Determining critical surface features affecting fatigue behavior of additively manufactured Ti-6Al-4V

Abstract

The fatigue life of additively manufactured metallic parts in the unmachined surface condition is typically shorter than those in machined surfaces, primarily due to the presence of surface micro-notches. Methods relying on conventional surface roughness parameters to account for the severity of the unmachined surfaces tend to give a rather “averaged” description, overlooking the localized nature of fatigue damage, thus they do not adequately capture the criticality of individual, fatigue critical surface valleys and their morphology. This study utilized a fracture mechanics based approach to identify the key surface features influencing the fatigue performance of laser powder bed fused (L-PBF) Ti-6Al-4V. X-ray computed tomography was employed to detect surface and sub-surface flaws, and a new method to capture and quantify the geometry of surface micro-notches—such as width, depth, opening angle, and radius of curvature—was proposed. These geometrical features, when integrated with classical stress concentration formulae, were shown to adequately represent the fatigue criticality of each notch. The calculated stress concentration was used with a fatigue notch factor model to predict the fatigue life of unmachined L-PBF specimens with a reasonable accuracy.