Won Sik Lee

Softening Phenomenon during Compression Test in Nanograined Aluminum Alloys

Al-Mg and Al-Mg-Cu alloys are known well to reveal superplasticity in tension at high temperatures. In this study, deformation behaviors of those alloys nanograined were investigated under compression test at room temperature. During plastic deformation softening phenomena occurred obviously in nanograined Al-1.5wt%Mg and Al-0.7wt%Mg-1.0wt%Cu alloys while slight strain hardening appeared in nanograined pure Al. These results suggest that the softening strongly depends on composition of alloys. The softening takes place over strain rate range from 10-4 up to 10-1.

Ni Nanoparticles Deposition onto CNTs by Electroless Plating

CNTs were decorated with Ni nanoparticles to decrease floatation of CNTs and improve the wettability between CNT and Al melt by electroless plating method. The as-received size of multi-wall CNTs with 99.5% purity was 10~20nm in diameter and 20um in length. Before Ni deposition, the wet ball milling was tried to improve the dispersion of CNTs in the Ni sulfate solution for several hours. After wet ball milling, the Ni electroless platings have been performed for 1hours at electroless deposition temperature. The Ni deposited CNTs have been characterized in respect of dispersion and size changes of CNTs and Ni particles with field emission scanning electron microscopy(FESEM). The deposited Ni nanoparticles onto the CNTs were 50nm in diameter without ball milling, but they increased in size with increasing milling times up to 120nm. Also, the milled CNTs were damaged and changed from its original morphology due to the high ball milling energy. The addition of surfactant improved the distribution and spheroidization of precipitated Ni nanoparticles. From this study, the multi-wall CNTs have been deposited and decorated with spherical Ni nanoparticles by electroless deposition at a proper milling time and surfactant addition.

Fabrication of TiH2 Powders from Titanium Tuning Chip by Mechanical Milling

In present work, manufacturing technologies of titanium hydride powder were studied for recycling of titanium tuning chip and for this, attrition ball milling was carried out under H2 pressure of 0.5 MPa. Ti chips were completely transformed into TiH2 within several hundred seconds that is very short time comparing to that in the previous report. Dehydrogenation process TiH2 powders is consist of two reactions: one is reaction of TiH2 to TiHx and the other decomposition of TiHx to Ti and H2. The former reaction shows relatively low activation energy ranged from about 100 KJ/mol to 250 KJ/mol and it is suggested that the reaction is caused by introduction of defects due to milling. In case of TiH2 powders that hardly contains defects, decomposition of TiH2 to Ti and H2 occurs directly without the reaction of TiH2 to TiHx and activation energy is very large as much as 929 KJ/mol.

Thermal decomposition behavior of nano/micro bimodal feedstock with different solids loading

Debinding is one of the most critical processes for powder injection molding. The parts in debinding process are vulnerable to defect formation, and long processing time of debinding decreases production rate of whole process. In order to determine the optimal condition for debinding process, decomposition behavior of feedstock should be understood. Since nano powder affects the decomposition behavior of feedstock, nano powder effect needs to be investigated for nano/micro bimodal feedstock. In this research, nano powder effect on decomposition behavior of nano/micro bimodal feedstock has been studied. Bimodal powders were fabricated with different ratios of nano powder, and the critical solids loading of each powder was measured by torque rheometer. Three different feedstocks were fabricated for each powder depending on solids loading condition. Thermogravimetric analysis (TGA) experiment was carried out to analyze the thermal decomposition behavior of the feedstocks, and decomposition activation energy was calculated. The result indicated nano powder showed limited effect on feedstocks in lower solids loading condition than optimal range. Whereas, it highly influenced the decomposition behavior in optimal solids loading condition by causing polymer chain scission with high viscosity.

Cu Particles Deposition onto MWCNTs by Chemical Reaction

CNTs were decorated with Cu particles to decrease floatation of CNTs and improve the wettability between CNTs and Al melt by chemical reaction method. The as-received size of multi-wall CNTs with 99.5% purity was 10~20nm in diameter and 20um in length. Before Cu deposition, the purified CNTs were suspended in solvent solution and ultrasonically stirred to improve the dispersion of CNTs in the copper chloride solution. The metallic Zn and Zn/CNTs composite powders were added into the suspension to precipitate Cu onto the CNTs surface. The Cu deposited CNTs have been characterized in respect of dispersion and size changes of CNTs and Cu particles with field emission scanning electron microscopy(FESEM). The deposited Cu particles onto the CNTs surface were in the range of 100~300nm in diameter. Also, the application of high ultrasonic treatment improved the full coverage of CNTs surface with Cu nanoparticles. From this study, the multi-wall CNTs have been deposited and embedded with Cu particles by chemical reduction process.

Development of Recycled TiH2 Feedstock for Powder Injection Molding

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.

In-Mold Rheology Curves with Feedstock and Mold Dimension

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.

Microstructures and Oxygen Reduction of Recycled TiH2 Powders during Sintering

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.

Fabrication of Micro Spur Gear in Nano Grained Al Alloy

In present work, manufacturing technologies of micro parts by micro forging and pressing were studied using strain softening phenomenon in nano grained Al-1.5mass%Mg alloy. During compressive test at 300, the Al alloy showed stain softening phenomenon by grain boundary sliding regardless of strain rate. Micro spur gear with ten teeth (height of 200 μm and pitch of 250 μm) was fabricated with sound shape by micro forging. During micro forging, increase of applied stress induced by friction between material and die surface was effectively compensated by decrease of stress by strain softening behavior and as a result, flow stress increased only about 50 MPa more than that in compressive test. However, in micro pressing, flow of material did not occurred sufficiently to fill die and the resultant shape of gear was very poor.

Titanium Hydriding and Consolidation for Recycling of Titanium Tuning Chip

Manufacturing technology of bulk titanium by milling and sintering was studied for recycling of pure titanium tuning chip. Ti chips were milled at RT under H2 pressure of 0.5MPa. All titanium chips were pulverized and hydrided within several minutes, which is very short time comparing to other thermal processes. The hydrided powders were hot pressed at 750°C. After hot pressing, bulk sample was identified to α-phase Ti by XRD measurement and measured density(4.509g/cm3) was very close to theoretical density(4.512g/cm3) of pure Ti. Consequently, it is carefully suggested that hydriding and sintering process is efficient and practicable solution for recycling of pure titanium tuning chip.