Electrochemical Machining

ECM consists basically of the electrochemical dissolution of the surface metal of a workpiece by conversion of metal to its ions by means of an electric current. The whole process is accomplished in an electrolytic cell by applying a positive (anodic) potential to the workpiece and a negative (cathodic) potential to the tool used to shape the workpiece. ECM can be used for shaping, finishing for improving the quality of the surface, deburring, and radiusing. One kind of ECM is electropolishing. Figure 1 shows the various schematics for machining different geometries using ECM.

Faraday Law Corrosion
Fig. 1 Schematics of electrochemical machining (ECM) operations. (a) Die sinking. (b) Shaping of blades. (c) Drilling. (d) Milling. (e) Turning. (f) Wire ECM. (g) Drilling of curvilinear holes. (h) Deburring and radiusing. (i) Electropolishing

The rate of material removal in ECM is governed by Faraday's law, since it is a function of current. The primary variables that affect the current density and the material removal rate are:

• Electrolyte conductivity

• Electrolyte composition

• Electrolyte flow

• Workpiece material

The voltage across the gap influences the current and the material removal rate and is controlled in most ECM operations. However, for a constant voltage, the current also depends on the electrical resistance of the cutting gap. Resistance is much more difficult to control because it depends on the conductivity of the electrolyte and the distance across the gap.

The feed rate, or penetration rate, is also controlled in most ECM operations. At a constant voltage, the gap is inversely proportional to the feed rate. The distance across the frontal gap is a function of feed rate because, as the cathode is fed into the workpiece at a higher rate, the gap closes, causing resistance to drop. As resistance drops, amperage increases; therefore, machining rate also increases until an equilibrium is reached. At slower feed rates, the material removal rate decreases as the gap increases because the cathode is not keeping up with the workpiece surface. As the gap increases, the resistance rises and amperage drops. Frontal gaps are usually between 0.1 to 0.8 mm (0.005 to 0.030 in.), and side gaps, in the case of drilling, are about 0.5 to 1.3 mm (0.020 to 0.050 in.).

The feed rate also varies directly with the current. For example, a hole machined at 2.5 mm/min (0.100 in./min) at 10 V and 1000 A would require 2000 A if the feed were increased to 5.0 mm/min (0.200 in./min). This would also require a potential of about 20 V and would increase power consumption (V • I) from 10 to 40 kW.

The feed rate also depends on the application. Typical feed rates for different ECM operations on Inconel 718 are:

Operation

Feed rate

mm/min

in./min

Round holes (blind)

2.2

0.085

Simple cavities

2.2

0.085

19 x 103 mm2 (30 m.2) faces

1.3

0.050

Accurate estimates of feed rates usually require pilot testing in the desired ECM configuration.

Electrolyte conductivity also affects resistance across the gap. Increasing the concentration of an electrolyte causes conductivity to rise, which causes a decrease in resistance. Temperature increases of the electrolyte also increase conductivity. Therefore, electrolyte concentration and temperature must be controlled.

Electrolyte composition directly influences conductivity, material removal rates, and surface characteristics. The parameters used for a given application may not yield the same ECM results if a different type of electrolyte is used. The normal development of an operation usually begins with the selection of the correct electrolyte. The other parameters and the cathode are then adjusted to obtain the desired result.

Electrolyte flow rate is also a factor in ECM process control. The temperature increase of the electrolyte passing through the gap is dependent on the flow rate. In addition, the rate at which hydrogen bubbles are carried away is thought to influence conductivity. Pressure control is the method of controlling flow rate (especially when a centrifugal pump is used). The flow rate also affects the level of turbulence of the electrolyte as it passes through the gap, and this influences the surface finish. The flow rate must also be great enough to remove machining byproducts (sludge).

The workpiece material also affects the material removal rates. Theoretical removal rates for various metals are listed in Table 1. These removal rates are derived from Faraday's Second Law, which states that 1 Faraday (96,494 coulombs or ampere seconds) will liberate 1 g equivalent weight of a substance, or its atomic weight divided by the valence. For example, the gram equivalent weight of iron is the atomic weight divided by the valence of the dissolved iron, or 56 divided by 2 equals 28. Thus, 28 g of iron will dissolve during the passage of each Faraday of electricity. Table 1 lists the theoretical metal removal rates at 1000 A of current flow.

Table 1 Theoretical removal rates in electrochemical machining

Metal

Valence

Density

Removal rate (1000 A current, 100% efficiency(a))

Mass

Volume

g/cm3

lb/in.3

kg/h

lb/h

mm3 x 103/min

in.3/min

Aluminum

3

2.7

0.098

0.34

0.74

2.1

0.13

Beryllium

2

1.9

0.067

0.17

0.37

1.5

0.09

Copper

1

9.0

0.324

2.37

5.22

4.4

0.27

2

9.0

0.324

1.18

2.61

2.1

0.13

Iron

2

7.9

0.284

1.04

2.30

2.3

0.14

3

7.9

0.284

0.69

1.53

1.5

0.09

Magnesium

2

1.7

0.063

0.45

1.00

4.4

0.27

Molybdenum

3

10.2

0.369

1.19

2.63

2.0

0.12

4

10.2

0.369

0.89

1.97

1.5

0.09

6

10.2

0.369

0.60

1.32

1.0

0.06

Nickel

2

8.9

0.322

1.09

2.41

2.1

0.13

3

8.9

0.322

0.73

1.61

1.3

0.08

Niobium

3

8.6

0.310

1.16

2.55

2.3

0.14

4

8.6

0.310

0.87

1.92

1.6

0.10

5

8.6

0.310

0.69

1.53

1.3

0.08

Tantalum

5

16.6

0.600

1.35

2.98

1.3

0.08

Titanium

3

4.5

0.163

0.59

1.31

2.1

0.13

4

4.5

0.163

0.45

0.99

1.6

0.10

Tungsten

6

19.3

0.697

1.14

2.52

1.0

0.06

8

19.3

0.697

0.86

1.89

0.8

0.05

Commercial alloys

4340

2.18

0.133

17-4 PH

2.02

0.123

A-286

1.92

0.117

M-252

1.80

0.110

René 41

1.77

0.108

U-500

1.80

0.110

U-700

1.77

0.108

L-605

1.75

0.107

(a) It is not always possible to predict the valence at which some metals will dissolve nor how much current will flow through the gap. Also, practical factors, such as the shape of the electrode, can limit current flow.

(a) It is not always possible to predict the valence at which some metals will dissolve nor how much current will flow through the gap. Also, practical factors, such as the shape of the electrode, can limit current flow.

Removal rates for a given current are, of course, less than the theoretical limit, and variations in temperature, metallurgy, and electrochemical reactivity of the electrolyte affect the metal removal rate. The most accurate method of determining removal rates is by empirical testing.

The electrical current in a particular application is determined by the current density and the area of the gap between the anode (wheel) and the cathode (workpiece). Current density most often depends on the material being processed, although it is also affected by gap distance and voltage. To obtain maximum removal rates, the area should be as large as possible so that it will draw greater current.

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