顔写真

アオヤギ ケンタ
青柳 健大
Kenta Aoyagi
所属
金属材料研究所 材料プロセス・評価研究部 加工プロセス工学研究部門
職名
助教
学位
  • 博士(工学)(東北大学)

  • 修士(工学)(東北大学)

経歴 3

  • 2015年10月 ~ 継続中
    東北大学 金属材料研究所 助教

  • 2014年4月 ~ 2015年9月
    名古屋大学 グリーンモビリティ連携研究センター 特任助教

  • 2012年4月 ~ 2014年3月
    立命館大学 立命館グローバル・イノベーション研究機構 ポストドクトラルフェロー

学歴 3

  • 東北大学 工学研究科 知能デバイス材料学専攻

    ~ 2012年3月27日

  • 東北大学 工学研究科 知能デバイス材料学専攻

    ~ 2009年3月25日

  • 東北大学 工学部 材料物性学科

    ~ 2007年3月27日

所属学協会 7

  • 軽金属学会

  • 日本セラミックス協会

  • 日本結晶成長学会

  • 日本顕微鏡学会

  • 人工知能学会

  • TMS

  • 日本金属学会

︎全件表示 ︎最初の5件までを表示

研究キーワード 6

  • インプロセスモニタリング

  • 機械学習

  • モデルベース開発

  • X in the Loop Simulation

  • 付加製造

  • 材料組織

研究分野 2

  • ナノテク・材料 / 材料加工、組織制御 /

  • ナノテク・材料 / 構造材料、機能材料 /

論文 82

  1. Machine-Learning-Assisted Development of Carbon Steel With Superior Strength and Ductility Manufactured by Electron Beam Powder Bed Fusion

    Yunwei Gui, Kenta Aoyagi, Huakang Bian, Akihiko Chiba

    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 55 (1) 320-334 2024年1月

    DOI: 10.1007/s11661-023-07251-1  

    ISSN:1073-5623

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    In this study, based on a novel support vector machine optimization method, a wide processing window for manufacturing defect-free S25C carbon steel by electron beam powder bed fusion (EB-PBF) was identified. Samples with same energy densities exhibited similar microstructures and mechanical properties. One sample showed an optimum strength and elongation combination of 459.3 MPa and 57.6 pct. The pearlite region with irregular cementite particles was the first to crack during deformation, and the cracks gradually expanded into the surrounding area. Ferrite, cellular structures, and pearlite with parallel and straight cementite particles could effectively modulate the deformation by slip and enhance the plasticity of the S25C parts. After quenching, the strength improved to an unprecedented value of 1722.5 MPa owing to the presence of martensite and dislocation entanglements, with an elongation of 16.8 pct. The strength decreased after further tempering, and the plasticity evidently increased, with an optimum strength and elongation combination of 722.7 MPa and 44.2 pct, respectively. The microstructure of tempered sample contained lath martensite, cementite particles, and sparse dislocation lines. These results demonstrate that the current method can serve as a powerful tool for effectively optimizing the high-dimensional parameters of the EB-PBF process to produce carbon steel with excellent mechanical properties.

  2. Influence of polymer on the surface quality and microstructure in a novel additive manufacturing method using metal paste 査読有り

    Weicheng Heng, Kenta Aoyagi, Akihiko Chiba

    Materials Letters 352 135096-135096 2023年12月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.matlet.2023.135096  

    ISSN:0167-577X

    eISSN:1873-4979

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    A new additive manufacturing method using metal paste was developed, in order to reduce waste caused by powder recycling in conventional powder bed fusion and maintain a high density in the fabricated parts. The paste was composed of metal powder, polymer particles, and a volatile solvent. Experiments were conducted to investigate the influence of a low polymer content on the paste during processing, surface topology before and after melting, and microstructure of the single-layer samples. The results showed that polymer addition could prevent the formation of powder clots by improving homogeneity of the metal paste and suppressing defect generation. The polymer also led to a smoother surface by reducing the spatter size. Finer grains were observed in the sample produced at higher polymer content.

  3. Cracking behavior of Ti-48Al-2Cr-2Nb alloy in powder bed fusion electron beam melting process

    Seungkyun Yim, Kenta Aoyagi, Huakang Bian, Keiji Yanagihara, Yuchao Lei, Shin ichi Kitamura, Hironobu Manabe, Yohei Daino, Kenta Yamanaka, Akihiko Chiba

    Journal of Materials Processing Technology 320 2023年11月

    DOI: 10.1016/j.jmatprotec.2023.118104  

    ISSN:0924-0136

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    Powder bed fusion electron beam (PBF-EB) melting process with low preheating temperatures is one of the promising ways to fabricate TiAl components with fully lamellar grain satisfying excellent mechanical properties and complex design. In this study, the cracking behavior of a Ti-48Al-2Cr-2Nb alloy produced by electron beam surface melting at 850 °C preheating was investigated under broad building parameter ranges. The transverse crack was more sensitive than the longitudinal crack because of the high tensile strength along the scan direction in single-track melting. The increased crack density deteriorated the tensile strength of the single-track melted Ti-48Al-2Cr-2Nb alloy via interdendritic fracture. Cross-sectional image analysis revealed that the solidification crack was prevented after single-track melting of Ti-48Al-2Cr-2Nb at 850 °C preheating owing to improved liquid feeding, and the crack opening was preferentially observed in the β/B2 phase that was placed near the melt pool boundary region. Based on a Scheil[sbnd]Gulliver and thermal conduction simulation, the partitioned α and β dendrite regions could be due to the restricted peritectic transformation of L + β → α during solidification by a rapid cooling rate above 2.3 × 104 °C /s. The crack opening of Ti-48Al-2Cr-2Nb alloy at 850 °C preheating can be suppressed by increasing the line energy above 1.85 J/mm. The critical residual stress causing crack opening in the B2 phase at 1100 °C was determined to be 117 MPa via thermal-elastoplastic analysis. Therefore, cracking due to residual stress of the Ti-48Al-2Cr-2Nb alloy produced by PBF-EB can be predicted as the maximum residual stress in the solidification region.

  4. In-situ observation of powder spreading in powder bed fusion metal additive manufacturing process using particle image velocimetry

    Seungkyun Yim, Jiayu Sun, Koki Minowa, Hao Wang, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 78 2023年9月25日

    DOI: 10.1016/j.addma.2023.103823  

    eISSN:2214-8604

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    Providing a dense powder bed is necessary to ensure the rationality of the final product prepared by powder bed fusion-based additive manufacturing under broad building parameters. In this study, the powder spreading mechanism was elucidated using particle image velocimetry and discrete element simulations. The particle flow regimes in the spreading process were identified based on the particle displacement: alignment, rotation, and deposition. The alignment regime was dominant in gas-atomized stainless steel 304 powder piles, whereas the rotation regime was dominant in plasma rotating electrode processed stainless steel 304 powder piles. A high-fidelity spreading simulation model was developed to clarify the critical factors resulting in the different flow regimes of different stainless steel 304 powders. The dominant rotational regime of the plasma rotating electrode processed powder was substituted for alignment by increasing the cohesive force. The particle supply in the spreading process was further suppressed by increasing the cohesive force, owing to the formation of a strong force arch and agglomeration. The high cohesive force in the gas-atomized powder was mainly attributed to the electrostatic force caused by the thick oxide film. Therefore, it was proven that the oxide film thickness is a key factor in determining the powder spreading mechanism and powder bed quality in the powder bed fusion additive manufacturing process.

  5. Processing condition dependency of increased layer thickness on surface quality during electron beam powder bed fusion

    Yufan Zhao, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Journal of Materials Research and Technology 26 5264-5279 2023年9月1日

    DOI: 10.1016/j.jmrt.2023.08.288  

    ISSN:2238-7854

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    The energy beam and powder layer interaction influences the dynamic melt behavior and determines the surface quality in electron beam powder bed fusion (PBF-EB). It is generally believed that increasing the powder layer thickness favors production efficiency but is contradictory to improving the forming quality. How variations in the powder layer thickness affect the interaction between the electron beam and the powder bed, which influences the melt behavior and resultant surface quality, has not been well understood. In this study, cylindrical specimens with increased nominal layer thicknesses from 80 to 140 μm were prepared using PBF-EB. The study verified the processing feasibility of ensuring the forming quality under a high layer thickness. Within the processing regime of this study, a relatively large powder layer thickness expanded the processing window. According to the thermophysical-property analysis of the powder bed, the emissivity and thermal conductivity exhibited upward and downward trends, respectively, with increased powder layer thickness. The increased thickness reduced the fusion efficiency, restricting the height difference within the overall sample surface caused by overheating. The numerical simulation clarified the dependence of the layer thickness-effect on the processing conditions. The proportion of incomplete melted powder in the electron beam irradiating area increased at a high scan speed. Subsequently, the hindering effect on heat absorption and transfer caused by the powder layer and its increased thickness was fully manifested. That is, the evolution trend of melt behavior and surface morphology resulting from increased layer thickness is remarkable at high scan speeds.

  6. Effect of powder morphology on flowability and spreading behavior in powder bed fusion additive manufacturing process: A particle-scale modeling study

    Seungkyun Yim, Huakang Bian, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 72 2023年6月25日

    DOI: 10.1016/j.addma.2023.103612  

    eISSN:2214-8604

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    Improving the quality of the powder bed is of great interest in the powder bed fusion-based additive manufacturing (PBF-AM) process to reduce micro-defects in the final product. We investigated the influence of particle morphology on flowability and spreading behavior using a discrete element method (DEM) simulation with multi-sphere modeling. The addition of non-spherical particles effectively decreased the flowability under gravity and rotation, resulting in a transition from rolling flow to cascading flow in the rotating condition. In the powder spreading process, the quality of the powder bed was decreased by adding elongated particles, owing to the frequent force-arch formation and additional wall effect caused by strong particle interlocking. Furthermore, the high spreading velocity facilitated loose powder packing owing to the frequent force arch and wide rearrangement area. In conclusion, spherical particles with a low spreading velocity are desirable for improving the quality of the powder bed, such as packing density and surface roughness, in the PBF-AM process.

  7. Multiscale heat transfer affected by powder characteristics during electron beam powder-bed fusion 国際誌 国際共著 査読有り

    Yufan Zhao, Kenta Aoyagi, Yujie Cui, Kenta Yamanaka, Akihiko Chiba

    Powder Technology 421 118438-118438 2023年5月1日

    DOI: 10.1016/j.powtec.2023.118438  

    ISSN:0032-5910

    eISSN:1873-328X

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    The quality of powder bed fusion (PBF) products is related to the processing conditions and materials. Powder characteristics determine the packing features and affect the heat transfer processes. The influences of powder characteristics are still incompletely understood. Moreover, there is limited research regarding the influence of powder characteristics on heat transfer at various scales. In this study, two types of CoCrMo alloy powders with different thermal conductivities were applied for PBF with electron beam (PBF-EB). Based on multiscale simulations, the effects of powder characteristics on the upper surface quality and solidification grain structure were analyzed. Under high-power conditions, the powder with a high thermal conductivity alleviated overheating within the entire processing area. Under low-power conditions, it was more readily preheated within the sample area, improving fusion sufficiency. This study highlights an additional process parameter—powder characteristics—that must be considered when establishing process–structure–property relationships for PBF-EB.

  8. Influence of linear energy on side surface roughness in powder bed fusion electron beam melting process: Coupled experimental and simulation study

    Haejin Lee, Yeonghwan Song, Seungkyun Yim, Kenta Aoyagi, Akihiko Chiba, Byoungsoo Lee

    POWDER TECHNOLOGY 418 2023年3月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.powtec.2023.118292  

    ISSN:0032-5910

    eISSN:1873-328X

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    The optimization of the side surface roughness of the as-built part is one of great interest in the additive manufacturing field because of the post-processing difficulty. In this study, the generation mechanism of the side surface in the powder bed fusion electron beam melting process was investigated using modified Rene ' 80 alloy under different linear energies. The dimensional inaccuracy of the as-built part mainly attributes to part diameter variation rather than part height variation. The side surface roughness of the as-built part was increased with linear energy owing to the formation of wavy bulges. The multi-physics model was developed to investigate the side surface generation in the electron beam melting process. The right-angled stair edge was relaxed at high linear energy owing to the increased molten pool dimension. The molten pool convection via the Marangoni effect was increased at high linear energy because of the large difference in temperature and low viscosity. The formation of wavy bulges on the side surface originated from the severe heat accumulation at the last solidified region in each melting path under high linear energy. Therefore, it was confirmed that the side surface roughness in the powder bed fusion electron beam melting process can be improved by optimizing the linear energy to restrict the formation of wavy bulges and coarse particles.

  9. Development of macro-defect-free PBF-EB-processed Ti-6Al-4V alloys with superior plasticity using PREP-synthesized powder and machine learning-assisted process optimization

    Yunwei Gui, Kenta Aoyagi, Akihiko Chiba

    MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 864 2023年2月

    出版者・発行元:ELSEVIER SCIENCE SA

    DOI: 10.1016/j.msea.2023.144595  

    ISSN:0921-5093

    eISSN:1873-4936

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    The elimination of internal macro-defects is a key issue in Ti-6Al-4V alloys fabricated via powder bed fusion using electron beams (PBF-EB), wherein internal macro-defects mainly originate from the virgin powder and inappropriate printing parameters. This study compares different types powders by combining support vector machine techniques to determine the most suitable powder for PBF-EB and to predict the processing window for the printing parameters without internal macro-defects. The results show that powders fabricated via plasma rotating electrode process have the best sphericity, flowability, and minimal porosity and are most suitable for printing. Surface roughness criterion was also applied to determine the quality of the even surfaces, and support vector machine was used to construct processing maps capable of predicting a wide range of four-dimensional printing parameters to obtain macro-defect-free samples, offering the possibility of subsequent development of Ti-6Al-4V alloys with excellent properties. The macro-defect-free samples exhibited good elongation, with the best overall mechanical properties being the ultimate tensile strength and elongation of 934.7 MPa and 24.3%, respectively. The elongation of the three macro-defect-free samples was much higher than that previously reported for additively manufactured Ti-6Al-4V alloys. The high elongation of the samples in this work is mainly attributed to the elimination of internal macro-defects.

