Suppression of Cusp magnetic field on formation and progression of stray grain in gradual change section during single-crystal-superalloy growth

Stray grains (SG) are the prevalent defects in nickel base single-crystal (SC) superalloy blades, which introduce new grain boundaries and significantly degrade the service performance. In this study, it is found that Cusp magnetic field (CMF) has an inhibiting effect on the SG formation and propagation in gradient cross-sections during directional solidification (DS) of SC superalloy. Moreover, the impact of withdrawal rate (V_w) and temperature gradient (G) on the suppression of SG by CMF is explored. The results suggest that the CMF inhibits the formation and propagation of SG in gradient cross-sections under all circumstances. Under the same G, an increase in the V_w strengthens the CMF effect in reducing SG formation. At the same V_w, an increase in the G diminishes the CMF suppressing effect. The mechanism of CMF inhibiting SG formation was investigated by numerical simulation, and the accuracy of simulation parameters was verified by experiments. This is ascribed to the decrease of radial temperature disparities at the liquid-solid (L-S) interface resulting from the flow-structure alterations and the fluid-velocity intensification near the L-S interface. This work provides the theoretical guidance for using CMF to suppress the SG formation in the SC blade.