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 elec-

trodes 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 quan-

tity of electricity (It) is proportional to the substances’ chemical equiv-

alent 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 elec-

trolytic 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 inten-

sify the mass and charge transfer through the sublayer near the anode.

The electrolyte removes the dissolution products, such as metal hydrox-

ides, 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 work-

piece 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 con-

stant dc (continuous or pulsed) voltage. The electrolyte-feeding unit sup-

plies 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

* 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 (

1. Ensure a uniform and high-speed anodic dissolution

2. Avoid the formation of a passive film on the anodic surface (elec-

trolytes 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 tem-

perature 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 elec-

trolyte 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 distribu-

tion 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 on-

times) can significantly improve the current efficiency and surface qual-

ity. 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 manu-facturing, 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 com-

puter-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 resist-

ance, chemical resistance to the electrolyte, low water absorption, and

resistance against wear in the machine guides and fixtures. Teflon, ure-

thane, 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 chan-

nel 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 sur-

face relative to the tool and hence the gap thickness is represented by

the coordinate y. The workpiece surface moves away from the tool sur-

face 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