高性能表面层制造:基于可控表面完整性的精密制造

高性能表面层制造是具有特殊功能性表面层结构零件的精密制造,体现了高性能零件性能与几何参数一体化制造的特点。依据功能性表面层结构零件的性能要求所设计的几何参数和材料特性,选择表面层加工制造方法,确定加工工艺载荷的物质与能量输入条件,通过减控加工工艺的几何、结构、物理、化学等多源耦合约束,构建主动协调的材料加工载荷的应力场、温度场和化学位场等(多)场环境,相应地揭示零件表面完整性变化关系内禀的加工过程印记,利用可控的表面完整性与高性能零件性能的关联模型,实现具有特殊功能性表面层的精密制造。高性能表面层加工制造原理的核心是表面完整性的形成机制、评价方法和调控作用,所提出的高性能表面层精密制造的体系框架,以基于知识方法取代实验迭代的试错法,可解决高性能制造的加工制造反问题。     

State-of-the-art of non-destructive measurement of sub-surface material properties and damages

The possibilities of common non-destructive measuring techniques are reviewed in this paper for their applications in precision engineering. The grazing X-ray technique is believed to be a powerful improvement of the conventional X-ray techniques under both the diffraction mode and the fluorescent mode. Information of the crystallographic structure and chemical composition can be obtained to a nanometre resolution. Ultrasound can be used in scanning acoustic microscopy to give information on the physical or even chemical nature of superficial layers. Raman spectroscopy has now become an important tool for studying superficial structures, chemical composition and stresses in cyrstalline and amorphous materials: it is recommended to use this method especially for the investigation of monocrystalline silicon and germanium. The instrumented microindentation technique is a quasi- non-destructive technique for evaluating mechanical material properties like hardness and Young's modulus in a nanometre range. It can be used on any material that does not require special surroundings like a vacuum. Photothermal microscopy has been developed recently for the non-destructive testing of the local thermal properties of materials. By using the Mirage effect and its local measurement above the surface, a non-destructive depth profiling of surface damages can be obtained.     

Increase in the efficiency of ultrasonic processing of components made of brittle materials

A study of the ultrasonic dimensional processing of KU-1-grade quartz glass, SO-115-grade glass ceramics, tungsten carbide, and KP-1-grade alumina ceramics at different amplitudes and frequencies of the ultrasound effect has been carried out. The dependence of the value of defects (cleavages) on the amplitude-frequency characteristics of the ultrasound and the structural features of the material being processed has been determined. It has been determined that, by applying certain oscillation frequencies of the tool during processing at lower amplitudes, one can reduce the sizes of cracks (cleavages) and the magnitude of roughness while preserving the process performance. It has been shown that the hardening of working surface of the tool based on the exposure of low-temperature plasma of combined discharge makes it possible to increase its age resistance by up to three times.     

Physiologic consequences of high frequency jet ventilation

High frequency jet ventilation provides effective gas exchange at frequencies of 60 to 900 minute-1. The selection of mechanical ventilatory support is based on multiple factors, including potential physiologic advantages and disadvantages. Although HFJV has demonstrated no clear significant advantage over CMV in many clinical applications, because it produces significantly lower peak airway pressures than CMV, cardiac compromise and barotrauma might be prevented. In addition, in situations in which tissue movement should be minimized, e.g., microneurosurgery, HFJV may prove beneficial.     

低频力学谱测试技术的研究

本文研究了基于Win98/2000的低频力学谱仪的先进测试技术——虚拟仪器、相位差的数字相关计算、内耗固有背景的处理、Windows操作系统下的精确定时采样、直接数字频率合成产生低频信号等。文中还给出了基于Win98／2000的低频力学谱仪的组成和典型实验数据。本文研制的低频力学谱仪在测量精度、测试自动化程度及数据处理等方面的性能与以往的固体内耗仪相比都大大提高了。     

高速铣削时硬质合金刀具与NAK80模具钢化学性能匹配研究

高速铣削加工时温度较高,易引起刀具与工件之间产生化学反应,加剧刀具与工件磨损,缩短刀具寿命。实现刀具材料和工件材料的化学性能匹配,可有效保证模具型面加工质量和提高生产效率。化学性能匹配主要从刀具材料与工件材料的氧化以及扩散方面进行研究。通过理论计算得到刀具与工件材料的化学反应自由能;结合高温氧化试验,研究NAK80模具钢的高温氧化产物并探究其抗氧化性能;基于变系数浓度扩散模型,利用元素扩散试验获得了NAK80模具钢与刀具材料间的元素扩散强度。结果表明,NAK80模具钢的氧化程度随温度升高而加剧,温度高于1300K可发生完全氧化;NAK80模具钢中的Fe元素扩散深度随硬质合金刀具中Co元素含量的增加而增加。     

