Laser keyhole welding of galvanized high-strength steel in a gap-free lap joint configuration

Abstract

The use of high-strength galvanized steels has increased significantly in various industries. However, welding of galvanized steels in a gap-free lap joint configuration presents a difficulty in dealing with the highly-pressurized zinc vapor resulting from the lower boiling point of zinc as compared to the melting point of steel. When failing to vent the highly-pressurized zinc vapor, a large amount of spatter and blowholes are produced in the weld during the welding process, which leads to significant degradation of the mechanical properties of the weld. In order to vent the zinc vapor, it is common for the industries to set up a small gap at the faying surface of two metal sheets prior to the welding process. This gap is costly to ensure the dimensional integrity of the flanges. Industry is continuously seeking for a single laser beam welding technique without the pre-or post weld processing.In this study, a novel laser welding process is proposed to lap join galvanized steels in a gap-free configuration. A side shielding gas is introduced to suppress the unstable laser-induced plasma, thus improving the coupling of the laser beam energy into the welded materials. At the same time, the drag force generated by the highly-pressurized zinc vapor, which results in the turbulent molten pool, is balanced by the external force from the side shielding gas. Under these welding conditions, the stable keyhole is maintained during the laser welding process, which provides a channel for the highly-pressurized zinc vapor to escape from the faying surface of two metal sheets. Completely defect-free lap joints are obtained by this laser welding process. In addition, a high-speed CCD camera is used to monitor the real-time dynamic behavior of the molten pool under different welding conditions. Furthermore, influences of thermocapillary force, recoil pressure and buoyancy force on the dynamic behavior of the molten pool are studied.The use of high-strength galvanized steels has increased significantly in various industries. However, welding of galvanized steels in a gap-free lap joint configuration presents a difficulty in dealing with the highly-pressurized zinc vapor resulting from the lower boiling point of zinc as compared to the melting point of steel. When failing to vent the highly-pressurized zinc vapor, a large amount of spatter and blowholes are produced in the weld during the welding process, which leads to significant degradation of the mechanical properties of the weld. In order to vent the zinc vapor, it is common for the industries to set up a small gap at the faying surface of two metal sheets prior to the welding process. This gap is costly to ensure the dimensional integrity of the flanges. Industry is continuously seeking for a single laser beam welding technique without the pre-or post weld processing.In this study, a novel laser welding process is proposed to lap join galvanized steels in a gap-free configuratio...