Study On Behaviors Of Directional Solidification And Single Crystal Growth Of Ni-based Superalloys Based On Scale Effects Caused By Nonlocal Fluid Dynamics

Ni-based single crystal superalloys are the main material for turbine blades of aerospace engines.The directional solidification preparation process of this material is the core technology of the aviation industry.In the directional solidification process of single crystal alloys,it is a long-term research topic for researchers to control the grain structure to obtain single crystal blades with excellent performance.The numerical simulation of the directional solidification is divided into macro scale and micro scale.The macroscopic simulation is mainly based on the continuum theory to study flow,heat transfer and mass transfer in the solidification field.The microscopic simulation is mainly to study the nucleation and growth of grains by some methods,such as cellular automaton.Compared with the micro scale numerical simulation,the macro scale numerical simulation has a more mature theory,more reliable results,and more easily applied in practice.At present,the study of liquid phase flow during solidification is mainly focused on the filling process,while the study of the micro flow in the frontier part of the directional solidification field is less.The micro flow in the solidification front,especially between dendrites,is related to the heat and mass transfer of liquid alloys.However,the flow between dendrites is strongly influenced by micro/nano scale effects.The simulation method based on the classical fluid mechanics theory is not applicable.It is necessary to develop a fluid dynamics model that is considered the influence of scale effects in order to accurately simulate the part of the hydrodynamic behavior.The nonlocal continuum theory has been widely used in fields of elastic mechanics and solid mechanics in micro/nano scale,however the application in micro/nano scale fluid mechanics is still limited.Based on the nonlocal fluid dynamics theory,this paper establishes a cross-scale flow model of the solidification front.To study the influence of nonlocal hydrodynamic effects on the dendrite growth of Ni-based alloys,the main work includes the following two aspects.Firstly,based on the classical hydrodynamic model,the liquid phase flow is simulated in the two-dimensional macroscopic solidification field under different withdrawal rates by setting different sidewall boundary conditions and to study thechange of the temperature field under the action of flow.Then the migration of the solidification interface is analyzed under different withdrawal rates.Secondly,based on the nonlocal fluid dynamics theory,a cross-scale model is established to simulate the two-dimensional flow of the dendrite interface.The SIMPLE algorithm is used to solve the correction field of the microflow velocity at the solidification interface,then to predict the growth trend of dendrite.The results show that the directional solidification of single crystal alloys at macro scale is affected by liquid phase flow,heat transfer and mass transfer.The liquid phase flow can promote the transmission of temperature and solute in the solidification field,and the uneven distribution of temperature and solute can also promote the liquid phase flow.Different withdrawal rates have completely different effects on the liquid phase flow near the solidification interface,and the liquid phase flow near the solidification interface has an important influence on the microflow between dendrites.The liquid phase flow between dendrites promotes asymmetric growth of dendrites under nonlocal scale effects.Different flow directions lead to different directions of asymmetric growth of dendrites.Single dendrite will greet the macroscopic flow growth caused by the nonlocal scale effect.Considering the nonlocal scale effect,the appropriate withdrawal rate can be used to reduce the influence of the micro flow on the asymmetric growth of dendrites and to control the solidification interface.