In addition, higher strength steels may require special techniques or treatments like tempering after welding. Spot weldability of HSLA (high-strength low-alloy) steels is directly related to composition and type of microalloying elements. It is advisable to check with the supplier before specifying spot welding here. Stainless steels are spot weldable, some grades more readily than others. Austenitic grades of the 300 series are the most commonly welded types, followed by ferritic. Martensitic stainlesses are the least common because welded joints are always much more brittle. All stainless steels require careful adjustment of welding parameters and/or special methods to obtain optimum quality welds.
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.
Although aluminium has a thermal conductivity and electrical resistance close to that of copper, the melting point for aluminium is lower, which means welding is possible. However, due to its low resistance, very high levels of current need to be used when welding aluminium (in the order of two to three times higher than for steel of equivalent thickness). In addition, aluminium degrades the surface of copper electrodes within a very small number of welds, meaning that stable high quality welding is very hard to achieve. For this reason, only specialist applications of aluminium spot welding are currently found in industry. Various new technology developments are emerging to help enable stable high quality spot welding in aluminium. See extra info on Tecna Spot Welder Parts.
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!