The influence of surface topography on the x-ray intensity in electron microprobe analysis (EDS/WDS)

Copper surfaces were ground or polished to various surface roughnesses and analysed by WDS and EDS. Both K and L photons were analysed at the accelerating voltages 5, 15 and 45 kV. The influence of topography on the K-peak intensity was negligible, while the L-peak intensity was rather sensitive to the surface roughness. For low and medium accelerating voltages the L-peak intensity was lower from a rough than from a smooth surface because of shadowing effects. For high accelerating voltages it was instead enhanced by the surface roughness because of an edge effect which outweighs the shadowing. If small areas are analysed (as always in WDS-analyses) the variation of the local orientation will cause some scattering in the measured intensities of low energy photons. High energy photons are less affected by the surface roughness. Because of the larger escape length in the material these photons can reach also the detector from areas which are shadowed. Generally, if the surface roughness cannot be avoided, high energy photons should be preferred for the analysis or the peak to background ratio method should be used. The accelerating voltage should in all cases be kept as low as possible in order to minimise the influence of surface roughness.     

高速铣加工壁板类零件转角

高速切削加工技术是一项全新的先进制造技术,其具有高的生产效率、加工精度、表面质量和生产成本低等优点。航空制造业中主承力结构件多数为整体坯料"掏空"的整体结构件,代替传统的拼接结构。因此在对材料去除率大、加工质量要求高、加工周期长的整体结构件加工中更能体现其独特的优势。探讨适合高速切削特点高速铣削零件转角的工艺改进方案,提高零件的加工质量缩短加工周期,能优质高效地完成生产任务。     

Review on mechanism and process of surface polishing using lasers

Laser polishing is a technology of smoothening the surface of various materials with highly intense laser beams. When these beams impact on the material surface to be polished, the surface starts to be melted due to the high temperature. The melted material is then relocated from the ‘peaks to valleys’ under the multidirectional action of surface tension. By varying the process parameters such as beam intensity, energy density, spot diameter, and feed rate, different rates of surface roughness can be achieved. High precision polishing of surfaces can be done using laser process. Currently, laser polishing has extended its applications from photonics to molds as well as bio-medical sectors. Conventional polishing techniques have many drawbacks such as less capability of polishing freeform surfaces, environmental pollution, long processing time, and health hazards for the operators. Laser polishing on the other hand eliminates all the mentioned drawbacks and comes as a promising technology that can be relied for smoothening of initial topography of the surfaces irrespective of the complexity of the surface. Majority of the researchers performed laser polishing on materials such as steel, titanium, and its alloys because of its low cost and reliability. This article gives a detailed overview of the laser polishing mechanism by explaining various process parameters briefly to get a better understanding about the entire polishing process. The advantages and applications are also explained clearly to have a good knowledge about the importance of laser polishing in the future.     

Predictive carbon nanotube models using the eigenvector dimension reduction (EDR) method

It has been reported that a carbon nanotube (CNT) is one of the strongest materials with its high failure stress and strain. Moreover, the nanotube has many favorable features, such as high toughness, great flexibility, low density, and so on. This discovery has opened new opportunities in various engineering applications, for example, a nanocomposite material design. However, recent studies have found a substantial discrepancy between computational and experimental material property predictions, in part due to defects in the fabricated nanotubes. It is found that the nanotubes are highly defective in many different formations (e.g., vacancy, dislocation, chemical, and topological defects). Recent parametric studies with vacancy defects have found that the vacancy defects substantially affect mechanical properties of the nanotubes. Given random existence of the nanotube defects, the material properties of the nanotubes can be better understood through statistical modeling of the defects. This paper presents predictive CNT models, which enable to estimate mechanical properties of the CNTs and the nanocomposites under various sources of uncertainties. As the first step, the density and location of vacancy defects will be randomly modeled to predict mechanical properties. It has been reported that the eigenvector dimension reduction (EDR) method performs probability analysis efficiently and accurately. In this paper, molecular dynamics (MD) simulation with a modified Morse potential model is integrated with the EDR method to predict the mechanical properties of the CNTs. To demonstrate the feasibility of the predicted model, probabilistic behavior of mechanical properties (e.g., failure stress, failure strain, and toughness) is compared with the precedent experiment results.     

