3.2.1. Alloying Elements Redistribution in WNZ

Figure 5 shows the second electron image and EDS mappings of the main alloying elements in two adjacent grains near the central line in the post-solid-solution treated sample corresponding to Figure 4a. As seen from Figure 5a, there are obviously a large number of the second phases at the grain boundary between these two grains. Figure 5b–d show the grains are composed of Zn, Mg, and Si, and Figure 6 shows that the right grain has a higher level of Mg and Zn, but a lower level of Si. In other words, the composition of grains in WNZ has been changed and the elements diffusion occurred during stirring or post-solid-solution. The line scanning in Figure 6 reveals the content of alloy elements is higher gradually from 6061 side to 7A52 side, which is the basic of the gradual increase of precipitation hardening in WNZ after post-aging from 6061 side to 7A52 side, as shown in Figure 4b–d.

**Figure 5.** The second electron image of the weld joint and the corresponding EDS mappings. (**a**) The second electron (SE) image, (**b**–**d**) the mapping of Zn, Mg and Si.

**Figure 6.** The line profiles of the main alloying elements (Zn, Mg, and Si) obtained along the yellow line in Figure 5a.

#### 3.2.2. Grains

Figure 7 shows the orientation maps of grains in and adjacent to different zones locating by hardness measurement. The left is 6061 alloy and the right is 7A52 alloy. The as-rolled 6061 alloy plate comprises of recrystallized approximately equiaxed grains, and the as-rolled 7A52 alloy plate is composed of deformed elongate grains. After welding, grains in WNZ are equiaxed with a mean size of 5 μm, resulting from dynamic recrystallization, independent of the pre-heat-treatment. In TMAZ on the advancing side (6061), the as received equiaxed grains were severely elongated, and bended too far along the direction of metal flow. On the other hand, in TMAZ on the retreating side (7A52), there are more equiaxed recrystallized grain when compared with the 7A52 matrix. On the retreating side, it was found that the transition of grains is pretty smooth from TMAZ to WNZ, in comparison to the advancing side. The width of the TMAZ on both sides is approximately 1 mm.

**Figure 7.** Electron back-scattered diffraction (EBSD) orientation map of grains in FSW joint of dissimilar 7A52 and 6061 alloys.

After post-solid-solution treatment, abnormal grain growth was found in both the WNZ for (T6 + welding + SS) sample and (SS + welding + SS) sample; this phenomenon has also been reported in literatures [32,33]. Obviously, the pre-treatment has significant influence on the region of abnormal grain growth. In (T6 + welding + SS) sample, abnormal grain growth only occurred in WNZ, but in (SS + welding + SS) sample, abnormal coarse grains were found in WNZ and TMAZ. The size of these coarse grains is of the order of millimeter or sub-millimeter.
