Application of finite element, phase-field, and CALPHAD-based methods to additive manufacturing of Ni-based superalloys

Abstract

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni–Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

Homogenization kinetics of a nickel-based superalloy produced by powder bed fusion laser sintering

“…We found that the streaks in the as-built IN625 data have a nominal length in q on the scale of 0.1 Å-1, which translates to a segregated region width of ≈ 6 nm. This result is in good agreement with our CALPHAD-based solidification simulations on as-built AM IN625[33], which show that most of the elemental segregation occurs within several nm from the center of the interdendritic regions.…”

Formation of the Ni3Nb δ-Phase in Stress-Relieved Inconel 625 Produced via Laser Powder-Bed Fusion Additive Manufacturing

“…In wrought form, the mechanical strength is derived primarily from solid solution strengthening via alloying with Cr, Mo, and Nb. Additional strengthening has been reported for certain compositions and conditions via precipitation of the metastable body-centered tetragonal D0 22 Ni 3 Nb c¢¢-phase. [9][10][11] Formation of the orthorhombic D0 a Ni 3 Nb d-phase is also possible in any alloy where precipitation strengthening by c¢¢ is possible, including IN625 [9][10][11] and Inconel 718.…”

“…Recent Calphad and phase-field modeling has also suggested significant micro-segregation in IN625 produced using L-PBF AM. [22] Zhang et al did not investigate secondary phases, but investigated the homogenization behavior of the highly micro-segregated solidification microstructure. They reported that a homogenization heat treatment of 30 to 60 minutes at 1423 K (1150°C) was sufficient to relieve the solidification micro-segregation.…”

Influence of Postbuild Microstructure on the Electrochemical Behavior of Additively Manufactured 17-4 PH Stainless Steel

“…Therefore, the high cooling rates (up to 10 7 °C/s) and multiple re-melting events that occur during the AM build process are likely to produce microstructures that are dramatically different than those observed in conventional materials. Specifically, prominent micro-segregation and/or unexpected phases are present, particularly in components where no post-build thermal processing is performed [5–7]. …”