| Resistance Welding |
By “electric resistance welding” we mean welding processes in which welding is performed by heating and exerting pressure on the parts to be welded.
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The most important factors affecting resistance welding are:
- Weld Current
- Forge Pressure at Electrodes
- Weld Time
- Electrode Condition
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A distinction is made between the following resistance welding processes:
- Spot Welding
- Projection Welding
- Seam Welding
- Butt Welding
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Spot welding is the most common welding method using resistance heating. The two parts to be welded are held under pressure between two copper electrodes. This pressure brings the work faces firmly into contact and provides a good contact between the component and the electrodes. A high current is passed through the contact area to heat the two metals where they are held in contact each other.
Force is maintained during the forge time, and after the current is cut off, while the nugget solidifies.
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Projection welding is a derivation of spot welding in which small areas are raised on the metals to be welded. Current is concentrated over the projection area and consequently the heat makes soften the projection that collapses. Projection welders are basically presses, with the electrodes of a spot welder replaced by flat electrodes.
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Seam Welding
Seam welding is a development of the conventional spot welding machine for fast joining on a production basis. The electrodes of the spot welder are replaced by two rotating copper alloy wheels of high electrical conductivity. Two forms of weld joint can be under taken. In the first, a continuous series of equally spaced spot welds are produced, and in the second case, the individual spot welds overlap each other to form a water and gas tight seam.
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Butt welding is similar to resistance spot welding except that the two components to be welded take the place of the electrodes. The ends of the components are prepared so that they form a good contact and then butted together under a forged pressure.
Flash butt welding is a process developed from Butt welding. With this process, preparation of the weld faces is not so important. The two ends of the components to be welded are machined flat, without a smooth finish. Current is applied and the two weld faces are brought together under a forge pressure. Contact is made at a number of points across the weld area. Short circuiting then occurs, and the contact areas are melted. Contact is therefore broken and re-established as the two weld faces continue to move together, till the final upsetting force is applied to achieve the total compenetration of the two surfaces.
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An important variable in resistance welding is the electrode, in that the current density is governed by the tip size. The tip size is related to the nugget size required for a particular sheet thickness and can be easily calculated.
The current density is important; if the tip size increases, the same amount of current is passed through a larger weld area resulting in a lower heat generation and, subsequently, a softener weld.
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