机械外文翻译--超声波加工过程

附 录Micro removal of ceramic material (Al2O3) in the precision Ultrasonic machiningAbstractUltrasonic machining process is an efficient and economical means of precision machining of ceramic materials. However, the mechanics of the process with respect to crack initiation and propagation, and stress development in the ceramic workpiece subsurface are still not well understood. This article presents experimental simulation of the process mechanics in an attempt to analyze the material removal mechanism in machining of ceramic (Al2O3). It is found that low-impact force causes only structural disintegration and particle dislocation. The high-impact force contributes to cone cracks and subsequent crater damage. 1999 Elsevier Science Inc. All rights reserved.Keywords: Ceramic material (Al2O3); Utrasonic machining1. IntroductionAdvanced engineering ceramics plays an increasingly important role in the modern manufacturing industries, especially in aerospace, automotive, electronics, and cutting tool industries because of its superior properties such as chemical inertness, high strength and high stiffness at elevated temperatures, high strength to weight ratio, high hardness, corrosion resistance, and oxidation resistance 1. The main barrier hindering further application of advanced ceramics is the inability of the present manufacturing processes to economically and efficiently machine (especially precision machine) ceramics.Currently, grinding is one of the most commonly used processes in the precision machining of ceramic, however,its high energy consumption results in high machining cost. Moreover, grinding also causes workpiece surface damage2. Laser beam machining (LBM) has the potential to be aviable alternative for ceramic machining, but surface quality of machined parts is relatively poor 3. Electrical discharge machining (EDM) is another alternative for ceramic machining,and many engine parts have been successfully machined by EDM 4. Unfortunately, EDM can machine only electrically conductive materials. Other methods such as electron-beam and ion-beam cutting, and microwave cutting have also been proposed, but require additional research and development efforts 57.Ultrasonic machining (USM), offers an effective alternative for precision machining of ceramics due to its many unique characteristics 8,9. Unlike EDM, wire-EDM or electro-chemical machining (ECM), which are all suited only for machining electrically conductive materials, USM can machine all hard and brittle materials 10.Furthermore, USM does not cause any thermal or chemical alterations in the subsurface characteristics of the machined material. Such alterations are inevitable in EDM, ECM, wire-EDM, LBM and many traditional machining. Additionally, USM also produces a better precision surface finish compared to other material removal processes. In most USM practices, an average workpiece surface finish of 0.50 mm can be obtained. With appropriate measures, a surface finish of 0.25 mm can also be achieved 11.USM has a great potential for applications in precision machining of ceramics, however, the material removal mechanism especially with respect to the microstructure and properties of the work material is not well understood 12.The stresses developed in the subsurface are of critical importance when machining brittle ceramics as the inherent microstructural variations and subsurface flaw characteristics influence the resultant stress distributions in the subsurface 13,14. It is necessary to Corresponding author investigate and understand the micro-material removal mechanism in ultrasonic mechanismfor improving its efficiency and precision in machining of ceramics.In USM, the tool strikes the workpiece about 20,000 times in a second, a machining action occurs as the tool vibrates the fine abrasive particles flowing through a very small gap (ten to hundred microns) and propels them against the workpiece