Investigation of transport phenomena and defect formation in pulsed laser keyhole welding of zinc-coated steels

Lasers are being used to weld zinc-coated steels due to high welding speed, high aspect ratio, and narrow heat affected zone. However, escape of high-pressure zinc vapour in the welding process can damage the weld pool continuity and cause large voids and serious undercuts in the final welds. In this paper, a mathematical model and the associated numerical techniques have been developed to study the transport phenomena and defect formation mechanisms in pulsed laser keyhole welding of zinc-coated steels. The volume-of-fluid (VOF) method is employed to track free surfaces. The continuum model is used to handle the liquid phase, the solid phase and the mushy zone of the metal. The enthalpy method is employed to account for the latent heat during melting and solidification. The transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse processes are calculated. The escape of zinc vapour through the keyhole and the interaction between zinc vapour and weld pool are studied. Voids in the welds are found to be caused by the combined effects of zinc vapour-melt interactions, keyhole collapse and solidification process. By controlling the laser pulse profile, it is found that the keyhole collapse and solidification process can be delayed, allowing the zinc vapour to escape, which results in the reduction or elimination of voids. The good agreement between the model predictions and the experimental observations indicates that the proposed model lays a solid foundation for future study of laser welding of zinc-coated steels.