A resistance welding process is a group of welding processes wherein coalescence is produced by the heat obtained from the resistance of the work to the flow of electric current in a circuit of which the work is a part and by the applications of pressure.
The main factors or variables that are responsible for resistance welding are Heat Generation of the place where two pieces are to be welded, and the applied pressure at the place where the welding joint is to be formed.
The general formula for Heat generation is (H= I2R T) where “H” is the heat for electrical resistance welding that is generated by passing a large electrical current with low voltage through two pieces of metal that are touching each other.”H” is calculated in joules, “I” is the current in Amperes, and “R” is the resistance in Ohms, “T” is the time of current flow through the pieces to be welded.
The temperature is regulated in resistance welding by controlling the magnitude and timing of the welding current and the welding current is obtained from a step-down transformer. The magnitude of current may be controlled through taps on the primary voltage supplied to the main transformer.
Current Supply System
There are three types of current supply systems that are used in the resistance welding process:
- Alternating current systems.
- Direct-current systems.
- Stored-energy current systems.
High-frequency resistance welding is used for applications for continuous seam or butt seam welding. The welding current frequencies are of the order of 450,000 cycles/second.
With a Direct current system, the energy is delivered directly from the power line ad rectifier to do direct current on the secondary side of the welding transformer.
Stored energy systems are mainly stored batteries, electromagnetic types, homopolar generators, and capacitor types. Capacitor stored energy involves charging a group of capacitors from a high voltage rectifier unit.
The total resistance of the system “R” between the electrodes consists of the resistance of the workpiece, the resistance between the faying surfaces of two metal pieces to be welded together, and the total resistance between the electrodes and the work.
To avoid overheating the welding electrodes the resistance between the electrodes and the resistance of the workpiece should be as low as possible.
The contact resistance between the electrodes and workpiece can be reduced by keeping the electrode tip and the workpiece surface properly cleaned and using the electrodes of highly conductive materials like Copper alloys. It can be minimized by controlling the shape and size of the electrodes and using proper pressure between the electrodes and the workpieces also.
The time “T” is set up in four definite segments or periods: Squeezing time, Welding time, Holding time, and Off time.
Squeezing time is the time between the initial application of the electrode pressure on the work and the initial application of current to make the weld. During this period the upper electrode comes in contact with the workpiece and develops full electrode force. At the end of squeeze time, the welding current is applied.
Weld time is the welding current flows through the circuit like enters from one electrode, passes through the workpieces, and goes out from the second electrode.
During holding time the force acts at the point of welding after the last impulse of the welding current ceases.
Off time is the interval from the end of the hold time to the beginning of the squeeze time for the next welding cycle. All these segments of times are controlled automatically in automatic machines, whereas in manually operated machines, only the weld time is controlled automatically.
The force, applied to the workpiece by the electrodes during the welding cycle, carries the current which passes through and generates heat at the place where the two workpieces are in pressed contact.
Resistance welding variables
- Weld time and current
- Welding m/c characteristics and types.
- Condition of electrodes and arms.
- Surfaces of material
- Pressure of electrodes
Advantages of Resistance welding
- High productivity and less-skilled workers can do the job
- Semi-automatic equipment.
- Dissimilar metals can be welded also
The disadvantage of Resistance welding
- Equipment is costly
- Skilled personnel is needed for the maintenance work.
- Special surface preparation is needed in some materials.