Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T6

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1 Materials Transactions, Vol. 48, No. 7 (2007) pp to 1937 #2007 The Japan Institute of Metals Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T6 Saad Ahmed Khodir* and Toshiya Shibayanagi Joining and Welding Research Institute, Osaka University, Ibaraki , Japan Dissimilar aluminum alloys such as 2024-T3 and 7075-T6 plates 3 mm thickness were friction stir butt welded. The welding was carried out at a constant welding speed of 100 mm/min and rotation speeds of 400, 800, 1200, 1600 and 2000 min 1. Effects of rotation speeds and fixed location of two alloys on microstructures especially the homogeneity of elemental distribution in the stir zone (SZ), hardness distributions, and tensile properties of the joints were investigated. The homogeneity of constituents of the two alloys in the SZ was analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS). At the lowest rotation speed of 400 min 1 there was no mixing of two alloys in the SZ and a border between them was observed regardless of the fixed location. Increase of rotation speed more than 400 min 1 brought about a mixed structure likewise onion ring with periodic change of equiaxed grain size and heterogeneous distribution of alloying elements in the SZ. At 2000 min 1 of rotation speed the SZ mainly composed of the material fixed on the advancing side. The hardness increased in the zones occupied by 2024-T3 Al alloy and fluctuations took place in the SZ due to the onion ring at the higher rotation speeds till 1600 min 1. At 2000 min 1 of rotation speed the hardness of SZ mainly depend on the material fixed on the advancing side. Hardness minima was in the heat affected zone (HAZ) on the side of AA2024-T3 Al alloy and the values slightly increased as the rotation speed increased. In the case of transverse tensile test, defect-free joints were fractured at the HAZ on 2024-T3 side and a maximum tensile strength of the joints was achieved at 1200 min 1 of rotation speed when 2024-T3 Al alloy was located on the advancing side. On the other hand, the tensile properties of SZ in the longitudinal direction showed higher values when 7075 Al alloy was located on the advancing side. [doi: /matertrans.mra ] (Received March 1, 2007; Accepted April 26, 2007; Published June 13, 2007) Keywords: Dissimilar joining 2024-T3 Aluminum alloy, 7075-T6 Aluminum alloy, friction stir butt welding, rotation speed, fixed location, energy dispersive X-ray spectroscopy, hardness distributions, tensile properties 1. Introduction High strength aluminum alloys such as 2xxx and 7xxx series are difficult to be welded by conventional fusion welding. Over the last decade, friction stir welding (FSW) has offered excellent welding quality to the joining of many alloys such as aluminum alloys, 1,2) magnesium alloys, 3) Cu alloys, 4) and steel alloys. 5,6) FSW is a solid state welding process in which a high temperature deformation is induced into base materials by a rotating tool composed of two parts called shoulder and probe. The frictional heat generated by the welding tool makes the surrounding material softer and allows the tool to move along the joint line. Temperature during welding does not exceed the melting point of base metals. 7) Nomenclature dictates that, the side of base metal (BM) having the same sense of rotation and welding speed vectors is named as advancing side while the opposite side where the rotation and welding are in the anti-sense of the directions is named as retreating side. 8) FSW joint is known to generally possess four zones such as intensively deformed zone called stir zone (SZ), thermomechanically affected zone (TMAZ), heat affected zone (HAZ), and non affected BM. 9) After succeeding in welding of similar metals, attempts have been made to join dissimilar alloys such as aluminum alloys to other dissimilar aluminum alloys, 10) copper alloys, 11) steels, 12) and magnesium alloys 13) for optimizing mechanical properties and chemical property such as corrosion resistance. Concerning dissimilar aluminum alloys joints by FSW, there are a few information about joining of 2024-T3 to T6 aluminum alloys. L. Cederqvist et al. 14) studied some *PhD Student, Joining and Welding Research Institute, Osaka University factors affecting the mechanical properties such as sheer stress and hardness of FSW lap joints of 2024-T3 and AA7075-T6 aluminum alloys. The factors studied were welding speed, rotation speed, number of passes, distances between passes, and tool dimensions while no information about microstructures were studied. P. Cavaliere et al. 15) investigated microstructure and mechanical properties of dissimilar FSW butt joints of 2024-T3 to 7075-T6 aluminum alloy plates of 2.5 mm thick. But no information about the effect of welding conditions on the properties was reported. Also, the location of both alloys was not indicated either located on the advancing side and which was on the retreating side. Therefore additional research must be carried out on dissimilar joints of 2024-T3 with 7075T-6 Al alloys, since the properties of dissimilar joints depend on many conditions such as rotation speed, welding speed, probe geometry, thickness of the work pieces, fixed location of materials, and material processed, etc. The present study aims to investigate the effects of rotation speed and fixed location of material on microstructure, hardness distribution, and tensile properties for dissimilar 2024-T3 and 7075-T6 aluminum alloys joints produced by FSW. In particular relationship between rotation speed and homogeneity of elemental distribution in SZ has been investigated. 2. Experimental Procedures Dissimilar 2024-T3 and 7075-T6 Aluminum alloys of 3 mm thick plates were friction stir butt welded using a tool made of tool steel (SKD61) composed of 12 mm diameter shoulder and 4 mm diameter threaded probe. The chemical composition and tensile properties of the base metals are

