Microscale simulation of stray grain formation in investment cast turbine blades

The formation of stray grains during the casting process can severely reduce the performance of single crystal turbine blades. In this study the mechanism by which stray grains form was investigated using a combined cellular automaton-finite difference (CA-FD) model. The model was first validated by a quantitative comparison of solute profiles and dendritic tip undercooling with those obtained from prior models. It was then applied to simulate dendritic growth in the platform region of turbine blades and to study the influence of withdrawal velocity and thermal gradient on the formation of stray grains. The simulation results show that increasing either the withdrawal velocity or the inclination angle of isotherms increases the undercooling within the platform region, which in turn favors the formation and growth of stray grains. Of these two factors, the isotherm angle has a larger influence than withdrawal velocity. The predicted dendritic microstructures show good correlation with prior experimental results and it is demonstrated that the developed CA-FD model offers the capability to understand the casting process for single crystal components of nickel-based superalloys at a microstructural level.