발간논문

Home > KJMM 논문 > 발간논문

The degree to which parameters affect the spheroidization heat treatment of steel was calculated by setting the spheroidization heat treatment conditions of Cr-Mo steel and using data analysis such as S/ N ratio and ANOVA. After analyzing the transformation temperatures of the steel, A_c1 and A_c3, using a DSC, the conditions were set accordingly. The surface hardness was measured for the conditions and used as an evaluation index. The correlation was analyzed by comparing the spheroidized volume fraction and the surface hardness, and the Pearson correlation coefficient was -0.88, proving that a correlation existed between the two values. Using S/N ratio and ANOVA, the degree to which each control parameter affects the decrease in the surface hardness was analyzed, qualitatively and quantitatively. For the S/N ratio, priority affecting the surface hardness for each control parameter was analyzed. The 1^st heating temperature was found to have a more preferential effect on the surface hardness than the 1^st heating time and the 2^nd heating temperature. Using ANOVA, the 1^st heating temperature was determined to be a very significant factor with the greatest influence, contributing 73.2% to the surface hardness. Intercritical annealing is a suitable spheroidization heat treatment condition, so if the surface hardness of the steel needs to be reduced using Intercritical annealing, the 1st heating temperature and time should be designed as the priority. (Received 31 January, 2023; Accepted 3 April, 2023)

keyword : spheroidization intercritical annealing medium carbon steel S/N ratio·ANOVA

The effects of Mg content and cooling rate on the T5 treatment of Al-Si-Mg alloys have been investigated using Avrami kinetics and transmission electron microscopy. The alloys were super-heated at 800 ℃ for 1 hr followed by degassing treatment at 720 ℃ for 15 min, and poured into a metallic mold preheated to 250 ℃. The as-cast alloys were then T5-treated at a temperature of 190 ℃. As the Mg content and/ or cooling rate increased, the strength of the T5-treated alloys increased while the elongation decreased over the entire aging time. However, some T5 treatment conditions were effective to at enhancinge elongation as well as strength. The alloy with low Mg content showed excellent elongation in the early stage of aging. B, but as the aging time increased, h. However, the elongation decreased rapidly so that there was showed no significant difference from with the alloy with high Mg content in the over-aging stage. This was due to a coarse and agglomerated precipitate structure, which were likely to be formed by the rapid precipitation and agglomeration of the β′ phase. The alloy with a high cooling rate exhibited superior strength with similar elongation over the entire aging time compared to the slowly cooled alloy. The reason for the decrease in strength for in the slowly cooled alloy was attributed to the coarsening of precipitates and suppression of the formation of fine precipitates during cooling. (Received 3 February, 2023; Accepted 10 April, 2023)

keyword : T5 treatment, Al-Si-Mg alloy, precipitate, elongation, avrami kinetic

In this study, we fabricated transparent heaters composed of an ultrathin Cu-layer heating element sandwiched between a ZnO underlayer and an Al₂O₃ overlayer. With the Cu layer thickness fixed at 8.5 nm, the thicknesses of the ZnO and Al₂O₃ layers were independently varied to reach the optimum antireflecting condition (maximum transmittance of 88.3% and average visible light transmittance of 79.8% were achieved). The sheet resistances for the ZnO/Cu/ Al₂O₃ heaters can be varied by simply modulating the Cu layer thicknesses. In order to assess the flexibility of the transparent heaters, we constructed a ZnO/Cu/ Al₂O₃ structure on flexible polyimide substrates, and the thermal, electrical, optical and mechanical characteristics were evaluated. Because of the planar heating element of the Cu layer, the thermal response was found to be extremely high, i.e., less than 10 s were required to reach 90% of the target temperatures. Once the target temperatures were reached, the heater temperatures were highly stable with no degradation of electrical and optical properties. Furthermore, the heating capability was maintained under severe mechanical deformation, e.g., at a bending radius of 4 mm. The structure also exhibited highly sustainable optoelectronic properties under repetitive mechanical deformation, confirming the potential for commercialization. Finally, we demonstrated that ZnO/Cu/ Al₂O₃ rolled around a human finger exhibited highly uniform heating characteristics, rendering the heaters suitable for wearable, healthcare electronics. (Received 21 February, 2023; Accepted 30 March, 2023)

keyword : transparent planar heater, magnetron sputtering, e-beam evaporation, sheet resistance, optical transmittance

