*3.1. Typical Interfacial Microstructure of the Al*/*Zn-22Al*/*Cu Brazed Joint*

Optical microscope images of the interfacial microstructure of brazed joints by different brazing methods are shown in Figure 3a,b are the weld microstructure obtained by the common brazing process and ultrasonic assisted brazing process respectively.

**Figure 3.** (**a**) brazing Al/Cu without ultrasonic; (**b**) semi-solid brazing Al/Cu with ultrasonic.

There are obvious differences in microstructure between the two kinds of brazing samples. The microstructure of brazed joint of common brazing sample in Figure 3a is mainly dendrite and fringe eutectic. The number of fringe eutectic tissues on the Cu- substrate- side (Abbreviated as Cu side) is larger than that on the Al side, which conforms to the diffusion law of Cu element.

In Figure 3b the microstructure of the TSBUAUN sample is mainly spherulite and fringe eutectic. The density of spherulite on the Al side is higher than that on the Cu side. Through graphic analysis of the corroded semi-solid specimen, it can be found that the core of the spherulite is more easily corroded than other parts of the semi-solid specimen.

Meanwhile, from TSBUAUN sample we can see there are obvious differences in corrosion resistance of spherulite cores on both sides of Cu and Al. Cu side spherulite core has better corrosion resistance than that of Al side. Under the condition of Al side supercooling, the weld formed dense α-Al columnar crystal and Large dendrites. Active heat dissipation on the Al side aggravated the imbalance of weld temperature and was beneficial to dendrite growth. The average length of dendrites can reach 200 to 300 microns under certain conditions.

Under the action of appropriate Al side heat dissipation rate and appropriate ultrasonic vibration duration semi-solid state in brazed joints was achieved. The diameter of semi-solid spherulites is mainly distributed between 25 and 40 microns, and the average size is about 35 microns.

By comparing Figure 3a,b, it was found that there was an obvious transition layer (about 10μm) on the Cu side of the common brazing sample, while the transition layer of the ultrasonic assisted brazing sample was very thin (0.5–0.8 μm).

The optical microstructure of the brazed joint of common brazing samples (Figure 4a), SEM microstructure (Figure 4b) and line-scan component analysis (Figure 4c) showed that the microstructure of common brazing was mainly composed of α-Al phase dendrite and Zn-Al eutectic. The rose-like intermetallic compounds are mainly distributed at grain boundaries. According to SEM/EDS results, the following two intermetallic compounds were identified: CuZn5-ε phase (light gray zone) and Al5Cu4Zn-T' phase (dark gray zone).

**Figure 4.** The dendrite microstructure OM diagram, SEM morphology diagram and line scan diagram of the brazed joint area of common sample: (**a**) OM morphology of dendrite in brazed zone; (**b**) SEM morphology of dendrite structure in brazed zone; (**c**) Linear scanning composition of dendrites in brazed joint microstructure; (**d**) Line scan for Al composition; (**e**) Line scan for Zn composition.

Under the ultrasonic-assisted semi-solid forming process, due to non-equilibrium solidification, striated eutectic was produced at the rapid cooling rate during the brazing process, and the number of eutectic tissues on the Cu side was significantly larger than that on the Al side. Due to the forced convection generated by ultrasonic waves, the dendrite-Al phase fragmentation was promoted, and the typical spherical structure composed of semi-solid particles was formed, as in Figure 5.

**Figure 5.** Spherulite OM morphology, SEM morphology, EDS layered image and composition distribution of ultrasonic-assisted semi-solid sample: (**a**) OM morphology of spherulite in the brazed joint zone; (**b**) SEM morphology of dendrite structure in the brazed joint zone; (**c**) EDS hierarchical images in spherulite; (**d**) Zn distribution in spherulite; (**e**) Al distribution in spherulite; (**f**) Cu distribution in spherulite; (**g**) Ni distribution in spherulite.
