Andrew Wessman

Interests

Teaching

Additive ManufacturingMaterials Science and MetallurgyAerospace Materials

Research

Metal Additive ManufacturingAlloy and Process DesignMechanical Behavior of MaterialsPhase Transformations and Materials CharacterizationMaterials for Challenging Environments

Courses

Scholarly Contributions

Journals/Publications

• Lam, M., Cruz, C., Loustaunau, A., Koumpias, A., Haselhuhn, A., Wessman, A., & Tin, S. (2025). Fatigue mechanisms at 450°C of a highly twined (>70%) and HIP-densified IN718 superalloy additively manufactured by laser beam powder bed fusion. International Journal of Fatigue, 190. doi:10.1016/j.ijfatigue.2024.108629
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  Fatigue resistance at elevated temperatures is crucial for certifying aerospace structures additively manufactured by the laser beam powder bed fusion (PBF-LB) method with IN718 superalloy. This study employed a multi-step, supersolvus heat treatment process with hot isostatic pressing (HIP), called RHSA, to minimize pores and brittle phases. Stress intensity factor (△K) calculations using data from X-ray computed tomography and shape factors referencing finite element analysis (FEA) studies confirmed the suppression of △K below the threshold of conventional IN718 (∼5 MPa√m), shifting fatigue behavior to grain-structure-dominated. Despite a very high twin boundary (TB) fraction (>70%), fatigue tests at 450°C and R = 0.1 demonstrated low scatter. Slip trace analysis and high-resolution electron backscatter diffraction (EBSD) revealed that TB-induced strain concentration became prominent only at high △K, causing cracking at 45⁰ to the loading direction. The randomly oriented TBs with higher angles (60⁰) compared to high-angle grain boundaries (HAGBs) (30–40⁰) likely enhanced slip resistance and provided a net strengthening effect, which can explain the lower-than-average TB% along fracture paths. These insights suggest that a high TB fraction is not detrimental if fatigue stress is not excessive, alleviating concerns about annealing twins during defect minimization in AM IN718, allowing novel processes to improve fatigue resistance in PBF-LB IN718.
• Lam, M., Koumpias, A., Haselhuhn, A., Wessman, A., & Tin, S. (2025). An additively manufactured IN718 strengthened by CSL boundaries with high-temperature tensile and short-term creep resistance up to 800°C. Materials Science and Engineering: A, 922. doi:10.1016/j.msea.2024.147654
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  Emerging aerospace applications such as reusable rockets, ramjets, and next-generation turbine engines require non-cracking additive manufactured (AM) alloys with short-term creep resistance beyond the usual 650°C service temperature of the IN718 superalloy. Integrating previous phenomena reported in multi-step heat treatment processes (designed as RHSA), IN718 produced by laser beam powder bed fusion (PBF-LB) was subjected to an optimized scheme with stress relief (SR), hot isostatic pressing (HIP) and solution (SS) all above the supersolvus temperatures to minimize brittle phase formation. The high-temperature tensile and creep behaviors were investigated at up to 800°C. The supersolvus RHSA process produced a densified, low-texture microstructure with a high fraction of coincident site lattice (CSL) boundary (64 %). The low-Σ CSL boundaries resisted grain boundary (GB) cracking at high temperatures. GB strain measured by EBSD and a creep stress exponent of 10.9 indicates primarily power law creep at 700°C, with increasing contribution by grain boundary sliding (GBS) at 800°C. Stress-assisted coarsening of γ'' and δ precipitations was also observed at 800°C with preferential alignment to the maximum shear stress directions. The particle coarsening rates exceeded the CALPHAD-based calculations with only diffusion contributions while more in line with the stress-assisted rate (Rδ) equation determined experimentally from another study. The creep resistances in terms of the Larson Miller parameter (LMP) were comparable to hot-rolled IN718 while higher than cold-rolled or AM IN718 as well as IN625 previously reported. The relatively high creep properties are attributed to the high proportion of the CSL boundary that simultaneously limits GB cracking at high temperatures and intragranular deformation.
• Rieffer, A., & Wessman, A. (2025). Interfacial Stability of Additively Manufactured Alloy 625???GRCop-42 Bimetallic Structures. Journal of Manufacturing and Materials Processing, 9(2).
