Elemental Effect on Gta Spot Weld Penetration in Cast Alloy 718 - WJ_1990_08_s285

7/29/2019 Elemental Effect on Gta Spot Weld Penetration in Cast Alloy 718 - WJ_1990_08_s285

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W E L D I N G R E S E A R C HSUPPLEMENT TO THE WELDING JOURNAL, AUGUST 1990Sponsored by the American Welding Society and the Welding Research Council

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Elemental Effects on GTA Spot W e ldPenetration in Cast Alloy 718High sulfur content ma y enhance pene tration, but the e ffecton solidification cracking must be established

BY R. A. SPICER, W. A. BAESLACK III, AND T. J. KELLY

In t roduc t ionDur ing the past two decades, numerous invest igators have reported the effects of minor alloying and impurity elements on the weld pool g eom etry in austenitic stainless steels and oth er iron-ba sedalloys (Refs. 1-16). Early studies generallyat tr ibuted these ef fects to changes in arcbehavior (Refs. 3-5) and variations in surface tension at the l iquid-vapor interface(Refs. 6, 7). More recently, Mills (Refs. 8,9) , Heiple and coworkers (Refs. 10-13)and others (Refs. 14, 15) have demonstrated the strong inf luence of f luid f lowon GTA weld pool geometry . Heip le andcoworkers (Refs. 10-13) fur ther showedthe dominance of surface tension ef fectsin dr iving this f luid f low (i.e., Marangon iconvect ion). Their studies showed thatsmall concentrat ions of Group VI elements (e.g., S, O, Se) can significantlychange weld pool shape in austenit icstainless steels due to their preferential

R. A. SPICER is with Howm et Corp., Whitehall,Mich. W. A. BAESLACK III is with the Department of Welding Engineering, The Ohio StateUniversity, Columbus, Ohio. T. ). KELL Y is withGE Aircraft Engines, Cincinnati, Ohio.

segregat ion to the weld pool surface (i.e.,surface act ivity) and strong inf luence onsurface tension gradients across this surface. Relatively small additions of nonsur-face act ive elements, such as Al and Ce,have also been shown to inf luence f luidf low in the weld poo l , and thereby weldpool shape, by combin ing wi th sur faceact ive e lements to form compounds thatare not surface active (Refs. 11 , 17).Although the study of elemental ef fectson weld pool shape in austenit ic Fe-Cr-Nialloys has been quite extensive, the documentat ion of these phenomena in more

com plex Ni-based superalloys, such as Al-

K E Y W O R D SG T A WWeld Poo l Geomet rySpot Weld Penet rat ionCast Alloy 718Impurity LevelsElemental EffectsAlloying ElementsAl loy Weldabi l i t ySurface Active ElementsHigh/Low Level Al loy

loy 718, has been very l imited. In addi t ion,basic informat ion regarding the surfaceact ivity of common alloying or impurityelements in Ni, and the inf luence of suchsurface active elements on surface tensionvs . temperature relat ionships are notreadily available in the literature.Studies by S avage, etal. (Ref. 16), o n th eNi-based alloy Inconel 600 conclud ed thatthe presence of relatively small quantit iesof oxygen in argon shielding gas significant ly decreased GTA weld penet rat ion.This ef fect was at tr ibuted to the format ionof an oxide on the weld pool surface that

altered the anode spot, and thereby influenced heat t ransfer into the weld poo l .Subsequent work by Savage, et al. (Ref.18), more systematically exam ined effectsof min or alloying elements on the shape ofG TA weld pools in Inconel 600. Througha full-factorial statistical analysis of six minor elements (S, P, Si, M n , Ti an d Al), theyfou nd Si and Ti to strongly increase bo ththe weld pool cross-sect ion and depth ofpenet rat ion. Al though S d id not show af irst-order ef fect , i ts presence did enhance the ef fect of Si. In this ear ly work,e lementa l e f fects on weld poo l geo metrywere at t r ibuted pr imar i ly to e lementa land oxide ef fects on the anode potent ialdrop.

WELDING RESEARCH SUPPLEMENT | 285-s


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Fig. 7 - Light macrographsshowing cross-sections ofCTA spot welds producedin cast Alloy 718 heats:A-No. 5;B-No. 8;C-No. 12. (3X).

