Device and method for applying group frequency ultrasounds to conduct magnesium alloy semi-continuous casting

The invention discloses a device and method for applying group frequency ultrasounds to conduct magnesium alloy semi-continuous casting. The device comprises a tundish, a first ultrasonic generation device is fixed to the side wall or a bottom plate of the tundish, a second ultrasonic generation device is arranged in the tundish, and the front end faces of ultrasonic radiation rods of the two ultrasonic generation devices are located in the tundish. The method includes the steps that firstly, magnesium alloy melt is smelted; secondly, the tundish, a liquid moving pipe and the two ultrasonic radiation rods are preheated; thirdly, the refined magnesium alloy melt is conveyed to the tundish, two ultrasonic generators are started, ultrasonic waves are transmitted through the two ultrasonic radiation rods, and group frequency ultrasonic waves are formed and are applied to the magnesium alloy melt; fourthly, a crystallizer is started; and fifthly, after the group frequency ultrasonic waves are applied, a flow contr