DD6合金选晶器定向凝固的数值模拟与实验

使用有限元数值模拟和实验方法对DD6合金选晶器定向凝固过程的温度影响、晶粒组织以及晶粒生长进行了研究。结果表明,温度场模拟与选晶器定向凝固过程的温度变化基本相符,晶粒组织生长的模拟能够反映晶粒竞争生长的实际情况,与实验试样基本一致。由于取向与温度梯度不一致晶粒的位置被取代,使得引晶段中的晶粒平均半径随着高度增加而增大,晶粒密度不断减小,两者有负指数关系。对比了螺旋段不同高度截面EBSD晶粒取向实验成像与晶粒模拟结果,组织模拟与实验结果较为吻合。通过分析螺旋通道中的晶粒竞争生长关系,位于螺旋通道内侧和上部的晶粒更具有位置优势,容易被选晶器选出。     

镍基单晶高温合金DD3制备过程中晶粒演化和选晶行为的EBSD研究

采用选晶法在改进的Bridgman定向凝固炉上进行镍基单晶高温合金DD3定向凝固实验,研究了选晶法制备镍基单晶高温合金过程中晶粒组织演化和螺旋选晶行为,采用电子背散射衍射(EBSD)技术对晶粒的竞争过程进行分析.结果表明,选晶器引晶段的主要作用是优化晶粒取向,获得取向良好的<001>定向凝固组织.在距激冷板36.1 m...     

单晶高温合金螺旋选晶器选晶行为的模拟和试验研究

通过试验和模拟的方法分析了选晶器引晶段的晶粒组织演化规律和不同情况下等温线的形态分布,以及倾斜等温线下晶粒竞争生长的过程,同时研究了螺旋段的选晶行为。结果表明,引晶段的不同高度具有不同的组织形态,引晶段主要靠晶粒间的竞争生长作用优化晶粒取向,不同形态的等温线下晶粒的优先生长方向不同。螺旋段主要靠螺旋通道对枝晶生长的几何约束选择晶粒,确保只有一个晶粒进入型腔生长成为单晶组织。     

镍基单晶高温合金螺旋选晶过程的数值模拟

采用ProCAST & CAFE模型,模拟了选晶法制备镍基单晶高温合金DD3过程中的晶粒组织演化。结果表明,选晶器引晶段的主要作用是优化晶粒取向,随着距引晶段底部距离的增加,<001>晶向与热流方向偏离角较大的晶粒逐渐被淘汰,到达距引晶段底部26 mm处时,<001>取向的晶粒与热流方向之间的夹角平均值小于10°。螺旋选晶段的主要作用是获得单一晶粒,其对晶粒的取向没有优化作用。螺旋选晶过程中,靠近螺旋通道外侧的晶粒因受到螺旋通道较强的几何阻碍逐渐失去生长空间,而靠近螺旋通道内侧的晶粒通常被选为最终的单晶。螺旋选晶段对晶粒的选择或淘汰与该晶粒在螺旋通道内所处的位置（螺旋通道的内侧或外侧）有密切关系,而与晶粒的晶体学取向没有必然联系     

单晶高温合金定向凝固螺旋选晶器数值模拟与实验研究

通过Pro CAST模拟软件数值模拟了温度场对螺旋选晶器引晶段晶粒生长的影响。在模拟和实验的基础上,研究了引晶段晶粒取向、密度和尺寸的变化规律。结果表明,随着引晶段高度增加,晶粒的取向得到优化,晶粒密度减小,晶粒尺寸增大,为引晶段的高度设计提供了理论支持。从宏观和微观角度解释了螺旋选晶器螺旋段晶粒竞争生长的演化过程,为实际生产提供了理论依据。     

基于ProCAST的大尺寸空心定向叶片数值模拟

采用ProCAST软件研究了DZ409合金大尺寸空心定向叶片精密铸造过程的温度场,分析了固液界面前沿的变化,研究了定向凝固过程中浇注温度、型壳温度、拉晶速率对温度场、凝固参数、应力场的影响,并通过G（温度梯度）/R（凝固速率）分析优选工艺模拟凝固组织。结果显示,当浇注温度和模具温度均为1 560℃时,铸件不同位置的温度梯度与G/R值较大,有利于定向凝固柱状晶的生长。低拉晶速率可以获得较大的温度梯度、G/R值,有利于定向柱状晶粒生长。通过选择合适的浇注温度和拉晶速率,可显著改善叶身的晶粒取向。对模拟得到的较优工艺进行试验验证,发现晶粒取向与模拟结果一致。     

不同制备方法对单晶高温合金取向特征的影响（英文）

在高温度梯度定向凝固炉中分别采用选晶法和籽晶法制备单晶高温合金,并采用XRD测定单晶的取向特征。结果表明,采用选晶法制备单晶高温合金时,在激冷板上随机形核的晶粒经过起晶段的竞争生长和螺旋段的选晶作用后,最终获得具有择优取向001的单晶。采用籽晶法制备单晶高温合金时,通过重熔籽晶的外延生长,所获得的单晶取向与籽晶取向保持一致。     

