基于改进元胞自动机模型的三元合金枝晶生长的数值模拟

在二元合金元胞自动机模型的基础上,通过耦合多元合金热力学相平衡求解器PanEngine, 建立了三元合金改进的元胞自动机模型,可模拟初生相枝晶的生长过程. 模型考虑了曲率过冷和成分过冷对界面平衡溶质成分的影响,通过不同组元的无量纲溶质过饱和度方程和界面溶质守恒方程之间的耦合来求解界面生长速率,并通过PanEngine计算界面处的液相线温度. 采用本模型模拟了Al-7%Si-xMg三元合金自由枝晶的生长形态, 结果表明Mg含量的增加会抑制枝晶一次臂的生长和二次臂的产生.同时模拟了不同抽拉速度下 Al-7%Si-0.5%Mg合金柱状枝晶的竞争生长过程,随着抽拉速度的增大,柱状枝晶一次枝晶臂间距逐渐减小, 与Hunt理论模型符合较好.     

相场法模拟多元合金过冷熔体中的枝晶生长

在二元合金相场模型研究的基础上，进行扩展获得了多元合金相场模型.以Al-Si-Mg三元合金为例，采用该相场模型实现了逼真地模拟多元合金凝固过程的等轴枝晶生长，得到了二次或更高次晶臂生长等复杂的枝晶形貌.随着第三组元Mg含量的减少，枝晶的二次枝晶越发达，枝晶中溶质的偏析越严重，枝晶尖端的生长速率和半径越大，与丁二腈-丙酮体系中枝晶尖端生长速率、半径随溶质浓度变化关系的理论计算和实验结果相符合.另外，枝晶初生晶臂中心的溶质浓度最低，在被二次晶臂包围的界面区域的溶质浓度最高；固液界面区域具有较大的浓度梯度，其中枝晶尖端前沿的梯度最大. 关键词： 相场法 多元合金 凝固过程 枝晶生长     

Phase-field simulation of dendritic growth in a binary alloy with thermodynamics data

This paper simulates the dendrite growth process during non-isothermal solidification in the Al-Cu binary alloy by using the phase-field model. The heat transfer equation is solved simultaneously. The thermodynamic and kinetic parameters are directly obtained from existing database by using the Calculation of Phase Diagram （CALPHAD） method. The effects of the latent heat and undercooling on the dendrite growth, solute and temperature profile during the solidification of binary alloy are investigated. The results indicate that the dendrite growing morphologies could be simulated realistically by linking the phase-field method to CALPHAD. The secondary arms of solidification dendritic are better developed with the increase of undercooling. Correspondingly, the tip speed and the solute segregation in solid-liquid interface increase, but the tip radius decreases.     

相场法模拟NiCu合金非等温凝固枝晶生长

利用相场模型与溶质场、温度场进行耦合计算,以Ni-40.83%Cu合金为例模拟了二元合金枝晶生长过程.系统研究相场模型中相场和温度场耦合强度对枝晶形貌和浓度分布的影响.模拟结果表明:随着耦合强度的增加,相场受温度场的影响加大,界面前沿变得不稳,扰动被放大,主枝上出现了二次枝晶.同时,枝晶尖端的生长速率增大,而枝晶尖端的曲率半径减小,枝晶前沿的溶质富集现象也更严重;另外,计算结果与Ivantsov理论符合较好. 关键词： 相场法 NiCu合金 枝晶生长 Ivantsov理论     

二元合金凝固过程微尺度热质传输的影响分析

针对二元合金凝固的微观偏析现象建立一维平面枝晶模型,考虑溶质在固相中有限扩散,液相完全扩散的情况。通过模拟计算,比较分析了Al-Cu和Fe-C合金微观偏析的特点,并讨论了不同凝固速度以及溶质扩散系数随温度变化与否对微观偏析的影响。分析结果表明,温度场的影响是不可忽略的,应该在研究中予以考虑。     

强迫对流影响二元合金非等温凝固枝晶生长的相场法模拟

在二元合金相场模型的研究基础上,建立了耦合溶质场、温度场和流场的相场模型,采用Simple算法求解质量和动量守恒方程,用交替隐式有限差分法求解温度控制方程,模拟了流场作用下二元合金等温和非等温凝固过程中枝晶的生长过程,研究了流场对枝晶生长形貌、溶质场和温度场分布情况的影响,将流场作用下二元合金等温和非等温凝固枝晶生长过程进行比较,分析了由于凝固潜热的释放对流场作用下凝固枝晶生长的影响． 关键词： 相场法 对流 非等温凝固 枝晶生长     

