Microstructures and mechanical properties of magnesium alloy and stainless steel weld-joint made by friction stir lap welding

Friction stir lap welding was conducted on soft/hard metals. A welding tool was designed with a cutting pin of rotary burr made of tungsten carbide, which makes the stirring pin possible to penetrate and cut the surface layer of the hard metal. Magnesium alloy AZ31 and stainless steel SUS302 were chosen as soft/hard base metals. The structures of the joining interface were analyzed by scanning electron microscopy (SEM). The joining strength was evaluated by tensile shear test. The results showed that flower-like interfacial morphologies were presented with steel flashes and scraps, which formed bonding mechanisms of nail effect by long steel flashes, zipper effect by saw-tooth structure and metallurgical bonding. The shear strength of the lap joint falls around the shear strength of butt joint of friction stir welded magnesium alloy.

► Friction stir lap welding technology with cutting pin was successfully employed to form lap joint of magnesium and steel.
► The cutting pin made the lower steel participate in deformation and the interface was no longer flat.
► A saw-toothed structure formed due to a mechanical mixing of the magnesium and steel was found at the interface.
► A high-strength joint was produced which fractured in the magnesium side.