Fatigue life extension of additively manufactured Nickel-base 718 alloy by nanostructured surface

In this study, the beneficial effect of ultrasonic nanocrystal surface modification (UNSM) treatment at room and high temperatures (RT and HT) on the mechanical properties and fatigue performance of Nickel-base 718 alloy that is fabricated by laser directed energy deposition (LDED) process was investigated. UNSM treatment at RT and HT demonstrated a significant enhancement in fatigue performance by approximately of 43% and 57%, respectively. This can be attributed to the introduction of a high compressive residual stress (CRS) and the generation of nanostructured surface layer. Moreover, it was found that the surface roughness reduced, while the hardness increased after UNSM treatment at RT and HT. Furthermore, it was found by analysing the fatigued surfaces that the main crack initiation site was located at the subsurface for the UNSM-treated samples, while it was located at the top surface for the untreated ones. Introduction of a high CRS by UNSM treatment at RT and HT shifted a crack initiation site into subsurface.