Electrochemical machining of metal

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

E. Rumiantsev, A. Davydov, Electrochemical Machining of Metals, Mir, Moscow, 1989. 

These effects are important for intense metal dissolution during pitting and electrochemical machining of metals where very high anodic current densities are effective. For anodic potentiostatic transients, the precipitation of a salt film slows down an initially higher dissolution rate and starts diffusion-limited electropolishing of the metal surface. 

Since the mid-1970s there has been a considerable amount of material published on the influence of ultrasound upon the electrochemistry of metal systems. Most of this work was carried out in former Eastern block countries and concentrated on such electrochemical processes as corrosion, electrodeposition, and electrochemical dissolution. Recently there has been an upsurge in the interest shown in sonoelectrochemical processes using both non-metal and metal systems worldwide. There have been a considerable number of publications in the employment of ultrasound in areas as diverse as semiconductor production to sono-electrochemical machining and metal finishing. A review by R. Walker [27] into the use of ultrasound in metal deposition systems, provides an introduction into the fundamental effects of ultrasound in plating and metal finishing. 

Ultrasound has also been successfully employed for the production of novel inorganic compounds such as Si semiconductors in which case it is found that ultrasound increases yields of products and in some cases influences the reaction mechanism itself. For the traditional electrochemical process of metal electroplating the presence of ultrasound increases the thickness of the material deposited, as well as increasing the efficiency of the reaction. Ultrasound has also been successfully employed in the machining of complex electronic circuitry using electrochemical etching techniques. A majority of papers published in the literature still... 

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. 

The enormous scope of the subject of corrosion follows from the definition which has been adopted in the present work. Corrosion will include all reactions at a metal/environment interface irrespective of whether the reaction is beneficial or detrimental to the metal concerned —no distinction is made between chemical or electropolishing of a metal in an acid and the adventitious deterioration of metal plant by acid attack. It follows, therefore, that a comprehensive work on the subject of corrosion should include an account of batteries, electrorefining, chemical machining, chemical and electrochemical polishing, etc. 

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 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. 

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... 

Electrodeposition and electrodissolution of metals has an important role in the fabrication of metal articles with shapes that are difficult to make by conventional methods32, We exemplify with two types of processing electroformation and electrochemical machining. 

Metal objects with complex shapes can be formed by electrochemical machining (electroerosion), especially important when mechanical machining is not possible. The object is the anode, where dissolution occurs, and the tool is the cathode, having the form of a mould for the object. The cathode has small holes from which jets of electrolyte exit so that there is a layer of electrolyte between anode and cathode (Fig. 15.9). An extremely important example is the manufacture of components such as blades for turbines. 

Electrochemical machining (ECM) is a method of metal machining that aims at producing parts of specified shape, dimensions, and surface finish. The process is based on the removal of metal by electrochemical dissolution ([1-15] and references cited therein). Special machines have been developed to realize this aim. A complete ECM installation (Fig. 1) consists of the machine, the power supply, the electrolyte circulation system (tank, pump, heat exchanger, and sludge removal unit), and the control system (control of current, voltage, feed rate, gap width, and electrolyte temperature, pH value, pressure, and concentration short-circuit protection). 

Another technique used in metal finishing is electrochemical machining, which employs anodic current densities of up to 5 MA rrT2 The principle of this technology is to advance a shaped tool, which serves as the cathode, towards the anodic workpiece. As the interelectrode separation narrows, the workpiece dissolves... 

Since the electrodes are in close proximity in electrochemical machining, a major problem is that the ions produced can interact with one another, producing slurries of the metal hydroxide. 

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