One of the major objectives of solidification modeling is to determine, prior to pouring, whether porosity, such as massive cavities and dispersed pores, are likely to appear in the casting. The numerical solutions of solidification heat transfer alone, however, cannot provide such information. In order to predetermine the presence of porosity, various criteria functions have been proposed by a number of investigators. These criteria functions are associated with cooling rate, thermal gradient, solidus velocity and local solidification time, etc. Since these parameters can be derived from numerical solutions, the reliability of porosity prediction largely depends on the accuracy of the numerical solutions employed. Thermal contact and phase change affect the numerical solutions significantly, and hence the local values of the predicted parameters. Consequently, these phenomena must be given special attention. This paper addresses some important aspects of thermal contact and phase change in determining the values of criteria functions. The free thermal contraction method is used to describe the variation and distribution of the heat transfer coefficient at the casting/mold interface. The phase change problem is treated by the heat source/sink algorithm. The sensitivity of criteria functions and the role of computational error are also discussed.

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