Multi-physical simulation of hydrogen porosity suppression in high-power laser beam welding with an oscillating magnetic field

• Applicant: Dr.-Ing. Marcel Bachmann
• Subject Area: Joining and Separation Technology ; Production Automation and Assembly Technology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547924277

Hydrogen porosity is hardly to avoid during laser beam welding (LBW) of aluminum alloys. The contactless magnetohydrodynamic (MHD) technique provides a promising method to decrease the hydrogen porosity ratio in the weld. Given the relatively small dimension of the hydrogen porosity (lower than 50 um), the experimental or numerical study of the suppression mechanism of the hydrogen porosity in the welding process is still facing challenges. This project represents the first attempt to simulate the transient microscopic behavior of hydrogen bubbles while considering macro fluid behavior within the weld pool during the LBW process by utilizing a coupled Eulerian-Lagrangian method. Furthermore, given the ambiguous mechanism behind hydrogen porosity suppression using a contactless magnetohydrodynamic (MHD) technique, this project also endeavors to provide the first quantified demonstration of the suppression mechanism for hydrogen porosity. Based on a validated 3D multi-physical model of LBW, considering crucial physical factors such as laser propagation, laser-material interaction, recoil pressure, and Marangoni shear stress etc., the temperature field, liquid metal flow and the solidification front geometry will be calculated, providing a solid basis for the hydrogen porosity modeling. The transportation and concentration behavior of the hydrogen will be calculated based on the above-mentioned physical variables. A stochastic hydrogen bubble model based on the calculated hydrogen distribution will be developed. The mesh-free discrete particle method will be used to simulate the release of the hydrogen bubbles near the solidification front at a microscopic level and the movement and capture of the bubble in the whole weld pool without introducing unaffordable computational cost. To clarify the suppression mechanism of the magnetic field on the hydrogen porosity, the transient induced eddy current and volumetric Lorentz force will be coupled with the 3D transient model for LBW. The electromagnetic lifting force produced due to the pressure difference around the bubbles will be applied to the hydrogen bubbles to consider the additional lifting speed. Other physical forces, such as buoyancy, drag force, and the braking force from the mushy zone, will also be implemented in this model. The above-mentioned models will be validated and recalibrated by experimental results. In this project, it is hypothesized that the hydrogen distribution (bubble formation) and the bubble movement are significantly influenced by the magnetic field-induced Lorentz force. To validate these two hypotheses, the influence of the magnetic field on the hydrogen element distribution and the released hydrogen bubble density will be studied and the bubble escaping speed, the bubble lifting routine and the local solidification rate will also be compared between the cases with/without the MHD effect, so that the suppression mechanism of hydrogen porosity will be illustrated clearly.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Michael Rethmeier