Influence of platform position on stray grain nucleation in Ni-based single-crystal dummy blade clusters

Stray grains are the most severe of the solidification defects that occur in the industrial single-crystal blade preparation process. In this study, a single-crystal dummy blade cluster with different crystal orientations controlled by the seeding method was prepared, and the influence of the position of the circular platform (relative to the sample and furnace body) on stray grain nucleation was investigated. Results show that the microstructure of the circular platforms could be divided into the center, expansion, and stray grain regions. The inside of the circular platform facing the center of the cluster is more prone to stray grain formation than the outside of the circular platform facing the furnace body. With an increase in the distance between the circular platform and the bottom of the dummy blade cluster, the stray grain region expands, whereas the expansion region narrows. The stray grain is slightly aggravated with increase of the misorientation. Finally, the mechanism underlying the influence of platform position on the formation of stray grains in single-crystal dummy blade clusters is discussed based on the temperature evolution during directional solidification.

     

Multiscale modelling and simulation of single crystal superalloy turbine blade casting during directional solidification process

As the key parts of an aero-engine,single crystal（SX）superalloy turbine blades have been the focus of much attention.However,casting defects often occur during the manufacturing process of the SX turbine blades.Modeling and simulation technology can help to optimize the manufacturing process of SX blades.Multiscale coupled models were proposed and used to simulate the physical phenomena occurring during the directional solidification（DS）process.Coupled with heat transfer（macroscale）and grain growth（meso-scale）,3D dendritic grain growth was calculated to show the competitive grain growth at micro-scale.SX grain selection behavior was studied by the simulation and experiments.The results show that the geometrical structure and technical parameters had strong influences on the grain selection effectiveness.Based on the coupled models,heat transfer,grain growth and microstructure evolution of a complex hollow SX blade were simulated.Both the simulated and experimental results show that the stray grain occurred at the platform of the SX blade when a constant withdrawal rate was used in manufacturing process.In order to avoid the formation of the stray crystal,the multi-scale coupled models and the withdrawal rate optimized technique were applied to the same SX turbine blade.The modeling results indicated that the optimized variable withdrawal rate can achieve SX blade castings with no stray grains,which was also proved by the experiments.     

Temperature evolution and grain defect formation during single crystal solidification of a blade cluster

In order to investigate the asymmetry of thermal conditions during directional solidification, the temperature evolution and correspondingly developed undercooling in a simplified single crystal blade cluster were numerically simulated. Simulation results demonstrate that the temperature distribution at the blade platforms is obviously asymmetrical. On the outside of the blade which directly faces the heating element, the liquidus (TL) isotherms progress relatively smoothly. On the inside of the blades facing the central rod, however, the TL-isotherms are in concave shape and the slope goes upwards to the platform extremities. The average undercooling extent ?T and undercooling time ?t at the inside are much higher than those at the outside. It was then predicted that the inside platform extremities have significantly higher probabilities of stray grain formation compared to the outside ones. A corresponding experiment was carried out and the metallographic examination exhibited the same side- and height-dependence of stray grain formation in the blades as predicted. On the inside of the blades, all platforms are occupied by stray grains, while the platforms on the outside are nearly stray grain free. The simulation result agrees very well with the experimental observation.     

高温合金单晶叶片定向凝固过程的宏微观数值模拟

基于有限元和Panda热动力学数据库建立了单晶叶片真空熔模铸造定向凝固过程的数理模型,对不同工艺下单晶叶片试样凝固过程中的温度场、糊状区演变及枝晶二次臂间距进行了仿真,研究了缺陷形成机理和规律。计算结果与实验吻合良好。计算结果显示,拉速大时二次臂细小,但杂晶产生的趋势加大;拉速小时杂晶不易形成,但二次臂增粗。对实际空心薄壁复杂单晶叶片定向凝固过程的模拟研究表明,二次臂间距在叶身部分分布比较均一, 3.5 mm/min抽拉时有可能在缘板处产生杂晶。采用变拉速工艺,不仅可避免杂晶缺陷,还能保证工件大部分枝晶细小,提高生产效率和成品率     

一种镍基单晶叶片叶身-缘板转接区铸造缺陷的形成机理

某种铸态镍基单晶叶片腐蚀后,观察到叶身-缘板转接区存在一种形状规则的条带状杂晶缺陷,且具有方向性,其长度方向平行(或垂直)于叶身和缘板的枝晶生长方向。采用光镜法(OM)、电子探针(EPMA)和电子背散射衍射技术(EBSD),分别研究了缺陷的组织、成分、取向,采用ProCAST模拟研究了缘板区域的温度场和过冷度分布。研究结果表明,叶身-缘板转接区杂晶缺陷由多个柱状晶粒构成;柱状晶粒的组织和成分与缘板基体一致,与基体的取向差形成小角度和大角度晶界;叶身-缘板转接区的过冷度比缘板边缘小,为缘板上的最后凝固区域。在此基础上,提出叶身-缘板转接区杂晶缺陷的形成机理,为消除该缺陷提供了理论基础。     

