Resistance spot welding joins overlapping sheet by squeezing it between electrodes and passing a large electric current through the stack for a short time.

Electrical resistance generates heat approximately according to

$$Q=I^2Rt,$$

where $I$ is current, $R$ is effective resistance and $t$ is current duration.

Because the current is concentrated through the contact region, a local molten weld nugget forms between the sheets. Electrode force maintains contact and contains the molten region while it solidifies.

For example, if the effective resistance is $100\ \mu\Omega$, current is $10\ \mathrm{kA}$ and current time is $0.20$ s, the ideal electrical heat estimate is

$$Q=(10{,}000)^2(100\times10^{-6})(0.20)=2000\ \mathrm J.$$

Real heat distribution is more complicated because resistance and temperature change during the weld.

Too little energy can create a small or incomplete nugget. Too much can expel molten metal, indent the surface severely or overheat electrodes. Electrode wear changes contact geometry and therefore process behavior over repeated cycles.

Spot welding is especially suited to high-volume sheet assembly because each weld is fast and needs no separate filler. Its design limitation is equally important: it requires electrode access from both sides of the joint.