Abstract:High-intensity ultrasound at a resonant frequency of 20kHz and power of 500 W was introduced into different solidification stages of hypoeutectic Al-5%Si alloy, and the formation mechanism of primary α(Al) phase and its interdendritic structure was explored. The results show that porosity in the solidified structure reduces remarkably, and the primary α(Al) phase transforms from coarse dendrites under static into equiaxed grains or fragmented dendrites by introducing power ultrasound during different solidification stages. Meanwhile, the interdendritic structure is also refined by ultrasound. When the power ultrasound is applied into liquid phase, it accelerates the nucleation of primary α(Al) phase by enhancing wetting status between the crystal embryos and impurities in the alloy melt. If ultrasound is employed during nucleation and growth of primary α(Al) phase, the cavitation-induced nucleation and cavitation-induced fragmentation take the responsibility of most remarkable grain refinement. Once the ultrasound is introduced in eutectic transformation, the dendritic fragmentation results from mechanical effect accounted for the reducing in grain size of primary α(Al) phase. Both microhardness and yield strength can be improved after ultrasound is applied at each solidification stage of Al-5%Si alloy, indicating that applying power ultrasound is an effective way to moderate the mechanical properties of Al-Si alloys.

Key words: ultrasonic field; eutectic solidification; dynamic nucleation; cavitation effect; structure formation; mechanical property