Influencing mechanism of pre-existing nanoscale Al5Fe2 phase on Mg–Fe interface in friction stir spot welded Al-free ZK60–Q235 joint

Abstract

Al-free ZK60 magnesium (Mg) alloy sheet was selected as substrate material of Mg–steel pinless friction stir spot welding (FSSW), avoiding the effect of the Al element in the substrate on the alloying reaction of Mg-iron (Fe) interface. The sound FSSW joint of ZK60 Mg alloy and Q235 steel with a hot-dipped aluminum (Al)-containing zinc (Zn) coating was successfully realized. The detailed microstructural examinations proved that Al5Fe2 phase at the Mg-Fe interface came from the pre-existing Al5Fe2 phase in the coating and acted as the transition layer for promoting the metallurgical bonding of Mg and Fe. The interfaces with well-matched lattice sites among Fe, Al5Fe2 and Mg were formed during FSSW. A low energy interface with good match of lattice sites ((002)Al5Fe2//(110)Fe, [110]Al5Fe2//[1¯13]Fe) between Al5Fe2 and Fe was identified. For the interface between Al5Fe2 and Mg, an orientation relationship of (62¯2)Al5Fe2//(31¯1¯2¯)Mgand[15¯8¯]Al5Fe2//[24¯23]Mg was observed. The tensile–shear load of the ZK60–steel joint could reach 4.6 kN. Moreover, the joint fracture occurred at the interface between the Al5Fe2 layer and the Mg alloy substrate, suggesting the brittle fracture characteristic.