Friction heating is generated by rubbing the faying surfaces to be joined together, under an axial force. This rubbing action causes softening of a region either side of the weld interface which is gradually extruded or expelled away from the interface as the welding force is maintained. The circumferential collar of extruded material from either component rolls back from the weld interface forming a rams horn configuration, known as a bifurcated flash formation.
In production, this flash formation is very often sheared off whilst still hot and soft. The appearance of an abnormal flash can indicate that the parameters used for a particular weld are not ideal.
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