Review : Microstructural Control and Functional Enhancement of Light Metal Materials via Metal Additive Manufacturing

Additive manufacturing (AM) has been attracting a great deal of attention in both academia and industry in recent years as a technology that could bring innovation to manufacturing. AM was originally developed as a method specialized in fabricating three-dimensional structures by the additive manner. However, in reality, a huge number of parameters involved in AM has a significant effect on the microstructure and the resulting physicochemical properties of the metallic material. Therefore, in very recent years, metal AM is being recognized as a technology for controlling the microstructure of metals rather than its shape. In addition, AM can even customize the microstructure of each site by applying locally controlled heat energy. The ability to simultaneously control complex shapes and microstructures will add even higher value to light-weight metal materials. This paper describes the potential of metal AM to control material and shape properties that dictates the essential mechanical properties of the product with introducing latest results.Ishimoto Takuya, Nakano Takayoshi. Review : Microstructural Control and Functional Enhancement of Light Metal Materials via Metal Additive Manufacturing. MATERIALS TRANSACTIONS 64, 10 (2023); https://doi.org/10.2320/matertrans.MT-MLA2022007

Tailoring the crystallographic texture of biomedical metastable β-type Ti-alloy produced via laser powder bed fusion using temperature-field simulations

Recently, the use of laser powder bed fusion (LPBF) to create crystallographic textures, such as single-crystal-like and polycrystalline textures, has attracted attention. However, the relationship between the LPBF conditions and the resulting texture is unclear. This study investigates the effects of the LPBF conditions (laser power and scanning speed) on the texture by estimating the solidification behavior using temperature-field simulations. Herein, we show for the first time that laser power and scanning speed negatively and positively affect the solidification rate R, respectively, and do not affect the thermal gradient G significantly. Thus, when the laser power decreases and scanning speed increases, the G/R ratio decreases and polycrystal formation is enhanced. This is consistent with practical observations.Ishimoto T., Suganuma R., Nakano T.. Tailoring the crystallographic texture of biomedical metastable β-type Ti-alloy produced via laser powder bed fusion using temperature-field simulations. Materials Letters 349, 134835 (2023); https://doi.org/10.1016/j.matlet.2023.134835

Stability of crystallographic texture in laser powder bed fusion: Understanding the competition of crystal growth using a single crystalline seed

In metal additive manufacturing, crystallographic orientation control is a promising method for tailoring the functions of metallic parts. However, despite its importance in the fabrication of texture-controlled functional parts, the stability of the crystallographic texture is not widely discussed. Herein, the crystallographic texture stability under laser powder bed fusion was investigated. Two methodologies were employed. One is that a laser scanning strategy was alternately changed for a specific number of layers. The other is a “seeding” experiment in which single-crystalline substrates with controlled crystallographic orientations in the building (z-) direction and the xy-plane (perpendicular to the building direction) were used as the starting substrate. The transient zone width, where the crystallographic orientation was inherited from the layer beneath, was analyzed to evaluate the texture stability. The crystallographic direction of the seed within the xy-plane, rather than the building direction, determined the transient zone width, i.e., the texture stability. In particular, the texture in the newly deposited portion was stable when the laser scanning direction matched the orientation in the underneath layer, otherwise the crystal orientation switched rapidly, such that the orientation was parallel to the scanning direction. Interestingly, the crystallographic orientation along the building direction in the underneath layer hardly impacted the stability of the texture. Therefore, for the first time, it has been clarified that the orientation in the scanning direction, rather than the building direction, was preferentially stabilized, whereas the orientation in the other directions secondary stabilized.Ishimoto T., Hagihara K., Hisamoto K., et al. Stability of crystallographic texture in laser powder bed fusion: Understanding the competition of crystal growth using a single crystalline seed. Additive Manufacturing, 43, 102004. https://doi.org/10.1016/j.addma.2021.102004

Modified cellular automaton simulation of metal additive manufacturing

Metal additive manufacturing (AM) technologies are attracting attentions not only as a fabrication process of complicated threedimensional parts but also as microstructure controlling processes. In powder bed fusion (PBF)-type AM, crystallographic texture can be controlled by scanning strategies of energy beam. To optimize microstructures, computer simulations for predicting microstructures play very important roles. In this work, we have developed simulation programs to explain the mechanism of the crystal orientation control. First, we simulated the shape of melt pool by analyzing the heat transfer using apparent heat conductivity when the penetration of laser beam through keyholes was taken into consideration because of the evaporation and accompanying convections. It was assumed that the primary crystal growth direction can be determined by the temperature gradient, and the crystals grow keeping the growth direction as generally recognized. The shapes of simulated melt pools agree well with experimental observations. The modified cellular automaton simulations successfully reproduced two typical textures with different preferential orientations along the building directions of (100) and (110) when the bidirectional scanning with and without a rotation of 90°, respectively, was accomplished between the layers.Kubo J., Koizumi Y., Ishimoto T., et al. Modified cellular automaton simulation of metal additive manufacturing. Materials Transactions 62, 864 (2021); https://doi.org/10.2320/matertrans.MT-M2021009

