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.
Copper and its alloys can also be joined by resistance spot welding, although spot welding copper cannot be easily achieved with conventional copper alloy spot welding electrodes, as heat generation in the electrodes and work piece are very similar. The solution to welding copper is to use an electrode made of an alloy with a high electrical resistance and a melting temperature far in excess of the melting point of copper (much greater than 1080°C). Electrode materials typically used for spot welding copper include molybdenum and tungsten. See extra details on Auto Body Spot Welder.
Portable spot welders are compact, lightweight and handy for difficult-to-access joints. As such, they are most commonly used in coachbuilding. They have an average welding capacity of 2 + 2mm and are relatively inefficient due to the fact that the electrodes aren’t cooled between cycles. Automatic adjustment makes it easier to adjust the spot parameters according to the thickness of sheet and welding head used.