Types of Welding Process:

The simplest welding procedure would be one in which the two parts to be joined contain their surfaces prepared to contours matching along atomic precision. In vacuum,  such surfaces brought together so as to enable electrons to be shared among atoms across the interface could result in an ideal weld. The preparation of surfaces along this degree of cleanliness and precision is not feasible at present, although this is approached in space technology while metals might be in contact in the ultra-high vacuum of outer space. Slight rubbing of surfaces under these conditions might induce welding by satisfying the first two conditions above at restricted points of contact, the third already being supplied by the vacuum. While such conditions of cleanliness and vacuum might be visualized for special micro-welding applications, alternative solutions ought to be found for practical welding.

The problem of attaining atomic contact among the parts to be joined is solved in one of two ways. Pressure might be applied, so that abutting surfaces are plastically deformed giving the necessary intimacy of contact at least at asperities as shown in Figure 1(a). The deformation also helps to satisfy the cleaning needs by rupturing films. Along ductile metals the plastic deformation ,might be accomplished cold, but less malleable metals might be first softened by heat. In other hand, the surfaces to be joined might be bridged along liquid metal. The required adjustments in structure and contour are then influenced as the molten metal solidifies (Figure 1(b)). The majority of welding procedure employ the latter method and their variety is an indication of the various ways by which this is possible to produce locally the heat needed for melting.