PHASE TRANSFORMATIONS IN INCONEL 718 SUPERALLOYS DURING ADDITIVE MANUFACTURING

• Published in: Calphad 2021-06, Vol.73, p.103
• Main Authors: Zhao, Yunhao, Chen, Qian, Hao, Liang-Yan, To, Albert C, Xiong, Wei
• Format: Article
• Language: English
• Subjects: Additive manufacturing; Alloy powders; Computer simulation; Cooling; Cooling rate; Delta phase (crystals); Dilatometry; Finite element method; Heat treating; Heat treatment; Kinetics; Laser beam melting; Mathematical models; Mechanical properties; Microstructure; Model accuracy; Nickel base alloys; Optimization; Phase diagrams; Phase transitions; Post-processing; Precipitates; Quenching; Software; Superalloys
• ISSN: 0364-5916; 1873-2984

The mechanical properties of additive manufacturing (AM) components are determined by microstructure evolution during and after the building processes. Ideally, one wants to minimize the efforts of post-heat treatment after the AM building process to achieve the best performance of the AM components, or at least, understand and control microstructure for an effective design during the post-processing steps. Therefore, phase transformation modeling for the AM melting process becomes essential to a sustainable AM design with reduced costs and development cycles. In this work, a mean-field type of ICME simulation framework is established by coupling the finite element analysis (FEA) and the CALPHAD approach with experimental calibration. The modeling is applied to the phase transformations in Inconel 718 alloys during the powder-bed laser melting process. For the sake of improving modeling accuracy, the experimental continuous cooling transformation (CCT) diagram is directly coupled into the phase transformation modeling. The CCT diagram is determined by employing electron microscopy and quenching dilatometry. The experimental results indicate that the δ phase is the main precipitate phase during the cooling, and the precipitation kinetics are influenced by homogenization steps. The precipitation kinetics of the δ phase determined by experiments support the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model analysis. In addition, phase transformations at constant cooling rates are simulated using the TC-Prisma module in the Thermo-Calc software package aided by critical phase transformation experiments. The interfacial energies of precipitates are calibrated by fitting the simulations with experimental data and agree with the JMAK model analysis. This work demonstrates the application of CALPHAD approach in AM modeling, and can assist in process optimization of AM Inconel alloys. The integration of finite element modeling and CALPHAD-based phase transformation provides a guidance to the future simulation of the in-situ laser melting in AM. We are grateful for the financial support of the NASA Early Stage Innovations program (Grant Number: NNX17AD11G). The authors thank the support of the Thermo-Calc company for the CALPHAD computation using the Thermo-Calc software and databases.