  10. Effect of mechanical ball milling on the electrical and powder bed properties of gas-atomized Ti-48Al-2Cr-2Nb and elucidation of the smoke mechanism in the powder b e d fusion electron beam melting process

    Seungkyun Yim, Kenta Aoyagi, Keiji Yanagihara, Huakang Bian, Akihiko Chiba

    JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY 137 36-55 2023年2月

    出版者・発行元:JOURNAL MATER SCI TECHNOL

    DOI: 10.1016/j.jmst.2022.07.024  

    ISSN:1005-0302

    eISSN:1941-1162

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    Smoke is unexpected powder-splashing caused by electrostatic force and is one of the main problems hindering the process stability and applicability of the powder bed fusion electron beam (PBF-EB) technology. In this study, mechanical stimulation was suggested to suppress smoke of gas-atomized (GA) Ti-48Al-2Cr-2Nb powder using Al2O3 and WC ball milling. The deformation mechanism of the GA powder depending on the ball milling media was discussed based on the developed particle morphology distribution map and contact mechanics simulation. It was revealed that the rapid decrement of flowability and packing density after WC ball milling owing to the formation of angular fragments by the brittle fracture. The variation of surface and electrical properties by mechanical stimulation was investigated via XPS, TEM, and Impedance analysis. The electrical resistivity of the ball-milled powders gradually decreased with increasing milling duration, despite the increased oxide film thickness, and the capacitive response disappeared in Al-60 and WC-30 via metal-insulator transition. This could be due to the accumulation of strain and defects on the oxide film via mechanical stimulation. The smoke mechanism of ball-milled powders was discussed based on the percolation theory. In the smoke experiment, smoke was more suppressed for WC-10 and WC-20 than that for Al-40 and Al-50, respectively, despite the longer charge dissipation time. This could be due to the high probability of contact with conductive particles. For the Al-60 and WC-30 powders, smoke was further restricted by the formation of a percolation cluster with metal-like electrical conductivity. We believe that this study will contribute to a better understanding of the smoke mechanism and process optimization of the PBF-EB. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

  11. Competition between solid solution and multi-component Laves phase in a dual-phase refractory high-entropy alloy CrHfNbTaTi

    Cheng Yang, Huakang Bian, Fan Zhang, Yujie Cui, Yuchao Lei, Yuichiro Hayasaka, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Materials and Design 226 2023年2月

    DOI: 10.1016/j.matdes.2023.111646  

    ISSN:0264-1275

    eISSN:1873-4197

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    Solid solution phases dominate the thermodynamic competition with intermetallic phases in high-entropy alloys (HEAs), known as the high-entropy effect. However, owing to inevitable local substitutions in multi-component intermetallics (MCIMs), imposed thermodynamic changes are often overlooked. This study investigated a partially disordered MCIM, C14 Laves phase, in a dual-phase CrHfNbTaTi refractory HEA. The chemical formula and site preferences were determined based on the observation of spatial atomic arrangement. Furthermore, a parameter kdis was proposed to describe the disorder degree of MCIMs. As kdis increased from 0 to 0.2 due to local substitutions, the formation enthalpy of the C14 Laves phase significantly increased, resulting in higher energy and lower stability. Therefore, when feasible composition regulation and site occupancy design are adopted to manipulate the kdis of the potential MCIM, the high-entropy solid solution is able to be superior to the MCIM even at intermediate temperatures. The study provides insights into the high-entropy effect by considering MCIM enthalpies, providing a valuable phase-regulation strategy for future design of HEAs.

  12. Developing auto process mapping technique for powder bed fusion using an electron beam

    Kenta Aoyagi, Manabu Ono, Keiji Yanagihara, Kimio Wakoh, Akihiko Chiba

    Progress in Additive Manufacturing 2023年

    DOI: 10.1007/s40964-023-00535-3  

    ISSN:2363-9512

    eISSN:2363-9520

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    Powder bed fusion using an electron beam offers promise for manufacturing intricate metal parts. However, process optimization for defect-free parts proves costly and time-consuming. Many studies have investigated process optimization and defect prediction methods, but automating process optimization remains a significant challenge. This study developed and validated software to automatically determine i + 1-th trial conditions based on the results of the i-th trial experiment. Two algorithms were implemented and evaluated:—a dynamic programming approach and a selecting boundary conditions approach. The latter method considerably reduced the time required to determine the next conditions compared to the former approach. Considering a process mapping experiment requiring real-time trial condition determination during the build, we chose the selecting boundary conditions approach. The selecting boundary conditions approach was used to conduct a process mapping experiment to validate the software for constructing a process map using machine learning. The model and hyperparameters were optimized using sequential model-based global optimization with a tree-structured Parzen estimator. The process map underwent four updates using the developed software to determine i + 1-th trial conditions and construct a process map from the results of the i-th trial experiment.

  13. Hot Deformation Behavior and Microstructure of Cast Ni-Based Superalloy IN-100 Based on the Processing Map

    Yusaku Hasebe, Takehito Hagisawa, Cheng Yang, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 54 (11) 4456-4471 2023年

    DOI: 10.1007/s11661-023-07178-7  

    ISSN:1073-5623

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    Hot compression tests were performed on a cast Ni-based superalloy IN-100 at various temperatures and strain rate ranges. From the flow stress-true strain curves and the microstructure observations, a processing map of the hot deformation was constructed based on the dynamic material model. At 1473 K, flow stresses showed a plateau region resembling the characteristics of a dynamic recovery, although values varied with the strain rate. The crystal orientation rotated from the initial <001> direction parallel to the compression axis to the <101> direction, which is the primary slip direction of the fcc system. Kink band formation was observed perpendicular to the compression axis, and fine discontinuous dynamic recrystallization at the boundary of the kink bands was observed for the slow strain rate. As the strain rate increased, meta-dynamic recrystallization was observed after the compression test. At 1373 K, the flow stress first increased sharply and then decreased gradually with increasing strain over the whole strain rate. Twin deformation was suggested to occur prior to the appearance of dynamic recrystallization under 1373 K for the low strain rate range. The stable region of hot deformation was considered to be located at approximately 1423 K and 0.003 s−1.

  14. Microstructure evolution and hardness of S30C carbon steel produced by powder bed fusion using an electron beam and subsequent heat treatments

    Yunwei Gui, Huakang Bian, Kenta Aoyagi, Akihiko Chiba

    MATERIALS LETTERS 328 2022年12月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.matlet.2022.133096  

    ISSN:0167-577X

    eISSN:1873-4979

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    The microstructure and hardness of S30C carbon steel fabricated via powder bed fusion using an electron beam (PBF-EB) and the influence of subsequent heat treatments were firstly investigated. The microstructure of as-built S30C, comprising ferrite and pearlite with an amount of dislocations. The microstructure and hardness showed a height-dependent relationship along building direction. The quenched sample showed lath martensite with maximum hardness (HV 643 +/- 18), which transformed to tempered martensite after further tempering. This study provides a research basis for the application of PBF-EB technology in the development of high-performance carbon steel.

  15. Si-addition contributes to overcoming the strength-ductility trade-off in high-entropy alloys

    Daixiu Wei, Wu Gong, Tomohito Tsuru, Ivan Lobzenko, Xiaoqing Li, Stefanus Harjo, Takuro Kawasaki, Hyeon-Seok Do, Jae Wung Bae, Christian Wagner, Guillaume Laplanche, Yuichiro Koizumi, Hiroki Adachi, Kenta Aoyagi, Akihiko Chiba, Byeong-Joo Lee, Hyoung Seop Kim, Hidemi Kato

    International Journal of Plasticity 159 103443-103443 2022年12月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.ijplas.2022.103443  

    ISSN:0749-6419

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    Face-centered cubic single-phase high-entropy alloys (HEAs) containing multi-principal transition metals have attracted significant attention, exhibiting an unprecedented combination of strength and ductility owing to their low stacking fault energy (SFE) and large misfit parameter that creates severe local lattice distortion. Increasing both strength and ductility further is challenging. In the present study, we demonstrate via meticulous experiments that the CoCrFeNi HEA with the addition of the substitutional metalloid Si can retain a single-phase FCC structure while its yield strength (up to 65%), ultimate strength (up to 34%), and ductility (up to 15%) are simultaneously increased, owing to a synthetical effect of the enhanced solid solution strengthening and a reduced SFE. The dislocation behaviors and plastic deformation mechanisms were tuned by the addition of Si, which improves the strain hardening and tensile ductility. The present study provides new strategies for enhancing HEA performance by targeted metalloid additions.

  16. Factors determining the flowability and spreading quality of gas-atomized Ti-48Al-2Cr-2Nb powders in powder bed fusion additive manufacturing

    Seungkyun Yim, Kenta Aoyagi, Huakang Bian, Yujie Cui, Akihiko Chiba

    POWDER TECHNOLOGY 412 2022年11月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.powtec.2022.117996  

    ISSN:0032-5910

    eISSN:1873-328X

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    A high-quality powder bed is necessary for obtaining high-quality products using powder bed fusion-based ad-ditive manufacturing. In this study, mechanical ball milling was suggested to improve the flowability and spreadability of gas-atomized (GA) Ti-48Al-2Cr-2Nb powder in the powder bed fusion additive manufacturing process. The deformation mechanism of the GA powder significantly differed depending on the ball milling media. Surface grinding mainly occurred during Al2O3 ball milling, while the pulverization was dominant in WC ball milling because of the higher impact force. The flowability of the GA powder under dynamic conditions was improved after Al2O3 and WC ball milling owing to the decreased cohesive force. The high cohesive force in the GA powder under dynamic conditions was attributed to the electrostatic force caused by charge accumulation on the oxide film. The packing density of GA powder was increased after WC ball milling despite the formation of numerous irregular particles owing to the synergetic effect of rapid energy dissipation, minimized wall effect, and void filling effect. Furthermore, the spreadability of the GA powder was improved by ball milling because of the decrease in the cohesive force. Therefore, it was demonstrated that the flowability and spreadability of the GA powder can be improved via ball milling because of changes in particle size, shape, and surface properties.

  17. A survey on basic influencing factors of solidified grain morphology during electron beam melting

    Yufan Zhao, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Materials &amp; Design 221 110927-110927 2022年9月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.matdes.2022.110927  

    ISSN:0264-1275

  18. A survey on basic influencing factors of solidified grain morphology during electron beam melting

    Yufan Zhao, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Materials and Design 221 2022年9月

    DOI: 10.1016/j.matdes.2022.110927  

    ISSN:0264-1275

    eISSN:1873-4197

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    Structure-function integration relies on the flexibility of in-situ solidification microstructure control of additive manufacturing (AM)-built metallic components. Thus, the basic factors influencing the solidified grain morphology must be further investigated, as the mechanisms of grain morphology evolution under the supernormal metallurgical conditions of high energy beam irradiation have not been sufficiently clarified. In this study, experimental and simulated single-track melting of Co-Cr-Mo alloy with electron beam were performed to probe the basic influencing factors of solidified grain morphology. With the aid of numerical modeling and data analysis, the importance of each variable to the grain morphology and correlation between variables, including process parameters, solidification parameters, and fluid velocity to the grain morphology, were revealed. Under unsteady solidification of the dynamic moving molten pool, prediction of the solidification criterion based on the fundamental solidification parameters was not as accurate as expected. The processability and thermophysical properties of the material jointly determined the limitations of controlling the solidification parameters only by the power and scan speed. Furthermore, the effect of melt flow on the grain morphology was discussed. The study results may inspire efficient methods for AM-solidified crystalline control through simpler and operable process optimization.

  19. Non-equilibrium solidification behavior associated with powder characteristics during electron beam additive manufacturing

    Yufan Zhao, Huakang Bian, Hao Wang, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Materials &amp; Design 221 110915-110915 2022年7月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.matdes.2022.110915  

    ISSN:0264-1275

    eISSN:1873-4197

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    For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification, microsegregation and the formation of interdendritic phases significantly affects the material properties. Notably, the powder characteristics influence the heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of the powder feedstock on solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was fabricated using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, samples built using the two powders were characterized and compared. By performing multi-scale numerical simulations, melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that after appropriate size control, compared with the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate was facilitated, thereby suppressing microsegregation. These findings contribute to PBF knowledge related to feedstock, proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

  20. 電子ビーム粉末床溶融結合法により作製したAlSi10Mg合金積層造形体のT6処理に伴う組織変化

    山川 剛平, 卞 華康, 青柳 健大, 山中 謙太, 千葉 晶彦

    軽金属 72 (6) 321-326 2022年6月15日

    出版者・発行元:一般社団法人 軽金属学会

    DOI: 10.2464/jilm.72.321  

    ISSN:0451-5994

    eISSN:1880-8018

  21. Detection, classification and prediction of internal defects from surface morphology data of metal parts fabricated by powder bed fusion type additive manufacturing using an electron beam

    Yunwei Gui, Kenta Aoyagi, Huakang Bian, Akihiko Chiba

    ADDITIVE MANUFACTURING 54 2022年6月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.addma.2022.102736  

    ISSN:2214-8604

    eISSN:2214-7810

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    In powder bed fusion type additive manufacturing using an electron beam (PBF-EB), various process parameters have a significant influence on the performance of manufactured parts. To expand the use of PBF-EB technology in the material industry, one of the problems is the generation of internal defects (pores, unmelted powder, among others) during the process. In this study, we determined a quantitative criterion (Sdr < 0.015 for an even surface; Sa > 80 mu m for an uneven surface; Sdr > 0.015 and Sa < 80 mu m for a porous surface) for classifying surface quality based on surface flatness, and we revealed that different surface qualities (even, uneven, and porous) include different types of internal defects. The parts with even surfaces are free of internal defects and have the highest density(7.7962 g/cm3). Parts with uneven surfaces have a large number of spherical pores owing to their excessive energy input, while parts with porous surfaces have a considerable number of irregularly shaped defects and unmelted powders owing to their insufficient energy input. When the energy input is excessively high, the combination of the Marangoni effect, vapor recoil pressure, and electron beam agitation leads to a high velocity flow of liquid, which tends to form bumps, resulting in an uneven surface. Conversely, if the energy input is too low, the depth of the melt pool is too small to penetrate the thickness of the powder layers, resulting in incomplete melting of the powder at the bottom of the layers and the formation of defects due to lack of fusion between the layers. In addition, five types of machine learning technologies (logistic regression, support vector machine, decision tree, XGBoost, and naive Bayes) were applied to the PBF-EB process parameters optimization of the S30C alloy. A support vector machine has the highest model performance, and we use it to construct a processing map corresponding to the internal defects and determine the PBF-EB process window for the S30C alloy. The optimal PBF-EB process parameter ranges for S30C alloy were predicted as follows: current = 2.5-10 mA, scan speed = 200-1000 mm/s, line offset = 0.11-0.25 mm, or current = 2.5-10 mA, scan speed = 200-750 mm/s, line offset = 0.27-0.33 mm. Moreover, a new framework for constructing a process map of PBF-EB fabricated parts was proposed, which is an effective method to accelerate PBF-EB for manufacturing parts without internal defects.