Cu₃Sb_1-x-yGe_xIn_ySe₄ (0.02 ≤ x ≤ 0.12; 0.04 ≤ y ≤ 0.08) permingeatite compounds doped with Ge and In were prepared using solid-state synthesis. The phases and microstructures were analyzed, and the charge transport and thermoelectric properties were evaluated according to the Ge and In doping content. Most of the samples contained a single phase of permingeatite with a tetragonal structure; however, secondary phases (Cu_0.875InSe₂, In₂Se₃, and InSb) were detected in the samples with x = 0.12 and y = 0.08. Both the a-axis and c-axis lattice constants of permingeatite were increased by Ge and In doping, with a = 0.5651-0.5655 nm and c = 1.1249-1.1255 nm, but the change in lattice constant due to the change in doping amount was insignificant. The melting point of the sample double-doped with Ge and In was determined to be 736 K, which was lower than the melting point (741 K) of pure Cu₃SbSe₄. This lowering of the melting point was attributed to the formation of a solid solution. The electrical conductivity exhibited degenerate semiconductor behavior, decreasing with increasing temperature. As the Ge and In doping content increased, the carrier concentration and electrical conductivity increased; however, when x ≥ 0.12, the electrical conductivity did not increase further. The Seebeck coefficient exhibited positive values and p-type conduction characteristics. In addition, intrinsic transitions did not occur in the measurement temperature range, and the Seebeck coefficient increased as the doping level increased. The power factor exhibited a positive temperature dependence, and Cu₃Sb_0.86Ge_0.08In_0.06Se₄ exhibited the highest value of 0.89 mWm^-1K^-2 at 623 K. As the temperature increased, the thermal conductivity tended to decrease because of the decreased lattice thermal conductivity and slightly increased electronic thermal conductivity. All the samples exhibited minimum thermal conductivities of 0.94- 1.11 Wm^-1K^-1 at 523 K. At high temperatures, the double doping of Ge and In improved the thermoelectric performance; thus, the dimensionless figure of merit obtained at 623 K for Cu₃Sb_0.86Ge_0.08In_0.06Se₄, was 0.47. (Received 13 February, 2023; Accepted 21 March, 2023)

keyword : thermoelectric, permingeatite, double doping, dual doping, co-doping

Wearable strain sensors with high and broad sensitivity, high stretchability and excellent mechanical endurance will be widely useful in smart wearable electronics. In this work, we developed a stretchable strain sensor fabricated with a simple stencil printing technique. The stretchable strain sensor was fabricated using a multi-walled carbon nanotubes (MWCNTs)-Ecoflex composite paste on an Ecoflex substrate. In particular, using IPA solvent, CNT particles were uniformly dispersed in the Ecoflex binder. The effect of the amount of Ecoflex resin on the stretchability and sensitivity of the sensor were also investigated. It was found that as the amount of Ecoflex resin increased, the stretchability of the sensor increased. The fabricated stretchable strain sensor showed a maximum stretchability of 1,000% with a wide sensitivity range from 3 to 12,287. The hysteresis tests indicated that the hysteresis of the fabricated stretchable strain sensor was very small, the electrical resistances of the sensors quickly returned to original value after tests. The strain sensor showed excellent mechanical durability during cyclic repeated tensile tests of 400,000 cycles. The results of the cross-cut adhesion tests indicated that the adhesion strength between the sensor composite layer and Ecoflex substrate was excellent. We also demonstrated the potential application of the stretchable sensor in wearable electronics by bending tests on a human finger and wrist. (Received 24 November, 2022; Accepted 14 March, 2023)

keyword : stretchable strain sensor, high sensitive, CNT, Ecoflex, printed electronics

MgO nanowires were grown by a thermal evaporation method at different N₂/O₂ gas ratios in order to investigate the effect of oxygen concentration on the growth and luminescence properties of the MgO nanowires. A thermal evaporation process was conducted at 1000℃ and under a pressure of 500Torr. No nanowires were grown in a pure N₂ gas atmosphere. Nanowires were formed at oxygen concentrations above 25% in a mixture of N₂ and O₂ gases. X-ray diffraction analysis showed that the MgO nanowires had a cubic crystal structure. Compared to the nanowires formed at high oxygen concentration, the nanowires grown at low oxygen concentration had larger diameters and rougher side surfaces. Nanowires with very smooth side surfaces were formed at high oxygen concentrations. The difference in surface roughness was supposed to be due to the change in the growth habit of nuclei. Two visible emissions were observed in the cathodoluminescence spectra of the MgO nanowires. One was an emission peak centered near 400 nm and the other was an emission peak with a central wavelength of 500 nm. As the oxygen concentration increased, the emission intensity of the 400 nm band decreased and the emission intensity of the 500 nm band increased. The maximum emission at 500 nm was observed from the nanowires formed in a pure O₂ atmosphere. The full width at half maximum of the emission peak at 500 nm was narrower than that of the emission peak at 400 nm. (Received 20 February, 2023; Accepted 10 April, 2023)

keyword : magnesium oxide, nanowires, thermal evaporation, oxygen concentration, cathodoluminescence

