Abstract: During aircraft, ship, and automobile manufacturing, lap structures are frequently produced among Al alloy skins, wall panels, and stiffeners. The occurrence of welding defects severely decreases mechanical properties during friction stir lap welding (FSLW). This study focuses on investigating the effects of rotation rate, multipass welding, and cooling methods on lap defect formation, microstructural evolution, and mechanical properties. Hook defects were eliminated by decreasing welding speed, applying two-pass FLSW with a small welding tool, and introducing additional water cooling, thus leading to a remarkable increase in effective sheet thickness and lap width. This above strategy yielded defect-free joints with an ultrafine-grained microstructure and increased tensile shear force from 298 to 551 N/mm. The fracture behavior of FSLW joints was systematically studied, and a fracture factor of lap joints was proposed to predict their fracture mode. By reducing the rotation rate, using two-pass welding, and employing additional water cooling strategies, an enlarged, strengthened, and defect-free lap zone with refined ultrafine grains was achieved with a quality comparable to that of lap welds based on 7xxx Al alloys. Importantly, this study provides a valuable FSLW method for eliminating hook defects and improving joint performance.