The selective laser melting (SLM) process was used to fabricate an Alloy718 specimen. The microstructure and creep properties were characterized in both the as-built and post-processed SLM materials. Post-processing involved several heat treatments and a combination of hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. The experimental results showed that the originally recommended heat treatment process, STA-980 °C, for cast and wrought materials was not effective for SLM-processed specimens. Obvious grain growth structures were obtained in the STA-1180 °C/1 h and STA-1180 °C/4 h specimens. However, the grain size was uneven since heavy distortion or high-density dislocation formed during the SLM process, which would be harmful for the mechanical properties of SLM-fabricated materials. The HIP+ direct aging process was the most effective method among the post-processes to improve the creep behavior at 650 °C. The creep rupture life of the HIP+ direct aging condition approached 800 h since the HIP process had the benefit of being free of pores, thus preventing microcrack nucleation and the formation of a serrated grain boundary.

This study confirmed that Al₂O₃ particles were formed in IN718 alloys during the fabrication by both selective laser melting (SLM) and electron beam melting (EBM). Different heat pattern and atmospheres in SLM and EBM result in different distribution and volume fraction of Al₂O₃ particles. The Al₂O₃ oxides would act as nucleation sites for the precipitation of Nb/Ti carbides, leading to the formation of unique core-shell composites with Al₂O₃ in the center and Ti/Nb at the periphery. In order to investigate the oxygen content introduced during SLM and EBM, the volume fraction of Al₂O₃ formed in spark plasm sintering (SPS)-fabricated substrate, by consolidating the pre-oxidized IN718 raw powders at 800 °C, was utilized. The oxygen contents introduced to IN718 substrates during SLM and EBM fabrication were calculated to be 0.030 wt% and 0.099 wt%, respectively.

The study focuses on obtaining Inconel 625-NbC composites for high-temperature applications, e.g., jet engines, waste-to-energy combusting systems or gas engine turbines, and characterizing them in terms of their microstructure and hardness improvement. Synthesis was performed utilizing Spark Plasma Sintering (SPS) at 1150 °C under the load of 45 MPa in medium vacuum (under 10⁻³ MPa) for a total time of 60 min. Four sets of samples with different Inconel 625 to NbC weight ratios were prepared (5, 10, 20, and 30 wt.%), followed by a reference sample containing no ceramic reinforcement. Obtained materials were hot-rolled at 1150 °C with a 10% reduction step and later cut and polished to perform characterization utilizing scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) module and microhardness testing device equipped with Vickers indenter. Hardness was improved proportionally to NbC addition achieving an increase of up to 20% of reference values. Additional heat treatment was conducted on the hot-rolled samples at 1200 °C in an argon atmosphere to further observe the interaction between reinforcement and alloy. Their microstructure revealed the coarsening of precipitates within the metal matrix and partial reinforcement dissolution, which proved to be crucial to obtaining the highest quality composites with homogenous hardness improvement.