High- and low-cycle fatigue behavior of additively manufactured Ti6Al4V and influence of surface finish

Ti6Al4V has emerged as one of the most promising materials for Additive Manufacturing (AM). In this work, three series of AM Ti6Al4V samples were employed to assess the material fatigue behavior and its dependency on surface finish. One series was left in as-built conditions, one was machined, and one was finely polished. No samples underwent heat treatment, and their dimensions were chosen to be representative of the characteristic strut size of AM reticular structures. The as-built and machined series were characterized in terms of roughness, monotonic, Low-Cycle Fatigue (LCF) and High-Cycle Fatigue (HCF) behaviors. The polished series was intended to determine the fatigue limit under ideal surface finish conditions, undergoing roughness and HCF tests only. This allowed the determination of the fatigue limit reduction factors due to surface finish, providing designers with a practical tool for evaluating the effect of surface roughness on the fatigue performance of AM Ti6Al4V. Besides, scanning electron microscopy and fracture mechanics analysis through Kitagawa-Takahashi diagrams, based on Murakami's theory, were performed. Different laws correlating defect size and fatigue limit of defective components were compared. By transforming roughness into an equivalent defect, further insight into the effect of surface finish on fatigue performance was gained.