Electric Resistance Welding & Its Types

Electric resistance welding is a type of pressure welding. It is used for joining pieces of sheet metal or wire. The welding heat is obtained at the location of the desired weld by the electrical resistance through the metal pieces to a relatively short duration, low voltage (from 6 to 10 volts only) high amperage (varying from 60 to 4000 amperes) electric current. The amount of current can be regulated by changing the primary turns of the transformer. When the area to be welded is sufficiently heated, the pressure varying from 25 to 55 MPa is applied to the joining area by suitable electrodes until the weld is solid. The various types of electric resistance welding are as follows:

1. Spot welding:

It is used for welding lap joints, joining components made from plate material having 0.025 mm to 1.25 mm in thickness. The plates to be joined together are placed between the two electrode tips of copper or copper alloy. It may be noted that:

(a) The electrode tip diameter (d) should be equal to √t where t is the thickness of plate to be welded.

(b) The distance between the nearest edge of plate and center of weld should be at least 1.5d.

(c) The spacing between two spot welds should not be less than 3d.

2. Roll spot and seam welding:

When the spot welds on two overlapping pieces of metal are spaced, the process is known as roll spot welding. If the spot welds are sufficiently made close, then the process is called seam welding. This process is best adopted for metal thickness ranging from 0.025 mm to 3 mm.

3. Projection welding:

It is similar to spot welding except that one of the metal pieces to be welded has projections on its surface at the points where the welds are to be made. In other words, it is a multi-spot welding process. It may be noted that:

(a) When two pieces of different metals are to be welded by projection welding, then the projections should be made on the metal piece with the higher conductivity.

(b) When the two metal pieces are of different thickness, the projections should be made on the thicker metal piece.

4. Butt welding:

The butt welding is of two types, i.e., upset butt welding and flash butt welding. The upset butt welding is especially adopted to rods, pipes and many other parts of uniform section. The flash butt welding is extensively used in the manufacture of steel containers and in the welding of mild steel shanks to high-speed drills and reamers.