Effect of Conventional Heat Treatments on the Microstructure and Microhardness of IN718 Obtained by Wrought and Additive Manufacturing

Abstract

IN718 is a Ni-based superalloy usually manufactured by conventional processes such as wrought or casting. However, recently additive manufacturing (AM) technologies, such as Powder bed fusion (PBF), are used to produce IN718 parts. Heat treatments, also designed for conventional processes, are usually used in AM parts to improve mechanical properties. Unlike traditional techniques, AM processes involve rapid cooling rates, large thermal gradients, and multiple reheat cycles, which might cause high residual stresses and elemental segregations in the printed parts affecting their final mechanical properties. In this work, a detailed comparative study of microstructural features was carried out in both wrought-and PBF-produced IN718. It was found differences in size, shape, and location of MC carbides. According to experimental results and the phase fraction diagram obtained from Thermo-Calc, these MC carbides cannot be dissolved in a conventional solution heat treatment. In consequence, these carbides continue their evolution during complete aging heat treatment, affecting the material hardness. Nevertheless, similar hardness in the wrought and AM sample was obtained after applying a modified aging treatment proposed in this work.

Microstructure and Microhardness Evolution of Additively Manufactured Cellular Inconel 718 after Heat Treatment with Different Aging Times

“…In our previous investigation of bulk samples of IN718 obtained by AM [17], similar phase transformations were observed after performing the respective heat treatments. In bulk samples, heat transfer during the AM process allows a slower cooling rate than in relatively thin cell structure samples.…”

“…Although it is not shown, defects such as porosity, lack of fusion, and inclusions were found. Solidification pools have a similar appearance to those obtained from "bulk" samples reported in our previous research work [17] using the same process parameters. The main difference is the well formation of the melting pool tracks in the bulk samples, which is attributed to the more significant amount of powder in bulk compared to thin cellular architecture samples.…”

“…Phase identification was based on the morphology, location, and chemical composition of the precipitates observed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). This methodology has been used before [17]. The SEM and EDS studies were carried out using an Oxford instrument model Ultimax 100 (Abingdon, UK) attached to an FE-SEM model JSMH-7200F (Tokyo, Japan).…”

“…Moreover, it controls the size and concentration of secondary phases, such as δ and Laves phases, and carbides. For these purposes, some standards [16,17] propose a heat treatment consisting of three steps: (1) solid solution treatment (ST) (900-1100 • C for 60 min) to establish a single-phase FCC γ matrix and dissolve the constituents and precipitates; (2) first aging treatment (700-760 • C for 480 min), to precipitate the primary γ and γ phases; (3) second aging treatment, (620-650 • C for 480 min), to promote the formation of fine γ and γ precipitates and the formation of primary carbides in grain boundaries. However, it is well known that the effect of this conventional heat treatment is different when it is applied to AM parts [17,18].…”

Tailored Time–Temperature Transformation Diagram for IN718 Alloy Obtained via Powder Bed Fusion Additive Manufacturing: Phase Behavior and Precipitation Dynamic

“…In a previous work published by our team [13], it has been demonstrated that the effect of conventional heat treatment [7,8] is different when it is applied to AM-produced parts of IN718. For this reason, we decided to go further in the investigation, looking for a TTT diagram that represents the phase transformation as a function of the time and temperature for AM-produced IN718.…”