Semi-solid processing of engineering alloys by a twin-screw rheomoulding process

Based on the extensive experience in injection moulding of polymeric materials, a twin-screw rheomoulding process has been developed in our laboratory for near net-shape production of engineering components. The rheomoulding equipment consists of a liquid metal feeder, a twin-screw extruder with closely intermeshing, self-wiping and co-rotating screws, a shot assembly and a central control unit. The fluid flow in the twin-screw rheomoulding process is characterised by high sheer rate and high intensity of turbulence. The experimental results of rheomoulded Sn-15wt.% Pb and Mg-30wt.% Zn alloys have demonstrated that the twin-screw rheomoulding process is capable of producing small and near mono-sized solid particles distributed uniformly in a fine-grained eutectic matrix. Compared with other existing semi-solid metal processing techniques, the twin-screw rheomoulding process has the following advantages: small and spherical solid particles of near mono-size, chemical and microstructural uniformity throughout the component, accurate control over a large range of solid volume fractions, lower overall component cost due to low cost of feedstock materials, and shorter cycle time. (C) 2001 Elsevier Science B.V. All rights reserved.