1种新的模拟铝合金铸态组织的随机性方法

采用1种新的随机性模拟方法并与宏观传热过程相耦合，对铝合金的微观组织进行了模拟研究。计算中采用了简化的枝晶形状。建立了简化的枝晶形状的物理与数学模型，并提出了1种形状函数来描述晶粒的外部轮廓。基于简化的晶粒形状，采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程。连续形核模型被用来处理异质形核现象，在生长模型中则考虑了枝晶尖端生长动力学和择优生长方向。开发了等轴晶生长的模拟程序，并进行了二维计算。进行了浇注金属型和砂型试样的模拟验证实验。结果表明，对两种不同的工艺，所得到的晶粒组织不同，金属型铸造时得到的晶粒尺寸较小，砂型铸造的较大。模拟结果与金相观察结果相符。     

Numerical simulation of directional solidification of single crystal turbine blade casting

Abstract

As the key parts of turbine engines, single crystal superalloy turbine blades directly determine the engine's performance and service time. In this paper, a mathematical model based on the modified cellular automaton and finite difference method was developed for the three-dimensional simulation of solidification process of single crystal turbine blade castings. Using a ray tracing method, the complex heat radiation among the multiple blade castings and the furnace wall was considered in the model. The microstructure evolution was simulated with the modified cellular automaton method. A discrete layer by layer calculation method was proposed to couple the macro- and microsimulations.

Simulation results show that with proper varying withdrawal rates, it is possible to increase the productivity and avoid the grain defects at the same time for single crystal blade castings. Experiments with constant and varying withdrawal rates were carried out to validate the proposed model.     

New approach to quantifying solidification path in modelling of macrosegregation in multicomponent Al alloys

Abstract

The continuum model Alsim, which computes the macroscopic transport of mass, enthalpy, momentum, and solute in aluminium castings, is being coupled with a new module based on the microsegregation model Alstruc in order to update the solidification path in macrosegregation computations for industrial multicomponent aluminium alloys. The coupling deals with two main challenges; the first being the need for accessing thermodynamical phase diagram data within reasonable limits of computational time and storage. This is solved in Alstruc by approximating the phase diagram data by simple, analytical expressions. The second challenge is that many advanced microsegregation models are restricted to 'closed' volume elements of constant total alloy composition. This is solved by calling the Alstruc module at each nodal point and time step in Alsim and thereby allowing for an 'open system'. The usefulness of the new module is preliminary assessed.     

局部加压铝合金的凝固变化及其数值模拟

局部加压方法是压铸中用于消除铸件厚壁处缩孔缺陷的一种新工艺.该研究通过试验模型测量局部加压过程中金属型、铸件和加压杆的温度变化曲线,分析了传热与凝固现象.结果表明,局部加压使铸件的凝固时间大为缩短,加压杆前端的温度大幅上升,而金属型的温度则无明显变化.利用变网格技术,根据加压深度通过适时修改网格文件、初始条件和边界条件的方法,模拟了铝合金在局部加压过程中的凝固变化,并与试验结果进行了对比和讨论.     

均衡凝固技术在高铬白口铸铁衬板上的应用

运用均衡凝固技术进行高铬白口铸铁球磨机衬板压边冒口的设计及应用,收到了比按照顺序凝固技术进行设计更为理想的效果:铸件无缩孔、缩松现象,操作简单,工艺出品率最高可达到80%以上,大大降低了生产成本,取得了较好的经济效益.近十年来已稳定生产衬板类铸件5000余吨,为探索白口铸铁的生产工艺开辟了新的途径,同时也表明均衡凝固技术作为工艺设计的理论和方法对铸件生产具有普遍意义.     

铝合金铸件中的凝固缺陷形成机理及预测

孔洞与热裂是铸件在凝固过程中最常遇到的典型缺陷。两者均形成于合金凝固末期,与液相补缩不足相关,但又有各自的形成机理。铝合金熔体对氢气有很大的溶解度,凝固过程中又因为溶解度的剧降而析出。孔洞是液相中过饱和的气体压力与凝固收缩引起的压力降共同作用的结果。热裂则是在凝固末期由于铸件收缩受阻而产生的应力以及液相补缩不足而导致的,其不仅与合金性质、铸造条件有关,并且受铸件形状的影响。文章基于近年来该方面的理论研究成果,总结了关于孔洞与热裂的形成机理以及几种目前所应用的经典预测模型,并对这几种模型的理论基础以及所考虑的关键参数进行了分析与讨论,在此基础上提出了未来新模型的研究方向和亟需解决的关键问题。     

