A Modified Mechanism of Porosity Formation in A356 Alloy Permanent Mold Castings

摘要

Porosity formation in A356 aluminum alloy permanent-mold castings was studied experimentally and theoretically. The thermal parameters of thermal gradient, solidus velocity and local solidification time as well as the porosity content were measured. In the theoretical analysis, the hydrogen diffusion kinetics is taken into consideration. By using a semi-empirical method, the equation describing the porosity content V_p(percent) of A356 aluminum casting is derived and formulated as: V_p(percent)=K'[Ho]{1-exp[-t_f/t_(tau)~(3/2)]}/P_g where K' is a characteristic constant, [H_0] is the initial hydrogen content in, ml/100g Al in the A356 melt, t_f is the local solidification time, t_(tau) is the relaxation time, and P_g is the pore gas pressure. The predicted porosity content predicted by a previous model is found much higher than the experimental data near the free end of the casting. It is shown in the study that the modified model can satisfactorily predict the porosity content at any location of A356 alloy casting.