ELECTROCHEMICAL MACHINING (ECM)Joseph McGeough
Institute for Integrated Micro and Nano Systems
University of Edinburgh
Edinburgh, EH9 3JL, United Kingdom
Michael Faraday early metallurgic researches, from 1818 to 1824, anticipated the developments which have led to widespread use today of alloy steels. Much effort has been expended to improve their performance for their service as cutting tools in machining. The aim has always been to yield higher rates of machining and to tackle recently developed harder materials on the principle that the tool material must be harder than the workpiece which is to be machined. Much progress has been made; however, in recent years some alloys, which are exceedingly difficult to machine by the conventional methods, have been produced to meet a demand for very high-strength, heat resistant materials. Moreover, these new materials often have to take a complex shape. A search has had to be made for alternative methods of machining since the evolution of suitable tooling has not kept pace with these advances.
Electrochemical machining (ECM) has been developed initially to machine these hard to machine alloys, although any metal can so be machined. ECM is an electrolytic process and its basis is the phenomenon of electrolysis, whose laws were established by Faraday in 1833. The first significant developments occurred in the 1950s, when ECM was investigated as a method for shaping high strength alloys. As of the 1990s, ECM is employed in many ways, for example, by automotive, offshore petroleum, and medical engineering industries, as well as by aerospace firms, which are its principal user.
Metal removal is achieved by electrochemical dissolution of an anodically polarized workpiece which is one part of an electrolytic cell in ECM. Hard metals can be shaped electrolytically by using ECM and the rate of machining does not depend on their hardness. The tool electrode used in the process does not wear, and therefore soft metals can be used as tools to form shapes on harder workpieces, unlike conventional machining methods. The process is used to smooth surfaces, drill holes, form complex shapes, and remove fatigue cracks in steel structures. Its combination with other techniques yields fresh applications in diverse industries. Recent advances lie in computer-aided tool design, and the use of pulsed power, which has led to greater accuracy for ECM-produced components.
Theoretical backgroundSince electrolysis is the basis of ECM, it must be understood before going further through the characteristics and other details of the process.
Electrolysis is the name given to the chemical process which occurs, for example, when an electric current is passed between twoconductors dipped into a liquid solution. A typical example is that of two copper wires connected to a source of direct current and immersed in a solution of copper sulphate in water, as shown in Figure 1. An ammeter, placed in the circuit, will register a flow of current; from this indication, the electric circuit can be deduced to be complete. A significant conclusion that can be made from an experiment of this sort is that the copper sulphate solution obviously has the property that it could conduct electricity. Such solution is termed an electrolyte. The wires are called electrodes, the one with positive polarity being the anode, and the one with negative polarity the cathode. The system of electrodes and electrolyte is referred to as the electrolytic cell, whilst the chemical reactions which occur at the electrodes are called the anodic or cathodic reactions or processes.
|Fig. 1. Electrolysis of copper sulphate solution. |
Electrolytes are different from metallic conductors of electricity in that the current is carried not by electrons but by atoms, or group of atoms, which have either lost or gained electrons, thus acquiring either positive or negative charges. Such atoms are called ions. Ions which carry positive charges move through the electrolyte in the direction of the positive current, that is, toward the cathode, and are called cations. Similarly, the negatively charged ions travel toward the anode and are called anions. The movement of the ions is accompanied by the flow of electrons, in the opposite sense to the positive current in the electrolyte, outside the cell, as shown also in Figure 2 and both reactions are a consequence of the applied potential difference, that is,voltage, from the electric source.
|Fig. 2. Electrolytic dissolution of iron. |
A cation reaching the cathode is neutralized, or discharged, by the negative electrons on the cathode. Since the cation is usually the positively charged atom of a metal, the result of this reaction is the deposition of metal atoms.
To maintain the cathodic reaction, electrons are required to pass around the external circuit. These are obtained from the atoms of the metal anode, and these atoms thus become the positively charged cations which pass into solution. In this case, the reaction is the reverse of the cathodic reaction.
The electrolyte in its bulk must be electrically neutral; that is, there must be equal numbers of opposite charges within it, and thus there must be equal amounts of reaction at both electrodes. Therefore, in the electrolysis of copper sulphate solution with copper electrodes, the overall cell reaction is simply the transfer of copper metal from the anode to the cathode. When the wires are weighted at the end of the experiment, the anodic wire will be found to have lost weight, whilst the cathodic wire will have increased in weight by an amount equal to that lost by the other wire. Some examples of the reactions occurring in these processes are shown in the Appendix.