material. Material removal takes place in the presence of abrasive slurry. Therefore, it is very difficult to monitor directly or indirectly the presence and progress of involved physical processes. One of the alternatives is to experimentally simulate the conditions of each physical processes to study their respective contribution. This article presents the results of experiments conducted to simulate the kinematics of the impact mechanism similar to the actual ultrasonic machining process. An impact test system was designed to simulate the impact mechanics of single diamond grit impelled by the contact force of the vibrating tool to strike an Al2O3 workpiece. The impact results on Al2O3 are analyzed. The experimental set-up and parameter selection for the impact test are described in second section. The experimental results are given in third section. The microstructural analysis for understanding crack propagation and surface fracture isPresented in fourth section. The last section summarizes the conclusions of this study.2. System set-up for the impact testThe testing unit (Fig. 1a) in the experiments consists of three rigid tubular rods bolted to a solid mild steel base, and clamped to a triangular plate at the top. The length of these rods which determine the maximum drop heights are 3 m.An “N” Brale diamond tipped hardness tester (Wilson Instruments,PA) is used to simulate a single abrasive particle. Once the tool assembly, comprising of the diamond tip fitted to a plexiglas frame (Fig. 1b), is positioned at the predetermined drop height, an electromagnetic release mechanism which also activates a trigger device controlling the start of the time measurements releases the indenter. This setup permits the indenter assembly to accelerate due to gravity without any disturbances or delays. The tool head provides a base for a unidirectional ENTRAN-EGB 125 accelerometer, which is a small-sized low-weight device capable of measuring accelerations up to 5000 g. Signals transmitted by a miniature semiconductive wheatstone bridge of the accelerometer device are amplified by a storage oscilloscope, and the data is stored in a 386 PC connected to the data acquisition system. Because the impact force is a function of the mass of the indenter, the weight of the tool assembly was designed for this test.2.1. Experimental parameter selectionIn most cases for an USM machining, the diameter of a vibrating tool ranges from 4 to 6 mm. The size of the abrasive particles are between 50 and 75 mm. In practice forces applied in ultrasonic machining are within a range of 4.0 and 6.0 N 11. Approximately, 100 particles are assumed to be covered under the tool during each down strike, and the mass of the indenter assembly is estimated by Newtons second law of motion, Mass (M) 5 0.51 (kg)/ 9.806(m/s2) 5 52.0 g. To simulate the effect of a particle striking on a workpiece surface in an USM machining process, a lightweight plexiglas frame was designed to hold the indenter assembly. The assembly with total weight of 50.0 g drops from a precalculated height to strike the workpiece. The drop heights for the indenter are between 45250 cm to obtain the impact velocities observed during the actual USM processes. A set of five drop heights were used in this experiment and a time device is automatically triggered when the indenter is released from its predetermined drop position. Fine grained (5 mm) Alumina (Kadco Ceramics, NJ) is impacted at two locations under predetermined impact velocities; the impact locations are far apart to avoid any interaction effects. The diameter of the indenter tip is selected close to the size of particles used in USM machining.The test is aimed at analyzing the dynamic parameters of a single particle during impact and the fracture characteristics of the damage. The impact parameters are calculated based on free fall up to the point of impact and the characteristics during