强制对流和自然对流作用下枝晶生长的数值模拟

建立了一个基于格子玻尔兹曼方法 （LBM） 的二维模型,对强制对流和自然对流作用下合金凝固过程中的枝晶生长行为进行了模拟研究. 与传统的基于求解Navier-Stokes方程计算流场的方法不同,本模型采用基于分子动理论的LBM对凝固过程中的传输现象进行数值计算. 用三组粒子分布函数分别建立了计算流场、由对流和扩散所控制的浓度场和温度场的LBM演化方程. 通过求解LBM演化方程获得固/液界面前沿的浓度和温度分布. 然后,基于溶质平衡方法计算了枝晶生长的驱动力. 为了对模型进行验证,将模拟在强制和自然对流作用下枝晶上游尖端的稳态生长特征分别与Oseen-Ivantsov 解析解和修正的Lipton-Glicksman-Kurz 模型预测结果进行了比较, 模拟结果和理论预测结果符合良好. 模拟结果还表明,对流使热量和溶质从上游传输到下游,从而加速了枝晶在上游方向的生长,而抑制了下游方向的生长,形成了非对称的枝晶形貌. 关键词： 微观组织模拟 枝晶生长 对流 格子玻尔兹曼方法     

元胞自动机方法模拟枝晶生长

为了能够准确反映具有明显界面的枝晶生长过程，利用元胞自动机方法模拟了凝固过程中固液两相具有不同热物理性质的枝晶生长.在模拟过程中发现，凝固过程中溶质易于富集在枝晶臂之间，同时随着凝固时间的延长，界面前沿的溶质浓度梯度和温度梯度逐渐下降.利用模拟所得的枝晶尖端半径与理论计算的相比较，发现随着枝晶尖端速度的增加，枝晶尖端半径逐渐下降，模拟与理论结果符合较好，偏差小于1μm. 关键词： 枝晶生长 凝固 偏析 模拟     

Effect of boundary heat flux on solidification in a forced liquid metal flow: a phase-field simulation

A phase-field model coupling with velocity field is employed to study the effect of boundary heat flux on the microstructure formation of a Ni-40.8%Cu alloy with liquid flow during the solidification, and an anti-trapping current is introduced to suppress the solute trapping due to the larger interface width used in simulations than a real solidifying material. The effect of the flow field coupling with boundary heat extractions on the microstructure formation as well as distributions of concentration and temperature fields are analyzed and discussed. The forced liquid flow can significantly affect the heat and solute diffusions, thus influencing morphology formation, concentration and temperature distributions during the solidification. The solute segregation and concentration diffusion are changed by boundary heat extractions, and the morphology, concentration and temperature distributions are significantly influenced by increasing the heat extraction, which relatively makes the effect of liquid flow constrained. By increasing the initial velocity of liquid flow, the lopsided rate of the primary dendrite arm is enlarged and the growth manner of dendrite arms gets changed, and the transition of the microstructure from dendrite to cellular moves to the large heat extraction direction. Therefore, there exists the competition between the heat flux, temperature gradient and forced liquid flow that finally determines the microstructure formation during directional solidification.     

对流作用下枝晶形貌演化的数值模拟和实验研究

合金凝固过程中存在于枝晶尖端液相区的强制对流和自然对流均能改变溶质扩散层厚度,从而会对枝晶形貌产生较大影响.在元胞自动机模型基础上,耦合液体流动方程、热传导方程和溶质对流扩散方程,建立了新的计算微观组织演化的数值模型,并利用该模型研究了强制对流和自然对流对枝晶生长的影响.三维数值模拟结果再现了强制对流作用下等轴枝晶的生长过程,揭示了强制对流对枝晶生长速率和尖端半径的影响特点.同时利用该模型模拟了NH₄Cl-H₂O溶液定向凝固过程中自然对流对柱状晶生长的影响,并采用相应的实验进行验证.模拟结果与实验结果符合良好,从而证明该模型是可靠的,可推广到实际合金系中. 关键词： 元胞自动机 对流 4Cl-H₂O溶液')" href="#">NH₄Cl-H₂O溶液 定向凝固     

