Entwicklung des Auftauch-Verfahrens zur Herstellung einkristalliner Turbinenschaufeln
Final Report Abstract
To overcome the shortcomings of the conventional and other modified Bridgman processes and to improve the quality of the SC castings of superalloys, in the present project a novel Downward Directional Solidification Process (DWDS) was developed. In the initial stage of the project, the equipment for the DWDS process was designed and assembled, which has the ability to produce SC castings of different alloys. After setup, a series of experiments were conducted to investigate the process parameters and their effects on the microstructure of the novel process for producing SC components. During the experiments, different dynamic baffle materials were examined to determine their applicability for the process. In addition to these, ceramic shell molds having 1 mm wall-thickness which have a sufficient strength and small heat resistance were ascertained as the optimal mold for the DWDS process. Using pure Al, SC blades can be produced, which verified the potential of the novel casting process. Based on the knowledge achieved in the initial stage, cylindrical SC bars of CMSX-6 and CMSX-4 superalloys were directional solidified. The measured thermal gradients at the solidification front achieved approximately 200‒240 °C/cm which were 10‒12 times larger than those in the Bridgman process. As a result, the microstructures were greatly refined. The volume fraction and size of micropores was reduced. The alloying elements’ degrees of segregation were alleviated. It indicates that in comparison to the Bridgman process, a better quality of SC castings can be achieved by using the DWDS process. To investigate the challenges of the new DWDS process in detail, some fundamental studies were conducted in the project: Using direction solidification quenching method, the density profile in the interdendritic regions of the mushy and the solidification behavior of the DWDS solidified SC superalloys were studied. The results of the density examination suggested that the freckle defect caused by density gradients can be essentially avoided when using the DWDS process. In order to investigate the solidification conditions of the DWDS process, the effect of the withdrawal rate on the microstructure of the DWDS solidified bars was investigated. With increasing withdrawal rates, gradually refined microstructures and reduced segregation of the alloying elements were observed in the DWDS solidified samples. The evolution of the primary dendrite arm spacing as a function of the withdrawal rate can be reasonably described by the theoretical model of Bouchard and Kirkaldy, which considers a convection factor, and when the dendrite-calibrating factor was assumed to be 13.5. In the final stage of the project, using the optimized parameters of the DWDS process, SC blades of CMSX-4 superalloy were directional solidified. In comparison to the conventional Bridgman process, these blades have a better quality such as significantly refined microstructures, smaller volume fraction of micro-pores, and decreased micro-segregation of the alloying elements. In addition, after macro-etching, typical freckle defects were not found on the surfaces of the DWDS solidified blades but visible on the surfaces of the Bridgman solidified blades. It further suggests that using the DWDS process, the freckle defects can be completely eliminated.
Publications
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Innovations in casting techniques for single crystal turbine blades of superalloys, in: E. Huron, et al. (Eds.), Superalloys 2016, Warrendale, PA, 2016
D. Ma, F. Wang, Q. Wu, S. Bogner, A. Bührig-Polaczek
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Experimental trials of the Thin Shell Casting (TSC) technology for directional solidification, IOP Conf. Series: Mater. Sci. Eng. 27 (2011) 012036
D. Ma, H. Lu, A. Bührig-Polaczek
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Effect of local cooling rates on the microstructures of single crystal CMSX-6 superalloy: A comparative assessment of the Bridgman and the downward directional solidification processes, J. Alloy. Compd. 616 (2014) 102‒9
F. Wang, D. Ma, J. Zhang, L. Liu, S. Bogner, A. Bührig-Polaczek
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Effect of solidification parameters on the microstructures of superalloy CMSX-6 formed during the downward directional solidification process, J. Cryst. Growth 389 (2014) 47‒54
F. Wang, D. Ma, J. Zhang, L. Liu, J. Hong, S. Bogner, A. Bührig-Polaczek
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Microstructural evolution of aluminium-copper alloys during the downward directional solidification process, Int. J. Mater. Res. 105 (2014) 168‒176
F. Wang, D. Ma, J. Zhang, L. Liu, J. Hong, S. Bogner, A. Bührig-Polaczek
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Investigation of segregation and density profiles in the mushy zone of CMSX-4 superalloys solidified during downward and upward directional solidification processes, J. Alloy. Compd. 620 (2015) 24‒30
F. Wang, D. Ma, J. Zhang, A. Bührig-Polaczek
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Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy, Sci. Technol. Adv. Mat. 16 (2015) 025004
F. Wang, D. Ma, A. Bührig-Polaczek
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Solidification behavior of a Ni-based single crystal CMSX-4 superalloy solidified by downward directional solidification process, Mater. Charact. 101 (2015) 20‒5
F. Wang, D. Ma, J. Zhang, S. Bogner, A. Bührig-Polaczek
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Comparative investigation of the downward and upward directionally solidified single-crystal blades of superalloy CMSX-4, Metall. Mater. Trans. A 47 (2016) 2376‒86
F. Wang, D. Ma, S. Bogner, A. Bührig-Polaczek
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Influence of the Size Effect on the Microstructures of the DWDS- and Bridgman-Solidified Single-Crystal CMSX-4 Superalloy, Metall. Mater. Trans. B 47 (2016) 76‒84
F. Wang, D. Ma, Y. Mao, S. Bogner, A. Bührig-Polaczek
