Flash (butt) welding is an electrical resistance based butt welding process used for joining components end-to-end across the full joint area.
A range of section sizes and complex shapes can be joined (for example from bicycle wheel rims to railway rails). The parts to be joined are clamped and brought together slowly while a flashing voltage is applied. The process can be manual, but most welding machines are automatic or semi-automatic.
Where small contacts are made between the components, there is a high current density and the material resistance heats, melts and blows out of the joint in a shower of melted particles, giving the characteristic flashing action. This flashing progressively introduces heating and a softened zone at the component ends, while eliminating oxides and contaminants from the interface. After a pre-set 'burn-off' length of the parent material, a forge force is applied to the parts to consolidate the joint. This produces a forge butt weld with no melted metal remaining in the joint.