Dynamic-loading behavior and anisotropic deformation of pre- and post-heat-treated IN718 fabricated by laser powder bed fusion

Abstract

The microstructures of additively manufactured (AM) metal components have been shown to be heterogeneous and spatially variable when compared to conventionally manufactured counterparts. Consequently, the mechanical properties of AM-metal parts are expected to vary locally within their volume. For AM structural components intended to operate in extreme environments, including high-strain-rate loading scenarios, there is a need to quantify variability of mechanical behavior within the same AM-build domain at quasi-static and dynamic strain-rates as well as the effect of heat treatment on the mechanical properties. The objective of this study is to investigate the effect of loading direction and direct-age hardening heat treatment on quasi-static and dynamic mechanical response within an Inconel 718 volume produced by laser powder bed fusion using manufacturer-recommended processing parameters. Uniaxial compression tests and a split-Hopkinson pressure bar (SHPB) were used to investigate the quasi-static and dynamic response, respectively, of as-built and heat-treated specimens extracted along the three principal processing directions. Electron backscatter diffraction measurements were made for representative specimens within the build domain to correlate microstructural features to observed location-specific mechanical deformation. Results from both quasi-static and dynamic loading show that the recommended processing parameters yield a homogeneous stress-strain response throughout the material volume in the as-built condition. In the direct-age-hardened condition, statistically significant differences were observed in the stress–strain responses between the build direction and sweep and transverse directions (although the latter two exhibited no statistically significant differences). Deformed specimen geometries showed a systematic and repeatable preferential deformation along the build direction, regardless of condition or loading strain rate when loading was applied in either of the two orthogonal processing directions. The deformation dependence is found to be related to the underlying, process-induced crystallographic texture and grain morphology. This study provides important insight that can be used for qualifying large AM Inconel 718 parts for critical structural applications used in extreme conditions.