  22. A method to manipulate non-steady-state columnar-to-equiaxed transition in powder bed fusion additive manufacturing using an electron beam

    Yuchao Lei, Kenta Aoyagi, Akihiko Chiba

    ACTA MATERIALIA 227 2022年4月

    出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD

    DOI: 10.1016/j.actamat.2022.117717  

    ISSN:1359-6454

    eISSN:1873-2453

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    Site-specific control of solidification grain structure is one of the largest attractiveness of manufacturing metallic parts with powder bed fusion additive manufacturing. In this study, we manufacture non-weldable superalloy Alloy713ELC with powder bed fusion additive manufacturing using an electron beam (PBF-EB) and achieve various bulk solidification grain structures, i.e. near fully equiaxed structure, interlocked zigzag structure, and columnar structures with various grain widths, through controlling process parameters under a line order scan strategy. An analytical transient model, which is capable of simulating heat transfer in PBF-EB single-layer melting under the experimental conditions, is established and validated by compared to numerical models of computational fluid dynamics and finite element method in PBF-EB single-track melting. The evolutions of solidification grain structure are rationalized using microstructural characterization and simulations based on various models. It is found that the mechanisms of columnar grain refinement and columnar-to-equiaxed transition (CET) are related to the Walton and Chalmers selection effect, which is governed by the spatial and temporal variations of solidification direction, and to the effect of convection within mushy zone. Based on the grain structure evolution mechanisms, we propose a method to manipulate CET or to achieve a novel interlocked zigzag grain structure in PBF-EB. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  23. Ball-milling treatment of gas-atomized Ti–48Al–2Cr–2Nb powder and its effect on preventing smoking during electron beam powder bed fusion building process

    Seungkyun Yim, Huakang Bian, Kenta Aoyagi, Keiji Yanagihara, Shin ichi Kitamura, Hironobu Manabe, Yohei Daino, Yuichiro Hayasaka, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 51 2022年3月

    DOI: 10.1016/j.addma.2022.102634  

    eISSN:2214-8604

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    Smoking is the explosive powder-scattering by Coulomb interaction, and it is a difficult issue to avoid in electron beam powder bed fusion (EB-PBF) process. In this study, we investigated the effectiveness of mechanical ball milling to prevent powder-bed smoking in EB-PBF using gas-atomized Ti–48Al–2Cr–2Nb powder. The surface of the gas-atomized powder was confirmed to be composed of outer porous TiO2 + Al2O3 and inner dense Al2O3 rich bilayer oxide film that gives rise to a high electrical resistance. The electrical resistivity after ball milling with 300 rpm (BM300) was significantly decreased due to the formation of the degenerated TiO2 and TinO2n–1-type suboxide. The electrical equivalent circuit of the powder bed of BM300 was essentially a single resistor-capacitor (RC) circuit, owing to the mechanically deformed outer oxide film. Besides, through ball milling at 300 rpm for 30 min, the relaxation time of the gas-atomized powder was significantly decreased 1/16 times, and its electrostatic force decreased approximately 1/57 times. Finally, the suppressing effect of smoking on the EB-PBF building process was experimentally examined using ball-milled powders. It was clearly demonstrated that mechanical ball milling is an effective method to restrict smoking during the EB-PBF building process without any chemical treatment and composition change.

  24. Spreading behavior of Ti–48Al–2Cr–2Nb powders in powder bed fusion additive manufacturing process: Experimental and discrete element method study

    Seungkyun Yim, Huakang Bian, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 49 2022年1月

    DOI: 10.1016/j.addma.2021.102489  

    eISSN:2214-8604

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    The quality of powder beds is a key factor governing the density of bulk parts produced by the powder bed fusion–additive manufacturing (PBF-AM) process. Conventional assessments of powder flowability cannot be used to evaluate spreadability of the spreading process owing to different dynamic conditions used. Therefore, factors that have dominant effects on the powder bed properties must be investigated. In this study, two types of Ti–48Al–2Cr–2Nb powders, produced by the plasma rotating electrode process (PREP) and gas atomization (GA), were used to compare spreading behaviors and powder bed properties. The static and dynamic flowabilities of the PREP powder were better than those of the GA powder, while the packing density and surface roughness of the powder bed were worse. During the spreading process, the recorded recoating angle gradually increased for the GA powder, while it remained constant for the PREP powder. The increase in the recoating angle for the GA powder was due to the preferential segregation of fine particles during the spreading process. The discrete element method (DEM) model was calibrated by the fitting static and dynamic angle of repose. Based on the DEM model, three particle-flow regimes were identified: the vortex region, free flowing region, and shear stress region. In the shear stress region, the contact force between the GA powder particles was lower than that of the PREP powder, resulting in high packing density. Artificial manipulation of the particle size and shape in the DEM simulation revealed that the predominant factor governing the powder bed properties between the GA and PREP powders was the particle size distribution. We believe that our study provides an improved understanding of the powder-spreading mechanism and optimization of the powder bed properties.

  25. Synergetic strengthening in HfMoNbTaTi refractory high-entropy alloy via disordered nanoscale phase and semicoherent refractory particle

    Cheng Yang, Huakang Bian, Kenta Aoyagi, Yuichiro Hayasaka, Kenta Yamanaka, Akihiko Chiba

    Materials & Design 212 110248-110248 2021年12月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.matdes.2021.110248  

    ISSN:0264-1275

    eISSN:1873-4197

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    We propose a novel strengthening strategy that involves the introduction of a high-entropy nanoscale phase (HENP) and semicoherent refractory nanoparticle into the A2 matrix of an as-cast equimolar HfMoNbTaTi refractory high-entropy alloy (RHEA). Based on the design concept of low difference in elemental melting points, equiaxed grains with slight segregation were obtained. The HENP was characterized as a disordered near-coherent structure with a composition significantly similar to the matrix that differed from traditional coherent intermetallics. The heterogeneous nucleation was attributed to the aggregation of the largest Hf atoms and interstitial complexes in the locally distorted matrix. Moreover, the refractory nanoparticle was inferred to be hafnium nitride, possessing high thermal stability and establishing a semicoherent interface with the matrix. Notably, the interface was lined with a Ti film of one or two atomic layers. Furthermore, owing to the high melting point and precipitation strengthening, a highest yield strength (851 MPa) was obtained at 1200 °C compared with those reported cast RHEAs. At room temperature, a high yield strength (1713 MPa) was revealed mainly due to the combined effect of solid-solution strengthening and precipitation strengthening. The present study provides a new pathway for the future design of HEAs.

  26. Effects of process parameters and cooling gas on powder formation during the plasma rotating electrode process

    Yujie Cui, Yufan Zhao, Haruko Numata, Kenta Yamanaka, Huakang Bian, Kenta Aoyagi, Akihiko Chiba

    Powder Technology 393 301-311 2021年11月

    DOI: 10.1016/j.powtec.2021.07.062  

    ISSN:0032-5910

    eISSN:1873-328X

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    The plasma rotating electrode process (PREP) is rapidly becoming an important powder fabrication method in additive manufacturing. However, the low production rate of fine PREP powder limits the development of PREP. Herein, we investigated different factors affecting powder formation during PREP by combining experimental methods and numerical simulations. The limitation of increasing the rotation electrode speed in decreasing powder size is attributed to the increased probability of adjacent droplets recombining and the decreased tendency of granulation. The effects of additional Ar/He gas flowing on the rotational electrode on powder formation is determined through the cooling effect, the disturbance effect, and the inclined effect of the residual electrode end face simultaneously. A smaller-sized powder was obtained in the He atmosphere owing to the larger inclined angle of the residual electrode end face compared to the Ar atmosphere. Our research highlights the route for the fabrication of smaller-sized powders using PREP.

  27. Controlling factors determining flowability of powders for additive manufacturing: A combined experimental and simulation study

    Yufan Zhao, Yujie Cui, Yusaku Hasebe, Huakang Bian, Kenta Yamanaka, Kenta Aoyagi, Takehito Hagisawa, Akihiko Chiba

    Powder Technology 393 482-493 2021年11月

    DOI: 10.1016/j.powtec.2021.08.006  

    ISSN:0032-5910

    eISSN:1873-328X

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    Powder flowability affects dynamic powder behavior during additive manufacturing and varies depending on particle size distribution, particle morphology, and surface features. In this study, Inconel 718 alloy powders were prepared by gas atomization (GA), plasma atomization (PA), and plasma rotating electrode process (PREP). The flowability was experimentally evaluated by the avalanche angle. With the help of numerical simulations, the individual effects of particle size distribution, particle morphology, and particle surface features on flowability were analyzed. The results showed that the PREP powder possessed the best flowability among the three powders. The PSDs were slightly different, but these differences had almost no effect on their flowability. Nevertheless, the excellent flowability of the PREP powder was due to the high particle sphericity and the thin surface oxide film. If the environmental factors are well controlled during the handling, PREP has a natural advantage over GA and PA in terms of flowability.

  28. The microstructure and mechanical properties of selective electron beam melting manufactured 9–12Cr ferritic/martensitic steel using N- and Ar-atomized powder

    Tack Lee, Kenta Aoyagi, Huakang Bian, Kenta Yamanaka, Shigeo Sato, Akihiko Chiba

    Additive Manufacturing 45 102075-102075 2021年9月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.addma.2021.102075  

    ISSN:2214-8604

    eISSN:2214-8604

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    Low-carbon 9-12Cr heat-resistant ferritic/martensitic steels were fabricated by SEBM using two types of powders (Ar gas- and N gas-atomized powders). Both the as-built Ar- and N-components showed a tempered and fine lath-like ferritic/martensitic microstructure, high tensile strength (approximately 1300 and 1167 MPa for the as-built N-component and the as-built Ar-component, respectively), and tensile elongation of approximately 8%. The high N content (0.44 wt%) of the as-built N-component resulted in a refined lath martensite phase and promoted the precipitation of a fine M2X phase which give rise to higher strength. SEBM is a suitable and rapid method for fabricating low-C martensite steels with a fine lath martensitic microstructure and enhanced mechanical properties for use in power plants without employing welding.

  29. Smoke Suppression in Electron Beam Melting of Inconel 718 Alloy Powder Based on Insulator–Metal Transition of Surface Oxide Film by Mechanical Stimulation

    Akihiko Chiba, Yohei Daino, Kenta Aoyagi, Kenta Yamanaka

    Materials 14 (16) 4662-4662 2021年8月18日

    出版者・発行元:MDPI AG

    DOI: 10.3390/ma14164662  

    eISSN:1996-1944

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    In powder bed fusion–electron beam melting, the alloy powder can scatter under electron beam irradiation. When this phenomenon—known as smoking—occurs, it makes the PBF-EBM process almost impossible. Therefore, avoiding smoking in EBM is an important research issue. In this study, we aimed to clarify the effects of powder bed preheating and mechanical stimulation on the suppression of smoking in the powder bed fusion–electron beam melting process. Direct current electrical resistivity and alternating current impedance spectroscopy measurements were conducted on Inconel 718 alloy powder at room temperature and elevated temperatures before and after mechanical stimulation (ball milling for 10–60 min) to investigate changes in the electrical properties of the surface oxide film, alongside X-ray photoelectron spectroscopy to identify the surface chemical composition. Smoking tests confirmed that preheating and ball milling both suppressed smoking. Furthermore, smoking did not occur after ball milling, even when the powder bed was not preheated. This is because the oxide film undergoes a dielectric–metallic transition due to the lattice strain introduced by ball milling. Our results are expected to benefit the development of the powder bed fusion–electron beam melting processes from the perspective of materials technology and optimization of the process conditions and powder properties to suppress smoking.

  30. Effect of multi-stage heat treatment on mechanical properties and microstructure transformation of Ti–48Al–2Cr–2Nb alloy

    Seungkyun Yim, Huakang Bian, Kenta Aoyagi, Akihiko Chiba

    Materials Science and Engineering A 816 2021年6月1日

    DOI: 10.1016/j.msea.2021.141321  

    ISSN:0921-5093

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    Refining the grain size of γ-TiAl alloys to improve their strength and ductility is of academic interest. Thus, we report a multi-stage heat treatment method consisting of solution treatment, cyclic heat treatment, annealing, short heat treatment, and aging to refine the microstructure of the Ti–48Al–2Cr–2Nb alloy. The solution-treated microstructure was refined from 1100 to 191 μm by cyclic heat treatment, promoting feathery γ packets. Fine duplex grains below 23 μm were obtained through discontinuous coarsening, which was accelerated in the cyclic-heat-treated alloys owing to the high driving force resulting from the presence of feathery γ packets and fine interlamellar spacing. Through the short heat treatment and aging at single α and α + γ fields, a tailored duplex structure with a grain size of 24 μm and interlamellar spacing of 42 nm was achieved. Through microstructure refinement, tensile strength and elongation were improved to 697 MPa and 2.1%, respectively, compared to those of the conventional forged specimen (622 MPa and 1.3%, respectively). We believe that our study provides a simple pathway to refine the grain size and interlamellar spacing of γ-TiAl alloys. 2

  31. Effects of the aluminum concentration on twin boundary motion in pre-strained magnesium alloys

    Yujie Cui, Kenta Aoyagi, Huakang Bian, Yuichiro Hayasaka, Akihiko Chiba

    Journal of Materials Science and Technology 73 116-127 2021年5月20日

    DOI: 10.1016/j.jmst.2020.07.027  

    ISSN:1005-0302

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    We investigated the effects of Al concentration on the reciprocated motion of twin boundaries in pre-strained Mg-Al-Zn alloys through a combination of applied compression and tension, in-situ electron-backscattering diffraction observations, and high-angle annular dark-field scanning transmission electron microscopy observations. The twin growth was restricted by increased Al concentration, which resulted in the occurrence of smaller-sized twins. The reverse motion of twin boundaries was also restricted, resulting in the formation of higher fractions of secondary twins and 2–5° boundaries during reverse tension. The secondary twins and 2–5° boundaries mainly contributed to the increased ultimate tensile strength of the pre-strained Mg alloys. This effect is more significant in Mg alloys with larger pre-compression. Moreover, the increased amount of the Al solute atoms, rather than the precipitates, mainly contributed to the increased strengthening effect on the twin boundary motion. Our research contributes to development of high-strength Mg alloys by stabilizing twin boundaries.

