Of particular note is the ability of AM to create multi-material parts, however, doing so typically requires multiple feedstock materials, as well as specialized machines designed to handle the different feedstocks, increasing costs and limiting capabilities. In addition, despite the benefits of AM and the increasing surges in popularity of the various techniques, there is still much about the science of the build processes that remains unknown. Filling in existing knowledge gaps areas can only serve to improve current and future processes and ensure the production of repeatable and reproducible parts.
One drawback to current AM parts is the variety of defects formed during the build process. The defects present are typically considered to be deleterious features, particularly in structural parts, and the literature has focused on either reducing or eliminating defects from final parts (during the actual additive process or with post-processing methods) or the effect of defects on mechanical properties, notably fatigue. Here, defects are used to provide crucial insight into the physics of the electron beam powder bed fusion Ti-6Al-4V, insights which can be extended to other alloy systems and processing techniques. The information provided helps fill knowledge gaps that still exist and generate suggested methodologies to acquire more in-depth information moving forward.
In addition, the high temperatures involved with electron beam melting can lead towards the preferential vaporization of select elements with higher vapor pressures compared to other alloying elements (e.g., chromium in stainless steels, magnesium in aluminum alloys, aluminum in titanium alloys), especially when processed under vacuum. For the alloy studied here, Ti-6Al- 4V, preferential vaporization leads to a loss of aluminum during the build process. Again, literature from the past years has focused on the characterization of materials states related to the thermomechanical cycles experienced during the AM process (i.e., the liquid-solid, solid-solid transformations resulting from the layer-by-layer nature of the process) through build heights, often neglecting this elemental loss and assuming compositionally homogeneous parts. While post processing may be able to homogenize local composition, this work shows that differing scan strategies and scan parameters can globally affect composition even within the same build process. The resulting preferential vaporization is therefore shown to have a significant effect on both the local and global materials state (e.g., microstructure, mechanical properties) of the parts.
Finally, the preferential vaporization was controlled through site-specific scan strategies to generate parts from a single feedstock material that contained two distinct compositions. The spatial control of composition led to a spatial control of materials state, including subsequent microstructural evolution controlled through heat treatments. The resulting chemical and microstructural variations were analyzed with a variety of characterization techniques, demonstrating that the chemistry control through the modification of processing parameters was significant enough to enable the tailoring of microstructural and mechanical properties, both elastic and plastic. This work thus offers a new technique to create multi-material parts that sidesteps current limitations and can easily be expanded to other alloy systems.
Citation: O'Donnell, K. (2023). The use of defects and compositional variations to elucidate physical phenomena in electron beam melted Ti-6Al-4V across scan strategies (Doctoral dissertation, Iowa State University).
Metal additive manufacturing (AM) has received increased interest throughout the aerospace industry. One avenue for the quality control of metal AM resides in nondestructive evaluation (NDE) capabilities. A literature survey of the implementation of NDE methods for metal AM was conducted. X-ray Computed Tomography (XCT) is the most commonly used NDE method for metal AM and the most appropriate modality for the goals of this research. Automated mechanical polish Serial Sectioning (SS) is hypothesized to characterize the material’s near-ground state. Procedures are developed to correlate SS results with the XCT representation over the same volumetric region. Using this method for comparison, the capabilities of XCT for the NDE of metal AM can be quantitatively
measured. Improvements and essential details to consider during the XCT of metal AM are presented. Variability between the SS and XCT measurements for porosity is demonstrated progressively, and details regarding data considerations are explained. The average measurement difference of the largest pores between XCT and SS data varied by 5%-25% in equivalent spherical diameter. The sampling density and contrast-to-noise ratio of each XCT scan are discussed as critical parameters for this investigation. A method for estimating the error of the metrics generated from an XCT scan by using the sampling density and contrast-to-noise ratio is investigated, simulated, modeled, and then corroborated against SS data. Corroboration was successful for this error estimation method for 37 out of 50 pores within XCT data. Finally, a review of the findings from the associated studies and each chapter is presented with an overall conclusion to this research.
