Three-dimensional Phase-field Simulation On Grains Growth In Al-Zn Alloy

Dendrite is the basic structure in solidification structure of metals,which determines the mechanical properties of castings.Therefore,it is important to study the growth process of alloy dendrites.As the application of computer numerical simulation in the research of metal solidification molding,the numerical simulation of microstructures has the advantages of high efficiency and economic.In recent years,it has become a hot topic in metal forming and has made many important progress.At present,the main methods of microstructure numerical simulation include deterministic method,stochastic method and phase field method.The phase field method has been widely applied to simulate solidification microstructure because it doesn't track solid liquid interface and is easy to be coupled with other external fields.In this paper,the solidification process of Al-Zn alloy is studied by Karma phase field model.1.By consulting relevant literature,the value of phase field parameters w₀ and?₀ was determined.The growth process of Al-Zn alloy equiaxed grains was simulated,which proved that the phase field parameters used in the simulation were accurate,and the results were in agreement with the results of Karma.2.The growth of equiaxed grains under different supersaturation and supercooling conditions is simulated.It is found that the growth velocity of dendrite tip is proportional to the degree of supercooling and supersaturation when the dendrite growth is stable,and the radius of the dendrite tip is inversely proportional to the supercooling degree.It is found that the changes in the anisotropic parameters of the interface can lead to the dendrite orientation transformation phenomenon.Through a large number of simulations,the dendrite orientation transformation of Al-Zn alloy is reproduced.According to the difference of the range of anisotropy of the interface energy,the equiaxed grains of the Al-Zn alloy can be divided into three types:<100>dendrite,<110>dendrite and mixed dendrite.By coupling thermal noise in the phase field,the effect of thermal noise on the growth of equiaxed crystal is studied.Thermal noise does not affect the steady state behavior of the dendrite tip,it can accelerate the generation of secondary dendrite.Thermal noise has no significant effect on the secondary dendrite spacing.The increase of the amplitude of the thermal noise will increase the amplitude of the secondary dendrite.3.The dendrite growth process in directional solidification was investigated,and the effects of temperature gradient,pulling speed and noise are analyzed in detail.The increase of the pulling speed and the temperature gradient can accelerate the growth of dendrites in directional solidification,and also make the secondary dendrites more developed,reduce the spacing between the secondary dendrites and the solute segregation is reduced.When the temperature gradient is large,the solid-liquid interface will move forward in a plane way.The thermal noise does not affect the steady state behavior of the directional solidification dendrite tip.It only promotes the formation and growth of secondary dendrites and forms an asymmetric dendrite.The influence of dendrite orientation on the dendrite morphology of directional solidification is studied.It is found that the length of the inclined dendrite in the direction of X axis is less than the normal dendrite.With the increase of the inclination angle,the length of the inclined dendrite decreases gradually in the direction of the X axis.On the inclined dendrite,in the direction of temperature gradient the growth velocity of the secondary dendrites is accelerated,the growth velocity of secondary dendrites on the other side is decreases.Compared with normal dendrites,tilted dendrites are at a disadvantage in competition and are usually eliminated.When the tilt angle is large,the main crystal initial may be their two or three dendrite dendrite elimination.When the tilt angle is large,the initial main crystal may be eliminated by its secondary dendrites or tertiary dendrites.