Faculty Profiles - KOBASHI, Makoto

Papers - KOBASHI, Makoto

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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

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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

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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

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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

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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

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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

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PubMed

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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

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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

CiNii Books

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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

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

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

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Papers - KOBASHI, Makoto