2,974 research outputs found
Astroturfing
No full textM.F.A."This thesis represents the first third of a fictional novel entitled Astroturfing"--p. iiby Eric M. Zrinsk
Review of the book "A World Divided. The Global Struggle for Human Rights in the Age of Nation-States" by Eric D. Weitz
No full textReview of the book A World Divided. The Global Struggle for Human Rights in the Age of Nation-States, by Eric D. Weitz. Princeton and Oxford: Princeton University Press, 2019, ISBN: 978-0-691-14544-0, 544 pp.Review of the book "A World Divided. The Global Struggle for Human Rights in the Age of Nation-States" by Eric D. Weitz, Princeton and Oxford: Princeton University Press, 2019, ISBN: 978-0-691-14544-0, 544. The author gratefully acknowledges the European Social Fund (ESF) and the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for supporting this publication through research grant SFRH/BD/136170/2018
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Mechanical testing of aluminum alloy AA7020 at elevated temperatures
Get PDFThe present study aims to better understand the mechanisms that might cause such behavior. AA7020 aluminum sheet material was mechanically tested in tension at 500 and 535 °C at strain rates from 10⁻³ to 3×10⁻¹ s⁻¹ to evaluate tensile elongation and plastic flow behavior. The high elongations reported by Taylor et al. were not reproduced. Tensile elongations ranged from 56 to 97%, with the maximum achieved at 535 °C and 10⁻¹ s⁻¹. For the present study, tensile tests were conducted only after each specimen reached its specified test temperature. The very high elongations produced by Taylor et al. are most likely from their tension tests beginning before a specimen had reached its specified test temperature. Data from the present study suggest that AA7020 aluminum quickly reaches approximately steady-state flow governed by five-power creep when deformed after reaching a temperature of T ≥ 500 °C. The test data indicate very little strain hardening, a strain-rate sensitivity of m = 0.14 to 0.16, and an activation energy for creep of Qc = 246 kJ/mol. The observed tensile elongations are consistent with the measured values of strain-rate sensitivity. Producing the very high elongations reported by Taylor et al. would likely require tensile straining to begin significantly before a specimen reaches a temperature of 500 °C or higher.Mechanical Engineerin
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Tear resistance and stress relaxation behavior of high-strength AA7075-T6 aluminum alloy sheet at warm temperatures near 200 °C
Get PDFThe use of lightweight metals is of great interest to the automotive industry for improving vehicle efficiency. The automotive industry is particularly interested in lightweight metals with high strength-to-density ratios that can perform well in crash scenarios. To this end, high-strength aluminum alloys are investigated for body-in-white applications. Currently, 6xxx series aluminum alloys are the most commonly used aluminum materials for the body-in-white. AA7075-T6 offers comparable density to and significantly higher strength than the 6xxx series aluminum alloys. However, AA7075-T6 demonstrates low ductility and formability at room temperature, which are barriers to producing and mechanically fastening components for the body-in-white. Prior investigations by Rader et. al. demonstrated that the ductility of AA7075-T6 sheet is approximately doubled at warm temperatures near 200 °C, allowing it to be stamped to complex geometries. However, joining of AA7075-T6 sheet remains a significant challenge. The present study investigates the material behaviors necessary to determine the potential for joining of AA7075-T6 sheet at warm temperatures by self-piercing riveting. Self-piercing riveting is commonly used with the lower strength 6xxx series aluminum alloys in the automotive industry. Tear-resistance and stress-relaxation experiments are conducted to evaluate the potential for successfully implementing of self-pierce riveting in AA7075-T6 sheet at warm temperatures. Tear energy measurements at warm temperatures appropriate for retrogression heat treatments in AA7075-T6 are compared to measurements at room temperature. A four-fold increase in the tear energy of AA7075-T6 at warm temperatures suggests a high possibility for success with self-piercing riveting at these temperatures. Rapid stress relaxation of up to 45% of the flow stress produced during deformation at warm temperatures indicates a potentially significant reduction in residual stresses after deformation, which should reduce spring-back after forming and reduce the chance of delayed cracking in AA7075-T6 sheet after the application of a self-piercing rivet. These experimental results support the potential to successfully apply a self-piercing rivet to AA7075-T6 sheet at a warm temperature while simultaneously retrogressing the material. Because a subsequent reaging heat treatment is known to restore full strength following a retrogression heat treatment, the retrogression riveting and reaging process is proposed as a method to successfully join AA7075-T6 sheet material while retaining the full strength of the T6 condition.Mechanical Engineerin
