Tree-ring formation during vacuum arc remelting of INCONEL 718: Part II. Mathematical modeling
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I. INTRODUCTION
VACUUM arc remelting (VAR) is a secondary remelting process for producing homogeneous ingots of reactive and segregation sensitive alloys. It is widely used for the production of aerospace disc alloys, such as INCONEL* 718. The *INCONEL is a trademark of Special Metals Corporation, Huntington, W.V.
VAR process was described in detail in Part I. The key features are as follows. (1) A large electric current is passed through a consumable electrode to generate a metal vapor arc; this arc produces sufficient thermal energy to melt the tip of the electrode. (2) Drops of molten metal fall from the consumable electrode into a molten pool in the ingot, which forms in a water-cooled copper crucible. (3) The molten metal solidifies inwards from the mold wall, forming a solid ingot with the melt pool located directly under the electrode. (4) The ingot microstructure forms in the mushy zone between the melt pool and the fully solidified ingot. The pool dynamics and ingot cooling directly affect changes in the thermal (or solutal) field in and near the mushy zone. The pool dynamics are controlled by the process X. XU, Research Associate, is with IMMPETUS, The University of Sheffield, Sheffield S1 3JD, United Kingdom. W. ZHANG, Development Engineer, is with CHAM Ltd., Wimbledon, London SW19 5AU. P.D. LEE, Senior Lecturer, is with the Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted April 4, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
parameters, including the arc behavior. It is believed that a homogeneous, defect-free microstructure will give the best properties for subsequent forging and engineering performance. Numerical simulations of the VAR process have been developed over the last 20 years to provide progressively more insight into the relationship between the process variables and the heat transfer and fluid flow occurring during VAR. Early VAR process models considered only thermal transport,[1] evolving to incorporate fluid flow and electromagnetic effects[2,3] and later turbulence.[4,5] More recent transient models tracked the evolution of the molten pool along the length of the ingot.[6] These macroscopic models are useful for inferring the probable effects of altering the process parameters upon the solidification microstructure. Direct simulation of microstructure by coupling these macroscale process models to a mesoscale or microscale models has helped elucidate the key solidification mechanisms in other processes (e.g., References 7 and 8). Recently, several authors have predicted the grain structure in castings using the cellular automaton (CA) method.[9,10] The current authors have applied a discrete CA model to predict the formation-grain structure in a VAR ingot using the nominal production parameters.[11,12] In addition to predicting the average structure, there is a particular interest in understanding how defects form and, hence, eliminating them or reducing their numb
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