Effect of Casting Geometry on the Freckle formation During Single Crystal Solidification of Superalloys

Freckles are presently one of the main casting defects in the single crystal (SC) components of superalloys. In the present work, a series of SC components with complex geometry, including turbine blades, were directionally solidified. On the transverse sections of the components with curved contour, freckles are exclusively found on the outward curving surfaces having positive curvature (angle<180 degrees), because the surface effect zones of the neighboring sides are overlapped, providing more favorable convection condition. In comparison, the surfaces with negative curvature (angle>180 degrees) 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. It is found that the casting shape has more significant influence on the freckle formation than the local thermal condition. The results of the different generations of single crystal superalloys show that the increase of Re content can significantly increase the freckles in the casting.