Machining of a film-cooling hole in a single-crystal superalloy by high-speed electrochemical discharge drilling

Single-crystal superalloys are typical advanced materials used for manufacturing aeroengine turbine blades. Their unique characteristics of high hardness and strength make them exceedingly difficult to machine. However, a key structure of a turbine blade, the film-cooling hole,needs to be machined in a single-crystal superalloy; such machining is challenging, especially considering the increasing levels of machining efficiency and quality demanded by the aeroengine industry. Tube electrode high-speed electrochemical discharge drilling(TSECDD), a hybrid technique of high-speed electrical discharge drilling and electrochemical machining, provides high machining efficiency and accuracy, as well as eliminating the recast layer. In this study, TSECDD is used to machine a film-cooling hole in a nickel-based single-crystal superalloy(DD6). The Taguchi methods of experiment are used to optimise the machining parameters. Experimental results show that TSECDD can effectively drill the film-cooling hole; the optimum parameters that give the best performance are as follows: pulse duration: 12 ls, pulse interval: 30 ls, peak current:6 A, and salt solution conductivity: 3 m S/cm. Finally, a hole is machined by TSECDD, and the results are compared with those obtained by electrical discharge machining. TSECDD is found to be promising for improving the surface quality and eliminating the recast layer.     

氙离子推力器阳极工质分配方式优化

使用两种数值方法研究微尺度下惰性气体的流动现象,验证表明壁面速度滑移条件的N-S（Navier-Stokes）方法能够较好地模拟离子推力器气体分配环内工质流动特性.为了降低氙工质注入放电室过程中气流速度,并改善其周向均匀性,针对现有气体分配环构型提出符合4级环切场离子推力器放电特性的改进方案:增加2级供气通道,进行双侧45°角开孔并增加气孔数目.数值计算表明:结构优化后氙气进入放电室周向均匀性提高37%,气流速度降低32%；推力器装配优化方案气体分配环进行实验,大束流工况下束离子电离能耗自183W/A下降至167W/A,离子推力器放电室性能得到了提高.      

Effect of no-load rate on recast layer cutting by ultra fine wire-EDM

Via material erosion in wire electrical discharge machining (WEDM), recast layers form on the surfaces of workpiece. In addition, ultra fine Wire-EDM can be usually cut once. To reduce the thickness of the recast layer as much as possible, the wire electrical discharge-electrochemical machining (WEDCM) method was proposed, which is based on the micro conductivity of the dielectric and microelectrolytic characteristics by adjusting the no-load rate of the pulse in the machining process. Furthermore, a state discrimination and servo control system based on discharge current was designed. The experiment results of different no-load rates show that the electrolytic effects increase as the no-load rate increases, and the main machining process is spark discharge erosion with a no-load rate in the range of 10% to 80%. At 90% no-load rate, the amount of recast layer formation in the forward direction of the wire electrode is almost the same as that of electrolytic dissolution, and it can be practically processed without a recast layer. Compared with 10% no-load rate, the kerf width only increases by 7.5%.     

电极面型对横向激励高气压CO2激光器放电的影响

计算并讨论了均匀场电极中电极板安装结构、电极板厚度对实际电场分布影响。计算表明在基本参数类似的条件下,近罗可夫斯基电极表面电场强度具有17%的非均匀性,但在垂直放电方向的对称面上却有更好的均匀性。对放电区辉光的时间积分照片和像增强型CCD相机记录的瞬态照片进行了分析,结果表明:电极表面的电场强度分布,仅对放电的初始发展具有影响,而放电区中央的电场分布以及预电离强度,对最终放电区的均匀性具有决定性的影响。     

Multi-channel discharge characteristics cutting by ultra-fine wire-EDM

Compared with a copper wire electrode, molybdenum wire with a poor conductor is usually used as the electrode in high speed wire-cut electrical discharge machining(HSWEDM), so the resistance of an ultra-fine wire cannot be ignored. To study the differences of discharge characteristics between the ultra-fine wire and the conventional diameter wire, the continuous discharge waveform of two kinds of wire electrodes was compared. It was found that there was a multichannel discharge phenomenon in the...     

