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New technologies need to be developed in the ironmaking process to reduce carbon dioxide emissions. To achieve this, in recent years steelmakers have been working to reduce coke usage by increasing thermal energy efficiency. However, it is very difficult to reduce coke consumption while maintaining production, because the level of energy efficiency of steelmakers in Korea is already among the world’s highest. Therefore, we devised an idea that can dramatically reduce carbon dioxide emissions by agglomerating iron ore without using thermal energy. This is a method that uses a curing reaction of sodium silicate and carbon dioxide gas contained in water glass after briquetting a mixture of ultra-fine ore and water glass. Carbon dioxide gas is not emitted because coke is not required. In this study, to investigate the applicability of this idea, the strength characteristics of the briquettes were measured and compared according to the preparation and reaction conditions. As a result, the highest compressive strength was obtained when the binder content was 5 wt%, the flow rate of the reaction gas was 20 L/min, and the reaction time was 10 seconds. Also, a shatter index similar to that of commercial sintered ore was measured when the molding pressure was 4 ton/cm² or more. (Received November 20, 2019; Accepted May 10, 2020)

keyword : ultra-fine ore, agglomeration process, carbon dioxide emission reduction

The powder-alloyed metallic sheet foam manufacturing process has the advantage of being able to control pore shape, size, and distribution more easily and homogeneously than conventional foam manufacturing processes. The effects of strain rate and tensile direction on the mechanical properties of Ni- Cr-Al superalloy foam fabricated by powder alloying method were investigated. As a result of structural characteristics obtained by X-ray tomography and scanning electron microscopy, average pore sizes were measured to be 2762.4 μm (normal direction), 2709.1 μm (rolling direction, RD), and 2518.4 μm (transverse direction, TD) respectively. The γ-Ni matrix and γ’-Ni₃Al (which was evenly distributed in the strut) were identified as the main constituent phases of the Ni-Cr-Al foam used in this study. Tensile tests were conducted with strain rates of 10^-2 ~ 10^-4 s^-1 along the rolling and transverse directions. The results showed that the tensile strength in the RD direction was 1.84~2.01 MPa, and in the TD direction was 1.2~1.27 MPa. The elongation in the TD direction was higher (30~36%) than in the RD direction (17~22%). It is noteworthy that the effect of quasi-static strain rate on the tensile strength and elongation was negligible. However, the loading direction was found to change mechanical properties significantly. This study also discussed the deformation behavior of the Ni-Cr-Al superalloy foam through observations of the fracture surface, and realtime observations during tensile tests in different directions. (Received February 5, 2020; Accepted May 14, 2020)

keyword : Ni-Cr-Al superalloy foam, structural characteristics, microstructure, tensile properties, strain rate

Pores and cracks are known as the main defects in metal additive manufacturing (MAM), including directed energy deposition(DED). A gaseous fume is often produced by laser flash (instantaneous high temperature) during laser processing, which may cause various defects such as porosity, lack of fusion, inhomogeneity, low flowability and composition change, either. However the cause and harmful effects of fume generation in DED are known little. In laser processing, especially laser welding, many studies have been conducted on the prevention of fume because it generates defects that hinder uniform reactions between the laser beam and the materials. Generally, the fume occurs with easily vaporizing low melting point components or sensitive oxidizing elements. Unsuitable conditions are also known to have an effect, including laser power, travel speed, powder feed rate and shielding gas supply. Practically, there are many more fume generating factors in the DED process, and the lack of understanding requires a lot of trial and error. In this article the laser-related and weld metallurgy literatures were reviewed, focusing on the prevention of fume in powder DED. The causes of the fume, were explained to result from the stages of cavitation bubbles generated by the laser induced plasma and the nanoparticles released. Additionally, the effects of alloying components and environmental conditions for fume generation in the DED process were investigated, and suggestions are proposed to prevent fume. (Received March 20, 2020; Accepted April 12, 2020)

keyword : metal additive manufacturing, defects, fume prevention, cavitation bubble, nanoparticles

