Non-pressure welding or fusion welding

‘Non-pressure welding’ is defined as the intimate joining of metal by the application of heat without the use of pressure. Local fusion of the surfaces to be joined and the filler metal takes place by the application of intense heat. The method of welding depends upon how the heat is produced, namely, electric arc welding, gas welding, electric resistance welding, etc. Filler metal used in the process must have the same composition as that of the parent metals to be joined. On cooling and solidification, a fine grained cast structure is developed having properties nearly the same as that of the mother metal.



 Electric arc welding

Electric arc welding is further subdivided into two main categories namely.

  • Metal arc welding
  • Carbon arc welding

Fig.


Electric Arc Welding



In both processes either the arc is unshielded or shielded by means of a flux or an inert gas environment.

Out of these two processes ‘Metal arc welding’ with flux coated consumable electrode is by far the most common and cheapest process of welding.



 Metal arc welding (Unshielded)

In this process, heat is obtained from an arc formed between the work and a metal electrode coated with flux. The metal electrode also supplies molten filler metal to the joint. Electric energy is converted into intense heat in the arc, which attains a temperature around 5,5000C. This intense heat in the arc forms a molten pool in the metal being welded and at the same time, melts the tip of the electrode. As the arc is maintained, molten filler metal from the electrode tip is transferred across the arc, where it fuses with the molten base metal.

In actual operation, an arc is started by striking the work with an electrode and quickly withdrawing it a short distance. As the electrode end is melted by the intense heat, most of it is transferred across the arc in the form of small globules to a molten pool. A small amount is lost by being vapourised  and some globules are deposited alongside the weld as spatter. The arc is maintained by uniformly moving the electrode towards the work at a rate equal to the amount of electrode metal transferred to the weld. At the same time, the electrode is gradually moved along the joint being welded.



 Types of electrodes

There are three types of metal electrodes or rods in use, namely

  • Bare
  • Fluxed
  • Heavy flux coated


Bare electrodes have a limited use for the welding of wrought iron and mild steel. Because the deposited metal is affected by the oxygen and nitrogen in the air forming undesirable oxides and nitrides in the weld metal, it introduces weakness in the joint.

However, 95 % of the total manual welding production today is with coated electrodes. Heavy flux coating on the electrode removes the oxides and nitrides in the form of slag which floats on the surface of the weld. It also prevents oxidation of the surface metal during cooling. Secondly, it also provides a protective gas shield. Besides these duties, coating produces ionized vapours to stabilize the arc and governs the melting rate and the amount of penetration into the parts being welded.


Heavy coating also permits the use of larger welding rods, higher current and greater welding speeds.