Additive manufacturing (AM) is a new manufacturing method that allows engineers to design complex parts that are difficult to manufacture by conventional methods. Nickel-based superalloy Inconel 718 (IN718) is one of the most used alloys in metal AM because of its wide range of applications seen in aerospace and energy industries. The mechanical damage resulting from fretting wear on these high-value components is exceptionally severe, given that they undergo both cyclic loading and slip concurrently at the contact interface, especially under elevated temperatures. This work attempts to find an optimized heat treatment cycle for Laser-based powder bed fusion (L-PBF) processed AM Inconel 718 alloy and evaluate the effect of various fretting wear test parameters, under varied test temperatures to compare its fretting wear damage susceptibility.
In the pursuit of an optimized heat treatment cycle, a series of thermal treatments were conducted at diverse homogenizing and solutionizing temperatures spanning from 980°C to 1130°C. Specifically, two different holding times, namely 1 hour and 0.5 hours, were applied for selected instances, followed by subsequent double aging (DA). Heat treatment (HT) of 1080°C/1hr. and DA was selected for further wear testing based on the improved hardness and decreased anisotropy in the microstructure of the sample. As the assembly in the gas turbine engines involves contact among different kinds of materials, initial fretting wear tests were performed on wrought, AM as-built, and AM HT samples against AISI 52100 steel balls as counter material. There was variation observed in the progression of fretting wear in different materials under respective fretting regimes. Abrasive wear was found to be the predominant wear mechanism for the contact pairs with the coefficient of friction (COF) values for all samples being found in the range of 0.5 to 0.8. At ambient conditions, it was found that the fretting resistance of wrought and AM HT IN718 was found to be similar, and the AM as-built IN718 samples showed the least wear resistance due to its lower hardness. To understand the influence of built orientation on the wear properties of this material, fretting wear tests were performed on AM samples of three built orientations of horizontal (at 0° to AM built-up direction (BD)), angular (at 45° to BD) and vertical (at 90° to BD) using Alumina ball. There was no significant difference observed in the fretting wear properties of AM IN718 with respect to built orientations. The high-temperature fretting tests conducted at 250°C and 500°C revealed a decline in the coefficient of friction (COF) values, reaching 0.3 as the temperature increased. Fretting loops for the tests were predominantly in gross slip regimes at high temperatures. The wear track characterization indicated a heightened presence of Fe and Ni oxides on wear tracks, accompanied by the formation of Cr oxides. The wear mechanisms exhibited a transition from abrasive and delamination to adhesive and oxidation wear for the additively manufactured (AM) IN718 samples. Comprehensive comparisons between additively manufactured (AM) as-built and AM heat-treated (HT) IN718 are reported, focusing on cracks induced by contact cyclic shear stress. This analysis aids in assessing the material's susceptibility to catastrophic failures.

Keywords: Fretting wear; additive manufacturing; high temperature, Inconel 718; heat treatment.