Development of twin screw Rheo extrusion technology

Abstract

Twin Screw Rheo Extrusion (TSRE) is a novel semisolid extrusion process developed at BCAST for producing simple profiles such as rods and wires of light alloys directly from melts with refined microstructures and improved mechanical properties. The process represents a shortened manufacturing route with great savings in investment, energy consumption and operation space. Research was carried out to investigate the feasibility of processing magnesium and aluminium alloys, to obtain the operations for the optimized microstructures and mechanical properties of the final product and to understand the mechanisms governing the evolution of microstructures. Experiments were conducted using an AZ91D magnesium alloy and several aluminium alloys on two specially made twin screw rheo extrusion machines and a range of conditions were tested. Results showed that the TSRE process was feasible for the AZ91D magnesium alloy and aluminium alloys, although modifications were required for processing aluminium alloys as the twin screw material used was found to react with aluminium. Analysis revealed that the extruded samples of both alloys had a uniform fine microstructure in both transversel and longitudinal directions and liquid segregation was limited, due to the application of intensive shearing during slurry making and extrusion. Low extrusion temperature was found to refine the structure and suppress the formation of the eutectic. The eutectic was easily dissolved upon heat treatment resulting in reasonable mechanical properties. Numerical analysis on thermal management was carried out and the results showed that a steady state thermal profile with a temperature gradient between the slurry feeding point and extrusion die could be established, promoting nucleation and preventing the formed solid particles from extensive growth during extrusion, which was confirmed by microstructural observations.