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Recent studies (Refs. 19, 20) of heat-affected zone (HAZ) l iquat ion cracking incast Alloy 718 using the spot-Varestrainttest have indicated composit ional ef fectson gas tungsten arc (GTA) spot weld geometry . Al though the top sur face widthand depth- to-penet rat ion of spot weldspro duc ed in ident ical heats w ere generallyfound to be qui te consis tent (5-10%),signif icant heat- to-heat var iat ions wereobserved . Such var iat ions in weld poolsize and shape are potent ial ly im portan t inthat they promote dif ferences in the HAZtemperature gradient , which may influence q uant itat ive cracking indices such asmaximum crack length and total cracklength. Potent ial elemental ef fects on G TAweld pool geometry in Alloy 718 are also

important when consider ing the extensivefusion welding of both cast and wroughtforms of the alloy by the aircraf t andaerospace industr ies, and the increaseduse of ful ly automated welding systems.The present study examined the in f luence of al loying and impu rity levels on th edepth of joint penetrat ion and top surfacewid th of GTA spot welds produce d in astatistically designed matrix of 20 cast Alloy 718 heats. Although this matr ix wasoriginally designed to analyze f irst-ordercomposit ional ef fects on suscept ibi l i ty toHAZ liquation cracking (Ref. 19), the alloying elements evaluated included severalpreviously shown to exhibit a f irst-orderef fect on weld pool geometry in high-nickel alloys (e.g., S, Si).

Table 1Compositions (wt-%) of Cast Alloy 718 Heats Used to Evaluate Elemental Effects onGTA Spot Weld Penetration*3*)Heat Nb Fe M o Hi1234567891011121314151617181920

0.10.1LAPLAP0.10.10.10.1LAP0.1LAP0.1LAPLAPLAPLAP0.10.1LAPLAP

0.35LAPLAP0.350.350.350.35LAP0.35LAP0.35LAPLAPLAPLAP0.350.35LAP0.35LAP

LAP


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at the weld center l ine, groundSiC paper and elec

in 10% oxalic acid. Theof the fus ion zone and the

of penet rat ion ( f rom the locat ion ofby magnify ing to 12X using a bin

a filar

and DiscussionMacrographs of three GTA spot weldhigh, in termedie and low depths of penet rat ion aren in Fig. 1. Cond uct ion- l im i ted, three-is evident for all

2 gives the dep th of(D), top sur face width (W)d D/W ratio for each Alloy 718 heat. As

of penetrat ion valuesa max imum of99 m m) to a m in imum of 0.057. (1.45 mm). Values of surface width

to a lesser extent from a max imumin. (8.58 mm) to a min imum ofin. (6.86 mm). Finally, D/W ratios

a maximum of 0.465 to aof 0.203.

Table 3 shows the ef fect of each eleon the t h ree we ld geom

As n o t e d ,e + indicates that a high level of thean increase in the pa(e.g., dep th of penet rat ion) at a

a indicates that a lowof the element promotes an increase

the parameter . The levels of statisticalare also indicated. Responsesare below exper imenta l er ror areNS and are generally consid

to be insignificant.Figure 2 graphically compares the influ

of selected elements ( those whicha cor re lat ion above exper imenon the GTA spo t we ld dep th of

by compar ing averages offor heats containing

vs. low elemental levels. In addi t ion,of elemental ef fectsn the GTA spo t we ld D/W ratio is pre

in Fig. 3.Table 2 and Figs. 2 and 3 show that Sthe greatest absolute ef fect in

in dep th of penet ratop sur face width and D/W rat io.ur also sho we d the strongest statistical

a 95% cor re lat ion wi thof penet rat ion and a 99.5% cor re

D/W ratio Table 3. Top surin absolute

a 90% cor re lat ion Tae 3.Table 3 and Fig. 2 indicate that the

of weld penet rat ion ef fect ive ly deed w i th an increase in B, Hf and Zr,did not signifi

the top sur face width,an effect just above experi-

Table 2Depth of Penetration (D), Top Surface Width (W) and Depth of Penetration/TopSurface Width Ratio (D/W) for GTA Spot Welds in Cast Allo y 718

