高温合金定向凝固界面换热系数逆向求解与验证

通过实验采集了DD6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确定了定向凝固过程中界面换热系数。采用这些界面换热系数进行了ProCAST数值模拟,得到了凝固过程位移场,并与真实叶片实测位移场进行比较,证实仿真结果与试验结果吻合较好,验证了反求界面系数的可靠性。     

采用正交设计优化单晶合金定向凝固数值模拟参数

采用正交设计方法研究单晶高温合金定向凝固过程数值模拟所需参数对温度场和冷却曲线的影响.结果表明:单晶高温合金凝固曲线对模拟参数十分敏感,特别敏感于铸件与铸型之间的界面换热系数.通过正交设计获得边界条件参数使模拟结果与试验结果吻合良好.确定符合DD6单晶高温合金凝固过程的数值模拟边界条件.     

单晶叶片铸造过程中界面换热系数的确定

为有效提高铸造数值模拟的准确性,以单晶空心涡轮叶片为典型件,设计试验方案,采用钨铼热电偶进行温度场数据采集工作.通过实测温度场、数值模拟温度场与界面换热系数反向求解相结合的方法,得出界面间换热系数随时间变化的经验公式.通过逆运算,得到较为精确的涡轮叶片铸造过程界面换热值.结果表明,铸造过程数值模拟精度得到了有效提高.     

高温合金叶片定向凝固过程中辐射换热的计算

提出改进型Monte Carlo射线追踪法进行辐射换热计算,通过子空间的划分,并结合定向凝固的特点回避了传统辐射换热计算中角系数的直接计算,节省了计算时间.把该方法嵌入自行开发的三维模拟软件对柱状晶和单晶叶片的定向凝固过程进行了温度场模拟,并对实际叶片凝固过程进行温度场测试,模拟结果与实测结果较好吻合.     

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。     

铸型界面换热系数的测定方法研究

为有效提高数值模拟的准确性．通过实测反求法对材料界面换热进行了测定．提出界面间换热是随温度变化的函数值。按照实验方案．在保证测量仪器．仪表精度的基础之上．采用特制热电偶进行了温度场数据的采集工作。通过实测温度场数据、数值模拟与凝固过程中界面换热系数的反向求解相结合的方法．得出铸件／铸型界面间的换热随温度变化的函数值。通过应用ProCAST软件反求模块进行逆运算，确定了现有某铸铁材料与树脂砂铸型材料间的界面换热值。计算结果表明，整个凝固过程中热电偶处的温度模拟结果与实验结果的最大相对误差在±10℃内．数值模拟精度得到了有效的提高，说明了此界面换热测定方法的可行性。     

抽拉速度对单晶叶片定向凝固过程的影响

以DD6高温合金作为单晶叶片试件合金材料，对固定抽拉速度、简单阶梯抽拉速度和多级阶梯抽拉速度下单晶叶片试件温度场、糊状区和凝固组织生长情况进行了数值模拟，并进行了定向凝固验证试验。结果表明：采用低速固定抽拉和多级阶梯抽拉速度能获得单晶完整性好的叶片试件；高速固定抽拉和简单阶梯拉速下单晶叶片试件在缘板产生杂晶；凝固组织数值模拟结果与定向凝固试验结果基本吻合。     

熔模铸造ZL114A铝合金凝固过程界面换热系数研究

为获得ZL114A铝合金在凝固过程中温度场的分布规律,根据实际工况设计了测温实验方案,利用热电偶和热成像仪得到了金属及其型壳在凝固过程中温度场的变化曲线,并根据实际测得的温度曲线借助ProCAST模拟软件中的反算模块对铸件与型壳间的界面换热系数进行了反求,得到了更加符合实际的界面换热系数。随后对其进行验证,用该界面换热系数所模拟求得的金属液温度曲线与实测值最大温差为10℃,型壳温度曲线与实测值最大温差为15℃,该方法及结果为铝合金熔模精铸模拟界面换热系数的设置提供了参考依据。     

数值模拟技术在制备燃气轮机叶片中的应用

从单晶叶片的选晶过程、HRS定向凝固工艺制备航空叶片及LMC工艺制备重型燃气轮机叶片3方面,简要介绍了数值模拟技术的研究进展。描述了数理模型的建立,对比分析了两种不同的定向凝固工艺的优缺点。螺旋选晶器的设计对单晶叶片的制备影响较大,其设计失效可能会导致选晶效率差,出现杂晶缺陷等。通过对航空叶片温度场及微观组织的模拟,结合试验研究,优化了工艺,成功制备出单晶叶片。重型燃气轮机叶片制备更为复杂,通过数值模拟缩短了研发周期,节约了成本。     

铝铸锭凝固边界热交换规律及温度场模拟

研究铝铸锭凝固边界热交换的变化规律及数学模型,并对不同浇注温度下凝固过程的温度场进行模拟。利用实时数据采集系统获得凝固过程中铸锭和金属模温度变化历史数据,采用非线性反算法和一维传热差分法对试验数据处理,建立界面换热模型并将其应用于凝固温度场模拟中。结果表明：在铸锭表面凝固前后凝固界面热流密度可分段用指数函数来描述其变化规律,而所建立的热交换系数与边界温度的对应关系可更好地反映实际的传热情况。模拟结果与实验测温结果相符,验证了该铸件/铸型边界热交换规律的可靠性。     

