Laser welding of galvanized DP980 steel assisted by the GTAW preheating in a gap-free lap joint configuration

With the increasing need for reducing the vehicle’s weight, improving fuel efficiency and safety, as well as the corrosion resistance, more and more galvanized high-strength steels have been used in the automotive industry. However, the successful laser welding of galvanized steels in gap-free lap joint configuration is still a big challenge. The high-pressurized zinc vapor is readily developed at the interface of the two metal sheets due to the lower boiling point of zinc (around 906 °C) than the melting point of steel (over 1300 °C). The failure of mitigating the high-pressurized zinc vapor at the interface leads to the formation of different weld defects such as the spatters and blowholes, which not only damage the weld surface quality but also deteriorate the mechanical properties of welds. Until now, there is no open literature on successfully obtained the defect-free welds of galvanized high-strength steels by using only laser welding. In this paper, a new welding procedure based on using GTAW as an auxiliary preheating source with a fiber laser as a main heat source is introduced. The controlled heat management during the preheating by GTAW will transform the zinc coating at the top surface of galvanized DP 980 steel metal sheet into the zinc oxides, which will dramatically improve the coupling of the laser power to the welded material. By the formation of the zinc oxides generated by the preheating of GTAW, the keyhole is readily formed, which provides the weld with a full penetration and allows the high-pressurized zinc vapor at the interface to be vented out through the keyhole. The completely defect-free laser welds have been obtained by using this welding method. Furthermore, a charge-coupled device video camera with the frame rate of 30 frames per second is used to on-line monitor the molten pool. By the analysis of the molten pool images, it is revealed that when the welding process is stable, the keyhole is kept open. However, the keyhole is readily collapsed when the welding process is unstable. In addition, the microhardness and tensile shear tests are carried out to evaluate the mechanical properties of laser welded joints. It is demonstrated that the welds of high strength are obtained in the laser welds.