Improvement effects of vibration on cutting force in rotary ultrasonic machining of BK7 glass

Rotary ultrasonic machining (RUM) exhibits a high potential for a significant reduction in the cutting force, which directly associates with tool wear, machining accuracy, machining temperature, and surface integrity. However, the improvement mechanisms of the ultrasonic vibration on the cutting force are still not fully recognized, restricting the currently optimization methods for further reducing the cutting force occurred during the RUM process. In this research, by incorporating the kinematics principles of the abrasive, the evolution features of the material strain rate in the loading phase were first discussed with respect to the indentation mechanics theory. Taking these features into account, the RUM scratching tests were carried out on the polished specimen surfaces under various process parameters to capture the integrated damage patterns evoked in the high strain rate stage. Following, the comparative indentation tests were respectively conducted on the RUM scratches and the gentle polished surfaces. The indentation-induced damage structures and the load–displacement curves were characterized and assessed to investigate the improvement mechanisms of the superimposed ultrasonic on the cutting force in formal RUM process. It was found that superimposing an ultrasonic vibration led to the incipient cracks nucleated in the abrasive loading phase, and their propagations would increase the material removal rate (MMR) obtained in formal RUM process. Furthermore, the incipient cracks provided a shielding effect to the indentation force, which was a dominant factor in diminishing the cutting force of the diamond tool. The nucleation of the incipient cracks resulted in more energy dissipation after the abrasives penetrating into the hard substrate of the material, which would lead to a higher residual stress on final RUM surface. In addition, a failure pattern (plastic deformation or brittle fracture) evolution model involved in abrasive loading phase was developed with respect to the strain rate effects of the material.     

Experimental observation of tool wear in rotary ultrasonic machining of advanced ceramics

As one of the cost-effective machining methods for advanced ceramics, rotary ultrasonic machining (RUM) has attracted much attention and there exist numerous publications on the process. However, few investigations on tool wear in the RUM process have been reported. This paper, for the first time in literature, presents an experimental observation on tool wear in RUM of silicon carbide (SiC). It first reviews some related wear mechanisms for grinding wheels and some techniques for studying the wheel wear mechanisms. After describing the experimental procedures, it presents and discusses the results on tool wear and cutting forces in RUM of SiC. It also discusses some practical implications of the findings from this study.     

C/SiC复合材料磨削的表面/亚表面损伤

采用树脂结合剂金刚石砂轮对二维正交编织结构C/SiC复合材料进行了平面磨削加工实验。通过对磨削加工表面形貌、磨削表面中碳纤维区域的粗糙度、磨削亚表面形貌的分析与测量,对C/SiC复合材料磨削表面/亚表面损伤进行了研究。结果表明:磨削表面中碳纤维损伤形式以阶梯状脆性断裂为主。对于编织方向平行于进给速度方向的纤维区域,脆性断裂尺寸、表面粗糙度受工艺参数影响较小;而对于编织方向垂直于进给速度方向的纤维区域,脆性断裂尺寸、表面粗糙度随进给速度增大无明显变化,但随磨削深度增大而明显增大。碳纤维区域亚表面损伤形式主要为阶梯状脆性断裂,而SiC区域亚表面损伤形式主要为脆性断裂及微裂纹,且损伤程度在实验参数范围内无明显差异。     

振幅对超声辅助磨削C/SiC复合材料表面形貌的影响

目的 通过超声振动辅助磨削加工技术加工C/SiC复合材料可以改变材料的去除方式,通过改变超声振幅能够提高材料去除率并获得较好的表面质量,从而成为C/SiC复合材料的新型加工方式。方法 采用超声辅助磨削技术对C/SiC复合材料进行加工,通过改变超声振幅,观察C/SiC复合材料在不同切削角度下的纤维去除机理、纤维断裂形式,测量不同切削角度下工件表面粗糙度Sa。结果 磨削过程中C/SiC复合材料的去除方式以脆性去除为主,纤维损伤形式以纤维断裂、纤维破碎为主。增大超声振幅后,纤维断裂形式增大并伴随出现基体破碎现象。随着超声振幅的增大,不同切削角度(0°、45°、90°、135°)下测得的表面粗糙度Sa显著减小,降低约15%~41%。结论 由于超声振动的作用,C/SiC复合材料在不同切削角度(0°、45°、90°、135°)下的材料去除方式发生改变,相比于常规磨削的纤维断裂形式,施加超声振动后,磨削过程中产生的纤维折断和基体破碎被去除,在提高材料去除率的同时,表面质量明显提高。随着超声振幅的增大,不同切削角度(0°、45°、90°、135°)下的表面粗糙度Sa都减小,且减小程度也不同,减小程度由大到小的顺序为45°>135°>90° >0°。     

