Ideally, equal thicknesses of two sheet metal parts to be joined produces an evenly distributed weld nugget within the two layers. When this is not practical, materials of different thicknesses can also be joined and produce a centered weld nugget by using a larger electrode on the thicker member. At a ratio above about 3-to-1 (thickest to thinnest member), spot welding becomes difficult. At this point, another joining method should be considered–for example, projection welding. Note that weld deformation is always greater on the thinner member. For this reason, stiffeners and brackets spot welded to cosmetic parts should be thinner than or equal in thickness to the exposed surface material.
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.
Spot welding is a resistance welding process that is used primarily for welding two or more metal sheets together by applying pressure and heat to the weld area. It works by contacting copper alloy electrodes to the sheet surfaces, whereby pressure and electric current are applied and heat is generated by the passage of current through resistive materials such as low carbon steels. Read extra info on https://www.weldingsuppliesdirect.co.uk/welding-equipment/spot-welding.html.
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!