非定向熔模铸造过程中高温合金叶片晶粒组织的计算机模拟

论述了近年来发展起来的非定向凝固高温合金叶片熔模铸造过程中晶粒组织形成的计算机模拟方法，包括凝固过程中晶粒组织形成的物理模型，模拟中所采用的二维和三维胞状自动机（CA）技术。     

二维晶粒演变过程的模拟及分形研究

以蒙特卡罗随机模拟方法为基础,通过对原有算法进行改进,对二维晶粒的演变过程进行了计算机模拟和分形研究,获得了统计等效组织模型．分析表明,模拟的晶粒在演变过程中具有分形特性,即晶粒形态具有与时间无关的相似性,与晶粒形核和生长的物理机制相一致,证明了模拟过程的合理性．从而为后续热变形显微组织演变的模拟中晶核的空间分布和生长提供较为精确的显微组织基础．     

凝固过程微观组织模拟研究进展

综述了凝固过程微观组织模拟的研究现状,重点介绍了微观组织演变过程中晶粒形核、长大、相关的理论模型和常用的数值模拟方法,并对微观组织模拟的未来发展进行了评价和展望.     

基于计算机模拟的正常晶粒长大的三维特征和二维截面表征

用适当的三维ＭｏｎｔｅＣａｒｌｏ技术模拟了较完整的单相材料正常晶粒长大过程，获得了晶粒长大动力学和拓扑学的全面信息，对正常晶粒长大的三维特征和相应的二维截面有征进行对应分析表明：二维截面上测定的时间指数，具有三维系统一致随时间向稳态晶粒长的大理论值（ｎ＝０．５）趋近的特征，由二维截面单位面积上的晶粒人与时间的定量关系可以推断三维系统晶粒长大是否达到准确稳态，二维截面上的晶粒尺寸分布不能代表真实的三     

氧化物冶金中的数值模拟及研究趋势

通过综述当前氧化物冶金技术的理论研究、夹杂物数值模拟的研究和凝固过程模拟方法的研究进展,提出利用proCAST软件,在元胞自动机-宏观有限元耦合算法(CAFE)模型的基础上开发模拟凝固过程中非金属夹杂物诱发晶内铁素体形核和细化晶粒组织的过程,预测氧化物冶金中非金属夹杂物的种类、尺寸和细化晶粒组织的工艺参数.     

基于CA-FE的双辊连铸纯铝凝固组织模拟

鉴于双辊薄带连铸等轴晶区半固态铸轧组织对产品性能的重要性,应用Procast软件中的CA-FE方法对工业纯铝在水冷钢辊连续铸轧中的凝固过程进行了模拟,建立了宏观温度场、浓度场和微观生长过程耦合的凝固组织模拟模型。以CA(Cellular Automaton)模型为基础,建立了晶粒生长过程的局部演变规则,在晶粒尺度上模拟了其凝固过程。应用建立的微观组织模型,研究了工艺参数对凝固组织的影响。模拟结果与实验结果基本吻合,检验了模型的正确性与适用条件。     

6.5%Si高硅钢铸锭凝固组织的CAFE法模拟研究

应用CAFE法对6.5%Si高硅钢铸锭凝固组织进行了模拟研究,确定出了适合高硅钢组织模拟的模型。进一步用该模型研究了过热度及冷却条件对凝固组织的影响,结果表明,随着过热度的降低,凝固组织中柱状晶比例和晶粒平均面积均减小。水冷条件下,高硅钢铸锭凝固组织几乎全是柱状晶,而且晶粒粗大。空冷条件下,等轴晶区扩大,但柱状晶仍占主要部分。缓冷条件下,等轴晶区占主要部分,晶粒平均面积和空冷条件下相当。     

焊接接头组织模拟进展

介绍了焊接接头组织模拟的现状,阐述了组织模拟的主要方法:Monte Carlo方法、Cellular Automaton方法及相场法,分析了不同方法在模拟凝固及固态相变过程中的晶粒生长、组织形貌和各相分数等不同方面的特点和存在的不足.     