Oxidation behavior of CVI,MSI and CVI+MSI C/SiC composites

The oxidation behavior of chemical vapor infiltration(CVI),molten silicon infiltration(MSI)and CVI+MSI C/SiC composites at 500-1 400℃was studied.The oxidation below 900℃increased successively for CVI,CVI+MSI and MSI composites.However,the oxidation of CVI composite above 1 000 ℃was much faster thanthat of MSI and CVI+MSI composites. As active carbon atoms produced by siliconization of fibers during MSI process were oxidized first and decreased initial oxidation temperature.The initial oxidation temperature of MSI,MSI+CVI and CVI composites was 526,552 and 710℃,respectively.New active carbon atoms were generated due to the breaking of 2D molecular chains during oxidation,so the activation energy of three C/SiC composites was decreased gradually at 500-800℃with oxidation process,exhibiting a self-catalytic characteristic.     

Rotary ultrasonic machining of ceramic matrix composites: feasibility study and designed experiments

Ceramic matrix composites (CMC) are enabling materials for a number of high-temperature and demanding applications in aerospace, power generation, ground transportation, nuclear, environmental, and chemical industries. Tremendous progress has been made in technology development, manufacturing, commercialization, and applications of CMC over the last few years. However, significant challenges (such as the lack of specifications, databases, and in-service repair methodology, and high machining cost) still remain for their widespread applications. In this paper, rotary ultrasonic machining (RUM) is introduced into drilling holes on CMC panels for the first time. The feasibility to machine CMC using RUM is investigated. Cutting forces and material removal rates (MRR) are compared for machining of CMC with and without ultrasonic vibration and for two types of CMC materials and one typical advanced ceramic material (alumina). Chippings at the hole exit are also observed under a microscope. Furthermore, the paper presents the results of a designed experimental investigation into RUM of CMC. A three-variable two-level full factorial design is employed to reveal main effects as well as interaction effects of three RUM process parameters (spindle speed, feedrate, and ultrasonic power). The process outputs studied include cutting force, MRR, and hole quality (in terms of chipping dimensions).     

Brazing C/SiC composites and Nb with TiNiNb active filler metal

Abstract

C/SiC composites and Nb were vacuum brazed with the Ti_39·4Ni_39·4Nb_21·2 alloy being the active filler metal. The mechanical properties of the filler material, the microstructure and the strength of brazing joints were investigated. The results showed that the filler TiNiNb alloy has a tensile strength of 860?MPa, an elongation of 51% and an elastic modulus of 78?GPa. Both Ti and Nb elements in the filler reacted with C/SiC during the brazing process, and a well bonded C/SiC–Nb joint was obtained. The ductile filler metal released the thermal stress in the joint. When the brazing was performed at 1220°C for 20?min, the shear strength of brazed joints reached 149, 120 and 73?MPa at 20, 600 and 800°C respectively.     

C/SiC复合材料表面状态对其与Nb钎焊的影响

采用等原子比Ti-Ni复合箔对C/SiC复合材料与Nb进行了真空反应钎焊,研究了焊前C/SiC表面状态对接头界面组织和力学性能的影响.结果表明,线切割态的C/SiC与Nb的接头界面(Ti,Nb)C反应层呈现锯齿状,而抛光和砂纸打磨状态的C/SiC接头界面处的反应层平直.锯齿状的界面反应层降低了界面处的应力集中程度,有助于提高接头的力学性能,使得线切割态的C/SiC与Nb的接头强度明显高于抛光和打磨状态的接头,达到188MPa.线切割态的C/SiC与Nb的接头断裂同时发生在C/SiC母材、界面和钎缝中,而抛光和打磨状态的C/SiC与Nb的接头断裂主要发生在界面处.     

