Microstructure and mechanical properties of rheo-diecast (RDC) aluminium alloys

A new semisolid metal (SSM) processing technology, the rheo-diecasting (RDC) process, has been developed for manufacturing near-net shape components, of high integrity, directly from liquid Al-alloys. The RDC process innovatively adapts the well-established high shear dispersive mixing action of the twin-screw mechanism to the task of in situ creation of high quality SSM slurry under high shear rate and high intensity of turbulence. This is followed by direct shaping of the semisolid slurry into a near-net shape component using the existing cold chamber diecasting process. The RDC process has been applied to process various Al-alloys, including cast alloys and wrought alloys. In this paper, we present the microstructure and mechanical properties of the RDC Al-alloys under as-cast, and various heat treatment conditions. Although the focus is on A357 alloy, we also present experimental results on other Al-alloys. The results indicate that the RDC samples have close-to-zero porosity, fine and uniform microstructure throughout the entire sample in the as-cast condition. Compared with those produced by conventional high-pressure diecasting (HPDC) and any other SSM processing techniques, RDC samples have much improved tensile strength and ductility. It is also found that heat treatment of the RDC M-alloys, under both T5 and T6 conditions, can substantially improve strength but with a slight sacrifice of ductility. (c) 2005 Elsevier B.V. All rights reserved.