Latest 30 articles

2025.04.10

Research on improving the creep properties of Ti alloys has been published in Journal of Metallurgical and Materials Transactions A.

Prince Valentine Cobbinah*, Sae Matsunaga, Yoshiaki Toda, Ryosuke Ozasa, Takuya Ishimoto, Takayoshi Nakano, Tsutomu Ito, Yoko Yamabe-Mitarai:*
On the enhanced creep performance in Ti6246 achieved through Laser Powder Bed Fusion (LPBF) processing,
Metallurgical and Materials Transactions A, (2025), in press.
DOI: https://doi.org/10.1007/s11661-025-07759-8

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Abstract
The high susceptibility of the Ti-6Al-2Sn-4Zr-6Mo wt pct (Ti6246) alloy to microstructural changes stands as a challenge when processed by the laser powder bed fusion (LPBF) technology. However, leveraging the capabilities of the LPBF process to successfully control the microstructure (and/or crystallographic texture) of the Ti6246 could improve mechanical properties, particularly at elevated temperatures. In this study, the creep performance (at 500 °C) of Ti6246 fabricated from three different LPBF processing conditions and heat-treated (HT) at 885 °C were investigated. In the as-built state, all the LPBFed-Ti6246 exhibited columnar microstructures with crystallographic lamellar-like microstructure (CLM), a near-single crystal-like microstructure (SCM), and polycrystalline microstructure (PCM) textures, respectively. At low applied stresses (100-300 MPa), diffusional creep was the dominant deformation mechanism and its resistance depended on grain size. The reference β-forged-HT Ti6246, characterized by large equiaxed grains, exhibited the lowest strain rate compared to the columnar microstructure of SX1 (CLM)-HT, SX2 (SCM)-HT, and SX3 (PCM)-HT. Conversely, dislocation slip governed deformation at high applied stresses (400-580 MPa) and its efficacy depended on the α/β interfaces in the microstructures. Disjointed columnar grains in SX1 (CLM)-HT and the deformation of the polycrystalline grains in SX3 (PCM)-HT indicated that the melt pool boundaries were unstable in the LPBFed-Ti6246. SX2 (SCM)-HT exhibited the longest creep life due to the relatively stable melt pool boundaries and the near < 001 > SCM crystallographic texture parallel to the applied stresses. Shallow ductile dimples and tears and the observation of laser scan tracks characterized the fracture surfaces of the LPBFed-Ti6246. These indicated that failure occurred by intergranular ductile fracture resulting from the formation of microvoids at the melt pool boundaries.

2025.03.18

Research on computer simulation of AM has been published in Journal of the Japan Society of Powder and Powder Metallurgy.

Shuhei Mino, Masayuki Okugawa, Takayoshi Nakano, Yuichiro Koizumi:
Raking and Fusing Behaviors during Fabrication of Multiple-layers in Powder Bed Fusion: An integrated discrete Element and computational thermal fluid Dynamics Study
Journal of the Japan Society of Powder and Powder Metallurgy, 72, (2025), 16P-T6-11; S1465-S1469.
https://doi.org/10.2497/jjspm.16P-T6-11

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Abstract
In the powder bed fusion (PBF) type additive manufacturing (AM), understanding the relationship between the quality of the powder bed and the powder spreading process is crucial to avoiding defect formation. In this study, we investigated the powder-raking behavior during the multiple-layer fabrication process by discrete element method (DEM) and computational thermal-fluid dynamics (CtFD) simulations. The integrated PBF process simulation revealed that the gap height between the powder spreading blade and the build platform increases nonlinearly with the number of stacking layers, and accordingly, the powder-covered area ratio increases in the formed powder beds and affects the melt pool shapes. The powder raking behavior and melting and solidification behavior are related to each other, and both the powder raking and the irradiation conditions need to be optimized to obtain a high-quality part in the PBF process.

Keywords
additive manufacturing; powder bed fusion; discrete element method; computational thermal-fluid dynamics

2025.03.15

A paper on Young's modulus prediction for Ti alloys was published as an OA article in Additive Manufacturing (IF=10.3).

Shota Higashino, Daisuke Miyashita , Takuya Ishimoto, Eisuke Miyoshi, Takayoshi Nakano, Masakazu Tane*:
Low Young's modulus in laser powder bed fusion processed Ti-15Mo-5Zr-3Al alloys achieved by the control of crystallographic texture combined with the retention of low-stability bcc structure,
Additive Manufacturing, 102 (2025), 104720; 1-13.
https://doi.org/10.1016/j.addma.2025.104720

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Abstract
Metastable β (body-centered cubic)-phase Ti alloys, quenched from a high-temperature β-phase field, have attracted great interest as biomedical implants, owing to their low Young's modulus. Recently, the application of additive manufacturing (AM) to β-phase Ti alloys has gathered much attention, because the AM process can form anisotropic crystallographic texture in which an elastically soft direction is preferentially oriented, resulting in low Young's modulus in a specific direction. However, the effects of anisotropic texture and microstructure formed by the AM process on anisotropic elastic properties have not been clarified in detail. In the present study, we measured all the independent elastic stiffness components of β-phase Ti-15Mo-5Zr-3Al (mass%) alloys, prepared by bidirectional scanning with (XY-scan) and without (X-scan) an interlayer rotation of 90° in laser powder bed fusion (LPBF), one of the AM processes, using resonant ultrasound spectroscopy. The measurements revealed that the LPBF-processed Ti alloys exhibited strong elastic anisotropy and a low Young's modulus (below 60 GPa) in the <100>-oriented direction of the alloy prepared by the XY-scan. Furthermore, micromechanics calculations based on Eshelby's inclusion theory revealed that the single crystal constituting the alloys prepared by LPBF had almost the same elastic stiffness as that of a single crystal prepared by the floating zone melting, which indicated that the metastable β phase was retained by suppressing an easily occurring β- to ω-phase transformation during LPBF. These results indicate that texture control combined with retention of the metastable β phase by LPBF achieves biocompatible low Young's modulus.

Keywords
Laser powder bed fusion; Elastic properties; Titanium alloys; Crystallographic texture; ω phase transformation

2025.03.15

The one-process realization of alloying, microstructure control, and shape control by metal 3D printers was published in Materials & Design.

Yong Seong Kim, Ozkan Gokcekaya*, Kazuhisa Sato, Ryosuke Ozasa, Aira Matsugaki, Takayoshi Nakano*:
In-situ alloying of nonequiatomic TiNbMoTaW refractory bio-high entropy alloy via laser powder bed fusion: Achieving suppressed microsegregation and texture formation,
Materials & Design, 252, (2025), 113824; 1-18.
https://doi.org/10.1016/j.matdes.2025.113824

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Abstract
High-entropy alloys (HEAs) have attracted considerable attention owing to their excellent properties. However, the severe segregation of the constituent elements remains a common challenge in refractory HEAs. Recently, an approach to suppress segregation was proposed using laser powder bed fusion (LPBF) owing to the ultra-high cooling rates during solidification. Despite the advantages of LPBF, the persistent microsegregation between the dendritic and interdendritic regions of refractory HEAs and costly gas atomization process hinder the further development. To address these challenges, a novel nonequiatomic TiNbMoTaW refractory HEA was designed to minimize the difference between the liquidus and solidus temperatures to prevent segregation and phase separation for a better biological performance. In-situ alloying was implemented instead of costly and time-consuming gas atomization process. The segregation of constituent elements was suppressed by remelting, resulted in epitaxial growth and development of crystallographic texture, consequently reducing residual stress. The mechanical properties were improved due to the increase of solid solution strengthening and densification. It showed superior mechanical strength and equivalent biocompatibility compared to conventional biomaterials, indicating its superiority as a biomaterial. This study represents the first successful control of crystallographic texture through in-situ alloying of BioHEAs for next-generation biomaterials.

