Phase-field Method And Experimental Study On Preferred Growth Orientation Of Dendrites In Directional Solidification Of Al-Zn Allo

Numerical simulation of microstructure is of great significance to the development and application of metal materials.It is the main development direction of computer application in the field of material science.With the rapid development of computer technology,higher and higher requirements are put forward for microstructure simulation technology.At present,scholars at home and abroad have done a lot of research work on different materials.Among them,metal solidification process is a typical problem.For the numerical simulation of microstructure,the deterministic method,stochastic method and phase field method are usually used.As a powerful tool to describe the evolution of complex phase interface in nonequilibrium state,the phase field method can simulate the complex morphology of dendrite growth during metal solidification without tracking the complex solid-liquid interface.It is the most advanced research direction in the world of solidification structure simulation.In this paper,through phase field simulation and experimental verification,the dendrite growth during directional solidification and the morphological diversity of dendrite microstructure were studied,which laid a good foundation for the prediction of mechanical properties of castings.Based on the comprehensive analysis of the existing binary alloy phase field models and the Ginzberg Landau phase transition theory,the improved binary alloy phase field model is derived,and the relationship between the phase field parameters and the thermophysical parameters of the material under the constraint of thin interface is established.At the same time,the effects of tensile speed,noise,Zn concentration and anisotropic strength on the simulation results are studied;Taking Al-Zn alloy as the research object,the experimental verification research was carried out by directional solidification at different tensile speed and different Zn concentration.At the same time,the solidified samples of the alloy were analyzed and studied.The experimental results show that the dendrite morphology,the preferred growth orientation of dendrites and their variation with parameters are basically consistent with the simulation results of phase field method.(1)The growth of columnar dendrites during directional solidification of Al-Zn binary alloy under different tensile speed and noise was simulated,and the phenomena of lateral branching competition growth and solute segregation in the growth process of columnar dendrites were reproduced.The effect of tensile speed on dendrite growth orientation was experimentally confirmed.It is found that the primary dendrite arm spacing decreases with the increase of loading rate at the same tensile strain;When higher stress is applied,the arm spacing of secondary dendrites becomes larger.The tensile speed causes solute enrichment at the secondary dendrite gap,thus causing dendrite competitive growth and fusion.The noise affects the symmetrical growth of secondary dendrites on both sides of the main dendrite,making the morphology difference of both sides of dendrites greater,and has a subtle effect on the morphology of dendrite tip.The experimental results are in good agreement with the simulation results.(2)The effects of different Zn concentrations on the growth morphology of columnar dendrites during directional solidification of Al-Zn binary alloy were simulated and verified by experiments.During directional solidification,the solid-liquid interface energy of Al-Zn changes continuously with the increase of Zn content.With the increase of Zn concentration,the growth direction of dendrite on {100} plane will change continuously from ?100? to?110?.When the concentration of Zn is between 25% and 55%,there are dendrites and algal crystals with dendrite growth orientation of ?320?.The reason for this dendrite orientation transformation is that the Al-Zn alloy system has a close row of hexagonal metal Zn phase in the primary body centered cubic al phase,which enhances the anisotropy of solidliquid interface energy.(3)The effects of different anisotropic parameters on the growth morphology of columnar crystals during directional solidification of Al-Zn binary alloy were simulated.With the change of anisotropy parameters,the transformation of columnar crystal growth direction from ?100?dendrite to seaweed crystal,and then to ?110? preferred growth direction is similar to that observed in the experiment.