*2.1. Experimental Equipment*

A new self-made L-shaped ultrasonic rod was used in the experiment, as shown in Figure 1. The L-shaped ultrasonic rod was composed of a transducer, a first-level horn that transmitted longitudinal mechanical vibrations in the horizontal direction, a second-level horn that transmitted bending vibrations in the vertical direction, and a ceramic radiator. The material used for the horn was a TC4 titanium alloy, and the material used for the ceramic radiator in direct contact with the metal melt was nano-silicon nitride ceramic. The working frequency of the ultrasonic wave was 21 ± 0.2 kHz and the power range was 0–1000 W.

**Figure 1.** L-Shaped ultrasonic rod device.