Keywords
High entropy alloys; Additive manufacturing; In-situ alloying; Crystallographic texture; Segregation

 

 

2025.03.14

The results of joint research on AM with Abe Lab. at the University of Tokyo were published in Additive Manufacturing (IF=10.3).

Han Chen, Daisuke Egusa*, Zehao Li, Taisuke Sasaki, Ryosuke Ozasa , Takuya Ishimoto, Masayuki Okugawa, Yuichiro Koizumi, Takayoshi Nakano, Eiji Abe*
Phase-separation induced dislocation-network cellular structures in Ti-Zr-Nb-Mo-Ta high-entropy alloy processed by laser powder bed fusion,
Additive manufacturing, 102, (2025), 104737; 1-14.
https://doi.org/10.1016/j.addma.2025.104737

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Abstract
Hierarchical structures, such as cellular structures, elemental segregations, and dislocation-network, are often proposed to enhance the mechanical properties of high-entropy alloys (HEAs) fabricated via additive manufacturing (AM). The formation of cellular structures is often attributed to elemental segregation during the solidification process or thermal strain resulting from the AM process. Here, we present a novel cellular structure where phase-separation and dislocation-network coupled in Ti-Zr-Nb-Mo-Ta HEA processed by laser powder bed fusion (L-PBF). Electron microscopy observations and X-ray diffraction (XRD) analyses show that this unique cellular structure consists of Zr-rich and Ta-rich body-center cubic (BCC) phases as the cell-wall and the cell-core, respectively, with their lattice constant difference of about 1 %. Moreover, a higher density of dislocations forming distinct networks is detected within this cellular structure, whose density reached 8 × 10 14 m 2 . Ma chine learning analysis reveals that the dislocations preferentially occur on the Zr-rich BCC side, thus accom modating the strains significant around the boundaries between the two BCC phases. With the aid of thermodynamic simulations, we propose a formation mechanism of the present cellular structure, which is governed by the elemental partitioning behavior of Zr and Ta during a solid-state phase separation under rapid cooling. Boundaries with this phase separation are introduced as semi-coherent interfaces with misfit disloca tions, introducing a high-density dislocation in the present material. This novel cellular structure can signifi cantly enhance the strength of AM HEAs, providing valuable insights for developing high-performance AM metals through the design of hierarchical microstructures.

Keywords
High-entropy alloy; Laser powder bed fusion; Cellular structure; Phase separation; Dislocation-network; Electron microscopy; Machine learning

2025.03.13

The Signature Pavilion "Future of Life" website has been renewed and reopened, and an article about Nakano Lab. has been posted on the "Future Technology" Seeds page.

An Innovative Bone Medical Device That Can Induce Strong Bones from the Early Stages of Regeneration

Click here to go to the top page→
https://expo2025future-of-life.com/
Click here to go to the Seeds of Future Technology top page→
https://expo2025future-of-life.com/about/future-technology-seeds/

2025.02.01

A paper on the microstructural effects of post-processing during laser and electron beam lithography has been published in the CIRP Journal of Manufacturing Science and Technology (IF=4.6).

Jibin Boban*, Afzaal Ahmed, Ozkan Gokcekaya , Takayoshi Nakano:
Ultra-precision surface treatment of beta-Titanium alloy printed using laser and electron beam melting sources, CIRP Journal of Manufacturing Science and Technology, 58, (2025), 01.006; 1-19.
https://doi.org/10.1016/j.cirpj.2025.01.006

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Abstract
Additive Manufacturing (AM) is a near net shape fabrication technology offering exceptional design freedom for complex part production. However, the inadequate surface quality and poorly generated micro-features adversely affect the functional performance of metal AM parts thereby restricting the direct adoption in biomedical implantation applications. Ultra-precision diamond turning (UPDT) can be regarded as a possible solution to overcome the aforementioned challenges in metal AM. However, the machinability of metal AM parts at ultra-precision level is highly sensitive to the material specific attributes and microstructure generated by the thermal characteristics of the process. In light of this, the present study follows a novel direction by investigating the dependence of distinct material characteristics imparted by two different AM powder melting sources on the ultra-precision post-treatment performance. Experiments were conducted on laser and electron beam printed beta-Ti alloy (Ti-15Mo-5Zr-3Al) which has potential importance in biomedical applications. The results demonstrate that the microstructural variations in respective samples affect the process performance and final surface integrity. The samples printed using laser powder bed fusion (LPBF) achieved a final surface finish (Sa) of ~66.3 nm after UPDT relative to the electron beam powder bed fusion (EPBF) samples (~104.3 nm). The cutting forces tends to exhibit sharp dip in forces in case of LPBF samples when micro-cutting was done perpendicular to the beam scanning direction. The chip morphology analysis corresponding to the LPBF and EPBF samples sub stantiates the generation of chips with segmentation/serrations on the free chip surface and parent material adhesion on the tool-chip contact surface. Further, precise microfeature generation was successfully accom plished on both the samples with minimal dimensional deviations on LPBF sample. Thus, the outcomes of the study establish the potential of UPDT in elevating the bioimplant surface standards of beta-Ti alloy with superior performance in LPBF samples.

Keywords
Additive manufacturing; Powder bed fusion; Diamond turning; Titanium; Microstructure

2025.01.15

A joint research with Yamaguchi University School of Medicine that elucidated that methylglyoxal inhibits fracture treatment through osteoblast differentiation was published in Biochemical and Biophysical Research Communications.

Tetsuya Seto, Kiminori Yukata, Shunya Tsuji*, Yusuke Takeshima, Takeshi Honda, Akihiko Sakamoto, Kenji Takemoto, Hiroki Sakai, Mayu Matsuo, Yurika Sasaki, Mizuki Kaneda, Mikako Yoshimura, Atsushi Mihara, Kazuya Uehara, Aira Matsugaki, Takayoshi Nakano, Koji Harada, Yoshiro Tahara, Keiko Iwaisako, Ryoji Yanai, Norihiko Takeda, Takashi Sakai, Masataka Asagiri*:
Methylglyoxal compromises callus mineralization and impairs fracture healing through suppression of osteoblast terminal differentiation,
Biochemical and Biophysical Research Communications, 747, (2025), 151312; 1-8.
https://doi.org/10.1016/j.bbrc.2025.151312

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Abstract
Impaired fracture healing in diabetic patients leads to prolonged morbidity and increased healthcare costs. Methylglyoxal (MG), a reactive metabolite elevated in diabetes, is implicated in various complications, but its direct impact on bone healing remains unclear. Here, using a non-diabetic murine tibial fracture model, we demonstrate that MG directly impairs fracture healing. Micro-computed tomography revealed decreased volumetric bone mineral density in the callus, while callus volume remained unchanged, resulting in a brittle bone structure. This was accompanied by reduced expression of osteocalcin and bone sialoprotein, both critical for mineralization. Biomechanical analysis indicated that MG reduced the mechanical resilience of the fracture site without altering its elastic strength, suggesting that the impairment was not primarily due to the accumulation of advanced glycation end-products in the bone extracellular matrix. In vitro studies confirmed that non-cytotoxic concentrations of MG inhibited osteoblast maturation and mineralization. Transcriptomic analysis identified downregulation of Osterix, a key transcription factor for osteoblast maturation, without altering Runx2 levels, leading to decreased expression of key mineralization-related factors like osteocalcin. These findings align with clinical observations of reduced circulating osteocalcin levels in diabetic patients, suggesting that the detrimental effects of MG on osteoblasts may extend beyond bone metabolism. Our study highlights MG and MG-sensitive pathways as potential therapeutic targets for improving bone repair in individuals with diabetes and other conditions characterized by elevated MG levels.