Review : Research and Development of Titanium-Containing Biomedical High Entropy Alloys (BioHEAs) Utilizing Rapid Solidification via Laser-Powder Bed Fusion

High entropy alloys (HEAs) have been developed as a new class of structural materials that consist of multicomponent elements with an approximately equiatomic ratio for increasing the mixing entropy to stabilize the solid solution phase. HEA for biomedical applications (BioHEA) was first developed in Japan; HEA comprising nonbiotoxic elements was specifically designed, demonstrating excellent mechanical properties and biocompatibility. However, elemental segregation, often observed in BioHEAs, hinders the inherent functions derived from high entropy effects and solid solution hardening. In this review article, elemental homogenization and functionalization of BioHEAs utilized by ultra-rapid cooling via laser-powder bed fusion and the characteristics of these BioHEAs, especially focusing on their excellent properties for biomedical applications, are introduced.Ozasa Ryosuke, Matsugaki Aira, Ishimoto Takuya, et al. Review : Research and Development of Titanium-Containing Biomedical High Entropy Alloys (BioHEAs) Utilizing Rapid Solidification via Laser-Powder Bed Fusion. MATERIALS TRANSACTIONS 64, 31 (2023); https://doi.org/10.2320/matertrans.MT-MLA2022011

A Novel Ex Vivo Bone Culture Model for Regulation of Collagen/Apatite Preferential Orientation by Mechanical Loading

The anisotropic microstructure of bone, composed of collagen fibers and biological apatite crystallites, is an important determinant of its mechanical properties. Recent studies have revealed that the preferential orientation of collagen/apatite composites is closely related to the direction and magnitude of in vivo principal stress. However, the mechanism of alteration in the collagen/apatite microstructure to adapt to the mechanical environment remains unclear. In this study, we established a novel ex vivo bone culture system using embryonic mouse femurs, which enabled artificial control of the mechanical environment. The mineralized femur length significantly increased following cultivation; uniaxial mechanical loading promoted chondrocyte hypertrophy in the growth plates of embryonic mouse femurs. Compressive mechanical loading using the ex vivo bone culture system induced a higher anisotropic microstructure than that observed in the unloaded femur. Osteocytes in the anisotropic bone microstructure were elongated and aligned along the long axis of the femur, which corresponded to the principal loading direction. The ex vivo uniaxial mechanical loading successfully induced the formation of an oriented collagen/apatite microstructure via osteocyte mechano-sensation in a manner quite similar to the in vivo environment.Watanabe R., Matsugaki A., Ishimoto T., et al. A Novel Ex Vivo Bone Culture Model for Regulation of Collagen/Apatite Preferential Orientation by Mechanical Loading. International Journal of Molecular Sciences, 23, 13, 7423. https://doi.org/10.3390/ijms23137423

Reduction of spatter generation using atmospheric gas in laser powder bed fusion of Ti-6Al-4V

Laser powder bed fusion (LPBF), a typical additive manufacturing (AM) process, is a promising approach that enables high-accuracy manufacturing of arbitrary structures; therefore, it has been utilized in the aerospace and medical fields. However, several unexplained phenomena significantly affect the quality of fabricated components. In particular, it has been reported that the generation of spatters adversely affects the stability of fabrication process and degrades the performance of the fabricated components. To realize high-quality components, it is essential to suppress the generation of spatters. Thus far, the suppression of spatter generation has been attempted based on the process parameters; however, this has not been adequately discussed in terms of the fabrication atmosphere. Therefore, in this study, we focused on the fabrication atmosphere and investigated spatter generation using gas with different physical properties rather than conventionally used argon. It was observed that the spatter generation during the fabrication of the Ti6Al4V alloy could be significantly suppressed by changing the atmospheric gas, even under constant LPBF process parameters. We proved that the fabrication atmosphere is an important factor to be considered, apart from the process parameters, in AM technology.Amano H., Yamaguchi Y., Ishimoto T., et al. Reduction of spatter generation using atmospheric gas in laser powder bed fusion of Ti-6Al-4V. Materials Transactions 62, 1225 (2021); https://doi.org/10.2320/matertrans.MT-M2021059

Lattice distortion in selective laser melting (SLM)-manufactured unstable β-type Ti-15Mo-5Zr-3Al alloy analyzed by high-precision X-ray diffractometry

