基于计算机模拟的焊缝金属组织凝固过程中枝晶生长分析

模拟焊缝凝固过程的枝晶生长,得到焊接熔池中心柱状晶和等轴晶溶质浓度分布、生长形态。熔池凝固结晶过程中,晶界偏析很大,枝晶竞争生长且相互作用。采用元胞自动机方法模拟枝晶生长可行,但仍需考虑多方面因素,并且在熔池内复杂多角度枝晶状态方面做更细致和深入的研究。     

铝带铸轧凝固过程中的枝晶生长模拟

采用元胞自动机方法(Cellular Automation Method) 模拟铝带铸轧过程中晶粒的生长,模拟过程中用简化的二元合金模拟了过冷熔体中枝晶的生长,且过冷熔体中枝晶的生长模型采用LGK模型.在枝晶生长的基础上模拟了铝带在铸轧过程中冷却及铸轧速度引发的过冷度对晶粒生长的影响.结果表明,在形核率一定的情况下,过冷度越大,晶粒越大.     

三元合金凝固过程枝晶生长数值模拟

建立了使用元胞自动机方法结合合金凝固过程动量、能量和质量传输计算三元合金枝晶形貌与偏析发展的数学模型.把该数学模型应用到了Fe-C-Si三元合金凝固过程,枝晶臂的生长、粗化过程,以及枝晶间的微观偏析得到了再现.同时该数学模型也描述了凝固过程熔体流动对Fe-C-Si合金凝固过程枝晶形貌发展的影响.     

Fe-0.6%C合金凝固过程枝晶生长数值模拟

采用元胞自动机方法,结合合金凝固过程中的动量、能量和质量传输,建立了计算枝晶形貌与偏析发展的数学模型。把该数学模型应用到Fe-0.6%C合金凝固过程,枝晶臂的生长、粗化和柱状晶向等轴晶转变过程得到了再现。同时该数学模型也描述了凝固过程熔体流动对Fe-0.6%C枝晶形貌发展的影响。     

焊缝金属凝固元胞自动机模型的研究

元胞自动机能够基于现象本身的物理机制，对于难以用数学方法定量描述的体系如微观组织的演变等具有良好的适用性。成功地将元胞自动机方法用于焊缝金属凝固组织的模拟中，不仅使数值方法应用于焊接领域的范围和深度进一步扩大，同时，也为更深入地揭示焊接熔池结晶中各种物理现象、探讨其机制打下基础。     

定向凝固过程中NH_4Cl-H_2O枝晶生长的数值模拟

建立了一种改进的元胞自动机模型(MCA),通过考虑成分过冷、曲率过冷、择优取向系数、温度梯度和抽拉速度等因素,模拟了不同温度梯度方向、不同择优取向及不同抽拉速度对柱状晶形态的影响.模拟结果较好地刻画了温度梯度和生长方向夹角的变化对一次枝晶臂间距的影响,不同择优生长取向的柱状晶竞争生长过程以及柱状晶尖端分叉机制.为了验证模型的可靠性,使用NH_4Cl-H_2O透明合金进行了定向凝固实验,实验结果和模拟结果吻合较好.     

元胞自动机方法在枝晶生长模拟中的应用

尽管大量关于枝晶生长模拟的研究都使用了相场方法,但近年来元胞自动机(CA)方法在枝晶生长模拟方面也取得了很大进展.介绍了CA方法模拟枝晶生长的原理和模型,评述了CA方法在枝晶模拟方面的发展历程,指出了存在的问题的和进一步研究的方向.     

用CA方法模拟过冷熔体中自由树枝晶的生长

基于溶质扩散和界面能的作用，建立了过冷熔体中自由树枝晶的生长模型，考虑了成分过冷、曲率过冷和界面能各向异性，用胞元自动机（CA）方法模拟了枝晶生长、界面扰动以及分枝的竞争演化，对枝晶尖端生长速度和过冷度的关系进行了模拟计算，并与L-G-K理论模型进行了定量的比较。     

元胞自动机法模拟铝合金三维枝晶生长

以元胞自动机模型为基础,基于晶粒形核和生长的物理过程及热质传输过程,建立了铝合金凝固过程微观组织形成及枝晶形貌演化的三维元胞自动机模型.与传统的元胞自动机不同,该模型不仅考虑了温度场扩散而且考虑了固液相中的溶质扩散、曲率过冷等重要因素.枝晶尖端生长速度与局部过冷度的关系采用KGT(Kurz-Giovanola-Trivedi)模型,温度场和浓度场计算采用有限差分法.使用该模型模拟了单晶生长和多晶生长.模拟结果表明,所建立的模型能够合理反映质点形核、单晶粒生长和多晶粒生长,微观组织形貌的模拟计算结果合理.     

基于元胞自动机法的等轴枝晶生长数值模拟

基于溶质扩散和界面能的作用,考虑成分过冷、曲率过冷、界面能各向异性和界面扰动等因素,建立了单个等轴枝晶的生长模型.采用元胞自动机(cellular automata)方法模拟了枝晶生长、界面扰动和分枝的竞争演化.对液相中的溶质传输进行了计算.通过模拟发现,凝同过程中溶质容易富集在枝晶臂之间,同时,随着凝固时间的延长,界面前沿的溶质浓度梯度逐渐下降.     