2 Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T Table 1 Chemical composition of base metals. Materials Chemical compositions (mass %) Si Fe Cu Mn Mg Cr Zn Ti Al AA2024-T Bal. AA7075-T Bal. Table 2 Mechanical properties of base metals. Materials Mechanical properties at room temperature Yield stress (MPa) Tensile stress (MPa) Elongation (%) AA2024-T AA7075-T Table 3 Welding Conditions. Sample number Welding speed mm/min Rotation speed (min 1 ) Probe diameter (mm) Shoulder Diameter (mm) Advancing side Retreating side AA2024-T T T6 AA2024-T listed in Tables 1 and 2, respectively. Tool axis was tilted by 3 degrees with respect to the vertical axis. Welding speed was kept constant at 100 mm/min and rotation speeds were set at 400, 800, 1200, 1600 and 2000 min 1. The location of two alloy plates was changed for each welding condition. Table 3 summaries welding conditions adopted in the present study. Specimens for macro and micro-structural observations were machined from the welded plates. The observation was performed on a cross-section in the weld region after mechanical polishing using emery papers followed by polishing using a suspension containing alumina with 0.3 mm diameter and a colloidal silica suspension. Surface of specimens was chemically etched by Keller s reagent (1 ml HF, 1.5 ml HCl, 2.5 ml HNO 3, and 95 ml H 2 O). Microstructure change throughout the weld zone was observed with an optical microscope. The homogeneity of constituents of two alloys in SZ was analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS). Microhardness measurement was carried out after 3.5 Ms (forty days) from the welding. This time was fixed for all the samples. The measurements were carried out along a midthick line on the cross section transverse to the welding direction with an internal spacing of 0.5 mm under the load of 0.98 N for 15 s of loading time. To determine the effect of rotation speed and fixed location of two alloys on tensile properties, tensile test specimens were machined both in the transverse and in the longitudinal directions to the weld line. Fig. 1 Shape and location of tensile test specimen in the welded joint. Shape of tensile test specimen and location in the welded joint are shown in Fig. 1. The tensile test was carried out at room temperature with a strain rate of 1: s 1. The tensile properties were evaluated as average values using three tensile specimens machined from the same joint for each welding condition. 3. Results and Discussion 3.1 Macrostructure of joints Figures 2 and 3 show macroscopic appearances of the cross-section of the dissimilar joints produced at different rotation speeds T3 Al alloy was set on the advancing side in Fig. 2 and the alloy was set on the retreating side in Fig. 3. In case of 400 min 1 a border appears between two alloys in the weld zone, indicating that no mixing of two alloys occurred regardless of the location of materials as

3 1930 S. A. Khodir and T. Shibayanagi Fig. 2 Macrostructures of joints welded at different rotation speed where AA2024-T3 fixed on the advancing side: (a) 400 min 1, (b) 800 min 1, (c): 1200 min 1, (d) 1600 min 1, and (e) 2000 min 1. Fig. 3 Macrostructures of joints welded at different rotation speed where AA2024-T3 fixed on the retreating side: (a) 400 min 1, (b) 800 min 1, (c): 1200 min 1, (d) 1600 min 1, and (e) 2000 min 1. shown in Figs. 2(a) and 3(a). The grey and brighter zones corresponded to 2024-T3 and 7075-T6 Al alloys, respectively. In cases of 800, 1200, and 1600 min 1 of the rotation speeds, as shown in (b), (c), and (d) of Figs. 2 and 3 onion ring patterns were obviously observed in SZ irrespective to the fixed locations of materials respectively. The onion ring was more clearly visible on the advancing side than retreating side. These clear visible bands on the advancing side than retreating side, which is usually appeared not only in dissimilar aluminum alloys friction stir welded joints but also in similar ones. It would be rather difficult to understand the formation mechanism of onion ring. Some reports 16,17) explained the reasons of onion ring formation. They suggested that the onion ring was formed as a result of the extrusion of cylindrical sheets of material per each revolution of welding tool during its forward motion. The tool appears to wait for a very short time to produce frictional heat and extrude a cylindrical shaped material around the tool to the retreating side of the joint. They also found that the spacing between bands was equal to the pitch of forward motion of the tool in one rotation. In addition to these explanations, based on our