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Introduction To Spot Welding -Working Principle of Projection welding

Introduction To Spot Welding

Spot welding is one of the type of welding processes.
In electric resistance welding (ERW) methods, a high current is passed through the metal pieces to be joined together and the heat is produced due to the resistance in the electric circuit. This heat energy is utilized to increase the temperature of a localised spot of the work pieces to produce coalescence, and then applying pressure at this spot till welding takes place. Electric resistance welding process is a pressure welding process and not a fusion welding process. The output of heat, in this process can be easily calculated. Heat generated is proportional to I2Rt, where I is value of current, R is resistance and t is the time during which current flows.

The following ERW processes are in vogue:
1. Spot welding process,
2. Seam welding process,
3. Butt welding process, and
4. Flash butt welding process.

Spot Welding Process

Spot welding consists of joining two pieces by placing them between two electrodes and passing a heavy current through them for a very short duration. This causes the material just below the electrodes to heat up quickly due to the intervening resistance to the flow of electric current. When coalescence temperature is reached, the current is switched off and a pressure is applied on the two electrodes. The pressure is released when the spot weld cools off. The portion of the material just below the electrodes gets pressure welded. The weld joint is usually in the form of a round spot (if the electrodes have circular tips), hence the name spot weld.

Spot Welding Working Diagram
Spot Welding Working Diagram
The electrodes are usually made of copper and are water cooled. One of them may be fixed and the other one is movable. Normally A.C. power is used along with a step down transformer. The two terminals of secondary winding of transformer are connected to the two copper electrodes to complete
the circuit.

Usually spot welding (as also other ERW machines) are automatic and work on the following weld cycle:
1. Squeeze the two metal pieces together with a light pressure.
2. Pass heavy electric current for a very brief time to obtain coalescence temperature,
3. Apply pressure and hold for sometime.
4. Remove pressure.

The whole cycle takes just a few seconds. Welding current may heat up the spot in less than a second.
This process is extremely suitable for mass production work and is extensively used for fabrication of automobile bodies, railway coaches, steel furniture etc.

Projection Welding Basic
One variation of spot welding process is called “Projection welding” process. In this process, at least one metal part has projections or depressions (made by some previous pressing operation). The other part and these projections contact each other. If welding is required to be done at these projected locations, an arrangement of electrodes such as shown in Fig. may be used.

Diagram Of Projection Welding
Diagram Of Projection Welding
The weld cycle remains same and on passing current all projections will heat up and get welded. Projections need not be round; they can be of any shape. Projection welding is extremely suited for mass production work, where a number of spot welds are required close to one another.

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