| 1. |
- Jaladurgam, Nitesh Raj, 1993
(författare)
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Heterogeneous deformation of multi-phase engineering materials - an in-situ neutron diffraction study
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas turbines are complex power generation systems used in aerospace or land-based-power stations. Materials such as Ni-base superalloys are involved in the combustion zone of these machines, which continuously experiences harsh environments with loading at high temperatures. Moreover, the continuous demand for increasing operating temperature to achieve higher efficiencies and reduced emission levels opens the scene to new heat resistant materials like the state-of-the-art high entropy alloys (HEAs), which require a thorough understanding of the structure-process-property relationships. The microstructures of these advanced multi-phase, multi-component alloys are complex, and the deformation is generally heterogeneous both with respect to the different phases and to the crystallographic orientation within each phase. Hence, it is important to understand their behavior and performance during processing and service. In-situ neutron diffraction is a unique technique to probe the deformation behaviour during service/processing-like conditions, including plastic deformation at various temperatures, in order to provide insights into the structure-property relations. In the first part of this work the deformation mechanisms of a newly developed Ni-base superalloy was investigated using in-situ neutron diffraction and electron microscopy at room temperature. In addition, elasto-plastic self-consistent (EPSC) crystal plasticity simulations are used to obtain insights into the operating deformation mechanisms. In the second part, the as-cast eutectic high entropy alloy AlCoCrFeNi2.1 was studied using in-situ neutron diffraction at temperatures from 77 to 673 K. These investigations provide unique insights into the complex heterogeneous deformation behavior of these high-performance multi-phase engineering materials.
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| 2. |
- Shoja, Siamak, 1980
(författare)
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Microstructure and plastic deformation of textured CVD alumina coatings
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is known that the wear performance of α-alumina coatings produced by chemical vapor deposition (CVD) is significantly influenced by the type and degree of texture. However, the main reasons behind this behavior are not fully understood. This thesis contains studies of two related topics for increasing the understanding of α-alumina coatings. The first topic concerns the microstructure and texture development of CVD α-Al2O3 coatings, and the second topic concerns calculations and analysis of the Schmid factors ( m ) for coatings with different textures. By combining different analysis methods (such as XRD, SEM, FIB/SEM, TKD, TEM, STEM, XEDS), and theoretical and experimental Schmid factor analysis by MATLAB and EBSD, the microstructure and plastic deformation of α-alumina coatings were investigated. The microstructures of three different CVD α-Al2O3 layers deposited onto a Ti(C,N,O) bonding layer were studied. Grain boundary diffusion of heavy elements from the substrate to the bonding layer/α-Al2O3 interface was observed. This may be the cause of a disturbance in the early growth of α-Al2O3. Additionally, it was found that the number of interfacial pores at the bonding layer/α-Al2O3 interface increased by introducing the H2S gas. The H2S gas also promoted an earlier development of the (0001) texture. The orientation of the grains was developed to the desired texture both as a gradual change over several grains and as an abrupt transformation from one grain to another. The probability of plastic deformation in different wear zones on the rake face of a cutting tool was investigated theoretically and experimentally by analyzing Schmid factors for textured α-Al2O3 coatings. Schmid factor diagrams were constructed using MATLAB/MTEX and used to extract frequency distributions for different slip systems and textures. The results were compared with lateral distribution maps of Schmid factors obtained from experimental coatings. It was observed that basal slip is most easily activated in the transition zone, followed by prismatic slip systems 1 and 2 in coatings with an (0001)-texture. The homogeneous plastic deformation behavior observed in this coating is also connected to mostly high Schmid factors in the m -value distribution. The differences between the m -value distributions for the three slip systems are not that pronounced in the (01-12) and (11-20) textures, and the distributions are relatively wide. The low wear rate and more homogeneous deformation of the coating with (0001) texture compared to the other coating textures may be the result of the high plasticity, offered by the easy activation of basal slip and prismatic 1 slip, and the low spread of Schmid factor values at the transition zone. In conclusion, the results presented in this thesis form a knowledge platform that can be used to understand the microstructure and wear mechanisms of textured CVD α-alumina coatings.
