ORIGINAL ARTICLE

High-temperature mechanical properties investigationof Al-6.5 % Cu gas tungsten arc welds madewith scandium modified 2319 filler

N. Kishore Babu & Mahesh Kumar Talari & D. Pan &

J. Wei

Received: 5 March 2012 /Accepted: 7 June 2012# Springer-Verlag London Limited 2012

Abstract The present study has investigated the influ-ence of scandium additions on the structure and me-chanical properties of AA2219 gas tungsten arc (GTA)weldments. Controlled amounts of scandium modifiedAA2319 fillers were introduced into the molten poolof AA2219 by predeposited cast inserts under differentwelding conditions in GTA welding. Full penetrationGTA welds were prepared using alternating current. Itwas observed that grain size decreased with increasingamounts of scandium and gradual transformation fromcolumnar to equiaxed grain morphology. The observedgrain refinement was shown to result in an appreciableincrease in fusion zone hardness, strength, and ductility.Room temperature hardness and tensile properties atdifferent temperatures (room temperature, 100, 150,200 and 250 °C) of weldments in the as-welded condi-tion were observed and correlated with microstructure.The results show that the welds subjected to post-weldaging treatment have displayed superior hardness andtensile strength.

Keywords Grain refinement . Scandium . Tensileproperties . AA2219 alloy

1 Introduction

AA2219 (Al-6.5 % Cu) alloy is widely used in defense andaerospace industries because of high strength, high corro-sion resistance and good weldability. These AA2219 alloycomponents can be joined using a variety of welding meth-ods including gas tungsten arc (GTA), gas metal arc (GMA),plasma arc, electron beam, laser beam, friction stir, and spotwelding [1]. GMAwelding is employed for joining relative-ly thicker sections and GTA welding for thin sheets. Thehigh thermal conductivity, high reactivity, and high coeffi-cient of expansion make welding of aluminum alloys diffi-cult. One of the main problems associated with the weldingof aluminum alloys is caused by high-oxidizing tendency ofaluminum during heating to high temperatures and forma-tion of oxide film. A significant difference in the meltingtemperatures of aluminum oxide and aluminum itself resultsin the surface of molten pool being covered by an oxide filmduring welding [2]. Thus, alternating current or variablepolarity is preferred during arc welding of aluminum alloysas electron emission destroys the oxide layer present on thesurface of base metal during electrode positive cycle.

However, Al–Cu alloys have poor as welded jointstrength due to dissolution of strengthening precipitates asa result of the quick heating and cooling cycles during thewelding. Previous studies have shown that Cu segregationin the as-welded microstructure is responsible for decreasingthe strength and this segregation in the fusion zone rendersthe weld metal nonresponsive to natural aging [3, 4]. ThisCu segregation can be avoided by employing electron beamwelding in which cooling rates are high and pulsed currenttechniques in GTA welding with good weld pool agitation.Higher cooling rates employed in pulsed current GTAweld-ing also reported to reduce the grain size which can improvestrength and ductility [5].

N. K. Babu (*) :D. Pan : J. WeiSingapore Institute of Manufacturing Technology,71 Nanyang Drive,638075 Singapore, Singaporee-mail: kishorebn@simtech.a-star.edu.sg

M. K. TalariFaculty of Applied Sciences, Universiti Teknologi MARA,40450 Shah Alam, Malaysia

Int J Adv Manuf TechnolDOI 10.1007/s00170-012-4297-7

Mechanical properties of the welded joints are signifi-cantly influenced by the grain size of the weld metal. Pre-vailing thermal conditions during the solidification of theweld metal result in coarse columnar grains in the weldfusion zones. The grain size of fusion zone can be modifiedby employing various welding parameters, such as highcooling rate, addition of grain refiners, gas impingementand physical agitation through pulsed current GTA weldingand torch vibration [6]. Inoculation by adding grain refineradditions is most popular method employed to achieve fineequiaxed grain structure in aluminum alloy castings.