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The construction and use of plasticity models to predict elevated temperature forming of magnesium ZEK100 alloy sheet material
Get PDFtextMechanical EngineeringMagnesium (Mg) alloys provide material properties that make them attractive for structural components. In particular Mg alloys can be used to produce components with lighter weight than most alloy sheets currently used. However, the insufficient ductility of Mg alloy sheet materials at room temperature can require these to be formed at elevated temperatures to achieve suitable formability. In this research, wrought Mg alloy ZEK100 is studied at 300 °C and lower temperatures. Behavior at these lower temperatures is compared to behavior of 450 °C and 350 °C. A goal of this study is to determine the possibilities for future forming technologies at these lower temperatures. The deformation mechanisms at these temperatures are examined, including their relation to plastic anisotropy. Knowledge of the active deformation mechanisms is used to formulate descriptive models of plastic deformation. Material constitutive models are constructed and used in finite element method (FEM) simulations of gas pressure bulge tests. Finally, results of FEM simulations are compared with experimental results, and the accuracies of the material constitutive models are validated.Mechanical Engineerin
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The construction and use of physics-based plasticity models and forming-limit diagrams to predict elevated temperature forming of three magnesium alloy sheet materials
Get PDFtextMagnesium (Mg) alloy sheets possess several key properties that make them attractive as lightweight replacements for heavier ferrous and non-ferrous alloy sheets. However, Mg alloys need to be formed at elevated temperatures to overcome their limited room-temperature formabilities. For example, commercial forming is presently conducted at 450°C. Deformation behavior of the most commonly used wrought Mg alloy, AZ31B-H24, and two potentially competitive materials, AZ31B-HR and ZEK100 alloy sheets, with weaker crystallographic textures, are studied in uniaxial tension at 450°C and lower temperatures. The underlying physics of deformation including the operating deformation mechanisms, grain growth, normal and planar anisotropy, and strain hardening are used to construct material constitutive models capable of predicting forming for all three Mg alloy sheets at 450°C and 350°C. The material models constructed are implemented in finite-element-method (FEM) simulations and validated using biaxial bulge forming, an independent testing method. Forming limit diagrams are presented for the AZ31B-H24 and ZEK100 alloy sheets at temperatures from 450°C down to 250°C. The results suggest that forming processes at temperatures lower than 450°C are potentially viable for manufacturing complex Mg components.Materials Science and Engineerin
Electron Backscatter Diffraction Patterns from Titanium-added Interstitial-free Steel Containing Subgrains
No full text<h3><strong>Associated Publications</strong></h3>
<ol>
<li>Bennett IV, T.J. and Taleff, E.M. Dynamic Grain Growth Driven by Subgrain Boundaries in an Interstitial-Free Steel During Deformation at 850 °C. <em>Metall Mater Trans A</em> 55, 429–446 (2024). <a href="https://doi.org/10.1007/s11661-023-07256-w">https://doi.org/10.1007/s11661-023-07256-w</a>.</li>
<li>Bennett IV, T.J. and Taleff, E.M. Imaging and Segmenting Grains and Subgrains using Backscattered Electron Techniques. Under review (2024).</li>
</ol>
<h3><strong>Data Description</strong></h3>
<p>These data were collected by Thomas J. Bennett IV on July 28, 2022.</p>
<p>The electron backscatter diffraction (EBSD) data and associated electron backscatter diffraction patterns (EBSPs) contained herein were acquired from a titanium-added interstitial-free (Ti-IF) steel sheet material containing numerous subgrains. The Ti-IF steel specimen that provided these data was ramped to 850 degrees Celsius over 30 minutes, held at this temperature for one hour, and then deformed at a constant true-strain rate of 10^-4 s^-1. Upon reaching a final true strain of 0.225, the specimen was air quenched while maintaining a constant stress to preserve subgrains formed during high-temperature deformation. The tensile specimen was cut from a Ti-IF steel sheet received in a hard as-rolled condition with the tensile axis parallel to the sheet rolling direction. EBSPs were acquired from a section cut from the center of the deformed gage region using a JEOL JSM-IT300HR SEM equipped with an EDAX Velodity EBSD camera at the Center for Integrated Nanotechnologies.</p>
