Transport phenomena and defect formation in laser welding of zinc-coated steels

Zinc-coated steels are used extensively in the auto industry because they are inexpensive, durable and have high corrosion resistance. Lasers are being used to weld zinc-coated steels due to high welding speed, small seam and narrow heat affected zone. However, it is difficult to laser weld lap-joint zinc-coated steel sheets under a very small gap condition between the metal interfaces since there is a considerable amount of zinc vapor generated. This vapor must be vented out; otherwise it will be trapped in the weld pool leading to different welding defects, such as large voids at the tip of the weld and porosities in the form of small bubbles in the weld. These defects can significantly decrease the strength of the weld. In this paper, a mathematical model and the associated numerical techniques have been developed to study the transport phenomena in laser 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, solid phase and 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 vapor through the keyhole and the interaction between zinc vapor and weld pool are studied. The aforementioned weld defects are found to be caused by the combined effects of zinc vapor-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 vapor to escape, which results in the reduction or elimination of weld defects.