GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR

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1 Trans. Indian Inst. Met. Vol.57, No. 5, October 2004, pp TP 1902 GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR S.R.Koteswara Rao 1, G.Madhusudhan Reddy 2, K.Srinivasa Rao 1, P.Srinivasa Rao 3, M.Kamaraj 1, K.Prasad Rao 1 1 Dept of Metallurgical and Materials Engg., IIT Madras, Chennai Defence Metallurgical Research Laboratory, Hyderabad LHWC/LPSC,ISRO, Bangalore sajjarkr@yahoo.com (Received 6 February 2004 ; in revised form 22 April 2004) ABSTRACT Weld metals of AA 2219 (Al-6%Cu), welded with 2319 filler, exhibit yield strength values of about 40% compared to their base metal counterparts. This study is an attempt to improve the yield strength of 2219 welds by using fillers containing scandium. Pitting corrosion behavior of the welds has also been studied. Welds were made with Al-Cu, Al-Cu-Sc, Al-Mg, and Al-Mg-Sc fillers. Manual AC-TIG process has been employed to weld 8.5mm thick 2219 T87 alloy plates. Tensile and corrosion behavior of welds was studied and results were substantiated by metallography. Metallographic studies showed extensive grain refinement in welds made of scandium containing fillers. Presence of scandium, improved the resistance to cracking in case of welds made of Al-Mg fillers. Improvement in mechanical properties has been observed due to the presence of scandium and the rise in percent elongation was significant. Crack free welds with significant improvement in mechanical properties were obtained with a weld metal composition having small amounts of Mg and Sc. Scandium addition did not alter the pitting corrosion resistance of the weld metals. Magnesium containing weld metals were found to be inferior in terms of pitting corrosion resistance. 1. INTRODUCTION Heat treatable aluminum alloy AA2219, with % copper as the main alloying element is widely used for aerospace applications. These applications involve structural components such as rocket shells, cryogenic tanks, engine casings and some other smaller components. Alloy 2219 is preferred mainly due to its good strength at high temperature as well as cryogenic temperatures(200 to 200 C). Good specific strength, high ductility, formability and reasonable corrosion resistance are other advantages. Though, welding processes such as electron beam welding and friction stir welding are proved to result in sound and high strength welds, the cost and technology involved and limitations on their applicability in some of the situations, make it necessary to use Gas Tungsten Arc Welding (GTAW) process in fabrication. GTAW is by far the cheapest and easy to use process, which can provide quality welds in case of aluminum alloys. However, certain disadvantages do exist in using fusion welding processes on the heat treatable aluminum alloys. Solidification cracking and loss of strength in the weld metal and heat affected zone are the two most important problems. One of the important aspect is the amount of alloying elements in an alloy. At a very low level of alloying elements the amount of liquid films at grain boundaries is very low and hence the low susceptibility to cracking. At a very high levels of alloying elements, for example in the case of 2219 alloy with about 6% copper, there is excess liquid available during the solidification which flows in to the cracks that form and heals the cracks. Even in case of highly weldable

