In designing for flash welding the main requirement is that the parts to be welded have the same cross section at the joint. One part may need to be modified or machined to achieve matching sections. This allows balanced heating of the ends of the components being joined. There are special cases where this is not possible, e.g. T-joints, but in such cases the highest joint quality may not be achievable.
When designing a part where the finished dimension is important, the length lost during flashing must be taken into account. Parts to be welded must be shaped to allow them to be clamped in the welding machine with the joint line transverse to the direction of the forging force.
When welding complex rolled or extruded sections in a butt or mitre joint, it is essential to design the dies to provide sufficient clamping and support of the section. This ensures even heating of the joint and proper support of the surrounding metal during the upset (forging) stage of the weld.
When rings are flash welded there can be a problem of current shunting through the body of the ring. There is a critical ratio of circumference to cross sectional area which must be exceeded to avoid this, the critical value depending on the conductivity of the ring material. However, relatively small diameter heavy section steel rings such as chain links of all sizes can be welded.