Quasi-in-situ EBSD Study on the Microstructure and Tensile Properties of Selective Laser Melted Inconel 718 Alloy Processed by Different Heat Treatments

The effects of various heat treatments on the microstructures and mechanical properties of as-built selective laser melted Inconel 718 alloy were investigated through conventional and quasi-in-situ tensile tests. The corresponding heat treatment processes include direct aging (DA), solution + aging (SA), and homogenization + aging (HA). The DA and SA samples preserve the melt pool configuration and grain size stability, while the precipitated phase characteristics reveal the refinement of the long-strip Laves phase and the appearance of the δ phase, respectively. The HA process induces recrystallization and grain coarsening. The specimens exhibit enhanced strength concomitant with diminished elongation, which is likely attributed to the reduction of the geometrically necessary dislocation density and the intensified precipitation of the γ′′ phase after heat treatment. Tensile plastic deformation displays notable strain concentration along grain boundaries. The dimensional alterations in precipitated phases were measured to quantitatively determine the impact of grain boundary, dislocation and precipitation strengthening on the yield strength after heat treatment. Precipitation strengthening encompasses coherent, order, and Orowan strengthening. A remarkable agreement is revealed between theoretical predictions and experimental results. Insights are offered for optimizing heat treatment processes to comprehend microstructural evolution effect on the mechanical properties of additive-manufactured metals.