Phase Transformations During Continuous Cooling in Inconel 718 Alloys Manufactured by Laser Powder Bed Fusion and Suction Casting

TL;DR

This study develops continuous-cooling-transformation diagrams for Inconel 718 alloys made by laser powder bed fusion and suction casting, revealing how homogenization and manufacturing methods influence phase precipitation and hardness during cooling.

Contribution

It provides new CCT diagrams for Inconel 718 that account for manufacturing methods and homogenization, highlighting the effects on precipitate formation and mechanical properties.

Findings

NbC and delta phases form after cooling, no gamma prime observed.

Homogenization time and manufacturing method affect Nb homogeneity and precipitation.

Subgrains form after cooling at 0.1 K/s, increasing hardness.

Abstract

Understanding alloy phase transformations during continuous cooling is important for post-processing design and optimization. In this work, continuous-cooling-transformation (CCT) diagrams of Inconel 718 alloys manufactured by laser powder bed fusion (LPBF) and suction casting are developed under different homogenization conditions. Unlike the available CCT diagrams in the reported studies, no gamma double prime and gamma prime precipitates can be observed. NbC and delta are determined to be the precipitates after cooling from the gamma matrix. Importantly, homogenization time and manufacturing methods are found to affect the Nb homogeneity in the matrix near NbC particles and thus significantly influence the precipitation process of the delta phase, which has a high content in Nb. In the alloys with high Nb homogeneity, the nucleation process mainly contributes to the precipitation, whereas in the alloys with low Nb homogeneity, the precipitation is primarily associated with the growth process. Subgrains are found to form after cooling at 0.1 K/s and can cause the highest hardness in samples. This work provides a new viewpoint on the study of processing-structure-property relationships during cooling in Inconel 718 and is beneficial to the development of alloy post-processing strategies.