The total resistance of the system between the electrodes consists of
(i) The resistance of the work piece R1
(ii) The contact resistance between the electrodes and the work, R2, and
(iii) the resistance between faying surfaces of the two metal pieces to be welded together, R3.
-In order to obtain a sound weld and to avoid overheating of the welding electrode as, R1, and R2 should be kept as low as possible with respect to resistance R3.
*R1, the resistance of the work piece, depends upon the nature of the material and its thickness. It cannot be changed otherwise. If the work piece material has low electrical resistance, such as aluminum, it requires very high currents in order to produce the required welding temperature and hence proper weld.
*R2, the contact resistance between the electrode and the work piece can be minimized by
(i) Keeping the electrode tip and the work piece surface properly cleaned.
(ii) Using the welding electrodes of highly conductive materials such as Cu- Cd or Cu-Cr alloys.
(iii) Controlling the shape and size of the electrodes.
(iv) Using the proper pressure between the electrodes and the work pieces.
*R3, the resistance between the contacting surfaces of the two work pieces, varies with the quality of the surfaces. Surfaces that have not been cleaned and possess seal, bright or other contaminants on them offer more resistance to the flow of welding current.
Smooth work piece surfaces and high electrode pressures reduce resistance R3. Overheating of the welding electrodes is avoided by circulating either water or a refrigerant through them
The main aim is to obtain a sound weld without overheating either the electrodes or the work-pieces.