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| 3. |
- Fazi, Andrea, 1992
(författare)
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Cold spray Cr-coated Optimized ZIRLO claddings: an option for accident tolerant fuels
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accident tolerant fuel development has started with the aim of providing nuclear fuels able to endure severe accident conditions. Research in this field has also sparked a wave of material renewal in the nuclear industry that had being delayed for the last few decades. Climate change is an ever-growing public concern, and policies about greenhouse gas emissions are becoming more stringent both at the national and international level. Nuclear energy produces very low carbon emissions and the successful development of new accident tolerant materials might play a role in making this technology a viable solution to this global issue. Cr-coated zirconium claddings are one of the most promising candidates as near-term response to the need for accident tolerant materials. These coatings can be produced via a range of different technologies, but the two main designs being currently developed are obtained with physical vapour deposition or cold spray (CS) deposition. In cold spray, high pressure gas is fed through a nozzle together with Cr powder. The Cr particles are accelerated up to 1200 m/s and directed to the substrate, in this case cladding tubes made of Optimized ZIRLO™ alloy. The resulting Cr-coated zirconium cladding is the subject of this work. As-fabricated samples and autoclave tested material are characterized with atom probe tomography and a range of electron microscopy techniques. The scope of the investigation is to evaluate the performances of Cr-coated claddings under operating conditions. Additionally, the nature of the adhesion in cold spray coating and the effects of this deposition method on the substrate are explored. A 10-20 nm thick intermixed bonding region was observed at the Cr/Zr interface of the as-fabricated cladding. This region is deemed to play an important role in the strong adhesion of CS coatings. When exposed to operating conditions, ZrCr2-Laves phase was found to nucleate in the intermixed bonding region. CS deposition involves severe plastic deformation of both coating and substrate. As a result, a 1-2 μm thick nanocrystalline layer can be found in the substrate adjacent to the Cr/Zr interface. After autoclave exposure, a Zr-Cr-Fe phase was discovered precipitating in this nanocrystalline layer at the Zr grain boundaries.
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| 4. |
- Kahlin, Magnus
(författare)
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Fatigue Performance of Additive Manufactured Ti6Al4V in Aerospace Applications
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive Manufacturing (AM) for metals includes is a group of production methodst hat use a layer-by-layer approach to directly manufacture final parts. In recent years, the production rate and material quality of additive manufactured materials have improved rapidly which has gained increased interest from the industry to use AM not only for prototyping, but for serial production. AM offers a greater design freedom, compared to conventional production methods, which allows for parts with new innovative design. This is very attractive to the aerospace industry, in which parts could be designed to have reduced weight and improved performance contributing to reduced fuel consumption, increased payload and extended flight range. There are, however, challenges yet to solve before the potential of AM could be fully utilized in aerospace applications. One of the major challenges is how to deal with the poor fatigue behaviour of AM material with rough as-built surface.The aim of this thesis is to increase the knowledge of how AM can be used for high performance industrial parts by investigating the fatigue behaviour of the titanium alloy Ti6Al4V produced with different AM processes. Foremost, the intention is to improve the understanding of how rough as-built AM surfaces in combination with AM built geometrical notches affects the fatigue properties.This was done by performing constant amplitude fatigue testing to compare different combinations of AM material produced by Electron Beam Melting(EBM) and Laser Sintering (LS) with machined or rough as-built surfaces with or without geometrical notches and Hot Isostatic Pressing (HIP) treatment. Furthermore, the material response can be different between constant amplitude and variable amplitude fatigue loading due to effects of overloads and local plastic deformations. The results from constant amplitude testing were used to predict the fatigue life for variable amplitude loading by cumulative damage approach and these predictions were then verified by experimental variable amplitude testing.The constant amplitude fatigue strength of material with rough as-built surfaces was found to be 65-75 % lower, compared to conventional wrought bar, in which HIP treatments had neglectable influence on the fatigue strength. Furthermore, the fatigue life predictions with cumulative damage calculations showed good agreement with the experimental results which indicates that a cumulative damage approach can be used, at least for a tensile dominated load sequences, to predict the fatigue behaviour of additive manufactured Ti6Al4V.