Microstructure Simulation of Al-Cu Alloy Based on Inverse Identified Interfacial Heat Transfer Coefficient

The interfacial heat transfer coefficient (IHTC) is taken as one of the most important factors affecting the accuracy of the simulation. In the present paper, the IHTC variation with temperature was obtained by an inverse heat conduction method. Then, a 3D cellular automaton-finite element method was adopted to predict the microstructure of an Al-Cu alloy based on the identified IHTC. It was found that the IHTC was of prime importance for the precise simulation of solidification microstructure, especially in the grains distribution. In addition, the simulated results using the IHTC variation with temperature were found to exhibit a better agreement with the experimental results than those using the constant value.     

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.

     

镁合金铸造成形过程宏观／微观模拟

通过研究镁合金压铸过程中界面热,采用热传导反算法确定压铸过程的界面换热系数,研究镁合金压铸过程中工艺参数及凝固过程对铸件界面换热系数的影响规律,建立镁合金压铸过程界面换热边界条件的处理模型,以实现镁合金压铸过程中凝固过程的准确预测。通过实验研究镁合金压铸过程中凝固组织,建立了镁合金压铸过程中形核模型。采用CA方法,建立了镁合金枝晶生长模型,以实现镁合金凝固组织的预测。采用相场方法研究了镁合金枝晶生长形貌。     

定向凝固共晶生长的元胞自动机数值模拟

本文在已有的二元初生相元胞自动机（CA）方法的基础上,针对二元共晶凝固过程提出了改进的元胞自动机（MCA）模型.该模型考虑成分过冷和曲率过冷对界面形态的影响,通过界面溶质浓度守恒来获得共晶α相和β相生长速率,模拟了层片的湮灭、分叉与稳态生长.为了验证模型的可靠性,对常见的CBr₄-C₂Cl₆共晶透明合金进行了模拟,研究了抽拉速率对共晶层片间距大小的影响,模拟结果与文献中的实验结果吻合良好;同时模拟了共晶层片间距调整过程的形貌演化以及层片振荡不稳定性现象.本文将MCA模型扩展到三维定向凝固过程中,研究了共晶形态的层棒状转变机制.     

Application of inverse method to estimation of boundary conditions during investment casting simulation

Inverse method was used in single crystal superalloy DD6 processing simulation during solidification. Numerical modeling coupled with experiments has been used to estimate the interface heat transfer coefficient (IHTC) between the surface of slab casting and inner mold. Calculated temperature dependent values of IHTC were obtained from a numerical solution. The calculated temperatures agreed well with the measurement of cooling profile.     

Study on interfacial heat transfer coefficient at metal/die interface during high pressure die casting process of AZ91D alloy

The high pressure die casting (HPDC) process is one of the fastest growing and most efficient methods for the production of complex shape castings of magnesium and aluminum alloys in today's manufacturing industry. In this study, a high pressure die casting experiment using AZ91D magnesium alloy was conducted, and the temperature profiles inside the die were measured. By using a computer program based on solving the inverse heat problem, the metal/die interfacial heat transfer coefficient (IHTC) was calculated and studied. The results show that the IHTC between the metal and die increases right after the liquid metal is brought into the cavity by the plunger, and decreases as the solidification process of the liquid metal proceeds until the liquid metal is completely solidified, when the IHTC tends to be stable. The interfacial heat transfer coefficient shows different characteristics under different casting wall thicknesses and varies with the change of solidification behavior.     

Measurement of temperature inside die and estimation of interfacial heat transfer coefficient in squeeze casting

As an advanced near-net shape technology, squeeze casting is an excellent method for producing high integrity castings. Numerical simulation is a very effective method to optimize squeeze casting process, and the interfacial heat transfer coefficient (IHTC) is an important boundary condition in numerical simulation. Therefore, the study of the IHTC is of great significance. In the present study, experiments were conducted and a"plate shape" aluminum alloy casting was cast in H13 steel die. In order to obtain accurate temperature readings inside the die, a special temperature sensor units (TSU) was designed. Six 1 mm wide and 1 mm deep grooves were machined in the sensor unit for the placement of the thermocouples whose tips were welded to the end wall. Each groove was machined to terminate at a particular distance (1, 3, and 6 mm) from the front end of the sensor unit. Based on the temperature measurements inside the die, the interfacial heat transfer coefficient (IHTC) at the metal-die interface was determined by applying an inverse approach. The acquired data were processed by a low pass filtering method based on Fast Fourier Transform (FFT). The feature of the IHTC at the metal-die interface was discussed.     

一种求解水平连铸中瞬态界面换热系数的新方法

为有效提高温度场数值模拟的准确性，对界面换热系数的模型进行了改进和参数化，并基于这个3阶段演变模型，提出了数值模拟与优化计算相结合的反向求解方法。针对水平连铸中出现的非均匀间隙情况，获得了与位置相关的界面换热系数。算例采用了两种合金Sn-5Pb和Sn-15Pb作为算例进行计算。优化结果表明，对于两种合金3个热电偶处的模拟温度与实测值最大相对误差不超过5％。     

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.

     

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