Gas occlusions in casted saturated composites
Jacek Jackowski Politechnika Poznańska, Instytut Technologii Materiałów, ul. Piotrowo 3, 61-138 Poznań
Annals 2 No. 4, 2002 pages 180-184
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abstract Porosity is highly undesired in any casting, inclusive of composite ones. The paper presents considerations related to formation, behavior, and volume of occlusions in saturated composite castings. Formation of the occlusions during saturation process is inevitable. Assuming the occlusion is subject to metallostatic pressure equal to reinforcement saturation pressure, volume of the occlusion is minimal, in accordance with the equation of gas state. However, under real conditions, such isolated regions may arise within a composite casting (according to casting structure or solidification conditions), in which pressure exerted on the occlusion might be considerably lower (Fig. 1), that induces their volume to increase. Model research of saturation process of an aluminosilicate reinforcement SIBRAL with low melting alloys (Wood's alloy or bismuth-free alloy) has shown, that initial occlusion volume, under the pressure approximating its normal value, amounts about to 30%, while porosity of composite samples (φ40x40 mm) saturated with Wood's alloy amounts to 2÷4%, irrespective of the way of their saturation (Fig. 2). Porosity of the samples saturated with bismuth-free low melting alloy was remarkably higher (Tables 1 and 2). Distribution of porosity of composite material has been examined with regard to the height and diameter of the samples (Figs 3 and 4). It was ascertained that for composite samples with Wood's alloy matrix porosity of the most compact parts of the samples is equal to or approximates zero. In case of the matrix of bismuth-free alloy (solidification shrinkage about 2%) the most compact parts of the samples showed 2% porosity. Variations of porosity in selected samples indicate formation of isolated zones (in result of taking over the punch pressure by solidified matrix layers). In consequence, decompression of the occlusions resulted in local porosity of the material, reaching a little less than twenty per cent. Solidification shrinkage of the matrix extensively increases porosity of the parts of the samples that solidify latest of all. More careful examination of behavior of the occlusions occurring in saturated composite casting requires metallographic research methods.