Strategy and mechanism to improve the fatigue properties of Ti6Al4V ELI alloy by microstructure modulation combined with surface strengthening process

The hot isostatic pressing (HIP) technique allows obtaining powder metallurgy Ti6Al4V parts with high densities, but its microstructure is not ideal, resulting in a lower fatigue life than expected. In this work, the ideal microstructure was obtained by heat treatment, but this process produces thermally induced porosity (TIP). After heat treatment, the fatigue strength of the bimodal Ti6Al4V alloy reached 680 MPa at 107 cycles, whereas the HIP Ti6Al4V alloy with a fully equiaxed microstructure was lower than 590 MPa. This is because the bimodal Ti6Al4V alloy has better resistance to fatigue crack initiation. This result confirmed that TIPs do not have a significant negative impact on fatigue properties. To further improve the fatigue performance, this study induced gradient nanostructured (GNS) in Ti6Al4V alloy by ultrasonic surface rolling process (USRP), and the fatigue strength of the USRP-treated specimen reached 750 MPa at 107 cycles. Combining microstructure observations and theoretical analysis, the surface hardening mechanism of the USRP-treated Ti6Al4V alloy was quantitatively described. The results showed that the plastic deformation layer is approximately 130 mu m, but grain boundary strengthening maintained its effect to a depth of about 200 mu m whilst dislocation strengthening maintained its effect to a depth of approximately 350 mu m. Under the comprehensive effect of surface hardening and compressive residual stress field, the fatigue crack initiation period of the USRP specimen was significantly extended. This may be the primary reason behind the enhanced fatigue performance obtained through USRP treatment.