Effect of ultrasonic vibration and solution heat treatment on microstructures and tensile properties of AZ91 alloy

Ultrasonic vibration with frequency of about 20 kHz and power of 350 W was introduced into molten AZ91 alloy. After ultrasonic vibration, morphology of the phase Mg17Al12 formed during the cooling and solidification varied from coarse plates to lamellar precipitates. With application of solution heat treatment, the phase Mg17Al12 was mostly dissolved in both the alloy with ultrasonic vibration and without ultrasonic vibration. The ultimate tensile strength and the elongation to fracture of the alloy were significantly increased after ultrasonic vibration. After solution heat treatment the tensile strength of the alloy without ultrasonic vibration increased remarkably while the alloy with ultrasonic vibration increased slightly.