Most flash welding machines operate automatically, so the machine is set up by running test welds at a range of settings and evaluating their quality (See also the FAQ: What factors affect flash (butt) weld quality?
Surface preparation is not generally required, although the areas clamped in the dies must not be heavily coated with oxide or paint, etc which would impede current flow into the components. The surfaces to be welded must allow some initial electrical contact, after which flashing will purge any remaining contaminants.
There must be a heat balance between the parts to be welded. With parts in the same material and similar section, a symmetrical set-up in the welding machine is satisfactory. The clamp gap allows heating and upset of the weld between the clamps but should not be so large that the alignment of the parts cannot be maintained. When welding materials of dissimilar electrical and thermal conductivity, the heating effect and the burn-off rate during flashing are likely to be different and this must be allowed for by setting the appropriate stick-out length from the clamps each side.
A shielding gas is not generally used, although it may sometimes improve weld quality by displacing air from the joint gap. A flammable gas (eg natural gas) is often used in strip line welding machines in steel mills, where the gas burns in the weld area, protecting the flashing surfaces from oxidation. An argon or helium shield is normally used when flash welding titanium.
A pre-heating stage - passing current through the parts to be welded under pressure - may sometimes be introduced into a flash welding procedure, particularly for thicker sections. The raised temperature makes flashing easier to start and maintain, and helps produce a well distributed upset zone. The number and duration of preheats are normally preset.
The flashing stage controls the heat input into the weld and prepares the component ends for the forging (upset) stage. The combination of forward speed, flashing voltage and flashing length is chosen to give uniform flashing without breaks or short circuits (butting up or freezing). Flashing speed is usually accelerating and may be servo controlled for best results.
If flashing voltage is too high or forward speed too low, deep craters can be formed on the flashed surfaces which increase the risk of weld interface flaws.
The upset force and length of upset are important in achieving correct forging of the weld. This eliminates melted metal and contaminants from the joint, producing a sound, solid phase weld (forge weld).
In-machine post-weld resistance heating of highly hardenable materials may be beneficial in reducing the risk of cracking.