Study On Laser Welding Of Galvanized Steel In A Zero-gap Overlap Joint Configuration Using A Topical Negative Pressure Method

The level of development of the automotive industry is on behalf of the industry level of the whole country. Since the new century, the automobile industry has developed rapidly, and its position in the national economy are becoming increasingly prominent.Galvanized steel plates are widely used in the automobile body because of its excellent corrosion resistance, while laser welding technology will also be favored by the auto industry for the high quality, rapid speed and flexible processing, presenting a tendency that the automobile body spot welding would be replaced by laser welding. However, Technical problems happen while laser welding galvanized steel in a zero-gap overlap joint configuration due to the boiling point of zinc(about 1200K), being below the melting point of steel(1800K). Highly pressurized zinc vapor is easily generated to the faying surface of two metal sheets during the laser welding process. The zinc vapor vents out through the weld pool and results in the formation of blowholes and pores which reduces the weld strength and affects the aesthetics of the weld seam. A method introduced and widely in use today to overcome this problem is to set a gap between the sheets prior to the welding, which provides a channel for the escape of the zinc vapor. But additional process is necessary to produce the special gap and increase the cost.Targeted at the problem during the laser welding galvanized steel, in this paper, we describe a method employing a suction device adapted to provide a negative pressure zone on the surface of the keyhole to allow the highly-pressurized zinc vapor to escape. The influence on the processing property was studied and a neatly defect-free lap joint and the best process parameters were been obtained by the experiments. A high-speed video camera was used to record the dynamic behavior of the laser-induced plasma plume and the zinc vapor. Moreover, the microstructure and properties of the lap joint obtained in different parameters were studied.The results of the study showing that a neatly defect –free lap joint can be acquired by the method providing a negative pressure zone on the surface of the keyhole and no porosity or cracks were found inside the weld. The mechanical properties could be satisfied. The high speed camera photos show that the plasma was very stable and the keyhole was kept open during the welding process. By the analysis of the process experiment,and the high-speed video result, the method was believed to accelerate the escape of the zinc vapor and the metal vapor rapidly from the keyhole, thus make the keyhole more pourable and stable. The keyhole during the laser welding galvanized steel in a zero-gap configuration process acts as a channel to venting out the zinc vapor.