Citation: Jolley, B. R. (2023). The Correlative Analysis of Porosity in Additively Manufactured Metals Via X-Ray Computed Tomography and Mechanical Polish Serial Sectioning (Doctoral dissertation, The University of Utah).
Towards model-based state estimation and control of the metal powder bed fusion process
We demonstrate the efficacy of applying training datafree algorithms to the in-situ PBF thermal process monitoring and control problems. Our process monitoring algorithm is the Ensemble Kalman Filter (EnKF), which is a type of state estimator that uses a particle swarm to generate self-tuned, approximately 2-norm optimal, model-based estimates of the relevant process signatures. Here, the signatures are all temperatures in the PBF build. Our control algorithm is Model Predictive Control (MPC), which uses model-based predictions of future process signatures (here, temperatures) to determine a sequence of process inputs that regulates them in a locally-optimized way while respecting process constraints. Our EnKF and MPC implementations are built on linearized finite element method descriptions of heat conduction for both the L-PBF and E-PBF processes with a variety of current, emerging, and proposed hardware configurations. We derive bounded sufficient conditions for the controllability and observability of these models. EnKF and MPC performance are validated in simulation, and the EnKF is also validated with experimental data collected from an open architecture L-PBF machine.
Wood, N. (2023). Towards model-based state estimation and control of the metal powder bed fusion process (Doctoral dissertation, The Ohio State University).
Carbon fiber reinforced composites (CFRPs) have become an industry standard in most applications requiring high strength to weight ratios. CFRPs have been manufactured in a multitude of ways, leaving manufacturers with a certain degree of control over cost and final strength. One consequence of the manufacturing is entanglement within the fibrous microstructure. Entanglement occurs when neighboring carbon fibers become nonparallel to each other, or off-axis from their intended orientation. It is thought that the presence of entanglement has a significant influence on the property scatter of composite materials. Therefore, the proper quantification of the microstructural features of carbon fibers is necessary for the advancement of microstructural modeling. The proposed work presents 1) a novel cluster metric which detects fiber bundles within a CFRP in an automated way, 2) and experiment isolating the entanglement within three composite microstructures when all other manufacturing parameters remain constant, and 3) a survey of manufacturing methods to understand which metrics differ and which remain the same across all methods. In this work, a case is made for the use of these metrics to link a fibrous microstructure back to the manufacturing process that created it. Studying these metrics could provide a basis for discussion of the differences in manufacturing methods and their effects on the microstructure, as well as provide a mathematical description of the way real fibers are organized with the potential for informing artificial microstructural generation.
Citation: Schey, M. (2023). Effects of Manufacturing on the Entanglement of Fiber Reinforced Composites (Doctoral dissertation, University of Massachusetts Lowell).
This presentation covered a wide variety of subjects from NASA’s activities along the path to Qualification & Certification of Additive Manufactured (AM) Parts for NASA Applications including important AM defect types and how to detect them, key quality assurance products from MSFC-STD-3716, MSFC-STD-3717, MSFC-STD-6030, MSFC-STD-6033 and AM Handbook, and NASA's the challenges and best practices for nondestructive evaluation (NDE) of metal AM parts.
Learn more by watching the Webinar Recording or review the Presentation PDF.
Citation: Burke, E., Jones, J. S., Lanigan, E., Leckey, C. A. C., & Wells, D. N. NASA's Agency Wide Efforts to improve Non-destructive Evaluation Methods for Additive Manufacturing and In-Space Inspection. In CNDE Webinar Presentation.
GRCop are a family of Cu-Cr-Nb alloys with a Cu matrix and Cr2Nb dispersoids. These alloys were developed for high temperature mechanical properties with high Thermal conductivity. This work describes the role of Robo-Met and other techniques in assessing the mechanical properties of these alloys.
