Modeling the formation and collapse of a keyhole during laser welding process

This dissertation presents a mathematical model that is capable of simulating the fluid flow, heat transfer, and mass transfer during the keyhole-mode of the laser welding process. A keyhole is gradually formed in the substrate when a high-density laser beam impinges onto the substrate surface, while the keyhole collapses after the laser power is ceased. During the keyhole formation process, molten metal in the keyhole is squeezed upward by recoil pressure and accumulated around the entrance of the keyhole. During the keyhole collapse process, molten metal backfills the keyhole and, depending upon how the keyhole is filled, there is a possibility of forming pores in the weld. Both multiple reflections on the keyhole wall and the inverse Bremsstrahlung absorption in the plasma are considered in the model. The model, using the volume of fluid (VOF) technique and the continuum formulation, is developed to study: (1) the process of keyhole formation and collapse; (2) the formation and prevention of pores in the weld; (3) the interaction between filler metal and weld pool.; The formation of pores in the weld is found to be caused by two competing mechanisms: (1) the backflow speed of the molten metal to fill the keyhole and (2) the solidification rate of the molten metal. Two methods are proposed and confirmed by simulations to reduce/eliminate the porosity. They are the optimization of laser power profile to prolong solidification time and the application of an electromagnetic force to increase the backfill speed during keyhole collapse process. In laser welding, filler metal can be used to improve weld quality through the change of fusion zone composition and the modification of weld bead shape. Effects of different filler metal parameters on weld composition and weld profile are investigated in this study.