Friction stir welding of 2060-T8 Al-Li alloy. Part II: Tensile fracture behavior

For friction stir welded (FSW) joints of conventional precipitation-hardened aluminum alloys, the lowest hardness zone (LHZ) was located at heat affected zone (HAZ) and determined the tensile fracture behavior with the fracture occurring at the LHZ. For FSW 2060-T8 joints, however, the LHZs were located at the nugget zone (NZ) under 400 rpm-200 mm/min, but at the HAZ under 800 rpm-200 mm/min, 1200 rpm-200 mm/min and 1200 rpm-50 mm/min. While the FSW 2060-T8 joints normally fractured along the LHZ under 400 rpm-200 mm/min and 1200 rpm-50 mm/min, abnormal fracture occurred at the thermo-mechanically affected zone (TMAZ) under 800 rpm-200 mm/min and 1200 rpm-200 mm/min. The FSW 2060-T8 joints with 800 rpm-200 mm/min and 1200 rpm-200 mm/min presented obvious diversity of crystal orientation and grain morphology, and subsequent deformation in the LHZs, the lower TMAZ and upper TMAZ. The large Schmid factor, coarse grains and delta'/beta' precipitates caused the preferential deformation and planar slip at the lower TMAZ, weakened the mechanical properties of the TMAZ, and therefore resulted in the abnormal failure at the TMAZ of FSW 2060-T8 joints under 800 rpm-200 mm/min and 1200 rpm-200 mm/min.