Faculty Profiles - KOBASHI, Makoto

Machine learning surrogate modeling toward the design of lattice-structured heat sinks fabricated by additive manufacturing Reviewed

Asuka Suzuki, Hideto Nakatani, Makoto Kobashi

Materials & Design Vol. 230 page： 111969 2023.6

Mechanical coupling and tuned anisotropic elasticity: Numerical and experimental material design for shear-normal and shear-shear interactions Reviewed International coauthorship

Molavitabrizi, D; Suzuki, A; Kobashi, M; Mousavi, SM

MATERIALS & DESIGN Vol. 230 page： 111950 2023.6

Fabrication of Al-CNT composite with high hardness and electrical conductivity by controlling Al4C3 formation Reviewed

Kim, D; Hirayama, Y; Liu, Z; Takagi, K; Kobashi, M

JOURNAL OF ALLOYS AND COMPOUNDS Vol. 942 page： 169102 2023.5

B2-structured Fe<inf>3</inf>Al alloy manufactured by laser powder bed fusion: Processing, microstructure and mechanical performance Reviewed International coauthorship

A. M. Vilardell, L. Pelcastre, D. Nikas, P. Krakhmalev, M. Kato, N. Takata, M. Kobashi

Intermetallics Vol. 156 page： 107849 2023.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

Prealloyed Fe3Al was successfully manufactured by laser powder bed fusion. The best set of process parameters led to parts with a relative density of 99.5 %, a surface roughness, Sa, of 31.5 ± 5.6 μm and a hardness of 319 ± 14 HV0.1. Its microstructure as well as its mechanical properties at room and high temperatures were analyzed. The results of the chemical composition showed minor variations in aluminum content oscillating between 21 and 28 at.% along the melt pool. Additionally, elongated grains were observed to grow parallel to the building direction, as well as the development of a weak {001} texture along the building direction. The mechanical properties were influenced by the temperature. Compression tests showed a loss in strength with the increase in temperature, from a yield strength of 621 ± 40 MPa at room temperature to 89 ± 20 MPa at 650 °C. Reciprocating sliding wear tests showed that fragmentation of the intermetallic at room temperature occurs, whereas plastic deformation dominated at higher temperatures. For all temperatures, tribochemical wear was also present due to the oxidation of wear debris.

DOI： 10.1016/j.intermet.2023.107849

Exchange-Coupled SmCo5/Fe Nanocomposite Magnet Prepared by Low Oxygen Powder Metallurgy Process Reviewed

Ultra-high coercivity Sm-Co bulk magnets with remarkable thermal stability Reviewed

Park, K; Hirayama, Y; Wang, J; Kobashi, M

SCRIPTA MATERIALIA Vol. 218 2022.9

Iron aluminides as an alternative binder for cemented carbides: A review and perspective towards additive manufacturing Reviewed International coauthorship

Vilardell, AM; Cinca, N; Tarres, E; Kobashi, M

MATERIALS TODAY COMMUNICATIONS Vol. 31 2022.6

Laser powder bed fusion of polyamide-composite for antibacterial applications: Characterization and properties Reviewed International coauthorship

Vilardell, AM; Yadroitsava, I; Wolf, WKC; Du Plessis, A; Tshibalanganda, M; Kouprianoff, DP; Garcia-Giralt, N; Kobashi, M; Yadroitsev, I

MATERIALS TODAY COMMUNICATIONS Vol. 31 2022.6

Park K., Hirayama Y., Yamaguchi W., Kobashi M., Takagi K.

IEEE TRANSACTIONS ON MAGNETICS Vol. 58 ( 2 ) 2022.2

Anisotropic Sm-Co nanopowder prepared by induction thermal plasma Reviewed

Kwangjae Park, Yusuke Hirayama, Masaya Shigeta, Zheng Liu, Makoto Kobashi, Kenta Takagi

Journal of Alloys and Compounds Vol. 882 page： 160633 - 160633 2021.11

Dual plateau stress of C15-type topologically close-packed lattice structures additive-manufactured by laser powder bed fusion Reviewed

Xiaoyang Liu, Asuka Suzuki, Naoki Takata, Makoto Kobashi, Masaki Kato

SCRIPTA MATERIALIA Vol. 202 2021.9

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Imitating crystal structures offers new avenues to design lattice structures composed of unit cells with nodes (act as atoms) and struts (act as atomic bonds). We found out unique dual plateau regions in the stress-strain curves of lattice structures inspired by topologically close-packed (TCP) structure of C15 Laves phase (AB(2)-type intermetallic compounds) and manufactured via the laser powder bed fusion. X-ray computed tomography of compressively deformed lattice structures as well as finite element analysis revealed that the existence of two kinds of units with different relative density caused the dual plateau regions. In addition, the C15 lattice structures exhibited a hybrid deformation mode of bending- and stretch-dominated deformations and can absorb high total energy of 5.0 MJ.m(-3) in two stress levels. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.scriptamat.2021.114003

Highly Conductive Al/Al Interfaces in Ultrafine Grained Al Compact Prepared by Low Oxygen Powder Metallurgy Technique Reviewed

Kim, D; Hirayama, Y; Liu, Z; Kwon, H; Kobashi, M; Takagi, K

NANOMATERIALS Vol. 11 ( 5 ) 2021.5

Managing both high strength and thermal conductivity of a laser powder bed fused Al-2.5Fe binary alloy: Effect of annealing on microstructure Reviewed

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 805 page： 140591 2021.2

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

The microstructure of Al-2.5Fe (wt%) binary alloy samples additively manufactured by laser powder bed fusion (L-PBF) was systematically characterized, and its change due to subsequent annealing at 300 degrees C and 500 degrees C was examined. The as-fabricated samples featured homogeneous distribution of numerous fine particles of the metastable Al6Fe phase in the solidification microstructure where melt pools formed, and a relatively coarsened cellular structure was observed around the boundaries between the melt pools formed at different locations (melt pool boundaries). After the annealing at 300 degrees C, a slight growth of the nano-sized Al6Fe phase occurred, and coarsened plate-like.-Al13Fe4 phases were locally formed in the cellular structure at melt pool boundaries. Annealing at 500 degrees C induced a pronounced transformation of the metastable Al6Fe phase to the stable.-Al13Fe4 phase in the microstructure where melt pools formed, although no significant change in the microstructure of the a-Al matrix was detected. The thermal conductivity and tensile properties of as-fabricated and subsequently annealed specimens were measured. The as-fabricated specimens exhibited a high tensile strength of similar to 320 MPa and a thermal conductivity of similar to 150 W m(-1) K-1. Annealing at 300 degrees C improved the thermal conductivity to similar to 185 W m(-1) K-1 without a loss of tensile strength, whereas annealing at 500 degrees C significantly decreased the tensile strength. These results are utilized to discuss a balance between mechanical properties and thermal conductivity, which provided novel insights for managing both high strength and thermal conductivity of L-PBFbuilt Al-Fe binary alloys by controlling microstructure.

DOI： 10.1016/j.msea.2020.140591

Fabrication of Metal Matrix Composite Containing Manganese Nitride Showing Giant Negative Thermal Expansion by Compressive Torsion Processing Reviewed

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion

Liu Xiaoyang, Wada Takafumi, Suzuki Asuka, Takata Naoki, Kobashi Makoto, Kato Masaki

MATERIALS & DESIGN Vol. 199 2021.2

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion

Liu Xiaoyang, Wada Takafumi, Suzuki Asuka, Takata Naoki, Kobashi Makoto, Kato Masaki

MATERIALS & DESIGN Vol. 199 2021.2

Koshi Takenaka, Kentaro Otsuka, Yoshihiko Okamoto, Yuji Kume, Makoto Kobashi

MATERIALS TRANSACTIONS Vol. 62 ( 5 ) page： 590 - 595 2021

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS & MATERIALS

Antiperovskite manganese nitride Mn3.1Zn0.5Sn0.4N was composited with Al by compressive torsion processing (CTP). This class of nitrides is anticipated for use as a thermal-expansion compensator because of giant negative thermal expansion (NTE). It is difficult to form a composite with molten metal that causes a chemical reaction. To date, only limited success has been achieved. Composites formed by CTP at 573K exhibit lower thermal expansion and electrical resistivity than those produced by electric-current sintering. That achievement is attributable to suppressed chemical erosion of the filler by treatment at a lower temperature while the strong mechanical energy strengthens the interface between the filler and the matrix and reduces pores in the material. Compressive torsion processing is a promising method for forming metal matrix composites containing giant NTE fillers.

DOI： 10.2320/matertrans.MT-M2020363

Induction-activated self-propagating, high-temperature synthesis of nickel aluminide

Shekari M., Adeli M., Khobzi A., Kobashi M., Kanetake N.

ADVANCED POWDER TECHNOLOGY Vol. 28 ( 11 ) page： 2974 - 2979 2017.11

Change in microstructure of selectively laser melted AlSi10Mg alloy with heat treatments

Takata Naoki, Kodaira Hirohisa, Sekizawa Keito, Suzuki Asuka, Kobashi Makoto

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 704 page： 218 - 228 2017.9

Compressive properties of porous Ti-Al alloys fabricated by reaction synthesis using a space holder powder

Takata Naoki, Uematsu Keisuke, Kobashi Makoto

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 697 page： 66 - 70 2017.6

Pressure-less melt infiltration characteristics of Mg melt into Ti powder preforms with various pore structures: Toward the application to the binder jetting additive manufacturing

Zhuoyu Li, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Materials Processing Technology Vol. 331 page： 118509 - 118509 2024.10

Enhancing Capillary Pressure of Porous Aluminum Wicks by Controlling Bi-Porous Structure Using Different-Sized NaCl Space Holders

Hongfei Shen, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Materials Vol. 17 ( 19 ) page： 4729 - 4729 2024.10

Publishing type：Research paper (scientific journal) Publisher：MDPI AG

Capillary pressure and permeability of porous media are important for heat transfer devices, including loop heat pipes. In general, smaller pore sizes enhance capillary pressure but decrease permeability. Introducing a bi-porous structure is promising for solving this trade-off relation. In this study, the bi-porous aluminum was fabricated by the space holder method using two different-sized NaCl particles (approximately 400 and 40 μm). The capillary pressure and permeability of the bi-porous Al were evaluated and compared with those of mono-porous Al fabricated by the space holder method. Increasing the porosity of the mono-porous Al improved the permeability but reduced the capillary pressure because of better-connected pores and increased effective pore size. The fraction of large and small pores in the bi-porous Al was successfully controlled under a constant porosity of 70%. The capillary pressure of the bi-porous Al with 40% large and 30% small pores was higher than the mono-porous Al with 70% porosity without sacrificing the permeability. However, the bi-porous Al with other fractions of large and small pores did not exhibit properties superior to the mono-porous Al. Thus, accurately controlling the fractions of large and small pores is required to enhance the capillary performance by introducing the bi-porous structure.

DOI： 10.3390/ma17194729

Variation in microstructural features of melt-pool structure in laser powder bed fused Al–Fe–Cu alloy at elevated temperatures

Yue Cheng, Takanobu Miyawaki, Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Journal of Materials Research and Technology Vol. 32 page： 4048 - 4058 2024.9

Thermal stability of refined Al<inf>6</inf>(Fe, Mn) phase formed in laser powder bed fusion process

Wenyuan Wang, Yifan Xu, Misaki Deguchi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato, Masatoshi Mitsuhara, Hideharu Nakashima

Journal of Alloys and Compounds Vol. 992 2024.7

Publishing type：Research paper (scientific journal)

The addition of Mn in Al–Fe alloys stabilizes the Al6Fe metastable phase (orthorhombic structure, oC28) owing to the partial occupation of Fe sublattice sites by Mn atoms (formation of Al6(Fe, Mn) phase), thereby achieving superior high-temperature strength of Al–Fe–Mn ternary alloys that are manufactured via laser powder bed fusion (L-PBF) process. This study aimed to fundamentally understand the thermal stability of the refined Al6(Fe, Mn) phase formed in the L-PBF fabricated Al–2.5%Fe–2%Mn alloy in terms of a thermodynamic approach and the kinetics of morphological changes in the Al6(Fe, Mn) phase at elevated temperatures ranging from 200 to 500 ℃. The L-PBF processed samples showed a high hardness of approximately 125 HV due to the formation of numerous dispersoids of the Al6(Fe, Mn) phase with sizes of several tens of nanometers in the α-Al matrix containing concentrated solute elements of Fe and Mn. The hardness and microstructure were almost unchanged even after exposure to a high temperature of 200 ℃ for an extended period of 1000 h. Upon exposure to 300 ℃, nucleation and growth of the Al6(Fe, Mn) phase occurred locally, particularly at grain boundaries in the α-Al matrix. Such a local growth contributed to a reduction in hardness upon thermal exposure. Simultaneously, numerous nanoscale precipitates were formed in the α-Al matrix, resulting in a suppressed reduction in hardness. Contrary to the result of the thermodynamic calculations, the θ-Al13Fe4 phase was scarcely found even after long-term exposure to 500 ℃. The θ-phase formation was accompanied by the dissolution of the refined Al6Fe metastable phase for strengthening, which significantly reduced the hardness during thermal exposure. The suppressed θ-phase formation could be associated with the thermodynamically stable Al6(Fe, Mn) phase developed during the L-PBF process, which predominantly contributed to the high microstructural stability of the L-PBF fabricated Al–Fe–Mn alloy at elevated temperatures.

