Investigation on fatigue performance and microstructure of split sleeve cold expansion of TC4 holes

The exceptional performance of pivotal structural components serves to enhance the aircraft's efficient and secure operational profile. The structural components are connected and assembled primarily through fastener holes. However, discontinuities in structural components can lead to stress concentrations around holes, resulting in fatigue failure of the structural components. This is significant in enhancing the fatigue performance of the hole structure. In this study, TC4 titanium alloy was strengthened by split sleeve cold expansion (SCE) technique and its effect on residual stress, microstructure and fatigue fracture was investigated. The results indicate that SCE induces residual compressive stress at the hole edges, thereby inhibiting fatigue crack propagation. Geometrically necessary dislocations are formed at the interface between the alpha phase and beta phase to accommodate the strain gradient. Low angle grain boundaries (LAGBs) impede dislocation movement and accumulate dislocation density, leading to an increase in LAGB misorientation and subsequent transition to high angle grain boundaries. The SCE process led to the formation of subgrain boundaries and refined the grains in the material. The fatigue life of the material was extended by 6.2 times, with peak compressive residual stresses up to 244 MPa at the inlet side. The combined effects of residual compressive stresses, back stress and grain refinement contribute to improving the fatigue life of the hole structure.