Numerical Simulation Study Of Macrosegregation Of Ingots Based On Mesoscopic Scale

Macrosegregation is a common defect during the casting solidification process.The research of macrosegregation has the disadvantages of high cost,long cycle by experimental methods.In recent years,the numerical simulation of macrosegregation has become a tendency due to the rapid development of computer technology,at the same time,the coupling model which considering the dendritic nucleation and growth has been prominent in many macrosegregation models.This model is of great practical value in exploring the formation mechanism of macrosegregation and seeking the solution to reduce the defects.In this paper,a cellular automaton(CA)method for simulating the nucleation and growth and a lattice Boltzmann method(LBM)for calculating macroscopic transport processes are coupled from the mesoscopic scale.First of all,the reliability of the model was verified.Then the coupling model was applied to simulate the macrosegregation of different solidification processes such as transgranular,columnar to equiaxed transition(CET)and global equiaxed grains.Finally,the effects of pouring temperature,wall heat transfer coefficient and convection intensity on macrosegregation were analyzed.Through the above study,the following conclusions are drawn:(1)The LBM model was used to simulate the Poiseuille flow and flow past a circle cylinder,Rayleigh-Bénard natural convection and solute diffusion,the results are in good agreement with the analytical solution and experimental results,which verify the reliability of the model.The coupling model was used to simulate the single dendritic growth under the natural convection,the result agrees well with the analytical solution,which indicates that the model can be applied to simulate the dendrite growth process.(2)The solidification process of transgranular of Fe-0.34 wt.%C alloy under the natural convection was simulated,the results show that the natural convection,which is caused by thermo-solutal buoyancy,is the most important factor that cause macrosegregation.The influences of pouring temperature and wall heat transfer coefficient on macrosegregation of transgranular were analyzed.The results show that the higher pouring temperature and the larger heat transfer coefficient,the more serious of macrosegregation.(3)The CET solidification process was simulated,the top positive segregation and multiple A-segregation bands were found in the results.The effects of wall heat transfer coefficient and natural convection intensity on macrosegregation were analyzed,it is found that the increase of heat transfer coefficient is beneficial to the nucleation of equiaxed and the reduction of the top positive segregation,but the number and the degree of the Asegregation will also increase.Increasing the natural convection intensity is conducive to increasing the density of equiaxed and the deflection angle of A-segregation,but excessive flow strength will reduce the uniformity of the grain size and increase the top positive segregation.(4)The solidification process of the global equiaxed was simulated.No top positive segregation and A-segregation are found,only one positive segregation band appears at the central axis,the grain size are also more uniform and fine.