Aluminum alloys are most commonly grain refined byAl–Ti, Al–Ti–B, Al–Zr, and Al–scandium (Sc) masteralloys, which provides intermetallic particles of TiAl3,ZrAl3, ScAl3 serving as nucleant substrates during solidifi-cation [7]. Commercial filler metals for the joining of alu-minum alloys generally have nominal amounts of differentgrain refining elements (Ti, V, Zr, and Sc). More recently,Kishore Babu et al. [8] also observed that significant grainrefinement occurs in AA6082 welds when the filler metals(AA4043 and AA5356) modified with Tibor™. They alsoobserved that the grain refinement is mainly caused by thegrain nucleation associated with constitutional undercoolingduring solidification [8].

The actual research on Sc to achieve grain refinementwas first started from the former Soviet Union [9–11]. Scadditions to the base alloy as well as to welding filler alloyshave been shown to have a beneficial influence on weld-ability, grain refinement, and hot cracking resistance ofaluminum alloys [11–15]. Scandium forms a high meltingeutectic phase with aluminum (at ~655 °C with 0.55 wt%Sc),ScAl3, having an L12 crystal structure with a lattice parametermismatch with Al of only 1.6 %, which can produce signifi-cant aging response [16]. Mousavi et al. [15] studied the effectof scandium additions on the grain refinement and solidifica-tion behavior of castings and GTA welds of aluminum alloy7108. They found that scandium had a clear grain refiningeffect on AA7108 in both welds and castings, with smallergrains achieved at high concentrations up to 0.20 wt%. Sim-ilarly, Ramaniah et al. [17] studied the effect of grain refiners(Zr, Tibor™, and Sc) on partially melted zone of AA6061GTA welds. They found that partially melted zone crackingresistance was improved because of grain refinement. Normanet al. [18] also reported that addition of Sc to 7000 series alloyshad resulted in considerable improvement in the mechanicalproperties. Koteswara Rao et al. [19] have studied the influ-ence of Sc and Mg on thermomechanical treatments of Al–Cu

alloy GTAwelds. They reported that artificial aging treatmentafter compressive deformation resulted in a significantimprovement in tensile properties of fusion zone. Thishas been attributed to the grain refinement, strengthening pre-cipitates, and strain hardening of the fusion zone caused by theaddition of scandium and magnesium to the conventional fillerof AA2319 [19]. Ramanaiah and Prasad Rao [20] investigatedthe effect of effect of Sc, Zr, and Tibor modified AA4043 filleron corrosion behavior of AA6061 GTAwelds. They found thatpitting corrosion resistance fusion zone of AA6061 alloy wasrelatively more when filler metal modified with Sc, Zr, andTibor. They also reported that highest pitting corrosion resis-tance was found with filler material containing 0.5 % Sc [20].

Excellent mechanical properties displayed by Al–Cualloys are mainly due to the finely dispersed coherent CuAl2precipitates, which are formed during the aging process.However, these alloys have tendency to over age duringprolonged aging durations or when exposed to high temper-atures. Loss of strength at elevated temperatures is moresevere in weld metal of these alloys due to as-cast micro-structures in the weld. Naga Raju et al. [21] have studied theinfluence of Sc, Mg, and Zr on high temperature stability ofage hardenable AA2219 aluminum–copper (6.3 %) alloy.They found that the 0.8 % Sc+0.45 %Mg+0.2 % Zr addi-tion was more significant in improving the high temperaturestability of AA2219 alloy. They also proposed the pinningof the grain boundaries by the finer precipitates like Al3Scand Al3 (Sc, Zr) is mainly responsible for the high temper-ature stability of AA2219 alloy [21]. Seshagiri et al. [22]and Koteswara Rao et al. [23] studied the effect of Scaddition on room mechanical properties of AA2219 weld-ments. They found that the room temperature mechanicalproperties of welds were significantly improved with theaddition of Sc. However, to the authors’ knowledge, thereare no reports on the high-temperature tensile properties ofAA2219 GTA welds prepared with Sc modified fillers. Theaim of the present investigation, therefore, was to evaluate themicrostructure, hardness, age hardening, and tensile properties(room and high temperatures) of AA2219 GTAwelds.

