抄録
Ultrasound has been successfully applied to the grain refinement of various cast metals through frequent nucleation, as well as degassing, and inclusion removal from molten metals. In the present study, in order to broaden applications of semi-solid castings, grain refinement and formation of granular primary solid solution were examined by applying ultrasonic radiation to a molten aluminum alloy (AC4CH) surface during the solidification period, instead of inserting the ultrasonic radiator into molten metal. The ultrasound was radiated from a vibrating end surface with a vibration amplitude of 60μm and resonant frequency of 20kHz. Prior to the solidifying experiments of molten aluminum alloy, a transparent substance of succinonitrile was visibly solidified with ultrasonic radiation. According to successive observations, solid crystals generated near the heaved liquid surface were found to settle at the bottom by the forced convection due to the ultrasonic radiation, which is known as acoustic streaming. Primary aluminum solid solution vigorously nucleated right next to the vibrating surface, so that granular primary crystals accumulated at the mold bottom, instead of forming a dendrite structure. The granularity of primary solid solution depends on the irradiation period of ultrasound, that is, slow solidification rate is desired in the solid/liquid coexisting temperature zone. In the compressive shaping of semi-solid slurry, the form ability improves with the degree of primary crystal granularity.
