Overlap welded joint strength of 2.0 GPa-strength steel sheets using single-mode laser wobbling

Abstract

During laser overlap welding of high strength steels, a wide interface-bead width is a prerequisite for ensuring joint strength. However, a wide weld bead is accompanied by thermal effects such as thermal deformation and softening of the heat-affected zone owing to the high heat input during welding. Hot-press-forming steel with a strength of 2.0 GPa is the highest strength steel sheet in the automotive industry. When laser-welded, the minimum hardness in the heat-affected zone is less than 2/3 of the base metal hardness. In this study, single-mode laser and beam wobbling was employed to obtain a proper bead width while minimizing the heat input in the lap welding of steel sheets with a strength of 2.0 GPa. Two strategies—high frequency wobbling/high travel speed and low frequency wobbling/low travel speed—were evaluated with a laser power fixed at 1 kW. In the high frequency wobbling/high travel speed condition, the load-carrying at the overlap joint increased as the travel speed and wobbling frequency decreased. However, even in the case with the maximum fracture load, the fracture location in the tensile–shear test was the weld metal. The low frequency wobbling/low travel speed strategy was more effective in ensuring joint strength, and the fracture location in the tensile–shear test moved to the heat-affected zone. An equivalent tensile strength of 1 GPa or more was achieved by selecting appropriate parameters. Under optimal conditions, multiple weld penetrations and sufficient interface beads were confirmed on the cross section.