What is flash defect in FSW?

In welding, flash refers to the burr on the retreating side due to the rotation of FSW tool.
The flash is the main defect that can be encountered during a friction stir welding operation. Flashes occur mainly when too much heat is applied to FSW butt or lap welds. This intense heat is due to a high welding effort or too much penetration. Moreover, flashes could be also due to the gap between the welded parts. Indeed, in presence of gap bridging, the FSW tool plunges more (thanks to the force control) in order to fill the void between the parts and to avoid any hole defect (wormhole).

These flashes are visible to the naked eye on the joint edges. It is more a question of an aesthetic imperfection. Indeed, this loss of plasticised material on the edge of the shoulder has no impact on the strength of the weld.