Applicability of shrinkage porosity prediction for casting with Niyama criterion

The macroscopic shrinkage cavity and porosity of the TiAl alloy crisscross and clintheriform casting was predicted by the finite element casting simulation software ProCAST with Niyama criterion. The prediction results were tested by experiment. The results show that for the Ti-47Al-2Cr-2Nb alloy crisscross and clintheriform casting, the Niyama criterion values are mainly affected by the casting section thickness (cooling rate). The criterion values increase with the augment of the casting section thickness. The region that the most serious of the macro shrinkage and porosity is not the region that the minimum of the criterion value. While the region that the minimum of the criterion value has only the microporosity but no macro shrinkage and porosity, so it cannot confirm the location and bulk of the macro shrinkage and porosity on the basis of the criterion values. Analyzing makes clear that Niyama criterion is founded on interdendritic microporosity, and its function values are well applied to assessing the tendentiousness of microporosity. It can well predict the microporosity. Because the formation mechanism of macro shrinkage and porosity is different from microporosity, it isn't suitable for predicting the macro shrinkage and porosity of complex castings with great disparity in casting section thickness.