Process Optimization and Microstructure Control for Twin Screw Rheo-Extrusion of an AZ91D Magnesium Alloy

Abstract

Twin screw rheo extrusion (TSRE) is a continuous semisolid forming process. The process takes advantage of the thixotropic properties of semisolid metal slurry and the high shear, controllable forward driving and self-sweeping capabilities of a counter rotating twin screw mechanism, offering a one-step melt-to-product forming technology with significant savings on energy consumption and manufacturing cost. The present work was carried out to investigate the feasibility of the process for processing light alloys using an AZ91D magnesium alloy and to obtain optimized operation conditions for microstructure control. A range of processing parameters was tested for slurry making, feeding and extrusion and their effect on microstructure development was examined. Experimental results showed that slurry with non-dendritic primary phase particles of various volume fractions, typical of semisolid microstructure, was obtained prior to extrusion. The particles were further refined during extrusion due to dynamic shearing applied by the twin screw mechanism under controlled thermal schemes and the final microstructure obtained after extrusion was dominated by uniformly distributed equiaxed, non-dendritic spherical grains and the average grain size was in the range of 35-80 mm, depending on processing parameters. This paper presents the performance of the TSRE process, focussing on process optimization and microstructure control. Mechanisms of microstructural refinement and particle morphology evolution are discussed.