Jo Dec 3, 2022
Ti2AlNb‒based alloys, so-called ordered orthorhombic (O (Cmcm)) phase alloys, have a chemical composition of Ti ‒ (18 ‒ 30) Al ‒ (12.5 ‒ 30) Nb (mole fraction, %). Since the discovery of orthorhombic (O) phase, Ti2AlNb‒based alloys have received special attention as a promising candidate for advanced aerospace and automotive application due to their high specific strength, excellent creep and oxidation resistance at elevated temperatures as well as good workability.
Up to now, many studies on the fabrication of Ti2AlNb-based alloys by casting, rolling and forging have been conducted, in which the major focus was set on the elimination of disadvantages such as microstructure segregation and inhomogeneity. It is because the thermodynamic properties of Ti, Al and Nb elements including a melting point, a density, a diffusion coefficient, etc. are very different.
Recently, powder metallurgy (PM) method such as spark plasma sintering (SPS) has been applied to the fabrication of Ti2AlNb‒based alloys, which made it possible to obtain fine and homogeneous microstructure. Several endeavors have been devoted to preparing PM Ti2AlNb‒based alloys from pre‒alloyed powder and elemental powders by vacuum hot pressing, and from pre‒alloyed powder by hot isostatic pressing (HIP). However, these as‒sintered compacts showed the microstructure with coarse grain size (>40μm), due to higher sintering temperature and longer sintering time. Therefore, the strength of these Ti2AlNb‒based alloys was not so high.
In recent years, several studies on the fabrication of high strength and ductility titanium alloys with nanostructured (NS) and ultrafine grained (UFG) microstructure have been reported. One process to achieve this microstructure consists of the preparation of NS or UFG powder by high energy ball milling (HEBM) and the consolidation of the powder into dense compacts by PM methods such as HIP and SPS. For instance, using the high energy ball-milled powder as a starting material, an ultrafine grained Ti‒6Al‒4V with high mechanical properties can be obtained.
In order to produce a high strength and ductility Ti‒22Al‒25Nb alloy from pre alloyed powder, Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has introduced HEBM with subsequent SPS. The process is as follows. Firstly, the Ti‒22Al‒25Nb pre‒alloyed powder with a weight of 75g was put in a stainless steel vial with bearing steel balls. HEBM was carried out in a high-energy planetary ball mill (QM-3SP4) under a high purity argon atmosphere for 20h at a rotation speed of 300 rpm. The HEBMed powder was directly loaded into a high-strength graphite mould, and subsequently consolidated by a LHPD250 SPS apparatus (FCT Co. Ldt., Germany) at SPS temperatures of 950℃ for SPS time of 10 min. After sintering, the sintered compact was cooled to the room temperature (RT) in the furnace. Finally, a sintered compact, with dimensions of φ50 × 15 mm, was obtained.
The Ti‒22Al‒25Nb alloy sintered at 950℃ for 10 min under the pressure of 50 MPa from the 20 h‒HEBMed powder showed the microstructure with a large amount of UFG/NS O-phase. The yield strength, tensile strength and elongation to failure were 1 092 MPa, 1 105 MPa and 9.4%, respectively. Compared with the referenced Ti‒22Al‒25Nb alloys, the Ti‒22Al‒25Nb alloy fabricated by HEBM and subsequent SPS exhibited higher strength and good ductility at the room temperature and high temperature. This indicates that HEBM with subsequent SPS is feasible for PM Ti‒22Al‒25Nb alloy with enhanced tensile properties.
More information about this is found in his paper “Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering” published in the SCI Journal “Journal of Alloys and Compounds”.
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Jo Nov 28, 2022
The research team led by Kim Hyok Chol, an institute head at the Faculty of Electrical Engineering, and Kim Nam Chol, a researcher at the Faculty of Mechanical Science and Technology, who designed and manufactured our style two-way high-speed circuit breaker for underground electric vehicles, is working for further achievements.
A two-way high-speed circuit breaker was designed and fabricated by combining a digital cut-off controller.
The device can achieve reliable high-speed cutoff even under severe operating conditions of underground electric vehicles. Accurate cutoff setting current and low mass and volume are other advantages of this device. In particular, successful bidirectional blocking is guaranteed.
The high-speed circuit breaker consists of a maintenance and release device for high-speed interruption, a blanking room, a locking control unit, and a contact system, which can sense both the current entering the electric vehicle and the current entering the grid during regenerative braking, so that the circuit can be shut down within 18 ms once an accident occurs.
The shutoff coil of the hold and trip device has no vibration-induced malfunction because there is no current flow during normal operation, and it takes less than 1ms to output a shutoff signal immediately after the current-cut signal from the current sensor enters the shutoff control device.
The turn-off controller has a double current sensing and a circuit to turn off analog and digital double, and the current setting has been simplified on a button.
The contactor is a two-point open type with an inclination angle of 45° and there is an arc-extinguishing component and a main contractor at each contactor. There is little electrical wear at the main contactor because there is no structural repulsion during turn-on.
