Principles of Electrochemical Machining : One of the most promising unconventional machining processes

Electrochemical Machining is one of the most effective unconventional machining processes. It can be considered as a process quite similar to reverse of electroplating with some modifications. Electricity conduction through metals involve movement of electrons whereas in electrolyte electricity is conducted by motion of ions and ions have mass. Thus conduction via electrolyte involves movement of mass. This principle is used to remove mass from an electrode during ECM process. Work piece is connected to anode and the tool to the cathode and the gap between the two is filled with electrolyte or rather the electrolyte is pumped through the gap. The anode starts to dissolve as current is passed through the cell. Rate of dissolution is more if the gap between the tool and work piece is smaller. If the tool is given downward motion, work surface tends to take same shape as the tool. Tool is given a constant feed motion. 

                                                Scheme of Electrochemical Machining

 Generally electrolyte is pumped at high pressure through the tool and the gap between the tool and work piece. The essential condition for ECM is that anode should be dissolved without any deposition at cathode. Current passed through the set-up is in range of few thousand amperes and the voltage in the range of 8-20 V. The gap measures about 0.1 to 0.2 mm. Material removal takes place at a rate of about 1600 mm³/min for 1000amp. One remarkable advantage of the process is that the mrr is independent of work material hardness. Any complicated profile can be machined on the work piece with this process. And above all there is no tool wear in the process. Tool and work are subjected to high pressure of the electrolyte fluid flowing across the gap. Besides that no other force acts on the tool (of course the feed force will be there). 

Area of tool where electrochemical machining is not desires is insulated to minimize astray (unwanted) machining. Insulation in form of reinforced solid plastic or synthetic rubber should be securely bonded to tool surface with either epoxy resin cement or plastic screws. Boundary of insulation should not be subjected to high velocity electrolyte flow that may tend to tear the glued layer of insulator. 

Electrochemistry of the process:  

Electrolysis process is governed by following laws of Faraday: 

1. Amount of chemical change produced by electric current; amount of material dissolved or deposited is proportional to quantity of electricity passed.
2.  Amounts of different substances dissolved by same quantity of electricity are proportional to their chemical equivalent weights:

so, mathematically , 

m  a  It  

And also

 m a e

So, m a It e 

Here m is the mass of material deposited or dissolved in gm.

I is current in ampere.

e is the gram equivalent weight of the material.

t is time in seconds

Introducing Faraday's Constant (F)  in above equation we get following relation:

 m =     (I t e) / F 

F = 96,500 Coulomb, and it is amount of charge required to deposit 1gm equivalent of any material:

 When any metallic body is submerged in an electrolyte, metallic atoms leave the body to become ions and some ions may enter the body to become atoms. Thus the point on surface of metallic body and the adjacent point in the electrolyte have a potential difference and this potential difference is known as electrode potential.

 In case of iron (Fe) and Copper (Cu) electrodes dipped in NaCl solution:

Fe «   Fe ²⁺  +  2e ^-               (-0.409 V)

Cu«    Cu ²⁺  +  2e^-              (+ 0.304V)

 Difference between the electrode potential is  :  0.304- (- 0.409)   =  0.713 V.

 Electrode potential value depends on electrode –electrolyte pair. If A and B are two electrodes submerged in an electrolyte and the electrode potentials of A and B are V_A and V_B   then the emf of the cell will be equal to difference between the electrode potentials i.e. VA - V_B. 

Let us consider the case of Fe and Cu inserted into NaCl solution. Fe is connected to the anode and Cu is connected to the cathode. 

At anode:        Fe  ® Fe ⁺⁺    +  2e^- 

At cathode:    H₂O   +  2e^-  ®  H₂   ­ +    2 (OH)^- 

Fe ⁺⁺    ions will be attracted towards the cathode  (copper electrode) and  (OH)^- ions will be attracted  towards anode. Following reaction will take place between these ions: 

   Fe ⁺⁺    +   2 (OH)^-   ®   Fe (OH)₂ 

Electrolyte selection should be such that there is no deposition at either electrode. 

e        =  A / Z  = Atomic Weight /  valency of the ion produced 

Thus recalling the relation we have derived above: 

m =     (I t e) / F   can be rewritten as :

m =   I t A / F Z 

or , m /t  =  IA / FZ    gm /sec 

Material Removal Rate Calculation in Case of Alloys: 

Let us consider a case of an alloy made of number of constituents with atomic weights A₁, A₂, A₃, A₄…. An and valencies of their ions be Z₁, Z₂, Z₃,Z₄……..Z_n and their proportions in the alloy be x₁ % , x₂% x₃% x₄% ……x_n%  respectively. Let r be the overall density of the alloy. And V be the total volume of the alloy. 