采用温度回升法对任意结晶区间的铸件凝固结晶潜热的数值计算

铸件凝固结晶潜热的释放行为与凝固传热、溶质传输及铸造合金种类与成分等诸多因素有关，合金凝固过程中其温度、固相体积分数与液相成分(T-fs—CL)三者之间一般存在着非线性强耦合关系．采用合金凝固传输统一模型及温度回升(补偿)法提出的处理任意结晶温度区间(包括零结晶区间)凝固潜热释放问题的数值迭代计算方法，对不同合金成分与不同固相反扩散效应的二维Al—Cu合金铸件定向凝固传输过程进行了数值计算，表明该方法对于从纯金属到共晶成分的不同成分合金及从Scheil模型到Lever-Rule模型之间的任意凝固模式均是有效的．将该方法推广应用于叶片铸件三维凝固传输过程的T-fs—CL耦合数值模拟仍显示出高的计算效率．通过对计算结果进行的三维图像数据处理，展示了铸件几何形状对凝固传输行为的重要影响．     

大型铸件成形过程数值模拟及工艺优化

用工程湍流模式计算大型铸件充型凝固过程三维速度场、温度场时,对提高模拟计算精度和效率进行了探讨。应用新的计算方法对大型不锈钢和合金铸铁件进行数值模拟,优化其铸造工艺。     

铸件计算机凝固模拟的现状、发展与前景

计算机凝固模拟已经取得令人注目的成就，可以协助设计人员改进工艺设计，提高铸件质量。但是凝固模拟软件需要在铸件凝固的物理模型，计算机数值计算，铸件的三维成形，初始条件和边界条件确定，热物性数据的测量，铸件缺陷的预测及显示等多方面进行改进以更大程度上满足铸造生产的实际需要。在未来很长时期内，凝固模拟作为一个有力的工具，仍然需要有实际铸造工艺设计经验的专家使用才能发挥其作用与潜力。另一方面，铸造生产厂家必须严格生产过程控制，正确使用凝固模拟软件，加强对技术人员的培训和不断积累使用凝固模拟软件的经验，这是生产高质量铸件的需要，也是成功应用凝固模拟软件的重要条件。     

Iterative reverse deformation optimization design of castings based on numerical simulation of solidification thermal stress

In the casting process, in order to compensate for the solidification shrinkage to obtain higher dimensional accuracy of the casting, it is often necessary to modify the original design of castings, and a suitable compensation method has a decisive impact on the dimensional accuracy of the actual casting. In this study, based on solidification simulation, a design method of reverse deformation is proposed, and two compensation methods, empirical compensation and direct reverse deformation, are implemented. The simulation results show that the empirical compensation method has problems such as difficulty in determining the parameters and satisfaction of both the overall and local accuracy at the same time; while based on the simulation results for each node of the casting, the direct reverse deformation design achieves the design with shape. In addition, the casting model can be optimized through iterative revisions, so that higher dimensional accuracy can be continuously obtained in the subsequent design process. Therefore, the direct reverse deformation design is more accurate and reasonable compared to empirical compensation method.

     

Global heat transfer model and dynamic ray tracing algorithm for complex multiple turbine blades of Ni-based superalloys in directional solidification process

High-quality solidification microstructure during directional solidification relies on precise temperature gradient control, so accurate calculation of the temperature field is critical. In this study, a 3D transient global heat transfer model of directional solidification by the Bridgman method based on the finite difference method is developed. The radiation heat in this model is calculated by the discrete transfer method, and a modified method of external surface area for irregular geometric models is proposed to reduce the zigzag shape caused by finite difference grids. Considering the radiative heat transfer between any surface elements of all materials in the directional solidification furnace, a dynamic ray tracing algorithm is developed to simulate the entire process of directional solidification. Then, the simulated results are compared with the theoretical results and experimental results, respectively. Finally, based on the present model and method, the simulation program developed is applied to the directional solidification of actual castings. The simulated results are in good agreement with the experimental results, which indicate that the model and method developed in this study is effective and practical.     

A Free Thermal Contraction Method for Modelling the Heat-transfer Coefficient at the Casting/Mould Interface

Abstract

This study is intended to explore a simple and inexpensive method that is able to model the variation and distribution of the heat transfer coefficient at the casting/mould interface. It has been assumed that, in a rigid mould, the magnitude of interface gap size primarily depends on the thermal contraction of cast metal as it solidifies. Consequently, a free thermal contraction method has been developed to describe the thermal contact phenomena at the casting/mould interface, based on the above assumptions. This method has been used in the solidification simulations for three sand castings of different shapes. The numerical solutions of the simulations agree closely with the experimental results of the thermocouple measurements in the castings and their moulds. For the casting and mould materials involved, the empirical parameters in the equation of the heat transfer coefficient are the same for each of these cases, i.e. independent of the geometric shape of the castings.     

