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Vacuum Brazing Furnace

Flux-free Vacuum Brazing of Aluminum Alloys

The brazing of aluminum alloys usually requires the use of flux, and the use of flux brazing is likely to cause the following problems:

(1) It is inevitable that there will be flux inclusions, which will affect the compactness of the brazing joint;

(2) The corrosive flux residue is highly corrosive, if it is not cleaned after welding, it will easily cause the brazing joint to corrode and damage quickly;

(3) Non-corrosive flux residues adhere firmly to the components, affecting the appearance of solder joints and the subsequent surface treatment process.

Therefore, the industry has been devoting itself to research on flux-free brazing of aluminum alloys. There have been many research results released: such as welding in the atmosphere protection furnace: (1) the surface of the aluminum alloy is coated with a special nickel layer, and mass production has been realized; (2) or a specially designed multi-layer composite board is used. So far, only vacuum brazing is the only flux-free aluminum alloy brazing method that can be generalized.

The dense oxide film on the surface of aluminum alloy cannot be removed by vacuum alone. Therefore, aluminum alloy vacuum brazing uses a special “flux” – magnesium (Mg) metal activator to remove and destroy the aluminum alloy oxide film and promote the spreading and flow of the solder.

The removal process of aluminum alloy vacuum brazing oxide film:

The expansion coefficient of aluminum is three times that of its oxide film, and when heated above 400°C, the thermal stress is sufficient to cause partial cracking of the oxide film;

High vacuum conditions and magnesium vapor protect the exposed clean surfaces from oxidation. The protective effect of magnesium vapor is mainly to eliminate the remaining H₂O, O₂ and other components that are unfavorable to aluminum alloy brazing in the vacuum:

Mg+1/2O₂→ MgO

Mg+H₂O → MgO+H₂

Magnesium vapor infiltrates through the cracks in the oxide film, interacts with aluminum and silicon under the film to form a low melting point Al-Si-Mg alloy, and local melting occurs, thereby destroying the bond between the surface oxide film and the base material. The adsorption of liquid solder reduces the strength of the oxide film, breaks and disperses and dissolves into the solder, and the liquid solder can wet the base metal.

Vacuum setting

The controlled vacuum environment during the aluminum vacuum brazing cycle is essentially non-adjustable and is a result of system design and practical application. The quality of the product is directly related to the vacuum environment, and care should be taken to ensure that water, grease and oil enter the vacuum chamber to a minimum. The working vacuum of aluminum alloy vacuum brazing should be less than 5×10-3Pa.

Vacuum brazing temperature

In order to ensure the quality of vacuum brazing products, it is very important to strictly control the temperature.

Below 380°C, the temperature control requirements are not too strict, and the heating rate can be increased appropriately. But it is necessary to ensure that the temperature difference of the products in the furnace does not exceed +/-25°C.

At 380°C-540°C, the heating speed is relatively slow to ensure that the temperature difference of the product in the furnace does not exceed +/-10°C. 540°C heat preservation ensures the temperature uniformity of the product, and at the same time, the evaporation of Mg is stable at this temperature, providing sufficient protection for the aluminum alloy.

At 540°C-615°C, a large amount of magnesium evaporates, and the uniformity of the temperature is strictly controlled, and then kept at the vacuum brazing temperature for a certain period of time, so that the solder has enough time to spread and form a joint.

Precautions for vacuum brazing:

In addition to meeting the requirements of the ultimate vacuum degree, the aluminum alloy vacuum brazing furnace also needs to pay special attention to the air leakage rate. In the case of high air leakage rate, the vacuum degree in the furnace can always be maintained at our required level through the high-capacity vacuum pumping function. But the workpiece in the furnace is in the air flow that leaks into and takes away, there is very big harm to brazing.

Aluminum alloy vacuum brazing is very sensitive to changes in various conditions, so strict standards must be established in the production process. Any changes must be verified before mass production to avoid mass scrapping. Such as brazing furnace maintenance, alternate welding of different materials, storage environment or cycle changes of raw materials, etc.

Do not attempt to fill the gap with solder flowing from one end, such as with a solder ring. The content of Mg in the solder is limited, and the scope of action is very small. With joints in this way, the rate of defective brazing will be high.