  32. Critical factor triggering grain boundary cracking in non-weldable superalloy Alloy713ELC fabricated with selective electron beam melting

    Yuchao Lei, Kenta Aoyagi, Kinya Aota, Kosuke Kuwabara, Akihiko Chiba

    Acta Materialia 208 116695-116695 2021年4月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.actamat.2021.116695  

    ISSN:1359-6454

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    Grain boundary cracking in non-weldable superalloy fabricated with selective electron beam melting is affected by the interaction of multiple factors including mechanical and compositional effects. In this study, we construct process maps in a wide range high-dimensional parameter space for the non-weldable superalloy Alloy713ELC through employing a machine learning approach, and we could fabricate many cracked and crack-free samples under the optimized conditions by excluding the extrinsic effect of process defects on cracking. Comparing between cracked and crack-free samples reveals that the samples with fine columnar grains can be cracked while those with coarse columnar grains can be crack-free, and that the cracking propensity in the optimized samples within a process window with scan speed ≤ 800 mm/s can be ranked by using a quasi-total plastic strain index (QTPSI), which is calculated via thermo-mechanical analysis. The total plastic strain level is a critical cracking factor and a larger scan speed tends to elevate the total plastic strain level, exhibiting a larger deviation beyond the QTPSI. Besides, the non-weldability in Alloy713ELC significantly attributes to its thermal expansion effect, which correlates to the large Al content. This thermal expansion effect combined with the liquation effect and the strain-age cracking effect reveals the intrinsic cause of non-weldability in Alloy713ELC.

  33. Effect of niobium addition on tensile properties and oxidation resistance of a titanium-based alloy

    Yujie Cui, Kenta Aoyagi, Yuichiro Koizumi, Cheng Yang, Huakang Bian, Yuichiro Hayasaka, Tadashi Fujieda, Akihiko Chiba

    Corrosion Science 180 109198-109198 2021年3月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.corsci.2020.109198  

    ISSN:0010-938X

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    Previous research has inadequately elucidated the influence of Nb addition on the tensile properties and oxidation resistance of Ti-based alloys. Herein, the tensile properties and oxidation resistance of Ti–6Al–2Sn–4Zr–2Mo–0.1Si–1.0B–xNb (x = 0, 0.2, and 1.0) alloys were investigated. The oxidation resistance was significantly improved by Nb addition without significantly influencing the tensile properties. The substitution of Ti4+ by Nb5+ in TiO2 and promoted formation of a Sn-rich layer between the oxidation layer and substrate caused an increase in the oxidation resistance, thus highlighting the effectiveness of Nb addition in enhancing the oxidation resistance.

  34. Thermal properties of powder beds in energy absorption and heat transfer during additive manufacturing with electron beam

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Powder Technology 381 44-54 2021年3月

    DOI: 10.1016/j.powtec.2020.11.082  

    ISSN:0032-5910

    eISSN:1873-328X

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    Powder bed properties affect the quality of the parts manufactured by powder bed fusion (PBF). On PBF with laser, researchers had thoroughly studied the impact of powder layer on consolidation. In terms of interaction mechanisms with materials, electron beam is different from laser. Thus, an in-depth discussion is required for PBF with electron beam. In this study, numerical simulations were performed to reconstruct the powder bed and analyze the subsequent consolidation. The powder layer's presence altered the emissivity and thermal conductivity to be different from those of a solid. Concerning the powder with different particle size distributions, the differences in thermal properties led to an alteration in the molten pool formation. The emissivity increased, and the thermal conductivity decreased as the fraction of fine powders increased. With the increased fraction of fine powders under a given layer thickness, relatively high energy should be input to ensure exceptional forming quality.

  35. Elucidating the effect of preheating temperature on melt pool morphology variation in Inconel 718 laser powder bed fusion via simulation and experiment

    Qian Chen, Yunhao Zhao, Seth Strayer, Yufan Zhao, Kenta Aoyagi, Yuichiro Koizumi, Akihiko Chiba, Wei Xiong, Albert C. To

    Additive Manufacturing 37 101642-101642 2021年1月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.addma.2020.101642  

    ISSN:2214-8604

    eISSN:2214-8604

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    In laser powder bed fusion (L-PBF) additive manufacturing, the mechanical performance, microstructure and defects of fabricated parts are closely associated with the melt pool morphology, e.g., its dimension and shape through the building process. Past studies have largely focused on how the process parameters such as laser power and scan speed affect melt pool characteristics. In this study, the melt pool morphology variation as a function of preheating temperature in the conduction, transition, and keyhole regimes and the underlying mechanisms in each regime are investigated through ex-situ sample characterization and computation thermal fluid dynamics (CtFD) simulation. Single tracks with different combinations of laser power and scan speed are deposited on an Inconel 718 bare plate preheated to a temperature range of 100–500 °C in the experiment. Significant changes are observed in melt pool morphology as a function of preheating temperature from optical measurements of melt track cross sections. The depth of melt pool in the three regimes increases monotonically with preheating temperature, e.g., at 500 °C, the experimental melt pool depth is increased by 49% in conduction regime, 34% in transition regime and 33% in keyhole regime, respectively, while the variation of melt pool width in each regime does not all follow an increasing trend but depends on the melt pool regimes. Melt pool width variation in the conduction and transition regimes is found to depend on the enhanced heat conduction directly related to temperature dependent thermal properties. Through validated CtFD simulations, it is found that in the keyhole regime the evaporation mass, recoil pressure, and laser drilling effect is enhanced with higher preheating temperature, which gives rise to a deeper melt pool. The simulations also reveal that preheating temperature significantly elongates the melt track length due to the increased flow rate and strong recoil pressure that accelerates the backward flow.

  36. Centrifugal granulation behavior in metallic powder fabrication by plasma rotating electrode process

    Yufan Zhao, Yujie Cui, Haruko Numata, Huakang Bian, Kimio Wako, Kenta Yamanaka, Kenta Aoyagi, Akihiko Chiba

    Scientific Reports 10 (1) 2020年12月1日

    DOI: 10.1038/s41598-020-75503-w  

    eISSN:2045-2322

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    In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rotating electrode process (PREP) have been highly recommended for powder-type additive manufacturing. Most research on PREP is aimed at establishing relationship between PREP parameters and powder size. However, almost no dedicated research on granulation behavior has been conducted so far. In the present study, PREP experiments of Ti64 and SUS316 alloys were carried out. Numerical modeling based on computational thermo-fluid dynamics was developed to analyze the granulation behavior. In particular, the roles of the additionally introduced gas blast and the morphology of the electrode end surface in fluid granulation were preliminarily investigated. The study showed that in addition to the electrode's rotating speed and diameter, manipulating the plasma arc current (i.e., the melting rate) could also be an effective way to control the PREP-powder size. According to the simulation, there were competing actions of the gas blast affecting the powder size. The gas blast created disturbance on the fluid and deepened the depression of the electrode end surface, which facilitated powder refinement. However, the cooling effect enhanced the fluid stability and hindered fluid granulation. The conclusions indicated the possibility of using various methods to manipulate PREP-powder size.

  37. Role of operating and environmental conditions in determining molten pool dynamics during electron beam melting and selective laser melting

    Yufan Zhao, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 36 2020年12月

    DOI: 10.1016/j.addma.2020.101559  

    eISSN:2214-8604

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    Electron beam melting (EBM) and selective laser melting (SLM) are representative powder bed fusion additive manufacturing methods. Because EBM and SLM have different operating and environmental conditions, such as ambient pressure of the chamber, initial temperature, and heat source, they have different molten pool dynamics. In this study, single-bead melting experiments using EBM and SLM were performed in conjunction with computational thermal-fluid dynamics simulations in high-energy conditions to highlight the differences in the molten pool dynamics of EBM and SLM. The experimental results reveal that SLM is more likely to melt in the keyhole mode than EBM under nominally identical line energy. The simulations showed that the instantaneous maximum temperature of the SLM molten pool is much lower than that of the EBM molten pool. An increase in the preheating temperature is found to strengthen the vapor recoil pressure; however, the vapor recoil pressure under vacuum is maintained at a considerably low level in EBM. Compared to EBM, the high atmospheric pressure and multiple laser reflections during SLM significantly enhance the effect of the vapor recoil pressure on the melt surface. The findings of this study can be useful for the formulation of appropriate processing strategies for the two processes.

  38. Manufacturing of a nanosized TiB strengthened Ti-based alloy via electron beam powder bed fusion 査読有り

    Yujie Cui, Kenta Aoyagi, Yufan Zhao, Kenta Yamanaka, Yuichiro Hayasaka, Yuichiro Koizumi, Tadashi Fujieda, Akihiko Chiba

    Additive Manufacturing 36 2020年12月

    DOI: 10.1016/j.addma.2020.101472  

    eISSN:2214-8604

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    © 2020 Elsevier B.V. We manufactured a Ti-based alloy via electron beam powder bed fusion (PBF-EB) and systematically investigated its microstructure and mechanical properties. We compared its properties with those of a conventionally forged alloy via electron backscattered diffraction, high-angle annular dark-field scanning transmission electron microscopy observations, and tensile tests. The formation mechanism, orientation relationship with the matrix, and crystal structures of the nanosized TiB particles were verified. The formation of nanosized TiB particles was mainly attributed to high cooling rates during PBF-EB manufacturing. In addition, the segregation of the solute Zr and Si atoms into the α-phase/TiB interface restricted TiB precipitate growth, which contributed to the formation of fine TiB particles in the alloy manufactured via PBF-EB. The orientation relationships between the TiB particles and α-phase in the PBF-EB-manufactured alloy are different from those in the hot-forged alloy. Moreover, for the first time, the simultaneous existence of the B27 and twinned B27 structures was verified in the nanosized TiB particles. Our study provides general guidelines for the manufacturing of materials strengthened by refined precipitates using PBF-EB or other additive manufacturing methods.

  39. Effects of plasma rotating electrode process parameters on the particle size distribution and microstructure of Ti-6Al-4 V alloy powder

    Yujie Cui, Yufan Zhao, Haruko Numata, Huakang Bian, Kimio Wako, Kenta Yamanaka, Kenta Aoyagi, Chen Zhang, Akihiko Chiba

    Powder Technology 376 363-372 2020年10月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.powtec.2020.08.027  

    ISSN:0032-5910

    eISSN:1873-328X

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    The starting powder quality significantly influences the process window optimization and properties of parts fabricated by additive manufacturing. In this study, we investigated the influence of plasma rotating electrode process (PREP) parameters on the particle size distribution and microstructure of Ti-6Al-4 V alloy powder. The martensite size in the powder decreased with increasing rotating electrode speed owing to the higher cooling rate. Numerical simulations using computational thermal fluid dynamics were found to be feasible for quantitative evaluation of the temperature variation, cooling rate, and powder size during PREP, which provide a new strategy to study the mechanism of powder formation. In addition, a statistical model combining principal component analysis and the Monte Carlo methods was proposed to evaluate the relationships between PREP parameters and average powder diameter based on the limited collected experimental data. The proposed statistical model can also be applied in research fields where multivariable problems exist.

  40. Significance of powder feedstock characteristics in defect suppression of additively manufactured Inconel 718 査読有り

    Yufan Zhao, Kenta Aoyagi, Yohei Daino, Kenta Yamanaka, Akihiko Chiba

    Additive Manufacturing 34 2020年8月

    DOI: 10.1016/j.addma.2020.101277  

    eISSN:2214-8604

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    © 2020 Elsevier B.V. The characteristics of powder applied in electron beam powder-bed fusion (EB-PBF) play a vital role in the process stability and final part performance. We use two types of Inconel 718 alloy powders for experiments, namely, (i) imperfect spherical and (ii) spherical powders. They have similar particle size distributions but are different in geometry and built-in defect. The forming qualities concerning surface topography, density, and internal defect of the EB-PBF-built samples prepared using two types of powders are characterized under the same processing conditions. In particular, the forming qualities are further compared under the optimal process condition to highlight the decisive role of powder features. Notably, different powder geometries with distinct surface feature inevitably affect the heat transfer during melting. The significance of powder feedstock characteristics in defect suppression is clarified with the aid of numerical simulations. The experimental results show that compared to spherical powders, fabrication using imperfect spherical powders is more likely to evoke lack–of–fusion and excessive melting under low and high energy conditions, respectively. Thus, spherical powders have a broader process window in ensuring a higher density and smoother surface than that of imperfect spherical powders. Moreover, in the sample built with spherical powders, the high cooling and solidification rates evaluated by numerical simulations result in the suppression of the interdendritic voids.

  41. Fabricating 9–12 Cr ferritic/martensitic steels using selective electron beam melting

    Tack Lee, Huakang Bian, Kenta Aoyagi, Haruki Ohnishi, Takehisa Hino, Yujiro Nakatani, Akihiko Chiba

    Materials Letters 271 2020年7月15日

    DOI: 10.1016/j.matlet.2020.127747  

    ISSN:0167-577X

    eISSN:1873-4979

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    9–12 Cr ferritic/martensitic steel is successfully fabricated using selective electron beam melting (SEBM). The microstructure of as-built components is compared to the ones fabricated by a conventional method, and it is revealed that the SEBM-built components have tempered martensite structure with fine equiaxed prior austenite grains (60.1 ± 8.3 μm) containing thin layers covered by W-rich phases and large fraction (20% over) of sub-blocks corresponding to K-S orientation relationships due to characteristics of SEBM as a rapid solidification process. The SEBM provides a new production method to fabricate heat-resistant components of 9–12 Cr ferritic/martensitic steels with unique microstructure.

  42. Hot deformation characteristics and dynamic recrystallization mechanisms of a Co–Ni-based superalloy 査読有り

    Lingxiao Ouyang, Rui Luo, Yunwei Gui, Yun Cao, Leli Chen, Yujie Cui, Huakang Bian, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Materials Science and Engineering A 788 2020年6月24日

    DOI: 10.1016/j.msea.2020.139638  

    ISSN:0921-5093

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    © 2020 Elsevier B.V. The hot deformation behavior of a Co–Ni-based superalloy was systematically investigated using thermal compression tests. Stress–strain curves showed a typical dynamic softening after peak stress, especially at high temperatures and low strain rates. An Arrhenius-type constitutive equation was developed to reveal the relationship between the flow stress, strain rate, and temperature, while a processing map was constructed based on the calculations from the stress-strain curves combined with microstructural observations to determine the optimum thermal deformation conditions. The extent of recrystallization was found to increase with increasing temperature, a decreasing strain rate, or an increasing strain. A complete dynamic recrystallization (DRX) condition was reached at 1050 °C/0.01 s−1/0.7. In addition, pre-existing annealing twins were replaced by discontinuous dynamic recrystallization (DDRX) grains along the twin boundaries and the twin-DRX (TDRX) grains in the twin interior. In the case of an un-twinned matrix, a combined DDRX and continuous DRX (CDRX) process occurred at high strain rates, in contrasted with a single DDRX process taking place at low strain rates.

