3.1.2. Microstructure

Figure 5 shows the sample of the keyhole-free FSSW joint of the dissimilar 6082 Al alloy and DP600 galvanized steel. It can be found that the keyhole was eliminated by the pin retraction technology in the welding process. Figure 6 shows the macroscopic morphology of the cross-section of the dissimilar Al/steel keyhole-free FSSW joint. The cross-sectional specimens can be divided into four zones: WNZ, TMAZ, HAZ, and BM. There is no keyhole on the cross-sectional specimens. A continuous inhomogeneous interface was formed at the Al/steel interface due to the effect of the stirring of the pin and the extrusion and friction of the shoulder of the tool. The walking path of the pin is shown by the blue arrow. The thickness of the steel plate was thinned from 1 mm to 0.48 mm, as shown in the thin area. There is a gap at the edge of the interface of the keyhole-free FSSW joint, which was the weak link in the spot-welded joints.

**Figure 5.** Sample of a dissimilar Al/steel keyhole-free FSSW joint.

**Figure 6.** Macroscopic morphology of the cross-section of a dissimilar Al/steel keyhole-free FSSW joint.

Figures 7 and 8 show all regional microstructures of the steel and Al alloy in the keyhole-free FSSW joint, respectively. Figures 7a and 8a show the fine equiaxed grains of the WNZ, which formed due to the recrystallization caused by the large plastic deformation and the friction heat [35]. The grain size of the WNZ decreases from 20 μm of BM to 1 μm. Figures 7b and 8b show the elongated texture of the TMAZ, which was caused by large thermoplastic deformation [36]. Compared with the BM, the microstructure of the HAZ is slightly larger than that of the BM, as a result of the friction heat, as shown in Figure 7c,d and Figure 8c,d.

**Figure 7.** The microstructure of the steel of the keyhole-free FSSW joint: (**a**) weld nugget zone (WNZ), (**b**) thermo-mechanically affected zone (TMAZ), (**c**) heat-affected zone (HAZ), and (**d**) base metal (BM).

**Figure 8.** *Cont*.

**Figure 8.** The microstructure of the Al alloy of the keyhole-free FSSW joint: (**a**) WNZ, (**b**) TMAZ, (**c**) HAZ, and (**d**) BM.

Figure 8d shows the microstructure of the 6082 Al alloy of BM zone, which has a typical basal texture. The grains of WNZ experienced a high temperature and strain rate under the combined action of the stirring pin and axial shoulder friction force, which caused the recrystallization of the grain and the redistribution of the enhancement phase and finally obtained the dynamic recrystallization grain, as shown in Figure 8a. The grain size of the WNZ decreases from 30 μm of BM to 5 μm. The width of the TMAZ is narrower, and the zone is partially plastic deformation due to the indirect agitation of the stirring pin. The temperature in the TMAZ is very close to that in the WNZ, so the grains of the recrystallization are shown in Figure 8b. The HAZ is only affected by the thermal cycle, so only the grain growth can be seen in Figure 8c.

Compared to Figures 7 and 8, the deformation of steel is more intense than that of Al. As shown in Figure 7b, the grains of steel at the TMAZ, located underneath the pin, exhibited greater elongation directly beneath the full penetration of the pin than that shown in Figure 8b. This phenomenon is caused by the difference of the compressive stirring force. The stirring force at the TMAZ in Figure 7b is higher than that in Figure 8b, so the grain deformation of steel is more serious than that of Al.
