An Approach to Improving the Mechanical Properties of Friction Stir Spot Welded Joints on the Basis of Hook Formation Mechanism

Background: Partial metallurgical bond (namely 'hook') is formed between the overlapped metal sheets duringfriction stir spot welding (FSSW). The geometry of hook is found to significantly affect the mechanicalperformance 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. Thecross-sectional morphology of each spot weld was investigated to clarify the material flow behavior and deducethe formation mechanism of hook. The tensile shear strength and fracture features were examined to reveal theeffect of hook geometry on the mechanical properties. Results: The weld geometry affects the tensile shear strength of FSSWed joints by determining their fracturemodes. The formation mechanism of hook is deduced by a material flow model. In the tool-plunging stage, thefaying 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 themechanical properties of welds. If the hook endpoint has not yet reached the low-pressure zone at the end ofwelding process, welds with ideal hook geometry can be obtained. Target friction stir spot welds were producedby the use of a tool possessing smaller pin diameter.