Selective laser melting (SLM) can produce a layer of a metal powder and then fabricate a three-dimensional structure by a layer-by-layer method. Each layer consists of several lines of molten metal. Laser parameters and thermal properties of the materials affect the geometric characteristics of the melt pool such as its height, depth, and width. The geometrical characteristics of the melt pool are determined herein by optical microscopy and three-dimensional bulk structures are fabricated to investigate the relationship between them. Powders of the commercially available Fe-based tool steel AISI H13 and Ni-based superalloy Inconel 738LC are used to investigate the effect of material properties. Only the scan speed is controlled to change the laser parameters. The laser power and hatch space are maintained throughout the study. Laser of a higher energy density is seen to melt a wider and deeper range of powder and substrate; however, it does not correspond with the most highly densified three-dimensional structure. H13 shows the highest density at a laser scan speed of 200 mm/s whereas Inconel 738LC shows the highest density at 600 mm/s.

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• Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions
  Jungsub Lee, Jungho Choe, Junhyeok Park, Ji-Hun Yu, Sangshik Kim, Im Doo Jung, Hyokyung Sung
  Materials Characterization.2019; 155: 109817.     CrossRef
• Correlation between Microstructure and Mechanical Properties of the Additive Manufactured H13 Tool Steel
  Woojin An, Junhyeok Park, Jungsub Lee, Jungho Choe, Im Doo Jung, Ji-Hun Yu, Sangshik Kim, Hyokyung Sung
  Korean Journal of Materials Research.2018; 28(11): 663.     CrossRef
• Effect of Porosity on Mechanical Anisotropy of 316L Austenitic Stainless Steel Additively Manufactured by Selective Laser Melting
  Jeong Min Park, Jin Myoung Jeon, Jung Gi Kim, Yujin Seong, Sun Hong Park, Hyoung Seop Kim
  Journal of Korean Powder Metallurgy Institute.2018; 25(6): 475.     CrossRef
• Evaluation of the Accuracy of Dental Prostheses manufactured by Metal 3D Printer
  Junho Hwang, Yun-Ho Kim, Hyun-Deok Kim, Kyu-Bok Lee
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• A study about sculpture characteristic of SKD61 tool steel fabricated by selective laser melting(SLM) process
  Jaecheol Yun, Jungho Choe, Ki-Bong Kim, Sangsun Yang, Dong-Yeol Yang, Yong-Jin Kim, Chang-Woo Lee, Chang-Woo Lee
  Journal of Korean Powder Metallurgy Institute.2018; 25(2): 137.     CrossRef

In this study, H13 tool steel sculptures are built by a metal 3D printing process at various laser scan speeds. The properties of commercial H13 tool steel powders are confirmed for the metal 3D printing process used: powder bed fusion (PBF), which is a selective laser melting (SLM) process. Commercial H13 powder has an excellent flowability of 16.68 s/50 g with a Hausner ratio of 1.25 and a density of 7.68 g/cm³. The sculptures are built with dimensions of 10 × 10 × 10 mm3 in size using commercial H13 tool steel powder. The density measured by the Archimedes method is 7.64 g/cm³, similar to the powder density of 7.68 g/cm³. The hardness is measured by Rockwell hardness equipment 5 times to obtain a mean value of 54.28 HRC. The optimum process conditions in order to build the sculptures are a laser power of 90 W, a layer thickness of 25 μm, an overlap of 30%, and a laser scan speed of 200 mm/s.

