Resistance welding is one of the oldest of the electric welding process is used by industry today. The weld is made by the combination of the heat, pressure and time.
All resistance welding spot welding, seam welding, projection welding, etc. are worked on the same principle of heat generation due to electric resistance.
Resistance Welding Working Principle / Diagram / Process:
When a current passes through an electric resistance, it produces heat.
This is the same principle which is used in the electric coil.
The amount of heat produced depends on the resistance of the material, surface conditions, a current supplied, time duration of current supplied, etc.
This heat generation takes place due to the conversion of electric energy into thermal energy. The current is the same in the whole circuit.
Heat is proportional to the product of resistance by the square of the current and by time.
therefor The heat generation equation is (H=I2Rt).
H=Heat generated in joules. I=Current in amperes (Amps). R=Resistance in omes. T=flow time of the current in seconds. The highest Heat is developed where the resistance is at its maximum, exactly where it is needed for this process.
Resistance Welding Equipment / Parts :
To start the welding process or supply the power for welding, first, we switch on.
The initiation switch is connected to the timer and from the timer, this is connected to two other devices contractor point and AC power supply there switch gets on.
Contractor points work like the connection between AC power and Step down transformer. This works as an intermediate switch.
Here voltages are more which is further decreased in the step-down transformer.
Step Down Transformer:
The step-down transformer work is to reduce the voltage in the secondary winding as compared to the primary winding.
The secondary winding is connected to the step-down transformer for receiving the power for the electrode and also to reduce their electrical resistance between them.
An electrode is a conductor through which the current passes and here this is used for making the joint between two workpieces.
Air is coming through the port and this air further utilized for making pressure.
This is used for supplying the pressure by the use of the air valve.
Resistance Welding Working:
The main requirement of the process is the low voltage and high current power supply.
This is obtained by means of a step-down Transformer with a provision to have different tapping on the primary side, as required for different materials.
The secondary windings are connected to the electrodes which are made of copper to reduce their electrical resistance.
The time of the Electric Supply needs to be closely controlled so that the heat released is just enough to melt the joint and the subsequent fusion takes place due to the force (Forge welding) on the joint.
The force required can be provided either mechanically, hydraulically or pneumatically as shown in the figure.
To precisely control the time sophisticated electronic timers are available.
The critical variable in the resistance welding process is the contact resistance between the two workpiece plates and their resistance themselves.
The contact resistance is affected by the surface finish on the plate since the rougher surface has higher contact resistance.
The contact resistance also will be affected by the cleanliness of the surface.
Oxides or other contaminants if present, should be removed before attempting resistance welding.
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