Additive Manufacturing: Solid-State Phase Transformations and Microstructural Evolution: Ni-based Superalloys
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Additive Manufacturing Committee, TMS: High Temperature Alloys Committee, TMS: Phase Transformations Committee
Program Organizers: Bij-Na Kim; Andrew Wessman, University of Arizona; Chantal Sudbrack, National Energy Technology Laboratory; Eric Lass, University of Tennessee-Knoxville; Katerina Christofidou, University of Sheffield; Peeyush Nandwana, Oak Ridge National Laboratory; Rajarshi Banerjee, University of North Texas; Whitney Poling, General Motors Corporation; Yousub Lee, Oak Ridge National Laboratory

Tuesday 2:00 PM
March 16, 2021
Room: RM 5
Location: TMS2021 Virtual

Session Chair: Katerina Christofidou, The University of Sheffield; Chantal Sudbrack, National Energy Technology Laboratory


2:00 PM  Invited
Impact of Post-processing on the Performances of Laser Additively Manufactured High-γ’ Ni Superalloys: Ning Zhou1; Austin Dicus1; Stephane Forsik1; Tao Wang1; Gian Colombo1; Mario Epler1; 1Carpenter Technology
    Laser additive manufacturing (AM) of high-γ’ volume fraction superalloys produces as-built microstructures consisting of dendritic grains and extremely fine γ’ precipitates. Hot isostatic pressing (HIP) and post-built heat treatments are critical steps to generate the microstructure necessary for good elevated temperature properties while mitigating the impact of residual stress and strain-age cracking associated with the precipitation of γ’. Thermodynamic modeling and multiple experimental heat treatments were performed on several experimental high-γ’ Ni superalloys to optimize the size and distribution of grain boundary carbides, control the grain size and obtain the desired γ’ morphology. The best mechanical properties were obtained after HIP and a subsolvus solution treatment without additional aging heat treatment and reached 1200 MPa ultimate tensile strength and 1120 MPa yield strength at 760°C. Extensive experimental characterization was performed to understand the relationship between processing, microstructure and mechanical properties to balance high-temperature strength and ductility.

2:30 PM  
Improving the Creep Properties on gamma prime-strengthened Nickel-based Superalloy by Selective Laser Melting: Marcus Lam1; 1Monash University
    Selective laser melting (SLM) of γ'-strengthened nickel-based superalloy can simplify the production process of gas turbine components and enable more efficient designs. These advantages however are offset by the reported inferior high temperature creep properties. The as-SLM microstructure is vastly different than the conventionally casted in terms of γ' volume fraction and grain size, and not surprisingly subsequent heat treatment using the conventional scheme produced a different, and very often, inferior microstructure for creep resistance. In this research, we are reporting a study of post heat treatment schemes that produced several microstructural features favorable to creep resistance, such as large grains, torturous grain boundaries and micro-pores minimization. The improvements result in creep property exceeding the requirements of the conventionally casted superalloys in terms of stress rupture time, proving that the creep properties of SLM-produced superalloys can be comparable to the conventional with better post heat treatment design.

2:50 PM  
New Superalloy ABD-900AM for Additive Manufacturing: The Role of Heat Treatment on Mechanical Properties: Yuanbo Tang1; Joseph Ghoussoub1; John Clark2; Andre Nemeth2; Roger Reed1; 1University of Oxford; 2OxMet Technologies
    The role of heat treatment on the new superalloy ABD-900AM is studied. The alloy was designed specifically for additive manufacturing, which presents superior processibility. The as-fabricated microstructure is investigated with advanced characterisation techniques, which includes X-ray synchrotron diffractometry and transmission kikuchi diffraction. The extreme cooling rates arising during the process suppress the gamma prime phase formation; thus the details of heat treatment are vital in tailoring the properties. The strength developed are marginally improved by super-solvus rather than sub-solvus heat treatment, but the ductility is then sacrificed. The tensile behaviour is superior to the heritage alloy IN939 which is comparable in gamma prime content; this is due to its finer scale precipitation and higher refractory content. The high dislocation density inherit by AM is sufficient to promote recrystallization during super-solvus heat treatment, which removes the texture and facilitates grain growth; however, this effect is not found for the sub-solvus case.

3:10 PM  
Microstructure and Texture Evolution During Printing and Post Processing of Ni-based Superalloy: Colleen Hilla1; Wei Zhang1; Michael Mills1; Alber Sadek2; Hyeyun Song3; 1The Ohio State University; 2Edison Welding Institute ; 3Edison Welding Institute
    Texture and microstructure impact the mechanical behavior of Ni-based superalloys. By leveraging unique thermal conditions of additive manufacturing, different microstructures can be explored for their effects on mechanical properties. A directionally solidified microstructure with a square/lattice cross section is seen in some laser powder bed (LPBF) builds. This study aims to identify the factors that produce this microstructure, analyze the stability during post processing and determine the effect on mechanical behavior. The microstructure and texture of Rene 65 printed by LPBF were analyzed using EBSD. Track-by-track heat conduction modeling was used to isolate the conditions under which this microstructure forms. Heat treatments were carried out in the subsolvus and supersolvus conditions. Both heat treatments led to precipitate formation, but only the supersolvus heat treatment caused recrystallization. Tensile and creep tests were completed in the as-built and heat treated conditions, and the effect of microstructure on the mechanical properties was analyzed.

