Influencing mechanism of Al-containing Zn coating on interfacial microstructure and mechanical properties of friction stir spot welded Mg-steel joint

2.4 mm thick AZ31 Mg alloy sheet and 1.5 mm Q234 steel sheet with a hot-dipped Al-containing Zn coating were friction stir spot welded (FSSW) using a pinless tool. It was shown that the Al-containing Zn coating played a crucial role in joining the Mg alloy and steel during FSSW. The Zn coating observably improved the Mg-steel interfacial wettability during FSSW, and the Al5Fe2 phase in the Zn coating on the steel substrate surface promoted the metallurgical bonding of Mg alloy and steel. It was confirmed that the Al5Fe2 phase on the steel surface resulted from the reaction between the steel substrate and the Al in the Zn coating during hot-dipping, and was not related to the Al-containing Mg alloy substrate. The tensile-shear load of the FSSW Mg-steel joint reached 4.3 kN. The fracture of the joint occurred along the interface on the steel substrate side. The interface between the Al5Fe2 layer and Mg alloy substrate was the weakest region of the Mg-steel joint.