中英文翻译
2020-03-03 21:48:09 来源：范文大全收藏下载本文
特种加工工艺
介绍
传统加工如车削、铣削和磨削等，是利用机械能将金属从工件上剪切掉，以加工成孔或去除余料。特种加工是指这样一组加工工艺，它们通过各种涉及机械能、热能、电能、化学能或及其组合形式的技术，而不使用传统加工所必需的尖锐刀具来去除工件表面的多余材料。
传统加工如车削、钻削、刨削、铣削和磨削，都难以加工特别硬的或脆性材料。采用传统方法加工这类材料就意味着对时间和能量要求有所增加，从而导致成本增加。在某些情况下，传统加工可能行不通。由于在加工过程中会产生残余应力，传统加工方法还会造成刀具磨损，损坏产品质量。基于以下各种特殊理由，特种加工工艺或称为先进制造工艺，可以应用于采用传统加工方法不可行，不令人满意或者不经济的场合：
1.对于传统加工难以夹紧的非常硬的脆性材料； 2.当工件柔性很大或很薄时； 3.当零件的形状过于复杂时；
4.要求加工出的零件没有毛刺或残余应力。
传统加工可以定义为利用机械（运动）能的加工方法，而特种加工利用其他形式的能量，主要有如下三种形式： 1.热能； 2.化学能； 3.电能。
为了满足额外的加工条件的要求，已经开发出了几类特种加工工艺。恰当地使用这些加工工艺可以获得很多优于传统加工工艺的好处。常见的特种加工工艺描述如下。
电火花加工
电火花加工是使用最为广泛的特种加工工艺之一。相比于利用不同刀具进行金属切削和磨削的常规加工，电火花加工更为吸引人之处在于它利用工件和电极间的一系列重复产生的（脉冲）离散电火花所产生的热电作用，从工件表面通过电腐蚀去除掉多余的材料。
传统加工工艺依靠硬质刀具或磨料去除较软的材料，而特种加工工艺如电火花加工，则是利用电火花或热能来电蚀除余料，以获得所需的零件形状。因此，材料的硬度不再是电火花加工中的关键因素。
电火花加工是利用存储在电容器组中的电能（一般为50V/10A量级）在工具电极（阴极）和工件电极（阳极）之间的微小间隙间进行放电来去除材料的。如图6.1所示，在EDM操作初始，在工具电极和工件电极间施以高电压。这个高电压可以在工具电极和工件电极窄缝间的绝缘电介质中产生电场。这就会使悬浮在电介质中的导电粒子聚集在电场最强处。当工具电极和工件电极之间的势能差足够大时，电介质被击穿，从而在电介质流体中会产生瞬时电火花，将少量材料从工件表面蚀除掉。每次电火花所蚀除掉的材料量通常在10-5～10-6mm3范围内。电极之间的间隙只有千分之几英寸，通过伺服机构驱动和控制工具电极的进给使该值保持常量。 化学加工
化学加工是众所周知的特种加工工艺之一，它将工件浸入化学溶液通过腐蚀溶解作用将多余材料从工件上去除掉。该工艺是最古老的特种加工工艺，主要用于凹腔和轮廓加工，以及从具有高的比刚度的零件表面去除余料。化学加工广泛用于为多种工业应用（如微机电系统和半导体行业）制造微型零件。
化学加工将工件浸入到化学试剂或蚀刻剂中，位于工件选区的材料通过发生在金属溶蚀或化学溶解过程中的电化学微电池作用被去除掉。而被称为保护层的特殊涂层所保护下的区域中的材料则不会被去除。不过，这种受控的化学溶解过程同时也会蚀除掉所以暴露在表面的材料，尽管去除的渗透率只有0.0025～0.1 mm/min。该工艺采用如下几种形式：凹坑加工、轮廓加工和整体金属去除的化学铣，在薄板上进行蚀刻的化学造型，在微电子领域中利用光敏抗蚀剂完成蚀刻的光化学加工(PCM)，采用弱化学试剂进行抛光或去毛刺的电化学抛光，以及利用单一化学活性喷射的化学喷射加工等。如图6.2a所示的化学加工示意图，由于蚀刻剂沿垂直和水平方向开始蚀除材料，钻蚀（又称为淘蚀）量进一步加大，如图6.2b所示的保护体边缘下面的区域。在化学造型中最典型的公差范围可保持在材料厚度的±10%左右。为了提高生产率，在化学加工前，毛坯件材料应采用其他工艺方法（如机械加工）进行预成形加工。湿度和温度也会导致工件尺寸发生改变。通过改变蚀刻剂和控制工件加工环境，这种尺寸改变可以减小到最小。
电化学加工
电化学金属去除方法是一种最有用的特种加工方法。尽管利用电解作用作为金属加工手段是近代的事，但其基本原理是法拉第定律。利用阳极溶解，电化学加工可以去除具有导电性质工件的材料，而无须机械能和热能。这个加工过程一般用于在高强度材料上加工复杂形腔和形状，特别是在航空工业中如涡轮机叶片、喷气发动机零件和喷嘴，以及在汽车业（发动机铸件和齿轮）和医疗卫生业中。最近，还将电化学加工应用于电子工业的微加工中。
图6.3所示的是一个去除金属的电化学加工过程，其基本原理与电镀原理正好相反。在电化学加工过程中，从阳极（工件）上蚀除下的粒子移向阴极（加工工具）。金属的去除由一个合适形状的工具电极来完成，最终加工出来的零件具有给定的形状、尺寸和表面光洁度。在电化学加工过程中，工具电极的形状逐渐被转移或复制到工件上。型腔的形状正好是与工具相匹配的阴模的形状。为了获得电化学过程形状复制的高精度和高的材料去除率，需要采用高的电流密度（范围为10～100 A/cm2)和低电压 （范围为8～30V）。通过将工具电极向去除工件表面材料的方向进给，加工间隙要维持在0.1 mm范围内，而进给率一般为0.1～20 mm/min左右。泵压后的电解液以高达5～50 m/s的速度通过间隙，将溶解后的材料、气体和热量带走。因此，当被蚀除的材料还没来得及附着到工具电极上时，就被电解液带走了。