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Jo Nov 25, 2022
Self-mixing (SM) technique has been a promising optical measurement technique for a few decades.
This technique is developing toward increasing the measurement distance and the limit of displacement measurement, enhancing the measurement accuracy and improving the real-time quality of the measurement. In addition, it is also important to drop the development cost of a sensor.
Estimation of the optical feedback factor C is the key to SM displacement measurement with a better resolution than half a wavelength. But this estimation is very complex and tricky, and it thus leads to its time-consuming nature, which is one of the important causes that increase the cost and labor for the development of a self-mixing sensor. There have been many methods reported for the C estimation, but they are very time-consuming, very sensitive to noise, or can only work in a certain feedback regime.
Kim Chol Hyon, a researcher at the Faculty of Physical Engineering, has proposed a method of the C estimation using the derivative of an SM signal. So to speak, he has solved a problem of fast estimation of the parameter of an SM interferometric system with the derivative in the jumping point of an SM signal. This method is very simple, and, what is more, it can work in all regimes including weak, moderate and strong feedback regimes.
The estimation precision of the proposed method is not only comparable to that of the data fitting technique but also much simpler than that. Therefore, it is effective in low-cost high-precision SM displacement measurement.
You can find the details of this in his paper “Fast estimation method of feedback factor based on the derivative of the self-mixing signal” presented to the SCI Journal “Applied optics”.
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Jo Nov 24, 2022
Water spraying itself is a dynamic course, and it is affected not only by the structure of an ultrasonic gas atomizer and the compressed air pressure but also by the movement of the compressed air flow in an atomizer.
The fundamental principle of formation of fine droplets in the ultrasonic gas atomizer is that a liquid sheet is disrupted into droplets by the energy of high-velocity air. If the ultrasonic energy is supplied, finer droplets with high sphericity and uniform size distribution can be achieved.
Kim Kang Dok, a section head at the Faculty of Mining Engineering, has observed water atomization of an ultrasonic gas atomizer by numerical simulation with CFD software ANSYS Fluent, and found out about the generation of ultrasound and water fog in the ultrasonic gas atomizer with a resonance chamber together with ultrasound characters (frequency, pressure level, power level).
Recently, experimental researches to improve the features of ultrasonic atomizers by air spray have been conducted, but only a little about numerical simulation was presented except for some experimental data.
Thus, he has simulated the characters of ultrasonic air atomization using the finite volume method based on the Roe solver in the structure similar to the Hartmann resonance chamber. Then, he has looked closely at the occurrence of oscillation and the factors affecting the oscillation, and investigated the distribution character of water droplets.
The experimental conditions for the simulation were compressed air pressure 3.5~4atm, air amount 0.1㎥/min, acoustic power in the cavity resonator 242dB, acoustic power at the outlet 172dB.
The range of frequency is 20~120 kHz, where 21~28 kHz has the highest density. The acoustic power decreases with the distance when the pressure of the compressed air is 0.3MPa.
The determination of the distribution characteristics of the atomizer indicated that condensation of water droplets decreased rapidly from the distance of 3m. Thus, it is clear that the rational spray distance of the atomizer is 3m. The results show that the number density of water particles is 1.52×1010 in the center, but it is 6×108 in the diffusion region.
You can find more information about this in his paper “Numerical simulation on the generation of ultrasound and Formation of Water fog in the Ultrasonic gas atomizer” presented to the SCI Journal “ULTRASONICS”.
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Jo Nov 21, 2022
Chlorination leaching, one of non-cyanidation processes, which was in use until the end of 19th century after its development in the middle of the 19th century, was deprecated after the cyanidation process was born. However, in the eighties of the 20th century it was found that chlorination leaching had a good effect on fine grains of gold ores, and researchers’ interest in it revived, leading to some successes.
Compared to cyanidation leaching, chlorination leaching of gold by chlorine or hypochlorite has faster gold leaching rate and less pollution. What is more, it is cheaper and simpler.
Kim Chang Sok, a researcher at the Science Engineering Institute, has conducted thermodynamic and experimental research on chlorination leaching of refractory gold concentrates. Then, he has observed the influence of the redox potential on gold leaching and confirmed reasonable conditions for the potential.
The results demonstrate that for a higher leaching rate of gold, leaching time should be more than two hours and redox potential should be controlled at 1.0V or more. Under the conditions of the initial concentration of active chlorine 0.5%, the value of pH 4.0, the leaching time at least two hours and the redox potential more than 1.0V, the leaching rate of gold could reach over 96%.
You can find more information about this in his paper “Research on Chlorination Leaching of Pressure Oxidized Refractory Gold Concentrate” presented to the SCI Journal “Hydrometallurgy”.
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Jo Nov 17, 2022
Several works have revealed the inclusion properties between the Orlicz spaces, which are complicated in the proof process and very difficult to apply.
Kim Jin Ju, a lecturer at the Faculty of Applied Mathmatics, has formulated a necessary and sufficient condition for the existence of inclusion properties between weighted and weighted weak Orlicz spaces, which is a more generalized space than an Orlicz space, using the characteristic function of a sphere.
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