Se-Weon Choi

Surface Protection of Magnesium Alloys via Pack Cementation Coatings with Aluminum Powder and Chlorides

Magnesium alloys have been received an attention for structural applications due to their low density compared to other alloys, and intensive studies have been focused for enhancing mechanical strength and surface protection as well. Especially, for environmental reasonings, coating processes in a dry condition have been recently received a great attention. In this study, diffusion coatings via Al powders with an aluminum chloride activator have been investigated in order to examine surface protection effect on magnesium alloys. The commercial AZ31 magnesium alloy has been subjected to diffusion coatings in an Al alloy powder for various time frames. An intermediate layer of Mg17Ag12 was successfully synthesized via diffusion annealing. The underlying mechanisms for surface layer formation are discussed together with growth kinetics and microstructural observations

The Relationship between Dendrite Arm Spacing and Cooling Rate of Al-Si Casting Alloys in High Pressure Die Casting

The effects of cooling rate on the solidification behavior of Al-8.5%Si-3%Cu and Al-11%Si-3%Cu alloys were studied during high pressure die casting (HPDC). The HPDC experiment was conducted by using the dies with three steps for three different cooling rates. Because of both high temperature and pressures involved, it was difficult to obtain the temperature profile directly from HPDC specimen. Therefore, in this study, cylindrical bar castings with different diameter were poured to acquire the cooling curves at the solidification range of 15°C/s up to 100°C/s and then the microstructures were compared to estimate the cooling rate in HPDC. The solidification characteristics including liquidus/solidus temperature and dendrite arm spacing of each alloy and each cooling rate was analyzed and the results showed strong proportional relationship between dendrite arm spacing (DAS) and cooling rate in HPDC. The results were also compared with the actual die casting specimens and MAGMA simulation.

Fabrication and Evaluation of Tantalum Compacts for Sputtering Target Application

In this study, tantalum (Ta) compacts were fabricated in a spark plasma sintering (SPS) process and their microstructure and mechanical properties were investigated. Ta compacts with a density of 99% were successfully fabricated by controlling the sintering conditions of the current and the temperature. The density and hardness were increased as the sintering temperature increased. The compound was observed at the surface of the compacts due to the contact between the Ta powder and graphite mold during the sintering process. The main fracture mode showed a mixed type with intergranular and transgranular modes having some roughness. ࠀ

Effects of Eutectic Si and Secondary Dendrite Arm Spacing on the Mechanical Properties of Al-Si-Cu Cast Alloys

The present study aims at investigating the effects of eutectic Si and Secondary dendrite arm spacing (SDAS) on mechanical properties of Al-Si-Cu alloy. Heat treatment and controlling of solidification rate affect to microstructure of Al-Si-Cu alloy. Al-Si-Cu alloy was dissolved in an electric furnace. The alloy cast in STD61 mold which had been pre-heated to 95 °C and 200 °C. Eutectic Si and SDAS were finer as cooling rate increased. Image analysis technique has been utilized to examine the microstructure. Microstructure observation results showed that T6 heat treatment has a strong influence eutectic Si particle morphology. The mechanical properties, such as tensile strength, yield strength, elongation, were improved by ASTM E8 standard. Tensile properties of the Al-Si-Cu alloys prepared by different cooling rates were the same as each other by T6 heat treatment. †(Received December 19, 2013)

Difference in Solidification Process between Al-Mg Alloy and Al-Si Alloy in Die-Casting

The effect of the alloy systems Al-Mg alloy and Al-Si alloy in this study on the characteristics of die-casting were investigated using solidification simulation software (MAGMAsoft). Generally, it is well known that the casting characteristics of Al-Mg based alloys, such as the fluidity, feedability and die soldering behaviors, are inferior to those of Al-Si based alloys. However, the simulation results of this study showed that the filling pattern behaviors of both the Al-Mg and Al-Si alloys were found to be very similar, whereas the Al-Mg alloy had higher residual stress and greater distortion as generated due to solidification with a larger amount of volumetric shrinkage compared to the Al-Si alloy. The Al-Mg alloy exhibited very high relative numbers of stress-concentrated regions, especially near the rib areas. Owing to the residual stress and distortion, defects were evident in the Al-Mg alloy in the areas predicted by the simulation. However, there were no visible defects observed in the Al-Si alloy. This suggests that an adequate die temperature and casting process optimization are necessary to control and minimize defects when die casting the Al-Mg alloy. A Tatur test was conducted to observe the shrinkage characteristics of the aluminum alloys. The result showed that hot tearing or hot cracking occurred during the solidification of the Al-Mg alloy due to the large amount of shrinkage.

