Possible formation of extra amount of Fe vacancies during the massive-like phase transformation of carbon steel has been examined by combinations of analytical modeling, atomistic simulations and phase-field modeling. Formation of a vacancy in a perfect crystal does not decrease its volume by as much as the volume of an atom, effectively increasing lattice constant per atom. Consequently, transformation strain due to solid-state transformation from BCC-structured δ phase to FCC-structured γ phase leads to formation of extra amount of Fe vacancies more than in thermal equilibrium. This is to relieve transformation strain due to nucleation and growth of γ phase, which would promotes the massive-like transformation. The extra vacancies may also promote diffusion of substitutional alloying elements across δ/γ interface or partitioning of the alloying elements during the massive-like transformation, at least in the nucleating γ phase and in the vicinity of δ/γ interface until transformation strain is relieved.

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Additive manufacturing（AM）technologies have attracted much attention because it enables us to build 3D parts with complicated geometry easily and control material properties significantly via the control of microstructures. In the powder- bed fusion（PBF）type AM process, 3D parts are fabricated by repeating a process of melting and solidifying metal powders by laser or electron beams. In general, the solidification microstructures can be predicted from solidification conditions defined by the combination of temperature gradient G and solidification rate R on the basis of columnar-equiaxed transition（CET）theory proposed by Hunt [Mater. Sci. Eng. 65, 75（1984）]. However, it is unclear whether the CET theory can be applied to the PBF type AM process because of the high G and R, even for general 316L stainless steel. In this study, to reveal relationships between microstructures and solidification conditions, we have analyzed solidification microstructures of 316L steel induced by electron- beam irradiation and evaluated solidification conditions at the solid/liquid interface using a computational thermal-fluid dynamics （CtFD）method. It was found that equiaxed grains were often formed under high G conditions contrary to the CET theory. CtFD simulation revealed that there is a fluid flow up to a velocity of about 400 mm s -1 , and suggested that equiaxed grains are formed owing to the effect of fragmentations and migrations of dendrites.

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Abstract: Solute segregation significantly affects material propert ies and is an essential issue in powder-bed fusion（PBF）type additional manufacturing（AM）of nickel-based superalloys. In the present study, we have investigated the effects of solidification conditions of temperature gradients and solidification rates on segregation behavior in Hastelloy-X Ni-based superalloy using multi- phase field（MPF）simulation. To evaluate liquation crack susceptibility, the solidus temperatures were calculated using predicted compositions of the segregated regions. MPF simulations revealed that the segregations of Mo and Cr and the depletion of Co become more remarkable with increasing cooling rate, and this solute segregation causes the decrease of a solidus temperature up to 17.7 K. In the PBF type AM process, metallic powders are solidified under a unique condition of a high temperature gradient G and a high solidification rate R, and suggested that strong solute segregation and liquation cracks occur.

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Abstract Metal additive manufacturi ng is expected to achieve a manufacturing revolution by merging the advance technologies such as information technology and image-sensing technology. The concept on the design of process parameters in metal additive manufacturing by means of machine learning is discussed in the present work. In order to realize the discussed concept, the process parameter in laser additive manufacturing is tried to be predicted from the images of molten pool geometry by the analysis of inverse problems with machine learning. It is found that the prediction of pitch width, which is one of the process parameters, can be achieved by probabilistic classification.

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Preparation of composite powders with uniform dispersion, high sphericity, suitable particle size and distribution is a great concern in fabricating high-performance components via additive manufacturing. Herein, we developed a novel technology, namely freeze-dry pulsated orifice ejection method（FD-POEM）, to prepare monodispersed spherical particles. Taking ZrO 2 nano-powders as an example, high-concentration slurries with homogeneous dispersion were prepared. The influence of slurry concentrations or dispersant types on the dispersion behavior of slurry and the properties of FD-POEM particles was thoro ughly investigated. This work demonstrates the great potential of FD-POEM as a promising method to fabricate monodispersed spherical composite powders.

