On the microstructural and mechanical properties of post-treated additively manufactured Inconel 718 superalloy under quasi-static and cyclic loading

Inconel 718 specimens were additively manufactured via selective laser melting (SLM) and subjected to different post-process heat treatments. Performance under monotonic and cyclic loading in the low-cycle fatigue (LCF) regime was investigated at room temperature (RT). Hot isostatic pressing (HIP) was employed in order to reduce the porosity typically present after SLM. Prior functional encapsulation with a Ni-20Cr coating applied by cathodic arc deposition (Arc-PVD) was applied to achieve full densification. The results show that after Arc-PVD and HIP, the mechanical properties are not only affected by reduced porosity but also by substantial microstructural changes. Precipitates, i.e. Laves phase, already present in the as-built condition can be dissolved by solution heat treatment. HIP leads to recrystallization and thus significantly changes microstructural appearance. Elongated grains and sub-micron sized cell structures stemming from SLM processing are eliminated by HIP. Aging leads to evolution of strengthening γ″ precipitates. Concurrently, δ phase forms upon HIP and aging. In comparison to aged conditions that were not subjected to HIP, microstructure upon HIP and aging results in improved ductility under monotonic loading, however, concomitantly deteriorates fatigue properties at RT.