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Electrochemical Machining
1 Introduction
Electrochemical machining (ECM) is a modern machining process that
relies on the removal of workpiece atoms by electrochemical dissolution
(ECD) in accordance with the principles of Faraday (1833). Gusseff
introduced the first patent on ECM in 1929, and the first significant
development occurred in the 1950s, when the process was used for
machining high-strength and heat-resistant alloys.
4.1.2 Principles of electrolysis
Electrolysis occurs when an electric current passes between two electrodes dipped into an electrolyte solution. The system of the electrodes
and the electrolyte is referred to as the electrolytic cell. The chemical
reactions, which occur at the electrodes, are called the anodic or
cathodicreactions. ED of the anodic workpiece forms the basis for ECM
of metals.
The amount of metal dissolved (removed by machining) or deposited is
calculated from Faraday’s laws of electrolysis, which state that
1. The amount of mass dissolved (removed by machining), m, is directly
proportional to the amount of electricity.
m ∝ It
2. The amount of different substances dissolved, m, by the same quantity of electricity (It) is proportional to the substances’ chemical equivalent weight e.
m∝e
3- Theory of ECM
ECM uses a direct current at a high density of 0.5 to 5 A/mm2
and a low
voltage of 10 to 30 V. The machining current passes through the electrolytic solution that fills the gap between an anodic workpiece and a
preshaped cathodic tool. The electrolyte is forced to flow through the
interelectrode gap at high velocity, usually more than 5 m/s, to intensify the mass and charge transfer through the sublayer near the anode.
The electrolyte removes the dissolution products, such as metal hydroxides, heat, and gas bubbles, generated in the interelectrode gap.
McGeough (1988) claimed that when a potential difference is applied
across the electrodes, several possible reactions occur at the anode and
the cathode. Figure 4.1 illustrates the dissolution reaction of iron in a
sodium chloride (NaCl) water solution as an electrolyte. The result of
electrolyte dissociation and NaCl dissolution
4- ECM equipment
the main components of the ECM machine: the feed
control system, electrolyte supply system, power supply unit, and workpiece holding device. As shown in Fig. 4.3, the feed control system is
responsible for feeding the tool at a constant rate during equilibrium
machining. The power supply drives the machining current at a constant dc (continuous or pulsed) voltage. The electrolyte-feeding unit supplies the electrolyte solution at a given rate, pressure, and temperature.
Facilities for electrolyte filtration, temperature control, and sludge
removal are also included. ECM machines are capable of performing a
wide range of operations such as duplicating, sinking, and drilling.
Semiautomatic and fully automated facilities are used for large-size
machining, such as deburring in the automotive industry. ECM
machines, in contrast to conventional machine tools, are designed to
stand up to corrosion attack by using nonmetallic materials. For high
strength or rigidity, metals with nonmetallic coatings are recommended
at www.unl.edu/.
* Power supply. The dc power supply for ECM has the following
features:
1. Voltage of 2 to 30 volts (V) (pulsed or continuous)
2. Current ranges from 50 to 10,000 amperes (A), which allow current
densities of 5 to 500 A/cm2
3. Continuous adjustment of the gap voltage
4. Control of the machining current in case of emergency
5. Short circuit protection in a matter of 0.001 s
6. High power factor, high efficiency, small size and weight, and low cost
* Electrolytes. The main functions of the electrolytes in ECM are to
1. Create conditions for anodic dissolution of workpiece material
2. Conduct the machining current
3. Remove the debris of the electrochemical reactions from the gap
4. Carry away the heat generated by the machining process
5. Maintain a constant temperature in the machining region
* The electrolyte solution should, therefore, be able to (www.unl.edu/)
1. Ensure a uniform and high-speed anodic dissolution
2. Avoid the formation of a passive film on the anodic surface (electrolytes containing anions of Cl, SO4, NO3, ClO3, and OH are often
recommended)
3. Not deposit on the cathode surface, so that the cathode shape
remains
unchanged (potassium and sodium electrolytes are used)
4. Have a high electrical conductivity and low viscosity to reduce the
power loss due to electrolyte resistance and heat generation and to
ensure good flow conditions in the extremely narrow interelectrode gap
5. Be safe, nontoxic, and less erosive to the machine body
6. Maintain its stable ingredients and pH value, during the machining
Period
7. Have small variation in its conductivity and viscosity due to temperature rise
8. Be inexpensive and easily available
The most common electrolytes used are sodium chloride (NaCl), sodium
nitrate (NaNO3), and, sodium hydroxide. Industrial ECM operations
usually involve using mixed electrolytes to meet multiple
requirementsas shown in Table 4.1. The selection of the ECM electrolyte
depends on
the workpiece material, the desired dimensional tolerance, the surface
finish required, and the machining productivity. During ECM, the electrolyte plays an important role in dimensional control.