  43. Process optimization and mechanical property investigation of non-weldable superalloy Alloy713ELC manufactured with selective electron beam melting

    Yuchao Lei, Kenta Aoyagi, Yujie Cui, Dong-Soo Kang, Kosuke Kuwabara, Kinya Aota, Akihiko Chiba

    Materials Science and Engineering: A 787 139485-139485 2020年6月

    出版者・発行元:Elsevier BV

    DOI: 10.1016/j.msea.2020.139485  

    ISSN:0921-5093

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    An efficient optimization method based on a support vector machine (SVM) is used to optimize multiple process parameters of selective electron beam melting (SEBM) for a non-weldable nickel-base superalloy Alloy713ELC. The global optimum condition and the near optimum conditions are extracted to fabricate SEBM samples. All the SVM optimized conditions lead to near net shaped samples with even top surfaces. The sample fabricated under the global optimum condition for sample dimension of 10 mm exhibits pore-less cross-sections, columnar grains with fine γ′ precipitates and fine substructure, a small amount of grain boundary crack and excellent room temperature tensile properties. The samples fabricated under the global optimum condition and a near optimum condition with increased beam current for sample dimension of 15 mm exhibit excellent creep properties under 980 °C. In both the two situations for sample dimensions of 10 mm and 15 mm, SEBM samples with mechanical properties superior to conventional cast alloys can be achieved by testing only 1–3 SVM optimized conditions. We demonstrate the current method is effective for optimizing SEBM process, especially when multiple parameters need to be considered simultaneously. Besides, this method can rapidly provide not only a batch of conditions leading to samples with good top surfaces but also the optimum conditions leading to good building quality and superior mechanical properties.

  44. Isothermal γ → ε phase transformation behavior in a Co-Cr-Mo alloy depending on thermal history during electron beam powder-bed additive manufacturing 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Journal of Materials Science & Technology 50 162-170 2020年3月

    DOI: 10.1016/j.jmst.2019.11.040  

    ISSN:1005-0302

    詳細を見る 詳細を閉じる

    Powder bed fusion with electron beam (PBF-EB), allows Co-Cr-Mo (CCM) implants with patient-customization to be fabricated with high quality and complex geometry. However, the variability in the properties of PBF-EB-built CCM alloy, mainly due to the lack of understanding of the mechanisms that govern microstructural heterogeneity, brings limitations in extensive application. In this study, the microstructural heterogeneity regarding the γ-fcc → ε-hcp phase transformation was characterized. The phase transformation during PBF-EB was analyzed depending on the thermal history that was elucidated by the numerical simulation. It revealed that isothermal γ → ε transformation occurred during the fabrication. Importantly, the difference in γ/ε phase distribution was a result of the thermal history determining which method phase transformation was taking place, which can be influenced by the PBF-EB process parameters. In the sample with a low energy input (Earea = 2.6 J/mm2), the martensitic transformation was dominant. As the building height increased from the bottom, the ε phase fraction decreased. On the other hand, in the sample with a higher energy input (Earea = 4.4 J/mm2), the ε phase formed via diffusional-massive transformation and only appeared in a short range of the lower part away from the bottom.

  45. Impacts of pre-strain on twin boundary mobility of magnesium 査読有り

    Yujie Cui, Huakang Bian, Yunping Li, Yufan Zhao, Kenta Aoyagi, Akihiko Chiba

    JOURNAL OF ALLOYS AND COMPOUNDS 816 152496-152496 2020年3月

    出版者・発行元:ELSEVIER SCIENCE SA

    DOI: 10.1016/j.jallcom.2019.152496  

    ISSN:0925-8388

    eISSN:1873-4669

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    Twin boundary mobility (TBM) has significant influences on the mechanical properties of magnesium (Mg). In the current study, the effects of pre-strain on TBM in pure Mg are systematically investigated by evaluating the change of yield stress during compression and tension, and microstructure observations by in-situ electron back-scattered diffraction. Owing to lower dislocation density and less restriction from other boundaries, smaller twins in pure Mg subjected to 2% compressive strain exhibit higher TBM than larger twins in pre-4% and pre-8% compressed pure Mg. Decreased TBM with increasing pre-strain is primarily because of the larger number of dislocations, which impede twin boundary motion. These findings indicate that altering twin size and dislocation density by pre-strain is an effective way to tailor TBM, providing a basis for developing new industrial Mg alloys. (C) 2019 Elsevier B.V. All rights reserved.

  46. Enhanced oxidation resistance of a titanium-based alloy by the addition of boron and the application of electron beam melting 査読有り

    Yujie Cui, Kenta Aoyagi, Yuichiro Koizumi, Tadashi Fujieda, Akihiko Chiba

    ADDITIVE MANUFACTURING 31 2020年1月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.addma.2019.100971  

    ISSN:2214-8604

    eISSN:2214-7810

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    Refined TiB precipitates significantly enhance the oxidation resistance of Ti-6Al-2Sn-4Zr-2Mo-0.1Si-1.0B alloy fabricated by electron beam powder bed fusion (EB-PBF). Refined TiB precipitates in the EB-PBF-built alloy enable finer oxide formation than the larger precipitates in the forged alloy, and the resulting oxidation layers are more compact. Evaporation of scattered B2O3 generated by the refined TiB precipitates in the EB-PBF-built alloy do not significantly accelerate detachment of the oxidation layer from the substrate. However, collective evaporation of B2O3 generated by larger TiB precipitates in the forged alloy accelerate detachment. The oxidation layer on the EB-PBF-fabricated alloy was more stable, preventing further oxidation and improving oxidation resistance.

  47. Microstructure refinement for superior ductility of Al–Si alloy by electron beam melting 査読有り

    Huakang Bian, Kenta Aoyagi, Yufan Zhao, Chikatoshi Maeda, Toshihiro Mouri, Akihiko Chiba

    Additive Manufacturing 32 (100982) 2019年12月

    DOI: 10.1016/j.addma.2019.100982  

    ISSN:2214-8604

    eISSN:2214-8604

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    Refining the microstructure to improve the ductility of an Al‒Si alloy is challenging. In this paper, we report for the first time a novel microstructure refinement approach for AlSi10Mg (wt%) alloys using electron beam melting (EBM) technology, without the addition of any modification elements. The synergetic effect of superheating, fast cooling, and preheating contributes to a refined Si phase with a fine granular structure (0.5–2 μm) within bimodal Al grains (40 μm grains and 0.5–2 μm sub-grains). The results provide good evidence for the proposed refinement mechanism. A maximum ductility of approximately 32.7 % with a tensile strength of approximately 136 MPa was achieved for the as-built AlSi10Mg EBM alloy. After solution heat treatment and T6-like aging, nano-Si precipitates formed which strengthened the alloys. The pathway developed in this study for refining the Al–Si alloy microstructure to improve the tensile ductility will provide a feasible and fast manufacturing method for improving the microstructure and mechanical properties of other low-melting temperature alloys in the near future using EBM technology.

  48. Microstructure evolution and mechanical property of a precipitation-strengthened refractory high-entropy alloy HfNbTaTiZr 査読有り

    Cheng Yang, Kenta Aoyagi, Huakang Bian, Akihiko Chiba

    MATERIALS LETTERS 254 46-49 2019年11月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.matlet.2019.07.027  

    ISSN:0167-577X

    eISSN:1873-4979

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    The mechanical properties and microstructure evolution of a precipitation-strengthened refractory high-entropy alloy, HfNbTaTiZr, with equiaxed grains are reported. The as-cast alloy shows a high yield strength of 356 MPa at 1200 degrees C, and 1597 MPa with good ductility at room temperature. The enhancement in strength is attributed to precipitation strengthening. Afterwards, microstructure evolution was investigated under several annealing treatment conditions: 1000 degrees C for 24 h, 1200 degrees C for 24 h, 1400 degrees C for 24 h, and 1450 degrees C for 168 h. The hcp-1 phase appears at 1000 degrees C and gradually dissolves up to 1200 degrees C. When the temperature rises to 1400 degrees C, the hcp-1 phase disappears, and fine nanoprecipitates begin to form. After annealing at 1450 degrees C for one week, however, this alloy exhibits phase decomposition. With the formation of two kinds of new fcc phase, fine nanoprecipitate only remains in bcc matrix, and another new hcp-2 phase forms in the grain. (C) 2019 Elsevier B.V. All rights reserved.

  49. Manipulating local heat accumulation towards controlled quality and microstructure of a Co-Cr-Mo alloy in powder bed fusion with electron beam 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    MATERIALS LETTERS 254 269-272 2019年11月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.matlet.2019.07.078  

    ISSN:0167-577X

    eISSN:1873-4979

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    Controlled forming quality and solidification microstructure are crucial issues for the application of additive manufactured metal. The local heat accumulation through a line order hatching strategy was considered when a Co-28Cr-6Mo alloy was fabricated by powder bed fusion with electron beam. The study revealed that by weakening the local heat accumulation, overheating as a source of unevenness was easily avoided. Transient geometry of the molten pool and associated temperature gradient direction were determined by the local heat accumulation, which subsequently alters the solidification microstructure. The numerical simulations were performed to clarify the mechanisms involved. (C) 2019 Elsevier B.V. All rights reserved.

  50. Microstructural control of alloy 718 fabricated by electron beam melting with expanded processing window by adaptive offset method 査読有り

    Xiao Ding, Yuichiro Koizumi, Kenta Aoyagi, Tadashi Kii, Nobuyuki Sasaki, Yuichiro Hayasaka, Kenta Yamanaka, Akihiko Chiba

    MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 764 138058 2019年9月

    出版者・発行元:ELSEVIER SCIENCE SA

    DOI: 10.1016/j.msea.2019.138058  

    ISSN:0921-5093

    eISSN:1873-4936

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    This paper presents microstructures of Inconel 718 fabricated by electron beam melting (EBM), a powder bed fusion (PBF), additive manufacturing (AM) process, under various conditions with an extended range of electron beam power (P) and scanning speed (V). An adaptive offset method (AOM) was used to optimize the beam scanning line offset adaptively to fabricate blocks without macroscopic defects. In the AOM, the line offset is changed depending on the geometry of melt-pools of adjacent scanning lines. The AOM is valid as long as melt pool depth is larger than layer thickness, and it greatly broadens the process window for building dense and even parts. The broadened process window extends the range of the solidification condition and the variation of the resultant microstructures. Fully columnar grains and mixtures of columnar and equiaxed grains were formed. Mixtures of columnar grains and equiaxed grains were formed under two different types of extreme conditions. The first type is with low line energy/small line offset, which causes equiaxed grains associated with a small number of defects. The second type is with high line energy/large line offset, which causes equiaxed grains via the columnar-to-equiaxed transition (CET). The fully columnar grains are preferably oriented to < 001 > direction in the build direction. The blocks with the different microstructure exhibited similar strengths but different elongations. A processing map was established by plotting the points indicating the build condition and resultant microstructure in the P-V space as a guide for controlling microstructure.

  51. On microstructural homogenization and mechanical properties optimization of biomedical Co-Cr-Mo alloy additively manufactured by using electron beam melting 査読有り

    Daixiu Wei, Ainiwaer Anniyaer, Yuichiro Koizumi, Kenta Aoyagi, Makoto Nagasako, Hidemi Kato, Akihiko Chiba

    ADDITIVE MANUFACTURING 28 215-227 2019年8月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.addma.2019.05.010  

    ISSN:2214-8604

    eISSN:2214-7810

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    The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM) technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal gamma-fcc -> epsilon-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant epsilon or predominant gamma phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The gamma -> epsilon strain-induced martensitic transformation is the major deformation mode of the gamma phase, meanwhile the formation of stripped epsilon phase at {111}(gamma) habit planes contributed to a good combination of strength and ductility. Nevertheless, the e phase was deformed mainly by (0001)(epsilon) <11 <(2)over bar>0 >(epsilon) basal and {1 (1) over bar 00}(epsilon) <11 <(2)over bar>0 >epsilon prismatic slip systems, exhibiting very limited ductility and strength. In addition, the epsilon grains act as secondary hardening factor in the samples consisting of dual gamma/epsilon phase, leading to a non-uniform deformation behavior.

  52. Comprehensive study on mechanisms for grain morphology evolution and texture development in powder bed fusion with electron beam of Co–Cr–Mo alloy 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Daixiu Wei, Kenta Yamanaka, Akihiko Chiba

    Materialia 6 100346 2019年6月

    出版者・発行元:Elsevier {BV}

    DOI: 10.1016/j.mtla.2019.100346  

    eISSN:2589-1529

  53. Simple method to construct process maps for additive manufacturing using a support vector machine 査読有り

    Kenta Aoyagi, Hao Wang, Hideki Sudo, Akihiko Chiba

    ADDITIVE MANUFACTURING 27 353-362 2019年5月

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.addma.2019.03.013  

    ISSN:2214-8604

    eISSN:2214-7810

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    We propose a simple method to construct a process map for additive manufacturing using a support vector machine. By observing the surface of the built parts and classifying them into two classes (good or bad), this method enables a process map to be constructed in order to predict a process condition that is effective at fabricating a part with low pore density. This proposed method is demonstrated in a biomedical CoCr alloy system. We show that the proposed method is effective at reducing the number of experiments necessary to tailor an optimized process condition. This study also shows that the value of a decision function in a support vector machine has a physical meaning (at least in the proposed method) and is a semi-quantitative guideline for porosity density of parts fabricated by additive manufacturing.

  54. Molten pool behavior and effect of fluid flow on solidification conditions in selective electron beam melting (SEBM) of a biomedical Co-Cr-Mo alloy 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Daixiu Wei, Kenta Yamanaka, Akihiko Chiba

    ADDITIVE MANUFACTURING 26 202-214 2019年3月

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.addma.2018.12.002  

    ISSN:2214-8604

    eISSN:2214-7810

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    Selective electron beam melting (SEBM) is a type of additive manufacturing (AM) that involves multiple physical processes. Because of its unique process conditions compared to other AM processes, a detailed investigation into the molten pool behavior and dominant physics of SEBM is required. Fluid convection involves mass and heat transfer; therefore, fluid flow can have a profound effect on solidification conditions. In this study, computational thermal-fluid dynamics simulations with multi-physical modeling and proof-of-concept experiments were used to analyze the molten pool behavior and resultant thermal conditions related to solidification. The Marangoni effect of molten metal primarily determines fluid behavior and is a critical factor affecting the molten pool instability in SEBM of the Co-Cr-Mo alloy. The solidification parameters calculated from simulated data, especially the solidification rate, are sensitive to the local fluid flow at the solidification front. Combined with experimental analysis, the results presented herein indicate that active fluid convection at the solidification front increase the probability of new grain formation, which suppresses the epitaxial growth of columnar grains.

