Materials Science Forum Vols. 510-511

Abstract: Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure, extension of the die life, and cost saving owing to its low energy consumption compared with the conventional extrusion processes. Especially, thixoextrusion process is expected to be very effective for hard-to-form materials with high strength. The present study focuses on 7075 Al wrought alloy to investigate the potential industrial applications of the thixoextrusion process. The microstructural evolution of 7075 Al wrought alloy for thixoextrusion was investigated with respect to isothermal holding temperature and time in the partially remelted semisolid state. The results showed that the liquid fraction increased with increasing isothermal holding temperature and time while the average grain size was inversely proportional to isothermal holding temperature and time up to 5min. However, there was no big change of liquid fraction and average grain size with respect to isothermal holding temperature and time. The important fact that the liquid fraction and average grain size were almost uniform after 5 min holding time is considered very useful for thixoextrusion in terms of process control.

Abstract: Nano-TiC powders were fabricated for mechanical alloying (MA) by shaker mill using the TiH2 powders mixed carbon powder. For mechanical alloying, titanium hydride was easily breaking alloy and easily decomposed titanium particles and hydrogen gas. The decomposition titanium powders in TiH2 powders were very fine particle size and unstable phase. And it easily reacted to carbon ;(TiH2 + C 􀃆 TiC + H2). The effects of mechanical allyoing, morphology, phase and particle size were evaluated with X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), TG/DTA and particle size analyzer. As-milled powders for 10 hours were successfully synthesized powders of TiC phase, mean particle 300nm size. And as-milled powder for 1 hour was included unstable phase, was annealed for 1 hour at 400°C-1300°C in 1x10-3torr vacuum atmosphere. Unstable phase was changed to recrystallize phase by heat treatment. After heat treatment for 1hour using as-milled powders, it was included many types of titanium oxide at temperature below 1000°C, was formed single phase of TiC at temperature over 1000°C.

Abstract: Small amounts of minor alloying elements such as RE and Sr were added to Mg- 8wt%Al-5wt%Zn (AZ91D+4%Zn), and their effects on the microstructure, mechanical properties and corrosion resistance were investigated. The microstucture of the investigated alloys could be characterized by dendritic Mg, Mg17Al12, a quasi-crystalline Zn-rich phase, and Al4RE (if RE is added). Although the tensile strength of alloys was not improved, the creep strength was significantly enhanced by the additions of minor alloying elements. No apparent influence of the additions could be found on the corrosion resistance.

Abstract: Spent foundry sand (SFS) from cast iron industry mixed with loess was characterized to recycle spent foundry sand as ceramic support materials. Since SFS mostly consists of sand including binding agents and residue [1], it was used as permeable media, and loess, which has highly adsorptive and reactive properties, was used as support materials Leaching tests such as Korean Leaching Test (KLT) and TCLP (Toxicity Characteristics Leaching Procedure) showed that spent foundry sands from cast iron industry were not hazardous. Turbidity in furan sand was much higher than that in other sands, and turbidity was almost not changed with increasing amount of loess. The order of ORP (oxidation-reduction potential) in spent foundry sands was green sand > furan sand > CO2 sand. ORP was decreased with the amount of sand in furan sand and CO2 sand but was increased with the amount of sand in green sand. Hence, mixture of SFS and loess can be used as ceramic support materials with reductive or oxidative capability.

Abstract: Chitosan-DNA nanoparticles were synthesized from the complexation of chitosan solution and the plasmid pCD-GFP encoding green fluorescent protein through a coacervation process. The nanoparticles formed were of diameter inferior to 100 nm. The transfection efficacy of chitosan-DNA nanoparticles in MGC803 cells was higher than in SMMC-7721 cells, whereas the plasmid GFP was not expressed in MG63 cells.

Abstract: Environmental regulations in the refrigeration technology make the harmful refrigerants such as CFCs (chlorofluorocarbons) and HCFCs (hydro-chlorofluorocarbons) replaced by eco-refrigerants such as CO2 , and the substitution of CO2 increases the pressure of multi-void condenser tube. As a result, the stiffness of the conventional 1000 series Al alloys can be open to question. The present study is mainly aimed at developing a direct extrusion process and neocondenser tube material for the application of eco-friendly refrigerants with higher working pressure than the conventional CFC and HCFCs refrigerants. The mechanical properties and burst strength for condenser tube were greatly affected by the large grains formed by brazing condition. As a result of the burst testing, 3003 Al alloy proved to be more suitable than 1100 Al alloy for the application of condenser tube to eco-friendly refrigerant usage because 3003 Al alloy can be durable in the heat exchanger required high pressure of working. Furthermore, optimum dies design has to be introduced for control metal flow and metal welding during direct extrusion process.

Abstract: In this study, we examined brazeability of Al5052 alloys using a Ag-28Cu insert metal in vacuum condition. A high frequency induction-heating vacuum hot press was used for the brazing. Under a static pressure of 0.5 MPa and a vacuum of 5×10-5 Torr, the temperature of hot press was raised up to brazing temperature (480-590°C) at a speed of 10°C/min and held at the brazing temperature for various times. Interface behaviors of the brazed joints were observed after brazing at 480-590°C by optical microscopy and Electron Probe Micro Analyzer (EPMA). Also, bonding strength of the brazed joints was examined. During vacuum brazing of Al5052 alloys with Ag-28Cu insert metal, eutectic melts were formed by eutectic reaction between Al5052 and Ag-28Cu insert metal at about 490°C. It was found that reaction layers of two phases are formed at the interface between Al5052 and Ag-28Cu insert. EPMA analysis revealed that two phases in the reaction layers consist of Ag-rich phase and Cu-rich phase. The bonding strength of the joints increased with brazing temperature and time. The highest bonding strength was 141 MPa at 580°C, 10min.

Abstract: Hexagonal boron nitride ceramics with sintering additives were synthesized by uniaxial hot-pressing. Hot pressing was carried out at 1800oC for 2 h under the uniaxial pressure of 30 MPa under a nitrogen atmosphere. Thermal conductivity perpendicular and parallel to the direction of the hot-pressing axis was measured by the laser-flush method.