枝晶生长和气泡形成的数值模拟

本文建立了二维的格子玻尔兹曼方法-元胞自动机(lattice Boltzmann method-cellular automaton, LBM-CA)耦合模型, 对凝固过程中枝晶生长和气泡形成进行模拟研究. 本模型采用CA方法模拟枝晶的生长, 根据界面溶质平衡法计算枝晶生长的驱动力. 采用基于Shan-Chen多相流的LBM模拟气泡在液相中的生长和运动. 在LBM-CA的耦合模型中包含了固-液-气三相之间的相互作用. 应用Laplace定理和模拟气-液-固三相之间的润湿现象对模型进行了验证. 应用所建立的LBM-CA耦合模型模拟研究了气-液相互作用系数对单气泡生长的影响. 发现单气泡的生长速度和平衡半径随气-液相互作用系数的增大而增大. 定向凝固过程中枝晶和气泡生长的模拟结果再现了枝晶的择优生长、 气泡的优先形核位置、气泡的长大、合并、在枝晶间受挤变形以及在液相通道中的运动等物理现象, 与实验结果符合良好. 此外, 初始气体含量越高, 凝固结束时气泡的体积分数也相对较高. 本模型的模拟结果可以揭示在凝固过程中气泡形核、 生长和运动演化以及与枝晶生长相互作用的物理机理.     

基于元胞自动机方法的定向凝固枝晶竞争生长数值模拟

通过耦合温度场模型、溶质扩散方程以及枝晶生长动力学方程等重要因素,建立了一种改进的元胞自动机模型.该模型通过采用偏心算法消除网格各向异性,实现了二维尺度上任意角度枝晶生长的模拟,同时适用于模拟三维尺度上枝晶的生长过程.利用建立的模型开展了定向凝固枝晶竞争生长过程的数值模拟.为了体现本模型的有效性,模拟了透明合金的竞争生长过程,并与实验结果符合良好.镍基高温合金汇聚竞争和发散竞争的模拟结果清楚地展现了不同抽拉速度和枝晶优先生长角度下枝晶的竞争生长过程,并且模拟结果与理论模型相符合.三维枝晶生长的模拟结果表明本模型可以用来模拟三维枝晶一次臂间距的调整过程.     

液态三元Fe-Cr-Ni合金中快速枝晶生长与溶质分布规律

采用落管方法实现了液态三元Fe-Cr-Ni合金的深过冷与快速凝固,合金液滴的冷却速率和过冷度均随液滴直径的减小而迅速增大.两种成分合金近平衡凝固组织均为粗大板条状α相.在快速凝固过程中,不同直径Fe_(81.4)Cr_(13.9)Ni_(4.7)合金液滴凝固组织均为板条状α相,其固态相变特征很明显,随着过冷度增大,初生δ相由具有发达主干的粗大枝晶转变为等轴晶.Fe_(81.4)Cr_(4.7)Ni_(13.9)合金液滴凝固组织由α相晶粒组成,随着过冷度增大,初生γ相由具有发达主干的粗大枝晶转变为等轴晶,其枝晶主干长度和二次分枝间距均显著下降,晶粒内溶质的相对偏析度也明显减小,溶质Ni的相对偏析度始终大于溶质Cr.理论计算表明,与γ相相比,δ相枝晶生长速度更大.在实验获得的过冷度范围内,两种Fe-Cr-Ni合金枝晶生长过程均由热扩散控制.     

Influence of initial solid–liquid interface morphology on further microstructure evolution during directional solidification

Preparation of the initial solid–liquid interface on which growth is started is a very critical step in directional solidification experiments. Dedicated experiments concerning preparation of the initial solid–liquid interface morphology and its influence on further directionally solidified microstructure were performed on Cu-20 wt% Sn peritectic alloy in a Bridgman-type furnace. To verify the morphology of the initial solid–liquid interface, steady-state directional dendritic growth was interrupted by thermal stabilization ranging from 0 to 1 h prior to quenching. With thermal stabilization duration increase, the solid–liquid interface morphology degenerated from dendritic to cellular and finally to planar. To verify the influence of the initial state on further solidification microstructure, directional solidification experiments were performed at a low pulling rate of 1 μm/s with different initial solid–liquid interface morphologies. The initial state affects solute redistribution and formation of peritectic coupled growth structure in the subsequent directional solidification process.     

深过冷三元Ni-Cu-Co合金的快速枝晶生长

研究了深过冷条件下三元Ni₈₀Cu₁₀Co₁₀合金的快速枝晶生长, 采用电磁悬浮无容器处理方法获得了335 K(0.2T_L)的最大过冷度. X射线衍射分析与差示扫描量热分析均表明,凝固组织为α-Ni单相固溶体. 随过冷度增大, 凝固组织显著细化, 并且当过冷度达110 K时，凝固组织的形态由粗大形枝晶转变为等轴晶. 深过冷条件下溶质截留效应增强, 使得微观偏析程度减小. 对不同过冷度下合金枝晶的生长速度进 关键词： 深过冷 枝晶生长 快速凝固 溶质截留     

