Resistance Projection Welding:
The resistance projection welding process is similar to spot welding except that the welding pressure, and welding current. Hence, the welding heat is localized by making projections or embossments on one or both of the workpieces to be joined. Such projections are made at all points where a weld spot is desired.
The projections have a diameter on the face equal to about the thickness of the stock and extend about 60 percent of the stock’s thickness above the stock.
The operation of resistance projection welding:
The welding current is passed through the joint, welding heat is generated at these projections. Under the welding pressure the projections flatten allowing the two surfaces to be joined to come together. The melted projection becomes the weld.
The number of projections made in a joint should permit proper contact between the workpieces at the projections. The ideal number is three, as the two sheets will always be in contact at three points. The maximum number of projections that can be satisfactorily handled is about six.
Special attention must be paid to the selection of the correct pressing force at the beginning of the welding process. The use of excessive force causes the projection to collapse before the weld pool is created, which increases the contact surface and reduces current density. Variation in the tensile strength of the workpiece may make welding more difficult, because it may result in projections of different sizes, in addition to the way they flatten during welding.
When welding several projections at the same time, problems may occur in the heat balance of the joint or in the flattening of the projections. Problems can often be avoided by increasing the distance between projections. The recommended distance is four times the diameter of the projection.
Welding soft materials may be difficult if the workpiece thickness is less than 0.50 mm, because projections may collapse before welding current is applied.
For a successful projection weld the projections made on the parts should have the following characteristics:
1. The projections should be stiff enough to take the squeeze force before the current is passed.
2. The projections should have sufficient mass to heat a spot in the plane surface to welding temperature
3. The projections should collapse during welding without splashing between the sheets being welded.
4. The projections should be properly formed without any partial shearing.
5. It should be possible to form the projections without disturbing the other portions of the component.
Advantages of Projection Welding:
1. More than one spot weld can be made in a single operation, so the operation is very fast.
2. Welding current and pressure required is less
3. It helps in obtaining a satisfactory heat balance in welding difficult-to-weld combinations of metals and thickness.
4. Closer spacing of welds is possible
5. Electrodes can be shaped to act as assembly fixtures for mass welding of parts
6. Uniform welds with good finish are produced.
7. Suitable for automation
8. Filler metals are not used. Hence clean weld joints are obtained
Disadvantages of projection welding:
1. Projections cannot be made in thin workpieces.
2. Thin workpieces cannot withstand the electrode pressure
3. Additional operation is required after the welding process is over.
4. Equipment is costlier
1. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly. Also, used for welding parts of refrigerators, condensers, refrigerator racks & grills, bushings, studs, nuts, handles, etc.