Control and Optimization of Stray Grain Defect during LMC Directional Solidification Process of Single-Crystal Blade

Single-crystal blades produced through the liquid metal cooling (LMC) process exhibit excellent high-temperature mechanical properties due to the elimination of grain boundaries. However, the formation of stray grain reintroduces grain boundaries, compromising the integrity of the single-crystal structure and the service performance of the blades. To address this, a predictive model for the temperature distribution and solidification structure during the LMC directional solidification process of single-crystal blades was developed. The solid/liquid interface stability, dendrite growth behavior and stray grain formation tendency in DD26 alloy single-crystal castings were investigated, and the formation mechanism of stray grain defect was systematically analyzed. The results show that as the withdrawal rate v increases, the mushy zone thickness decreases from 5.0 mm (v₁) to 3.4 mm (v₉), the solid/liquid interface transitions from a convex shape, which promotes the divergent growth of dendrites, to a flat interface, and further develops into a concave shape that promotes the convergent growth of dendrites. A large number of stray grains, misoriented with the preferred crystal orientation (14.17°), appear at the end of the platform, and the dendrite spacing within the stray grain regions significantly decreases. The critical withdrawal rate v_cr for stray grain formation is determined to be 4.8 mm·min^-1, corresponding to a critical nucleation undercooling $\Delta T_{cr}^{⁎}$$\Delta T_{cr}^{⁎}$ of 9.0 °C. The optimized variable withdrawal rate process effectively controls the dendrite tip undercooling below the threshold $\Delta T_{cr}^{⁎}$$\Delta T_{cr}^{⁎}$, resulting in blades with an intact single-crystal structure.