Application of a Heat Conductor Technique in the Production of Single-Crystal Turbine Blades
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INTRODUCTION
SINGLE-CRYSTAL solidification of turbine blades for advanced gas turbines is a key technology for the production of reliable and high-efficiency gas turbines. The excellent properties of single crystals originate from the absence of grain boundaries, which eliminate sites liable to result in crack nucleation under thermomechanical fatigue conditions. Under creep conditions, the absence of grain boundaries not only removes the sites liable to host fracture initiation but also to remove a potential mechanism for high-temperature deformation, in particular grain boundary sliding.[1] Because of everincreasing demands on casting quality, the production of single-crystal turbine blades has been thoroughly investigated. Unfortunately, control of the industrial casting process is still not to a satisfactory level, so that a significant number of the components produced have to be rejected because of the grain defects, which form a high-angle boundary with the primary crystal.[2] Grain defects mostly occur as a consequence of the preferred solidification of geometrical features of the component. Experimental investigation has shown that the macroscopic curvature of the liquidus isotherm becomes markedly concave while traversing extreme enlargements in the cross section of the component, e.g., transition from the blade portion to the shroud portion.[3] This leads to the formation of an isolated, thermally undercooled region of melt, which may lead to heterogeneous nucleation and hence the formation of stray grains. As blade designs become more complex and the demand for larger single-crystal castings dramatically increases, the formation of grain defects caused by geometrical features becomes an increasingly serious challenge. This key problem cannot be effectively
DEXIN MA, Research Scientist, and A. BU¨HRIG-POLACZEK, Director, are with the Foundry Institute, RWTH University Aachen, 52072 Aachen, Germany. Contact e-mail: [email protected] Manuscript submitted January 20, 2009. Article published online August 14, 2009. 738—VOLUME 40B, OCTOBER 2009
overcome by conventional process modification, such as optimization of withdrawal rate or baffle design. The authors have recently developed a heat conductor (HC) technique to improve the thermal condition at the transition point and thus to suppress stray grain formation at the extremities of the platform.[4] The assumption was, as shown in Figure 1(a), that a hot spot exists at the inner corner of the cross-sectional transition (position A), resulting from the poor local cooling condition. This hot spot hinders single-crystal growth from the blade portion into the platform. On contrast, the outer corner (position B) cools more rapidly because the local shell mold is much thinner. As a result, because of the concave curvature of the liquidus isotherm, the temperature at position B falls faster below the liquidus temperature, and the dendrite tips of the primary crystal are unable to reach this isolated undercooled region, where stray grains eventually occur. In the HC t
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