Electro -chemical Machining - MECH14mech14.weebly.com/uploads/6/1/0/6/61069591/me_ntmp... · Electro -chemical Machining presented by Dr. P. Saha D epartment of Mechanical Engineering

Electro-chemical Machining

presented by

Dr. P. SahaDepartment of Mechanical Engineering IIT Kharagpur

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2/13/2018 P Saha, Mech Engg, IIT Kharagpur 2

Surface

Treatments

Coating Machining

Passivation

AnodisingElectro-

Chemical

Machining

Thin

Coating

Thick

Coating

Electroplating Electroforming

Electro Chemical Processing

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http://www.philamuseum.org/collections/permanent/50519.html


A Basic Electro-chemical Cell

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Basic electrochemistry of a plating bath

Anode reaction:

M Mn+ + ne-

Mn+ + Zm- MmZn

Mn+ + n(OH)- M(OH)n

Electrolyte (ionization)

MmZn Mn+ + Zm-

H2O H+ + (OH)-

Cathode reaction:

Mn+ + ne- M 2 H+ + 2e- H2

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Concept of Electro-chemical Machining

• Reverse of

electroplating

• Job becomes anode,

which gets eroded.

• Tool remains unaltered

(neither deposition nor

erosion)

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Outcome:

No damage to the tool

( neither deposition nor erosion)

Basic Electrochemistry in Electrochemical Machining

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ECM setup

with fixture

Electrolytic

tank

DC power

supply

(Continuous

or Pulsed)

Sludge

Electrolyte

Pump

Centrifuge

Basic scheme of an ECM machine

Electrolyte

+ Sludge

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Schematic Diagram of an ECM Machine

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Various Voltage Drops at the Electrodes and Electrolyte

CURRENT DENSITY :

10 to 100 A/cm2

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Faraday’s Laws of Electrolysis

When an electric current passing through an electrolyte results in

chemical reaction.

1. The amount of any substance deposited or dissolved (W), produced by current is proportional to the quantity of electricity passed (Q) through the electrolyte.

W Q∞ .Q I t=where, ‘I’ is the current and ‘t’ is the time for which current flows

where, A and v are Atomic weight and Valency of the substancerespectively.

W ECE∞ /ECE A v=

1. The quantities of substances liberated or deposited on the electrode by the passage of a given quantity of electricity (current) are proportional to the electrochemical equivalent weights of those substances.

and

and

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Combining the two laws:

or

(1/ ) ( / )W F A v Q=

(1/ ) ( / ) .W F A v I t=where, F is the Faraday’s Constant, F = 96,500 Coulombs

So the removal rate,

ˆ ( / ) ( / . )W W t A F v I= = gm-equivalent/s

cm3/s( / . ) ( / )MRR A F va d In ρ=

where, ρ is density of the material, g/cm3)

( / . ) ( / )CFeed rate A F v J ρ= cm/s

where Jc

is the current density = I / Surface area

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So material removal rate at any point is proportional to current density.

gm (equivalent)/s

For an ALLOY, with ‘n’ number of elements present in it;

1

ˆ (100. / )[1/ / ]n

i i ii

W I F X v A=

= ∑

So the removal rate,

Valency, atomic weight and their percentage presence are

represented by v1to v

n, A

1to A

nand X

1to X

n

respectively;

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Transfer of tool profile to workpiece is possible in ECM

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?

Whether the dissolution reactions In ECM proceed if

only an appropriate constant voltage is provided to the electrodes and

there is zero feed of the cathode tool with respect to the anode workpiece ?