DOI： 10.1016/j.jallcom.2024.174593

Elucidating dominant flow channel size for capillary performance of open-cell porous wicks Reviewed

Hongfei Shen, Asuka Suzuki, Naoki Takata, Makoto Kobashi

International Journal of Heat and Mass Transfer Vol. 223 page： 125217 - 125217 2024.5

Four-dimensional electrochemical impedance spectroscopy: Role of microstructure on corrosion behaviour of Al-Si alloys additive-manufactured by laser powder bed fusion Reviewed

Hoshi, Y; Miyazawa, R; Otani, Y; Kim, D; Takata, N; Kobashi, M

CORROSION SCIENCE Vol. 232 2024.5

Effect of Cr content on thermally activated deformation in single-crystal micropillars of Fe-Cr binary alloys Reviewed

Zhu, TQ; Takata, N; Kim, D; Kobashi, M; Yoshino, M; Tsukamura, M

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 900 2024.5

In-Situ SEM Observation and DIC Strain Analysis for Deformation and Cracking of Hot-Dip ZnMgAl Alloy Coating Reviewed

Takata, N; Yokoi, H; Kim, D; Suzuki, A; Kobashi, M

CORROSION SCIENCE AND TECHNOLOGY-KOREA Vol. 23 ( 2 ) page： 113 - 120 2024.4

Microstructural origins of high strength of Al–Si alloy manufactured by laser powder bed fusion: In-situ synchrotron radiation X-ray diffraction approach Reviewed

Naoki Takata, Mulin Liu, Masahiro Hirata, Asuka Suzuki, Makoto Kobashi, Masaki Kato, Hiroki Adachi

Journal of Materials Science and Technology Vol. 178 page： 80 - 89 2024.4

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

The microstructural factors contributing to the high strength of additive-manufactured Al–Si alloys using laser-beam powder bed fusion (PBF-LB) were identified by in-situ synchrotron X-ray diffraction in tensile deformation and transmission electron microscopy. PBF-LB and heat treatment were employed to manufacture Al–12%Si binary alloy specimens with different microstructures. At an early stage of deformation prior to macroscopic yielding, stress was dominantly partitioned into the α-Al matrix, rather than the Si phase in all specimens. Highly concentrated Si solute (∼3%) in the α-Al matrix promoted the dynamic precipitation of nanoscale Si phase during loading, thereby increasing the yield strength. After macroscopic yielding, the partitioned stress in the Si phase monotonically increased in the strain-hardening regime with an increase in the dislocation density in the α-Al matrix. At a later stage of strain hardening, the flow curves of the partitioned stress in the Si phase yielded stress relaxation owing to plastic deformation. Therefore, Si-phase particles localized along the cell walls in the cellular-solidified microstructure play a significant role in dislocation obstacles for strain hardening. Compared with the results of the heat-treated specimens with different microstructural factors, the dominant strengthening factors of PBF-LB manufactured Al–Si alloys were discussed.

DOI： 10.1016/j.jmst.2023.08.050

CALPHAD-aided design of high-strength Al-Si-Mg alloys for sufficient laser powder bed fusion processability Reviewed

Yuki Otani, Keito Saki, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Journal of Alloys and Compounds Vol. 977 page： 173449 - 173449 2024.3

Microstructural factors controlling crack resistance of Zn–Al–Mg alloy coatings prepared via hot-dip galvanizing process: Combined approach of in-situ SEM observation with digital image correlation analysis Reviewed

Dasom Kim, Naoki Takata, Hiroki Yokoi, Asuka Suzuki, Makoto Kobashi

Journal of Materials Research and Technology Vol. 29 page： 1535 - 1541 2024.3

Laser-beam powder bed fusion of Al-Fe-Cu alloy to achieve high strength and thermal conductivity Reviewed

Cheng, Y; Miyawaki, T; Wang, WY; Takata, N; Suzuki, A; Kobashi, M; Kato, M

ADDITIVE MANUFACTURING LETTERS Vol. 8 page： 100191 2024.2

Al-Si 合金中のSi 含有量による炭素繊維／溶湯界面の化合物制御 Reviewed

古川雄一, 冨田高嗣, 三輪拓海, 鈴木飛鳥, 高田尚記, 小橋眞

鋳造工学 page： 68 - 72 2024.2

Austenite Reversion Behavior of Maraging Steel Additivemanufactured by Laser Powder Bed Fusion Reviewed

Takata, N; Ito, Y; Nishida, R; Suzuki, A; Kobashi, M; Kato, M

ISIJ INTERNATIONAL Vol. 64 ( 2 ) page： 303 - 315 2024

Combustion Foaming of Porous (Al, Fe)3Ti with L12 Ordered Structure

Asuka Suzuki, Terumasa Fujishiro, Naoki Takata, Makoto Kobashi

MATERIALS TRANSACTIONS Vol. 65 ( 4 ) page： 414 - 421 2024

Evaluation of reactive wetting kinetics of carbon fibers by molten Al-Ti alloy and its application to the fabrication of Al/carbon fiber composites Reviewed

Asuka Suzuki, Renkichi Ishiguro, Takumi Miwa, Naoki Takata, Makoto Kobashi, Yuichi Furukawa

Journal of Alloys and Compounds Vol. 968 page： 172168 2023.12

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

Wetting and interfacial reactions between molten Al and carbon fibers (CFs) are crucial for the fabrication of high-thermal conductive Al/CF composites through liquid-state processes including casting. The formation of Al4C3 which has a low thermal conductivity and high reactivity with water needs to be suppressed while improving wettability. The addition of Ti into molten Al forms TiC and contributes to significant improvement in wettability. To control a thin morphology of TiC, the kinetics of reactive wetting need to be clarified. In the present study, the reactive wetting kinetics between molten Al or Al-Ti alloy and pitch-based CF were investigated using the dipping coverage method combined with microstructural observations. The addition of Ti into molten Al drastically accelerated the wetting and reduced the apparent activation barrier for reactive wetting, which was related to the change in the interfacial products from the Al4C3 phase to the TiC phase. The Al4C3 phase grew while eroding the CFs, whereas the TiC phase formed in a thin-layered morphology around CFs and suppressed the diameter reduction of CF. The reduction in the apparent activation barrier and the morphology of carbides indicated a change in the dominant phenomena for reactive wetting from Al4C3 growth to TiC formation (controlled by diffusion in liquid Al). Based on the kinetics evaluations, CF were hybridized with pure Al and Al-Ti alloy through a casting process, and their thermal conductivities were evaluated. The addition of CF into pure Al degraded the thermal conductivity due to the eroded CF and coarsened Al4C3 phase, whereas the addition of CF into Al-Ti alloy enhanced the thermal conductivity. This study provides new insights into reactive wetting phenomena related to the fabrication of high-thermal conductive metal matrix composites.

DOI： 10.1016/j.jallcom.2023.172168

Evaluation of reactive wetting kinetics of carbon fibers by molten Al-Ti alloy and its application to the fabrication of Al/carbon fiber composites

Suzuki, Asuka, Ishiguro, Renkichi, Miwa, Takumi, Takata, Naoki, Kobashi, Makoto, Furukawa, Yuichi

Journal of Alloys and Compounds Vol. 968 page： 172168 2023.12

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

単結晶マイクロピラー圧縮試験を用いたAl-Fe合金積層造形体の高強度支配因子の検討 Reviewed

長子明弘, 朱天斉, 高田尚記, 鈴木飛鳥, 小橋眞

軽金属 Vol. 73 ( 11 ) page： 523 - 529 2023.11

Wear properties of aluminum alloys fabricated by laser powder bed fusion Reviewed International coauthorship

Gong, S; Takata, N; Kobashi, M; Shin, SE

TRIBOLOGY INTERNATIONAL Vol. 187 page： 108769 2023.9

Design of Al-Fe-Mn alloy for both high-temperature strength and sufficient processability of laser powder bed fusion Reviewed

Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

ADDITIVE MANUFACTURING Vol. 68 page： 103524 2023.4

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

This paper presents a novel design concept for Al alloys with both sufficient laser powder bed fusion (L-PBF) processability and high-temperature mechanical performance using an Al-Fe-Mn ternary alloy with a refined alpha-Al/Al6M (M = Fe, Mn) two-phase microstructure. The near-eutectic (liquid -> alpha-Al(fcc) + Al6M) composition of Al-2.5Fe-2Mn (mass%) was designed to avoid the formation of coarse primary intermetallics (having a detrimental effect on the PBF processability for densification) based on non-equilibrium liquidus projection thermodynamic calculations. The designed Al-2.5Fe-2Mn alloy powder exhibited sufficient L-PBF processability, and fully dense centimeter-sized samples (relative densities of above 99%) were successfully prepared. The L-PBF Al-2.5Fe-2Mn alloy samples exhibited a significantly refined solidification microstructure consisting of Al6M and alpha-Al phases in the melt-pool regions. Fe and Mn alloying elements contributed to the formation of nanoscale Al6M-phase particles and a high solid solution concentration in the alpha-Al matrix. The strength of the L-PBF Al-2.5Fe-2Mn alloy was slightly reduced at higher temperatures, but remained above 240 MPa at an elevated temperature of 300 degrees C. This superior high-temperature mechanical performance was attributed to the high thermal stability of the refined alpha-Al/Al6M two-phase microstructure and the formation of nanosized Al6M-phase precipitates.

DOI： 10.1016/j.addma.2023.103524

Microstructural origin of anisotropic tensile ductility of Al-Si alloy manufactured by laser powder bed fusion Reviewed

Yuki Otani, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Scripta Materialia Vol. 226 2023.3

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

Aluminum alloys fabricated by laser powder bed fusion (L-PBF) exhibit anisotropic tensile ductility. To identify microstructural origins in melt-pool (MP) structures (formed by the L-PBF process) contributing to the anisotropic ductility, the local strain distribution inside the MP structure in deformation was quantified by a combination of digital image correlation (DIC) strain analysis and in-situ SEM observations of tensile tests. It was found that higher strain was localized in locally coarsened microstructures (softer regions) along melt-pool boundaries (MPBs). The strain localization played a significant role in crack initiation (and propagation) around MPBs, contributing to fracture. The strain localization at MPBs changed depending on the geometrical relation of the MPBs with the loading direction (LD), which was controlled by the LD with respect to the building direction of L-PBF samples. The three-dimensional MPB distribution related to the LD would be responsible for the anisotropic ductility.

DOI： 10.1016/j.scriptamat.2022.115259

Austenite Reversion Behavior of Maraging Steel Additive-manufactured by Laser Powder Bed Fusion Reviewed

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

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN Vol. 109 ( 3 ) page： 201 - 214 2023.3

Language：English Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550 degrees C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (gamma) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the gamma-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing gamma phase) localized at cell boundaries would play a role in the nucleation site for the formation of gamma phase at 550 degrees C, resulting in the enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

DOI： 10.2355/tetsutohagane.TETSU-2022-066

Effects of Mn and Cu Additions on Solidification Microstructure and High-Temperature Strength of Cast AlFe Binary Alloy Reviewed

Naoki Okano, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Materials Transactions Vol. 64 ( 2 ) page： 492 - 499 2023

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

In order to investigate the effects of Mn and Cu additions on solidification microstructure and high-temperature strength of cast AlFe alloys, we have fabricated various AlFe-based alloys with compositions of Al1%Fe, Al1%Fe1%Mn, Al1%Fe1%Cu, and Al1%Fe1%Cu1%Mn (mol%) solidified at different cooling rates (0.3 K·s11 and 145 K·s11). In the Al1%Fe binary alloy, the coarsened ª-Al13Fe4 phase with a needle-shaped morphology was often observed in the furnace-cooled sample (0.3 K·s11), whereas the cast sample (145 K·s11) exhibited several elongated ¡ phases surrounded by fine ¡/Al6Fe eutectic microstructure. Such a solidification microstructure was observed in the cast Al1%Fe1%Cu alloy, whereas the Al23CuFe4 phase was locally formed in the finally solidified zone in the furnace-cooled sample. In the Al1%Fe1%Mn alloy, the Al6(Fe, Mn) phase was formed regardless of the cooling rate. Finer ¡/Al6(Fe, Mn) two-phase eutectic microstructure was almost entirely occupied in the cast sample. The fine eutectic microstructure was observed in the cast Al1%Fe1%Cu1%Mn alloy as well. Compression tests for cast alloy specimens revealed that the Al1%Fe1%Cu1%Mn alloy exhibited the highest strength level among the studied alloy specimens, indicating the combined addition of Mn and Cu elements could be effective in improving the high-temperature strength of the cast AlFe alloys.

DOI： 10.2320/matertrans.MT-LA2022010

Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micro pillars Reviewed

Naoki Takata, Shintaro Uesugi, Tianqi Zhu, Socihiro Takeyasu, Asuka Suzuki, Makoto Kobashi

MATERIALS TRANSACTIONS Vol. 64 ( 8 ) page： 1952 - 1958 2023

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS & MATERIALS

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10 mu m, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (tau(i)) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of tau(i)/G=0.33(d/b)(-0.63). The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.26 similar to 0.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples<bold>.</bold>

DOI： 10.2320/matertrans.MT-L2023004

Introducing Hatch Spacing into Deposited Energy Density toward Efficient Optimization of Laser Powder Bed Fusion Process Parameters Reviewed

Mai Kunieda, Asuka Suzuki, Naoki Takata, Masaki Kato, Makoto Kobashi

Materials Transactions Vol. 64 ( 6 ) page： 1099 - 1106 2023

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

The optimization of processing parameters is indispensable for the laser powder bed fusion (L-PBF) process. The deposited energy density (DED) is one of the process indexes for the L-PBF process and has a simplified formula of P v 0.5, where P is the laser power, and v is the scan speed. This parameter describes the change in the relative density and the melt pool morphology with laser power and scan speed well, whereas it does not include the effect of other processing parameters, e.g., hatch spacing (S). In the present study, an attempt was made to incorporate the effect of S into DED. Al12Si (mass%) alloy cube samples were fabricated by L-PBF under various P, v, and S, for evaluating the relative density and the melt pool morphology. The melt pool depth and width of L-PBF-manufactured Al12Si alloy increased linearly with P v 0.5 and did not exhibit a clear correlation with S. Based on the experimental observation, the effect of hatch spacing on DED was estimated to be S 0.5, and a new index of P v 0.5 S 0.5 was proposed. This index described the change in the relative density of the L-PBF-manufactured Al12Si alloy with laser conditions (P, v, and S) well when the thermal conduction mode melting was dominant. This study also indicated the limitation of the applicability of P v 0.5 S 0.5 under the keyhole or transition mode melting.

