Process optimisation of squeeze cast magnesium-zinc-rare earth alloys and short fibre composites

The work reported in this thesis demonstrates the potential of the squeeze casting process for the production of castings using magnesium aIloys and its composites. In particular, the studies involving composites are focused on fabrication through squeeze infiltration. These show the ability of the squeeze casting process to produce castings of high metallurgical integrity. The work offers a clear understanding of a number of key parameters for the squeeze casting process which are prerequisites for the production of high strength castings of magnesium aIloys and composites. A better understanding of the behaviour of the squeeze cast material tested at both ambient and elevated temperature has been achieved. A methodology, which aIlows the identification of optimum squeeze casting conditions, has been developed. This has been successfully used in the identification of casting conditions which produce the best tensile properties at both test temperatures. Two casting programmes, namely: primary and secondary programmes, were designed to evaluate the controlling parameters for squeeze cast magnesium alloys and composites. The investigation was conducted with two magnesium alloys: ternary RZ5DF (Mg-4.2o/oZn-RE) alloy and commercial RZ5 (MgZn- RE-Zr) alloy. Different preform systems were investigated during the primary casting programme and the results showed that 14% volume fraction alumina fibres with 5 % silica binder provided the most satisfactory results in terms of ease of fabrication, improvement in strength and cost. Applied pressures of 0.1 to 120 MPa were studied with and without the addition of fibre reinforcement. Pressures of 60 MPa and 80 MPa were found to yield optimum tensile properties in the RZ5DF alloy and its composite respectively. It was also found that a preform temperature of 600°C or above was necessary to achieve minimum resistance to magnesium infiltration at the preform surface. Other process settings, such as applied pressure duration, were also investigated. The influence of pouring and die temperature on the tensile properties was studied during the secondary casting programme. It was found that a higher pouring and intermediate die temperature provided the highest tensile properties. The mechanical properties of castings were tested at both ambient and elevated temperatures. It was found that fibre reinforcement improved the mechanical properties of the materials at ambient temperature but the most significant improvement was observed at 250°C. The effect of grain refinement (zirconium) addition on the squeeze cast magnesium alloys and composites was also investigated. The results indicated that the tensile properties in the zirconium-free RZ5DF alloy were comparable to those of the RZ5 alloy grain refined with zirconium. The influence of zirconium addition on the tensile properties of RZ5DF and RZ5 MMC was similarly reported. Heat treatment improved the properties of the alloys by a small margin and adversely affected the properties of the composite. The overall results showed that there is an opportunity to achieve a significant saving in material and process cost when producing Mg-Zn-RE alloys and composites by the squeeze casting process.