  55. Electron beam melting of boron-modified Ti–2Al–2Sn–4Zr–2Mo–0.1Si alloy with superior tensile strength and oxidation resistance at elevated temperatures 査読有り

    Tadashi Fujieda, Yujie Cui, Kenta Aoyagi, Yuichiro Koizumi, Akihiko Chiba

    Materialia 4 367-372 2018年12月

    DOI: 10.1016/j.mtla.2018.10.013  

    eISSN:2589-1529

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    © 2018 Acta Materialia Inc. The effectiveness of applying an electron beam melting (EBM) additive manufacturing process to Ti–6Al–2Sn–4Zr–2Mo–0.1Si–1B alloy by comparing the EBM alloy with a corresponding wrought alloy was demonstrated. The EBM alloys exhibited microstructures that had much finer whisker-like TiB precipitates with nanometer width dispersed more uniformly than those of the wrought alloy formed due to the rapid solidification process. The EBM alloys had higher tensile strength than the wrought alloy at elevated temperatures up to 873 K. Furthermore, the oxidation resistance at 1073 K of the EBM alloys was significantly improved because of the dense oxide film formation.

  56. 金属付加製造の現場と粉末床溶融結合における組織制御 招待有り 査読有り

    小泉 雄一郎, 山中 謙太, 青柳 健大, 千葉 晶彦

    スマートプロセス学会誌 7 (6) 216-222 2018年11月

    ISSN:2186-702X

  57. Significant impact of yttrium microaddition on high temperature tensile properties of Inconel 713C superalloy 査読有り

    Dong Soo Kang, Yuichiro Koizumi, Kenta Yamanaka, Kenta Aoyagi, Huakang Bian, Akihiko Chiba

    MATERIALS LETTERS 227 40-43 2018年9月

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.matlet.2018.03.106  

    ISSN:0167-577X

    eISSN:1873-4979

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    High temperature tensile properties of Inconel 713C without and with Yttrium microaddition have been investigated. Yttrium addition affected the solidification structure with promotion of equiaxed grain formation in the center of mold. Ultimate tensile strength increased from approximately 892 to 1006 MPa while the elongation increased from approximately 13% to 21% by 0.05 mass% Yttrium addition. 0.05 mass% Yttrium addition in Inconel 713C alloy showed optimum high temperature tensile property. (C) 2018 Elsevier B.V. All rights reserved.

  58. サポートベクターマシンによる金属積層造形レシピ開発の効率化 招待有り

    青柳 健大

    第22回結晶工学セミナー 「結晶工学×データサイエンス」 -最先端事例から学ぶクリスタルインフォマティクス- 33-44 2017年12月

  59. Characterization of powder bed generation in electron beam additive manufacturing by discrete element method (DEM) 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    MATERIALS TODAY-PROCEEDINGS 4 (11) 11437-11440 2017年

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.matpr.2017.09.023  

    ISSN:2214-7853

    eISSN:2214-7853

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    The dynamic behavior of a collection of powder during raking process was simulated by utilizing discrete element method (DEM), aiming at obtaining powder packing features of powder layer, which are valuable for electron beam additive manufacturing. For a better matching between simulation and real process, the mechanical model of particulate system was modified. The simulation results revealed that powder packing density increased with increasing layer thickness, and the powder with better flowability was favored for higher packing density and uniformity of powder layer. The mixture of powder with large and small mean size can increase packing density in some degree, but it was found to be detrimental to the powder flowability. (C) 2017 Elsevier Ltd. All rights reserved.

  60. 革新接合技術開発への道しるべ - 電子ビーム積層造形 査読有り

    小泉 雄一郎, 青柳 健大, 山中 謙太, 千葉 晶彦

    溶接学会誌 86 (8) 570-578 2017年

    DOI: 10.2207/jjws.86.570  

    ISSN:0021-4787

  61. Modeling and Simulation of Electron Beam Additive Manufacturing for Biomedical Co-Cr-Mo Alloy 査読有り

    Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Kenta Yamanaka, Akihiko Chiba

    Proceedings of the Visual-JW2016 3 48-49 2016年8月

  62. Polytype control by activity ratio of silicon to carbon during SiC solution growth using multicomponent solvents 査読有り

    Atsushi Horio, Shunta Harada, Daiki Koike, Kenta Murayama, Kenta Aoyagi, Takenobu Sakai, Miho Tagawa, Toru Ujihara

    JAPANESE JOURNAL OF APPLIED PHYSICS 55 (1) 01AC01-1-01AC01-5 2016年1月

    出版者・発行元:IOP PUBLISHING LTD

    DOI: 10.7567/JJAP.55.01AC01  

    ISSN:0021-4922

    eISSN:1347-4065

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    We report on the relationship between grown polytypes and the activity ratio of silicon to carbon during SiC solution growth using multicomponent solvents. From the thermodynamic calculation as well as crystallization experiments, we revealed that a high activity ratio (a(Si)/a(C)) in the solution leads to the growth of low-hexagonality polytypes, and low a(Si)/a(C) results in the growth of high-hexagonality polytypes. 4H-SiC is stable when a(Si)/a(C) is relatively low (similar to 10(1) > a(Si)/a(C)), 3C-SiC is stable when a(Si)/a(C) is relatively high (similar to 10(4) < a(Si)/a(C)), and 6H-SiC is stable in the intermediate a(Si)/a(C) range (similar to 10(2) < a(Si)/a(C) < similar to 10(3)). From these results, the Cr-Si solvent at high temperatures is expected to be suitable for 4H-SiC growth, and Sc-Si and Fe-Si solvents at relatively low temperatures are expected to be suitable for 3C-SiC growth. (C) 2016 The Japan Society of Applied Physics

  63. Spatial distribution of carrier concentration in 4H-SiC crystal grown by solution method 査読有り

    Zhen Jiang Wang, Takahiko Kawaguchi, Kenta Murayama, Kenta Aoyagi, Shunta Harada, Miho Tagawa, Takenobu Sakai, Tomohisa Kato, Toru Ujihara

    Materials Science Forum 858 57-60 2016年

    出版者・発行元:Trans Tech Publications Ltd

    DOI: 10.4028/www.scientific.net/MSF.858.57  

    ISSN:0255-5476

    eISSN:1662-9752

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    We investigated the spatial distribution of carrier concentration in n-type 4H-SiC grown by the solution method using the peak frequency of the longitudinal optical phonon-plasmon coupled (LOPC) mode of the Raman spectra on the surface. The carrier concentration at the position of the smooth terrace was higher than the carrier concentration at the position where the macrosteps were formed. This indicates the nitrogen incorporation efficiently occurs on the smooth surface where the density of macrosteps is relatively low. The different incorporation of nitrogen depending on the surface morphology can be understood from the view point of the adsorption time of impurity on the terrace. The present result implies that the uniform surface morphology is necessary to achieve uniform doping concentration in SiC crystal.

  64. High-speed solution growth of single crystal AlN from Cr-Co-Al solvent 査読有り

    Shota Watanabe, Masashi Nagaya, Yukihisa Takeuchi, Kenta Aoyagi, Shunta Harada, Miho Tagawa, Toru Ujihara

    Materials Science Forum 858 1210-1213 2016年

    出版者・発行元:Trans Tech Publications Ltd

    DOI: 10.4028/www.scientific.net/MSF.858.1210  

    ISSN:0255-5476

    eISSN:1662-9752

    詳細を見る 詳細を閉じる

    We achieved high-speed solution growth of AlN single crystal from Cr-Co-Al solvent, which was designed on the basis of thermodynamic calculation. To avoid the unintentional precipitations at the interface between solution and gas as well as to increase the growth rate, we designed the solution growth condition, in which the solution near the interface is undersaturated and the solution near the growth position is supersaturated. To realize this kind of ideal condition, we selected Co and Cr as solvent materials due to their high affinity with nitrogen or aluminum. Finally, we achieved a growth rate as high as 200 μm/h in maximum.

  65. Reaction path for formation of Cu2SnSe3 film by selenization of Cu-Sn precursor 査読有り

    Zeguo Tang, Kenta Aoyagi, Yuki Nukui, Kiichi Kosaka, Hikaru Uegaki, Jakapan Chatana, Daisuke Hironiwa, Takashi Minemoto

    SOLAR ENERGY MATERIALS AND SOLAR CELLS 143 311-318 2015年12月

    出版者・発行元:ELSEVIER

    DOI: 10.1016/j.solmat.2015.07.025  

    ISSN:0927-0248

    eISSN:1879-3398

    詳細を見る 詳細を閉じる

    Reaction path for fabrication of Cu2SnSe3 (CTSe) film by selenization of Cu-Sn precursor was investigated via in-situ X-ray diffraction (XRD) as well as glazing incident XRD (GIXRD) measurements. Cross-sectional scanning electron microscopy (SEM)-energy dispersive spectrometry (EDS) and transmission electron microscope (TEM) analyses revealed the element and phase distribution along the depth direction. Based on these results, a proposed growth model was concluded below: first, the Se atoms from evaporation source reacted with Cu and Sn atoms to produce Cu2-xSe and SnSe2 phases. Noticeably, resulting film presented bilayer feature with Cu2-xSe located at the surface and SnSe2 located at bottom. Second, CTSe phase formed at the interface of Cu2-xSe and SnSe2 as the increasing temperature. The Cu2-xSe was depleted by Sn-related secondary phases when the Cu/Sn ratio was smaller than 1.72. The secondary phases of SnSe2 and SnSe were coexisted with CTSe phase independent of Cu/Sn ratio in metallic precursor, which was attributed to the weak diffusion ability of Sn and Sn-related secondary phases in the CTSe film. The origins for high carrier concentration in CTSe films were ascribed to the Cu2-xSe and intrinsic acceptor concentration and effective approach to reduce the value was explored. An attempt of solar cell with CTSe as absorber was performed and photocurrent of 9.9 mA/cm(2) was detected. (c) 2015 Elsevier B.V. All rights reserved.

  66. Dislocation conversion during sic solution growth for high-quality crystals 査読有り

    Shunta Harada, Yuji Yamamoto, Shi Yu Xiao, Daiki Koike, Takuya Mutoh, Kenta Murayama, Kenta Aoyagi, Takenobu Sakai, Miho Tagawa, Toru Ujihara

    Materials Science Forum 821-823 3-8 2015年

    出版者・発行元:Trans Tech Publications Ltd

    DOI: 10.4028/www.scientific.net/MSF.821-823.3  

    ISSN:0255-5476

    eISSN:1662-9752

    詳細を見る 詳細を閉じる

    Solution growth of SiC has attracted significant attention due to its potential for the production of high-quality SiC wafers. We have recently investigated the dislocation propagation behavior during SiC solution growth with the aim of reducing the dislocation density. Threading dislocations were found to be converted to defects on the basal planes during solution growth. Utilizing this dislocation conversion phenomenon, we have proposed a dislocation reduction process during solution growth and achieved high-quality 4H-SiC crystal growth. Here we confirm the potential of SiC solution growth for the production of high-quality SiC wafers.

  67. Control of interface shape by non-axisymmetric solution convection in top-seeded solution growth of SiC crystal 査読有り

    Daiki Koike, Tomonori Umezaki, Kenta Murayama, Kenta Aoyagi, Shunta Harada, Miho Tagawa, Takenobu Sakai, Toru Ujihara

    Materials Science Forum 821-823 18-21 2015年

    出版者・発行元:Trans Tech Publications Ltd

    DOI: 10.4028/www.scientific.net/MSF.821-823.18  

    ISSN:0255-5476

    eISSN:1662-9752

    詳細を見る 詳細を閉じる

    We achieved the convex growth interface shape in top-seeded solution growth of SiC applying non-axisymmetric solution convection induced by non-axisymmetric temperature distribution. The detailed solution flow, temperature distribution and carbon concentration distribution were calculated by 3-dimensional numerical analysis. In the present case, the solution flow below the crystal was unidirectional and the supersaturation was increased along the solution flow direction. By the rotation of the crystal in the unidirectional flow and the temperature distribution, we successfully obtained the crystal with the convex growth interface shape.

  68. Research on solvent composition for different surface morphology on C face during 4H-SiC solution growth 査読有り

    Shi Yu Xiao, Natsumi Hara, Shunta Harada, Kenta Murayama, Kenta Aoyagi, Takenobu Sakai, Toru Ujihara

    Materials Science Forum 821-823 39-42 2015年

    出版者・発行元:Trans Tech Publications Ltd

    DOI: 10.4028/www.scientific.net/MSF.821-823.39  

    ISSN:0255-5476

    eISSN:1662-9752

    詳細を見る 詳細を閉じる

    Surface morphology of the SiC crystal grown on the C face of the 4H-SiC seed crystal by TSSG method using pure Si, Si-1at%Ti-C, Si-5at%Ti-C and Si-20at%Ti-C solvents was investigated. The surface morphology of the crystal grown from pure Si solvent was smooth. By the addition of Ti to the solvent, the surface morphology became rougher. The RMS value is not proportional to the concentration of Ti. The formation of macrosteps with several micrometers was observed when the addition of Ti increased to 5at% indicating the possibility of the threading screw dislocation conversion on the C face of the 4H-SiC crystal.

  69. Effect of rear-surface buffer layer on performance of lift-off Cu(In,Ga)Se-2 solar cells 査読有り

    Kenta Aoyagi, Akihiro Tamura, Hideyuki Takakura, Takashi Minemoto

    JAPANESE JOURNAL OF APPLIED PHYSICS 53 (5) 05FW05 2014年5月

    出版者・発行元:IOP PUBLISHING LTD

    DOI: 10.7567/JJAP.53.05FW05  

    ISSN:0021-4922

    eISSN:1347-4065

    詳細を見る 詳細を閉じる

    The effect of an Au and MoOx rear-surface buffer layer inserted between Cu(In,Ga)Se-2 (CIGS) and ZnO:Al on solar cell performances was examined. The lift-off CIGS solar cell without a rear-surface buffer layer showed particular characteristics of two series-connected diodes in the reverse direction, and its short-circuit current density was almost zero. In contrast, the Au or MoOx rear-surface buffer layer improved these characteristics. Although the lift-off CIGS solar cell with the Au rear-surface buffer layer showed shunt characteristics and low efficiency, the efficiency of the lift-off CIGS solar cell with the MoOx rear-surface buffer layer was approximately 50% of that of substrate-type CIGS solar cells. Diode parameters of lift-off CIGS solar cells were determined by fitting analysis of current density-voltage curves using a proposed new equivalent circuit model for lift-off CIGS solar cells. (C) 2014 The Japan Society of Applied Physics

  70. Thermo-physical properties of Cu2ZnSnS4 single crystal 査読有り

    Akira Nagaoka, Kenji Yoshino, Kenta Aoyagi, Takashi Minemoto, Yoshitaro Nose, Tomoyasu Taniyama, Koichi Kakimoto, Hideto Miyake

    JOURNAL OF CRYSTAL GROWTH 393 167-170 2014年5月

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.jcrysgro.2013.11.077  

    ISSN:0022-0248

    eISSN:1873-5002

    詳細を見る 詳細を閉じる

    The thermo-physical properties of quaternary compound Cu2ZnSnS4 (CZTS) have been evaluated from high-quality single crystals grown by the solution method. The Debye temperature, one of the important properties, was determined from the measured heat capacity value below 7 K using the Debye function. The experimental Debye temperature is 302 K, which is a reference value for the thermo-physical understanding of CZTS. The present result can be used to stimulate future experimental and theoretical work. (C) 2013 Elsevier B.V. All rights reserved.

