Grain refinement of magnesium alloys by ultrasonic vibration

Cavitation by ultrasonic vibration has been shown to be able to alter the solidification process of a wide range of materials. The influence of ultrasonic vibration on the grain refinement of different magnesium materials has been studied using an ultrasonic system with a frequency of 20 kHz. Ingot samples of pure magnesium, AZ31 (Mg-3%Al-1%Zn), and AZ91 (Mg-9%Al-1%Zn) with a diameter of 70 mm were produced with and without ultrasonic treatment. The melt temperature used was 730 degrees C and the sonicating time was 180 seconds. Substantial grain refinement was observed in all ingot samples cooled in a hot clay-graphite crucible under ultrasonic vibration. The average grain size of the 70 mm diameter ingot samples of AZ31 was found to decrease from about 2 mm to 150 mu m or finer. Similar observations were made for AZ91. For pure magnesium, columnar grains (average length: 30 mm) were changed to nearly equiaxed grains (average grain size: 310 pm). The grain refinement mechanisms by ultrasonic vibration are discussed on the basis of the cavitation study established on water because of the similar densities and viscosities of water and molten magnesium.