Quantification of porosity and microstructure and their effect on quasi-static and dynamic behavior of additively manufactured Inconel 718

Abstract

The objective of this investigation is to quantify the evolution of microscopic features, including porosity, precipitates, and grain structure with respect to post-processing heat-treatments and their influence on the quasi-static and dynamic behavior of additively manufactured Inconel 718 built by laser powder bed fusion technique. In laser powder bed fusion, porosity, phase formation, and grain configuration could arise from different sources, including building powder or from the processing (e.g., laser pattern and solidification). The effect of these microscopic features on mechanical properties has drawn the scientific community's attention to developing thermal post-processing procedures targeting porosity closure, microstructural recrystallization, and precipitation of strengthening phases, thus improving material performance. One major challenge is to understand whether one can generalize the effect of certain microscopic features on mechanical properties under different states of stress and whether observed trends remain the same across strain rates? To address these questions, a comprehensive characterization agenda, which includes X-ray Micro-Computed Tomography, Scanning Electron Microscopy, Electron Backscatter Diffraction and mechanical testing at two strain rates (10−4 s−1 and 103 s−1) were implemented on as-built, stress relieved, hot isostatically pressed, and solution treatment plus aging test samples. Results show that, for the given specimen size, precipitate morphology and distribution changes from heat-treatment have a more significant effect than the porosity. In addition, it is demonstrated that there is a considerable enhancement in the magnitude of the plastic flow stress after applying the stress relief heat-treatment, while the subsequent heat-treatments do not have the same effect. When the state of stress is changed to compression, a different trend is observed that is relatively consistent increases in the magnitude of the flow stress. However, an opposite observation is seen under dynamic loading conditions indicating compressive mechanical behavior is more sensitive to the underlying microstructure than tensile behavior. Furthermore, strengthening mechanisms as a result of implemented heat-treatment is also discussed.