Flash Butt Welding-Flashing
The phenomenon of flashing is of extreme importance in achieving satisfactory welds. Flashing can begin only after a molten bridge has been formed somewhere along the interface. If the parts fit accurately, difficulty in initiating the process may be experienced and it may be necessary to level the butting surfaces to give local contact. This procedure is much different from that for resistance pressure welding where a good fit is desirable to exclude the atmosphere and prevent surface contamination. In the flash welding, surface contamination is removed in the spatter during flashing and molten metal is expelled in the final upset or forging operation.
Once a molten bridge has been formed it is set in motion over the interface by the electrodynamic forces resulting from the distortion of the current path through the work pieces. While it traverses over interface at a speed of 10 cm / s, more heat is generated in the bridge by the joule effect and is lost to the colder metal at each end. Assuming the bridge does not freeze, it will be broken in one of two ways. It may either explode because of the combined influence of overheating and the pinch effect or it may race to the edge of the work piece where it becomes extended in the form of an arc between the work faces until it ruptures. In either event, much of the metal contained in the bridge is propelled out of the joint as spatter.
Because, there is inevitably considerable inductance in the secondary circuit, collapsing of the electrical field at rupture results in a voltage surge which is capable to initiate an arc, although the open circuit voltage cannot sustain it and it is rapidly extinguished.
The Considerable heat is generated in these brief arcing periods which accounts for the efficiency of the process compared with resistance butt welding.