Effect of surface states of steel on microstructure and mechanical properties of lap joints of magnesium alloy and steel by friction stir welding

AZ31 Mg alloy (top sheet) and steel (zinc coated steel and brushed finish steel) were friction stir lap welded, and the effect of surface states of steel on the microstructure and mechanical properties of joints was examined. The failure loads of the zinc coated steel joints could reach 2.3 kN. Mg alloy and brushed finish steel could not be welded in current experimental conditions. The presence of zinc coat promoted the formation of liquid low melting Mg-Zn eutectic products at the interface. The liquid products, as well as broken oxide films and surface contaminants, were forced out of the joining interface by the high pressure produced by the tool. As a result, fresh interfaces were exposed, which led to the mutual diffusion between magnesium alloy and steel. Findings of the present study suggest that the presence of zinc coat significantly improves the weldability in friction stir lap welding of magnesium alloy and steel.