Freckle Formation in IN718 Under Vertical and Tilted Growth Conditions

Abstract. In industrial processes involving molten metals, freckles are a commondefect in nickel-based superalloy castings, which negatively impact mechanicalperformance. Castings with freckles are often discarded, leading to significanteconomic losses, as these defects cannot be eliminated through post-casting orthermomechanical treatments. Despite advancements in understanding themechanisms behind these defects, reliable theories and empirical guidelines toinform engineering design remain scarce. This study investigates the formation ofsegregated channels during the directional solidification (DS) of Ni-5wt%Nb alloy,focusing on the influence of growth front inclination on macro-segregation andsolute-driven convection. A two-phase solidification model, coupled with fluidflow, heat transfer, and solute transport, is employed to simulate the solidificationof Ni-5wt%Nb alloy in a 3D cylindrical geometry with a diameter of 25 mm and alength of 150 mm using the finite volume method. Two cases are considered: (i)vertical solidification (0° tilt) and (ii) inclined growth (35° tilt) to replicate theexperimental conditions reported by Auburtin et al. [Met and Mat Trans B, 2000].The results demonstrate that no segregated channels form in the verticalsolidification case, whereas segregated channels appear in the 35° tilt case,aligning with experimental observations. The analysis of segregation mapsconfirms that the inclined growth front enhances solutal convection, leading to theformation of segregated channels. A comparison with a published experimentalsetup, which closely resembles the classical Bridgman furnace used in industry,demonstrates good agreement with the simulation results.