What is hooking defect in FSW?

Hooking is a defect in the surface of the material that can occur in FSW welding operations. The material rises on the edges of the feed zone and it thins on the receding side. This internal defect can be detected by macrography.
The hooking is mainly due to a lack of stirring creating a reduction of the welded section.
This defect of non-welded zones leads to a decrease in the mechanical resistance of the weld to tensile stress. To avoid hooking, it is important to use the appropriate FSW tools and make the appropriate settings.