Electrophysical and Electrochemical Processing
Electrophysical and Electrochemical Processing
a general term used to denote methods of processing or working structural materials directly by means of an electric current, by electrolysis, and by combinations of such methods with mechanical action. Ultrasonic, plasma, and various other methods are also subsumed within the category. The development of electrophysical and electrochemical processing methods and their use in production marked a major advance in the technology of processing materials, inasmuch as electricity became a working agent instead of an auxiliary means of mechanical processing (used to move a workpiece or tool). The increasing use of such methods in industry is due to their high productivity and to the feasibility of performing production operations that are not possible with mechanical methods. The great variety of electro-physical and electrochemical processing methods may be arbitrarily divided into electrophysical (electroerosive, electromechanical, and beam), electrochemical, and the combination methods (Figure 1).
Electroerosive machining. In electroerosive machining, particles
are removed from the surface of a material by means of an electric discharge or pulse. For a given voltage (or distance) between electrodes immersed in a liquid dielectric, the dielectric will break down as the electrodes are drawn closer (or the voltage is increased); as a result, an electric discharge occurs, creating a high-temperature plasma in its path.
Since the duration of the electric pulses used in such machining does not exceed 10–2 sec, the heat evolved cannot propagate deep into the material, and even a small amount of energy is sufficient to heat, melt, and vaporize a small amount of material. Moreover, the pressure developed by the plasma particles upon impact on an electrode can eject not only melted material but also material that has been merely heated. Since electrical breakdown usually occurs along the shortest path, the regions closest to the electrodes are eroded first. Thus, when an electrode of some given shape (the tool) is brought close to another electrode (the workpiece), the surface of the latter takes on the shape of the former’s surface (Figure 2). The productivity of the process and the surface quality obtained depend mainly on the parameters of the electric pulses (duration, repetition frequency, and pulse energy). Electroerosive machining comprises electrospark and arc-discharge methods.
Electrospark machining was proposed by the Soviet scientists N. I. Lazarenko and B. R. Lazarenko in 1943. The technique uses a spark discharge, in which the temperature reaches 10,000°C in the path and substantial hydrodynamic forces are developed. However, since the individual pulses are of very short duration, the energy content is small, and, as a result, the effect of each pulse on the material’s surface is not great. The method can produce a high-quality surface but does not afford high productivity. Tool wear with the method is also relatively high (equaling as much as 100 percent of the volume of material removed). Electrospark machining is used mostly for the precision machining of small parts and apertures and for cutting contours in hard-alloy dies with a wire electrode (see below).
Arc-discharge machining uses pulses of an arc discharge. It was proposed by the Soviet specialist M. M. Pisarevskii in 1948 and was introduced into industry in the early 1950’s. In contrast with a spark discharge, an arc discharge has a low plasma temperature of 4000°–5000°C, which makes it possible to increase the duration of the pulses, reduce the interval between pulses, and thereby direct considerable power (several tens of kilowatts) to the machining area, thus increasing machining productivity. The cathode disintegration typical of an arc discharge results in wear on the tool (which in this case is connected to the anode) that is lower than that typical of electrospark machining, amounting to between 0.05–0.3 percent of the volume of material removed; in some cases the tool may exhibit no wear at all. The more economical arc-discharge method is used mainly for rough machining and for three-dimensional machining of shaped surfaces. Each technique complements the other.
Electroerosive techniques are especially effective for machining hard materials and complicated irregular objects. In the machining of hard materials by mechanical methods, tool wear becomes very important. An advantage of electroerosive methods (and, in general, of all electrophysical and electrochemical processing
methods) is that the tools are made of very cheap, easily machined materials. Tool wear may be insignificant in many cases. For example, in the fabrication of certain types of dies by mechanical methods, the cost of the tool accounts for more than 50 percent of the production costs of machining; when such dies are machined by electroerosive methods the tool cost does not exceed 3.5 percent.
