Superior high-cycle fatigue property through in-situ precipitation of α' martensite in additively repaired titanium alloy

TC4-DT titanium alloy is commonly used in aircraft airframe structures due to its high strength-to-weight ratio. However, these parts are susceptible to fatigue failure during their service life. In this study, the damaged TC4-DT titanium alloy components were repaired using laser directed energy deposition (LDED) by depositing TC4 powders with a side repairing method, and the effects of the repair process on microstructural evolution, residual stress together with high-cycle fatigue strength were comprehensively investigated through multiscale characterization techniques. Results showed that the high-cycle fatigue strength of the repaired specimen was significantly increased by 11.9% compared with the substrate material, from 404.3 MPa to 452.6 MPa, and this was ascribed to its unique microstructural characteristics, especially the in-situ precipitation of α ` martensite resulting from the rapid cooling involved in the LDED process. Furthermore, the dense repaired microstructure, the surface residual compressive stress, and the numerous dislocations induced by phase transformation, combined with the Hall-Petch effect caused by the formation of α lath, hindered the dislocation motion and delayed the strain localization, which significantly inhibited crack initiation and greatly contributed to the fatigue performance. This work demonstrates the potential of additive manufacturing to repair alloys with superior mechanical properties for structural applications.