Resistance welding
From Wikipedia, the free encyclopedia
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.
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[edit] Spot welding
- main article: Spot welding
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 first in the higher resistance of the surfaces in contact with each other. As the heat dissipates into the work, the rising temperature causes a rising resistance, and the heat is then generated by the current through this resistance. The surface resistance lowers quickly, and the heat is soon generated only by the materials' resistance. The water cooled copper electrodes remove the surface heat quickly, since the copper is an excellent conductor. However the heat in the center has nowhere to go, as the metal is a poor conductor of heat by comparison. Therefore the heat remains in the center, melting the metal from the center outward. As the heat dissipates throughout the workpiece in less than a second the molten, or at least plastic, state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. Some coatings, such as zinc, cause localized heating due to its high resistance, and may require pulsation welding to dissipate the unwanted surface heat into the copper tips.
If too much heat is applied, or more often applied too quickly, the molten area may extend to the outside, and with its high pressure, typically 30,000 psi, will escape the containment force of the tips with a hazardous burst of molten metal, referred to as expulsion. When this occurs, the metal will be thinner, and therefore have less strength than a good weld with no expulsion. It is an unfortunate circumstance that the common method of checking a weld is a peel test, technically referred to as "coach peel", as expulsion weakens the material by thinning, and makes it pass the peel test easier. A better test is the tensile test, which is much more difficult to perform, and requires calibrated equipment.
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.
[edit] Seam welding
- main article: Seam welding
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.
[edit] Other methods
Other resistance welding methods include flash welding, resistance 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
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[edit] External links
Resistance Welding Manufacturing Alliance
"Making Resistance Spot Welding Safer," from the Welding Journal. Pdf file.
