Influence of Al-Si coating on resistance spot welding of Mg to 22MnB5 boron steel

Weld appearance, interfacial microstructure and mechanical properties with and without Al-Si coating were compared on resistance spot welding (RSW) of Mg to steel. No reaction layer was observed in Mg/bare steel joint. In Mg/coated steel joint, Al-Si eutectic layer improved the weld appearance and reduced welding defects, also was squeezed to border region and reacted with Mg as a metallurgical seal to ensure the clean contact between molten Mg and original intermetallic compounds (IMC) layer. Some of molten Al-Si eutectic diffused into Mg side facilitating a columnar-to-equiaxed dendrites evolution. The original IMC layer was influenced by both of growth mechanism and dissolution mechanism, which resulted in various interfacial reaction layers at different regions: new reaction layers at weld center and intermediate regions; original but grown IMC layer at weld border region. The metallurgical bonding between Mg and steel was enhanced by these new reaction layers because crack propagated inside of Mg alloy rather than the interface at weld center and intermediate regions. With Al-Si coating, the welding quality was improved 85% in peak load, a factor 6 increase in fracture elongation and a factor 9 increase in fracture energy absorption.