Effect of processing parameters on microstructure and mechanical properties of friction stir welded 7A04 aluminum alloy joints

7A04 aluminum plates with a thickness of 3mm were welded by friction stir welding (FSW). The microstructure, precipitates mechanical properties and fractured surfaces of the joints were investigated. The results show that the microstructure of the nugget zone consists of fine equiaxed grains due to dynamic recrystallization with precipitates dissolved,and the average grain size of the nugget zone decreases with welding speed increasing. According to the microhardness test, the hardness of the nugget zone is lower than that of the parent material and the lowest hardness locates at HAZ. The ultimate tensile strength (UTS) reaches its peak value (454.2 MPa), at a welding speed of 120 mm/min, which is 95% of the parental material. The elongation of the joint is 3.97% under this parameter. The SEM observed fracture morphologies indicate that the joint fails through a mixture ductile and brittle mode.