Process Simulation for the Directional Solidification of a Tri-Crystal Ring Segment via the Bridgman and Liquid-Metal-Co
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EASED temperature capability within the hot section of modern turbine engines relied on the use of single-crystal (SX) materials for blade applications. Typical SX geometries consist of a relatively small cross section and long axial dimension, as this geometry is favorable for the directional-solidification process. The Bridgman process is the conventional processing approach for directional solidification of SX castings. In order to increase turbine inlet temperatures further, SX materials were considered for other engineering components within the turbine-section possessing significantly different geometries than those typical of SX materials.[1,2] The directional solidification of these geometrical configurations was not considered previously, and it is apparent that fundamental challenges exist for the control of heat transfer and dendrite growth due to the relatively large cross section and short axial dimension. J.D. MILLER, Materials Research Engineer, formerly with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109, is now with AFRL/RXLM, Air Force Research Laboratory, Wright Patterson Air Force Base, OH 45433. Contact e-mail: [email protected] T.M. POLLOCK, Professor, formerly with the Materials Science and Engineering Department, University of Michigan, is now with the Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted September 28, 2011. Article published online March 21, 2012 2414—VOLUME 43A, JULY 2012
The liquid metal cooling (LMC) process is an emerging, high-gradient directional-solidification process (Figure 1).[3–9] The benefits of the process derive from the enhanced heat extraction of the casting by a liquidmetal coolant during solidification, which allows for SX growth in larger cross sections in which the thermal gradient would be too low for the Bridgman process.[10,11] This process is capable of providing improved mechanical performance of engineering components due to refined dendritic structure and corresponding defect scales.[7–9,12–17] Application of the process to thick sections or complex geometries is ongoing.[4–14] Simulation of the LMC process is particularly challenging due to heat transfer at the baffle-coolant-mold interface, interpenetration of the casting/mold and liquid-metal coolant domains, and the morphology of the floating baffle. Previously, the solidification and mechanical performance of bi-crystal castings were investigated transverse to the growth direction. The motivation for this work was to determine the effect of a low-angle boundary on the creep rupture strength of the material.[1,18] Alloys were designed to maintain SX rupture strength with low-angle boundaries up to 18 deg, depending on temperature and stress level.[1] The design of such alloys allowed the presence of low-angle boundaries in engineering components without substantial mechanical-property degradation.[18] Recently, a design for a large-diameter SX ring was proposed.[2] A ring whose crystallographic orient
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