Citation: Smith, J., Scannapieco, D. S., Ellis, D. L., & Lewandowski, J. J. (2023, October). Mechanical Characterization of As-built and Post-processed In-situ Alloyed Additively Manufactured GRCop-42. In Materials Science and Technology Conference 2023.
ORNL has a strong background in materials characterization. Multiple facilities are outfitted with light and electron microscopes, x-ray diffraction, mechanical tensile/compressive testing, and so on. Two unique capabilities acquired specifically for analyzing AM components include the Malvern Panalytical particle microscope and the UES Robo-Met serial sectioning unit.
Citation: Hyer, H. (2023). Status of Additive Manufacturing Capabilities for Processing Refractory Alloys Under the Mo-99 Program (No. ORNL/TM-2022/2721). Oak Ridge National Lab.(ORNL), Oak Ridge, TN (United States).
Defects in parts produced by additive manufacturing, instead of simply being perceived as deleterious, can act as important sources of information associated with the complex physical processes that occur during materials deposition and subsequent thermal cycles. Indeed, they act as materials-state ‘fossil’ records of the dynamic AM process. The approach of using defects as epoch-like records of prior history has been developed while studying additively manufactured Ti–6Al–4V and has given new insights into processes that may otherwise remain either obscured or unquantified. Analogous to ‘epochs,’ the evolution of these defects often is characterized by physics that span across a temporal length scale. To demonstrate this approach, a broad range of analyses including optical and electron microscopy, X-ray computed tomography, energy-dispersive spectroscopy, and electron backscatter diffraction have been used to characterize a raster-scanned electron beam Ti–6Al–4V sample. These analysis techniques provide key characteristics of defects such as their morphology, location within the part, complex compositional fields interacting with the defects, and structures on the free surfaces of defects. Observed defects have been classified as banding, spherical porosity, and lack of fusion. Banding is directly related to preferential evaporation of Al, which has an influence on mechanical properties. Lack-of-fusion defects can be used to understand columnar grain growth, fluid flow of melt pools, humping, and spattering events.
Citation: O’Donnell, K., Quintana, M. J., Kenney, M. J., & Collins, P. C. (2023). Using defects as a ‘fossil record’to help interpret complex processes during additive manufacturing: as applied to raster-scanned electron beam powder bed additively manufactured Ti–6Al–4V. Journal of Materials Science, 1-24.
Statistically equivalent, artificial microstructures with similar fiber morphologies to as-manufactured scans are commonly used in micromechanical modeling. Features such as fiber clusters and matrix pockets are important as they may influence macroscale failure. In this study, a method of generating statistically equivalent artificial microstructures to experimental scans using local fiber volume fraction, fiber clusters, and matrix pockets was examined. 3000 artificial microstructures were created with a generator by randomly sampling input parameters which changed the fiber morphology. Fiber cluster and matrix pocket areas, densities, and orientations were used to characterize microstructures by sorting neighboring fiber triads. Experimental scans were used validate inputs from the artificial microstructure generator. Results showed the microstructures generated produced descriptors within range of the experimental scans. Microstructures were generated to match different descriptors of scanned specimens. First only local volume fraction was matched, and results compared to scans, then all descriptors were matched and compared.
Citation: Husseini, J. F., Pineda, E. J., & Stapleton, S. E. (2023). Generation of artificial 2-D fiber reinforced composite microstructures with statistically equivalent features. Composites Part A: Applied Science and Manufacturing, 164, 107260.
Carbon Fiber Reinforced Plastics (CFRPs) are widely used due to their high stiffness to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of compacting a dry fibrous reinforcement prior to resin infusion. This process is meant to decrease the average inter-fiber distance, effectively increasing the fiber volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRP laminates, varying only the debulking step in the manufacturing process. High resolution serial sections of all three laminates were taken for analysis. Using these scans, the fiber positions were reconstructed. Statistical descriptors such as local fiber and void volume fractions, fiber orientation, and void distribution and morphology were then generated for each sample. Fiber clusters present within the material were identified and analyzed for each level of debulking applied. Using these descriptors, the effects of debulking on the morphology and organization of the composite microstructure was evaluated.