2 TRANS. INDIAN INST. MET., VOL. 57, NO. 5, OCTOBER 2004 alloys such as 2219, the joint strength is only 40% when compared to the T87 base metal strength. This is true both in autogenous welds as well as those welded with the matching filler 2319, which contains slightly higher contents of Ti and Zr. The loss of strength is due to the melting and quick resolidification, which renders all the strengthening precipitates to dissolve and the material is just as good as solution treated. Due to the poor weld metal strength, the sections of a structure, which are to be welded has to be twice or thrice thicker compared to the rest of the area. Most of the times, this is achieved by starting with a thick sectioned plate and reducing the thickness in sections, which are away from the welding area. Chemical milling or CNC pocket milling is employed to cut away the portions and reduce the total weight of the tank or a shell. Obviously the whole fabrication becomes costly and cumbersome. If the strength of the weld metal/joint can be increased by about 20 to 30% the cost and weight savings would be significant. Study of recent literature indicates that scandium can provide the necessary grain refinement as well as strengthening in the weld metals. As far as wrought alloys are concerned, the strengthening effect of scandium is well documented. Seidman et al 1 have investigated various aluminum scandium alloys at room temperatures as well as at temperatures up to 500 C and found that room temperature flow stress of aluminum increases significantly from about 20 MPa for pure aluminum to about 200 MPa with an addition of 0.3% scandium. More interestingly they also showed that this strength value would not change in case of any creep experiment conducted below 300 C. Apart from being added to pure aluminum, scandium also has been added to many existing aluminum alloy systems, Al-Mg system being the most widely experimented with. Aiura et al 2 have added scandium to 5083 alloy and investigated the effect. Small additions of scandium (0.2%) resulted in about 25% increase in yield strength of an extruded product. It has been shown by Kendig et al 3 that an Al-Mg alloy when added with suitable amounts of scandium and zirconium gives an yield strength up to 640 MPa. They have attributed this increase in strength to three mechanisms: sub-micron grain size, precipitation strengthening due to Al 3 (Sc,Zr), and solid solution strengthening, being effective in that order. Ocenasek et al 4 have investigated the effects of scandium additions to alloy 5754(Al-Mg-Mn-Cr). Fuller et al 5 have shown that small amounts of Zr additions to scandium containing alloys would delay the onset and kinetics of over aging at 350 to 375 C. However, they conclude that addition of Zr does not have any effect on the hardness of the alloy. Many similar reports on different aluminum alloy systems prove the effectiveness of scandium in grain refinement, strengthening and creep resistance. Importance of keeping Mg as one of the alloying elements in aluminum scandium alloys is highlighted by Marquiss et al 6. Davidov et al 7 have enlisted the principles of making scandium additions to different existing aluminum alloy systems. They bring out, from literature as well as their own work the point, that out of the alloying elements generally used in aluminum alloys (Mg, Zn, Li, Cu, Si), Zn, Mg and Li do not react with scandium and hence it is more expedient to add scandium to these alloying systems. Very few researchers have investigated the effect of scandium additions on the corrosion behavior of these alloys. Ahmad et al 8 have added scandium (0.1% to 0.3%) to an Al-2.5% Mg alloy (5052) and found that scandium addition does not introduce any appreciable loss in corrosion resistance in 3.5% NaCl solution. From the survey of literature it can be seen that the introduction of scandium is redefining aluminum alloys by raising the strength levels and creep properties. Some other studies have indicated positive results in case of strength at cryogenic temperatures and fracture toughness 9. A kind of summary is given by Irving 10 on the use of scandium in the weld filler materials. In case of 6061 alloy cracking got reduced to zero with a small addition of scandium. In case of Al-Mg-Sc alloys, Lathabai et al 11 showed that the cracking susceptibility does not change at a scandium level of 0.26%, during a varestraint test, and they have suggested that weld metals might need higher levels of scandium content when compared to their wrought alloy counterparts to obtain a good strengthening effect and resistance to solidification cracking. Aluminum alloys belonging to 2000 and 7000 series have been investigated by Norman et al 12. Wedge shaped castings of the above alloys with different scandium contents are made, slices of material are cut from thicker as well as thinner sections of the wedge to study the effect of cooling rate. The thinner section of the wedge would 452