DOI： 10.2320/matertrans.MT-ME2022002

Effect of solute Ni on thermal activation process of plastic deformation in Fe-18Cr single-crystal micropillars Reviewed

Tianqi Zhu, Hongmei Li, Naoki Takata, Makoto Kobashi, Masataka Yoshino

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 857 2022.11

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

The aim of the present study was to identify the effect of solute Ni element on the plastic deformation and its thermal activation process of 18Cr ferritic stainless steels. Single-crystal micropillars with different diameters of approximately 2 and 5 mu m were fabricated on the surface of Fe-18%Cr-Ni alloy sheets containing different contents of Ni (0-2 mass%). Compression tests were performed on the micropillars at different initial strain rates to investigate the strain rate sensitivity (m) of stress for the slip initiation and the specimen-size dependence of the m value. In micropillars with a small diameter of 2 mu m, the Fe-18%Cr binary alloy specimen exhibited a relatively higher strain-rate sensitivity of initial slip stress (m = 0.12), whereas m was reduced to 0.01 by adding 2% Ni. The same trend was observed in micropillars with a larger size of approximately 5 mu m. The experimental results were utilized to evaluate the activation volume of plastic deformation. The activation volumes of the Fe-18%Cr alloy and alloy containing 1% Ni followed the specimen-size dependence trend of activation volume in bcc metals; however, the alloy containing 2% Ni exhibited larger activation volumes, suggesting a different thermal activation process presumably due to the significant interaction that occurred between dislocations and solute Ni atoms in bcc solid-solutions.

DOI： 10.1016/j.msea.2022.144076

Design of heat-resistant Al–Mg –Zn–Cu–Ni quinary alloy: Controlling intermetallic phases and mechanical performance at elevated temperature

Ruoqi Li, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Materials Science and Engineering: A Vol. 857 2022.11

Publishing type：Research paper (scientific journal)

Owing to good formability, wrought Al alloys are used in a wide range of industries, including turbochargers in automobile engines. The development of Al alloys with increased high-temperature strength is necessary to improve the fuel efficiency of engines. In this study, a modified heat-resistant Al–Mg–Zn–Cu–Ni quinary alloy with superior high-temperature strength and creep rupture life at 200 °C (potential temperature in service for compressor radial impellers) was designed. The thermodynamic assessments provided a modified alloy composition of Al–5Mg–3.5Zn–2Cu–2Ni (at %) exhibiting α-Al phase in equilibrium with T-Al6Mg11Zn11 and Al3(Ni, Cu)2 phases at elevated temperatures. The experimental alloy showed granular T and Al3(Ni, Cu)2 phases often located on grain boundaries in the α-Al matrix, although the undissolved Al3Ni phase (formed in solidification) locally remained. Apart from the T phase, the η-Zn2Mg phase containing Cu element preferentially precipitated on grain boundaries. A large number of fine precipitates were dispersed homogeneously in the grain interior after the aging at 200 °C or 300 °C. Compared to the base alloy with a ternary composition of Al–5Mg–3.5Zn (at %) and conventional Al alloys, the designed quinary alloy (pre-aged at 200 °C for 10 h) showed a high strength at elevated temperatures above 200 °C. The creep-rupture life at 200 °C/105 MPa was remarkably extended to 665 h by the addition of Cu and Ni elements into the base ternary alloy. The rupture life was more than ten times longer than one (57 h) of the ternary alloys. The alloy modification based on the thermodynamic calculations provides an effective approach for designing lightweight Al alloys with superior high-temperature strength and creep rupture life.

DOI： 10.1016/j.msea.2022.144055

Machine-learning assisted optimization of process parameters for controlling the microstructure in a laser powder bed fused WC/Co cemented carbide Reviewed

Asuka Suzuki, Yusuke Shiba, Hiroyuki Ibe, Naoki Takata, Makoto Kobashi

Additive Manufacturing Vol. 59 2022.11

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

A Single laser scan was performed on a sintered WC/Co composite and the process conditions were optimized to realize a WC/Co two-phase microstructure in a WC/Co composite fabricated by laser powder bed fusion (L-PBF). Laser irradiation on the sintered WC/Co composites generated two different microstructural regions. One is composed of WC and Co phases (WC/Co two-phase region), and the other includes W2C and W3Co3C phases (WC decomposition region). In the L-PBF-fabricated WC/Co composites, these two microstructural regions are distributed in a complex manner and their fractions varies depending on the laser parameters. Using a convolutional neural network model that is trained with the micrographs of single-laser-scanned WC/Co composites, the fractions of the two microstructural regions in the WC/Co composites fabricated by the L-PBF under various laser conditions were quantified. Multiple linear regression analyses revealed that the laser power and spot size contribute more than the scan speed, which has been verified by numerical analysis based on the moving heat source model. A support vector machine (SVM) suggested that low laser power and low scan speed are required for suppressing the WC decomposition in the consolidated samples. The WC decomposition has been successfully suppressed in the samples fabricated under SVM-recommended conditions.

DOI： 10.1016/j.addma.2022.103089

Optimizing Laser Conditions and Controlling Microstructure/Properties of Laser Powder Bed Fused Al-Si Alloys

SUZUKI Asuka, TAKATA Naoki, KOBASHI Makoto, KATO Masaki

Journal of the Japan Society of Powder and Powder Metallurgy Vol. 69 ( 10 ) page： 417 - 425 2022.10

Language：Japanese Publisher：Japan Society of Powder and Powder Metallurgy

In the laser beam powder bed fusion (PBF-LB) processes, laser parameters need to be customized for fabricating dense components. Al-Si alloys are representative Al alloys used for the PBF-LB processes. The Al-Si alloys fabricated by the PBF-LB processes exhibit characteristic microstructures and mechanical properties, which are affected by the laser parameters. This paper describes the effect of laser conditions on the relative density, microstructure, and mechanical properties of Al-12Si (mass%) alloy fabricated by the PBF-LB process. The first topic is a machine-learning-assisted exploration of laser parameters (laser power and scan speed) for fabricating dense Al-12Si alloy parts. Using a neural network model, the laser condition range for fabricating dense Al-12Si alloy parts is predicted. A methodology for accurately predicting the laser condition range for fabricating dense components is introduced. The second topic is controlling the microstructures and mechanical properties of the dense Al-12Si alloys. Variations in α-Al crystal grain size/orientation, the width of primary α-Al phase, and solute Si content in α-Al matrix with laser conditions are introduced. The changes in mechanical properties with laser conditions and correlation with microstructural features are also described.

DOI： 10.2497/jjspm.69.417

Controllable tensile performance of additively manufactured Al–Fe alloy Reviewed

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Materials Science and Engineering: A Vol. 855 2022.10

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

Achieving both high strength and ductility is challenging in additive manufacturing of Al alloys using the laser powder bed fusion (L-PBF) process by altering the laser conditions to control the microstructural characteristics. The current work focuses on the influence of varying laser energy density (ED), which is adjusted by different sets of laser power (P) and scan speed (v), on the microstructure-tensile property relationships of the L-PBF-fabricated Al–2.5%Fe alloy under varying laser conditions (P = 204–230 W, v = 450–600 mm/s) with a fixed powder layer thickness (t = 30 μm) and hatch distance (h = 30 μm). With increasing ED (= P/v·t·h), a slight increase in the average size of the columnar α-Al grains was found in the experimental samples concurrently with a higher fraction of <001>-oriented grains. Nanosized particles of the Al6Fe phase with an increased tendency to interconnect were observed in the matrix inside the melt pool, and the θ phase formed within the submicron-sized cellular structure along the melt pool boundaries (MPBs). A low tensile ductility, induced by the local microstructural inhomogeneities across MPBs, was found in all specimens tensile-deformed along the building direction, which was characterized by a preference fracture along the MPBs. At higher applied ED, the tensile ductility of the experimental Al–2.5Fe alloy was significantly improved with only a slight decrease in tensile strength. The difference in concentration of Fe across the MPBs in the matrix was also moderately diminished, which is consistent with the reduced local difference in microhardness across the MPBs under higher ED conditions. Therefore, the present results demonstrate that local microstructural inhomogeneities across MPBs can be controlled by the laser conditions, thereby producing both high strength and good ductility in the L-PBF processed Al–Fe alloys.

DOI： 10.1016/j.msea.2022.143893

Meso-structural Control on Metal Surfaces Based on Powder Metallurgy and Its Application to Metal/Polymer Joining

Suzuki Asuka, Takata Naoki, Kobashi Makoto

Journal of the Society of Powder Technology, Japan Vol. 59 ( 10 ) page： 511 - 517 2022.10

Language：Japanese Publisher：The Society of Powder Technology, Japan

This paper introduces meso-scale structure synthesis on metal surfaces through reaction processes from raw metal powders. The mesostructures are applied to metal/polymer anchor joining. The first topic is the synthesis of Fe-based porous structure on Fe substrate through the combustion reaction of the Fe-Ti-B system. The structure could be controlled in view of the phase equilibria at the maximum temperature of the reaction. A Fe/epoxy resin joint via the porous structure exhibits high tensile strength in a direction perpendicular to the joint interface. The second topic is the fabrication of mesostructure protruded on an Al alloy surface by laser scanning on Al-Ti-C powders. The meso-structural morphology is related to the formation of TiC and Al3Ti phases, which can be understood based on the solidification sequence after the laser irradiation. An Al alloy/CFRTP joint via the mesostructure exhibits high joint strength compared with previously reported similar anchor joints.

DOI： 10.4164/sptj.59.511

Aluminum matrix composites reinforced with multi-walled carbon nanotubes and C-60 manufactured by laser powder bed fusion Reviewed International coauthorship

Sangmin Yoo, Se-Eun Shin, Naoki Takata, Makoto Kobashi

JOURNAL OF MATERIALS SCIENCE Vol. 57 ( 38 ) page： 17984 - 17999 2022.10

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

Carbon nano-materials are widely used as reinforcing materials for aluminum-based composites due to their excellent metallurgical properties. This study presents an approach for manufacturing Al/C composites utilizing a high energy ball milling and laser powder bed fusion (LPBF) process. We prepared Al/C-60 and Al/MWCNT composites under 25 different conditions to observe their microstructure and mechanical property trends according to laser power and scan speed. The relative density of LPBFed composites increased as the energy density increased due to the oxide layer present on the aluminum surface and the high laser reflectance. It was found that, as the energy density increased, the increase in nanohardness and elastic modulus of the LPBFed composites was due to the high relative density. However, the high laser power condition of 179 W resulted in the softening of materials due to grain coarsening, resulting in a decrease in nanohardness and elastic modulus. Under the same laser conditions, the nanohardness of the LPBFed Al/2 vol.% MWCNT composites was approximately 0.5 GPa higher than that of the LPBFed Al/1 vol.% C-60 composites, which is explained by the high relative density and grain refinement strengthening.

DOI： 10.1007/s10853-022-07119-6

Enhancing the solid-state joinability of A5052 and CFRTP via an additively manufactured micro-structure Reviewed

Keiichiro Iwata, Asuka Suzuki, Seung Gwang Kim, Naoki Takata, Makoto Kobashi

Journal of Materials Processing Technology Vol. 306 2022.8

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

An Al alloy (A5052) sheet was joined with a carbon fiber-reinforced thermoplastic (CFRTP) sheet by hot pressing without melting the CFRTP sheet (that is, solid-state joining). A micro-structure was manufactured via laser irradiation of Al-Ti-C powders bedded on the A5052 sheet. The laser irradiation to Al-Ti-C powders generates numerous particle-shaped protrusions additively on the A5052 sheet, which facilitate the infiltration of CFRTP and mechanically interlock with the CFRTP sheet. To determine the optimal conditions for joining A5052 and CFRTP consisting of a polyamide 6 (PA6) matrix, the effects of joining temperature during hot pressing on the interfacial structure and joint strength were investigated. The maximum joint strength was attained at the joining temperature of 210 °C. At lower temperatures, PA6 was not sufficiently softened to infiltrate the micro-structure, resulting in lower joint strengths. At higher temperatures, carbon fibers (CFs) were exposed on the CFRTP surface and inhibited the penetration of PA6 and CFs into the micro-structure, thereby lowering the joint strength. To infiltrate PA6 and CFs into the micro-structure, hot pressing was performed in a gradient temperature field. The proposed process enabled appropriate infiltration of CFs and PA6 into the micro-structure and led to high and stable joint strength. It is expected that using the additively manufactured micro-structure approach proposed herein, the operating windows of various cutting-edge joining processes can be expanded to more moderate and lower energy conditions.

DOI： 10.1016/j.jmatprotec.2022.117629

Microstructural Variations in Laser Powder Bed Fused Al-15%Fe Alloy at Intermediate Temperatures Reviewed

Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

MATERIALS Vol. 15 ( 13 ) 2022.7

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

The samples of the Al-15Fe (mass%) binary alloy that were additively manufactured by laser powder bed fusion (L-PBF) were exposed to intermediate temperatures (300 and 500 degrees C), and the thermally induced variations in their microstructural characteristics were investigated. The L-PBF-manufactured sample was found to have a microstructure comprising a stable theta-Al13Fe4 phase localized around melt-pool boundaries and several spherical metastable Al6Fe-phase particles surrounded by a nanoscale alpha-Al/Al6Fe cellular structure in the melt pools. The morphology of the theta phase remained almost unchanged even after 1000 h of exposure at 300 degrees C. Moreover, the nanoscale alpha-Al/Al6Fe cellular structure dissolved in the alpha-Al matrix; this was followed by the growth (and nucleation) of the spherical Al6Fe-phase particles and the precipitation of the theta phase. Numerous equiaxed grains were formed in the alpha-Al matrix during the thermal exposure, which led to the formation of a relatively homogenous microstructure. The variations in these microstructural characteristics were more pronounced at the higher investigated temperature of 500 degrees C.

