Experimental and Computational Study of Fluid Flow and Heat Transfer in the Lost Foam Casting Process

Abstract

The Lost foam casting (LFC) process has been considered as one of the most significant modern developments in casting technology. The use of polymer foam patterns allow complex shapes to be created by integrating several parts in one casting. Even though the LFC process has been incorporated in casting production around the world, a fundamental understanding of the interaction between the molten metal and foam pattern is limited. Computational modeling, which has proven to be very successful in the simulation and optimization of traditional sand casting, has been hindered by limited knowledge of LFC process. The LFC process, therefore, has yet to be optimized to achieve reductions in cost and time. This research study consists of two major parts: an experimental study of the transport phenomena between the molten metal front and foam pattern, and a computational simulation of the foam decomposition by improving the basic LFC model in the commercial package FLOW-3D. In the experimental study, a cylindrical polymer foam pattern and heated steel block were used to study the endothermic losses associated with the thermal degradation of the polymer pattern at the metal front. Thermocouple readings were analyzed to determine the kinetic zone temperature and the heat transfer coefficient between the advancing metal front and the receding foam pattern. Flow visualization was also used to verify the measurements in the kinetic zone. The results showed that the endothermic degradation of the polystyrene pattern at the metal front introduced a steep thermal gradient in the metal and a consistently increasing heat flux and heat transfer coefficient as the foam decomposes. The values of heat transfer coefficient, initially 150 W/m²?K gradually increased to 220 ~ 300 W/m²?K to the end of the process. The kinetic zone temperature was measured to be in the range of 150 to 290°C with an average of 200°C and a gaseous gap size of 1 to 4 cm which is further confirmed by the visualization. In the numerical study, a computational fluid dynamics (CFD) model has been developed to simulate the flow of molten aluminum and the heat transfer at the interfacial gap between the metal front and the foam pattern. The commercial code FLOW-3D provides a basic LFC model that can track the front of the molten metal by a Volume of Fluid (VOF) method and allow complicated parts to be modeled by the Fractional Area/Volume Ratios (FAVOR) method. The code was modified by including the effects of varying interfacial heat transfer coefficient. The modification was validated against experimental studies and the comparison showed improved agreement over the basic model.