微观偏析建模与仿真的研究进展

微观偏析数值模拟是材料成形过程数值模拟研究的一个重要分支。论述了微观偏析数值模拟研究的重要性和研究进展。分析了晶粒域微观偏析模型和界面域微观偏析模型的建模思想、各自特点及其与相关模型的耦合方法。指出了当前微观偏析数值模拟研究中存在的问题，展望了这一领域未来的研究前景和发展方向。     

合金凝固微观偏析模型化研究进展

全面综述了合金凝固微观偏析模型化方面的研究,介绍了该领域内的代表性工作以及最新的研究进展,包括典型的二元合金微观偏析模型、多元合金偏析模型以及微观偏析模型化与相图计算耦合方面的工作。重点关注了枝晶形貌、固相反扩散、热力学数据获取效率等因素对微观偏析模拟的影响,并指出了该领域需要进一步研究的问题。     

等轴晶移动对宏观偏析影响的数值模拟

建立了自由等轴晶移动对宏观偏析影响的数学模型,对铸锭凝固过程中的对流和溶质分布进行了数值模拟.在模型中按照临界固相分数将糊状区分为紧密树枝晶和自由等轴晶两个不同的区域.对带冒口铸钢锭的宏观偏析进行了数值模拟,并同实验结果进行了比较.与假设糊状区内固相静止的模型相比,考虑等轴晶移动的模型得到的溶质分布结果与实验结果更接近.在凝固过程中,等轴晶随液体流动并在铸锭的底部中心聚集,在凝固后的铸锭中形成锥形的负偏析.还发现,在铸锭的中心靠上的区域形成正偏析,在铸锭的外部区域形成负偏析.     

Fe-C二元合金凝固过程枝晶偏析的数值模拟研究

利用元胞自动机法对Fe-C二元合金过冷熔体中枝晶的生长过程进行了模拟,其中明确地将液相溶质浓度与固相溶质浓度区分开,并探讨了冷却速率、液相溶质扩散系数以及固相溶质扩散系数对枝晶偏析的影响规律。模拟结果表明:随着冷却速率的增大,固相中溶质浓度波动不断加剧,且枝晶偏析程度增大,但偏析程度的增大趋势逐渐减缓;当液相扩散系数变化时,液相溶质扩散系数越小,其微观偏析越严重,此外,固相中溶质的波动随液相扩散系数的增大而减小;当固相溶质扩散系数变化时,在枝晶中心附近,微观偏析程度基本保持一致,而在远离中心点的外围区域,微观偏析程度的波动十分明显。     

等轴晶移动条件下金属凝固温度场、流场及溶质分布数值模拟

本文利用作者提出的固相移动条件下金属凝固传热、传质及动量传输数学模型对砂型铸造 Al-4.5%Cu 合金铸锭凝固过程进行了数值模拟研究与试验验证。模拟冷却曲线及宏观偏析与试验结果基本相符。     

板料成形数值模拟中网格结点编号优化算法研究

提出一种用于板料成形数值模拟的网格结点编号优化算法.该算法用各个结点的相邻单元结点编号总和除以该结点相邻单元数,所得的结点商作为重新结点编号的依据,对板料成形模拟的四边形单元结点编号进行优化,减少了带宽,减少有限元的计算贮存量,缩短了板料成形数值模拟的计算时间.     

金属钨坯改性元素的偏析

液－液混合法是提高灯用钨丝质量的重要人法。本文不用扫描电镜（ＳＥＭ）对影响钨丝质量的金属钨坏晶界、气孔的显微组织进行了研究；对晶界、气孔及基体内杂质含量采用扫描电镜Ｘ射线能谱分析进行测量。结果证明：液－液混合钨坯的显微组织及杂质成分的分布是很均匀的：但是，杂质偏析不仅在晶界和气孔，而且在边部气孔都能被检测出来。这种微弱的第二相偏析是钨坏结构与性能改变的重要原因。掺杂比超过１／１时，杂质偏析出现极限值。     

基于多元非线性回归分析的Cu-Ag合金偏析预测模型研究

目的研究Cu-Ag合金金属型铸造过程中,铸造工艺对宏观偏析的影响规律。方法通过数值模拟技术和实际浇注试验,对Cu-Ag二元合金金属型铸造过程中的宏观偏析缺陷进行分析,研究主要凝固参数,即冷却速率和温度梯度对宏观偏析程度的影响规律。在此基础上,利用多元非线性回归分析方法对所获得数据进行分析,明确主要凝固参数与宏观偏析程度之间的定量关系,建立Cu-Ag合金金属型铸造过程中的宏观偏析预测模型。结果采用研究获得的最佳浇注工艺方案,即浇注温度为1100℃、浇注时间为120 s、铸型温度200℃、涂料厚度为1.5 mm进行实际浇注,所获得铸件中的宏观偏析缺陷得到了明显的改善。结论降低浇注速度可以有效提高冷却速度和温度梯度,从而有效减小宏观偏析的倾向。     

真空二极管阴极表面场致发射模型的研究

为研究真空二极管阴极表面场致发射模型的算法及数值模拟参数对场致发射过程的影响,建立了计算阴极表面电场强度的有限差分近似模型和高斯定理模型,并基于高斯定理模型自行编程对不同间隙距离的二极管进行模拟计算。模拟得到了场致发射过程中阴极表面电场随时间的演变特性及阴极表面稳态电场与外加电场之间的关系,还将阴极表面稳态电场的模拟结果与理论分析结果进行了对比。研究结果表明,空间网格划分数目的多寡对基于高斯定理场致发射模型计算得到的阴极表面电场的影响不大;每个宏电子包含真实电子的数目、二极管间隙距离、二极管外加电场强度等参数均会对数值模拟结果产生显著影响。     

