Abstract
Electron beam melting (EBM) of Alloy 718 is of rapidly growing interest as it allows cost-effective production of complex components. However, the inherent flaws in the component in as-built state are of concern in view of the severe working conditions in which Alloy 718 components typically operate. The present work entails an investigation of changes in microstructure that accompany some post-treatments that are being widely considered to address defects in EBM processed Alloy 718. The effect of two different post-treatments, namely hot isostatic pressing (HIP) and a combined HIP + heat treatment (HT) carried out inside the HIP vessel, have been studied and results from as-built and post-treated specimens were compared in terms of porosity/lack-of-fusion, microstructure, phase constitution (NbC content, δ-phase) and micro-hardness. Post-treatment resulted in reduction in defect content by more than an order of magnitude. HIPing led to complete dissolution of δ phase. In comparison to as-built material, HIPed specimens exhibited significant drop in hardness. However, a sharp ‘recovery’ of hardness to yield values higher than in as-built condition was observed after HIP + HT and can be attributed to precipitation of γ′′ phase.
Highlights
Additive manufacturing (AM) is the process of fabrication of parts through layer-by-layer addition of material
The present study comprehensively investigates the influence of two different post-treatments, namely hot isostatic pressing (HIP) and HIP + heat treatment (HT)
The above variation in microstructure with build height in case of selective laser melting (SLM) was attributed to the difference in cooling rate, which is higher near the bottom, i.e., closer to the base plate, and lower near the top region
Summary
Additive manufacturing (AM) is the process of fabrication of parts through layer-by-layer addition of material. This allows manufacture of extremely complex parts intended for structural and functional applications, such as turbine blades with unique cooling channels. Alloy 718 ( known as Inconel 718) in wrought and cast form has been widely studied, there is a need for a detailed analysis of AM built material as the latter exhibits different microstructures and properties in comparison to those manufactured through the conventional manufacturing routes. The present study comprehensively investigates the influence of two different post-treatments, namely HIP and HIP + heat treatment (HT). The latter involved HIP, solution treatment, and aging carried out as a single cycle within the HIP vessel. The influence of post-treatment on grain structure, defect concentration, phase constitution, and micro-hardness have been investigated
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