contact are estimated either directly from measurements or indirectly from calculations. When graphically represented over time, the characteristics of a single particle during impact are illustrated by different parameters such as force, velocity, or distance. An understanding of the mechanics of the material removal process based on observations of the microstructure were performed with the aid of a high resolution scanning electron microscope. Fig. 2. a) Impact force and velocity over time; b) Impact force and depth over time; c) Impact velocity and depth over time.3. Results: Impact force, velocity, and penetration depthThe output from the accelerometer and the time measurementsprovide specific impact characteristics such as the force exerted, the impact velocity, and the depth of penetration. The results of the different impact characteristics, are presented in Table 1. The impacts are identified by the workpiece number (p01, p02, and p03), and the impact number (i01, . . . i05). The maximum points sampled indicate the number of data points collected at a sampling rate of a million points per second. Relative time values are computed with the aid of computer software, which identifies the peak position and triggers a relative time clock to store data up to 500 points before the maximum depth (peak) and up to 6000 points after the peak. The end time in seconds is the instant when the indenter leaves the workpiece surface after impact. Fig. 2 (a, b, and c) represent the comparative results of the impact parameters of force, velocity, and depth of penetration at a drop velocity of 1.92 m/sec. Fig. 2a compares the impact velocity and force during penetration; at the point of contact the velocity is maximum, and as the acceleration at this point is zero, the impact force drops to zero. The velocity reduces on impact and finally reaches zero at a point when the indenter is momentarily stationery, this point corresponds to the instant when the force exerted on the indenter by the material is maximum. This instant is also described by the maximum depth of penetration as described in Fig. 2b and c. The velocity on rebound increases as described by the curve until a combination of the acceleration due to gravity and material resistance reduces it to zero. A summary of results of the impact characteristics are presented in Table 2.For linearly increasing values of the maximum free fall velocity there is a proportional increase in the maximum force experienced by the diamond tip during impact, and a corresponding increase in the penetration depth. Fig. 3 describes the effect of the maximum free fall velocity on the penetration depth. The depth shows a steep increase at intermediate velocities and a relatively gradual increase at the low and high velocities. Fig. 3. Effect of maximum free fall velocity on penetration depth. Fig. 4. Schematic of the crack morphology in brittle materials 15. Fig. 5. Surface characteristics at an impact velocity of 0.98 m/sec. (3 750.)4. Microstructure analysisAs the crack propagation and subsurface fractures are mainly responsible for material removal, the SEM technique is particularly useful in its ability to give a vivid and more definitive representation of the crack types and their modes of propagation with respect to the microstructural characteristics of the material.In a study reported by Smith et al. 15 on indentation fractures in brittle materials, median cracks growing perpendicular to the surface just below the tip of the indenter and are contained within the subsurface, but eventually propagate to the surface. Lateral cracks grow parallel to theimpact surface, and propagate toward the top at high-impact velocities. The characteristics of these cracks are described schematically in Fig. 4, which shows a sectional view of the impacted area.The SEM micrographs of the damaged area at different impact velocities are presented in Fig. 59. Fig. 5 shows