In-situ and ex-situ comparison of oxidation of Inconel 718 manufactured by selective laser melting and conventional methods up to 650 °C

Abstract

This work reveals the initial stages of Inconel 718 (IN718) oxidation manufactured by selective laser melting (SLM). In-situ and ex-situ investigations were conducted on SLM-IN718 and a commercially obtained Inconel 718 sample (Comm-IN718) to compare their oxidation behavior. In-situ and ex-situ testing was performed using a unique synchronized system integrating precise thermal control with Raman spectroscopy, optical microscopy and scanning probe techniques. X-Ray diffractometry (XRD) and Scanning Electron Microscopy (SEM) were used for microstructure analyses before and after heating. Raman spectroscopy indicated that, in both samples, there is a threshold temperature for oxidation at around 300 °C. The highest rate of oxidation was observed up to 550 °C as different oxides form. After 550 °C, there was no noticeable formation of new compounds up to 650 °C. Extended heating at 650 °C resulted in further changes in the Comm-IN718 sample, however the SLM-IN718 sample was unchanged by longer heating times. XRD of cooled samples did not detect any oxides because of the structurally disordered nature of the oxides. After being cooled, Comm-IN718 showed undesirable oxides like iron oxides in an inhomogeneous crystalline oxide layer. For SLM-IN718 the continuous unbroken oxide layer on the cooled SLM-IN718 sample imparted more effective protection of the alloy.