Effect of solutionizing temperature on the microstructural evolution during double aging of powder bed fusion-additive manufactured IN718 alloy

Quantitative microstructural analysis was carried out to determine the size and volume fraction of strengthening phases: gamma double prime (gamma'') and gamma prime (gamma'); in additively manufactured (AM) Inconel 718 (IN718) with an emphasis on the utilization of Nb for gamma'' formation during aging treatment. Powder bed fusion AM (PBF-AM) technique was used to generate coupons using indigenously synthesized IN718 alloy powder. Microstructural studies using transmission electron microscope (TEM) and small angle x-ray scattering (SAXS) were carried out on powder bed fusion-additive manufactured (PBF-AM) coupons in as-built condition (as processed) and in three different heat-treated conditions: solutionizing at three different temperatures (980, 1030 and 1080 degrees C) followed by double aging treatment. As-built microstructure consists of dendritic structure accompanied with Nb segregation along the interdendritic regions resulting in the formation of a Nb rich interdendritic phases dispersed within the interdendritic Nb-rich gamma matrix. The inhomogeneous Nb distribution resulted in preferential precipitation of coarse delta during solutionization at 980 degrees C and further coarse gamma'' during direct aging and aging subsequent to solutionizing at 1030 degrees C. As-cast structure and related elemental segregation got modified during solutionization at 1080 degrees C and resulted homogeneous gamma'' precipitation. Though the average size of gamma'' precipitates remained same in all aged samples, volume fraction increased with increasing solutionization temperature. SAXS studies were carried out on PBF-AM as well as conventionally produced wrought samples with the similar heat treatment conditions and confirmed that PBF-AM built IN718 aged samples solutionized at 1030 degrees C and 1080 degrees C resulted in gamma'' precipitates with average precipitate size and volume fraction similar to that of the wrought-IN718 sample solutionized at 1080 degrees C.

Automated summary

Quantitative microstructural analysis was conducted on additively manufactured Inconel 718 to determine the size and volume fraction of strengthening phases. The study focused on the effect of Nb on the formation of γ'' phase during aging treatment. The results showed that the average size of γ'' precipitates remained the same in all aged samples, but the volume fraction increased with increasing solutionization temperature.