Solidification behaviour under intensive forced convection

Solidification behaviour of AZ91D Mg-alloy and 357 Al-alloy under intensive forced convection in the rheo-die-casting process, was investigated experimentally to understand the effects of the intensity of forced convection and shearing time 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 'effective and continuous nucleation'. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a slow coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. In addition, it was found that intensive forced convection partially suppresses the formation of the primary phase during the primary solidification, and that this effect is more pronounced in 357 Al-alloy than in AZ91D Mg-alloy.