Electro-chemical Machining (ECM) Lecture Notes - PDF

Electro-chemical Machining presented by Dr. P. Saha Department of Mechanical Engineering IIT Kharagpur  A Basic Electro-chemical Cell 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 2  Electro Chemical Processing Su...

Electro-chemical Machining presented by Dr. P. Saha Department of Mechanical Engineering IIT Kharagpur  A Basic Electro-chemical Cell 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 2  Electro Chemical Processing Surface Coating Machining Treatments Thin Thick Coating Coating Passivation Electroplating Electroforming Anodising Electro- Chemical Machining 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 3  Basic Electrochemistry of a Plating Bath Anode reactions: M  Mn+ + n e- Mn+ + Zm-  MmZn Mn+ + n (OH)-  M(OH)n Electrolyte (ionization) MmZn  Mn+ + Zm- H2O  H+ + (OH)- Cathode reactions: Mn+ + n e-  M i 2 H+ + 2e-  H2 h 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 4  Concept of Electrochemical Machining Reverse of electroplating Job becomes anode, which gets eroded. Tool remains unaltered (neither deposition nor erosion) 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 5  Basic Electrochemistry in Electrochemical Machining Outcome: No damage to the tool ( neither deposition nor erosion) 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 6  Basic scheme of an ECM machine ECM setup DC power with fixture Electrolyte supply (Continuous or Pulsed) Electrolyte + Sludge Pump Electrolytic Centrifuge tank Sludge 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 7  Schematic Diagram of an ECM Machine 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 8 Various Voltage Drops at the Electrodes and Electrolyte CURRENT DENSITY : 10 to 100 A/cm2 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 9  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 and Q  I.t where, ‘I’ is the current and ‘t’ is the time for which current flows 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 (ECE) of those substances. W  ECE and ECE  A / v where, A and v are atomic weight and valency of the substance respectively. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 10 Combining the two laws: W  (1/ F ) ( A / v) Q or W  (1/ F ) ( A / v) I.t where, F is the Faraday’s Constant, F = 96,500 Coulombs So the removal rate, Wˆ  (W / t )  ( A / F.v) I [gm-equivalent /s] and MRR  ( A / F.v) ( I /  ) [cm3/s] where,  is density of the material, g/cm3) Feed rate  ( A / F.v)( J C /  ) [cm/s] where Jc is the current density = I / Surface area [A/cm2] 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 11  For an ALLOY, with ‘n’ number of elements present in it; Valency, atomic weight and their percentage presence are represented by v1 to vn, A1 to An and X1 to Xn respectively; So the removal rate, n Wˆ  (100.I / F )[1/  X i vi / Ai ] [gm-equivalent /s] i 1 So material removal rate at any point is proportional to current density. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 12  Transfer of tool profile to workpiece is possible in ECM 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 13  ? 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 work piece ? 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 14 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 / (gh/a), where, V is the applied voltage and g= specific resistance (dh/dt) = (1/F) (ECE) (V/ gh ) (6) When, (1/F) (ECE) (V/ g), is constant = C (dh/dt) = C / h (7) The initial gap, h0 increases to h1 within time, t: ∫h0h1 hdh = ∫0t Cdt h12 – h02 = 2Ct (8) The gap increases parabolically, machining rate decreases accordingly, 10-Feb-21 and finally ceases P Saha, to zero Mech Engg, IIT Kharagpur 15 WITH FEED = S Eqn. 7 is written as: (dh/dt) = C / h – s (9) For Removal rate (MRR) in steady state, (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 So 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) 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 16  Equilibrium Gap ELECTRODE GAP takes an EQUILIBRIUM GAP finally, whatever the initial gap may be. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 17  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. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 18 Higher material removal is achieved by higher current, if inter-electrode gap condition is maintained. Electrolyte flow is necessary to avoid · ion concentration · the deposition in the tool · reaction precipitants (sludge) · overheating of the electrolyte 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 19  Estimation of the minimum flow rate of the electrolyte required in ECM 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 20  If the electrolyte flow is restricted by the design constraint of the machine  maximum permissible current for avoiding boiling of electrolyte canbe found out  so we can calculate corresponding MRR  max. permissible feed rate  equilibrium gap for this feed rate 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 21  Typical values of parameters and conditions of ECM Power supply Type: Direct Current Voltage: 5 to 30 V (continuous or pulsed source) 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 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 22  An ECM Die Sinking Machine with Power supply Courtesy AEG-Elotherm-Germany 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 23  Working Parameters Frontal Working Gap : 0.05 to 0.3 mm Feed rate : 0.1 to 20 mm/min Electrode material : Brass, Copper, Bronze Tolerance 2-dimensional shapes : 0.05-0.2 mm 3-dimensioanl shapes : 0.1 mm Surface Roughness (Ra) : 0.1 to 2.5 m Example of cathode tool (above) and anode work piece (below). 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 24  Typical tools for ECM Corresponding forms on jobs achieved through Die-sinking ECM 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 25  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 m to Ra= 2.5 m. ECM generates no residual stress The specific energy consumption of ECM is relative high and equal from 200 J/mm3 to 600 J/mm3. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 26  Tool Design in ECM Design of ECM tools for considerations like Electrolyte flow Insulation Strength Fixing arrangement Electrolyte flow in such a manner so as to avoid acvitation, vortex, stagnation etc. Flow path should have a corner radius of 07. to 0.8 mm 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 27  Tool Design in ECM 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 28  Considerations for Tool Design in ECM to Take Care Various Phenomenon Occuring in Inter-electrode Gap 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 29 Considerations for Tool Design in ECM due to Phenomenon Occurring in Inter-electrode Gap 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 30 Considerations for Tool Design in ECM due to Phenomenon Occurring in Inter-electrode Gap 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 31  Considerations for Tool Design in ECM: Effect of tool inclination Two surfaces inclined at angles  and g with downward feed direction (S  ) will produce unequal interelectrode gap of h and y, respectively. [ since the approach rate of the tool to the two inclined surfaces are different]. The machining on surface inclined at an angle g will be more as compared to the other. So the surface needs a correction x so as to maintain an equilibrium gap of z. 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 32 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 33 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 34  Surface Finish Issues in ECM As ECM is an molecule-by- molecule dissolution process, can we get a nano-level surface finish through ECM? If not, why? 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 35 Some special applications of Electrochemical Machining Electro chemical cutting off Electro chemical milling Electro chemical turning of thin plate Electro chemical turning 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 37  Electro Chemical Shaping 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 38  Full Form Electro Chemical Shaping Design of cathode tool shape to obtain profile of turbine blade 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 39 Electro Chemical Machining of Irregular Shaped Hole 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 40  Deburring by Electrochemical Means Entrance burr Hanging burr Exit burr Roll over burr Feather burr Tear burr Flash burr Different types of burrs during manufacturing 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 41  Electro-Chemical Deburring Simple operation Often static electrode Gradual and preferential smoothening Widely accepted in industry 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 42  Electro-Chemical Deburring Table : Machining conditions for deburring different materials Material Electrolyte Applied Current Time, Voltage, Density, V A/cm2 s High carbon steel 5-15% NaNO3 Low-carbon steel 2-2.5% NaNO3 Copper alloys 5-15% NaNO3 12-24 5-10 5-100 Aluminum alloys 5-20% NaNO2 Stainless steels 5% NaNO3 0.5% NaCl 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 43  Electro-Chemical Deburring Deburring of surface irregularities on external spline of case- hardened steel shift hub sleeve. (Configuration of both external spline carrying surface irregularities and cathode tool is shown) 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 44  Small Hole Drilling Techniques by Electrochemical Machining 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 45  Small Hole in Fuel Injector Nozzles Nozzle outlet diameter around 100 micron 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 46  Electro Chemical Hole Drilling In conventional electrochemical drilling  hollow metal tube as cathode Electrolyte flows at high velocity Cathode penetrates the hole Hole diameter  Slightly greater than cathode outside diameter Reversal of the electrolyte flow can often produce considerable improvement in machining accuracy 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 47  Deep Hole Drilling by Electro Chemical Action 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 48  At - A - Glance In conventional electrochemical drilling  hollow metal tube as cathode Electrolyte flows at high velocity Cathode penetrates the hole Hole diameter  Cathode outside diameter To Drill a small hole – Electrostream drilling Application – small cooling hole in super alloys Negatively charged acid electrolyte stream with high velocity 0.127 mm < Hole diameter < 0.890 mm 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 49  Electro-stream Drilling with Zero feed rate Dwell drilling => Shallow and less accurate holes Limited depth of hole Work piece configuration or machine capability does not allow nozzle movement Maximum depth upto 5 mm High applied voltage 150V – 800V 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 50  Electro-Stream (Capilary) Drilling Electro-stream Drilling with finite feed rate  Maximum depth 19mm H2SO4 /HCl electrolyte HCl for Aluminum & Titanium H2SO4 for Carbon steel, Cobalt alloy, SS Titanium wire is placed inside sleeve Individual wire runs in each nozzle Drilling inclined holes 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 51  Electro-Stream (Capilary) Drilling….. Advantages of ES Drilling · High depth-to-diameter ratios are possible · Many holes can be drilled simultaneously · Blind and intersecting holes ES Drilling in inaccessible positions can be machined · Burr-free holes are produced · Absence of recast and metallurgical defects Use of small titanium wire to charge electrolyte · Powder metallurgy hard materials can be drilled 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 52 Shaped Tube Electrolytic Machining (STEM) 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 53  Shaped Tube Electrolytic Machining Small dia (0.5-6.0 mm) deep holes in electrically conductive material High aspect ratio (1:100), round and shaped holes Super alloy machining Acid-based electrolyte (10% H2SO4 + Water) Reaction products gets dissolved Shaped tube coated on exterior tube except tip Low voltage 5-15V At higher voltage  Higher MRR, Damage to electrolyte coating, Boiling of the electrolyte 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 54 Applications of STEM 1. Turbine blade cooling holes 2. Fuel nozzles 3. Any hole where EDM recast is not desirable 4. Starting holes for wire EDM 5. Drilling holes for corrosion resistant metals of low conventional machinability 6. Drilling oil passages in bearings where EDM causes cracks 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 55  Thank You 10-Feb-21 P Saha, Mech Engg, IIT Kharagpur 56