Keywords
Fracture healing; Osteoblasts; Diabetes; Methylglyoxal

2025.01.14

Acta Biomaterialia (IF=9.4) published an article on induction of bone orientation by zebra coating with MPC polymer.

Tadaaki Matsuzaka, Aira Matsugaki*, Kazuhiko Ishihara, Takayoshi Nakano*:
Osteogenic tailoring of oriented bone matrix organization using on/off micropatterning for osteoblast adhesion on titanium surfaces, Acta Biomaterialia, 192, (2025), 39644943; 487-500.
https://doi.org/10.1016/j.actbio.2024.12.017

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Abstract
Titanium (Ti) implants are well known for their mechanical reliability and chemical stability, crucial for suc cessful bone regeneration. Various shape control and surface modification techniques to enhance biological activity have been developed. Despite the crucial importance of the collagen/apatite bone microstructure for mechanical function, antimicrobial properties, and biocompatibility, precise and versatile pattern control for regenerating the microstructure remains challenging. Here, we developed a novel osteogenic tailoring stripe- micropatterned MPC-Ti substrate that induces genetic-level control of oriented bone matrix organization. This biomaterial was created by micropatterning 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer onto a titanium (Ti) surface through a selective photoreaction. The stripe-micropatterned MPC-Ti substrate establishes a distinct interface for cell adhesion, robustly inducing osteoblast cytoskeleton alignment through actin cyto skeletal alignment, and facilitating the formation of a bone-mimicking-oriented collagen/apatite tissue. More over, our study revealed that this bone alignment process is promoted through the activation of the Wnt/ β-catenin signaling pathway, which is triggered by nuclear deformation induced by strong cellular alignment guidance. This innovative material is essential for personalized next-generation medical devices, offering high customizability and active restoration of the bone microstructure.

Keywords
Photoreactive; MPC polymer; Stripe-micropatterned titanium substrate; Osteoblast orientation; Apatite/collagen orientation; Bone regeneration

 

2025.01.08

The paper on "Superposition of 'material (crystal orientation)' and 'shape' by AM to express mechanical functions", a result of JST-Crest, has been accepted by Acta Materialia (Elsevier, IF=8.3). The paper was published as an OA paper.

Takuya Ishimoto, Naotaka Morita, Ryosuke Ozasa, Aira Matsugaki, Ozkan Gokcekaya, Shota Higashino, Masakazu Tane, Tsuyoshi Mayama, Ken Cho, Hiroyuki Y. Yasuda, Masayuki Okugawa, Yuichiro Koizumi, Masato Yoshiya, Daisuke Egusa, Taisuke Sasaki, Eiji Abe, Hajime Kimizuka, Naoko Ikeo, Takayoshi Nakano*:
Superimpositional design of crystallographic textures and macroscopic shapes via metal additive manufacturing--Game-change in component design, Acta Materialia, 286, (2025), 120709; 1-12.
DOI: https://doi.org/10.1016/j.actamat.2025.120709

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Abstract
This study demonstrates the control of the crystalline orientation through metal additive manufacturing, enabling the development of component design guidelines that incorporate the inherent anisotropy of the me chanical properties (e.g., Young's modulus) in crystalline materials. We, for the first time, successfully fabricated a <111>//build direction (BD)-oriented single-crystalline-like texture in an alloy with a cubic crystal structure via laser powder bed fusion (LPBF) and completed a series of three single-crystalline-like microstructures with <001>, <011>, and <111>//BD orientations in a single material. The <001>and <111>directions exhibited the lowest and highest Young's moduli, respectively, demonstrating a wide range of control over the anisotropy of the mechanical properties of the product. To achieve a <111>//BD-oriented single-crystalline-like texture, a novel three-layer cyclic strategy of "uni"directional laser scanning at 120◦ angular intervals was developed by considering the easy growth direction and crystal symmetry. To the best of our knowledge, no previous study has reported this unique strategy. By superimposing the realized <111>orientation and shape-based anisotropy, products exhibiting high Young's modulus anisotropy, which cannot be expressed by shape and texture alone, were obtained via the LPBF single process. This achievement holds promise for the realization of a new component design guideline that integrates texture (mechanical properties) design for each internal location--modifiable through scanning strategies--with traditional shape optimization techniques typically used in computer-aided design. This approach enables tailored mechanical performance through optimized design strategies.

Keywords
Laser powder bed fusion; Shape anisotropy; Crystallographic orientation; Superimposition; 3D puzzle; Young's modulus

2024.12.18

The Journal of Materials Engineering and Performance published an article on the effect of energy density on the functionality of Ti6Al-4V alloys formed by AM.

H. Yilmazer, Y. A. Sadikoglu, S. Kucuk, O. Gokcekaya, I. C. Turu, T. Nakano, B. Dikici*:
The effect of energy density on microstructural, mechanical, and corrosion characteristics of Ti6Al-4V alloy fabricated via selective laser melting (SLM) and electron beam melting (EBM) techniques,
Journal of Materials Engineering and Performance, (2024), 1-13.
https://doi.org/10.1007/s11665-024-10512-8

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Abstract
In this study, the effect of production parameters on Ti-6Al-4V alloys fabricated using selective laser melting (SLM) and electron beam melting (EBM) techniques was investigated. Through the variation of energy volume (12.5, 25, 37.5 J mm−3), these two additive manufacturing methods were compared in terms of microstructure, mechanical, and corrosion properties. Density was calculated using Archimedes' technique, while microstructure was characterized through optical microscopy (OM) and scanning electron microscopy (SEM). Mechanical properties were determined via micro-Vickers hardness and tensile tests. Electron backscatter diffraction (EBSD) and x-ray diffraction (XRD) analyses were performed on EBM and SLM samples for a comprehensive understanding. Corrosion susceptibilities of the alloys were evaluated using potentiodynamic scanning (PDS) tests in a 3.5% NaCl solution at room temperature. Microstructural analysis revealed that SLM-produced parts predominantly consisted of the α′ (martensite) phase, whereas EBM-produced parts primarily comprised the α phase with a small amount of the β phase. The strength values of all SLM samples exceeded 930 MPa, surpassing those of wrought Ti-6Al-4V ELI. However, only EBM samples fabricated with a 37.3 J mm−3 energy volume approached this standard. Corrosion susceptibility generally increased with higher energy volume in both EBM and SLM samples, with porosity volume and grain size variations influencing corrosion behavior.

