| This exposition presents a novel thermodynamical and microstructural modification to light alloys, such as aluminum alloys and magnesium alloys, by ultrasonic vibrations.; An experimental apparatus which supplied a powerful 1500 Watts at 20 kHz of ultrasonic power was designed and built. Thermodynamic simulations were carried out using the Scheil model to determine the temperature versus solid fraction curve of the alloys. Thermal analysis shows that, with ultrasonic vibration, the steady growth temperature and the minimum supercooling temperature have been elevated; the recalescence time decreased significantly, which indicates a much slower growth rate of primary fcc aluminum grains.; Upon ultrasonic vibration, in A356 alloy, fine globular primary aluminum grains were obtained at an unprecedented level of 20 to 40 mum; superfine globular grains less than 20 mum in size were obtained in the area near the ultrasonic radiator; the morphology of eutectic silicon in the alloy was modified from a coarse acicular plate-like form to a finely dispersed rosette-like form, with significantly reduced length, width, and aspect ratio; fine globular grains were also obtained in other aluminum alloys, including A354, 319, 6063, 6061, 2618 alloys; 670°C is the optimum casting temperature for grain refinement of 2618 alloy; not only did magnesium AM60B alloy experience a reduction in size of primary alpha-Mg grains from 760 mum to about 25∼48 mum in diameter, which is much better than other traditional grain refinement methods, but also the volume fraction of eutectic phases was reduced significantly.; The mechanisms for ultrasonic influence on solidification have been studied. It was concluded that acoustically induced heterogeneous nucleation, rather than dendrite fragmentation, played a dominant role in the formation of a globular microstructure; high acoustic amplitude/intensity favors the formation of small, spherical primary aluminum grains; the casting temperature of 630°C brings about best grain refinement result for A356 alloy; the primary aluminum grain size in a casting increases with the increasing distance from the acoustic radiator. |
