Comparison of Isothermal Oxidation Performance of IN939 Produced by Casting and Additive Manufacturing

The focus of this study was to compare the isothermal oxidation behavior of IN 939 nickel-based superalloys produced by selective laser melting and casting. Oxidation experiments were performed on both heat-treated and non-heat-treated, as cast and additively manufactured samples, to reveal the role of heat treatment and manufacturing methods on oxidation behavior. As cast samples underwent a two-step aging at 1080 and 843 °C, while a one-step aging was carried out for additively manufactured samples at 845 °C. The microstructure of the as cast IN 939 exhibited a dendritic structure with gamma prime precipitates. Following the heat treatment, primary and secondary gamma prime precipitates were formed. Additively manufactured IN 939 exhibited clearly visible melt pools and no trace of gamma prime precipitates. After heat treatment the melt pools disappeared, and gamma prime precipitates formed. Oxidation experiments were performed at 800, 900 and 1000 °C. All samples exhibited similar weight gain characteristics and obeyed a parabolic rate law. Spallation did not occur at 800 and 900 °C, whereas at 1000 °C all samples experienced spallation. The activation energies of all samples, calculated for three temperatures (800, 900, and 1000 °C), were similar, ranging between 260.99 and 287.51 kJ/mole. XRD and EDS analyses indicated that the oxide scale formed on all IN 939 samples was mainly Cr₂O₃ and TiO₂ in rutile form. The internal oxidation and nitridation zones were investigated using SEM and image analysis. The results showed that at 1000 °C, internal oxidation and nitridation extended deeper into the bulk material for additively manufactured samples due to the finer and columnar grains along the building direction which contained extensive amounts of precipitates compared to cast microstructure.