The study of the microstructure and mechanical properties of friction stir spot welding joint between AZ31B magnesium alloy and steel

摘要

We used a friction stir spot welding without pin method to carry out the welding between a 1mm sheet of AZ31B magnesium alloy and a galvanized steel plate, then we analyzed the microstructure and mechanical properties of the joint. We found that the structure on both sides of the joint is a finer equiaxial structure which dynamically recrystallized under the action of friction heat and force. Moreover, the microstructure of the magnesium alloy is composited by two phases of white α-Mg and black β-Mg_(17)Al_(12). Under 1800rpm, the magnesium alloy microstructure is over-sintering. The shear strength of the joint first increases with increasing stirrer rotation speed but decreases later. The shear strength of the joint reaches its maximum of 4.85 KN at the rotation speed of 1500 r/min. It was found that a new phase, Mg_(0.97)Zn_(0.03) was generated by Magnesium alloy and the galvanized layer of steel plate. The joint fracture is a cleavage fracture in microcosm, in the form of a joint surface fracture, with cracks producing in the cellular dendrites zone of weld nugget and expanding along the joint surface rapidly until it fracture completely.