Numerical Model Study Of Multi-Grain Motion Bahavior Inmelt Based On LBM-CA Method

The movement of large number of grains in alloy melt is a common phenomenon in the solidification process of alloy castings,which has an important influence on the distribution of microstructure,composition and service performance of heavy castings.At present,the study on the microstructure of alloy solidification is mostly limited to the growth behavior of stationary dendrites,and there are few studies involving moving dendrites.In this paper,the movement behavior of multi-grain in melt is studied,which is a key basic problem for further understanding the formation mechanism of macro-segregation in industrial ingots.In this paper,a new model for solving the solute field of moving grains is proposed.The CA-LBM-Ladd coupling model which can reasonably simulate the motion behavior of multiple dendrites is established.First of all,the reliability of our model was verified.Then the model is applied to simulate the growth and movement of single and multiple dendrites under gravity.Finally,the effects of preferential growth direction,solid phase density and buoyancy terms on grain motion are analyzed.The conclusions are as follows:(1)The growth of single dendrite under natural convection is calculated and the simulation results are compared with the analytical solutions,which verifies the accuracy of CA-LBM model for calculating the microstructure.The simulation of the drop of the circular particles in the infinitely long pipe and the translation and rotation of the dendrites proves the accuracy of the Ladd method for the boundary treatment;The motion of single dendrites under the gravitational field is calculated,which verifies the reliability of the model in calculating the solute field with moving dendrites.(2)The growth and motion of single dendrites under gravity were simulated.The results show that when the preferred crystal growth angle of the dendrites is not 0,the dendrites will rotate during the falling process due to the asymmetry of growth.The rotation will cause the dendrites to reach a stable growth direction.The rotation speed of dendrite increases with the initial orientation,and the falling velocity decreases as the initial orientation increases.The influence of solid phase density on the movement of dendrites is greater.At the same solidification time,the greater the density of dendritic solid phase the faster the dendritic falling speed.(3)The growth and movement of multiple dendrites under gravity are simulated.The simulation results show that the growth and movement patterns of multi-grain are quite different from those of single dendrites,which are laid in two aspects: Firstly,the flow field merges togather changing the original motion state of dendrites,which slows down the falling speed of dendrites to a certain extent.Secondly,themerging of solute field between dendrites changes the original growth mode of boundary dendrites and increases their rotational speed.(4)The effects of thermal buoyancy and solute buoyancy on the multiple dendritics motion of Al-4.7%Cu single-phase solid solution alloy were simulated.The study found that the downward convection caused by solute discharge will increase the falling speed of dendrites.The resulting upward flow field caused by release latent heat reduces the rate of falling of the dendrites.