sodium nitrate solution is preferable, because the local metal
removal rate is high at the small gap locations where both the current
density and the current efficiency are high. Additionally, the local
removal rate is low at the larger gap locations where both the current
density and current efficiency are low. This results in the gap distribution tending toward uniformity.
The current efficiency in ECM depends on the anodic material and the
electrolyte. When the pulsed voltage is applied instead of the commonly
used continuous voltage, proper use of pulse parameters (e.g., pulse ontimes) can significantly improve the current efficiency and surface quality. Depending on the tool shape and type of the machining operation,
several methods of supplying electrolyte to the machining gap
The choice of the electrolyte supply method depends on the
part geometry, machining method, required accuracy, and surface finish.
Typical electrolyte conditions include a temperature of 22 to 45°C, a
pressure between 100 to 200 kPa, and a velocity of 25 to 50 m/s.
3 -Tools
The design of a suitable tool for a desired workpiece
shape, and dimension forms a major problem. The workpiece shape is
expected to be greater than the tool size by an oversize. In determining
the geometry of the tool to be used under steady-state conditions, many
variables should be specified in advance such as the required shape of
the surface to be machined, tool feed rate, gap voltage, electrochemical
machinability of the work material, electrolyte conductivity, and anodic
and cathodic polarization voltages. With computer integrated manufacturing, cathodes are produced at a lower cost and greater accuracy.
Computer-aided design (CAD) systems are used first to design a cathodic
tool. This design is programmed for CNC production by milling and
turning. After ECM, the coordinate measuring machine inspects the
workpiece produced by this tool and sends data back to the CAD computer-aided manufacturing (CAM) unit for analysis of the results.
Iterations of the cathodic tool are made so that the optimum tool design
is selected.
The material used for ECM tools should be electrically conductive
and easily machinable to the required geometry. The various materials
used for this purpose include copper, brass, stainless steel, titanium, and
copper tungsten. Tool insulation controls the side electrolyzing current
and hence the amount of oversize. Spraying or dipping is generally the
simplest method of applying insulation. Durable insulation should
ensure a high electrical receptivity, uniformity, smoothness, heat resistance, chemical resistance to the electrolyte, low water absorption, and
resistance against wear in the machine guides and fixtures. Teflon, urethane, phenol, epoxy, and powder coatings are commonly used for tool
insulation (Metals Handbook, 1989).
5- Basic working principles
The simplest case to consider is that of plane-parallel electrodes normal
to the feed direction as described by Tipton (1971) and shown in Fig. 4.6.
Consider an electrolyte of conductivity k and density re that flows at a
mean velocity u, in the direction of increasing x, in a channel. The channel is assumed to extend to the left of the origin x = 0 where the tool and
workpiece start, so that the flow has reached a steady state and the inlet
conditions can be neglected. All properties of the system are assumed
to be independent of the z direction. The position of the workpiece surface relative to the tool and hence the gap thickness is represented by
the coordinate y. The workpiece surface moves away from the tool surface in the direction of increasing y at a rate proportional to the current
The current efficiency g is defined as the ratio of the observed amount
of metal dissolved to the theoretical amount predicted from Faraday’s
laws for the same specified conditions of electrochemical equivalence,
current, etc. Apparent current efficiency values may be due to
1. The choice of wrong valence
2. Passivation of the anodic surface
3. Grain boundary attack, which causes the removal of metal grains by
electrolyte forces
4. Gas evolution at the anode surface
It is convenient to write the machining constant C for the particular
workpiece-electrolyte combination (m2
⋅ min−1