Freckle Formation Affected by Geometry Features of Single Crystal Superalloy Castings

In comparison to our conventional knowledge, some new features of freckle appearance have been observed, indicating new aspects of freckle formation in the Single Crystal (SC) turbine blades made of superalloys. The casting shape has more significant influence on the freckle formation than the local thermal condition. On the transverse sections of the components with curved contour, freckles were exclusively found on the outward curving surfaces having positive curvature, because the surface effect zones of the neighboring sides are overlapped, providing more favorable convection condition. In comparison, the surfaces with negative curvature remained freckle free, because the surface effect of the neighboring sides is divergent from each other. In the longitudinal direction, the freckle formation can be promoted by contracting contour and suppressed by expanding one, respectively.