Abstract
Hypereutectic Al-Si alloys are promising candidates of light-weight wear resistant materials due to the hard primary silicon particles embedded in the eutectic matrix. These alloys are therefore expected to be applied to tribological components such as automotive cylinder liners. However, they do not exhibit sufficient performance heavy-duty tribological parts because coarse primary silicon grains reflect poor ductility and machinability. Both refinement and increase in the amount of primary silicon grains are required in order to use them as tribological components widely. Sono-solidification, in which ultrasound is radiated into molten metal during the solidification, improves the morphology of coarse primary silicon, as well as crystallizes non-equilibrium α-Al grains in hypereutectic Al-Si alloys. With sono-solidified Al-Si alloy billets, centrifugal thixocasting was examined for the segregation of refined globular primary silicon at an inner surface. The present study clarifies that the sono-solidification increases the amount of primary silicon in Al-18%Si alloy by 2.5 times more compared with that of equilibrium condition along with both refinement of globular primary silicon and crystallization of non-equilibrium α-Al grains. When sono-solidification billets are heated up to the semi-solid temperature, the α-Al grains are ready to disappear, but the refined primary silicon grains remain without coarsening their sizes. Centrifugal thixocasting causes segregated refined primary silicon grains at the inner surface of cylindrical castings within a short solidification period.
