Microstructure and mechanical properties of resistance spot welded in welding-brazing mode and resistance element welded magnesium alloy/austenitic stainless steel joints

An AZ31 Mg alloy and 316L austenitic stainless steel (ASS) were joined via resistance spot welding (RSW) and resistance element welding (REW). The RSW joints were found to be produced through welding-brazing mode, in which the Mg alloy melted and spread on the solid steel, forming a nugget only in the Mg alloy. The microstructure of the nugget consisted of columnar dendritic structure, indicating that columnar-to-equiaxed transition was interrupted. Shrinkage porosity and cracking were also observed in the RSW nugget. In contrast, a two-zone nugget was formed during REW, consisting of a peripheral nugget on the ASS side and the main nugget. Compared with the RSW joints, the REW joints were obtained at lower welding currents, and they exhibited superior mechanical performance, with 63% higher peak load and 9 times higher energy absorption. Irrespective of the welding current, the RSW joints failed in interfacial failure mode, while the failure mode of the REW joints transited from interfacial to pull out with increased welding current.