Efficient refinement of spherical grains by LSPSF rheocasting process

Abstract

A new rheocasting process, low superheat pouring with a shear field (LSPSF), to achieve sound semisolid slurry is reported. Results show that a fully grain refined spherical structure can be obtained using proper processing conditions within 25 s. The efficient grain refinement should be attributed to the continuous nucleation events, enhanced crystal separation from the nucleation sites and crystal survival, as a consequence of low superheat pouring combined with localised rapid cooling and positive mixing–shearing during the initial stage of solidification. The discussion based on the morphological instability theory indicates that the combined effects of both low cooling and high grain density can enhance the stabilisation of solid/liquid interface and promote grains to grow spherically to a larger size scale. Suppressing dendrite growth and coarsening through the overlapping of diffusion fields and the Gibbs–Thomson effect are the main dynamic conditions that lead to the formation of spherical primary a α-Al particles.