Microstructural Evolution, Microhardness, and Residual Stress in FGH4097 Superalloy Subjected to Cold Expansion

Cold expansion is widely used to improve the fatigue resistance of aerospace structures with holes. To address the strengthening requirements of holes in FGH4097 superalloy components, this study systematically investigated the effects of cold expansion on microhardness, residual stress, and microstructural evolution. An L9(34) orthogonal design was used to evaluate the effects of the mandrel-hole interference level, lead angle, lubrication condition, and mandrel speed on the microhardness and residual stress of FGH4097. Electron backscatter diffraction, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used to characterize the deformation microstructure and explore strengthening mechanisms. Results show that cold expansion produced a pronounced radial microhardness gradient and compressive residual stress field. The maximum near-hole microhardness was 538 HV, approximately 36% higher than that of the base material, and the largest compressive residual stress of −629.4 MPa was observed in the same specimen. Range analysis identified the mandrel-hole interference level as the dominant processing parameter for both microhardness and residual stress. The near-hole region exhibited increased local misorientation, microstrain, and dislocation density, and dense dislocation tangling and piling-ups at γ/γ′ interfaces were observed. The microhardness increase can primarily be attributed to dislocation multiplication, dislocation interactions, and the impediment of dislocation motion by γ′ precipitates.