Experimental Research And Numerical Simulation On Primary Recrystallization Of Nickel-based Single Crystal Superalloy

Single crystal superalloy turbine blade castings are key hot parts for modern advanced aero-turbines.Recrystallization(RX)defect could degrade the mechanical properties significantly and should be avoided during directional solidification and subsequent heat treatment.It is of great importance to reduce the RX defect and increase the yield rate of single crystal blades castings by conducting both experimental study and numerical simulation on RX mechanism of single crystal superalloy and RX evolution process.A thermo-elastic-plastic model based on macroscopic scale for engineering application was developed by considering the anisotropic mechanical properties of Ni-based single crystal superalloys.Key mechanical parameters of DD6 were obtained using the regression method.The simulation results based on the developed model showed that the geometric features of castings are more significant than the crystal orientation.Semi-quantitative prediction of the vulnerable sites of RX was made in cored rod sample castings,and the prediction results agreed well with the experimental results.Recrystallization mechanism(nucleation and grain growth)was systematically investigated through hot compression and indentation experimentation.It was found that deformation temperature plays a great role in RX sensitivity under low plastic strains.Meanwhile,the as-cast inhomogeneity significantly influenced the RX microstructural evolution.The higher the deformation amount,the faster the grain boundary moved and the higher the RX nucleation rate.The recovery treatment could hardly prevent RX of deformed samples.The stacking faults in the deformed microstructure could facilitate RX nucleation,and coherent γ′particle would greatly retard the motion of RX grain boundaries.In addition,the anisotropic mechanical properties in single crystal materials could affect the distribution of RX regions.A modified Cellular Automaton(CA)method was developed for simulating RX microstructures of single crystal superalloys.The model considered the influence of as-cast dendritic microstructures and deformation temperatures.The elastic-plastic model was used to achieve the driving force of RX,and some key parameters(eg.activation energy for grain boundary motion)were obtained.The RX microstructures under isothermal and standard solution treatment were simulated,and the results agreed well with the experimentation.RX kinetics in as-cast single crystal superalloys was discussed,and the cause for irregular RX grain boundaries was also analyzed.Two simplified geometries were designed to represent an analogue of real complicated turbine blades.Directional solidification was conducted,and standard heat treatment was carried out to induce RX in castings.Simulation was performed to predict the plastic distribution and the sites prone to RX,and the prediction agreed well with the experimental results.It was found that tiny structures from foundary procedures(eg.gibbosity by core supports)could induce RX.The features of stress concentration,such as holes,grooves and thin walls,would not necessarily lead to RX.However,sites where two or more of above features appear could result in RX.In addition,temperature and plastic strains were predicted during directional solidification of a real hollow single crystal turbine blades.The sites prone to RX were analyzed and compared with the real production.Finally,based on the above analysis,some suggestions were proposed to reduce RX in single crystal blades.