Transactions of The Indian Institute of Metals
Microstructural characterization of dissimilar friction stir welds between AA2219 and AA5083 J.J.S. Dilip1, M.Koilraj2, V.Sundareswaran2, G.D. Janaki Ram1 and S.R. Koteswara Rao3 1
2 3
Materials Joining Laboratory, Dept. of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600 036, India Department of Mechanical Engineering, Anna University, Chennai-600025 Tagore Engineering College, Rathinamangalam, Cheenai-600 048, India
Email:
[email protected] Received 25 March 2010 Revised 28 July 2010 Accepted 29 July 2010 Online at www.springerlink.com © 2010 TIIM, India
Keywords: friction stir welding; dissimilar welding; aluminum alloys
Abstract Fusion welding of dissimilar aluminum alloys is very challenging. In the present work, Al-Cu alloy AA2219-T87 was friction stir welded to Al-Mg alloy AA5083-H321. Weld microstructures, hardness, and tensile properties were evaluated in as-welded condition. Microstructural studies revealed that the nugget region was primarily composed of alloy 2219, which was placed on the advancing side. No significant mixing of the two base materials in the nugget region was observed. Hardness studies revealed that the lowest hardness in the weldment occurred in the heat-affected zone on alloy 5083 side, where tensile failure were observed to take place. Tensile tests indicated a joint efficiency of around 90%, which is substantially higher than what can be achieved with conventional fusion welding. Overall, the results show that satisfactory butt welds can be produced between AA2219-T87 and Al-Mg alloy AA5083-H321 sheets using friction stir welding.
1.
Introduction
One of the main problems in fusion welding of aluminum alloys is weld solidification cracking (hot cracking). Solidification cracking in aluminum alloys is extremely sensitive to weld metal composition, which depends on the composition of the filler metal, composition of the base metal, and amount of dilution. Therefore, one must carefully choose the filler metal composition and/or welding parameters such that the resultant weld composition is not susceptible to solidification cracking. This can be done relatively easily in the case of fusion welding of similar aluminum alloys. Guidelines for selection of filler metals exist for different classes and types of aluminum alloys. However, when it comes to fusion welding of dissimilar aluminum alloys, solidification cracking is not easy to deal with. For many dissimilar aluminum alloy combinations, no filler metals exist that can produce crack-free welds. Even in cases where there is a reasonable filler metal option, one cannot achieve satisfactory joint efficiencies. For these reasons, fusion welding of dissimilar aluminum alloys is generally avoided in industry. Solid-state welding processes are ideally suited for welding of dissimilar aluminum alloys. Because these processes do not involve melting, the issue of weld solidification cracking does not arise. Similarly, solid-state welding processes overcome a variety of other problems in fusion welding of aluminum alloys such as porosity, segregation, brittle intermetallic formation, and heat-affected zone liquation cracking. Among the solid-state welding processes, friction stir welding (FSW) is very attractive for
welding of dissimilar aluminum alloys as it is suitable for producing welds in a variety of joint configurations, including butt joints. FSW has been extensively researched in the recent past. Defect-free friction stir welds have been demonstrated in a variety of engineering materials, including aluminum alloys, magnesium alloys, steels, titanium alloys, etc. Significant insights have been gained into the thermo-mechanical phenomena involved in the process. Sizable amount of information has been generated on the effects of process parameters, microstructural evolution, and structure-property correlations. Recent works by Mishra and Ma [1] and Nandan et al. [2] have comprehensively reviewed these developments. A number of researchers have explored FSW for welding of dissimilar metals and alloys. Several different dissimilar aluminum alloy combinations have been successfully friction stir welded with reasonably good joint