The Role of Electrochemical Machining (ECM) in Industrial Metallurgy

By
Dr Thoguluva Raghavan Vijayaram
BE (Mechanical Engineering, Madurai Kamaraj University, India),
ME (Metallurgical Engineering, Bharathiyar University, India),
PhD (Mechanical Engineering, Universiti Putra Malaysia, Malaysia),
Rector Grant Researcher in Metallurgy (Genoa University, Italy),
Chartered Engineer (M123412-3, IIE, Calcutta, India)
MIIF, MISTE, MIIPE, MIE (Calcutta, India)
Senior Lecturer in Manufacturing Engineering and Researcher in Metallurgy,
Department of Manufacturing Process and System, Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka, Malaysia.
E-mail: thoguluva@utem.edu.my

Electrochemical Machining (ECM) is one of the newest and most useful machining process of metal removal by the controlled dissolution of the anode of an electrolytic cell. This process is particularly suited to metals and alloys which are difficult or impossible to machine by mechanical machining. The working principle is based on Michael Faraday’s classical laws of electrolysis, requiring basically the two electrodes, an electrolyte, a gap and a source of D.C. power of sufficient capacity. ECM is basically the reverse of electroplating operation. Almost any conducting material can be machined by this method. Non conducting materials cannot be machined. A schematic sketch of the ECM process is shown in Figure-1.

Figure-1 Electrochemical Machining

In the actual process, the cathode is tool-shaped, more or less like the mirror image of the finished work piece. The work piece is connected to the positive supply. The tool or cathode, connected to the negative terminal, is advanced towards the anode (work piece) through the electrolyte that completes the electrical circuit between the anode and cathode. Metal is then removed from the work piece through electrical action and the cathode (tool) shape is reproduced on the work piece. The electrolyte bath is pumped at high pressure through the gap between the work piece and the tool and must be circulated at a rate sufficiently high to conduct current between them and carry heat. The electrolysis process that takes place at the cathode liberates hydroxyl ions (negatively charged) and free hydrogen. The hydroxyl ions combine with the metal ions of the anode to form insoluble metal hydroxides and the material is thus removed from the anode. This process continues and the cathode (tool) reproduces its shape in the work piece (anode). The tool does not contact the work piece producing no direct friction and therefore does not wear and no heat buildup occurs. Figure-2 shows a typical set-up of ECM process. Some of the important ECM elements are electrolyte, cathode tool, anode work piece, and D.C.power supply.

Figure-2 Scheme of ECM

The electric current is of the order of 50 to 40000 Ampere at 5 to 30 V D.C. for a current density of 20 to 300 Ampere/Square cm, across a gap of 0.05 to 0.70 mm between the tool and the work piece. The electrolyte flows through this gap at a velocity of 30 to 60 meter/second forced by an inlet pressure of about 20 kgf /square centimeter. Suspended solids are removed from the electrolytes by setting, centrifuging, or filtering, and the filtered electrolyte is recirculated for use. It is interesting to note that the salt is not being consumed and the metal is being machined at the expense of electrical energy and with a small amount of water. The electrolyte acts as a carrier of current. The common electrolytes used are sodium chloride, sodium nitrate, potassium chloride, sodium hydroxide, sodium fluoride, sulfuric acid, and sodium chlorate. These solutions on reaction produce an insoluble compound in the form of sludge. The electrolyte carries the current between the tool and the work piece. It removes the machined products and other insoluble products from the cutting region. It dissipates the heat produced in the operation. The electrolyte should posses good electrical conductivity, non-toxicity, chemical stability, non-corrosive property, low viscosity, and high specific heat. The most commonly use electrolyte is the solution of sodium chloride in concentration varying from 0.10 to 0.25 kg / litre of water.
The general requirements on the tool material in ECM are mentioned below:

The tool material should be a good conductor of electricity.
It should be rigid enough to take up the load due to fluid pressure.
It should be chemically inert to the electrolyte.
It should be easily machinable to make it in the desired shape.
Copper, Brass, Titanium, Copper-Tungsten, and Stainless steels are most commonly used electrode materials when the electrolyte is made of sodium or potassium.
The other materials which can be used as tool materials are aluminium, graphite, bronze, platinum, and tungsten carbide.
The cavity or hole that is made, exactly reproduces the tool shape. Thus the accuracy of the tool shape directly affects the work piece accuracy.

Factors govern the accuracy of parts produced by ECM:

Machining Voltage
Feed rate of the electrode (tool)
Temperature of the electrolyte
Concentration of the electrolyte

Now, electrochemical machining systems are available as numerically-controlled machining centers with the capability of high production rates, high flexibility, and the maintenance of close dimensional tolerances. The electrolyte in flowing through the machining gap creates a thin boundary layer of slowly moving fluid next to the anode. Ions of work material leaving the metal surface must traverse this slowly moving boundary layer primarily by a process of diffusion. The rate at which ion can move through the boundary layer influences the rate of metal removal. The ideal electrolyte would provide a uniformly thin layer over the entire surface of the work piece, irrespective of pressure and fluid velocity variations. High velocity flow (30 to 60 meter /second) over the electrode surface is one of the key factors in ECM. This is necessary in order to prevent crowding of hydrogen gas and debris of machining. If this is not fulfilled, bubbles of hydrogen gas will fill the machining gap and machining will stop in that area. It also flushes the metallic particles suspended in the electrolyte, leading to local heating or arcing, and ultimately damage of the tooling and the work piece.

Metal removal rate (MRR) is an important parameter in ECM. The overall machining rate is governed by Faraday’s Laws of Electrolysis. Because of the tendency for the electrolyte to erode away sharp profiles, electro chemical machining is not suited for producing sharp square corners or flat bottoms. Controlling the electrolyte flow may be difficult, so irregular cavities may not be produced to the desired shape with acceptable dimensional accuracy.

Designs should make provision for a small taper for holes and cavities to be machined. The shaped tool should be either in the form of a solid or tubular form, and generally made of brass, copper, bronze, or stainless steel. The tool should be rigid enough to take up the load due to fluid pressure. The tool should be easily machinable to make it in the desired shape. The accuracy of the tool shape directly affects the work piece accuracy. Under ideal conditions with properly designed tooling, ECM is capable of holding tolerances of the order of plus or minus 0.02 mm and less. In general, tolerance can be maintained on a production basis in the region of plus or minus 0.02 to 0.04 mm. As a general rule, the more complex the shape of the work, the more difficulties to hold tight tolerances.

The main applications of ECM process are in machining of hard-heat-resisting alloys, for cutting cavities in forging dies, for drilling holes, machining of complex external shapes like that of turbine blades, aerospace components, machining of tungsten carbide and that of nozzles in alloy steels. Almost any conducting material can be machined by this method. The material removal rate by this process is quite high for high strength-temperature-resistant (HSTR) materials compared to conventional machining processes. Tool wear is nearly absent and extremely thin metal sheets can be easily worked without distortion. It is also used to machine automotive components like engine castings, and gears. It is also used for machining and finishing forging-die cavities (die sinking) and to produce small holes. Versions of this process are used for turning, facing, milling, slotting, drilling, trepanning, and profiling, as well as in the production of continuous metal strips and webs. More recent applications of ECM include micromachining for the electronics industry.

About the Author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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