Simulation of stray grain formation in Ni-base single crystal turbine blades fabricated by HRS and LMC techniques

The simulation models of the thermal and macrostructural evolutions during directional solidification of Ni-base single crystal(SX) turbine blades under high rate solidification(HRS) and liquid metal cooling(LMC) have been constructed using Pro CAST software, coupled with a 3D Cellular Automaton Finite Element(CAFE) model. The models were used to investigate the tendencies of stray grain(SG) formation in the platform region of turbine blades fabricated by HRS and LMC techniques. The results reveal that the LMC technique can prohibit SG formation by smoothing the concaved isotherm and in turn alleviating the undercooling in the platform ends to let the dendrites fill up the undercooled zone before SG nucleation. The simulation results agreed well with the experimental results, indicating that these models could be used to analyze the macrostructural evolution or to optimize process parameters to suppress SG formation. Using these models, the critical withdrawal rate for casting SX turbine blades without SG formation were determined to be around 75 μm·s~(-1) and 100 μm·s~(-1) for HRS and LMC respectively, suggesting that LMC can be used as an efficient technique in fabricating SX turbine blades without any SG defect formation.     

Analysis of Competitive Growth Mechanism of Stray Grains of Single Crystal Superalloys during Directional Solidification Process

在高梯度定向凝固装置中采用枝晶方向与热流偏离的籽晶制备AM3晶体,分析晶体生长过程中杂晶的形核原理和不同晶粒的竞争淘汰机制。结果表明,枝晶方向与热流偏离造成的过冷可促使杂晶的形成。枝晶生长方向与热流偏离较大的晶粒在一定条件下可淘汰枝晶方向与热流接近的晶粒;晶粒的竞争与淘汰过程受晶粒间枝晶的相对位向、晶粒生长方向与热流方向的夹角大小、晶粒的晶体取向等因素有关。     

Effects of Substrate Crystallographic Orientations on Microstructure in Laser Surface-Melted Single-Crystal Superalloy:Theoretical Analysis

A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed.And the effect of welding conditions on the tendency of stray grains formation during solidification was researched.In the present work,these analytical methods were further extended.Combined with an original vectorization method,a 3D Rosenthal solution was used to determine thermal conditions of the welds.Afterward,the dendrite growth orientation,the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted.Finally,the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot.The results indicate that substrate orientation has some impacts on the crystal growth pattern,dendrite growth velocity,distribution of thermal gradient and stray grain.Based on the research methods proposed in this work,any substrate crystallographic orientation can be studied,and predicted stray grains distribution can be visualized.     

单晶高温合金铸件结构对定向凝固过程中温度场的影响

为了提高单晶涡轮叶片的气冷效果,其冷却通道内部结构越来越复杂,导致制备过程中凝固缺陷出现频率增高,凝固缺陷的形成与定向凝固过程中温度场演变密切相关。采用ProCAST数值模拟的方法研究了铸件结构对定向凝固过程中温度场的影响。结果表明,单晶高温合金铸件尺寸突变和渐变都会导致液固界面位置和温度梯度的变化;相比渐变截面结构,突变截面结构容易形成较大的凝固界面曲率,且边角处形成较大的过冷,更容易导致凝固缺陷的形成。     

Thermomechanical analysis of multi-bead pulsed laser powder deposition of a nickel-based superalloy

In laser powder deposition (LPD) repair of nickel-based turbine blades, hot cracking is the most common defect. The cracking has been found to be associated with thermal stress concentration and low-melting constituents on the grain boundaries. For directionally solidified and single-crystal blades, a positive correlation is established between cracking and the “stray grain” formation. Control of the deposit molten pool shape has been proposed as an effective method to limit the stray grains. However, in multi-bead, multi-layer LPD with a pulsed laser, due to a much more complex bead geometry, appearance of stray grains seems to be random. To obtain insight into cracking and stray grain formation during multi-bead LPD process on nickel-based superalloys, a 3-D transient finite element (FE) model has been developed and a thermomechanical analysis is performed. Time-dependent temperature and thermal strain fields have been predicted. The developed thermal model has been applied to predict and analyze the distribution of thermal stress concentration and the tendency of stray grain formation. The reliability and accuracy of the model are verified experimentally by the measured temperature field profile and the observation of microstructure.     

考虑炉壁温度变化的高温合金叶片定向凝固过程模拟

考虑炉壁温度的变化,基于Monte Carlo射线追踪法建立了高温合金叶片定向凝固过程的温度场计算改进模型.通过法向射线加密提高加热炉炉壁的辐射计算精度,并采用叶片三维有限差分网格和加热炉二维网格混合使用的方法提高计算效率.叶片和炉壁的温度曲线模拟与实验结果吻合良好,改进模型反映了抽拉过程中炉壁温度的变化及其对叶片内部温度分布的影响,提高了叶片温度的模拟精度.     

Stray grain formation in the seed region of single-crystal turbine blades

Seed crystals are frequently used to provide an off-axial 〈001〉 crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth. For more information, contact P.D. Lee, Department of Materials, Imperial College London, Exhibition Road, London SW7 2BP, U.K.; e-mail p.d.lee@imperial.ac.uk.     