efficiencies [3-10]. In most of these investigations, significant mechanical mixing of the two alloys was noticed in the stir zone or weld nugget with complex vortex, whorl, and swirl features characteristic of chaotic-dynamic mixing. Also, it has been shown that the locations of two dissimilar alloys exerted a significant effect on material flow pattern and the resultant weld quality [1]. Many investigators reported that it is beneficial to place the stronger of the two materials on the advancing side [4,11]. In their studies on FSW of cast aluminum alloy A356 to wrought aluminum alloy AA6061, Lee et al. [12] found that the stir zone predominantly consisted of the material that was placed on the retreating side. Whereas, Priya et al. [9], working on FSW of AA2219 to AA6061, observed that the material placed on the advancing side dominated the stir
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zone. Similarly, pin positioning with respect to the joint line was shown to be critical during FSW of dissimilar metals and alloys. For example, Kimapong and Watanabe [13] achieved stronger aluminum/steel dissimilar welds by offsetting the tool towards aluminum. Similarly, Cavaliere and Panella [14], in their studies on FSW of AA2024 to AA7075, found that offsetting the tool into the AA2024 side (advancing side) significantly improved the joint fatigue properties. From the foregoing, it appears that success in dissimilar FSW requires careful judgment with respect to material placement, tool positioning, and process parameters depending on the properties of the materials to be joined. Al-Cu alloy AA2219 and Al-Mg alloy AA5083 are two widely used aluminum alloys, especially in aerospace industry. Joining between these materials is often called for. However, practically no information is available in open literature on FSW of this particular material combination. Therefore, an attempt is made here to investigate FSW of AA2219-T87 to AA5083-H321. Our study focuses on weld microstructures and tensile properties.
2. Experimental work The materials used in this study are 5 mm thick sheets of Al-Cu alloy AA2219-T87 (Al-6.5wt.%Cu-0.3wt.%Mn) and Al-Mg alloy AA5083-H321 (Al-4.5wt.%Mg-0.7 wt.%Mn). The sheets were cut and machined into rectangular coupons (150 mm long and 75 mm wide) (coupon width along the sheet rolling direction) for friction stir welding. Welding was carried out in butt joint configuration using a commercially available friction stir welding machine (make: RV Machine Tools, Coimbatore, India) by employing the following process parameters: rotational speed - 650 rpm, welding speed - 55 mm/minute, axial load - 1 ton (9.8 kN). These process parameters were arrived at after conducting extensive welding trials. Alloy 2219 was placed on the advancing side. A straight-cylindrical FSW tool made of M2 grade tool steel (pin length: 4.8 mm, pin diameter: 4.8 mm, shoulder diameter: 14.6 mm) was employed for friction stir welding. The pin was positioned at the center of the joint line. Weld microstructures in as-welded condition were examined using stereo, optical, and scanning electron microscopes (FEI Quanta 200). Studies were carried out using short-transverse, long-transverse, and top surface sections in both unetched and etched conditions (modified Keller’s reagent). For transmission electron microscopy (TEM), thin sections cut from the weld nugget (using a low speed diamond saw) were mechanically ground to a thickness of 100 microns, from which 3 mm diameter disks were obtained (mainly consisting of alloy 2219 region of the weld nugget). The disks were subsequently thinned by electropolishing on a twin jet polisher at 15 V, “ 20 °C, in a 25% nitric acid + 75% methanol solution. Samples were examined using a Phillips CM12 transmission electron microscope operated at 200 kV. Transverse tensile specimens with a gage length of 25 mm and a width of 6 mm (overall length: 100 mm) were prepared from the weld coupons in as-welded condition. Room-temperature tensile tests were conducted on three samples as per ASTM E8 on a universal tensile testing machine. Vickers microhardness measurements were made across the weldment using a load of 100 g applied for 15 s.
3.
