Phase-field Method Research On Microstructure Of Solidification Under Forced Flow

The final properties of the solidification material depend on the microstructures established during the solidification process, so to control the formation of the microstructures effectively in solidification has important theoretical and practical significance. Convection is difficult to avoid in solidification processes, it will influence the formation of the microstructures remarkably by altering the distributions of concentration and temperature in the vicinity of the solid/liquid interface. With the development of computational science, coupling with flow filed of phase field simulation for solidification microstructures has become a hot research scholars at home and abroad. Dendrite is common microstructure in solidification process. In this Dissertion, the dendritic growth process in solidification is simulated by the phase field model coupling with flow field, the mechanism of the dendritic growth under forced flow during solidification is discussed, the dendritic growth behavior of tips under without flow and forced flow is quantitatively analysed, and a favorable base for prediction and improvement of mechanical property of material is established.Based on the Wheeler model, coupling with the temeperature field and flow field, the phase field model for dendritic growth of pure material under forced flow is developed; Further coupling with concentration field, the phase field model for dendritic growth of alloy under forced flow is developed. The governing equations are discretized on uniform grids using the Finite Difference method. The thermal governing equation is numerically solved using an alternating direct implicit method, which improve the calculating efficiency and avoid the restriction of the time step. The Visual C++ code is used to complete the phase-field simulation program, when visible software Tecplot is integrated with the simulation program, the visible system of the phase-field simulation is established.Using the pure material dendritic growth phase-field model, taking pure Ni for example, the growth process of the single and multi-dendrites under forced flow are simulated. In the case of single dendrite growth, the upstream arms of dendrite grow fast, the downstream arms of it grow slow, upstream side and downstream side of the lateral principal branches has significant differences. For multi-dendrites growth, the growth of dendrite arms on the preferred directions toward the out of upstream area is promoted, which lead to the principal branches and side branches are developed; The growth of dendrite arms on the other preferred directions is inhibited, principal branches and side branches degenerate. The simulated results show that the solidification features are consistent with those observed based on the dendrite growth in succinonitrile under gravity convection.Using the alloy dendritic growth phase-field model, taking Ni-Cu alloys for example, the growth process of the single and multi-dendrites under forced flow are simulated. In the case of single dendrite growth, because of fluid flushing, the temperature and the concentration of the upstream dendrite arm are low, the actual supercooling of it is great, which lead to the dendrite grow fast; Heat and solute enrich in the downstream, the temperature and the concentration of the downstream dendrite arm are high, the actual supercooling of it is small, which make the dendrite grow slow. For multi-dendrites growth, the growth of dendrite arms in internal area is mainly controlled by adjacent dendrite and those in external area away from border is mainly controlled by flow field. The simulated results show that the solidification features are consistent with those observed based on the dendrite growth in Sn-Bi alloys under direct current electric field.Using the dendritic growth phase-field model for the magnesium alloys with hcp structure, the growth process of the single and multi-dendrites of AZ91D alloys under without flow and forced flow are simulated. These results indicate that the dendrite displays obviously six-fold symmetry anisotropy and the growth is same in each direction under without flow. When dendrite exists in a forced flow, the growth of three directions on the upstream side are much faster than those of three directions on the downstream side, there exists much differences in the length of dendrite arm. In the case of the multi-dendrites, at the early stage, the crystals grow independently and is regular hexagon; With the increase of time, the regular hexagon become to dendrite morphology. The dendrite arms grown face to face influence mutually and grow competitively, the dendrite arms of different dendrite mutually inhibited, ultimately, to form asymmetric dendrite morphology. The simulation results as same as the experimental results.The function of anisotropic interface energy with the form of?=?0[1+?kcos(k?)] is regularized, and the regularized phase field model is developed. By comparing the simulation morphology for dendrite growth of pure Si under without flow with experimental results, the dendrite growth behaviours of Ni-Cu alloys with strong surface energy under without flow and forced flow is predicted. The results show that the variation of interface orientation discontinuity and the corners form on the main stem and side branches of dendrite during the dendrite growth process of pure Si with strong interface energy anisotropy, the simulation results is similar to the experimental results. During the dendrite growth process of Ni-Cu alloys with strong interface energy anisotropy (?4), as?4 increases, the growth velocity of dendrite increases when the?4 is lower than the critical value; As the?4 crossed the critical value, the growth velocity dropped down by about 4.34%; With again increase in?4, the growth velocity reached a maximum value and then tends to decrease. Introducing interface kinetic anisotropy (?), when there only exists?, the melt solidifies growth along the <110> orientation and the crystals grow into a square-like. Under the strong interface energy anisotropy or two kinds of anisotropies conditions, the melt solidifies in a dendrite pattern growth along the <100> orientation. Under forced flow with flow velocity of 6.43 m/s, the upstream dendrite arm grows faster because of the undercooled melt flushing, the tip velocity at steady state increases by about 12.61% compared to the case without flow. The heat and solute are congregated in the downstream, which makes the dendrite arm grows slowly, the tip velocity at steady state decreases by about 14.62% compared to the case without flow.