Three-dimensional Phase-field Method Research On Grain Growth Of Directional Solidification Of Binary Alloy

The theory of directional solidification occupy the frontier and core position in the field of solidification, which laid the foundation of the modern theory of solidification, and corresponding directional solidification technology has become an important direction of the subject and a very active field of technology in the field of material preparation and forming.The research of the solid-lipuid interfacial morphology under the experimental condition is hard, still facing complex and difficult problems, and numerical simulation has a unique advantage in dealing with the microstructure evolution, especially the phase-field method,don’t need to track complex interface, is a hot research on the field of the study of the solidification microstructure.Based on KKS phase-field model, the dendritic solid-liquid interface migration normal vector is employed as a independent variable to derive phase-field governing functions; the angle between the direction of dendritic growth interface migration normal and the optimal growth direction is used as a independent variable to derive anisotropic equations of interfacial free energy, and coupled with the temperature field and solute field, established a new three-dimensional phase-field model in this paper. Taking Al-Cu binary alloy for example, three-dimensional simulation of directional solidification is conducted by using the new phase field method, reproduced the process of evolution of solid-liquid interface shape that from the flat interface to cellular crystal interface during the solidification, the variation pattern of the leading edge of solid-liquid interface and the coarsening mechanism of cellular in the process of directional solidification are investigated, and the influence of undercooling degree on interface morphology and the competitive growth of grains of different orientations are analyzed. The results of investigation show that cellular crystal coarsening is result of the interaction with melting and merging during the directional solidification process. With the decrease of the undercooling degree, the solid-liquid interface morphology tends to develop into planar interface during the directional solidification process. There are two competing mechanisms during the growth of grains of different orientations: Fist, tilted dendrites in directional solidification affected by surrounding solute and temperature tend to grow in the vertical direction when the secondary dendrite is underdeveloped, Second, tilted dendrites in directional solidification stopped by the second dendrites are eliminated gradually when the secondary dendrite is developed.A new calculation method named zone partitioning sequential calculational method was proposed to expand the simulation area of phase-field method. Phase-field method can beused in the simulation of metal structure changes during the solidification process, but it can only be used to simulate a small area of the metal due to the large amount of calculation. The new method can divide the simulation area into several parts and calculate them one by one in a certain order. In order to test and verify this method,and compared the simulational results with the results that are adoptded common approach. Results showed that:This new method can not only reduce the storage capacity of single calculation but also the transfer error between the region and the region has little effect on the grain growth, and it has certain accuracy and reliability; With this method can breakthrough the limit of the amount of computer calculational node, realize a greater regional area, and expand the scope of application of the phase-field method.