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| 5. |
- Gunnerek, Rasmus, 1992
(författare)
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Increased Productivity of Ferrous Alloys Produced by Powder Bed Fusion - Laser Beam
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low production speeds limit wider adoption of powder bed fusion – laser beam (PBF-LB) in cost sensitive industries as it correlates directly with high production costs. The main process parameters such as scan speed, hatch distance and layer thickness can be scaled up to increase build speed, but it often comes at a trade off in lower quality, i.e. surface roughness, porosity/density and mechanical properties. The objective of this thesis study was to investigate how large build rates influences microstructure and processability of two low alloy steels (4130 and 4140) and 316L stainless steel by detailed characterization of defects and porosity present in relation to process parameters applied. The initial study found that achieving consistent processability of low alloy steels across layer thicknesses, as indicated by the as-printed density, is better correlated with surface energy density (SED) J/mm2 rather than volumetric energy density (VED) J/mm3. Regions with high densities above 99.8% exhibited similar ranges of SED at different layer thicknesses, explained by the resulting melt pool depths being more similar than at similar ranges of VED. The second study addressed the impact of three- and four-factor increase in hatch distance and layer thickness compared with state of the art. It was observed that the influence of VED on density was poorly described as differences of up to 7.5% in density were measured at the same VED. More accurate representations of impact on main print parameters on density was found by regression analysis which also captured the interaction between laser power, scan speed and hatch distance at different layer thicknesses. Build rate increase can be realized by numerous combinations of basic laser parameters, however this results in distinctive porosity characteristics and even at the same levels of build rate increase, pore characteristics, such as orientation, aspect ratio and size, can differ significantly.
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| 6. |
- Hanning, Fabian, 1988
(författare)
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Weld Cracking in Precipitation Hardening Ni-based Superalloys
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Manufacturing of hot structural components for aero engines requires the materials being used to be weldable. The high demands on strength and temperature resistance make nickel based superalloys the material of choice for this application. Alloy 718 has been the standard grade for several years, providing high strength at elevated temperatures while being weldable due to the relatively slow precipitation kinetics of its hardening phase gamma double prime. Increasing operating temperatures as well as intermittent cycling of land-based gas and steam turbines motivate research on highly temperature stable alloys such as nickel based superalloys. Increased temperature stability of precipitation hardening superalloys is generally achieved via the gamma prime phase, which in contrast to gamma double prime causes a very rapid hardening effect in the material. Rapid hardening of the gamma prime phase can cause strain age cracking (SAC), a cracking phenomenon occurring during heating towards the post weld heat treatment when stress relaxation mechanism coincide with the precipitation of hardening phases. With the general mechanism of SAC being established, detailed knowledge about the material response is necessary to be able to predict the welding behaviour and to prevent SAC. This is especially relevant with regard to newly developed alloys such as Haynes® 282®, where limited weldability data is available. This work hence sets focus on investigating the weldability of the relatively new superalloy Haynes® 282®. It was found that the welding response of Haynes® 282® is generally good, with the heat input during welding being identified as main effect on the cracking response under the studied conditions. Solidification cracks were observed in the material, while neither heat affected zone liquation cracks nor SAC could be confirmed. A simulative Gleeble test was developed to provide more data on ductility in the SAC temperature range and its dependence on ongoing precipitation reactions during thermal exposure, correlating the loss in ductility with hardness evolution in the material.
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| 7. |
- Jacobsson, Jonny, 1982
(författare)
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Weldability and Testing Methodology in Precipitation Hardening Superalloys
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Usable weldability data is desired in the manufacturing industry, especially within the aerospace industry where fabrication of structural jet engine components are realized. Welding of precipitation hardening superalloys such as Waspaloy®, Alloy 718, ATI® 718Plus™ and Haynes® 282® in particular can lead to solidification cracking in the fusion zone, liquation cracking in the heat affected zone and/or to solid state cracking. This concern requires some kind of weldability testing such as Varestraint testing to improve the fundamental knowledge on how to prevent this type of cracks from occurring. It was found that the micro- hardness for all four alloys is approximately 250HV in the weld metal while the parent metal differs more, 208HV for Haynes® 282®, 243HV for Alloy 718, 340HV for Waspaloy® and 384HV for ATI® 718Plus™. The hardness in the HAZ reaches about 400HV for Waspaloy® and ATI® 718Plus™, while Alloy 718 and Haynes® 282®approach 250-350HV. The grain size is smallest for ATI® 718Plus™ (8.3 µm) and Alloy 718 (16µm) followed by Haynes® 282® (64µm) and Waspaloy® (90µm). Simulation using JMatPro suggested a larger amount of γ' in ATI® 718Plus™ compared to Alloy 718. In Haynes® 282®, the sigma-phase and M6C levels are higher compared with those in Waspaloy®, for which M23C6 was found instead. Based on measurements, system analyses and design of experiment it was concluded that the lowest variation in evaluating weld cracking can be achieved with the method using penetrant combined with the use of one operator. The welding speed affected the variation in weld cracking most followed by current, die mandrel radius and the bending stroke rate. Testing parameters with lowest standard deviation/mean Total Cracking Length (TCL)-values are here found for welding speed of 1mm/s, weld current of 70A, die mandrel radius of 60mm and bending stroke rate of 10mm/s. The compression strains in the lower part of the specimen during the bending at Varestraint testing have no significant impact on the weld cracking. Based on Varestraint testing of Alloy 718 and Waspaloy®, there was similar cracking response at 1.1% to 4.3% augmented strain and if extrapolated downwards the critical strain from crack initiation approach zero. Similar to Alloy 718 and Waspaloy®, it was also found that ATI® 718Plus™ and Haynes® 282® both seemed to have a level of critical augmented strain of around 1% while at the highest strain level of 8.6% Haynes® 282® showed somewhat higher susceptibility values. The lower susceptibility to hot cracking in ATI® 718Plus™ compared to Alloy 718 and Haynes® 282® is supposed to be associated with the smaller grain size of ATI® 718Plus™ despite of its higher hardness. The HAZ liquation cracking in Haynes® 282® seems to be connected to Ti-Mo based MC-type carbides.
