The Influence of Surface Roughness on Freckle Formation in Directionally Solidified Superalloy Samples

A series of directional solidification experiments with superalloy CMSX-4 was carried out with various surface profiles. The results obtained clearly demonstrate the influence of surface features on the occurrence of freckles. The rough surfaces exhibit less freckling tendencies than smooth surfaces. For a notch depth larger than the critical layer beneath the surface, the freckle formation can be completely suppressed. The freckle formation is not only a surface defect but can initiate the occurrence of internal equiaxed and stray grains. The results of the present study can be applied to completely prevent freckle formation and the accompanying internal grain defects by using oversize with an appropriately roughened surface.

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