Influence of Annealing Treatment on Microstructure Evolution and Mechanical Property of Friction Stir Weld AZ31 Mg Alloys

In order to improve microstructure distribution and mechanical properties of Mg alloy joint by annealing treatment, die-casting AZ31 Mg alloy was successfully welded at rotation speed of 1 400 rpm and travel speed of 200 mm/min. The welded joints were annealed at 150–300 °C for 15–120 min and then were subjected to transverse tensile. The microstructure of annealed joints was analyzed by optical microscopy and electron backscatter diffraction. The experimental results indicate that (0001) texture intensity in stir zone significantly reduces and sharp transition of grain size is relieved in the interface between stir zone and thermo-mechanically affected zone after annealed at 200 °C for 30 min. Meanwhile, the elongation is increased from 7.5% to 13.0% and strength is increased slightly. It is because that annealing treatment can inhibit twin transformation and retain its ability to coordinate deformation during tensile deformation, which contributes to the improvement of plasticity. In addition, annealing treatment can increase the width of interfacial transition zone and lead to gradual transition of grain size between the SZ and TMAZ, which balances dislocation diffusion rate in different zone.