Multi-process regulation of surface residual stress for improving SCC resistance of Al-Zn-Mg-Cu alloy high-lock nuts

Aerospace high-lock nuts, as core aircraft fasteners, face severe stress corrosion cracking (SCC) risks during long-term service, threatening flight safety. This study investigates the effects of closing, stress relief aging and shot peening on residual stress, SCC resistance and microstructure of high-strength Al-Zn-Mg-Cu alloy M8 high-lock nuts. Residual stress was measured via X-ray diffraction, stress corrosion behavior was evaluated through cyclic immersion tests, and microstructural characteristics were analyzed using OM, SEM, EBSD and TEM. The results show that the combined process of closing + stress relief aging + shot peening achieves a uniform and stable residual compressive stress layer with an average value of −88.2 MPa, and the best stress corrosion resistance, with a corrosion area fraction of less than 5% and shallow corrosion pits with a depth of about 162.7 μm. Mechanistically, the synergistic effect of uniform high residual compressive stress and optimized grain boundary microstructure effectively inhibits the transition from pitting to SCC. The proposed process parameters are fully compatible with existing industrial production lines and can be directly applied to the manufacturing of Al-Zn-Mg-Cu alloy aerospace high-lock nuts, providing a reliable technical solution for improving the service life of aerospace fasteners.