Microstructural Development in As Built and Heat Treated IN625 Component Additively Manufactured by Laser Powder Bed Fusion

The RL10 engine program is exploring the use of IN625 Ni-base superalloy components that are additively manufactured using laser powder bed fusion (LPBF). IN625 alloy powders are commercially available for LPBF to produce dense, complex parts/components. In this study, IN625 components, with both simple and complex geometries with overhangs, were manufactured via LPBF, and subjected to a heat-treatment consisting of a stress relief, hot isostatic pressing (HIP), and a solution anneal. The microstructure was examined with optical, scanning electron, and transmission electron microscopy. Changes in phase constituents and microstructure were documented as a function of heat treatment and component geometry (i.e., bulk section built on support structure versus thin, overhang section built on top of the previous powder bed). The as-built microstructural features included large columnar grains, a sub-grain cellular-solidification structure, approximately ~ 1 µm in diameter, and solute enriched cell boundaries decorated with A₂B Laves phases. After heat treatment, the bulk section consisted of recrystallized equiaxed grains with annealing twins, and the sub-grain cellular-solidification structure was found to be completely dissolved. However, in the thin, overhang section, the sub-grain cellular-solidification structure persisted within columnar grain structure, which exhibited no recrystallization. An alternate HIP cycle with a higher temperature was employed to produce desired microstructure (i.e., recrystallized grains without sub-grain cells and Laves phases) in components with geometrical complexity for successful testing of RL10 engine.