Resistance welding
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Resistance welding refers to a group of welding processes that produce coalescence of faying surfaces where heat to form the weld is generated by the resistance of the welding current through the workpieces. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the electrode materials, electrode geometry, electrode pressing force, weld current and weld time, etc. Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current (100–100 000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.
Spot welding is a popular resistance welding method used to join two to four overlapping metal sheets which are up to 3 mm thick each. In some applications with only two overlapping metal sheets, the sheet thickness can be up to 6 mm. Two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated in the air gap at the contact points. At the contact points between electrodes and workpiece the heat dissipates throughout the copper electrodes quickly, since the copper is an excellent conductor. However at the air gap between metal sheets the heat has no where to go, as the metal is a poor conductor of heat by comparison. Therefore the heat remains in the one location, which melts the metal at that spot. As the heat dissipates throughout the workpiece over a second or so, it cools the spot weld, causing the metal to solidify.
The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods. This limits the usefulness of the process. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld stainless steel.
Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.
[edit] References
- Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN 0-8493-1773-8.
- O'Brien, R.L. (Ed.) (1991). Welding Handbook Vol. 2 (8th ed.). Miami: American Welding Society. ISBN 0-87171-354-3
 Metalworking
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| Welding | |
| Arc welding: Shielded metal (MMA) | Gas metal (MIG) | Flux-cored | Submerged | Gas tungsten (TIG) | Plasma | |
| Other processes: Oxyfuel | Resistance | Spot | Forge | Ultrasonic | Electron beam | Laser beam | |
| Equipment: Power supply | Electrode | Filler metal | Shielding gas | Robot | Helmet | |
| Related: Heat-affected zone | Weldability | Residual stress | Arc eye | Underwater welding | |
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See also: Brazing | Soldering | Metalworking | Fabrication | Casting | Machining | Metallurgy | Jewelry |
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