Employing post-combustion technology in the converter, using the sensible heat of the hot metal and the oxidation reaction heat as a heat source, is known to compensate for insufficient heat in the converter process. However, most studies on post-combustion have been conducted using subsonic nozzles, whereas actual converter processes use supersonic nozzles. Therefore, research on the combustion behavior of supersonic jets is needed. In this study, experiments and analyses were conducted using a converter simulator and a supersonic nozzle to investigate the effect of nozzle height on the post-combustion behavior. The reaction was set to complete combustion, with an O₂ gas flow rate of 150 L/min blown through the upper lance and a CO gas flow rate of 300 L/min blown at the bottom of the simulator to represent the surface of the molten metal. The combustion reaction of CO gas was calculated to be rate-controlled by reactant mixing. The nozzle heights were set to 250, 380, and 530 mm from the surface of the molten metal. Post-combustion analysis showed that the lowest gas velocity was observed under the condition of the highest nozzle height of 530 mm, and the high temperature and reaction zones were widely distributed in the lower region. Therefore, to facilitate heat compensation to the molten metal, it is necessary to control the gas velocity of the molten steelgas interface slowly. (Received 29 December, 2022; Accepted 13 March, 2023)

keyword : converter process, converter simulator, numerical simulation, post-combustion, nozzle height

A356 cast aluminum alloy contains 7 at.% Si and 0.3 at.% Mg, producing an approximately 50% eutectic microstructure. This high Si content and various casting conditions play a significant role in strengthening A356 alloy, by controlling the eutectic morphology and precipitates of other intermetallic compounds. Understanding how Si-related precipitates and clusters are soluble in the α-matrix is necessary to provide high strength and fatigue resistance to A356 alloys. The aging heat-treatment temperature in the A356 alloy most likely promotes the formation of these precipitates and clusters. The A356 samples were differently aged at temperatures of 110 ℃ and 130 ℃ for 2 h, and were labeled 110A, and 130A, respectively. 110A was found to have improved mechanical properties, such as high strength and elongation, compared to 130A, which may be attributed to the formation of secondary phases in the α-phase matrix. Scanning and transmission electron microscopy and atom probe tomography analyses demonstrated Ti₂Si precipitation and various-sized cluster formations in 110A. In contrast, 130A had fewer clusters than 110A. Therefore, different aging heat-treatment temperatures relate to a change in the behavior of atoms, affecting the mechanical properties. (Received 27 December, 2022; Accepted 16 March, 2023)

keyword : aging heat treatment, cast aluminum, atom probe tomography, precipitation, clustering

The physical properties of metal-based structural materials, such as hardness, strength and toughness, are directly or indirectly affected by residual stress inside or on the surface of the given part. Repeated rapid heating and cooling during the additive manufacturing process causes thermal gradients and expansion and contraction in the material, which causes residual stress. Tensile residual stresses are known to exist on the surface of additive manufactured products and should be kept to a minimum as they affect the mechanical properties and lead to product deformation and product failure. Therefore, it is important to evaluate the residual stress after making the product and to control it under the desired conditions. There are limitations to using the destructive method commonly used for residual stress evaluation with additive manufacturing products, due to difficulties in repeated measurements, product size, and cost issues. Therefore, it is necessary to apply a non-destructive evaluation method and verify the validity of the method. In this study, A356.2 aluminum alloy powders were used for additive manufacturing using the powder bed fusion process, and the surface residual stress generated during the process was measured. X-ray diffraction (XRD) methods were used to observe the surface residual stress. After XRD measurement, analyses were performed using the Williamson- Hall plot, sin²ψ, and cosα methods. The residual stress measurement results of samples manufactured through the LPBF process and the characteristics and limitations of each method were discussed. (Received 6 February, 2023; Accepted 3 April, 2023)

keyword : additive manufacturing, powder bed fusion, residual stress, XRD, aluminum