抄録
In laser welding of steel sheets, hydrogen dissolves into the weld metal. Since the elongation at break decreases when hydrogen dissolves into steel, the weld may fracture if significant deformation is applied immediately after welding. For this reason, in press forming of laser welded TWB, it is necessary to ensure sufficient time between welding and forming. On the other hand, there is no simple method for non-destructively measuring the amount of hydrogen remaining in the laser weld bead. However, if the analytical solution of the diffusion equation describing hydrogen evolute from the weld bead and the diffusion coefficient are known, it is possible to estimate the required waiting time between welding and press forming. While there are numerous methods for measuring the diffusion coefficient, no effective method for measuring the diffusion coefficient at the laser welds is currently known. Therefore, the authors attempted to determine the diffusion coefficient at the laser welds. Hydrogen dissolved during laser welding localizes within the weld metal. It then diffuses into the heat-affected zone. However, due to the thinness of the steel sheet, the hydrogen is released into the atmosphere before it can diffuse widely into the base metal. Therefore, if the diffusion coefficient is determined based on the diffusion behavior of hydrogen dissolved during welding, it can be considered as the diffusion coefficient of the welds. The authors derived an equation representing the amount of hydrogen diffusing from the laser-welded metal and determined the diffusion coefficient to reproduce the evolution behavior from the welds. The determined values are diffusion coefficients at
50 ℃, with 5.76×10-5 mm2/s for 0.18C steel welding and 1.28×10-4 mm2/s and 1.54×10-4 mm2/s for two types of 0.14C steel welding, respectively.