Results
3.1 Microscopy The microstructures of alloy 2219 and alloy 5083 base materials are shown in Fig.1. Both the base materials contained a large number of undissolved second-phase intermetallic particles, as can be seen in Fig.1c and Fig.1d. The second-phase particles (10-15 ìm in size) present in alloy 2219 were confirmed to be Al2Cu (θ) eutectic particles based on X-Ray Energy Dispersive Spectroscopy (EDS) analysis (Fig.1e). The second-phase present in alloy 5083 were found to be iron/manganese aluminides (Fig.1f). Compared to alloy 2219, alloy 5083 contained fewer and finer second-phase particles. Figure 2 shows the macrostructures of the weld on shorttransverse, long-transverse, and top surface sections. Various regions of interest on these macrographs are shown at a higher magnification in Fig.3. Three distinct microstructural zones – stir zone or weld nugget (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) – were present in the weld. With respect to the original joint line, the weld nugget was found to be shifted slightly towards the advancing side of the weld. The shape of the stir zone is neither basin-type nor elliptical-type, which are the two most common stir zone shapes. The stir zone appeared more like a rectangular box, with a “flow arm” extending towards the advancing side of the weld top surface. It is known that the shape of the stir zone can widely vary depending on the process parameters, type of materials, tool design, etc. [1,2]. Judging from its etching response, the stir zone appeared to be homogeneous, consisting mainly of alloy 2219. However, some streaks of alloy 5083 were found to occur in the flow arm. On the weld top side, the stir zone existed below a layer of alloy 5083, which extended across the joint line (Fig.3, Point “a”). Similarly, on the weld bottom side, a layer of alloy 2219 on the advancing side and a layer of alloy 5083 on the retreating side existed underneath the stir zone, which were separated by the stir zone itself (Fig.3, Point “b” on the retreating side, for example). The stir zone showed a characteristic “onion-ring” pattern, typically found in friction stir welds (Fig.3, Point “b”), and was composed of very fine recrystallized grains (Fig.3, Point “c”). The weld longtransverse section showed a central dark-etched region corresponding to the weld nugget, with a layer of lightetching alloy 5083 on top and bottom sides. This can be understood from the shape of the weld nugget as seen on the weld short-transverse section. The weld long-transverse section more clearly revealed the onion-ring pattern in the stir zone (Fig.3, Point “d”). The weld top surface section (at a depth of less than 0.5 mm from the actual weld top surface) showed alloy 5083 over two thirds of the weld width. Towards the advancing side, a region fine recrystallized grains corresponding to the weld nugget appeared. In this region, streaks of alloy 5083 were found to occur (Fig.3, Point “e”), which correspond to the flow arm seen on the weld shorttransverse section. Overall, no macroscopic defects or weaklybonded regions were observed in the weld region, which extended through the full thickness of the sheets. The TMAZ on the advancing side showed highly deformed grains, with a clearly discernible SZ/TMAZ and TMAZ/HAZ boundaries (Fig.4a). However, on the retreating side, these interfaces were rather diffused, especially the latter (Fig.4b). In the HAZ, on either side of the weld nugget,
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Fig. 1 : Microstructures of base materials: (a) Optical, alloy 2219, (b) Optical, alloy 5083, (c) SEM-BSE, alloy 2219, (d) SEM-BSE, alloy 5083, (e) EDS spectrum of a typical second-phase particle in alloy 2219, (f) EDS spectrum of a typical second-phase particle in alloy 5083.
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Fig. 2 : Macrostructures of the dissimilar weld.
Table 1 : Results of tensile testing (average of three tests) Material/ Condition
0.2% UTS Proof Stress (MPa) (MPa)
Elongation (%)
Base metal 5083-H321
261
292
26
Base metal 2219-T87
365
456
20
Dissimilar friction stir weld
228
265
13
Fig. 3 : Optical micrographs corresponding to Points “a” to “e” in Fig.2.
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Fig. 4 : TMAZ microstructures: (a) Advancing side, (b) Retreating side.
(c)
(d)
Fig. 5 : (a) SEM-BSE image of the weld nugget in as-polished condition. (b) EDS spectrum obtained on a coarser second-phase particle in the weld nugget. (c) SEM-BSE image of a light-etching streak in the flow arm, (d) SEM-BSE image of the onion-ring pattern (long-transverse section).
