In its widest possible definition, welding refers to a fusion of different or similar materials (for example the welding of plastic), through the application of heat. Even glass can be “welded” but “perfect” welding occurs between metals.

Two or more sheet metal parts are welded together by fusion: the heated edges of the two products are cast together and in just three seconds at room temperature, a single part is generated.

Undoubtedly, this is the most costly production phase in that production times are very long and welding is used, for example, in the production of structural elements (elevator doors etc.).
  
To achieve a welded product which is resistant and imperfection-free, the fusing area should be protected from oxidation, and any debris present on the metal should be removed. To avoid oxidation, welding should be carried out in an oxygen-free atmosphere: for this purpose substances such as gas, borax, silicates and carbonates should be admitted to the welding area, creating a “protective cloud” around the fusion bath and facilitating the elimination of debris.

Another problem can occur from distortion: if, when heated, the metal expands, and if an expanding metal is restrained, it changes form, immediately undergoing a change in shape caused by internal residual tensions. Changes in shape create constant problems in the welding process, so that it is imperative to control this phenomenon in order to achieve good results.

In controlling changes in shape, it is necessary to consider several factors: the degree of restraint required for the part in question, the time necessary for welding, the dimensions of the welded product and the welding speed.

Following the welding operation, the welded join is cleaned (a process which is often not taken seriously, or carried out in a superficial way).

This is a fundamental operation in that oxide deposits caused by welding, if not removed, could lead to the emergence of cracks. Aggressive agents could penetrate these cracks and cause damage, owing to conditions which are favourable to them, such as stagnation and the absence of oxidation. Another phenomenon which could occur is iron contamination. For this reason it is advisable to clean using tools which do not transfer iron particles onto stainless steel. Tools made from stainless steel or neutral metals should be used, in this instance. If cleaning is carried out with liquids or pickling or passivation chemicals, it is necessary to ensure that these substances are completely removed by rinsing abundantly so that any possible residue does not cause corrosion.

In recent years, more refined techniques and sophisticated technology have been invented which have brought welding processes and safety to a high level.