Tailoring Microstructure and Deformation Behaviour of Additively Manufactured Inconel 718 by Advanced Thermal Post-Processing Sequences

A study of thermal post-processing of laser powder bed fusion (powder bed fusion by laser beam for metal, PBF-LB/M)-built Inconel 718 components is presented, evaluating the effects on their mechanical properties, microstructure and deformation behaviour. For this, a variety of heat treatments is conducted to define the impact of the individual processes as well as of combinations of differently sequenced heat treatments. To be precise, solution treatment, hot isostatic pressing and ageing procedures are used as well as subsequently combined for a comprehensive investigation on process successions and the improvement of mechanical properties. The ultimate tensile strength, part density and microhardness are recorded for a quantification of metallographic and mechanical properties, as the heat treatments improve material properties. While an improvement of the material properties for the employment of the heat treatments is achieved, an outstanding hot-isostatic pressing sequence leads to a significant improvement in part density, hardness and mechanical properties with the highest measured tensile strength improving by 50%. Furthermore, microstructural and fractographic analysis is used for a discussion on the mechanisms of thermal post-processing, revealing phase precipitations and microstructural changes. An element analysis for phase identification and the characterization of precipitations is conducted, using an SEM/EDX measurement of the heat-treated samples. Microstructural changes, grain boundary migration as well as Nb-precipitations are shown for the heat-treated samples. Deformation mechanisms of the material states and the microstructure are discussed and reveal differences in fracture behavior caused by the development of shear bands and lengthened grain boundaries. Based on the fractographic analysis, the heat-treated samples could be classified according to whether their microstructures promoted ductile or more brittle fracture behaviour after heat treatment.