Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Due to the extensive applications in automotive and aerospace industries of Al-7Si-Mg casting alloys, its understanding of the dendrite microstructural formation is of great importance to control the desirable microstructure and thereby to modify the performance of castings. In this work, through analyzing the measured cooling curves in different cooling conditions of Al-7Si-0.36Mg ternary alloy during sand casting, a theoretical nucleation model correlated maximum nucleation undercooling with the nucleation density is proposed. Besides, a 2D and 3D cellular automaton (CA) model allowing for the quantitatively predicting dendrite growth of ternary alloys is presented. This model introduces a new tracking neighboring rule algorithm to eliminate the effect of mesh dependency on dendrite growth. The thermodynamic and kinetic data needed in the simulations is obtained by coupling with Pandat software package in combination with thermodynamic/kinetic/equilibrium phase diagram calculation databases. This model has also taken account the multi-component diffusion, constitutional undercooling, curvature undercooling, dendrite preferential growth angles as well as the effect of interactions between the alloying elements etc. This model is applied to quantitatively simulate the dendrite growth with various crystallographic orientations of Al-7Si-0.36Mg ternary alloy in 2D and 3D during polycrystalline solidification, and the predicted secondary dendrite arm spacing (SDAS) shows a reasonable agreement with the experimental results. The experimental observed complicated and diverse dendrite microstructure occurring in solidification process can be well reproduced by this 3D-CA model which has considered the effects of various preferred growth orientations, the interactions of adjacent dendrites as well as the influence of S/L interface anisotropies. The simulated results effectively demonstrated the abilities of this model in prediction of dendrite microstructure in ternary alloys.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Gas turbine plays an important role in energy and aviation, among which the turbine blades are the key components. Ni base superalloys are the preferred material to manufacture blades due to their high temperature strength, microstructural stability and corrosion resistance. As a conventional directional solidification method, high-rate solidification (HRS) is used to produce columnar grain and single crystal blades. However, there are several problems when HRS is scaled to cast industrial gas turbines (IGT) components. In recent years, several possible techniques are being proposed for large IGT blades. The liquid-metal cooling (LMC) is one of the best methods among them, which improves heat extraction by immersing the casting and the mold into a container of metal coolant with low melting temperature as they are withdrawn from the heating zone. Unfortunately, the trial and error method is time and money cost and lead to a long R&D cycle. Therefore, numerical simulation plays an important role to optimize the process, and enhance the productivity in LMC directional solidification. In this work, mathematical models for dynamic heat radiation and convection boundary of LMC process are established to simulate the temperature fields. Cellular automaton (CA) method and KGT growth model are used to describe the nucleation and growth. The pouring experiments are carried out. The accuracy of the model is validated by the cooling curves and microstructure. Moreover, the liquid- metal cooling directional solidification process is discussed in more detail, including primary dendrite arm space (PDAS), secondary dendrite arm space (SDAS), mushy zone and microstructure, etc.. Simulation and experiment results are compared in the work. This study indicates that simulation and experimental results agree with each other well. The maximum error of temperature is less than 5 percent and the morphologies of grains are similar. The withdrawal rate has an important influence on the shape of mushy zone and dendritic structure. A concave mushy zone is formed and the grain tends to convergent under an excessive withdrawal rate. However, the mushy zone has a convex shape and the grain is divergent under a smaller withdrawal rate. A proper withdrawal rate is found to obtain smooth mushy zone, improve the parallelism of grains, and refine the dendritic structure.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The aero turbine is spun by high-temperature and high-pressure burning gases. The practice has proven that the directional solidification (DS) turbine blade with perfect column grains has still excellent high-temperature performance in this kind of working environment. This means that the size and orientation of column grains have great influence on the high-temperature property and performance of turbine blades. On the other hand, the high-quality blade is not easy to be produced in DS process due to the difficulty of obtaining the desired temperature field needed to produce the grains with ideal morphology. In addition, the growth of columnar grains in the wide- chord hollow guide blade is obstructed by the complex camber and the platform. How to produce turbine blades with desired microstructures is the key problem in the DS process. Numerical simulation of the DS process is an effective way to investigate the growth and the morphology of the grains and hence to optimize the process. In this work, a mathematical-physical model for simulating the DS process of wide-chord blade is established in which nucleation and grain growth in the blade in the DS process are modeled by the cellular automation (CA) method with multi-scale dynamic bidirectional coupling technology. Some general analytic indicators are proposed to assess the morphology of mushy zone and grains in a blade quantitatively. Based on the simulated results by using the usual starter blocks 1, 2 and 3, a new starter block is designed considering numerically controlled cutting. Temperature fields and grains in DS processes and corresponding indicators at different withdrawal rates for above 4 starter blocks are numerically predicted to investigate the influences of varying these technological parameters, and hence to determine the influence mechanism to the DS process. For comparison, the DS validation experiments by using starter blocks 1, 2 and 3 have been carried out. The numerical and experimental results agree well, their morphologies including those faulty grains are similar. It is found that higher withdrawal rate leads to larger concavation of mushy zone, but the effect of chill is stronger than that of withdrawal rate if the contact area between casting and chill plate is large enough. Better grain structure in a blade is achieved by starter block 3 than by starter blocks 1 and 2. By starter block 4, the amount of column grains is larger and the amount of lateral grain boundaries is smaller, compared with that of starter blocks 1, 2 and 3. Therefore higher withdrawal rate could be adoptable without excessive concavation of mushy zone, resulting in parallel column grains, finer dendrites in the blade, and much higher blade productivity. Optimum withdrawal rates are also determined for starter blocks 3 and 4.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：现代航空发动机及工业燃气轮机的快速发展对单晶叶片的性能提出了更高的要求。选晶器是单晶叶片铸件的重要部件，其关键的结构包括引晶段和螺旋段。本研究从实验及模拟角度对比研究了引晶段的晶粒密度和取向随晶粒生长高度（研究位置距试样底面的高度）的变化规律，并给出了引晶段参数的设计准则。采用EBSD晶体取向成像技术获得了引晶段截面的晶粒形貌和取向极图；采用CA-FD方法，针对单晶定向凝固过程进行数理建模与仿真，实现了凝固过程的三维宏观温度场与微观组织生长的模拟计算。从宏、微观角度解释了定向凝固过程的晶粒竞争演化行为，揭示了引晶段晶粒竞争生长的本质规律，为引晶段设计提供理论支持。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：螺旋选晶器是生产单晶叶片及确保晶体完整性的关键部件，其主要结构包括引晶段和螺旋段。本工作研究了螺旋段在选晶过程中的作用。基于金相及EBSD实验结果，指出在晶粒竞争生长过程中，晶粒二次臂取向和初始位置分布特征综合决定了晶粒的竞争优势。总结提出了选晶过程的两个几何限制机理: 水平方向的二次臂竞争促进作用和竖直方向的一次臂限制作用。两种模型成功解释了螺旋段对选晶过程的作用。采用修正的元胞自动机(MCA)技术，对螺旋段的晶粒竞争生长过程进行了模拟。研究了晶粒结构及晶体取向随定向凝固过程的变化规律，并与实验结果进行了对比，两者吻合较好。基于模拟与实验的研究，获得了螺旋参数对选晶行为的影响规律及作用机理，提出了选晶器螺旋段的设计准则。
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