Keywords
Corrosion; EBM; Energy density; Phase transformation; SLM; Ti-6Al-4V

2024.12.05

The Journal of Alloys and Compounds published an OA paper on the results of joint research with Northeastern University, China.

Chenyang Wu, Xiaoli Zhao*, Takayoshi Nakano, Mitsuo Niinomi, Nan Jia, Deliang Zhang:
Grain size dependence of deformation behavior in Ti-15Mo alloy prepared by powder metallurgy
Journal of Alloys and Compounds, 1010, (2025), 177825: 1-12.
https://doi.org/10.1016/j.jallcom.2024.177825

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Abstract
To the best of our knowledge, this is the first report demonstrating grain refinement to 4 μm via a mass-production method for the Ti-15Mo alloy. Grain sizes ranging from 4 to 38 μm were achieved by controlling thermomechanical processing in powder metallurgy, combined with heat treatment using recycled coarse powders for additive manufacturing. The critical grain size for deformation twinning was investigated, alongside an analysis of the deformation behavior and mechanical properties of the Ti-15Mo alloy with various grain sizes. Upon refining the grain size to 7 μm, deformation twinning is inhibited, shifting the plastic deformation mechanism from mechanical twinning to dislocation slip. The yield strength can be adjusted between 921 and 715 MPa, with elongation ranging from 18.4% to 34.4%, by varying the grain size distribution ratio of small to large grains relative to 7 μm from 1.5 to 0.42. This strengthening effect primarily arises from dislocation strengthening, Mo solid solution, texture strengthening, and modifications in the Hall-Petch constant due to changes in deformation behavior during grain refinement.

Keywords
Ti-Mo alloy; Power Metallurgy; Grain refinement; Mechanical properties; Mechanical twinning

2024.12.05

A paper on microstructural changes and high temperature mechanical properties of titanium alloys was published in Materials Transactions.

Tomoki Kuroda, Haruki Masuyama, Yoshiaki Toda, Tetsuya Matsunaga, Tsutomu Ito, Makoto Watanabe, Ryosuke Ozasa, Takuya Ishimoto, Takayoshi Nakano, Masayuki Shimojo, Yoko Yamabe-Mitarai:
Microstructure Evolution and High-Temperature Mechanical Properties of Ti-6Al-4Nb-4Zr Fabricated by Selective Laser Melting,
Materials Transactions, 64 [1], (2024), 2022021; 348-356.
DOI: https://doi.org/10.2320/matertrans.MT-MLA2022021

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Abstract
Ti-6Al-4Nb-4Zr (mass%) was prepared by selective laser melting (SLM) under various conditions, and the microstructure evolution resulting from SLM processing and subsequent heat treatments was investigated. The effects of the unique SLM-induced microstructure on the high-temperature compressive strength and creep properties of the samples were then elucidated. Under rapid cooling conditions, the martensitic structure formed in a scale-like pattern, with a 100 µm in size, consistent with the laser scanning pattern. By contrast, under slow cooling conditions, the α/β lamellar structure formed in β grains with a 300 µm grain size instead of in a scale-like pattern. The martensitic structure drastically changed to a Widmanstätten structure during heat treatment. The equiaxed α phase also formed at the interface of the scale-like patterns. By contrast, the α/β lamellar structure did not exhibit a change in response to heat treatment. The compressive strength of the SLM samples was governed by the martensite α size and the grain size, both of which depended on the cooling rate. The dominant creep deformation mechanism at 600°C and under a loading stress of 137 MPa was grain boundary sliding. The creep life depended on the grain size. The HIP treatment improved the creep life because it eliminated pores introduced by the SLM process.

Keywords
selective; lasermelting; heat-resistant; Ti alloys; compression; strength; creep

2024.11.21

A paper on our recent research on titanium alloys was published in the December issue of Materials Transactions.

Mitsuo Niinomi*, Takayuki Narushima, Takayoshi Nakano:
Recent Research and Development in the Processing, Microstructure, and Properties of Titanium and Its Alloy
Materials Transactions, 65 [12], (2024), 1600-1611.
https://doi.org/10.2320/matertrans.MT-M2024082

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Abstracts
The special issue on recent research and development in the processing, microstructure, and properties of titanium and its alloy contains four review articles on metal additive manufacturing (AM) focusing on the processing, microstructural and/or crystallographic control, and biomedical applications of titanium and its alloys, and seventeen regular articles on metal AM, refining, microstructural evolution, and mechanical and fatigue properties related to the microstructure, and biomedical applications of titanium and its alloys, which have been published in Materials Transactions in 2023. This study briefly addresses this issue.

Keywords
titanium and its alloys; additive manufacturing (AM); phase transformation; microstructure; mechanical properties; titanium based intermetallics; biomaterials; recycling

2024.11.13

A paper on the successful laser fabrication of high-melting-point Cr in cyber/physical space has been published as an OA paper in the "Journal of Materials Research and Technology".

Sung-Hyun Park, Ozkan Gokcekaya*, Tatsuya Nitomakida, Takayoshi Nakano*:
Effects of heat accumulation strategies on defects and microstructure of pure chromium fabricated by laser powder bed fusion: An experimental and numerical study,
Journal of Materials Research and Technology, 33, (2024), 11.049; 1-12.
DOI: https://doi.org/10.1016/j.jmrt.2024.11.049

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Abstract
The process of employing laser powder bed fusion (L-PBF) process to refractory materials, such as chromium (Cr), remains challenging because of its high ductile-brittle transition temperature. Therefore, a strategy is required to increase the processing temperature to prevent defects. The focus of this study is to clarify the effect of the preheat temperature and scan length variations on defects during the L-PBF process with an experimental and numerical study. Applying a high preheat temperature and short scan length was effective in mitigating the defects. By tuning the heat accumulation strategies, the determined relative density measured by the Archimedes principle and optical measurement of pure Cr parts increased from 97.2% to 99.8% and 97.4% to 99.2%, respectively. The numerical study indicated that deepened and elongated melt pool geometry owing to heat accumulation promoted epitaxial growth. Strong crystallographic texture formation with epitaxial growth led to higher grain size, lower high-angle grain boundary misorientation, Kernel average misorientation, and Taylor factor, which resulted in densification by preventing defects. The hardness of L-PBFed Cr samples gradually decreased with the stronger crystallographic texture formation and compressive yield strength exhibited the same phenomenon. However, the high densification sample with strong crystallographic texture promised the highest compressive strength and strain because it allows single and multi-slip operation during the compressive deformation without premature fracture. This study is a pivotal moment in heat accumulation strategies to achieve high densification for brittle materials fabricated by the L-PBF process while proposing an approach to ensure the reliability of structural applications.

Keywords
Chromium; Laser powder bed fusion; Densification; Numerical simulation; Process-microstructure-property relationship

2024.10.07

The results of joint research with Prof. Adachi et al. of Nagoya University were published in Materials Characterization.

