Papers by Author: Se Hyun Ko

Abstract: This study investigates the effect of Cu-Carbon Nanotube (Cu-CNT´s) composite powders on the mechanical properties of an Al-Si9.5-Cu4-Fe1.3 wt.% (LM24) aluminium matrix composite (AMC). Carbon nanotubes (CNT’s) can exhibit exceptional mechanical properties, e.g. stiffness up to 1000 GPa and strength in the order of 100 GPa. In recent years there has been significant scientific interest in improving properties in conventional alloys, via fabricating CNT metal matrix composites in order to attempt to harness their extraordinary attributes. In this study mechanically alloyed Cu-CNTS powders were added to molten LM24. The melt was processed using ultrasonic cavitation and subsequently high pressure die casting to form as-cast tensile specimens. SEM results indicate that CNT’s can be successfully introduced into the melt using this method. Compared to the unreinforced alloy, the CNT additions resulted in an increment (~20±10 MPa) to both ultimate tensile strength and yield strength, with a corresponding decline (~1±0.5l %) in elongation. This observed increase in strengthening may be attributed to the CNT’s pinning and hindering both grain boundary and dislocation migration during applied loading. Interestingly, no significant difference in properties were found with an increase in the CNT content (from 0.05 to 0.1 wt.%) potentially indicating a saturation limit.

Abstract: In this paper, it was aimed to develop a new Al-Zn-Mg base aluminum alloy having high electrical conductivity, strength, and formability simultaneously. As a result, Al-Zn-Mg base low aluminum alloy sheet can be effectively strengthened without significant thermal conductivity loss by multiply alloying precipitation hardening elements and properly controlling production process parameters.

Abstract: The aim of this work is to investigate the possibility of improvement of surface roughness in the wire-EDMed sample by usinig ultrasonic wave and abrasive particle suspension. For this study, two-type ultrasonic polishing methods were used. Removal of the white layers on surface of EDMed sample depended largely on the movement of the suspended particles : the white layers are removed by the impact of free moving particles due to ultrasonic wave, so-called micro chipping mechanism. Even though the surface roughness was improved very slowly, 3-dimensional fine polishing was achieved in wire-EDMed micro PIM molds. These results indicated that micro polishing by ultrasonic wave using suspension is applicable to improvement of surface roughness of micro parts with 3-dimensional complex shape.

Abstract: In this work, in-mold rheology curves were generated by injection molding machine and the effects of feedstock and mold dimension on in-mold rheology curve were studied. The rheology within mold in µMIM process depends on the shape and/or size of cavity and process conditions rather than intrinsic viscosity of feedstock. The optimum injection speed was determined in region of Newtonian flow of in-mold rheology curve. The dimensional deviation was below about 5 µm in the micro part injected with the optimum speed.

Abstract: Powder Injection molding (PIM) is a cost-effective process for the fabrication of complex shaped parts, and has a great potential in many applications. In this work, an improved wax-based binder was developed for the powder injection molding of TiH₂ powder fabricated by recycling of Ti chip. Fine TiH₂ powders of about 350 nm in particle size were produced by attrition milling of Ti chip in less than five minutes, resulting from simultaneous self-propagating High temperature Synthesis (SHS) and fracturing. TiH₂ feedstock, a mixture of binder and powders, was fabricated with critical powder loading of 68 vol.%. The rheological characteristics of the feedstock were investigated for subsequent processing step. Viscosity of the feedstock showed pseudo-plastic flow behavior and to optimize injection molding parameter, in-mold rheology curve was generated. The results indicated that the recycled TiH₂ feedstock can be used for the fabrication of the complex shaped parts with good shape.

Abstract: Vibration characteristics of 5μm- thick Ni film were investigated with applying acoustic wave to the Ni diaphragm of 2mm x 2mm unit size. In the modal analysis, the first resonance mode of the diaphragm showed an out-of-plane piston-like movement and the first natural frequency was 1,643 Hz, whereas in this experiment, the first natural frequency appears at about 1,300 Hz under sound pressure of 0.2 Pa. The amplitudes of diaphragm increase with increase of sound pressure level in the applied frequency range from 300 Hz to 1,000 Hz, indicating that area of diaphragm influences directly the amplitude.

Abstract: Using commercial Ti powders and recycled TiH2 powders made from titanium turning chips, the effects of hydrogen on microstructures and oxygen reduction were investigated. Each powder was hot-pressed at 750oC and 1100oC, which are below and above β-transition temperature (880oC), respectively. The TiH2 and Ti samples of sintered at 1100oC showed equiaxed α phase. However, TiH2 sample sintered at 750oC consist of primary α phase, secondary α phase and titanium hydride (TiH2), though Ti sample represented single α phase. The formation of complex microstructure in TiH2 sample sintered at 750oC is due to hydrogen remained after sintering. The oxygen contents after sintering in TiH2 samples were relatively lower than Ti samples, although those before sintering were higher than Ti. This result can be explained that hydrogen gas generated from dehydrogenation of TiH2 leads to the reduction of oxygen contents and protection of re-oxidation during sintering.

Abstract: Some nano grained Al materials were produced by mechanical milling/alloying followed by vacuum hot pressing: nano grained pure Al, Al-1.5Mg and Al-0.7Mg-1.0Cu alloys in wt%. The nano bulk materials had average grain sizes of 90-150 nm and ball-on-disk wear equipment was used to investigate the effects of grain size comparing to coarse grained pure Al and T6-treated Al 6061 alloy. In comparison of coarse and nano grained pure Al materials, nano grained specimens showed much higher wear resistance and size of wear debris was very fine as much as 100 nm at applied load of 100g. Wear in nano grained materials proceeded by micro fracturing mechanism like abrasion in low applied and sliding velocity and the mechanism resulted in high wear resistance. Nano grained Al-1.5Mg and Al-0.7Mg-1.0Cu alloys showed much superior wear characteristics due to nano grains and high hardness. As a result, size of grains was a predominant factor for high resistance at low applied load and/or sliding velocity, while hardness at higher applied load or velocity.

Abstract: The effects of use of open cell Al foam and thermoelectric element in a hydrogen storage system were investigated. Extremely different heat conductivities were observed in two storage systems with or without open cell Al foam. By applying the open cell Al in this hydrogen storage system, the reaction sensitivity of the temperature and equilibrium pressure was far rapidly increased than that without the open cell Al foam. During increasing the temperature by heating element, heating rate of hydride powders was very fast in the storage system including Al foam, while temperature of powders was almost not changed in system without foam. Also, in case of using thermoelectric element, heating and cooling rate was very sensitive in the system with Al foam and heating-cooling cyclic behavior within the system controlled by thermoelectric element is seemed to be satisfied for some applications in industry.

Abstract: The open cell Al (or Al foam) was employed in the alloy hydride system due to the rapid heat conductivity of Al. The hydrogen storage alloy powders were packed in this storage system made of Al open cell, and the changes of temperature and equilibrium pressure of hydrogen desorbed were measured. By applying the open cell Al in this hydrogen storage system, the reaction sensitivity of the temperature and equilibrium pressure was far rapidly increased than that without the open cell Al.