

-. w $7 5?-

a* a j * -

v .'vg-af -100 M : ; * . - :100 M

Fig. 1 Microstructure around the weld pool boundary quenched during the CTA welding of 2219 aluminum alloy. Aschematic sketchB- micrograph taken at position X; C - micrograph taken at position Y. The brackets in B and C denote regions of partially melting grains and grodendrites, respectively

will be descr ibed later in this paper, themechanisms of dendr i te f ragmentat ion,gra in detachment and heterogeneousnucleat ion can be well explained by themicrost ructure of the weld in g zone. Also,there are situat ions where more than oneof these mechanisms are possible, and ame thod is neede d for ident ify ing wh ichmechanism is responsible for the nucleat ion and gra in ref in ing produced in thewe ld meta l .This paper deals with the importantsubject of weld meta l nuc leat ion andgrain ref ining, with emphasis on the

microst ructure of the weld ing zone, i tsconnect ion wi th the mechanisms ofnucleat ion and grain ref ining, and theident if icat ion of the r ight mechanism. Theeffect of welding var iables (e.g., the heatinput and welding speed, the oscil lat ionand pulsat ion of the arc, the gas coolingof the weld pool sur face, etc . ) on weldmetal nucleat ion and grain ref ining wil l beaddressed in subsequent reports. Theult imate goal of this and subsequentstudies is to help achieve ef fect ive con t rol or ref inement of the weld metal grainstructure.

Exper imenta l ProcedureThe mater ia l used for weld ing wascommerc ia l 6061 aluminum sheets of 1.6-m m (>i6-in.) th ickness. The nom inal chem ica l composi t ion of 6061 a luminum al loyis Al, 0.6Si, 1.0Mg, 0.3Cu, 0.2Cr, i n wt -%.Commercial 2219 aluminum sheets of thesame thickness were also used, the nominal chemical composi t ion being A l,6.3Cu, 0.3Mn, 0.06Ti , in wt -%.A programmable gas tungsten arc(GTA) weld ing machine was used in con junct ion wi th a 4-pole magnet ic arc oscil-

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for osc i l la ted arc weld ing. Quench

This allowed the arc to be ext in

The test welds, i.e., the welds for

re 7.6 mm /s (18 ipm) , 100 A and 11 V,The test welds were made coaxially

the former could be conta ined

den

vol tage w ere 5.9 mm /s (14 ipm),

vol tage we re 0.64 mm /s (1.5

The specimens were sect ioned, po l

N 0 3 and 5 ml HF to reveal the grain

acid, 150 ml sulfuric acid, 275 ml water

part icles

Figure 1 show s the micros tructure o f

1A shows the locat ions where thes in Figs. 1B and C w ere taken.re 1B show s the micros tructure along

the leading por t ion of the po ol bo undary.As indicated by the brackets in the micrograph, the grains near the pool boundarywere much smaller than those in the basemetal, thus indicat ing part ial melt ing ofgrains. In fact , the closer to the poolboundary, the smaller the grain size andthe volume fract ion of solid. The veryf ine solidif icat ion structure near the topof the photo is the l iquid metal in theweld pool which sol id i f ied upon quenching.

Figure 1C shows the microstructurealong the t ra i l ing por t ion of the poolboundary. The brackets in the micrograph ind icate the region of growingdendrites. Partially melting grains andgrowing dendrites are the character ist icsof the melt ing and solidif icat ion of al loysdur ing weld ing, respect ive ly . The h igherthe a l loy ing content , or more appropr i ately, the wider the solidif icat ion temperature range, the more clear the part ial lymelt ing gra ins and the gro win g dendr i tesare.Figure 2A shows the microstructure ofa smal l por t ion of the weld pool meta lahead of the trai l ing port ion of the poolboundary , i.e., posi t ion " Z " in Fig. 1A. Asshown, a heterogeneous nucleus is evi dent at the or igin of the equiaxed dendr i te , which nucleated f rom upon thenucleus and grew in the weld p oo l dur ingquenching. This heterogeneous nucleus isshown at higher magnifications in Figs. 2Band 2C. The energy d ispers ive sp ect rometer analysis of the nucleus is shown inFig. 2D. As shown, the nucleus is r ich int itanium, just l ike heterogeneous nuclei inaluminum cast ings and ingots (AI3Ti or itssimilar compounds). The microcavit iesinside and along the grain boundary wereoriginally occupied by the eutect ic phase,which dissolved dur ing etching. Similarresults were observed in 2219 aluminumwe lds. I t should b e po inte d ou t that this isthe f irst t ime that heterogeneous nuclei inthe we ld poo l were observed .

