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Interdendritic fluid flow Streat, Norman

Abstract

Fluid flow through liquid interdendritic channels of a partially remelted lead-tin casting has been measured directly, with gravity as the driving force. The results were shown to be consistent with Darcy's Law. The permeability of the dendritic array was found to be a function of the square of the primary dendrite spacing, and was observed to increase with time due to coarsening of the dendritic structure. The formation of casting defects in lead-tin alloys was studied with isothermal and unidirectional solidification experiments. Solute convection was observed when the liquid close to the bottom of the solid-liquid region was less dense than the liquid above, using radioactive tracer techniques. Macrosegregation was shown to be related to the solidification conditions, and channel-type defects, resembling freckles and A segregates, were formed when the rising interdendritic liquid dissolved dendrite branches in its path. A simple mathematical model is proposed, which predicts the composition profiles in vertical, directionally solidified lead-tin castings, as a function of the structure, growth rate, and temperature gradient. The model is shown to agree qualitatively with the experiments, and can be used to recommend specific changes in casting practice to reduce gravity segregation effects.

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