3 KOTESWARA RAO, et.al. : GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR have undergone a cooling rate equivalent to the weld metals in TIG or MIG welding and the material taken from that section would simulate the weld metals. This method is adopted to reduce the time involved in making new fillers. They have reported significant increase in ultimate tensile strength, about 100MPa in case of 2024 alloy and up to 200 MPa in case of 7475 alloy. Some work in this area has been done at Defense Metallurgical Research Laboratory by Mukhopadhyay et al 13 on 7010 alloy. They have concluded that scandium greatly increases the hot cracking susceptibility and about 25% rise in weld joint strength in the as welded condition. The present work aims at finding out the effect of scandium and Zr additions, to some of the existing filler alloys. In the presence of large amount of copper in alloy 2219, how effective these additions are, in improving the mechanical properties of these welds, and what is their effect on corrosion behavior are the issues being considered. 2. EXPERIMENTAL The chemical compositions of the base material (AA2219) and the fillers used in this study are given in Table 1. Manual AC-GTAW process is employed to weld two 8.5mm thick 2219 (T87) plates to form a joint. Joint design used is V-groove butt with 60 included angle and high purity argon is used for shielding and back purging. The electrodes and base plates are acid pickled, water and acetone washed and finally wire brushed just before welding. Three passes were used to complete the weld. Welding parameters used are given in Table 2. It can be noted that filler materials selected were 2319 and 5356 and their equivalents containing about % scandium, which are designated as 2319Sc and Filler alloy 5025 was commercially available. The filler 2319Sc is made through the melting route. Ingots are rolled in to sheets of thickness 2.5mm, homogenized and cut into thin slices which can be used like stick electrodes for manual GTA welding. Welds are also made with a combination of 2319Sc and 5025 filler wires, by pre-placing weighed amounts of these wires in a groove and making a bead on plate run. This resulted in a weld metal with an approximate composition of Al-6Cu-0.25Sc-0.4Mg. Specimens cut from the welds were prepared for metallographic examination on the surface as well as in the through thickness direction. Etching the polished specimens with freshly prepared Keller s reagent revealed the microstructure. Table 1 CHEMICAL COMPOSITIONS OF FILLERS AND BASE MATERIAL USED Filler Material Cu Mg Sc Zr Ti Fe Si Mn Cr V 2219(Base Material) Sc Table 2 WELDING PARAMETERS USED Process Welding Gas Flow Welding Joint Current (A) Rate(cft/h) Speed(mm/min) Configuration Manual V-Groove AC-TIG Butt (3 Passes) 230 (back purging)

4 TRANS. INDIAN INST. MET., VOL. 57, NO. 5, OCTOBER 2004 Fig.1 : Microstructures of 2219 welds made with different fillers. 454

5 KOTESWARA RAO, et.al. : GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR Etched specimens were primarily examined for shape and size of grains. Some of the typical microstructures are presented in Fig.1. Micro-hardness survey of the weldments is done using Shimadju micro hardness tester. Values are obtained across the weld up to a distance of 20 mm from the fusion line. At least 10 values are taken in each area i.e in the weld; interface and the HAZ and average values are presented as a graph in Fig.2. Tensile testing of sub-size specimens made according to the ASTM E8 specification is done on an Instron universal testing machine. Tensile testing was not done on welds made with 5356 filler as visible cracks were observed at the fusion boundary. Values of yield strength (proof), UTS and percent elongation are presented in Table 3. All values presented are average values of at least 3 specimens. Software based PAR Basic electrochemical system has been used for conducting potentio-dynamic polarization tests to study the general and pitting corrosion behavior of base metal and welds. Saturated calomel electrode (SCE) and carbon electrode were used as reference and auxiliary electrodes respectively. All the experiments were conducted in 3.5% NaCl solutions with ph adjusted to 10. The potential scan was carried out at mv/sec with initial potential of 0.25V(SCE) to final potential of pitting. The exposure area for these experiments was 1 cm 2. The potential at which current increases drastically is treated as critical pitting potential (E pit ). Specimens exhibiting more positive potential were considered as those with better pitting corrosion resistance. Fig.2 : Joint hardness profiles of welds made with different fillers Table 3 TENSILE TESTING RESULTS (AVERAGE OF 3 TESTS) S.No Filler Material Proof UTS % Region of Strength (MPa) (MPa) elongation failure T87, Base Material Weld Sc Weld 4* Interface Sc+0.4%Mg Weld *Results are scattered, due to the fissures at the weld interface. Best values are presented 455

