Investigation of wire selection for CMT plug joining Mg AZ31-to-galvanized steel

In this study, the effect of wire selection (i.e., nickel, steel, copper and magnesium wires) on the feasibility of cold metal transfer plug welding of 3 mm thick Mg AZ31-to-1 mm thick galvanized mild steel was investigated. Welding tests with the use of various wires were conducted and the welded galvanized steel-Mg AZ31 joints were mechanically tested and analyzed with SEM, EDS, and micro-hardness analyses. The results indicated that among all wires tested in this study, sound Mg AZ31-galvanized steel joints with Mg AZ61 wire were obtained. Mg-Zn eutectic and thin Fe-Al intermetallic at the brazed interface between the weld metal and galvanized steel base metal were observed. A thin fusion zone between Mg AZ31 weld metal and base metal was formed, which led to the weld metal Mg AZ31 pulled out fracture mode. To summarize the tests conducted in this study, although good quasi-static tensile strengths were obtained for galvanized steel-Mg AZ31 joints made with NRNiCu-7 (Ni-based) wire, local cracks for galvanized steel-Mg AZ31 joints were often induced in the weld metals, which potentially would cause a concern in fatigue performance of the joints. Based on these results, it is recommended to use Mg AZ61 wire to join galvanized steel and Mg AZ31 by cold metal transfer plug welding.