ZL201合金半固态压铸成形过程数值模拟

为了解半固态压铸过程中浆料充型规律及其流动特点，本文采用AnyCasting铸造仿真软件特有的半固态触变功能模块(Bingham粘度模型)对半固态ZL201铝合金的触变充型过程进行数值模拟，研究慢、快压射速度及切换时间对半固态触变压铸充型过程的影响，对最优充型条件下的铸件微观组织及力学性能进行模拟研究，并进行试验对比.数值模拟结果显示，ZL201合金半固态触变压铸成形在浆料温度600 ℃、模具温度240 ℃时、低速压射速度0.1 m/s、且在1.5 s后进行速度切换、高速压射速度为1 m/s时，所得铸件维氏硬度最大可达72HV，平均抗拉强度为208 MPa.按照该工艺条件成型的成形件显微组织致密，测得其平均抗拉强度为212.5 MPa、平均硬度值为70.8HV, 性能较高，与模拟结果符合较好.     

0Cr19Ni9 TIG焊接接头的抗蚀性及电弧重熔

研究了低碳奥氏体不锈钢０ｃＲ１９Ｎｉ９手工钨极氩弧焊（ＴＩＧ）焊接接头表面抗蚀性能及电弧重熔对其抗蚀性能的影响。实验结果表明,受焊接热循环的作用,焊接接头热影响区的抗蚀性能,以及焊缝金属的抗蚀性能,较母材有所降低。但经微束等离子弧表面重熔后,由于接头表面重熔层的快速凝固作用,细化了支组织,减小了显微的,抑制了碳铬化合物在晶界的沉淀析出,焊接接头的抗蚀性能得以显著提高。     

Crystal growth in two-layer fluid systems in microgravity

A numerical study of the crystal growth in zero gravity with the use of an additional liquid layer for the ampoule-free variant of zone melting (floating-zone method) has been conducted. The crystallization process was investigated both without the magnetic-field action and with a static magnetic field at two directions of the magnetic field vector. A classification of the flow structure depending on the dominance of the thermocapillary effect on the free surface or at the interface between the liquids has been made. The radial microsegregation of the dopant for the above crystal growth methods with several combinations of physical parameters of the liquids has been investigated. The dependences of the radial microsegregation of the dopant on the flow conditions and the strength and sense of the magnetic field have been determined. It has been shown that the strongest effect of a decrease in the microsegregation of the dopant is achieved for flows with a dominance of the thermocapillary effect on the free outer boundary of the liquid two-layer system at a certain magnetic field strength.     

Computer simulation of solidification and microsegregation in presence of particles

A computer simulation was used to study the influence of particles on microsegregation during solidification. Particles in the melt can affect the solidification microstructure by changing the cooling curves, acting as barriers to solute diffusion, reducing the metal volume, affecting the coarsening process, etc. The computer simulation model used in the present work calculates the effect of particles of different thermal properties on the cooling curves and the consequent changes to microsegregation and dendrite arm spacing. The microsegregation calculations are valid for cases where the interparticle spacing is much smaller than the dendrite arm spacing. By using the simulation, it was possible to study various contributing factors in isolation as well as in combination so that the relative significance of each factor could be evaluated. It was observed that the reduction of liquid volume by the presence of the particles was the largest contributing factor to the influence of particles on the solidification microstructure. Thus, the changes in matrix microstructure of cast metal matrix composites depend more on the volume fraction of the reinforcing particles than on the properties of the particles themselves. Irrespective of the thermal properties of the particles, the dendrite arm spacing and microsegregation in the composite matrix was seen to be less than in the unreinforced alloy solidified under the same external conditions.

MST/3405     

Numerical modelling of fatigue crack initiation and growth of martensitic steels

This paper presents a numerical simulation of micro‐crack initiation that is based on Tanaka‐Mura micro‐crack nucleation model. Three improvements were added to this model. First, multiple slip bands where micro‐cracks may occur are used in each grain. Second improvement deals with micro‐crack coalescence by extending existing micro‐cracks along grain boundaries and connecting them into a macro‐crack. The third improvement handles segmented micro‐crack generation, where a micro‐crack is not nucleated in one step like in Tanaka‐Mura model, but is instead generated in multiple steps. High cycle fatigue testing was also performed and showed reasonably good correlation of proposed model to experimental results. Because numerical model was directed at simulating fatigue properties of thermally cut steel, edge properties of test specimens were additionally inspected in terms of surface roughness and micro‐structural properties.     

Effects of Pulsed Magnetic Field on Microsegregation of Solute Elements in a Ni-Based Single Crystal Superalloy

The effects of a pulsed magnetic field(PMF) on the microsegregation of solute elements during directional solidification of a Ni-based single crystal superalloy were experimentally investigated,and the results show that the PMF significantly affects the microsegregation of Al,Ti,Co,Mo and W elements in the alloy.However,the distribution behavior differs for both positive and negative segregation elements.With the PMF,the microsegregation of negative segregation elements,Co and W,was restrained effectively,while that of positive segregation elements,Al,Tiand Mo,was aggravated.A segregation model was established to reveal the distribution mechanism of the elements with PMF.It is considered that,under the action of PMF,the jumping of solute atoms from the liquid phase to solid phase is hindered,but the jumping of solute atoms from the solid phase into liquid phase is promoted during solidification.As a result,the effective distribution coefficient of the solute atoms is reduced,which leads to the reduction of microsegregation of negative segregation elements and aggravation of microsegregation of positive segregation elements.