Study On Laser Welding For Galvanized Steel In A Gap-free Lap Joint By Using Assistant Gas To Generate Negative Pressure

High strength galvanized steel sheet is widely used in automobile industry because of its excellent corrosion resistance and stamping performance.With the characteristics of fast welding velocity,high precision,low heat input,small thermal deformation and flexible processing,laser welding has gradually become the main way of the body connection.However,because of the lower evaporation temperature of zinc(around 906°C)than the melting point of substrate steel(over 1500°C),the high-pressurized zinc vapor was produced between the two sheets,resulted in the unstable process of laser welding,produced welding defects and seriously affected the mechanical property of lap joint.For the quality problem caused by the zinc vapor,the main solutions were producing the escape channel,alloying coating and removing the coating before welding.These methods required additional processes,raised production costs or caused new quality problems.In this paper,a novel laser welding technology with controlling the morphology and stability of the keyhole and form a stable zinc venting channel by local negative pressure above keyhole was developed to solve the weld defects caused by zinc vapor.The effect of the method on the escape of zinc vapor was clarified from the aspects of mechanism,welding process and properties and a complete laser welding implement device was built.The state and behavior of zinc vapor in the keyhole and molten pool under different environmental pressures and different negative pressure forming methods were analyzed,the influence of different environmental pressures on the behavior of zinc vapor and the complete information of the escape of the zinc vapor were obtained.The mechanism of effective escape of zinc vapor through the keyhole was revealed.The effect of negative pressure on the escape of zinc vapor and the influence of welding stability were clarified.The welding stability,appearance and mechanical properties of laser welding of galvanized steels in a gap-free lap joint were studied and the superiority of the negative pressure method was verified.The stable laser welding process of galvanized and the lap joints with excellent properties were obtained.Optimum process parameters were obtained under the different negative pressure environments.The effect of the negative pressure method on the morphology and stability of the keyhole and the mechanism of the effective escape of zinc vapor were clarified through the real-time monitoring of welding process by high-speed video camera and the distribution of residual zinc elements in lap joint.