灰铸铁凝固过程结晶潜热处理模型

基于潜热处理的温度回升法,针对灰铸铁凝固过程中具有初晶和共晶两个阶段的凝固特性,提出了适合灰铸铁的带共晶的潜热处理方法。根据Fe-C-Si三元合金相图,采用杠杆定律来计算初晶和共晶的分配系数。将该方法应用于实际灰铸铁件的凝固模拟计算,并将计算结果与不区分初晶与共晶过程的潜热处理方法的计算结果相比较。该方法的计算结果更接近实际凝固情况。     

可视化铸造技术的研究与应用

结合水模拟、计算机模拟和X射线实时观察三个不同发展阶段,叙述了铸件充型与凝固由不可见到可见的过程,初步提出了可视化铸造的概念.它是指将计算机模拟与X射线实时观察相结合,进行铸件浇注系统优化设计的新技术.结合实例说明可视化铸造技术的应用将会克服传统的浇注系统设计引起的卷气、夹杂、冒口尺寸过大等缺陷,实现铸件的近终形铸造.     

计算机对“T”型铸钢件接头凝固特性的研究

本文针对普通铸钢件,用数值模拟法计算研究了不同比例、不同尺寸的“T”型接头在普通砂型铸造条件下凝固时间分布规律、凝固系数变化和拐角冷铁对其凝固过程的影响。得到有益于认识铸件中此类接头凝固特性和热节位置的变化规律,对工艺设计者认识和防止成型过程中产生凝固收缩缺陷是很必要的。     

球墨铸铁的凝固形貌--球墨铸铁基础理论的最新发展(四)

介绍了球墨铸铁的微观和宏观凝固特征,薄壁和厚壁球铁的凝固形貌特点,以及球铁铸件壁厚与凝固时间的关系,同时分析了石墨球数和共晶晶粒数对球墨铸铁凝固品质的影响。     

Modeling characteristics for solidification in single-crystal,investment-cast superalloys

Macroscopic traits of solidification such as thermal gradient, solidification rate, and other criteria functions were extracted from computer modeling data of a single-crystal, investment-cast superalloy. The thermal histories were used to predict the presence or absence of grain defects in the shaped castings. Freckle defects were found on castings with various processing parameters, even though the traditional microstructural defect map predicted no freckles and a consistent microstructure. The findings suggest that the traditional defect map criteria are insufficient to accurately describe the formation of these and other single-crystal grain defects. Hence, additional criteria for porosity formation and other phenomena were investigated and found to be sensitive to changes in solidification conditions and grain defect formation. The additional functions were found to be necessary to fully capture the total energy of all solidification phenomena. When these functions are used in conjunction with the traits of the microstructural defect map, the prediction of defects in single-crystal castings can more accurately describe solidification under the stringent requirements of single-crystal turbine components.     

运用均衡凝固理论解决英国铸件的缩孔问题

用实例给出运用均衡凝固理论,特别是薄、宽、短矩型冒口颈解决英国铸造厂球墨铸铁件的缩孔问题.表明均衡凝固理论作为工艺设计的理论和方法对生产高品质铸件有普遍意义.对计算机凝固模拟进行了讨论.计算机凝固模拟软件可以帮助设计人员改进工艺设计,提高铸件品质.但是认为购置和应用了计算机凝固模拟软件就能产生立竿见影的效果是对计算机凝固模拟软件的误解.在未来很长时期内,计算机凝固模拟作为一个有力的工具,仍然需要有实际铸造工艺设计经验的专家使用才能发挥其作用与潜力,才能达到预期效果.计算机凝固模拟软件与铸造工艺设计的专家系统软件和人工智能软件相结合将能够更好地解决铸造工艺设计问题.     

Ti-6Al-4V合金异形铸件离心力场下的充型与凝固

建立了离心力场下Ti-6Al-4V合金异形铸件充型及凝固过程的数学模型,通过引入附加项将离心力场模型化,采用流函数和涡函数代替描述方程中的压力项,并建立了数学模型的差分格式,总结了离心力场对Ti-6Al-4V合金异形铸件充型和凝固过程的影响规律.结果表明,在离心力场作用下,型腔内的流动由正向流动和反向流动两部分组成,初始温度分布的非对称性使凝固过程中的温度分布始终处于一种非对称状态,从而导致铸件的最后凝固区域与中心线发生偏离.     

灰铁凝固过程中缩孔缩松的预测

通过热分析实验及对灰铁试件的浇注实验,分析了碳当量及孕育对灰铁凝固过程的影响.在动态膨胀叠加法的基础上,综合考虑了初始温度、碳当量、孕育、初晶和共晶体积的收缩与膨胀,建立了灰铁缩孔预测模型.开发了灰铁凝固过程数值模拟分析系统,并对实际的灰铁浇注件进行验证分析,预测结果与实验结果吻合较好.