Estimation of Residual Hydrogen in Laser Weld by Two-dimensional Differential Calculation

Abstract

It has been reported that hydrogen dissolves in the weld bead in laser welding. If the hydrogen concentration in the weld bead is high, there is concern that the weld bead may fracture during press forming of tailored blanks. Therefore, assuming that 1ppm of hydrogen is dissolved in the weld bead at welding, the change in hydrogen concentration over time is estimated by a two-dimensional difference calculation. In the calculations, hydrogen is assumed to follow the Fick’s diffusion equation, and the diffusion coefficient at room temperatures is the value determined by the authors for a 980MPa steel laser welds in the past. The diffusion coefficients at high temperatures were determined from several references. As a result, it was estimated that about 30% of the hydrogen had effused from the surface by the time the weld bead cooled to room temperature. The mechanical properties of the weld bead are considered to be governed by the maximum hydrogen concentration at the center of the weld bead cross-section, but this value cannot be measured directly. Therefore, assuming that all the hydrogen contained in the entire model is in the weld bead and considering the average hydrogen concentration in the weld bead, the ratio of the maximum hydrogen concentration to the average hydrogen concentration is around 1 up to 6h after welding, indicating that the average hydrogen concentration is a good indicator of the maximum hydrogen concentration. It was also confirmed that the logarithm of the average hydrogen concentration is proportional to time as time passes after welding and that the hydrogen concentration distribution in the thickness direction behaves as a solution of a one-dimensional diffusion equation. These results suggest that the hydrogen concentration in the laser weld bead can be considered as an analytical solution of the one-dimensional diffusion equation.