Vacuum brazing of aluminum and aluminum alloy

Compared with other metals, aluminum and aluminum alloy is difficult to braze, and its welding performance is poor. The main reason is that aluminum has A great affinity for oxygen, and it is easy to form A dense and chemically stable aluminum oxide film with A high melting point (~ 2050^C) on the surface. At the brazing temperature of 500~ 600C, the film thickness increases sharply to 1000 ~ 2000 A, which hinders the wetting and bonding of the filler metal and the base material.When vacuum brazing without flux, it is difficult to remove this kind of oxidation film only by vacuum action.Therefore, when vacuum brazing aluminum alloy, in order to remove the oxidation film and prevent aluminum oxidation.

Some metal activators must also be assisted.Metal activators are elements that have a higher vapor pressure and a greater affinity for oxygen than aluminum, such as copper, barium, strontium, bismuth, and magnesium.Practice shows that magnesium is the best chemical agent because of its high vapor pressure and easy volatilization in vacuum.Therefore, it is now widely used.

Magnesium as an activator can be steamed into the brazing area, but the equipment and technology are more complex, not promoted.For example, small grains of pure magnesium are placed directly near the joint, because the evaporation of magnesium occurs when the temperature is much lower than that of brazing, it is difficult to spread magnesium vapor in welding parts with complex structure, and the effect is not ideal.- generally, adding magnesium as an alloy element into the brazing alloy can not only ensure the mutual adaptation between the evaporation of magnesium and the melting of the brazing alloy, but also reduce the melting point of the brazing alloy and improve the fluidity and wettability of the brazing alloy.However, the content of magnesium should not be too much, in order to prevent the dissolution of the parent material, generally should be controlled to 1%~ 1.5%.The results show that adding bismuth with mass fraction of 0.1% at the same time of adding magnesium can reduce the magnesium adding personnel, reduce the surface tension, improve wettability and reduce the requirement of vacuum degree.

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