冰层辅助对电火花-电解复合穿孔的影响

电火花-电解复合穿孔(ECDD)加工方法有望实现难加工材料涡轮叶片气膜冷却孔无重铸层高效加工,为了进一步提升小孔孔壁的加工质量,提出了在工件底部填充冰层的电火花-电解复合穿孔加工新方法。分析了冰层辅助对复合加工过程中两极之间电流电压的波形、复合穿孔的加工效率、小孔的出入口孔径、孔壁重铸层去除等的影响,进行了冰层辅助与无冰层辅助电火花-电解复合穿孔对比试验。试验表明:冰层辅助加工可以在小孔穿透之后形成充分的反向冲液,有效地解决小孔穿透之后的漏液问题。在增加底部停顿时间的基础上,即小孔穿透后管电极到达预设深度继续停留一段时间,延长管电极对孔壁的电解作用时间,可以显著提高重铸层的去除效果,有望实现小孔整个孔壁重铸层的完全去除。     

超声磨料对TC4钛合金电火花加工表面质量的影响

为改善TC4钛合金电火花加工表面质量,减少表面微裂纹、重熔层等表面缺陷,提出了基于电极振动的超声磨料电火花复合加工方法。在煤油工作液中混入12g/L的SiC磨料粉,进行了有无超声磨料作用的窄脉冲(脉宽小于1μs)电火花加工的表面粗糙度对比实验研究。实验结果表明:SiC磨料的超声振动作用使零件表面粗糙度Ra由0.5μm降到0.2μm左右;重熔层厚度、表面裂纹的扫描电子显微镜(SEM)照片显示,超声磨料作用使重熔层厚度减薄20～30μm,表面微裂纹得到有效控制;机理分析认为工作液高频振动及磨料对工件的冲击作用,是改善TC4钛合金电火花加工表面质量的主要原因。研究表明磨料的超声振动作用可显著改善TC4钛合金电火花加工的表面质量。     

Breakthrough detection in electrochemical discharge drilling to enhance machining stability

Film cooling holes are widely used in aero-engine turbine blades. These blades feature large numbers of holes with complex angles and require a high level of surface integrity. Electrochemical discharge drilling(ECDD) combines the high efficiency of electrical discharge drilling(EDD) with high quality of electrochemical drilling(ECD). However, due to the existence of a variety of energy for material removal, accurate and timely detection of breakthroughs is fraught with difficulties. An insuffic...     

Investigation of MHD power generation with supersonic non-equilibrium RF discharge

Magnetohydrodynamic (MHD) power generation with supersonic non-equilibrium plasma is demonstrated. Capacitively coupled radio frequency (RF) discharge (6 MHz, maximum continual power output of 200 W) was adopted to ionize the Mach number 3.5 (650 m/s), 0.023 kg/m3 airflow. In a MHD channel of 16 mm × 10 mm × 20 mm, MHD open voltage of 10 V is realized in the magnetic field of 1.25 T, and power of 0.12 mW is extracted steadily and con-tinuously in the magnetic field of 1 T. The reasons for limited power generation are proposed as:low conductivity of RF discharge; large touch resistance between MHD electrode and plasma;strong current eddies due to flow boundary layer. In addition, the cathode voltage fall is too low to have obvious effects on MHD power generation.     

钛合金齿轮LPES法表面强化的电场与流场

针对TC4钛合金齿轮复杂曲面液相等离子体电解渗透(LPES)表面强化放电困难的问题,基于仿真分析和实验验证的方法,建立了齿轮表面强化系统仿真模型,进行了强化系统电场和流场仿真,确定了齿轮复杂表面放电机理,研究了电极系统参数和入口流速对强化层形成的影响。结果表明:齿轮复杂表面放电困难的根本原因在于电场的分布不均。采用啮合形阳极时的电场和强化层均匀性较好。电极距离过小容易造成强化系统的短路,过大时会降低强化层的均匀性和厚度。合理的系统电解液流速对放电的稳定性和强化层的形成均具有重要的意义。相较于未处理时的基体,强化后的齿轮表面耐磨性有了明显提升。     