高温合金定向凝固界面换热系数逆向求解算法与应用

使用双铂铑(Pt-Rh₃₀/Pt-Rh₆（B）)热电偶采集单晶叶片定向凝固过程温度场数据,基于数值模拟逆向求解出定向凝固过程中高温合金DD6与模壳之间的界面换热系数,并分析了界面换热系数随界面温度变化的关系。采用得到的界面换热系数,基于ProCAST数值模拟,得到的仿真温度场数据与实测温度场数据吻合良好,单晶叶片定向凝固过程数值模拟精度得到有效提高。     

Single Crystal Castability and Undercoolability of PWA1483 Superalloy

Both of the single crystal (SX) castability and undercoolability of PWA1383 superalloy were investigated during the directional solidification and isothermal cooling. In all the six SX parts of a casting cluster, no stray grains were found, revealing a defect-free SX structure. This excellent SX castability of the superalloy was attributed to its good undercoolability. The melting point (T_L) and the critical nucleation temperature (T_N) of the alloy were measured to be 1327 °C and 1306 °C, respectively. The statistic average of the critical nucleation undercooling ΔT_N?=?T_L?-?T_N of the alloy was determined to be about 21 K, exhibiting a relatively great capacity to be deeply cooled to a temperature below the melting point without the onset of solidification.     

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.     

一种常温间接测量液态高温合金表面张力的方法

由于液态合金与铸型界面间的相互作用,液态的自由表面与铸型表面接触处形成弯月形的空间曲面.基于此,提出了一种测量液相线温度下表面张力的方法.对于具有一定宽度结晶范围的高温合金,对定向凝固过程分析发现,凝固后的自由表面弯月形曲面在能够较为精确反映液相线温度下液态合金的弯月形曲面.在常温下测量弯月形曲面,根据Young-Laplace关系计算,就可以得到该合金液相线温度下的表面张力.通过对单晶高温合金DD6进行实验,得到液相线温度下DD6的表面张力约为1.34J·m~(-2).     

Manufacturing Processes for Long-Life Gas Turbines

Dual-alloy turbine wheels produced by solid-state diffusion bonding of vacuum investment cast blade rings of one superalloy to preconsolidated powder metal hubs of a second superalloy have the long cyclic lives characteristic of wrought or powder superalloys combined with the high creep strength and net-shape blades characteristic of cast superalloys. A wide variety of superalloys and turbine configurations are compatible with this technology. Improved temperature capability turbine blades and vanes of the MAR-M 247 alloy made by directional solidification casting processes are now in volume production for Garrett gas turbines. Single-crystal alloys derivative to MAR-M 247 further extend the temperature capability of turbine blades and have been successfully engine tested. These blades are produced by a relatively simple modification of the processes used to manufacture directionally solidified blades.     

一种柱状晶辅助选晶制备镍基单晶高温合金的方法

提出了一种采用柱状晶辅助选晶的制备镍基单晶高温合金的方法。采用金相显微镜和电子背散射衍射（EBSD）等方法表征了单晶制备过程中显微组织及晶体取向。结果表明:柱状晶部分熔化,柱状晶边缘回熔界面附近形成大量杂晶,但只有<001>方向偏离定向凝固方向较小的晶粒能够长大。采用柱状晶选晶的方法能够在螺旋选晶器出口处获得<001>方向偏离定向凝固方向小于8°的单晶铸件。     

Origin of stray grain formation in single-crystal superalloy weld pools from heat transfer and fluid flow modeling

Stray grain formation in laser and electron beam welds on single-crystal alloy CMSX-4 was investigated through heat transfer and fluid flow simulations. The results were combined with a single-crystal growth model and stray grain calculations to investigate the influence of welding parameters on stray grain formation. Stray grain contents were also experimentally measured on laser and electron beam welds prepared over a wide range of parameters. The experimental and simulation results each demonstrate that stray grain content initially increases and then decreases with increasing travel speed. Increases in beam power produce an increase in the stray grain content. The results also demonstrate that restriction of growth along 〈1 0 0〉 directions decreases the magnitude of the temperature gradient and increases the growth rate along the dendrite growth direction (relative to the solidification interface normal). This effect promotes stray grain formation in the pool by increasing the extent of constitutional supercooling.     

新型可脱离式隔热板对燃气轮机叶片定向凝固过程的影响

基于有限元模拟软件,对实际工艺参数条件下的大型空心燃气轮机叶片凝固全过程中的温度场、温度梯度、凝固界面的演变进行了模拟仿真,以此为基础研究了雀斑缺陷形成机理及其规律。模拟结果与实验结果取得了良好的吻合。模拟结果表明,在叶片榫头处温度梯度突然降低是导致此处局部缺陷产生的主要原因。并针对该实验现象设计了一种新型隔热板,大幅度提高叶片凝固界面前沿的温度梯度,从而达到避免叶片榫头处产生雀斑缺陷的目的。