Se Hyun Ko

Study of Mechanical Properties of an LM24 Composite Alloy Reinforced with Cu-CNT Nanofillers, Processed Using Ultrasonic Cavitation

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.

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.

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.

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.

Size Effect in Sintering of Micro Metal Injection Molded Part

Micro gears with different dimension were fabricated by micro metal injection molding and sintering behavior of the micro gear was investigated. For this, ultraprecision micro molds were prepared by micro electroforming. The sintered micro gears had outer diameter of 470μm and 250 μm, respectively. From the sintering results at 1200°C and 1250°C, the grain growth and the degree of thermal etching depend dominantly on the size of micro part rather than micro feature size. The sintering temperature of micro parts should be lower than that of macro components.

Fabrication of High Aspect Ratio Microscale Rod Arrays Using Metal Injection Molding

In this work, micro rod arrays of 50 um and 200 um in diameter, respectively, were fabricated by 316L metal injection molding. Acryl sacrificial mold was used for the 50 um rod array. Aspect ratios were about 8 and 6 in 50 um and 200 um rod arrays, respectively and the bending of the rods occured due to high aspect ratio in debinding and sintering steps. Also, severe grain growth occurred at rod surface by sintering for 3 hrs at 1300°C and the average size was measured to be about 70 um.