The effect of mold precession on channel and macro-segregation in ammonium chloride-water analog castings
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I.
INTRODUCTION AND PREVIOUS WORK
IN a majority of alloy systems liquid density variations with composition exceed those resulting from thermal expansion, i.e., dp/dC > dp/dT. A consequence of this effect is a driving force for convection if the liquid is inhomogeneous, so called solutal convection, and in castings it is the cause of "A" segregation channels in steel ingots or "freckel" in electroslag ingots. Liquid inhomogeneity develops in a partly solidified ingot because interdendritic liquid has a different composition to the bulk liquid. Microsegregation in the solid-liquid array of dendrites causes the solute enriched or depleted liquid to rise or fall, and as this liquid percolates through the dendrite assembly local melting leads to the development of well defined channels. Not only is the presence of such extended inhomogeneity undesirable in a casting, but the rapid flow along such channels provides a mechanism for long range macrosegregation which may require severe cropping of an ingot at considerable expense in energy and material. The above effect has been clearly demonstrated and examined by McDonald and Hunt, t'z Mehrabian, Keane, and Flemings, 3 and Copley e t a l 4 using ammonium chloridewater mixtures, s In the analog mixture there are some notable differences from metallic alloys, in thermal conductivity, solid-liquid densities, and particularly the proportion of solid in the dendrite assembly, but the influences of gravity and of temperature gradient are e v i d e n t - - a less dense channel segregate rising and melting its way toward the hotter bulk liquid. Copley e t a l 4 with a simulated E. S.R. arrangement (bottom cooling) also demonstrated how the direction of channel flow remained vertical despite the inclination of the casting and further illustrated this in a steel ingot. Some reduction in the development of channels and consequential macrosegregation can be achieved by suitable combinations of solutes which will minimize the density variations, 6 but this is possible only over limited composition ranges and in practice, the choice of solutes made ALEX SAMPLE, Graduate Student, and ANGUS HELLAWELL, Professor, are both with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted November 6, 1981. MEq'ALLURGICALTRANSACTIONS B
to reduce this channel development may not be compatible with the requirements of the product. In situations where the macroscopic growth front is concave toward the liquid as in continuously cast billets or E. S.R. ingots, a further consequence of such fluid flow is to enhance macrosegregation at the center of an ingot. Kou, Poirier, and Flemings 7 have shown that in such a situation, if the solute is the more dense, the development of freckel channels can be retarded or eliminated by centrifuging the ingot about a vertical axis at rates of the order of 102 rpm, thereby spinning the more dense liquid toward the mold walls.
II.
AIM AND DESIGN OF PRESENT STUDY
Solute rich channels must develop a
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