DOI： 10.3390/ma15134497

Fast scanning calorimetry study of Al alloy powder for understanding microstructural development in laser powder bed fusion Reviewed

Naoki Takata, Mulin Liu, Hongmei Li, Asuka Suzuki, Makoto Kobashi

MATERIALS & DESIGN Vol. 219 2022.7

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

We examined the variation in the solidification microstructure of AlSi10Mg alloy powder over a wide range of cooling rates controlled by using the fast scanning calorimetry (FSC) technique. Two exothermic peaks derived from the primary solidification of the alpha-Al phases and the alpha-Al/Si eutectic reaction were detected. These exothermic peaks became broader and shifted to lower temperatures at higher cooling rates. Focus ion beam (FIB) milling enables microstructural characterization of AlSi10Mg powder solidified at controlled cooling rates above 10(2) degrees C center dot s(-1) in the FSC equipment. The sample solidified at 4 x 10(4) degrees C center dot s(-1) exhibited a fine microstructure consisting of a primary alpha-Al phase surrounded by lamellar-shaped Si phases. The fraction of the Si phase continuously decreased with increasing cooling rate, indicating higher content of solute Si element in the alpha-Al phase solidified at a higher cooling rate. The secondary dendrite arm spacing (k) of the primary alpha-Al phase decreased in the sample solidified at a higher cooling rate (dT/dt). The relation follows a general equation of lambda = A (dT/dt)(-n) (A: 38.4, n: 0.33) in wide range of cooling rate (10(-1) similar to 10(4) degrees C center dot s(-1)). The results were utilized to discuss the cooling rate of Al alloys during the laser powder bed fusion (L-PBF) process. (c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

DOI： 10.1016/j.matdes.2022.110830

Design and controlling microstructure of cast-type aluminum eutectic alloys with an allowable service temperature of 300°C

Takata Naoki, Okano Naoki, Aikawa Motonari, Okano Taiki, Suzuki Asuka, Kobashi Makoto

Journal of Japan Institute of Light Metals Vol. 72 ( 6 ) page： 381 - 387 2022.6

Image-based finite element analysis of anomalous thermal conductivity in Al-10%Si-0.4%Mg alloy additive-manufactured by laser powder bed fusion

Suzuki Asuka, Miyasaka Tatsuya, Takata Naoki, Kobashi Makoto

Journal of Japan Institute of Light Metals Vol. 72 ( 5 ) page： 164 - 171 2022.5

Language：Japanese Publisher：The Japan Institute of Light Metals

Laser powder bed fused Al-10Si-0.4Mg (mass%) alloy exhibits low and anisotropic thermal conductivity. To elucidate dominant microstructural features generating the characteristic thermal conductivity, thermal conduction analysis was performed by an image-based finite element method (FEM) using scanning electron microscope (SEM) images. The serial sectioning images of as-fabricated and 300ºC/2 h heat treated Al-10Si-0.4Mg alloys were obtained by focused ion beam SEM (FIB-SEM) and were used for constructing three-dimensional models. The homogenization method was applied to calculate average thermal conductivity of these models. When the thermal conductivity of α-Al phase was set at the thermal conductivity of pure aluminum, calculated thermal conductivity of as-fabricated model was much higher than experimentally measured thermal conductivity of as-fabricated Al-10Si-0.4Mg alloy, indicating that α-Al phase in the as-fabricated Al-10Si-0.4Mg alloy had much lower thermal conductivity than pure aluminum. In addition, the anisotropy of thermal conductivity appeared only in the as-fabricated model in which the α-Al/Si interfacial thermal resistance was taken into account. These results were used for discussing dominant contributors of low and anisotropic thermal conductivity of laser additive manufactured Al-10Si-0.4Mg alloy.

DOI： 10.2464/jilm.72.164

Negative strain rate sensitivity of yield strength of Al-Si alloy additive-manufactured using laser powder bed fusion Reviewed

Observation of compressive deformation behavior of Al-12%Si alloy lattice structures additive-manufactured by laser powder bed fusion using mechanical stress-induced luminescent material

Kato Masaki, Suzuki Asuka, Mcmillan Matthew, Liu Xiaoyang, Takata Naoki, Kobashi Makoto

Journal of Japan Institute of Light Metals Vol. 72 ( 5 ) page： 246 - 250 2022.5

Naoki Takata, Mulin Liu, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Scripta Materialia Vol. 213 page： 114635 - 114635 2022.5

Novel approach in fabricating microchannel-structured La(Fe,Si)13Hy magnetic refrigerant via low-contamination route using dissolutive mold Reviewed

Kaoru Imaizumi, Asaya Fujita, Asuka Suzuki, Makoto Kobashi, Kimihiro Ozaki

Materials & Design Vol. 217 page： 110651 - 110651 2022.5

Microstructure and magnetocaloric property of homogeneous La(FexSi1−x)13 compounds fabricated by laser fusion using a powder mixture of Fe and LaySiz compounds Reviewed

Kaoru Imaizumi, Asaya Fujita, Asuka Suzuki, Makoto Kobashi, Kimihiro Ozaki

Journal of Alloys and Compounds Vol. 901 page： 163706 - 163706 2022.4

Improvement of magnetocaloric effect in La(FexSi1-x)13 by dealing with inhibitory microstructures at high Fe concentration Reviewed

Kaoru Imaizumi, Asaya Fujita, Asuka Suzuki, Makoto Kobashi, Kimihiro Ozaki

Acta Materialia Vol. 227 page： 117726 - 117726 2022.4

Design and Microstructural Control of Heat-Resistant Aluminum Wrought Alloys Based on Calculated Phase Diagrams Reviewed

Takata Naoki, Suzuki Asuka, Kobashi Makoto

Materia Japan Vol. 61 ( 4 ) page： 195 - 201 2022.4

High-temperature strength sustained by nano-sized eutectic structure of Al–Fe alloy manufactured by laser powder bed fusion Reviewed

Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Materials Science and Engineering: A Vol. 838 page： 142782 - 142782 2022.3

Solidification microstructure and room-temperature fracture toughness of Al-based cast alloys prepared through eutectic reactions in Al-Mg-Zn ternary system

Naoki Okano, Motonari Aikawa, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Journal of Japan Institute of Light Metals Vol. 72 ( 3 ) page： 79 - 87 2022.3

Change in microstructural characteristics of laser powder bed fused Al–Fe binary alloy at elevated temperature Reviewed

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Journal of Materials Science & Technology Vol. 97 page： 38 - 53 2022.1

Influence of Added Fourth Elements on Precipitation in Heat-Resistant Al–Mg–Zn Ternary Alloys Reviewed

Hiroki Ishii, Rikito Takagi, Naoki Takata, Asuka Suzuki, Makoto Kobashi

MATERIALS TRANSACTIONS Vol. 63 ( 4 ) page： 513 - 521 2022

Effect of heat treatment on solute Si content in Al-12%Si alloy additive-manufactured by laser powder bed fusion

Naoki Takata, Mulin Liu, Asuka Suzuki, Makoto Kobashi, Masaki Kato, Shumpei Iwai, Hiroki Adachi

Keikinzoku/Journal of Japan Institute of Light Metals Vol. 72 ( 5 ) page： 178 - 184 2022

Publishing type：Research paper (scientific journal)

We have investigated microstructural characteristics and solute Si element in the α-Al (fcc) matrix of Al-12%Si binary alloy additive-manufactured by laser powder bed fusion (L-PBF) process in terms of different locations in the melt-pool structure (inside melt-pool or around melt-pool boundaries). Their variations by post heat-treatments at various temperatures were examined. The fraction of Si phase and lattice parameter of α-Al phase were quantified by XRD measurements combined with sample rotation and oscillation systems. In the L-PBF manufactured sample, the columnar α -Al phases contained highly concentrated Si element in solution (above 3 mol%) inside melt-pools, whereas the measured Si content was approximately 2 mol% in the relatively coarsened α-Al phase along melt-pool boundaries. The location dependence of solute Si content would be associated with a continuous change in growth rate of solid phase in solidification by local heating in the L-PBF process. The solute Si content was significantly reduced by heat-treatments and its location dependence appeared less pronounced. The as-manufactured sample exhibited a lower lattice parameter of α-Al phase with concentrated Si in solution in comparison with the heat-treated samples. The fraction of Si phase became higher in the heat-treated samples. These results were rationalized by microstructural change at elevated temperatures.

DOI： 10.2464/jilm.72.178

Application of Machine Learning to Process Parameter Optimizations of Metal Additive Manufacturing

SUZUKI Asuka, IBE Hiroyuki, TAKATA Naoki, KOBASHI Makoto, KATO Masaki

Journal of Japan Thermal Spray Society Vol. 59 ( 4 ) page： 210 - 216 2022

Language：Japanese Publisher：Japan Thermal Spray Society

This paper introduces machine learning approaches for process parameter optimizations of the laser powder bed fusion (L-PBF) process, which is one of the most representative additive manufacturing technologies. The first topic is an exploration of laser conditions for obtaining dense Al-12 mass%Si alloy parts using a neural network. A neural network architecture and training manner for precisely predicting optimized laser conditions using limited data are introduced. The second topic is laser parameter optimization for controlling the microstructure of L-PBF-fabricated WC/Co composites. Two characteristic microstructural regions (WC/Co two-phase region and WC decomposition region) are formed in the L-PBF-fabricated WC/Co composites. The WC decomposition region is formed by the melting of the WC particles and has a detrimental effect on the mechanical properties. Therefore, its formation should be suppressed. To quantify the amount of the WC decomposition region formed, a convolutional neural network model, in which micrographs of single-laser-scanned WC/Co composites were trained, was constructed. The support vector machine was also used for optimizing the laser parameters. These machine learning methods can make an efficient estimation of optimized laser parameters for the L-PBF process.

DOI： 10.11330/jtss.59.210

Control of microstructural characteristics and mechanical properties of AlSi12 alloy by processing conditions of laser powder bed fusion Reviewed

Asuka Suzuki, Tatsuya Miyasaka, Naoki Takata, Makoto Kobashi, Masaki Kato

Additive Manufacturing Vol. 48 page： 102383 2021.12

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

To elucidate the possibility of controlling the microstructure and mechanical properties of Al-Si alloys additive-manufactured by laser powder bed fusion (L-PBF), the relationship among the laser conditions, microstructural characteristics (crystal grain, cellular microstructure, and solute Si in alpha-Al matrix), and tensile properties of L-PBF manufactured AlSi12 alloy samples was investigated. The crystal grain size and fraction of 001 ori-ented alpha-Al crystal grains toward the building direction increased as the laser power became higher and scanning velocity became slower. The width of cellular microstructure (sub-cell size) where the primary alpha-Al phase was surrounded by alpha-Al/Si eutectic microstructure increased as the laser power became higher and scanning velocity became slower. The refinement of microstructure was attributed to the solidification under high cooling rate, which was achieved by low laser power and high scanning velocity. The area fraction of Si phase analyzed from FE-SEM images tended to decrease as the laser power became higher and scanning velocity became slower, suggesting the possibility of solute Si concentration in alpha-Al matrix controlled by laser conditions. The STEM/EDS analysis revealed high concentration of solute Si in the alpha-Al matrix (above a solubility limit of 1.5 mol% in the Al-Si binary system) of L-PBF manufactured AlSi12 alloy samples and its concentration became higher in the sample manufactured using a higher laser power. The tensile tests of as-built AlSi12 specimens fabricated under various laser conditions were also performed. High strength and high ductility tended to be achieved when high power laser was applied in slow laser scanning velocity. The 0.2% proof stress was relatively higher when the laser scanning velocity was faster. The mechanical properties correlated with crystal grain size and sub-cell size rather than solute Si in the alpha-Al matrix. These results provide new insights to control mechanical performance of AlSi12 alloy via L-PBF processing parameters.

DOI： 10.1016/j.addma.2021.102383

Precipitation morphology and kinetics of T-Al6Mg11Zn11 intermetallic phase in Al–Mg–Zn ternary alloys Reviewed

Naoki Takata, Rikito Takagi, Ruoqi Li, Hiroki Ishii, Asuka Suzuki, Makoto Kobashi

Intermetallics Vol. 139 page： 107364 - 107364 2021.12

Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al–Si–Cu Alloy Reviewed

Ruoqi Li, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Yuji Okada, Yuichi Furukawa

Materials Vol. 14 ( 23 ) page： 7155 - 7155 2021.12

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

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15(Fe, Mn)3Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.

DOI： 10.3390/ma14237155

Enhancement in strength and ductility of laser powder bed fused Al–12Si alloy by introducing nanoscale precipitates

Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Additive Manufacturing Letters Vol. 1 page： 100008 - 100008 2021.12

Controlling WC/Co two-phase microstructure of cemented carbides additive-manufactured by laser powder bed fusion: Effect of powder composition and post heat-treatment Reviewed

Hiroyuki Ibe, Yuta Kato, Junya Yamada, Masaki Kato, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Materials & Design Vol. 210 page： 110034 - 110034 2021.11

Control of surface micro−structure for Al alloy/polymer joining fabricated by laser process using Al−Ti−C powders: Effect of powder composition. Reviewed

Seung-Gwang Kim, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Advanced Joining Processes Vol. 4 page： 100068 - 100068 2021.11

Inhomogeneous Microstructure and its Thermal Stability of AlSi10Mg Lattice Structure Manufactured via Laser Powder Bed Fusion

Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Materials Science Forum Vol. 1016 page： 826 - 831 2021.10

Language：English Publishing type：Research paper (international conference proceedings)

The inhomogeneous microstructure and its change by annealing for an AlSi10Mg lattice structure with a body-centered cubic unit cell additively manufactured via laser powder bed fusion (LPBF) were investigated. The as-built lattice structure exhibited a cellular microstructure consisting of a number of primary α-Al phases decorated with α-Al/Si eutectic structure. The developed microstructure varied depending on the locations of the node and strut parts of the lattice structure. At the location near the bottom surface of the node part, the cellular microstructure became coarser and more equiaxed than those at the location near the top surface. At the location near the bottom surface of the strut part, the columnar α-Al phases were often elongated along the direction of the strut part. After the annealing at 300 °C for 2 h, numerous Si particles finely precipitated within the primary α-Al phases and coarsening of the eutectic Si phases occurred. After the annealing at 530 °C for 6 h, the microstructural characteristics changed significantly. A significant coarsening of the Si particles and the formation of Fe-containing intermetallic phase (Β-AlFeSi) with a plate-shaped morphology occurred. The microstructures became homogeneous in the whole area of the lattice structure annealed at 530 °C for 6 h.

DOI： 10.4028/www.scientific.net/MSF.1016.826

Effect of added stabilizing elements on thermal activation process of plastic deformation in 18Cr ferritic stainless steel Reviewed

Hongmei Li, Naoki Takata, Makoto Kobashi, Masataka Yoshino

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 824 2021.9

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

The effects of a trace amount (similar to 0.1 at.%) of added stabilizing elements (Nb, Ti, and Zr) on the tensile properties of 18Cr ferritic stainless steel sheets at strain rates ranging from 10(-3)-10(2) s(-1) (from ordinary quasi-static deformation to dynamic deformation) were investigated. The additional Ti or Zr elements formed coarse carbide or nitride precipitates, which consumed interstitial solute elements (C and/or N) in the ferrite matrix. In the Nb-added steel, however, only fine NbN precipitates with a few hundred nanometers in size were observed locally, implying a small amount of solute Nb in the ferrite matrix. Ti or Zr additions had only a slight effect on the deformation resistance of 18Cr ferritic steel sheets and strain rate sensitivity (m) of the flow stress, however, the Nb addition increased the deformation resistance and significantly reduced the strain rate sensitivity. The measured activation volume (v*) values of the Ti- and Zr-added steels (62 b(3) and 66 b(3)) were almost equivalent to one of the 18Cr base steel (64 b(3)). This result suggested that a general thermally activated process for screw dislocation motion is controlled by double-kink nucleation, which is slightly affected by Ti or Zr additions. The high v* value (106 b(3)) of the Nb-added steel indicated that the thermally activated process of plastic deformation would be associated with the substitutional solute Nb atoms (act as short-range obstacles) interacting with the moving dislocations. These results showed that the Nb element played a distinct role in plastic deformation from the other stabilizing elements of Ti and Zr.

