<p>High-locking nuts are critical components in the aerospace industry, typically undergoing closure and aging treatment to optimize their mechanical properties and service reliability. This study systematically investigated the microstructural characteristics and mechanical behavior of 7075 Al alloy high-locking nuts under three conditions: non-processed, nut closure, and aging after closure. The results demonstrate that the aging treatment homogenized the distribution of refined grains within the closure region, increased the proportion of high-angle grain boundaries and stabilized the microstructure. These microstructural changes enhanced the nuts’ mechanical resistance and suppressed crack propagation initiated during closure. At the same time, during aging, coarse second-phase particles partially dissolved and preferentially redistributed along grain boundaries, leading to grain boundary strengthening. The synergistic effect of grain refinement and second-phase precipitation increased the hardness of the high-locking nuts by 14.80%. Furthermore, while the closure treatment ensured the required locking performance, the subsequent aging treatment effectively reduced residual stresses and dislocation density within the closure region. By combining the closure treatment with T73 aging, this study reveals the mechanism by which post-closure aging regulates the mechanical behavior of high-locking nuts, providing a scientific basis for process optimization.</p>