Citation: Schey, M., Beke, T., Owens, K., George, A., Pineda, E., & Stapleton, S. (2023). Effects of debulking on the fiber microstructure and void distribution in carbon fiber reinforced plastics. Composites Part A: Applied Science and Manufacturing, 165, 107364.
The newly patented micro-multilayer multifunctional electrical insulation (MMEI) system was developed for future electric aircraft applications which critically require lightweight but high voltage (HV), high temperature, and corona or partial discharge (PD) resistant insulation. During the initial development stages, the concept and practicability of the MMEI system were successfully validated with its exceptionally high dielectric breakdown voltages. The multilayer structures were optimized in terms of material type, individual layer thickness, and overall layer configuration along with potential mechanisms identified for its superior performance. Subsequently, scalability, manufacturability, and commercial applicability of the MMEI system were demonstrated with the 1 meter long, 3-phase HV, high power (HP) bus bar prototypes. Two prototypes, one with the conventional SOA insulation system including Mica sheet and the other with an optimized MMEI, were designed, fabricated, and tested successfully. Both prototypes passed both HiPot and PD tests up to the highest test voltage available, 15 kVAc, although the latter showed a slight increase in PD activities at 12.5 kV. However, the prototype with MMEI was 15% lighter or 12% thinner than the other one. Current efforts to significantly enhance the PD resistance of the MMEI system by employing semiconductive shielding layers, which can be also multifunctional, e.g., electromagnetic interference shielding, moisture blocking, heat dissipation, for various HV applications are also discussed in this paper.
Citation: Shin, E. S. E. (2023, June). Feasibility of Micro-Multilayer Multifunctional Electrical Insulation (MMEI) System for High Voltage Applications. In 2023 IEEE Electrical Insulation Conference (EIC) (pp. 1-6). IEEE.
This work explores the correlation between the characteristics of the cast structure (dendrite growth pattern, dendrite morphology and macro-texture) and strain hardening capacity during high temperature deformation of Mg-5Sn-0.3Li-0 and 3Zn multi-component alloys. The three dimensional (3D) morphology of the dendrite structure demonstrates the transition of the growth directions from <112¯3>,<112¯0> and <112¯2> to< 112¯3> and <112¯0> due to the addition of Zn. The simultaneous effects of growing tendency and the decrement of dendrite coarsening rate at the solidification interval lead to dendrite morphology transition from the globular-like to the hyper-branch structure. This morphology transition results in the variation of the solidification macro-texture, which has effectively influenced the dominant deformation mechanisms (slip/twin activity). The higher activity of the slip systems increases the tendency of the dendrite arms for bending along the deformation direction and fragmentation. Apart from this, the dendrite holding hyper-branch structure with an average thickness below 20 µm are more favorable for fragmentation. The dendrite fragmentation leads to considerable softening fractions, and as an effective strain compensation mechanism increases the workability of dendritic structure.
To obtain the Three-dimensional (3D) morphologies of the α-Mg dendrite, as-solidified microstructures were analyzed through the automatic serial sectioning machine (UES Inc., Robo-Met.3D) and 3D analysis software (FEI, Avizo Fire 7). The samples were mechanically polished and hundreds of consecutive images were obtained by optical microscopy. Three-dimensional images were obtained by image segmentation by thresholding, image processing using noise reduction filters, and 3-D surface generation. The mean curvatures between solid (dendrites) and the liquid phases were calculated at the solidification interval, where the average solid fraction was about 50%.
Citation: Jalali, M. S., Zarei-Hanzaki, A., Mosayebi, M., Abedi, H. R., Malekan, M., Kahnooji, M., ... & Kim, S. H. (2023). Unveiling the influence of dendrite characteristics on the slip/twinning activity and the strain hardening capacity of Mg-Sn-Li-Zn cast alloys. Journal of Magnesium and Alloys, 11(1), 329-347.
