Microstructure and mechanical properties of dissimilar friction stir lap welding between AZ31 Mg alloy and DC01 steel

AZ31 magnesium alloy and DC01 steel were friction stir lap welded (FSLW) with Z-axis force controlled at 7.5 kN. The microstructural features and element distributions of the joint were studied in detail. Mg17(Al, Zn)12 particles with significantly high concentration of Zn were found to precipitate in the stir zone on the AZ31 side. A bowl-shaped region formed with a maximum depth of similar to 770 mu m on the DC01 side, with the grain size decreasing rapidly from the base material to the area at the interface (similar to 120 nm) due to the operation of dynamic recrystallization. Significant amount of Mg and Al atoms was found in DC01 grains near interface, which can be attributed to local mechanical intermixing between AZ31 and DC01 and the enhanced solid solubility via severe deformation. A thin Al-rich layer of similar to 50 nm thick formed within DC01 grains upside at the interface with the Al atoms crossed the interface from AZ31, while abundant nanoscale Mg particles precipitated out at the boundaries of nano-sized DC01 grains due to low solid solubility of Mg in Fe. The tensile shear strength of the joint reached 295.6 N/mm, with the crack passing through the root of hook-like structure and the stir zone of AZ31 instead of the interface, indicating a high bonding strength of such an interface microstructure.