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 toolplunging
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 lowpressure
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.