作为一种非机械式金属去除加工方法，ECM可以以高切削量加工任何导电材料，而无须考虑材料的机械性能。特别是在电化学加工中，材料去除率与被加工件的硬度、韧性及其他特性无关。对于利用机械方法难于加工的材料，电化学加工可以保证将该材料加工出复杂形状的零件，这就不需要制造出硬度高于工件的刀具，而且也不会造成刀具磨损。由于工具和工件间没有接触，电化学加工是加工薄壁、易变形零件及表面容易破裂的脆性材料的首选。 激光束加工
LASER是英文Light Amplification by Stimulated Emiion of Radiation 各单词头一个字母所组成的缩写词。虽然激光在某些场合可用来作为放大器，但它的主要用途是光激射振荡器，或者是作为将电能转换为具有高度准直性光束的换能器。由激光发射出的光能具有不同于其他光源的特点：光谱纯度好、方向性好及具有高的聚焦功率密度。
激光加工就是利用激光和和靶材间的相互作用去除材料。简而言之，这些加工工艺包括激光打孔、激光切割、激光焊接、激光刻槽和激光刻划等。
激光加工（图6.4）可以实现局部的非接触加工，而且对加工件几乎没有作用力。这种加工工艺去除材料的量很小，可以说是“逐个原子”地去除材料。由于这个原因，激光切削所产生的切口非常窄。激光打孔深度可以控制到每个激光脉冲不超过一微米，且可以根据加工要求很灵活地留下非常浅的永久性标记。采用这种方法可以节省材料，这对于贵重材料或微加工中的精密结构而言非常重要。可以精确控制材料去除率使得激光加工成为微制造和微电子技术中非常重要的加工方法。厚度小于20 mm的板材的激光切割加工速度快、柔性好、质量高。另外，通过套孔加工还可有效实现大孔及复杂轮廓的加工。
激光加工中的热影响区相对较窄，其重铸层只有几微米。基于此，激光加工的变形可以不予考虑。激光加工适用于任何可以很好地吸收激光辐射的材料，而传统加工工艺必须针对不同硬度和耐磨性的材料选择合适的刀具。采用传统加工方法，非常难以加工硬脆材料如陶瓷等，而激光加工是解决此类问题的最好选择。
激光切割的边缘光滑且洁净，无须进一步处理。激光打孔可以加工用其他方法难以加工的高深径比的孔。激光加工可以加工出高质量的小盲孔、槽、表面微造型和表面印痕。激光技术正处于高速发展期，激光加工也如此。激光加工不会挂渣，没有毛边，可以精确控制几何精度。随着激光技术的快速发展，激光加工的质量正在稳步提高。
超声加工
超声加工为日益增长的对脆性材料如单晶体、玻璃、多晶陶瓷材料的加工需求及不断提高的工件复杂形状和轮廓加工提供了解决手段。这种加工过程不产生热量、无化学反应，加工出的零件在微结构、化学和物理特性方面都不发生变化，可以获得无应力加工表面。因此，超声加工被广泛应用于传统加工难以切削的硬脆材料。在超声加工中，实际切削由液体中的悬浮磨粒或者旋转的电镀金刚石工具来完成。超声加工的变型有静止（传统）超声加工和旋转超声加工。
传统的超声加工是利用作为小振幅振动的工具与工件之间不断循环的含有磨粒的浆料的磨蚀作用去除材料的。成形工具本身并不磨蚀工件，是受激振动的工具通过激励浆料液流中的磨料不断缓和而均匀地磨损工件，从而在工件表面留下与工具相对应的精确形状。音极工具振动的均匀性使超声加工只能完成小型零件的加工，特别是直径小于100 mm 的零件。
超声加工系统包括音极组件、超声发生器、磨料供给系统及操作人员的控制。音极是暴露在超声波振动中的一小块金属或工具，它将振动能传给某个元件，从而激励浆料中的磨粒。超声加工系统的示意图如图6.5所示。音极/工具组件由换能器、变幅杆和音极组成。换能器将电脉冲转换成垂直冲程，垂直冲程再传给变幅杆进行放大或压抑。调节后的冲程再传给音极/工具组件。此时，工具表面的振动幅值为20～50μm。工具的振幅通常与所使用的磨粒直径大致相等。
磨料供给系统将由水和磨粒组成的浆料送至切削区，磨粒通常为碳化硅或碳化硼。另外，除了提供磨粒进行切削外，浆料还可对音极进行冷却，并将切削区的磨粒和切屑带走。
Nontraditional Machining Procees Introduction
Traditional or conventional machining, such as turning, milling, and grinding etc., uses mechanical energy to shear metal against another substance to create holes or remove material.Nontraditional machining procees are defined as a group of procees that remove exce material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tool as it is used in traditional manufacturing procees.
