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Material extrusion (ME) using a filament including metal powders has recently attracted considerable attention because it allows the production of metal parts at low cost. However, like other additive manufacturing processes, metal ME suffers from the problem of internal pores. In this study, warm isostatic pressure (WIP)—a post-process used to downsize or remove the pores in polymer ME—was employed in metal ME to improve the mechanical properties of the finished part. It was confirmed experimentally that the tensile strength and the strain at the ultimate tensile strength were increased by WIP. However, from hardness tests, two different results were obtained. On a microscopic scale, there was no change in hardness because the temperature of the WIP process was not high enough to change the microstructure, while on a macroscopic scale, the hardness changed owing to the collapse of the pores within the material under the indenter load. In specimens with relatively large pores, the hardness sensitivity increases with a larger indenter. Finally, factors affecting the WIP process parameters in metal ME were discussed.

Cellular structures achieved using high-precision additive manufacturing techniques have enabled extraordinary results, such as negative Poisson ratio, negative thermal expansion, and optical metamaterials. However, the use of cellular structures in mechanical applications is disadvantageous because of 1) limited design options, causing limited mechanical performances and 2) the absence of systematic design methodologies. Hence, we propose a novel structure inspired by the polycrystalline structure to enhance design variability as well as mechanical reliability. This is called meta grain structure (MGS). A systematic design methodology is also proposed to obtain the optimal design of MGS that satisfies the target mechanical property. A database with 138 different MGSs is built and tested numerically, so that the correlation between the relative densities of grains in the MGS and the corresponding mechanical properties is analyzed. In addition, the optimum design parameters of the MGS that satisfy the target Young’s modulus are obtained using particle swarm optimization (PSO) and machine learning (ML)-based regression models. The proposed design methodology enables us to build a unique structure that offers various design options and overcomes the current limitations of lattice structures. Furthermore, a systematic inverse design methodology using PSO and ML-based regression models helps us to create an MGS that satisfies the required mechanical performance

Development of an efficient and stable electrocatalyst for the oxygen evolution reaction (OER) is crucial to generate hydrogen via water splitting as a sustainable fuel. Nickel iron layered double hydroxides (NF-LDHs) are considered the most promising electrocatalysts for alkaline water oxidation among various low-cost transition metal-based electrocatalysts although exact mechanisms are still on debate. Herein, we disclose that quaternary multimetallic Ni–Fe–Al–Co LDHs (NFAC-MELDHs) function as one of the best catalysts for alkaline as well as seawater oxidation due to the synergetic effects among the four different redox-flexible metals. The multimetallic Ni–Fe–Al–Co LDHs are prepared via the metal–organic framework (MOF)-derived electrochemical incorporation of fourth transition metal (Co) into ternary Ni–Fe–Al LDHs grown by a hydrothermal reaction. Moreover, we reveal an exact electrocatalytic mechanism for the OER in NFAC-MELDHs via ex situ spectroscopies in combination with density functional theory (DFT) calculations. Redox-flexible Fe is identified with a real active site in synergy with the neighboring metals stabilizing adsorption of oxygen intermediates and simultaneously facilitating charge transfer. In consequence, NFAC-MELDHs exhibit one of the lowest overpotentials of 220 and 280 mV for affording a current density of 100 mA cm−2 in alkaline and simulating seawater solutions, respectively. More importantly, activity and stability merits in electrocatalysis for the OER are improved in the sequence of unary, binary, ternary, and quaternary LDHs, implying that catalyst design using multimetals for LDHs is a highly promising strategy.

In this study, the residual stress reduction through low energy density reheating method in selective laser melting (SLM) process and the correlation between microstructure and mechanical properties were investigated. Inconel 718 superalloy specimens were manufactured via SLM and subjected to reheating (RH) strategies with different energy density, which was based on laser rescanning method. The microstructures were characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The microstructure was observed in the vertical section, parallel to the building direction. With a scanning speed decrease in the RH conditions, the low angle grain boundary (LAGB) and average misoriented region fraction progressively increased. RH conditions exhibited higher Vickers hardness, tensile strength, and yield strength than those of the as-built condition, which increased slightly when the scanning speed was increased. Thermal residual stress was generally higher at z-axis than that at the x-axis. The lowest residual stress value was observed in the highest scanning speed RH condition, and as the scanning speed decreased, it became similar to that of the as-built condition.