  71. Optimum bandgap profile analysis of Cu(In,Ga)Se-2 solar cells with various defect densities by SCAPS 査読有り

    Masashi Murata, Daisuke Hironiwa, Naoki Ashida, Jakapan Chantana, Kenta Aoyagi, Naoya Kataoka, Takashi Minemoto

    JAPANESE JOURNAL OF APPLIED PHYSICS 53 (4) 04ER14 2014年4月

    出版者・発行元:IOP PUBLISHING LTD

    DOI: 10.7567/JJAP.53.04ER14  

    ISSN:0021-4922

    eISSN:1347-4065

    詳細を見る 詳細を閉じる

    The bandgap of a Cu(In,Ga)Se-2 (CIGS) absorbing layer is varied from 1.0 to 1.7 eV by changing the composition ratio of gallium (Ga), realizing an optimum design for solar cell absorbers. In this study, the effects of a graded bandgap profile on the cell performance of a CIGS solar cell are investigated using a device simulator. Moreover, optimum bandgap profiles with various defect densities are simulated. In the case of low defect densities, when the lowest bandgap, Eg(min), is inside the space-charge region (SCR), the double-graded structure is effective for achieving high efficiency. However, when Egmin is outside the SCR, the negative gradient from Eg(min) to the CIGS surface acts as a barrier that impedes the collection of photogenerated electrons, thereby increasing the recombination rate and decreasing cell efficiency. In the case of high defect densities, to decrease the recombination current and improve the efficiency, a more positive gradient from the back contact to the surface is needed. (C) 2014 The Japan Society of Applied Physics

  72. Nano-structure around 90° domain wall and elastic interaction with misfit dislocation in PbTiO<SUB>3</SUB> thin film 査読有り

    Takanori Kiguchi, Kenta Aoyagi, Yoshitaka Ehara, Hiroshi Funakubo, Tomoaki Yamada, Noritaka Usami, Toyohiko J. Konno

    Key Engineering Materials 566 167-170 2013年

    出版者・発行元:None

    DOI: 10.4028/www.scientific.net/KEM.566.167  

    ISSN:1013-9826

    eISSN:1662-9795

  73. Analysis of Lattice Defects in an Epitaxial PbTiO3 Thick Film by Transmission Electron Microscopy 査読有り

    Kenta Aoyagi, Takanori Kiguchi, Yoshitaka Ehara, Hiroshi Funakubo, Toyohiko J. Konno

    ELECTROCERAMICS IN JAPAN XV 566 171-174 2013年

    出版者・発行元:TRANS TECH PUBLICATIONS LTD

    DOI: 10.4028/www.scientific.net/KEM.566.171  

    ISSN:1013-9826

    eISSN:1662-9795

    詳細を見る 詳細を閉じる

    The microstructure of an epitaxial PbTiO3 thick film was investigated by using transmission electron microscopy (TEM). An analysis of bright-field TEM (BFTEM) images revealed the existence of displacements along the [001] direction of PbTiO3. High-resolution TEM (HRTEM) observation indicated that stacking faults parallel to the (001) plane of PbTiO3 are formed in the thick film. Local strain fields around the stacking faults were quantified by geometric phase analysis of the HRTEM image. The measured strain suggested the presence of a pair of extrinsic and intrinsic stacking faults. The distance between an extrinsic stacking fault and an intrinsic one corresponds to two unit cells along the [001] direction of PbTiO3. The formation of these stacking faults is considered to be associated with the strain relaxation of the film.

  74. Molecular Dynamics Simulation of 90˚ Ferroelectric Domains in PbTiO<SUB>3</SUB> 査読有り

    Takeshi Nishimatsu, Kenta Aoyagi, Takanori Kiguchi, Toyohiko J. Konno, Yoshiyuki Kawazoe, Hiroshi Funakubo, Anil Kumar, Umesh V. Waghmare

    Journal of the Physical Society of Japan 81 (12) 124702 2012年12月

    出版者・発行元:None

    DOI: 10.1143/JPSJ.81.124702  

    ISSN:0031-9015

    eISSN:1347-4073

  75. Stacking faults in an epitaxially grown PbTiO3 thick film and their size distribution 査読有り

    Kenta Aoyagi, Yumiko Kodama, Takanori Kiguchi, Yoshitaka Ehara, Hiroshi Funakubo, Toyohiko J. Konno

    MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS 177 (7) 528-531 2012年4月

    出版者・発行元:ELSEVIER SCIENCE BV

    DOI: 10.1016/j.mseb.2012.01.013  

    ISSN:0921-5107

    詳細を見る 詳細を閉じる

    The microstructure of an epitaxial PbTiO3 thick film, grown on a SrRuO3/SrTiO3 substrate at 600 degrees C by pulsed-MOCVD method, was investigated by using transmission electron microscopy. A number of extrinsic or intrinsic stacking faults were observed in the epitaxial PbTiO3 thick film and they were parallel to the (001) plane of the PbTiO3. We also investigated the size distribution of these stacking faults. The width of these stacking faults along the [100] axis of the PbTiO3 was very small, ranging from 2 to 13 nm. It was also revealed that the size distribution of stacking faults depends on the position in the film: near the surface, near the substrate, near threading dislocations, and near 90 degrees domain boundaries. (c) 2012 Elsevier B.V. All rights reserved.

  76. 3D structures of alloys and nanoparticles observed by electron tomography 招待有り 査読有り

    K. Sato, K. Aoyagi, T. J. Konno

    Optical Measurements, Modeling, and Metrology Springer 5 1-9 2011年12月

    DOI: 10.1007/978-1-4614-0228-2_1  

    ISSN:2191-5644

    eISSN:2191-5652

    詳細を見る 詳細を閉じる

    3D structures of bulk alloys and nanoparticles have been studied by means of electron tomography using scanning transmission electron microscopy (STEM). In the case of nanoparticles of Fe-Pd alloy, particle size, shape, and locations were reconstructed by weighted backprojection (WBP), as well as by simultaneous iterative reconstruction technique (SIRT). We have also estimated the particle size by simple extrapolation of tilt-series original data sets, which proved to be quite powerful. We demonstrate that WBP yields a better estimation of the particle size in the z direction than SIRT does, while the latter algorithm is superior to the former from the viewpoints of surface roughness and dot-like artifacts. In contrast, SIRT gives a better result than WBP for the reconstruction of plate-like precipitates in Mg-Dy-Nd alloys, in respect of the plate thickness perpendicular to the z direction. We also show our recent results on the 3D-tomographic observations of microstructures in Ti-V-Al, Ti-Nb, Cu-Ag, and Co-Ni-Cr-Mo alloys obtained by STEM tomography.

  77. Diffraction contrast analysis of 90˚ and 180˚ ferroelectric domain structures of PbTiO<SUB>3</SUB> thin films 査読有り

    Kenta Aoyagi, Takanori Kiguchi, Yoshitaka Ehara, Tomoaki Yamada, Hiroshi Funakubo, Toyohiko J. Konno

    Science and Technology of Advanced Materials 12 (3) 034403 2011年6月

    出版者・発行元:None

    DOI: 10.1088/1468-6996/12/3/034403  

    ISSN:1468-6996

  78. Configuration and local elastic interaction of ferroelectric domains and misfit dislocation in PbTiO3/SrTiO3 epitaxial thin films 査読有り

    Takanori Kiguchi, Kenta Aoyagi, Yoshitaka Ehara, Hiroshi Funakubo, Tomoaki Yamada, Noritaka Usami, Toyohiko J. Konno

    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 12 (3) 034413 2011年6月

    出版者・発行元:TAYLOR & FRANCIS LTD

    DOI: 10.1088/1468-6996/12/3/034413  

    ISSN:1468-6996

    eISSN:1878-5514

    詳細を見る 詳細を閉じる

    We have studied the strain field around the 90 degrees domains and misfit dislocations in PbTiO3/SrTiO3 ( 001) epitaxial thin films, at the nanoscale, using the geometric phase analysis (GPA) combined with high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The films typically contain a combination of a/c-mixed domains and misfit dislocations. The PbTiO3 layer was composed from the two types of the a-domain (90 degrees domain): a typical a/c-mixed domain configuration where a-domains are 20-30 nm wide and nano sized domains with a width of about 3 nm. In the latter case, the nano sized a-domain does not contact the film/substrate interface; it remains far from the interface and stems from the misfit dislocation. Strain maps obtained from the GPA of HRTEM images show the elastic interaction between the a-domain and the dislocations. The normal strain field and lattice rotation match each other between them. Strain maps reveal that the a-domain nucleation takes place at the misfit dislocation. The lattice rotation around the misfit dislocation triggers the nucleation of the a-domain; the normal strains around the misfit dislocation relax the residual strain in a-domain; then, the a-domain growth takes place, accompanying the introduction of the additional dislocation perpendicular to the misfit dislocation and the dissociation of the dislocations into two pairs of partial dislocations with an APB, which is the bottom boundary of the a-domain. The novel mechanism of the nucleation and growth of 90 degrees domain in PbTiO3/SrTiO3 epitaxial system has been proposed based on above the results.

  79. TEM Observation on Ferroelectric Domain Structures of PbTiO3 Epitaxial Films 査読有り

    Kenta Aoyagi, Takanori Kiguchi, Yoshitaka Ehara, Hiroshi Funakubo, Toyohiko J. Konno

    ELECTROCERAMICS IN JAPAN XIV 485 179-+ 2011年

    出版者・発行元:TRANS TECH PUBLICATIONS LTD

    DOI: 10.4028/www.scientific.net/KEM.485.179  

    ISSN:1013-9826

    eISSN:1662-9795

    詳細を見る 詳細を閉じる

    The ferroelectric domain structure of PbTiO3 (PTO) films was investigated by using transmission electron microscopy (TEM). In the film with PTO/SrTiO3 (STO) structure, 180 degrees domains are formed near the SrTiO3 (STO) substrate and the domain length of 180 degrees domains is 100 nm. However, 180 degrees domains are not formed in the film with Pt/PTO/SrRuO3 (SRO)/STO structure. These results show that 180 degrees domains are formed in order to minimize depolarizing field energy, and that the domain length of 180 degrees domains is determined by the competition among the depolarizing field energy, domain wall energy, Coulomb interaction and elastic interaction.

  80. Geometric Phase Analysis of Nano-Scale Strain Fields Around 90° Domains in PbTiO3/SrTiO3 Epitaxial Thin Film 査読有り

    Takanori Kiguchi, Kenta Aoyagi, Toyohiko J. Konno, Satoru Utsugi, Tomoaki Yamada, Hiroshi Funakubo

    Materials Research Society Symposium Proceeding 1199 1199-F09-08-01-1199-F09-08-06 2010年7月

    DOI: 10.1557/PROC-1199-F09-08  

    ISSN:0272-9172

  81. Three-dimensional shapes and distribution of FePd nanoparticles observed by electron tomography using high-angle annular dark-field scanning transmission electron microscopy 査読有り

    Kazuhisa Sato, Kenta Aoyagi, Toyohiko J. Konno

    JOURNAL OF APPLIED PHYSICS 107 (2) 024304-1-024304-7 2010年1月

    出版者・発行元:AMER INST PHYSICS

    DOI: 10.1063/1.3280026  

    ISSN:0021-8979

    詳細を見る 詳細を閉じる

    We have studied three-dimensional shapes and distribution of FePd nanoparticles, prepared by electron beam deposition and postdeposition annealing, by means of single-axis tilt tomography using atomic number contrasts obtained by high-angle annular dark-field scanning transmission electron microscopy. Particle size, shape, and locations were reconstructed by weighted backprojection (WBP), as well as by simultaneous iterative reconstruction technique (SIRT). We have also estimated the particle size by simple extrapolation of tilt-series original data sets, which proved to be quite powerful. The results of the two algorithms for reconstruction have been compared quantitatively with those obtained by the extrapolation method and those independently reported by electron holography. It was found that the reconstructed intensity map by WBP contains a small amount of dotlike artifacts, which do not exist in the results by SIRT, and that the particle surface obtained by WBP is rougher than that by SIRT. We demonstrate, on the other hand, that WBP yields a better estimation of the particle size in the z direction than SIRT does, most likely due to the presence of a "missing wedge" in the original data set.

