Sung-Kil Hong

Microstructure and tensile properties of bi-materials with macro-interface between unreinforced magnesium and composite

The microstructure and tensile properties of bi-materials consisting of the unreinforced magnesium, the macro-interface and composite reinforced with SiC whiskers were investigated. The macro-interface has diffuse interface characteristics, its thickness is about 7∼10 μm. It has been found that the tensile strength of the bi-materials at R.T. is lower than that of its corresponding pure magnesium and the composites, and remains unchanged until 423 K. Above 423 K, the tensile strength decreases with increasing temperature and becomes the same as that of magnesium. The elongation is in between the pure magnesium and the composite at R.T. and elevated temperatures. These results were discussed based on the experimental observation of fracture surface by FE-SEM.

Dynamic Young’s modulus and internal friction in polycrystalline copper

An attempt has been made to measure the temperature dependence of dynamic Young’s modulus together with the related variation of internal friction in polycrystalline copper. A mechanical spectroscopy study was employed using a standard servo hydraulic fatigue testing machine equipped with a scanning laser extensometer. Dynamic Young’s modulus and internal friction were measured over a temperature range of 298 to 873 K at very low frequencies of 0.1, 0.05, and 0.01 Hz. One internal friction peak was observed over the ranges 450 to 700 K, together with marked decreases in the dynamic Young’s modulus in the same temperature range. From a quantitative analysis of the experimental data with the relaxation strength, relaxation time, and activation energy, it is concluded that the peak phenomenon is due to grain-boundary sliding relaxation.

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.