• Budinoff, H., Wessman, A., & Chauhan, K. D. (2024). Using online learning modules to improve students’ use of technical standards in additive manufacturing courses and projects. Proceedings of American Society for Engineering Education Conference and Exhibition. doi:10.18260/1-2--44580
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  Using online learning modules to improve students' use of technical standards in additive manufacturing courses and projects
• Hasan, N., Saha, A., Wessman, A., & Shafae, M. (2024). Machine learning-based layer-wise detection of overheating anomaly in LPBF using photodiode data. Manufacturing Letters, 41. doi:10.1016/j.mfglet.2024.09.169
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  Overheating anomaly detection is essential for the quality and reliability of parts produced by laser powder bed fusion (LPBF) additive manufacturing (AM). In this research, we focus on the detection of overheating anomalies which can lead to various defects in the part including geometric distortion, and poor surface roughness, among others, using photodiode sensor data. Photodiode sensors can collect high-frequency data from the melt pool, reflecting the process dynamics and thermal history. Hence, the proposed method offers a machine learning (ML) framework to utilize photodiode sensor data for layer-wise detection of overheating anomalies. In doing so, three sets of features are extracted from the raw photodiode data: MSMM (mean, standard deviation, median, maximum), MSQ (mean, standard deviation, quartiles), and MSD (mean, standard deviation, deciles). These three datasets are used to train several ML classifiers. Cost-sensitive learning is used to handle the class imbalance between the “anomalous” layers (affected by overheating) and “nominal” layers in the benchmark dataset. To boost detection accuracy, our proposed ML framework involves utilizing the majority voting ensemble (MVE) approach. First, the top three ML classifiers are identified from an initial pool of classifiers, based on their performance in k-fold cross-validation. Next, final predictions are generated using majority voting from the individual predictions of the top three classifiers. We performed 100 iterations to generate statistically reliable results. This proposed method is demonstrated using a case study including an open benchmark dataset of photodiode measurements from an LPBF specimen with deliberate overheating anomalies at some layers. The results from the case study demonstrate that the MSD features yield the best performance for all classifiers, and the MVE classifier (with a mean F1-score of 0.8654) surpasses the individual ML classifiers. Moreover, our machine learning methodology achieves superior results (9.66% improvement in mean F1-score) in detecting layer-wise overheating anomalies, surpassing the existing methods in the literature that use the same benchmark dataset. Finally, based on our results, we provide useful insights and recommendations for future research on applying machine learning techniques for defect detection in AM.
• June, D., Mayeur, J., Gradl, P., Wessman, A., & Hazeli, K. (2024). Effects of size, geometry, and testing temperature on additively manufactured Ti-6Al-4V titanium alloy. Additive Manufacturing, 80. doi:10.1016/j.addma.2024.103970
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  This article presents a comprehensive study concerning size and geometry effect on the ambient and high-temperature mechanical behavior of additively manufactured laser powder bed fusion Ti-6Al-4V alloy. Key mechanical property metrics are presented, including strain hardening rate, yield strength, and Young's modulus as a function of thickness and testing temperature (ambient, 250 °C, and 450 °C). The effect of specimen size on manufacturing-induced microstructural feature formation is demonstrated and discussed. A detailed analysis regarding Young's modulus, yield strength, and strain hardening rate variation at elevated temperatures is also presented. Size effect and temperature-sensitive deformation mechanisms are linked to the underlying microstructural deformation mechanisms activated at respective temperatures. Counter-intuitively, this study determined that irrespective of the geometry, strain hardening rates increased as temperature increased. An in-depth microstructural examination is presented to explain the stated observation. The texture of Interrupted tensile test samples examined for ambient, 250 °C, and 450 °C post-yield was observed to be unchanged, indicating the strain hardening behavior was not texture dependent. Schmid factor analysis, coupled with experimental findings from previous work, was implemented to generate a hypothesis. This hypothesis suggests that the drop in critical resolved shear stress for basal and pyramidal slip systems, as temperature increases, paired with the high dislocation density of laser powder bed fusion Ti-6Al-4V, leads to dislocation entanglement and results in increased strain hardening at elevated temperatures.
• Lauretta, D. S., Connolly Jr, ,., Aebersold, J. E., Alexander, C., Ballouz, R., Barnes, J. J., Bates, H. C., Bennett, C. A., Blanche, L., Blumenfeld, E. H., Clemett, S. J., Cody, G. D., DellaGiustina, D. N., Dworkin, J. P., Eckley, S. A., Foustoukos, D. I., Franchi, I. A., Glavin, D. P., Greenwood, R. C., , Haenecour, P., et al. (2024). Asteroid (101955) Bennu in the laboratory: Properties of the sample collected by OSIRIS-REx. Meteoritics \& Planetary Science, 59(9), 2453-2486.