recent work in FSW of similar AA2024-T3, 18) the formation of onion ring depends on the welding parameters such as rotation speed, welding speed and type of backing materials. The clearer onion rings on the advancing side than retreating side could be attributed to the different rotating and travelling directions of the probe on both sides. The tangential component of the rotation has the same direction as the travelling direction on the advancing side, while the two directions are in opposite way on the retreating side. Thus steeper gradient of plastic strain caused by the severer deformation mode eventually resulted in rather visible distinct between bands of onion ring on the advancing side. The composition and structure of onion ring bands associated with dissimilar joints will be explained later in the present study. Thickness of bands in onion ring is wider in the center of SZ and becomes narrower as going on towards to the periphery both in the directions of advancing and retreating sides. The thickness of bands near the center of SZ and advancing or retreating sides ranges from mm and mm, respectively. At a rotation speed of 2000 min 1 SZ decreases its area and becomes irregular shape with unclear onion ring especially when the AA2024 Al alloy was located on the retreating side as shown in (e) of Figs. 2 and 3. The size of SZ increased with increasing rotation speed till 1200 min 1 of rotation speed due to increasing strain and heat input. However, larger rotation speed above 1200 min 1 resulted in decreasing area of SZ. This could be attributed to the excess amount of metal slash removed from the weld zone at higher rotation speeds. These features are considered as peculiar cases appeared in dissimilar joints of Al alloys. In addition severe macro cracks formed among the metal slash layers in the top surface region of joints welded at the rotation speeds of 1600 and 2000 min 1 regardless of the location of alloy plate, yet cracks more severs when 2024-T3 Al alloy was located on the retreating side as shown in Fig. 3(d) and (e). This may be attributed to different metal flow modes which in turn affected by the material which located in advancing or retreating side. All joints are free from other defects such as porosity and tunnel like defects. These observed features in macrostructures suggest that rotation speed and fixed location of materials should be important factors affecting the evolution process of microstructure and mechanical properties of dissimilar aluminum alloys joints produced by FSW. 3.2 Microstructure of joints Figure 4 represents an example of microstructures of different regions in the joint welded at a rotation speed of 1200 min 1 with 2024-T3 Al alloy on the retreating side, which corresponds to the macrostructure of Fig. 3(c). The microstructure of 7075-T6 BM is shown in Fig. 4(a) representing elongated grains along the rolling direction with a random distribution of second phase particles

4 Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T Fig. 4 Microstructures at different location of joint welded at 1200 min 1 of rotation speed where AA2024-T3 fixed on the retreating side. recognized as small black particles. These grains with several hundred microns long and approximately 40 mm wide are in the direction normal to the welding direction. The second phase particles in this alloy were reported as Al 2 Cu 2 Fe, Al 23 CuFe 4,orAl 2 CuMg. 19) Figure 4(b) represents the microstructure of HAZ on the 7075-T6 Al alloy side which is similar to that of BM with slightly darker contrast. Microstructure of TMAZ near the top surface on the advancing side of 7075-T6 Al alloy is shown in Fig. 4(c). The grains were bended by about 70 degrees by the rotating welding tool, indicating the severe plastic deformation in this region. No recrystallization occurred in this region. On the other hand, Fig. 4(d) represents the microstructure of SZ near 7075-T6 Al alloy side where is consisted of equiaxed grains with much smaller size compared to the elongated grains in BM. The microstructure composed of lamellar like onion ring structure with an alteration of grey and slightly dark grey bands in turn. The lamellar bands have not only different contrasts but also different grain size. The difference in grain size inside the bands is observed obviously at higher magnification as shown in Fig. 4(e) which is taken from the center of SZ. The size of grey band is larger than that of the other band. The recrystallized grain size of the grey bands ranges from 3.9 to 7.7 mm, while the other bands ranging from 3.2 to 5.6 mm. Onion ring pattern was also observed on the retreating side of SZ as shown in Fig. 4(g), but the width of each band is smaller than that shown in Fig. 4(d). Figure 4(f) represents the microstructure near the bottom surface of the joint. It contains three different microstructures. The first is a mixture of recrystallized fine grains of two base alloys frequently appeared on the upper side. The second is located on the lower left side and is the residual elongated grains of the 7075-T6 Al alloy BM. The third one on the right side is the elongated grain of 2024-T3 Al alloy BM. A clear kissing bond was observed between them. This defect was only observed in the joints welded at 1200, 1600 and 2000 min 1 of rotation