PFC gas is primarily used during the etching process in the manufacture of ULSIs and in cleaning after CVD processes. PFC is classified as a greenhouse gas that stays in the atmosphere for a long time and has a high GWP. High capacity and high integration have been achieved in recent years as semiconductor device structures have been replaced by vertical layer structures, and the consumption of PFC gas has exploded due to the increase in high aspect ratio and patterning processes. Therefore, many researchers have been working on methods to decompose, recover, and reuse the gas after the etching process to reduce the emissions of PFC gas. In this study, etching and recovery processes were performed using C₅F₈ in L-FC which is in liquid phase at room temperature. Among the L-FCs, C₅F₈ gas has a high C/F ratio, similar to that of the C₄F₈ gas, which is a conventional PFC gas. In addition, to confirm its reusability, the recovered C₅F₈ was injected back into the chamber, and the electron temperature, plasma density, and ion energy distribution were analyzed. Based on these experimental data, the reliability of the etch processes performed with recovered C₅F₈ gas was evaluated, and the possibility of reusing the recovered C₅F₈ gas was confirmed. (Received March 27, 2020; Accepted April 14, 2020)

keyword : liquid fluorocarbon, plasma etching, global warming potential

In this study, solidification cracking behavior and susceptibility in dissimilar cladding of Inconel alloy FM 52, 308L stainless steel to carbon steel, was investigated by submerged arc welding and transverse-Varestraint testing with gas tungsten arc welding. The effect of cladding conditions on cracking behavior and susceptibility was extensively evaluated, and metallurgical factors affecting susceptibility were clarified. Depending on the cladding sequence (cladding combination A: Inconel 52→308L, cladding combination B: 308L→Inconel 52), opposite types of solidification cracking behavior were observed. Specifically, solidification cracking was observed only for cladding combination A. Using transverse-Varestraint tests, the solidification brittle temperature range (BTR) was determined to be 298 K for cladding combination A and 200 K for cladding combination B. The reason for solidification cracking in cladding combination A could be its higher solidification susceptibility (i.e., a larger BTR (298 K)) compared with cladding combination B (BTR: 200 K). To elucidate differences in solidification cracking susceptibility, a numerical simulation of non-equilibrium solidification segregation for impurity elements (P, S) was performed, based on velocity dependent solidification theories and the finite differential method. Different segregation behaviors were calculated upon the cladding combinations. The severe segregation of P and S during solidification was found to be one of the important metallurgical factors for the large BTR of cladding combination A, compared with cladding combination B. (Received February 28, 2020; Accepted April 17, 2020)

keyword : dissimilar cladding, varestraint test, solidification cracking susceptibility, impurity elements, solidification segregation

Electropulse-induced heating has attracted attention due to its high energy efficiency. However, the process gives rise to a nonlinear temperature variation, which is difficult to predict using a traditional physics model. As an alternative, this study employed machine-learning technology to predict such temperature variation for the first time. Mg alloy was exposed to a single electropulse with a variety of pulse magnitudes and durations for this purpose. Nine machine-learning models were established from algorithms from artificial neural network (ANN), deep neural network (DNN), and extreme gradient boosting (XGBoost). The ANN models showed an insufficient predicting capability with respect to the region of peak temperature, where temperature varied most significantly. The DNN models were built by increasing model complexity, enhancing architectures, and tuning hyperparameters. They exhibited a remarkable improvement in predicting capability at the heating-cooling boundary as well as overall estimation. As a result, the DNN-2 model in this group showed the best prediction of nonlinear temperature variation among the machinelearning models built in this study. The XGBoost model exhibited poor predicting performance when default hyperparameters were applied. However, hyperparameter tuning of learning rates and maximum depths resulted in a decent predicting capability with this algorithm. Furthermore, XGBoost models exhibited an extreme reduction in learning time compared with the ANN and DNN models. This advantage is expected to be useful for predicting more complicated cases including various materials, multi-step electropulses, and electrically-assisted forming. (Received April 2, 2020; Accepted April 28, 2020)

keyword : machine learning, magnesium, electropulse, electrically-assisted forming

