Effect of Aging Time on Tensile Properties of 7075 Aluminum Alloy

A solid solution treatment (SST) followed by single-stage aging (0–30 h, 140 °C) was performed on 7075 aluminum alloy specimens with longitudinal axes oriented at 0°, 45° and 90° to the rolling direction. The mechanical properties and microstructure were analyzed by tensile testing, optical microscope (OM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the average tensile strengths of the as-received 7075 aluminum alloy in the three directions were 304 MPa (0°), 295 MPa (45°) and 297 MPa (90°), respectively, with an anisotropy index (AI) of 0.97, indicating that the as-received samples exhibited negligible anisotropic mechanical properties. After SST, elongated grains with coarse size were formed, which is primarily attributed to the inheritance of the deformed fiber texture introduced by hot rolling. EBSD analysis of the 30 h aged specimens revealed that, within the same analyzed area, the total grain-boundary length in the 45° direction (16.4 cm) was much larger than that in the 0° (10.4 cm) and 90° (13.5 cm) directions. As the grain morphology showed no significant change between the SST and aged conditions, this grain-boundary distribution was representative of the microstructural state established during SST and persisted throughout the artificial aging process, contributing to the anisotropic mechanical properties. During artificial aging, prolonged aging time significantly facilitated the precipitation, with the 30 h aged sample exhibiting a significantly higher density of precipitates compared to the 6 h aged sample, leading to enhanced mechanical properties. The tensile strengths of the 30 h aged samples increased to 165 MPa, 236 MPa and 196 MPa in the three directions, respectively. Meanwhile, due to the fixed crystallographic orientation relationship between the precipitates and the Al matrix, the precipitates tended to form on specific planes, which enhanced the anisotropic mechanical properties. Consequently, the AI value increased from 0.97 (as-received) to 1.43 (30 h aged) with prolonged aging time.