界面能各向异性对定向凝固枝晶生长的影响

采用元胞自动机 (cellular automaton, CA) 模型研究了界面能各向异性对二维定向凝固枝晶生长的影响. 模拟结果显示当晶体的择优生长方向与热流方向一致时, 随着界面能各向异性强度的增大, 凝固组织形态由弱界面能各向异性时的海藻晶转变为强界面能各向异性时的树枝晶. 同时, 界面能各向异性强度会影响稳态枝晶尖端状态的选择, 界面能各向异性越强, 定向凝固稳态枝晶尖端半径越小, 尖端界面前沿的液相浓度和过冷度越小. 稳态枝晶生长的尖端状态选择参数与界面能各向异性强度也存在标度律的指数关系, 而枝晶一次间距则受界面能各向异性强度影响较弱. 当晶体的择优生长方向与热流方向呈-40°夹角时随着界面能各向异性强度的增大, 凝固组织形态由海藻晶逐渐转变为退化枝晶, 后又逐渐转变为倾斜枝晶. 关键词： 元胞自动机 枝晶 界面能各向异性     

Heat transfer and diffusion-controlled kinetics of liquid–solid phase in titanium matrix composite during selective laser melting

This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed.     

Orientation evolution of single-crystal superalloys under different solidification interface

The single-crystal superalloys with different deviation angle are prepared by using the liquid metal cooling high-temperature gradient directional solidification furnace, and the orientation evolution of single-crystal superalloys under different solidification interface is studied. The results indicate that the increase of the deviation angle will make the planar interface unstable. With the increase of the solidification rate, the growth direction of the crystal will deviate from the direction of heat flow to the preferred orientation direction. Under the developed dendrite crystal growth, the branches of the dendrite will follow the preferred orientation, nearly free from the effect of the heat flow.     

The effect of ultrasonic processing on solidification microstructure and heat transfer in stainless steel melt

The heat transfer in the ultrasonic processing of stainless steel melt is studied in this thesis. The temperature field is simulated when the metal melt is treated with and without ultrasound. In order to avoid the erosion of high temperature melt, ultrasound was introduced from the bottom of melt. It is found that the temperature of melt apparently increases when processed with ultrasound, and the greater the ultrasonic power is, the higher the melt temperature will be; ultrasonic processing can reduce the temperature gradient, leading to more uniform temperature distribution in the melt. The solidification speed is obviously brought down due to the introduction of ultrasound during solidification, with the increasing of ultrasonic power, the melt temperature rises and the solidification speed decreases; as without ultrasound, the interface of solid and mushy zone is arc-shaped, so is the interface of liquid and mushy zone, with ultrasound, the interface of solid and mushy zone is still arc-shaped, but the interface of liquid and mushy zone is almost flat. The simulation results of temperature field are verified in experiment, which also indicates that the dendrite growth direction is in accord with thermal flux direction. The effect of ultrasonic treatment, which improves with the increase of treating power, is in a limited area due to the attenuation of ultrasound.     

Interactions between a cavitation bubble and solidification front under the effects of ultrasound: Experiments and lattice Boltzmann modeling

The phenomena of melting and dendritic fragmentation are captured by using an in-situ device during the ultrasound-assisted solidification of a succinonitrile-acetone (SCN-ACE) alloy. The experimental results show that the dendrite arms detach from primary trunk due to the melting of the solid phase, which is caused by a moving ultrasound cavitation bubble. To quantify the interactions between the ultrasound cavitation bubble and the solidification front, a coupled lattice Boltzmann (LB) model is developed for describing the fields of temperature, flow, and solid fraction, and their interactions. The multi-relaxation-time (MRT) scheme is applied in the LB model to calculate the liquid-gas flow field, while the Bhatnagar–Gross–Krook (BGK) equation is executed to simulate the evolution of temperature. The kinetics of solidification and melting are calculated according to the lever rule based on the SCN-ACE phase diagram. After the validation of the LB model by an analytical model, the morphologies of the cavitation bubble and solidification front are simulated. It is revealed that the solidification interface melts due to the increase of the temperature nearby the cavitation bubble in ultrasonic field. The simulated morphologies of the cavitation bubble and solidification front are compared well with the experimental micrograph. Quantitative investigations are carried out for analyzing the melting rate of the solidification front under different conditions. The simulated data obtained from LB modeling and theoretical predictions reasonably accord with the experimental results, demonstrating that the larger the ultrasonic intensity, the faster the melting rate. The present study not only reveals the evolution of the solidification front shape caused by the cavitation bubbles, which is invisible in the ultrasound-assisted solidification process of practical alloys, but also reproduces the complex interactions among the temperature field, acoustic streaming, and multi-phase flows.