Projection welding is a variation of the spot welding process in which raised projections on one or both sheets localize and concentrate the welding energy. This technique allows for closer weld spacing on thicker materials than are possible with the conventional approach. Projection welding is commonly used to attach connector points such as studs and nuts to sheet metal assemblies particularly in the automobile industry. Materials suitable for the projection welding process include aluminium, low carbon steel, and stainless steel.
Conventional spot welding involves passing a high-tension electric arc between two electrodes on either side of the material to be welded. This causes localized melting of the two materials, thus creating a spot weld. Projection welding uses the same basic principle but utilizes a shallow projection on one or both surfaces at the weld points. These projections face inwards towards the inside surfaces of the weld sheets and concentrate the heat generated during the weld discharge. This maximizes the melt pool potential and allows for thicker materials to be welded with the same current settings.
The projections in the work pieces are typically of spherical or blunt conical cross section. Where both sheets are embossed, carefully calculated patterns of projections ensure accurate indexing. The projections are also often used to accurately line up work pieces. As with conventional spot welding, the rotating electrodes are kept static while the work pieces are advanced between them. The advance speed has to be carefully tailored to ensure that the projections correspond with the welding arc timing as they pass between the electrodes.
The automobile construction industry uses projection welding extensively in the assembly of body pans. Connector points such as integral nuts and studs used for attaching seats, dashboards, seat belts, and interior trim are generally attached using projection welding. This technique also offers the additional benefit of minimal shrinkage and distortion at the weld site. Postproduction work pieces are easier to plate or paint with little grinding or finishing necessary. Materials of dissimilar thickness are also easier to weld using this method.
Projection welding is a suitable process for a number of metals including low carbon steel, stainless steel, and aluminium. The increased efficiency means that sheet thickness of up to 0.125 inch (3 mm) can be successfully welded. This process does have a couple of downsides such as extra steps in the production process and limitations regarding the materials that can be used. The benefits, however, generally outweigh the disadvantages; projection welding is a good alternative to conventional spot welding.