The flowability of particulate TiB₂-containing metal matrix composite

Abstract

This paper reports a study of the flow behaviour of a particulate TiB₂-containing metal matrix composite (MMC) and a SiC particulate MMC containing the same matrix alloy, both in comparison with the unreinforced matrix alloy, Al-7Si-0.4Mg.

Both the MMCs exhibited a similar extreme sensitivity to surface turbulence during casting, to the point at which it was not easily possible to produce good-quality castings using a conventional gravity-poured filling system. Further results obtained from fluidity measurements showed that the fluidity is not so different to conventional casting alloys, although the fluidity of the TiB₂ MMC was slightly less good than the Al-SiC metal matrix composite, which in turn was poorer than the matrix alloy. A surprising result was that increasing the casting temperature did not significantly improve the fluidity of either of the MMCs.

The relatively low fluidity and the high density of major defects appears to be associated with the high viscosity of the mixture and the fact that the mixture exhibits thixotropy, i.e. the apparent viscosity decreases with increasing rate of shear. Thus to assist the flowability of the mixture a number of experiments have been carried out to assess the effect of high rates of imposed shear, applied by vibration and stirring actions. Results show that shearing is only marginally effective in fluidity enhancement but it has no influence on casting quality. The only method which was eventually identified as giving a practically defect-free casting was the low-turbulence casting technique adapted from the Alcan-Kingston laboratory development.