the surface characteristics at 7503, of the damage under an impact velocity of 0.98m/sec. It can be seen that fragmentation, chipping, and microfracture due to the impact force have mainly contributed toward material removal. Some material removal may also have resulted from dislodged particles as observed by the presence of cavities in the microstructure. With an impact velocity of 1.15m/sec., the presence of intergranular microcracks can be observed in Fig. 6. However, most of the material is removed from the impact force. At an impact velocity of 1.5 m/sec., cracks projecting from the deformation zone are clearly visible in Fig. 7. The presence of these cracks indicate lateral and median crack propagation to the surface. The presence of intergranular microcracks are also visible and are indicated by arrowheads.Because material removal involves both plastic flow and fracture, a combination of compressive and tensile stresses, respectively, are responsible for material removal 11. Median cracks perpendicular to the surface and propagating below the deformation zone, result from the compressive stresses due to the impact force of the indenter. Lateral Fig. 6. Surface characteristics under an impact velocity of 1.15 m/sec. (3350.) Fig. 7. Surface characteristics under an impact velocity of 1.5 m/sec. (3650.) Fig. 8. Surface characteristics under an impact velocity of 1.9 m/sec.(31200.) Fig. 9. Surface characteristics from an impact velocity of 2.05 m/sec. (3750.)cracks propagate to the surface due to high impact forces 15 and merge with the median cracks (Fig. 8) describes the deformation zone with cracks propagating from the epicenterat an impact velocity of 1.5 m/sec. These cracks originate in the subsurface and propagate to the top, contributing to material removal by dislodging large sections of the material. The network of intergranular microcracks also contribute to the material removal process. Fig. 9 describes the surface characteristics of the damage under an impact velocity of 2.05 m/sec. The dislodged particles in the crater range from small grain sized particles to chunks of dislodged material in the 4050 mm. range. In an effort to relate the surface area of the damage to the impact velocity, a measurement technique was applied to estimate the area of each crater. The results of the test shown in Fig. 10 describes an increasing trend for increasing impact velocities. This may be attributed to the material removed when lateral and median cracks merge at the surface. Thus, the impact zone material removal occurs mainly from a combination of particle coalescence and microstructural disintegration. Below the plastic zone, at the lowimpact forces, material removal occurs from particles dislodged from the surface. Fragmentation from the impact force of the diamond tip, and intergranular microcracks also result in material removal. At the higher impact velocities between 1.92 and 2.1 m/sec lateral and median cracks merge at the surface to dislodge large particles of material in the range between 20 to 50mm. At these loads the network of intergranular microcracks also play a significant role inthe material removal process.5. ConclusionsThis study was aimed at understanding the mechanism of the material removal process in fine grain Alumina during precision ultrasonic machining. Microstructural analysis of the damaged surface due to the dynamic impact of an abrasive particle indicates the presence of two phenomena that contribute to material removal; the deformation at the point of impact, and the brittle structure below the impact zone. From the dynamic impact tests, material removal in the USM process appears to be a function of the impact velocity, which is determined by the frequency and amplitude of the vibrating tool. Material in the impact zone is removed by fragmentation and by chipping microfracture due to the high compressive stresses developed in the region. At low-impact velocities, material removal in the brittle substructure occurs mainly due to structural disintegrations and particle dislocations. At higher velocities, material is removed by a network of intergranular microcracks and from the propagation of lateral and median cracks. These cracks merge at the surface dislodging large sections of the material.AcknowledgmentsThe authors thank the Nebraska Research Initiative Fund for their financial support. The authors also gratefully acknowledge the assistance of Ms. L. Shi and Mr. N. Saxena in the preparation of this article.