A three-phase model of hydrogen pore formation during the equiaxed dendritic solidification of aluminum-silicon alloys
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I. INTRODUCTION
DEFECTS formed during the solidification of metals, such as segregation, inclusions, and porosity, dramatically affect the engineering properties of finished metal products.[1] Hence, predicting and reducing the occurrence of defects is of great importance to the metals industry. Many of these defects form as a result of several interacting mechanisms. The use of computer simulation in the study of solidification permits the examination of process interactions impossible to evaluate using analytical solutions, allowing the dominant mechanism to be determined for specified situations. Microporosity is a casting defect that can reduce mechanical properties, such as fatigue life.[2] Microporosity forms for two reasons: the precipitation of gas dissolved in the molten metal during solidification and the inadequate feeding of volumetric shrinkage. Bubbles formed due to these two causes remain when solidification is complete, appearing as tiny cavities in the casting, hence, the name “microporosity.” This article focuses upon the simulation of microporosity formation in a binary alloy system of aluminum 7 wt pct silicon with dissolved hydrogen. This binary alloy system is the basis for several important commercial-casting alloys, and hydrogen is the most significant pore-forming gas in aluminum.[1,3] The formation of this type of defect is governed by the interaction of the solidification dynamics, diffusion, fluid flow, solution chemistry, and nucleation mechanics; this study concentrates on the subsystem in which the interactions among solidification dynamics, solution chemistry, diffusion, and nucleation mechanics are considered. A schematic of the growth of the solid and gas phases in an Al-7 wt pct Si binary alloy system is shown in Figure 1, illustrating the competing phenomena. Initially, the material is a single liquid phase consisting of aluminum with dissolved silicon and hydrogen. Primary aluminum grains R.C. ATWOOD, Postgraduate Student, and P.D. LEE, Senior Lecturer, are with the Department of Materials, Imperial College, London SW7 2BP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted June 11, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
nucleate and grow as the liquid cools below the liquidus temperature. Since the solubility of both silicon and hydrogen is lower in the solid than in the liquid, these constituents accumulate in the remaining liquid phase. The change in temperature also alters the solubility of each constituent in the aluminum. The hydrogen concentration, silicon concentration, and the temperature change all contribute to the degree of hydrogen supersaturation in the liquid. When this supersaturation becomes great enough, nucleation of gas bubbles occurs. Once the bubbles have formed, they grow due to the diffusion of hydrogen from the surrounding liquid. The hydrogen diffusion is impeded by the solid, in which its diffusivity is lower. The transport of liquid metal to feed the volume change due to solidification is also impeded as the amount of so
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