Numerical Simulation Of Nucleation And Growth Behavior In Solidification Process Of Ferritic Stainless Steel With Cellular Automaton

The solidification process of liquid metal involves complex physical change mechanisms at the macro and micro scales,therefore,studying the solidification process of metallic materials is of great significance for deep understanding the principles of structure formation and improving the process of material preparation.The solidification of liquid metal is a complex process controlled by the interplay of uneven distribution of temperature field,turbulence effect of melt flow field,the process of dendrites nucleation and growth.Different numerical models can reveal the actual reaction processes,reproduce the distribution of macroscopic physical field and the temporal evolution of the microstructure during solidification.In the present work,based on the research of vibration-excited liquid metal nucleation technology,a two-dimensional microscopic dendrite growth cellular automaton(CA)model of ferritic stainless steel Cr17 was constructed and coupled with the macroscopic temperature field to study the nucleation and growth behavior of dendrites in the microscopic region during the vibration chilling process.The main research content and results of the thesis are as follows:A coupled model to connect the explicit finite difference method(FDM)and the cellular automata model(CA-FDM)was established for the simulation of microscopic dendrite growth.The growth kinetics takes solute diffusion as the main driving force,and considers the influence of solute component undercooling,curvature undercooling,and interface energy anisotropy function on process of dendrite growth.Optimized the curvature calculation method and adopted the Zigzag capture rule,which aims to reduce the grid anisotropy introduced by CA algorithm.Coupling the calculation results of the macroscopic temperature field,the heterogeneous nucleation model,and the microscopic dendrite growth algorithm,the influence of the process parameters of the vibration-excited liquid metal nucleation technology on the evolution of solidification microstructures are investigated,the competitive dendrites growth process in the microscopic region on the surface of the crystal nucleus generator was reproduced.The influence of the different surface undercooling and vibration frequency of the crystal nucleus generator on the proportion of the internal equiaxed crystal area,the average grain size,and the length of the columnar crystal area was analyzed.It can be concluded that,as the vibration frequency of the crystal nucleus generator increases,the irregular turbulence effect in the melt gradually increases over time,which promotes the process of dendrite fragmentation and migration and increased the potential heterogeneous nucleation site,thus the process of the columnar-to-equiaxed transition(CET)was promoted.Under the vibration frequency of 1000 Hz,the undercooling on the surface of the crystal nucleus generator was maintained at 300?400 K,the temperature gradient at melt is uniform,a wider composition undercooled zone is formed at the front of columnar dendrites,which conducive to the process of the columnar-to-equiaxed transition.When the degree of undercooling continues to increase,the temperature gradient in the melt was increased,and the equiaxed dendrites nucleation area at the front of the columnar crystal was decreased,which is beneficial to the directional growth of the columnar dendrites.When the degree of undercooling is constant,increasing the vibration frequency is conducive to the expansion of the equiaxed dendrites area,and the development of the columnar dendrites area is restricted.The main reason can be summarized,the convective stirring effect in the melt was intensified while the external force of vibration increased,the effect of fluid flow could break off the primary dendrites arm and the fragments of dendrites were drawn into the melt,which increased the potential heterogeneous nucleation site,meanwhile,the thermal conductivity of the melt increased gradually,which resulted in the uniform distribution of temperature gradient,larger parts of the region entered an undercooled state.