Microstructure evolution induced by inoculants during the selective laser melting of IN718

This study focuses on the microstructure evolution induced by eutectic WC-W2C inoculants during the selective laser melting (SLM) of IN718. The as-built microstructure observed using an electron microscope indicates that grain nucleation occurred on the surface of inoculants and that the diffusion layer between inoculants and IN718 composed of a mixture of IN718 and inoculants. After the post heat treatment of the as-built SLM specimens, more grains nucleated around the inoculants, and Nb-rich precipitates were formed along the grain boundaries. With an increase in the post heat-treatment temperature, the microstructure evolution became more pronounced. To elucidate the underlying mechanism, both theoretical and experimental analyses were performed. In summary, eutectic WC-W2C inoculants could provide heterogeneous nucleation sites for grain formation owing to the low wetting angle and the semi-coherent interface with the matrix. Theoretical analysis suggests that the difference in the thermal expansion coefficient between inoculants and IN718 did not provide a significant amount of residual stress. Thus, it can be concluded that heterogeneous nucleation is the primary mechanism by which inoculants can influence the microstructure in the present study.