Faculty Profiles - KOBASHI, Makoto

Papers - KOBASHI, Makoto

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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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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

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 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

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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

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

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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

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

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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

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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

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

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

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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

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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

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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

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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

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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

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Papers - KOBASHI, Makoto