Process optimization of gravity sand casting process for thin-walled castings of Al–7Si-0.5 Mg alloy using a coupled simulation-experiment method
Zhijie Guo, Dong-Rong Liu, Hongyu Bao, Z. Pu, Yicheng Feng, Erjun Guo
Abstract
Optimizing casting parameters serves as a crucial method for reducing porosity and misrun defects during solidification of thin-walled castings. A heat-momentum-transfer coupled model is hired to simulate the solidification and fluid dynamics in thin-walled castings (ProCAST). Locations in a casting with Niyama values below 2 (°C s) 0.5 cm -1 are regarded as having higher tendency of forming shrinkage porosity. Numerical simulations are employed in conjunction with experimental characterizations to systematically investigate the effects of pouring temperature, sand-mold preheating temperature, and filling time on the porosity distribution and filling length of Al-7Si-0.5Mg alloy. The simulated results indicate that the pouring temperature has the most significant impacts on the fluidity and porosity formation in the thin-walled castings. High pouring temperature accelerates the gas absorption and causes the formation of coarse microstructure, finally resulting in the occurrence of gas pore and shrinkage porosity. Increasing the sand-mold preheating temperature reduces the chilling effect of mold and enhances the fluidity, however, slightly increases the porosity. Quick pouring minimizes the heat loss and increases the fluidity. The optimal processing parameters are pouring temperature of 720 °C, sand-mold preheating temperature of 110 °C, and the filling time of 2.5 s. Two mechanisms stand out with respect to the porosity formation. The refined equiaxed dendrites form skeleton along the filling path that stops the further feeding from the surrounding liquid, which results in the shrinkage porosity. In the case of higher pouring temperature, the formation of gas pores is also related to the gas absorption during pouring and the gas entrapment during solidification.