Faculty Profiles - KOBASHI, Makoto

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

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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