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there were no noticeable changes in the grain structure compared to the respective unaffected base materials. For Scanning Electron Microscopy (SEM), as-polished weld samples and back-scattered electron (BSE) imaging were mainly used. The weld nugget region showed a large number of second-phase particles, distributed more or less uniformly (Fig.5a), occurring in two different size ranges. The coarser particles were in the size range of 2-5 µm. The finer particles were of sub-microscopic size, which could not be clearly revealed in SEM examination. EDS analysis revealed that the coarser second-phase particles in the weld nugget contained Al and Cu (Fig.5b), closely matching with the stiochiometry of Al2Cu. In the matrix region of the weld nugget, an average composition close to that of alloy 2219 was measured using EDS. The streaks of light-etching regions observed in the flow arm region of the weld nugget were confirmed to be alloy 5083 with the help of BSE imaging and EDS analysis (Fig.5c, location corresponds to that of Fig.4a). SEM-BSE examination of the onion-ring patterns, did not show any perceivable changes in matrix chemistry across the rings (Fig.5d, location corresponds to that of Fig.3d). However, alignment of second-phase particles conforming to the onionring patterns was observed. Similarly, in the TMAZ on the advancing side, a strong alignment of Al 2Cu second-phase particles was observed (Fig.6a). The TMAZ on the retreating side also showed alignment of manganese aluminides secondphase particles (Fig.6b), although the alignment was not as pronounced as in the advancing side TMAZ. Alloy 2219 region of the weld nugget was examined under TEM to investigate the effects of welding on strengthening precipitates. TEM examination clearly revealed the grain size (1 to 2 µm) in the 2219 region of the weld nugget (Fig.7). A large number of uniformly distributed, fine second-phase particles (less than 250 nm in size) were observed in the 2219 region of the weld nugget (these correspond to the submicroscopic particles noted in Fig.5a). TEM-EDS analysis showed that these particles were rich in Al and Cu, confirming them to be Al2Cu (θ). However, the meta-stable, coherent, disc-shaped, Al2Cu (θ′) strengthening precipitates, typically seen in AA2219-T87 were found to be absent in the weld nugget. No dislocation structures were observed in the weld nugget. 3.2 Hardness and tensile tests Vickers microhardness tests were conducted across the weld (mid-section, 0.25 mm spacing) to ascertain possible microstructure/property variations among the various regions of the weldment. The results are pictorially shown in Fig.8. As can be seen, on the advancing side, there was a significant drop in hardness from the 2219 unaffected base material to the weld nugget boundary. The weld nugget hardness was considerably lower than that of the 2219 base material. On the retreating side, only slight drop in hardness was observed from the 5083 unaffected base material to the weld nugget boundary. The weld nugget showed higher hardness compared to the 5083 base material. As can be expected, 2219 base material showed significantly higher hardness compared to 5083 base material. The results of weld transverse tensile tests are listed in Table 1. All the three weld specimens failed in the 5083-side HAZ (Fig.9). The tensile properties of the two base materials are also listed in Table 1. Alloy 2219 is significantly stronger compared to alloy 5083. The welded specimens showed slightly lower strength compared to 5083 base material specimens.
Fig. 6 : SEM-BSE images at the TMAZ/SZ interface in aspolished condition. (a) Advancing side, (b) Retreating side.
Fig. 7 : TEM micrograph taken in alloy 2219 region of the weld nugget.
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Fig. 8 : Microhardness profile across the weld.
Fig. 9 : Typical failed tensile test samples.
4.