DOI： 10.1016/j.msea.2021.141866

Influence of added fourth elements on precipitation in heat-resistant Al-Mg-Zn ternary alloys

Hiroki Ishii, Rikito Takagi, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Journal of Japan Institute of Light Metals Vol. 71 ( 7 ) page： 275 - 282 2021.7

Thermal activation process of plastic deformation in Fe-18Cr single-crystal micropillars with high-density dislocations Reviewed

Hongmei Li, Tianqi Zhu, Naoki Takata, Makoto Kobashi, Masataka Yoshino

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 819 2021.7

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

Micropillar compression testing was performed on bcc solid-solution single-crystal micropillars with two diameter (d) ranges (2-3 and 5-6 mu m) prepared on the surfaces of fully annealed and subsequently 10% coldrolled 18Cr ferritic steel specimens. This was done to investigate the effect of initial dislocations on the plastic deformation of micron-sized crystals. The 10% cold rolling process significantly increased the stress level (provided by an activated single slip system) required to initiate slip in the annealed micropillars with different diameter ranges. In the case of the annealed micropillars, the strain-rate sensitivity (m) of the yield stress becomes lower for larger specimens (the m value decreased from 0.12 to 0.04). However, the 10% cold-rolled micropillars exhibited a much lower strain-rate sensitivity regardless of the specimen size. The activation volume (v*) of the annealed micropillars showed a significant specimen size dependency (v*proportional to d1.47), consistent with published data on the micropillar compression experiments of single-crystals of the bcc metals and alloys. For the 10% cold-rolled micropillars, the high v* values (88b3 and 112b3) showed a slight dependency on the specimen size (v*proportional to d0.33). The v* values appeared somewhat higher but were equivalent to those of bulk-size specimens, suggesting that the initial high-density dislocations would provide sufficient sources for dislocation multiplication in the micron-sized specimens under compression. The observed high v* values might be associated with the strong dislocation interactions in the alpha-(Fe, Cr) solid-solution matrix.

DOI： 10.1016/j.msea.2021.141459

In Situ X-Ray Observations and Microstructural Characterizations for Understanding Combustion Foaming and Reaction Processes to Synthesize Porous Al3Ti Composites From Al-Ti-B4C Powders Reviewed

Inukai Takamasa, Suzuki Asuka, Takata Naoki, Kobashi Makoto, Okada Yuji, Furukawa Yuichi

ADVANCED ENGINEERING MATERIALS Vol. 23 ( 5 ) 2021.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

The combustion foaming behaviors to synthesize closed-cellular porous Al3Ti composites are observed in situ using X-ray radioscopy. The in situ observation of the thermal explosion (TE) mode foaming reveals that the combustion foaming process consists of six steps. The shape of the sample changes uniformly in the first three steps before the sample foams. First, the sample expands uniformly 1) gradual expansion) and maintains an almost constant volume for a fixed period 2) temporary steady state at a constant volume). During these stages, bubbles are not generated inside the sample. Subsequently, the sample shrinks uniformly 3) slight shrinkage). After these three steps, the bubbles are rapidly generated inside the sample 4) rapid foaming). Immediately after foaming, the sample shrinks drastically while bubbles are ruptured and combined 5) severe shrinkage and bubble coarsening). Finally, the sample solidifies as a porous metal 6) solidification). The sample fabricated under the self-propagating high-temperature synthesis (SHS) mode includes graded microstructures corresponding to steps (1)-(4). The microstructures of the samples foamed under the TE and SHS modes are characterized. These results are used for discussing the elementary reaction steps arising at steps (1)-(4).

DOI： 10.1002/adem.202001284

Critical resolved shear stress of activated slips measured by micropillar compression tests for single-crystals of Cr-based Laves phases Reviewed International coauthorship

Xue Yunlong, Takata Naoki, Li Hongmei, Kobashi Makoto, Yuan Liang

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 806 2021.3

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

Single-crystal micropillar compression tests were performed in this study to investigate the mechanical properties and deformation mechanisms of Cr-based Laves phases. The results showed that, the activated slip system in the C15-Cr2Nb was exclusively the basal slip {111}< 110 >, transiting from the pyramidal slip {11 (2) over bar2}<(11) over bar 23 > to the prismatic slips {10 (1) over bar0}< 0001 > and {11 (2) over bar0}< 1 (1) over bar 00 > in the C14-Cr2Ta with the increased inclination angle. The average compressive strength of the C15-Cr2Nb changed between 10.0 GPa and 11.7 GPa, and that of the C14-Cr2Ta ranged within 11.6-12.4 GPa. The compressive strength changed due to the evolutions of activated slip systems in both Laves phases. The critical resolved shear stress (CRSS) of the C15-Cr2Nb was estimated as 4.4-4.6 GPa, which was lower than 5.1-6.1 GPa in the C14-Cr2Ta, and the higher CRSS of the C14-Cr2Ta resulted from higher shear modulus. The mechanical properties and deformation mechanisms obtained were helpful for better understanding and designing the Cr-based Laves phase alloys.

DOI： 10.1016/j.msea.2021.140861

Processability and Optimization of Laser Parameters for Densification of Hypereutectic Al-Fe Binary Alloy Manufactured by Laser Powder Bed Fusion Reviewed

Wenyuan Wang, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

CRYSTALS Vol. 11 ( 3 ) 2021.3

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

Centimeter-sized samples of hypereutectic Al-15 mass% Fe alloy were manufactured by a laser powder bed fusion (L-PBF) process while systematically varying laser power (P) and scan speed (v). The effects on relative density and melt pool depth of L-PBF-manufactured samples were investigated. In comparison with other Al alloys, a small laser process window of P = 77-128 W and v = 0.4-0.8 ms(-1) was found for manufacturing macroscopically crack-free samples. A higher v and P led to the creation of macroscopic cracks propagating parallel to the powder-bed plane. These cracks preferentially propagated along the melt pool boundaries decorated with brittle theta-Al13Fe4 phase, resulting in low L-PBF processability of Al-15%Fe alloy. The deposited energy density model (using P center dot v(-1)(/2)) would be useful for identifying the optimum L-PBF process conditions towards densification of Al-15%Fe alloy samples, in comparison with the volumetric energy density (using P center dot v(-1)), however, the validity of the model was reduced for this alloy in comparison with other alloys with high thermal conductivities. This is likely due to inhomogeneous microstructures having numerous coarsened theta-Al13Fe4 phases localized at melt pool boundaries. These results provide insights into achieving sufficient L-PBF processability for manufacturing dense Al-Fe binary alloy samples.

DOI： 10.3390/cryst11030320

Joint strength of Fe/epoxy resin hybrid structure via porous Fe/TiB2 composite layer synthesized by in-situ reaction process Reviewed

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion Reviewed

Xiaoyang Liu, Takafumi Wada, Asuka Suzuki, Naoki Takata, Makoto Kobashi, Masaki Kato

MATERIALS & DESIGN Vol. 199 page： 109416 2021.2

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion Reviewed

Xiaoyang Liu, Takafumi Wada, Asuka Suzuki, Naoki Takata, Makoto Kobashi, Masaki Kato

MATERIALS & DESIGN Vol. 199 page： 109416 2021.2

Suzuki Asuka, Noritake Kazuki, Takata Naoki, Kobashi Makoto

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY Vol. 288 2021.2

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

Fe/epoxy resin hybrid structures were fabricated via three-dimensional open-cellular porous Fe/TiB2 composite layers by an in-situ reaction synthesis process. Pores in the porous layer were filled with an epoxy resin to form an interpenetrating phase layer (IPL) consisting of epoxy resin and Fe/TiB2 composite. The relationship between the tensile joint strength perpendicular to the joint interface and structures of the porous layer was investigated. The tensile joint strength increased with increasing the roughness of an end surface of the porous layer but decreased when the surface roughness was higher than approximately 30 mu m. The decrement in the joint strength was caused by a transition of the fracture position from an epoxy resin/IPL interface to Fe/IPL, interface due to poor adhesiveness of the porous layer/Fe interface. A high-resolution X-ray computed tomography (CT) was utilized for investigating change in area fraction of the epoxy resin with depth from the end surface of the IPL. Based on the X-ray CT and fractography, it was revealed that the joint strength could be understood in view of epoxy resin/IPL interfacial strength at the fracture position estimated from the mixture law when the adhesiveness of the porous layer/Fe interface was enough. Based on the results above, an in-situ reaction induced from a functionally graded powder condition was applied for enhancing joint strength.

DOI： 10.1016/j.jmatprotec.2020.116843

Effect of trace solute titanium on plastic deformation of alpha-(Fe, Cr) single-crystal micropillars fabricated from 18Cr ferritic stainless steel Reviewed

Li Hongmei, Zhu Tianqi, Takata Naoki, Kobashi Makoto, Yoshino Masataka

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 803 2021.1

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

The effect of a trace amount (similar to 0.1 at.%) of solute Ti on the compression response of cylindrical single-crystal micropillars fabricated from a 18Cr ferritic stainless steel was investigated. The micropillars of the solution treated steel samples were fabricated on a special grain with different diameter ranges (d = 2-3 mu m or 5-6 mu m). By this process, the flow stress required for slip initiation (corresponding to the yield stress) and its strain rate dependence could be determined. The compression tests on the micropillars revealed that Ti addition (solute Ti in the alpha-(Fe, Cr) ferrite matrix) has a slight effect on the activated slip system and stress level required for the slip initiation of 18Cr steel specimens with two different sizes. In addition, the strain rate sensitivity (m) of the yield stress at an offset strain of 0.2% (0.2% proof stress) in the small micropillars (d = 2-3 mu m) of the micron sized 18Cr steel specimens decreased slightly (from 0.12 to 0.08), owing to the Ti addition. However, the m value (0.04) of the large micropillars (d = 5-6 mu m) of the 18Cr steel is unaffected by Ti addition and is equivalent to the m value measured for millimeter-sized specimens (conventional bulk specimens). The low activation volume (v*) values (18-19 b3) were obtained for the small micropillars of both the 18Cr and Ti-added steels. For the large specimens, the v* value of the 18Cr steel decreased slightly from 50 b3 to 46 b3 owing to the Ti addition and is close to the value obtained from the millimeter-sized specimens. These results indicate that the trace amount of solute Ti has a slight effect on the thermal activation process for controlling the dislocation motion in the 18Cr ferritic stainless steels. The intriguing low activation volume in the small micropillars is discussed in comparison with the results of micron-scale specimens of various bcc metals and alloys.Y

DOI： 10.1016/j.msea.2020.140455

Microstructure of Al-2.5 Fe Binary Alloy Fabricated by Laser Powder Bed Fusion Reviewed International journal

Xing Qi, Naoki Takata, Asuka Suzuki, Makoto Kobashi, Masaki Kato

Materials Science Forum Vol. 1016 page： 1175 - 1180 2021.1

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Trans Tech Publications Ltd

In the present study, microstructural and crystallographic features of an Al-Fe binary alloy with a near eutectic composition (Al-2.5wt%Fe) fabricated by laser powder bed fusion (LPBF) process were examined. The LPBF processing for the Al-Fe alloy powder was conducted at room temperature using a 3D Systems ProX 200 operating at a laser power of 204 W and a laser scan speed of 0.6 m/s and 0.8 m/s to fabricate cube samples with high relative density above 99 %. The fabricated sample exhibited characteristic microstructure consisting of a number of melt pools in which the regions had locally melted and rapidly solidified by laser irradiation during the LPBF process. Numerous fine particles of Al-Fe intermetallics with a mean size below 100 nm were found within the α-Al matrix in the observed melt pools, whereas relatively coarsened particles were localized around melt pool boundaries. Electron backscatter diffraction (EBSD) analyses revealed a number of columnar α-Al grains with a mean size of approximately 10 µm. The LPBF-fabricated Al-Fe alloy exhibits a high hardness of approximately 90 HV, which is more than twice higher than the conventionally casted Al-Fe alloy.

DOI： 10.4028/www.scientific.net/MSF.1016.1175

Mechanical behavior of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu lattice structures manufactured by laser powder bed fusion and designed for implant applications Reviewed International coauthorship

Vilardell A. M, Takezawa A, du Plessis A, Takata N, Krakhmalev P, Kobashi M, Albu M, Kothleitner G, Yadroitsava I, Yadroitsev I

JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS Vol. 113 page： 104130 - 104130 2021.1

Manufacturing and characterization of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu by laser powder bed fusion Reviewed International coauthorship

Vilardell A. M, Yadroitsev I, Yadroitsava I, Albu M, Takata N, Kobashi M, Krakhmalev P, Kouprianoff D, Kothleitner G, du Plessis A

ADDITIVE MANUFACTURING Vol. 36 2020.12

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

Biofunctionalization of Ti6Al4V alloy with metallic agents like Ag or Cu is a promising approach to add antibacterial properties and thus to reduce the risk of implant failure. This research investigates the in-situ alloying of Ti6Al4V(ELI) with 3 at.% Cu powders using Laser Powder Bed Fusion (L-PBF). The morphology and geometrical characteristics of the single tracks and layers were studied. Laser powers of 170 W and 340 W, and scanning speeds ranging from 0.4 to 1.4 m/s and 0.8–2.8 m/s were implemented. Single track results showed balling effect and humping at high scanning speeds, 1.4 m/s and 1.6 m/s, for each laser powder respectively. Conversely, keyhole formation occurred at lower scanning speeds of 0.4–0.6 m/s for 170 W laser power, and below and 0.8 m/s for 340 W laser power. For both laser powers, single layers resulted in smoother surfaces at lower scanning speeds. These results were used for the development of optimal process parameters for 3D cubes with 99.9 % density. Optimal process parameters were found for 170 W and 340 W laser powders at 0.7−0.9 and 1.0–1.2 m/s scanning speeds, respectively. In-situ alloying by L-PBF was challenging and a homogeneous distribution of Cu within the alloy was hard to achieve. The increase in laser power from 170 to 340 W resulted in small increase in homogenization. Microstructural analyses after stress-relieving treatment showed the presence of α’ and β phases, as well as CuTi2 intermetallic precipitates. The finer microstructure together with CuTi2 intermetallic precipitates resulted in an increase in hardness. This study demonstrates the potential for printing in-situ alloyed Ti6Al4V(ELI)- 3 at.% Cu for biomedical applications. However, further studies are required to determine the effectiveness of antibacterial properties.

