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Materials 2016, 9(11), 929; doi:10.3390/ma9110929

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys

1
Shanghai Key Lab of Advanced High-Temperature Materials and Precision Forming, Shanghai Jiao Tong University, Shanghai 200240, China
2
Foundry Institute, RWTH Aachen University, Aachen 52056, Germany
3
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
*
Authors to whom correspondence should be addressed.
Academic Editor: Mark T. Whittaker
Received: 31 August 2016 / Revised: 6 November 2016 / Accepted: 9 November 2016 / Published: 16 November 2016
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Abstract

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades. View Full-Text
Keywords: freckles; convection resistance; interface; directional solidification freckles; convection resistance; interface; directional solidification
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Hong, J.; Ma, D.; Wang, J.; Wang, F.; Sun, B.; Dong, A.; Li, F.; Bührig-Polaczek, A. Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys. Materials 2016, 9, 929.

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