Highly conductive materials like aluminum require very high power to form quality spot welds. However aluminum alloys are routinely spot welded (see Table I for weldability). Here, cleanliness is much more of a concern than with low-carbon steels because of aluminum’s rapid surface oxidation characteristics. For optimum quality and weld performance, expensive cleaning procedures to remove surface oxide are required. For demanding applications, equipment to monitor surface resistivity from lot to lot is necessary to assure consistency of quality. This leads to a related consideration. If aluminum has been chosen for an important reason, such as lightweight or high strength-to-weight ratio, the added expense of ensuring a high-quality weld should be justified. If it has not, re-evaluation of the original material selection is in order or, perhaps, another assembling method should be considered.

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. Read more details at Tecna Spot Welder Parts.

To make sure your welding settings are correct, you should carry out a test run using metal offcuts, followed by a destructive test where you separate the welded parts with a hammer and a forked chisel. After separation, a hole must be made in one of the two conjoined sheets – a process known as “unbuttoning” in metalworking jargon – as proof that the settings are right for the metals and thicknesses involved. Apologies for the abstruse technical term! When welding radiators, make sure you also carry out a leak test.