Study on the Residual Stress and Fatigue Performance of Cold Expansion Hole on 7050-T7451

Cold expansion was employed on 7050-T7451 aluminum. Residual stresses were measured, fatigue tests were carried out for joints with different expansion, and the fracture surfaces of specimens were observed by scanning electron microscope (SEM). The results show that residual compressive stresses generate by cold expansion process. The maximum values of radial residual compressive stresses locate the middle of the inner wall of the hole and residual compressive stresses extend radially up to one diameter from the edge of the hole. Axial residual compressive stresses distribute near the exit. The fatigue lives N50 and N95,95 for specimen with strengthened hole with 5% expansion are respectively 3.54 and 3.68 times compared with non-strengthened hole. Fatigue fracture morphology shows the characteristics of mixed transgranular fracture and the propagation speed of the crack along horizontal direction is larger than vertical direction.