枝晶生长模型对双辊薄带连铸组织模拟的影响

枝晶生长模型的选择对于准确预测金属凝固组织有着决定性作用。LGK和KGT是目前应用较广的模型,分别使用LGK和KGT模型对凝固组织进行模拟比较,选择适用于双辊薄带连铸过程的枝晶生长模型。研究结果表明,KGT模型的模拟结果显示了晶粒吞噬、柱状晶向等轴晶转变(CET)等金属凝固现象,而LGK模型则无此现象。因此KGT模型能更加准确地预测双辊薄带连铸的凝固组织,试验结果的验证也证实了这一结论。     

Micromodel of Simulation on Twin-Roll Continuous Casting Thin Strip Solidification Structure

在研究双辊连铸铝合金薄带工艺凝固过程的基础上,基于金属凝固的基本原理,建立双辊连铸铝合金薄带凝固的异质形核、枝晶尖端的生长动力学(KGT)、柱状晶向等轴晶生长的转变(CET)的解析模型,同时建立基于元胞自动机(CA)的双辊连铸铝合金薄带凝固组织的仿真模型,为双辊连铸薄带凝固组织形成的仿真模拟奠定基础,从而为双辊薄带连铸工艺提供一定的理论指导。同时利用铝合金双辊薄带连续铸轧组织凝固过程验证了数学模拟的可行性。     

真空自耗电弧熔炼γ-TiAl合金铸锭凝固组织模拟

采用CAFE模型对真空自耗电弧熔炼γ-TiA1合金铸锭凝固组织进行了数值模拟,在CAFE模型中分别采用高斯分布连续形核模型和扩展KGT模型描述晶粒形核和枝晶尖端生长速率.研究了熔炼速率、界面传热系数和形核参数对铸锭凝固组织的影响.结果表明,增大熔炼速率或者增大最大形核密度,均有利于促进等轴晶形成,抑制柱状晶晶粒长大;增大平均形核过冷度或者增大界面传热系数,均有利于促进柱状晶的形成和晶粒长大;标准方差过冷度对铸锭凝固组织几乎没有影响.     

铝双辊连续铸轧凝固微观组织的数值模拟(英文)

在研究双辊连铸纯铝薄带凝固过程的基础上,基于金属凝固的基本原理,并运用现代计算机仿真技术建立双辊连续铸轧纯铝薄带凝固的异质形核,枝晶尖端的生长动力学(KGT),柱状晶向等轴晶生长的转变(CET)的解析模型;建立基于元胞自动机(CA)的双辊连铸纯铝薄带凝固组织的仿真模型,为双辊连铸薄带凝固组织的仿真模拟奠定基础,从而为双辊薄带连铸工艺提供一定的理论指导。同时,利用双辊薄带连续铸轧纯铝凝固微观组织过程验证数学模拟的可行性。     

铝合金凝固过程柱状晶向等轴晶转变的研究

研究分析了不同保温温度、冷却水温等条件对铝合金等轴晶形核率、柱状晶形核率、柱状晶向等轴晶转变(CET)位置的影响规律.试验结果表明:降低保温温度、升高冷却水温,会增大等轴晶区的面积；降低冷却水温、降低保温温度会增大铸件柱状晶和等轴晶的形核率.还根据获得的CET位置实验结果,讨论了国内外文献提出的CET位置判据对本研究的适用性,并针对本文的实验条件,提出了具有更高预测精度的CET转变判据.     

ULCB钢焊接熔池凝固过程数值模拟

采用元胞自动机法建立ULCB钢焊接熔池凝固过程的宏微观耦合模型,模拟非均匀温度场下焊接熔池凝固过程的组织形貌演变和溶质场分布,分析不同形核参数和合金成分对焊接熔池枝晶形貌的影响。结果表明,熔池边缘形成柱状晶,熔池中心形成等轴晶,柱状晶和等轴晶相互抑制长大;枝晶生长过程中始终存在着枝晶偏析现象;形核密度和合金成分影响枝晶的形核和生长,随着形核密度和合金成分的增加,焊缝中等轴晶区域增加,晶粒细化。     

Al—Cu合金等轴枝晶组织形成的模拟及计算机可视化

通过解用热焓法处理结晶潜热的二维不稳定热传导方程模拟凝固温度场,利用连续形核模型和枝晶尖端生长的动力学模型,即KGT模型模拟Al-4%Cu合金等轴晶的形成,将凝固温度场的模拟和晶粒组织形成的模拟相耦合,计算了该合金的凝固等轴晶组织特征值,包括晶粒密度和平均晶粒尺寸,并在计算机上实现了晶粒形成过程的可视化;与实验结果进行比较表明,模拟结果与实验结果基本吻合。     

