The quality of a flash weld is influenced by materials and process factors.
Specific material factors arise when the parent material is highly inhomogeneous, for example steels containing long stringer inclusions or those that are heavily banded or have rolling defects. The upset at the weld line alters the fibre direction of the structure and this may lead to reduced ductility and the formation of hook cracks.
Flashing voltage and speed are generally selected to give good flashing action and sufficient heat build-up in the weld area. When heating is too low it is not possible to produce sufficient upset to achieve a sound forged weld. When the flashing is coarse or intermittent, there is a risk of forming inclusions and cratering in the weld region, leading to flat spot flaws. The flashing time must also be set such that the material is softened sufficiently to allow proper upset, without becoming so soft that it collapses. Overheated metal that is not properly forged out during upset may be susceptible to embrittlement due to grain boundary liquation of sulphides, for example. Fracture in this area is known as break-up.
The current after flashing, during the first part of the upset, must be of sufficient duration to allow the upset to be fully achieved, but not cause overheating of the weld area.
The upset force, upset distance and rate of upset must be set so as to introduce sufficient deformation while the material is plastic to expel the molten material and contaminants from the joint interface.