*3.5. BM Grain Structure and Texture*

For all the welded T-joint, the SZ is mixed regions of AA2024-T4 and AA7075-T6. Degree of mixing enhances as the plasticity of the two Al-alloys increases during the FSW. The plasticity of both alloys is related to the amount of heat input introduced to the work-piece. Actually, plasticity increases as the rotational welding speeds increase at the constant welding parameters. Selected areas of both Al-alloys in the SZ have been analyzed in terms of inverse pole figure (IPF) coloring maps and texture and compared with the as-received features of BMs. Figure 10 shows the IPF coloring maps with respect to the rolling direction (RD) and grain boundary (GB) maps of the BMs (a) AA2024-T4 and (b) AA7075-T6 Al alloys. It can be observed that the BMs are almost similar in terms of grain structure and low angle boundaries (LABs) distributions. The grain shapes are that of the typical rolled materials characterized by large and elongated grains. The GB maps are almost free of LABs, which indicate that both materials were in full-recovered state. The AA2024-T4 Al-alloy showed an average grain size of 47 μm and the AA7075-T6 Al-alloy revealed relatively finer average grain size of 43 μm, as obtained from the grain size distribution histogram shown in Figure 11a,b, respectively. The misorientation angle distribution displays very low density of LABs for the as-received materials, as shown in Figure 11a,b. In terms of texture the 101 and 111 pole figures (PFs) illustrated in Figure 12 fairly shows the rolled texture of the fcc metals although the number of grains obtained in the analyzed areas are limited number.

**Figure 10.** IPF coloring OIM maps relative to rolling direction (RD) and the grain boundary maps with high angle grain boundaries (HAGBs) > 15 in black lines and low angle boundaries (LAGBs) < 15 in red lines, (**a**) AA2024-T4 and (**b**) AA7075-T6.

**Figure 11.** Grain size distribution and misorientation angle distribution of BMs, where (**a**) AA2024- T4and (**b**) AA7075-T6.

**Figure 12.** 101 and 111 pole figures (PFs) of BMs, where (**a**) AA2024-T4 and (**b**) AA7075-T6.

*3.6. Grain Structure and Texture of Friction Stir Welded T-Butt Joints*

The SZ of the T-butt joint Friction stir welded AA2024-T4 and AA7075-T6 has been investigated using EBSD by acquiring data at AA2024-T4 and AA7075-T6 regions. The IPF coloring maps with respect to the ND and their corresponding grain boundary maps are presented in Figure 13. It can be observed that the grain structure in both regions is dynamically recrystallized of equiaxed grains. Geometric dynamic recrystallization has been suggested by number of researchers to be the main recrystallization mechanism in the SZ of the welded aluminum [38,39]. In terms of grain size, it can be noted that in comparison to the BMs it is extremely fine, however a variation in the grain size can be observed between AA2024-T4 and AA7075-T6 regions, Figure 13a,b, respectively. The AA2024-T4 region exhibits coarser grain size relative to the AA7075-T6. The GB maps of the EBSD data obtained from the SZ of the T-butt Friction stir welded joint between AA2024-T4 and AA7075-T6 are presented in Figure 14. The maps are consisted of HAGB > 15◦ of the major density and a lower density of the LAB (5–15◦). The examined SZ areas of AA2024-T4 and AA7075-T6 dominate mainly by HAGBs > 15◦ with a fraction of 0.801 and 0.924, respectively. While the value of LAGBs 5–15◦ displays with a fraction of 0.113 and 0.071, respectively.

**Figure 13.** IPF coloring maps and their corresponding grain boundary maps for SZ of T-butt Friction stir welded joint AA2024-T4 and AA7075-T6 produced at 600 rpm, where (**a**) at AA2024-T4 region and (**b**) at AA7075-T6 region in the joint produced at 600 rpm. The positions at which the EBSD data collected are indicated with two red rectangles on the macrograph of the joint below the figure. The IPF maps key coloring, grain boundary legend and FSW axes are also presented below the figure.

