4 October 2019

What is electrochemical machining | Working principle | Application | Advantages and Disadvantages

Electro-chemical machining is one of the newest and most useful processes of metal removal by the controlled dissolution of the anode of an electrolyte cell.

The process is suited to metal and alloys which are difficult or impossible to the machine by mechanical processes.

This is based on Michael Faraday's classical laws of electrolysis, requiring basically the two-electrodes, electrolytes, a gap and a source of D.C power of sufficient capacity.

Working principle of electrochemical machining

In this machining process workpiece acts as anode and tool acts like cathode and the electrodes should be placed closely with a gap of about 0.5 mm. Anode and cathodes should be immersed in an electrolyte. A potential difference is maintained in between the electrodes as a result ions existing in the electrolytes and it migrates towards the electrodes. Positively charged ions are attracted toward the cathode and negatively charged ions are attracted towards the anode and thus the flow of current is initiated in the electrolyte. The setup is kept stationary and the tool is fed linearly the desired amount of metal is removed through the electrolysis process.

To keep the tool safe from damage by a continuous supply of electrolyte is ensured by pumping at high pressure. The temperature generated is very low and no spark produced this there may not any scope of metallurgical changes in the work material. The tool and the workpiece do not come in direct contact with each other so in this machining process negligible wear and tear are observed. By using this process dimensions up to 0.05 mm can be easily machined. 

Function of electrolyte 

The electrolytes used in this machining process are sodium chloride, sodium nitrate, potassium chloride, sodium hydroxide, sodium fluoride, sulfuric acid and sodium chlorate. 

The main function of electrolyte in this process is following below. 
  • To carries the current between the tool and workpiece.
  • Removes products of machining and other insoluble products from the cutting region.
  • To dissipates heat produced in the operation.
  • To keed the reaction continuous. 
What should be the criteria for selecting Electrolyte?
  • Required Machining rate
  • Required Dimensional Accuracy
  • Surface Texture and Integrity
The essential characteristics of Electrolytes are following below. 
  • Good electrical conductivity.
  • Non-toxicity and chemical stability.
  • Non-corrosive property.
  • Low viscosity
  • High specific heat.
The properties of Electrolytes are following below. 
  • High Electrical Conductivity.
  • High Current Efficiency for machining.
  • Good Surface finish and integrity are necessary.
  • Composition of the electrolyte and structure of the material controls the final surface texture. 

Tool material for electrochemical machining 

Copper, brass, titanium, copper-tungsten and stainless steels are most commonly used electrode material when the electrolyte is made of slats of sodium or potassium. Some material such as aluminium, graphite, bronze, platinum and tungsten carbide are also used for tool material.

Requirements of the tool material in electrochemical machining are following below.
  • Tool material is a conductor of electricity.
  • Because of the fluid pressure, it should be rigid enough to take up the load.
  • The electrolyte should be chemically inert.
  • Making it in the desired shape should be easy to machine.

Accuracy of electrochemical machining

Under ideal conditions with properly designed tooling, is capable of holding a tolerance of the order of (+0.02 to -0.02 mm) and less. Internal radius is greater than 0.2 mm and  0.05 mm an external radius. The taper is of the order of 0.010 mm for 10 mm depth and side over-cut is about 0.1 to 0.2 mm. Based on the work material, the surface finish of this process varies from 0.2 to 0.8 microns.

Factors which affect the accuracy are following below. 
  • Machining Voltage.
  • The feed rate of the electrode. 
  • The temperature of the electrode.
  • The concentration of electrolyte.

Application of electrochemical machining

The main application is machining of hard heat-resisting alloys. This process also used to make an aerospace component, machining of tungsten carbide and that of nozzles in alloy steels. Almost any conducting material can be machined by this method. The main process performed by the electrochemical machining process is electrochemical cutting, broaching, and drilling. 

Typically this method is used for mass production and works with extremely hard materials that are difficult to handle using other techniques but are limited to the use of electrically conductive materials.

Advantages of electrochemical machining 

  • The metal removal rate is quite high for high-strength-temperature-resistant (HSTR) materials as compared to the conventional process.
  • Accurate machining.
  • Residual stress is low.
  • Surface finish is in the order of 0.2 to 0.8 microns.
  • No direct contact between tool and workpiece.
  • Negligible wear and tear of tool material.
  • Environmental friendly.
  • Possible to machine non-rigid and open workpiece.
  • It can be a machine configuration which is beyond the capability of the conventional machining process.
  • Extremely thin sheets of metal can be worked easily without distortion.
  • A job with complex shapes can be machined easily and accurately.
  • Several holes can be done at once during drilling.
  • It is a time-saving machining process as compared to conventional machining.
  • Deburring can be done in hard to access areas.
  • Fragile and brittle materials can be machined easily without cracking or breaking.
  • The metallic workpiece is not damaged due to thermal stresses.

Disadvantages of Electrochemical Machining :

  • Power consumption is more.
  • Initial tooling can be timely and costly.
  • Non-conducting materials can not be machined.
  • The process is costly because of expensive equipment.
  • Continuous supply of electrolyte is necessary.
  • Steady voltage should be maintained during the whole process.
  • Corrosion and rust of the machine can be a hazard.
  • If hydrogen is liberated at the tool surface then it is possible to suffer from hydrogen-embitterment of the surface.
  • There may be a possibility of damages because of sparks.
  • Conventional machining produced more improved fatigue properties than electrochemical machining.

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