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| 8. |
- Kanesund, Jan-erik
(författare)
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Influence of deformation and environmental degradation of Inconel 792
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Industrial gas turbines are often used as a mechanical drive for pumps and compressors or in power generation as an electric power supply. The gas turbine has for many years been a popular engine due to its flexibility with respect to different types of fuel and due to a design, that enables a high power-to-weight ratio. A simplified description of a gas turbine is that the engine consists of a cold and hot section. The turbo compressor section belongs to cold section and the combustion chamber together with the turbine section belongs to the hot section. In the hot section of a gas turbine, the condition is extremely severe because of an aggressive environment characterized by high temperatures, increased temperature gradients, high pressure and centrifugal forces resulting in large stresses on individual components together with an oxidizing and corroding atmosphere. Materials used in the high temperature section (hot gas path) of a modern gas turbine are different types of superalloys, as single crystal, directionally solidified or polycrystalline alloys, depending on temperature and load conditions. In the first turbine stage, temperature is very high due to exposure to the combustion gas. To handle the problem with creep, single crystal superalloys are often used in this section. In the second row of turbine blades, the temperature of the gas is lower and polycrystalline superalloys are typically used. IN-792 is a cast polycrystalline superalloy with high strength, good resistance to hot corrosion and a cheaper option than single crystals. In the hot section of gas turbine, IN-792 is a suitable material for components such as turbine blades and vans where a complex load condition, high temperature and severe environment prevails. Due to startup and shutdown of the gas turbine engine during service, the components in the hot section are exposed to cyclic load and temperature. This will generate mechanical and thermal fatigue damage in gas turbine components. Steady state temperature gradient arises by the cooling system acting at cold spots during service to introduce tensile stress, which indirectly gives rise to creep damage in the component. This work includes tree studies of deformation and damage mechanisms of superalloy IN-792. The first study is made on test bars exposed to thermomechanical fatigue in laboratory environment, the second and the third study is made on turbine blades used during service. In the second study, the machines are placed off-shore and exposed to marine environment. In the third study the machine is landbased and exposed to an industrial environment. In the second study, the deformation and damage mechanisms are compared between the turbine blades used during service and the test bars exposed to thermomechanical fatigue testing in the first study.
