Review on Fatigue of Additive Manufactured Metallic Alloys: Microstructure, Performance, Enhancement, and Assessment Methods

Additive manufacturing (AM), which is a process of building objects in a layer-upon-layer fashion from designed models, has received unprecedented attention from research and industry because it offers outstanding merits of flexibility, customization, reduced buy-to-fly ratio, and cost-effectiveness. However, the fatigue performance of safety-critical industrial components fabricated by AM is still far below that obtained from conventional methods. This review discusses the microstructural heterogeneities, randomly dispersed defects, poor surface quality, and complex residual stress generated during the AM process that can negatively impact the fatigue performance of as-printed parts. The difference in microstructural origin of fatigue failure between conventionally manufactured and printed metals is reviewed with particular attention to the effects of the trans-scale microstructures on AM fatigue failure mechanisms. Various methods for mitigating the fatigue issue, including pre-process, inter-process, and post-process treatments, are illustrated. Empirical, semi-empirical, and microstructure-sensitive models are presented to predict fatigue strength and lifetime. Summary and outlooks for future development of the fatigue performance of AM materials are provided.