Vacuum brazing of aluminum and aluminum alloys often uses magnesium metal as an activator. Among the metal activators that can accelerate aluminum brazing, Mg has a high vapor pressure and is easy to evaporate under vacuum, which helps to remove Al2O3...

Vacuum brazing of aluminum and aluminum alloys often uses magnesium metal as an activator. Among the metal activators that can accelerate aluminum brazing, Mg has a high vapor pressure and is easy to evaporate under vacuum, which helps to remove Al2O3, and has a low price, so it has become a commonly used activator in aluminum alloy vacuum brazing. Used as metal activators are some elements with higher vapor pressure and greater affinity for oxygen than aluminum, such as antimony, bismuth, magnesium, etc.

The amount of magnesium added in the solder has a significant effect on the wettability of the solder. With the increase of the amount of magnesium, the flow coefficient of the solder increases. However, with the increase of magnesium content, the corrosion of solder to aluminum is also intensified, which is due to the formation of Al-Mg-Si ternary eutectic; and if the magnesium content is too high, the solder is easily lost and damages the surface of the weldment . Studies have shown that the addition of bismuth with a mass fraction of about 0.1% in the aluminum-silicon solder can reduce the amount of magnesium added to the solder, reduce the surface tension of the solder, improve the wettability, and reduce the vacuum degree. Require.

Vacuum aluminum brazing is suitable for butt joints, T-shaped joints and similar joints, because these joints have better openness and the oxide film in the gap can be easily removed. The oxide film in the lap joint is more difficult to discharge, so it is not recommended. The spreading ability of the solder in vacuum brazing is worse than that in dip brazing, so a larger brazing gap should be used.

The process of vacuum brazing of aluminum is basically the same as that of other metals. Because its film removal relies on the action of magnesium activator, for welding parts with complex structures, in order to ensure the full effect of magnesium vapor on the base metal, supplementary process measures for partial shielding are often taken, that is, the welding parts are first put into stainless steel boxes (collectively referred to as process box), and then placed in a vacuum furnace for heating and brazing, which can significantly improve the quality of the brazing. If necessary, a small amount of pure magnesium granules can be added in the box to enhance the effect. The vacuum brazed aluminum parts have smooth surface and dense brazing seam, and do not need to be cleaned after brazing.