Segregation Development in Multiple Melt Vacuum Arc Remelting
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remelting (VAR) is a semicontinuous process widely used to improve the cleanliness and refine the structure of ingots of specialty steels, superalloys, and Ti-based alloys.[1] The process is performed in vacuum via melting the consumable electrode and collecting the solidifying metal in a water-cooled copper crucible (Figure 1). The heat required for remelting is released by an electric arc between the consumable electrode and liquid metal. The solidifying liquid metal in the crucible is affected by electromagnetic and buoyancy forces, resulting in the formation of flow patterns, which penetrate the mushy zone and result in ingot scale macrosegregation. Macrosegregation compromises the quality of produced ingots and leads to formation of many types of defects,[1] including beta fleck in Ti-10V-2Fe-3Al.[2] Macrosegregation is the result of two physical phenomena: solute partitioning and fluid flow. Partitioning by itself results in a microscale redistribution of solute DMYTRO ZAGREBELNYY, formerly Graduate Student with the School of Materials Engineering, Purdue University, West Lafayette, IN 47907, is now Metallurgist/Process Engineer with Republic Special Metals, Canton, OH 44706. MATTHEW JOHN M. KRANE, Associate Professor, is with the School of Materials Engineering, Purdue University, West Lafayette, IN 47907. Contact e-mail: [email protected] This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online August 7, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
between solid and liquid phase at the solid-liquid interface during alloy solidification. However, it is the fluid flow that transports the interdendritic liquid out of the mushy zone and replaces it with fluid of a different composition. This solute advection is the prime cause of macrosegregation. The flow in the pool during VAR is mostly determined by buoyancy and electromagnetic forces. Density differences caused by nonuniform liquid composition and temperature give rise to the buoyancy force. The electromagnetic (Lorentz) force is a result of the strong electrical current passing through the liquid pool. At low values of arc current, the Lorentz force is weak, and the flow is dominated by buoyancy forces, but as the arc current increases, the strength of electromagnetic force increases as well, resulting in its domination of pool flow. In the case of Ti-10-2-3 alloy, the solutal buoyancy is relatively weak,[3] and, at lower powers, the thermal buoyancy forms the clockwise flow cell in Figure 2(a). Hot liquid at the top of the pool is cooled by the watercooled crucible and moves downward along the liquidus interface (seen in Figure 2 as the fraction solid = 0.01 line). At higher powers, the Lorentz force acts across the liquid pool pushing the liquid from the surface downward toward the centerline of the ingot, displacing the cold liquid from the bottom of the pool upward along the liquidus interface. Therefore, a higher curren
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