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

Galvanized steel plates are widely used in car body because of its excellent corrosionresistance, while laser welding technology will also be favored by the auto manufacturer forthe high quality, rapid speed and flexible processing, presenting a tendency that the carbody spot welding would be replaced by laser welding. However, Technical problems arisewhen laser welding galvanized steel in a zero-gap overlap joint configuration due to theboiling point of zinc (about1200K), being well below the melting point of steel(1800K).Highly pressurized zinc vapor is easily generated on the faying surface of two metal sheetsduring the welding process. The zinc vapor vents out through the weld pool and results inthe formation of blowholes, spatter and pores which dramatically reduces the weld strengthand affects the aesthetics of the weld seam. A method introduced and widely in use todayto overcome this problem is to set a gap between the sheets prior to the welding, whichprovides a channel for the escape of the zinc vapor. But additional process is necessary toproduce the special gap and increase the cost.In order to resolve this problem during the laser welding galvanized steel, in this paper,we describe a method employing a suction device adapted to provide a negative pressurezone on the surface of the keyhole to allow the highly-pressurized zinc vapor to escape. Theinfluence on the processing property was studied and a neatly defect-free lap joint and thebest process parameters were been obtained by the experiments. A high-speed video camerawas used to record the dynamic behavior of the laser-induced plasma plume and the zincvapor. The distribution of zinc element in the joints was analyzed using synchrotronradiation X-ray and the temperature field during the welding process was measured usingthe thermocouples. At the same time, the microstructure and properties of the lap jointobtained in different parameters were studied. Fluent software was applied to simulate theair flow distribution on the surface of the weld and the behavior of the plasma under theside-suction.A neatly defect–free lap joint can be acquired by the side-suction method and noporosity or cracks were found inside the weld. The mechanical properties could be satisfied.The best process parameters were the laser power of3.4kW and the welding speed of2.4m/min. When laser welding galvanized steel, laser was placed in front of the gas nozzle,which was above the molten pool. Less residual zinc was found in the weld when using the side-suction during thewelding process, showing that the suction method can facilitate the escape of the highly-pressured zinc vapor from the keyhole and stabilize the molten pool. Experimental resultsobtained with the high speed camera photos show that the plasma was very stable and thekeyhole was kept open during the welding process. The simulating results revealed that anegative pressure region nearby the outlet of the nozzle was performed when using theFluent software, and when studying the plasma, there was a great agreement between thesimulating result and the real morphology taking from the High-speed video.By the analysis of the process experiment, the high-speed video result and residualzinc mapping in this paper, the side-suction method was believed to accelerate the escape ofthe zinc vapor and the metal vapor rapidly from the keyhole, thus make the keyhole morepourable and stable. The keyhole during the laser welding galvanized steel in a zero-gapconfiguration process acts as a channel to venting out the zinc vapor.