*3.5. E*ff*ect on Mechanical Property*

Hardness is a comprehensive index to measure the strength, plasticity, fracture toughness, deformation resistance, and other mechanical properties of aluminum alloy. Figure 12 shows the hardness test results, along the radial direction of the ingots, under different processing conditions. It can be seen from the figure that the hardness values of every position of the ingots, with ultrasonic treatment, is higher than that of the ingot without ultrasonic treatment, and the values tend to be consistent. The hardness values rose greatly in the center of ingots treated with the L-shaped ultrasonic wave guide rod and the straight-rod ultrasonic wave guide rod, increasing by 4.8%, and 5.6%, respectively. Previous studies have shown that, when ultrasonic treatment is applied to the solidification process of the aluminum alloy, the cavitation effect and acoustic steam increase the nucleation rate, refine the grain, and make the solidification structure more compact and uniform. At the same time, porosities, inclusions, and other defects are also effectively suppressed, and the volume fraction and size of the secondary phase is reduced, the morphology and distribution of the secondary phase are improve, and the composition segregation can also be restrained to a certain extent. The comprehensive effect of these factors effectively improved the mechanical properties of ultrasonic ingots. Compared with other positions, the effect of grain refinement in the center of ingots was better, the secondary phase was smaller and more evenly distributed, and the hardness of aluminum alloy was greatly improved. At the edge of the ingots, the fine equiaxed crystal structure increased the resistance of dislocation movement, thus, increasing the strength. However, due to the existence of serious negative segregation, the decrease of Cu content resulted in the decrease of the amount of Al2Cu in the matrix, and the hardness values of aluminum alloy were not much higher than that at the 1/2 radius of the ingots. The mechanical properties and isotropy of the ingots, treated by ultrasonic, improved, which is beneficial to the subsequent processing.

**Figure 12.** Hardness test results along the radial direction of the ingots under different processing conditions.

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