Discussion
The results of this investigation show that friction stir welding can produce satisfactory butt welds between AA2219-T87 and AA5083-H321 sheet materials, demonstrating the unique capabilities of the process in dealing with joining of dissimilar aluminum alloys. Though friction stir welding of dissimilar aluminum alloys has been demonstrated in quite a few earlier investigations, one interesting aspect of the welds made in this study is absence of significant metal mixing. Shigematsu et al. [10] reported similar observations in friction stir welds between alloy 5083 and alloy 6061. In many of the earlier studies, the nugget region was found to be characterized by “chaotic mixing” of the two metals being welded [3,5,8]. Also, the two metals were shown to occur as alternating layers in the onion rings. In the current study, however, no such chaotic mixing was observed, nor the onion rings showed the two metals in alternating layers. Thus, the current study shows that with proper choice of welding parameters one can produce sound friction stir welds even without significant mixing of the two metals being welded. The nugget region was found to be primarily composed of alloy 2219, which was on the advancing side of the weld. In other words, the material placed on the advancing side is pulled into the nugget region significantly more than that on the retreating side. Similar observations were reported by Priya et al. [9]. It is known that during friction stir welding the conditions are such that the material placed on advancing
side experiences higher temperatures and greater deformations than that on the retreating side [1,2]. While these factors can account for this observation, further work is necessary for attempting a detailed explanation in this regard. In any case, it appears that one can achieve higher joint efficiencies by placing the stronger of the two base materials on the advancing side. The weld nugget showed very fine recrystallized grains, a characteristic feature of FSW. The size of the Al2Cu (θ) second-phase particles in the weld nugget was found to be significantly smaller when compared to that in the 2219 base material. This is known to be a consequence of the severe plastic deformation involved in the process. Fragmentation of hard, brittle second-phase particles into smaller pieces during FSW has been widely acknowledged in literature [1,2]. During FSW, temperatures in the nugget region can exceed the solvus temperature of Al2Cu (θ), although the residence times are very brief. However, within the available time, smaller second-phase particles can be expected to undergo greater amount of dissolution than coarser second-phase particles. This can explain why in the nugget region some of the second-phase particles are finer and some are coarser. In the case of metastable coherent Al 2 Cu strengthening precipitates (θ′), which are much smaller than the stable incoherent Al 2Cu (θ) second-phase particles, the timetemperature situation in the weld nugget can result in complete dissolution. Reprecipitation of strengthening precipitates during cooling cannot occur due to the rapid cooling rates involved in FSW. This explains the absence of strengthening precipitates in the weld nugget, as revealed in TEM examination. The TMAZ and HAZ regions on 2219 side can be expected to experience some coarsening of the strengthening precipitates [1,2]. Similarly, the HAZ on the 5083 side can be expected to undergo some amount of softening due to annealing effects [1,2]. The results of hardness tests confirm the above effects. The lowest hardness in the weldment was observed in the HAZ of alloy 5083. In dissimilar welding, a rule-of-thumb for qualification of welds is that failure should occur in the weaker of the two base materials, away from the weld. Friction stir welding did not quite meet this requirement in the current case, with all the failures occurring in the 5083-side HAZ, where the lowest hardness levels in the entire weldment were recorded. The weld specimens showed slightly lower strength compared to 5083-H321 base material specimens. Loss of cold work in the HAZ due to annealing effects can account for this. Nevertheless, the current study shows that one can achieve sound butt joints between 2219-T87 and 5083-H321 sheet materials with a joint efficiency of around 90% (based on alloy 5083) by using FSW, which is much higher than what can be achieved with conventional fusion welding processes.
5. Conclusions z
z
Friction stir welding can produce satisfactory butt welds between AA2219-T87 and AA5083-H321 sheets with a joint efficiency of around 90% (based on alloy 5083). For this specific material combination, failures occur in the heat-affected zone of alloy 5083. Significant mixing of the two base materials in the nugget region does not seem to be a necessary condition for producing good friction stir welds. The welds produced did not show significant mixing of the two base materials.
z
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In friction stir welding of dissimilar aluminum alloys, the material placed on the advancing side dominates the nugget region. By placing the stronger of the two base materials on the advancing side, one can achieve higher joint efficiencies.
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