According to a study published in Materials and Design, several parameters affect the final product. Flash time is the time that the arc is present. Upset time is the amount of time that the two pieces are pressed together. Flash time needs to be long enough to sufficiently heat the metal before it is pressed together. However, if it is too long, too much of the base metal begins to melt away. The upset time is critical in creating the desired mechanical properties of the finished weld. During the upset, any impurities in the base metal are pressed out creating a perfect weld. If the upset time is too short, all of the impurities may not be pushed out of the base metal creating a defective weld. The upset time is also crucial in the strength of the finished weld because it is during the upset that coalescence occurs between the two pieces of metal. If the upset time is too short, the two pieces of metal may not completely bond.
Very often flash butt welding is controlled by distance rather than time such that the flashing would occur for a pre-determined length, say 5mm, before the upsetting cycle starts. Upsetting may then also be controlled by distance. A parameter would be set to apply the upsetting force until a certain distance has been upset. It is generally the upsetting distance that is more important than the upsetting time.
At the end of upsetting there is commonly a 'hold time' during which the joint is held still to allow the joint to cool and the two pieces of metal to completely bond.