Grain Selection in Spiral Selectors During Investment Casting of Single-Crystal Turbine Blades: Part I. Experimental Inv
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o achieve increased efficiency and performance in aerospace propulsion and power generating gas turbines, while reducing cost and emissions, requires materials with high-temperature capability. In a gas turbine, turbine blades experience the most severe environment, since they are located behind the combustor and rotate at high speeds. Therefore, alloys possessing high strength at elevated temperature, such as Ni-base superalloys, are used to manufacture turbine blades. Due to the complex blade geometries and intricate cooling channels, turbine blades are produced via investment casting. Significant improvements in casting techniques have evolved over the last 30 years, the most significant achievement being the introduction of the directional solidification (DS) casting process, which has culminated in single-crystal (SX) technology for making turbine blades.[1] To obtain columnar dendrites in DS conditions, modified versions of the Bridgman furnaces were used. A typical modified Bridgman furnace for investment casting of the SX turbine blade is schematically shown in Figure 1.[2] It comprises three zones: (1) a vacuum induction-melting unit, (2) a resistance-heated furnace chamber, and (3) a withdrawal chamber. The preheated ceramic mold is open at the bottom and rests on a waterH.J. DAI, former Postdoctoral Student, is now a Visiting Researcher, with the Department of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom. N. D’SOUZA, Casting Specialist, is with Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, United Kingdom. H.B. DONG, Reader, is with the Department of Engineering, University of Leicester. Contact e-mail: [email protected] *This article is partially based on a presentation given in Superalloys 2008, Champion, PA. Manuscript submitted October 18, 2010. Article published online June 28, 2011 3430—VOLUME 42A, NOVEMBER 2011
cooled copper chill plate. Once the molten charge is poured into the ceramic mold cavity, the mold and water-cooled chill plate are lowered gradually from the furnace, thereby producing a thermal gradient, which enables the liquid metal to solidify upward from the bottom of the mold. To grow SX components, a grain selector is used to ensure that only one grain can be selected and grow into the component. The impetus for SX technology arises from the fact that the creep life of SX blades can be improved significantly due to the removal of grain boundaries that exist in a polycrystalline morphology, such as equiaxed or columnar.[3–8] Another advantage for having a SX structure is that grain boundary strengthening additions, such as B and Hf, can be removed, which raises the incipient melting temperature and improves the operating window in the solution heat treatment. The solution heat treatment is required in SX components to eliminate as-cast microsegregation as well as to obtain an optimum shape, size, and morphology of the strengthening c¢ precipitates.[9–14] Different designs of grain selectors were employed; typical examples are shown in Figure 2.[15] As indicated in this fig
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