Solidification and Segregation Behaviors of Superalloy IN718 at a Slow Cooling Rate

Abstract

The solidification and micro- and macro-segregation behaviors of as-cast INCONEL 718 (IN718) alloy at different temperatures under a slow cooling rate (5 °C/min) were investigated in this study. The results indicate that the solid-liquid interface grows into reticulation of hexagons during solidification. The variation trend of the solid fraction and transition rate of the solid phase with solidification time can be well characterized by the Boltzmann and Gaussian distribution, respectively. The order of segregation degree of negative segregation elements is: Fe > Cr > Al. Nb is the most principal positive segregation element, which is abundant in the long-term unsolidified remaining liquid. At the terminal stage of solidification, the increasing tendencies of the Nb and Mo contents in the liquid and the residual liquid density with decreasing temperature reverse due to the formation of the Laves phase. The freckles are most likely to form in the early stages of solidification, at which the liquid fraction is between 0.3 and 0.2, and the temperature range is about 1320 °C to 1310 °C. The information produced is expected to characterize the solidification and segregation behaviors of IN718 alloy when cooled at a slow rate characteristic of larger ingots typical of those required for industrial gas turbines and aircraft engines.

Fabrication and Characterization of the Newly Developed Superalloys Based on Inconel 740

“…As a result of these changes, the Ni/Nb ratio was 1.6-2.1. The solidification of Nb-bearing austenitic superalloys like Alloy 718 begins with the formation of Nb-lean austenitic dendrites, while interdendritic eutectic-type solidification constituents involve among others NbC and Nb-rich Laves phase precipitates [46][47][48][49]. In this study, for all modified variants the segregation coefficient of Nb was lower than 1, which confirms that dendritic regions were depleted in this alloying element.…”

Laser direct energy deposition – the effect of process parameters on inconel 718

“…Two eutectic reactions occur during the nonequilibrium solidification of Inconel 718. These depend on solidification conditions and degree of elemental segregation (Shi, Duan, Yang, Gua, & Gua, 2018).…”

“…According to solidification theory (DuPont, Notis, Marder, Robino, & Michael, 1998;Knorovsky, Cieslak, Headley, Romig, & Hammetter, 1989), primary γdendrites form first, whilst Nb and C begin to segregate due to their low maximum solubility in the molten liquid (Ding, et al, 2015;Shi, Duan, Yang, Gua, & Gua, 2018). As solidification progresses, Nb and C segregate further until reaching their maximum solubilities within the γ-matrix.…”

“…As Laves formation is promoted by increased Nb and Mo segregation, this can be reduced by using a dwell time between depositing adjacent tracks and subsequent layers. This increases cooling rates, reducing Nb and Mo segregation, and hence Laves formation (Bambach, Sizova, Silze, & Schnick, 2018;Shi, Duan, Yang, Gua, & Gua, 2018). Alternatively a suitable heat-treatment can be applied following depositionthese have already been proved to reduce Laves proportions (Cao, Bai, Liu, Hou, & Guo, 2020;Fayed, et al, 2021;Liu, Lyu, Liu, Lin, & Huang, 2020;Sreekanth, Hurtig, Joshi, & Andersson, 2021).…”

“…It will have also significantly reduced the cooling rate of the material for each layer, again altering the microstructure. A slower cooling rate increases Nb and Mo segregation during solidification, promoting Laves phase and carbide formation (Shi, Duan, Yang, Gua, & Gua, 2018;Bambach, Sizova, Silze, & Schnick, 2018).…”