Abstract: Manufacturing defects have been an unavoidable feature of additive manufacturing (AM) processed metals:typically these comprise gas pores and lack of fusion defects. The applications of optimized process parameters and post-AM heat treatment are able to reduce these defects to a certain degree. Unfortunately, to date there is no effective way to completely eliminate them. Defects can have a detrimental effect on the fatigue strength and life of a material since they act as potential crack initiation sites due to high stress concentration, posing a significant threat to the structural integrity of AM processed components. The research progress of AM defect behavior is summarized from five aspects:static defect characterization, dynamic defect evolution, defect classification, defect-fatigue strength design and defect-fatigue life evaluation. First, the inherent manufacturing defects induced by AM are characterized and quantified using X-ray computed microtomography in a three-dimensional and non-destructive way. Special studies on the spatial defect or crack evolution behavior are also reviewed by using a novel in situ synchrotron X-ray computed microtomography during cyclic loading in an in situ, real-time and dynamic way. AM defects are characterized by global distribution, diverse morphologies and large size spans. Six ranking strategies, having varying levels of complexity, are proposed to estimate the threat posed by different defects. Within the framework of defect tolerance and damage tolerance, some methods are developed to evaluate the fatigue strength and lifetime in terms of the defect geometry at the surface, subsurface, and in the interior of the materials. Finally, it is pointed out that the data-driven high-throughput testing approach and machine learning algorithms as well as the multi-scale & multi-physics numerical simulation are of vital significance for the integrated exploration on the process design-defect characterization-performance evaluation of AM processed metals.

Key words: additive manufacturing, in-situ X-ray tomography, defect characterization and classification, crack initiation and propagation, fatigue strength and life assessment