speeds when 7075-T6 Al alloy was fixed on the advancing side. It may be resulted from insufficient metal flow of materials due to the lower temperature generated from the friction between the rotating probe and two alloy plates or from the stirring action of the probe which becomes abnormal at higher rotations speed when 7075-T6 Al alloy was located on the advancing side. The Fig. 5 Optical microstructures of SZ of the joints where AA2024-T3 fixed at advancing side: (a) 400 min 1, (b) 800 min 1, (c) 1600 min 1, and (d) 2000 min 1. microstructure of BM of 2024-T3 Al alloy is shown in Fig. 4(h), representing elongated grains with a slightly different dimension and a higher concentration of second phase particles than that in 7075-T6 Al alloy. Figure 5 shows magnified grain structures for different rotation speeds in the center region of SZ where 2024-T3 Al alloy was fixed on the advancing side. Average grain size increases with increasing rotation speed. On the other hand, as represented by previous works on dissimilar friction stir welded joints, 19 21) the SZ zone had dynamically recrystallized fine structure with a periodic change of grain size which is clearly observed Fig. 5(c). The average grain sizes of smaller and larger grain areas are 4.2 mm and 5.7 mm, respectively. Increasing rotation speed resulted in finer and more homogenous distribution of second phase particles in SZ as shown in Fig. 5. Quantitative analysis of average grain size in SZ measured by linear intercept method is shown in Fig. 6 for two types of fixed locations of Al plates. Grain size dependence on the rotation speed is almost identical to each other. This might be attributed to little change of temperature in SZ regardless of the fixed location of materials. The grain size increases with

5 1932 S. A. Khodir and T. Shibayanagi Fig. 6 Relationship between rotation speed and average grain size in stir zone. Fig. 8 SEM and EDS images of SZ of the welded joint welded at 1200 min 1 of rotation speed: (a) SEM image of SZ, (b) EDS image of Cu, (c) EDS image of Zn, and (d) EDS image of Mg. Fig. 7 SEM and EDS analysis of SZ of the joint welded at 400 min 1 of rotation speed: (a) SEM image of boundary line in the center of SZ, (b) EDS image of Cu, (c) EDS image of Zn, (d) EDS line analysis of main alloying elements. increasing rotation speed due to increasing heat input. 9,18) For example, in case of 2024 Al alloy fixed on the advancing side, average grain size increases linearly from 3.1 mm at 400 min 1 to 5.9 mm at 2000 min 1. This is consistent with the report by Ying Li et al. 20) on dissimilar FSW of 2024 Al alloy to 6061 Al alloy where the average grain size in SZ increases with increasing rotation speed. 3.3 EDS analysis of joints Figure 7 shows a result of SEM-EDS analysis performed at the center position of SZ for the joint welded at 400 min 1 with 2024 Al alloy fixed on the advancing side. A clear border laying between two regions having clearly different contrasts is observed by a back-scattered electron image (BEI) as shown in Fig. 7(a). The corresponding X-ray images of Cu-K and Zn-K are shown in (b) and (c), respectively. Since Cu and Zn are the main alloying elements in 2024-T3 and 7075-T6 Al alloys, respectively. As shown in (b), Cu is enriched in the left side region suggesting that this region is occupied by 2024-T3 Al alloy. The other area on the right side of border is abundant of Zn as seen in (c) indicating that this region is occupied by 7075-T6 Al alloy. Quantitative EDS line analysis of Cu and Zn was then carried out along the center line in SZ, transverse to the welding direction and the result is shown in Fig. 7(d). The content of Cu fluctuated from 5.1 to 6.3 mass% in the left side region of the border and decreased to % just after crossing the border in to retreating side. On the other hand, the composition of Zn increased sharply when a crossing the border from 0.2% on the advancing side to about % on the other side. The composition on the left side related to 2024-T3 Al alloy while that at the right related to 7075-T6 Al alloy. These differences in chemical composition observed on both sides of border suggest that there was no mixing occurred between the two materials in the SZ in case of lowest heat input. Figure 8 shows a result of SEM-EDS analysis for SZ near the advancing side of the joint welded at 1200 min 1 of rotation speed where 2024 Al alloy fixed on the retreating side. The BEI image of SZ clearly shows the onion ring pattern in the middle part of the image as shown in Fig. 8(a), while almost uniform image was obtained in the higher or lower parts. The X-ray images of Cu, Zn, and Mg, are shown in Fig. 8(b), (c), and (d), respectively. Corresponding to the onion ring of different contrast regions in Fig. 8(a), each alloying element, especially Zn is heterogeneously distributed likewise periodic fluctuation. The brighter bands in Fig. 8(a) were abundant of Zn while the darker bands were rich with Cu and Mg. In contrast, both of Cu and Mg have lower contrast than that for Zn as shown in Fig. 8(b), (d). Both 2024-T3 and 7075-T6 Al alloys contain Cu and Mg but Zn exists only in 7075 Al alloy. Therefore, the darker bands observed in Fig. 8(a) are related to 2024-T3 Al alloy while the brighter ones are related to 7075-T6 Al alloy.

