Effects of Withdrawal Velocity and Casting Geometry on Freckle Formation of Nickel-Based Single Crystal Superalloys

To further understand the mechanism of freckle formation and explore effective control methods. The effects of withdrawal velocity and casting geometry structure on freckle formation were studied by a casting with a sudden reduction in the cross section in this research. Optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the microstructural of freckles, and electron backscattered diffraction (EBSD) was used to measure the crystallographic orientation of the freckled area. Numerical simulation software ProCAST was used to analyze the morphology of the mushy zone during directional solidification. The results suggest that with the increase of the withdrawal velocity, primary dendrite arm spacing would be reduced and the permeability K of the mushy zone would decrease, which inhibits the formation of freckles. On the contrary, the sloping angle of the mushy zone would increase with the rise of the withdrawal velocity, which promotes the freckle formation. These inhibitory effect and promotion effect jointly influence the formation of freckles. The slanted convection channel result from the lateral heat loss makes it easier for freckles to occur in the higher isotherms of the mushy zone. Reduced size of the casting diminishes the volume of liquid phase in the mushy zone and reduces the tendency of freckle formation. A modified freckle formation criterion based on Rayleigh number considering the casting geometry and the morphology of the mushy zone is proposed according to this experimental and simulation results, and the original calculation methods of these parameters are given.