Porosity can form during investment casting as a result of the solidification conditions. Significant porosity can result in agglomerations (pore clusters) which are a primary crack initiation source and can result in reduced fatigue life of Nickel-based superalloys. Such clusters of porosity are today conventionally measured using 2D micrographs. An approach to accurately predict and analyze casting porosity from 2D cuts while facing in reality a 3D shape problem is currently missing. In this work, an approach that combines automation and simulation to assess the representative pore dimension, their interconnectivity and the porosity position is presented. On a LPT blade made of IN100, the porosity percentage and Feret diameter have been measured in multiple cuts and the porosity was found to be in the range of 0.88 to 5.4 pct. From the same micrographs the critical defect sizes were estimated with an automated tool to be in the range of 200 to 1800 μm. The simulated shrinkage porosity for the same part, was predicted to be in the range of 1.67 to 2.33 pct. This study shows that the scaling factor between the pore Feret diameter and the critical pore size is equal to 2.9, in accordance to the proportionality of critical pores size clusters calculated from a 2D and 3D dataset. Finally, the simulated casting porosity was compared to that measured on cast blades in critical regions and the predictive accuracy is discussed in detail.
Citation: Bellomo, N. P., Öztürk, I., Günzel, M., Reed, R., Sundar, V., Ammar, A., & Schwalbe, C. (2023). Identifying Critical Defect Sizes From Pore Clusters in Nickel-based Superalloys Using Automated Analysis and Casting Simulation. Metallurgical and Materials Transactions A, 1-11.
We investigated the microstructural evolution and precipitation behavior of a multicomponent Al83Zn5Cu5Mg5Li2 alloy using transmission electron microscopy (TEM) and atom-probe tomography (APT) during solution treatment, cooling, and natural aging. The as-cast alloy consisted of high volume fractions of V-Al5Cu6Mg2 and η-Mg(Zn,Cu,Al) phases, which were partially dissolved during solution treatment at 450 °C. Slow furnace cooling (FC) after solution treatment resulted in the formation of polygonal V-phase, elongated η-phase, and platelet Y-phase precipitates. The Y-phase precipitates consisted of one Mg-rich central layer and three adjacent Zn/Cu-rich layers, both lying on the {111}Al planes. Instead of Y-phase precipitates, air cooling (AC) induced the formation of S- and Zn-phase precipitates with well-developed orientation relationships. No precipitation occurred during water quenching (WQ). The microhardness increased significantly during the natural aging of the AC and WQ alloys, whereas no hardening was observed in the FC alloy. TEM and APT analyses demonstrated that the natural aging induced the formation of fine solute clusters with a chemical composition of 52.3Al–21.3Li–18.3Zn–4.9Mg–3.1Cu (at%), contributing to the natural age-hardening of AC and FC alloys. The solute clusters were fully coherent with the Al matrix and exhibited an interfacial region rich in Zn and two central regions rich in Li and Zn.
Citation: Seo, N., Jeon, J., Lee, S. H., Euh, K., Kim, S. H., Ahn, T. Y., ... & Jung, J. G. (2023). Revealing complex precipitation behavior of multicomponent Al83Zn5Cu5Mg5Li2 alloy. Journal of Alloys and Compounds, 169192.
Thermal spray coatings are microscale coatings used to improve the surface properties of surfaces they are applied to. Statistical quantification can improve understanding of a thermal coating's microstructure; however, doing so in an automatic and autonomous
manner from image data is non-trivial. A new methodology is developed to characterize thermal spray coatings from serial sectioning images (or other analogous image data) using manifold surfaces in higher dimensions, as well as associated operators on such data.
Citation: Ahadzie, Senyo Evan. "Thermal Spray Coating Characterization via Higher Dimensional Surfaces." PhD diss., Rice University, 2022.