Abstract
Ultrasonic treatment was conducted to study its effect on the size and morphology of the α-Al phase of the A356 Al–Si–Mg alloy. Treatments were carried out in the temperature range from 700 to 614 °C, i.e. from above the liquidus temperature (619 °C) to semisolid temperatures. The treatment times varied from 15 to 180 s. The results showed that ultrasonic treatment could control the alloy microstructure when applied in the liquid or semisolid states, and was most efficient when applied at a few degrees above the liquidus temperature, e.g., 620 °C, where globule size of 57 μm and roundness between 0.62 and 0.70 were obtained. Ultrasonic treatment times in the order of 15–30 s were sufficient to dramatically refine the size and alter the morphology of the α-Al phase. The refining effect observed in samples treated in semisolid state was due to solid fragmentation and dispersion, while in the liquid state, refining occurred most likely as a result of activation of non-equilibrium nucleation events which was confirmed from experimental simulations performed using succinonitrile polymer.