In resistance welding, the metals are heated by an electrical current and parts are joined by pressure. Copper electrodes are placed on either side of the pieces to be joined. The electrode tips contact the metal sheets and push them together for a short period of time to ensure the weld tip force is reached and the weld gun is fully closed. This is called “squeeze time” and can run between 12 to 30 cycles depending on the weld gun size.
Next, the current flows between the tips, generating heat at the part interface. Heat at the weld tip interfaces is conducted away by water cooling in the weld tips. Halfway through the weld time, the metals at the part interface melt and form a weld nugget.
The weld time is complete when the current stops flowing. The gun stays closed for about four to five cycles while the heat is absorbed away from the weld nugget, causing it to harden.
An advantage of resistance welding is that no other materials are needed, such as fluxes or filler material, which keeps the process cost-effective. All operations are automatic, and process variables are preset and held constant.
The different forms of resistance welding are differentiated by the types and shapes of weld electrodes. Resistance spot welding, for example, uses the face geometries of the electrodes to focus current and apply force to the desired location. Resistance seam welding is a subset of spot welding, and it uses wheel-shaped electrodes to apply force and current.
Depending on the settings, the resulting welds may be individual spot welds at defined intervals, or a complete welded seam, with a series of overlapping spot weld nuggets, each nugget cooling before the following nugget is initiated. This provides fluid-tight lap welds often used for pressurized vessels with mild internal pressure, such as exhaust system catalytic converters for vehicles.
Resistance projection welding uses projections, embossments or intersections to localize the weld at specified points. When current is supplied, the projections collapse and the force of the electrodes form the weld nugget. This method produces multiple welds at one time. Projection welding is used effectively for joining at complicated locations and can weld metals of almost all thicknesses.
Flash welding is a form of resistance welding that creates heat at the faying surfaces, where one of the surfaces that are in contact at a joint, and not only by resistance to the flow of electric current but also by arcing (flashing) across the interface. Force is applied at a pre-determined time and the surfaces are moved together at a controlled rate.
In upset welding, also called butt welding, the pieces to be joined are already in firm contact with one another, and no flashing occurs. The pressure is applied before the current is started, and maintained until the process is finished. Rather than melting followed by coalescence as with other resistance welding techniques, this method forges the parts together. In other words, a butt weld is a single-stage operation of both current and pressure.