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| 9. |
- Malladi, Sri Bala Aditya, 1993
(författare)
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Microstructural development in laser-based powder bed fusion - from ferritic stainless steel to medium entropy alloys
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ever since the advent of additive manufacturing (AM), the interest in AM technologies has skyrocketed due to its intrinsic ability to produce near net shaped components. Laser-based powder bed fusion (LB-PBF), being one of the most widely adapted AM technologies, has been especially a game changer due to its ability to produce components of complex geometries with improved designs and to reduce not only the final weight of the products but also the amount of the waste produced. However, the rapid melting and solidification in the LB-PBF process usually result in anisotropy in properties with respect to different printing directions, due to the epitaxial growth of columnar grains. One way to address this issue is by developing the alloy systems tailored to the LB-PBF process conditions, thereby achieving more control over the solidification dynamics to produce parts with more isotropic properties. One possible solution to inhibit the columnar growth of grains is by inducing in-situ inoculation via alloy design, thereby reducing the undercooling required for the growth of columnar grains. The first part of this work focuses on in-situ inoculation for ferritic stainless steels via alloy design to achieve the columnar to equiaxed transition. Three different ferritic steel grades based on SS441 were studied, with the aim of exploring the effectiveness of TiN as an inoculant in the LB-PBF process. The results showed that a substantial reduction in grain size with concomitant changes in the solidification behaviour occurred in the alloys pre-alloyed with inoculant forming elements, compared to the alloy without those inoculant forming elements. The second part of this thesis explores the possibility of improving the strength of the equiatomic CoCrNi medium entropy alloys by the addition of nitrogen. The motivation is to take advantage of the rapid melting and solidification that are intrinsic to the LB-PBF process and to stabilize nitrogen as an interstitial solid solution strengthening element. Two different grades of CoCrNi, one without nitrogen and one pre-alloyed with nitrogen, have been studied. Printing with optimized parameters resulted in parts with densities greater than 99.9% with cellular solidification structure. Heat treatments of printed CoCrNi specimens resulted in the nucleation of chromium rich oxides, while no nitrogen rich phases were observed. The expected interstitial solid solution strengthening resulted in improved yield and ultimate tensile strength values from about ~730 and ~970 MPa to ~890 and ~1110 MPa, respectively.
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| 10. |
- Miwen, Yuan, 1990
(författare)
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Mechanical Properties, Microstructures and Defects of Tool Steels Fabricated by Additive Manufacturing
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive Manufacturing (AM) has been gaining significant interest in the manufacturing of metallic materials in the recent years. From a techno-commercial standpoint, tooling is one of the areas with low production volumes but very high demands on part performance and complexity in design, all of which are characteristic for metal AM. In this thesis study, the focus has been on AM of tool steel intended for hot work applications. Tool steels manufactured through laser beam powder bed fusion (LB-PBF) and directed energy deposition (DED) are studied. A modified H13 hot work tool steel was produced by means of laser beam powder bed fusion (LB-PBF). The effect of two post processing routes, direct tempering (DT) of as-built part and conventional austenitizing followed by tempering (QT), was evaluated with respect to their impact on microstructure and mechanical properties in terms of hardness, tensile properties and impact toughness. The typical microstructure observed in DT condition retained the melt pool boundaries and cellular structure from the as-printed state as well as tempered martensite. A more uniform microstructure including tempered martensite with carbides possibly along lath boundaries was obtained in the QT condition. While comparable hardness and tensile properties were obtained in these two conditions, QT sample exhibited significantly higher impact toughness compared to DT sample due to its higher work hardening ability and strain rate sensitivity originating from the varied microstructure. For hot-work applications, resistance of the steel to softening at high temperatures is critical and determines the life of the tool. After long term exposure at elevated temperatures, the response of modified H13 after DT and QT treatments was evaluated by means of hardness measurements and microstructural observation. Thermal softening resistance was strongly influenced by the post-AM treatment and the conditions of subsequent exposure at high temperatures. As expected, lesser softening was observed for 550 ℃ exposure than that for 600 ℃. The decrease of hardness was more severe for QT samples. It is hypothesized that finer grain size of ferrite, less coarsened carbides and the cellular structure being preserved contribute to DT samples being more resistant to thermal softening. Evolution of carbides was analyzed by a combination of experiment and simulation using JMatPro software. For AM tool steels having high hardness, high strength and poor ductility, defects constitute a particularly serious concern. Uddeholm Vanadis 4 Extra (V4E) cold work steel was deposited on a hot work tool steel with a modified H13 composition (Uddeholm Dievar) by DED with varying number of layers. In the as-built state, defects including pores and cracks were found in the deposited zone. The number of both kinds of defects increased with the building height. Three types of pores were identified: large irregular ones, keyhole and shrinkage pores. Thin Si oxide film (~20 nm) was detected on the internal surface of the larger irregular pores by means of EDX and AES, below which a layer enriched in alloying elements and C was detected. The formation of this type of pore was supposed to be associated with the elemental segregation on the pore surface and insufficient heat input. Solidification cracking was observed as well, especially in the 3rd and 4th layers. Two factors are considered to contribute to higher cracking susceptibility: a) large temperature range for the solidification of V4E steel, simulated by ThermoClac software; b) change in microstructure with the building height from cellular to columnar dendrite.
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