ソノ凝固によるAC2B合金半凝固鋳造材の引張特性改善

抄録

  In order to improve the ductility of AC2B aluminum alloy, which is generally applied to automotive cylinder-heads, microstructure and tensile properties were studied using rheocast disc samples with sono-solidified slurry. Rheocast AC2B alloy with sono-solidified slurry shows superior tensile properties compared to those with normally solidified slurry. However, eutectic silicon morphology becomes slightly coarser in rheocast AC2B alloy with sono-solidified slurry, although sono-solidification provides the required granular primary α-Al grains for rheocasting. Primary silicon particles grown as the nucleus of AlP are recognized in the eutectic region of rheocast AC2B alloy with sono-solidified slurry, and they cause the coarse eutectic silicon morphology. Such coarse eutectic silicon morphology can be modified by the addition of strontium, which remarkably improves ductility. Impurity of iron in rheocast AC2B alloy reduces ductility owing to the easy crack initiation at the plate-like intermetallic compound of β-Al₉Fe₂Si₂. The direct observation of sono-solidifying model liquid revealed that new crystals are generated at the collapsed sites of cavitation, requiring ultrasound to be irradiated to molten metal above the melting point for the effective grain refinement. Hence, by applying ultrasonic vibration to molten AC2B alloy above the melting point of α-Al₈Fe₂Si, tensile properties of rheocast alloy are improved by the appearance of granular intermetallic compound of α-Al₈Fe₂Si, instead of plate-like β-Al₉Fe₂Si₂.