In this article we will discuss about:- 1. Meaning of Electro-Chemical Machining (ECM) 2. Applications of ECM Process 3. Advantages 4. Disadvantages and Limitations 5. Characteristics.
Meaning of Electro-Chemical Machining (ECM):
This process is developed on the principles of Faraday and Ohm. In this process, an electrolytic cell is formed by the anode (workpiece) and the cathode (tool) in the midst of a flowing electrolyte. The metal is removed by the controlled dissolution of the anode according to the well-known Faraday’s Law of Electrolysis.
When the electrodes are connected to about 20 V electric supply source, flow of current in the electrolyte is established due to positively charged ions being attracted towards the cathode and vice versa. Current density depends on the rate at which ions arrive at respective electrodes which is proportional to the applied voltage, concentration of electrolyte, and gap between the electrodes.
Due to electrolysis process at the cathode, hydroxial (-vely charged) ions are released which combine with the metal ions of anode to form insoluble metal hydroxides. Thus the metal is mainly removed in the form of sludge’s and precipitates by electro chemical and chemical reactions occurring in the electrolytic cell. This process continues like this till the tool has reproduced its shape in the workpiece.
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This process is thus reverse of electroplating, but the metal removed from the work before being deposited on the tool is pumped in the flowing electrolyte. It is a production process for machining conducting materials and gives the highest chip removal rates with reasonable surface finish on repetitive work.
By this process, even hardest possible material can be machined. This process is ideally suited for the production of deep holes and profiled cavities in electrically conducting materials. Examples of application of this process include aircraft engine parts, turbine blades, grinding of carbide tools and dies, gun drilling etc.
The metal removal is carried out by maintaining an electrolyte between the work (anode) and tool (cathode) in a very small gap of 0.1 to 0.2 mm between the two by pumping electrolyte through the gap. The work is generally kept stationary and the tool is fed in a linear direction. It is essential that the feed drive system to the tool be free from stick-slip under high forces.
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The electrolyte used is generally an aqueous solution of common salt or dilute acid which dissolves the particles. The metal from the work is removed due to the ion migration towards the tool and the deposition of the ions on the tool is prevented by pumping a strong stream of electrolyte at high pressure upto 14 kg/cm2.
Velocity of electrolyte flow through the gap between tool and workpiece is of the order of 30—60 m/sec. All this calls for rigid machine structure to resist the hydrostatic and hydro- dynamic forces. A great deal of hydrogen is evolved at the cathode, and high pressure of electrolyte across the gap helps dissipate the gas, thereby avoiding polarisation. In this process no spark is produced and the temperatures generated are low which do not cause metallurgical changes in the workpiece material.
When sufficient electrical energy (about 6 eV) is available between tool and workpiece, s metallic ion may be pulled out of the workpiece surface. The positive metallic ions will react with negative ions present in the electrolytic solution forming metallic hydroxides can other compounds, and thus the metal will be anodically dissoluted with the formation of sludges and precipitates. Current density in the gap between tool and workpiece is of the order of 0.4 to 8 A per mm2.
It would be noted from Fig. 10.22 (b), that unlike EDM, work need not be submerged in the electrolyte but it is pumped around the work-piece at high speed. There is no tool wear unlike EDM process, giving a very long tool life.
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As tool does not wear away, serve control system to maintain constant gap between tool and workpiece is not required. The temperature of electrolyte is maintained between 25 and 60°C to retain conductivity within reasonable limits.
The advantages of the process are that as the tool does not come in contact with the work, practically no wear takes place. However, electrolyte may cause some chemical corrosion of the tool. Machining is done at low voltages and compared to other processes the metal removal rate is high. Dimensions upto 0.05 mm can be easily controlled.
By this process any metal or alloy which is good conductor of electricity can be given any complicated profile in a single step operation. The chemical composition and structure, melting point, hardness, toughness or brittleness of the material to be machined have no influence on the machining process. Due to low temperature developed at the time of normal machining of workpiece, no thermal damage is done to the workpiece structure.
