Transport Phenomena and Keyhole Dynamics during Pulsed Laser Welding

Numerical and experimental studies were conducted to investigate the heat transfer, fluid flow, and keyhole dynamics during a pulsed keyhole laser welding. A comprehensive mathematical model has been developed. In the model, the continuum formulation was used to handle solid phase, liquid phase, and mushy zone during melting and solidification processes. The volume-of-fluid method was employed to handle free surfaces. The enthalpy method was used for latent heat. Laser absorptions (Inverse Bremsstrahlung and Fresnel absorption) and thermal radiation by the plasma in the keyhole were all considered in the model. The results show that the recoil pressure is the main driving force for keyhole formation. Combining with the Marangoni shear force, hydrodynamic force, and hydrostatic force, it causes very complicated melt flow in the weld pool. Laser-induced plasma plays twofold roles in the process: (1) to facilitate the keyhole formation at the initial stage and (2) to block the laser energy and prevent the keyhole from deepening when the keyhole reaches a certain depth. The calculated temperature distributions, penetration depth, weld bead size, and geometry agreed well with the corresponding experimental data. The good agreement demonstrates that the model lays a solid foundation for the future study of porosity prevention in keyhole laser welding.