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  Member of OSIRIS-REx Sample Analysis Team, cited as part of a mega-multi author appended list. Provided sample measurement support and assitance with developing a non-contaminating thermal property measurement technique.
• Chakraborty, A., Muhammad, W., Masse, J., Tangestani, R., Ghasri-Khouzani, M., Wessman, A., & Martin, É. (2023). Role of alloy composition on micro-cracking mechanisms in additively manufactured Ni-based superalloys. Acta Materialia, 255. doi:10.1016/j.actamat.2023.119089
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  Inherent micro-cracking mechanisms in two contrasting high-gamma-prime (high-γ’) Ni-based superalloys processed by laser powder bed fusion (LPBF) were investigated. RENÉ 65 (R65) has a 42% γ’ volume fraction including Al, Ti, and Nb, while RENÉ 108 (R108) has a higher γ’ volume fraction (63%) including Al, Ta, and lesser Ti. Quantitative analysis showed R108 exhibits 329% higher micro-cracking density than R65. All micro-cracks propagate along high angle boundaries (HABs) and exhibit interdendritic morphologies suggesting solidification cracking is the dominant micro-cracking mechanism. Moreover, secondary phases detrimental for solid-state cracking are not observed at the HABs. Atom probe tomography (APT) showed preferential segregation of Hf, Mo, C, B at the R108 HAB and Zr, Ti, Mo, C, B at the R65 HAB. The Kou solidification cracking criterion showed that Hf and Zr partitioning along the grain boundaries increases micro-cracking susceptibility. Gamma prime did not form during the LPBF process and titanium has a higher tendency than tantalum to partition in the last liquid to solidify.
• Hasan, N., Habibor Rahman, M., Wessman, A., Smith, T., & Shafae, M. (2023). Process Defects Knowledge Modeling in Laser Powder Bed Fusion Additive Manufacturing: An Ontological Framework. Manufacturing Letters, 35. doi:10.1016/j.mfglet.2023.08.132
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  Comprehensive knowledge of the laser powder bed fusion (LPBF) process defects, their causal factors, and relationships can enable proactive prevention and/or mitigation of those defects to ensure the production of high-quality products. However, this knowledge is scattered in a plethora of research articles, and there is a need for a formal and structured knowledge base to document the LPBF process defects knowledge and model the complex network of relationships among those defects and their causal factors. In response, this paper proposes an ontological framework to systematically structure and represent the knowledge of LPBF defects in a sustainable, reusable, and extensible way. In doing so, we first conducted a detailed literature review and analysis of current LPBF defect surveys to systematically develop a consistent and comprehensive classification of defects and potential causal factors. Then, to effectively represent the gathered knowledge, the ontological framework was designed to: (1) organize and formalize knowledge on LPBF defects and the causal factors, (2) model the complex network of causal links and cascading effects among the defects and causal factors, (3) enable easy querying of the stored knowledge, and (4) include ontological entities that are suitable for extension and reuse. A prototype LPBF defects ontology was developed in Web Ontology Language (OWL)/Resource Description Framework (RDF) formalism using the Protégé tool to effectively realize those design requirements. The developed ontology covers thirty-one unique defects and knowledge of their causal factors, including defect-to-defect causal relationships and hierarchical categorization under four major categories of high-level defect classes. Similarly, forty-five unique causal factors were categorized under twelve major categories. The proposed ontological framework and knowledge model offer a pathway to (1) provide a comprehensive knowledge base on LPBF defects for in-depth tutoring and training of novice researchers and practitioners; (2) help investigators to identify root causes of detected defects for informed corrective action; (3) guide process planning tasks from a defect control perspective; and (4) support future application and reuse of the knowledge. This study also provides several examples to illustrate the modeling of cascading effects in causal relationships, discovering knowledge using an ontology reasoner, visualizing the complex network of causal links using OntoGraph, and retrieving stored knowledge by answering competency questions using SPARQL queries. In the future, the reasoning capabilities of our proposed ontology can also be leveraged to develop expert systems for optimizing the AM workflow and quantitatively predict and diagnose LPBF defects. © 2023 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the Scientific Committee of the NAMRI/SME.