A peculiar lattice distortion in a selective laser melting (SLM)-manufactured unstable β-type Ti-15Mo-5Zr-3Al was observed for the first time, through high-precision X-ray diffraction (XRD) analyses. After SLM, Ti-15Mo-5Zr-3Al exhibited a body-centered-tetragonal structure instead of a body-centered-cubic structure; the c-axis was 0.63% shorter than the a-axis. The XRD analyses also revealed tensile residual stresses of 210 ± 12 MPa at the specimen surface. A numerical simulation indicated rapid cooling during the SLM, which could have caused the residual stresses. A comparison of the partially stress-released SLM specimen and an electron beam melting-manufactured specimen with negligible residual stress suggested that the residual stress caused by the rapid cooling in SLM induced the lattice distortion. This finding is not consistent with the previous understanding that residual stress changes the lattice parameter without lattice distortion. This study provides new insight into lattice distortion generated by a combination of SLM-specific ultrarapid cooling and unstable phases.Takase A., Ishimoto T., Suganuma R., et al. Lattice distortion in selective laser melting (SLM)-manufactured unstable β-type Ti-15Mo-5Zr-3Al alloy analyzed by high-precision X-ray diffractometry. Scripta Materialia, 201, 113953. https://doi.org/10.1016/j.scriptamat.2021.113953

Surface residual stress and phase stability in unstable β-type Ti–15Mo–5Zr–3Al alloy manufactured by laser and electron beam powder bed fusion technologies

The differences between the physicochemical properties of the laser and electron beam powder bed fusion (L- and EB-PBF) methods are yet to be explored further. In particular, the differences in the residual stress and phase stability of alloys with unstable phases remain unexplored. The present work is the first to systematically investigate how the heat source type and process parameters affect the surface residual stress and phase stability of an unstable β-type titanium alloy, Ti–15Mo–5Zr–3Al. The surface residual stress and β-phase behavior were studied using high-precision X-ray diffraction (HP-XRD). Significant differences were observed between the two methods. The L-PBF-made specimens exhibited tensile residual stresses of up to 400 MPa in the surface area. HP-XRD analysis revealed a stress-induced lattice distortion, interpreted as a transitional state between the β-phase and α”-phase. In contrast, the EB-PBF-made specimens showed no significant residual stress and had an undistorted β-phase coexisting with the hexagonal α-phase caused by elemental partitioning. This study provides new insights into the previously neglected effects of L-PBF and EB-PBF in unstable β-type titanium alloys.Takase A., Ishimoto T., Suganuma R., et al. Surface residual stress and phase stability in unstable β-type Ti–15Mo–5Zr–3Al alloy manufactured by laser and electron beam powder bed fusion technologies. Additive Manufacturing, 47, 102257. https://doi.org/10.1016/j.addma.2021.102257

Ibandronate Suppresses Changes in Apatite Orientation and Young's Modulus Caused by Estrogen Deficiency in Rat Vertebrae

Bone material quality is important for evaluating the mechanical integrity of diseased and/or medically treated bones. However, compared to the knowledge accumulated regarding changes in bone mass, our understanding of the quality of bone material is lacking. In this study, we clarified the changes in bone material quality mainly characterized by the preferential orientation of the apatite c-axis associated with estrogen deficiency-induced osteoporosis, and their prevention using ibandronate (IBN), a nitrogen-containing bisphosphonate. IBN effectively prevented bone loss and degradation of whole bone strength in a dose-dependent manner. The estrogen-deficient condition abnormally increased the degree of apatite orientation along the craniocaudal axis in which principal stress is applied; IBN at higher doses played a role in maintaining the normal orientation of apatite but not at lower doses. The bone size-independent Young's modulus along the craniocaudal axis of the anterior cortical shell of the vertebra showed a significant and positive correlation with apatite orientation; therefore, the craniocaudal Young’s modulus abnormally increased under estrogen-deficient conditions, despite a significant decrease in volumetric bone mineral density. However, the abnormal increase in craniocaudal Young's modulus did not compensate for the degradation of whole bone mechanical properties due to the bone loss. In conclusion, it was clarified that changes in the material quality, which are hidden in bone mass evaluation, occur with estrogen deficiency-induced osteoporosis and IBN treatment. Here, IBN was shown to be a beneficial drug that suppresses abnormal changes in bone mechanical integrity caused by estrogen deficiency at both the whole bone and material levels.Ishimoto T., Saito M., Ozasa R., et al. Ibandronate Suppresses Changes in Apatite Orientation and Young's Modulus Caused by Estrogen Deficiency in Rat Vertebrae. Calcified Tissue International, 110, 6, 736. https://doi.org/10.1007/s00223-021-00940-2