6 Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T Table 4 EDS quantitative analysis of zones corresponding to Fig. 7 for the rotation speed of 1200 min 1. Alloying Content/mass% elements Cu Zn Mg Mn AA2024-T3 fixed in the retreating side Table 4 summarizes the results of EDS quantitative analysis for the main alloying constituents such as Cu, Zn, Mg and Mn in the two materials. The measurement was carried out at the positions indicated in Fig. 7(a). The positions 1 to 9 were in the onion ring while the positions 10 to 18 were taken from the homogenous contrast region which is located beneath the onion ring. The mass percentages of Cu and Mn at the positions 1, 3, 5, 7, and 9 were almost corresponding to their contents of 2024-T3 Al alloy while concentrations of Zn and Mn at the positions 2, 4, 6 and 8 were close to 7075-T6 Al alloy. The Mg contents at the locations 1, 3, 5, 7 and 9 was slightly higher than that of 2024-T3 Al alloy and was lower than that of the 7075-T6 Al alloy at locations 2, 4, 6 and 8. The redistribution of Mg suggests to the diffusion of Mg inside the SZ. In addition to the difference in compositions at these positions, grain size was also different in each region. The grain size in the regions 1, 3, 5, 7 and 9 where the composition is close to that of 2024-T3 Al alloy was lower than that at the regions 2, 4, 6 and 8 corresponding to T6 Al alloy region. The mass percentages of the Cu, Zn, Mg and Mn at the positions 10 to 18 are almost the same levels but different from any of two alloys. These results clearly suggest that a new alloy was produced in this area during FSW. Also, the distributions of grain size were equal than that was in the onion-ring. Therefore, it should be concluded that the onion ring pattern reflects the difference in chemical composition and grain size between the lamellar bands in the nugget. Figure 9 shows the SEM and the EDS images of SZ near retreating side of the joint welded at 2000 min 1 of rotation speed where 2024 Al alloy plate was fixed on the advancing side. The onion ring pattern was not clear in the BEI image as shown in (a). In the present study, when onion ring pattern was observed in the structure, it reflects the difference in chemical composition between its bands as clearly shown in Fig. 8 and Table 4. In case of 2000 min 1 of rotation speed, the onion ring pattern was only observed in small area inside the SZ near the boundary between the SZ and the TMAZ on the advancing side as shown in Fig. 2(e). The image of Fig. 9(a) was taken from the area include SZ and TMAZ located on the retreating side in which onion ring pattern was not clear. The composition in this area might be homogenous or come from either 2024 or 7075 Al alloy. Since, the backscattered electron image mode (BEI) reveals the compositional differences as the contrasts, therefore the onion ring was not observed in the BEI image as shown in (a). This suggests will confirm by EDS line analysis in the next Figure. The X-ray intensities of Cu and Mg in the SZ were higher than that of Zn as shown in (b), (d) and (c), respectively. Fig. 9 SEM and EDS images of SZ of the welded joint welded at 2000 min 1 of rotation speed where AA2024 fixed at advancing side: (a) SEM image of SZ, (b) EDS image of Cu, (c) EDS image of Zn, and (d) EDS image of Mg. Fig. 10 EDS quantitative analysis at mid thickness transfers to the welding directions of the joint welded at 2000 min 1 of rotation speed where AA2024-T3 fixed in the advancing side. Figure 10 shows the results of quantitative EDS line analysis for Cu and Zn using k- line of each element. The analysis was carried out along the center line on the cross section of SZ transverse to the welding direction. Starting from the advancing side, the content of Cu beyond 2:5 mm

7 1934 S. A. Khodir and T. Shibayanagi from the weld center is almost the same as its content in the BM of 2024-T3 Al alloy. But it begins to decrease at the position 2:5 mm and reaches a minimum value of 3.7% at 2:0 mm. At about 1:25 mm the content of Cu increased to that level in 2024-T3 Al alloy and kept almost constant till 2.0 mm from the weld center line. After 2.0 mm from the welding center line, the Cu content decrease again to be the same as its content in the BM of 7075-T6 Al alloy. On the other hand, the Zn content beyond 2:5 mm from the weld center is almost the same as its content in the BM of 2024-T3 Al alloy. At about 2:0 mm, it increases and reaches a maximum value of 6% and then turn to drop at about 1:5 mm to its value in BM of 2024-T3 Al alloy. Beyond this position, it kept almost constant till 2.0 mm from the center. After 2.0 mm from the center line, it content sharply increases again to the same value of 7075-T6 Al alloy BM. The variations of Cu and Zn contents in the region from 2:0 to 1:25 are due to the formation of onion ring in that area as shown in Fig. 2(e). These differences in the compositions of Cu and Zn observed in the SZ suggested that the microstructure of the SZ mainly depend on the material fixed at the advancing side. In contrast, the microstructure of SZ in the dissimilar joints