• Hilla, C., Wessman, A., Aman, R., Eff, M., Hayes, R., DiMarco, B., Herderik, E., Zhang, W., & Mills, M. (2023). Effect of Solutionizing Heat Treatment on Microstructure and Mechanical Behavior of Additively Manufactured Medium Gamma Prime Nickel Superalloy. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 54(6). doi:10.1007/s11661-023-07035-7
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  Additive manufacturing (AM) of γ′-strengthened Ni-based superalloys is appealing for use in fabrication of high-temperature structural components. As AM produces unique microstructures and mechanical behaviors, a better understanding of microstructure development during post-printing heat treatment is important. An extensive set of experimental data of Rene65 printed by powder bed fusion-laser beam is reported. Effects of heat treatment on microstructure are characterized by scanning electron microscopy and electron-backscattered diffraction. Elevated temperature tensile testing, tension creep, and compression creep are conducted with samples loaded parallel and transverse to the build direction. Recrystallization occurs, resulting in an equiaxed grain structure, only with supersolvus heat treatments. There is no effect of supersolvus hold time on grain growth, a behavior different from that of wrought Rene65. Subsolvus heat treatments result in a coarse bimodal precipitate structure, while rapid cooling from supersolvus results in a fine homogenous structure. Comparable tensile behavior is seen regardless of heat treatment, apart from differences in elongation to failure due to loading direction. Creep behavior is improved with supersolvus heat treatment, although increased hold time has a detrimental effect. Based on the experimental results, the relation of microstructures to mechanical behaviors for additively manufactured Rene65 is discussed.
• Tiparti, D., Wessman, A., Cormier, J., & Tin, S. (2023). Comparison of the stress relaxation and creep behavior of conventionally forged and additively manufactured René 65. Journal of Materials Science, 58(13). doi:10.1007/s10853-023-08399-2
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  René 65 is a high strength cast & wrought (CW) Ni-base superalloy with thermomechanical performance tailored toward intermediate temperature and stress conditions (677 °C/690 MPa). René 65 Turbine disk rotor forgings are typically subject to a sub-solvus heat treatment utilized to retain a uniform and fine-grained microstructure that confers a high level of fatigue performance and strength at the expense of high-temperature creep resistance. However, the processing of René 65 via laser powder bed fusion (LPBF) can enable this material to exhibit intermediate or coarse-grained microstructures after heat treatment that possess improved creep resistance. The effect of the difference in the processing route in conjunction with heat treatment on the resulting microstructure is presented and correlated to the creep behavior (at 677 °C/690 MPa) and stress relaxation behavior (700 °C). A good correlation of relaxation and creep behaviors was found, with LPBF processed René 65 exhibiting overall improved resistance to high-temperature deformation. The primary factor with regards to microstructural difference toward the stress relaxation/creep behavior of René 65 has been identified to be the size and volume fraction of tertiary γ’ present resulting from the chosen processing pathway.
• Chakraborty, A., Tangestani, R., Batmaz, R., Muhammad, W., Plamondon, P., Wessman, A., Yuan, L., & Martin, É. (2022). In-process failure analysis of thin-wall structures made by laser powder bed fusion additive manufacturing. Journal of Materials Science & Technology, 98, 233-243.
• Chakraborty, A., Tangestani, R., Muhammad, W., Sabiston, T., Masse, J., Batmaz, R., Wessman, A., & Martin, É. (2022). Micro-cracking mechanism of RENÉ 108 thin-wall components built by laser powder bed fusion additive manufacturing.
• Wessman, A., Tin, S., Cormier, J., & Tiparti, D. (2022). Comparison of the Stress Relaxation and Creep Behavior of Conventionally Forged and Additively Manufactured René 65 Corresponding Author: Dhruv Tiparti. Social Science Research Network. doi:10.2139/ssrn.4229945
• Varney, T. C., Quammen, R. N., Telesz, N., Balk, T. J., Wessman, A., & Rottmann, P. F. (2021). Effects of Pore Geometry on the Fatigue Properties of Electron Beam Melted Titanium-6Al-4V. Metallurgical and Materials Transactions A, 52(5), 1836-1849.
• Katsari, C. M., Wessman, A., & Yue, S. (2020). Heat Treatment Optimization of a#Ó2-Strengthened Nickel-Based Superalloy Based on Central Composite Design. Metallurgical and Materials Transactions A, 51, 5806 - 5817.
