1. Prepared by: DD urgesh Kumar

3.  Electrochemical machining is a metal machining technology based on electrolysis where the product is processed without contact and thermal influence. The metal work piece is partially dissolved (Machined) through electricity (Electro) and chemistry (Chemical) until it reaches the required complex 3D end shape.  Unlike previous ECM techniques, the current ECM technique benefits from pulsating power supplies and vibrating axis. This concept enables processing products with a minimum process gap varying by single micrometers. The shape accuracy of the end product depends on the size of the gap.

4.  Electrochemical Machining (ECM) has established itself as one of the major alternatives to conventional methods of machining difficult - to - cut materials of and/or generating complex contours, without inducing residual stress and tool wear.  It has been applied in diverse industries such as aerospace, automotive and electronics, to manufacture airfoils and turbine blades, die and mold, artillery projectiles, surgical implants etc.  Moreover with recent advances in machining accuracy and precision, based on the development of advanced electrochemical metal-removal processes, demonstrate that the ECM can be effectively used for micromachining components in the electronics and precision industries.

5.  Electrochemical machining is developed on the principle of Faradays and Ohm.  Operation principle of ECM basically considered as the reverse of electroplating with some modification.  The electrolysis principle has been in use for long for electroplating where the objective is to deposit metal on the workpiece.  In this process, an electrolyte cell is formed by the anode (work piece) and the cathode (tool) in the midst of a following electrolyte.  The metal is removed by the controlled dissolution of the anode according to the well known Faradays law of electrolysis.

6.  When the electrode are connected to about 20 v electric supply source, flow of current in the electrolyte is established due to positively charged ion being attracted towards cathode and vice-versa.  Due to electrolysis process at cathode hydroxyl ion are released which combine with the metal ions of anode to form insoluble metal hydroxide.  Thus the metal is removed in the form of sludge and precipitated in electrolytic cell. This process continues till the tool has produced its shape in the work piece.

7. These results are embodied in Faraday’s laws of electrolysis:  Faraday’s 1 st Law of Electrolysis: The amount of chemical change produced by an electric current, that is the amount of any material dissolved or deposited is proportional to the quantity of electricity passed.  Faraday’s 2 nd Law of Electrolysis: The amounts of different substances dissolved or deposited by the same quantity of electricity are proportional to their chemical equivalent weights.

8. WORKWORK TOOLTOOL WORKWORK TOOLTOOL Initial Stage Of ECMSteady Stage Of ECM Electrolyte  As the tool approaches the work piece it erodes the negative shape of it. Thus complex shapes are made from soft copper metal and used to produce negative duplicates of it.  This process is called Electrochemical Sinking

9.  The tool may also be connected to a CNC machine to produce even more complex shapes with a single tool.  Electrochemical machining with CNC controlled motion of universal electrodes.

10.  Power supply  Electrolyte filtration and delivery system  Tool feed system  Working tank  The electrochemical machining system has the following modules:

11. Low voltage high current power supply PS -ve +ve Constant feed to the tool Tool Pressure Gauge Flow meter Flow control valve Pressure relief valve Pump Filters sludge Spent electrolyte centrifuge

13.  During ECM, there will be reactions occurring at the electrodes i.e. at the anode or work piece and at the cathode or the tool along with within the electrolyte.  Let us take an example of machining of low carbon steel which is primarily a ferrous alloy mainly containing iron.  For electrochemical machining of steel, generally a neutral salt solution of sodium chloride (NaCl) is taken as the electrolyte. The electrolyte and water undergoes ionic dissociation as shown below as potential difference is applied. NaCl ↔ Na + + Cl - H 2 O ↔ H + + (OH) -

14.  As the potential difference is applied between the work piece (anode) and the tool (cathode), the positive ions move towards the tool and negative ions move towards the work piece.  Thus the hydrogen ions will take away electrons from the cathode (tool) and from hydrogen gas as: 2H + + 2e - = H 2 ↑ (at cathode )  Similarly, the iron atoms will come out of the anode (work piece) as: Fe = Fe 2+ + 2e -  Within the electrolyte iron ions would combine with chloride ions to form iron chloride and similarly sodium ions would combine with hydroxyl ions to form sodium hydroxide. Na + +OH - = NaOH

15.  In practice FeCl 2 and Fe(OH) 2 would form and get precipitated in the form of sludge. In this manner it can be noted that the work piece gets gradually machined and gets precipitated as the sludge.  Moreover there is not coating on the tool, only hydrogen gas evolves at the tool or cathode. This figure depicts the electro-chemical reactions schematically.  As the material removal takes place due to atomic level dissociation, the machined surface is of excellent surface finish and stress free.

