Electrochemical Machining

Electrochemical Machining (ECM)

hanshenk — Tue, 16 Mar 2010 22:30:26 +0000

Electrochemical Machining (ECM)

Electrochemical Machining is an unconventional metal machining technology based on electrolysis in which the product (workpiece) is connected in an electrical circuit as anode and the tooling (electrode) as cathode. In electrochemical machining the product is processed without contact or any thermal influence.

The workpiece, that has to be electrical conductive, is dissolved locally through electricity (Electro) and chemistry (Chemical) until it reaches the required complex 3D end shape.

The tool (electrode) is placed close to the workpiece and in the gap between the tool and the workpiece an electrolyte is flushed for the conductance of the current and for the removal of reaction products like metal hydroxides, gas and heat.

The highest current density is reached at places where the tool is the closest to the workpiece; the shape of the tool is copied in the workpiece.

The desired anodic reaction at the workpiece is the dissolution of metal to metalions.

Me â†’ Meⁿ⁺ + n e^-

Other side reactions are possible at the anode, leading to a lower current efficiency, for example the oxidation of water:

2H₂O â†’ 4H⁺ + O₂ + 4 e^-

During electrochemical machining the reaction at the cathode usually is the reduction of water, leading to the evolution of hydrogen gas:

2H₂O + 2 e^- â†’ 2OH^- + H₂

In the electrolyte bulk the metal ions can react with the hydroxyl ions to form metal hydroxides. Whether the metal ions react to metal hydroxides depends on the type of metal ions and the properties of the electrolyte (pH).

Meⁿ⁺ + n OH^- â†’ Me(OH)_n

The metal hydroxides can be removed from the electrolyte by, for example, microfiltration.

The current through the electrolyte in the working gap is conducted by the anions and cations in the electrolyte.

The temperature of the electrolyte increases by the movement of the ions, this is called Joule heating.

Copper (II)

hanshenk — Tue, 16 Mar 2010 22:19:28 +0000

Copper (I)

hanshenk — Mon, 15 Mar 2010 10:35:28 +0000

Cathode

admin — Thu, 11 Mar 2010 16:27:38 +0000

The cathode is the electrode at which reduction takes place. In electrochemical machining the cathode is the negative electrode and is used as tool to machine the positive workpiece.

The choice of cathode material is defined by the required properties like hardness, machinability, (electro-) chemical resistance, electric conductivity, erosion resistance and by cost. Frequently used cathode materials are Stavax and Monel.
In some ECM processes, when using bipolar pulses, the cathode material has to be inert to the negative pulse (where the tool is working as anode). Using an inert material like platinum can solve the problem, but will raise the costs and also platinum is not completely inert in the ECM process. Another possibility is to coat the tool electrode with a conductive and inert coating.

Coating

admin — Thu, 11 Mar 2010 16:27:02 +0000

Parts of the cathode can be covered with an electrically insulating coating to minimize stray current and to increase copying accuracy of the ECM process.

If in electrochemical machining a counterpulse is used (for cleaning the cathode or for improving the anodic dissolution reaction), an electrically conductive coating can be used on the cathode to protect the cathode from dissolving.

The current density

admin — Thu, 11 Mar 2010 16:26:40 +0000

The current density - J is the ratio of the current I to the machining area A. The current density is commonly expressed in amperes per square centimetres. Electrochemical machining is performed at high current density, usually tens or even hundreds of amperes per square centimetre.

Assuming an ohmic resistance of the electrolyte, the current I can be expressed as function of potential difference over the electrolyte Uelectrolyte and the resistance of the electrolyte Relectrolyte. The resistance of the electrolyte is directly proportional to the distance of the gap s and inversely proportionally to the electrolyte conductivity and the working area A.

Electrochemical Drilling (ECD)

admin — Thu, 11 Mar 2010 16:26:11 +0000

Electrochemical Drilling (ECD) is an electrochemical machining technique used to manufacture, for example, cooling channels in products which can be characterised by the large length/diameter relation, where lengths of up to 800mm are not uncommon. As tool electrode a hollow tubular cathode is used. The cathode can be made of titanium having an insulating exterior coating, but also tubes made of glass are used.
The salt solutions commonly used in electrochemical machining can not be used in electrochemical drilling because the sludge produced in the process would make it impossible to manufacture holes with large length/diameter ratio. To ensure that the metal ions produced in the process stay soluble in the electrolyte, in electrochemical drilling acids (nitric acid, sulphuric acid) are used as electrolyte.

Electrodischarge Machining (EDM)

admin — Thu, 11 Mar 2010 16:25:41 +0000

Electrodischarge Machining, also called spark erosion, is one of the most accurate manufacturing processes available where the material is removed by electrical discharges. The disadvantage is that the electrical discharge between the work piece and the electrode can change the micro-structure of the product. At the same time the electrode is susceptible to wear. Furthermore, EDM is identified by its slow machining speed.

Aluminium

admin — Thu, 11 Mar 2010 16:25:11 +0000

Aluminium is a ductile, soft and light weight silvery white or dull grey metal. The metal is strongly corrosion resistant due to the thin layer of oxide that forms on the surface when the metal is exposed to air. Despite of that oxide layer on the surface, the pure metal aluminium can be electrochemical machined well, but because the metal is very good machinable with conventional methods ECM is only recommended when difficult shapes that canâ€™t be machined otherwise are required. An other disadvantage of electrochemically machining aluminium is that the current efficiency will be high for all current densities (potentials), which means that the copying accuracy will be low. Under electrochemical machining conditions aluminium will dissolve as trivalent Al3+, which will react in the electrolyte to aluminiumhydroxide (Al(OH)3) or oxide (Al2O3). The aluminium(hydr)oxide can be removed from the electrolyte, for example by filtering.

Chromium

admin — Thu, 11 Mar 2010 16:24:48 +0000

Chromium is a hard, steel-gray, shiny, metal that breaks easily. The pure metal chromium can be electrochemical machined, but more often chromium will be machined as alloying element in steels or other alloys. Using chromium in steel increases the hardness and the corrosion resistance of the steel.

Chromium dissolves in the trans-passive state during electrochemical machining; the protecting oxide film dissolves at sufficient high potential, leading to the dissolution of the metal. Chromium will dissolve with high current efficiency independent of potential or electrolyte type, copying accuracy is hard to control.

Under electrochemical machining conditions chromium can dissolve as trivalent chromium and as hexavalent chromium. The hexavalent chromium will appear in the electrolyte as chromate ion (CrO42-) or dichromate ion (Cr2O72-) depending on the pH, and causes a health risk because of its carcinogenic character. For that reason the chromate has to be reduced to the much less hazardous trivalent chromium, for example by adding iron(II)sulphate. (see: chromate reduction)