Showing papers by "Hai-Lung Tsai published in 1993"

Modeling of solute redistribution in the mushy zone during solidification of aluminum-copper alloys

Abstract: A mathematical model has been established to predict the formation of macrosegregation for a unidirectional solidification of aluminum-copper alloys cooled from the bottom. The model, based on the continuum formulation, allows the calculation of transient distributions of temperature, velocity, and species in the solidifying alloy caused by thermosolutal convection and shrinkage-induced fluid flow. Positive segregation in the casting near the bottom (inverse segregation) is found, which is accompanied by a moving negative-segregated mushy zone. The effects of shrinkage-induced fluid flow and solute diffusion on the formation of macrosegregation are examined. It is found that the redistribution of solute in the solidifying alloy is caused by the flow of solute-rich liquid in the mushy zone due to solidification shrinkage. A higher heat-extraction rate at the bottom increases the solidification rate, decreasing the size of the mushy zone, reducing the flow of solute-rich liquid in the mushy zone and, as a result, lessening the severity of inverse segregation. Comparisons between the theoretical predictions from the present study and previous modeling results and available experimental data are made, and good agreements are obtained.

Modeling of the formation of microporosity in alloys

Abstract: A complete mathematical model, consisting of a continuum model and a gas evolution model, was established for the prediction of shrinkage- and gas-caused porosity in castings. The model allows simultaneous calculation of transient temperature, velocity, pressure, and porosity distributions in a solidifying casting. Fluid flow caused by both natural convection and solidification contraction, as well as change of global domain due to shrinkage, were considered. A parametric study was performed to investigate the effects of various solidification variables in the formation of microporosity, including the use of chilling, initial gas content, characteristics of heat transfer between the casting and the mold, casting size, and riser height. It was found that a low initial gas content in the molten metal and casting conditions that facilitate solidification-rate decrease the formation of microporosity. The calculated porosity distribution in 1% Cr-steel alloys compares favorably with published experimental data.