Electromagnetic stirring with alternating current during electroslag remelting
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I.
INTRODUCTION
THE more
severe standards required by current applications of metallurgical products have led to the development of processes which can deliver large ingots without the common defects found in conventional casting. Among these processes, the group based on the remelting of consumable electrodes has become popular in the production of large ingots of special alloys. In particular, electroslag rernelting (ESR) ti,2,3~ and vacuum arc remelting (VAR) tnj are the most widely used consumable electrode remelting processes. Both share a melting technique based on the passage of an electric current through a consumable electrode, resulting in molten metal in the form of droplets which then solidify against a watercooled mold, building up an ingot. This particular heat extraction regime permits a controlled freezing of limited amounts of liquid at a time, as opposed to the freezing mechanism of conventional casting. Even though ingots produced by either ESR or VAR are structurally sounder than those produced by conventional techniques, freckles and other macroscopic segregations can develop when ingot size and/or melting rate are increased, depending upon the particular alloy. Additionally, the heat extraction patterns caused by the mold configuration favor a completely columnar structure, and therefore, equiaxed zones do not develop during freezing under normal operation. It is in this context that electromagnetic stirring (EMS), a technique already commonly used in processes like continuous casting, during the remelting operation can be a useful technology for modifying the structure and controlling macrosegregation. It has already been established that the liquid pool of a VAR t51 or direct current (DC) ESR [6,71 process can be stirred by using the interaction of an external field with the melting current. Indeed, this technique is widely used to homogenize alloy additions in the VAR process
ALEC MITCHELL, Professor, and BERNARDO HERNANDEZMORALES, Graduate Student, are with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC V6T IW5, Canada. Manuscript submitted August 29, 1989. METALLURGICAL TRANSACTIONS B
for titanium and zirconium alloys. However, it has not been applied to alternating current (AC) ESR systems. Mechanical forces in liquid conductors arise whenever an induced or natural current interacts with induced or natural magnetic fields.rS~ These forces act on directions that are perpendicular to both current and magnetic field as expressed by Fm= J • B
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This electromagnetic body force may either enhance or damp fluid flow caused by thermal gradients or surface forces, as well as the turbulent component of the flow. Many metallurgical processes employ strong currents which induce magnetic fields, resulting in liquid metal movement which might be deleterious. On the other hand, intentional electromagnetic stirring produced with external devices, either permanent magnets or solenoids, has been used as an aid in promoting equiaxed structures or
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