Effects of Fe, Mn Contents on the Al Alloys and STD61 Steel Die Soldering

Recently, various attempts to produce a heat sink made of Al 6xxx alloys have been carried out using die-casting. In order to apply die-casting, the Al alloys should be verified for die-soldering ability with die steel. It is generally well known that both Fe and Mn contents have effects on decreasing die soldering, especially with aluminum alloys containing substantial amounts of Si. However, die soldering has not been widely studied for the low Si aluminum (1.0~2.0wt%) alloys. Therefore, in this study, an investigation was performed to consider how the soldering phenomena were affected by Fe and Mn contents in low Si aluminum alloys. Each aluminum alloy was melted and held at 680 o C. Then, STD61 substrate was dipped for 2 hr in the melt. The specimens, which were air cooled, were observed using a scanning electron microscope and were line analyzed by an electron probe micro analyzer. The SEM results of the dipping soldering test showed an Al-Fe inter-metallic layer in the microstructure. With increasing Fe content up to 0.35%, the Al-Fe inter-metallic layer became thicker. In Al-1.0%Si alloy, the additional content of Mn also increased the thickness of the inter-metallic layer compared to that in the alloy without Mn. In addition, EPMA analysis showed that Al-Fe inter-metallic compounds such as Al2Fe, Al3Fe, and Al5Fe2 formed in the die soldering layers.

Effect of Mn on the Interaction between Die Casting Steel and Al Alloy

The formation of intermetallic reaction layers was investigated for die soldering between a STD61 steel and aluminum alloy. It is generally well known that Mn contents have advantageous effect on decreasing die soldering especially with aluminum alloys containing substantial amount of Si. However, die soldering has not widely studied for the low Si aluminum (1~2wt.%) alloys. Each aluminum alloy had melted then STD61 substrate was dipped in the melt for 2hr. The result from dipping soldering test showed the Al-Fe intermetallic layer in the microstructure. In Al-1wt.%Si alloy, additional content Mn also increased the thickness of the intermetallic layer compare to the alloy without Mn.

Interface properties and phase formation between surface coated SKD61 and aluminum alloys

Abstract The intermediate phase formation and surface protection effects between SKD61 die mold alloys and aluminum alloys were investigated during a simulated die-casting process. The surface coatings of SKD61 alloy were carried out via Si pack cementation coatings at 900 °C for 10 h and the ɛ-FeSi phase formed. When the coated SKD61 alloy was dipped in the liquid aluminum alloy (ALDC12), the surface coated SKD61 alloys showed better surface properties compared with uncoated SKD61 alloys, i.e., the intermediate phases (FeSiAl compound) were not produced for the coated SKD61 alloy. The coating layer of ɛ-FeSi served as a diffusion barrier for the formation of FeSiAl compounds.

Evaluation of Ni-YSZ Anode fabricated by Spark Plasma Sintering for SOFC Application

SOFC (Solid Oxide Fuel Cell) Ni-YSZ anode was fabricated by the spark plasma sintering (SPS) process and its microstructure and electrical properties were investigated in this study. The spark plasma sintering process was carried out at for holding time of 5 min under 40 MPa. To fabricate Ni-YSZ anode, the SPS processed specimens were reduced at under atmosphere. The reduced specimens showed relative density of according to sintering temperature. And also, the electrical conductivity of reduced specimens after sintering at 900 and showed (S/cm) values at the measuring range of .

The Effect of Alloying Elements on Thermal Conductivity and Casting Characteristic in High Pressure Die Casting of Aluminum Alloy

Recently, demand of aluminum alloys for use in high thermal conductivity application is increases but the most aluminum die casting alloys exhibit very lower thermal properties because of their high concentrations of alloying elements. However, those alloying elements are essential to obtain sufficient fluidity and mechanical strength. Therefore, the purpose of this study is to analyze the effect of alloying elements in die casting alloys, Si, Cu, Mg, Fe and Mn, in thermal conductivity, die casting characteristics and mechanical properties and find out the appropriate amount of each alloying element for development of heat sink component. The results showed that Mn had the most deleterious effect in thermal conductivity and Si and Fe contents were important to improve strength and limit casting defects, such as hot tearing and die soldering. The alloy with 0.2~1.0wt%Cu, 0.3~0.6wt%Fe and 1.0~2.0wt%Si showed very good combination of high thermal conductivity and good casting characteristics.