Basic Requirements:

Each welding procedure must fulfill a number of conditions. Most of the significant, energy in some form, usually heat, ought to be supplied to the joint so that the parts might be united by being fused together. The heat might be generated by a flame, an arc, resistance to an electric current, radiant energy or by mechanical means. In a restricted number of procedures such like pressure welding the union of the parts is accomplished without melting, but energy is expended in forcing together the parts to be joined and heat might be used to bring the weld region to a plastic condition. Generally ,Fusion is considered as synonymous with melting, but in the context of welding this is desirable to distinguish at once among these words. By common usage the word fusion implies melting with subsequent union, and this is possible for the parts of a joint to be melted but not fused together.

In order to unite two surfaces satisfactorily they are to be free from oxide or other contaminants. Cleaning the surfaces before welding ,might be helpful but is not usually sufficient and it is a feature of every welding procedure that the contaminated surface film is dissolved or dispersed. The chemical action of a flux or the sputtering of an arc or even mechanical means such as rubbing and rupturing may do it. The contaminants, which ought to be removed from the surface, are of three kinds - organic films, adsorbed gases and chemical compounds of the base metal, usually oxides. Heat effectively eliminate thin organic films and adsorbed gases so that with the majority of welding procedure where heat is employed this is the remaining oxide film, which is of greatest significance.

Once removed, surface films and specifically nitrides ought to be prevented from forming throughout the process of welding. In almost every welding procedure, therefore, there ought to be some way of excluding the atmosphere while the procedure is carried out. If a flux is utilized for cleaning the fusion faces of the joint, this also performs the function of shielding. If a flux is not utilized, a blanket of an inert gas, or a gas, which does not form refractory compounds along the base metal, may provide shielding. The atmosphere might also be excluded mechanically by welding with the faces to be joined in close contact and the final protection from the atmosphere is achieved by removing it completely by welding in a vacuum. Where the welding operation is performed at high speed and with such restricted heating that there is no time for appreciable oxidation, shielding might be unnecessary. This is possible with a few processes, though, for any contaminated molten metal to be expelled before the joint is completed or for the properties of the weld metal to be corrected by making alloying additions to the weld pool.

One further significant requirement is that the joint produced by the welding procedure should have satisfactory metallurgical properties. In this methods which involve melting of some of the part of the joint it is often essential to add deoxidants or alloying additions, just as is done in the foundry. Frequently the material to be welded ought to have a controlled composition. Some alloys are unweldable by almost any procedure, but a great many are only appropriate for welding, if their composition is controlled within the close limits. These considerations are the fundamental of welding metallurgy.

So we might be say in short that every welding procedure must fulfill four requirements :

1. Avoidance of atmospheric contamination or its influence, and

2. A supply of energy to built an union by fusion or pressure or both,

3. A mechanism for removing superficial contamination from the joint faces,

4. Control of weld metallurgy.