Effects of Stress Amplitude and Cold Expansion Strengthening on the Fatigue Property of TC4 Titanium Alloy

Effects of stress amplitude and cold expansion strengthening treatment on TC4 titanium alloy by performing stress-controlled high-cycle fatigue (HCF) tests are studied. Results show that the fatigue life increases with the decrease in stress amplitude. The test at the small stress amplitude of 200 MPa tends to enter the very high-cycle fatigue domain with a life at the order of 10 7 . Moreover, the fatigue life gets significantly increased by about 60-5% after cold expansion strengthening treatment with the expansion ratio (ER) equal to 2% at the stress amplitude of 400 and 250 MPa. The fracture morphology is characterized by scanning electron microscope (SEM). The crack generally initiates at the inner edge of the hole wall for the specimen without cold expansion treatment and the number of crack initiation site decreases with the decrease in stress amplitude; however, the crack nucleates at the entrance of mandrel. Moreover, the hardness of strengthened layer was increased by 33% determined by nanoindentation tests. Furthermore, the finite element analysis is performed to simulate the cold expansion process and determine the stress distribution around the hole to provide an explanation for the improved fatigue life.