From the above discussion, the micro-structure of the welding zone of an alloycan be shown schematically in Fig. 3. Asshown, it consists of part ial ly melt inggrains along the leading port ion of thepool boundary, growing dendr i tes a longthe t ra i l ing por t ion of the pool boundaryand , in some alloys, heterogeneous nucleiin the weld poo l .Mechanisms of W eld M etal N ucleation

Based on the m icrost ructure obse rved,three d i f ferent mechanisms of weld meta lnucleat ion and grain ref ining can be introduc ed , as sh ow n in Fig. 3. In order to helpdiscuss these mechanisms, weld poolconvect ion wi l l be br ie f ly descr ibedfirst.As demonst rated recent ly by Kou, etal . (Refs. 11 , 12), convect ion exists in theweld pool due to var ious dr iv ing forces,

such as the buoyancy force, the e lect romagnet ic force and the surface tensiongradient at the pool surface. Figure 4, forinstance, shows the co nvect ion pat tern ina GTA weld pool in a 3.2-mm (Vi-in.)thick 6061 aluminum sheet, due to theelectromagnet ic force. The calculatedconvect ion pattern is in the plane perpendicular to the weld ing d irect ion andpassing through the heat source. Thevelocity magnitude is indicated by thelength of the arrows. The decrease in thevelocity of the l iquid metal towards thepool boundary suggests the existence ofnegat ive velocity gradients and, therefore, shear forces at the pool boundary.The p urp ose of Fig. 4 is to illustrate thatconsiderable convect ion can exist in theweld pool. I t is not implied that weld poolconve ct ion in a th inner workp iece , e.g.,1.6 mm CAs in.) thick, should be ident ical.Dendrite Fragmentation

Weld pool convect ion can in pr inc ip lecause fragmentat ion of the t ips of thedendrites in the m ushy zo ne, as il lustratedin Fig. 3. These dendrite f ragments arecar r ied in to the bulk we ld p o o l , where, i fab le to surv ive the weld pool temperature, they can act as nuclei for ne w grainsto form. I t is interest ing to note that thismechanism has been quoted very frequent ly as the grain ref ining mechanismfor weld meta ls , even though there hasbeen no proof at al l .Grain Detachment

In the case where part ial ly melt inggrains are loosely held together by thel iqu id f i lms between them, weld poolconvect ion can make them detach themselves f rom the pool boundary and car rythem into the weld poo l , as also illustrated in Fig. 3. Like dendrite f ragments,these partially melting grains, if they survive in the weld pool, can act as nucleifor the format ion of new grains in theweld metal. Pearce, ef al . (Ref. 13), haverecent ly sugg ested that this is the mech anism responsible for grain ref ining in GTAwelds of 7004 a luminum al loy made wi thweld pool s t i r r ing.Heterogeneous Nucleation

According to the nucleat ion theory,atoms in a l iqu id meta l have to ove rcom ea cr it ical energy barr ier (AG *) so that theycan form solid nuclei that are larger than acrit ical size (r*), in order to be stable andable to grow into solid grains (Ref. 14).Unfortunately, this cr it ical energy barr ierusually is high and dif f icult to ov erc om e, ifthese nuclei are to be formed solely bythe atoms in the l iquid themselves, i.e.,the so-cal led homogeneous nucle i .

For this reason, const itut ional super-

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con

gnif icant nu mb er of solidcon

i.e., he te ro

s been p rodu ced in welds of a luminum

The mechanism of grain detachment is

. As show n in Step 1 of Fig.wi de single-pass prew eld e xhib

such that i ts fusion boundaries are

et al. (Ref.

oo l boundary is no w sur roundedructure of the pre we ld.on the other hand, the equiaxed grains

Heterogeneousnucleus


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T e s t W e ld P r e we ld ( s i n g le p a s s )

S T E P 1O V E R L A PW E L D I N G

M e c h a n i s m : g r a i n d e t a c h m e n tT e s t W e ld P r e we ld ( s i n g le p a s s )

M e c h a n i s m : d e n d r i t e f r a g m e n t a t i o n orheterogeneous nucleat ionT e s t W e ld P r e w e l d ( m u l t i p a s s to d i s s o l v e n u c l e i )

S T E P 2O V E R L A PW E L D I N G

^ ) & ^ ^ ) ^ & ^ & ^'wwgmrmn^M e c h a n i s m : d e n d r i t e f r a g m e n t a t i o n

T e s t W e l d P r e w e l d ( m u l t i p a s s to d i s s o l v e n u c l e i )mm^f>>>>y^%M e c h a n i s m : h e t e r o g e n e o u s n u c l e a t i o n

STEP 3ELECTRONMICROSCOPY

E q u i a x e d g r a i n\ e x a m i n e d w i t h SEMa n d EDS.