• Katsari, C., Wessman, A., & Yue, S. (2020). Taguchi Design for Heat Treatment of Rene 65 Components. Journal of Materials Engineering and Performance, 29(6). doi:10.1007/s11665-020-04783-0
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  Rene 65 is a nickel-based superalloy used in aerospace components such as turbine blades and disks. The microstructure in the as-received condition of the superalloy consists of ~ 40% volume fraction of gamma prime precipitates, which gives such a high strength that thermomechanical processing is problematic. The goal of this study was to improve the processability of Rene 65 by developing a heat treatment to lower the strength through changes in the size distribution and volume fraction of those precipitates. Gamma prime in this alloy is observed in three sizes, ranging from a few μm to tens of nm. For the design of the heat treatments, Taguchi’s L8 matrix design of experiments was used. The four factors that are examined are cooling rate, hold temperature, hold time and cooling method to room temperature. The levels of the factors were two (high and low) with replication. Microstructures were characterized by scanning electron microscopy and mechanical properties by Vickers microhardness testing. Regression analysis on the results revealed that the most significant factor for this design is hold temperature. The softest sample and the hardest sample have a significant difference microstructurally, with the latter having a trimodal distribution of precipitates which is believed to cause the strength.
• Martin, E., Muhammad, W., Detor, A. J., Spinelli, I., Wessman, A., & Wei, D. (2020). "Strain-annealed" grain boundary engineering process investigated in Hastelloy-X. Acta Materialia, 9, 100544.
• Poorganji, B., Ott, E., Kelkar, R., Wessman, A., & Jamshidinia, M. (2019). Review: Materials Ecosystem for Additive Manufacturing Powder Bed Fusion Processes. JOM.
• Wessman, A., Ott, E., & Kelkar, R. (2019). Heat treatment of additively manufactured metal components: For parts made by direct metal laser melting, heat treatment is important to set the final microstructure, which impacts mechanical properties. Advanced Materials and Processes, 177(2).
• Oruganti, R., Shukla, A., Nalawade, S., Sarkar, S., Sivakumar, K., Vishwanath, T., Sondhi, S., Wessman, A., Wei, D., Powell, A. M., Bain, K., Schaeffer, J. C., Peck, A., Arnett, M., Shastry, G., & Mastromatteo, F. (2018). A Microstructure-Based Model for Creep of Gamma Prime Strengthened Nickel-Based Superalloys. Journal of Engineering Materials and Technology. doi:10.1115/1.4040554
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  This paper outlines a microstructure-based model relating gamma prime microstructure and grain size of Ni-base alloys to their creep behavior. The ability of the model to explain creep of multiple superalloys with a single equation and parameter set is demonstrated. The only parameters that are changed from alloy to alloy are related to the gamma prime characteristics and grain size. This model also allows prediction of creep performance as a function of heat treatment and explains some apparently contradictory data from the literature.
• Stinville, J., Martin, E., Karadge, M., Ismonov, S., Soare, M., Hanlon, T., Sundaram, S., Echlin, M., Callahan, P., Lenthe, W., Miao, J., Wessman, A., Finlay, R., Loghin, A., Marte, J., & Pollock, T. (2018). Competing Modes for Crack Initiation from Non-metallic Inclusions and Intrinsic Microstructural Features During Fatigue in a Polycrystalline Nickel-Based Superalloy. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 49(9). doi:10.1007/s11661-018-4780-3
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  Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.
• Stinville, J., Martin, E., Karadge, M., Ismonov, S., Soare, M., Hanlon, T., Sundaram, S., Echlin, M., Callahan, P., Lenthe, W., Miller, V., Miao, J., Wessman, A., Finlay, R., Loghin, A., Marte, J., & Pollock, T. (2018). Fatigue deformation in a polycrystalline nickel base superalloy at intermediate and high temperature: Competing failure modes. Acta Materialia, 152. doi:10.1016/j.actamat.2018.03.035
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  The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries.
• Smith, T. M., Esser, B. D., Antolin, N., Carlsson, A., Williams, R. E., Wessman, A., Hanlon, T., Fraser, H. L., Windl, W., McComb, D. W., & Mills, M. J. (2016). Phase transformation strengthening of high-temperature superalloys. Nature Communications. doi:10.1038/ncomms13434
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  Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new 'phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ' to η phase along stacking faults in γ' precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.
