DEFINITION AND CLASSIFICATION

Welding is a process of permanent joining two materials (usually metals) through localised coalescence resulting from a suitable combination of temperature, pressure and metallurgical conditions. Depending upon the combination of temperature and pressure from a high tem­perature with no pressure to a high pressure with low temperature, a wide range of welding processes has been developed.

Classification of Welding Process

American Welding Society has classified the welding processes as shown in Fig. 1.1. Various welding processes differ in the manner in which temperature and pressure are combined and achieved.

Welding Processes can also be classified as follows (based on the source of energy):

1. Gas Welding

— Oxyacetylene

— Oxy hydrogen

2. Arc Welding

— Carbon Arc

— Metal Arc

— Submerged Arc

— Inert-gas-Welding TIG and MIG

— Plasma Arc

— Electro-slag

3. Resistance Welding

— Spot

— Seam

— Projection

— Butt Welding

— Induction Welding

4. Solid State Welding

— Friction Welding

— Ultrasonic Welding

— Explosive Welding

— Forge and Diffusion Welding

5. Thermo-chemical Welding

— Thermit Welding

— Atomic H2 Welding (also arc welding)

6. Radiant Energy Welding

— Electron Beam Welding

— Laser Beam Welding

In order to obtain coalescence between two metals there must be a combination of prox­imity and activity between the molecules of the pieces being joined, sufficient to cause the formation of common metallic crystals.

Proximity and activity can be increased by plastic deformation (solid-state-welding) or by melting the two surfaces so that fusion occurs (fusion welding). In solid-state-welding the surfaces to be joined are mechanically or chemically cleaned prior to welding while in fusion welding the contaminants are removed from the molten pool by the use of fluxes. In vacuum or in outer space the removal of contaminant layer is quite easy and welds are formed under light pressure.