Principle of Resistance Welding
- Figure shows the basic arrangement for the resistance welding. The two metal sheets to be welded are clamped tightly together with external pressure, between the two electrodes.
- The welding current flows through these electrodes and through the metal pieces to be welded.
- Due to the flow of welding current through the electrodes and through the metal pieces, the temperature of the common surface of sheets in the region shown by the dotted lines will increase, until the two pieces of metal fuse together under the pressure of the electrodes.
- The heat is produced due to the current flowing through the resistance offered by the metal sheets. Therefore this process is known as “Resistance welding.
- The heat produced which is utilized for welding is given by :
Where i = Welding current
R =
Resistance between the metal pieces to be weided
tw
= Welding time
- Ideally all the heat should be produced and concentrated at the spot to be welded i.e. the entire resistance should occur at this place.
- But practically the temperature rise occurs at the following parts other than the spot to be welded:
- Throughout the metal sheets.
- In the contacts between the sheets and the electrodes.
- In the two electrodes themselves. Therefore it is possible that the electrodes themselves may be welded to the materials or a hole may result in the material. In order to avoid the welding of electrodes to the sheet, the electrodes are made up of metals having high electrical and thermal conductivity (i.e. low resistivity and therefore less temperature) and they are generally water cooled.
- This will further reduce the heating of electrodes and their possibility of getting weld to the metal pieces. In order to allow the water circulation these electrodes are hollow from inside.
- By proper choice of electrode pressure, magnitude and duration of welding current it is possible to weld the metal sheets of thickness varying from about 10 cm to several centimeters.