Enhanced mechanical properties in 7075 Al alloy fasteners processed by post ECAP-CU aging

The reasonable selected aging scheme plays an important role in regulating the mutual influence between precipitation phenomena, grain size, dislocation density and mechanical properties in precipitation-hardenable alloys after deformation. In this research work, 7075 aluminium (Al) alloy fasteners with fine-grain structure were manufactured at room temperature using equal channel angular pressing (RT-ECAP) and cold upsetting (CU) composite deformation. Post-aging heat treatment was conducted to study the precipitation behavior of 7075 Al fasteners after composite deformation using additional characterization techniques such as X-ray diffraction, differential scanning calorimetry, transmission electron microscopy. The mechanical properties of 7075 Al specimens were investigated using a universal tensile testing machine and a microhardness tester. The synergistic effects of ECAP-CU and post-aging on the microstructure and mechanical properties of 7075 Al alloy fasteners were investigated, the contribution of each strengthening mechanism was quantitatively calculated and finally the optimal post-deformation aging regime was determined. The results show that after ECAP-CU composite deformation and combined with post-aging, work-hardening and age-strengthening are maximally enhanced with limited dynamic recovery. The size, volume fraction of precipitates and dislocation density of 7075 Al alloy after AA82 (353K-20min) post-aging were 60.7 +/- 5.1 nm, 3.9 % and 1.7 x 1015 m-2, respectively. The microhardness of the samples after ECAP-CU combined with the AA82 aging regime (218.5 +/- 0.9HV) was approximately 77.9% higher than the microhardness of the initial specimens (122.8 +/- 2.0HV). In addition, the strength of the 7075 Al alloy in composite deformation combined with the AA82 post-aging regime was quantitatively calculated to reach 678 MPa within a finite error. In the ECAP-CU composite plastic deformation process combined with the AA82 low-temperature short-time post-aging treatment, the synergistic effects of work hardening and precipitation strengthening are maximized, significantly enhancing the mechanical properties of Al alloy fasteners, which can be used as an effective low-temperature short-time deformation post-aging scheme. This study provides a new idea for ECAP-CU composite deformation combined with low-temperature short-time post-ageing heat treatment to achieve further strengthening of 7075 aluminum alloy fasteners.