Microstructure and mechanical performance of friction stir welded joints of AZ31 magnesium alloy

The weld formation, microstructure and mechanical performance of friction stir welded joints of AZ31 Mg alloy were investigated. The results show that the plastic flow of the material welded is improved and the cavities disappear with increasing the rotation speed. With increasing the welding speed, the grain growth accompanied by the dynamic recrystallization in the weld nugget is restrained, and the structure of the grain becomes finer. The optimum technological parameters for the friction stir welding of 4mm AZ31 Mg alloy are as follows: rotation speed 1 000 r/min, and welding speed 45 mm/min. The tensile strength coefficient of AZ31 Mg alloy is up to 63. 7%. The brittle fracture of the joints belongs to the mixed fracture mode, the upper part of the weld is often brittle fracture, and the lower part is slight ductile fracture. The microhardness of the weld nugget is the lowest, that of the thermomechanical affected zone slightly increases, and that of the heat-affected zone is equal to that of the base metal.