GRAIN REFINEMENT OF AZ91 IN NEW RHEOCASTING

摘要

The addition of SiC particles were found to grain refine AZ91. Many of these particles were found in the centre of what appears to be an Al_8Mn_5 phase, which precipitates before the formation of α-Mg. It appears that the precipitation of the Al_8Mn_5 may either reduce the number of SiC particles that are available for nucleation, or the SiC particles act to nucleate the Al_8Mn_5 which in turn nucleates the α-Mg. It was also found that the SiC particles decreased the grain size in New Rheocasting (NRC) when higher pouring temperatures were used. It was found that the degree of inhomogeneity increased considerably in the NRC castings when the grain size was large. This appeared to be due to the 'sponge' effect, where liquid was squeezed from the centre of the highly dendritic network when the grain size was large.