Projection welding can be used on low-carbon, low-alloy and stainless steels, as well as on aluminum. Typically, thicknesses up to 0.125 in. (3.18 mm) can be joined. Thin workpieces–from 0.010 in. (0.25 mm) up to 0.022 in. (0.56 mm)–may require special equipment. Below 0.010 in. (0.25 mm), resistance spot welding is recommended, because on this thin material the projections would collapse before the fusion temperature is reached. While projection welding can be less expensive than resistance spot welding, workpiece alignment is more critical, and heights of projections with simultaneous welds need to be closely controlled– typically, within 0.003 in. (0.08 mm) of each other.
One alternative to plug welding is “MIG spot welding”. It is similar to plug welding, although a hole is not drilled in the front sheet of metal. Instead the power of the MIG is relied upon to fully melt the top sheet and penetrate into the back sheet. This technique would require less preparation work than plug welding, but the two sheets need to be in tight contact and high amps used to complete the weld or else the weld could be very weak. Plug welding is a much more suitable technique for all but the most experienced welders.
Where is Spot Welding Used? Spot welding has applications in a number of industries, including automotive, aerospace, rail, white goods, metal furniture, electronics, medical building and construction. Given the ease with which spot welding can be automated when combined with robots and manipulation systems, it is the most common joining process in high volume manufacturing lines and has in particular been the main joining process in the construction of steel cars for over 100 years. See more details at Spot Welder.
What type of sheets can be welded? Rust-free, non-painted sheets of the same or different metals can be welded provided they are compatible alloys with a very similar melting point. Metals such as stainless steel, aluminium, steel alloys and galvanized steels can be spot welded, subject to operating adjustments (current, welding time, intensity of compression). Note that the coating on galvanized metals tends to clog the electrodes – which must be cleaned regularly!