Laser-tungsten inert gas hybrid welding of dissimilar metals AZ31B Mg alloys to Zn coated steel

Laser-tungsten inert gas (TIG) hybrid welding has been developed for joining Mg alloys to Zn coated steel in a lap joint configuration. The joint could not be produced in laser or arc welding only, while acceptable joints without obvious defects were obtained with a relatively wide processing window in the hybrid process. Two reaction layers were observed to form at the interface and were identified as Mg–Zn eutectic structure (α-Mg + MgZn) and Fe₃Al phase by TEM analysis. In some cases, Al₆Mn phase also formed adjacent to the Fe–Al reaction layer. The tensile-shear strength attained the maximum value of 68 MPa, representing 52.3% joint efficiency relative to Mg base metal. The element Al from AZ31B Mg alloys diffused to the liquid/solid interface and then reacted with the elements from steel, such as Fe and Mn, contributing to the metallurgical bonding at the interface. The weak bonding between Mg–Zn reaction layer and newly formed Fe–Al layer resulted in the interfacial failure.