Purpose When a pure titanium component is fabricated in a selective laser melting (SLM) process using titanium powder, the oxygen concentration of the SLM sample increases compared to the initial powder. The purpose of this paper is to study the reason for increasing oxygen concentration after SLM. Design/methodology/approach To understand this phenomenon, the authors analyzed the oxidation behavior during the SLM process thermodynamically. Findings Based on the laser parameters used in this study, the temperature of the Ti melt during the SLM process was expected to rise to 2,150°C. Based on the thermodynamic analysis, the equilibrium oxygen partial pressure for oxidation was 2.32 × 10−19 atm at 2,150°C when the dissolved oxygen concentration in the titanium is 0.2 wt.%. However, the oxygen partial pressure inside the SLM chamber was 1 × 10−3 atm, which is much higher than the equilibrium oxygen partial pressure. Therefore, oxidation occurred during the SLM process, and the oxygen concentration of the SLM sample increased compared to the initial powder. Originality/value Most studies on fabricating Ti components using additive manufacturing (AM) have been focused on how the changes in the microstructures and mechanical properties depend on the process parameters. However, there are a few studies that analyzed the oxygen concentration change of Ti during the AM process and its causes. In this study, the authors analyzed the oxidation behavior during the SLM process thermodynamically.

Laser beam melting (LBM) based on the complete-melting energy density for commercially pure titanium (CP-Ti) was investigated. Process parameters were set corresponding to the thermodynamically calculated energy density for full melting of CP-Ti. From the calculated energy density, power of 50–250 W and scan speed of 371–1855 mm/s were derived with the fixed other conditions. Microstructural study on formation of pores was conducted to understand the change in part’s density. Fundamental study with wide range of LBM process parameters, particularly based on the complete-melting energy density, could confirm the effects like high density and porous structure.

To improve the reliability and functionality of medical commercially pure titanium (CP-Ti) part fabricated by electron beam additive manufacturing (EBAM), we studied a post-treatment like cyclic heat treatment. For uniform microstructure by inducing the repetitive nucleation during phase transformation, the specimens were repeatedly heat-treated from 25 °C to 1,000 °C in Ar atmosphere. Microstructural and mechanical studies were carried out with as-built specimen and heat-treated specimens for 1, 3, and 5 cycles. Grain size was uniformly refined from 61.2 μm to 19.4 μm, in addition the elastic modulus was lowered from 136.8 GPa to 64.4 GPa. Particularly, the martensitic α′′ phase with orthorhombic structure in few tens of nm size was observed after cyclic heat treatment which was considered to reduce the elastic modulus. Although the yield and tensile strengths were slightly decreased after cyclic heat treatment, it was still higher value compared with the wrought CP-Ti part. For the patient-specific CP-Ti part, consequently, the cyclic heat treatment could be considerable post-process without any mechanical process.

Purpose: This in vitro study aimed to determine the efficacy of a damping capacity assessment in evaluating the implant stability in a simulated peri-implant bone loss model. Materials and Methods: The same type of implant was placed sequentially in 0.5-mm-depth increments in polyurethane bone of a constant density, resulting in 11 specimens with varying surrounding bone levels. The implant stability was evaluated by a damping capacity assessment consisting of six consecutive impacts in one set. The damping results, including the contact time and stability index, were measured by three repeated sets of stability tests for each specimen. All implant micromotions were recorded in real time using a laser scanning vibrometer during these stability tests. The micromotions were analyzed in terms of three parameters: maximum displacement, expected mobility, and vibration frequency. Additionally, two other stability indices were acquired three times each for reference. Pearson correlation analysis was used to confirm the correlations among all the variables; P < .05 was considered statistically significant. Results: As the peri-implant bone level increased, the contact time results decreased gradually from 502 to 290 μs, and the stability index increased from 55 to 78. The implant micromotions of all specimens showed a damped sine waveform graph, which can be divided into impact displacement and self-vibration patterns by the contact end points. As the implant stability increased, these contact end points converged toward the third peak, the maximum displacement and expected mobility decreased, and the vibration frequency increased (ρ = –0.85, –0.88, and 0.99, respectively). Two other stability indices reflected the implant stability due to peri-implant bone loss. The statistical analysis indicated significant correlations among all measured variables; in particular, the three stability indices exhibited high correlations with each other (ρ = 0.99, –0.99, and –1.00, respectively). Conclusion: Within the limitations of this in vitro study, the implant stability measured by a damping capacity assessment was suitable for investigating the extent of implant micromotions, which were determined by 0.5-mm horizontal changes in the peri-implant bone level.

The effects of laser energy density on microstructural, mechanical properties, and chemical composition of stainless steel 316 L (SUS316L) parts fabricated by selective laser melting (SLM) technology were studied. Total 36 specimens were fabricated under the range from 1.3 to 46.7 J/mm3. The process conditions to achieve over 99.5% ± 0.1% relative planar density were obtained, and the mechanism of tunable mechanical properties was investigated by understanding the correlation between the microstructure and chemical composition according to the energy density. As the energy density increased, tensile properties were decreased with grain growth and the concentrations of light elements were increased by accelerating dissolution. As the concentrations of light elements increased, the mechanism of destructive behavior changed from ductile fracture to brittle fracture and it caused that the hardness was improved. Consequently, it was necessary to control the energy density from 9.34 to 23.98 J/mm3 in order to fabricate SUS316L parts of high strength and high elongation characteristics by SLM method.

