Tensile strength, hardness, and joint efficiency of similar AA2024 and AA7075 friction stir welded joints with post-weld treatment

Although Friction Stir Welding (FSW) reduces defects associated with conventional fusion welding of high-strength aluminium alloys such as AA7075 and AA2024, the resulting joints may still exhibit microstructural heterogeneity, residual stresses, and reduced joint efficiency. Limited studies have investigated cryogenic treatment as a post-weld technique to enhance the performance and structural integrity of similar AA7075 and AA2024 joints. Therefore, this study investigates the effects of cryogenic treatment on the thermal behavior, mechanical properties, and joint efficiency. Similar joints were fabricated using FSW parameters with three different shapes of pin tools. The joints were exposed to cryogenic treatment in liquid nitrogen. The mechanical characteristics of the weldments were evaluated using Vickers microhardness and tensile tests. Thermal imaging indicated that the maximum welding temperatures persisted significantly lower than the melting temperatures of both alloys, with a peak registered temperature of 311.1 °C for AA7075, thereby promoting dynamic recrystallization within the stir zone. Mechanical testing displayed that cryogenic treatment greatly improved the ultimate tensile strength, increasing from 430 MPa to 485 MPa for AA7075 joints and from 360 MPa to 405 MPa for AA2024. Furthermore, in the stir zone, the hardness was upgraded by 15%, reaching 220 HV for AA7075 and 150 HV for AA2024. The joint efficiency was also enhanced by 12.5–12.7%, reaching 83.52% for AA2024 and 82.96% for AA7075. The findings show that the combination of FSW and cryogenic treatment is a simple method to improve the mechanical reliability and structural integrity of aluminium alloy.