6 May 2017

Electric arc welding

Introduction :

In electric arc welding, generation of heat by an electric arc is one of the most efficient methods. The electric arc welding process makes use of the heat produced by the electric arc to fusion-weld metallic pieces. This is one of the most widely used welding processes because of the ease of use and high production rates that can be achieved economically.

Principle of arc :

An arc is generated between two conductors of electricity, cathode and anode when they are touched to establish the flow of current and then separated by a small distance.
An arc is sustained electric discharge through the ionized gas column is called plasma between two electrodes.
The electrons liberated from the cathode move towards the anode and are accelerated in their movement. When they strike the anode at high velocity, a large amount of heat is generated.
In order to produce the arc, the potential difference between two electrodes should be sufficient to allow them to move across the air gap. 
For the convenience of explanation, we have chosen a direct current arc for the above description. But even with an arc of the alternating current ( AC ), it would be similar, with the main difference that the cathode and anode would change continuously and as a result, the temperature across the arc would be more uniform compared to a DC arc. 

How does it work?

The arc welding is a fusion welding process in which the heat required to fuse the metal is obtained from an electric arc between the base metal and an electrode.

First of all, metal pieces to be weld are thoroughly cleaned to remove the dirt, dust, grease, oil etc. After that, the workpiece should be firmly held in suitable fixtures. Insert a suitable electrode in the electrode in the electrode holder at the angle of 60 to 80 degree with the workpiece.

Select the proper current and polarity. The spot is marked by the arc at the places where welding is to be done. The welding is done by making contact of the electrode with the workpiece and then separating the electrode to a distance to produce an arc.

When the arc is obtained, heat is produced and melts the work below the arc, and forming a molten metal pool. A small amount of depression is formed in the work and the molten metal is deposited around the edge of this depression. After the completion of welding, the electrode holder should be taken out quickly to break the arc and the supply of current is switched off.

Electric arc welding

Arc welding equipment :

The main requirement in an electric arc welding is the source of electric power. They are essential for two types :

Alternating current ( AC ) machines 

  • Transformer 
  • Motor or engine-driven alternator
Direct current ( DC ) machines 
  • Transformed with DC rectifier
  • Motor or engine-driven alternator
The arc welding machines can also be divided into two types :
The first one is the constant current welding machines and the second one is droop curve machines.

Advantages of electric arc welding :
  • low cost.
  • Simplicity and portability of the tool.
  • Versatile process.
  • Wide choice of the electrode.
  • Higher welding speed.
  • No flux required.
Limitations of electric arc welding :
  • Wastage of material.
  • Less productive due to continuous wire process.
  • Proper alignment and care of electrode required.
  • Radiation impacts more extreme.