焊接熔池凝固过程组织演变模拟

以枝晶生长动力学和晶粒生长能量最小原理为基础,建立了宏观传热与微观传质、形核、生长相耦合的焊接熔池CAMC晶粒生长二维数学模型.模型以CA方法模拟晶粒生长主干,MC在内部辅助晶粒生长以体现枝晶分枝机制,同时考虑非均匀形核对熔池结晶的影响因素,模拟了焊接熔池组织形成过程.结果表明,CAMC模型能够定量地描述熔池晶粒数目、尺寸和形貌演变,可以较准确地反映焊接熔池微观组织结构和熔池凝固过程中晶粒择优生长、柱状晶向等轴晶转换等物理机制.     

焊缝金属凝固组织元胞自动机模拟

应用元胞自动机方法进行了焊缝金属凝固组织的模拟.所建立的焊缝凝固组织二维元胞自动机模型考虑了晶粒的概率性成核、曲率过冷、温度过冷、成分过冷、潜热的释放、溶质浓度的再分布以及焊接熔池晶粒的联生长大等影响因素.模型在统一网格下分别采用差分法计算温度和溶质的扩散,应用元胞自动机方法模拟晶粒形核及生长.模拟结果能够定性地再现焊缝金属晶粒择优取向与竞争长大机制.结果表明,元胞自动机方法较好地反映了焊缝金属凝固的特点,是焊缝凝固组织模拟的新途径.     

Modeling of solidification grain structure for Ti-45% Al alloy ingot by cellular automaton

A cellular automaton model for simulating grain structure formation during solidification processes of Ti45% Al(mole fraction) alloy ingot was developed, based on finite differential method for macroscopic modeling of heat transfer and a cellular automaton technique for microscopic modeling of nucleation, growth, solute redistribution and solute diffusion. The relation between the growth velocity of a dendrite tip and the local undercooling,which consists of constitutional, thermal, curvature and attachment kinetics undercooling is calculated according to the Kurz-Giovanola-Trivedi model. The effect of solidification contraction is taken into consideration. The influence of process variables upon the resultant grain structures was investigated. Special moving allocation technique was designed to minimize the computation time and memory size associated with a large number of cells. The predicted grain structures are in good agreement with the experimental results.     

Multi-scale coupling simulation in directional solidification of superalloy based on cellular automaton-finite difference method

Casting microstructure evolution is difficult to describe quantitatively by only a separate simulation of dendrite scale or grain scale, and the numerical simulation of these two scales is difficult to render compatible. A three-dimensional cellular automaton model couplling both dendritic scale and grain scale is developed to simulate the microstructure evolution of the nickel-based single crystal superalloy DD406. Besides, a macro–mesoscopic/microscopic coupling solution algorithm is proposed to improve computational efficiency. The simulation results of dendrite growth and grain growth of the alloy are obtained and compared with the results given in previous reports. The results show that the primary dendritic arm spacing and secondary dendritic arm spacing of the dendritic growth are consistent with the theoretical and experimental results. The mesoscopic grain simulation can be used to obtain results similar to those of microscopic dendrites simulation. It is indicated that the developed model is feasible and effective.     

Dendritic grain growth simulation in weld pool of nickel base alloy

Dendritic grain growth at the edge of the weld pool is simulated using a stochastic numerical model of cellular automaton algorithm. The grain growth model is established based upon the balance of solute in the solid/liquid interface of the dendrite tip. Considering the complicated nucleation condition and competitive growth, the dendrite morphologies of different nucleation condition are simulated. The simulated results reproduced the dendrite grain evolution process at the edge of the weld pool. It is indicated that the nucleation condition is an important factor influencing the grain morphologies especially the morphologies of secondary and tertiary arms.     

Simulation of microstructure evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model. The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth. For dendritic structures formed under a high thermal gradient, the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth condition. Furthermore, it has been observed that, with increasing pulling velocity, there exists a cell/dendrite transition region consisting of cells and dendrites,which varies with the thermal gradient in a contradicting way, i.e. increase of the thermal gradient leading to the decrease of the range of the transition region. The simulations agree reasonably well with experiment results.     

Two-dimensional cellular automaton model for simulating structural evolution of binary alloys during solidification

1Introduction During solidification of cast metallic materials,simulations of microstructural evolution,which track kinetics in a local fashion,are of interest for two reasons.First,from a fundamental point of view,it is desirable to better understand the…     

Cellular automaton simulation of peritectic solidification of a C-Mn steel

A cellular automaton model has been developed to simulate the microstructure evolution of a C-Mn steel during the peritectic solidification.In the model,the thermodynamics and solute diffusion of multi...     

“节点修正”FVM-CA方法熔积凝固组织模拟

基于Nastac模型建立了一种优化的元胞捕捉机制,即"节点修正"法元胞自动机(CA)模型.较传统的CA模型能够简便有效地模拟任意主轴方向的柱状晶生长;将其与有限体积法(finite volume method,FVM)的宏观温度场模拟相结合,构建了一种改进的FVM-CA宏微观耦合计算方法.用该改进模型对碳钢电弧熔积组织演变过程进行了模拟,并探讨了熔积速度对熔池枝晶间距及生长方向的影响.试验结果证明了计算模型的合理性,该工作将为熔积增材成形金属零件的组织预测与控制提供科学基础.