Optimizing the mechanical properties of friction stir welded dissimilar joint of AM60 and AZ31 alloys by controlling deformation behavior

The special microstructure distribution of the friction stir welded (FSW) Mg alloys joints is usually responsible for the non-uniform deformation and poor mechanical properties. Hence, it was possible to improve the joint mechanical properties by modifying the microstructure. In the present study, two kinds of AM60/AZ31 Mg alloys dissimilar FSW joints with different microstructure were obtained using different rotation rates (800 rpm and 1600 rpm). The results of mechanical tests showed that the tensile strength and elongation of the joint at 1600 rpm were 13.5% and 115% higher than those at 800 rpm, respectively. The microstructure observation indicated that the grain size of various zones in the joint at 1600 rpm was obviously larger than that at 800 rpm, especially at the AM60 side. However, there was no obvious difference in the texture distribution of the two welds. In addition, microstructure observation after fracture showed that there were more twins in AM60 side of the joint with coarse grains. These coarse grains promoted the activation of basal slip and twinning in the regions far away from the nugget zone (NZ) interface. This weakened the strain localization of the joint to some extent. Meanwhile, the coarse grain joint had lower concentration degree of the strain and dislocation near the NZ boundary compared with the fine grain joint. These factors led to better mechanical properties of the joint with coarse grains. This work could provide some guidelines for improving the inhomogeneous deformation behavior of dissimilar Mg alloys FSW joints.