Electro-chemical machining (ECM) is a non-traditional machining process that removes metal by dissolving it in an electrolyte with the use of electric current. In ECM, the workpiece acts as an anode and is dissolved by the electrolyte, while a tool with the desired shape acts as a cathode. Key factors in ECM include the electrolyte, which carries current and removes dissolved material, the tool and workpiece materials, and a DC power supply. ECM can machine hard metals and complex shapes with high accuracy and no tool wear. Common applications of ECM include machining turbine blades, aerospace components, and other difficult-to-machine metals.
2. Introduction
Electrochemical Machining (ECM) is one of the
newest and most useful non-traditional machining
(NTM) process belonging to Electrochemical
category.
Electrochemical machining (ECM) is used to remove
metal and alloys which are difficult or impossible to
machine by mechanical machining process.
The reverse of electroplating.
This machining process is based Michael Faraday’s
classical laws of electrolysis, requiring basically two
electrodes, an electrolyte, a gap and a source of D.C
power of sufficient capacity.
4. Working of ECM
In the actual process of ECM, the cathode is tool
shaped(mirror image of work-piece) and anode is
the work-piece.
A gap(0.05 to 0.7 mm) is provided between the tool
and work-piece and electrolyte flows through the
gap at a velocity of 30 to 60 m/s and it completes
the electrical circuit.
Electrolyte is pumped at high pressure of 20
kgf/cm2(1.96 MPa) through the gap.
Electrolyte must be circulated at a rate sufficiently
high to conduct current between them and to carry
heat.
Metal is removed from the work-piece by dissolution
5. The electric current is of the order of 50 to 40,000 A at 5
to 35 V D.C for current density of 20 to 300 A/cm2.
Power of 3 KWh is needed to remove 16 cm3 of metal
which is almost 30 times the energy required in the
conventional process(when the material is readily
machinable).
6. Electrolysis process
In the electrolytic circuit the electron is flow from
the work-piece through the power supply to the
tool.
The electrolysis process that takes place at the
cathode liberates hydroxyl ions(OH-) and free
hydrogen.
The hydroxyl ions combine with the metal ions of
the anode to form insoluble metal hydroxides and
material is thus removed from the anode.
At cathode, the following reactions takes place:
1. M++ e- M(M denotes metal)
2. 2H++ 2e- H2(Hydrogen evolution)
7. At anode, the following reactions takes place with
a halogen electrolyte:
1. M M++ e-(Metal dissolution)
2. 2H2O O2+ 4H++ 4e- (Oxygen evolution)
3. 2Cl- Cl2+ 2e- (Halogen gas evolution)
As an example, in machining of iron in NaCl
electrolyte, at the cathode the reaction products
are FeCl2, Fe(OH)2, Fe(OH)3 which forms a layer
and this is how iron is removed by electrolytic
action.
Current of 1000 A would dissolve iron at the rate
of about 15 g/min and generate hydrogen at a
rate of about 300 cm3/min.
8. Elements of ECM
Important elements of ECM are:
1. Electrolyte
2. Tool (cathode)
3. Work-piece (anode)
4. D.C power supply
Electrolyte:
Common electrolytes used are sodium chloride, sodium
nitrate, sodium hydroxide, sodium fluoride, sodium
chlorate, potassium chloride and sulphuric acid.
These solution on reaction produce an insoluble
compound in the form of sludge.
9. The main functions of an electrolyte in ECM are:
1. It carries the current between tool and work-piece.
2. It removes the products of machining and other
insoluble products from the cutting region.
3. It dissipates heat produced in the operation.
The essential characteristics of electrolyte includes:
1. Good electrical conductivity.
2. Non-toxicity and chemical stability.
3. Non-corrosive property.
4. Low viscosity and high specific heat.
High velocity flow over the electrode surface is one of
the key factors of ECM. It is necessary to prevent
crowding of hydrogen gas and debris of machining.
10. Tool (cathode):
The most commonly used tool material are copper,
brass, titanium, copper tungsten and stainless steels
when electrolyte is made of salts of sodium and
potassium.
Titanium has been found to be the most suitable tool
where the electrolyte has the tendency to anodize the
tool as in case of sulphuric acid.
The other tool materials are aluminium, graphite,
bronze, platinum and tungsten carbide.
The accuracy of tool shape directly affects the work-
piece accuracy.
Electro-forming and cold forging are two methods of
tool shaping.
11. The general requirements of tool material in ECM are:
1. It should be conductor of electricity.
2. It should be rigid enough to take up the load due
to fluid pressure.
3. It should be chemically inert to the electrolyte.
4. It should be easily machinable to make it in the
desired shape.
Work-piece (anode):
Work-piece should be conductor of electricity. So it is
almost limited to metals only.
12. Material removal rate:
Material removal rate (MRR) is an important characteristic
to evaluate efficiency of a non-traditional machining
process.
In ECM, material removal takes place due to atomic
dissolution of work material which is governed by Faraday’s
laws of Electrolysis.
MRR = =
where
m = ItA/Fv = mass of material dissolve
I = current ; A = Atomic weight ; v = valency
F = Faraday’s constant = 96500 coulumbs
ρ = density of the material
m
tρ
IA
Fρν
13. Power Supply:
1. Type: direct current
2. Voltage: 2 to 35 V
3. Current: 50 to 40,000 A
4. Current density: 20 A/cm2 to 300 A/cm2
14. Accuracy of ECM:
There are number of factors which govern the
accuracy of the parts produced by ECM. The major
ones are:
1. Machining voltage.
2. Feed rate of electrode (tool).
3. Temperature of electrolyte.
4. Concentration of electrolyte.
Under ideal conditions with properly designed tooling,
ECM is capable of holding tolerances of ±0.02 mm.
Surface finish in ECM is of the order of 0.2 to 0.8
micron.
No burrs and sharp edges are left on the work-piece.
15. Application
The most common application of ECM is high accuracy
duplication. Because there is no tool wear, it can be
used repeatedly with a high degree of accuracy.
It is commonly used on thin walled, easily deformable
and brittle material because they would probably
develop cracks with conventional machining.
It is used in machining of hard-heat-resisting alloys.
It is used in cutting cavities and holes in various
products, machining of complex external shapes like
that of turbine blades, aerospace components and
machining of tungsten carbide and nozzles of alloy
steels.
Any conducting material can be machined by this
method.
16. Advantages
There is no cutting forces therefore clamping is not
required except for controlled motion of the work piece.
It can machine configurations which is beyond the
capability of conventional machining processes.
Very accurate (tolerance of ±0.02 mm).
Relatively fast.
Can machine harder metals than the tool.
Extremely thin materials can be easily worked without
distortion.
Tool wear is nearly absent.
Better surface finish (0.2 to 0.8 micron).
17. Disadvantages
High energy consumption.
Non conducting material cannot be machined.
Corrosion and rust of ECM machine can be
hazardous but preventive measures can help in this
regard.