of A356 and AA6061 aluminum alloys 21) was reported to be affected mainly by the material fixed on the retreating side. This result indicates that the different combination of aluminum alloys causes different metal flows which in turn affect dominant structure in SZ. From the results mentioned above, it should be emphasize that the effect of rotation speed on microstructures of the weld joints brought about four types of structure in SZ. The first was non-mixed structure from two alloys which was observed at 400 min 1. The second and third structures were the onion ring and homogenous contrast region which is located beneath the onion ring, respectively. These two types were observed in SZ of joints welded at rotations speeds from 800 to 1600 min 1. The last one was obtained at 2000 min 1 of rotation speed in which the microstructure of the SZ mainly occupied by the material located on the advancing side. This heterogeneous in microstructures through the SZ at different rotation speeds might be attributed to the insufficient welding time required for completing diffusion of alloying elements in SZ. 3.4 Hardness of joints Figure 11 shows microhardness profiles of joints after 3.5 Ms (forty days) of natural aging. The measurement was carried out across the BMs on both sides, on the transverse cross-section welded at different rotation speeds. The T3 Al alloy was fixed on the advancing side. SZ shows a great change of the hardness distribution with increasing rotation speed. For example, hardness beyond 9 mm from the welding center line of the joint welded at 400 min 1 is the same as that of the BM of 2024-T3 Al alloy naturally aged. At about 9 mm from center, hardness starts to decrease and reaches a minimum value of 112 Hv at about 4 mm. Beyond that position when going closer to the near the welding center position, hardness value turns to increase but the values are still lower by 23 Hv than that of the BM of 2024-T3 Al alloy. This may be attributed to lower temperature during FSW than the solution temperature of precipitates which brings about Fig. 11 Hardness profiles at mid thickness transfers to the welding directions of the welded joints for different rotation speeds where AA2024- T3 fixed on the advancing side. an insufficient driving force for larger precipitates to be dissolved during joining, and in consequence to reprecipitate during natural aging. Hardness at the center sharply increased to about 148 Hv in SZ and reaches to a maximum value of 152 HV at 3 mm in TMAZ of 7075-T6 Al alloy side. Beyond 4.0 mm from the center, the hardness value begins to decrease and a minimum value about 120 Hv was obtained at about 7.5 mm. the hardness turn increase again beyond this point and eventually reached to 163 Hv which is a mean value of the BM of T6 Al alloy. The minimum hardness value in the HAZ of 2024 Al alloy located at about 4:0 mm from the center while the HAZ of 7075-T6 Al alloy had a minimum hardness value at about 7.5 mm from the center. This may be attributed to the deference of precipitation behaviors between two alloys under a given thermal history of the joining. The abrupt increase of hardness just beyond the center into retreating side in the joint welded at 400 min 1 is caused by the change of materials from 2024-T3 to 7075-T6 Al alloy, where no mixed structure was obtained as shown in Fig. 7(a). In case of the rotation speed of 1200 min 1 the hardness increased and fluctuated from 143 to 150 Hv in the SZ due to the formation of onion ring pattern where bands of a periodic alteration of composition from 2024-T3 and 7075-T6 Al alloys are observed. Increasing rotation speed has a significant influence on the hardness distribution of SZ but is less effective for HAZs in the two alloys. Also, increasing rotation speed has a significant influence on the TMAZ of 2024-T3 Al alloy side but slightly effective on that of the other side. Softened zones exist on both sides of HAZs of the two materials. In these zones some precipitates might have coarsened and lost their coherency due to thermal history. But the minimum values on the 7075-T6 Al alloy side are higher than that those on the 2024-T3 Al alloy side. This could be attributed to the higher hardness value of 7075-T6 Al alloy BM (163 Hv) than that of the 2024-T3 Al alloy (145 Hv). Both HAZs of the two alloys slightly harden as the rotation speed raises. At 2000 min 1 the hardness in the SZ is slightly decreased but the distribution is more homogenous than that at 1200 min 1. This is because of the structure of the SZ that is mainly composed of 2024 Al alloy as shown in Fig. 10. For age hardening aluminum alloys, the peak temperature