Fig. 6 A step-by-step method for identifying the nucleation mechanism

Prewe ld

Tes t We ld

W e l d i n g d i r e c t i o n

Fig. 7 - Changing the microstructure around the weldpool and dissolving heterogeneous nuclei byoverlap welding

men , or already have lost them duringpolishing. Further polishing, usually elec-trochemically, can help reveal the nucleib e lo w the surface of the specimen. Insome alloys (e.g., 5052 aluminum), thecoarsening ef fect dur ing solidif icat ion isso severe that dendrite arms are fused orsintered together and the dendrit ic structure becomes rather ill def ined , thus making the origins of the dendrites ratherdif f icult , if not impossible, to locate. Forthese alloys, and possibly alloys in whichheterogeneous nuclei are too t iny todetect , the over lap weld ing suggested inStep 2 is more usefu l .Identification of Nucleation Mechanisms inAluminum Welds

The procedure suggested above wasapplied to ident ify the grain refiningmechanism in GTA welds of commerc ia l6061 aluminum alloy made with circulararc oscillation at 35 Hz.Since arc oscillation is expec ted topromote we ld poo l convec t ion , bo thgra in d etachment and dendr i te f ragmentat ion are likely to be operat ing. In orderto test the mechanism of grain detachment , over lap weld ing was carr ied outusing single-pass prewelds. Figures 9Aand 9B show the grain structure of thetest weld outside and inside the over lapregion, respect ively. As shown, equiaxedgrains existed in the test weld both out side and inside the overlap region, thusconf i rming that gra in detachment wasno t the mechanism responsible fo r grainref ining.In order to f ind out whether dendr i tef ragmentat ion or heterogeneous nucleat ion was responsible fo r grain ref ining,over lap weld ing was carr ied out usingmultipass prewelds. Figure 10A showsthe grain structure of the test we ld in theregion where it over lapped w i t h a pre we ld made w i t h six melting passes. Bycompar ing it wi t h the micrograph shownin Fig. 9A, it can be seen that the numberof equiaxed grains decreased. As s h o wnin Fig. 10B, the test weld in the over lapregion was essentially free of equiaxedgrains, whe n the p r e we ld wasmade wi t hten melting passes. This suggested thatheterogeneous nucleat ion was responsib le fo r grain refining in the oscillated arcwe ld of 6061 aluminum. This f inding issurpr ising from the v iewpo in t t ha t commercial 6061 aluminum is not k n o w n toconta in t i tan ium, at least not accord ing tothe specif icat ions of the alloy (Ref. 21).Detailed chemical analysis, shown inTable 1, did indicate a t i tanium level of0.043 wt -%, however . It should be point ed out that chemical analysis on 6061aluminum f rom a different supplier alsoshowed similar results. In fact, similar andsomewhat lower t i tan ium levels were

respect ive ly observed in commerc ia l5052 and 2014 aluminum alloys (Table 1),



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mul

AI 3Ti , to reform after being disI t is worth ment ioning that the colum

for the test weld remainedepi

other w ords , the10 is not the result of enhanced

In order to further conf irm that the

re 11 show s the hetero gene ouswe ld .

11D. I t is worth not ing thatdr ite is not located at

all direct ions and

K

1500

1 3 0 0

TIOO

0.12%.9 0 0

fi

At.%Ti5 10 20 3

11iL, /7-*pta*-F

Liq.

Liq. + AI

Liq .+TiAl ,

9 3 8 KII

A I + T i A l ,I

L iq .+Ti Al;

- 3 5 %

T i A I 3

36.5%S

Xeio0

K t

x

v5*""'- '"^"^


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l lVn^jFig. 10 Grain structure of oscillated arc welds of 6061 aluminum in the region of overlapping. A a preweld made with six melting passes; B-apreweld made with ten melting passes

the d irect ion of the maximum temperature gradien t. It is also w or th not ing thatthe c lear ev idence of heterogeneousnuclei in this study conf irms the recentstudies of Kerr, ef al. (Refs. 10, 13), inwhich the ev idence of heterogeneousnuclei was shown but was not as clear.

The results descr ibed above are co n sistent with the heterogeneous nucleif ound in t he we ld poo l by quench ing ,e.g.. Fig. 2. However, i t should be pointed out that the existence of heterogeneous nuclei in the weld pool is only anecessary, but not suff icient, condit ion

for grain ref ining of the weld metal. Thisand the ef fect of weld ing parameters onweld meta l nuc leat ion and gra in ref in ingwil l be addressed in subsequent reports.F igure 12A shows the heterogeneousnucleus in a grain in the overlap regionbe tween the t es t we ld and a p rewe ld

Fig. 11 - Heterogeneous nucleus in a sm all grain in an oscillated arc w eld of 6061 aluminum alloy. A-330X; B-2000X; C- 15000X; D-analysisED S



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r . _ * % V - * . ** *

* . v * jr 7 *V *"% * \

A .' * ;- 'H 1 * * ; .

