Effects of Process-Induced Supersaturation on the Oxidationbehavior of Alloy718 Fabricated By Additive Manufacturing Technologies
Authors: Kuo Yenling , Hayashi Shigenari , Kakehi Koji 
Published: December 2020

Alloy718 is widely used in gas turbines and related aerospace applications due to its excellent mechanical properties and structural stability at elevated temperatures. Alloy718 is a precipitation-strengthened superalloy which is generally manufactured through casting, forging, and heat treatment. Over the past decade a variety of additive manufacturing (AM) technologies involving laser and electron beam have become increasingly attractive fabrication methods due to their several advantages over conventional manufacturing methods, such as a large degree of design ﬂexibility, and reductions in the lead time and investment cost. However, the selective laser melting process (rapid heating and cooling) will make matrix gets supersaturated in alloying elements, in particular Nb, Ti and Al, which have significant effects on the formation of the strengthening phases in austenite matrix, e.g., γ′′ (Ni3Nb,), γ′ (Ni3Al,Ti) phases.Up to now, the specific influences of supersaturation on the oxidation behavior of the scales in a Cr-forming Ni-based alloy have not been dealt with in depth.

In this study, the oxidation behavior of Alloy718 specimens produced by selective laser melting (SLM) and electron beam melting (EBM) process were comparatively examined. A slower cooling rate and a preheating process in the EBM process contribute relatively lower degree of saturation in 𝛾 matrix. On the other hand, the high thermal gradient and rapid solidification rate in SLM process lead to supersaturation in 𝛾 matrix. All alloys were studied in two versions to compare the order of supersaturation: “As-received” and heat treated (HT). The heat treatment procedure was carried out at 980 °C for 1 h/air cooling to room temperature and a two-step aging treatment consisting of 718 °C for 8 h/furnace cooling to 621 °C and holding at 621 °C for 10 h before AC to room temperature. The oxidation kinetics as well as the surface morphology of the oxide scales were characterized by means of weight gain measurements, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), electron probe microanalysis (EPMA), X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis techniques.

The experimental results indicated that the oxidation performance of as-received SLM samples were better than those of as-received EBM samples due to the formation of thinner Cr2O3 scale without spallation. In addition, the morphology of their chromia scale was different. Coarse and columnar chromia grains developed on SLM samples, whereas the chromia scale of EBM samples consisted of extremely fine grains. After heat treatment process, the oxidation kinetics of heat-treated samples were increased compare to as-received samples. Moreover, the fine-grained chromia scale which is similar to that formed on EBM samples was formed on heat-treated SLM samples. The mentioned results indicate that the degree of supersaturation in the matrix did affect the morphology of chromia scale resulting in the oxidation kinetics.