Joining of dissimilar AZ31B magnesium alloy and SS400 mild steel by hybrid gas tungsten arc friction stir welding

The joining of dissimilar materials, magnesium alloy (AZ31B) and mild steel (SS400), was performed using a hybrid gas tungsten arc-friction stir welding (HGTAFSW) method that applied a preceding gas tungsten arc welding (GTAW) preheating heat source to a mild steel plate surface during friction stir welding (FSW). The mechanical and microstructural characteristics of the HGTAFS welds were evaluated and compared to those of FS welds to confirm the effect of the additional GTAW preheating heat source. The tensile strength of the HGTAFS welds was approximately 91% of that of the magnesium alloy base metal but higher than that of the FS welds. This was attributed to the enhanced material plastic flow and partial annealing effect in the magnesium alloy and mild steel materials by GTAW reheating of the mild steel side, which induced a significant increase in the elongation of the welds. The concentration profiles indicated that no intermetallic FeAl and FeAl3 compounds had formed according to the phase diagram, which led to a decrease in joint strength. Overall, the use of HGTAFSW by applying a GTAW preheating heat source to a mild steelplate surface resulted in a mechanically sounder and metallurgically defect-free welds compared to FSW.