This study analyzed the variation of mechanical properties and its causes with increasing the laser power in the fabrication of pure titanium by selective laser melting (SLM). SLM samples were fabricated using commercially pure titanium grade 1 powder when the scan speed was 1000 mm/s and the laser power as 120, 200, 280, 360, and 440 W, respectively. As the laser power increased, the hardness and strength of the samples increased gradually. During the SLM processing, the concentrations of oxygen and nitrogen in the SLM samples were increased, which resulted in the increase of hardness and strength. The SLM equipment used in this study removed oxygen in the chamber by flowing high purity argon gas and fabricates the sample while preserving the oxygen concentration in the atmosphere to 0.2%. Evaluating the possibility of oxidation and nitriding during the SLM process by thermodynamic analysis, it was found that the process occurred under conditions in which temperature and residual oxygen and nitrogen partial pressure led to oxidation and nitriding.

Pure titanium components fabricated by casting have a coarse grain microstructure. To improve the mechanical strength of pure titanium components by refining the grain size, the cast samples were r...

Titanium parts in special industries such as vacuum, catalyst and medical application require a variety of surface properties with a unique structure for high efficiency. In this study, the unique surface of commercially pure titanium (CP Ti) parts fabricated using Electron Beam Melting (EBM) in the process of additive manufacturing is post-treated by the electrolytic polishing process under conditions ranging from 10 to 30 V and 10–600 s in order to study the surface characteristics. The possibility of controlling microstructural surface shape according to electrolytic polishing process conditions is presented from the increase of surface area through the formation of micro-dimples on protruding residual powder on the surface to the flattening of entire surface.

Microstructural characterization was carried out during the pre-oxidation of Ni–20Cr–8Al alloy foam using transmission electron microscopy (TEM). During the pre-oxidation at 1000 °C for 1, 30, and 60 min in air, the sequential formations of NiO, NiCr2O4, Cr2O3, and α-Al2O3 1-μm-thick oxide layers were, respectively, characterized. Initially, during pre-oxidization, the layers formed abnormally in an island growth mode, but they grew to be morphologically uniform after 30 min. Pores were found after only 1 min in the middle region of the oxide layers, near the Cr2O3 layer, and then these developed into critical micro-cracks after 60 min. Copyright © 2017 John Wiley & Sons, Ltd.

Chemical composition of pure titanium, particularly oxygen, severely affects mechanical properties of product. Oxygen refining during electron-beam additive manufacturing (EBAM) of pure titanium products was thermodynamically analyzed. The oxygen concentration of the titanium sample fabricated by EBAM was 0.172%, which was lower than that of the initial titanium powders (0.228%). This was mainly because the TiO2 native oxide layer formed on the initial powders reduced during EBAM, thereby decreasing the oxygen concentration. Analysis of temperature and oxygen partial pressure in the EBAM building chamber confirmed the occurrence of reduction and hence the oxygen refining effect.

This study investigated the corrosion behavior of the additive-manufactured CoCr parts to understand how the surface modification with plasma electrolytic polishing (PEP) influenced the corrosion resistance of CoCr alloy. As the open circuit potential (OCP) of the mechanical polishing (MP)-treated samples showed an anodic shift compared to that of the PEP-treated one and the oscillation in the OCP, this study inferred that the PEP process forms more corrosion-resistant oxide film based on the transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS) results. After the PEP treatment, the continuous oxide film was observed on the polished sample's surface, which resulted in a stable OCP. Also, the EIS results showing the higher oxide resistance and charge transfer resistance, apparent two-time constants, and phase angle over 80 degrees indicated that the PEP-induced oxide film was more corrosion resistant, compared to the film formed by the MP process.

In the additive manufacturing process, fine powders having spherical shape are used due to their high flowability and packing density. When powders are produced by the water atomization method, it is advantageous to prepare fine powders, but powders having irregular shape are prepared, which is not suitable for the AM process. Therefore, in this study, 316 stainless steel powder fabricated by water atomization method were spheroidized using inductively-coupled thermal plasma. The initial water atomized 316 stainless steel powders had an irregular shape; however after spheroidization, their morphology was changed to a perfect spherical shape, which results in the drastic improvement of flowability. Examining the composition changes after spheroidizing, it was confirmed that the concentrations of oxygen and Cr were decreased. The reason for the decrease in oxygen concentration was that the temperature in the plasma process is extremely high and the oxygen partial pressure in the plasma chamber is low. The decrease in Cr concentration was caused by the evaporation of Cr during the spheroidization process, and the vapor pressure and evaporation coefficient of Cr in 316 molten liquid were calculated to analyze the reason.

