Spot welding creates overlapping seams which, when immersed in electroplating solutions, trap solution residues through capillary action. This creates two problems. First, the residue often leaves plating salt deposits which are unsightly and which, in extreme cases, may require touch up or manual removal at increased cost. Second, the metal in the seam is unprotected and can corrode severely in harsh environments. When designing spot welded assemblies for electroplating, consideration must be given to plating drainage, enclosed seams and pockets, overlapping seams and other areas where solutions may be trapped or where special cleaning or processing techniques may be required. When these operations are combined, early consultation with an experienced supplier is crucial.
If the sheets are the same thickness then the power setting used for plug welding would be the same as you would use for 1.5 times the thickness of one of the sheets. This is the sort of penetration you would expect from a plug weld. The molten pool is just breaking out of the reverse of the back sheet. The heat marks indicate the weld has arced against the back sheet rather than at the side of the hole. If you don’t get these marks then consider a little seam welding just to be sure. There is a special clamp designed for plug welding that makes life really easy. The parts you see in the photograph are attached to a normal mole grip. This clamp came in a set of three random welding clamps all of which are extremely useful.
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. See extra info at Tecna 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!