• Smith, T. M., Esser, B. D., Antolin, N., Viswanathan, G., Hanlon, T., Wessman, A., Mourer, D. P., Windl, W., McComb, D. W., & Mills, M. J. (2015). Segregation and η phase formation along stacking faults during creep at intermediate temperatures in a Ni-based superalloy. Acta Materializa. doi:10.1016/j.actamat.2015.08.053
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  In this paper, the local compositional and structural changes occurring in association with stacking faults in a Ni-base superalloy are characterized and related to the possible rate-controlling processes during creep deformation at intermediate temperatures. These rate-controlling processes are not presently understood. In order to promote stacking fault shearing, compression creep tests on specially prepared single crystals of an exploratory Ni-base superalloy were conducted at 760 °C in the [0 0 1] orientation. Scanning transmission electron microscopy (STEM) imaging was coupled with state-of-the-art energy dispersive X-ray (EDX) spectroscopy to reveal for the first time an ordered compositional variation along the extrinsic faults inside the γ′ precipitates, and a distinct solute atmosphere surrounding the leading partial dislocations. The local structure and chemistry at the extrinsic fault is consistent with the η phase, a D024 hexagonal structure. Density Functional Theory (DFT) and high angle annular dark field (HAADF)-STEM image simulations are consistent with local η phase formation and indicate that a displacive–diffusive transformation occurs dynamically during deformation.
• Laurence, A., Wessman, A., Mompiou, F., Petinari-Sturmel, F., Cormier, J., Franchet, J., Hantcherli, M., Villechaise, P., & Billot, T. (2014). Impact of the Solution Cooling Rate and of Thermal Aging on the Creep Properties of the New Cast & Wrought Rene 65 Ni-Based Superalloy. Superalloy 718 and Derivatives 2014. doi:10.7449/2014/superalloys_2014_333_348
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  Aeroengines turbine disks can encounter temperature in excess of 700°C-750°C for hundreds to thousands of hours in the rim sections during service operations, exposures which may affect the viscoplastic properties of the alloys due to strengthening particles' growth.The present work aims at quantifying and analyzing the impact of y'-size evolutions and of intermetallic grain-boundary precipitation during long-term thermal exposures at 700°C and 800°C on the creep properties at 700°C of the newly developed Rene 65 alloy.In an aim to separate both effects, various sub-solvus solution heat treatments with different cooling rates were used to achieve y' sizes comparable to a given thermal exposure in the 700-800°C range, without the precipitation of secondary intermetallic particles at grain boundaries.Using this methodology, it is shown unambiguously that the precipitation of TCP phases at grain boundaries is deleterious to the creep properties at 700°C/800 MPa.

Proceedings Publications

• Beamer, C., Wessman, A., & Godfrey, D. (2023). Use of High-Pressure Heat Treatment (HPHT™) for L-PBF F357. In Hot Isostatic Pressing- HIP 2022.
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  Recent advancements in hot isostatic pressing (HIP) equipment now offer the ability to integrate HIP and heat treatment in the HIP furnace with the aid of controllable high-speed cooling and in-HIP quenching. This approach not only offers improvement in productivity but provides a path to prevent anomalies during heat treatment including thermally induced porosity (TIP) and part quench cracking or distortion. This manuscript will cover the approach of High Pressure Heat Treatment (HPHTTM) applied to SLM Solutions laser powder bed fusion (L-PBF) printed high strength aluminum alloy F357. Microstructure, tensile properties, and part distortion are evaluated. The results capture a post process method making it possible to prevent hydrogen blistering, mitigate defects present in the L-PBF material, offer strength properties exceeding that of MMPDS cast properties, and minimize geometric distortion of complex part geometries.
• Gonzalez, J., Zhang, Y., Wessman, A., & Klemm-Toole, J. (2023). Understanding Annealing Behavior During Post-Built Heat Treatment of Ni-Based Alloys Across Additive Manufacturing Processes. In Superalloys 718 and Derivatives 2023.
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  Ni-based alloys are used for high temperature structural components that span from small, highly complex, with fine feature resolution to large, simple shapes with low dimensional tolerances, necessitating the use of processes spanning from laser powder bed fusion (LPBF) to wire arc additive manufacturing (WAAM). However, there is very little understanding about how annealing behavior during post-build heat treatments varies between additive manufacturing processes. In this work, we explore the annealing behavior of IN625 and Haynes 282, manufactured with WAAM and LPBF, under the same annealing conditions. The results of hardness measurements after annealing indicate that for both IN625 and Haynes 282, the LPBF samples show larger decreases in hardness between the as-built condition and after annealing at 1200 °C for 1 h compared to the WAAM samples. LPBF IN625 and Haynes 282 samples annealed at 1200 °C for 1 h, all show complete and partial recrystallization, respectively, whereas none of the WAAM samples annealed at this temperature show recrystallization. For a given alloy, both LPBF and WAAM samples annealed at 1200 °C show particles with compositions consistent with MC carbides that are predicted from thermodynamic simulations. The MC particles present are of similar size and distribution in both LPBF and WAAM samples indicating a similar capacity for these particles to pin moving boundaries during recrystallization. In concert, these results suggest that LPBF samples have more stored energy in the as-built condition compared to their WAAM counterparts, and therefore have a higher driving force for recovery and recrystallization.