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Laser powder bed fusion（LPBF）, one of additive manufacturing（AM）techniques, is a promising technology that enables arbitrary structures to be fabricated with high accuracy and used in the aerospace and medical fields. Also, LPBF is an attractive method to improve the functionality of fabricated parts from metallographic control. However, when considering the high functionality of fabricated parts, the optimal process parameters such as laser power, scan speed, and pitch width are limited and may not be compatible with the conditions for further high functionality. In this study, we focused on the fabrication atmosphere, which has not been discussed widely, and investigated the atmosphere’s effect on the fabricated parts. As a result, we confirmed a change in the metallurgical structure by changing the atmosphere gas. Furthermore, we found that the fabrication atmosphere may be a new important factor as well as process parameters in AM technique.

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Laser powder bed fusion（LPBF）is a type of additive manufacturing technology capable of fabricating 3-dimensional parts with a complex shape from powdered metallic materials. Among LPBF compatible metals, β-type titanium alloys are considered ideal candidates for custom implants because of their low Young’s modulus. However, the effects of the LPBF process on its residual stresses are not well understood. In this study, we applied the X-ray diffraction（XRD）method to investigate the relationship between residual stresses and laser beam scan strategy using the LPBF-made β-type Ti-15Mo-5Zr-3Al parts. We successfully measured the residual stresses in Ti-15Mo-5Zr-3Al fabricated by the two scan strategies, X-scan and XY-scan. The tensile residual stress over 200 MPa was detected on both parts, and no significant difference was observed between the two scan strategies.

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The influences of input energy density determined by the processing parameters on structural integrity and microstructure of b-containing Ti-44Al-4Cr alloy rods fabricated by electron beam melting process were investigated. We found that it is important to control the input energy density to obtain the rods with good dimensional accuracy. Moreover, we also found that the microstructure of the rods depends strongly on the input energy density. The rods fabricated at higher energy densities show a uniform α 2 /β/γ mixed structure. On the other hand, ultrafine α 2 /γ lamellar grains and β/γ cells which are discontinuously precipitated at the grain boundary of the lamellar grains can be seen at lower energy densities. The unique ultrafine lamellar grains are originated from the massive a grains formed upon rapid cooling. The strength and ductility of the alloys are closely related to the volume fractions of the ultrafine lamellar grain and the β/γ cells, respectively.

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Electron beam powder bed fusion has attracted much attention as an important technique for fabricating three-dimensional Ti- 6Al-4V structures for biomedical applications. Since long bones possess uniaxial anisotropy, three-dimensional macrostructures with unidirectionally elongated pores are promising candidates for biomaterials. The Young’s modulus of three-dimensional macrostructures with unidirectional pores is dominated by porosity and/or pore size. As a technique to further control the Young’s modulus, the combination of uniaxial anisotropy of macrostructure and microstructure is expected. In this article, the grain structure and the variants selectivity were analyzed through phase transformation using the SEM-EBSD method. The microstructure analysis demonstrates that there is no variant selectivity through phase transformation, but that electron beam scanning promotes the formation of elongated β-phase columnar grains along the building direction above the α/β transition temperature.

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The metal implant fixation with bone tissues is an important factor for excellent clinical outcomes in total joint arthroplasty. Various porous surfaces produced by additive manufacturing have been developed to enhance bone-implant fixation, and clinically used. However, in some cases, poor fixation of the additively manufactured implants has been reported, because they are not bioactive. In the present study, we focused on the nano-hydroxyapatite as a promising coating material for additively manufactured metal implants. Regardless of surface topographies, nano-hydroxyapatite could be coated on additively manufactured metal surfaces, and nano-hydroxyapatite coated Ti-6Al-4V alloys had high bioactivity.

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Drug-releasing implants are attracting attention as a local drug delivery system（LDDS）with high drug efficiency and few side effects because they can administer drugs locally and continuously. Since the surface structure of implants is important for drug- releasing implants, many research reports have been made using metallic surfaces with nanostructures such as TiO 2 nanotubes as the platform for LDDS. In present study, we investigated drug release characteristic of collagen coated Type 316L stainless steel with self-organized nanopores formed by anodic polarization. Regardless of whether or not nanopore structure, the collagen coating treatment increased the amount of drug loaded on samples. The samples with nanopore structure suppressed burst release and increased the rate of slow release.