多晶材料晶粒生长的MonteCarlo计算机模拟方法：II.模拟异常晶粒生长

晶粒生长的显微结构的演化是一种受诸多因素影响的复杂过程，前文已简述模拟二维正常晶粒生长所采用的基本蒙特卡罗（ＭｏｎｔｅＣａｒｌｏ）方法，异常晶粒生长的最直接原因是总体系能的改变。而导致体系能变化的因素很多，本文在重点分析由于晶界能和迁称率的各向异性引起体系能量变化的基础上，介绍模拟异常晶粒生长的基本方法，为解决如何实际一长环境复杂性引入生长模型中及如何进一步模拟生长的问题提供重要思路。     

基于CAFE模拟双辊连铸纯铝凝固微观组织

鉴于双辊薄带连铸等轴晶区半固态铸轧组织对产品性能的重要性,基于热传输、流动传输、溶质传输等基本传输过程以及晶粒生长物理过程,建立了双辊连续铸轧纯铝凝固过程的宏观温度场、浓度场、微观组织及枝晶形貌演化的三维数学模型.采用CA-FE法对双辊连续铸轧纯铝在水冷钢辊连续铸轧中的凝固过程进行了模拟,采用光学显微镜研究了铸轧工艺参数对凝固组织的影响.数值模拟结果表明,所建立的数学模型能够合理描述晶粒沿任意角度生长的过程,温度场、溶质场和微观组织形貌的模拟计算结果合理.模拟结果与实验结果基本吻合,检验了模型的正确性.     

A computer simulation of the influence of processing conditions on as-cast grain structures

A computer model for simulating solidification and involving a Monte Carlo procedure has been used to qualitatively predict the influence of solidification conditions on the grain structures of small castings. In agreement with experimental observations, it was found that increasing either superheat or mould temperature results in coarsening of the grain structure. This agreement supports the contention that the principal mechanism of equiaxed grain formation in small castings is heterogeneous nucleation. The model also illustrates the importance of simulating equiaxed grain movement in the bulk liquid in order to obtain realistic predictions of grain structures.     

在脉冲电流或脉冲磁场作用下LY12合金的凝固组织

在LY12铝合金凝固过程中施加脉冲电流或脉冲磁场，研究脉冲电流或脉冲磁场对这种合金凝固组织的影响。结果表明，脉冲电流和脉冲磁场均可使晶粒明显细化，等轴化，在脉冲电磁艇下金属凝固组织的晶粒细化机制为：在脉冲电磁场的作用下剧烈的强迫对流促进晶粒从型壁上游离，大大增加金属熔体的形核率，导致晶粒细化；温度相对均匀使得游得的晶粒得以保存下来，抑制了晶粒在某个方向上的优先,从而抑制了树枝晶的形成。     

Numerical modeling and simulation of a diffusion-controlled liquid–solid phase change in polycrystalline solids

A fully implicit two-dimensional moving-mesh finite element simulation model was developed to study the influence of grain boundaries in polycrystalline solids on diffusion-controlled liquid–solid transition during transient liquid phase (TLP) bonding. The new model, which was developed without the non-trivial symmetry assumption in existing numerical models for the process, was found to conserve solute and its calculated solutions were unconditionally stable and in good agreement with experimental results. Contrary to the assumption that increased grain boundary diffusion coefficient would significantly accelerate the rate of liquid–solid interface migration, numerical calculations and experimental verification showed that enhanced intergranular diffusivity had a minimal effect on the time required to achieve complete diffusion-induced solidification in cast superalloys. The results indicate that reducing the number of grain boundaries in structural alloys through directional solidification casting techniques did not constitute a disincentive to efficient application of TLP bonding to this class of materials.     

