발간논문

Home > KJMM 논문 > 발간논문

Hot stamping technology has been steadily developed because it provides both excellent formability and high strength. With the development of TWB hot stamping technology, it is now possible to freely apply the required strength and thickness in the right place. In this study, the microstructure and collision performance of TWB hot stamped parts were evaluated according to their combined strength. Through dilatometry analysis, the hot stamping heat treatment temperatures of 22MnB5 steel and 30MnB5 steel were set at 950 ℃ and 870 ℃. The 5MnB8 steel was composed of Bainite + Martensite when heat-treated at 950℃ and provided a strength of 980 MPa grade. When heat-treated at 870 ℃, it was composed of Ferrite + Bainite + Martensite and provided a strength of 780 MPa grade. Simulated rear impact testing showed the 30MnB5- 5MnB8 TWB combination had the best performance, because the 5MnB8 part of the 780 MPa grade absorbed enough energy and the 30MnB5 part of the 1.8 GPa grade fully served as an anti-intrusion. (Received 2 June, 2022; Accepted 29 August, 2022)

keyword : hot stamping, Tailor Welded Blank, TWB, Al-10 % Si coating, rear impact test, 30MnB5

The strain aging and hydrogen embrittlement behavior of two bake hardening (BH) steels with different microstructures were investigated in this study. The single-phase BH steel was composed of fully ferrite, while the dual-phase BH steel consisted of ferrite and 3 % martensite. The BH index and aging index (AI) of the two BH steels were measured to compare the effect of microstructure on strain aging behavior. As a small amount of martensite is present in the dual-phase BH steel, the BH index value of the dual-phase BH steel was higher than that of single-phase BH steel, and the AI showed a relatively low value. After the BH steels were pre-strained up to 20% and electrochemically pre-charged with hydrogen, on the other hand, a slow strain rate test (SSRT) was carried out to examine the effect of pre-strain on hydrogen embrittlement behavior. The silver decoration technique and thermal desorption analysis (TDA) were performed to understand the hydrogen embrittlement mechanism from the standpoint of hydrogen trapping. Regardless of pre-strain the dual-phase BH steel had lower hydrogen embrittlement resistance than the single-phase BH steel because martensite formed in the dual-phase BH steel acts as a reversible hydrogen trap site. The hydrogen embrittlement resistance of the two BH steels decreased with increasing amount of pre-strain which increases dislocation density. Based on these results, it can be said that the introduction of martensite and dislocation in BH steels have a negative impact on hydrogen embrittlement resistance. (Received 4 July, 2022; Accepted 31 August, 2022)

keyword : bake hardening steel, strain aging, hydrogen embrittlement, microstructure

The direct energy deposition (DED) method is an emerging manufacturing technique that has been recently highlighted due to its capabilities to coat materials with complex geometry for high temperature and highly corrosive environmental applications, such as steam turbine blade. Amongst metallic materials, SS304L has been reported to have superior corrosion resistance in such environments. Therefore, 3D printing of complex-shaped parts using SS304 maybe a promising strategy to introduce 3D printing techniques for nuclear industry application. To obtain robust and stable products that can withstand extreme environments, optimization of the SS304 manufacturing process conditions is required. This study investigated the effects of process parameters including laser power, scan speed, coaxial gas flow, and feeder rate using the DED method with a 4 way nozzle. It was found that laser power has an effect on grain growth behaviors. If the other variables are fixed and the scan speed is above a certain value, then the scan speed does not affect surface oxidation. The feeder rate is related to height. As the feeder rate increases, the height increases. Coaxial gas works to prevent oxidation and damage to the optic components and is not a critical factor in determining the size of the deposited material. These results can be utilized in future studies to accelerate the process design time of other 3D printed materials and future applications using SS304L (Received 4 July, 2022; Accepted 29 August, 2022)

keyword : 3D printing, additive manufacturing, direct energy deposition (DED), SS304L, 4-way nozzle