6156-T4铝合金焊接加筋薄壁结构剩余强度评估的CTOD方法

为保证飞机设计满足损伤容限性能要求,有必要对其进行剩余强度评估。针对6156-T4铝合金焊接连接薄壁结构进行了R曲线和剩余强度试验,判断了裂纹扩展经过筋条时的裂纹扩展路径,测量了母板和筋条两个方向的裂纹扩展速率,并采用不同的断裂准则和分析方法对单及双跨度多个初始裂纹长度焊接加筋薄板的剩余强度进行了预测。结果表明:裂纹扩展在经过筋条时,同时沿着母板和筋条继续扩展,筋条上的裂纹扩展方向垂直于母板且两个方向的裂纹扩展速率基本相同;采用净截面屈服准则进行剩余强度预测时会低估这种韧性较好的焊接连接薄壁结构的剩余强度;基于SINTAP-FITNET评价体系,以裂纹尖端张开位移(CTOD-δ5)作为裂纹尖端弹塑性表征参量进行剩余强度预测时,预测结果比采用K曲线预测方法精度高。     

基于粘弹性材料的五轴铣削加工被动抑振技术

针对薄壁件在铣削过程中极易出现颤振,且动力学特性快速变化导致切削稳定性多变的特点,研究基于粘弹性材料的被动阻尼技术,以增加薄壁零件阻尼、提升切削稳定性。提出阻尼层、约束层以及质量层的设计方案,模态试验表明该技术能大幅提升薄壁件阻尼,且对多阶振动模态有抑制作用。最后,将其应用于薄壁S样件的五轴铣削加工试验,多组切削参数下的试验结果表明,工件切削振动幅值最大可下降约97%。     

高空低气压下电晕放电电磁脉冲辐射特性模拟实验

为研究高空低气压下飞行器表面电晕放电电磁脉冲辐射特性,以尖端导体电晕放电为研究对象,进行了低气压下电晕放电电磁脉冲辐射特性模拟实验,给出了辐射电场幅值与气压的关系表达式,并基于气体放电和电磁场理论总结了电晕放电电磁脉冲辐射特性随电压极性、气压以及测试距离的变化规律。结果表明:电晕放电电磁脉冲辐射电场时域波形呈衰减振荡形式,持续时间约为600 ns,首脉冲方向随电压极性的不同而不同,且辐射电场分量主要集中在与放电针相同的极化方向;辐射电场频谱主要分布在500 MHz以内,在35 MHz和170 MHz处出现稳定的波峰;在气压为4~30 kPa范围内,随着气压的降低,放电电磁脉冲辐射电场增强;随着测试距离的增加,电晕放电电磁脉冲辐射电场逐渐衰减。该研究结论可为低气压下飞行器电晕放电电磁脉冲辐射特性研究提供有益参考。     

超高温NiAl合金锥形薄壳件制备成形一体化新工艺

针对NiAl合金板坯制备及板坯成形锥形薄壳件存在的材料流动及组织性能控制困难的问题,提出了一种制备成形一体化新工艺。该工艺是将塑性成形和反应合成在同一工步中,即先将Ni/Al叠层箔置于模具中进行塑性成形,随后对成形的Ni/Al叠层箔原位加热加压反应合成NiAl合金薄壳件。采用三维扫描仪及Geomagic Studio/Qualify对锥形薄壳件的形状精度进行了分析,采用扫描电子显微镜( Scanning Electron Microscope, SEM )和电子背散射衍射( Electron Backscatter Diffraction, EBSD )技术对其微观组织进行了表征,并对构件的高温力学性能进行了测试。结果表明,采用该新工艺制备的锥形薄壳件成形效果良好,整体型面偏差尺寸在±0.1 mm以内;构件沿轴截面壁厚平均偏差为0.012 mm,沿横截面壁厚平均偏差为0.072 mm,构件轴截面壁厚分布均匀性及组织成分均匀性高于横截面。构件高温力学性能稳定,1000 °C时平均屈服强度为77.8 MPa,平均抗拉强度为82.6 MPa。NiAl合金构件的微观晶粒形貌与Ni箔的初始热处理状态及Ni/Al箔的初始厚度有关,未退火的Ni箔将延缓粗晶区晶粒的长大,减小初始箔材厚度可实现晶粒的进一步细化。     

