Digital Twin Analysis of Solidification Process in Powder-Bed Fusion of Inconel 718 Ni-Based Superalloys

Solute segregation in the powder-bed fusion (PBF) additive manufacturing (AM) process significantly affects material properties¹⁾. Therefore, a comprehensive understanding of solute segregation is essential for controlling the mechanical properties of PBF fabricated parts. We have investigated the segregation behavior of Ni-based superalloys under solidification conditions in the PBF process using phase field (PF) simulations weakly coupled with computational thermal-fluid dynamics (CtFD) simulations, suggesting that solute segregation at the interface affects material properties such as strength and toughness ^2.3)_. In this study, the effect of temperature change on segregation behavior under solidification conditions in the PBF process is investigated by PF simulations weakly coupled with CtFD calculations.<br/> The subject of this study is IN718, a precipitation hardening Ni-based superalloy that has been frequently applied to the PBF pr beam irradiation experiments were performed on the IN718 cubic samples fabricated by PBF-LB machine (EOSINT M290, EOS). The beam power, scanning speed, and scanning line spacing were set to 360 W, 1000 mm s^-1, and 80 μm, respectively, and the laser was scanned in 10 reciprocating scans parallel to the X direction (left-right direction, directly facing the device).The solute distributions in the laser-irradiated region were analyzed using a scanning electron microscope (SEM) equipped with the energy dispersive X-ray spectroscopy (EDS). Corresponding CtFD simulations were also carried out using a commercial 3D thermal-fluid analysis software (Flow Science FLOW-3D® with Flow-3D Weld module) to obtain the temperature distributions during the laser-beam irradiation. Beam power, scanning speed, and scanning line spacing were the same as in the experiment. The number of lasers was set to six. Two-dimensional MPF simulations weakly coupled with CtFD simulation were performed to analyze the microstructure formation and the solute segregations using the Microstructure Evolution Simulation Software (MICRESS). The Gibbs free energy and diffusion coefficient of the system were calculated using the Thermo-Calc TCNI10 thermodynamic database and the MOBNI5 mobility database. The solidus temperature and the equilibrium phases using the compositions of each local region were calculated to reveal the effects of the solute segregations on the properties of the parts.<br/> Cross sectional observation of the laser-irradiated part showed a dendritic cellular structure formed by directional solidification. The primary dendrite arm spacing (PDAS) increased in the cross section formed by the 10th laser irradiation compared to the 1st laser irradiation, and the Nb segregation at the cell interface became more pronounced. CtFD-coupled PF simulations were similar to the experimental results, with an increase in PDAS and more pronounced Nb segregation in the solidification microstructure formed at the 6th laser compared to the 1st laser. This is considered to be because repeated laser irradiation increased the substrate temperature and decreased the temperature gradient and solidification rate in the melt region. Thermal equilibrium calculation suggests that the precipitation of the γ” phase at the cell interface formed by the 6th laser increased compared to that in the 1st laser scan track, resulting in a stronger susceptibility to ductility lowering cracking.<br/>This work was partly supported by CREST Nanomechanics: Elucidation of macroscale mechanical properties based on understanding nanoscale dynamics for innovative mechanical materials (Grant Number: JPMJCR2194) from the Japan Science and Technology Agency (JST) and JSPS KAKENHI (21H05192, 21H05193)<br/><b>References</b>:<br/>1) P. Kontis et al., Acta Mater. <b>177</b>, pp. 209–221 (2019).<br/>2) K. Sawaizumi et al., JIM 2023 autumn, Proceedings, P84.<br/>3) M. Okugawa, K. Saito, H. Yoshima, K. Sawaizumi, S. Nomoto, M. Watanabe, T. Nakano, Y. Koizumi, Addit. Manuf., <b>84</b>, (2024), 104079.