6 TRANS. INDIAN INST. MET., VOL. 57, NO. 5, OCTOBER RESULTS AND DISCUSSION 4.1 Al-Cu-Sc Welds From Fig. 1 (a) and (b) it is very clear that addition of Sc has resulted in fine equiaxed grains at the center of the weld metal. The grain structure presented in Fig.1 (b) is actually taken from the root pass area and shows that there is no grain coarsening due to the subsequent welding passes. However, the effect does not get translated into increase in strength as can be seen from tensile testing results presented in Table 3. The rise in yield strength is just about 10% and is not significant mainly because of the low value of Hall-Petch constant for most aluminum alloy systems. However, there is a significant improvement in the ductility of the weld metal due to grain refinement. Percent elongation improved by about 70%, as can be seen from the same table. As for pitting corrosion resistance (Fig. 3 and Fig. 4) welds made of 2319Sc show better corrosion resistance compared to the 2319 welds. This could be possibly due to the finer grain size resulting in lesser segregation of copper to the grain boundaries. It can also be seen (Fig. 3), that the Sc weld metals also showed better corrosion resistance, when compared to 5025 and 5356 weld metals. 4.2 Al-Mg-Sc Welds Though it is well known that the Al-Mg fillers are not compatible with Al-Cu alloys as for welding, they have been considered for this study to find the effectiveness of Sc in resisting the solidification cracking. Form Table 1 it can be seen that the two Al-Mg filler alloys used, differ in that the 5025 alloy is a scandium version of 5356 (0.27%Sc and 0.17%Zr added to the 5356 alloy). When the welds are made using 5356 filler, visible cracks appear at the fusion boundary where the base metal (2219) gets diluted with the filler alloy. This is mainly due to the fact that Mg addition in an Al-Cu alloy greatly increases the melting range and leads to severe solidification cracking. Consequently, alloys such as 2024, 7075 that contain both Cu and Mg as alloying elements, are not used in fusion welded structures. But, when 5025 filler, which is an alloy derived from 5356 by adding small amounts of scandium and zirconium, is used for welding, such cracks are not visible. Scandium addition results in fine equiaxed grains in the weld metal (Fig.1 (d)). However, close observation of the microstructure at the fusion line reveals segregation and micro fissures at the grain boundaries in some regions. Therefore, during the tensile testing of the weld joints, the results are highly scattered and the ductility is very poor in Fig.3 : Variation in weld metal hardness with different fillers 456

7 KOTESWARA RAO, et.al. : GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR Fig. 4 : Pitting Potentials (E pit ) of Weld metals of different fillers spite of higher hardness of the weld metal as shown in Fig. 5. Correlation between the hardness of the weld metal and strength of the weld metal is not observed which is mainly due to the presence of micro fissures at the fusion line as shown in Fig.1 (f). Though the scandium additions help in reducing the severity of cracking, it could not completely avoid it and hence the filler 5025, though it offers high strength does not suit the welding applications of 2219 alloy. This is mainly due to the very small ratio of Cu to Mg (6:4), which results in severe cracking in the absence of Sc and Zr. The high weld metal hardness, particularly at the interface of welds made with 5025 (132VHN) as shown in Fig.2, indicates that, smaller amounts of Mg and Sc when mixed with 2219 composition could result in higher strength weld metals with lesser or no cracking. This observation is derived from the fact that at the fusion boundary the dilution of the weld metal is high and that area contains more of copper and less of Mg in contrast to the center of the weld. Even the center of the weld, where the Mg content is higher exhibits a higher hardness value (112VHN) than any other weld metal considered as can be seen from Fig. 5. Therefore there is a definite raise in weld metal hardness when Mg is present in the weld metal, and an optimum level of Mg and scandium contents that would result in crack free welds with better tensile properties have to be found out. As far as corrosion is concerned pitting corrosion resistance is not affected by the small addition of Sc as can be seen from Fig. 5 and Fig Al-Cu-Sc-Mg welds From the tensile test results (Table 3), it is evident that a small addition of Mg to the weld metal increases the yield strength while maintaining the good ductility level. Consistent tensile test results and careful metallographic observations revealed that using this composition produces crack free welds. The literature clearly shows that the effect of scandium on Al-Mg alloy system is very significant 6,7. One of the reasons proposed for this effect is that Mg acts as nucleant for precipitation of Al 3 Sc particles, which are responsible for high level of strengthening in scandium containing aluminum alloys. Further study is required in this area, to optimize the contents of Mg and Sc in 2219 welds. Optimization is necessary because, higher Mg content 457

