Characterization Of Microstructures And Performances In Undercooled Melt Solidifying Process With Phase-field Method

The transformation must be happened from liquid to solid in the process of manufacturing for most materials, especially for metal materials. The central task of solidification is the formation mechanism and control method of the microstructure, and studies of the solid/liquid interfacial morphology is an important part of solidification theory, then the interface formation determines the final microstructures and materials properties. Many factors affect the formation of solidification, so the numerical simulation offers researchers a convenient way to visualize the evolution process and reveal the formation mechanism of microstructures during solidification in recent years. As one of the numerical simulation methods to elucidate the complex microstructure evolution, the phase-field method has been widely accepted by researchers.Based on Ginzburg Landau theory and entropy function, a phase field model of binary alloy is established and an explicit difference method with uniform grid is used to solve the phase field and solute field controlled equation in this paper. Through the studies of stable conditions of convergence in equations’numerical solutions, the choice of the space steps and time step and the restrictive relationship among physical parameters is determined, and the alternating direction implicit method for solving temperature field controlled equation is also employed to avoid the restrictions of time step. Using the compatibility of VC++platform, the C Programming Code is implemented to complete the phase-field simulation. The image generation and dynamic displaying of salutation results are visualized by using Tecplot software, and the tip velocity of dendrite/cell, the curvature radius and the solidifying rate are calculated in the process of solidification. The simulation results consist of following aspects.The crystalline anisotropy coefficient y is a crucial parameter that determines the interfacial morphology and the tip operating state for an isothermal solidification process of Cu-Ni binary alloy. For the<100> dendrites growth, the slope of the interface has discontinuities and appears faceted when y>1/15, then the initial phase model becomes inapplicable and must to be regularized. For the atypical dendritic growth of <110> directions, the interfacial morphology presents a perfectly symmetrical snowflake structure when γ=0.02. When γ＞0.02, the main branch of [110] is more developed than the [101] and [011] branches, and the symmetry of dendrite morphology is disrupted, and interfacial morphology changes to a feathery shape even a needle shape, then the level of solute trapping is severe along with [110]. The interface dynamic coefficient β also has a significant effect on dendritic growth of <110> dendrites. With increasing of β, the coalescence of some side-branches are observed, and the crystal growth patterns are formed include a sector form and a plate formation, but the symmetry of dendrite morphology is not disrupted, and the solute trapping appears obvious. When β=0.60, the level of solute trapping is almost complete.Some phenomena have been found by contrasting simulation results of isothermal and non-isothermal solidification process for Ni-Cu alloy. Release of latent heat results in a high temperature distribution in solid area, and the highest temperature appears in the center area of some developed side-branches, that is, recalescence occurs. Release of latent heat reduces supercooling of liquid, so dendritic branches and the solid ratios are comparatively small comparing to isothermal solidification, and the gap becomes bigger according to time up. Moreover, the solute concentration is low in the region of solid/liquid interface at high temperature, and the level of solute trapping is strong. Under the conditions of a small interface thickness, the thermal noise is enlarged so easily, and the tip stability of the dendrite growth is declined, then the crystal growth is restrained. The influence of diffusion coefficient DT to crystal growth is achieved by thermal diffusion layer mainly. With the increment of DT, the latent heat diffuses easily, and the extent of the solid temperature is slight. The solute gradient coefficient δ would affect the dendrite feature hardly, but with elevating of solute gradient, the distribution of solid solute is consistent, and the severity of solute segregation is reducedDuring the directional solidification process of binary alloys, with the increment of interface speed Ⅴ, the transition from plane to cells/fine cellular structures, then to planar structures(plane-cell-plane) will happen, and the level of solute trapping is severe following with the concentration of solid/liquid trends to equal in the interfacial region. The effect of crystalline anisotropy on the interfacial morphology and tip velocity is obvious at low interface speed. The solid-liquid interface shape with seaweed microstructure is achieved at a small anisotropy coefficient y. And the interfacial morphology changes through transition from seaweed to cellular dendrite or cell with the increment of y, and cell tip velocity increases correspondingly, but the stability of the cell tip velocity becomes down. Then, the operating behavior for planar growth is hardly any affected by the crystalline anisotropy. In addition to the interface speed, some physical properties have a significant impact on the solute trapping effect. With increasing of the solute concentration gradient δ and the interface dynamic coefficient β, the solute concentration in solid increases correspondingly, thus the level of solute trapping is strong.The effect of melt convection on the dendrtic growth is obvious. In the presence of fluid flow, the symmetry of dendrite morphology is disrupted. The primary dendrite growing upstream is most luxuriated, and the branches in a horizontal direction normal to the flow come next, then the downstream branch is slowest. Many grains growing along with the preferred orientation morphology under forced flow is studied. It is demonstratcd that the dendrite growth is controlled mainly by flow and the upstream tips are highest, but the grains impact each other when they are closer. The effect of crystalline anisotropy on the interfacial morphology is investigated under forced flow. The simulation results indicate that, the main braches are all obese at different directions at low γ; with increasing of y, they become slender with abundant in side-branches, and the necking is evident. When γ=0.07, the variation of dendrite morphology distortion happens, and the slope of the interface has edges and corners and appears faceted growth; then, the downstream dendrite generates tip-splitting, and the dendritic root trends fracture. Convection affects the interfacial morphology of binary alloys in the process of solidification also. With the enhancement of convection, the upstream tip velocity increases approximately linear, and the level of solute trapping is severe, but the horizontal tips are nearly steady. While the stable growth velocity of downstream tip decreases accordingly at feeble convection, and then increases suddenly when the speed of the fluid flow crosses the critical value.