The movement of particles in liquid metals due to gravity
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I.
INTRODUCTION
W H E N a metal solidifies solid grains form and grow in the liquid. These grains may move in the liquid as a result of gravity forces, sinking or floating if the particle density is greater or less than the local liquid density, respectively. The terminal velocity of a spherical particle moving under gravity through the liquid for conditions of low Reynolds number is given by Stokes Law. v = 2ga2(pl
-
P2)
9~7 where
v = a = p~P2 = 7/=
terminal velocity of the particle radius of sphere the densities of the sphere and liquid, respectively coefficient of viscosity of the liquid
The movement and final configuration of the grains in the melt may also depend on fluid flow, due to temperature gradients in the melt. In addition, the effective density of the solid grains may not be clearly defined due to the presence of low density constituents associated with heterogeneous nucleation, shrinkage or gas porosity, and segregation. Segregation may also cause density gradients in the liquid. Grains are generally not spherical and move under higher Reynolds number conditions which makes calculations based on Stokes Law uncertain. The effect of some of these factors on grain movement is demonstrated in observations of the distribution of oxide particles in large steel ingots.2 The solid oxide particles have a lower density than that of the liquid and should float to the top surface of the melt. Experimental observations show that the oxides are concentrated in a particular pattern in the lower part of the ingot, the pattern being associated with the fluid flow configuration in the melt. The present investigation was undertaken to determine experimentally the movement of solid particles in a melt under gravity forces and to compare the results with Stokes Law. F. WEINBERG is Professor and Head, Department of Metallurgical Engineering, The University of British Columbia, Vancouver, B.C. V6T IWS, Canada. Manuscript submitted June 13, 1983. METALLURGICALTRANSACTIONS B
II.
EXPERIMENTAL
Two separate experimental procedures were used to investigate particle movement in the melt. In one set of experiments the distribution of solid grains in a solid-liquid tin matrix was examined as a function of the amount of solid present and the grain settling time. In the second set of experiments the movement of small pieces of copper in a lead-tin liquid matrix was examined as a function of the density difference between the copper and the melt. To examine the distribution of solid grains in a solidliquid matrix, a mold was filled with pure tin (99.99 pct) and partially solidified. During solidification the closed mold was rotated about a horizontal axis to distribute the solid grains uniformly throughout the melt. After a given period of time the rotation was stopped; radioactive 2~ was added to the melt and rotation then resumed for a short period. The mold was then held steady in the vertical position for a given time to allow the solid particles to settle, and the entire mold then rapidly quenched. The mold consisted o
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