Mechanism of Zn in the Joint of Magnesium and Steel by Resistance Spot Welding

2.0 mm thick AZ31B magnesium alloy and 1.0 mm thick SPHC galvanized steel were welded by KDWJ-17 three-phase secondary rectification resistance welding machine for welding test. The organizational structure and distribution of composition of the joints were analyzed. The role of Zn in the joint's formation was analyzed by atomic diffusion and thermodynamics. The Mg-Zn compound of the joint was speculated by the test and Mg-Zn binary phase diagram. The results show that the presence of Zn elements can form a concentration gradient in the joints and promote the diffusion of Al element. The extruded Zn-Mg compound can play a role in protection as mechanical seal.