Development of the rheo-diecasting process for Mg-alloys

Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolid metal (SSM) processing is one in which a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancing the effective nucleation and promoting spherical growth during solidification. Experimental investigation and theoretical analysis of solidification under forced convection allowed us to identify the conditions for achieving enhanced effective nucleation and promotion of spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new SSM processing technology, rheo-diecasting (RDC), has been developed for the production of components with high integrity. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity, fine and uniform microstructure and much improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry.