Improvement of welding stability and mechanical properties of galvanized DP800 steel lap joint by high-speed tandem beam laser

Abstract

Dual beam laser welding has shown promise to improve the weldability of galvanized dual phase steels. In this study, the lap joints of galvanized dual phase steel were welded with tandem beam laser at speed up to 100 mm/s. The power ratio for the dual beams was set to 50%:50%, 40%:60%, 30%:70%, and 20%:80%, respectively. A high-speed camera was used to observe the dynamic process of zinc vapor, molten pool, and keyhole behaviors under different welding conditions. The corresponding metallographic and mechanical analysis were conducted to reveal the effects of variable welding thermal cycles on the microstructures and properties of the welded joints. The results indicate that the fusion zone (FZ) is composed of blocky-lath martensite, whereas the heat-affected zone (HAZ) is characterized by the lath microstructure containing tempered martensite (TM) and ferrite (F). The power ratio of dual beams significantly affects the microstructures and hardness profiles of welded joints. It also influences the zinc-vapor and molten pool dynamic behaviors, thus affecting the weld bead formation. The weight of spatter decreases with the decrease of leading beam power in tandem beam laser welding process. In addition, the mechanical properties of the welded lap joints are significantly affected by the high-speed tandem dual beam laser welding process parameters. The failure load of tensile-shear testing reaches the maximum value when the beam power ratio is 30%:70%.