Enhancing oxidation and corrosion performance of laser PBF Inconel 718 alloy by structural-phase rearrangement and surface nanocrystallization using thermomechanical post-treatment

Inconel 718 test parts were manufactured using a laser powder bed fusion (LPBF) additive manufacturing method, which is a promising technology for the production of superalloy endproducts with complex shapes for aerospace applications. The combined thermo-mechanical posttreatments (hot isostatic pressing (HIP), heat treatment (annealing combined with aging), and surface mechanical treatment) were applied to enhance the operational properties of the LPBF-built IN 718 alloy parts. The variation in microstructure and high-temperature performance of LPBF IN 718 alloy with different heat treatments and surface severe plastic deformation (SPD) by shot peening was compared, followed by exposure to air oxidation at 650 °C for 40 h. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies revealed the microstructure and phase state formed after thermo-mechanical post-processing, which provides the increased surface hardness of LPBF samples. The heat treatments increase the oxidation resistance of the studied superalloy manufactured by LPBF. However, homogenization-induced grain growth in the HIPHA process results in a lower anti-oxidation resistance enhancement than that after the HIPA heat treatment. Further surface SPD processing by the shot peening method significantly improves the oxidation resistance of the studied Ni-based superalloy owing to the accelerated formation of protective scales that were favoured by the SPD-induced nanostructure in the near-surface layers of LPBF samples. The corrosion resistance was also improved after the SPD post-processing.