3:30 PM  Invited
Applying Additive Manufacturing Itself as a High-throughput Tool to Accelerate Heat Treatment Design of Additively Manufactured Alloys: Yunhao Zhao1; Noah Sargent1; Kun Li1; Wei Xiong1; 1University of Pittsburgh
    Laser-based additive manufacturing often introduces complex phase transformations in alloys with cyclic heating and cooling processes. Therefore, post-heat treatment often has to be applied to achieve the desired microstructure for improved mechanical performance. This becomes particularly important in the alloys which rely on precipitation strengthening for mechanical strength. In this research, we developed a gradient temperature heat treatment on a bar shape Inconel 718 alloy sample prepared by the laser powder bed fusion. Such a high-throughput experimental concept has been successfully validated through the process-structure-property study using the combined approach of electron microscopy and microhardness. We observed that the precipitation strengthening is predominant for the studied superalloy by laser powder bed fusion, and the grain size variation is insensitive on temperature between 605 and 825 Celsius. This work demonstrated that additive manufacturing itself could be an ideal high-throughput tool for accelerated alloy development and heat treatment optimization.

4:00 PM  
Simulation of Solid State Precipitation during Post Process Annealing of Additively Manufactured alloy 625: Bala Radhakrishnan1; Younggil Song1; John Turner1; 1Oak Ridge National Laboratory
    During laser powder bed fusion processing of alloy 625, solutes such as Mo and Nb segregate to the inter-dendritic regions of the solidification microstructure. The delta phase is known to precipitate in these solute-rich regions during post-process annealing at 870K. We present phase field simulations of post-process anneal using an in-house developed phase field code at ORNL. The phase field code is linked to alloy thermodynamics through Calphad based free energies of the matrix and precipitate phases in the alloy. The simulations indicate that in the absence of dislocations in the matrix, the gamma” phase forms during the 870K anneal. However, when the increase in the matrix energy due to the observed dislocation density in the AM material is included, the delta phase forms at the expense of the gamma” phase in accordance with experimental results. Research sponsored by the Exascale Computing Program at ORNL under contract DE-AC05-00OR22725.

4:20 PM  
Assessing Compositional Gradients in DED Inconel 718 Builds via Directional Reflectance Microscopy: Ekta Jain1; Yeoh Yong Chen1; Bernard Gaskey1; Guido Macchi2; Antonio Mattia Grande2; Matteo Seita1; 1NTU Singapore; 2Politecnico di Milano, Italy
    Directed-energy-deposition (DED) is an additive manufacturing technology that enables rapid production and repair of metallic parts. The inherent multi-directional solidification and non-uniform cooling rate throughout the process, however, may yield parts with heterogeneous microstructures, which exhibit large scatter in mechanical properties. Assessing this heterogeneity in additively manufactured metals is challenging using conventional characterization techniques due to their low-throughput and limited field-of-view. Here, we employ an optical technique, directional reflectance microscopy (DRM), to rapidly assess large-scale microstructure heterogeneity information from Inconel 718 builds produced by DED. DRM relies on acquiring a series of optical micrographs of the chemically etched sample surface under different illumination angles. Using DRM, we successfully map the spatial distribution of niobium across the builds by quantifying local changes in surface reflectance, which stem from differences in δ-precipitates (Ni3Nb) size and distribution. These results demonstrate the capability of inferring compositional information using a low-cost and high-throughput optical technique.

4:40 PM  
Effect of Stress-relief Treatments on The Microstructure and Mechanical Response of Additively Manufactured IN625 Thin-walled Elements: Arunima Banerjee1; Mo-Rigen He1; William Musinski2; Paul Shade2; Marie Cox2; Edwin Schwalbach2; Kevin Hemker1; 1Johns Hopkins University; 2Air Force Research Laboratory
    Additively manufactured (AM) metallic structures are rife with non-equilibrium elemental segregations and high levels of residual stress due to the repeated thermal cycles that occur during printing. These characteristics pose challenges in fabricating AM parts with adequate material properties but can be mitigated through post-processing heat treatments. This study was undertaken to measure the effect of two stress-relief treatments on the microstructure and attendant mechanical response of thin-walled Inconel 625 T-elements fabricated by DMLM. Strain contour maps indicate similar plastic localizations around the nodes of the elements for both stress-relief conditions, suggesting a minor effect of stress-relief temperature on the mechanical response. Electron microscopy analysis of the stress-relieved elements revealed columnar grains oriented along <001> and a well-developed subgrain structure with an average size of 400 nm. Si-rich eta-type and Nb-rich MC-type precipitates decorate the subgrain boundaries, alluding to the importance of powder composition on the formation of secondary phases.