Channel segregation during solidification and the effects of an alternating traveling magnetic field
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9/26/04
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Channel Segregation during Solidification and the Effects of an Alternating Traveling Magnetic Field M. MEDINA, Y. DU TERRAIL, F. DURAND, and Y. FAUTRELLE The model proposed by Felicelli, Heinrich, and Poirier is used to simulate the solidification of a small two-dimensional domain of Pb-10 wt pct Sn alloy in the presence of electromagnetic stirring by different traveling fields, with or without gravity. Results show (a) enrichment of the bulk liquid by mush solute draining; (b) spontaneous formation of vertical channels, acting as ducts, significantly modified by the electromagnetic flow; and possibly (c) a finer periodic structure of subchannels. Only the last feature is sensitive to the mesh size and permeability value. Scaling analysis is used to balance Darcy, buoyancy, and electromagnetic phenomena. Attention is focused on the gradient zone at the solidification front. Electromagnetic forces can change the flow structure in the bulk liquid. In this way, they modify the pressure differences at the solidification front, changing the channel segregation pattern. Although they cannot eliminate the channels, they can control their positions and partly prevent the unsteadiness of buoyancy effects.
I. INTRODUCTION
CHANNEL segregates are concentration defects arising during the solidification of solute-rich alloys. In most cases, they are revealed by macrographic examinations. They are long in shape, forming channels in the solid-liquid mush, typically several centimeters long in the vertical direction and a few millimeters wide across the horizontal section, where they appear as “freckles.” Channel segregates have been observed in different solidification products including turbine blades made from nickel-base alloys, vacuum-arc-remelted ingots of nickel-base alloys, and high-carbon alloy steels cast into rolls for rolling mills. They are considered as severe defects leading to rejection.[1–5] Similar defects can be reproduced in the laboratory on model systems such as ammonium chloride solutions,[6,7] or on model alloys.[7–10] In particular, Sarazin and Hellawell[7] performed a detailed study on the directional solidification of small ingots of Pb-Sn, giving populations of channel segregates representative of industrial defects. Concerning the mechanisms of formation, channel segregates originate from convective solute transport caused by density differences.[2,6,8,11] More precisely, their discrete space localization should be related to time-space modifications in the convective solute transport. Section V presents a theoretical analysis of this aspect based on Bernoulli integration along a flow loop. We shall mainly focus on the gradient zone fringing the mushy zone, characterized by a steep gradient in solid fraction and associated with a modification in the density gradient. It has been suggested[7,12] that this zone should be active in the toroidal “plume” vortex operating at the volcano mouth of the channel, precisely where the real M. MEDINA, Professor, is with the Consejo de I
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