Erosion–corrosion performance of high-Cr cast iron alloys in flowing liquid–solid slurries

In many slurry transportation systems, such as in FGD (Flue Gas Desulphurization) and chemical processing applications, corrosion and erosion are the two main mechanisms of material degradation of the pump wet-end components including pump casing, impeller and liners. The performance of a selected material is mostly dependent upon its relative corrosion and erosion resistance to the service environment. In these cases erosion, corrosion and the related synergistic effects can be very complicated since they are affected by numerous factors including solid and slurry properties, chemical contents, hydraulic conditions and temperatures. In this experimental study, sliding Coriolis erosion testing has been performed with various corrosion factors such as pH value, chlorides content and temperature to evaluate the erosion–corrosion resistance of some high-alloyed white cast irons containing different levels of chromium and other elements. Optical microscope and SEM-EDS have also been used to examine microstructure and surface conditions of tested materials. Results indicated that material loss due to corrosion factors increased as acidity-chlorides and temperature increased. At relatively high corrosion intensity, the white cast irons with higher alloy content (especially chromium) clearly showed improved corrosion resistance and combined erosion–corrosion resistance over those with lower alloy content. Under certain corrosion and hydraulic conditions, particle size is perhaps the single most influential factor on erosion–corrosion rate of the high-Cr cast iron alloys. Relatively large particles are much more effective than small ones at removing both the corroded surface layer and the fresh material, causing substantially higher rate of material loss. Some other related factors have also been addressed.     

Synthesis of silicon carbide ceramics from a polysilastyrene at low temperatures

The conventional route for preparation of silicon carbide ceramics is by the use of pressureless sintering, hot pressing, or hot isostatic pressing of silicon carbide starting powders. High sintering temperatures (2073–2473 K) and the addition of sintering additives are normally used to enhance densification. These sintering additives, however, form second phases at grain boundaries which impair the mechanical properties of the material, particularly at high temperatures. It is therefore desirable that new processing routes are developed that overcome these difficulties. A proposed route is to use a polymeric pressure which can provide a Silicon carbide matrix as binding agent for silicon carbide powders, thus making the requirement for high temperatures and sintering additives unnecessary. This paper reports observations of the direct transformation of a polymeric precursor into amorphous Si–C, and crystalline SiC at low temperatures, and the use of this precursor as a binder for the production of SiC powder/ex-precursor SiC composites.     

Feasibility of tunable MEMS photonic crystal devices

Periodic photonic crystal structures channel electromagnetic waves much as semiconductors/quantum wells channel electrons. Photonic bandgap crystals (PBC) are fabricated by arranging sub-wavelength alternating materials with high and low dielectric constants to produce a desired effective bandgap. Photons with energy within this bandgap cannot propagate through the structure. This property has made these structures useful for microwave applications such as frequency-selective surfaces, narrowband filters, and antenna substrates when the dimensions are on the order of millimeters. They are also potentially very useful, albeit much more difficult to fabricate, in the visible/near-infrared region for various applications when the smallest dimensions are at the edge of current micro-lithography fabrication tools. We micro-fabricated suspended free standing micro-structure bridge waveguides to serve as substrates for PBC features. These micro-bridges were fabricated onto commercial silicon-on-insulator wafers. Nanoscale periodic features were fabricated onto these micro-structure bridges to form a tunable system. When this combined structure is perturbed, such as mechanical deflection of the suspended composite structure at resonance, there can be a realtime shift in the material effective bandgap due to slight geometric alterations due to the induced mechanical stress. Extremely high resonance frequencies/device speeds are possible with these very small dimension MEMS.     

Silicon Nitride Rolling Bearings for Extreme Operating Conditions

The high hardness and excellent high-temperature performance of ceramics are attractive for critical rolling-bearing applications, such as in high-thermal-efficiency engines. In addition, the lower density of ceramics permits lower centrifugal loads, less skidding at high speeds, and, consequently, results in longer bearing component life than conventional bearing steels or alternative super-alloys or cermets. Silicon nitride has the lowest friction and elastic modulus of the available ceramics, which reduce bearing contact stresses. This paper reviews the merits and demerits of silicon nitride as a bearing material, its processing, and different mechanisms for solid-lubricant replenishment in severe environments. Test data and application design guidelines accumulated to dale are also provided.     

