Principle of Electrical Resistance Welding
In resistance welding, a heavy current above 100 A at low voltage (typically 1 volt) is passed through the joint for a very short time. This high ampere current heats the joint. The heat developed by the resistance to flow of current in electrical resistance welding can be expressed as:
H = I2Rt
where H = total heat produced in the work in joules.
I = Electric current in amperes.
t = time for which the electric current is passing through the joint is seconds.
R = Resistance of the joint.
Alternating current is found to be most suitable electrical resistance welding as it can provide any desired combination of voltage and current by means of a suitable transformer. In order to obtain good weld and avoid overheating it is necessary to maintain the contact resistance uniform which depends upon the surface conditions.
The pressure necessary to effect the weld varies from 2.5 to 5.5 Kgf/mm2. The electrical voltage required varies from 2 to 10 volts depending upon the composition, thickness, area etc. of metal plates to be welded.
Electrodes for Resistance Welding
Since the electrodes in resistance welding have to carry a large amount of current, pressure and also help to remove the heat from the weld zone electrodes should have high electrical conductivity as well as high hardness. Hence copper in alloyed form is used for making electrical resistance welding electrodes.
Advantages of Resistance Welding
- No filler rod or flux is required.
- It can be used for large production at high speeds.
- Little skill is required to operate the resistance welding machine.
- Resistance welds are made very quickly.
- Resistance welding operations are automatic.
- It is possible to weld dissimilar metals as well as metal plates of different thickness.
- No consumable materials are used in this process except electric power. Hence it is a very economical process.
Disadvantages of Resistance Welding
- The initial cost of equipment is high.
- Skilled persons are required for maintenance.
- A job of big thickness cannot be welded.
- Some electrical resistance welding operations are limited to only lap joints.
Applications of Electrical Resistance Welding
- Making metal furniture.
- Making containers for different types of equipment.
- Joining sheets, tubes and bars.
- Welding automobile parts.