Faculty Profiles - KOBASHI, Makoto

Papers - KOBASHI, Makoto

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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

Web of Science

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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

Scopus

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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

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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

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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

Web of Science

PubMed

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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

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Papers - KOBASHI, Makoto