1. Introduction

Semi-solid metal (SSM) processing is a new technology for near net-shape production of engineering components, in which metal alloys are processed at a temperature in the interval between solidus and liquidus (Flemming, 1991; Kirkwood, 1994). The critical characteristics of semi-solid alloys are their globular or non-dendritic grain structures formed during solidification under forced convection (Flemming, 1972). SSM slurries exhibit distinctive deformation characteristics, namely, thixotropy and pseudoplasticity (Joly, 1976). The advantages of SSM processing compared with conventional die-casting processes are lower operating temperatures, laminar flow during mold filling and reduced solidification shrinkage. Consequently, one may expect longer die life, shorter cycle time, improved mechanical properties, reduced gas entrapment and porosity and lower tendency to hot tearing (Flemming, 1991; Kirkwood, 1994).

Since the early 1970s, many alternatives to the original MIT rheocasting process (Flemming, 1976) have been developed, such as thixoforming (Kirkwood, 1994) and thixomolding (Pasternak, 1992). However, the high cost of pre-processed raw materials and the difficulties in process control are the main obstacles to their acceptance by industry. Based on the extensive experience in processing of polymeric materials offered by the polymer processing community (Rauwendaal, 1985), a twin-screw rheomolding technology for SSM processing has been developed recently (Fan, 1999). In this process, the liquid alloy is converted into semi-solid slurry under high shear rate and high intensity of turbulence provided by a twin-screw extruder, and the semi-solid slurry is subsequently injected...