The semi-solid process is viewed as a promising manufacturing method for producing near net-shape metal products with low porosity and shrinkage. However, semisolid slurry has inferior fluidity compared to the liquid state, resulting in the low formability of the semisolid process. Hence, improving the fluidity of semisolid slurry is one of the most important issues for the semisolid process. To resolve this problem, we attempted to improve fluidity by applying shear stress on slurry at the gate of the mold. In this study, the effect of shear rate on the fluidity of the AC4CH aluminum alloy semisolid slurry was investigated. The shear rate at the gate was controlled by changing the thickness of the gate from 1.0mm, 2.2mm 3.1mm and 4.0mm. The fluidity was evaluated by injecting into a spiral cavity through a narrow gate, as flowed length into the cavity. The microstructures were observed by optical microscopy, and the roundness and diameter of solid particles in the specimens were calculated by image analysis. The results showed that fluidity increases with increasing gate velocity, or increasing shear rate at the same injection velocity. On the other hand, both the mean roundness and diameter of the solid particles in the specimen decreased with increasing shear rate by decreasing gate thickness. This means that slurry composed of fine, spherical solid particles can be obtained with a high shear rate. Additionally, both the mean particle roundness and diameter correlated with fluidity. These results suggest that the fluidity of slurry can be improved by increasing the gate velocity and the shear rate because the viscosity of the slurry decreases. As just described, the fluidity of the semisolid slurry can be improved by controlling the gate velocity and shear rate. This method is expected to enable the production of semisolid forming products with complex configurations.

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  In this study, die casting machine test was conducted to establish the quantitative evaluation of the formation of cast defect and to elucidate the squeeze effect on the formation of porosity in ADC12 aluminum die casting alloy. Considerable porosities were seen to be distributed parallel to the surface of the permanent mold in the samples, and the larger spherical or Chinese-like shrinkage showed a strong tendency to distribute at the center of the samples. On the contrary, dispersal of only a few porosities was found immediately below the surface, since density of the aluminum sample is higher around the casting surface than that in the internal area. From the density analysis, the porosities were calculated to be 3.1 to 4.6% in volume in the sample, and the gas porosity was estimated to be 1.5% or lower out of the total porosity, indicating that more than 98% of the porosities are produced due to solidification shrinkage even in the die cast process. On the other hand, the volume fraction of the shrinkage porosity tended to decrease with increasing ratio of modulus of the gate and that of the sectioned area of products (M_g/M_e). A modified version of Chvorinov's rule was adopted to explain the influence of the modulus on the formation of shrinkage porosity, and the optimization of die casting conditions was discussed.

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  Surface defects of die castings, such as surface folds, cold shut and misrun etc., are thought to occur due to the solidification of molten metal during mold filling. However, die casting is a process in which mold filling and solidification are completed under high pressure, thus varying the casting pressure may also have some effects on surface defects.
  In this study, we investigated the effects of solidification on mold filling and pressure transmission and elucidated the influences of casting pressure on the quality of die castings.
  In the mold filling process of JIS AC4C alloy, flow resistance was found to increase with the increase of solid phases, thus decreasing the velocity of molten metal flow. Cold shut occurred due to the rapid drop of molten metal temperature as a result of the decrease in the flow velocity. A similar phenomenon was also observed for JIS AD12.1 alloy, although this alloy shows a skin-formation type solidification. The pressure transmission of molten metal was found to weaken gradually with the increase of solid phases and eventually completely stop and give rise to casting defects. In addition, resistance to molten metal flow due to back-pressure, shape resistance of the mold cavities, etc. were seen. These results indicate that a comparatively higher plunger pressure is necessary to obtain die castings without defects.

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