8 TRANS. INDIAN INST. MET., VOL. 57, NO. 5, OCTOBER 2004 Fig. 5 : Polarization curves for 2319, 2319Sc even as it hardens the weld metal, could well result in cracking also. At the same time Sc content as it increases, at some level cannot contribute any further to grain refinement, strength and solidification cracking resistance. This particular weld metal composition with small amounts of Mg and Sc in Al-Cu, is not only found to be superior in mechanical properties, but also offers good pitting corrosion resistance as can be seen from Fig. 3. It offers a pitting potential value of 572 mv, which is comparable to the copper containing alloys and is far better than welds made of Al-Mg fillers 5025 and It looks as long as the Mg content is kept low the corrosion resistance can be maintained in 2219 alloy welds. that Cu reacts with Sc to form W-phase, details of which are not available in the literature. Higher scandium content could result in better mechanical properties as some scandium would be available to form Al 3 Sc apart from W-phase. However, when a small amount of Mg is added, it seems to facilitate the formation of hardening precipitates, and at the same level of scandium there is appreciable 4.4 Mechanical Properties and Corrosion A small addition of Scandium to the existing filler composition of 2319 does not improve the strength significantly. One reason could be the amount of scandium in the welds (0.25%) is less than that is required to form Al 3 Sc particles, which are known to impart strength to the matrix. It is also known Fig.6: Polarization curves for 5356,

9 KOTESWARA RAO, et.al. : GAS TUNGSTEN ARC WELDED AA 2219 ALLOY USING SCANDIUM CONTAINING FILLERS - MECHANICAL AND CORROSION BEHAVIOR strengthening of the weld metal (Table 3). Higher Mg and Sc contents have to be investigated to further improve the weld metal strength and to be optimized with respect to corrosion and solidification cracking resistance. In case of Al-Cu weld metals (2319) scandium addition improved pitting corrosion resistance (Fig. 4). Addition of scandium does not alter the pitting corrosion resistance of Al-Mg weld metals (Fig. 6). However, with addition of Mg to Al-Cu-Sc welds, there is a marginal decrease in the pitting resistance as indicated by pitting potential values of 2319Sc (-572 mv) weld and 2319Sc+5025 weld (-586mV), as shown in Fig CONCLUSIONS 1. Small addition of scandium resulted in fine equiaxed grain structure in 2219 aluminum alloy weld metals. 2. Hardness values of weld metals increased by the addition of scandium, particularly in the presence of Mg. 3. Fillers containing high Mg (5356,5025) content resulted in cracking or micro fissures at the fusion boundary, in spite of the presence of scandium. Sound welds could be obtained when weld metal Mg content was kept low, with addition of small amount of Sc. 4. Addition of 0.25% Sc to Al-Cu filler did not result in improvement of strength of the weld metal. However, addition of 0.4% Mg to Al- Cu-Sc filler improved the strength significantly. Scandium addition in general resulted in significant increase in percent elongation. 5. Addition of Sc improved the pitting corrosion resistance in case of Al-Cu welds and it did not significantly affect the pitting resistance of Mg containing welds. REFERENCES 1. David N S, Emmanuelle A M, and David C D, Acta Materialia 50 (2002) Tadashi A N, Sugawara, and Yasuhiro M, Materials Science and Engineering A280 (2000), Kendig K L, and Miracle D B, Acta Materialia 50 (2002), Vladivoj O, and Margarita S, Materials Characterization 47 (2001) Christian B F, David N S, and David C D, Acta Materialia, 51 (2003) Emmanuelle A M, David N S and David C D, Acta Materialia 51 (2003), Davidov V G, Rostova T D, and Zakharov V V, Materials Science and Engineering, A280 (2000) Zaki A, Anwar U H, and Abdul A B J, Corrosion Science 43 (2001) Marcia S D, and Dennis L D, Evaluation of Sc-Bearing aluminum alloy C557 for aerospace applications, NASA Document no. NASA/TM , (2002). 10. Irving B, Welding journal, 76 (1997) Lathabai S, and Lloyd P G, Acta Materialia 50 (2002) Norman A.F, Material Science and Engineering, A00 (2003) Mukhopadhyay A K, Reddy G M, Prasad KS, Kamath S V, Dutta A and Mondal C, In Proc Symp Advances in the metallurgy of Aluminum alloys, (ed) Tiryakioglu M, Indianapolis, USA (2001) p