Effect of Ultrasounds Treatment on Alloys for Semisolid Application

Semisolid metallic alloys are commercially produced by means of mechanical or electromagnetic stirring. Among the mechanical devices, the rotating pin immersed in a solidifying alloy seems to be easier to manage in industrial practice although it can induce some porosity, depending on the shape of the pin. As known, ultrasounds are mechanical waves which, when applied to liquid metals, increase the number of solidification nuclei, so that the cast products show superior mechanical performances, as a consequence of the finer grain structure. In this paper the use of ultrasound waves applied to different alloys during solidification was studied in order to obtain feedstock for semisolid die-casting application. A dedicated ultrasound power unit, together with a proper sonicator pin, was designed and manufactured by the authors and, subsequently, the effect of the ultrasonic treatment on the microstructure of A356 aluminum alloy and ZA27 zinc alloy was investigated. All the produced samples were characterized by metallographic analyses to measure the globule size and shape factor, which are main criteria for thixo-microstructure assessment. The results were compared to those obtained with traditional mechanical stirring, showing the higher capability of ultrasound treatment in producing better semisolid microstructure. An optimized combination of process parameters seems to be necessary to get a reasonable thixotropic structure in treating ZA27 alloy. Less severe production conditions are needed in the case of aluminum alloy, revealing the potentiality of ultrasounds as an alternative treatment to traditional mechanical stirring, with the further advantage of alloy degassing and grain refinement, without the use of expensive addictions (TiB2). Trials were finally performed on a continuous casting pilot plant in combination with electromagnetic stirring to produce semisolid billets.