Fei Sun*, Yoshitaka Adachi, Kazuhisa Sato, Takuya Ishimoto, Takayoshi Nakano, Yuichiro Koizumi:
Quantitative revealing the solute segregation behavior at melt pool boundary in additively manufactured stainless steel using a novel processing method for precise positioning by HAADF-STEM,
Materials Characterization, Materials Characterization, 217, (2024), 114435: 1-6.
https://doi.org/10.1016/j.matchar.2024.114435

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Abstract
Laser-powder bed fusion (LPBF) enables the fabrication of complex metallic components by manipulating various laser scan strategies to control microstructure and texture. Multiple thermal cycling and rapid solidification lead to non-equilibrium, non-uniform microstructure, and micro-segregation at the melt pool boundary (MPB), whose accurate location is still invisible by transmission electron microscopy (TEM), and quantitative concentration remains imprecise. In this study, we proposed a novel method to make it clear by controlling the crystallographic texture of 316 L stainless steel through unique LPBF processing parameters to obtain a single-crystal-like microstructure of the cellular structures along the laser scanning direction. The accurate location of the track-track MPB is distinguishable by means of the transverse and longitudinal cellular dislocation structures on both sides. The edge-on state of the track-track MPB makes the quantitative concentration analysis precisely using high-angle annular dark-field scanning TEM with energy-dispersive X-ray spectroscopy, which is in good agreement with the Scheil-Gulliver solidification simulations.

Keywords
Additive manufacturing; Melt pool boundary; Cellular structure; Segregation

2024.10.04

Materialia (Elsevier) published a paper on non-stoichiometric TiNbMoTaW high-entropy alloys that can be fabricated by an in-situ alloying method.

Yong Seong Kim, Ozkan Gokcekaya, Aira Matsugaki, Ryosuke Ozasa, Takayoshi Nakano*:
Laser energy-dependent processability of non-equiatomic TiNbMoTaW high-entropy alloy through in-situ alloying of elemental feedstock powders by laser powder bed fusion, Materialia, 38, (2024), 102241: 1-10.
https://doi.org/10.1016/j.mtla.2024.102241

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Abstracts
Pre-alloyed powder, which is primarily used in laser powder bed fusion (LPBF), has the disadvantages of requiring time and high manufacturing costs. To overcome these limitations, in-situ alloying, which mixes pure elemental powders and alloys them in real-time during the LPBF process, has attracted attention. In particular, manufacturing high entropy alloys (HEA) containing high-melting-point refractory elements through in-situ alloying presents considerable challenges. In this study, a non-equiatomic single body-centered cubic (BCC) solid-solution HEA was fabricated via in-situ alloying with Ti, Nb, Mo, Ta, and W powders through the LPBF process. Specifically, by applying a high volumetric energy density (VED), we successfully mitigated the segregation of constituent elements, leading to an enhanced crystallographic texture. Consequently, the reduction in the residual stress and high-angle grain boundary (HAGB) density progressed, contributing to an increased relative density. Thus, this study marks a pioneering endeavor for in-situ alloyed HEA fabrication via LPBF, illustrating the efficacy of in-situ alloying utilizing mixed powders.

Keywords
High entropy alloys; Additive manufacturing; In-situ alloying; Segregation; Cracking

2024.04.16

A paper on grain size dependence of martensitic transformation in beta titanium alloys has been published as OA in Materials Science and Engineering A (IF=6.4).

Chenyang Wu, Xiaoli Zhaoa*, Mengrui Zhang, Hideki Hosoda, Takayoshi Nakano , Mitsuo Niinomi, Nan Jia, Zhiwen Shao , Deliang Zhang:
Strong grain size effect on tensile behavior of the body-centered-cubic Ti-30Zr-5Mo alloy with stress-induced α' martensitic transformation,
Materials Science and Engineering A, 900, (2024), 146455, 1-13.
https://doi.org/10.1016/j.msea.2024.146455

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Abstract
In this study the grain size effect on mechanical properties of a body-centered-cubic Ti-30Zr-5Mo alloy was investigated. Double yielding behavior in the stress-strain curves and four-stage behavior in the strain hardening rate curves can be seen in all Ti-30Zr-5Mo materials with different average grain sizes ranging from 6 to 475 μm, which is attributed to the occurrence of the stress-induced α′ transformation. The static Hall-Petch coefficient (k) for phase transformation was calculated to establish the relationship between grain size and trigger stress of the various materials. With the increase of strain, the hindrance of αʹ/β grain boundaries and αʹ/αʹ grain boundaries to dislocations gradually replaced β/β grain boundaries, thus the work hardening ability and k value changed. β grains were segmented by α′ martensite, resulting in a dynamic Hall‒Petch effect. Combined with a large stress field in the fine-grained materials with an average grain size of 6 μm, the highest work hardening rate with a value of 13 GPa was obtained. As the β grain size increased, the ultimate strength gradually decreased, while both trigger stress of the stress-induced αʹ transformation and elongation fluctuated. The trigger stress can be adjusted between 211 and 464 MPa by controlling the grain size. The grain size has little effect on the amount of the stress-induced αʹ phase. With a high trigger stress of 464 MPa in the material with the finest grains, the excellent ductility of 21% is obtained. The best comprehensive mechanical properties with a strength-ductility index value of 252 MPa is obtained in the material with an average grain size of 113 μm.
　
Keywords
Ti-30Zr-5Mo alloy; Grain size; Stress-induced phase transformation; αʹ martensite; Mechanical properties; Grain refinement; α′ phase; Titanium alloys

2024.04.13

A paper on the deformation behavior of two-dimensional special lattice structures fabricated by AM has been published as OA in Materials & Design (IF=8.4).

Zana Eren, Ozkan Gokcekaya, Demet Balkan, Takayoshi Nakano, Zahit Mecitoğlu:
Comparison of in-plane compression of additively manufactured Ti6Al4V 2D auxetic structures: Lattice design, manufacturing speed, and failure mode,
Materials & Design, 241, (2024), 112885, 1-19.
https://doi.org/10.1016/j.matdes.2024.112885

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Abstract
The metal-based 2D auxetic lattice structures hold the potential for multifunctional tasks in aerospace applications. However, the compression response of those manufactured by powder bed fusion process is underexplored. This study proposes a comprehensive comparison of in-plane quasi-static compression performance of 2D auxetic lattice structures, utilizing three designs (anti-tetrachiral (ATC), double arrow-headed (DAH), and tree-like re-entrant (TLR)), manufactured with stiff Ti6Al4V by the electron beam powder bed fusion process (PBF-EB) with various manufacturing speeds. The results revealed unique failure patterns and superior energy absorptions among 2D lattice structures in the literature. TLR design enhanced energy absorption by overcoming failures between DAH columns and exhibited the lowest standard deviations in specific energy absorption (SEA) values (9.75 -12.62 ). Besides, Kernel average misorientation (KAM) values followed the order of DAH, TLR, and ATC, and inversely correlated with SEA values. ATC structures with the lowest KAM outperformed DAH and TLR by 47.5 and 6.44 , respectively. Scan speed variations affected SEA and porosity values differently for each lattice design while exhibiting similar microstructure characteristics. The findings in this study propose a significant contribution to the development of aerospace sandwich structures where harsh environments exist and employment of 2D topologies are required.

Keywords
Auxetic structures; Additive manufacturing; Energy absorption; Compressive failures; Residual stress

2024.04.03

A computer simulation study on the segregation behavior of a nickel-based heat-resistant alloy (Hastelloy-X) has been published as an OA paper in the journal "Additive Manufacturing" (IF=11).