Ionic liquids have emerged in hydrometallurgy as potential extractants for metals. In this work, the interactions of ALiCY (R₄N⁺A^-) synthesized by Aliquat336 and Cyanex272, and a mixture of ALiCY and TBP, were analyzed using extraction data and FT-IR spectroscopy. Co(II) and Ni(II) were extracted from two HCl concentrations (1.0×10^-4 and 6.8 mol·L^-1) using ALiCY and its mixture with TBP. The extraction results indicated that ALiCY has a dual function, as a cationic extractant at low HCl concentration and an anionic extractant at high HCl concentration depending on the types of metal complexes. The addition of TBP to ALiCY had a negative effect on the extraction of Co(II) from the 6.8 mol·L^-1 HCl, while the effect was negligible from the 1.010^-4 mol·L^-1 HCl. The solvation of R₄N⁺ by TBP significantly affected the extraction behavior of ALiCY for metal ions. The formation of hydrogen bonding between TBP and Cyanex272 was verified. These findings shed light on the behavior and interaction occurring between bif-ILs and solvating extractants for the extraction of metals in acidic media. (Received January 20, 2020; Accepted April 29, 2020)

keyword : ionic liquids, solvent extraction, hydrometallurgy, aliquat336

Technologies for pedestrian safety are increasingly emphasized by Automakers in advance of autonomous driving vehicles. A Night Vision System attached behind the front grille can reduce fatal accidents, especially during the nighttime, however, consumers may hesitate to adopt such systems on account of their high price. High-cost Germanium is used in commercial Night Vision System windows, and therefore replacing it with a cheaper infrared window material can lead to a more affordable system. To achieve this, Zinc Sulfide (ZnS), which has about 70% transmittance in the Long-Wavelength Infrared region of 8~12 μm, was selected for the window substrate material. In this study, we designed, fabricated and characterized a single layer cost-effective anti-reflection coating on a ZnS window substrate using Calcium Fluoride (CaF2). The CaF2 coating was fabricated by E-beam evaporation technique, with Quarter wavelength anti-reflection thickness (QAR). It was characterized by FT-IR, SEM and a thermal camera test module. We found that CaF2 both side coated the ZnS window and exhibited about 10~15% higher transmittance than the ZnS window substrate. In addition the CaF2 coating stably bonded to the ZnS substrate without any internal defects. A thermal camera based window test also showed better detection performance with the CaF2 Coating than a bare ZnS substrate window, which was calculated using the output voltage of the microbolometer thermal sensor. (Received April 21, 2020; Accepted May 11, 2020)

keyword : e-beam evaporation, anti-reflection coating, night vision system, long-wave infrared

In this study, we report how Cu doping can modify the thermoelectric performance of p-type Bi_0.5Sb_1.5Te₃ and Bi_0.4Sb_1.6Te₃ thermoelectric alloys, including their electronic and thermal transport properties. For electronic transport, the power factors of both Bi_0.5Sb_1.5Te₃ and Bi_0.4Sb_1.6Te₃ compositions were increased by Cu doping. The origins of the enhanced power factors were examined using a single parabolic band model, by estimating the changes in deformation potential, effective mass, nondegenerate mobility and weighted mobility in both valence and conduction bands. The weighted mobility of the valence band was increased by Cu doping and increased Sb ratio, while the weighted mobility of the conduction band decreased, suggesting bipolar conduction was greatly reduced. For thermal transport, Cu_0.0075Bi_0.4Sb_1.6Te₃ and Bi_0.4Sb_1.6Te₃ had a lower lattice thermal conductivity than Cu_0.0075Bi_0.5Sb_1.5Te₃ and Bi_0.5Sb_1.5Te₃, respectively, due to an increase in Umklapp scattering. In addition, Cu doping suppressed bipolar thermal conductivity at high temperatures, by increasing hole concentration. It was also confirmed that Cu-doped samples had a lower lattice thermal conductivity than undoped samples due to additional point defect scattering. As a result, the thermoelectric figure of merit (zT) was greatly enhanced by 0.0075 mol of Cu doping, from 0.80 to 1.11 in Bi_0.5Sb_1.5Te₃, while the zT is increased from 1.0 to 1.05 for Bi_0.4Sb_1.6Te₃. (Received May 4, 2020; Accepted May 14, 2020)

keyword : Bi, sub, 2, sub, Te, sub, 3, sub, Cu doping, thermoelectric properties, density-of-states effective mass, thermal conductivity