Multi-scale Coupling Numerical Simulation Of Directional Solidification Process Of Aeronautical Nickel-Based Superalloy Blades

Precision casting directional solidification technology is the main technology of integral casting complex shape superalloy blades,which is widely used in aeronautical high temperature complex parts forming and manufacturing.At present,the experimental study of directional solidification process can not directly reflect the mechanism of process-physical field-solidification structure.To a certain extent,the numerical simulation study can obtain some rules of process influence and mechanism of structure evolution in directional solidification process,but from process to solidification structure of castings is a multi-scale coupling.The multi-scale relationship of directional solidification process is often neglected in single-scale numerical simulation studies,which may lead to inaccurate characterization of the evolution mechanism of directional solidification structure.Accordingly,in view of the actual process of directional solidification of nickel-base superalloy blades in aeronautical precision casting,the macro-scale heat transfer,micro-scale structure growth and meso-grain evolution growth behavior and mechanism of directional solidification process were systematically studied by numerical simulation and experimental methods,and the multi-physical field and multi-scale were deeply explored.A multi-scale numerical prediction and analysis system based on Huazhu CAE directional solidification process was developed.The system was applied to process optimization of directional solidification process of Aeronautical single crystal blades and scientific guidance for actual production.The main research work and achievements are as follows.Multiscale experiments of directional solidification are carried out.On the macro scale,the key position of heat transfer for directional furnace equipment and the distribution of temperature field in casting system are studied.The results show that the temperature field inside the insulating plate is relatively stable for different locations of the insulating plate,and the more unstable the temperature field is for different locations of the water-cooled copper ring,the farther from the heating zone and the insulating zone downwards.The evolution of columnar crystal structure in directionally solidified nickel-base superalloy was studied on the micro-scale.The results show that the maximum dendrite size of local cross-section of columnar crystal in directionally solidified nickel-base superalloy exceeds 500 microns,and the primary dendrite spacing is 0.189 mm.On the mesoscopic scale,the directional temperature field of mesoscopic grain is analyzed.The results show that with the solidification growth along the Z-axis,the number of grains in each section decreases gradually,but the grain size increases gradually.A multi-scale coupled mathematical model for directional solidification process was constructed.On the macro scale,the macro scale heat transfer numerical model of directional solidification process is modified;on the micro scale,the numerical model of micro-structure growth is improved;on the meso scale,the numerical model of multi-grain meso-and micro-structure evolution is established,which is suitable for the limited area;finally,the macro heat transfer model and meso-and micro-grain evolution are synthesized.Considering the coupling growth of primary dendrite and secondary eutectic,a multi-scale coupling model for direct solidification structure evolution of full-scale castings was constructed.In the study of macro-scale heat transfer in directional solidification process,a time-varying step characteristic ray dynamic tracing algorithm for complex castings in directional solidification furnace was proposed,and the accuracy of numerical calculation of radiant heat on the outer surface of differential grids was improved.The numerical simulation of macro-temperature field of several blades in directional solidification and drawing stage was carried out.The simulation results show that the final temperature field of the die shell presents a symmetrical central radial distribution.The temperature field of the outer surface of the die shell radially outwards is larger than that of the inner surface of the die shell radially.In the research of directional solidification micro-scale structure,the CA numerical simulation method was improved.The 27-point solute field discrete scheme was used to solve the process of micro-dendrite growth.The numerical simulation analysis of micro-dendrite growth was carried out.The results show that when the inlet velocity is 5.0×10-4 m·s-1,the undercooling increases.The effect of asymmetric growth of dendrite morphology decreases gradually.With the increase of inlet velocity,the effect of asymmetric growth of dendrite morphology increases with the increase of supercooling degree at 4 K.The dendrite arm morphology of dendrite with larger orientation is more luxuriant,which is more likely to hinder the growth of other dendrites nearby.In the study of directionally solidified mesoscale structure,a grain boundary tracing simulation algorithm for calculating solid fraction quantitatively at the same mesoscopic grid scale is proposed,and the influence of nucleation parameters on equiaxed to columnar crystals of directionally solidified structure is studied.The simulation results show that when the ratio of average undercooling to undercooling variance is calculated.The nucleation parameters in the melt will gradually have little effect on the structure of the castings.The effects of cooling rate and temperature gradient on solidified grain size and structure(ECT)distribution were studied.The simulation results show that with the increase of cooling rate,the equiaxed grain size changes from coarse to fine,and the number of grains increases significantly.At the same time,the increase of temperature gradient can promote the rapid growth of columnar crystals,thus inhibiting the growth of columnar crystals.Number of grain nucleation.In the macro-meso/micro multi-scale coupling study of directional solidification,the evolution of solidification structure in the induction and selection stages of single crystal blades was numerically simulated and analyzed.The simulation results show that at low pulling speed,the number of eliminated grains can be increased,but the selection distance will be extended.The numerical simulation of directional solidification structure of full-size single crystal blades shows that the formation of heterocrystals can be effectively inhibited when the blade cross-section size is reduced.Finally,the numerical calculation module of directional solidification multi-scale simulation software based on Huazhu CAE is designed and developed.The numerical simulation analysis of directional solidification shrinkage holes shows that the shrinkage holes distribute at the top edge and the crown edge of the induction section,and the shrinkage holes only distribute at the crown.The process of directional solidification structure evolution of single crystal blade was optimized by multi-scale simulation.The simulation results directly showed that the concave shape of macro-isotherm was easy to form large undercooling on the platform of single crystal castings with complex shape,resulting in heterocrystal defects,while the horizontal or convex shape did not lead to heterocrystals.