Masayuki Okugawa*, Kenji Saito, Haruki Yoshima, Katsuhiko Sawaizumi, Sukeharu Nomoto, Makoto Watanabe, Takayoshi Nakano, Yuichiro Koizumi*:
Solute segregation in a rapidly solidified Hastelloy-X Ni-based superalloy during laser powder bed fusion investigated by phase-field and computational thermal-fluid dynamics simulations,
Additive Manufacturing, 84, (2024), 104079, 1-13.
https://doi.org/10.1016/j.addma.2024.104079

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Abstract
Solute segregation significantly affects material properties and is a critical issue in the laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ni-based superalloys. To the best of our knowledge, this is the first study to demonstrate a computational thermal-fluid dynamics (CtFD) simulation coupled multi-phase-field (MPF) simulation with a multicomponent-composition model of Ni-based superalloy to predict solute segregation under solidification conditions in LPBF. The MPF simulation of the Hastelloy-X superalloy reproduced the experimentally observed submicron-sized cell structure. Significant solute segregations were formed within interdendritic regions during solidification at high cooling rates of up to 1.6 × 106 K s−1, a characteristic feature of LPBF. Solute segregation caused a decrease in the solidus temperature (TS), with a reduction of up to 38.4 K, which increases the risk of liquation cracks during LPBF. In addition, the segregation triggers the formation of carbide phases, which increases the susceptibility to ductility dip cracking. Conversely, we found that the decrease in TS is suppressed at the melt-pool boundary regions, where re-remelting occurs during the stacking of the layer above. Controlling the re-remelting behavior is deemed to be crucial for designing crack-free alloys. Thus, we demonstrated that solute segregation at the various interfacial regions of Ni-based multicomponent alloys can be predicted by the conventional MPF simulation. The design of crack-free Ni-based superalloys can be expedited by MPF simulations of a broad range of element combinations and their concentrations in multicomponent Ni-based superalloys.

Keywords
Laser powder-bed fusion; Hastelloy-X nickel-based superalloy; Solute element segregation; Computational thermal-fluid dynamics simulation; Phase-field method

2024.03.27

Smart Materials in Manufacturing (Elsevier) published a paper as OA on the unique microstructure and hardness variation of Ti-6Al-2Sn-4Zr-6Mo alloys by metal additive manufacturing.

Prince Valentine Cobbinah*, Sae Matsunaga, Yoshiaki Toda, Ryosuke Ozasa, Masayuki Okugawa, Takuya Ishimoto, Yuheng Liu, Yuichiro Koizumi, Pan Wang, Takayoshi Nakano*, Yoko Yamabe-Mitarai*:
Peculiar microstructural evolution and hardness variation depending on laser powder bed fusion-manufacturing condition in Ti-6Al-2Sn-4Zr-6Mo,
Smart Materials in Manufacturing, 2, (2024), 100050, 1-10.
https://doi.org/10.1016/j.smmf.2024.100050

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Abstract
This study aims to comprehensively analyze the phase and microstructure evolution and related hardness variations of the Ti-6Al-2Sn-4Zr-6Mo wt.% (Ti6246) alloy produced by laser powder bed fusion (LPBF) under various laser conditions and to gain insight into the mechanisms of these changes using numerical thermal analysis. Higher laser volumetric densities (VEDs) resulted in a finer α/α' microstructure and increased hardness, exhibiting a positive correlation with the VED, except under extremely high conditions. This contrary trend, reported for the first time, is attributed to the solid-phase transformation from the β phase to metastable α' martensite during LPBF induced by rapid cooling. Despite the finer microstructure, the samples under very high VED conditions showed lower hardness, deviating from the overall trend. The X-ray diffraction peaks in the high-VED samples suggested a partial decomposition of α' to α + β owing to laser-induced reheating of the underlying layers, which is considered a contributing factor to the hardness reduction. The numerical analysis showed that the underlying layer was exposed to high temperatures for a relatively long time under high-VED conditions. It was revealed that the hardness of LPBF-fabricated Ti6246 was influenced by unique thermal processes: rapid cooling and reheating of the pre-solidified part, leading to the formation of a metastable α' phase and partial decomposition into α + β. These findings provide insights for tailoring Ti6246 with desired physical properties via LPBF.

Keywords
LPBF; Ti6246; Polycrystalline microstructure; Metastable phase; Rapid cooling; Thermal history

2024.03.01

A paper proposing the world's first (R≥2.0) super high entropy alloy (SHEA) was published as OA in Materials Chemistry and Physics (IF=4.6).

Tadaaki Matsuzaka, Akira Hyakubu, Yong Seong Kim, Aira Matsugaki, Takeshi Nagase, Takuya Ishimoto, Ryosuke Ozasa, Hyoung Seop Kim, Tomoji Mizuguchi, Ozkan Gokcekaya, Takayoshi Nakano:
Development of an equiatomic octonary TiNbTaZrMoHfWCr super high-entropy alloy for biomedical applications,
Materials Chemistry and Physics, 316, (2024), 129120; 1-7.
DOI: https://doi.org/10.1016/j.matchemphys.2024.129120
　
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Abstract
A super-high-entropy alloy (SHEA) with ΔSmix ≥ 2.0R (where R is the gas constant) was designed to produce metallic materials with superior mechanical properties to conventional alloys. As an alternative to conventional quinary high-entropy alloys (HEAs), herein, octonary SHEAs for biomedical applications (BioSHEAs) are proposed for the first time, and the TiNbTaZrMoHfWCr BioSHEA was fabricated. Arc-melted BioSHEA exhibited an extremely high yield strength of 1953 ± 84 MPa, which was approximately 550 MPa higher than that of the quinary TiNbTaZrMo BioHEA. This yield strength is considerably higher than that estimated by the rule of mixtures for pure metals, confirming the achievement of significant solid-solution strengthening induced by a supermulticomponent solid solution composed of elements with different atomic radii. Its biocompatibility was comparable to that of pure Ti and the quinary BioHEA, and superior to that of SUS316L. This study demonstrates the validity of a novel entropy-based guideline for increasing the mixing entropy to achieve metallic materials with ultrahigh strength.
　
Keywords
Super-high-entropy alloys (SHEAs); Octonary system; Supermulticomponent; Solid-solution strengthening; BioSHEAs

2024.02.23

An international collaborate research on the microstructure of pure Cr produced by L-PBF and the effect of HIP on wear has been published in the International Journal of Refractory Metals and Hard Materials (IF=3.6).