• Zhang, Y., Hasan, N., Middendorf, J., Spears, T., Smith, T., Zhang, F., Shafae, M., & Wessman, A. (2023). Correlating Alloy Inconel 718 Solidification Microstructure to Local Thermal History Using Laser Powder Bed Fusion Process Monitoring. In Superalloy 718 and Derivatives 2023.
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  Additive manufacturing processes such as laser powder bed fusion produce material by localized melting of a powder feedstock layer by layer. The small melt pools and high energy density generate very different microstructures in nickel superalloys when compared to more traditional cast or wrought processing, including features such as cellular structures and epitaxial grain growth. The features of these microstructures vary depending on local thermal history, alloy chemistry, and processing parameters. There is a need to develop a systematic understanding of the influence the local thermal conditions during solidification have on the resulting microstructure. Such understanding will be useful in predicting and ultimately avoiding microstructural defects such as undesirable phases or non-optimal grain structures. In this work, in-situ Longwave Infrared imaging of a laser powder bed fusion process is used to characterize the local thermal conditions throughout additively manufactured builds for alloy IN718 processed using systematically varied process parameters. This information is then correlated to observations of the microstructural features of these alloys in the as-built condition. This correlation analysis shows clear influence of the local thermal conditions during solidification on the dimensions of the dendritic microstructures formed during the build process for IN718. These dendritic structures arise due to segregation of elements such as niobium during solidification, an observation which can be predicted using a Scheil modeling approach.
• Wessman, A., Cormier, J., Hamon, F., Rainey, K., Tin, S., Tiparti, D., & Dial, L. (2020, September). Microstructure and Mechanical Properties of Additively Manufactured Rene 65. In Superalloys 2020.
• Katsari, C., Che, H., Guye, D., Wessman, A., & Yue, S. (2018). Microstructural characterization and mechanical properties of rene 65 precipitates. In Superalloy 718 and Derivatives 2018.
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  Nickel based superalloys are used in the aerospace industry as turbine rotor material due to their high strength and excellent fatigue resistance at high temperature. High strength is required for the high stress and high temperature environments in which these alloys operate. This is achieved by their microstructure of gamma matrix/gamma prime precipitates. The latter can cause thermomechanical processing issues since they can make the material extremely hard. The purpose of this work is to characterize the gamma prime precipitates which are present in Rene 65, a newly developed cast and wrought nickel-based superalloy by General Electric and ATI. The main precipitate that was examined was the gamma prime phase [Ni3 (Al, Ti)], which largely determines the mechanical properties of the alloy. The gamma prime in this particular alloy was found to form in three different sizes; primary, secondary and tertiary. After various heat treatments, the differences in the volume fraction and morphology of each precipitate type is described and the effect of the precipitates on hardness is determined.
• McAllister, D., Lv, D., Feng, L., Deutchman, H., Wessman, A., Wang, Y., & Mills, M. (2018). Characterization and modeling of deformation mechanisms in Ni-base superalloy 718. In Superalloy 718 and Derivatives 2018.
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  Although the relationships between processing and the resulting properties are relatively well known for alloy 718, a better understanding of the deformation mechanisms activated across its usable temperature range is needed to create more mechanistically accurate property models. In this work, direct atomic-scale imaging with high angle annular dark field scanning transmission electron microscopy (STEM) has been complemented by phase field modeling informed by generalized stacking fault surface calculations using density functional theory. This coupled experiment/modeling approach has shed light on the complex shearing processes occurring in alloy 718 following a standard commercial heat treatment, which produces both monolithic y and y" particles, as well as composite particles. Deformation at room temperature occurs through complex shearing of γ" into intrinsic stacking fault configurations that were restricted to the precipitates. Exploration of possible shearing sequences with the aide of phase-field dislocation dynamics has revealed that precipitate shearing by motion of coupled 1/2 dislocations of non-parallel Burgers vectors on the {111} glide plane is the dominant deformation mechanism at lower temperature. A “fast-acting” yield strength model is discussed which takes into account microstructure variations and deformation mechanism transitions. Deformation at higher temperature (427 and 649 °C) has revealed a distinct transition in deformation modes, including stacking faults extending into the matrix, as well as microtwinning. The possible origin of the temperature and rate dependence of the stacking fault and microtwinning modes and temperature will be discussed.
