Development of Dendritic Structure in the Liquid-Metal-Cooled, Directional-Solidification Process
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THE need for large power generation turbine components with high-temperature capabilities has provided a driving force for the use of a directional solidification process with higher cooling rates compared to the conventional Bridgman process. A higher thermal gradient during solidification reduces segregation and allows for increased temperature capability due to the more complete homogenization of high refractory content superalloys.[1,2] The liquid-metal cooling (LMC) process provides improvements in thermal gradients for casting cross sections as large as 50 mm2.[2] The benefits of the LMC process include reduced dendrite arm spacings and finer and more homogeneously distributed carbide precipitates, porosity, and c/c¢ eutectic.[1–3] In comparison to the Bridgman casting technique, enhanced heat extraction in the LMC process was shown to increase cooling rates by 1.5 to 7.5 times for large scale cross sections, applicable to industrial gas turbine blades.[3] However, the potential benefit of the LMC process for smaller cross sections, such as aircraft engine turbine blades, has not yet been evaluated, although it is anticipated that the microstructure refinement and reduced defect occurrence will provide superior mechanical behavior. The dendritic structure, generally characterized by the primary dendritic arm spacing (PDAS) and secondary
dendritic arm spacing (SDAS), influences the strength, ductility, and homogenization kinetics of the alloy. The solidification parameters that produce the finest dendritic spacings using liquid tin-assisted casting for small scales have not been fully investigated. Despite the benefit of the high thermal gradient, the enhanced heat extraction in the LMC process promotes increased curvature of the solidification front. This can cause variability in the microstructure scale within individual castings.[4] Thus, in order to better understand the evolution of dendrite morphology under the unique processing conditions of LMC, characterization of dendritic structure throughout the volume of the casting is essential. The evolution of dendritic structure as a function of processing conditions is evaluated in detail in this study. Recently, solidification modeling was used in conjunction with experimentation to optimize processing parameters and determine the critical heat-transfer steps during the conventional Bridgman single-crystal solidification process.[4,5] This article aims to further develop solidification modeling capabilities that can describe structure evolution under conditions that are unique to the LMC process. The sensitivity of microstructure to processing conditions is evaluated in detail with a solidification model, and the implications for the mechanical properties are discussed. II.
C.L. BRUNDIDGE, Graduate Student, is with the Department of Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] J.D. MILLER, Research Scientist, is with the AFRL/RXLMP, Air Force Research Laboratory, Materials and Manufacturing Directo
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