To rapidly sinter a bondline and obtain mechanical stability at high temperature and high thermal conductivity, 1.5 μm Ag particles with enhanced surface area were synthesized by a wet-chemical method, and a sinter-bonding paste containing these Ag particles was obtained. Some particles were present in the form of agglomerates of spike stems and short-branch dendrites, while others existed as spheres with rough nodule surfaces or relatively smooth surfaces. To determine an effective sinter-bonding process, a significantly short thermo-compression bonding (10 s) under 5 MPa in air and subsequent annealing in nitrogen were performed. The thermo-compression bonding at 250 ℃ resulted in a low shear strength of 8.15 MPa in the formed bondline. Although the annealing at 250℃ increased its strength, it did not reach 20 MPa, which is required for practical applications. Interestingly, the 10 s bonding at 300℃ exhibited sufficient shear strength of 21.96 MPa, and when annealed for 30 min at 300℃, the excellent strength of 37.75 MPa was obtained. The bondline porosity of 12.16% immediately after the thermo-compression bonding, decreased to 9.13% after annealing for 30 min. The densification in bondline by the annealing also induced a change in the fracture path as well as enhancement in the shear strength. Thus, the suggested subsequent annealing is an effective method for sinter-bonding, similar to the pressureless sinter-bonding process. Consequently, the synthesized Ag particles exhibited superior sintering properties and the suggested combination process shows potential for tremendously improving chip sinter-bonding productivity. (Received 7 July, 2022; Accepted 17 August, 2022)

keyword : Ag particle, enhanced surface area, sinter-bonding, thermo-compression, annealing

A small amount of Pd(II) present in spent electroplating solutions is concentrated by cementation. In this work, a process was developed to recover pure palladium metal by selective reduction from the hydrochloric and the sulfuric acid leaching solutions of cemented zinc. For this purpose, hydrazine and ascorbic acid were employed as reducing agents and the effect of several variables such as the acidity of the leaching solutions, molar ratio of reducing agents to Pd(II), reaction time and temperature was investigated. Since neither of the reducing agents can reduce Pd(II) from strong acidic solutions, it was necessary to decrease the acidity of the leaching solutions by dilution with distilled water. Our results indicated that it was possible to recover Pd metal with high purity by selective reduction after diluting the hydrochloric and sulfuric acid leaching solutions of cemented zinc with distilled water. When the molar ratios of the reducing agents to Pd(II) were the same, the reduction percentage of Pd(II) from the sulfuric acid solution was higher than that from the hydrochloric acid solution, due to the much higher complex formation constant of PdCl42- than Pd(SO4)22-. The reduction of Pd(II) by hydrazine occurred within 15 mins at room temperature, while a higher temperature (60 ℃) and longer reaction time (1h) were necessary to reduce Pd(II) with ascorbic acid. The reduction of Pd(II) from sulfuric acid solution was more efficient than from hydrochloric acid solution. It was possible to recover Pd metal with high purity from the leaching solution of cemented zinc using the reduction method. (Received 22 May, 2022; Accepted 7 September, 2022)

keyword : palladium, electroplating solution, reduction, hydrazine, ascorbic acid

In this study, single-crystalline methylammonium lead tribromide (MAPbBr₃, CH₃NH₃PbBr₃) flakes with a higher theoretical specific capacity than methylammonium lead iodide (MAPbI₃, CH₃NH₃PbI₃), the prototype of organic/inorganic hybrid perovskite, were synthesized by the inverse temperature crystallization method. The combination of the additive-free electrophoretic deposition (EPD) system and the single crystal fabrication method provided a fundamental understanding of the electrochemical properties associated with the lithium-ion storage mechanism in which the formation of the lithiated-phase (Lix:CH₃NH₃PbBr₃) and the conversion reaction causes significant irreversible capacity, reducing battery cycle stability. The conversion reaction in MAPbBr₃ was observed to be the main factor for high irreversible capacity, while the capacity due to the alloying reaction was more significant. The occurrence of the capacity due to the conversion reaction in the slurry system demonstrates that the additives worked as a buffer to relieve the stress associated with the formation of the lithiated phase, which was not observed in the MAPbBr₃ EPD film. As part of an investigation of the active material/electrolyte in terms of cycling stability, the problem of structural instability was addressed by replacing the lithium salt and organic solvent that are components of the electrolyte. Our findings shed light on the intrinsic electrochemical properties of MAPbBr₃ during lithiation/delithiation in the charging/discharging process, eliminating the complex effect caused by the MAPbBr₃/additive mixture. Structural stability at the MAPbBr₃/electrolyte interface was probed by varying the solvent and lithium salt. (Received 4 March, 2022; Accepted 1 September, 2022)

keyword : perovskite, electrophotretic deposition, lithium ion battery, conversion reaction

