Effect of mould baffle technology on stray grain formation in single crystal blades by integral fabrication based on 3D printing

Stray grains, the most serious casting defect, mainly occur in the platform because of the abrupt transition of the cross-section in the directional solidification of superalloy single-crystal blades. A new mould baffle technology based on 3D printing and gelcasting is proposed herein to reduce the formation of stray grains in the platform. The influence of the proposed mould baffle technology on the temperature field in the platform during solidification was investigated by simulation and experiment. The numerical simulation results indicate that the proposed mould baffle technology can effectively hinder the radiation and heat dissipation at the platform extremities, and therefore, reduce undercooling in the platform and the formation of stray grains during directional solidification. Casting trials of a hollow turbine blade were conducted using CMSX-4 superalloy. The trial results demonstrate the potential of the proposed approach for manufacturing single-crystal superalloy blades.

     

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     

Directional solidification casting technology of heavy-duty gas turbine blade with liquid metal cooling (LMC) process

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling(LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength(the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature(1,520 ℃ for upper region and 1,500 ℃ for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring(3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation 001, resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate.     

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.     

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.     

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.     

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。     

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.     

Development of heat conductor technique for single crystal components of superalloys

Abstract

A new heat conductor (HC) technique was developed to minimise stray grain formation arising from the geometrical effect during single crystal solidification of superalloys. Graphite is recommended as HC material because of its excellent thermophysical properties. Computer simulation and temperature measurements have shown that this technique is able to produce a clear improvement of thermal condition in the critical region of the components. Both the extent and the period of undercooling at the platform extremity significantly decreased. Structural investigation revealed a notable reduction in stray grain quota, providing evidence for the effectiveness of the HC technique for improving the quality of single crystal components.     

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…     

Stray grain formation,thermomechanical stress and solidification cracking in single crystal nickel base superalloy welds

Abstract

In the present study, the effects of stray grain formation and thermomechanical stresses on solidification cracking in welds of single crystal Ni-base superalloys have been investigated. Welds were made in an asymmetric crystallographic orientation under three different processing conditions. As welding speed and power increased, stray grain formation became extensive, but only on one side of the weld. Solidification cracking also became more extensive and occurred mostly along the stray grain boundaries. The three welding processes have been simulated using the finite element method (FEM). The calculation results showed that thermomechanical stresses increase with welding speed and power, leading to increased susceptibility to cracking. These results agree well with experimental observations.     

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.     

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。     

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。     

Formation of low angle boundaries in Ni-based superalloys

Abstract

An experimental study has been used to determine the influence of liquid undercooling ahead of dendrite tips on dendrite growth and the accompanying evolution of misorientation resulting in formation of low angle grain boundaries during solidification of a Ni-base superalloy, CMSX-4. The experiment was designed whereby two dendrite envelopes formed by initial branching of the primary front converged along horizontal platforms perpendicular to the withdrawal direction. The magnitude of the undercooling was increased by increasing the length of the platform, ranging from 10 to 60 mm. The position of the dendrite envelope was calculated using a computational thermal model, ProCAST. Misorientation, measured using Electron Back Scattered Diffraction (EBSD), was generally <2·5°, which was consistent with previous studies of dendrite growth in the constrained condition. However, in the longest platform a monotonic increase in misorientation in one dendrite envelope developed leading to formation of a grain boundary of 10°. This monotonic increase was not dependent on the local undercooling ahead of the dendrite envelope and was instead caused by mechanical moments arising from extensive lateral growth of unsupported tertiary dendrite branches growing laterally across the platform.     

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.     

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.     

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.     

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。