2 Experimental

Sheets of 3 mm thick, AA2219 Aluminum alloy in T31condition were used in the present study. ConventionalAA2319 were used as filler metal. The chemical composi-tion of base metal and filler metal are given in Table 1. The

Table 1 Composition of basemetal and filler (weight percent) Si Cu Fe Mn Mg Zn Ti Zr Al

AA2219 0.05 6.5 0.10 0.32 0.02 0.04 0.04 0.11 Balance

AA2319 0.20 6.8 0.30 0.20 0.02 0.10 0.10 0.10 Balance

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modified filler metals (cast inserts) were prepared by addingmaster alloy (Al-2 % Sc) to AA2319. Cast inserts will bemade by adding different amounts of Al-2%Sc master alloyto the molten base alloy, chill casting the mixture in a coppermold, and then machining 2×2 mm square inserts from theingot (illustrated in Fig. 1). Figure 2 shows the opticalmicrograph of the Al-2 % Sc master alloy added to the fillershowing the morphology of Al3Sc. The filler metals weremodified with different Sc levels (0.25 % Sc, 0.50 % Sc, and0.75 % Sc) using casting route. Hereafter, unless otherwisestated, all the compositions are stated in weight percent.Just prior to welding, the base material coupons werewire-brushed and degreased with acetone and castinserts were preplaced along the weld joint and tacked.The schematic of weld preparation is shown in Fig. 3.The welding parameters are given in Table 2. Theseinserts were machined from modified 2319 filler, select-ed to provide a controlled variation in scandium contentin the weld metal. Argon was used as the shielding gasand mixture of argon and helium (50–50 %) used forauxiliary trailing and backing gas shields. The flow rateof the shielding gas was 15 and 20 l/min for the trailingand backing gas shields, respectively. Welded plates

were subjected to post-weld aging treatment at 190 °Cfor different periods of time (up to 120 h) to investigatethe age hardening response of as-welded plates.

The composition analyses of weld metal were deter-mined using spectroscope. The samples for light micros-copy were suitably sectioned, mounted, mechanicallypolished and etched. For etching, a solution containing5 ml HNO3, 3 ml HCl, 2 ml HF in 190 ml water wasused. Scanning electron micrographs were taken using asecondary electron image mode at 15 and 20 kV. Fortransmission electron microscopy, thin slices of thick-ness 0.125 mm were cut from the specimens on a lowspeed saw and they were further thinned by mechanicalpolishing to 0.03 mm. Disks of 3 mm diameter,punched off from these slices, were electropolished asolution of 30 vol.% nitric acid in methanol at −30 °Cand 16 V. The thin foils were examined in a JEOL CX-200 transmission electron microscope operating at160 kV. Micro hardness measurements were carried onthe base metal, heat-affected zone (HAZ), and weldmetal by a diamond pyramid indenter under a load of200 g for 15 s. A microhardness traverse was madeacross the weldments at 0.5 mm intervals. Tensile speci-mens were prepared in accordance with ASTM E 8 Mfrom as-welded coupons. For tensile tests, transverseweld specimens with a gauge length of 25 mm, gaugewidth of 6 mm and thickness of 3 mm were used.Tensile tests were carried out on base metal as well asweldments at a constant displacement rate of 3 mm/minat different temperatures (room temperature, 100, 150,200, and 250 °C).