In an effort to minimize irreversible capacity loss and volume expansion, research on Si nanocomposite materials with a SiOx/Mg₂SiO₄/SiOx structure through a magnesiothermic reduction process (MTR) has attracted much attention from researchers. Mg₂SiO₄(forsterite) has been shown to improve the initial coulombic efficiency (ICE) by minimizing the irreversible capacity loss due to pulverization and highvolume expansion of the Si-based anode complexes. In this study, forsterite was synthesized as the main phase by Mg vapor control in the MTR process. We used an electrophoretic deposition system to investigate the intrinsic electrochemical properties of forsterite, which served as a buffer for the improvement of ICE, associated with the lithiation/delithiation process. Importantly, a stable specific capacity of up to 200 mAh/ g was achieved during the charging/discharging process, demonstrating its potential use as an anode electrode. We also found that no significant capacity was found by alloying with Si. In other words, there is a lithium storage mechanism unique to forsterite which is not related to the Si alloying reaction in the storage mechanism. The results presented here are the first demonstration of a forsterite lithium-ion battery; forsterite has only been considered as a buffer layer of the Si/SiO composite structure. Furthermore, the finding is of crucial importance as it provides the basis for various approaches to develop reversible and high power li-ion battery anodes by synthesizing the Si composite through MTR. (Received 5 March, 2022; Accepted 1 September, 2022)

keyword : forsterite, magnesiothermic reaction, li-ion battery, silicon composite

NbFeSb-based alloys are promising p-type half-Heusler materials with excellent thermoelectric performance, thermal stability, and naturally abundant constituent elements. Alloying and doping are powerful techniques for enhancing the thermoelectric properties of half-Heusler materials. This study experimentally investigated the effects of V alloying in NbFeSb codoped with Ti, Zr, and Sn. As the V content increases, the electrical conductivity of Nb_0.8-xV_xTi_0.15Zr_0.05FeSb_0.98Sn_0.02 (x = 0, 0.05, 0.1, 0.2, and 0.3) decreases monotonically because of a simultaneous reduction in carrier concentration and mobility, reducing the power factor up to -39% from 4.13 (x = 0) to 2.52 mW m^-1K^-2 (x = 0.3) at 298 K. Moreover, the lattice thermal conductivity decreases with increasing x by as much as -40% at the maximum V content of x = 0.3, demonstrating that V addition considerably enhances phonon scattering even in the presence of substitutional dopants Ti, Zr, and Sn. The differences in the mass and size of V and Nb atoms cause a substantial decrease in lattice thermal conductivity. According to our study, the addition of a small V content to Hf-free NbFeSbbased alloys can improve their thermoelectric properties, and a maximum dimensionless figure of merit zT of 0.89 was obtained in Nb_0.75V_0.05Ti_0.15Zr_0.05Sn_0.98Sb_0.02 at 973 K. (Received 19 July, 2022; Accepted 18 August, 2022)

keyword : thermoelectric properties, half-Heusler, NbFeSb, V, thermal conductivity

A gas turbine (GT) blade is a key hot-pass component for advanced GT engines, and should have stable properties under extreme conditions of 1,350°C and 3,600rpm, etc. The GT blade, after operating with nearly 800 equivalent start-stops (ES) or 24,000 equivalent operation hours (EOH), should be replaced due to degradation of properties and microstructure, particularly, the formation of cracks in the airfoil tip and platform region. To date, the assessment of materials, prototype blade, etc, has been extensively studied in a laboratory simulated environment; however, evaluation of the full-scale blades in a service environment has been rarely reported. Here, the properties and microstructures of an F-class GT first stage blade, with a service history of 800ES were investigated. The results showed decreasing tensile properties at the airfoil part due to its higher temperature exposure. The microstructural characterization results revealed that the finer grain size and dendrite interfaces facilitated the formation of Cr-enriched M₂₃C₆ along grain boundaries, as well as the spherical in the airfoil part, resulting in a decrease in tensile properties. The results obtained here provide precious background for assessing the serviced blade and developing advanced blades. (Received 26 July, 2022; Accepted 31 August, 2022)

keyword : Ni-Base superalloy, gas turbine blade, tensile, microstructure, carbide, gamma prime