基于CAFE模型研究过冷度/形核数对H13钢微观组织的影响

基于CA-FE方法对工业H13钢进行凝固过程数值模拟,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。以元胞自动机模型为基础,耦合有限元模型,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,研究了过冷度、形核数对凝固组织的影响。结果表明,过冷度处于0.1~5℃,晶区主要是中间等轴晶区为主,柱状晶区占有比极小,且生长趋势受到等轴晶的抑制;当过冷度处于5~7℃,柱状晶生长速度增大,占有比增加,但是中间等轴晶内部组织粗化,形状因子增大,晶粒尺寸和晶粒度偏差表现为先增大,后减小;然而,过冷度达到7~15℃,柱状晶生长受到内部等轴晶的抑制,柱状晶占有优势,且随过冷度增加,晶粒尺寸得到细化;且形核数增加,柱状晶区减小,等轴晶区增大,同时晶粒先增大后减小。     

基于热溶质对流及晶粒运动的柱状晶-非球状等轴晶混合三相模型

基于Eulerian-Eulerian方法,阐述了简化枝晶状等轴晶、柱状晶以及金属液三相完全混合的凝固模型.模型考虑了等轴晶的移动及柱状晶对等轴晶的捕获,跟踪了柱状晶尖端的位置并考虑了等轴晶和柱状晶的相互竞争生长,因此该模型具备了预测柱状晶向等轴晶转变(CET)的能力;为了在不过量增加计算量的前提下提高模型的精度,模型对等轴晶采取了简单的枝晶化处理,即采用简化方法描述等轴晶包络线内固相分数.分别模拟了3.25和25 t钢锭的凝固过程,成功预测了大型钢锭凝固过程所形成的底部锥形负偏析、“类-A型”偏析以及CET等现象.分析认为长细形状铸锭中出现的顶部负偏析区,是由于凝固后期所形成的局部小钢锭及等轴晶在其内部的沉积聚集而成.     

Numerical Simulation of Titanium Alloy Ingot Solidification Structure during VAR Process Based on Three-Dimensional CAFE Method

建立三维多尺度数学模型计算Ti-6Al-4V合金铸锭真空自耗电弧熔炼(VAR)过程中的温度场、流场及凝固组织的形成。该模型包括宏观质量、动量及能量守恒方程和介观晶粒形核生长模型。在传热与流动计算的基础上,模拟铸锭VAR过程中的三维凝固组织的形成。对比计算结果与实验观察可知,两者在晶粒结构与晶粒生长方式方面吻合较好。当考虑VAR过程中熔池表面的辐射换热后,铸锭顶部的柱状晶被很好地呈现。最后,考察了自然对流对铸锭凝固组织的影响,计算结果表明自然对流对柱状晶-等轴晶转变(CET)及晶粒尺寸影响较大,表现为促进CET及细化晶粒。     

Understanding compacted graphite iron solidification through interrupted solidification experiments

While the manufacture of compacted graphite (CG) iron castings has seen significant expansion over the recent years, the growth of CG during iron solidification is still not fully understood. In this work, effort was expanded to experimentally reveal the evolution of graphite shape during early solidification and its relationship to the solid fraction. To this purpose, interrupted solidification experiments were carried out on hypereutectic irons with three magnesium levels. The graphite shape factors were measured and analysed as a function of chemical composition and solid fraction. Scanning electron microscopy was carried out to establish the fraction of solid at which the transition from spheroidal graphite (SG) to CG occurs. It was confirmed that solidification started with the development of SG for all CG irons. The SG-to-CG transition was considered to occur when the SG developed a tail (tadpole graphite). The findings were integrated in previous knowledge to attempt an understanding of the solidification of CG iron.     

Effect of solidification velocity on weld solidification process of alloy tool steel

Abstract

In order to clarify the effect of solidification velocity on the weld solidification process of alloy tool steel during the welding, the information about microstructure evolution was obtained by the concurrent experiments of liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. It was found from the experiments that the solidification mode was transferred from an FA to an A mode at the high solidification velocity. The effect of solidification velocity on the phase selection during solidification between the primary δ-ferrite and γ-austenite was theoretically proved by the Kurz, Giovanola and Trivedi (KGT) model. It is thus explained that the solidification cracking susceptibility of the weld metal of alloy tool steel was enhanced due to the δ to γ transition of the primary phase.     

