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 electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Spot welding is used in preference to more costly mechanical fastening, such as riveting and screwing, when disassembly for maintenance is not required. While weld strength at each weld spot 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, making the process suitable for only certain applications. 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.
Spot welding is typically used when welding steel sheet metal. Thicker stock is difficult to heat up from a single spot, as the heat can flow into the surrounding metal too easily. Spot welding can be easily identified on many sheet metal goods, such as metal pails. Aluminum alloys can also be spot welded. However, their much higher thermal conductivity and electrical conductivity mean that up to three times higher welding currents are needed. This requires larger, more powerful, and more expensive welding transformers.
Due to changes in the resistance of the metal as it starts to liquefy, the welding process can be monitored in real-time to ensure a perfect weld every time, using the most recent advances in monitoring/feedback control equipment. The resistance is measured indirectly, by measuring the voltage at and current through the electrodes.
The voltage needed for the welding depends on the resistance of the material to be welded, the sheet thickness and desired size of the nugget. When welding a common combination like 1.0 + 1.0 mm sheet steel, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V at the end of the weld. This drop in voltage stems from the resistance reduction caused by the steel melting. The open circuit voltage from the transformer is much higher than this, typically in the 5-10 V range, but there is a very large voltage drop in the electrodes and secondary side of the transformer when the circuit is closed.
Perhaps the most common application of spot welding services is in the automobile industry, where it is used almost universally to weld the sheet metal forming a car. Spot welders can also be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use for robots being painting).
Another place where spot welding is used is in the orthodontist's clinic, where small scale spot welding equipment is used when resizing metal "molar bands" used in orthodontics.
Like spot welding, seam spot 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.
Spot Welding Resources
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