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Abstract Cast aluminum alloys are widely applied in aero and auto industries because of their good mechanical properties, light weight and high strength to mass ratio. The castability and mechanical property of aluminum alloy degrade due to the porosity defect. In order to decrease the porosity of cast aluminum and improve casting quality, researchers engage in knowing the mechanism of porosity formation by experiments and physical modeling.The main purpose of this study is to introduce kinetic effect of hydrogen diffusion to the previous model of interdendritic fluid flow based on the Darcy''s law as well as surface tension in porosity formation. The volume percent of porosity (Vp%) is formulated in this study. Previous data from Chieu''s plate sand castings and Jain''s cylindrical metal mold were used to verify this theory. It is found that local pressure drop, thermal gradient and pressure of surface tension effect are important factors in porosity formation. In this study the hydrogen content exerts the influence on the porosity formation, and it is assumed that the diffusion coefficient of hydrogen is a function of the solidus velocity. Since hydrogen solubility in the solid is much less than it is in the liquid phase, it is proposed that hydrogen concentration increases in the remaining liquid because of the rejection of hydrogen in the liquid. When the dissolved hydrogen overcomes the atmospheric and hydrostatic pressures, and the pressure of surface tension effect, porosity will form.
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