Phase-field simulation of semisolid metal processing under conditions of laminar and turbulent flow

Interface stability theory predicts that the effect of stirring during solidification will decrease the stability of the solid-liquid interface and promote dendrite formation. In contrast, during semi-solid metal processing, non-dendritic particles form when stirring a casting. This apparent conflict has been resolved in the present work by taking into account the effect of turbulence. A model of stagnant diffusion boundary layer, developed by Vogel & Cantor, has been used to describe the effects of laminar flow, and has been modified to allow for the effects of turbulence. The growing of an Al5Fe2 spherical seed from an Al-Fe-Si melt under conditions of both laminar and turbulent flow has been simulated by means of a phase-field model. In pure laminar flow, the phase-field simulations agree well with the interface stability theory predictions. Under conditions of increased turbulence, the particles change from dendritic to rosettes and then to spherical morphologies as the extent of stirring increases; this result agrees well with experimental observations of semi-solid metal processing. The relative viscosities of slurries with such different particle morphologies were calculated and showed good agreement with experimental data. The effect of turbulence was explained in terms of constitutional undercooling and classical particle growth theory. (C) 2003 Elsevier Science B.V. All rights reserved.