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The freckle formation and distribution in large-size directional solidified nickel-based superalloy blades at different withdrawal rates were investigated. The microstructure of freckles was analyzed by optical microscope (OM), and scanning electron microscope (SEM). The crystallographic orientation in the freckle regions was characterized by electron backscattered diffraction (EBSD). The solute segregation patterns on the surface of freckle regions were obtained by X-ray fluorescence (XRF). Numerical simulations for the temperature field were performed to analyze the mushy zone evolution during directional solidification. The experimental results indicated that the root region is prone to form freckles. The freckle chains were only distributed on the outward surface of the root region at low and middle withdrawal rates, and the inward side was free of freckles. When a high withdrawal rate was applied, a few freckles occurred on the inward side of the root region. The number and length of freckle chains increased as the withdrawal rate increased. The XRF results indicated that channel segregation formed on the surface of the root region. The segregation channels predicted by the macrosegregation simulation agreed well with the experimental observation. The concave or inclined solidification interface increased the propensity for freckle formation.

The solutal convection is able to affect the dendrite growth and solute distribution during directional solidification. The solutal convection is raised by the solute difference-caused buoyancy. When solutal convection becomes intensive, solute plumes will occur, which may cause freckle defects in directional solidification of superalloys. The onset of solutal plumes depends on the solidification condition. In this study, the dendrite growth with solutal convection during directional solidification of nickel-based superalloy was investigated by two-dimensional phase-field simulations. The lattice Boltzmann method was used to solve the flow field driven by solute difference caused-buoyancy force. The pseudo-binary alloy approximation was adopted for simplification of the multicomponent alloy. The effects of the temperature gradient, pulling velocity, and isotherm inclination angle on dendrite growth and solutal convection were investigated. The dendrite growth dynamics and solutal convection pattern under different solidification conditions were analyzed. Dendrite growth velocity oscillation was observed. The effect of pulling velocity on dendrite growth velocity oscillation was analyzed.

A parallel three-dimensional (3D) cellular automaton (CA) model is developed for dendritic growth simulation during solidification of binary alloy. The CA model is implemented using the graphic processing unit (GPU). Compute Unified Device Architecture (CUDA) is combined with Message Passing Interface (MPI) to perform large-scale simulation on the multi-GPU architecture. A CUDA-Aware OpenMPI is used to reduce the communication cost between different GPU cards. The accuracy and the speedup ratio of the CA model are investigated. First, the simulated steady-state tip velocity under different undercooling is compared with the LGK analytical solution. Then, dendritic growth during directional solidification and equiaxed multi-dendrite growth are simulated. Dendritic growth simulation with a domain size up to 453 million grids is performed. Finally, the speedup ratio of the CA model using four GPUs is compared with that using four CPU processes via MPI, which leads to a maximal speedup ratio of 152.19. The developed CA model achieves high parallel performance compared with the MPI-based parallel model. The developed CA model is capable of predicting the columnar to equiaxed transition (CET) event during directional solidification. The results indicate that increasing cooling rate promotes the occurrence of the equiaxed dendrites in the melt ahead of the solidification front, and equiaxed dendrites are prone to nucleate in the columnar dendrite groove zone when a middle cooling rate is applied.