Faculty Profiles - KOBASHI, Makoto

Papers - KOBASHI, Makoto

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460°CにおけるFe-Si合金/純Zn浴の特異な固液界面反応

Vol. 105 ( 7 ) page： 693 - 700 2019

水蒸気プロセスにより作製したアルミニウム合金表面の層状複水酸化物皮膜の透過電子顕微鏡観察

Vol. 105 ( 2 ) page： 299 - 303 2019

溶融Zn-0.2Al浴に浸漬したFe-Si合金におけるFe2Al5 相の形成

Vol. 105 ( 7 ) page： 701 - 708 2019

460°CにおけるFe-Si合金/純Zn浴の特異な固液界面反応

Vol. 105 ( 7 ) page： 693 - 700 2019

Finite element simulation for selective laser melting of AlSi10Mg alloy powder bed layers

OIDE Kai, KOBAYASHI Takaya, TAKATA Naoki, KOBASHI Makoto

The Proceedings of The Computational Mechanics Conference Vol. 2019.32 page： 096 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In recent years, fabricating with a 3D printer has become common, but it is essential to understand the process of melting and solidification in order to obtain accurate information on its strength. It is not easy to obtain the information in the process of fabricating, so it has been estimated by visual observation of the fabricated samples. In this study, the process of selective laser melting (SLM) is reproduced by finite element simulation for AlSi10Mg alloy samples with different widths by Takata et al.(1), and the process is clarified though it is within the qualitative range.

DOI： 10.1299/jsmecmd.2019.32.096

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Structural design and bonding strength evaluation of Al/epoxy resin joint via interpenetrating phase layer Reviewed

Asuka Suzuki, Yuta Arai, Naoki Takata, Makoto Kobashi

Journal of Materials Processing Technology Vol. 262 page： 11 - 18 2018.12

Microstructural characterization of cellular AlSi10Mg alloy fabricated by selective laser melting Reviewed

Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Materials and Design Vol. 157 page： 478 - 491 2018.11

Size dependence of microstructure of AlSi10Mg alloy fabricated by selective laser melting Reviewed

Naoki Takata, Hirohisa Kodaira, Asuka Suzuki, Makoto Kobashi

MATERIALS CHARACTERIZATION Vol. 143 page： 18 - 26 2018.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE INC

In order to investigate the size dependence of microstructure of the AlSi10Mg alloy fabricated by selective laser melting (SLM) combined with a powder bed technique, plate-shaped AlSi10Mg alloy samples with various widths (ranging from approximately 10 mm to 0.3 mm) were produced in this study. All the fabricated samples exhibited characteristic microstructural morphologies consisting of melt pools with columnar alpha-Al grains surrounded by fine eutectic Si particles, whereas their sample sizes had a relatively small effect on the texture and average grain size of the alpha-Al matrix. However, the formation of fine Si precipitates inside the columnar alpha-Al grains was observed more often for the smaller-sized samples, which could be a possible dominant contributor to the observed softening of the sample by reducing sample size. The obtained results were utilized for discussing the size dependence of the strength of the SLM-manufactured porous alloy samples.

DOI： 10.1016/j.matchar.2017.11.052

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Kinetics of Zr-Al intermetallic compound formation during ultra-high vacuum magnetron sputter-deposition of Zr/Al2O3(0001) thin films

Tanaka Koichi, Fankhauser Joshua, Zaid Hicham, Aleman Angel, Sato Masaki, Yu Dian, Ebnonnasir Abbas, Li Chao, Kobashi Makoto, Goorsky Mark S, Kodambaka Suneel

ACTA MATERIALIA Vol. 152 page： 34 - 40 2018.6

Crystallographic Features of Microstructure in Maraging Steel Fabricated by Selective Laser Melting Invited Reviewed

Naoki Takata, Ryoya Nishida, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Metals Vol. 8 ( 6 ) page： 440 2018.6

Language：English Publishing type：Research paper (scientific journal) Publisher：MDPI

This study characterizes the microstructure and its associated crystallographic features of bulk maraging steels fabricated by selective laser melting (SLM) combined with a powder bed technique. The fabricated sample exhibited characteristic melt pools in which the regions had locally melted and rapidly solidified. A major part of these melt pools corresponded with the ferrite (alpha) matrix, which exhibited a lath martensite structure with a high density of dislocations. A number of fine retained austenite (gamma) with a < 001> orientation along the build direction was often localized around the melt pool boundaries. The orientation relationship of these fine gamma grains with respect to the adjacent alpha grains in the martensite structure was (111)(gamma)/ /(011)(alpha) and [-101](gamma) / /[-1-11](alpha) (Kurdjumov-Sachs orientation relationship). Using the obtained results, we inferred the microstructure development of maraging steels during the SLM process. The results depict that new and diverse high-strength materials can be used to develop industrial molds and dies.