The production procedures of electroerosive machining may be classified as piercing and profiling. Piercing can produce holes with a diameter of less than 0.3 mm, which is impossible with mechanical methods. In this case a thin wire serves as the tool. The technique reduces the cost of making holes in wire-drawing dies, including diamond dies, by 20–70 percent. Electroerosive methods can also be used to produce spiral apertures. Machining with a ribbon electrode (Figure 3) is commonly used in profiling. The ribbon, which is wound from reel to reel, bends around a template that reproduces the shape of a gear tooth. In rough-finishing operations the ribbon notches the workpiece to the required depth; the workpiece is then turned, and the slot is broadened to the required width. Machining with a wire electrode, in which the ribbon is replaced with a wire, is even more widely used. With this method, for example, it is possible to obtain simultaneously from one piece of material male and female dies with virtually a perfect fit. Electroerosive machining is also suitable for producing parts with complicated shapes. Variations on the basic techniques include dimensional machining, tool hardening, and the production of powders for powder metallurgy.
A Soviet electrospark machine tool, used to remove broken tools stuck in workpieces, was the world’s first electroerosive device (1943). Since then a wide range of electroerosive machine tools, varying in purpose, productivity, and design, have been produced in the USSR and abroad. Like metal-cutting machine tools, they may be classified according to purpose as universal types (for the performance of a variety of operations on a wide range of products), special-purpose tools for processing articles of the same type and various sizes (Figure 4), and special-purpose tools for processing articles of the same type and size. They may
also be classified according to the required accuracy of machining as general-purpose, increased-precision, and superhigh-precision. All electroerosive machine tools have a device for holding and moving the tool (or workpiece), a hydraulic system, and a device for the automatic adjustment of the interelectrode gap (between workpiece and tool). The generators for the spark or arc pulses are usually of separate manufacture and can function with different machines. The principal distinction between the devices for moving the tool (or workpiece) on electroerosive machine tools and those on metal-cutting machine tools is the absence of substantial power loads in the former and the use of electrical insulation between the electrodes. The hydraulic system comprises a tank for the working fluid (oil, kerosene, or the like), a hydraulic pump to pump the fluid through the interelectrode gap, and filters to remove erosion products from the hydraulic fluid being fed to the pump.
Arc-discharge machine tools differ from electrospark types only in the type of pulse generator used. Soviet industry manufactures generators for a variety of purposes. The developing technology of semiconductor devices has made it possible to design generators that can vary pulse parameters over a wide range. For example, in the Soviet ShGI-125-100 generator the pulse frequency ranges from 0.1 to 100 kilohertz, the pulse duration can be varied from 3 to 9,000 microseconds, the maximum power is 7.5 kilowatts, and the rated current is 125 amperes. The range of working voltages is 60–200 volts for electrospark machining and 20–60 volts for arc-discharge machining. Modern electroerosive machine tools are highly automated and are often operated in a semiautomatic mode.
Electromechanical processing. Electromechanical processing methods simultaneously bring mechanical and electrical action to bear on the material being processed in the machining zone. The category also includes methods based on the use of certain physical phenomena, such as hydraulic impact and ultrasound.
In electric-resistance machining, electric energy—a high-power AC or DC arc of up to 12,000 amperes at a voltage of up to 50 volts—is introduced to the zone of mechanical machining. For example, the arc may travel between a disk used to remove material from the machining zone and the workpiece (Figure 5). The technique is used for stripping castings, cutting, and other machining operations that are similar to almost all the types of mechanical machining operations with respect to the motions described. The advantages of the method include high productivity (up to 106 cu mm/min) for rough cutting, simplicity of tool design, and operation at relatively low voltages. Because of the low unit pressures on the tool—from 30 to 50 kilonewtons per sq m (0.3–0.5 kilogram-force per sq cm)—tools made of relatively soft materials can be used to machine hard materials. Disadvantages include the high degree of roughness of the machined surface and the thermal effects on metals during roughcutting.
Electroabrasive machining, a type of electric-resistance machining, consists in machining with abrasive tools (including diamond-abrasive tools) made of an electrically conducting material. The introduction of electric energy to the machining zone substantially reduces tool wear.
Electric-resistance machine tools do not differ in kinematics from analogous metal-cutting machine toolsl They are equipped with high-power current supply units.
Magnetic-pulse forming is used for the plastic deformation of metals and alloys—compressing and expanding tubes, shaping tubular and sheet workpieces, sizing, and the like. It is based on the direct conversion of the energy in a rapidly varying magnetic field into mechanical energy upon interaction with a conductor (the workpiece). In typical applications the field may be excited by the discharge of a bank of capacitors through an inductor (Figure 6). Advantages of the technique include the absence of moving and rubbing parts in the equipment, high reliability and productivity, ease of control and compactness, and the need for only one tool—a female die (the field acts as the other). Disadvantages include relatively low efficiency and the difficulty of machining very thick workpieces and workpieces having holes or recesses (which resist the flow of current).
