**2. Experimental Materials and Methods**

The substrate materials used in the experiment are 6060 Al provided by Henan Jiayuan Aluminum company (Zhengzhou, China) and T1 copper (Cu) plate provided Zhongtian Copper Industry Co. LTD (Dongguan, China), and each size is 60 mm × 20 mm × 2 mm. The detailed composition of the substrate materials are shown in Table 1. The filler metal is Zn-22Al provided by Xinrui China (Shengzhou, China) with a thickness of 0.3 mm and a size of 7 mm × 20 mm. Before the experiment, the surface of substrate metals and filler metal were polished, and the oxide film was removed. After grinding, the substrate metals were put into acetone and then washed with water bath ultrasonic for 15 min to avoid the impact of grease and impurities on the joint structure on the material surface. Then, the cut composite filler metal is coated with flux. The substrate metals and the composite filler metal are fixed in accordance with the structure shown in Figure 1.

**Table 1.** Chemical compositions of 6061 Al alloy, T1 Cu alloy, and Zn–22Al alloy.

**Figure 1.** Schematic representation of bonding process for vibration brazing.

Brazing was carried out in atmospheric environment. The melting point of the brazing alloy Zn-22Al is about 450 ◦C. The use of pure cesium salt of medium temperature flux in the brazing process can protect the brazing materials and help to break the oxidation film on the metals surface. The melting point of pure cesium salt is 445 ◦C, which plays a role in the vicinity of the filler metal liquid temperature line of 450 ◦C. The melting of pure cesium flux (Xinrui, China) before that of filler metal helps to protect the welding joint.

In the experiments, SP-25B model high frequency induction heating equipment which produced by Shuangping Supply Power Technologies Company (Shenzhen, China) was used as the brazing heat source. It consists of signal source, induction coil and cooling equipment. Meanwhile, thermocouple was used for temperature measurement.

In order to investigate the influence of Al side heat dissipation rate as while as ultrasonic duration on semi-solid forming and brazing joint microstructure [18,23–27], different experiments were designed. Firstly, under the condition of keeping the ultrasonic vibration duration unchanged, we carried out several groups of experiments under the condition of 2 K/s, 5 K/s, 7 K/s and 9 K/s (here K is the unit of thermodynamic temperature, named Kelvin, Abbreviated as K and K/s Represents the heat dissipation rate per second). Heat dissipation rate at Al side by adjusting the heat exchanger. Then, several groups of experiments under 0, 2, 4, 6, and 9 s ultrasonic vibration duration were carried out under the condition of the same heat dissipation rate on Al side.

The bonding cycle of heating, ultrasonic vibrating, solid/liquid ratio adjustment and Secondary ultrasonic dispersion and cooling are schematically illustrated in Figure 2.

**Figure 2.** Sequence diagram of ultrasonic assisted one-side core semisolid brazing process.

After brazing, the samples were cut off along the width and each the section was polished with a series of metallographic sandpapers. The corrosion status of each polished specimen was observed under an optical microscope (OM) (GX71, Olympus corporation, Tokyo, Japan). The corrosion method was as follows: aluminum substrate material was corroded by 2% hydrofluoric acid, and copper substrate material was corroded by 3 g of FeCl3 powder and 2 mL hydrochloric acid and 96 mL anhydrous ethanol. The corrosion time was about 5 s.

In order to further investigate the effect of semisolid process on microstructure and microstructure composition of brazed joints, the polished samples were observed by a scanning electron microscope (SEM, Quanta FEG 200, FEI Corporation, Hillsboro, OR, USA). Meanwhile, in order to explore the mechanism of the semi-solid process on the mechanical properties of brazed joints, the shear tensile strength (tensile rate of 1 mm/min) of each sample was measured on a universal mechanical tester.

In addition, experiments of common non-ultrasonic vibration brazing of Al-Cu were carried out to compare with the new method-TSBUAUN