16.  Reaction at cathode Reaction at Anode Fe Fe 2+ + 2e - Na + + e - Na Fe 2+ +2Cl Fecl2 Na+H2O Na(OH)2 Fe +2OH Fe(OH)2 2H + +2e - H2  It shows that only hydrogen gas will evolve at cathode and there will be no deposition.

17.  The voltage is required to be applied for the electrochemical reaction to proceed at a steady state. That voltage or potential difference is around 2 to 30 V. The applied potential difference, however, also overcomes the following resistances or potential drops.  They are:  The electrode potential  The activation over potential  Ohmic potential drop  Concentration over potential  Ohmic resistance of electrolyte

18. Anodic overvoltage Cathodic overvoltage Anode potential Activation over potential Ohmic potential concentration potential Ohmic drop activation overpotential cathodic potential Voltage Anode Cathode

19.  Power Supply Type direct current Voltage 2 to 35 V Current 50 to 40,000 A Current density 0.1 A/mm 2 to 5 A/mm 2  Electrolyte Material NaCl and NaNO 3 Temperature 20 o C – 50 o C Flow rate 20 lpm per 100 A current Pressure 0.5 to 20 bar Dilution 100 g/l to 500 g/l

20.  Working gap 0.1 mm to 2 mm  Overcut 0.2 mm to 3 mm  Feed rate 0.5 mm/min to 15 mm/min  Electrode material Copper, brass, bronze  Surface roughness, R a 0.2 to 1.5 μm

21.  There is no cutting forces therefore clamping is not required except for controlled motion of the work piece.  There is no heat affected zone.  Very accurate  Relatively fast  Can machine harder metals than the tool  The hardness, toughness and thermal resistance do not affect the material removal rate (MRR).  MRR is almost independent on the type of material

22.  Each product and material requires new research,production numbers are essential, as a special electrode must be developed for each product.  High power consumption but in general lower than other nonconventional machining techniques.  Design of electrode is complex and initially expensive, however “the electrode will not wear”.  ECM requires relatively high skilled staff.  Need more area for installation.  Electrolytes may destroy the equipment.  Not environmentally friendly (sludge and other waste).

23.  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 also used to make cavities and holes in various products.  Sinking operations (RAM ECM) are also used as an alternative to RAM EDM.  It is commonly used on thin walled, easily deformable and brittle material because they would probably develop cracks with conventional machining.

24.  ECM can machine any electrically conductive work material irrespective of their hardness, strength or even thermal properties.  Moreover as ECM leads to atomic level dissolution, the surface finish is excellent with almost stress free machined surface and without any thermal damage.  ECM is used for :  Die sinking  Profiling and contouring  Trepanning  Grinding  Drilling  Micro-machining

25. DIE SINKING 3D PROFILING TOOL WORK TOOL DRILLING TREPANNING WORK TOOL WORK

26.  The two most common products of ECM are turbine/compressor blades and rifle barrels. Each of those parts require machining of extremely hard metals with certain mechanical specifications that would be really difficult to perform on conventional machines.  Some of these mechanical characteristics achieved by ECM are: Stress free grooves. Any groove geometry. Any conductive metal can be machined. Repeatable accuracy of 0.0005”. High surface finish. Fast cycle time.

27.  Several sensors are used to control short circuit, turbulence, passivation, contact and overcurrent sensors. In case of contact, immense heat would be generated melting the tool, evaporating the electrolyte and cause a fire.  The worker must be insulated to prevent electrocution.  The tool and the work piece must be grounded before any handling is performed.

28.  Hydrogen gas emitted is very flammable, so it should be disposed of properly and fire precautions should be taken.  The waste material is very dangerous and environmentally unfriendly (metal sludge) so it must be recycle or disposed of properly.  Electrolyte is highly pressurized and worker must check for minor cracks in piping before operating.