Isothermal Solid-State Transformations of Inconel 718 Alloy Fabricated by Selective Laser Melting

Inconel 718 samples have been fabricated by selective laser melting (SLM) and subjected to a series of heat treatments to study the isothermal precipitation behaviors of delta, gamma ', and gamma '' phases. A time-temperature-transformation (TTT) diagram for as-built Inconel 718 is finally plotted. The morphology, size, and distribution of delta phase are studied, and the corresponding transformation kinetics is analyzed by Avrami equation. The results demonstrate that the transformation rate at 900 degrees C is the highest, though the nose of the TTT curve occurs at 860 degrees C after an incubation period of 10 min. As the aging temperature increases (620-820 degrees C), the precipitation rate of gamma '/gamma '' increases and the time to reach peak hardness decreases. The nose of the TTT curve for gamma '/gamma '' precipitation occurs at 820 degrees C after only 32 s. Nevertheless, when aged above 720 degrees C, the peak hardness decreases. In addition, the age hardening can be described by Avrami equation when delta phase is absent (620-770 degrees C). In summary, SLMed Inconel 718 exhibits a faster precipitation kinetics compared with wrought material, whereas their phase transition temperatures are similar.

Automated summary

Inconel 718 samples were subjected to heat treatments to study the precipitation behaviors of different phases. The study found that with increasing aging temperature, the time to reach peak hardness decreases, while the peak hardness decreases with higher aging temperature. The kinetics of precipitation in SLMed Inconel 718 is faster compared to wrought material, with similar phase transition temperatures.