8 Microstructure and Mechanical Properties of Friction Stir Welded Dissimilar Aluminum Joints of AA2024-T3 and AA7075-T Table 5 Mechanical properties and location of fracture of welded joints in the transverse direction to weld center line. Tensile properties at room temperature Welding Rotation speed speed AA2024-T3 located in advancing side AA7075-T6 located in Advancing side (mm/min) (min 1 ) Yield Tensile Yield Tensile Elongation Fracture Elongation Fracture strength strength strength strength (MPa) (MPa) (%) location (MPa) (MPa) (%) location At HAZ of At HAZ of T T At HAZ of At HAZ of T T At HAZ of 2024-T At the SZ At the SZ At the SZ At the SZ At the SZ and deformation rates during FSW strongly influence the microstructures aspects such as precipitations, grain size, and dislocations that control the weld properties. No previous temperature measurement was done on dissimilar joints of 2024-T3 and 7075-T6 Al alloys, but from our previous work 18) on FSW of similar 2024-T3 Al alloy, the temperature in the SZ at 400 min 1 of rotation speed was 723 K. The sharp increase of hardness just after the border shown in Fig. 7(a) could be attributed to the temperature in the SZ was sufficient at 400 min 1 for larger amounts of precipitates in the 7075 Al alloy region to be dissolved and reprecipitate again during the natural aging. But not enough for 2024 Al alloy region. Therefore, it should noticed that the temperature required for dissolution of precipitates in 7075 Al alloy is lower than that for 2024 Al alloy. Since the temperature of SZ increases with increasing rotation speeds, the temperature at higher rotation speed than 400 min 1 was sufficient for two alloys to force larger precipitates to be dissolved and reprecipitate in the SZ during the successive natural aging. The higher hardness measured in the TMAZ especially for 7075 al alloy side could be attributed to both higher dislocation density and precipitates introduced during cooling. The minimum hardness values in the HAZs for two alloys indicate that overaging process occurred in these regions. The slight increase of hardness in HAZ with increasing rotation speed could be attributed to the relatively increasing of heating or cooling rates during welding by increasing rotation speeds. Increasing heating rate reduce the time for precipitates to grow and hence leads to hardness increase in HAZ. Also, increasing cooling rate after welding increases the amount of supersaturated solute which will be available for further precipitation reaction at room temperature. Although, there are some previous works 22 24) dealing with the behaviors of the hardening precipitates during welding thermal cycles for each alloy, additional TEM observation will be carried in the future specially in the SZ containing mixed structures of two alloys to investigate the behaviors of the hardening precipitates. 3.5 Tensile properties of joints Table 5 summarizes the tensile properties and fracture locations measured for dissimilar joints welded at different rotation speeds, together with two types of materials locations. Tensile strength increased with increasing rotation speed till 1200 min 1 but still lower than those of two base metals. Elongation of the joints is much lower than that of 2024-T3 BM but slightly lowed than 7075-T6 BM and its maximum value was about 14.9% at 1200 min 1 when T3 Al alloy was fixed on the advancing side. The maximum tensile strength of the joints was 423 MPa which was achieved at 1200 min 1 of rotation speed when 2024-T3 Al alloy was fixed on advancing side. The lowest tensile strength was obtained for the joint welded at 2000 min 1 for both locations of Al plate locations due to the presence of severe cracks in the top surface region as shown in Figs. 2(e) and 3(e). Also, due to a kissing bond observed for the case of 1200 min 1 where 2024-T3 Al alloy was located on the retreating side as shown in Fig. 4(f), tensile strength of the joint decreased less than those when the 2024-T3 Al alloy was located on the advancing side. Tensile properties of the joints showed higher values when 2024-T3 was located on the advancing side due to higher hardness in the HAZ on the advancing side. The temperature measured in our previous work 18) on FSW of similar 2024-T3 Al alloy, was higher on the retreating side than that on the advancing side. Higher temperature resulted in softening retreating side and hence lower hardness was obtained in this side. Fracture occurred in the HAZ on the 2024-T3 Al alloy side regardless of its fixed location for all defect-free joints whereas SZ become the region of fracture for the joints included defects such as kissing bond at root or cracks at the top surface. Although, the border appeared between non mixed regions in the SZ of the joints welded at a rotation speed of 400 min 1, the fracture occurred in HAZ. This result suggests that the border exerts no fatal effect on the mechanical properties of the joints and thus the joints are sound for this rotation speed. Generally, fracture in FSW joints starts from the weakest point in the joint upon tensile test. It is well known that, hardness distributions of the joints indicate the presence of the softest or the weakest region in the joint. The hardness distribution of the joint welded at 400 min 1, reveals that the weakest region is located in HAZ on the advancing side as shown in Fig. 11. Therefore the fracture occurred in this region instead of the border between the non-mixed alloys regions appeared in the SZ. In order to investigate the effects of rotation speed and

