*3.2. E*ff*ect on Microstructure*

μ μ μ μ Figure 5 shows the microstructure of the ingots cross-section along the radius under different processing conditions. It can be seen from the figure that the grain size of the three ingots is decreasing from the center to the edge. The dendrites in the center of the conventional ingot are dominant, the secondary dendrite arms are large, and only a few fine grains exist. At the one half radius mark, the number of coarse dendrites decreases and the grain size is smaller. The grain is basically fine equiaxed at the edge. After ultrasonic treatment, the grain of the ingots is refined to a certain extent, mainly fine equiaxed grain with more uniform distribution, especially in the center and at the 1/2 radius. Obviously, the closer the distance from the ultrasonic tool head, the better the grain refinement effect, which shows that the effect of grain refinement of ultrasonic treatment is closely related to its range of action. From the distribution curve of grain size of the ingots, under different processing conditions in Figure 6, it can be seen that in the conventional ingot, the grain size in the center is the thickest with an average grain size of 251.74 µm. Under the action of the L-shaped ultrasonic wave guide rod and the straight-rod ultrasonic wave guide rod, the average grain sizes decreased to 190.62, and 185.81 µm, respectively, and the decreasing amplitude exceeded 60 µm. At the edge, the average grain size of the ingot treated by the L-shaped ultrasonic wave guide rod is 135.96 µm, and the refining effect is better than that of the ingot treated by the straight-rod ultrasonic wave guide rod. The mechanism of ultrasonic grain refinement can be attributed to the cavitation effect and acoustic steam of ultrasonic [29]. The L-shaped ultrasonic wave guide rod had end radiation, but also more side radiation, compared with the action of the straight-rod ultrasonic wave guide rod, which made the area of ultrasonic cavitation larger. The shock wave generated in the process of cavitation bubbles collapse had a strong crushing effect on the primary dendrite and growing dendrite structure in the aluminum alloy melt, which increased the number of crystal nucleus during the solidification process, and the cavitation effect led to instantaneous local lower cold, in part, and heterogeneous nucleation. At the same time, the instantaneous high-pressure, produced by the collapse and fracture of the cavitation bubbles, constantly impacted the surface of heterogeneous particles in the melt, increased the contact angle with aluminum liquid, improved its wettability, activated the heterogeneous particles, and promoted the nucleation core to increase during solidification and crystallization. A large amount of nuclei was generated in these areas, thereby, increasing the number of nucleation and refining microstructure [23]. There was still a certain sound flow effect at the edge of the ingots, which made the aluminum melt be stirred continuously and forced the grain to disperse evenly in the melt. At the same time, it made the temperature field and solute field more uniform in the mold, and the growth direction of the grain became relatively uniform, so as to achieve the effect of refining grain.

**Figure 5.** Microstructure of obtained under different processing conditions: (**a**) Center of ingot without ultrasonic; (**b**) 1/2 radius of ingot without ultrasonic; (**c**) edge of ingot without ultrasonic; (**d**) center of ingot by L-shaped ultrasonic; (**e**) 1/2 radius of ingot by L-shaped ultrasonic; (**f**) edge of ingot by L-shaped ultrasonic; (**g**) center of ingot by straight-rod ultrasonic; (**h**) 1/2 radius of ingot by straight-rod ultrasonic; and (**i**) edge of ingot by straight-rod ultrasonic.

**Figure 6.** Distribution of grain size of ingots under different processing conditions.
