Aluminum alloys are widely used in our daily life, and vacuum brazing is very important in the processing of some aluminum alloy products. Today, let's talk about the principle of aluminum alloy vacuum brazing and brazing materials.

Advantages of aluminum alloys

Aluminum alloys are widely used in aerospace, construction, electrical appliances, automobiles, ships and other sectors due to their light weight, good corrosion resistance, and excellent thermal conductivity. Structural material for waveguides and many other complex components. The processing technology of aluminum alloy plays a very important role in the manufacture of its products, and one of the important methods is the vacuum brazing process without any flux. Because of its many advantages: no complicated cleaning work before and after welding, simplified operation, avoidance of slag inclusion caused by flux, no flux residue in the structure to ensure its corrosion resistance and high productivity, from now and Considering the concept of environmental protection in the future, the application of aluminum alloy vacuum brazing will become more and more extensive.

The principle of aluminum alloy vacuum brazing

There is a dense and very stable oxide film Al2O3 on the surface of the aluminum alloy, which is the main obstacle for the molten solder to wet the base metal. It is difficult to remove the oxide film by simply relying on vacuum conditions, and some metal activators, such as magnesium Mg, bismuth Bi, etc., must be used at the same time. In the early days, some people thought that the purpose of removing the film could be achieved by the action of Mg. This is because:

From these reaction equations, it is believed that on the one hand, Mg reacts with O2 and H2O remaining in the vacuum to eliminate their harmful effects on aluminum; The Al2O3 on the surface of the material undergoes a reduction reaction to achieve the purpose of directly removing the oxide film. However, a large number of studies since then have shown that the oxide film of the base metal has not been completely removed, so a new point of view has been put forward for film removal. In addition to the role of Mg in eliminating O2 and H2O in the environment, Mg vapor penetrates into the material layer under the film and diffuses. Together with Si, the surface layer forms a low-melting Al-Si-Mg alloy and melts, thereby destroying the combination of the surface oxide film and the base metal, so that the molten solder can wet the base metal, spread on the base metal under the film, and The surface film is lifted and removed.

Although a lot of research and experiments have made aluminum alloy vacuum brazing widely used in the industry, there is no reasonable explanation for many scrap brazed joints in actual production. Aluminum alloy vacuum brazing is very sensitive to subtle changes in parameters. Although the components of the brazing alloy cladding prefabricated by different manufacturers are all within the specified range, there are often obvious differences in the brazed joints. So far, due to the lack of a real and accurate understanding of the vacuum brazing mechanism of aluminum alloys, the formulation of ideal vacuum brazing processes has been greatly restricted, and it is still carried out on the basis of the accumulated practical experience to a large extent.

Brazing filler metal for aluminum alloy vacuum brazing

Most brazing alloys are based on Al-Si systems, where w(si) is typically between 7% and 12%. This series of solders are excellent in brazing, strength and color consistency of base metal, platability and corrosion resistance, and are rare and excellent solders, especially these solders can be modified Greatly increases the toughness and flexural strength of brazed joints. When w(si) is 11.7%, the Al-Si system is a eutectic system, and the eutectic temperature is 577°C. The brazing filler metal of this composition is a standard filler metal commonly used in production, and is suitable for brazing with various melting points. High aluminum alloy, such as 3A21. Adding Mg and other elements to Al-Si solder can prepare new brazing alloys, but low boiling point elements such as Zn cannot be added to aluminum solder, otherwise, it will cause great pollution to the vacuum brazing furnace. The commonly used solder composition and brazing temperature range are shown in Table 1 below:

Brazing filler metal for vacuum brazing

In addition, brazing clad plates is also a commonly used method, that is, a layer of brazing alloy is coated on one or both sides of the structural parent metal. The relevant data are shown in Table 2 below:

Vacuum Brazed Clad Sheets Vacuum brazed clad sheets are much more convenient to use than wire, strip or powder, and can be easily used to make complex parts. When the clad metal on the brazing plate melts, it can directly wet the bonding base metal and fill the gap immediately. Joints are formed with only slight diffusion.

In order to reduce the melting point of aluminum solder, Al-Si-Ge solder was developed. This type of solder can reduce its melting point to 423°C at most, and the solder has good fluidity and excellent spreadability. Adding it will greatly increase the brittleness of the solder, the corrosion resistance will be deteriorated, and the color will be darker, so it cannot be used as a practical solder for vacuum brazing. According to relevant data, the performance of this system alloy has been greatly improved after being modified by Na, Sr, La and other elements, which proves to be a very meaningful solder alloy, and the brazing process is very good. If in-depth research, it should be able to get better comprehensive properties of the solder alloy.

In addition, there are also reports of Al-Si-Ge solder, the phase diagram of this system shows that there is a peritectic eutectic point in this system, w(Si)=13%, w(Be)=0.5%. Its composition is the peritectic eutectic point, and its temperature of 571°C is 6°C lower than that of the Al-Si eutectic. After the combined modification of Sr and La, the strength of the solder is significantly improved. However, its melting point is still high, and it is not suitable for brazing of high-strength aluminum alloys such as 6061. If the melting point can be further lowered, the solder of this system will be very promising.

At present, although some low-melting-point vacuum brazing solder alloys available in the laboratory have been formulated, the solder alloys with certain corrosion resistance and good mechanical properties still do not meet the requirements for industrial production.