Effect of heat treatment on the microstructure evolution and mechanical behaviour of a selective laser melted Inconel 718 alloy

Abstract This study aims to address the fact that after the selective laser melting (SLM) of an Inconel 718 alloy, the microstructures at each fabrication position are anisotropic due to the coupling effect of multi-dimensional heat conduction caused by the forming mode, and combined with the formation laws of each phase in the Inconel 718 alloy, the microstructure evolution mechanism and corresponding mechanical behaviour laws at each fabrication position under different heat treatment regimes (including precipitation ageing (DA), δ ageing + precipitation ageing (SA) and high-temperature microstructure homogenization + δ ageing + precipitation ageing (HSA)) were studied. It is found that with increasing heat treatment temperature (DA→SA→HSA), the microstructure of each fabrication position gradually changes from anisotropic to isotropic, including the weakening and disappearance of the primary fabricated characteristic microstructure ("perlage" molten pools and long strip molten channels) and the primary crystal structure, and the corresponding recrystallization degree significantly increases. Among them, the nucleation and growth of recrystallized grains occurs through the process of high temperature polygonization recovery and the grain boundary migration mode, and the orientation of the recrystallized grains is determined by the direction of the grain boundary migration during recrystallization. Additionally, with the continuous dissolution of a large number of intergranular Laves phases precipitates in the primary fabricated microstructure (AB) and the further uniform diffusion of solute from intergranular to intragranular, the number of second-phase strengthening particles (γ″ phase and γ' phase) precipitated in the crystal increases and the distribution uniformity is improved significantly. Furthermore, the intergranular δ phase precipitation will gradually change from intergranular to intragranular, and when the heat treatment temperature is close to the melting temperature of the δ phase, its shape changes from a primary short rod-like shape and needle-like shape to a bulk shape. Thus, under the influence of the precipitation-dissolution coupling between the strengthening phase in the crystal and the intragranular brittle phase, the comprehensive mechanical strength of the samples after heat treatment shows the trend of SA＞HSA＞DA＞AB, while the elongation shows the opposite trend (AB＞DA＞HSA＞SA) with the precipitation content of the δ phase, and the mechanical behaviours are all superior to those of the mechanical standards of forging and casting of Inconel 718 alloy.