Experimental and Numerical Investigation on the Formation Mechanism of Freckle Defects in a Novel Third-Generation Nickel-Based Single Crystal Superalloy Turbine Blade

This paper investigates the formation mechanism and key influencing factors of freckle defects that arise during the directional solidification of a novel third-generation nickel-based single crystal superalloy turbine blade. A combined experimental and multi-physics numerical simulation approach was adopted. The results reveal that freckle formation primarily results from the coupling effect of solute segregation and thermo-solutal convection during solidification, leading to dendrite fragmentation and subsequent aggregation of equiaxed grains. The resultant density inversion drives upward interdendritic flow, which plays a dual role: it promotes remelting and fragmentation of secondary dendrite arms, while simultaneously opening solute-enriched preferential flow channels that eventually develop into freckle defects. The severity of freckling is closely dependent on both the casting's position within the furnace and its local geometric characteristics. Castings located in regions with poorer heating conditions experience lower temperature gradients and slower solidification rates, significantly increasing their susceptibility to freckle formation. Similarly, on a given casting, the side subjected to less favorable heating is more prone to freckle initiation. This work provides a crucial theoretical foundation for understanding freckle formation in nickel-based single crystal superalloys and offers practical guidance for optimizing blade manufacturing processes, reducing solidification defects, and enhancing blade quality and service performance.