Papers by Author: Tae Hoon Choi

Abstract: Deformation characteristics and forming limit of ultrafine-grained bulk Al-Mg alloy were examined with upsetting process. The Al-7.5%Mg alloy produced by cryogenic milling and HIP was subjected to hot hydrostatic extrusion as a final consolidation in fabricating the ultrafine-grained bulk material. Upsetting was performed to study their mechanical characteristics in a practical forming process. The extruded specimen showed that the pores remained in spite of the HIP had been collapsed and almost eliminated. The effective removal of the distributed pores resulted in significant increase of formability by preventing early cracking. Metallographic investigations showed that the size of grain remained below a few hundred nanometer scale in the processes.

Abstract: Multi-filament fabrication process using repetitive hydrostatic extrusion of Cu/Al at high temperature was conducted to obtain micro-scaled Aluminum wires. In the process an aluminum rod claded with a copper tube was extruded repetitively three times where a number of the single extruded rods were bundled together and subjected to hydrostatic extrusion to obtain multi-filament wire bundle. Aafter final bundle extrusion the diameter of the aluminum rod was effectively reduced from 33.6 mm to 30μm.

Abstract: Many micro technology researches have been concentrated in the field of materials and a process field. But the properties of micro materials should be understood to give still more advanced results. Among the various material properties, mechanical material properties such as tensile strength, elastic modulus, etc., is the basic property. To measure mechanical properties in micro or nano scale, actuating must be very precise. Piezo is a famous actuator, frequently used to measure very precise mechanical properties in micro research field. But piezo has a nonlinearity called hysteresis. Not precision result is caused because of this hysteresis property in piezo actuator. Therefore feedback control method is used in many researches to prevent this hysteresis of piezo actuator. Feedback control method produces a good result in processing view, but causes a loss in a resolution view. In this paper, hysteresis is compensated by using an open loop control method. To apply the open loop control method to piezo actuated nano scale material testing machine, hysteresis property is modeled in a mathematical function, and a compensated control input is constructed using inverse function of original data. The reliability of this control method can be confirmed by testing nickel, aluminum, and copper micro thin foil that is used in MEMS material broadly. If these MEMS material properties are used in a MEMS research field, more economical and high performance MEMS materials can be obtained.

Abstract: This paper investigates the characteristics of a hydro-mechanical punching process. The hydro-mechanical punching process is divided into two stages: the first stage is the mechanical half piercing in which an upper punch goes down before the initial crack is occurred; the second stage is the hydro punching in which a lower punch goes up until the final fracture is occurred. Ductile fracture criteria such as the Cockcroft et al., Brozzo et al. and Oyane et al. are adopted to predict the fracture of a sheet material. The index value of ductile fracture criteria is calculated with a user material subroutine, VUMAT in the ABAQUS
Explicit. The hydrostatic pressure retards the initiation of a crack in the upper region of the blank and induces another crack in the lower region of the blank during the punching process. The final fracture zone is placed at the middle surface of the blank to the thickness direction. The result demonstrates that the hydro-mechanical punching process makes a finer shearing surface than the conventional one as hydrostatic pressure increases.

Abstract: Reducing the particle size of drug materials down to submicron is an important matter in pharmaceutical industry. Cryogenic milling technology is one of the mechanical milling processes, which is mostly utilized in refining grain size of metal and ceramics at extremely low temperature environment. This technique has not been readily studied in application to medical and biotechnology. This paper, therefore, describes the application of cryogenic milling process to reduce particle size of Ibuprofen. The shape and size of the Ibuprofen particle before and after the cryogenic ball milling process were analyzed. XRD analysis was performed to examine a change in crystallinity of Ibuprofen by the cryogenic ball milling process. The results showed that the size of Ibuprofen particles was reduced to 1/10 or less of its initial size. The results also showed that the degree of crystallinity of Ibuprofen was slightly reduced after cryogenic ball milling with nitrogen.

Abstract: The optimum design of a die shape for Pilger mill process was carried out using FEM analyses considering various processing factors. The important design parameters of the Pilger mill machine are feed rate and profile of the grooved die. Optimum design procedure was conducted in order to investigate effects on forming load and the deformed shape of a material depending on the die surface profiles. Profiles of the die surface for the optimum design were testified with linear, cosine and quadratic curves considering physical forming process. The results of the analyses provided that the model of the quadratic profile gave the lowest forming load and the proper deformed shape.