DOI： 10.1016/j.addma.2020.101436

Effect of Annealing on Anisotropic Tensile Properties of Al-12%Si Alloy Fabricated by Laser Powder Bed Fusion Reviewed

Liu Mulin, Wada Takafumi, Suzuki Asuka, Takata Naoki, Kobashi Makoto, Kato Masaki

CRYSTALS Vol. 10 ( 11 ) 2020.11

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

In this study, we systematically investigated microstructures and tensile properties of an Al-12mass%Si alloy additive-manufactured by laser powder bed fusion (LPBF) process and subsequently annealed at various temperatures. Microstructure of the as-fabricated sample was characterized by a number of melt pools consisting of alpha-Al phases surrounded by Si eutectic phases. Fine Si precipitates were observed in the alpha-Al phase in the sample annealed at 200 degrees C. The eutectic Si phase appears to agglutinate, resulting in a coarsened Si phase formed at high temperatures above 300 degrees C. The initial cellular microstructure completely disappears and a number of coarsened Si phases and plate-shaped intermetallic phases (beta-AlFeSi) were formed in the sample annealed at 530 degrees C. However, the grain morphology of the alpha-Al matrix slightly changed after the annealing at high temperatures. The as-fabricated specimen showed a high strength above 400 MPa and a low ductility of below 10% in total elongation. The tensile ductility varied depending on the tensile direction. The annealed specimens exhibited a lower tensile strength and larger elongation, whereas the direction dependence of the tensile properties was less pronounced in the specimens annealed at higher temperatures. The anisotropic tensile ductility can be rationalized by preferential fractures occurred around melt pool boundaries.

DOI： 10.3390/cryst10111007

Formation of multiple intermetallic phases in a hypereutectic Al-Fe binary alloy additively manufactured by laser powder bed fusion Reviewed

Wang Wenyuan, Takata Naoki, Suzuki Asuka, Kobashi Makoto, Kato Masaki

INTERMETALLICS Vol. 125 2020.10

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

An attempt was made to additively manufacture an Al-Fe binary alloy sample with a hypereutectic composition of 15 wt% Fe using a laser powder bed fusion (LPBF) process. The LPBF-built Al-15Fe alloy sample exhibited a microstructure consisting of a number of melt pools in which regions had locally melted and rapidly solidified due to scanning laser irradiation during the LPBF process. The alpha-Al (fcc) matrix consists of a number of elongated grains with a mean size of approximately 10 mu m. These microstructural features correspond well to previous results of Al alloys additively manufactured by the LPBF process. It was found that relatively coarsened stable theta-Al13Fe4 phases with a length of a few micrometers were localized along the melt pool boundaries. Numerous spherical particles of a metastable Al-Fe intermetallic phase were finely distributed within the nanoscale eutectic microstructure consisting of alpha-Al and metastable Al6Fe phases inside the melt pools. The metastable phase formation corresponds well to the previous results on the rapidly solidified Al-Fe alloys. The refined multiple intermetallic phases produced by the LPBF process contribute to a high hardness of approximately 200 HV. The refined microstructure appeared stable at an elevated temperature of 300 degrees C. The high microstructural stability would sustain sufficient strength in a hostile environment for long-term periods of service at elevated temperatures above 200 degrees C. The present results were utilized to discuss the formation sequence of multiple Al-Fe intermetallic phases in rapid solidification by the LPBF process.

DOI： 10.1016/j.intermet.2020.106892

Laser powder bed fusion of a near-eutectic Al-Fe binary alloy: Processing and microstructure Reviewed

Qi Xing, Takata Naoki, Suzuki Asuka, Kobashi Makoto, Kato Masaki

ADDITIVE MANUFACTURING Vol. 35 2020.10

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER

This study focused on additive manufacturing (AM) of the Al-Fe binary alloy samples with a near-eutectic composition of 2.5 mass% Fe using the laser powder bed fusion (LPBF) process. The melt pool depth, relative density, and hardness of LPBF-fabricated Al-2.5Fe alloy samples under different laser power (P) and scan speed (v) conditions were systematically examined. The results provided optimum laser parameter sets (P = 204 W, v <= 800 mm s(-1)) for the fabrication of dense alloy samples with high relative densities > 99%. Additionally, Pv(-1/2), which is based on the deposited energy density model, was found to be a more appropriate parameter for additively manufacturing Al-2.5Fe alloy samples, and using it to simplify the relative densities of the samples made the determination of a threshold value for the laser parameters required to fabricate dense alloy samples. The microstructural and crystallographic characterization of the LPBF-built Al-2.5Fe alloy samples revealed a characteristic microstructure consisting of multi-scan melt pools that resulted from local melting and rapid solidification owing to laser irradiation during the LPBF process. Furthermore, a number of columnar grains with a mean width of similar to 21 mu m elongated along the building direction were also observed in the alpha-Al matrix. Numerous nano-sized particles of the metastable Al6Fe intermetallic phase with a mean size < 100 nm were finely dispersed in the alpha-Al matrix. The hardness of the refined microstructure produced by the LPBF process was high at similar to 90 HV, which is more than twofold higher than that of conventionally casted alloys that contain the coarsened plate-shaped Al13Fe4 intermetallic phase in equilibrium with the alpha-Al matrix.

DOI： 10.1016/j.addma.2020.101308

Manufacturing Aluminum/Multiwalled Carbon Nanotube Composites via Laser Powder Bed Fusion Reviewed International coauthorship

Lee Eo Ryeong, Shin Se Eun, Takata Naoki, Kobashi Makoto, Kato Masaki

MATERIALS Vol. 13 ( 18 ) 2020.9

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

This study provides a novel approach to fabricating Al/C composites using laser powder bed fusion (LPBF) for a wide range of structural applications utilizing Al-matrix composites in additive manufacturing. We investigated the effects of LPBF on the fabrication of aluminum/multiwalled carbon nanotube (Al/MWCNT) composites under 25 different conditions, using varying laser power levels and scan speeds. The microstructures and mechanical properties of the specimens, such as elastic modulus and nanohardness, were analyzed, and trends were identified. We observed favorable sintering behavior under laser conditions with low energy density, which verified the suitability of Al/MWCNT composites for a fabrication process using LPBF. The size and number of pores increased in specimens produced under high energy density conditions, suggesting that they are more influenced by laser power than scan speed. Similarly, the elastic modulus of a specimen was also more affected by laser power than scan speed. In contrast, scan speed had a greater influence on the final nanohardness. Depending on the laser power used, we observed a difference in the crystallographic orientation of the specimens by a laser power during LPBF. When energy density is high, texture development of all samples tended to be more pronounced.

DOI： 10.3390/ma13183927

Morphology and mechanical properties of the T-Al6Mg11Zn11 phase in the eutectic microstructure of Al-Zn-Mg ternary alloys Reviewed

Takata Naoki, Okano Taiki, Aikawa Motonari, Suzuki Asuka, Kobashi Makoto, Hagihara Koji

INTERMETALLICS Vol. 124 2020.9

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

We designed three Al-based cast alloys strengthened by T-Al6Mg11Zn11 intermetallic phases using a eutectic reaction (liquid alpha(fcc) + T) in the Al-Mg-Zn ternary system and then fabricated them using solidification processes. Thermodynamic assessment provided target alloy compositions of Al-26.5Zn-21.5 Mg (at%), Al-23.5Zn-22.5 Mg (at%), and Al-13Zn-28Mg (at%) with the alpha-Al (fcc) phase in equilibrium with the T phase with high fractions (>60%). All of the alloys exhibited regularly arrayed fibers of the alpha phase embedded in a T-phase matrix in alpha/T two-phase eutectic microstructures. A slight effect of phase compositions on the fibrous eutectic morphology was observed, whereas the volume fraction of the T phase could be controlled according to the thermodynamic assessments. The fibrous alpha phases had faceted interfaces of {111} planes with a surrounding T-phase matrix, suggesting a low interfacial energy of the alpha/T interfaces. The fabricated eutectic alloys exhibited a high Vickers hardness (>230 HV) and high yield strength at elevated temperatures above 250 degrees C. The Mg-rich alloy with a composition of Al-13Zn-28Mg (at%) exhibited a relatively lower hardness, responsible for the low hardness of the T phase with a Mg-rich composition. The Mg-rich alloy exhibited the plasticity at a lower temperature of 200 degrees C but comparatively lower strength at elevated temperatures above 250 degrees C. These results indicated that chemical composition strongly affected the mechanical properties of the T phase in the eutectic alloys. The present results provide important insights into control of the mechanical properties of Al-based eutectic alloys reinforced by the T intermetallic phase.

DOI： 10.1016/j.intermet.2020.106881

Compressive Properties of Al-Si Alloy Lattice Structures with Three Different Unit Cells Fabricated via Laser Powder Bed Fusion Reviewed International journal

Liu Xiaoyang, Sekizawa Keito, Suzuki Asuka, Takata Naoki, Kobashi Makoto, Yamada Tetsuya

MATERIALS Vol. 13 ( 13 ) 2020.7

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

In the present study, in order to elucidate geometrical features dominating deformation behaviors and their associated compressive properties of lattice structures, AlSi10Mg lattice structures with three different unit cells were fabricated by laser powder bed fusion. Compressive properties were examined by compression and indentation tests, micro X-ray computed tomography (CT), together with finite element analysis. The truncated octahedron- unit cell (TO) lattice structures exhibited highest stiffness and plateau stress among the studied lattice structures. The body centered cubic-unit cell (BCC) and TO lattice structures experienced the formation of shear bands with stress drops, while the hexagon-unit cell (Hexa) lattice structure behaved in a continuous deformation and flat plateau region. The Hexa lattice structure densified at a smaller strain than the BCC and TO lattice structures, due to high density of the struts in the compressive direction. Static and high-speed indentation tests revealed that the TO and Hexa exhibited slight strain rate dependence of the compressive strength, whereas the BCC lattice structure showed a large strain rate dependence. Among the lattice structures in the present study, the TO lattice exhibited the highest energy absorption capacity comparable to previously reported titanium alloy lattice structures.

DOI： 10.3390/ma13132902

Formation Process of Microstructure in Laser Powder Bed Fusion with WC Cemented Carbide Powder

超硬合金粉末を用いたレーザ積層造形の微視組織形成過程

Vol. 67 ( 6 ) page： 313 - 319 2020.6

IBE Hiroyuki, KATO Yuta, YAMADA Jyunya, KATO Masaki, SUZUKI Asuka, TAKATA Naoki, KOBASHI Makoto

Journal of the Japan Society of Powder and Powder Metallurgy Vol. 67 ( 6 ) page： 313 - 319 2020.6

Language：Japanese Publisher：Japan Society of Powder and Powder Metallurgy

In this paper, the microstructure formation process of WC cemented carbide manufactured by Laser Powder Bed Fusion (LPBF) was investigated with WC/25 wt%Co agglomerated and sintered powder. The internal structure of the as-built cemented carbide specimen was categorized into two regions. The first region included many pores and a unique microstructure which was not observed from the samples fabricated with conventional manufacturing methods. In this region, W2C and W3Co3C were formed due to a WC decomposition and an evaporation of graphite and Co during laser irradiation. On the other hand, the second region showed the similar microstructure to the conventional method. The microstructure of the first region revealed that the powder was completely melted during a laser scanning process. The precipitation process of W2C, W3Co3C, and Graphite phase was considered with calculated liquidus projection of a C-Co-W ternary phase diagram.

DOI： 10.2497/jjspm.67.313

Other Link： http://id.ndl.go.jp/bib/030490134

Effect of Heat Treatment on Gradient Microstructure of AlSi10Mg Lattice Structure Manufactured by Laser Powder Bed Fusion Reviewed International journal

Liu Mulin, Takata Naoki, Suzuki Asuka, Kobashi Makoto

MATERIALS Vol. 13 ( 11 ) 2020.6

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

The present study addressed the effect of heat treatment process on microstructure of an AlSi10Mg lattice structure with a body-centered cubic unit cell manufactured via laser powder bed fusion (LPBF). The as-manufactured lattice specimen exhibited a unique cellular structure composing of primary a-Al phases bounded by a-Al/Si eutectic microstructure. A gradient microstructure (continuous microstructural changes) was found in the node and strut portions composed of the lattice specimen. The microstructure appears more equiaxed and coarser with approaching the bottom surface of both portions. The continuous microstructural changes contributed to a variation in hardness measured at different locations in the as-manufactured lattice specimen. Si particles finely precipitate in the primary a-Al phases, and eutectic Si particle coarsening occurs at an elevated temperature of 300 °C. The microstructural coarsening is more pronounced at a higher temperature. A number of significantly coarsened Si particles and a stable Fe-containing intermetallic phase (b-AlFeSi) were observed at all locations in 530 °C solution-treated specimen. The homogenous microstructure results in a constant hardness value independent of the location in the lattice specimen. These results provide new insights to control the compressive properties of the AlSi10Mg lattice structure manufactured via LPBF by subsequent heat treatment processes.

