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Introduction to vacuum brazing process of aluminum and aluminum alloys

Aluminum alloy liquid cooling plate

With the widespread application of liquid cooling technology in various industries, there is a huge demand for aluminum alloy liquid cooling plates. Among the welding processes of various aluminum alloys, the brazing process occupies a very important position and is widely used.

01 What is brazing process
Brazing is a method for thermal connection of materials or material coatings. Welding is achieved at the connection through the melting of the solder or the diffusion of the contact surface.
Brazing and fusion welding are incompatible. It uses technical materials whose liquidus temperature is lower than the solidus temperature of the base metal as the filler metal. The base metal and the filler metal are heated until the filler metal melts, and the liquid filler metal is used to wet the base metal. It fills the gap between the bonding surfaces and dissolves and diffuses with the base metal to achieve base metal connection.
Each filler metal should have two properties related to the base metal:
1>The melting point of the filler metal should be lower than the melting point of the base metal;
2> The filler metal is wetted on the base metal, that is to say, the interface gap between the filler metal and the base metal is alloyed through diffusion.
According to the working temperature (the lowest surface temperature when the solder can be melted and wetted), it can be divided into three types: soft soldering (<450℃), hard soldering (>450℃) and high-temperature brazing (>900℃) .

02Vacuum brazing in furnace brazing
Aluminum is relatively active and can easily form a dense oxide film on the surface when placed in the air. This oxide film has extremely poor reactivity and a high melting point, which has become the biggest obstacle to the connection between aluminum parts. This issue was also mentioned in the previous article about aluminum alloy diffusion welding. It is difficult to remove simply by heating and pressure, which also limits the development of aluminum alloy diffusion welding.
However, in the brazing process, additional filler metal is added. Therefore, oxides can be removed with the help of certain metal activators such as Mg, Bi, etc.
It is generally believed that the film removal mechanism of activator is:
1>The activator reacts with the remaining oxygen and water in the vacuum, eliminating their harmful effects on aluminum brazing;
2>Mg vapor penetrates into the surface material layer under the film and forms a low melting point Al-Si-Mg alloy together with the diffused Si. During brazing, the alloy melts and destroys the combination of the oxide film and the base material, allowing the molten brazing filler metal to Wet the base material, spread it on the base material under the film, and lift the surface film to remove it.
When vacuum brazing aluminum alloys, an appropriate vacuum brazing furnace should be selected based on production requirements, cost, weldment size and weldment structure.
The most important and most difficult to control process parameter in vacuum brazing is the degree of vacuum. To obtain a high-quality joint, it largely depends on the degree of vacuum. According to the experience of some workers for many years, if the brazing equipment has not been used for a long time, the vacuum furnace should be allowed to run for several hours before use. When in use, especially in mass production, the time interval between two uses should be shortened as much as possible, so that the vacuum degree of the vacuum furnace can easily reach the requirements quickly. Vacuum brazing is an excellent brazing method, but it also has the disadvantages of complex and expensive equipment, and the technical difficulty of maintaining the vacuum system.