**1. Introduction**

Due to their excellent corrosion resistance, high mechanical strength, and excellent electric and heat transfer performance, Aluminum and copper and their alloys have been widely used in electric power, cooling, chemical industry and aerospace fields [1–5]. In the process of equipment manufacturing, the connection problem of aluminum and copper dissimilar metals is often faced.

Zn-Al alloy is a promising brazing filler alloy for Al/Cu brazing [6–9]. The main advantage of the filler metal is that zinc metal and Cu and Al metal all had a very good solubility, which form good metallurgical combination easily, and the same time the Zn-Al alloy has good toughness and corrosion resistance. The Zn-Al filler [10] is a medium temperature solder and the eutectic temperature is about 380 ◦C, which can basically adapt all Al alloy brazing [11] connection and can avoid stress cracks resulted from the high temperature brazing and substrate metals ablation. Zn alloy has good thermal and electrical conductivity, which meeting the needs of Al/Cu connector performance. With the advantages of above-mentioned, the Zn-Al filler brazing alloy of Al/Cu dissimilar metals is increasingly become a hot research area.

From what has been discussed above, it can be seen that in brazing welding, semi-solid filler metal has many advantages: reducing connection temperature; getting isometric crystals and uniform microstructure of the joint whose strength can be achieved or close to the strength of the substrate

metals. In addition, semi-solid brazing can deposit a large amount of filler metal in a single-pass brazing, which avoids smoke and splash [12–17].

Existing research [12–14,18–20] shows that using semi-solid brazing can increase the uniformity of brazing seam joint structure, but the research on ultrasound-assisted brazing of Al/Cu dissimilar metals using non-prefabricated semi-solid Zn-22Al (Eutectic solder) as filler metal is rarely found.

Due to the heat and mass transfer characteristics [21], ultrasonic assisted method has been widely used in various kinds of metal welding [22] in recent years to improve the microstructure of the welding seam (including brazing seam).

So, a kind of non-vacuum ultrasound-assisted semi-solid Zn-22Al high-frequency induction brazing of Al/Cu is adopted in this paper, herein referred to as semi-solid-state brazing with ultrasonic-assisted unilateral nucleation (abbreviated as TSBUAUN). Then, the influence of aluminum side heat dissipation rate and ultrasonic duration on the microstructure evolution and physical properties of semi-solid forming brazed seam joint are studied, which are of great significance for promoting the industrial application of Al/Cu dissimilar metal connection.

The proposed TSBUAUN method can improve the uniformity of the brazing joint structure, and its forming mechanism can be extended to semi-solid brazing of other dissimilar metals, providing a method and theoretical support for new brazing technology and the design of dissimilar brazing solder.
