Microstructural evolution and mechanical properties of Alloy 718 fabricated by selective laser melting following different post-treatments

In this study, Alloy 718 specimens were fabricated via selective laser melting (SLM), and the effects of different post-treatments on microstructures and mechanical properties of the alloy were systematically studied. The post-treatment techniques investigated were solid solution + dual aging (SA) and hot isostatic pressing + solid solution + dual aging treatment (HIP + SA). The results indicate that the specimens treated using SA have a columnar crystal structure similar to the as-built sample, the microstructure is partially recrystallized, and the grain size increases slightly. Meanwhile, the grain morphology is dominated by equiaxed crystals in samples treated using HIP + SA, and the microstructure is completely recrystallized, with distinct grain coarsening and a large number of twins formed. Specimens treated using SA and HIP + SA show high tensile strength and yield strength and excellent elongation at room temperature, which can be primarily attributed to the strengthening of the γ′ and γ″ precipitated phases. The number of γ″ precipitated phases are reduced due to the consumption of Nb to form a large amounts of δ phase and (Ti, Nb)C carbides, resulting in the strength of HIP + SA specimens being lower than that of SA specimens.