This data is presented as misorientation angle distribution histogram for the two alloys in Figure 14a,b with the corresponding grain size distribution. The average grain size of the AA2024-T4 is about 11 μm while that of the AA7075-T6 is about 6 μm as given in Figure 14a,b, respectively. This variation can be attributed to the variation in the chemical composition of the two alloys and in consequently to the different types of precipitates in the two alloys. Ahmed et al. [2] in their study of similar and dissimilar Friction stir welded AA7075 and AA5083 reported a variation in the resulted grain size between the AA7075 and the AA5083 inside the same SZ of the dissimilar joints. In terms of the misorientation angle distribution which clearly showing an increase in the LABs density for the data obtained in the AA2024-T4. In terms of texture, the 101 and 111 PFs of this data also calculated and presented in Figure 15a,b. The two alloys show the simple shear texture with only about two times random. This is typical type of texture reported to be obtained in the SZ of the Friction stir welded aluminum alloys [27,40–42]. Crystallographic texture affects the mechanical properties of materials as it expresses the orientation of the crystallographic plans relative to the material reference axes [13]. This directly affects the alignment of the slip systems relative to the axis of the applied load. This why the isotropic materials that of random texture has the same mechanical properties in all direction while in contrast the anisotropic material that of strong texture its mechanical properties varied based on the direction of the applied load relative to the material reference axes [43–46].

**Figure 14.** (**a**) Grain size and misorientation angle distributions of the dissimilar T-butt Friction stir welded joint, where (**a**) AA2024-T4 region and (**b**) AA7075-T6 region in the joint produced at 600 rpm.

**Figure 15.** 101 and 111 PFs calculated from the data from SZ of T-butt Friction stir welded joint presented in Figure 13, where (**a**) AA2024-T4 and (**b**) AA7075-T6.

*3.7. T-Butt Joints Tensile Properties*

Figures 16 and 17 show photographs of typical fracture locations of Friction stir weldedAA2024-T4 and AA7075-T6 T-butt joints, which were pulled along skin and stringer, respectively. For the welded specimens at 400 rpm pulled along skin (Figure 17a) fracture occurs at the SZ in the RS. This may be attributed to reduction of hardness at the interface

between AA2024-T4 skin and AA7075-T6 stringer (Figure 7a) for the T-joint as a result of insufficient heat input for good mixing in the SZ of the dissimilar aluminum alloys. For 600 rpm, fracture occurs at HAZ region in the RS of AA2024-T4 skin, as shown in Figure 17b. This in agreement with the distribution of the lowest value of hardness in hardness map (Figure 7b) of the joint. For 800 rpm, fracture occurs at TMAZ of RS of the skin, as shown in Figure 17c. This may be ascribed to the lowering hardness map (Figure 7c) in the weld joint as a result of higher heat input.

**Figure 16.** Photographs of fracture locations of T-butt welded specimens along skin. (**a**) 400 rpm, (**b**) 600 rpm, and (**c**) 800 rpm.

**Figure 17.** Photographs of fracture locations of T-butt welded specimens along stringer. (**a**) 400 rpm, (**b**) 600 rpm, and (**c**) 800 rpm.

Furthermore, the fracture locations of T-butt welded specimens along stringer are shown in Figure 17. For 400 rpm, Figure 17a, fracture occurs at center of the SZ. It coincides with the variation of hardness values in hardness map (Figure 7a) due to insufficient mixing of AA2024-T4 and AA7075-T6 in the SZ at the lowest rotational speed. For 600 rpm, Figure 17b, the fracture occurs at HAZ in stringer part. It is in good agreement with the lowest hardness values in hardness map (Figure 7b) of stringer AA7075-T6. For 800 rpm, Figure 17c, the fracture occurs at SZ. It coincides with the reduction of hardness values in SZ in hardness map compared to BM AA2024-T4 (Figure 7c).
