Formation mechanism of spherical particles in undercooled melt

The microstructure evolution of Al-20%(mass fraction)Cu alloy under LSPSF (Low superheat pouring with a shear field) conditions was investigated. The quick formation mechanism of spherical particles in an undercooled melt was discussed by microstructure simulation technique and M-S interface stability theory. The results show that the morphology of primary α(Al) is determined by both the number of free crystals and the cooling intensity of melt. The first observable primary α(Al) of Al-20%Cu alloy is spherical shape, and grows spherically in subsequently lower cooling. The overlapping diffusion fields from adjacent growing crystals can induce a stabilizing effect on the morphological instability at the solid-liquid interface and promote the globular growth of primary α(Al). Restraining and coarsening caused by overlapping diffusion fields and Gibbs-Thomson effect are main dynamic conditions that promote the morphology transition of instabilized crystal from equiaxed shape into spherical shape.