Solidification behavior of Sn-15 wt pct Pb alloy under a high shear rate and high intensity of turbulence during semisolid processing

Abstract

Microstructural evolution under intensive forced convection is of importance to the development of new semisolid processing technologies. Although efforts have been made to understand the nucleation and growth behavior of the primary phase under forced convection, important aspects of microstructural evolution still remain as open questions. Experimental work was undertaken to investigate the effects of a high shear rate and high intensity of turbulence on the solidification behavior of a Sn-15 wt pct Pb alloy using a twin-screw extruder. It was found that increasing the shear rate and intensity of turbulence can give rise to substantial grain refinement, which can be attributed to the increased effective nucleation rate caused by the extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. It was also found that forced convection increases the growth velocity and stabilizes the solidification interface, resulting in a transition of the growth morphology from rosette to spheroid with increasing shear rate and intensity of turbulence. Discussions are made on the effects of flow conditions on the nucleation behavior, constitutional undercooling, stability of the solidification interface, and growth morphology.