Development and assessment of a new quick quench stir caster design for the production of metal matrix composites

Abstract

Examination of the liquid and semi-solid stir casting method to produce Al–SiC composites was the focus of this study. A significant part of the work consisted of the design, construction and validation of a specialised quick quench compocaster for this high temperature processing method. Stainless steel was chosen as the main crucible and stirrer material. The machine consisted of a four 45 ∘ flat bladed stirrer and a crucible in a resistance heated furnace chamber. A linear actuator was integrated to this rig to allow the crucible to be quickly extracted from the furnace for quenching. Stirring speed ranging from 200 to 500 rpm and different shear periods were investigated. Ten percentage volume of 30 μ m sized SiC particles was used. The main research challenge was to get a uniform distribution of SiC in the aluminium matrix. In the compocasting experiments it was found that the uniformity of SiC particles in the aluminium matrix were dependent on shear rate, shear period, cooling rate and volume fraction of primary solid. The quick quench compocaster system was successful in producing cast MMC samples. The use of clean heat-treated SiC particles and the quick quench method was sufficient to produce homogeneous composites. Castings from the liquidus condition were found to result in poor incorporation of SiC particles whereas castings from the semi-solid condition were found to produce a uniform distribution of SiC particles. However, quicker solidification, after cesation of mixing, was found to improve the uniformity of the SiC distribution significantly. Characterization of the MMC samples produced included microstructure recording and image analysis thereof. The matrix phase size, morphology and distribution of SiC particles throughout the stir castings were examined.