Abstract:
Recently Mg-based alloys have drawn increasing attention of designers and scientists owing to their low density, high strength-to-weight ratio, high damping capacity, excellent machinability, good castability and low inertia. Because of these aspects, Mg alloy developments have been traditionally driven by aerospace and automobile industrial requirements to operate under increasingly demanding conditions. However, magnesium alloys are not yet used to the same extent as their competitor such as aluminium alloys because of difficult to control their microstructure. To enlarge the usage of Mg alloys, further improvements of their mechanical properties are being demanded. Microstructure modifications like grain refinement and phase morphology are therefore required to improve the mechanical properties of as cast components. Generally there are two methods for grain refining in Mg alloys. One is the use of compounds or chemical elements that can act as effective grain refiners. The other is to use physical fields such as pulsed electric current, magnetic fields and ultrasonic vibration. The use of physical field usually has better effect on grain refining than that using grain refiners. Ultrasonic vibration is one of the simple and effective physical methods to refine grain size of Mg alloys during solidification process.
Ultrasonic processing of materials has been practiced for over a half century. It is a powerful technological tool to treat a material. It possesses the advantages of being environmentally favorable, cost effective, and ready to be combined with other physical processing technologies for liquid and solidified metal. The structure changes due to the ultrasounds in solidifying metal are generally due to the processes in the melt and the two-phase liquid-solid zone i.e. ultrasonic induced nucleation, dendritic fragmentation. The primary effects involve ultrasonic cavitation and acoustic streaming, associated with propagation of ultrasound waves in media.
