UBC Theses and Dissertations
Inverse segregation and centreline shrinkage Minakawa, Sakae
One of the major factors controlling macrosegregation and shrinkage porosity in castings is the extent of interdendritic fluid flow which occurs during solidification. Interdendritic flow results from movement of liquid to fill the voids left by solidification and thermal shrinkage, which in the case of many alloys cast against a cold chill, results in inverse segregation. In the present investigation, Inverse segregation has been examined theoretically. Quantitative values for the segregation both at and adjacent to the chill face have been determined using a mathematical model and computer calculations. The alloys considered were Al-Cu, Al-Zn and Sb-Bi, the latter being of particular significance since it expands during solidification. The model results show reasonable agreement with published data of chill face inverse segregation. In the previous models, the volume shrinkage used to calculate inverse segregation did not include the thermal contraction which, in the present model predictions, was found to have more significant role in the segregation than expected. The results from the present model differs significantly from those of the earlier models, particularly in the Al-Zn alloys. Centreline shrinkage in a steel plate casting, associated with back flow of interdendritic liquid, was examined using the interdendritic fluid flow model combined with a heat transfer model of the system. Regions of the plate where centreline porosity should occur were predicted with the new model and compared to published experimental results of porosity in steel plates. Excellent agreement was obtained between the predicted and experimental results. In the fluid flow model, Darcy's Law was used to determine the extent of flow in the interdendritic channels. The present results indicate Darcy's Law is valid in this application.
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