碳/碳复合材料SiC/W-Al-Si涂层微观结构及抗氧化性能

分别采用包埋法、料浆法在碳/碳（C/C）复合材料表面制备了碳化硅（SiC）内涂层、W-Al-Si合金外涂层,借助XRD和SEM分析了所得涂层的物相组成和微观结构,并测试了带有单一SiC涂层、SiC/W-Al-Si双涂层碳/碳复合材料试样在1500℃静态空气中的抗氧化性能。结果表明：富Si的SiC内涂层结构疏松,仅能为碳/碳基体提供数小时的防氧化保护;W-Al-Si合金外涂层主要由WSi2和W（Si,Al）2两相组成;SiC/W-Al-Si双涂层厚度约为100μm,其抗氧化性能明显优于单一SiC涂层,氧化19 h后涂层试样的质量损失未超过5%;有望进一步通过优化W-Al-Si外涂层料浆比例,避免因为与SiC内涂层热膨胀不匹配而产生透性裂纹,从而发挥出超过19 h后SiC/W-Al-Si双涂层的氧化防护潜力。     

SiC/ZrB2-SiC-B4C涂层的抗氧化和耐烧蚀性能研究

碳/碳化硅复合材料(C/SiC)在使用时经常受到高温氧化和烧蚀作用。本文采用化学气相沉积(CVD)和浆料刷涂-烧结法制备了双层SiC/ZrB2-SiC-B4C涂层,对比研究了无涂层,单层SiC涂层和双层SiC/ZrB2-SiC-B4C涂层C/SiC复合材料在1500℃下的氧化和在4.2 MW/m2热流密度下的烧蚀性能。结果表明,制备态ZrB2-SiC-B4C涂层致密、完整,表面平均粗糙度约为1 μm,孔隙率约为4.2 %。在1500℃氧化30 h后,SiC/ZrB2-SiC-B4C涂层C/SiC复合材料的质量损失率约为10%,涂层表面氧化膜致密,无明显裂纹。高温烧蚀20 s后,SiC/ZrB2-SiC-B4C涂层的线烧蚀率和质量烧蚀率分别为1.0±0.3 μm/s和1.1±0.2 mg/s,与单层SiC涂层相比分别降低了75.0 %和50.0 %,SiC/ZrB2-SiC-B4C涂层烧蚀后形成的ZrO2-SiO2氧化膜可以减缓火焰对复合材料的机械剥蚀作用。     

Fabrication of hybrid micro/nano-textured surfaces using rotary ultrasonic machining with one-point diamond tool

A novel rotary ultrasonic texturing (RUT) technique is proposed to fabricate hybrid periodic micro/nano-textures on flat surfaces. Different from conventional rotary ultrasonic machining, a tailored one-point diamond tool was manufactured and employed for RUT on surfaces of electroless nickel–phosphorus (Ni–P) plating. A one-dimensional longitudinal-vibration mode is used. The combined effect of ultrasonic vibration, rotation and feed motion leads to high-frequency periodic change of cutting edge׳s motion, which is the basic principle for the RUT process. Therefore, to accurately predict and control the texturing process, the cutting locus is firstly mathematically calculated. Hybrid periodic micro/nano-textures comprising linear grooves at the micrometer scale and sinusoidal grooves at the micrometer or nanometer scale were successfully fabricated on machined surfaces, which are in compliance with the results of the mathematical calculations. Different types of surface textures were generated by changing machining conditions. The surface generation mechanism of RUT is illustrated and discussed by analyzing the surface textural features, the cutting locus and the tool tip׳s geometry, including various tool faces, cutting edges, and the cutting corner. The requirements for RUT technique are concluded.     

熔硅浸渗工艺快速制备C/SiC复合材料的非等温氧化行为

以针刺整体毡为预制体,采用化学气相沉积（CVD）增密制备C/C多孔体,用熔硅浸渗（MSI）工艺快速制备C/SiC复合材料,通过非等温热重分析研究材料低温下的氧化反应动力学和反应机理。结果表明：C/SiC材料的非等温氧化过程呈现自催化特征,氧化机理为随机成核,氧化动力学参数为l：g（A/min^-1）=8.752,Ea=169.167 kJ/mol。MSI工艺中,纤维因硅化损伤产生的活性碳原子易先发生氧化,使C/SiC材料起始氧化温度仅为524℃,比C/C材料约低100℃,且氧化产生大量的裂纹和界面,使材料在氧化初期即具有大的氧化反应速率,C/C材料则出现氧化反应速率滞后现象。     

