Revealing stir zone heterogeneities in friction stir-welded and annealed AZ31 alloy

The difference in the microstructure, texture in the stir zone (SZ) of the AZ31 (Mg-3Al-1Zn, wt.%) alloy after friction stir welding (FSW) and subsequent annealing at 400 ℃ for 1 h was characterized by scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD) measurements at the surface and core regions. The findings indicate that FSW produced grain refinement where the mean grain size decreases from 19 µm (base metal) to 5.1 and 3.5 µm at the surface and core regions, respectively. The c-axis of the grains at the surface region was aligned with the normal direction (<0001>//ND) due to the additional strain of the tool shoulder. In contrast, the core region shows a typical shear texture, where the c-axis tends to be oriented parallel to the welding direction (<0001>//WD). The Vickers microhardness mapping across the SZ revealed that the core region was soften than the surface region due to the dynamic recrystallization and texture weakening. The microstructure of the SZ remains principally deformed after annealing treatment except for the development of massive Mg₁₇Al₁₂ precipitates and the abnormal grain growth of a few grains with <11−20>//WD orientation at the upper side of the surface region. The c-axis of the grains at the surface region was tilted about 10° toward WD, while an inclined <0001>//WD orientation about 30° from WD was developed at the core region. Consequently, the distribution of microhardness values across the SZ was more heterogeneous than the FSW sample. The results were discussed in the light of grain boundary misorientation, dislocation density and the pinning effect of Mg₁₇Al₁₂ precipitates. Additionally, Schmid factor analysis was used to examine the activation of the basal slip mode to characterize the associated mechanical response.