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• Spheroidization of Enamel Powders by Radio Frequency Plasma Treatment and Application to Additive Manufacturing
  Ki-Bong Kim, Dong-Yeol Yang, Yong-Jin Kim, Jungho Choe, Ji-Na Kwak, Woo-Hyung Jung
  Journal of Korean Powder Metallurgy Institute.2020; 27(5): 388.     CrossRef
• Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions
  Jungsub Lee, Jungho Choe, Junhyeok Park, Ji-Hun Yu, Sangshik Kim, Im Doo Jung, Hyokyung Sung
  Materials Characterization.2019; 155: 109817.     CrossRef
• Nano-mechanical Behavior of H13 Tool Steel Fabricated by a Selective Laser Melting Method
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• Correlation between Microstructure and Mechanical Properties of the Additive Manufactured H13 Tool Steel
  Woojin An, Junhyeok Park, Jungsub Lee, Jungho Choe, Im Doo Jung, Ji-Hun Yu, Sangshik Kim, Hyokyung Sung
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• Evaluation of Strain-Rate Sensitivity of Selective Laser Melted H13 Tool Steel Using Nanoindentation Tests
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• Comparison of Nano-Mechanical Behavior between Selective Laser Melted SKD61 and H13 Tool Steels
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• A study about sculpture characteristic of SKD61 tool steel fabricated by selective laser melting(SLM) process
  Jaecheol Yun, Jungho Choe, Ki-Bong Kim, Sangsun Yang, Dong-Yeol Yang, Yong-Jin Kim, Chang-Woo Lee, Chang-Woo Lee
  Journal of Korean Powder Metallurgy Institute.2018; 25(2): 137.     CrossRef

A cold-work tool steel powder is used to fabricate 3-dimensional objects by selective laser melting using a high-pressure gas atomization process. The spherical powder particles form continuous carbide networks among the austenite matrix and its decomposition products. The carbides comprise Nb-rich MC and Mo-rich M2C. In the SLM process, the process parameters such as the laser power (90 W), layer thickness (25 μm), and hatch spacing (80 μm) are kept fixed, while the scan speed is changed from 50 mm/s to 4000 mm/s. At a low scan speed of 50 mm/s, spherical cavities develop due to over melting, while they are substantially reduced on increasing the speed to 2000 mm/s. The carbide network spacing decreases with increasing speed. At an excessively high speed of 4000 mm/s, long and irregularly shaped cavities are developed due to incomplete melting. The influence of the scan pattern is examined, for which 1 × 1 mm² blocks constituting a processing layer are irradiated in a random sequence. This island-type pattern exhibits the same effect as that of a low scan speed. Post processing of an object using hot isostatic pressing leads to a great reduction in the porosity but causes coarsening of the microstructure.

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• Micro-Texture Analyses of a Cold-Work Tool Steel for Additive Manufacturing
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• Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions
  Jungsub Lee, Jungho Choe, Junhyeok Park, Ji-Hun Yu, Sangshik Kim, Im Doo Jung, Hyokyung Sung
  Materials Characterization.2019; 155: 109817.     CrossRef

In this study, STS316L powders prepared by gas atomization are used to manufacture bulk structures with dimensions of 10 × 10 × 10 mm³ using selective laser melting (SLM). The microstructures and hardness of the fabricated 316L stainless steel has been investigated with the laser beam overlap varied from 10% to 70%. The microstructures of the fabricated STS316L samples show a decrease in the balling and satellite of powders introducing defect in the bulk samples and the porosity caused by the gap between the molten metal pools disappearing as the overlap ratio increases, whereas a low overlap ratio results in significant balling and a large amount of isolated powders due to the increased gap between the melt pools. Furthermore, the highest value in Vickers hardness is obtained for the sample fabricated by 30% overlapped laser beams. These results show that the overlap ratio of laser beams in the SLM process should be considered as an important process parameter.

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• Influence of Powder Size on Properties of Selectively Laser-Melted- AlSi10Mg Alloys
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  Journal of Korean Powder Metallurgy Institute.2020; 27(2): 103.     CrossRef
• Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions
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• Microstructures and Characterization of Al-Si-Mg Alloy Processed by Selective Laser Melting with Post-Heat-treatment
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  Journal of Korean Powder Metallurgy Institute.2019; 26(2): 138.     CrossRef
• Correlation between Microstructure and Mechanical Properties of the Additive Manufactured H13 Tool Steel
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Synthesized monocrystalline nanodiamond (nD) particles are heat-treated at various temperatures to produce highly structured diamond crystals. The heat-treated nDs show different weight loss ratios during thermogravimetric analysis. The crystallinities of the heat-treated nDs are analyzed using Raman spectroscopy. The average particle sizes of the heat-treated nDs are measured by a dynamic light scattering (DLS) system and direct imaging observation methods. Moreover, individual dispersion behaviors of the heat-treated nD particles are investigated based on ultrasonic dispersion methods. The average particle sizes of the dispersed nDs according to the two different measurement methods show very similar size distributions. Thus, it is possible to produce highly crystallized nD powder particles by a heattreatment process, and the nD particles are relatively easy to disperse individually without any dispersant. The heattreated nDs can lead to potential applications such as in nanocomposites, quantum dots, and biomedical materials.

