Microstructure and Mechanical Properties of a Rheo-Diecast Mg–10Zn–4.5Al Alloy

Abstract

High zinc content Mg–Zn–Al alloys have the potential to be used as high pressure die casting (HPDC) alloys for applications up to 150C. However, these alloys show high tendency to hot tearing and microporosity. Rheo-diecasting (RDC) is an innovative semisolid HPDC process, which can effectively eliminate the formation of primary magnesium dendrites and convert them into fine, globular particles. This very positive change in the morphology and distribution of the primary phase reduces the subsequent formation of various casting defects. In this study, a composition of Mg–10Zn–4.5Al was selected from the Mg–Zn–Al system and processed with both HPDC and RDC. It is shown that, samples produced by the RDC process exhibit substantially reduced hot cracks, few gas pores, and a nearly uniform distribution of fine, globular primary particles. These microstructural changes resulted in much improved strength and elongation, which are comparable to those of AZ91D, while the processing temperature is much lower. It is concluded that high zinc content Mg–Zn–Al alloys have good potential to be exploited as commercially useful alloys by the RDC process.