Investigating the Influence of Scan Strategy on the Fatigue Resistance of Additively Manufactured Thin-wall Inconel 718

Investigating the influence of scan strategy on the fatigue resistance of additively manufactured thin-wall Inconel 718 Additive manufacturing (AM) holds tremendous promise for significantly reducing the cost to manufacture parts with complex geometries. One major hurdle to realizing this promise is the development of robust process-structure-property models that consider the unique processing conditions of AM. Unlike traditional materials, broadly applicable models are difficult if not impossible to generate due to the vast complexities present in AM which can vary layer by layer and are dependent on the local geometry. Instead, individual alloys, printing techniques, and process parameters must be studied. Results from such studies can then be leveraged to optimize AM processing routes for engineering applications as well as inform future studies for other alloys and print strategies. This proposed research seeks to elucidate the manner in which small scale features of additive parts (e.g. thin walls) possess disparate microstructure and properties from areas toward the center of the part for Inconel 718 (IN718) fabricated using laser powder bed fusion. Inconel 718 is a precipitation-strengthened Ni-based alloy used in a wide variety of applications both for NASA and throughout the aerospace industry due to its strength and high temperature resistance. Microstructural characterization will quantify the grain size/shape, phases present, segregation of elements, and porosity; all of which are expected to be a function of orientation. Mechanical characterization, primarily focusing on low-cycle fatigue testing, will quantify the impact of part geometry and build strategy on the fatigue resistance of IN718. Furthermore, to elucidate the impact of difficult to detect subsurface porosity on fatigue life, thin wall compact tension samples were fabricated in both horizontal and vertical orientations with modified build strategies to produce both specimens with subsurface porosity and those that are fully dense. These results will inform ongoing complementary NASA investigations intended to optimize the properties of AM IN718 parts with complex geometries.