*3.2. OM Investigations*

Optical microscopy investigations were performed on the transverse cross-sections of the FSWd T-joint specimen's perpendicular to the welding direction. The three zones (SZ, TMAZ, and HAZ) were arranged by SZ from the center line of the weld joint perpendicular to the welding direction in both sides ending HAZ in the BM. The width of each zone and its features are related to the heat input introduced to the work-piece during FSW process, as shown in the joints' macrographs in Figure 5. The variation in heat input in the present work came mainly from the applied different rotational speeds of 400, 600 and 800 rpm where the travel speed is kept constant at 50 mm/min, the heat input index is equal to (ω2/*v*) [10]. The microstructures of the cross-sectional dissimilar friction stir welded T-butt joints of AA2024-T4 and AA7075-T6 Al alloys show new formation of the grain structures in the SZ (Figure 6). For friction stir welded T-joint at 400 rpm good mixing between skin and stringer can be clearly seen, as given in Figure 6a. The mixing between skin and stringer in the SZ has improved as the rotation welding speed increased to 600 rpm, Figure 6c. FSW at 600 rpm welding condition converts the initial elongated grains in as AA2024 and AA7075 aluminum alloys to fine equiaxed recrystallized grains in the weld SZ. Moreover, the coarse precipitates given by the lowest rotational speed of 400 rpm become finer and more dispersed. This change in microstructure is ascribed to high temperature and high stirring action by the pin inside skin and stringer which positioned between the two pieces forming the skin (Figure 1a,b). Remarkable coarser grains with very good mixing are observed at the highest rotational speed of 800 rpm, as shown in Figure 6e,f. However, the high heat input presented a chance to generate coarse precipitates in both SZ and TMAZ. Similar results of coarsening the precipitates at such conditions have been noticed in other works [1,27]. It can be concluded that, the best mixing between skin and stringer is obtained at rotational speeds of 600 and 800 rpm. The T-joint produced at 600 rpm rotational speed exhibits finer grains and precipitates in the SZ than the other two joints. Detailed EBSD grain size measurements for the parent alloys and for the friction stir welded material are presented in subchapter 3.5. In the TMAZ no recrystallization is noticed because the frictional heat and plastic deformation are not high enough to cause recrystallization Figure 6b,d,f.

**Figure 6.** Optical microstructures of AA2024-AA7075 T-butt joint welded at different rotational speeds of (**a**,**b**) 400, (**c**,**d**) 600 and (**e**,**f**) 800 rpm. The positions of the different images are shown in the top schematic drawing.
