Numerical simulation of dendrite white spot formation during vacuum arc remelting of INCONEL718
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I. INTRODUCTION
VACUUM arc remelting (VAR) is one of the principal secondary remelting processes used to produce aerospace quality ingots of nickel-based superalloys from vacuum induction melted (VIM) cast electrodes. The performance requirements for turbine-disc alloys for aeroengines, such as INCONEL* 718, have become progressively more *INCONEL is a trademark of Special Metals Corporation, Huntington, WV.
demanding, resulting in tight specifications for alloy cleanliness and homogeneity. Improved control of the melting process has reduced the incidence of many melt-related defects, such as freckles,[1,2] however, the occurrence of defects, termed white spot, remains a concern to VAR processing. Generally, white spots appear as discrete lightly-etched areas, which contrast with the surrounding matrix. Typically, regions of white spot are lean in certain solute elements, such as niobium and titanium, and may have a coarser grain structure than the surrounding matrix. Considerable attention has focused on the classification of different types of white spot, identification of their origins, and characterization of their effects on mechanical properties. In April 1991, the White Spot Committee[3] grouped white spot in terms of its location in ingot, composition, solidification structure, and “cleanliness.” Using these groupings, they identified three distinct types of white spot and then termed them by the hypothesized mechanism by
W. ZHANG, Development Engineer, is with CHAM Ltd., London, SW19 5AU, United Kingdom. P.D. LEE, Senior Lecturer, and M. McLEAN, Professor, are with the Department of Materials, Imperial College, London, SW7 2BP, United Kingdom. Manuscript submitted March 8, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
which they form: discrete, dendritic, and solidification white spot.[3] Discrete and dendritic white spots are caused by solid debris, of composition different from the alloy, that falls into the solidifying VAR ingot and is trapped in the semisolid region. Figure 1 schematically illustrates the most common sources of intrinsic and extrinsic inclusions including the various types of white spot, splash, and the extrinsic defects, such as tungsten carbide and steel shot. If the solid debris is melted or dissolved in the VAR molten pool, then, it is likely to be innocuous. If it falls to the bottom of the pool and is incorporated in the semisolid region, it can either survive as a foreign inclusion or as a white-spot compositional heterogeneity. Solidification white spot, on the other hand, results from thermal perturbations occurring at the solidification front. Discrete white spot appears as bright-contrast grains that have distinct boundaries with the alloy. They are usually located between the mid-radius and the center of the billet cross section.[3] The grain size of these white spot particles is greater than, or at least as large as, that of the matrix. The white spot particles are strongly depleted of niobium and slightly depleted of titanium, aluminum, and molybdenum; they are enr
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