ORALS

Applying power ultrasound during solidification has proven to be an effective way to improve the microstructure and enhance the mechanical properties of metallic alloys. However, most previous investigations focus on the dendritically solidified Al or Mg based single phase alloys, the effect of power ultrasound on the multiphase alloys, including eutectic, pertiectic and monotectic alloys are still remains unclear. In present work, the solidification mechanism of these three kinds of metallic alloys is summarized on the basis of the authors’ work. For binary Sn-Pb eutectic alloy, differing from the regular lamellar eutectic formed during static solidification, there are two typical kinds of spherical eutectic cells formed within the ultrasonic field. Theortectical analysis shows that the local high undercooling induced by cavitation takes responsibility to the formation of anomalous eutectic, while the radial symmetry of both the concentration and temperature fields induced by acoustic streaming ensures that the solid-liquid interface is symmetric in three dimensions. As for binary Cu-Sn peritectic alloy, the ultrasonic field brings about a striking size refinement effect to the primary intermetallic compound by more than one order of magnitude. Meanwhile, it facilitates or even completes the usual peritectic transformation which occurs only to a very limited extent during static solidification. These lead to the remarkable improvement of mechanical properties for such Cu–Sn alloy, whose compressive strength and microhardness are both remarkably increased. Power ultrasound is also introduced to the liquid phase separation and dynamic solidification of ternary Cu-Sn-Bi monotectic alloy. It is found that as compared with the layered structure formed under static condition, the macrosegregation resulted from liquid phase separation is remarkably reduced with the increase of ultrasonic amplitude. This is mainly ascribed to the ultrasonically induced cavitation and acoustic streaming, which promotes the nucleation, the fragmentation, and the dispersion of secondary droplets. The finally solidified immiscible alloy exhibits obvious improvements in electrochemical corrosion resistance, microhardness and wear-resistance.

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