*3.2. Microstructural Features of the FSWed Dissimilar AA5083-AA5754 Joints*

Figure 4 shows a collage of the macro- and micrographs that reveal the main characteristics of the grain structures for the dissimilar AA5754-AA5083 joints welded using different combinations of rotation rates and travel speeds of the FSW tool. The macrographs in the middle point out to the locations (Figure 4), where the EBSD measurements were performed using 1 μm step size. Two locations (one at the top and the other at the bottom) were investigated in the NG zone, almost along the vertical centerline for each joint. Generally, the top locations in the NG zones (a, b, and c) showed always larger grain structures than those developed at the corresponding bottom locations (d, e, and f). This can be attributed to the high heat generated at the top surface due to the effect of both the shoulder and the pin, while near the bottom of the NG is only affected by the pin with lower heat experienced [12,24,32,40,41]. Furthermore, it is clear that the grain sizes in the NG zones showed a dependency on the rotation and travel speeds as well. As can be seen, the grain sizes of J1 manufactured at 600 rpm–60 mm/min (Figure 4a,d) were

coarser than their counterparts of J2 manufactured at 400 rpm–60 mm/min (Figure 4b,e), indicating a grain refining effect induced by the decrease in the rotation rate of the tool from 600 to 400 rpm. On the other hand, the decrease in the welding traverse speed from 60 mm/min to 20 mm/min at a constant rotation rate of 400 rpm had not resulted in a significant effect on the grain structure and the average grain size, as can be observed from Figure 4c,f. The variation in grain sizes from the top to the bottom locations through the thickness in the NG zones can be explained by the higher heat experienced at the top regions of the joints due to the friction-induced heat caused by the contact between the work-piece and the tool shoulder and pin during FSW, while the bottom regions are only affected by the pin and accordingly experience a lower heat [42,43]. Another factor that promotes a variation in heat from the top to the bottom of the NG zones is the thick section of the welded plates, which contribute to a higher cooling capacity during FSW [44]. It is also expected that the variation in heat from the top to the bottom through the thickness of NG zones can be affected by the rotation and travel speeds. Accordingly, higher heat input is excepted for the higher rotation speed and slower travel speed, which reflects the grain structure evolution in J1 that experiences the highest heat input (coarse grain structure) and in J2 that is exposed to the lowest heat input (finer grain structure). The obtained results here are in agreement with that reported in work conducted by Ahmed et al. [24] for the FSW of the thick section AA6082. They reported a significant reduction in the grain size towards the bottom part of the weld NG, which they attributed to the lower heat input experienced at the lower part due to the only pin effect relative to the top part of the NG, which was affected by both the pin and the shoulder of the tool. Besides, there was a significant reduction in the grain size by decreasing the heat input through the reduction of the tool rotation rate. The grain-size distributions represented in grain diameter based on the measured grain areas in the NG zones of J1, J2, and J3 are shown in Figure 5. The same data-sets represented in Figure 3 were utilized to calculate the grain-size distributions at the top locations (a, b, and c) and at the bottom locations (d, e, and f) for J1, J2, and J3, respectively. It was remarked that the average measured grain diameters in the NG zones at the top locations varied from 33, 25, to 24.5 μm, and at the bottom locations, changed from 19, 12, to 11.8 μm for J1, J2, and J3, respectively. Obviously, the grain sizes in the NG zones at the bottom locations were more than two times finer than those counterparts at the top locations. It should be noted here that the effect of reducing the tool rotation rate was more effective in controlling the grain size than increasing the traverse speed. Reducing the tool rotation rate from 600 rpm to 400 rpm resulted in a reduction of the average grain size at the top from 33 μm to 25 μm and at the bottom from 19 μm to 12 μm. On the other hand, decreasing the traverse speed from 60 mm/min to 20 mm/min almost did not affect the grain size parameters. In both cases, the average grain size was almost similar at the top locations, about 25 μm, and at the bottom locations, about 12 μm. In terms of grain orientation of the maps presented in Figure 4 and obtained at the top and the bottom locations of the NG from each weld, it could be considered randomly orientated with mixed <001> red, <101>green, and <111> blue orientations. It should be mentioned here that the data presented in Figure 4 is the as-collected data in which there was a difference between the FSW reference frame (TD, ND, WD) and the actual shear reference frame (θ, z, r), as quantitatively determined in a detailed study by Ahmed et al. [45,46] for the methodology to be applied to align the FSW reference frame with the shear reference frame to obtain the real FSW texture and orientations. Figure 6 shows the inverse pole figure (IPF) coloring maps with their corresponding (111) pole figures for the same data presented in Figure 4 after applying the required rotations to align the FSW reference frame with the shear reference frame. Now the IPF maps (Figure 6a−f) were dominated by the <111> blue orientations due to the alignment of the <111> poles with shear plan normal (r). In terms of texture, it could be observed from the (111) pole figures (PFs) that the texture was strong texture with up to 10 times random and was mainly of simple shear texture. The (111) PF of the J1 joint (Figure 6a,d) had the strongest texture with 10 times random at the top and 7 times random at the bottom of the NG. This could be attributed to the high amount of

deformation experienced due to the high tool rotation rate (600 rpm) and the fast welding speed (60 mm/min). The (111) PF of the J2 joint (Figure 6b,e) had slightly relatively less strong texture with 6 and 5 times random at the top and bottom of the NG, respectively. The (111) PF of the J3 joint (Figure 6c,f) showed strong texture with 7 times random at the top and only 3 times at the bottom. This indicates the effect of the FSW parameters on the strength of the texture components. In all cases, the textures were of the simple shear, which is the main type of texture reported in the NG of FSWed aluminum alloys [45,46].
