ISSUE 1/2013

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Tsunekawa, Y.; Okumiya, M.; Miyamoto, Y. Functionally graded thixocast pipes with sono-solidified hypereutectic Al-Si alloy billets

Hypereutectic Al-Si alloys are an attractive candidate for low-density wear-resistant materials owing to the dispersion of hard primary silicon particles, so that they are expected to be applied to light weight tribological components such as automotive cylinder blocks and liners. However, it is known that the coarse primary silicon particles reflect poor ductility and machinability. Both refinement and increase in the amount of primary silicon are required to expand them widely into practical uses. Sono-solidification, in which ultrasound is radiated into molten metal during the solidification, improves the morphology of coarse primary silicon. With sono-solidified Al-Si alloy billets, centrifugal thixocasting was performed for the segregation of fine globular primary silicon particles on the inner surface of cylindrical castings. This study clarifies that the amount of primary silicon increases in the sono-solidification compared with that in normal-solidification, in addition to the refinement of primary silicon particles and crystallization of a-Al phase. When sono-solidified billets are heated up to the semi-solid temperature, the a-Al phase is ready to disappear, but the fine primary silicon particles maintain their size without coarsening. Centrifugal thixocasting causes segregated fine primary silicon particles with lower density on the inner surface of cylindrical castings.

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