激光选区熔化316L不锈钢凝固特征与微观组织结构

对不同工艺参数下激光选区熔化(Selective Laser Melting, SLM)成形316L不锈钢微观组织结构进行表征,研究不同工艺参数下SLM成形316L不锈钢微观组织结构演化规律、单熔化道凝固特性。结果表明,SLM成形316L不锈钢具有跨尺度、非均质凝固组织特征,包括微米尺度柱状晶粒、小角晶界、熔池界面和纳米尺度亚结构。单熔化道的稳定成形是三维块体成形的基础,熔化道稳定性由激光工艺参数与金属粉体物理特性共同决定。不同的激光工艺参数显著影响SLM成形316L不锈钢微观组织结构,通过改变激光参数可实现微观组织结构的调控,在不同的激光逐层旋转角度下,SLM成形316L不锈钢晶粒尺寸随着扫描间距的增大而增大。强制定向热流使得外延生长机制主导凝固晶粒的生长,在不同的激光工艺参数下,沿增材方向的柱状晶粒形貌普遍存在。     

Critical Assessment 32: Controling the melting behaviour of cast structures through solid-state deformation

The role of solid-state deformation of cast structures in controlling the morphological transformations, accompanying their subsequent melting is critically assessed. When dendritic morphologies, present after casting, are subjected to solid-state deformation with a strain level sufficient to trigger recrystallisation, they are replaced by equiaxed grain structures. Heating of such structures to temperatures of the solidus-liquidus range initiates melting at triple junctions and along grain boundaries, causing disappearance of dendrites, which are replaced by globular features. The semisolid slurry formed this way exhibits thixotropic properties, positively affecting its flow behaviour and properties after solidification. Although a variety of conventional and exotic deformation techniques generate very fine grain structure after recrystallisation, it does not lead to substantial differences in morphological characteristics of the semisolid slurry.     

Mechanisms of Solidification Structure Improvement of Ultra Pure 17 wt% Cr Ferritic Stainless Steel by Ti,Nb Addition

The grain structures and the precipitates in the solidification microstructure of the ultra pure 17 wt% Cr ferritic stainless steels with different Ti and/or Nb micro-alloying were investigated both experimentally and theoretically.It was found by the grain structure observation that the addition of Ti or Nb to the steel reduced the grain size (D) and elongation factor (E),and improved the equiaxed grain proportion (P) and globularity factor (ξ).Among the four steels studied,the minimum grain size and maximum equiaxed grain proportion were obtained by jointly adding both Ti and Nb to the steel.The SEM observation indicated that several kinds of precipitations,such as TiN,MC (rich in Nb),Laves phase (Fe2Nb) and so on,formed in the corresponding steels.In addition,the results calculated using the Thermo-Calc software illustrated that TiN precipitates in the liquid at proper Ti and N contents.Meanwhile,the solidification interval (△T) was enlarged by the addition of Ti or Nb,and the effectiveness of Nb was stronger than Ti.Based on the experimental and calculation results,the mechanisms of grain refinement and increment in equiaxed grain proportion were discussed.     

Microstructure Evolution during Solidification of AZ91D Magnesium Alloy in Semisolid

The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.     

Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.Numerical simulations of directional solidification of binary Al–Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part II: Illustrative modelling results and parameter studies

A volume-averaging multiphase solidification model was introduced in Part I. In Part II, illustrative simulations are made for two benchmarks, a unidirectional solidification benchmark and a cylindrical ingot casting, using a binary Al–Cu alloy. For the case of unidirectional solidification the competing growth of columnar and equiaxed structures, evolution of different phase regions, solute redistribution, and the influence of grain sedimentation and melt convection are analyzed in detail. The columnar-to-equiaxed transition (CET) is investigated, with important insights derived from the CET prediction. The new features of the model and its applicability to industrial-type castings are demonstrated with simulations of a cylindrical ingot casting. This is done in both a 2D axisymmetric and a full 3D geometric domain to demonstrate the ability of the model to produce consistent results. The main features of the model that are verified include tracking of the columnar primary dendrite tip, nucleation of equiaxed grains ahead of the columnar tip front, hydrodynamic and solutal interactions between the equiaxed and columnar structures, the columnar-to-equiaxed transition (CET), melt convection and grain sedimentation, and macrosegregation and the final macrostructure. With appropriate modelling parameters the typical columnar-equiaxed macrostructure observed in experiments can be reproduced. Uncertainties due to model parameters and assumptions are addressed and discussed.