The Pressure Index of Porosity Formation for A356 Alloy Permanent Mold Castings with Variation of Hydrogen Contents in the Melt

摘要

Porosity formation in permanent mold A356 aluminum castings with variation of hydrogen content in the melt was studied experimentally and theoretically. The experimental results show that the porosity content of casting is a function if the initial hydrogen content in the melt. The model of porosity formation from this study indicates the important roles of thermodynamics, Darcy's law and thermal parameters on the porosity formation. The porosity content (V_p) can be modeled as: V_p (percent) = 14.2 centre dot ([H] - 0.021) / P* where [H] is the initial hydrogen content in ml/100g Al in the A356 melt. Also the pressure index (P*), a function of location x in the casting, is given as: P* = P_0 + rho_1gh + 4 sigma/C centre dot t_f~0.286 - 24pi tau~3 mu beta' DELTA T~0.428/C~2 (V~1.572/G~0.428) In (alpha/f~1(x) ) Where P_0 is the atmospheric pressure, rho_1gh is the hydrostatic pressure of the riser, C = 35.1 mum/min~0.286, DELTAT = 38 + 18.655e~(-0.244tf) deg C and sigma = 0.864 N/m. f_1(x) is the liquid volume fraction, and alpha is the liquid volume fraction at the end of mass feeding. G, V_s and t_f, are local thermal parameters of the casting. Other parameters are physical characteristics of A356 alloy as detailed in the text. It is shown in the study that satisfactory quantitative porosity content at any location of the casting can be predicted by the proposed model.