Jo Oct 20, 2023
Ti2AlNb-based alloys have expansive application prospects in advanced automotive and aerospace fields because of their excellent mechanical properties such as high specific strength, good creep resistance at elevated temperature, low thermal expansion and inimitable combination of strength and elongation.
The deformation process of Ti2AlNb-based alloys is inevitably performed at high temperatures because of their limited plasticity at room temperature. Therefore, predicting the hot deformation behavior of Ti2AlNb-based alloys is quite important for numerical modeling of hot working processes.
In recent years, some scholars have constructed some constitutive models to predict the hot deformation behavior of Ti2AlNb-based alloys. Unfortunately, however, few endeavors have been devoted to developing and improving ZA model and KHL model for P/M Ti-22Al-25Nb alloys.
Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has developed improved m-ZA and KHL models which can accurately predict the hot deformation behavior of a fine-grained (FG) Ti-22Al-25Nb alloy fabricated from mechanically alloyed powder by spark plasma sintering.
First, in order to obtain true stress-strain curves of P/M Ti-22Al-25Nb alloy, he conducted isothermal uniaxial compression tests at different deformation conditions of 950 ~ 1 070 ℃ and 0.001 ~ 1 s-1.
Second, using the friction-corrected experimental data, he developed a modified Zerilli-Armstrong model and a Khan-Huang-Liang model in the α2+β/B2 + O triple-phase and α2 + B2 two-phase fields, respectively, and evaluated the predictability of the models.
Finally, based on the analysis of the reason for large deviation, he modified Zerilli-Armstrong and Khan-Huang-Liang models to consider the coupled effects of deformation parameters.
Conclusively, he drew the following conclusions.
First, the m-ZA and KHL models for the P/M Ti-22Al-25Nb alloy showed relatively good predictability at the reference deformation conditions. However, the prediction accuracy of the models was lowered in other deformation conditions.
Second, the improved versions of the m-ZA and KHL models exhibited enhanced prediction accuracy. R2 and AARE of the improved constitutive models were 0.989 6 and 6.14%, 0.989 1 and 6.82%, respectively.
Finally, in comparison with other constitutive models, the improved versions of the m-ZA and KHL models are very appropriate for engineering applications including numerical simulation and control for hot working processes of P/M Ti-22Al-25Nb alloy.
The details of this are found in his paper “Modified Zerilli-Armstrong and Khan-Huang-Liang constitutive models to predict hot deformation behavior in a powder metallurgy Ti-22Al-25Nb alloy” in “Vacuum” (SCI).
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Jo Oct 17, 2023
Han Un Chol, a researcher at the Science Engineering Institute, studied the structural influence of a connection block on the pressure impulse generated by the detonator and transmitted to a large number of shock tubes via a numerical simulation by using ANSYS AUTODYN code.
He employed two modes of connection block for initiating a bundle of shock tubes for his numerical simulations and experiments: One is a lateral initiation mode widely used in blasting practice, in which each shock tube surrounds the detonator, extending in parallel to the axis of the detonator body. The other is a frontal initiation mode where entrances of shock tubes are aligned at certain distances from the firing end of the detonator.
He compared the strength of a pressure impulse within the shock tubes numerically obtained for frontal initiation mode with that for lateral initiation mode.
Then, for frontal initiation mode, he observed the relationship between structural factors such as strength of a pressure impulse, materials and thickness of connector block bodies, and standoff distance between the firing end of the detonator and the inlets of shock tube.
The results showed that his study is useful for designing connector blocks of a frontal initiation mode for simultaneously initiating a large number of shock tubes.
His paper was presented in the 10th International Conference on Advanced Technologies.
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Jo Oct 16, 2023
A lot of convex corner compensation methods of V-grooves structure thin membranes of a semiconductor pressure sensor have been introduced, but they still make it difficult to make thin membranes with square mass. In order to make silicon thin membranes in different thicknesses with compensated stress, thin corner compensation method for preserving the square shape have to be developed.