9 1936 S. A. Khodir and T. Shibayanagi Table 6 Mechanical properties and location of fracture of welded joints in the longitudinal direction to weld center line. Tensile properties at room temperature Welding speed Rotation speed AA2024-T3 located in advancing side AA7075-T6 located in Advancing side (mm/min) (min 1 ) Tensile Yield strength Tensile strength Elongation Yield strength Elongation strength (MPa) (MPa) (%) (MPa) (MPa) (%) materials locations on the tensile properties of SZ, the tensile properties were measured using specimens having the gauge section along the longitudinal direction and the results are listed in Table 6. The tensile strength and elongation increased with increasing rotation speed till 1200 min 1. Due to the cracks appeared on the surfaces at higher rotations speed more than 1200 min 1, the relationship between rotation speed and tensile strength is not defined well. Tensile strength and elongation measured for this type of specimens in the longitudinal direction were higher when the 7075-T6 Al alloy was fixed on the advancing side. This is because that the microstructure of the SZ was mainly composed of the 7075T6 Al alloy region with relatively higher strength when it was fixed on the advancing side. The maximum tensile strength and elongation of the joints were 491 MPa and 30%, respectively, that were achieved at 1200 min 1 of rotation speed when 7075-T6 Al alloy was fixed on the advancing side. Tensile properties and fracture location of the dissimilar FSW joints of hardening aluminum alloys depend on the softening behavior in the HAZ of the alloy with lower tensile properties, welding defects, and fixed location of materials. Softening in the HAZ and welding defects are functions of the welding parameters. When dissimilar joints are free from defects, their tensile properties are affected by the softening in the HAZ and the fixed location of materials. 5,18) As seen in Figs. 11, increasing tensile strength of joints with increasing rotation speeds from 400 to 1200 min 1 could be attributed to a slight increase of hardness in the HAZ on the 2024-T3 Al alloy side. Also, when the joints are free from defects, the joints are fractured in the HAZ on the side 2024-T3 Al alloy regardless of the fixed locations. The tensile properties of the SZ depend on the dominant structure in the SZ which in turn depend on the fixed location of materials. The tensile strength of the SZ was higher when 7075-T6 Al alloy of higher strength is fixed on the advancing side since the microstructure of the SZ in the joints is occupied by mainly depends on the material fixed on the advancing side. 4. Conclusions Effects of rotation speed and fixed locations of materials on microstructures, hardness distributions, and tensile properties of the friction stir welded dissimilar joints of aluminum 2024-T3 and 7075-T6 Al alloys were investigated. The following results were obtained; (1) At the rotation speed of 400 min 1 there was no mixing of the two alloys in the stir zone and a border was observed between them irrespective to the fixed location of materials. Increase of rotation speed turned to brought about the mixed structure and onion ring pattern was formed with a periodic change of equiaxed grain size and heterogeneous distribution of alloying elements in the SZ. At 2000 min 1 of rotation speed, the onion ring pattern was disappeared and the composition of SZ mainly composed of the material fixed on the advancing side. (2) Average grain size in the SZ increased with increasing rotation speed regardless of the fixed locations of materials. (3) At 400 min 1 of rotation speed higher hardness was obtained in the SZ of the 7075-T6 Al alloy side of the joint where no mixing took place between two alloys. At higher rotation speed than 400 min 1, hardness increased and fluctuated in the SZ due to the formation of onion ring pattern. Hardness minima were in HAZ on the 2024-T3 Al alloy side and its values slightly increased with increasing rotation speed. This tendency became evident when the 2024-T3 Al alloy was located on the advancing side. (4) At 2000 min 1 of the rotation speed, hardness of the SZ was slightly increased when 7075-T6 Al alloy was located on the advancing side. (5) In the case of transverse tensile test, the defect-free joints were fractured at the HAZ on the 2024-T3 Al alloy side while the defect-containing joints fractured in the SZ. Maximum tensile strength of the joints of 423 MPa was achieved at 1200 min 1 of rotation speed when 2024-T3 Al alloy located on the advancing side. (6) The tensile properties in the longitudinal direction of the SZ showed higher values when 7075-T6 Al alloy was located in the advancing side. Maximum tensile strength of 491 MPa was achieved at 1200 min 1 of rotation speed when 7075-T6 Al alloy was located on the advancing side. Acknowledgements This work was supported by a grant-in-aid for scientific research B (project No ) and a grant-in-aid for cooperative research project of nationwide joint-use Research Institutes on Development Base of Joining Technol-

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