Fig. 1 Schematic of cast insert production

Fig. 2 Optical micrograph of Al-2 % Sc master alloy showingmorphology of Al3Sc

Fig. 3 Schematic of weld preparation

Table 2 Weldingparameters Alternating current GTA welds

Arc voltage 15 V

Arc current 160 A

Ar shielding gas 20 l/min

Travel speed 20 cm/min

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3 Results and discussion

3.1 Microstructural investigation

The optical and SEM microstructure of base metal areshown in Fig. 4a and b. The microstructure showed roundprecipitate particles, distributed in the interior and along thegrain boundaries of the matrix. Microchemical analyses ofthese round shaped precipitates in the SEM micrographshowed that the precipitate regions are rich in Cu and Aland precipitates have been identified as CuAl2 (Table 3).The fine evenly distributed CuAl2 precipitates are the mainreasons for high strength of base metal.

Optical macrographs of the welds made using AA2319filler and modified filler with 0.50 % Sc are compared inFig. 5. These macrographs were taken from the top sectionsof the welds. The fusion zone of the welds with AA2319filler (Fig. 5a) is composed of coarse columnar grainsextending from the fusion boundaries to the weld center.Because of the presence of steep thermal gradients and theepitaxial nature of growth process in fusion welds, weldmetal solidification often takes place in a columnar mode[24]. The columnar grains in the fusion boundary also couldlead to poor mechanical properties [25]. It can be observedfrom micrographs (Fig. 5b) that weld metal inoculated with0.75 % Sc has resulted in not only fully equiaxed structuresbut also resulted in reduced grain size. The grain aspect ratio(length/width ratio) is highest in the welds made of AA2319filler and lowest in the welds made of AA2319+0.75 % Sc.Measurement of grain size in the welds made of AA2319filler showed an average columnar grain width ~35 μm andaspect ratios of between 5 and 11. However, AA2319+0.75 % Sc resulted in marked change in grain structure

Fig. 4 The optical microstructure of a base metal and b SEM micro-graph of base metal

Table 3 EDX quantification of precipitate particles in base metal

Element Weight (%) Atomic (%)

Al 44.52 65.39

Cu 55.48 34.61 Fig. 5 The fusion zone optical macrostructure of AA2219 welds madeusing different fillers a AA2319 and b AA2319+0.5 % Sc

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(Fig. 5b) and columnar grains have been replaced by equi-axed grains (~10 μm diameter).

The fusion zone optical microstructures of the weldsmade using AA2319 filler and filler metal modified withdifferent Sc additions are compared in Fig. 6. The gradualtransformation from columnar to equiaxed grain morpholo-gy and simultaneous reduction in grain size can be clearlyseen from the micrographs with the increasing Sc addition.The addition of Sc, in increasing amounts, resulted in sig-nificant grain refinement in the welds. In Fig. 6a Sc, freeweld shows completely columnar grains in the fusion zone.For Sc levels less than ~0.25 %, only a small reduction ingrain size was observed, compared to that seen with the Scfree weld (Fig. 6b). However, grain size of the fusion zonewas found to decrease (~10 μm) with the further increase ofSc from 0.50 to ~0.75 % in the weld metal (Fig. 6c and d).The effect of scandium additions on mean grain size of weldmetal is shown in Fig. 7. From the Fig. 7, it can be observedthat the slope of the plot is steep up to 0.5 % Sc and withfurther addition of Sc the slope is decreased. These obser-vations are consistent with the observations made by Nor-man et al. [18] during MIG welding of 7000 series alloysusing Al–Sc fillers. The minimum level of Sc required toachieve grain refinement, therefore, depends on the presenceof the L+Al3Sc phase field, which in turn depends on thealloy composition [18].

According to binary Al–Sc phase diagram, eutectic reactionoccurs at 0.55wt%Sc. For hypereutectic compositions, the firstphase to form during solidification is Al3Sc intermetallic

precipitate, which has low lattice parameter mismatch(~1.6 %) with Al. This proeutectic Al3Sc intermetallic precip-itate can act as efficient substrate for the heterogeneous nucle-ation of α-Al grains [26–28]. However in the present study,partial grain refinement was observed even with 0.25 % Sc andcould be attributed to the presence of other elements such as Tiand Zr in the base metal. Ti and Zr act as a strong grain growthrate limiter during solidification, which increases the grainrefining efficiency of the Al3(Sc,(TiZr)) phase by controlling

