Influence of grain inhomogeneity and precipitates on the stress rupture properties of Inconel 718 superalloy fabricated by selective laser melting

Optimizing the microstructure to enhance the high temperature stress rupture properties of Inconel 718 (IN718) alloy fabricated via selective laser melting (SLM) is an urgent issue. In this study, SLM-built IN718 specimens were heat-treated with hot isostatic pressing, solution and aging treatment, and their microstructure and high temperature stress rupture properties were investigated, focusing on the influence of grain structure and precipitates. The results show that the coarsening of γ″ precipitate with the aging time at 720 °C follows the Lifshitz-Slyozov-Wagner theory. As the γ″ precipitate size increases, the rupture life gradually increases, area reduction gradually decreases, and the uniform elongation first increases and then decreases. Furthermore, a wide distribution of grain size results in the fine grains regions with much higher geometrically necessary dislocations compared with coarse grain regions during deformation. Hence, the micro-cracks preferentially initiate and propagate along fine-grain boundary. The main deformation mechanisms of specimens aged at 720 °C for 4 h (HSA4) and 8 h (HSA8) with the small γ″ precipitate are dislocation slipping and micro-twinning mode during stress rupture test, while those of specimen aged at 720 °C for 32 h (HSA32) with relatively large γ″ precipitate are dislocation slipping and isolated faulting mode. The HSA8 specimen with the best comprehensive stress rupture properties has the average life of 130 h and uniform elongation of 7.5%, meeting the standard specification for wrought IN718. Finally, some potential strategies for further improving the stress rupture properties of SLM-built IN718 were proposed by comparing to forged-IN718.