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The structural regeneration of biological tissue is imperative for recovery of the organ function during tissue healing process. The mechanical and biological function of bone tissue is governed by the oriented microstructure of collagen/apatite matrix. Control of unidirectional cell alignment triggers the subsequent oriented bone matrix organization. However, the development of biomedical devices equipped with ideal design for microstructural recovery of bone tissue is not yet fully achieved. Mesenchymal stem cells （MSCs） play important roles in bone tissue regeneration, which have been considered as a promising therapeutic target easily isolatable form the patients. Here, we propose additive manufacturing（AM）technology as a powerful tool for control of the differentiation fate and the function of mesenchymal stem cells. MSCs aligned along the grooved structure fabricated by selective laser melting. Moreover, the aligned MSCs showed upregulated expression of BGLAP, an important osteogenic differentiation marker gene. The results indicate the technological advances of AM process, which realize the functional bone tissue regeneration from MSCs.

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Copper is an electrically and thermally conductive material widely used in industry, and copper-based materials are expected to facilitate a future society with a high degree of digital transformation. Laser beam powder bed fusion（LB-PBF）, a metal additive manufacturing method, is a promising technology that can provide functional products with arbitrarily complex shapes, controlled microstructures and crystal orientations. However, due to the low laser absorption and high thermal conductivity of copper, fabrication of copper-based functional materials by LB-PBF has been a major challenge. In this article, we fabricated zirconium-containing copper-based alloys with enhanced laser absorption using LB-PBF and optimized the process parameters to obtain functional high-density products. The alloys fabricated using LB-PBF achieved higher densities and were characterized by directional microstructures, and thus exhibited higher electrical conductivity than the as-cast alloys, as well as pure Cu fabricated using LB-PBF.

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Ceramic dental crowns composed of yttria-stabilized zirconia（YSZ）were fabricated by stereolithographic additive manufacturing（STL-AM）. The YSZ composite appeared translucent on crown surfaces. In graphic modeling, installation structures were designed to hold and mount the crown object with free surfaces. The nanoparticles were dispersed in acrylic resins to obtain a paste-like consistency for STL processing. An ultraviolet laser beam was scanned over the evenly spread out paste to create two-dimensional （2D）cross sections. Through layer stacking and interlayer bonding, three-dimensional （3D）components were successfully shaped. The dimensional tolerances for the horizontal and vertical features were optimized by systematic modulation of the irradiated power. The composite precursors were dewaxed and sintered in ambient air to create the filled ceramic crowns. The dense ceramic microstructures were observed using scanning electron microscopy（SEM）. The linear shrinkages were measured and incorporated into the model for structural design to realize high-dimensional accuracies.

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Ceramic electrodes with dendritic lattice patterns were fabricated successfully by stereolithographic additive manufacturing （STL-AM）. Solid electrolytes of yttria and scandia stabilized zirconia（YSZ and SSZ）were selected to oxygen separation in molten salt electrolysis for aluminum refining without carbon dioxide excretions. In the graphic design of dendrite patterns, the cylinder lattices were connected at 4, 6, 8 and 12 in coordination numbers, and the aspect ratio were optimized to obtain maximal specific surfaces. Spatial profiles of high temperature liquid propagations in dendritic patterns were visualized systematically by computer fluid dynamics（CFD）. The zirconia particles were dispersed into a photo sensitive resin to obtain the paste materials for the STL process. An ultraviolet laser beam was scanned on the spread paste to create two-dimensional （2D）cross sections. Through layer laminations and interlayer bonding, three-dimensional （3D）objects with dendritic structures could be fabricated. The composite precursors were dewaxed and sintered in the air atmosphere to obtain fine ceramic microstructures.

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Solid electrolyte components of lithium lanthanum zirconate（LLZ）were successfully fabricated using stereolithographic based Additive Manufacturing（STL-AM）for all solid batteries. Micro-embossed patterns were introduced into ultrathin electrolyte sheets to increase energy density and decrease internal resistance through computer-aided design, manufacture, and evaluation （CAD/CAM/CAE）. The two-dimensional （2D）patterns were precisely formed by UV-laser scanning on photosensitive LLZO nanoparticle-acrylic resin composite surfaces. Three-dimensional（3D）structures were fabricated automatically by continuous laminations of the drawn layers and interlayer chemical bonding. These STL components were processed using cold isostatic pressing（CIP）to closely infuse the acrylic matrix with LLZ nanoparticles. Microstructural densification can be realized after dewaxing and sintering in the air atmosphere.

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