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

• Ultrasonic vibration could refine the phase Mg17Al12 of AZ91 alloy. • Solution heat treatment was effective in dissolving the phase Mg17Al12. • The tensile properties of AZ91 alloy with ultrasonic vibration were improved. • The solution heat treatment could lead to the improvement of the tensile properties. Ultrasonic vibration with frequency of about 20kHz and power of 350W 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.