<p>The following conditions were used for EBSD data acquisition:</p>
<table>
<tbody>
<tr>
<td>Accelerating Voltage:</td>
<td>20 kV</td>
</tr>
<tr>
<td>Beam Current:</td>
<td>80%</td>
</tr>
<tr>
<td>Working Distance:</td>
<td>20.0 mm</td>
</tr>
<tr>
<td>Magnification:</td>
<td>200×</td>
</tr>
<tr>
<td>Dynamic Focus:</td>
<td>44 (out of 255, arbitrary units)</td>
</tr>
<tr>
<td>Specimen Tilt:</td>
<td>70 degrees</td>
</tr>
<tr>
<td>Scanning Grid Type:</td>
<td>Square</td>
</tr>
<tr>
<td>Step Size (x and y):</td>
<td>0.5 μm</td>
</tr>
<tr>
<td>Scan Size:</td>
<td>520 (across) × 340 (down) pixels</td>
</tr>
<tr>
<td>EBSD Camera Resolution:</td>
<td>446 × 446 pixels</td>
</tr>
<tr>
<td>EBSD Camera Binning:</td>
<td>1 × 1</td>
</tr>
<tr>
<td>EBSD Camera Exposure Time:</td>
<td>10 ms</td>
</tr>
<tr>
<td>Frame Averaging:</td>
<td>None</td>
</tr>
<tr>
<td>Specimen Tensile Direction:</td>
<td>Horizontal</td>
</tr>
<tr>
<td>Specimen Rolling Direction:</td>
<td>Horizontal</td>
</tr>
<tr>
<td>Specimen Long Transverse Direction:</td>
<td>Vertical</td>
</tr>
<tr>
<td>Specimen Short Transverse Direction:</td>
<td>Normal to plane</td>
</tr>
<tr>
<td>Pattern Center (EMSphInx Convention):</td>
<td>(x_pc, y_pc, L) = (-0.2 pixels, 112.76 pixels, 21736.4 μm)</td>
</tr>
<tr>
<td>EBSD Camera Elevation Angle:</td>
<td>3 degrees</td>
</tr>
<tr>
<td>EBSD Camera Screen Width:</td>
<td>32 mm</td>
</tr>
<tr>
<td>Pixel size on EBSD Camera Screen:</td>
<td>71.749 μm/pixel ( = 32000 μm / 446 pixels)</td>
</tr>
</tbody>
</table>
<p> </p>
<p><em>Note:</em> Conversions between different pattern center conventions may be found in the journal article below or at the following link: <a href="https://github.com/EMsoft-org/EMsoft/wiki/DItutorial">https://github.com/EMsoft-org/EMsoft/wiki/DItutorial</a>.</p>
<ul>
<li>Jackson, M.A., Pascal, E., and De Graef, M. Dictionary Indexing of Electron Back-Scatter Diffraction Patterns: a Hands-On Tutorial. <em>Integr Mater Manuf Innov</em> 8, 226–246 (2019). <a href="https://doi.org/10.1007/s40192-019-00137-4">https://doi.org/10.1007/s40192-019-00137-4</a>.</li>
</ul>
<h3><strong>File Descriptions</strong></h3>
<ul>
<li>Specimen_orientation.pdf - A schematic showing specimen reference directions and the orientation used for EBSD data acquisition.</li>
<li>Patterns.zip - A compressed archive containing Patterns.up2. This file contains 16-bit EBSPs and is 70,336,697,616 bytes (70.3 GB) uncompressed.</li>
<li>SHT_Indexed.ang - A file containing orientation data produced by indexing Patterns.up2 using EMSphInx. Orientations are represented by Euler angles (Bunge convention) and are to be interpreted using the EDAX Setting 2 convention (see MTEX documentation at <a href="https://mtex-toolbox.github.io/EBSDReferenceFrame.html">https://mtex-toolbox.github.io/EBSDReferenceFrame.html</a>).</li>
<li>SHT_Indexed.h5 - A file in HDF5 format containing orientation data and other relevant information produced by indexing Patterns.up2 using EMSphInx.</li>
<li>SHT_Indexed_IPFmap.png - An image of an inverse pole figure map colored with respect to the short transverse direction showing the data from SHT_Indexed.ang.</li>
</ul>
<p><em>Note:</em> The basic format of "up2" files is the following. The first 4 bytes provide the version number. The second 4 bytes are the width of the patterns. The third 4 bytes are the height of the patterns. The fourth 4 bytes are the starting position of the pattern image data.</p>
<h3><strong>Acknowledgments</strong></h3>
<p>The authors gratefully acknowledge support from the National Science Foundation under Grant DMR-2003312 and instrumentation under Grant DMR-9974476. The authors also gratefully acknowledge support from the U.S. Department of Energy, Office of High Energy Physics under Grant DE-SC0009960. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract 89233218CNA000001) and Sandia National Laboratories (Contract DE-NA-0003525). The authors thank Mr. Thomas Cayia (Arcelor Mittal) for providing the interstitial-free steel material used for this study.</p>
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The effect of microstructure on cavitation during hot deformation in fine-grained AA5083 aluminum alloy sheet material