DOI： 10.3390/met8060440

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高純度アルミニウム単結晶マイクロピラーの強度に及ぼす試験片寸法および形状の影響

Vol. 68 ( 5 ) page： 250 - 256 2018.5

Effect of Specimen Size and Shape on Strength and Deformation of High-Purity Aluminum Single-Crystal Micropillars Invited Reviewed

Soichiro Takeyasu, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Journal of The Japan Institue of Light Metals Vol. 68 ( 5 ) page： 250-256 - 256 2018.5

Language：Japanese Publisher：The Japan Institute of Light Metals

In the present study, we have examined the compression response of single-crystal cylindrical micropillars with different diameters (approximately ranging from 1 to 10 µm) and shape parameters prepared on the sample surface of 4N purity aluminum (Al) sheets with the recrystallized microstructure. The compression tests for micropillars with various sizes demonstrated the flow stress of micropillars increases with decreasing pillar diameter. The observed size dependence of resolved shear stress for slip corresponds well to the previous studies on micropillars prepared from 3N and 5N purity Al sheets. The measured shear stress resolved onto a primary slip system (τi) scaled by shear modulus (G) and the pillar diameter (d) scaled by Burgers vector (b) shows the following correlation: (τi/G)=0.33(d/b)−0.63. The compression response of micropillars with different d1/d2 (ratio of top diameter (d1) to bottom diameter (d2) of cylindrical micropillars) revealed higher d1/d2 than 0.5 is required for precisely measuring the strength of cylindrical micropillars, which was confirmed by the observations of compressed micropillars.

DOI： 10.2464/jilm.68.250

CiNii Books

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Effect of layer thickness on bonding strength of Al/epoxy resin joints via interpenetrating phase layer Reviewed

Asuka Suzuki, Yuta Arai, Naoki Takata, Makoto Kobashi

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY Vol. 254 page： 338 - 345 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Open porous Aluminum (Al) layer was fabricated on Al substrate, and epoxy resin (epoxy) was infiltrated into the layer and cured. Al and epoxy formed "interpenetrating phase layer (IPL)", and interlocked three-dimensionally each other. The volume fraction of epoxy in the IPL was fixed at 0.8, and the effect of the thickness of the IPL (t(IPL)) on the tensile strength (sigma(j)) was investigated. sigma(j) increased when tin was in the range of 0-2 mm, and decreased when t(IPL) was 5 mm and 10 mm. Based on observations of fractured surfaces, this decrease in cri was due to incomplete infiltration of epoxy. When t(IPL) was in the range of 0-2 mm, separation of Al and epoxy and rupture of epoxy occurred at the epoxy/IPL interface edge, which was the starting point for the fracture. For t(IPL) < 0.5 mm, the area fraction of epoxy at the epoxy/IPL interface was lower than 0.8, resulting in significant stress concentration. tin needs to be higher than the pore diameter in the Al substrate. By controlling t(IPL), Al/IPL/epoxy joints were obtained with a tensile strength of 38.0 MPa.

DOI： 10.1016/j.jmatprotec.2017.10.054

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Microstructure of intermetallic-reinforced Al-Based alloy composites fabricated using eutectic reactions in Al-Mg-Zn ternary system Reviewed

Naoki Takata, Taiki Okano, Asuka Suzuki, Makoto Kobashi

INTERMETALLICS Vol. 95 page： 48 - 58 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