Extremely hard and brittle materials are difficult to be machined by traditional machining procees.Using traditional methods to machine such materials means increased demand for time and energy and therefore increases in costs; in some cases traditional machining may not be feasible.Traditional machining also results in tool wear and lo of quality in the product owing to induced residual strees during machining.Nontraditional machining procees, also called unconventional machining proce or advanced manufacturing procees, are employed where traditional machining procees are not feasible, satisfactory or economical due to special reasons as outlined below: 1.Very hard fragile materials difficult to clamp for traditional machining; 2.When the workpiece is too flexible or slender; 3.When the shape of the part is too complex; 4.Parts without producing burrs or inducing residual strees.
Traditional machining can be defined as a proce using mechanical (motion) energy.Non-traditional machining utilizes other forms of energy; the three main forms of energy used in non-traditional machining procees are as follows: 1.Thermal energy; 2.Chemical energy; 3.Electrical energy.Several types of nontraditional machining procees have been developed to meet extra required machining conditions.When these procees are employed properly, they offer many advantages over traditional machining procees.The common nontraditional machining procees are described in the following section.Electrical Discharge Machining (EDM)
Electrical discharge machining (EDM) sometimes is colloquially referred to as spark machining, spark eroding, burning, die sinking or wire erosion.It is one of the most widely used non-traditional machining procees.The main attraction of EDM over traditional machining procees such as metal cutting using different tools and grinding is that this technique utilizes thermoelectric proce to erode undesired materials from the workpiece by a series of rapidly recurring discrete electrical sparks between workpiece and electrode.
The traditional machining procees rely on harder tool or abrasive material to remove softer material whereas nontraditional machining procees such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired shapes.So, the hardne of the material is no longer a dominating factor for EDM proce.