A freckle criterion for the solidification of superalloys with a tilted solidification front

A freckle criterion developed from M.C. Fleming’s macrosegregation theory was extended to consider the effect of the slope of the solidification front. Such slopes are always associated with the remelting of alloy ingots and are frequently encountered in the directional solidification of single crystal and directionally solidified blades. When experimental verification of the criterion was carried out using the horizontal directional solidification method, dendrite growth rate was found to be a key factor in the formation of freckles. The criterion derived considering the slope of the solidification front describes the occurrence of freckles relatively well. Critical values for freckle formation were obtained for a few nickel-base superalloys. For more information, contact Wan-Hong Yang, Special Metals Corporation, 3200 Riverside Dr., Huntington, WV 25705.     

The effect of solidification behaviour of austenitic stainless steel on the solidification cracking susceptibility

0Introduction Oneofthemainproblemsinweldingausteniticstain lesssteelsishotcracking[1,2].Inweldingofsingle phase austeniticstainlesssteels,thetendencyofhotcrackingis moreserious[3].Inordertopreventhotcrackingofthis kindofmaterial,itwasattemptedtogaintwo p…     

Modeling of alloy casting solidification

Alloy casting solidification processes involve many physical phenomena such as chemistry variation, phase transformation, heat transfer, fluid flow, microstructure evolution, and mechanical stress.1 Simulation technologies are applied extensively in casting industries to understand the effects of these phenomena on the formation of defects and on the final mechanical properties of the castings. As of today, defect prediction is still one of the main purposes for casting solidification simulation. In this paper, we will first present the commonly used microstructure simulation methods, then discuss the predictions of the major defects of a casting, such as porosity, hot tearing, and macrosegregation. The modeling of casting solidification can be chained with later stages of heat treatment such that the resultant microstructure, defects, and mechanical state will be used as the initial conditions of the subsequent processes, ensuring the tracking of the component history and maintaining a high level of accuracy across metallurgical stages.     

凝固微观组织的多层次模拟

通过引入溶质再分配、溶质扩散、界面能各向异性和界面曲率,构建了描述合金凝固微观组织形态演变的元胞自动机模型.在介观和微观尺度上的模拟结果表明,该模型可有效地解决微观组织形成的多尺度问题.模拟结果清晰地再现了与实测结果相一致的枝晶形态和生长现象.根据分形理论采用分维定量比较了模拟结果和实验结果,两者的计盒维数分别为1.703和1.694,阐述了分维定量表征枝晶形貌的物理含义.模拟结果表明熔体过冷度和树枝晶的计盒维数呈近似抛物线关系.     

Numerical simulation of mold-temperature-control solidification

A finite element method（FEM） for the numerical simulation of the columnar part of the mould-temperature-control solidification（MTCS） process was presented. The latent heat was taken into account and 3D transient heat transfer analysis was carried out by using the developed FEM software. The relative errors between the numerical and experimental data are less than 6%. Three MTCS cases were computed with this method. The first case only opens the cooling channels in the bottom of the mold. The second case individually controls the separate 7 groups of cooling channels by giving 7 control points. When the temperature of a control point reaches the preset value of 400℃, the corresponding channel will be opened. The third case opens all the cooling channels at the same time. The results indicate that in the second case, the solid-liquid interface keeps near-planar. The growth velocity of the solid-liquid interface is 0.3-0.4 mm/s, which is greater than 0.1-0.3 mm/s of the first case, performing better than the others. Thus the forming quality and efficiency part interior can be improved by mold-temperature-control and the numerical model is validated. The numerical simulation of MTCS can provide an available tool for the advanced investigation on the defect improvement and the crystal＇s quality.     

气压式熔融沉积快速成形系统

介绍了快速成形制造技术中熔融沉积造型的基本原理及其工艺特点，对气压式熔融沉积快速成形系统的基本结构、加工工艺等做了进一步的说明，详细阐述了系统的各项工艺参数对成形精度的影响，并通过工艺实验验证了从理论角度得出的各项参数之间关系。结果表明，以气压力为挤压动力有效可行，系统工艺简单，成形材料选择范围广泛，不仅可完成传统FDM的快速设计任务，还可完成制造人工生物活性骨的模型加工。