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Frontiers of Mechanical Engineering

Front. Mech. Eng.    2018, Vol. 13 Issue (1) : 3-16
Novel casting processes for single-crystal turbine blades of superalloys
Dexin MA()
Wedge Central South Research Institute, Shenzhen 518045, China
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This paper presents a brief review of the current casting techniques for single-crystal (SC) blades, as well as an analysis of the solidification process in complex turbine blades. A series of novel casting methods based on the Bridgman process were presented to illustrate the development in the production of SC blades from superalloys. The grain continuator and the heat conductor techniques were developed to remove geometry-related grain defects. In these techniques, the heat barrier that hinders lateral SC growth from the blade airfoil into the extremities of the platform is minimized. The parallel heating and cooling system was developed to achieve symmetric thermal conditions for SC solidification in blade clusters, thus considerably decreasing the negative shadow effect and its related defects in the current Bridgman process. The dipping and heaving technique, in which thin-shell molds are utilized, was developed to enable the establishment of a high temperature gradient for SC growth and the freckle-free solidification of superalloy castings. Moreover, by applying the targeted cooling and heating technique, a novel concept for the three-dimensional and precise control of SC growth, a proper thermal arrangement may be dynamically established for the microscopic control of SC growth in the critical areas of large industrial gas turbine blades.

Keywords superalloy      investment casting      Bridgman process      directional solidification      single crystal      turbine blade     
Corresponding Authors: Dexin MA   
Just Accepted Date: 30 October 2017   Online First Date: 13 December 2017    Issue Date: 23 January 2018
 Cite this article:   
Dexin MA. Novel casting processes for single-crystal turbine blades of superalloys[J]. Front. Mech. Eng., 2018, 13(1): 3-16.
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Dexin MA
Fig.1  (a) High-pressure turbine rotor assembled with (b) SC blades
Fig.2  (a) Circular-clustered wax assembly; (b) the corresponding shell mold manufactured for casting SC blades
Fig.3  (a) Schematic of a Bridgman furnace; (b) SC solidification with a grain selector
Fig.4  Schematic of the LMC process [11]
Fig.5  Schematic of the GCC process [18]
Fig.6  (a) Schematic of the SG formation on the blade platform; (b) the corresponding heat barrier and melt undercooling region
Fig.7  (a) Turbine blade of a superalloy with low undercoolability, exhibiting SG formation on the platform; (b) SG growth into the blade root
Fig.8  (a) SC blade fabricated from alloy CMSX-6; (b) the transverse section of the platform showing the three-dimensional growth of SC dendrites; (c) fragmented grain defect in the deeply undercooled edge A
Fig.9  (a) GC technique employed to avoid SG formation; (b) the subgrain boundaries between the bypassed grain and the primary grain from the airfoil
Fig.10  Procedure of shell mold manufacture for HC insertion. (a) Wax model; (b) attachment of HC after the first layer; (c) finished shell mold for dewaxing [29]
Fig.11  Comparison of simulated temperature development (top), typical surface structures (middle) and microstructure in the blade platforms (bottom) between the castings without HC (left: (a1), (a2), (a3)) and those with HC (right: (b1), (b2), (b3)). (a) Without HC; (b) with HC
Fig.12  Sketch of the cylindrical Bridgman furnace currently used for manufacturing SC blade clusters. (a) Transverse section; (b) longitudinal section
Fig.13  Sketch of the PHC system. (a) Transverse section; (b) longitudinal section
Fig.14  Illustration of the D&H process. (a) Dipping the shell mold into the melt bath; (b) mold filling; (c) pulling up the mold to initiate downward solidification
Fig.15  (a) Wall thickness of the shell molds used for the production of turbine blades through the conventional Bridgman process; (b) through the D&H process
Fig.16  (a) D&H experiment with a thin-shell mold to manufacture SC blade of superalloy CMSX-4; (b) as-cast blade; (c) transverse section
Fig.17  Microstructure of CMSX-4 blades produced through the (a) Bridgman and (b) D&H processes
Process G/(K?mm?1) λ1/μm γ/γ′/μm2 γ′/μm Porosity/vol. %
Bridgman 2.2 445.6 1544.2 0.65 0.13
D&H 14.2 299.3 346.9 0.30 0.02
Tab.1  Comparison of the process and structural parameters of the Bridgman and D&H processes
Fig.18  Schematic of TCH in the longitudinal (a) and cross section (b) of a large turbine blade, to precisely control the local SC solidification in the platform and airfoil area respectively.
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