Microstructural defects governing torsional fatigue failure of additively manufactured as-built and heat-treated Inconel 718

Abstract

Nickel-based superalloys, such as Inconel 718 (IN718), have the unique properties of high strength, oxidation, and corrosion resistance, hence necessitating their wide use in aircraft engine structural components, power-generation engines, pressure vessels, high performance automobiles, and rocket engines. Additive manufacturing (AM) of these metal superalloys is an emerging technology, which is being investigated in its capability in driving material microstructural development towards enhanced mechanical performance. With torsional fatigue as an often underlying cause for failure of components within these applications, this study investigates the microstructural defects induced through the additive manufacturing process that have driven Inconel 718 samples to failure when subjected to torsional fatigue loading conditions. As-built and heat-treated Inconel 718 torsional fatigue fracture surfaces were compared through use of a variety of material characterization techniques, to show similarities, or lack thereof, in the defects formed due to the AM process that have contributed to torsional fatigue fracture. Results from both as-built and heat-treated direct metal-laser sintered (DMLS) Inconel 718, manufactured using optimized processing parameters reveal that primary torsional fatigue cracks initiated at surface and sub-surface defects where lack of fusion regions and un-melted powder particles are apparent. Through these analyses, a correlation was made between the fracture mechanics response exhibited by each build orientation/post-processing condition and resulting torsional fatigue properties.