Dissimilar friction stir spot welding without key hole between AZ31B magnesium alloy and 5052 aluminum alloy

To study the impact of the welding process parameters on the spot weld surface molding, mechanical performance and the stir zone microstructure, we use friction stir spot welding without keyhole method which designed and developed by ourselves to carry on the lap welding between 1mm sheet of AZ31B magnesium alloy and 3mm sheet of 5052 aluminum alloy with magnesium on top and aluminum bottom. The result of the orthogonal test shows that the shoulder friction time have a greater impact on the forming of the surface of the welding joint; When the rotating speed of 1000 r/min, shoulder friction time of 4s, pin length of 3.5mm, the tensile shearing force of the welded joint achieves its maximum value of 2.46 KN. Through observating the microstructure of the welding joint's section stirring zone, we found the typical joint microstructure of Dissimilar metal. Through analyzing the microstructure of the tensile fracture, we found that fracture was cleavage fracture. We also found the fracture mechanism of the welding joint was cleavage fracture in microcosm but brittle fracture macrocosm.