Spot welding of very dissimilar metals, such as aluminum and steel, is generally not possible because of different melting characteristics and conductivities. Some types of coated low-carbon steels require special techniques. Steels plated with chrome and nickel for electrical conductivity can usually be resistance welded as readily as uncoated material. Aluminum, tin, zinc and terne-coated steels are also spot weldable with special precautions and welding equipment. Some coatings can emit poisonous fumes that must be safely handled when spot welded, thereby increasing cost. Spot welding of coated substrates creates burn marks in the coating which can be unsightly and may corrode in severe environments. Designers should carefully consider the product’s appearance and service requirements before specifying spot welding of pre-plated materials.
Start off by drilling 7.5mm holes in the front sheet of metal at a spacing of normally 25mm to 40mm (or whatever the original spot weld spacing was). Then clamp this sheet onto the back sheet. 7.5mm is a reasonably good hole size for 0.8 or 1.0mm sheet. Thicker sheet might require a slightly larger hole size. Try a little test piece out like this one before welding a whole sill onto a car and check the weld has penetrated through both sheets.
How Does Spot Welding Work? A form of resistance welding, spot welding is one of the oldest welding processes whereby two or more sheets of metal are welded together without the use of any filler material. The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint. See more info at Tecna Spot Welder.
Electric welding relies on the Joule Effect. This is the thermal result of the electrical resistance, occurring when an electric current passes through a conductive metal – in this case metal sheets for assembly. If that last sentence went over your head, here’s how it works: to weld two or more sheets together without adding a filler metal, they are tightly compressed between two heat-resistant electrodes (i.e. non-melting), generally made of copper, and a high-intensity current is applied to melt the plates together at that point. The result is a small merging of metal which constitutes a welding point. The welding time is very short, between one and two seconds, and the shape of the resulting welding spot depends on your choice of electrodes.
