The surface of the sheet fabricated by the vertical-type high-speed twin-roll casting exhibits various patterns consisting of metallic luster areas (shiny zones) and cloudy areas (un-shiny zones). This study aims to elucidate the causes of the structural differences between the shiny and un-shiny zones, using two representative alloys : pure aluminum and Al-5%Mg alloy.

  In the case of pure aluminum, the surface exhibited a scale-like pattern with dispersed un-shiny zones several millimeters in size. These un-shiny zones were found to form due to the transfer of roll polishing marks to the sheet surface when the thicker solidified shell of the un-shiny zones passed through the roll gap compared to the shiny zones. On the other hand, the Al-5%Mg alloy showed a periodic pattern with alternating shiny and un-shiny zones along the casting direction, where the solidified shell in the un-shiny zones was thinner and under residual tensile stress. Microstructural observations revealed that for pure Al, although there are differences in the growth of the solidified shell, the influence of the crystal orientation near the surface is greater than that of the cooling rate. On the other hand, in the case of Al-5%Mg, the growth of the solidified shell corresponded well with the cooling rate, indicating that differences in cooling rate also affected the formation of surface patterns.

  Semi-solid processing can decrease the porosity of casting products and increase dimensional accuracy. On the other hand, in HPDC, JIS-ADC12 aluminum alloy (equivalent to ASTM 383) is mostly used in Japan. This alloy has a narrow semi-solid temperature range and exhibits a skin-formation type solidification, making it difficult to apply the semi-solid process. In this study, we prepared ADC12 aluminum alloy slurry by applying mechanical vibration and evaluated the fluidity of the slurry, as well as the effects of shear rate and gate velocity on fluidity.

  The ADC12 aluminum alloy slurry made by the mechanical vibration method formed thin solidification shells and coarse primary α-Al particles before injection. The fluidity of the slurry increased with increasing gate velocity and shear rate. These solid particles became fine in the slurry at high shear rates and high gate velocities. However, in low injection velocity and narrow gate thickness conditions, there were some instances where the slurry could not flow into the die cavity or exhibited very low fluidity. On the other hand, the slurry could flow stably in high injection velocity conditions, allowing the solidification shells to flow into the cavity.

  By comparison with JIS-AC4CH (equivalent to ASTM 356) alloy slurry, the fluidity of the ADC12 alloy slurry was more affected by the gate velocity and shear rate. The AC4CH aluminum slurry had relatively homogeneously distributed solid particles, but the ADC12 alloy had both coarse primary α-Al particles and solidification shells. Solid particles became fine and spherical with the application of shear stress, which contributed to decreased viscosity of the slurry and increased fluidity. Additionally, in the ADC12 alloy, the application of high shear stress is believed to enable the solidification shell to be broken into fine and spherical pieces. Therefore, the improvement effect of increasing gate velocity and shear rate was higher in the ADC12 alloy.

  It is well-known that wettability largely affects the adhesion and coalescence behavior of non-metallic inclusions and bubbles. For the evaluation of the adhesion behavior of bubbles and particles, water model experiments are suitable because the contact angle between particles and water can be easily varied by using hydrophilic and water repellent agents. In this study, swirl flow was applied to the entry portion of the nozzle to form an air column at the nozzle center line. This air column was then destabilized to allow formation of fine bubbles from the shearing force created by the swirl flow in the nozzle. After these preparations, the adhesion and coalescence behaviors of the dispersed bubbles and particles were observed by a high-speed microscope and laser slit beam. The results clarified that the difference in the contact angle significantly affects the adhesion behavior of bubbles and particles even under poor wettability conditions. Moreover, it was found that the penetration length of the particles into the bubble can be explained by a simplified theoretical model based on the force balance between adhesion force generated by the surface tension of water and pushing force created by Laplace pressure.