*4.1. Microstructure*

The FSW joint was characterized by general unique zones as shown in Figure 9. Those zones included the stir zone (SZ) around the weld centre line, the thermo-mechanically affected zone (TMAZ) on both sides of the weld nugget/stir zone, the heat-affected zone (HAZ), which surrounded the TMAZ, and the non-affected base metal (BM) [12–18]. The regions originated from the material flow behaviour caused by the tool rotation. The macroscopic and microscopic patterns of the three specimens were similar, hence one pattern is presented to avoid duplication. Figure 9a shows the macroscopic view of the joint with its thermal zones. There are no defects visible on the joint. This suggests that the welding was performed successfully [21,29]. Figure 9b,c shows the micrographs of base metals AA1050-H14 and AA5083-H111, respectively. Figure 9d shows the microscopic view of the AA1050-H14 pulled towards the stir zone, and this morphology is called hook defect [30]. Figure 9e shows rotational traces of the materials at the stir zone. Figure 9f shows the magnified morphology of the stir zone. The magnified

view shows the refined grain structures at the stir zone. The average grain size for the base metal AA1050-H14 was about 29 μm while the average grain size of AA5083-H111 base metal was about 7.3 μm. The average grain size for the stir zone was about 9.35 μm. The average grain size for the stir zone is close to the average grain size for AA5083-H111 base metal.

**Figure 9.** (**a**) Macrostructure of the welded joint; (**b**) microstructure of AA1050-H14; (**c**) microstructure of AA5083-H111; (**d**) thermo-mechanical affected zone; (**e**) stir zone; (**f**) grains of the stir zone.
