The Influence of Surface Roughness on Freckle Formation in Directionally Solidified Superalloy Samples
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es are the typical grain defects found in Nibased superalloy castings. In directionally solidified superalloy components, freckles appear as long chains of equiaxed grains parallel to the direction of gravity. It is now generally agreed that their formation is caused by thermosolutal convection induced by density inversion in the mushy zone.[1,2] During the vertically upward directional solidification with a positive temperature gradient, dendrites will grow upward against the heat flux. In this process, heavier elements like Re and W will segregate at the dendrite core while lighter elements like Al and Ti will be rejected into the interdendritic liquid. This liquid then becomes lighter compared to the bulk liquid above it. As a result, the system becomes unstable with a heavier liquid above a less dense liquid. Thus, convection will be promoted due to the buoyancy force, creating a channel in the mushy zone for the emanating convectional jets. The channel will finally solidify as freckle chains that consist of small equiaxed grains. The freckles in directionally solidified single-crystal components, such as turbine blades, are considered as severe defects resulting in rejections.[1–6] In the directional solidification of superalloys using the Bridgman process, nearly all freckle chains were found to occur, with only few exceptions,[7] at the DEXIN MA, Senior Scientist, and A. BU¨HRIG-POLACZEK, Professor, are with the Foundry Institute, RWTH Aachen University, 52072 Aachen, Germany. Contact e-mail: [email protected] Manuscript submitted January 31, 2012. Article published online June 20, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
external surface of the castings (mold/casting interface).[1,2,8] The reason for the favorable appearance of freckles on the component surface is believed to be the significantly better flow permeability along the melt/ mold interface compared with the internal dendrite network. This favors the freckle formation on the casting surface. Therefore, the surface effect plays an important part in the thermosolutal convection and consequently in the freckle formation. The recent studies pertaining to freckle formation have concentrated on the development of computational process models[9–15] and the incorporation of Rayleighbased criteria to describe the onset of thermosolutal convection and the subsequent formation of grain defects.[16–22] In these models, the susceptibility of experimental alloys to freckle formation was predicted as a function of both composition and process parameters. However, the surface effect and the geometric aspect of the castings have not been taken into account. In order to better understand the freckle formation with regard to the surface situation, a series of directional solidification experiments was carried out in the current work with various surface profiles. The superalloy used here is CMSX-4 (Cr 6.5, Co 9.0, Mo 0.6, W 6.0, Al 5.6, Ta 6.5, Ti 1.0, Re 3.0, and Hf 0.1 in wt pct). The basic casting geometry is a 20 mm diameter cylindrical bar. Except for some ref
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