Non-destructive Observation of Internal Fatigue Crack around the Nugget of Friction Stir Spot Weld Using X-Ray µCT Scan

Abstract

Lap-shear joints of aluminum alloy A6061-T6 sheets were fabricated by a friction stir spot welding (FSSW) technique using a conventional tool with a screwed probe (probe tool) and a scroll-grooved shoulder tool without a probe (scroll tool). Tensile-shear fatigue tests were conducted using the fabricated FSSW joints and non-destructive observation using X-ray computed tomography (CT) scan was performed on the fatigued FSSW joints to investigate the fatigue crack initiation and propagation behavior around the nugget. The failure modes were dependent on the applied load and the tool geometry, in which shear failure through the nugget at higher load and base metal failure of the lower sheet at lower load occurred in the joint fabricated by a scroll tool (scroll tool joint). On the other hand, plug failure at higher load, base metal failure at middle load and plug failure combined with shear fracture at lower load occurred in the joint fabricated using a probe tool (probe tool joint). Fatigue crack initiated in the lower sheet at the edge of nugget and propagated along the nugget and in the thickness direction in the scroll tool joint. The fatigue crack initiation life was about 33~44% of the total fatigue life. In the probe tool joint, fatigue crack initiated in the upper sheet around the nugget in the early stage of fatigue life, and subsequently the other fatigue crack initiated in the lower sheet at the edge of the nugget. The fatigue crack initiation life in the lower sheet was about 55~66% of total fatigue life.