This study aimed to estimate the notch sensitivity of spheroidal vanadium carbide cast iron (SCI-VCrNi) in rotating bending fatigue test. We investigated the fatigue failure mechanism of SCI-VCrNi by comparison with FCD400 by fractography. As a result of fatigue test, the fatigue limit of SCI-VCrNi was found to be σ_w ＝ 350MPa, higher than FCD400. In addition, the notch factor and notch sensitivity of SCI-VCrNi was less than FCD400. As a result of observing the crack initiation points of SCI-VCrNi, the cluster of VC was found to be dominant. We then quantified the cluster of Vanadium Carbide (VC) using the √area parameter model, and as a result, confirmed that the notch specimen's √area was smaller than that calculated for smooth specimens.
  Moreover, the stress intensity factor range (ΔK) taking into account the considered stress concentration of notch specimens was higher than the smooth specimens. For this reason, the small clusters of VC were thought to become the crack initiation point in the notch specimen. We also observed a stripe pattern peculiar to cast iron in the fatigue state of the matrix, and thus studied the fracture surface of SCI-VCrNi. The results suggested that SCI-VCrNi in which crack progresses in the matrix has low notch sensitivity due to the low notch sensitivity of austenite steel. In addition, the VC of the fracture surface manifested broken and characteristic aspects, indicating that crack progresses in the matrix while breaking VC when the crack reaches VC, and as a result, VC becomes resistant to crack growth.

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  Semisolid injection molding is expected to be increasingly applied to high flammable magnesium alloys as a new forming process because it can be carried out under temperatures lower than that of die-cast. In this study, we investigated the effects of molding conditions on the tensile strength and internal casting defect of AZ91D magnesium alloy. Semisolid injection molding was conducted at the injection speeds of V ＝ 220, 300 and 400mm/s, and fraction solidf_s ＝ 0.0, 0.3, 0.4, and 0.5. The volume of the casting defects decreased with the reduction of the injection speed, but on the other hand, the mechanical strength reached maximum value at the injection speed of 300mm/s. The results of investigations showed that the solidification microstructure, α-Mg, and β-Mg₁₇Al₁₂ phase which were liquid during injection were refined with increasing injection speed, suggesting that the tensile strength increases with increasing injection speed if there are no casting defects. This may be because the heat-transfer coefficient between the mold and slurry increases with rising flow rate. On the other hand, the volume of casting defects increased with increasing injection speed and deteriorating tensile strength. When the injection speed increased, the effects of decreased strength due to the increase in the number of casting defects and effects of increased strength due to the refinement of the liquid phase counterbalanced each other. For this reason, it is considered that the mechanical strength reached the maximum value at the injection speed of 300mm/s. Thus, the tensile strength of semisolid products was affected not only by the number of casting defects but also by the microstructure of the residual liquid phase which was refined by the increase in injection speed.

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  Recycling of aluminum alloy components plays a significant role in achieving sustainable society which requires superior performance of cast aluminum alloys containing high level of impurity elements such as iron. Since impurities of iron critically reduce both tensile strength and elongation of cast Al-Si alloys with iron content of over 1mass%, the morphological change in microstructure of Al-7mass%Si alloys with different iron contents was examined by applying ultrasonic vibration during the solidification, that is, the sono-solidification. The microstructure of conventionally solidified billets for thixocasting mainly composed of large plate-like β-Al₅FeSi compound and dendritic primary a-aluminum solid solution is eminently modified into a fine and globular shape through the sono-solidification. After induction heating of the billet up to the semi-solid temperature of 583℃, it was immediately thixocast to observe their microstructure and mechanical properties. In addition to the refinement of β-Al₅FeSi compound and primary α-aluminum in Al-Si-Fe alloy billets through the sono-solidification, eutectic silicon needles also become finer by thixocating, because of release of less latent heat compared with conventional castings solidified from their liquid state. This kind of characteristic microstructure generated by thixocasting with sono-solidified billets, exhibits not only high tensile strength, but also marked elongation even at high level of impurity iron. An increase in elongation of thixocast hypoeutectic Al-Si alloys with sono-solidified billets reflects major usage of recycled aluminum alloy ingots instead of virgin ones for the production of highly insured cast aluminum alloy components.

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  Al-7%Si alloy was melted using 99.99%Al and 99.9999%Si, and specimens were prepared by adding phosphorus using a Cu-8%P alloy. Specimens were then cast into three casting molds with different cooling rates, and the relationship between the phosphorus content and cooling rate was clarified by measuring the eutectic silicon interlamellar spacing of each specimen. The mechanism by which phosphorus in the molten metal is partitioned to the α phase and silicon phase during solidification was considered by separating and quantitatively determining the phosphorus contents in the silicon phase and the α phase, and a parameter representing the eutectic structure was proposed in place of the modification/unmodification condition. The eutectic silicon interlamellar spacing is affected by the content of phosphorus in the silicon phase. The eutectic silicon interlamellar spacing is narrower (1―3μm) when the phosphorus content in the silicon phase is zero, and broader (4―12μm) when the phosphorus content is 3―5ppm. Partitioning of phosphorus in a molten metal to phosphorus (P_si) in the silicon phase and phosphorus (P_α) in the α phase is considered to occur in the following order : 1) solid solution or incorporation of phosphorus in the primary crystal α(P_α), 2) formation of AlP(P_si) for nucleation of the eutectic silicon, and 3) remaining eutectic α(P_α). The ratio eutectic silicon interlamellar spacing λ/dendrite arm spacing d₂ was proposed as a parameter expressing the eutectic structure. The formula (λ/d₂) ＜0.05 for the Al-7%Si-0%P alloy is considered an appropriate criterion for defining the improved structure.

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