Structure and mechanical properties of tribologically induced nanolayers

Ultra low wear rates are common for lubricated state-of-the-art mechanical devices used in machines and cars. The main paths of energy dissipation are heat and wear generation as well as a significant change of near-surface material within the range of a few hundred nanometers. Due to mechanical intermixing at the asperity level the involved materials change with respect to chemical composition, morphology and mechanical properties. By means of focused ion beam analysis the morphology of the near-surface material of tribologically stressed and unstressed samples was investigated. In addition, nanoindentation was applied to characterize the mechanical properties. In the presented study mechanical intermixing leads to nanocrystalline material which is softer than bulk material. Depending on the level of tribological stressing, the modified material exhibits smaller energy dissipation, thus lower friction and wear.     

立方氮化硼刀具在切削加工中的应用

以立方氮化硼（CBN）刀具为研究对象,简要介绍了CBN材料物理机械性能,以及其具有的硬度高、稳定性好、耐磨、加工质量高等优势,阐述了CBN刀具在现代机械加工中的应用,并提出我国机械行业CBN刀具在切削加工中的应用及发展。     

Process optimization for PA12/MWCNT nanocomposite manufacturing by selective laser sintering

Nanocomposites produced through the addition of carbon nanotubes to a polymeric matrix can improve the material properties. The mobility of the polymer chains is usually affected, and this is also related to the properties. Parts produced with the free-form fabrication process using the selective laser sintering (SLS) technique can be used in different high-performance applications as they do not require expensive tools for their manufacture. A specific field of interest is the aerospace industry which is characterized by a low production volume and the need for materials with a high performance to weight ratio. In this study, the free-form fabrication by SLS of parts made from nanocomposites comprised of polyamide 12 and multiwalled carbon nanotubes (MWCNTs) was investigated. Specimens were manufactured by SLS to identify the appropriate processing parameters to achieve high mechanical properties for aerospace applications. Laser energy density was adjusted to improve the material density, flexural modulus, and stress at 10 % elongation. Design of experiments was used to identify and quantify the effects of various factors on the mechanical properties. The results obtained showed that there was a limit to the amount of MWCNTs which could be mixed with the polyamide powder to improve the mechanical properties since a higher content affected the laser sintering process.     

液态二氧化碳冷却切削系统的研制与DT4切削中的应用

DT4工业纯铁是一种碳含量极低的铁基合金,由于其具有磁感应强度高、磁导率高、造价低廉等优点,在电磁领域和航空等工业中得到广泛应用。但DT4纯铁类材料的韧性和延展性较高造成其加工性能差,难以保证加工质量,故针对DT4加工精度低、粗糙度差的问题,采用低温强压二氧化碳作为冷却剂替代品的方法,进行了液态二氧化碳在强压喷射条件下的DT4纯铁材料的切削试验,分析了液态二氧化碳在强压喷射条件下对工业纯铁DT4加工的影响以及PCD刀具材料在干冰低温切削中的磨损抑制原因。试验结果表明,本文提出的液态二氧化碳喷射平台切实有效,而PCD刀具在此前提下能极大提升DT4表面质量并降低切削力。该方法在DT4的精密薄壁车削方面具有可观的应用价值前景。     

超声弹性成像在肿瘤良恶性鉴别中的应用

超声弹性成像技术自问世以来,迅速发展并得到广泛的临床应用,已成为超声诊断的重要工具。本文对超声弹性成像在乳腺、甲状腺、前列腺、肝脏肿瘤的良恶性鉴别以及子宫、肾脏、淋巴结、涎腺、胰腺、胆囊良恶性肿瘤评估中的应用进行综述。     

Comparison of crystallization characteristics and mechanical properties of polypropylene processed by ultrasound and conventional micro-injection molding

Ultrasound injection molding has emerged as an alternative production route for the manufacturing of micro-scale polymeric components, where it offers significant benefits over the conventional micro-injection molding process. In this work, the effects of ultrasound melting on the mechanical and morphological properties of micro-polypropylene parts were characterized. The ultrasound injection molding process was experimentally compared to the conventional micro-injection molding process using a novel mold, which allows mounting on both machines and visualization of the melt flow for both molding processes. Direct measurements of the flow front speed and temperature distributions were performed using both conventional and thermal high-speed imaging techniques. The manufacturing of micro-tensile specimens allowed the comparison of the mechanical properties of the parts obtained with the different processes. The results indicated that the ultrasound injection molding process could be an efficient alternative to the conventional process.