  82. Characterization of L1<SUB>0</SUB>-type FePd alloy nanoparticles by atomic-resolution HAADF-STEM and electron tomography

    K. Sato, K. Aoyagi, T. J. Konno, Y. Hirotsu

    Microscopy and Microanalysis 15 (SUPPL. 2) 1262-1263 2009年7月1日

    DOI: 10.1017/S1431927609093830  

    ISSN:1435-8115 1431-9276

    eISSN:1435-8115

︎全件表示 ︎最初の5件までを表示

MISC 34

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    趙宇凡, 崔玉傑, 卞華康, 山中謙太, 青柳健大, 千葉晶彦

    粉体粉末冶金協会講演大会(Web) 2021 2021年

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    趙宇凡, 趙宇凡, 王昊, 王昊, 青柳健大, 雷雨超, 卞華康, 崔玉傑, 山中謙太, 千葉晶彦

    粉体粉末冶金協会講演大会(Web) 2021 2021年

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    王昊, 宮城俊美, 青柳健大, 山中謙太, 千葉晶彦, 須藤英毅

    日本金属学会講演概要(CD-ROM) 164th 2019年

    ISSN:2433-3093

  4. Ti6Al4V合金の各種金属積層造形材の疲労強度に及ぼすガス欠陥の影響とHIP効果

    千葉晶彦, 青柳健大, 山中謙太, 李啓晟, 趙宇凡

    粉体粉末冶金協会講演大会(Web) 2019 2019年

  5. 金属付加製造の現状と粉末床溶融結合における組織制御

    小泉雄一郎, 山中謙太, 青柳健大, 千葉晶彦

    スマートプロセス学会学術講演会講演概要 2019 2019年

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    小泉雄一郎, WEI Daixiu, ZAO Yufan, 山中謙太, 青柳健大, 千葉晶彦

    東北大学金属材料研究所新素材共同研究開発センター共同利用研究報告書(CD-ROM) 2017 2018年

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    千葉晶彦, 青柳健大, 山中謙太

    日本金属学会講演概要(CD-ROM) 163rd 2018年

    ISSN:2433-3093

  8. 電子ビーム積層造形されたN添加Co-Cr-Mo合金の逆変態熱処理における結晶粒微細化の調査

    宮城俊美, 王昊, 青柳健大, 山中謙太, 小泉雄一郎, 小泉雄一郎, 千葉晶彦, 須藤英毅

    日本金属学会講演概要(CD-ROM) 163rd 2018年

    ISSN:2433-3093

  9. 材料組織制御のための電子ビーム積層造形プロセスデザイン

    小泉雄一郎, DING X., ZHAO Y., 青柳健大, 山中謙太, 千葉晶彦

    日本セラミックス協会秋季シンポジウム講演予稿集(CD-ROM) 31st 2018年

  10. 電子ビーム積層造形で3Dプリントされた生体用コバルト合金の疲労

    小泉雄一郎, 高島大洋, 山中謙太, 青柳健大, 斎藤毅, 千葉晶彦

    東北大学金属材料研究所新素材共同研究開発センター共同利用研究報告書(CD-ROM) 2016 2017年

  11. 電子ビーム積層造形した金属材料の凝固組織解析

    小泉雄一郎, 青柳健大, 山中謙太, 千葉晶彦, DING Xiao, ZHAO Yufan

    スマートプロセス学会総合学術講演会講演概要 2017 2017年

  12. 鋳造用耐熱合金IN-713Cの熱間加工特性

    川崎健太郎, 青柳健大, 山中謙太, 韓剛, 千葉晶彦

    日本金属学会講演概要(CD-ROM) 161st 2017年

    ISSN:2433-3093

  13. 強誘電体薄膜の90<SUP>。</SUP>ドメインと整合界面の弾性相互作用

    木口賢紀, 青柳健大, 白石貴久, 今野豊彦, 舟窪浩

    まてりあ 55 (12) 585-585 2016年12月1日

    出版者・発行元:日本金属学会

    DOI: 10.2320/materia.55.585  

  14. Inconel718合金の電子ビーム積層造形における材料組織制御

    小泉雄一郎, 丁笑, 孫世海, 孫世海, 齋藤毅, 青柳健大, 山中謙太, 山崎敏広, 千葉晶彦, 下野祐典, 野本祐春

    粉体粉末冶金協会講演概要集 2016 2016年

  15. 電子ビーム積層造形で3Dプリントされた生体用コバルト合金の疲労

    小泉雄一郎, 高島大洋, 山中謙太, 青柳健大, 斎藤毅, 千葉晶彦

    東北大学金属材料研究所新素材共同研究開発センター共同利用研究報告書 2015 2016年

  16. 電子ビーム積層造形された生体用Co-Cr-Mo合金の組織と繰り返し変形挙動

    小泉雄一郎, 高島大洋, 高島大洋, 山中謙太, 青柳健大, 齋藤毅, 千葉晶彦

    日本金属学会講演概要(CD-ROM) 158th 2016年

    ISSN:2433-3093

  17. 電子ビーム積層造形における凝固中の冷却速度評価

    王昊, 青柳健大, 小泉雄一郎, 山中謙太

    日本金属学会講演概要(CD-ROM) 159th 2016年

    ISSN:2433-3093

  18. 24aCL-5 PbTiO_3の昇温/降温分子動力学シミュレーション(24aCL 誘電体(酸化物,ペロヴスカイト,光散乱),領域10(誘電体,格子欠陥,X線・粒子線,フォノン))

    西松 毅, 青柳 健太, 木口 賢紀, 今野 豊彦, 川添 良幸, Kumar Anil, Waghmare Umesh V.

    日本物理学会講演概要集 67 (1) 984-984 2012年3月5日

    出版者・発行元:一般社団法人日本物理学会

    ISSN:1342-8349

  19. MOD法によるPMN薄膜の成膜条件の最適化による結晶性の向上

    三坂好央, 水谷雅巳, 木口賢紀, 西嶋雅彦, 青柳健大, 宇佐美徳隆, 今野豊彦

    日本セラミックス協会年会講演予稿集 2012 2012年

  20. BaTiO<sub>3</sub>の固溶によるBiFeO<sub>3</sub>薄膜の微細組織変化

    水谷雅巳, 木口賢紀, 西嶋雅彦, 青柳健大, 宇佐美徳隆, 今野豊彦, 篠崎和夫

    日本セラミックス協会年会講演予稿集 2012 2012年

  21. MPB近傍PZT薄膜の90°ドメインと帯電した菱面体晶ドメイン境界

    青柳健大, 木口賢紀, 今野豊彦, 和田亜由美, 江原祥隆, 舟窪浩

    日本セラミックス協会年会講演予稿集 2012 2012年

  22. PbTiO<sub>3</sub>薄膜における90°ドメイン底部境界構造の解明

    木口賢紀, 青柳健大, 今野豊彦, 江原祥隆, 舟窪浩

    日本セラミックス協会年会講演予稿集 2012 2012年

  23. MPB近傍PZT厚膜におけるインコメンシュレート相の発見

    青柳健大, 木口賢紀, 今野豊彦, 和田亜由美, 江原祥隆, 舟窪浩

    日本セラミックス協会年会講演予稿集 2012 2012年

  24. 菱面体晶系Pb(Zr,Ti)O<sub>3</sub>エピタキシャル薄膜微細組織の電子顕微鏡観察

    青柳健大, 木口賢紀, 今野豊彦, 和田亜由美, 江原祥隆, 舟窪浩

    日本セラミックス協会年会講演予稿集 2012 2012年

  25. MPB組成Pb(Zr,Ti)O<sub>3</sub>エピタキシャル厚膜における変調構造

    青柳健大, 木口賢紀, 今野豊彦, 和田亜由美, 江原祥隆, 舟窪浩

    日本セラミックス協会年会講演予稿集 2012 2012年

  26. PbTiO<sub>3</sub>膜における90°ドメイン境界近傍の無積層欠陥帯

    青柳健大, 兒玉裕美子, 木口賢紀, 今野豊彦, 江原祥隆, 舟窪浩, 山田智明, 山田智明

    日本セラミックス協会年会講演予稿集 2011 2011年

  27. ミスフィット転位上への強誘電体90°ドメインの核生成

    木口賢紀, 青柳健大, 今野豊彦, 江原祥隆, 舟窪浩, 山田智明

    日本セラミックス協会年会講演予稿集 2011 2011年

  28. Pb系強誘電体薄膜における極性構造の原子分解能観察

    木口賢紀, 青柳健大, 中村崇昭, 宇佐美徳隆, 今野豊彦, 江原祥隆, 舟窪浩, 山田智明, 山田智明

    日本セラミックス協会年会講演予稿集 2011 2011年

  29. PbTiO<sub>3</sub>膜における積層欠陥形成による歪み緩和

    青柳健大, 兒玉裕美子, 木口賢紀, 宇佐美徳隆, 今野豊彦, 江原祥隆, 舟窪浩, 山田智明, 山田智明

    日本セラミックス協会年会講演予稿集 2011 2011年

  30. 積層欠陥を含むPbTiO<sub>3</sub>厚膜の組織解析

    青柳健大, 兒玉裕美子, 木口賢紀, 宇佐美徳隆, 今野豊彦, 江原祥隆, 舟窪浩, 山田智明, 山田智明

    日本セラミックス協会秋季シンポジウム講演予稿集 24th 2011年

  31. ミスフィット転位との弾性相互作用によるPbTiO<sub>3</sub>薄膜90°ドメインの核生成・成長メカニズム

    木口賢紀, 青柳健大, 江原祥隆, 舟窪浩, 山田智明, 山田智明, 宇佐美徳隆, 今野豊彦

    日本セラミックス協会秋季シンポジウム講演予稿集 24th 2011年

  32. PbTiO<sub>3</sub>厚膜における格子欠陥のTEM解析

    青柳健大, 木口賢紀, 今野豊彦, 江原祥隆, 舟窪浩

    エレクトロセラミックス研究討論会講演予稿集 31st 2011年

  33. PbTiO<sub>3</sub>薄膜の90°ドメイン境界の構造とミスフィット転位との弾性相互作用

    木口賢紀, 青柳健大, 江原祥隆, 舟窪浩, 山田智明, 山田智明, 宇佐美徳隆, 今野豊彦

    エレクトロセラミックス研究討論会講演予稿集 31st 2011年

  34. Mg-Dy-Nd三元系合金における時効析出物の微細組織観察

    青柳健大, 佐藤和久, 木口賢紀, 今野豊彦, 西嶋雅彦, 平賀賢二

    日本金属学会講演概要 143rd 2008年

    ISSN:1342-5730

︎全件表示 ︎最初の5件までを表示

講演・口頭発表等 10

  1. Additive Manufacturingによる材料開発を加速する機械学習によるプロセス設計効率化の試み 招待有り

    青柳 健大

    大阪大学接合科学研究所東京セミナー 2018年11月26日

  2. 金属積層造形による組織制御のための凝固マップ作成 招待有り

    青柳 健大

    PUCA2018 2018年11月16日

  3. Development of Informatics Approach to Determine the Process Window for Electron Beam Melting Process: Demonstration in CoCrMo alloy 国際会議

    EBAM2018 2018年4月11日

  4. サポートベクターマシンによる金属積層造形レシピ開発の効率化 招待有り

    青柳 健大

    第22回結晶工学セミナー 「結晶工学×データサイエンス」 -最先端事例から学ぶクリスタルインフォマティクス- 2017年12月12日

  5. 積層造形プロセスにおける温度履歴のシミュレーション 国際会議 招待有り

    青柳 健大, 千葉 晶彦

    PUCA2017 - ESI Users' Forum Japan 2017 2017年11月16日

  6. モデルベース開発に基づく金属積層造形レシピ開発手法

    青柳 健大, 千葉 晶彦

    第68回塑性加工連合講演会 2017年11月11日

  7. CA法による凝固マップの作成と金属積層造形プロセス設計での利用方法

    青柳 健大, 千葉 晶彦

    日本金属学会 2017年秋期(第161回)講演大会 2017年9月7日

  8. 電子ビーム積層造形で作製した12Cr耐熱鋼の組織

    李 澤, 青柳 健大, 千葉 晶彦, 大西 春樹, 只野 智史, 日野 武久, 中谷 祐二郎

    日本鉄鋼協会 第174回秋季講演大会 2017年9月6日

  9. 高効率な金属積層造形プロセス最適化手法

    青柳 健大, 王 昊, 千葉 晶彦

    日本金属学会 2017年春期(第160回)講演大会 2017年3月15日

  10. 電子ビーム積層造形のための凝固組織シミュレーション 国際会議 招待有り

    青柳健大, 千葉晶彦

    PUCA2016 - ESI Users' Forum Japan 2016年11月24日

︎全件表示 ︎最初の5件までを表示

共同研究・競争的資金等の研究課題 3

  1. Al合金の電子ビーム積層造形とその相変態カイネティクス

    青柳 健大

    2021年4月1日 ~ 2024年3月31日

    詳細を見る 詳細を閉じる

    本研究の目的はAl合金を対象とし電子ビーム積層造形(以下、PBF-EB)プロセスにおける、溶融凝固挙動ならびにプロセス中の相変態挙動を明らかにすることである。プロセス中において生じるキーホール型欠陥や未溶融欠陥などのプロセス条件が最適でないことに起因する欠陥は偶発的に生じることが多く、コントロールしづらく、また核生成サイトになりえるため、プロセス特有の組織形成機構を明らかにする上では排除すべき対象である。2021年度は、欠陥形成の原因となりうる因子の分析、欠陥の入らない条件を探索するプロセス条件最適化手法の確立、最適化された条件での造形体作製、そして造形体の組織評価を実施した。 PBF-EBでは粉末表面でのチャージアップにより粉末が飛散するスモークという現象が起こりうる。そこで、原料粉末表面の構造をAESで分析し、粉末のスモークテストも実施することで、スモークのメカニズムを解析した。また、造形体表面と造形欠陥の関係を定量的に評価し、プロセス最適化手法の改良を行った。そして、そのプロセス最適化手法を用いて最適化された条件でAlSi10Mg合金を造形し、造形材の組織評価を行った。組織評価の際には、積層方向の高さによって熱履歴が異なるため、積層方向の高さ位置依存性も調査した。これにより、プロセス中のその場熱処理による組織変化挙動を明らかにするためのデータを収集した。さらに、造形まま材の機械特性評価も実施し、従来手法に比べて、延性に富むことを明らかにした。 プロセス最適化手法に関して、1本の論文投稿を行い、Additive Manufacturingに受理されている。

  2. 電子ビーム3Dプリント金属材料学の構築

    千葉 晶彦, 小泉 雄一郎, 山中 謙太, 青柳 健大

    提供機関:Japan Society for the Promotion of Science

    制度名:Grants-in-Aid for Scientific Research

    研究種目:Grant-in-Aid for Scientific Research (A)

    研究機関:Tohoku University

    2018年4月1日 ~ 2022年3月31日

    詳細を見る 詳細を閉じる

    PBF-EBで製造された溶接困難超合金のGB割れは、機械的および組成的効果を含む複数の要因の相互作用の影響を受ける。 本研究は機械学習に基づく方法を適用して、lloy713ELCの高次元PBF-EBパラメーター空間を最適化した。最適化されたサンプルの亀裂傾向は、熱機械分析によって定義および計算された準全塑性ひずみ指数(QTPSI)を使用してランク付けできることを示した。溶接可能な超合金Alloy718と比較することにより、非溶接性はその熱膨張効果に大きく起因する。この熱膨張効果と液化効果およびひずみ時効割れ効果の組み合わせにより、Alloy713ELCの非溶接性の本質的な原因が明らかになった。

  3. 金属用3Dプリンタで形状だけでなく内部組織を自在に制御するための材料科学

    小泉 雄一郎, 千葉 晶彦, 山中 謙太, 福山 博之, 青柳 健大

    提供機関:Japan Society for the Promotion of Science

    制度名:Grants-in-Aid for Scientific Research

    研究種目:Grant-in-Aid for Scientific Research (A)

    研究機関:Osaka University

    2017年4月1日 ~ 2021年3月31日

    詳細を見る 詳細を閉じる

    金属3Dプリンタの主流である粉末床溶融結合(Powder Bed Fusion: PBF)型付加製造(Additive Manufacturing: AM)にて、部材の形状だけでなく材料特性を決定する結晶組織形成の支配因子を、実験と数値シミュレーションを組み合わせて調べた。具体的には、金属粉末を溶融結合させるための電子ビームやレーザービームの出力と走査速度、そして走査線の間隔の組み合わせにより、造形部材の結晶粒組織を高配向の単結晶状にしたり、低配向の多結晶にしたりするための学術的指針を、PBFプロセス特有の凝固条件の発現とその条件での結晶成長挙動と合金の物理化学的特徴との相関に基づいて示した。