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 degrees 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 (sigma(f)) at 600 degrees C is 23% lower than that at RT. This was due to the lower work hardening rate at 600 degrees C, which facilitates easy crack initiation at LOFs that both favorably located and oriented. For stress amplitudes (sigma(a)) higher than sigma(f), however, dynamic recrystallization at the crack tip regions of the specimens fatigue tested at 600 degrees 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 degrees 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. (C) 2021 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

This study investigated the microstructures, mechanical properties, and high cycle fatigue behavior of Inconel 718 alloy produced by laser beam powder bed fusion (LB-PBF) process at room temperature and 600 degrees C. The results showed that the fatigue strength at 600 degrees C was lower than that at room temperature due to the lower work hardening rate. However, dynamic recrystallization at the crack tip regions in specimens tested at 600 degrees C resulted in a higher fatigue life for stress amplitudes higher than the fatigue strength. The microstructural characteristics induced by LB-PBF were effective in resisting the growth of fatigue cracks at high temperatures.