State-of-the-art survey of literature for finding an effective solution to mitigate the exit-hole defect in friction stir welded samples

Friction stir welding is a solid-state welding process among the most important processes used for welding materials that the conventional welding process cannot weld. The joining of two metals occurs by heat generated due to the friction between the non-consuming rotating tool surface and workpiece. This technology is extensively used in the automobile, railway, maritime and aerospace industries. There are some defects that occurred in this process such as the tunnel, kissing bond, incomplete root penetration, excessive flash and void, those can be overcome by modifying the process parameters namely tool rotational speed, welding speed, downward force, shoulder diameter, shoulder geometry, tool pin geometry, tool pin diameter, tool pin length and tilt angle. The inevitable defect is the exit-hole left after the retraction of a rotating tool at the end of the weld cycle. The present work reviews different repair techniques for the exit-hole of the welded joints. This article discusses various exit-hole repairing techniques pertinent to the method, process parameters, material, plate thickness and the defect that emerges using these techniques. The current survey also presents a few future research opportunities in this field that can be carried out to mitigate the exit-hole defect.