• Katsari, C., Che, H., Turner, B., Wessman, A., & Yue, S. (2017). Precipitation characteristics of gamma prime precipitate in rene 65. In Proceedings of MS&T17 Conference.
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  Superalloys have been used for many years in the aerospace industry due to their excellent creep performance in high temperature and high stress environments. The precipitation that contributes to creep resistance also increases the strength; however, this gives rise to thermomechanical processing issues. Therefore, the ultimate goal of this work is to understand the precipitation characteristics in order to design heat treatments that make thermomechanical processing easier without compromising the final microstructure and properties. The main precipitate that was examined was the gamma prime phase (γ') [Ni3 (Al,Ti)], which largely determines the mechanical properties of the alloy. The γ' was found to form in three different sizes; primary, secondary and tertiary. The differences in the volume fraction and morphology of each precipitate type was noted and the relationship between heat treatment and precipitates will be discussed.
• Powell, A., Bain, K., Wessman, A., Wei, D., Hanlon, T., & Mourer, D. (2016). Advanced supersolvus nickel powder disk alloy DOE: Chemistry, properties, phase formations and thermal stability. In Superalloys 2016.
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  GE Aviation under the US Air Force Hybrid Disk Contract (FA8650-10-C-5225) investigated a powder nickel alloy design space in search of an alloy composition to increase the state of the art temperature capability for rotating disk applications to 1450 oF (788 °C). The eleven alloy Design of Experiments (DOE) explored the relationship between chemistry and mechanical properties (creep, tensile, ductility, dwell LCF, cyclic fatigue crack growth & static fatigue crack growth) as well as identified key attributes around gamma prime precipitation temperatures, phase stability and long term exposures and stability of gamma prime. Regression analysis of the data obtained indicates that increasing W and decreasing Cr and Ti are desirable for improved high temperature strength and creep capability.
• Smith, T., Duchao, L., Hanlon, T., Wessman, A., Wang, Y., & Mills, M. (2016). Determination of orientation and alloying effects on creep response and deformation mechanisms in single crystals of Ni-base disk superalloys. In Superalloys 2016.
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  In this study, the effects of varying crystal orientation and composition on active deformation modes are explored for two different Ni-base disk alloys. Understanding these effects will allow for improved predictive modeling and consequently faster advancements in Ni-base alloy development. In order to investigate these effects, compression creep tests were conducted on [001] and [110] oriented single crystal specimens of the disk alloys ME3 and ME501, at different stress/temperature regimes. At 760 °C and below, a prominent creep anisotropy exists between the two orientations, with the [110] oriented samples exhibiting superior creep strength. At 815 °C, the creep anisotropy disappeared between the two orientations. Through bright field scanning transmission electron microscopy, it was determined that the existence of creep anisotropy is a result of differences in deformation modes between the different orientations and alloy compositions. Results of phase field modeling in which the interaction of dislocations with realistic precipitate structures is also conducted to further advance predictive creep deformation models.
• Wessman, A., Laurence, A., Cormier, J., Villechaise, P., Billot, T., & Franchet, J. (2016). Thermal stability of cast and wrought alloy Rene 65. In Superalloys 2016.
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  The cast and wrought nickel superalloy Rene 65 has been recently introduced to provide a gamma prime strengthened alloy with high performance at temperatures exceeding 700 °C at a lower cost than powder metallurgy alloys. While similar in chemistry to the PM alloy Rene 88DT, Rene 65 is typically used in a subsolvus form with finer grain size and a very different gamma prime distribution. This study provides an overview of the structure of Rene 65, and the effects high temperature exposures over long times have on the structure and mechanical properties of the alloy.

Presentations

• Wessman, A. (2024). Alloy and Process Development for Nickel Superalloys in Additive Manufacturing Beyond PBF-LB. ASTM International Conference on Additive Manufacturing 2024.
• Wessman, A. (2024). High Throughput Screening Methods in Alloy and Process Development of AM Aluminum. ASTM International Conference on Additive Manufacturing 2024.