Solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process

Rheo-diecasting (RDC) is a new semisolid processing technology for production of near net shape components. In this work, the solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process was investigated experimentally to understand the effects of the intensity of forced convection, shearing time and shearing temperature on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as continuous effective nucleation. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. Secondary solidification of the intensively sheared semisolid slurry takes place also through effective nucleation, but with dendritic growth. Increasing the intensity of forced convection enhances nucleation and promotes the formation of the primary phase during the secondary solidification in the shot sleeve. The final solidification microstructure is strongly dependent on the presence of turbulence rather than the shear rate. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.