SiC涂层对C/C复合材料高温氧乙炔焰烧蚀性能影响

采用化学气相反应法在C/C复合材料表面制备抗氧化SiC涂层,借助X射线衍射仪、扫描电镜及能谱等分析手段,研究涂层的结构;通过氧乙炔焰烧蚀试验考察SiC涂层对C/C复合材料高温耐烧蚀性能影响。结果表明:SiC涂层可明显提高C/C复合材料的高温短时耐烧蚀性能,经过20 s的高温氧乙炔焰烧蚀后,C/C复合材料试样的线烧蚀率和质量烧蚀率分别为13μm/s和6.6 mg/s,SiC涂层试样的线烧蚀率和质量烧蚀率分别为22μm/s和0.5 mg/s;在烧蚀中心区,涂层试样的烧蚀以升华分解为主,同时还伴有氧化烧蚀和微区机械剥蚀;在烧蚀过渡区,涂层的烧蚀机制以热氧化和燃气冲刷为主;而在烧蚀边缘区,涂层的烧蚀则主要表现为弱氧化烧蚀。     

Edge-chipping reduction in rotary ultrasonic machining of ceramics: Finite element analysis and experimental verification

Rotary ultrasonic machining (RUM) is one of the machining processes for advanced ceramics. Edge chipping (or chamfer), commonly observed in RUM of ceramic materials, not only compromises geometric accuracy but also possibly causes an increase in machining cost. In this paper, a three-dimensional finite element analysis (FEA) model is developed to study the effects of three parameters (cutting depth, support length, and pretightening load) on the maximum normal stress and von Mises stress in the region where the edge chipping initiates. Two failure criteria (the maximum normal stress criterion and von Mises stress criterion) were used to predict the relation between the edge chipping thickness and the support length. Furthermore, a solution to reduce the edge chipping is proposed based upon the FEA simulations and verified by experiments.     

Microstructure and mechanical properties of 3D fine-woven punctured C/C composites with PyC/SiC/TaC interphases

The 3D fine-woven punctured C/C-(P_yC/SiC/TaC) composites, composed of P_yC/SiC/TaC interphases and pyrocarbon (PyC) matrix, were synthesized by isothermal chemical vapor infiltration (ICVI) methods. The alternating layers and the structure of these composites were examined by polarized light microscopy (PLM), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). It is found that the P_yC matrix has rough laminar (RL) structure, the TaC layer has NaCl-type cubic structure, and the SiC layer has few wurtzite type 10H-SiC besides β-SiC structure. The effects of fiber coating and the bulk density on the tensile and flexural properties of composites along X or Y and Z direction were investigated. It is shown that fiber coated 3D woven punctured C/C composites have good tensile and flexural strength, and the maximum of flexural strength is 375 MPa in X or Y direction at density of 1.89 g/cm³, which is about three times higher than that of samples without TaC/SiC fiber coating. The flexural strength and bending strength increase with increasing the density of the composites. The analysis of fracture surfaces reveals that fibers and fiber bundles are pulled out in composites, indicating that the composite exhibits a non-linear failure behavior through propagation and deflection of the cracks.     

Effect of fiber characteristics on fracture behavior of C_f/SiC composites

Cf/SiC composites were prepared by precursor pyrolysis-hot pressing, and the effect of fiber characteristics on the fracture behavior of the composites was investigated. Because the heat treatment temperature of fiber T300 (below 1500℃) was much lower than that of fiber M40JB (over 2000℃), fiber T300 had lower degree of graphitization and consisted of more impurities compared with fiber M40JB, suggesting that T300 exhibits higher chemical activity. As a result, the composite with T300 showed a brittle fracture behavior, which is mainly ascribed to a strongly bonded fiber/matrix interface as well as the degradation of fibers during the preparation of the composite. However, the composite with M40JB exhibits a tough fracture behavior, which is primarily attributed to a weakly bonded fiber/matrix interface and higher strength retention of the fibers.     

针刺预制体成型参数与C/C复合材料多目标力学性能响应模型研究

基于响应曲面法,采用Design-expert系统研究了预制体针刺成型参数与C/C复合材料多目标性能的相关性,构建了响应曲面数学模型。分析结果表明,针刺C/C复合材料的拉伸强度、剪切强度、压缩强度、增强预制体体密度、拉伸强度与剪切强度比值各响应模型的显著性水平P均小于0.05,且各复相关系数平方和均大于0.82,模拟值与实测值吻合程度较高,可应用于针刺C/C复合材料各项目标性能的设计与预测。当针刺密度为12.18针/cm₂、针刺深度11.68mm、网胎面密度90.55g/m₂时,增强预制体体密度可达0.42g/cm₃,针刺C/C复合材料的综合力学性能最佳,其拉伸强度为116.49MPa、弯曲强度21.84MPa、剪切强度19.41MPa、压缩强度160.88MPa。     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.