Investigation on fatigue failure of split-sleeve cold expansion holes of 7075-T651 aluminum alloy

Abstract

7075-T6 is widely used in the manufacture of aircraft structural components, the majority of which are joined and assembled through hole structures. However, the geometric discontinuity near the fastener holes facilitates stress concentration and makes the joints susceptible to crack nucleation and fatigue failure. It is essential to improve the fatigue resistance of the hole structure. In this paper, the factors affecting the split-sleeve cold expansion on the fatigue behavior of fastener holes of 7075-T651 are investigated based on the compressive residual stress, microstructure, surface roughness, and fatigue fracture. Results revealed that cold expansion causes sub-grain refinement near the hole wall, which increases the surface micro-hardness and favors inhibition of crack growth. Grain refinement, plastic deformation on the surface of the holes, and residual compressive stress produced by the cold expansion process all work together to extend the fatigue life of the hole structure. The constituent particles and microstructure after expansion are important factors affecting fatigue performance, higher level of Fe content could result in the formation of a large number of iron-bearing particles leading to the acceleration of the failure of the workpiece.