3.1.1. Features in the Single-Sided FSW

Figure 5 shows the grain map and the corresponding grain size distribution, TEM and its diffraction pattern in the single-sided FSW. The average grain size in the NZ was 9.16, 8.49 and 7.38 μm on L1, L2 and L3 line respectively. It is often suggested that the microstructure in the NZ is the result of continuous dynamic recrystallization supported by dynamic recovery [6]. The recrystallized grain size is mainly affected by the heat input in the welding process [6]. More heat input generated in the upper NZ and less heat input in the bottom NZ, which resulted in the larger grain size (9.16 μm) in the upper NZ and smaller grain size (7.38 μm) in the bottom NZ.

In order to study the microstructural evolution, the BM was observed by TEM, as shown in Figure 6. The BM contains a high density of fine hardening precipitates, as reported in the literature [6]. In the 6005A-T6 single-sided FSW, no precipitation was observed in the NZ. It is inferred that precipitations in the NZ were dissolved due to the friction heat and plastic deformation.
