Heat Transfer Model of Directional Solidification by LMC Process for Superalloy Casting Based on Finite Element Method
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SINGLE crystal (SX) superalloy blades are the key hot components of advanced gas turbines and aeronautical engines due to their high-temperature strength, oxidation resistance, and fatigue resistance. Directional solidification (DS) process is used to control the grain orientation strictly and eliminate grain boundaries, which contributes to the excellent comprehensive properties of SX blades.[1,2] As a conventional DS method, high-rate solidification[3,4] (HRS) has been widely adopted in the industry. During the HRS process, the shell is gradually drawn into the cooling zone, after preheating and pouring are complete. Heat radiates step by step, eventually forming the DS process. However, owing to the low thermal gradient on the solidification interface,[5] the use of HRS can lead to a series of problems, such as stray grains and freckling when casting large components.[6,7] One method of achieving high and consistent thermal gradients is the utilization of liquid-metal cooling[8–10] (LMC) process. During the process of LMC, the shell is gradually immersed in the liquid-metal coolant at a certain rate, losing heat by both conduction and convection. The cooling rate and YUZHANG LU, Ph.D. Student, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Liaoning, P. R. China. Contact e-mail:[email protected] LIU CAO, Ph.D. Student, DUNMING LIAO, Professor, and TAO CHEN, Postdoctoral Fellow, are with the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (HUST), Wuhan 430074, Hubei, P. R. China. Manuscript submitted January 20, 2016 Article published online June 29, 2016 4640—VOLUME 47A, SEPTEMBER 2016
thermal gradient were significantly better under the LMC process than under HRS,[11,12] giving LMC an obvious advantage in the manufacturing of large SX castings.[13–15] The optimization of LMC process is difficult and costly by experimental methods, especially for the complexly shaped gas turbine blades, owing to the complicated technological parameters of solidification process. Therefore, numerical simulation is effective to optimize the technological process and also provides theoretical guidance. Napolitano et al.[16] evaluated a set of nickel-based superalloy SX investment castings for crystal perfection, reaching a conclusion that the appearance of stray grain on the platform is closely related to the shape of liquid/solid interface, and a higher withdrawal rate causes the curvature of liquid/solid interface increase, making stray grain appear easily on the platform. Kermanpur et al.[17] simulated the thermal field and the grain structure of a cored superalloy turbine blade during LMC process by commercial software ProCAST (ESI Group, Pairs, France), which were compared with experimental observation. Elliott et al.[18] simulated the LMC temperature field in two dimensions, concluding that heat transfer between the casting and the interior mold surface is the primary resistance in the LMC process, and heat extraction is en
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