以复合工作液为放电介质的低速走丝电火花线切割可行性研究

从典型放电波形分析了电火花线切割加工的极间放电特征,对高速走丝电火花线切割放电机理进行了探讨,通过对高速走丝线切割放电间隙、走丝速度对切割效率及表面质量影响的分析,得出了以复合工作液为放电介质的低速走丝电火花线切割是可行的结论。     

NS-DBD激励控制非细长三角翼前缘涡仿真研究

通过在三角翼前缘施加纳秒脉冲介质阻挡放电（NS-DBD）激励唯象学模型，进行了47°后掠角钝前缘三角翼流动控制的仿真。分析了不同迎角下升力和阻力系数的变化、流场结构的变化、以及激励诱导旋涡的演化过程。研究表明:施加无量纲激励频率F+=1.44的NS-DBD激励后，可明显提高三角翼失速前后的升力系数；同时阻力系数也有所增加，变化趋势与实验结果一致。激励在前缘分离剪切层处诱导产生流向涡，改变了前缘剪切层结构，使其向内卷吸；激励后时均流场形成了明显的负压峰值，前缘涡附着线外移，吸力面回流区减小。      

磁场辅助激光熔覆铝基金属玻璃覆层

为提高铝合金的表面性能,采用磁场辅助激光熔覆的方法在5083铝合金表面制备了Al-Ni-Y-Co-La五元金属玻璃熔覆层,并对其进行组织成分及性能分析。实验结果表明：熔覆层主要由非晶相、α-Al相以及Al₄NiY等金属间化合物组成,旋转磁场的搅拌作用使熔覆层非晶相含量由10.2%提高到30.7%,且能够有效抑制多道搭接和多层堆积过程中重熔区晶粒的生长,细化了熔覆层晶粒组织,降低了残余应力,提高了显微硬度及韧性,使其平均显微硬度从278 HV_0.1提高至335 HV_0.1,且波动较小,平均抗拉强度为303 MPa,为基体拉伸件的110.2%,平均伸长率为6.79%,为基体的33.1%。     

残余应力重分布引起的薄壁零件加工变形研究

现代航空工业中为减轻飞机重量,提高飞机的各项机械性能,整体构件越来越多地被使用。加工大型整体薄壁构件时,有90％以上的材料被切削加工去除,由于材料去除后零件刚度的降低以及应力的释放,造成过大的加工变形。本文用MSC．Marc有限元软件仿真了铝合金预拉伸板材料去除对于加工变形的影响,并分析了加工变形的成因。为验证有限元结果的正确性,在高速数控铣床上加工了同样的试件。结果表明仿真结果与实验结果一致,残余应力的释放与重分布是薄壁零件加工变形的主要原因。     

航空薄壁回转体零件热处理残余应力的建模与仿真

航空薄壁回转体零件由于自身结构的特殊性,使得残余应力对其变形和精度稳定性有着很大的影响.采用热弹塑性有限元模型对薄壁回转体铝合金零件的热处理过程进行建模和仿真,运用有限元软件,对2A12铝合金薄壁回转体零件热处理过程瞬态温度场和应力场进行模拟,分析了热处理各个阶段的应力分布形态,并进行实验验证,数值模拟得到的残余应力值与实验结果基本一致.     

飞秒激光能量密度对镍基合金重铸层和加工效率的影响

以航空发动机叶片制孔为导向,结合飞秒激光对单晶镍基高温合金材料的非热熔性损伤阈值(Φth1)和热熔性损伤阈值(Φth2)特征,研究了飞秒激光能量密度(0Φ44.2J/cm2)对制孔重铸层和加工效率的影响规律。研究结果表明:在Φth1ΦΦth2时,镍基合金经飞秒激光加工后加工侧壁没有出现明显的重铸物;在ΦΦth2时,加工侧壁开始出现重铸物,并随着能量密度的增加,重铸层厚度增大。在试验结果的基础上,建立了飞秒激光单脉冲加工深度与能量密度的定量关系。能量密度越高,飞秒激光单脉冲加工深度越大,加工效率越高。