EDM removes material by discharging an electrical current, normally stored in a capacitor bank, acro a small gap between the tool (cathode) and the workpiece (anode) typically in the order of 50 volts/10amps.As shown in Fig.6.1, at the beginning of EDM operation, a high voltage is applied acro the narrow gap between the electrode and the workpiece.This high voltage induces an electric field in the insulating dielectric that is present in narrow gap between electrode and workpiece.This causes conducting particles suspended in the dielectric to concentrate at the points of strongest electrical field.When the potential difference between the electrode and the workpiece is sufficiently high, the dielectric breaks down and a transient spark discharges through the dielectric fluid, removing small amount of material from the workpiece surface.The volume of the material removed per spark discharge is typically in the range of 10-5 to 10-6 mm3.The gap is only a few thousandths of an inch, which is maintained at a constant value by the servomechanism that actuates and controls the tool feed. Chemical Machining (CM)
Chemical machining (CM) is a well known non-traditional machining proce in which metal is removed from a workpiece by immersing it into a chemical solution.The proce is the oldest of the nontraditional procees and has been used to produce pockets and contours and to remove materials from parts having a high strength-to-weight ratio.Moreover, the chemical machining method is widely used to produce micro-components for various industrial applications such as microelectromechanical systems (MEMS) and semiconductor industries.
In CM material is removed from selected areas of workpiece by immersing it in a chemical reagents or etchants, such as acids and alkaline solutions.Material is removed by microscopic electrochemical cell action which occurs in corrosion or chemical diolution of a metal.Special coatings called maskants protect areas from which the metal is not to be removed.This controlled chemical diolution will simultaneously etch all exposed surfaces even though the penetration rates of the material removed may be only 0.0025-0.1mm/min.The basic proce takes many forms: chemical milling of pockets, contours, overall metal removal, chemical blanking for etching through thin sheets; photochemical machining (pcm) for etching by using of photosensitive resists in microelectronics; chemical or electrochemical polishing where weak chemical reagents are used (sometimes with remote electric aist) for polishing or deburring and chemical jet machining where a single chemically active jet is used.A schematic of chemical machining proce is shown in Fig.6.2a.Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop (as shown by the areas under the edges of the maskant in Fig.6.2b).Typically, tolerances of ±10% of the material thickne can be maintained in chemical blanking.In order to improve the production rate, the bulk of the workpiece should be shaped by other procees (such as by machining) prior to chemical machining.Dimensional variations can occur because of size changes in workpiece due to humidity and temperature.This variation can be minimized by properly selecting etchants and controlling the environment in the part generation and the production area in the plant.Electrochemical Machining (ECM)
Electrochemical metal removal is one of the more useful nontraditional machining procees.Although the application of electrolytic machining as a metal-working tool is relatively new, the basic principles are based on Faraday laws.Thus, electrochemical machining can be used to remove electrically conductive workpiece material through anodic diolution.No mechanical or thermal energy is involved.This proce is generally used to machine complex cavities and shapes in high-strength materials, particularly in the aerospace industry for the ma production of turbine blades, jet-engine parts, and nozzles, as well as in the automotive (engines castings and gears) and medical industries.More recent applications of ECM include micromachining for the electronics industry.
Electrochemical machining (ECM), shown in Fig.6.3, is a metal-removal proce based on the principle of reverse electroplating.In this proce, particles travel from the anodic material (workpiece) toward the cathodic material (machining tool).Metal removal is effected by a suitably shaped tool electrode, and the parts thus produced have the specified shape, dimensions, and surface finish.ECM forming is carried out so that the shape of the tool electrode is transferred onto, or duplicated in, the workpiece.The cavity produced is the female mating image of the tool shape.For high accuracy in shape duplication and high rates of metal removal, the proce is operated at very high current densities of the order 10-100 A/cm2,at relative low voltage usually from 8 to 30 V, while maintaining a very narrow machining gap (of the order of 0.1 mm) by feeding the tool electrode with a feed rate from 0.1 to 20 mm/min.Diolved material, gas, and heat are removed from the narrow machining gap by the flow of electrolyte pumped through the gap at a high velocity (5-50 m/s), so the current of electrolyte fluid carries away the deplated material before it has a chance to reach the machining tool.
