Additively Manufactured Inconel 718 Low-Cycle Fatigue Performance

Inconel 718 is one of the most used alloys within the aerospace gas turbine industry. The acceptance of Inconel 718 within the aerospace gas turbine industry has largely been due to its high strength and fatigue capabilities up to 677 degrees C (1250 degrees F). This alloy is traditionally produced through conventional manufacturing methods, such as casting, wrought, and sheet forming. The various traditional manufacturing methods of this alloy have been well understood and characterized for use in critical components. However, Inconel 718 can also be produced with non-traditional manufacturing methods, such as by additive manufacturing. Producing Inconel 718 by additive manufacturing has the opportunity to design more complex components that provide distinct advantages over conventionally produced components. However, prior to implementing additively manufactured Inconel 718 within the aerospace gas turbine industry, there needs to be a complete understanding of the material's performance. In an effort to completely characterize additively manufactured Inconel 718, this study focuses on the characterization of the alloy's low-cycle fatigue performance. Specimens were produced via the laser powder bed fusion process in a vertical orientation. Both as-printed surfaces and fully machined surface specimens were evaluated at 24 degrees C (75 degrees F) and 538 degrees C (1000 degrees F). Fractography analysis was then completed on the specimens to understand differences in the crack initiation and propagation with respect to test temperatures and surface conditions. Based on these tests, it was shown that the fatigue life knockdown due to the as-printed surface conditions was 62.8% at 538 degrees C (1000 degrees F) versus only 8.5% at 24 degrees C (75 degrees F). These findings are discussed in detail within this article, and future work is proposed.