ECM Process Capabilities

There are numerous parameters that influence the ECM process in terms of machining rate, surface finish, and other end product physical characteristics. In particular, the metal removal rate and the surface finish depend on current density, machining gap, feed rate, electrolyte composition, temperature, and flow rate or pressure of the electrolyte. The design of an ECM system for a specific application should take into account the range of values for these parameters.

The only requirement of the workpiece material for ECM is that it should be electrically conductive. Because the physical properties (rather than the chemical properties) of the material determine the machining rate, alloys that contain more than one phase of the same material usually present no problem for ECM. However, alloys with inclusions of different materials may be difficult to machine by ECM , or the surface may be unacceptable because of the preferential erosion of one of the materials (e.g., some high-silicon aluminum alloys) (Ref 4). One of the benefits of ECM is the higher machining rates (0.2 to 10 mm/min) for difficult-to-machine materials such as heat-resistant alloys and titanium alloys. Table 1 lists some theoretical removal rates.

Surface Integrity. In addition to high machining rates, ECM produces smooth, damage-free surfaces. The surface finish produced by ECM depends on the workpiece metal or alloy, the electrolyte, and the operating conditions. In general, ECM of nickel-base, cobalt-base, and stainless steel alloys produces smoother surfaces (0.1 to 0.4 /'m Ra) than the surfaces obtained with iron-base alloys and steels (0.6 to 1.5 /''m Ra), where Ra is the surface roughness in terms of arithmetic average. Most of the ECM in industry, at present, is carried out with NaCl electrolyte because NaCl is inexpensive and gives surface finishes in the ranges mentioned above. However, NaNO3 and KNO3, which are more expensive electrolytes, are found to give smoother surfaces for many metals, including iron, copper, nickel, aluminum, and cobalt. Table 2 lists various electrolytes for common work metals.

Table 2 Electrolytes for the electrochemical machining of various metals

Work metal

Electrolyte

Removal rate, mm3 x 103/min (in.3/min) per 1000 A

Major constituent

Concentration (max), kg/L (lb/gal) of H2O

Steel; iron-, nickel-, and cobalt-base alloys

NaCl or KCl

0.30 (21) 2

2.1 (0.13)

NaNO3

0.60 (5)

2.1 (0.13)

Steel; hardened tool steel

NaClO3

0.78 (61) 2

2.0 (0.12)

Gray iron

NaCl

0.30 (21) 2

2.0 (0.12)(a)(b)

NaNO3

0.60 (5)

2.0 (0.12)(a)(b)

White cast iron

NaNO3

0.60 (5)

1.6 (0.10)(c)

Aluminum and aluminum alloys(d)

NaNO3

0.60 (5)

2.1 (0.13)

NaCl or KCl

0.30 (21) 2

2.1 (0.13)

Titanium alloys

NaCl or KCl(e)

0.12 (1)

1.6 (0.10)

Tungsten

NaOH(f)

0.18 (11)(g) 2

1.0 (0.06)

Molybdenum

NaOH®1

0.18 (1-) 2

1.0 (0.06)

NaCl or KCl

0.30 (2-) 2

1.0 (0.06)

Copper and copper alloys(d)

NaCl or KCl

0.30 (2-) 2

4.4 (0.27)

NaNO3

0.60 (5)

3.3 (0.20)

Zirconium

NaCl or KCl

0.30 (2-) 2

2.1 (0.13)

(a) Feed rates limited by graphite particle size.

(a) Feed rates limited by graphite particle size.

(b) Maximum; can vary widely.

(c) Rough surface finish.

(d) NaNO3 electrolyte provides better surface finish.

(e) Voltage must be greater than 11.

(f) NaOH used up in process and must be replenished.

(h) pH of electrolyte decreases with use; maintain pH by adding NaOH or KOH.

The main parameter affecting surface roughness of the workpieces is the current density (Fig. 3). Increasing the grain size of a material has also been found to increase the surface roughness, as shown in Fig. 4 (Ref 5). Under normal operating conditions, ECM produces stress-free, burr-free surfaces with no burning or thermal damage to workpiece surfaces or other detrimental effects on materials. ECM-produced surfaces frequently have better wear, friction, and corrosion-resistant characteristics than surfaces obtained with mechanical finishing. ECM also eliminates the need for subsequent operations such as polishing (Ref 4).

Fig. 3 The effect of electrochemical machining current density (/) on surface roughness (Ra) of three steels. 1, steel with Mo; 2, steel with Mo + W; 3, steel with Ni + Nb. Electrolyte: 150 g/L NaCl + H2O

2.50

2.50

15 20 30 35 40

Grain Size (jim)

Fig. 4 Surface roughness (Ra) as a function of grain size (at 15 volts). Feed rate: (• ), 2.54 mm/min; (I ), 1.72 mm/min; (A ), 0.86 mm/min

However, the accuracy of the machining obtained by ECM is not very high, as ECM may not be able to produce clear cuts or sharp corners. The tolerances achieved by ECM are in the range of 0.02 to 0.2 mm. The positioning accuracy of ECM tools now reaches ±0.01 mm. ECM also requires special corrosion protection systems and waste (sludge) disposal techniques.

Electrochemical deburring (ECD) is used exclusively to deburr or radius workpieces (Fig. 5). ECD equipment is constructed with either single or multiple workstations. Some machines are designed with multiple workstations served from a single power supply. ECD equipment is extremely simple, with the electrolyte pump being the only moving part. The principle of ECD is to use a stationary tool, thus eliminating the need for feed mechanisms and control.

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