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2/13/2018 P Saha, Mech Engg, IIT Kharagpur 15mech14.weebly.com

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The rate of anodic dissolution:

dW = a. dh.ρ = (1/F) (ECE) I dt

Or, the rate of change of gap:

(dh/dt) = (1/F) (ECE) (I / a ρ)

Since,

Current, I = V / (γh/a),

where, V is the applied voltage and γ= specific resistance

(dh/dt) = (1/F) (ECE) (V/ γh ρ)

When, (1/F) (ECE) (V/ γρ), is constant = C

(dh/dt) = C / h (7)

The initial gap, h0 increases to h1 within time, t:

∫h0h1 h dh = ∫0

t C dt

h12 – h0

2 = 2 Ct (8)

The gap increases parabolically, machining rate decreases accordingly, and finally ceases to zero mech14.weebly.com

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WITH FEED = S

Eqn. 7 is written as:

(dh/dt) = C / h – s (9)

Under equilibrium condition, (dh/dt) = 0

So, feed rate, s = C/h, or h = C/ s

Again for constant feed rate,

h = constant =he, equilibrium gap = C/s

Hence the dimension-less parameters may be formed as:

h’ = (h/he) = (sh/C) and t’ = st/he = s2t/C

So a non-dimensional form of eqn. 9 is written as:

(dh’/dt’) = (1- h’) / h’ (10)

On integration between time, 0 to t’ and gap between h0’ to h1

’

t’ = [ (h0’- h1

’) + ln {( h0’- 1)/ (h1

’- 1) (11)

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Equilibrium Gap

ELECTRODE GAP takes an EQUILIBRIUM GAP finally,

whatever the initial gap may be.

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SOME IMPORTANT FEATURES OF ECM PROCESS

1. The process attempts to maintain a constant gap (whatever may be the initial gap and feed rate). So the process is self regulated.

2. Value of equilibrium gap depends on feed rate

3. Higher feed-rate provides smaller gap hence higher current density higher MRR and higher job accuracy

4. Surface finish is independent of feed-rate

5. ECM process is independent of hardness of the material:

For materials above 450 BHN, ECM is better than conventional machining.

Typically harder material helps to improve the rate (for higher ECE of constituting elements).

6. There is no tool wear.

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Higher material removal is achieved by higher current.

Electrolyte flow is necessary to avoid

· ion concentration

· the deposition in the tool

· reaction precipitants (sludge)

· overheating of the electrolyte

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Typical values of parameters and conditions of ECM

Power supply

Type: Direct Current

Voltage: 5 to 30 V (continue or pulse)

Current: 50 to 40,000 A

Current Density: 10 to 500 A/cm2

Electrolyte

Type and Concentration

Most used: NaCl at 60 to 240 g/l

Frequently used: NaNO3 at 120 to 480 g/l

Less Frequently used: Proprietary Mixture

Temperature : 20 to 50o C

Flow rate: 16 l/min/100A

Velocity : 1500 to 3000 m/min

Inlet Pressure: 0.15 to 3 MPa

Outlet Pressure: 0.1 to 0.3 MPa

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Working Parameters

Frontal Working Gap : 0.05 to 0.3mm

Feed rate : 0.1 to 20mm/min

Electrode material : Brass, Copper, Bronze

Tolerance

2-dimensional shapes : 0.05-0.2 mm

3-dimensioanl shapes : 0.1mm

Surface Roughness (Ra) : 0.1 to 2.5 µm

Example of cathode tool

(above) and anode work

piece (below).

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An ECM Die Sinking Machine with Power supply

Courtesy AEG-Elotherm-Germany

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2/13/2018 P Saha, Mech Engg, IIT Kharagpur 24mech14.weebly.com

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PARTS MACHINED BY ECM PROCESS

(Courtesy AEG-Elotherm-Germany)

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Summary: Main characteristics of ECM

MRR in ECM does not depend on the mechanical properties of the metal but depends on work piece composition

Accuracy of ECM depends on shape and dimensions of machining workpiece and approximately is from 0.05 mm to 0.3 mm at using continuous current, and from 0.02 mm to 0.05 mm at using pulsed ECM.

Sharp corners are difficult to create because of stray current

Surface roughness of machined surface is decreasing with increasing machining rate (for typical materials) and approximately is equal from Ra=0.1 µm to Ra= 2.5 µm.

ECM generates no residual stress

The specific energy consumption of ECM is relative high and equal from 200 J/mm3 to 600 J/mm3.

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Thank You

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Electro -chemical Machining - MECH14mech14.weebly.com/uploads/6/1/0/6/61069591/me_ntmp... · Electro -chemical Machining presented by Dr. P. Saha D epartment of Mechanical Engineering