附 录超声波加工过程是一个高效和经济的手段，用在精密加工的陶瓷材料上。然而，力学这一过程对裂纹产生和扩大，并强调发展陶瓷工件表层仍然没有得到很好的理解。本文介绍的模拟实验研究的过程力学，试图分析材料去除机理在加工陶瓷（氧化铝）过程中。结果发现，低冲击力只有助于结构性解体和粒子脱位。高冲击力有助于锥裂缝和随后的缝口损坏。 关键词：陶瓷材料（氧化铝） ;超声波加工 1 ：导言 先进的工程陶瓷发挥着越来越重要的作用，现代制造业，特别是在航空航天，汽车，电子和切割工具行业，由于其优越的性能，如化学惰性，高强度，高刚度在较高高温，高强度重量比，硬度高，耐腐蚀，抗氧化目前，磨削是一种最常用的精密加工陶瓷方法，然而，其能源消耗高，结果增加加工费用。此外，还造成磨削工件表面损伤 。激光加工（ LBM ）有可能成为可行的选择陶瓷加工，但表面质量机械零件的相对较差 。放电加工是另一个替代的陶瓷加工，许多发动机部件已成功应用电火花加工 。然而，电火花加工机只能用唯一导电材料。其他方法，例如电子束和离子束切割和微波切割也被提出，但需要额外的研究和开发工作。 超声波加工（超声波马达） ，提供了一个有效的用于精密加工陶瓷的方法由于其有许多独特的特点。电火花，线切割或电化学加工 ，这些都是只适合加工导电材料，超声波马达可用于所有硬脆材料 。此外，超声波加工会引起加工材料的任何热或化学的表层特征改变。这种改变在电火花加工，电子对抗，线切割，LBM和许多传统的机械加工中是不可避免的。此外，超声波加工作了一个更好的表面加工精度与其他材料去除过程相比。在大多数超声波加工法中，平均工件表面光洁度可达0.50毫米。有了适当的措施，表面光洁度也可以实现到0.25毫米 。超声波加工大的应用潜力在精密加工陶瓷中，然而，对材料去除率机制，尤其是对微观结构和工作性质的材料没有得到很好的理解。在强调发达国家在地下关于要性脆性陶瓷加工时的固有微观结构的变化和地下缺陷特征由此造成的影响应力分布在地下微观材料去除机理的超声机制为提高其效率和加工精度陶瓷。在超声波加工中，罢工工件约20000次每秒，发生运动的工具振动细磨料颗粒流经一个非常小的差距（ 10至100微米） ，并推动他们到工件材料。材料去除发生在在场的磨料泥浆。因此，它是非常困难的监测直接或间接的存在和进步涉及的物理过程。一个替代办法是模拟实验条件的每个物理过程，研究其各自的贡献。本文介绍的实验结果进行运动学模拟的影响类似机制的实际超声波加工过程。撞击试验系统的设计是为了模拟影响力学单钻石的砂砾促使接触力的振动工具罢工的氧化铝工件。 结果影响氧化铝进行的分析。实验设置和参数的选择影响试验中描述的第二部分。实验结果给出了第三节的微观结构分析理解和表面裂纹扩展第四部分中提出的。最后一节总结了这项研究的结论。 2 。系统设置的影响试验 测试单位（图1A ）在实验中包括三个刚性管杆螺栓，以坚实的钢架为基础，钳位到三角形板在顶部。这些长度棒确定的最大下降高度为3米 图 1 。表1 结果所产生的影响在不同的特性下降高地测试图 a为压降测试图 b为影响的立场 “ N ”形状钻石刀具的硬度仪（威尔逊仪器巴勒斯坦权力机构）用在模拟磨粒。当刀具被安装上，组合的钻石刀具安装上有机玻璃框架（图1B） ，在预定下降高度上定位，电磁释放装置激活触发装置来控制开始测量时刻的压头。此安装允许，由于加速重力没有任何干扰或延误。该工具头提供了一个基地，单向进入 ，安装装置加速度125，这是一个小型体重装置能测量加速度高达5000克信号转交的一个缩影半导体压阻桥梁的加速度装置扩增存储示波器，数据存储在一个相当于386个人脑的电脑数据采集系统中。由于冲击力影响是一个功能的质量压痕，这一试验的目的是为重量工具安装。2.1 实验参数的选择在大多数情况下的超声波马达加工,直径振动工具范围从4至6毫米。磨料颗粒尺寸的大小是50至75毫米。在实践中其适用于超声波加工的范围为4.0和6.0 大约100颗粒子被假定以所涵盖的工具在每次下跌受埙，和大众的压痕一起估计， 牛顿第二定律的运动，质量（男） 5 0.51 （公斤） / 9.806 （ m/s2 ） 5 52.0克 模拟粒子效果显着的工件表面加工工艺的超声波马达，轻巧有机玻璃框架的目的是要举行压大会大会的总重量五十点克下降了 precalculated高度罢工工件。下拉高地压为45-250厘米之间取得观察速度的影响在实际超声波马达进程了一套五份下降高地中使用了这个实验和时间的设备会自动触发时压释放其预定的下降立场。细粒度（ 5毫米）氧化铝（ Kadco陶瓷，新泽西州）是影响在两个地点的冲击速度下的预定;影响地点相距遥远，以避免任何互动的效果。直径的压痕提示选择密切的大小颗粒加工中使用的超声波马达。试验的目的是分析的动态参数单粒子在影响和断裂特征造成的损害。影响参数的计算基于自由跌落到弹着点和特点在联系，估计可以直接从测量或间接计算。当图形代表随着时间的推移，一个单一的特点粒子在影响是不同的参数说明如力量，速度，或距离。了解力学的材料去除过程的意见的基础上该组织进行的帮助下，高分辨率扫描电子显微镜。图 2 。a）冲击力和速度随着时间的推移; b ）影响力和深度随着时间的推移; c ）影响速度和深度随着时间的推移。 表2 综述结果的影响特点图a 冲击力和速度随着时间的推移图b 影响力和深度随着时间的推移图C 影响速度和深度随着时间的推移3 结果：冲击力，速度和穿透深度输出加速度和时间的测量提供具体的影响等特点工队施加的影响，速度和深度渗透。结果的不同影响特点， 列于表1 的影响是确定的工件号码（ p01 ， p02和p03 ） ，和影响数量（ i01 - i05 ） 。最高点取样表明数据点数目收集采样率万分每秒。相对时间价值观在计算机的帮助下，计算机软件，其中确定该峰的位置和相对时间触发时钟数据存储多达500点之后的最大深度（高峰）和高达6000点后的最高点。结束时间 秒的瞬间时，压叶片工件表面后产生的影响。图 2（ a ， B和C ）代表的比较结果参数的影响力，速度和深度渗透在下降的速度一点九二米/秒。图2A型比较冲击速度和渗透力;在联络点的速度是最大的，并作为加快在这一点上是零，冲击力降到零速度降低的影响，并最终达到零上点压时，这一点对应于即时生效时施加的压痕的材料是最大的。这也是即时所描述的最大深度的渗透所描述在图2b和c关于反弹的速度增加所描述的曲线之前的组合加速由于重力和材料电阻降低到零。汇总结果的影响特点列于表2 。线性增加值最大的自由落体速度有一个比例增加最多部分所经历的钻石冰山期间的影响，以及 相应增加深度。图3描述影响最大的自由降落速度的穿透深度。深度显示在急剧增加中间速度和相对逐步增加在低和高的速度。图3 ：影响最大的自由降落速度的穿透深度。图4 ：示意图裂纹形态的脆性材料 。 图5 ：表面特性在冲击速度为0.98米/秒。 4 ：微观结构分析 由于裂纹扩展和水下裂缝主要负责材料去除，扫描电镜技术是特别有用，它能够让一个生动和更明确的代表性裂缝类型及其传播模式方面的微观结构特征的材料。在一项研究报告史密斯等人。关于压痕骨折的脆性材料，中间垂直裂缝越来越表面略低于冰山的压并包含在地下，但最终传播到地表。横向裂缝增长并行影响面，和宣传的歌曲在高影响速度的特点，这些裂缝描述 schematically图 4 ，这显示了截面鉴于影响地区。SEM照片损坏的地区，不同的冲击速度介绍图59。图5显示表面特性在7503 米下的损害的冲击速度为0.98米/秒可以看出，分裂切和微由于撞击力主要贡献的材料去除率。一些材料去除率也可能造成脱落观察到的粒子的存在空洞的微观结构。冲击速度1.15米/秒该间存在裂纹可以看出图6。 然而，大多数材料是从冲击力在冲击速度为1.5米/秒裂缝预计从变形区是显而易见图7。 存在这些裂纹表明横向和中间裂纹扩展到地表。在场的情况下 间微也可见，由箭头并表示。由于材料去除涉及塑性流动和断裂，它结合了压缩和拉伸应力，分别负责材料去除 。中位数裂纹垂直于表面和宣传以下的变形区，因压强调由于撞击力的