Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200 rev min−1 and welding speed of 300 mm min−1, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5 MPa, 199.1 MPa and 7.3%, respectively. It was found that (300°C – 1 h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4 MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.

The fatigue fracture behaviour and thermographic analysis of friction stir-welded (FSW) AZ31 was studied. It showed that fatigue fracture at the advancing side (AS). Heat-affected zone (HAZ) contained a greater fraction of coarse grains and a small amount of twins. The grains in the thermo-mechanically affected zone were smaller, indicating that grains may originate from incompletely dynamic recrystallisation during the FSW. The nugget zone is composed of equiaxed grains caused by the dynamic recrystallisation. During cyclic deformation, hysteresis loops from the AS of the FSW joints with the stain amplitude were higher than the retreating side of the FSW joints, the temperature of HAZ at AS is higher than other regions.