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The only drawback of the process is that huge amount of energy is consumed (about 100 times that required for turning or milling steel). The electrical generating equipment generating electric supply at about 5-15 volt. D.C. and upto 10,000 Amp. represents a major art of the cost of ECM machine. Material removal rate is of the order of 1600 mm3/ min for each 1000 amp.
It is thus seen that Electrochemical machining (ECM) process uses electrical energy in combination with chemical energy to remove the material of workpiece. Electrochemical machining removes material of electrically conductor workpiece. The workpiece is made anode of the setup and material is removed by anodic dissolution.
Tool is made cathode and kept in close proximity to the workpiece and current is passed through the circuit. Both electrodes are immersed into the electrolyte solution. This works on the principle of reverse of electroplating. This works on the basis of the Faraday’s law of electrolysis. The cavity machined is the mirror image of the tool.
Whole process is carried out in a tank, (made of transparent plastic non-reactive to the electrolyte.) The tank is filled with electrolyte. The workpiece is made anode Electrolyte is pumped between workpiece and the tool Water is used as base of electrolyte in ECM. Normally water soluble NaCl and NaNO3 are used as electrolyte. Electrolyte facilitates as carrier of dissolved workpiece material. It is recycled by a pump after filtration. Material of workpiece is removed by anodic dissolution. Only electrically conducting materials can be processed by ECM.
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A specially designed and shaped tool is used. The tool is usually made of copper, brass, stainless steel, and it is a mirror image of the desired machined cavity. Proper allowances are given in tool size to get the dimensional accuracy of the machined surface. Servo motor is used to feed the tool to the machining zone. It is necessary to maintain a constant gap between the workpiece and tool so tool feed rate is kept accordingly while machining.
DC power source is used to supply the current. Tool is connected with the negative terminal and workpiece with the positive terminal of the power source. Power supply supplies low voltage (3 to 4 volts) and high current to the circuit.
Metal Removal Rate:
According to the Faraday’s first law of electrolysis, mass of ions liberated by the substance M = ZIt
where I = is the current flowing through the electrolytic cell in amperes, t = time in sec. and
Z = constant known as the electro-chemical equivalent of the substance. It is equal to the mass of the ions liberated by the substance by the passage of one amp. of current for one sec through the electrolytic solution or by the flow of one coulomb of charge.
According to the Faraday’s second law of electrolysis:
M = Equivalent weight of a substance dissolved or deposited
It would be noted that the gap is proportional to the applied voltage and inversely proportional to the feed rate. Also current density is proportional to the feed.
In actual ECM process, the actual metal removal rate may differ from theoretical calculations due to conditions not being ideal and other factors influencing the same. For example, in case of high currents, dissolution takes place at a higher potential difference and thus trivalent dissolution (in addition to divalent dissolution assumed) also takes place. Dissolution valency is also a function of electrolytes. For example, copper dissolves in monovalent form in chloride solutions, but in divalent state in nitrate solutions.
Suspended solids from electrolyte are removed by settling, centrifuging and filtering or by combination of these and the filtered electrolyte is used again and again.
Material Removal Rate:
It is a function of feed rate which dictates the current passed between the work and the tool. As the tool advances towards work, gap decreases and current increases which increases more metal at a rate corresponding to tool advance. A stable spacing between tool and work is thus established (known as equilibrium machining gap).
It may be mentioned that high feed rate not only is productive but also produces best quality of surface finish. However feed rate is limited by removal of hydrogen gas and products of machining. Metal removal rate is lower with low voltage, low electrolyte concentration and low temperature.
Surface Finish:
ECM can produce surface finish of the order of 0.4 pm by rotation of tool/work. Any defect on the tool face produces replica on workpiece. Tool surface should therefore be polished. The finish is better on harder materials. Cobalt alloys give mirror like finish and copper and aluminium give a matty finish. For optimum surface finish, careful electrode design, maximum feed rate, and surface improving additives in electrolyte are selected.
Low voltage decreases the equilibrium machining gap and results in better surface finish and tolerance control. Low electrolytic concentration decreases the equilibrium machining gap and thus better surface finish and tolerance. Low electrolyte temperature also promotes better surface finish.