Heat123 ,4567891011121314151617181920

f D e p t h of?trat ion, in. (mm)0.061 (1.55)0 .072(1 .83)0.086 (2.18)0 .100(2 .54)0.123 (3.12)0.157 (3.99)0 .057(1 .45)0.086 (2.18)0.072 (1.83)0.094 (2.39)0 .069(1 .75)0 .059(1 .50)0 .066(1 .68)0 .070(1 .78)0.088 (2.23)0.091 (2.31)0 .064(1 .63)0.092 (2.34)0.073 (1.85)0 .078(1 .98)

Top SurfaceW i d t h , in. (mm)0.300 (7.62)0.293 (7.44)0.300 (7.62)0.300 (7.62)0.312 (7.92)0.338 (8.58)0.270 (6.86)0.281 (7.14)0.281 (7.14)0.278 (7.06)0.281 (7.14)0.289 (7.34)0.293 (7.44)0.285 (7.24)0.285 (7.24)0 .319(8 .10)0.293 (7.44)0.304 (7.72)0.281 (7.14)0.285 (7.24)

D / WRatio0.2030.2450.2870.3330.3940.4650.2110.3060.2560.3380.2460.2040.2250.2460.3080.2850.2180.3030.2600.274

Table 3Summary of Statistical Analysis of Elemental Effects on GTA Spot Weld Penetrationin Cast Alloy 718W e l d

Characterist icD e p t h of penet ra t i on (D)W i d t h of top surface (W)D / W r a t io

CNS


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(0CC. c o

a.Q

I(3

SULFUR SIL ICON BORON HAFNIUM Z IRCONIUMElement Being Evaluated

Fig. J Bar graph comparing elemental effects on CTA spot weld depth-of-penetration/top surface width (D/W) ratio values in cast Alloy 718. High and low depth-to-width values for each respective e lement a re averages of values obtained for high and low level heats, respectively.

mental error as shown in Table 3. Theseelements, as wel l as Nb, sho we d a somewhat s t ronger effect in reducing the D /Wratio Table 3 and Fig. 3. The influenc e o fthese elements in reducing penetrat ionmay be related to their combinat ion withsurface act ive elements. Zr and Hf, beingreactive e lements, may comb ine w i th oxygen in a manner similar to the ef fect o f Alin Fe-based alloys. Obvio usly, such a proposed interact ion assumes oxygen to besurface act ive in the A lloy 718 weld poo l .Al though Si showed no cor re lat ion wi thdepth of penet rat ion and D /W rat io , ahigh Si level did co rrelate w ith an increasein top surface w idt h. I t is of interest to n otethat three heats, which contained highlevels of bo th S and Si, sho we d the highest depths of penetrat ion Table 2, suggest ing an interact ion between these elements in promoting an increase in penet ra t ion . Indeed, Savage and coworkers(Ref. 17) showed a similar effect of S p r o mot ing an increase in depth of penetrat ion in the presence of Si.

Conclus ionsResults of this present analysis show S toexhibit the strongest ef fect on weld poolgeometry in Al loy 718. Consider ing thedemo nst rated ef fect of S in promo t ingsurface-tension-dr iven f luid f low in Fe-Cr-

Ni alloys, it may seem logical to e xten d thisexplanat ion to an alloy containing the

same pr incipal elements but with an apprec iably larger propor t ion of Ni . However, the presence of numerous minor alloying elements in Alloy 718, which maydirect ly or indirect ly inf luence surface tension on the weld pool surface, or af fectother factors that may alter weld pool geometry, l ikely precludes the operat ion ofsingular com posit iona l ef fect on wel d po olgeom etry . Indeed, d if ferences in the p roport ions of major alloying elements (i.e.,Ni, Fe, Cr) and mino r alloying elem ent levels must account to a significant extent fordif ferences in elemental ef fects on weldpool geometry observed in the presentwork and those reported ear l ier in Inconel600. Ho we ver , del ineating these complexinteract ions wil l require a considerablydetailed invest igat ion.

From a more pract ical standpoint , i t hasbeen s ho wn that S con tent in cast Alloy718 does not increase the incidence ofHAZ l iquat ion cracks in GTA welds whenkept in the range evaluated in this work(Ref. 18). Th ere fore , if it can be establishedthat S does n ot re nde r cast Alloy 718 suscept ible to solidif icat ion cracking, it maybe desirable to maintain S levels on thehigh side of the alloy specification range inorder to assure adequate weld penetrat ion dur ing autogenous GTA weld in g. Thisef fect on GTA weld penet rat ion may bemore cr itical for wr o u g h t 718 product , butthe ef fect of sulfur on the wrought p r o d uct weldabil i ty must be determined be

fore S can be s pecif ied as an intent iona laddit ion to increase weld penetrat ion.