Amount of constituent 1 in the alloy is    (rV x₁ / 100) gm.

Similarly amount of constituent 2 in the alloy is  (rV x₂  / 100) gm.

 Charge required to deposit/dissolve all of constituent 1 from the alloy is: 

Therefore, total charge required to dissolve out all constituents from the alloy is:

So volume removed or dissolved per unit charge =

Factors influencing material removal in ECM

Material removal rate during ECM is affected by many factors. Some metals have more than one valence states. For example nickel ions can have valency 2 or valency 3. When current is low nickel dissolves in divalent form while on larger current supply nickel starts to dissolve in trivalent state. The rate of dissolution (gm/sec or mm³/sec) will decrease with increase of dissolution valency.  Sometimes the dissolution valency also depends on the type of electrolyte being used. For example copper is monovalent in chloride solution and divalent in nitrate solution.

 Atomic Weight , Valency of Dissolution and Density  of Important Elements 

Electrolyte conductivity changes as the electrolyte passes through the gap due to :

a)      Increase in its temperature

b)      Hydrogen Bubbles Formation

c)      Formation of Precipitates 

Surface Finish in ECM: 

Generally the surface finish in ECM is good as the machining process doesn't involve direct contact between tool and work piece. But there are some factors that are likely to affect the surface finish: 

i)                    Selective Dissolution

ii)                   Sporadic Breakdown of Anodic Film

iii)                 Separation of Flow and Formation of Eddies

iv)                 Evolution of H2 gases. 

Selective Dissolution: The alloy constituents have different electrode potential. In a single work piece there may be two constituents A and B such that V_dB > V_dA ; electrode potential of B is greater than the electrode potential of A . But during the process the whole anode surface is equipotent. Thus to create required V_dB , parts of constituents of B will protrude out of the anode. This results in surface roughness. 

Sporadic Breakdown of Anodic Film: This is due to gradual fall in potential difference between work surface and electrolyte in the region away from the machining area.  

 At point P1 the voltage Vp1 is below the dissolution potential of one phase. Voltage goes on decreasing along the distance away from the machining area and increasing number of phases stop dissolving. This also results on concentration of electric field in small proportions of the anode surface. As a result phases dissolve very rapidly and pits are formed on the surface. Beyond P2 no dissolution occurs. 

Flow Separation & Formation of Eddies: Presence of hills and valleys on anode surface causes separation of electrolyte flow and eddies are formed. These eddies separate from main flow and bring about large concentration of metal ions at some areas near the electrode surface. High concentration overpotentials caused in the eddies result in localized variations in removal rates and the surface becomes uneven. 

Hydrogen Gas Evolution: Hydrogen gas in electrolyte reduces conductivity of the solution. The conductivity decreases along downstream and results in deterioration of surface finish. 

Electrolyte in ECM  process serves following functions: 

a)      Completing the electric circuit and allowing a high current to pass.

b)      Sustaining required electrochemical reactions.

c)      Continuous dissolution of anode and no material deposition at the cathode.

The cationic constituents of common electrolyte used in ECM process are: Ammonia, alkali metals or hydrogen. Besides, electrolyte should have good chemical stability, inexpensive, safe and non-corrosive.

 Types of Electrolytes  (Ref: Ghosh & Mallik) 

 Effects of ECM on material properties:  

This process is a smooth and gentle one.  Residual compressive stress on the surface is low. Depth of work hardened surface layer is negligible about 0.001mm compared to 0.5- 1.5 mm in turning and milling operations. Magnitude of residual stress on the surface machined by ECM is almost zero while that on the surfaces machined by conventional processes is 50 Kg/cm². The fatigue strength of parts produced by ECM is however low. Additional processes like mechanical polishing, vapor blasting or glass bead blasting can be used to enhance the fatigue strength of ECM products. 

Summary of ECM Characteristics:

 Mechanics of Material Removal :      Electrolysis 

Medium :                                             Conducting Electrolyte 

Tool Materials:                                   Cu, brass, steel 

Material Removal to tool wear ratio:     Infinity 

Gap                                                       50-300 mm    

Maximum Material Removal Rate      15 x 10 ³ mm³/min 

Specific Power Consumption                7 W/mm³/min 

Critical Parameters                             Voltage, current, feed rate, electrolyte,

                                                            electrolyte conductivity 

Materials Application                         All conducting materials and alloys 

Shape Application                              Blind Complex Cavities, curved surfaces, through  

                                                            cutting, large through cavities 

Limitations                                         High Specific Energy Consumption (150 times that

                                                            required in conventional machining), not applicable

                                                            for non-conductors and for small jobs,  expensive.