Asli Gunay Bulutsuz*, Buse Gulec, Ozkan Gokcekaya*, Johannes Gardstam, Takayoshi Nakano, Hakan Yilmazer:
An investigation over microstructure and HIP processing effects on wear performance of pure chromium parts fabricated by laser powder bed fusion,
International Journal of Refractory Metals and Hard Materials, 120, (2024), 106616, 1-10.
DOI: https://doi.org/10.1016/j.ijrmhm.2024.106616

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Abstract
Chromium (Cr) and its alloys have long been valued for their exceptional properties, including corrosion resistance and high-temperature stability, rendering them indispensable in industrial applications such as chemical processing, energy production, and aerospace. However, due to the difficulties in Cr-based alloy manufacturing, there is a need for novel manufacturing methodologies. Additive Manufacturing (AM) emerges as a promising approach, particularly Laser Powder Bed Fusion (LPBF), which allows for intricate designs, reduced material waste, and simplified assembly. However, certain issues still need to be addressed, including defects like cracks and porosities in the manufactured parts. Post-processing techniques such as Hot Isostatic Pressing (HIP) have gained prominence for enhancing the material properties and quality of AM parts, including those produced using LPBF. HIP treatments are effective in eliminating internal pores, although some challenges remain, notably the presence of trapped argon and grain coarsening side effects of HIP process parameters. This study focuses on LPBF-processed pure chromium parts with crystallographic texture and investigates their properties after HIP treatment, including microstructure, hardness, and tribological performance. According to the obtained results: the HIP process reduced cracks, especially in the center region, but increased gaps in the side region. HIP also hindered grain realignment, limiting grain growth, and resulting in high HAGB density and low MUD values. Elevated HIP processing pressure negatively affected tribological performance due to increased grain size, and reduced hardness. This study, for the first time, realized the effect of HIP conditions on the microstructure and tribological performance of LPBF-processed pure Cr.

Keywords
Laser powder bed fusion; Pure Cr; Crystallographic texture; Hot isostatic pressing; Wear performance

2024.02.20

Research on the development of low elasticity antimicrobial Ti-Nb-Cu alloys for biological applications was published in Materials Today Communications (IF=3.8).

Qiang Li*, Qizhen Peng, Qi Huang, Mitsuo Niinomi, Takuya Ishimoto, Takayoshi Nakano:
Development and characterizations of low-modulus Ti-Nb-Cu alloys with enhanced antibacterial activities,
Materials Today Communications, 38, (2024), 108402, 1-8.
DOI: https://doi.org/10.1016/j.mtcomm.2024.108402
　
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Abstract
Avoiding infection is a requirement for the long-term stability and safety of implants, but most Ti alloys for implantation hardly inhibit the bacterial proliferations. Cu works as a β stabilizer in Ti alloys, and Cu ion can kill bacteria. To obtain antibacterial activities in low-modulus Ti alloys, Ti-35Nb-(0, 1, 2, 3, 4)Cu (wt%) alloys were prepared by non-consumable arc melting following by homogenization, hot rolling, and solution treatment. They were then subjected to phase analysis, microstructure observation, tensile test and dynamic polarization. The cytotoxicity and antibacterial activities were finally evaluated. The results show that Cu stabilizes the β phase and inhibits the generation of α" phase during quenching. Ti-35 Nb and Ti-35Nb-1Cu consist of β and α" phases, and Ti-35Nb-(2, 3, 4)Cu alloys have a single β phase. The Ti-Nb-Cu alloys exhibit the Young's moduli ranging from 57 to 72 GPa. All the alloys show passivation behavior with a low passive current density. Cytotoxicity is hardly observed in the alloys. The antibacterial rates of Ti-35Nb-(1, 2, 3, 4)Cu alloys against E. coli are 62.8%, 68.9%, 70.9% and 73.2%, respectively, and the antibacterial rates against S. aureus are 63.4%, 69.7%, 72.8% and 74.7%, respectively. The developed β-type Ti-35Nb-4Cu alloy shows a relatively low Young's modulus and good antibacterial properties, and is a candidate for biomedical applications.
　
Keywords
Biomedical Ti alloys, Mechanical properties, Corrosion resistance, Cytotoxicity, Antibacterial properties

2024.01.27

Bone (IF=4.1) published an OA article showing that bone matrix orientation has a stronger effect on bone mechanical function adaptation than bone density.

Jun Wang, Takuya Ishimoto, Tadaaki Matsuzaka, Aira Matsugaki, Ryosuke Ozasa, Takuya Matsumoto, Mikako Hayashi, Hyoung Seop Kim, Takayoshi Nakano*:
Adaptive Enhancement of Apatite Crystal Orientation and Young's Modulus Under Elevated Load in Rat Ulnar Cortical Bone,
Bone, 181, (2024), 117024: 1-10.
https://doi.org/10.1016/j.bone.2024.117024

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Abstract
Functional adaptation refers to the active modification of bone structure according to the mechanical loads applied daily to maintain its mechanical integrity and adapt to the environment. Functional adaptation relates to bone mass, bone mineral density (BMD), and bone morphology (e.g., trabecular bone architecture). In this study, we discovered for the first time that another form of bone functional adaptation of a cortical bone involves a change in bone quality determined by the preferential orientation of apatite nano-crystallite, a key component of the bone. An in vivo rat ulnar axial loading model was adopted, to which a 3-15 N compressive load was applied, resulting in approximately 440-3200 μɛ of compression in the bone surface. In the loaded ulnae, the degree of preferential apatite c-axis orientation along the ulnar long axis increased in a dose-dependent manner up to 13 N, whereas the increase in BMD was not dose-dependent. The Young's modulus along the same direction was enhanced as a function of the degree of apatite orientation. This finding indicates that bone has a mechanism that modifies the directionality (anisotropy) of its microstructure, strengthening itself specifically in the loaded direction. BMD, a scalar quantity, does not allow for load-direction-specific strengthening. Functional adaptation through changes in apatite orientation is an excellent strategy for bones to efficiently change their strength in response to external loading, which is mostly anisotropic.

Keywords
Functional adaptation; Bone strength; Apatite orientation; Bone quality: In vivo loading

2024.01.27

An international collaborate research on improving the antimicrobial properties of Ti by Zn coating with improved adhesion has been published in Materials Today Communications (IF=3.8).

Ming Li, Qiang Li*, Jiawei Yang, Mitsuo Niinomi, Takayoshi Nakano:
Preparation and Antibacterial Activity of Zn Coating on Pure Ti with Enhanced Adhesion,
Materials Today Communications, 38, (2024), 1-10
https://doi.org/10.1016/j.mtcomm.2024.108149

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Abstract
A titania nanotubes layer was prepared as an intermediate layer on pure Ti via anodic oxidation, and subsequently, a Zn-containing coating was electrochemically deposited. The influence of the deposition parameters on the Zn content was studied using an orthogonal experiment. The obtained coatings were characterized using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and their adhesion was measured using a scratch test. The deposition time had the greatest effect on the Zn content of the electrodeposited samples, whereas the deposition temperature had the smallest effect. After the electrodeposition, Zn was uniformly distributed on the surface and existed mainly as a simple substance. The adhesion was only 10.9 N in the Zn-free sample; it increased with increasing Zn content of the samples, and reached a maximum value of 22.4 N in the sample with a Zn content of approximately 16%. The samples with Zn-containing coatings exhibited strong antibacterial effects against E. coli and S. aureus, and the antibacterial effect increased with increasing Zn content. Cell viability was above 80%, indicating that the Zn-containing coating is a good candidate as an antibacterial coating for biomedical applications.

Keywords
Zn-containing coating; Electrochemical deposition; Orthogonal experiment; Adhesion; Antibacterial property; Pure Ti

2024.01.17

A paper was published as OA in The International Journal of Advanced Manufacturing Technology (IF=3.4).

