Abstract:In this paper, Mg-9Al-1Si alloy was prepared by gravity casting of copper metal mold, and the microstructure of the alloy was controlled by ultrasonic treatment at different time before solidification. The evolution of microstructure and morphology and its effect on mechanical properties of Mg-9Al-1Si alloy under different ultrasonic treatment time were studied by means of OM, XRD, SEM and tensile test. The results show that without ultrasonic treatment, α-Mg matrix is coarse dendrite, coarse Chinese character-like Mg2Si phase has obvious agglomeration, β-Mg17Al12 phase is intermittently reticulated along the dendrite boundary, ultrasonic treatment can significantly refine the microstructure of the alloy, the grain size of α-Mg matrix changes from coarse dendritic to flocculent, and the Chinese character Mg2Si phase is refined into strip or thin line, and the distribution becomes uniform. The reticular trend of β-Mg17Al12 phase weakens, and a large number of lamellar β-Mg17Al12 phases appear at the boundary. With the increase of ultrasonic time, the effect of ultrasonic cavitation and sound flow can obviously improve the microstructure of the alloy, but the effect of ultrasonic cavitation and acoustic flow on microstructure refinement is weakened for too long. Therefore, the mechanical properties of Mg-9Al-1Si alloy were significantly improved after ultrasonic treatment, and increased at first and then decreased with ultrasonic treatment time, in which the improvement of ultrasonic treatment of 20min was the highest, which was 72MPa and 2.31% higher than that of untreated alloy, respectively.