Get PDFtextAluminum alloys are of great interest to the automobile industry for vehicle mass reduction, which improves vehicle performance and reduces emissions. Hot forming processes, such as superplastic forming (SPF) and quick-plastic forming (QPF) have been developed to take advantage of the improved formability of certain aluminum materials at elevated temperature. Commercial fine-grained aluminum alloy AA5083 sheet is the most commonly used material in the SPF and QPF forming processes. Hot formability of AA5083 is often limited by material cavitation during forming, which makes understanding and controlling cavitation an issue of primary importance for improving hot sheet forming processes. The thermomechanical processing history of AA5083 can strongly affect superplastic performance, causing variations in formability between material lots. These variations are closely related to microstructure, and intermetallic particles are prime suspects for controlling cavitation behavior. However, there has been little more than anecdotal evidence available that these particles nucleate or influence cavitation. Interactions between intermetallic particles and cavities were, thus, analyzed using both two-dimensional (2-D) and three-dimensional (3-D) microstructure characterization techniques. Analysis of 3-D microstructures from AA5083 specimens deformed under conditions similar to the SPF and QPF processes provide conclusive proof that cavities form at specific types of intermetallic particles. Differences in cavitation between materials deformed under the SPF and QPF processes result from differences in deformation mechanisms. These differences are illustrated by the formation of filaments on fracture surfaces of superplastically deformed AA5083 specimens, which have been characterized.Mechanical Engineerin
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The tensile behavior of AA6013 at room temperature and 240 °C
Get PDFAutomotive manufacturers are pursuing technologies, such as lightweighting, that improve vehicle fuel-efficiency and reduce emissions. High-strength aluminum alloys might provide performance equal to the current ultra-high-strength steels while decreasing vehicle weight. High-strength 6xxx-series aluminum alloys, such as AA6013, are candidates for lightweighting structural components of vehicles because of their high strength-to-weight ratios compared to steel. In the peak-aged condition, these alloys often lack the ductility necessary to form complex part geometries at room temperature. Forming at elevated temperatures increases the ductility but can reduce strength. Retrogression forming and reaging (RFRA) is a relatively new technology for warm forming parts in high-strength aluminum alloys and then recovering strength to equal the peak-aged condition. Previous studies on aluminum alloy AA6013 performed by Rader et al. demonstrated a significant response to retrogression and reaging. New data for AA6013 are presented from tension tests at room temperature and 240 °C, an appropriate temperature for retrogression of this alloy. The effects of different heat treatments on room temperature properties are investigated. The effects of temperature and time at temperature on plastic deformation are investigated using experiments at 240 °C. Retrogression from the T6 temper reduced room-temperature strength by 3.5%, but subsequent reaging restored strength to within 2% of the original T6 temper. At 240 °C, the yield stress was 25 to 30% lower and elongations after rupture were 42% higher than at room temperature for the T6 temper. Stress relaxation at 240 °C decreased stress by 32 to 43% at a fixed elongation within approximately three minutes. These results suggest that RFRA could be viable for forming complex components in AA6013-T6Mechanical Engineerin
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The effects of ternary alloying additions on solute-drag creep in Al-Mg alloys
No full texttextEffects of ternary additions of Zn, Fe, and Cu on solute-drag creep and duc
tility in Al-Mg alloys are studied. The materials studied are, in wt. pct., Al
2Mg-5Zn, Al-3Mg-5Zn, Al-4Mg-5Zn, Al-3Mg-0.11Fe, Al-3Mg-0.27Fe, Al-3Mg
0.40Fe, Al-3Mg-0.50Cu, Al-3Mg-1.02Cu, Al-3Mg-1.52Cu, and Al-3Mg-2.15Cu.
Experimental data show that ternary Zn additions do not have an adverse ef
fect on solute-drag creep in Al-Mg alloys, but increase the sensitivity of stress
exponent, n, to Mg content. Transitions to power-law breakdown in the Al
xMg-5Zn materials are discussed. Ternary Fe and Cu additions increases n
during solute-drag creep. Ductilities of over 100% are consistently achieved in
the Al-xMg-5Zn and Al-3Mg-xFe materials. Age hardenability during natu
ral aging and simulated paint-bake cycle are studied for the Al-xMg-5Zn and
Al-3Mg-xCu materials. Zn creates a significant paint-bake response, while the
effect of Cu is small for a simulated paint-bake cycle.Materials Science and Engineerin
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