Microstructural characteristics and mechanical properties of fiber laser welded-brazed Mg alloy-stainless steel joint

AZ31B Mg alloy and 201 austenitic stainless steel were lap joined by a laser welding-brazing (LWB) process using a Mg-Al-Zn based welding wire. The influence of the process heat input on microstructure and mechanical properties of the dissimilar joint was investigated. Metallurgical bonding of immiscible Mg and Fe couple was achieved by the formation of continuous ultrathin reaction products at the fusion zone-steel interface, with a thickness varied from 0.5 to 3 μm, growing slowly with the increase of the heat input. The interfacial reaction products were identified as Al19Mn4 and Fe(Al) from the fusion zone to the steel, respectively. The tensile shear test indicated that joints produced at 5.4 kJ/cm heat input exhibited the highest mechanical resistance reaching 2472 N fracture load, representing a 75.4% joint efficiency relative to the Mg base metal. The failure occurred in the heat-affected zone (HAZ) of the Mg base metal, indicating a strong bond at the fusion zone-steel interface. The thermodynamic stability of the reaction products formed at the interface during the laser welding-brazing process was also evaluated in the temperature range of 400° to 1100°C using FactSage thermochemical software. The results were found to be in good agreement with the experimental results. The results contributed to a better understanding of microstructural evolution at different temperatures and compositions during the laser welding-brazing of Mg alloys to stainless steel sheet.