DEVELOPMENT OF THE RHEO-DIECASTING PROCESSFOR Mg- ALLOYS AND THEIR COMPONENTS

摘要

Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolidrnmetal (SSM) processing is one in which a suitable volume fraction of fine and spherical particlesrndispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancingrnthe effective nucleation and promoting spherical growth during solidification. Experimentalrninvestigation and theoretical analysis of solidification under forced convection allowed us tornidentify the conditions for achieving enhanced effective nucleation and promotion of sphericalrngrowth. Such conditions can be summarised as uniform temperature and chemistry throughoutrnthe whole volume of liquid alloy, high shear rate and high intensity of turbulence during thernsolidification process. Based on such understandings, a new SSM processing technology, rheodiecastingrn(RDC), has been developed for the production of components with high integrity.rnAZ91D Mg-alloy was used to optimise the rheo-diecasting process and component productionrntrials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity,rnfine and uniform microstructure and much improved strength and ductility. Rheo-diecastingrnprocess is particularly suitable for production of high-safety, airtight and highly stressedrncomponents in the automotive industry.