Yu Nam Chol, a researcher at the Science Engineering Institute, has designed a convex corner compensation pattern to make V-grooves structures thin membranes and proved the superiority of this method by means of the final analytical results.
His method ensured the membrane thickness of mass structure close to a right-angled pattern by combining several compensation patterns from 10㎛ to 20㎛. He made a membrane with mass whose a/h is less than 3.72 and b/h is less than 3.33, by applying corner compensation patterns.
You can find the details of this in his paper “A Compensating method of Convex Corner in Etching of (110) Si” in “Microsystem Technology” (SCI).
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Jo Oct 14, 2023
In recent years, carbon nanotubes (CNTs) are widely used as a high-sensitivity sensor material. The fine structure of the surface morphology largely affects the characteristics. Therefore, delicate observation of the microstructures within the range of nanometer is vitally important for achieving better CNT surface properties.
On the basis of theoretical analysis on the observation of the microstructure of CNT surface, Yu Nam Chol, a researcher at the Science Engineering Institute, has observed the image of CNT surface microstructure by Quanta 200 SEM to select a proper accelerating voltage.
The theoretical analysis of the effects of energy of incident electrons on the range of the electron-CNTs interaction and the resolution of SEM indicated that the most proper accelerating voltage for microstructure observation of CNTs surface by using an SEM (Quanta200) is within 5~10kV. Through the experiments based on it, he found that the accelerating voltage of 7.5kV provides the sharpest image of the microstructure of CNT surface.
You can find more information about this in his paper “Study on Accelerating Voltage of SEM in Observation of Carbon Nanotube Surface” in “Nanoscience and Nanometrology”.
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Jo Oct 12, 2023
Pak Se Yang, a researcher at the Science Engineering Institute, has proposed a new control technique to realize the maximum torque per ampere (MTPA) in direct torque control (DTC) of a permanent magnetic synchronous motor (PMSM).
Because MTPA condition is provided by the relation between d-axis and q-axis components of the stator current, rotor position has to be known for frame conversion. Therefore, he chose to employ the estimation method of rotor position for realization of MTPA.
For the sake of rotor position estimation, he formulated the recursive least square problem for online estimation of back electro-magnetic force by using a motor model based on the rotational reference frame, and then determined the rotor position. In order to minimize the torque ripple of conventional DTC system, he synthesized the 3 phase output voltages by using the space vector pulse width modulation (PWM) technique.
His paper was presented in the “10th International Conference on Advanced Technologies”.
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Jo Oct 9, 2023
Generally, rotary angular sensors can be classified into two types ― sensors based on changes in an electric field and a magnetic field, and optical sensors ― according to the physical principle used for measurement. Among them, optical sensors that operate on visible light or infrared light offer advantages of contactless measurement and insensitivity to electric and magnetic fields.
Yu Nam Chol, a researcher at the Science Engineering Institute, has proposed an absolute rotary angular sensor with a nonlinear transparent disc between light source and light dependent resistors (LDR).
In this sensor, the absolute rotary angle is measured by the output resistance of double LDRs which has a linear change by nonlinear transparency of the disc in a range of 0~360° according to the characteristics of LDR’s resistance via irradiance.
This sensor’s advantage is that it is immune to shock and vibration as it has a nonlinear transparent disc instead of a binary coded disc and it has a large gap between the disc and the optical sensor. It means that it is easy to make a sensor. The disc is made of fiber glass and covered up by a self-adhesive tape whose transparency changes nonlinearly in a range of 0~360°.
Another advantage of this sensor is that its resolution is determined by an AD converter in the signal processing circuit as the output signal of LDRs is analogue. Therefore, this rotary angular sensor is provided with a high resolution in a range of 0~360°.
For more information, please refer to his paper “Improving of Characteristics of Rotary Angular Sensor Using Nonlinear Transparent Disc” in “International Journal of Sensors and Sensor Networks”.
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