Dynamic solidification behavior during metal additive manufacturing directly influences the as-built microstructure, defects, and mechanical properties of printed parts. How the formation of these features is driven by temperature variation (e.g., thermal gradient magnitude and solidification front velocity) has been studied extensively in metal additive manufacturing, with synchrotron x-ray imaging becoming a critical tool to monitor these processes. Here, we extend these efforts to monitoring full thermomechanical deformation during solidification through the use of operando x-ray diffraction during laser melting. With operando diffraction, we analyze thermomechanical deformation modes such as torsion, bending, fragmentation, assimilation, oscillation, and interdendritic growth. Understanding such phenomena can aid the optimization of printing strategies to obtain specific microstructural features, including localized misorientations, dislocation substructure, and grain boundary character. The interpretation of operando diffraction results is supported by post-mortem electron backscatter diffraction analyses.
Laser based directed energy deposition system for operando synchrotron x-ray experiments
The adoption of metal additive manufacturing (AM) has tremendously increased over the years; however, it is still challenging to explain the fundamental physical phenomena occurring during these stochastic processes. To tackle this problem, we have constructed a custom metal AM system to simulate powder fed directed energy deposition. This instrument is integrated at the Cornell High Energy Synchrotron Source to conduct operando studies of the metal AM process. These operando experiments provide valuable data that can be used for various applications, such as (a) to study the response of the material to non-equilibrium solidification and intrinsic heat treatment and (b) to characterize changes in lattice plane spacing, which helps us calculate the thermo-mechanical history and resulting microstructural features. Such high-fidelity data are made possible by state-of-the-art direct-detection x-ray area detectors, which aid in the observation of solidification pathways of different metallic alloys. Furthermore, we discuss the various possibilities of analyzing the synchrotron dataset with examples across different measurement modes.
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- Award ID(s):
- 2046523
- PAR ID:
- 10394609
- Date Published:
- Journal Name:
- Review of Scientific Instruments
- Volume:
- 93
- Issue:
- 7
- ISSN:
- 0034-6748
- Page Range / eLocation ID:
- 075106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)Additive manufacturing (AM) comprises a group of transformative technologies that are likely to revolutionize manufacturing. In particular, laser-based metal AM techniques can not only fabricate parts with extreme complexity, but also provide innovative means for designing and processing new metallic systems. However, there are still several technical barriers that constrain metal AM. Overcoming these barriers requires a better understanding of the physics underlying the complex and dynamic laser–metal interaction at the heart of many AM processes. This article briefly describes the state of the art of in situ / operando synchrotron x-ray imaging and diffraction for studying metal AM, mostly the laser powder-bed fusion process. It highlights the immediate impact of operando synchrotron studies on the advancement of AM technologies, and presents future research challenges and opportunities.more » « less
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The continued development of metal additive manufacturing (AM) has expanded the engineering metallic alloys for which these processes may be applied, including beta-titanium alloys with desirable strength-to-density ratios. To understand the response of beta-titanium alloys to AM processing, solidification and microstructure evolution needs to be investigated. In particular, thermal gradients (Gs) and solidification velocities (Vs) experienced during AM are needed to link processing to microstructure development, including the columnar-to-equiaxed transition (CET). In this work, in situ synchrotron X-ray radiography of the beta-titanium alloy Ti-10V-2Fe-3Al (wt.%) (Ti-1023) during simulated laser-powder bed fusion (L-PBF) was performed at the Advanced Photon Source at Argonne National Laboratory, allowing for direct determination of Vs. Two different computational modeling tools, SYSWELD and FLOW-3D, were utilized to investigate the solidification conditions of spot and raster melt scenarios. The predicted Vs obtained from both pieces of computational software exhibited good agreement with those obtained from in situ synchrotron X-ray radiography measurements. The model that accounted for fluid flow also showed the ability to predict trends unobservable in the in situ synchrotron X-ray radiography, but are known to occur during rapid solidification. A CET model for Ti-1023 was also developed using the Kurz–Giovanola–Trivedi model, which allowed modeled Gs and Vs to be compared in the context of predicted grain morphologies. Both pieces of software were in agreement for morphology predictions of spot-melts, but drastically differed for raster predictions. The discrepancy is attributable to the difference in accounting for fluid flow, resulting in magnitude-different values of Gs for similar Vs.more » « less
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Abstract The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique.more » « less
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Abstract The rapid solidification associated with additive manufacturing (AM) leads to complex microstructures with peculiar features amongst which cellular solidification structures are the most remarkable. These metastable structures possess a clear segregation pattern dictated by the solidification pathway of the alloy and are bounded by dislocation walls. While they confer exceptional strength and ductility to AM 316L stainless steel, their effect on localized corrosion in chloride environments remains to be established. Here, we employ correlative electron microscopy to reveal coupled chemical, electrochemical, and crystallographic effects on localized corrosion attack and its development. We show that the Cr and Mo-depleted interior of the cellular solidification structures dissolves selectively, giving rise to an intricate damage morphology, that is directly related to the underlying crystallographic orientation. Whereas surface observations only reveal apparently shallow micrometer-size cavities, 3D tomography via focused ion beam serial-sectioning shows a high degree of connectivity between these features underneath the surface. We reveal this intricate morphology, propose a formation mechanism, and discuss alloy design guidelines to mitigate this phenomenon.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0081186