The surface finish in ECM process is affected by the factors like selective dissolution, (due to surface not being smooth). Sporadic breakdown of the anodic film, (which occurs due to gradual fall in the potential difference between the work surface and the electrolyte in the region away from the machining area), flow separation and formation of eddies (caused by the presence of hills and valleys on the anode surface which can be taken care of by properly designing the electrolyte flow path in a tool), and evolution of hydrogen gas which is collected by the flowing electrolyte and reduces the specific conductivity of the solution.
Applications of ECM Process:
(a) This is used for sharpening and internal finishing of surgical needles.
(b) Machining of hard, brittle, heat resistant materials without any problem. Machining of cavities and holes of complicated and irregular shapes.
(c) Drilling of small and deeper holes with very good quality of internal surface finish. It is used for making inclined and blind holes and finishing of conventionally machined surfaces.
(d) As electrochemical deburring process, it is used to finish rough surface.
Advantages of ECM Process:
(а) Machining of hard and brittle material is possible with good quality of surface finish and dimensional accuracy.
(b) Complex shapes can be easily machined.
(c) There is almost negligible tool wear so cost of tool making is only one time investment for mass production.
(d) There is no application of force, no direct contact between tool and work and no application of heat so there is no scope of mechanical and thermal residual stresses in the workpiece.
(e) Very close tolerance can be obtained.
Disadvantages and Limitations of ECM Process:
(i) All non-conducting materials cannot be machined.
(ii) Total material and workpiece material should be chemically stable with the electrolyte solution.
(iii) Designing and making tool is difficult but its life is long so recommended only for mass production.
(iv) Accurate feed rate of tool is required to be maintained.
Example 1:
In an ECM process for machining iron it is desired to obtain a metal removal rate of 1 cm3/min. Determine the amount of current required for the process, assuming that At. wt. of iron = 56 gm, valency at which dissolution occurs = 2, density of iron = 7.8 gm/cm3 and Faraday’s constant = 1609 amp-min.
Solution:
Given atomic weight of iron = At = 56 gm
Valency of iron dissolution
= v = 2, Density of iron = ρ = 7.8 gm/cm3
Example 2:
In an ECM process machining iron and using copper tool, and saturated solution of NaCl in water as electrolyte, the electrode area = 1 cm x 2 cm and the initial gap (h) for electrolyte to pass is equal to 0.020 cm. For the electrolyte, specific heat = 0.997 cal per gm per oC; density = l gm/cm3 and specific resistance = 3 ohm-cm. Calculate (i) the permissible fluid flow velocity if the maximum permissible temperature of electrolyte is the boiling point (95°C). The ambient temperature is 25oC and the applied voltage = 10 V; (ii) the maximum metal removal rate if the permissible current density has been 150 amp/cm2.
Given that permissible fluid velocity for rectangular electrode,
Solution:
V = 10 volts, l = 2.0 cm, re = 3 ohm-cm, h = 0.020 cm, ρe = 1 gm/cm2, Ce = 0.997 cal/gm °C, θB – θA = 95 – 25 = 70°C.
Figs. 10.27 to 10.30 show the applications of electrochemical machining processes.
Characteristics of ECM Process:
(a) Material Removal Mechanism:
Controlled removal of metal by anodic dissolution in an electrolytic medium.
(b) Tool:
Cu, brass or steel.
(c) Power Supply:
Constant voltage DC supply.
(d) Voltage and Current:
5 — 30 V d.c.,
50 — 40,000 Amp.
(e) Material Removal Rate:
1600 mm3/min.
(f) Specific Power Consumption:
7 W/mm3/min (around 150 times more in comparison to conventional methods).
(g) Electrolytic Solution:
Neutral salts, acids and alkalies.
(h) Accuracy and Surface Finish:
± 0.02 mm., 0.4 µm.
Applications:
Used for machining difficult-to-machine materials and complex- shaped parts.
Mechanical and Surface Properties of Metals:
Stress free machining, burr- free surface, reduced tool wear, no thermal damage.
Limitations:
High specific energy consumption, not suited for non-conducting pieces; high initial and working cost.