The Principle of Aluminum Alloy Vacuum Brazing

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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:

The viewpoint derived from these reaction equations is that on the one hand, Mg reacts with the remaining O2 and H2O 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.

Brazing filler metal for aluminum alloy vacuum brazing

Most brazing alloys are based on the Al-Si system, 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. Greatly increases the toughness and flexural strength of brazed joints. When the w(si) is 11.7%, the Al-Si system is a eutectic system, and the eutectic temperature is 577°C. The solder of this composition is a standard solder 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.

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