Fig. 6 The fusion zone opticalmicrostructure of AA2219welds made using differentfillers a AA2319, b AA2319+0.25 % Sc, c AA2319+0.5 %Sc, d AA2319+0.75 % Sc

Fig. 7 Variation of weld metal mean grain size with Sc content in the2319 filler

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the level of recalescence [29]. Norman et al. [28] have reportedthat the addition of Zr and Ti along with Sc has resulted in grainrefinement even at lower Sc levels. Furthermore, heteroge-neous nucleation of α–Al grains might have taken place bythe partially melted Al3Sc particles; those are still present in theweld pool due to fast cooling cycles during welding. However,in the 2319 filler modified with 0.5 % Sc and 0.75 % Sc alloys(Fig. 6c, d), there is a total absence of columnar grains, whichusually nucleate on the fusion boundary and grow into thecenter of the weld. This gives an indication of the very efficientgrain refining ability of Sc, when in excess of the eutecticcomposition. According to the phase diagram, for the 2319filler modified with 0.5 % Sc and 0.75 % Sc alloys, the firstphase to form on crossing the liquidus is the L12 Al3Sc phasewhich precipitates as primary intermetallic particles in theliquid.

The transmission electron microscopy (TEM) micro-graph of base metal (Fig. 8a) shows that the round-shapedCuAl2 precipitates are distributed homogeneously through-out the matrix. Figure 8b shows coarse precipitate particlesin welds madewith AA2319+0.50% Sc filler. Microchemical

analyses of these precipitates in the TEM micrograph showedthat the precipitate regions are rich in Cu and Al and precip-itates have been identified as CuAl2 (Fig. 8c). Due to the fast

Fig. 8 The TEM micrograph ofa AA2219 base metal, b weldsmade of AA2319+0.50 % Scshowing CuAl2 precipitates, cEDS spectrum, d welds made ofAA2319+0.50 % Sc showingshowing Al3Sc primaryparticle, e EDS spectrum

Fig. 9 Weld cross-section comparing base metal, HAZ, and weldmetal optical microstructure of AA2319 filler

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heating and cooling cycle in the welding, these CuAl2 precip-itates would not have dissolved completely and survived inthe coarsened condition. Figure 8d is a typical TEM micro-graph showing Al3Sc primary particle seen at the center of therefined grain in weld made using AA2319+0.50 % Sc filler.Microchemical analyses of these Al3Sc particles in the TEMmicrograph showed that the particles are rich in Sc, Cu, and Al(Fig. 8e). As discussed in the previous section, the presence ofa high density of these small Al3Sc primary particles is mainlyresponsible for the grain refinement of welds made usingscandium.

Figure 9 shows a weld cross-section comparing basemetal, HAZ, and weld metal microstructures of AA2319filler. Weld metal exhibits the usual coarse columnar den-dritic structure. No significant grain growth was observed inthe HAZ. A zone of fine equiaxed grains can be seen next tothe fusion boundary from the micrograph. Existence of fineequiaxed zone adjacent to the fusion line was reportedearlier in Al–Li alloy welds containing Zr and the regionhas been called as nondendritic equiaxed zone (EQZ) [30]

or the chill zone [31]. It is reported that the A13Zr particles,that are present in the base metal, could have acted asheterogeneous nucleation sites and resulted in EQZ at thefusion boundary. Due to the low temperature and sluggishfluid flow conditions at the fusion boundary, these A13Zrparticles survive without complete dissolution. However,due to high temperatures at the center of the weld, thesedispersoids dissolve and result in usual columnar dendriticgrowth [30].

