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Contemporary Machining Processes
1.6 ELECTROCHEMICAL MACHINING METHODS
Electrochemical machining (ECM) can be traced back to the first electrolytic polishing process proposed by the Russian scientist E.I. Shpitalsky in 1911 (Koryagin
et al., 2000) and developed by Gussev in 1929 (Ivanov et al., 2015). In the ECM process, controlled dissolution of an anodic workpiece takes place through electrolysis
governed by Faraday’s law, so that an approximately complementary image of the
cathodic tool is reproduced on the anodic workpiece, dissolving it electrolytically
without any deposition in the cathode (Jain and Pathak, 2017). Material is removed
through anodic dissolution, an electrochemical process in which metal is dissolved
from the anodic workpiece and gases are released on the cathodic tool as well as
on the anode surface, and various electrochemical reactions take place in the bulk
electrolyte (Bhattacharyya, 2015). In general, MRR depends on the electrochemical
properties of the workpiece material, electrolyte properties, and characteristics of
electric current. This can be seen from the equation (Ribeiro et al., 2013):
MRR = × ×
C I h
(1.6.1)
where C is a material constant, I is a supplied current, and η is a current efficiency.
Values of MRR typically range from 1 to 20 cm 3 /h depending on current density
(Ribeiro et al., 2013). The ECM process generally uses low values of DC, between
8 and 30 V, while a high current density of 10–100 Acm −2 is generated, because the
anode and a pre-shaped cathode are separated by a very small inter-electrode gap of
0.1–1.0 mm (Jain and Pathak, 2017).
Electrolytes are used to dissolve the anodic material and to flush away products of
the electrochemical reaction. In addition, they remove heat from the passage of the
current. There are four main groups of electrolytes (Leese and Ivanov, 2016):
1. Neutral aqueous salts
2. Aqueous acids
3. Aqueous bases or alkalis
4. Non-aqueous electrolytes
Aqueous salts are usually the first choice as they are generally inexpensive and tend
not to cause damage to machinery setup. In contrast, an acidic electrolyte could
corrode machinery over time. The most common electrolyte used for ECM is concentrated sodium chloride or sodium nitrate. For electrochemical micromachining
(ECMM), a less concentrated electrolyte is required to enhance the machining precision by restricting the current passage through increased electrolyte resistance.
Sodium chloride is regularly used to machine stainless steel when a bright surface
finish is required, since it does not form a passive layer on the stainless steel surface.
When close replication of the tool is more important than surface finishing of stainless steel, sodium nitrate is employed because it prevents stray corrosion, ensuring
precise tool replication. However, when aqueous salt solutions do not provide an
environment in which dissolution can occur, acidic or basic electrolytes can be used.
Contemporary Machining Processes
1.6 ELECTROCHEMICAL MACHINING METHODS
Electrochemical machining (ECM) can be traced back to the first electrolytic polishing process proposed by the Russian scientist E.I. Shpitalsky in 1911 (Koryagin
et al., 2000) and developed by Gussev in 1929 (Ivanov et al., 2015). In the ECM process, controlled dissolution of an anodic workpiece takes place through electrolysis
governed by Faraday’s law, so that an approximately complementary image of the
cathodic tool is reproduced on the anodic workpiece, dissolving it electrolytically
without any deposition in the cathode (Jain and Pathak, 2017). Material is removed
through anodic dissolution, an electrochemical process in which metal is dissolved
from the anodic workpiece and gases are released on the cathodic tool as well as
on the anode surface, and various electrochemical reactions take place in the bulk
electrolyte (Bhattacharyya, 2015). In general, MRR depends on the electrochemical
properties of the workpiece material, electrolyte properties, and characteristics of
electric current. This can be seen from the equation (Ribeiro et al., 2013):
MRR = × ×
C I h
(1.6.1)
where C is a material constant, I is a supplied current, and η is a current efficiency.
Values of MRR typically range from 1 to 20 cm 3 /h depending on current density
(Ribeiro et al., 2013). The ECM process generally uses low values of DC, between
8 and 30 V, while a high current density of 10–100 Acm −2 is generated, because the
anode and a pre-shaped cathode are separated by a very small inter-electrode gap of
0.1–1.0 mm (Jain and Pathak, 2017).
Electrolytes are used to dissolve the anodic material and to flush away products of
the electrochemical reaction. In addition, they remove heat from the passage of the
current. There are four main groups of electrolytes (Leese and Ivanov, 2016):
1. Neutral aqueous salts
2. Aqueous acids
3. Aqueous bases or alkalis
4. Non-aqueous electrolytes
Aqueous salts are usually the first choice as they are generally inexpensive and tend
not to cause damage to machinery setup. In contrast, an acidic electrolyte could
corrode machinery over time. The most common electrolyte used for ECM is concentrated sodium chloride or sodium nitrate. For electrochemical micromachining
(ECMM), a less concentrated electrolyte is required to enhance the machining precision by restricting the current passage through increased electrolyte resistance.
Sodium chloride is regularly used to machine stainless steel when a bright surface
finish is required, since it does not form a passive layer on the stainless steel surface.
When close replication of the tool is more important than surface finishing of stainless steel, sodium nitrate is employed because it prevents stray corrosion, ensuring
precise tool replication. However, when aqueous salt solutions do not provide an
environment in which dissolution can occur, acidic or basic electrolytes can be used.