Electrohydraulic forming (primarily a forging operation) uses the energy from a hydraulic impact with a powerful electric (spark) discharge in a liquid dielectric (Figure 7). The cavity between the workpiece and the die must be evacuated because, with the extremely high velocities with which the workpiece moves toward the die, air cannot escape from the cavity and would otherwise prevent the workpiece from fitting tightly against the die. The method is simple and reliable, but it is inefficient, requires high voltages, and does not always give consistent results.
Electromechanical processing also includes ultrasonic processing.
Beam processing. Beam methods of processing materials include those using electron beams and light beams (seeLASER TECHNOLOGY). Electron-beam machining is accomplished with a stream of high-energy electrons (with energies up to 100 kiloelectron volts). The technique can be used to machine all known materials (modern electron optics can concentrate an electron beam into a very small area to create intense power densities in the machining zone). Electron-beam machine tools are capable of cutting (including piercing holes) and welding with great accuracy (down to 50 angstroms). Their primary component is the electron gun. They also have devices for monitoring the machining and moving the workpiece as well as vacuum equipment. Because of their relatively high cost, low productivity, and technical complexity, they are used mostly for precision operations in microelectronics, the fabrication of wire-drawing dies with very small holes (diameters as small as 5 micrometers), and operations with very pure materials.
Plasma treatment is also classified as an electrophysical method.
Electrochemical processing is based on the laws of electrochemistry; the methods may be classified according to the principles used as anodic and cathodic processes (see) and according to the production potentials as surface and dimensional methods.
Surface electrochemical processing. The practical application of electrochemical methods began in the 1830’s (electroplating and electroforming; seeELECTROPLATING TECHNOLOGY). The first patent for an electropolishing method was issued to E. I. Shpital’skii in 1910. The essence of the method is that anode material dissolves (anodic dissolution) under the action of an electric current, and projections on the surface dissolve quickest, thereby smoothing the surface. Here, material is removed from the entire surface, unlike mechanical polishing, in which only the largest projections are removed. Electropolishing makes it possible to produce a surface with very low roughness. An important distinction between electropolishing and mechanical polishing is that the former method does not alter the structure of the material being processed. (See alsoPASSIVATION.)
Dimensional electrochemical processing. Dimensional electrochemical methods include anode-hydraulic machining and anode-mechanical metalworking.
Anode-hydraulic machining was first used in the late 1920’s in the Soviet Union to extract the remains of a broken tool from a workpiece. The rate of anodic dissolution depends on the distance between the electrodes; the smaller the gap, the faster the dissolution. Thus, when the electrodes are close to each other, the surface of the anode (the workpiece) will reproduce exactly the surface of the cathode (the tool). However, the dissolution process is impeded by the electrolysis products that accumulate in the machining zone and deplete the electrolyte. The products are removed and the electrolyte is renewed either by mechanical means (anode-mechanical metalworking) or by pumping electrolyte through the machining zone (Figure 8).
Proper selection of the electrolyte makes it possible to machine practically any current-conducting material and to achieve high productivity with a high-quality surface. The electrochemical machine tools used for anode-hydraulic machining are simple to operate and use low-voltage (up to 24 volts) equipment. However, the high current densities (up to 200 amperes per cm2) require high-capacity current-supply units and large quantities of electrolyte (as much as one-third of the shop area may be occupied by tanks for the electrolyte).
Combination processing methods exhibit the advantages of both electrophysical and electrochemical methods. A wide variety
of combinations exists. For example, the combination of anode-mechanical metalworking and ultrasonic processing improves productivity in some cases by a factor of 20. The electroerosive-ultrasonic machine tools available permit the two methods to be used separately or together.
REFERENCES
Vishnitskii, A. L., I. Z. Iasnogorodskii, and I. P. Grigorchuk. Elektrokhimicheskaia i elektromekhanicheskaia obrabotka metallov. Leningrad, 1971.Elektrofizicheskie i elektrokhimicheskie melody razmernoi obrabotki materialov. Moscow, 1971.
Cherepanov, Iu. P., and B. I. Sametskii. Elektrokhimicheskaia obrabotka v mashinostroenii. Moscow, 1972.
Novoe v elektrofizicheskoi i elektrokhimicheskoi obrabotke materialov. Leningrad, 1972.
D. L. IUDIN