SEM micrograph of the welds made using AA2319 fillerand AA2319+0.50 % Sc filler are shown in Fig. 10. Weldmetal with AA2319 filler shows long columnar primaryalpha phase grains with eutectic phase at the grain boundary,whereas, fine equiaxed grain structure can be seen in theweld metal micrographs with AA2319+0.50 % Sc filler.Eutectic morphology was found to be coarser and continu-ous when welded with AA2319 filler (Fig. 10a). In contrast,the addition of Sc, in increasing amounts, resulted in finereutectics (Fig. 10b).

3.2 Microhardness

Transverse micro hardness measurements are shown inFig. 11. The weld metal prepared using the 2319+0.75 %Sc casts insert exhibited higher hardness compared withother welds, and was attributed to finer grain structureassociated with higher titanium content. It is evident for allthe samples that the weld metal and HAZ hardness valuesare lower than the base metal hardness. The welds preparedusing the 2319 cast insert exhibited a lower hardness com-pared to other welds which can be attributed to the coarsegrain structure. Age hardening behavior of base metal andwelds with different fillers up to 120 h are shown in Fig. 12.The welds have responded to the post-weld aging treatmentand showed highest hardness around 30 h of aging. The

Fig. 10 SEM micrograph of the welds made using a AA2319, b, cAA2319+0.5 % Sc

Fig. 11 Transverse microhardness data for welds made from AA2319and Sc added AA2319 fillers

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welds prepared with AA2319 fillers exhibited limited agingresponse and the hardness increased from 107 to 130 HVafter aging. Furthermore, Sc added welds displayed higherhardness in the as-welded condition and the hardness in-creased further with the aging treatment. However, withprolonged aging time up to 120 h, hardness values are seento drop to lower values due to over aging. The cooling ratesexperienced by the weld metal in the fusion zone are muchhigher compared to the cooling rates during conventionalcasting and it can be expected that the weld metal wouldremain in super saturated condition, as time is not sufficientto form precipitates. Thus, the formation of CuAl2 precip-itates from the weld metal resulted in the increase in hard-ness during the postweld aging treatment.

3.3 Room temperature tensile properties

Results from tensile tests of base metal and welds with allthe fillers are compared in Table 4. The tensile data for eachcondition are an average of measurements made on threespecimens. The base metal exhibited high strength and

ductility compared with GTA welds and was attributed thefine grain size. It can be seen from Table 4 that the weld-ments prepared with 2319+0.75 % Sc cast insert exhibitedhigh strength and ductility compared with other welds, andcould be attributed the fine equiaxed grain structure. Amongthe inoculated welds themselves, the weldments preparedwith 2319 filler exhibit lower strength and ductility andcould be attributed to the presence of coarse columnargrains. Secondary electron images of fracture morphologiesof the base metal compared to the weld metals are illustratedin Fig. 13. The fracture surface of the base metal is found tohave mostly dimpled features, which is a characteristic ofductile fracture. Ductile and very fine dimpled fracturefeatures are observed in 2319+0.75 % Sc welds as shownin Fig. 13c. Fine dimples are characteristic features of duc-tile materials and hence 2319+0.75 % Sc welds exhibithigher ductility when compared to other conditions. How-ever, the tensile fracture surfaces of the weld with AA2319filler exhibited features with little deformation which couldbe attributed to the coarse columnar grain structure(Fig. 13b). The yield strength and ultimate tensile strength(UTS) of welds were observed to increase after the post-weld aging treatment as shown in Table 4. All the weldsmade using 2319 filler and 2319 with different amounts ofSc displayed significant improvement in strength, but at thecost of ductility. The peak strengths of GTA welds wereobtained at an aging temperature of 190 °C for 30 h. Furtheraging at this temperature would have led to coarsening ofprecipitates causing the weld strengths to deteriorate rapidly.It can be concluded that the welds can be aged directly afterwelding without the intermediate solution treatment. Solu-tion treatment of welded joints could lead to problems suchas distortion and additional cost. However, from the hard-ness and tensile results it looks like refining doesn’t seem tohave direct effect on age hardening response as, all weldswith 2319 filler and 2319 with different amounts of Sc haveshown similar response to the aging treatment.

