Interaction of porosity with a planar solid/liquid interface
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1/4/04
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Interaction of Porosity with a Planar Solid/Liquid Interface ADRIAN V. CATALINA, DORU M. STEFANESCU, SUBHAYU SEN, and WILLIAM F. KAUKLER In this article, an investigation of the interaction between gas porosity and a planar solid/liquid (SL) interface is reported. A two-dimensional numerical model able to accurately track sharp SL interfaces during solidification of pure metals and alloys is proposed. The finite-difference method and a rectangular undeformed grid are used for computation. The SL interface is described through the points of intersection with the grid lines. Its motion is determined by the thermal and solute gradients at each particular point. Changes of the interface temperature because of capillarity or solute redistribution as well as any perturbation of the thermal and solute field produced by the presence of non-metallic inclusions can be computed. To validate the model, the dynamics of the interaction between a gas pore and a solidification front in metal alloys was observed using a state of the art X-ray transmission microscope (XTM). The experiments included observation of the distortion of the SL interface near a pore, real-time measurements of the growth rate, and the change in shape of the porosity during interaction with the SL interface in pure Al and Al-0.25 wt pct Au alloy. In addition, porosity-induced solute segregation patterns surrounding a pore were also quantified.
I. INTRODUCTION
GAS evolution during solidification is responsible for two casting defects: macroporosity (gas porosity) and microshrinkage (microporosity). While both defects have a significant influence on mechanical properties, their mechanism of formation is quite different.[1] Macroporosity results when gas is rejected from the liquid and is entrapped in the solidifying metals as spherical gas bubbles of millimeter size. Microshrinkage occurs when liquid metal cannot reach interdendritic areas during casting solidification, and is caused by a combination of shrinkage and gas evolution. It is a standard foundry defect for mushy-freezing alloys, such as aluminum alloys, steel, superalloys, brass, bronze, and cast iron. Its size is of the order of 10 to 100 m. Previous experimental studies of porosity evolution in metals have primarily relied on quenching of directionally solidified samples followed by postsolidification analysis of the combined effect of gas and shrinkage porosity. For example, experiments performed on Cr, Ni, and Mo alloyed steel[2] revealed that elongated or ellipsoidal macroporosity occurred only when a nucleation agent such as Al2O3 was added to the melt. In the absence of nucleants, irrespective of the hydrogen content, only microshrinkage was obtained. The effect of gravity on pore distribution was studied by Kim et al.[3] in a A356 Al-Si alloy. They demonstrated that for upward solidification, the distribution of pores was concentrated at the hot (upper) end of the sample, implying that terminal floatation velocity was the dominant factor. Floatation did not permi
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