Squeeze casting of magnesium-based alloys and their metal matrix composites

Ha, How Ung (1988) Squeeze casting of magnesium-based alloys and their metal matrix composites. University of Southampton, Doctoral Thesis.

Abstract

An ingot shaped die set was used to study the effects of combinations of casting conditions on the cast structure of both AZ91 and AZ31. Pressing pressures of ∼ 100 MPa and ∼ 50 MPa were found to be sufficient to produce pore free castings in AZ91 and AZ31 respectively. The relationship between the magnitude of squeeze casting pressure and the total solidification time was established. Cup-shaped castings were produced to study the effect of liquid movement, as a result of the back extrusion, on the cast structures of AZ91 and AZ31. The effects of casting dimensions, i.e. Height/Diameter ratio, squat cylindrical castings of AZ31 were investigated. In both the cup-shaped and squat cylindrical castings studied, a pressure of 50 MPa was found to be sufficient to produce pore free castings. Under a high pouring temperature, ∼ 730^oC in this study, the solidification behaviour, of both AZ31 and AZ91, under the applied pressure, was very similar except the improved heat extraction rate under the applied pressure produced ingot type structure in AZ91 compared to the normally equiaxed structure under atmospheric conditions. With a pouring temperature below 680^oC a completely equiaxed grain was obtained. The grain size was found to increase with an increase in the applied pressure. However, the results showed that the pouring temperature was the most effective single casting parameter in controlling the cast structure. A lower pouring temperature always resulted in a refinement of the grain size. In the case of AZ31, with a pouring temperature of ∼ 630^oC, i.e. close to the liquidus temperature, the cast structure was found to consist of extremely fine cellular grains (90-111μm). In the absence of porosity, the relationships between (i) casting defects and (ii) grain size and the mechanical properties of AZ91 and AZ31 have been established. The mechanical properties studied included: tensile properties, fracture toughness and fatigue strength. In both AZ91 and AZ31 the mechanical properties of squeeze cast materials were always superior to those of the gravity cast materials. The casting conditions necessary to give a good combination of mechanical properties for AZ91 have been proposed. Metal matrix composite castings using saffil, glass/graphite mixture and stainless steel wire as the reinforcement phase and AZ91 as the matrix phase have been investigated. A pressure of ∼ 100 MPa was found to be sufficient to produce pore free MMC castings. Comparative studies of the mechanical properties such as hot hardness, fatigue (180^oC) and creep resistance (180^oC) between the MMC and matrix materials were carried out. (DX84421)

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