DOI： 10.3390/ma13112487

Anomalous strengthening by supersaturated solid solutions of selectively laser melted Al-Si-based alloys Reviewed

Takata Naoki, Liu Mulin, Kodaira Hirohisa, Suzuki Asuka, Kobashi Makoto

ADDITIVE MANUFACTURING Vol. 33 2020.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER

To identify the dominant contributing factor in the anomalously high strength of Al-Si-based alloys fabricated by selective laser melting (SLM), microstructural characteristics of a SLM-built Al-10Si-0.3 Mg alloy (AlSi10Mg) and their changes upon annealing at elevated temperatures were investigated. The as-built AlSi10Mg alloy exhibits a peculiar microstructure comprising of a number of columnar alpha-Al (fcc) phase with concentrated Si in solution. Numerous nano-sized particles were observed within the alpha-Al matrix. At elevated temperatures, a number of Si phase (diamond structure) precipitates consumed the solute Si in the columnar alpha-Al phase, but the microstructure of the alpha-Al matrix changed slightly. After annealing at elevated temperatures, the tensile strength of the as-built AlSi10Mg alloy substantially decreased accompanied by a reduction in the strain hardening rate. The supersaturated solid solution of the alpha-Al phase containing numerous nano-sized particles enhanced the strain hardening, resulting in the anomalous strengthening of the SLM-built AlSi10Mg alloy. The microstructural features were formed due to rapid solidification at an extremely high cooling rate in the SLM process, which provides important insights into controlling the strength of Al-Si-based alloys fabricated by SLM.

DOI： 10.1016/j.addma.2020.101152

Microstructure and compressive properties of porous hybrid materials consisting of ductile Al/Ti and brittle Al3Ti phases fabricated by reaction sintering with space holder Reviewed

Suzuki Asuka, Kosugi Naoki, Takata Naoki, Kobashi Makoto

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 776 2020.3

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

The strain-hardening of cell wall in porous metals decreases the energy absorption properties. In order to suppress the strain hardening, porous hybrid materials consisting of ductile Al and Ti phases and brittle Al3Ti phase were synthesized by reaction sintering between Al and Ti powders with NaCl space holder. Changes in the porous structure and microstructure with sintering were investigated. Area fraction of Al3Ti phase formed at Al/Ti interface increased following the Johnson-Mehl-Avrami-Koromogrov (JMAK) equation of three-dimensional diffusion-controlled growth with zero nucleation rate. The growth of Al3Ti phase led to the formation of pores and cracks in cell wall by Kirkendall effect and volume shrinkage. Effect of microstructure on compressive properties of the porous Al/Al3Ti/Ti hybrids were also investigated. Porous hybrids consisting of α-Al matrix exhibited the plateau region with positive slopes due to the strain hardening of α-Al phase. Porous hybrids consisting of Al3Ti matrix exhibited the plateau region with the slope of almost zero, resulting in high energy absorption capacity and efficiency. Fractography revealed that crack propagated in brittle Al3Ti phase but arrested at α-Al phase. These results were used to discuss the microstructure for improving energy absorption properties.

DOI： 10.1016/j.msea.2020.139000

In Situ Scanning Electron Microscopy Observation of Crack Initiation and Propagation in Hydroxide Films Formed by Steam Coating on Aluminum-Alloy Sheets Reviewed International journal

Li Hongmei, Takata Naoki, Kobashi Makoto, Serizawa Ai

MATERIALS Vol. 13 ( 5 ) 2020.3

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

Hydroxide film was formed on 6061 Al-alloy (Al-1.00Mg-0.62Si(wt.%)) sheets by steam coating with the temperature of 220 °C for 24 h. During bending test of the coated specimens, the crack initiation and propagation processes in the hydroxide film were investigated using in situ SEM observations. The hydroxide film formed exhibited a dual-layer structure composed of an inner amorphous layer and an outer polycrystalline γ-AlO(OH)-phase layer. On the compressively strained surface, lateral cracks are preferentially initiated inside the inner amorphous layer, and propagate either inside this layer or on its interface with the outer γ-AlO(OH) layer. Digital image correlation analyses of the in situ observed SEM images suggested that the concentrated tensile strain along the surface normal localized at some parts of the amorphous layer could contribute to the crack initiation. On the tensile-strained surface, a number of cracks were initiated inside the inner amorphous layer along the surface normal and propagate into the outer γ-AlO(OH) layer. No cracks were found along the interface of the amorphous layer with the Al-alloy substrate. As a result, the anticorrosion hydroxide film adhered on the Al sheet after bending deformation. Such strong adhesion contributes to the excellent corrosion resistance of the Al-alloy parts provided by the steam coating.

DOI： 10.3390/ma13051238

Effect of hot-press thermal history on joint strength of A5052/Polyamide-6 hybrid structure via a porous layer Reviewed

Kim Seung-Gwang, Suzuki Asuka, Takata Naoki, Kobashi Makoto

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY Vol. 276 2020.2

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

The influence of the joining temperature history on the mechanical behavior of an Al alloy (A5052)/Polyamide-6 (PA6) lap joint was investigated. PA6 is a semi-crystalline thermoplastic and its mechanical behavior depends on its crystalline properties, which are affected by thermal processing. The A5052/PA6 joining process was carried out at 215 °C using a hydraulic hot-press. The temperature was decreased from 215 °C to various pre-set temperatures at a cooling rate of about 2 °C/s followed by air cooling under the application of pressure. The crystallinity of PA6 and strength of the lap joint increased with a decrease in the joining temperature from 215 to 190 °C and remained constant at temperatures below 190 °C. The thermoanalytical examination revealed that the crystallinity increase was caused by slow cooling. The results demonstrated that slow cooling is an effectual approach to improve the bonding strength. The fractography analysis results showed that the fracture mode of PA6 changed from ductile to brittle with an increase in its crystallinity.

DOI： 10.1016/j.jmatprotec.2019.116388

Anomalous size-dependent strength in micropillar compression deformation of commercial-purity aluminum single-crystals

Naoki Takata, Soichiro Takeyasu, Hongmei Li, Asuka Suzuki, Makoto Kobashi

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 772 2020.1

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

An anomalously reduced size-dependent strength of commercial-purity aluminum (Al) single-crystal micropillars with diameters ranging approximately from 1 to 10 mu m is reported. High-purity Al (99.99%) single-crystal micropillars exhibited an obvious size dependence of the resolved shear stress for slip. The measured shear stress resolved onto a primary slip system (tau(i)) scaled by the shear modulus (G) and the pillar diameter (d) scaled by the Burgers vector (b) showed the following correlation: tau(i)/G = 0.33(d/b)(-0.63), which agreed well with previous works. However, the commercial-purity Al samples exhibited a lower power-law exponent (0.19) for their size-dependent strength, resulting in (tau(i)/G) = 0.006(d/b)(-0.19). TEM characterization revealed the local presence of Al-Fe intermetallic precipitates surrounded by relatively high-density dislocations in annealed commercial-purity Al samples. These results indicate the relatively high-density dislocations could be responsible for the reduced size-dependent strength, which was confirmed by the remarkably reduced size dependence of their resolved shear stress by prior cold rolling. The reduced size-dependent strength can be rationalized using the stochastic model of the dislocation source length. Thus, this study provides a new insight to allow the application of the micropillar compression test to commercially produced Al alloys.

DOI： 10.1016/j.msea.2019.138710

Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting

Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Journal of Materials Science and Technology Vol. 36 page： 106 - 117 2020.1

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

To identify the microstructural features of the lattice structures of Al alloys built via the selective laser melting (SLM) process, AlSi10Mg alloy with a body-centered cubic (BCC)-type lattice structure was prepared. Characteristic microstructures comprising melt pools with several columnar α-Al phases with <001 > orientations along the elongation direction and surrounded by eutectic Si particles were observed at all portions of the built lattice structure. In the node portions of the lattice structure, a gradient microstructure (continuous change in microstructure) was observed. The columnar α-Al phases were observed near the top surface of the node portion, whereas they became coarser and more equiaxed near the bottom surface, resulting in softening localized near the bottom surface. In the strut portions of the lattice structure, the columnar α-Al phases were elongated along the inclined direction of struts. This trend was more prevalent near the bottom surface. The α-Al phases became coarser and more equiaxed near the bottom surface as well. The aforementioned results were the basis of a discussion of the development of the gradient microstructure in lattice-structured Al alloys during the SLM process in terms of thermal conductivities at the boundaries between the manufactured (locally melted and rapidly solidified) portions and adjacent (unmelted) alloy powder.

DOI： 10.1016/j.jmst.2019.06.015

Influence of Brazing Time on Liquid Migration and Cell Structure of Porous Aluminum

Tomohito Kurosaki, Makoto Kobashi, Naoki Takata, Asuka Suzuki, Tadashi Minoda

MATERIALS TRANSACTIONS Vol. 61 ( 2 ) page： 368 - 374 2020

Strain Rate Sensitivity of Flow Stress Measured by Micropillar Compression Test for Single Crystals of 18Cr Ferritic Stainless Steel

Tianqi Zhu, Hongmei Li, Naoki Takata, Makoto Kobashi, Masataka Yoshin

ISIJ INTERNATIONAL Vol. 60 ( 4 ) page： 774 - 781 2020

Language：English Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

We have fabricated cylindrical single-crystal micropillars with different diameter (d) ranges of 2-3 mu m and 5-6 mu m on a specific grain in the 18Cr ferritic stainless steel with a ferrite single-phase microstructure. The initial strain rate at the onset of plastic deformation was controlled by variable loading rate in the used nanoindenter. The strain rate sensitivity of the stress required for the slip initiation were examined using the fabricated micropillars. The present compression tests addressed the shear stress on an activated single slip system of micron-scale single-crystals. Smaller-sized micropillars (d = 2-3 mu m) often exhibit intermittent strain bursts. The stress for slip initiation (after an elastic loading) changes depending on the initial strain rate, resulting in a high strain rate sensitivity (m) of 0.12. Larger-sized micropillars (d = 5-6 mu m) show a continuous yielding. A slight change in their yield stress depending on the initial strain rate provides a relatively low m of 0.04. It is similar to one of the millimeter-sized specimens measured by conventional tensile tests. These results provide new insights to optimize the specimen size for the micro pillar compression test applied for the 18Cr ferritic stainless steels.

DOI： 10.2355/isijinternational.ISIJINT-2019-448

Preparation and Magnetic Properties of Nano-Sized Iron Powder Particles Coated with Silica Film by Calcium Hydride Reduction Method

Matsumoto, A; Sugiura, T; Kobashi, M; Yamamoto, S

MATERIALS TRANSACTIONS Vol. 61 ( 7 ) page： 1404 - 1408 2020

Adhesion properties of hydroxide films formed on the Al-Mg-Si aluminum alloy sheets prepared by steam coating process

Hongmei Li, Naoki Takata, Makoto Kobashi, Ai Serizawa

Keikinzoku Yosetsu/Journal of Light Metal Welding Vol. 58 page： 102 - 106 2020

Publishing type：Research paper (scientific journal)

© 2020 Japan Light Metal Welding Association In this study, in order to evaluate the adhesion properties of the hydroxide film formed on 6061 alloy (Al-1Mg- 0.6Si (wt.％)) sheets prepared by steam coating, we examined the microstructure and cracking behavior of hydroxide film on Al alloy sheets prepared by steam coating at 220°C for 24 h. EBSD analyses revealed the steam-coated sample exhibits microstructure of a-Al matrix consisted of equiaxed grains with a mean grain size of approximately 20 mm. The studied sample exhibits high hardness of 79 HV due to the strengthening by the precipitates of nanoscale particles (Mg2Si). The observation for the surface morphologies revealed the hydroxide film consisted of a number of particles with a mean size of approximately 300 nm and cross-sectional microstructural observations identified a continuous hydroxide film with a thickness of approximately 1.2 mm. The XRD measurement demonstrated the formed hydroxide film consisted of the g-AlO(OH) phase. Minute microstructure analyses revealed that the hydroxide film exhibits a dual layer structure consisting of g-AlO(OH) crystalline layer on the surface and a continuous Al-rich layer on the Al alloy. The cracking and delamination behavior of the hydroxide film were examined by bending test. It was found that the local delamination of g-AlO(OH) phase would occur under compressive stress rather than tensile stress, whereas the delamination of the hydroxide film from the Al alloy substrate was not observed. These results indicate that the hydroxide film prepared by steam coating has a remarkable adhesion strength on the Al alloy substrate.

DOI： 10.11283/jlwa.58.102s

Topology optimization and characterization of Ti6A14V ELI cellular lattice structures by laser powder bed fusion for biomedical applications

アルミニウム合金粉末のレーザ積層造形に伴う組織形成の理解に向けて

Vol. 66 ( 12 ) page： 573 - 580 2019.12

アルミニウム合金粉末のレーザ積層造形に伴う組織形成の理解に向けて

Vol. 66 ( 12 ) page： 573 - 580 2019.12

Vilardell, AM; Takezawa, A; du Plessis, A; Takata, N; Krakhmalev, P; Kobashi, M; Yadroitsava, I; Yadroitsev, I

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 766 2019.10

Effects of Heat Treatments on Compressive Deformation Behaviors of Lattice‐Structured AlSi10Mg Alloy Fabricated by Selective Laser Melting

Asuka Suzuki, Keito Sekizawa, Mulin Liu, Naoki Takata, Makoto Kobashi

Advanced Engineering Materials Vol. 21 ( 10 ) page： 1900571 - 1900571 2019.10

Synthesis and structural control of Fe-based porous layer on Fe substrate for joining with resin parts using combustion reaction

Asuka Suzuki, Kazuki Noritake, Naoki Takata, Makoto Kobashi

Advanced Powder Technology Vol. 30 ( 10 ) page： 2101 - 2109 2019.10

レーザ積層造形技術が生み出すアルミニウム合金造形体の特異な組織形成

Vol. 91 ( 9 ) page： 612 - 617 2019.9

レーザ積層造形技術が生み出すアルミニウム合金造形体の特異な組織形成

Vol. 91 ( 9 ) page： 612 - 617 2019.9

CaH2還元によるSiO2被覆Feナノ粒⼦の合成と磁気特性

Vol. 66 ( 9 ) page： 429 - 433 2019.9

CaH2還元によるSiO2被覆Feナノ粒⼦の合成と磁気特性

Vol. 66 ( 9 ) page： 429 - 433 2019.9

Microstructure and cracking behavior of hydroxide films formed on aluminum-alloy sheets prepared by steam coating

Hongmei Li, Naoki Takata, Makoto Kobashi, Ai Serizawa

Materials Science and Engineering A Vol. 764 2019.9

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

© 2019 Elsevier B.V. To elucidate the adhesion properties of anticorrosion hydroxide films formed on aluminum (Al)-alloy sheets prepared by the steam coating process, cracks that propagated inside the hydroxide film formed on Al–Mg–Si alloy (6061 grade) sheets were characterized. Continuous hydroxide films with different thicknesses of 0.7 μm and 1.4 μm formed at different temperatures of 180 °C and 200 °C, respectively. Microstructural characterizations revealed that these hydroxide films exhibited a dual-layer structure consisting of a polycrystalline γ-AlO(OH) layer (on the surface side) and a continuous amorphous layer (on the Al-alloy side). Bending tests caused numerous cracks inside the hydroxide films under tension and compression. On the tensile-strained surface, numerous vertical cracks penetrated through the hydroxide films, although their delamination was not observed. On the compressive-strained surface, local delamination of the upper γ-AlO(OH) layer occurred. However, cracks were not observed in the lower amorphous layer on the Al-alloy substrate deformed under compression. These results indicate that the dual-layer-structured hydroxide film prepared by steam coating exhibits remarkable adhesion to the Al-alloy substrate.

