The solidification process of copper with and without La was observed by a high-temperature confocal laser scanning microscopy. The solidification microstructure was simulated by cellular-automaton and finite-element (CAFE) method and the source of equiaxed grains, the constitutional supercooling and nucleation rate were investigated. The results showed that some floats with regular shape appeared in copper melt with La addition, which is the source of equiaxed grains. The simulation results are in a good agreement with experimental results. The mechanism of columnar to equiaxed transition was that the spontaneous nucleation probability of copper melt was increased with La addition. The nucleation rate was increased due to the increase in critical nucleation temperature and constitutional supercooling, and the reduction of nucleation undercooling.
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