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

Abstract Inconel 718 test specimens were manufactured using a laser powder bed fusion (LPBF) additive manufacturing method, which is a promising technology for producing superalloy end-products with complex shapes for aerospace applications. To enhance the operational properties of the Inconel 718 alloy parts manufactured by the LPBF process, a combined thermomechanical post-treatment was applied in this work, which included hot isostatic pressing (HIP), heat treatment (homogenization annealing combined with double aging), and surface shot peening (SP). The variation in microstructure and high-temperature performance during exposure to air oxidation at 650 °C for 40 h was compared for the LPBF-manufactured Inconel 718 alloy specimens with various heat treatment conditions and surface severe plastic deformation (SPD) by SP. Studies using X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the microstructure and phase state formed after thermomechanical post-processing, which provides the increased surface hardness of LPBF samples. The heat treatments (HIP → annealing → aging (HIPHA) and HIP → aging (HIPA)) increase the oxidation resistance. 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 post-processing by the SP significantly improves the oxidation resistance owing to the accelerated formation of protective chromium-enriched scales that were favoured by the SPD-induced nanostructure in the near-surface layers of LPBF samples. The corrosion resistance in a 3.5% solution of sodium chloride was also improved after both thermal treatments applied after HIP and SP-finishing applied after thermal post-treatments.