Study on the properties and mechanism of magnesium alloy and Ni-plating stainless-steel by ultrasonic-laser welding-brazing

This study aims to provide a new process to improve the welding performance of steel/magnesium dissimilar materials, using an ultrasonic-laser welding-brazing method to joints AZ31B magnesium alloy on 304 stainless steel. The relationship between laser power, Ni-plating and element distribution, microstructure, and mechanical properties of welded joints was studied, and the formation mechanism of the interface zone between AZ31B and 304 joint was analyzed. The research results indicate that the best welding-brazing effect is achieved when the laser power is 2000 W, the minimum wetting angle is 45° and continuous joints without obvious defects. Excessive heat input can cause magnesium alloy evaporation and the formation of oxidation inclusions inside. Ni-plating can effectively improve the wetting and spreading ability of the molten magnesium alloy on steel surface under ultrasonic-laser welding-brazing process. The cross-sections of the Ni-plating fusion zone are formed two new phases compared with no plating, which are AlNi and Mg₂Ni, respectively. The hardness of the fusion zone is higher than that of the solidification zone of magnesium alloy and lower than that of stainless steel. The tensile shear linear load of Ni-plating is higher than that of no plating under the same processing parameters. The maximum linear load of fracture shear strength reaches 222 N/mm or above.