Fracture and fatigue in additively manufactured metals

Additive manufacturing (AM) of metallic components offers many advantages over conventional manufacturing methods, most notably design freedom at little material waste. Consequently, there is significant current interest in the manufacturing aspects of a wide variety of structural alloys. Concomitantly, establishing the processing – microstructure – mechanical performance relations, in conjunction with the attributes such as flaws, residual stresses, and mesostructures inherent to the AM processes, is critical for the widespread adoption of structural metallic components made using AM. Keeping this in view, a comprehensive review of the current understanding of the structure-property correlations in AM alloys is provided here. Unique aspects of the microstructures of the AM alloys, process-related attributes, and their effect on the tensile, fracture, fatigue crack growth, and unnotched fatigue properties are highlighted, with emphasis on the interplay between the microstructures and process attributes in determining the structural integrity of AM alloys in terms of properties such as near-threshold fatigue crack growth rate, fracture toughness, and fatigue strength. These aspects are contrasted with respective structure-property correlations in wrought or cast alloys. Strategies employed for improving the damage tolerance of the alloys through either improvisation of the processing conditions during AM or via post-processing treatments such as annealing, hot-isostatic pressing, and shot peening, are summarized. The existing gaps in understanding fatigue and fracture in AM alloys, which are critical for widespread deployment and reliable design of engineering components, are identified; such gaps are expected to provide future avenues for research in this area.