Varied heat treatments and properties of laser powder bed printed Inconel 718

Abstract

The effects of post-build heat-treatments on the microstructure, phase formation, recrystallization behavior, and mechanical properties of laser powder bed additively manufactured Inconel 718 superalloy were investigated. Several heat-treatment schedules were used, including simulated hot isostatic pressing (HIP), and variations of the standard double aging treatment with various soaking times and quench procedures. Their effects on the precipitation, grain morphology, grain size, texture sharpness and mechanical properties were all documented. For the as-built coupons, columnar grains with inter-dendritic micro-segregation were formed along the build direction (xz-plane) with equiaxed grains forming on perpendicular sections to the build direction (xy-plane). After prolonged soaking times during the simulated HIP process, the microstructure transitioned from heterogeneous columnar grains to homogenous recrystallized grains with MC-type carbide precipitates. This leads to changes of microhardness (281 HV2.0 to 171 HV2.0), Young's modulus (209 GPa–229 GPa) and texture intensity. However, aging treatments increased both hardness and Young's modulus possibly because of formation of γ″ and γ′ precipitates in the Ni-matrix and the small effective grain size. Phase analysis using XRD confirmed the evolution of the precipitate formation. The combination of additive manufacturing and post-build heat-treatments can result in optimized microstructures and mechanical properties for specific applications depending on part requirements and operating conditions.