Analysis of Cracking of 7075 Aluminum Alloy High-Lock Nuts

A 7075 aluminum alloy high-lock nut developed multiple cracks after 3 years of exposure to atmospheric conditions. To identify the root cause of the cracking, a comprehensive analysis was conducted, including chemical composition, macro- and micro-fracture analyses, microstructural analysis, mechanical performance verification, and residual stress testing. The results indicated that stress corrosion was the cause of the fractures. After assembly, the crimping part of the high-lock nuts exhibited significant residual tensile stress and stress concentration, which led to stress corrosion in the industrial atmospheric environment. A comparison of the residual tensile stress in high-lock nuts with wall thicknesses of 0.75 mm, 1.00 mm, and 1.25 mm revealed that the residual tensile stress at the crimping part decreased as the wall thickness increased. Additionally, stress corrosion testing demonstrated that high-lock nuts with a wall thickness of 1.25 mm did not undergo stress corrosion within 30 days.