Effects of ultrasonic treatment on microstructure and tensile strength of AZ91 magnesium alloy

Abstract

In this study, the effects of ultrasonic treatment on microstructural features and tensile strength of AZ91 magnesium alloy were investigated. AZ91 melts were subjected to ultrasonic waves of different power levels for 5 min using an ultrasonic device with frequency of about 20 kHz and maximum power of 600 W and cast in sand moulds. The results showed that ultrasonic treatment of the melt prior to casting had a significant effect on the size and sphericity of α-Mg dendrites as well as on the size, continuity, sphericity and distribution of intermetallic particles formed during cooling and solidification of the alloy. Increasing the applied ultrasonic power resulted in smaller, more rounded and better distributed grains and particles. The microstructural effects were mainly attributed to the cavitation and streaming phenomena which took place during ultrasonic treatment in the melt. Tensile strength of the alloy was significantly improved by ultrasonic treatment of the melt. Discontinuity and refinement of Mg 17Al 12 particles in the ultrasonically treated samples is thought to be the main reason for this improvement. The paper also examines different possible mechanisms responsible for microstructural modification of different phases under ultrasonic treatment conditions.