Welding procedure:

The two types of welding procedure described are fundamentally different, and the division among them forms the first breakdown in the classification of welding procedure. Those welding methods employing pressure to deform plastically the faying surfaces are frequently known as 'solid-phase' methods. There is no accepted term for the technique in which union is made through liquid metal however they might be called 'liquid-phase' methods.

For some years this has been customary to divide welding procedure into 'pressure' and 'fusion' welding methods. In the pressure-welding processes , pressure was used at some stage in welding, and while this classification included all of  the methods which could be really classed as solid-phase methods it also included various methods in which fusion takes place. This is a second reason for utilizing the word 'fusion' with care in the welding context.

Diffusion and Brazing bonding are not usually classed with welding procedure. In brazing, the gap among the parts to be joined is bridged by adding up a liquid metal having a lower melting point than that of the work. Two different techniques of brazing are possible. One employs a narrow gap among the parts to be joined into which the molten brazing metal is drawn by capillary action. It is the brazing process as normally understood. The second technique, called as 'braze-welding', resembles welding in that conventional welding procedure are utilized, gas, arc, etc., to supply heat but no use is made of capillary attraction. This is only in the use of filler metal with a melting point appreciably lower than the parent metal that the technique vary from welding.

Diffusion bonding is a specialized solid-phase joining method. In its simplest form in which the parts to be joined are heated up in vacuum under modest pressure to a temperature at which interfacial diffusion readily take place, the process has much in common with pressure welding which is explained later. To improve the process of bonding, a thin film of another metal, which diffuses readily into the parent metal, might be interposed at the joint interface.