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Nanopowders provide better details for micro features and surface finish in powder injection molding processes. However, the small size of such powders induces processing challenges, such as low solid loading, high feedstock viscosity, difficulty in debinding, and distinctive sintering behavior. Therefore, the optimization of process conditions for nanopowder injection molding is essential, and it should be carefully performed. In this study, the powder injection molding process for Fe nanopowder has been optimized. The feedstock has been formulated using commercially available Fe nanopowder and a wax-based binder system. The optimal solid loading has been determined from the critical solid loading, measured by a torque rheometer. The homogeneously mixed feedstock is injected as a cylindrical green body, and solvent and thermal debinding conditions are determined by observing the weight change of the sample. The influence of the sintering temperature and holding time on the density has also been investigated. Thereafter, the Vickers hardness and grain size of the sintered samples have been measured to optimize the sintering conditions.

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We report on a simple and robust route to the spontaneous assembly of well-ordered magnetic nanoparticle superstructures by irreversible evaporation of a sessile single droplet of a mixture of a ferrofluid (FF) and a nonmagnetic fluid (NF). The resulting assembled superstructures are seen to form well-packed, vertically arranged columns with diameters of 5~0.7 μm, interparticle spacings of 9~2 μm, and heights of 1.3~3 μm. The assembled superstructures are strongly dependent on both the magnitude of magnetic field and the mixing ratio of the mixture. As the magnitude of the externally applied magnetic field and the mixing ratio of the mixture increase gradually, the size and interspacing of the magnetic nanoparticle aggregations decrease. Without an externally applied magnetic field, featureless patterns are observed for the γ-Fe₃O₄ nanoparticle aggregations. The proposed approach may lead to a versatile, cost-effective, fast, and scalable fabrication process based on the field-induced self-assembly of magnetic nanoparticles.

Titanium alloys have high specific strength, excellent corrosion and wear resistance, as well as high heatresistant strength compared to conventional steel materials. As intermetallic compounds based on Ti, TiAl alloys are becoming increasingly popular in the aerospace field because these alloys have low density and high creep properties. In spite of those advantages, the low ductility at room temperature and difficult machining performance of TiAl and Ti3Al materials has limited their potential applications. Titanium powder can be used in such cases for weight and cost reduction. Herein, pre-forms of Ti-Al-xMn powder alloys are fabricated by compression forming. In this process, Ti powder is added to Al and Mn powders and compressed, and the resulting mixture is subjected to various sintering temperature and holding times. The density of the powder-sintered specimens is measured and evaluated by correlation with phase formation, Mn addition, Kirkendall void, etc. Strong Al-Mn reactions can restrain Kirkendall void formation in Ti-Al-xMn powder alloys and result in increased density of the powder alloys. The effect of Al-Mn reactions and microstructural changes as well as Mn addition on the high-temperature compression properties are also analyzed for the Ti-Al-xMn powder alloys.

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In this study, the electroless nickel plating method has been investigated for the coating of Ni nanoparticles onto fine Al powder as promising energetic materials. The adsorption of nickel nanoparticles onto the surface of Al powders has been studied by varying various process parameters, namely, the amounts of reducing agent, complexing agent, and pH-controller. The size of nickel nanoparticles synthesized in the process has been optimized to approximately 200 nm and they have been adsorbed on the Al powder. TGA results clearly show that the temperature at which oxidation of Al mainly occurs is lowered as the amount of Ni nanoparticles on the Al surface increases. Furthermore, the Ni-plated Al powders prepared for all conditions show improved exothermic reaction due to the selfpropagating high-temperature synthesis (SHS) between Ni and Al. Therefore, Al powders fully coated by Ni nanoparticles show the highest exothermic reactivity: this demonstrates the efficiency of Ni coating in improving the energetic properties of Al powders.

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