The Effect of Processing Conditions on Heat Transfer During Directional Solidification via the Bridgman and Liquid Metal
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of solidification models to optimize casting conditions and predict the formation of solidification defects and undesirable casting conditions has improved dramatically over the past decade as the maturity of process models has improved. However, in order to accurately predict solidification conditions, appropriate boundary conditions and correct thermophysical properties of process materials and alloy are required. This research focused on a modeling analysis of the directional solidification (DS) process and the sensitivity of the various boundary conditions employed in the model. In general, the DS process uses an investment casting mold that is withdrawn from a hot zone to a cold zone within a vacuum furnace at a particular speed—the withdrawal rate. This research was initiated to improve the fundamental understanding of heat transfer during DS, particularly in new approaches, such as the liquid metal cooling (LMC) DS process. JONATHAN D. MILLER, formerly Ph.D. Candidate with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, is now Senior Materials Research Engineer with the Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson AFB, OH 45433. Contact e-mail: [email protected] TRESA M. POLLOCK, Professor, formerly with the Department of Materials Science and Engineering, University of Michigan, is now with the Materials Engineering Department, University of California Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted May 16, 2012. Article published online September 25, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
The LMC DS process has been studied intermittently over the past 2 decades.[1–20] The LMC process uses a stirred liquid-metal coolant in the cold zone of the DS furnace that enables more efficient heat transfer through conduction and convection through the coolant, rather than radiation to the furnace walls in the conventional Bridgman DS process (Figure 1).[5,6] A floating baffle comprised of ceramic beads rests atop the coolant, in this case, tin, to shield the coolant from radiation heating.[4,14] The resultant dendrite structure is refined with reduced defect occurrence as compared to the Bridgman process.[2–4,9–19] The LMC process was simulated using analytical models for simple geometries,[20] finite-element (FE) models with location-dependent boundary conditions,[11–16] and computationally intensive models that incorporate interaction of the coolant and mold materials.[1] However, prediction of the microstructure scale from thermal information and optimization of the process conditions have been limited by a lack of the fundamental understanding of the various heat-transfer processes associated with the technique. Furthermore, the relationship between deviations in process conditions and shifts in heat-transfer mechanisms is not well understood and limits broad applicability of solidification models. For single-crystal turbine airfoils, a major challenge is the determination of the optimal process cond
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