DOI： 10.1016/j.msea.2019.138247

Joining of metals and polymers using powder metallurgy with laser irradiation

Seung-Gwang Kim, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Materials Processing Technology Vol. 270 page： 1 - 7 2019.8

Design of laser parameters for selectively laser melted maraging steel based on deposited energy density

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

Additive Manufacturing Vol. 28 page： 160 - 168 2019.8

Formation of Fe2Al5 Phase Layer on Fe-Si Alloy Sheets Hot-Dipped in Zn-0.2Al Alloy Melt

Naoki Takata, Kunihisa Hayano, Asuka Suzuki, Makoto Kobashi

Tetsu-to-Hagane Vol. 105 ( 7 ) page： 701 - 708 2019.7

Formation sequence of Fe–Al intermetallic phases at interface between solid Fe and liquid Zn–6Al–3Mg alloy

Hiroki Yokoi, Naoki Takata, Asuka Suzuki, Makoto Kobashi

Intermetallics Vol. 109 page： 74 - 84 2019.6

Effect of powder blending ratio on porous structure formed by combustion synthesis reaction between Ni and Al together with space holder Reviewed

Yunmao Shu, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Functionally Graded Materials Vol. 33 page： 38 - 43 2019.5

ろう付用Al–Si系合金材を用いたポーラスアルミニウムの接合におけるポーラスアルミニウムの気孔構造変化と液相の挙動

Vol. 69 ( 2 ) page： 93 - 100 2019.2

ろう付用Al–Si系合金材を用いたポーラスアルミニウムの接合におけるポーラスアルミニウムの気孔構造変化と液相の挙動

Vol. 69 ( 2 ) page： 93 - 100 2019.2

Fabrication of porous NiAl intermetallic compounds with a hierarchical open-cell structure by combustion synthesis reaction and space holder method Reviewed

Yunmao Shu, Asuka Suzuki, Naoki Takata, Makoto Kobashi

Journal of Materials Processing Technology Vol. 264 page： 182 - 189 2019.2

Transmission Electron Microscopic Observation of Layered Double Hydroxide Films Formed on Aluminum Alloys Prepared by Steam Coating Process

Takata Naoki, Li Hongmei, Kobashi Makoto, Shimada Yuta, Serizawa Ai, Ishizaki Takahiro

Tetsu-to-Hagane Vol. 105 ( 2 ) page： 177 - 182 2019.2

Language：Japanese Publisher：The Iron and Steel Institute of Japan

Microstructures and crystallographic features of layered double hydroxide (LDH) particles and the LDH film formed on the aluminum alloy sheet (alloy 7075) using the steam coating process were examined by transmission electron microscopy (TEM). The LDH powder particles exhibited a hexagonal-disc-shape morphology. Their disc plane and side planes were parallel to a basal plane and prism planes of the LDH hexagonal crystal, respectively. A cross-sectional thin TEM sample was prepared from the interface between the LDH film and the Al alloy substrate using a focused ion beam combined with pick-up technique. TEM observations revealed that the LDH film formed on the Al alloy exhibited a dual-layer structure, which consists of a continuous Zn-rich amorphous layer (including numerous nano-particles with fcc structure) on the Al substrate side and a LDH layer composed of a number of disc-shaped LDH particles.

DOI： 10.2355/tetsutohagane.TETSU-2018-064

Synthesis of porous Al/Al3Ti composite with hierarchical open–cell structure for combining with phase change material Reviewed

Asuka Suzuki, Shohei Miyake, Wataru Naruse, Naoki Takata, Makoto Kobashi

Journal of Alloys and Compounds Vol. 770 page： 1100 - 1111 2019.1

Microstructure and strength of a novel heat-resistant aluminum alloy strengthened by T-Al6Mg11Zn11 phase at elevated temperatures Reviewed

Naoki Takata, Masato Ishihara, Asuka Suzuki, Makoto Kobashi

Materials Science & Engineering A Vol. 739 page： 62 - 70 2019.1

Microstructure and Mechanical property of Porous Nickel aluminides Fabricated by Reactive Synthesis with Space Holder Powder

Yunmao Shu, Asuka Suzuki, Naoki Takata, Makoto Kobashi

MRS Advances Vol. 4 ( 25-26 ) page： 1515 - 1521 2019

Tensile Properties of a Heat-Resistant Aluminium Alloy Strengthened by T-Al6Mg11Zn11 Intermetallic Phase

Satoshi Nakatsuka, Masato Ishihara, Naoki Takata, Asuka Suzuki, Makoto Kobashi

MRS Advances Vol. 4 ( 25-26 ) page： 1485 - 1490 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

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

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

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

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

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

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

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

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

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

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

Implementation of DSC analysis in reaction kinetics during heating of Ti-50 at.%Al powder mixture Reviewed

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

金属粉末レーザ積層造形法によって作製されたAL-10Si-0.4Mg合金の組織と機械的性質 Invited Reviewed

高田尚記, 小平寛久, 関沢圭人, 鈴木飛鳥, 小橋眞

軽金属 Vol. 67 ( 11 ) page： 582-588 2017.11

化学反応を利用したアルミニウム基複合材料の製造方法 Invited Reviewed

小橋　眞、高田尚記、鈴木飛鳥

軽金属 Vol. 67 ( 11 ) page： 571-575 2017.11

Solid Fe/Liquid Zn Interfacial Reaction of Hot-Dip Galvanized Fe-Si Alloy Sheets Invited Reviewed

Naoki Takata, Kunihisa Hayano, Asuka Suzuki, Makoto Kobashi

Proceedings of GALVATEC2017 (11th International Conference on Zinc and Zinc Alloy Coated Steel Sheet) page： 177-181 2017.11

金属粉末レーザ積層造形法によって作製されたAL-10Si-0.4Mg合金の組織と機械的性質 Invited Reviewed

高田尚記, 小平寛久, 関沢圭人, 鈴木飛鳥, 小橋眞

軽金属 Vol. 67 ( 11 ) page： 582-588 2017.11

化学反応を利用したアルミニウム基複合材料の製造方法 Invited Reviewed

小橋眞, 高田尚記, 鈴木飛鳥

軽金属 Vol. 67 ( 11 ) page： 571-575 2017.11

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

A. Takezawa, Y. Koizumi, M. Kobashi

Additive Manufacturing Vol. 18 page： 194-202 2017.10

Induction-Activated Self-propagating, High-temperature Synthesis of Nickel Aluminide

M. Shekari, M. Adeli, A. Khobzi, M. Kobashi, N. Kanetake

Advanced Powder Technology Vol. 28 page： 2974-2979 2017.9

Induction-Activated Self-propagating, High-temperature Synthesis of Nickel Aluminide

M. Shekari, M. Adeli, A. Khobzi, M. Kobashi, N. Kanetake

Advanced Powder Technology Vol. 28 page： 2974-2979 2017.9

Change in Microstructure of Selectively Laser Melted AlSi10Mg Alloy with Heat Treatments Reviewed

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

Materials Science & Engineering A Vol. 704 page： 218-228 2017.8

Change in Microstructure of Selectively Laser Melted AlSi10Mg Alloy with Heat Treatments Reviewed

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

Materials Science & Engineering A Vol. 704 page： 218-228 2017.8

燃焼合成反応を利用したFe/TiB2複合材料の合成とポーラス化 Invited Reviewed

伊藤峻, 髙田尚記, 小橋眞

粉体および粉末冶金 Vol. 64 ( 6 ) page： 288-294 2017.6

燃焼合成反応を利用したFe/TiB2複合材料の合成とポーラス化 Invited Reviewed

伊藤峻, 髙田尚記, 小橋眞

粉体および粉末冶金 Vol. 64 ( 6 ) page： 288-294 2017.6

Compressive properties of porous Ti-Al alloys fabricated by reaction Reviewed

N. Takata, K. Uematsu, M. Kobashi

Materials Science & Engineering A Vol. 697 page： 66-70 2017.5

M. Adeli, S. H. Seyedein, M. R. Aboutalebi, M. Kobashi, N. Kanetake

JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY Vol. 128 ( 2 ) page： 867 - 874 2017.5

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

In this study, the reactions in Ti-50 at.%Al powder mixture upon heating at constant heating rates were studied using DSC analysis. Heating up the mixture resulted in melting of aluminum at temperatures close to its melting temperature, which was manifested as an endothermic peak in DSC curves. The melting of aluminum was the onset of an intense exothermic reaction, so-called combustion synthesis reaction which results in the formation of titanium aluminide. The shift in exothermic peak temperature in various linear heating rates was used to calculate the apparent activation energy according to Kissinger-type model-free methods. Heating rates of 10, 20, 30, 40, and 50 K min(-1) were used to estimate the activation energy based on DSC data. In order to study the effect of ball-milling on reaction behavior, the starting powder mixture was ball-milled for different times, and a comparison was made between non-ball-milled and ball-milled DSC curves. The results showed that ball-milling tends to diminish the endothermic peak of melting of aluminum and shift the exothermic reaction temperature to lower temperatures, apparently altering the mechanism from a solid-liquid to an almost solid-solid one. The activation energy of the process in the non-ball-milled state was found to be close to the activation energy of diffusion of aluminum in TiAl (220 kJ mol(-1)). However, the application of same model-free methods to the ball-milled samples showed unexpectedly increased values for activation energy. It would be appropriate to check other methods as well as for calculation of activation energy in the ball-milled systems.

DOI： 10.1007/s10973-016-5982-9

Porous metal produced by selective laser melting with effective isotropic thermal conductivity close to the Hashin-Shtrikman bound Reviewed

Compressive properties of porous Ti-Al alloys fabricated by reaction Reviewed

N. Takata, K. Uematsu, M. Kobashi

Materials Science & Engineering A Vol. 697 page： 66-70 2017.5

粉末冶金法を利用したポーラス金属の製造プロセスおよび気孔構造制御 Invited Reviewed

小橋　眞

粉体および粉末冶金 Vol. 64 ( 2 ) page： 61-67 2017.3

粉末冶金法を利用したポーラス金属の製造プロセスおよび気孔構造制御 Invited Reviewed

小橋眞

粉体および粉末冶金 Vol. 64 ( 2 ) page： 61-67 2017.3

Porous metal produced by selective laser melting with effective isotropic Reviewed

A. Takezawa, M.Kobashi, Y. Koizumi, M. Kitamura

International Journal of Heat and Mass Transfer Vol. 105 page： 564-572 2017.2

Akihiro Takezawa, Makoto Kobashi, Yuichiro Koizumi, Mitsuru Kitamura

INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER Vol. 105 page： 564 - 572 2017.2

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Additive manufacturing may be a novel method for fabricating porous materials. These materials can achieve effective performance because of their internal geometries. Metal-additive manufacturing is expected to utilize thermal conduction materials and devices. We have developed a porous metal with effective isotropic thermal conductivity by using metal-selective laser melting additive manufacturing. The internal pore structure was designed by topology optimization, which is the most effective structural optimization technique to maximize effective thermal conductivity. The designed structure was converted to a three-dimensional STL model, which is a native digital format of additive manufacturing, and assembled as a test piece. Effective thermal conductivity was measured by a steady-state method in which the effective thermal conductivity was calculated from a one-dimensional temperature gradient and the heat flux of the test pieces. The test pieces showed an effective thermal conductivity close to the Hashin-Shtrikman or Maxwell-Eucken bound, which is the theoretical limit of effective performance with an error less than 10%. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijheatmasstransfer.2016.10.006

Porous Ti-Al Intermetallic Based Alloys Fabricated by Pressure-Sintering Elemental Powders with a Space Holder Powder Reviewed

N.Takata, K.Uematsu, M.Kobashi

MRS advances Vol. 2 ( 26 ) page： 1387-1392 2017.1

Porous Ti-Al Intermetallic Based Alloys Fabricated by Pressure-Sintering Elemental Powders with a Space holder Powder Reviewed

Naoki Takata, Keisuke Uematsu, Makoto Kobashi

MRS ADVANCES Vol. 2 ( 26 ) page： 1387 - 1392 2017

Language：English Publishing type：Research paper (scientific journal) Publisher：CAMBRIDGE UNIV PRESS

In the present study, we have attempted to produce the porous Ti-Al intermetallic based alloys by pressure-sintering of the elemental powders of Ti and Al through a combustion reaction, with a NaCl space holder to achieve the development of a highly porous structure. The powders of Ti (particle size: <45 pm) and Al (particle size: <45 pm) were mixed to various total compositions of Ti-0-40 Al (at.%), and NaCl (particle size: 300-400 mu m) was mixed with the Ti/Al powder to give a highly porous structure. The mixed powder was sintered at 700 degrees C: under a constant pressure of 30 MPa to form cylindrical specimens with a diameter of 20 mm. The soaking of the specimens in pure water allows for the removal of NaCl (with a volume fraction of 60 %) and the formation of the pores to achieve a controlled relative density of 0.4. The present process can produce porous Ti Al alloys with various compositions. In as-sintered Ti-AI specimens, an inhomogeneous microstructure consisting of Ti particles surrounded by the a Ti Al alloy layer (various Ti-Al intermetallic phases) was observed. The as-sintered specimens exhibited the brittle behavior under compression, which is independent of their Al content. However, in a Ti-20Al alloy heat-treated at 1300 degrees C, a nearly Ti3Al single-phase microstructure appeared in the porous specimen. The heat-treated porous Ti-20Al alloy shows both a high plateau-strength level of approximately 100 MPa and a high plateau-end strain over 50%, resulting in a high absorption energy. The present results demonstrate that controlling the microstructure for the formation of the Ti(3)AI single-phase would he an effective method to achieve high energy absorption capacity of the porous Ti Al alloy.

DOI： 10.1557/adv.2017.123