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RESEARCH PAPERS: Phase Change and Multiphase Heat Transfer

An Experimental Investigation Into the Effects of Grain Transport on Columnar to Equiaxed Transition During Dendritic Alloy Solidification

[+] Author and Article Information
J. W. Gao, C. Y. Wang

Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802

J. Heat Transfer 121(2), 430-437 (May 01, 1999) (8 pages) doi:10.1115/1.2825996 History: Received December 10, 1997; Revised December 10, 1998; Online December 05, 2007

Abstract

An experimental study has been conducted to investigate the effects of grain transport on the columnar to equiaxed transition (CET) in dendritic alloy solidification. Using the aqueous ammonium chloride solution as a transparent model alloy, experiments were performed in a vertical test cell with cooling from the top, resulting in unidirectional columnar crystals growing downwards. Ahead of the columnar front, equiaxed nuclei were observed to originate mostly by fragmentation of the columnar dendrites in the presence of a thermally driven flow in the melt beneath the columnar mushy zone. Being heavier than the liquid, these fragments fall into the bulk melt where they may grow or remelt. The survived equiaxed crystals finally settle towards the floor and pile up to form an equiaxed bed. The CET occurs when the bottom equiaxed packed bed rises and eventually obstructs the columnar mushy zone growing from the upper surface. Therefore, the CET in the present configuration was predominantly controlled by the sedimentation of equiaxed crystals. A parametric study by varying initial concentration, cooling rate, and superheat was performed.

Copyright © 1999 by The American Society of Mechanical Engineers
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