Projection is a type of resistance welding. It is named so because different projections are formed on the workpiece for effective welding.
In this welding process, the design or shape of the part is used to make discreet individual contact to concentrate the current during the welding process.
In most cases, multiple small projections are formed over the surface of the parts to be welded.These projections can be round dimples, elongated ridges, dome type, circular, or the extended corners of weld nuts.
Two wires placed together at 90 degrees form a contact point contact. This would also be projection weld.
When the mating parts are brought together these projections concentrate the flow and generate the heat in these locations. When the projection get hot they collapse and the weld nuggets are formed at the place of projections. After cooling process, several weld nuggets are formed which holds the two workpieces together.
Projection welding is another variation of spot welding.It is similar to spot welding but it requires much less current than spot welding. Unlike spot welding, electrdodes are not used for for concentration of heat in projection welding instead the projection on the workpiece is used for this purpose.
Other factors must be controlled such as allignment, projection consistency, welder follow up and with nut welding threads protection while performing projection welding process.
In projection welding copper electrodes are used as copper is a very good conductor of heat and electricity.
The two metal pieces which are to be joined are kept between two elctrodes and the electrodes are kept between the platens as shown in the figure above. A large pressure force is to be applied to the electrodes by the two platens, upper and lower platen.
After that current is passed through the system. As the current passes, the heat is produced due to internal resistance possessed by the metal workpieces.
This heat produced by resistance is concentrated at the projections on the workpiece because when the two workpieces are placed together, the sheets touch only at the points of projections. Usually two or three projections are welded at the same time.
When welding workpieces of unequal thickness projection is made in thicker workpiece and for dissimilar materials welding projections are made in the one with high thermal conductivity.
When pressure is applied to the workpiece by the platens, these projections collapse and get fused and weld nuggets are formed at the place of projections. Hence a quality weld is formed and the two metal workpieces are welded successfully.
Advantages of Projection Welding:
1) This projection welding requires a very small supply of current. As less current passes through the electrodes, the life of electrodes increases. This process require also less pressure for welding.
2) Unlike spot welding, there is no limitation on the thickness of the metals to be welded.
3) Unlike spot welding, more than one spot can be made in single operation.
4) This process is suitable for automation.
5) Filler metals are not used in this process.Hence, clean weld joints are obtained in this process.
6) This process is also effective in welding joints which are in complicated locations.
7) This process gives a good heat balance while welding.
Disadvantages of Projection Welding:
1) Projection formation in projection welding is a complicated process and it takes time. Only a skilled person can form projections of different required shape like spherical projections.
2) This process is not applicable to all types of workpieces, the composition of workpiece has to be considered while choosing this welding process.
3) It can not be applied to some type of brasses and coppers.
4) Projections cannot be made in thin workpieces.
5) Equipments are costlier.
Applications of Projection Welding:
Projection Welding is most commonly used when special nuts, small fasteners, special bolts, studs and similar parts are used as projection on the portion of the part to be welded.
Projection Welding is also used for welding parts of refrigerator, condensers, refrigerator racks, grills, etc
Apart from that, projection welding is used in automobile workshops, ship building works and sheet metalworking.