定向凝固晶粒竞争生长的研究进展

竞争生长是材料微观组织演化过程中普遍存在的现象,不同组织间(包括相、枝晶、晶粒等)均可能存在竞争生长,这些竞争作用对最终微观组织的形成及力学性能具有重要影响。不同取向晶粒间的竞争生长控制是材料微观组织调控的重要环节,尤其是对定向单晶叶片的制备,不同取向晶粒间的竞争生长是决定能否成功获得完整单晶结构的关键因素。近年来,一方面单晶材料制备要求的不断提高,迫切需要有关晶粒竞争生长机制及规律方面的理论性指导,另一方面经典晶粒竞争生长理论预测结果与实验观测并不相符,有时甚至可能完全相反,这使得定向凝固过程中不同取向晶粒间的竞争生长问题日益成为一个热门的研究课题。本文首先回顾了Walton-Chalmers模型的晶粒竞争生长机制及其所受到的严重挑战,然后分别对定向凝固二维条件下汇聚生长和发散生长及三维条件下晶粒竞争生长的研究现状进行了述评,并介绍了本课题组近年来基于相场法数值模拟在定向凝固晶粒竞争生长研究方面的工作进展,最后对定向凝固过程中不同取向晶粒之间的竞争生长研究进行了总结与展望。     

不同制备方法对单晶高温合金取向特征的影响

在高温度梯度定向凝固炉中分别采用选晶法和籽晶法制备单晶高温合金,并采用XRD测定单晶的取向特征。结果表明,采用选晶法制备单晶高温合金时,在激冷板上随机形核的晶粒经过起晶段的竞争生长和螺旋段的选晶作用后,最终获得具有择优取向<001>的单晶。采用籽晶法制备单晶高温合金时,通过重熔籽晶的外延生长,所获得的单晶取向与籽晶取向保持一致。     

薄板试样定向凝固枝晶竞争生长

深入理解高温合金定向凝固杂晶演化具有重要的科学和工程意义,其中枝晶竞争生长是揭示晶界演化与淘汰的关键环节。定向凝固双晶竞争生长的实验研究证实了传统尖端过冷度理论的局限性,定量相场模拟的研究获得了大量统计规律,促进了人们对定向凝固双晶竞争生长的理解。本研究在薄板状试样的定向凝固过程中,同时获得了大量的不同取向竞争生长的枝晶,通过拼接金相显微组织同时获得多个晶界的取向演化规律。结果表明,枝晶列竞争生长主要通过晶界两侧不同取向一次枝晶臂及二次枝晶臂的交互作用实现。在汇聚生长过程中,择优枝晶一次臂阻挡非择优枝晶,并且择优枝晶自身无法形成三次臂,导致晶界方向沿择优枝晶生长方向。发散生长过程中,处于温度梯度方向一侧时,两列枝晶的竞争生长方式以非择优枝晶新生三次臂为主,晶界以沿择优枝晶方向生长为主;当两列枝晶处于温度梯度不同侧时,两列枝晶均有三次臂新生的概率,其晶界在两列枝晶取向之间。实验统计结果给出了晶界随两列竞争生长枝晶的取向的变化规律,与定量相场模拟结果吻合。商业软件ProCAST的CAFE计算晶粒定向竞争生长晶界演化统计结果与实验结果存在较大偏差,其主要原因是CAFE仅给出晶粒尺度的轮廓信息,未考虑溶质扩散及二次臂的竞争。本研究获得的薄板状试样枝晶竞争生长的统计规律,对认识高温合金定向凝固过程中晶界取向的演化具有重要的意义,对柱状晶叶片的制备具有指导意义。     

抽拉速率对螺旋选晶器选晶行为和晶体取向演变的影响

采用宏观-微观耦合的元胞自动机（CAFE）方法,研究了不同抽拉速率对螺旋选晶器的选晶行为和晶体取向演变的影响,并通过实验对仿真结果进行了验证。结果表明：在相同的抽拉速率下,随着与激冷面距离的增大,枝晶数目逐渐减少,一次枝晶臂间距逐渐增大。随着抽拉速度由2 mm/min增加到8 mm/min,一次枝晶臂间距逐渐减小,枝晶茎数增加,枝晶间距逐渐减小,枝晶组织逐渐细化。在模拟过程中,在螺旋选择器启动块的3种抽拉速率下,固态组织的数量、面积和颜色都是相似的;在螺旋部分,在5 mm/min提取速率下晶体的微结构数量比其他2个提取速率下的少,并且面积更大,颜色更浅。从选晶器的晶粒组织演变和金相组织分析结果可以得出,中等抽拉速率（5 mm/min）下选晶器的选晶效率优于其他2种抽拉速率。     

Progress on modeling and simulation of directional solidification of superalloy turbine blade casting

Directional solidified turbine blades of Ni-based superalloy are widely used as key parts of the gas turbine engines.The mechanical properties of the blade are greatly influenced by the final microstructure and the grain orientation determined directly by the grain selector geometry of the casting.In this paper,mathematical models were proposed for three dimensional simulation of the grain growth and microstructure evolution in directional solidification of turbine blade casting.Ray-tracing method was applied to calculate the temperature variation of the blade.Based on the thermo model of heat transfer,the competitive grain growth within the starter block and the spiral of the grain selector,the grain growth in the blade and the microstructure evolution were simulated via a modified Cellular Automaton method.Validation experiments were carried out,and the measured results were compared quantitatively with the predicted results.The simulated cooling curves and microstructures corresponded well with the experimental results.The proposed models could be used to predict the grain morphology and the competitive grain evolution during directional solidification.     

