Enhancement of Thermal Fatigue Strength by the Addition of Calcium to Hypoeutectic Aluminum-Silicon Alloys 2018-32-0027

Several elements affect the structure of eutectic silicon in hypoeutectic aluminum alloys [1, 2, 3, 4]. Among them, calcium has been investigated to a lesser extent compared to the typically used sodium and strontium. In order to enhance the thermal fatigue strength of a small engine, the morphology of eutectic silicon in hypoeutectic aluminum-silicon alloys is controlled by the addition of calcium. In addition, the castability and mechanical properties are investigated. Hence, samples containing different amounts of calcium are prepared at different cooling rates during solidification. The results revealed that, with the increase in the calcium amount and the cooling rate, eutectic silicon exhibits a fine morphology in cross-sectional images. Particularly, with the addition of at least 62 mass ppm of calcium in a specific range of cooling rates, refined eutectic silicon is obtained. In order to clarify additional effects of the added calcium, the amount of dissolved gas, fluidity, and porosity defects are evaluated. The amount of dissolved gas and the fluidity do not change in the range of the investigated calcium amounts. However, the sizes of each of the shrinkage porosities decrease and the region where porosities generated is spread, with spread-out regions. The sample strength with refined eutectic silicon is evaluated by tension and thermal fatigue tests. Refined samples exhibit greater elongation and thermal fatigue strength compared with those of eutectic silicon with a coarse morphology. The addition of calcium leads to the effective enhancement of the mechanical properties of hypoeutectic aluminum-silicon alloys.