Welding is a process of metallurgically joining two pieces of metals by the application of heat with or without the application of pressure and addition of filler metal. The joint formed is a permanent joint. Modern methods of welding may be classified under two broad headings.

    1. Plastic welding process
    2. Fusion welding process

In plastic welding process the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. This procedure is used in forge welding, resistance welding, spot welding in which pressure is required.

In the fusion welding, the material at the joint is heated to a molten state and allowed to solidify. This includes gas welding arc welding and Thermit welding.

The surfaces of the metal which are to be joined by any of the welding processes must be sufficiently clean to permit clean metallic surfaces to come in to contact. In some operations, materials known as fluxes are applied to the parts being welded to dissolve the oxides or to prevent the formation of oxides. Fluxes are different for different metals. For ferrous materials borax, sodium carbonate etc, have been found to give excellent results.

Types of Joints:

The welding joints are classified as Butt, Lap, Tee, Corner joints and edge joints. The choice of the type of joint is governed by the kind of metal to be welded, its thickness and technique of welding. Figure 1 shows the different types of joints used in welding.