Solidification shrinkage in metal matrix composites

Abstract

A new test is described using simple unfed cast discs which allows a quantitative assessment of the shrinkage behaviour of cast materials in terms of (i) internal or external porosity and (ii) concentrated or dispersed porosity. The test has been applied to two different particulate metal matrix composites, and an aluminium alloy (Al—7% Si—0.4 Mg) which was common to both as the matrix alloy has also been investigated. One composite contained 7 vol% TiB2 particles and the other contained 20 vol% SiC particles. The shrinkage behaviour of the MMCs was found to be different from that of the matrix alloy. For higher section thickness castings, the matrix alloy alone developed a concentrated central shrinkage cavity, whereas the MMCs showed different behaviour. A large amount of external shrinkage was generated in TiB2 MMC, while the amount of internal shrinkage was small. For the SiC MMC, the internal and external shrinkage was similar. Generally, the internal porosity in both MMCs was found to be finer and more uniformly dispersed. The reason for the greater proportion of internal porosity in the SiC MMC compared to its TiB2 equivalent appears to be associated with the partly non-wetted surfaces of the SiC particles, whereas the chance of pore nucleation on well-wetted TiB2 particles (formed by chemical reduction in the melt) was very low. The level of internal porosity in thin-section castings was low (approximately 0.5%) for all cast materials in this study, and even for heavy sections in the cast MMCs the porosity would be tolerable for many components because it was dispersed and therefore would not by itself cause a casting to leak.