Being a non-mechanical metal removal proce, ECM is capable of machining any electrically conductive material with high stock removal rates regardle of their mechanical properties.In particular, removal rate in ECM is independent of the hardne, toughne and other properties of the material being machined.The use of ECM is most warranted in the manufacturing of complex-shaped parts from materials that lend themselves poorly to machining by other, above all mechanical methods.There is no need to use a tool made of a harder material than the workpiece, and there is practically no tool wear.Since there is no contact between the tool and the work, ECM is the machining method of choice in the case of thin-walled, easily deformable components and also brittle materials likely to develop cracks in the surface layer.Laser Beam Machining (LBM)
LASER is an acronym for Light Amplification by Stimulated Emiion of Radiation.Although the laser is used as a light amplifier in some applications, its principal use is as an optical oscillator or transducer for converting electrical energy into a highly collimated beam of optical radiation.The light energy emitted by the laser has several characteristics which distinguish it from other light sources: spectral purity, directivity and high focused power density.
Laser machining is the material removal proce accomplished through laser and target material interactions.Generally speaking, these procees include laser drilling, laser cutting, laser welding, and laser grooving, marking or scribing.
Laser machining (Fig.6.4) is localized, non-contact machining and is almost reacting-force free.This proce can remove material in very small amount and is said to remove material “atom by atom”.For this reason, the kerf in laser cutting is usually very narrow , the depth of laser drilling can be controlled to le than one micron per laser pulse and shallow permanent marks can be made with great flexibility.In this way material can be saved, which may be important for precious materials or for delicate structures in micro-fabrications.The ability of accurate control of material removal makes laser machining an important proce in micro-fabrication and micro-electronics.Also laser cutting of sheet material with thickne le than 20mm can be fast, flexible and of high quality, and large holes or any complex contours can be efficiently made through trepanning.
Heat Affected Zone (HAZ) in laser machining is relatively narrow and the re-solidified layer is of micron dimensions.For this reason, the distortion in laser machining is negligible.LBM can be applied to any material that can properly absorb the laser irradiation.It is difficult to machine hard materials or brittle materials such as ceramics using traditional methods, laser is a good choice for solving such difficulties.
Laser cutting edges can be made smooth and clean, no further treatment is neceary.High aspect ratio holes with diameters impoible for other methods can be drilled using lasers.Small blind holes, grooves, surface texturing and marking can be achieved with high quality using LBM.Laser technology is in rapid progreing, so do laser machining procees.Dro adhesion and edge burr can be avoided, geometry precision can be accurately controlled.The machining quality is in constant progre with the rapid progre in laser technology.Ultrasonic Machining (USM)
Ultrasonic machining offers a solution to the expanding need for machining brittle materials such as single crystals, glaes and polycrystalline ceramics, and for increasing complex operations to provide intricate shapes and workpiece profiles.This machining proce is non-thermal, non-chemical, creates no change in the microstructure, chemical or physical properties of the workpiece and offers virtually stre-free machined surfaces.It is therefore used extensively in machining hard and brittle materials that are difficult to cut by other traditional methods.The actual cutting is performed either by abrasive particles suspended in a fluid, or by a rotating diamond-plate tool.These variants are known respectively as stationary (conventional) ultrasonic machining and rotary ultrasonic machining (RUM).
Conventional ultrasonic machining (USM) accomplishes the removal of material by the abrading action of a grit-loaded slurry, circulating between the workpiece and a tool that is vibrated with small amplitude.The form tool itself does not abrade the workpiece; the vibrating tool excites the abrasive grains in the flushing fluid, causing them to gently and uniformly wear away the material, leaving a precise reverse from of the tool shape.The uniformity of the sonotrode-tool vibration limits the proce to forming small shapes typically under 100 mm in diameter.
The USM system includes the Sonotrode-tool aembly, the generator, the grit system and the operator controls.The sonotrode is a piece of metal or tool that is exposed to ultrasonic vibration, and then gives this vibratory energy in an element to excite the abrasive grains in the slurry.A schematic representation of the USM set-up is shown in Fig.6.5.The sonotrode-tool aembly consists of a transducer, a booster and a sonotrode.The transducer converts the electrical pulses into vertical stroke.This vertical stroke is transferred to the booster, which may amplify or suppre the stroke amount.The modified stroke is then relayed to the sonotrode-tool aembly.The amplitude along the face of the tool typically falls in a 20 to 50 μm range.The vibration amplitude is usually equal to the diameter of the abrasive grit used.
The grit system supplies a slurry of water and abrasive grit, usually silicon or boron carbide, to the cutting area.In addition to providing abrasive particles to the cut, the slurry also cools the sonotrode and removes particles and debris from the cutting area.
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