A Method of Stray Grain Suppression for Single-Crystal Superalloy During Seed Melt-Back
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s a key component for advanced aero-engines and industrial gas turbines, Ni-based superalloy single-crystal turbine blades have been widely applied because of their excellent high-temperature strength and creep strength. In the industrial production, these single-crystal turbine blades are usually produced through a grain selector method.[1,2] However, an off-axial orientation dendrite is easily formed during directional solidification due to the random nature of the grain selector, which usually deviates from the orientation of [001] up to 12 deg to 15 deg.[3] Many investigations show that the off-axial orientation dendrite in single-crystal blades is seriously deleterious to their mechanical performance.[4,5] In the recent decades, the seeding technology has been applied to control the crystal orientation because the seed is a single crystal with desired
WEIDONG XUAN, Materials Research Engineer, JIAN LAN, Master, HUAN LIU, Ph.D. Student, CHUANJUN LI, Associate Professor, XINGFU REN, Senior Engineer, and YUNBO ZHONG, XI LI, GUANGHUI CAO, and ZHONGMING REN, Professors, are with the State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 30, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
orientation.[6–8] However, from the melt-back period to the initial withdrawal stage, a concave isotherm is plotted as the radial heat loss from the mold wall is greater than that in the center at a high withdrawal velocity, which results in the undesirable grain structure formation on the edge of sample, such as stray grains.[9–12] The stray grain as a major grain defect significantly affects the mechanical properties of single-crystal blades because of the high-angle boundaries (HABs) of primary grain resulted from the random orientations of stray grains. The stray grain formation during directional solidification has been investigated by many researchers,[9–20] and several methods are proposed to eliminate the formation of stray grains, such as using lower withdrawal velocity or improved thermal gradient,[13,21,22] and the application of spiral grain selector between the blade and the seed.[9–11] Unfortunately, up to now, the formation of stray grains during seed melt-back has not been solved completely. Recently, some studies indicate that the stray grain formation is suppressed through modifying the local heat transfer of sample to change the temperature condition and the solidification sequence during directional solidification.[13,23] Ma and Bu¨hrig-Polaczck[23] investigated the formation and growth of stray grains in the platform region and proposed applying heat conductor (HC) technology to improve the thermal condition in the shroud region of turbine blades. Experimental results indicate that the heat conductor technology entirely suppresses the formation of stray grains in the platform region because the maximum undercooling in t
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