In the present study, the osseointegration of additively manufactured screws with a micro-rough surface structure and conventionally machined Ti6Al4V ELI screws with a smooth surface structure is compared. Screws were implanted in rabbit femurs or tibiae for two weeks. The additively manufactured screws exhibited a micro-rough surface with attached semi-molten powder with a size of 50–100 μm and blunt threads, whereas the conventionally machined screws showed a smooth surface and sharp threads. The bonding strength of the additively manufactured screws was significantly higher than that of the conventionally machined screws. The additively manufactured screws showed higher bone-to-implant contact than the conventionally machined screws. Considerable amount of bone tissue on the surface of the additively manufactured screws remained after the push-out tests, and the rabbit tibiae with the additively manufactured screws were significantly damaged, indicating a higher osseointegration rate. Active osteogenesis was observed around the semi-molten powder of the additively manufactured screws, and new bone was well developed along the micro-rough surface. Overall, this study shows that the micro-rough surface improved the bone ingrowth and suggests that additively manufactured screws may be an alternative to conventionally machined screws.

A transmission electron microscopy analysis was conducted on the oxide layers of Mg–5.5Al–0.25Mn (AM60) alloys with different levels of CaO content formed in a thermogravimetric analyzer (TGA) system at 450 °C for 7 h. It was found that the oxide layer in AM60 + CaO grew as a columnar structure and, resultantly, that the oxide layers had different thicknesses with different levels of CaO content. The columnar oxide grains that were formed on the Mg + CaO alloy grew on the [1 0 0] MgO , resulting in a more compact and thicker initial oxide layer compared to the normal oxide layer. The columnar growth of the oxide layer in AM60 + CaO formed due to the Ca, which dissolved from CaO.

The refining effect of electron beam melting (EBM) during the additive manufacturing of Ti products was studied by comparing unused Grade-2 commercially pure (CP) Ti powder to an equivalent powder that had been recycled 50 times. The pre-heating and blasting processes that occur during recycling were found to degrade the perfectly spherical morphology of the powder, and the concentration of O and N were also increased by about 15%. This resulted in it changing to a Grade-3 CP Ti powder; however, the purity of the final product was improved to Grade-1 CP Ti by the slight refining effect of the EBM process.

Purpose The purpose of this study was to compare the daily food intake rate and the rate of screw loosening between 2 groups of rabbits with mandibular continuity defects: custom implant (CI) group and 5-hole mini-plate group. Materials and Methods Two types of cylindrical implants were printed, and their physical strength was compared. In this study using rabbits, 1 group (n = 5) received a CI for the reconstruction of a mandibular continuity defect (CI group) and the other group (n = 5) received a 5-hole mini-plate without a bone graft (reconstruction plate [RP] group). After reconstruction, the daily food intake rate and the rate of screw loosening were examined postoperatively. Histologic examination in the CI group was performed 3 months after the operation. Results The design that mimicked the mandible showed greater physical strength. The amount of time required to achieve 50% recovery was shorter in the CI group than in the RP group (P = .011). The total number of loosened screws in the CI group was lower than that in the RP group at 3 months postoperatively (P = .008). New bone formation in the porous CI was evident in the CI group. Conclusions Rabbits with mandibular continuity defects treated with CIs for reconstruction showed faster recovery of the daily food intake rate and fewer loosened screws than those treated with a 5-hole mini-plate without bone graft.

Hard carbon and microcrystalline graphite (MG) core-shell structured composite materials are prepared, and their electrochemical performances as an anode material for lithium-ion batteries are reported. The composite materials are obtained by coating a mixture of MG and pitch onto hard carbon particles, followed by heating at 1200 °C under an argon atmosphere for 1 h. The surface of the hard carbon is subsequently covered with a layer of the MG/pitch carbon composite. In the coating layer of the MG/pitch carbon composite, the MG particles are divided into nanoscale graphite sheets, and uniformly dispersed within the pitch of carbon matrix. The composite particles have a rounded shape, especially when the content of MG increases, which can improve their packing density compared to hard carbon having sharp edges. Anodes prepared from these composite materials exhibit enhanced electrochemical performances, including a high reversible capacity, high initial coulombic efficiency, high charging/discharging rate capability, and desirable cycling stability.

Three types of oxide layers Al2O3, Al1.98Cr0.02O3, and AlFeO3 phases on the surface of Fe–22Cr–6Al (FeCrAl) foam during pre-oxidation were characterized by means of transmission electron microscopy (TEM). The growth rate of each oxide layer was investigated using measurement of total weight gain. The Al2O3 layer mainly affected the growth rate of the total oxide layer, and it hindered the diffusion of Fe and Cr atoms from the matrix to the oxide layers.