A phase-field simulation of columnar-to-equiaxed transition in the entire laser welding molten pool

In order to simulate the columnar-to-equiaxed transition (CET) in the entire molten pool of Al–Cu alloy laser welding, a phase-field model was established by considering crystalline orientations and heterogeneous nucleation. Simulation results revealed that crystals initialized as planar ones from the molten pool edge and were then transformed into columnar dendrites during the solidification process. It was found that dendrites grew toward the center of the fusion zone irrespective of their crystalline orientations. Equiaxed grains grew ahead of columnar dendrites. They gradually formed a belt ahead of columnar dendrites and stopped columnar dendrites from growing. The highest number of equiaxed grains was found at the top edge of the cross-section of the molten pool due to the fastest pulling velocity. The variation of undercooling in the molten pool was analyzed to investigate CET during the solidification process. Heterogeneous nuclei were formed throughout the solidification process. They melted during the early stage of the solidification process and started to grow when the undercooling ahead of columnar dendrites was large enough. With the prolonged solidification, the number of heterogeneous nuclei gradually increased and the size of equiaxed grains decreased. Experimental results were consistent with simulations.