SEMI-SOLID CASTING OF AZ91D MAGNESIUM ALLOY FROM EXTRUDED BILLETS

摘要

Commercial magnesium castings in the AZ91D magnesium alloy are mostly pressure die cast from liquid metal at elevated temperatures, or produced using the proprietary Thixomolding process at a low percentage solid fraction. Semi-solid casting of magnesium billets from reheated feedstock is relatively unknown; unlike aluminum alloys, continuously cast bars suitable for this purpose are not readily available commercially. In this respect, the use of extruded billets is a clear alternative. This paper demonstrates the feasibility of casting AZ91D magnesium alloy from 76.2 mm (3 in) diameter extruded billets reheated to below the liquidus temperature to provide a varying amount of solid. Upon heating, the fine primary alpha phase is transformed into globular particles similar to what is commonly observed in heating continuously cast and electromagnetically stirred material with a fine non-dendritic structure. Subjected to a high shearing force during die casting, the thixotropic nature of the alloy allows complete die filling to be achieved readily. In the experiments, the feedstock material is contained in a crucible and heated using an in-house induction system specially designed to provide a variable energy source necessary to maintain an even temperature throughout the workpiece. The alloy temperature and input power are controlled by a thermocouple inserted at the center of the 152.4 mm (6 in) billet or slug. Slugs heated to different temperatures (and thus containing different percentages of solid) are cast into a number of complex box-like components. Various sections of the castings are cut and evaluated to establish variations in microstructure and mechanical properties. The influence of certain casting parameters on castability is examined, with the view to identifying those critical to die filling. Results are compared with those obtained from castings produced from liquid metal.