*3.3. E*ff*ect on the Secondary Phase*

Figure 7 shows the distribution of the secondary phase along the radial direction of the ingots under different processing conditions. The dark gray area is α-Al matrix, and the white part is the secondary phase. In the solidification process of the alloy, the matrix phase formed first, and then the alloy elements precipitated, due to the decrease in solubility of the matrix, forming binary or ternary eutectic phase structure with Al element. It can be seen from the figure that the grain size in the microstructure of the conventional ingot is obviously larger, and there are some dendrites, while the proportion of equiaxed grain in the ingots by ultrasonic treatment is larger, the grain becomes smaller and more uniform, the size of the secondary phase is also reduced, the morphology is improved, and the distribution is more uniform. All of these significantly improved the comprehensive mechanical properties of the ingots. In the center of the ingots, whether it is the conventional ingot, without ultrasonic treatment, or the ingots with ultrasonic treatment, the secondary phase agglomeration phenomenon at the grain boundaries is very obvious and there is still serious segregation. At the one half radius of the ingots, the ultrasonic treatment broke-up some of the clustered secondary phase, and branched intergranular secondary phase with a discontinuous distribution appeared. In particular, the crushing effect of the ingot, treated with the L-shaped ultrasonic wave guide rod, was found to be better than that of ingots treated with the straight-rod ultrasonic wave guide rod, and the coarse secondary phase on the grain boundary was reduced, but there are still obvious long strip and massive eutectic phases. At the edge of the ingots, there are relatively few agglomerations of the secondary phase, and the secondary phase is mainly in the form of fine mesh. The secondary phase of the ingot, treated by the straight-rod wave guide rod, is increasingly fine, distributed in an intermittent network at the grain boundary of equiaxed grain, and has been basically dissolved in the matrix in some areas.

**Figure 7.** Distribution of the secondary phase of the ingots 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.

Figures 8 and 9 show the distribution and the area percentage of the secondary phase in the grain at the 1/2 radius respectively. The coarse bars and blocks of the secondary phase in the conventional ingot without ultrasonic treatment have larger proportion and are dispersed in the grain. The secondary phase in the grain of the ingots by ultrasonic treatment is mainly granular and acicular, and the distribution is relatively uniform, but there are still a great deal of little block or short rod-like secondary phase. The application of ultrasonic treatment can refine the secondary phase in the grain, break and re-melt the primary dendrite in the solidification front of the melt, reduce its aggregation and growth, and promote its uniform distribution.

**Figure 8.** Distribution of the secondary phase in the grain at the 1/2 radius: (**a**) No ultrasonic treatment; (**b**) treatment by the L-shaped ultrasonic wave guide rod; (**c**) treatment by the straight-rod ultrasonic wave guide rod.

**Figure 9.** Area percentage of each secondary phase in the grain at the 1/2 radius.

Figure 10 shows the results of line scan analysis of main alloy elements in the ingots under different processing conditions. It can be seen from the figure that the main alloy elements Cu, Mg, and Si, in the ingots without ultrasonic treatment, have obvious segregation at the grain boundary, and the segregation of Cu element is particularly serious. Also, Cu, Mg, and Si are mainly enriched in the secondary phase, Al2Cu, which is white. In the ingots treated by ultrasonic, the concentration of main alloy elements decreased, solute diffusion was promoted, part of Al2Cu phase dissolved in the matrix, but a large amount number of Cu element were still segregated at the grain boundary.

Further point scanning analysis was carried out on the α-Al matrix structure of the ingots under different processing conditions. About 100 points in one grain were selected for scanning, and the average value of the measured 100 component points was taken as the main content of solute elements in the matrix. Table 3 shows the content of solute elements in the α-Al matrix structure of the ingots treated with different processing conditions.

**Figure 10.** Results and areas of line scan analysis of main alloy elements in the ingots under different processing conditions: (**a**) No ultrasonic treatment; (**b**) treatment by the L-shaped ultrasonic wave guide rod; and (**c**) treatment by the straight-rod ultrasonic wave guide rod.

**Table 3.** Content of solute elements in the α-Al matrix structure of the ingots treated under different processing conditions (Wt.%).