References1. Lucas, W ., a nd Eardley, J. A. 198 1. Effectof cast-to-cast material variat ions in TIC w e l d ing. Welding Institute Conference Report, N o .168.2. DeRosa, S., Robinson, j . L., Go oc h, T. G. ,and Nicholson, R. J. K. 1982. Cast-to-cast var i

at ions in penet rat ion dur ing mechanized welding of austenit ic stainless steels: progres s r ep ort.Welding Institute Research Report, No. 190.3. Ludwig, H. C. 1968. Current densi ty andanode spot size in the gas tungsten arc. Welding lournal 47(5);234-s to 240-s.4. Metca l f , | . C , and Qu igley, M. B. C. 1977.Arc and pool instabi l i ty in GTA welding. Welding Journal 56(5):133-s and 139-s.5. G lickstein, S. S., and Ye niscav ich, W. A.1977. A review of minor e lement ef fects on thewe ld ing a rc and we ld pene t ra t i on . WeldingResearch Council Bulletin, No. 226 .6. Bradstreet, B. ). 1968. Effect of surfacetens ion and meta l f l ow on we ld bead fo rmat i on . Welding Journal 47(7):314s to 322-s.7. Ishizaka, K. 1966. Interfacial tension theo ry o f a rc we ld ing phenomena f o rmat ion o ft he we ld ing bead . Welding Research Abroad12(3):68-86.8. Mil ls, G. S. 1977. Analysis of high man ganese sta inless steel weldabi l i ty pro blem . Welding Journal 56(6): 186-s to 188-s.9. Mil ls, G. S. 1979. Fundamental mechanisms of penet rat ion in GTA welding. WeldingJournal 58(1):22-s to 24-s.10 . Heiple, C. R and Roper, J. R. 1981. Eff ec t o f se len ium on GT AW fus ion zone geom et ry. Welding Journal bOioy.ltt-s to 145-s.11 . Heiple, C. R., and Roper, ). R. 1982.Mechanism for minor e lement ef fect on GTAfus ion zone geomet ry . Welding Journal 6 1

(4):97-s to 102-s.12 . Heiple, C. R Roper, ). R., Stagner, R. T.,and Aden, R. J. 1983. Surface act ive elementef fects on the shape of GTA , laser and elec t ronbeam we lds . Welding Journal 62(3):72-s to77-s.13. Heiple, C. R., and Burgardt, T. 1985. Effects of SO2shield ing gas addi t ion o n GTA we ldshape. Welding Journal 64(6): 159-s to 162-s.14 . Wa lsh, D. W. , an d Savage, W . F. 1985.Technical note: autogenous GTA weldments bead geometry var iat ions due to minor e lements. Welding Journal 64(2):59-s to 62-s.15. Pollard, B. 1988. The effects of minorelements on the welding character ist ics ofstainless steel. Welding Journal 67(9):202-s to

213-s.16 . Savage, W . F., Lundin, C. D. , and G oo dwi n , G. M. 1968. An effect of shielding gas onpenet rat ion in Inconel weldments. WeldingJournal 47(7):313-s to 322-s.17 . Heiple, C. R., and Roper, ). R. 1982.Mechanism for minor e lement ef fect on GTAfus ion zone geomet ry . Welding Journal 6 1(4):97-s to 102-s.18 . Sava ge, W . F., Nipp es , E. F., and G o o d wi n , G. M . 1977 . Effect of min or eleme nts onfusion zone dimensions of Inconel 600. Welding Journal 56(4): 126-s to 132-s.19 . Kelly, T. ). 1989. Elemental effects oncast 718 weldabi l i ty. Welding Journal 68 (2):44s to 51s.20 . Baeslack, W . A. I l l, 1988. U npu blishe dresearch, The Oh io State Universi ty, Colum bus.

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Elemental Effect on Gta Spot Weld Penetration in Cast Alloy 718 - WJ_1990_08_s285