Notch tensile response of mini-regions in AA2024 and AA2524 friction stir welds

Abstract

For the first time, experiments are conducted on solid-state FSW joints in a pair of aerospace alloys to determine how the process affects notch–strength and notch sensitivity. In this study, AA2524 and AA2024 FSW welds using notched tensile specimens are used to evaluate the notch tensile strength (NTS) of specific regions with different hardness values in the weld process zone. For the FSW process parameters used in this study, the response of the unnotched weld nugget specimen incurs a 4% loss in ultimate tensile strength (UTS) while retaining good ductility in comparison to the base metal. However, when a notch is introduced, the nugget experiences a reduction of ∼13% in the NTS while notch fracture strain (NFE) decreased by 40%. Fractographic results show that the existence of a banded microstructure in the nugget region is a primary factor affecting the reduction in NTS and NFE in the weld nugget. For notched specimens located at both the nugget–HAZ boundary and at the minimum hardness (HAZ), the measured reduction in NTS is directly correlated to a greater amount of inter-granular fracture in these regions due to over-aging in the welding process. Finally, when comparing local mechanical properties for both notched and unnotched specimens, the notch–strength ratio for all FSW zones in the AA2024 and AA2524 FSW joints, NSR FSW zone = (NTS/UTS) FSW zone > 1.00, demonstrating that the entire FSW process zone in AA2024 and AA2524 is notch insensitive.