Hook defect is one of the trickiest issues for friction stir lap welding, which remains to be resolved. In this study, a designed pin with an enlarged-end was proposed to control the interfacial hook defect by optimizing its morphology orientation. The insert aduncous structure at the advancing side is dissected by multilayer metallographic observation, which is the main character to terminate the inner-ward extension of the hook. The application of this tool was verified practical with a varied plunge depth of pin into the lower plate from 0.1 mm to 1.6 mm. The angle of the hook was small and terminated at the insert-structure or oriented toward the material convergence region. No joint fractured along the hook line and the highest joint efficiency reached 86％ of the parent 6082-T4 aluminum alloy.