Unnotched fatigue of Inconel 718 produced by laser beam-powder bed fusion at 25 and 600°C

Porosity—and hence poor fatigue performance—is a major concern for additively manufactured (AM) alloys using powders as the raw materials. Since the microstructures in these alloys are also often unique, a detailed understanding of the interactions between the fatigue cracks emanating from the pores and the microstructural features is necessary for the successful deployment of the AM alloys in industry. Keeping this in view, the microstructures and mechanical properties of the as-built Inconel 718 alloy, produced using laser beam powder bed fusion (LB-PBF) process, were investigated at room temperature (RT) and 600 °C. Emphasis was on the high cycle fatigue behavior evaluated using the rotating bend fatigue tests, and the role of the lack of fusion pores (LOFs) on the fatigue resistance. The experimental results show that the unnotched fatigue strength (σf) at 600 °C is 23% lower than that at RT. This was due to the lower work hardening rate at 600 °C, which facilitates easy crack initiation at LOFs that both favorably located and oriented. For stress amplitudes (σa) higher than σf, however, dynamic recrystallization at the crack tip regions of the specimens fatigue tested at 600 °C retards the short fatigue cracks (SFCs) and leads to a substantially higher fatigue life as compared to that at RT. Postmortem analyses were for understanding the initiation and growth mechanisms of SFCs and the roles of plasticity/oxidation induced crack closure on them. It shows that the LB-PBF induced microstructural characteristics such as solidification cells with high dislocation density are effective in resisting the growth of SFCs at 600 °C, as compared to RT. These results further highlight the roles of the unique microstructural aspects of additively manufactured alloys on the fatigue resistance, especially at high temperatures.