Machining, electrochemical

In electrochemicai machining, the removal of the metal to form a hole or to contour the surface is by anodic dissolution (Fig. 9.3). Clearly, for the process to have sufficient accuracy to be useful in engin ring, the metal removal must occur undentQtally controlled conditions. This is possible to good tolerances but requires the design of a cathode, known as the tool, for each job  

F t 9J An aircraft engine casing, (a) Before electrochemical machining, (b) Afler electrocheniical machining. (The cathode tool is also shown in (b).) (Courtesy Rolls-Royce Ltdt Aero Division.) 

Electrochemical machining (ECM) relies upon a high local current density and control of the anodic dissolution such that passivation (Chapter 10) does not 

In the electrochemical cell the workpiece is the anode and the tool is Che cathode The electrolyte is fed through the tool at a rapid flowrate (9-60 m M in such a way that the supply of electrolyte is uniform over the surface. The electrolyte flow pattern is as important in practice as the arrangement of conducting surfaces on the tool in determining the current density distribution and both factors must be considered in the design of the tool. 

Large volumes of electrolyte are required and, hence, it is normal to use an aqueous solution with a cheap electrolyte (i e. NaCl or NaNOj) or where essential to obtain metal dissolution, NaBr or NaF the workpiece must not passivate The electrode reactions are, at the anode  

In electrochemical machining, the removal of the metal to form a hole or other feature is by anodic dissolution (Fig. 8.3). Clearly for the process to have the 

Electrochemical machining is a recent innovation, the practice dating back less than twenty-five years. Much of the driving force for its development has come from the aerospace industry with its requirement to machine very hard alloys (e.g. those based on Ti and Fe/Co/Ni/Cr) to produce components able to perform a function reUably but also having minimum weight this specification often leads to components of very complex shape. The alloys which must be employed cause problems in conventional machining because of the low rate of metal removal and the short tool life. Electrochemical machining is, however, based on different properties of the metal independent of its hardness and it is only necessary to find an electrolyte where the alloy will dissolve anodically without passivation when the rate of metal removal may be estimated from Faraday s law. 

The main drawback to electrochemical machining lies in the need to design a tool (cathode) for each new job. Moreover the design process to obtain the correct current density distribution remains a skilled art rather than a science it is often necessary to test the tool and to modify it by trial and error. In addition the need to use large volumes of electrolyte solutions does not fit in well to a mechanical workshop. However carefully they are handled, they lead to corrosion in the environment. 

A schematic representation of a typical ECM apparatus is shown in Fig. 9.2 and consists of a workpiece to be machined (the anode), a properly shaped cathode tool which is movable and maintains a constant gap with the workpiece. The electrolyte fiows between the two electrodes, removing the products of electrolysis as well as heat. The power supply furnishes the high currents necessary to dissolve the anode. 

---

Manufacturing engineers wishing to use ECM processes in industry need to address the challenge of proper tool design. The cost of design can be as much as 20% of the cost of an electrochemical machine for complex components. PredictabiUty of overcuts obtained for specific appHcations and the particular electrolytes to be used for the alloy metals that have to be machined must also be considered along with specific controls and limits on the ECM equipment needed. 

J. A. McGeough, Principles of Electrochemical Machining, Chapman and Hall, London, 1974. 

FIGURE 16.5 Electrochemical machining of metals (1) workpiece (anode) (2) tool (cathode). 

The electrochemical machining (ECM) of metals rests on the selective local anodic dissolution of metal. It is used to give metal parts the required shape and size, to drill holes, create hollows, cut shaped slots, and fashion parts of a complex pattern (e.g., the blades of gas turbines). It is an advantage of this method that it can also be used for hard metals (high-alloy steels and other alloys, metals in the quenched state, etc.). 

Electrochemical machining is performed in concentrated solntions of salts alkali chlorides, snlfates, or nitrates. Very high current densities are nsed hundreds or thousands of kA/m when referring to the surface area of the anodic working sections. At a current density of 10" mA/cm, the rate of iron dissolution is about 0.15 mm/min. This should also be the rate of advance of the cathode in the direction of the anode. High rates of solution flow through the working gap are used to eliminate the reaction products and heat evolved (e.g., flow rates of 10" cm/s). 

Electrochemical machining is a process based on the same principles used in electroplating except that the workpiece is the anode and the tool is the cathode. Electrolyte is pumped between the electrodes and a potential is applied, resulting in rapid removal of metal. 

Other abrasive jet machining Electrostatic painting Electrical discharge machining Electrochemical machining... 

Electrochemical gas sensor, 13 589 Electrochemical grinding, 9 603 Electrochemical machining (ECM), 9 590-606... 

Holdup, 70 764-765 Hole drilling, via electrochemical machining, 9 598-600 Holes, valence band, 9 728 Hole-transport layer (HTL)... 

Metal-recovery operations, phosgene in, 18 810-811. See also Metals recycling Metal reductions hydrazine, 13 569 to liquid metal, 16 141-146 Metal refining, 16 149-151 barium application, 3 349 limestone in, 15 38-39 Metal removal, in electrochemical machining, 9 593-595 Metal-rich phosphides, 19 59... 

Pulsatile drug delivery systems, 9 57-61 Pulsating heat pipes (PHP), 13 235-236 Pulse combustion heat sources, 9 104-105 Pulse cycles, 9 778 Pulsed baffle reactors, 15 709-710 Pulsed discharge detector (PDD) gas chromatography, 4 614 Pulsed dye lasers, 23 144 Pulsed electrochemical machining (PECM), 9 604-605... 

Surface-enhanced resonance Raman scattering (SERRS), 21 327-328 advantage of, 21 329 Surface Evolver software, 12 11 Surface excess, 24 135, 136 Surface extended X-ray absorption fine structure (SEXAFS), 19 179 24 72 Surface filtration, 11 322-323 Surface finish(es). See also Electroplating in electrochemical machining, 9 591 fatigue performance and, 13 486-487 Surface finishing agents, 12 33 Surface force apparatus, 1 517 Surface force-pore flow (SFPF) model,... 

Titanium alloy, electrochemical machining of, 9 593-594 Titanium alloys... 

This may be why cells use electric signals because these are in principle highly efficient. Electrochemical machines (i.e., storage batteries) are just the type that enable large concentration gradients to be balanced by electrical potential jumps so as to preserve the continuity of the electrochemical potential, which is the requirement for reversibility. The concentration gradients of K+ and Na across cell membranes that... 

Finally, metal objects can sometimes be fabricated in their entirety by electrodeposition (electroforming), with much the same considerations as electroplating. Conversely, portions of a metal specimen can be selectively electrolyzed away (electrochemical machining). This technique is especially useful where the metal to be shaped is too hard or the shape to be cut is too difficult for conventional machining. The sample is made the anode, a specially shaped tool the cathode, and electrolyte solution (e.g., aqueous NaCl) is fed rapidly but uniformly over the surface to be machined. Current densities may reach several hundred amperes per square centimeter across the electrolyte gap of a millimeter or so. Excellent tolerances can be achieved in favorable circumstances.16... 

When a jet of fluid submerged in a medium of that fluid strikes a surface perpendicularly, it spreads out radially over that surface. Original interest in these systems was due to mass transfer investigations of downward directed jets of vertical-take-off aircraft [41], though other applications such as electrochemical machining are important. 

Because rhenium is very difficult to machine with carbide tools and other conventional methods, electrical-discharge machining (EDM), electrochemical machining (ROM), abrasive cutting, or grinding is... 

---