We designed two types of Al-based alloy composites reinforced by eta-Zn2Mg (hexagonal) and T-Al6Mg11Zn11 (cubic) intermetallic phases using monovariant eutectic reactions in the Al-Mg-Zn ternary system and then attempted to fabricate them using solidification. Thermodynamic assessment revealed two alloy compositions of Al-18Mg-36Zn and Al-22.5Mg-23.5Zn (at%) with the alpha-Al (fcc) phase reinforced with high fractions (> 50%) of the eta and T phases. Both alloys exhibited fine two-phase eutectic microstructures consisting of alpha/eta and alpha/T phases, respectively. The morphology of the alpha-Al phase in the eutectic colonies varied from lamellae to fibers upon changing the neighboring intermetallic phase. These eutectic microstructures exhibit high stability at an elevated temperature of 300 degrees C. In addition, the size and spacing of the alpha-Al phase could be controlled by changing the cooling rate during solidification. The fabricated composites exhibited high hardness values exceeding 220 HV, which were much superior to those of conventional Al alloys. The calculated solidification path and component partitioning in the liquid phase during solidification provided insights into the solidification segregation of Zn element in the Al-18Mg-36Zn alloy. The Zn-enriched regions would enhance the local microstructural change at elevated temperatures, which confirmed by the microstructural observations of the alloy exposed at 300 degrees C.

DOI： 10.1016/j.intermet.2018.01.018

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Enhanced Interfacial Reaction of Fe–Si Alloy Sheets Hot-Dipped in Zn Melt at 460°C Reviewed

Naoki Takata, Kunihisa Hayano, Asuka Suzuki, Makoto Kobashi

ISIJ International Vol. 58 ( 9 ) page： 1608 - 1615 2018

Design methodology for porous composites with tunable thermal expansion produced

A. Takezawa, M. Kobashi

Composites Part B Vol. 131 page： 21-29 2017.12

Design methodology for porous composites with tunable thermal expansion produced by multi-material topology optimization and additive manufacturing

Akihiro Takezawa, Makoto Kobashi

COMPOSITES PART B-ENGINEERING Vol. 131 page： 21 - 29 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

To realize negative thermal expansion (NTE), porous composites made of two materials with different coefficients of thermal expansion are being actively researched. NTE can be realized by taking advantage of the thermal deformation mechanisms of a composite material's internal geometry. However, in addition to negative thermal expansion, materials with anisotropic and large positive thermal expansion are also desirable for various applications. Also, additive manufacturing provides new ways to fabricate composites by layering multiple materials at arbitrary points in three-dimensional space. In this study, we developed a design methodology for porous composites, which showed defined thermal expansion characteristics, including negative and positive thermal expansion as well as isotropic and anisotropic thermal expansion. Our approach was tested based on the fabrication of a multi-material photopolymer by additive manufacturing. The internal geometries required to produce such characteristics were designed by topology optimization, which is the most effective structural optimization method for realizing macroscopic inward deformation and for maintaining stiffness. The designed structures were converted to three-dimensional models and fabricated by multi-material photopolymer additive manufacturing. Using laser scanning dilatometry, we measured the thermal expansion of these specimens, revealing well-ordered thermal expansion, from anisotropic positive thermal expansion to anisotropic negative thermal expansion, over a wide range of about -3 x 10(-4) K-1 to 1 x 10(-3) K-1. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.compositesb.2017.07.054

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Size Dependence of Microstructure of AlSi10Mg Alloy Fabricated by Selective Laser Melting Reviewed

N.Takata, H.Kodaira, A.Suzuki, M.Kobashi

Materials Characterization 2017.12

High-stiffness and strength porous maraging steel via topology optimization and selective laser melting

Akihiro Takezawa, Yuichiro Koizumi, Makoto Kobashi

Additive Manufacturing Vol. 18 page： 194 - 202 2017.12

Language：English Publishing type：Research paper (scientific journal)

© 2017 Elsevier B.V. Recent additive manufacturing technologies can be used to fabricate porous metals with precise internal pore structures and effective performance. We use topology optimization to derive an optimal pore structure shape with high stiffness that is verified experimentally. The design maximizes the effective bulk modulus and isotropic stiffness, and the performance is compared with Hashin–Shtrikman (HS) bounds. The optimized structure is fabricated via selective laser melting of maraging steel, which is a high-strength, iron-nickel steel that cannot easily be made porous with conventional methods. The optimal porous structure achieved 85% of the performance of the HS upper bound in numerical simulations, and at least 90% of them were realized in compressive testing. Finally, the performance is discussed relative to that of other metals.

DOI： 10.1016/j.addma.2017.10.004

Web of Science

Scopus

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Effect of layer thickness on bonding strength of Al/epoxy resin joints via interpenetrating phase layer Reviewed

A.Suzuki, Y.Arai, N.Takata, M.Kobashi

Journal of Materials Processing Technology Vol. 25 page： 338-345 2017.11

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Papers - KOBASHI, Makoto