. - - . ". i,

i j >\ v *v1 *j * s c" - . , * .< *

- " I J"'V,- '* , , * *

? 003

L

i -i1 !1 iI,j !

.1A ..

;

i ' ^ 16? i^-C.'

12 Heterogeneous nucleus in a grain in the overlap region between an oscillated arc weld of 6061 aluminum and a preweld made with six meltingA -330X; B EDS analysis

ler than that sho wn p reviously in11 , due to dissolut ion dur ing the

wn in Fig. 12B, the t i tanium c onten t o fwn previously in Fig.

. This is because the smaller the nucle, the mo re th e EDS analysis re f lected

1. The microst ructure of the weld ing

wn in this study. It is an impo rtant key

2. Heterogeneous nuclei in the weldin the we ld po ols of 6 061

3. The mechanisms of dendrite fragn detachment and hetero

4. A method for ident i fy ing the nucle

microscopy.5. As an i l lustrat ion of this me tho d, thenucleat ion mechanism responsible forgrain ref ining in an oscil lated arc weld ofcommercial 6061 aluminum alloy hasbeen ident i f ied as heterogeneous nucleat ion , rather than dendr i te f ragmentat ionor gra in detachment .6. The heterogeneous nucle i obse rvedin the weld metal of 6061 aluminum alloyare r ich in ti tanium , even tho ugh this alloyis not kn ow n to conta in t i tan ium.Detailed chemical analysis, which indicates a low but signif icant t i tanium levelof 0.043 wt-% in this alloy, conf irms thepresence of these t itanium r ich nuclei.The presen t stud y is significant in that itclear ly reveals the microstructure in theweld ing zone dur ing weld ing and t ies i twith nucleat ion and grain ref ining mechanisms, so that the important subject ofweld metal grain ref ining can be betterunderstood. Fur thermore, a method isprovided for ident ifying the r ight mechanism responsible for nucleat ion and grainref ining of the weld metal.

AcknowledgmentsThe authors gratefu l ly acknowledgesupport for this study from the Nat ionalScience Found at ion, under NSF Grant No .DMR 8419274 .

References1. Garland, J. C. 1974. Metal C onstructionan d British Welding /ournal. Vol. 21 , p. 121.2. Davies, C. )., and Garland, ). G. 1975.International M etallurgical R eviews. Review196, Vo l. 20, p. 83.

3. Kou, S., and Le, Y. 1985. MetallurgicalTransactions A. Vol. 16A, p. 1345.4. Petersen, W. A. 1973. Welding lournal52(2):74-s to 79-s.5. Hall, E. O. 1951. Proc. Phys. Soc. Vol.64B, p. 747.6. Garstone, J., and lohnson, F. A. 1963.British Welding lournal. Vol. 10, p. 224.7. Savage, W . F., Lundin, C. D., and Aron son, A. H. 1965. Welding Journal 44(4)^7Ss to181-s.8. Savage, W. F. 1980. Welding in theWorld 18(5/6):91.9. Kou, S., and Le, Y. 1985. MetallurgicalTransactions A. Vol. 16A, p. 1887.10. Ganaha, T Pearce, B. P., and Kerr, H.W. 1980. Metallurgical Transactions A. Vol.11A, p. 1351.11. Kou , S., and Sun, D. K. 1985. Metallurgica l Transactions A. Vol. 16A, p. 203.12. Kou, S., and Wa ng, Y. H. 1986. WeldingJournal 65(3):63-s to 70-s.13. Pearce, B. P., and Kerr, H. W. 1981.Metallurgical Transactions B. Vol. 12B, p. 479.14. Turnbull, D. 1950. Journal of ChemicalPhysics. Vol. 18, p. 198.15. Matsuda , F., Nakata, K., Tsukam oto, K.,and Arai, K. 1983. Transactions of JWRl. Vol.12, p. 93.16. Heintze, G. N., and McPherson, R.1986. Welding Journal 65(3):71-s to 82-s.17. Misra, M. S., Olson , D. L., and Edwards,G. R. 1983. Grain Refinement in Castings an dWelds. Ed. by G. ). Abbaschian and S. A.David, Metallurgical Society of AIME, p. 259.18. Mondolfo, L. F. 1976. AluminumAlloys Structure and Properties. Butterwo rths, London-Boston, p. 386.19. Arnb erg, L., Backerud, L., and Klang, H.1983. Grain Refinement in Castings an d Welds.Ed. by G. J. Abbaschian and S. A. David, TheMetallurgical Society of AIME, p. 165.20. Lee, D. S., and Basuran M. Ibid,p. 183.

21. Aluminum Standards and Data. 1982.The Aluminum Association, New York, N.Y.,p. 15.

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