Papers by Author: Tetsuichi Motegi

Abstract: In order to produce AZ91 and AZ31 magnesium alloy billets semisolid continuous casting by using an inclined cooling plate was carried out. The molten magnesium alloy in an electric furnace was flowed on the inclined cooling plate and then poured in a tundish. The slurry prepared using this process was solidified by drawing a dummy bar from a water-cooled copper mold, consequently producing a billet. As a result, we could produce the magnesium alloy billets by semisolid continuous casting. Magnesium alloy billets with refined and globular primary α crystals were successfully and continuously cast by optimizing the solidification conditions. In this case, the grain size of AZ91 billets was about 55μm and that of AZ31 billets was 50μm. The AZ91 billets produced by semisolid continuous casting were employed in thixocasting experiments. The experiments revealed that thixocasting could produce the mobile phone casings with spherical primary α crystals at 858K. Moreover, we proved that the difference of shot cycle did not influence the microstructures for up to 251 shots.

Abstract: Sulfur hexafluoride (SF6) is widely used by the magnesium industry as a component of cover gas mixtures, which reacts with the melt and forms a protective film on the melt surface. In this study the dew points of the air that acts as carrier gas for SF6 was varied from 203K to 273K, and the effect of the SF6 cover gas humidity and concentration on the efficient protection of molten AZ91D magnesium alloy was investigated in temperatures ranging from 903K to 973K. Gas chromatography (GC) combined with ion chromatography (IC) were used to identify the chemical constituents of the by-products present in the protective atmosphere, while. X-ray photoelectron spectroscopy (XPS) and Auger electron Spectroscopy (AES) were used in the characterization of the protective films formed on the melt. The results indicate that the quantity of SF6 to protect molten Mg from burning decreased with decrease in the humidity of the cover gas XPS showed that the films formed in all atmospheres with different dew points were composed of MgO, MgF2 and MgSO4. However, in cover gases with higher dew point, the amount of MgO of the film tended to increase, due to the dissociation of MgSO4 of the film into H2 and SO4-2 in the presence of moisture, which lead to the destruction of the protective film and oxidization of the melt.

Abstract: Ultrasonic vibration was applied to the surface of AZ91D/CNF slurries in order to homogeneously disperse the reinforcement through the semi-solid matrix, and eliminate the clusters of nano-fibres originated during previous processes. The method utilized to produce the slurry was a modified compocasting process that generates semi-solid magnesium alloy using an inclined cooling plate and mechanical stirring. The CNF reinforcement was added in quantities varying from 1 to 3vol% and different vibration times and amplitudes were applied to the slurries under isothermal conditions. Macro and microstructure combined analysis showed the occurrence of grain refinement in the whole ingot for higher wave amplitudes at shorter times, while for lower amplitudes longer times up to 10min. were required. Vibration times over 10min were not significant to further improve the results and times up to 4min did not seem to provide any result whatsoever. The best results, however, were obtained for 16 and 18μm amplitudes. Discontinuous application of ultrasound seems to produce better results than continuous vibration. The dispersion of reinforcement seems to occur by the smash of CNF clusters due to the movement of the solid fraction present in the Mg alloy slurry, and by cavitation, responsible also for some microporosity present in the microstructures.

Abstract: The influence of an inclined cooling plate utilized as a modification in the compocasting process of aluminum alloy is studied in this paper. Based on the crystal separation theory, molten A356 is poured on the inclined copper plate in order to produce solidification nuclei inside the fluid metal, which is, then, mechanically stirred while reinforcement is added through its surface. Carbon nano fibers (CNF) and particulate SiC were utilized as reinforcement, in quantities varying from 0.5 to 15vol%. Dispersion of the reinforcements was observed by macro and microstructure analysis. For CNF, addition of quantities up to 1vol% resulted in homogeneous dispersion through the matrix, although SEM analysis showed the presence of clusters of up to 50μm in some points of the samples. SiC was properly mixed into matrix until 10vol%. Globular crystals were obtained, but some coarsening occurred, especially for small quantities of reinforcement. Utilization of the inclined cooling plate produced good inclusion and homogenization for two different reinforcements through an A356 matrix. The best results were obtained for a 180mm cooling plate, with inclination of 60o. The pouring temperature utilized was 10oC over the melting point of the alloy, and temperature of the tundish was kept around 598oC during stirring.