Ozkan Gokcekaya*, Ali Günen, Ferhat Ceritbinmez, Abdollah Bahador, Takayoshi Nakano, Melik Çetin:
Wire-EDM performance and surface integrity of Inconel 718 with unique microstructural features fabricated by laser powder bed fusion,
The International Journal of Advanced Manufacturing Technology, (2024), 1-16
https://doi.org/10.1007/s00170-023-12924-7
　
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Abstract
Inconel 718 alloy is difficult to machine using conventional methods due to its physical properties. Thereby, additive manufacturing (AM) of IN718 components with near-net shapes has been extensively studied. Even though AM processes provide shape and size accuracy, there is still the need for the machining of the AM-processed components to achieve the final shape of a component. Laser powder bed fusion (LPBF) has been successfully utilized to fabricate near-net shape IN718 components; moreover, the microstructure of LPBF-IN718 was unique owing to the AM processing, resulting in differences in grain size, grain boundary characteristics, and grain orientations. Furthermore, these microstructural characteristics are expected to alter the machining performance of IN718. Therefore, this study investigated the wire electro-discharge machining (WEDM) performance of LPBF-718 samples compared to wrought IN718 while focusing on the unique microstructure characteristics of LPBF-IN718 samples (lamella, single-crystal, ploy-crystal). Three different cutting strategies (rough, semi-finish, and finish) were implemented to understand the performance of the multi-pass cutting phenomenon and its effect on the surface of IN718. For all samples, rough (single pass) cutting displayed high roughness, while finish (three passes) cutting exhibited good surface quality. Compositional analyses on the machined surface showed debris formation including Zn and Cu-containing recast material, indicating wire erosion. The surface of single-crystal LPBF-IN718 after the WEDM process was smooth owing to its large grain size and less amount of grain boundary, resulting in slow cutting speed but a good surface finish. Thus, this study, for the first time, investigated the effect of unique microstructural characteristics of LPBF-fabricated IN718 on WEDM performance and machined surface quality.

2024.01.04

The Journal of the Mechanical Behavior of Biomedical Materials (IF=3.9) published the results of our collaborate research with Shanghai University of Science and Technology and Dr. Niinomi on the suppression of infection by surface deposition of iodine.

Qiang Li*, Shuaishuai Li, Hao Sun, Mitsuo Niinomi, Takayoshi Nakano:
Preparation and Characterizations of Antibacterial Iodine-containing Coatings on Pure Ti,
Journal of the Mechanical Behavior of Biomedical Materials, 151, (2024), 106366, 1-8.
https://doi.org/10.1016/j.jmbbm.2023.106366

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Abstract
Iodine-containing coatings were prepared on pure Ti surfaces via electrochemical deposition to enhance their antibacterial properties. The factors influencing iodine content were analyzed using an orthogonal experiment. The electrochemically deposited samples were characterized using scanning electron microscopy with energy dispersive spectroscopy and X-ray photoelectron spectroscopy, and their antibacterial properties and cytotoxicity were evaluated. The results showed that changing the deposition time is an effective way to control the iodine content. The iodine content, coating thickness, and adhesion of the samples increased with deposition time. Iodine in the coatings mainly exists in three forms, which are I2, I3−, and pentavalent iodine. For samples with iodine-containing coatings, the antibacterial ratios against E. coli and S. aureus were greater than 90% and increased with increasing iodine content. Although the samples with iodine-containing coatings showed some inhibition of the proliferation of MC3T3-E1 cells, the cell viabilities were all higher than 80%, suggesting that iodine-containing coatings are biosafe.

2023.12.19

Metals and Materials International (IF=3.5) has published a study of beta-containing TiAl alloys as an OA paper.

Sung-Hyun Park, Ozkan Gokcekaya, Ryosuke Ozasa, Myung-Hoon Oh, Young-Won Kim, Hyoung Seop Kim, Takayoshi Nakano:
Microstructure and crystallographic texture evolution of β-solidifying γ-TiAl alloy during single- and multi-track exposure via laser powder bed fusion,
Metals and Materials International, (2023), 1-15
https://doi.org/10.1007/s12540-023-01579-4

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Abstract
The microstructural evolution and crystallographic texture formation of β-solidifying Ti-44Al-6Nb-1.2Cr alloy were identified under single- and multi-track exposures via laser powder bed fusion (L-PBF) for various process parameters. Under single-track exposure, the microstructure of the melt pool was divided into the band-like α2 phase in the melt pool boundary and β phase in the melt pool center. Numerical and thermodynamic simulations revealed that the underlying mechanism of phase separation was related to the variation in the cooling rate in the melt pool, whereas microsegregation induced a shift in the solidification path. Meanwhile, the crystallographic texture of the α2 phase region was identical to that of the substrate owing to the epitaxial growth of the β phase and subsequent α phase nucleation. In contrast, the β phase exhibited a ± 45° inclined <100> alignment in the melt pool, which was tilted to align along the build direction toward the center of the melt pool corresponding to the simulated thermal gradient direction. Furthermore, the narrow hatch space condition maintained the crystallographic texture to the subsequent scan, forming a continuous band-like α2 phase with a strong selection. However, the crystallographic texture in a wide hatch space condition manifested a random distribution and constituted a fine mixture of the β and α2 phases. For the first time, these results will offer an understanding of an anisotropic microstructure control via the L-PBF process and ensure the tailoring of the mechanical properties in the β-solidifying γ-TiAl-based alloys by approaching hatch spacing control.

Keywords
γ-TiAl alloy, Microstructural evolution, Crystallographic texture, Laser powder bed fusion

2023.11.23

A study on microstructural changes after heat treatment of beta TiAl powder for AM has been published as an OA article in Crystals (IF=2.7).

Sung-Hyun Park, Ozkan Gokcekaya, Ryosuke Ozasa, Ken Cho, Hiroyuki Y. Yasuda, Myung-Hoon Oh, Takayoshi Nakano*:
Microstructure evolution of gas-atomized β-solidifying γ-TiAl alloy powder during subsequent heat treatment,
Crystals, 13, (2023), 1629; 1-10.
https://doi.org/10.3390/cryst13121629

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Abstract
To promote the use of γ-TiAl alloys in various domains, such as the aerospace industry, it is pivotal to investigate the unusual phase transformation from rapidly solidified and metastable γ-TiAl toward the equilibrium state. In this study, the microstructure characteristics of gas-atomized β-solidifying Ti-44Al-6Nb-1.2Cr alloy powder, in terms of the effect of rapid solidification on microstructure evolution, were explored in comparison with cast materials. The phase constitution, morphology, and crystallographic orientation between phases were noted to be distinct. Furthermore, subsequent heat treatment was conducted at different temperatures using gas-atomized powder. The transition from the metastable to equilibrium state was observed, wherein firstly, the γ phase precipitated from the retained α2 phase, forming an α2/γ lamellar microstructure. In intensified heat-treatment conditions adequate for cellular reaction, β/γ cells were formed at the grain boundaries of α2/γ lamellar colonies. The findings highlight the overall phase transformation during rapid solidification and continuous microstructural evolution from the nonequilibrium to the equilibrium state. This research can bridge the gap in understanding the effect of the solidification rate on microstructural evolution and contribute to enhanced comprehension of the microstructure in other domains involving rapid solidification, such as the additive manufacturing of γ-TiAl alloys.

Keywords
gas atomization; rapid solidification; nonequilibrium state; β-solidifying γ-TiAl alloy; recrystallization

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