Modeling of grain selection during directional solidification of single crystal superalloy turbine blade castings

Single crystal superalloy turbine blades are currently widely used as key components in gas turbine engines. The single crystal turbine blade casting’s properties are quite sensitive to the grain orientation determined directly by the grain selector geometry of the casting, A mathematical model was proposed for the grain selection during directional solidification of turbine blade casting. Based on heat transfer modeling of the directional withdrawing process, the competitive grain growth within the starter block and the spiral of the grain selector were simulated by using the cellular automaton method (CA). Validation experiments were carried out, and the measured results were compared quantitatively with the predicted results. The model could be used to predict the grain morphology and the competitive grain evolution during solidification, together with the distribution of grain orientation of primary <001> dendrite growth direction, with respect to the longitudinal axis of the turbine blade casting.     

螺旋选晶器对单晶高温合金晶粒取向的影响(英文)

对于航天推进和发电燃气轮机,为了实现更高的效率和性能并同时降低成本和排放,需要发展先进的镍基单晶高温合金。随着合金的发展,合金的铸造技术已经从定向凝固(DS)发展到单晶(SX)。单晶取向与择优取向的偏差已经成为单晶高温合金最重要的铸造缺陷之一。在自行设计的高温度梯度定向凝固装置中,采用改进的Bridgeman法,用螺旋选晶器拉制出DZ417G合金的单晶。使用XRD和EBSD对其取向进行分析研究。结果表明:较小角度的螺旋选晶器可以有效地减少单晶取向与择优取向的偏差。     

Effect of spiral shape on grain selection during casting of single crystal turbine blades

Abstract

In the development of turbine blades, solidification structures have progressed from equiaxed to directionally solidified (DS) and then to single crystal (SX). The transition from DS to SX was achieved by introducing a grain selector which consists of two parts: a starter block referring to the grain orientation optimisation and a spiral part to ensure that only one grain can eventually survive and grow into the blade. With emphasis on the spiral selector, the microstructure evolution and grain competitive growth is visualised using a coupled macroscale ProCAST and mesoscale cellular automaton finite element (CAFE) model in this study. To improve the efficiency of the spiral grain selector and to save cost in casting, the effects of spiral geometries on the grain selection are investigated. Simulation results reveal that the spiral becomes more efficient in grain number selection with a smaller spiral thickness (d _T) and a larger spiral diameter (d _S).     

Grain structure and texture evolutions during single crystal casting of the Ni-base superalloy CMSX-4

The solidification grain structure and texture evolutions during single crystal (SX) casting of the advanced Ni-base superalloy CMSX-4 have been investigated. In order to understand the development of the solidification grain structure, SX casting experiments were carried out with a specially designed grain selector in a Bridgman directional solidification (DS) furnace. In addition to casting trials, the SX casting process was simulated by a 3-D cellular automaton-finite element (CAFE) model. The predicted solidification grain structure and the texture evolutions were validated by comparison with the microstructural observation and the electron back scattered diffraction (EBSD) results. It was shown that the overall grain structure, crystallographic texture evolution, and the location where the final selection of the single crystal occurs can be predicted well by the present CAFE model. The axial texture evolution of the single crystal was found to be significantly influenced by the grain density at the chill surface. The CAFE predictions also revealed that the geometry of the grain selector plays a significant role in the final selection of the single crystal.     

Structure simulation in unidirectionally solidified turbine blade by dendrite envelope tracking model （ Ⅱ）： model validation and defects prediction

1 Introduction Commonly, a numerical model should be validated before application. Whether a simulation model for dendritic grain growth is reliable strongly depends on two aspects. 1) The model can effectively describe the anisotropy of grain growth. Th…     

Mechanism of competitive grain growth in a curvilinear channel of crystal-sorter during the orientational solidification of nickel-based heat-resistant alloy

Using numerical simulation in the ProCAST program complex, the conditions of the solidification of heat-resistant nickel alloy in curvilinear channels of a ceramic mold have been investigated. It has been shown that, in practically important cases, the vector of the temperature gradient is oriented along the axis of the curvilinear channel. In a spiral crystal selector, a cyclic change in the preferred direction of growth occurs because of the cyclic change in the direction of the vector of the temperature gradient. The fact that the vector of the temperature gradient is almost always directed along the axis of the curvilinear channel makes it possible to govern the orientation of the vector of the temperature gradient in space and, therefore, to obtain a grain with the preferred crystallographic orientation. Based on the results of this investigation, a method of the grain selection with a desired azimuthal orientation is proposed.