3.4 High-temperature tensile properties

Yield strength (YS), UTS, and elongation of the weld tensilesamples at different temperatures are presented in Fig. 14a–c.

Fig. 12 Age-hardening behavior of base metal and welds made withdifferent fillers

Table 4 Tensile properties ofbase metal and welds made withdifferent fillers

AWAs-welded sample, PWAHpost-weld aged-hardened sample(aging time, 30 h)

Filler Yield strength (MPa) Ultimate tensile strength (MPa) Elongation (%)

AW PWAH AW PWAH AW PWAH

Base metal 257±5 262±8 362±10 394±8 12±2 11±1.5

AA2319 98±9 106±7 233±6 262±6 6±0.5 5±1

AA2319+0.25Sc 124±6 132±6 246±7 279±8 7.3±1 6.1±0.5

AA2319+0.50Sc 136±5 145±7 257±11 288±6 8.4±0.5 7±0.3

AA2319+0.75 Sc 145±7 156±8 268±12 298±9 9.7±1 7.4±1

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YS and UTS values continuously decreased with in-creasing test temperature and this decrease is observedto be rapid above 150 °C in both cases. The decrease intensile properties was attributed to rapid coarsening ofstrengthening precipitates [32]. On the other hand, elon-gation increased with temperature. It can be seen thatwith the base metal YS and UTS values are higher thanthe weld metal and could be attributed to the cold workedstructure. Among the weldments made of Sc-modifiedAA2319 filler, YS and UTS were highest for the weldmentsprepared with AA2319+0.75 % Sc and the lowest for thoseprepared using unmodified AA2319 filler.

For the welds prepared by Sc added AA2319 filler,decrease in YS and UTS with the increase in temperatureis found to be less compared to the welds prepared byAA2319 filler, which can be attributed to the presence ofAl3Sc precipitates that are stable at elevated temperatures[29]. Furthermore, Srinivasa Rao et al. [33] reported that theaddition of Sc to AA2219 alloys increased the eutectictemperature with a corresponding decrease in the freezingrange. Increase in the eutectic temperature could also aid tothe stability of the precipitate particles and reduce graincoarsening at elevated temperatures. As discussed earlier,Sc addition also resulted in modification of the eutecticmorphology resulting smaller disconnected eutectic par-ticles as against the welds prepared with AA2319 filler.Increase in elongation with Sc addition could be attributed

to the fine grain structure as well as fine disconnectedeutectic phases that are present in the weld metal.

4 Conclusions

Addition of Sc through the prealloyed AA2319 cast insertsin to the AA2219 weld pools resulted in fine equiaxedgrains in the fusion zone and improved yield strength,ultimate tensile strength of the joint with a significant im-provement in ductility. The increase in scandium in the weldmetal has resulted in significant grain refinement of weldmetal caused by heterogeneous nucleation of Al3Sc particlesduring the solidification. Microstructural examinationshowed that the grain size was the least with equiaxedmorphology in the welds made using the 2319+0.75 % Sccast insert compared to the coarse columnar grains in thewelds made from AA2319 cast insert. The welds preparedusing the 2319+0.75 % Sc cast inserts also exhibited highertensile strength, hardness and ductility compared with otherwelds, and was attributed to finer grain structure due tohigher Sc content. Improvement of ductility with Sc addi-tion was also attributed to the finely dispersed eutectic phasein the weld metal. Sc addition is proved to improve theelevated temperature tensile strength and was attributed tothe stable ScAl3 precipitates and increase of the eutectictemperature with Sc addition. All the welds have responded

Fig. 13 Tensile fracturesurfaces of a AA2219 basemetal, b AA2319 weld, cAA2319+0.75 % Sc weld

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to the post-weld aging treatment and improved hardness, YSand UTS values after 30 h aging.

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Fig. 14 Variation of a ultimate tensile strength vs. temperature, b yieldstrength vs temperature, and c elongation vs temperature for base metaland weldments in the as welded condition

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