Rong-zhen Xiao

Numerical Simulation of Recalescence of 3-dimensional Isothermal Solidification for Binary Alloy Using phase-field Approach

A accelerated arithmetic algorithm of the dynamic computing regions was designed, and 3-dimensional numerical simulation of isothermal solidification for a binary alloy was implemented. The dendritic growth and the recalescence of Ni-Cu binary alloy during the solidification at different cooling rates were investigated. The effects of cooling rate on dendritic patterns and microsegregation patterns were studied. The computed results indicate that, with the increment of the cooling rate, the dendritic growth velocity increases, both the main branch and side-branches become slender, the secondary dendrite arm spacing becomes smaller, the inadequate solute diffusion in solid aggravates, and the severity of microsegregation ahead of interface aggravates. At a higher cooling rate, the binary alloy presents recalescence; while the cooling rate is small, no recalescence occurs.

Phase-field simulation of forced flow effect on random preferred growth direction of multiple grains

Abstract The random distribution problem of dendrite preferred growth direction was settled by random grid method. This method was used to study the influence of forced laminar flow effect on multiple grains during solidification. Taking high pure succinonitrile (SCN) undercooled melt as an example, the forced laminar flow effect on multiple grains was studied by phase-field model of single grain which coupled with flow equations at non-isothermal condition. The simulation results show that the random grid method can reasonably settle the problem of random distribution and is more effective. When the solid fraction is relatively low, melt particles flow around the downstream side of dendrite, and the flow velocity between two dendrite arms becomes high. At the stage of solidification time less than 1800Δ t , every dendrite grows freely; the upstream dendrites are stronger than the downstream ones. The higher the melt flow rate, the higher the solid fraction. However, when the solid fraction is relatively high, the dendrite arm intertwins and only a little residual melt which is not encapsulated can flow; the solid fraction will gradually tend to equal to solid fraction of melt without flow.

Phase-field simulations of forced flow effect on dendritic growth perpendicular to flow

Abstract The effect of supercooled melt forced laminar flow at low Reynolds Number on dendritic growth perpendicular to melt flow direction was investigated with the phase-field method by incorporating melt convection and thermal noise under non-isothermal condition. By taking the dendritic growth of high pure succinonitrile (SCN) supercooled melt as an example, side-branching shape difference of melts with flow and without flow was analyzed. Relationships among supercooled melt inflow velocity, deflexion angle of dendritic arm and dendritic tip growth velocity were studied. Results show that the melt inflow velocity has few effects on the dendritic tip growth velocity. A formula of relationship between the velocity of the melt in front of primary dendritic tip and the dendritic growth time was deduced, and the calculated result was in quantitative agreement with the simulation result.

Corrosion and wear behaviors of Al-bronzes in 5.0% H2SO4 solution

Abstract Steady-state corrosion and wear behaviors of two Al-bronzes, Cu-14Al-X and QAl9-4, in 5.0% H2SO4 solution were investigated. It is found that wear loss of bronzes in 5.0% H2SO4 solution is lower than that in water or in air, namely, it exhibits negative synergy between corrosion and wear. Further analysis shows that corrosive solution plays an important role in cooling of specimen during the sliding wear to prevent the reduction of the surface hardness of specimen, induced by frictional heat. On the other hand, the bronzes suffer a de-alloying corrosion, and a noble copper subsurface and patina form on the specimen surface in corrosive solution, which has a passive function for further corrosion. The noble copper subsurface experiences strain hardening during the corrosion and wear, resulting in the increase of the surface hardness thus the increase in wear resistance.

Comparative analysis of isothermal and non-isothermal solidification of binary alloys using phase-field model

Abstract Based on the entropy function, a two-dimensional phase field model of binary alloys was established. Meanwhile, an explicit difference method with uniform grid was adopted to solve the phase field and solute field controlled equations. And the alternating direction implicit (ADI) algorithm for solving temperature field controlled equation was also employed to avoid the restriction of time step. Some characteristics of the Ni–Cu alloy were captured in the process of non-isothermal solidification, and the comparative analysis of the isothermal and the non-isothermal solidification was investigated. The simulation results indicate that the non-isothermal model is favorable to simulate the real solidification process of binary alloys, and when the thermal diffusivity decreases, the non-isothermal phase-field model is gradually consistent with the isothermal phase-field model.

Effect of forced lamina flow on microsegregation simulated by phase field method quantitatively

Abstract The influence of supercooled melt forced lamina flow on microsegregation was investigated. The concentration distribution at solid-liquid boundary of binary alloy Ni–Cu was simulated using phase field model coupled with flow field. The microsegregation, concentration maximum value, boundary thickness of concentration near upstream dendrite and normal to flow dendrite, and downstream dendrite were studied quantitatively in the case of forced lamia flow. The simulation results show that solute field and flow field interact complexly. Compared with melt without flow, in front of upstream dendrite tip, the concentration boundary thickness is the lowest and the concentration maximum value is the smallest for melt with flow. However, in front of downstream dendrite tip, the results are just the opposite. The zone of poor Cu in upstream dendrite where is the most severely microsegregation and shrinkage cavity is wider and the concentration is lower for melt with flow than that without flow.

Parameters affecting microsegregation in phase-field simulation

The influence of various material and computational parameters such as interface kinetic coefficient(β), surface energy(σ), anisotropy parameter(γ) and the noise amplitude(α) upon microsegregation patterns during the crystal growth was investigated by using the phase-field model which incorporated the concentration field equations. The computed results indicate that, when the appropriate value is assigned to α, the fluctuant scope of solute composition in the solid is steady, and the influence of α on microsegregation is small; the larger the interface kinetic coefficient β, the more acutely the solute composition in the solid fluctuates, but the severity of microsegregation in the front interface reduces; with the increment of anisotropy parameter γ, the fluctuation of solute composition in the solid becomes more acutely, and the severity of microsegregation in the front interface aggravates; the larger surface energy σ, the smaller the fluctuant scope of solute composition in the solid is, and the smaller the degree of microsegregation is.

Phase-field modeling of dendritic growth under forced flow based on adaptive finite element method

Abstract A mathematical model combined projection algorithm with phase-field method was applied. The adaptive finite element method was adopted to solve the model based on the non-uniform grid, and the behavior of dendritic growth was simulated from undercooled nickel melt under the forced flow. The simulation results show that the asymmetry behavior of the dendritic growth is caused by the forced flow. When the flow velocity is less than the critical value, the asymmetry of dendrite is little influenced by the forced flow. Once the flow velocity reaches or exceeds the critical value, the controlling factor of dendrite growth gradually changes from thermal diffusion to convection. With the increase of the flow velocity, the deflection angle towards upstream direction of the primary dendrite stem becomes larger. The effect of the dendrite growth on the flow field of the melt is apparent. With the increase of the dendrite size, the vortex is present in the downstream regions, and the vortex region is gradually enlarged. Dendrite tips appear to remelt. In addition, the adaptive finite element method can reduce CPU running time by one order of magnitude compared with uniform grid method, and the speed-up ratio is proportional to the size of computational domain.