A study on twin-screw rheo-diecasting of AZ91D Mg-alloy

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A newly developed one-step semisolid process by BCAST for semisolid processing of magnesium alloy was studied in this thesis - the Twin-Screw Rheo-Diecasting (RDC) of AZ91D Mg-alloy. The RDC process is an innovative process which combines the dispersive mixing power of the Twin-Screw Slurry Maker (TSSM) for creation of high quality semisolid slurry and the existing cold chamber High-Pressure Die-Casting (HPDC) process for component shaping. Magnesium alloys, due to their low density and superior strength/weight ratio, offer distinct advantages in weight savings, and are gaining increasing interests in applications. The research on the new RDC technology aimed to eliminate the limitations of the conventional HPDC and to meet the requirements from extensive application of Mg-alloys. In this thesis, the major tasks were to optimise of the RDC technology, to evaluate the microstructure and mechanical properties of RDC AZ91D Mg-alloy in both as-cast and heat treated conditions, and to understand the solidification process in the TSSM. The results of the RDC as-cast state indicated that the microstructure of primary a-Mg particles had a fine size (around 40μm), extremely spherical morphology and uniform distribution throughout the entire castings; the RDC AZ91D samples had extremely low levels of porosity. Due to the unique microstructure and much reduced level of defects, the RDC AZ91D alloy exhibited a substantial improvement in mechanical properties. In addition, a traditional full heat treatment was performed for RDC AZ91D alloy. Compared with HPDC alloy, the RDC AZ91D alloy was found to exhibit an accelerated dissolution of ß-Mg17A112during solution treatment, and a faster age-hardening kinetics of the ß-phase during subsequent ageing. The microstructural investigations showed that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt and the nuclei grew spherically. Ostwald ripening took place by dissolution of the smaller particles but at a very slow coarsening rate. Increasing the intensity of forced convection enhanced nucleation and reduced volume fraction of primary phase solidified in the slurry maker. This study has demonstrated that the novel RDC process possesses a number of advantages and it is suitable for production of high integrity Mg-alloy components