Faculty Profiles - KOBASHI, Makoto

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

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

Otani, Y; Saki, K; Takata, N; Suzuki, A; Kobashi, M; Kato, M

JOURNAL OF ALLOYS AND COMPOUNDS Vol. 977 page： 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

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

JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T Vol. 29 page： 1535 - 1541 2024.3

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