Background: Partial metallurgical bond (namely 'hook') is formed between the overlapped metal sheets during
friction stir spot welding (FSSW). The geometry of hook is found to significantly affect the mechanical
performance of FSSWed joints, while that how to adjust hook geometry to a better state remains to be studied.
Methods: The conventional FSSW joints under different plunge depths and dwelling time were obtained. The
cross-sectional morphology of each spot weld was investigated to clarify the material flow behavior and deduce
the formation mechanism of hook. The tensile shear strength and fracture features were examined to reveal the
effect of hook geometry on the mechanical properties.
Results: The weld geometry affects the tensile shear strength of FSSWed joints by determining their fracture
modes. The formation mechanism of hook is deduced by a material flow model. In the tool-plunging stage, the
faying interface is broken by upward-flowing materials, hook is therefore initiated and driven up gradually.
During the tool-dwelling stage, hook continues to migrate to the low-pressure zone, surrounding the stir zone.
Conclusion: The uncertainty of crack-propagating endpoint along hook makes it difficult to ensure the
mechanical properties of welds. If the hook endpoint has not yet reached the low-pressure zone at the end of
welding process, welds with ideal hook geometry can be obtained. Target friction stir spot welds were produced
by the use of a tool possessing smaller pin diameter.