What is vacuum brazing and how to weld with vacuum brazing?

For many chemical equipment manufacturers, brazing is rarely used, and welding engineers do not understand it. However, in air separation units, vacuum brazing is often used to weld aluminum plate-fin heat exchangers. So today it is necessary to introduce to you what vacuum brazing is.
Brazing definition: Brazing belongs to solid phase connection. It is different from other methods. The base material does not melt during brazing. A brazing material with a lower melting point than the base material is used. The heating temperature is lower than the solidus line of the base material and higher than the liquidus line of the brazing material. When the connected parts and the brazing material are heated to the melting point of the brazing material, the liquid brazing material is used to wet the surface of the base material, spread and dissolve and diffuse with the base material, and wet in the gap of the base material, capillary flow, fill the gap and dissolve and diffuse with the base material to achieve the connection between the parts.
Compared with the fusion welding method, brazing has the following advantages:  The heating temperature of brazing is low, and the influence on the structure and performance of the base material is small. The brazing joint is flat and smooth, with beautiful appearance and small deformation of the weldment. Especially when using uniform heating (such as furnace brazing) brazing method, the deformation of the weldment can be reduced to a minimum, and the dimensional accuracy of the weldment can be easily guaranteed. Some brazing methods can weld dozens or hundreds of welds at a time, with high production efficiency.
However, brazing also has its own disadvantages. The strength of the brazing joint is relatively low, the heat resistance is relatively poor, and the electrochemical corrosion caused by the large difference in the composition of the base material and the brazing filler metal leads to poor corrosion resistance and high assembly requirements.
Brazing in a vacuum furnace: Brazing in a furnace or welding room where air is extracted is an economical method for connecting many joints of the same or different metals. No brazing flux is used in the process.
So what is brazing flux?
The use of brazing flux during brazing can remove oxides on the surface of the brazing filler metal and the base material and improve wetting. Brazing flux can often reduce the interfacial tension of the liquid brazing filler metal. Therefore, the selection of appropriate brazing flux is very important to improve the wetting effect of the brazing filler metal on the base material.
Vacuum conditions are particularly suitable for joints with large and continuous brazing areas, such as these: a) It is difficult to completely remove solid or liquid brazing flux from the brazing interface during brazing. b) The shielding gas is not completely effective because the atmosphere cannot completely exhaust the gas hidden in the close brazing interface.
Vacuum brazing has the following advantages and disadvantages:
Vacuuming fundamentally removes all gases in the brazing area, so there is no need to purify the supplied atmosphere. The pressure used for industrial vacuum brazing is generally 0.065MPa or higher.
Some oxides of the parent material will decompose at vacuum brazing temperatures. The difficult problem of the brazing interface being sometimes contaminated by the exhaust of the parent material can be considered non-existent. At high temperatures, there is a low pressure around the parent material and the brazing material, which can remove volatile impurities and gases in the metal. This characteristic is also a disadvantage. Due to the low surrounding pressure, the brazing material, the parent material and the elements therein will evaporate at the brazing temperature.
Since some elements are easy to volatilize in a vacuum, vacuum furnace brazing is not suitable for brazing materials with high content of zinc, cadmium, manganese, phosphorus and other elements, nor for base materials containing a large amount of these elements.
Vacuum brazing production process:
1. Workpiece surface preparation: remove oxides, grease, dirt and paint on the workpiece surface
2. Assembly and fixation
3. Placement of brazing materials: Except for flame brazing and soldering iron brazing, most of them are pre-placed on the joint. When installing the brazing material, try to use the gravity of the brazing material and the capillary action of the gap to promote the brazing material to fill the gap. Paste brazing material should be applied directly to the brazing area, and powdered brazing material can be mixed with adhesive and adhered to the joint.
4. Applying flow barrier: In order to completely prevent the loss of brazing material, it is sometimes necessary to apply flow barrier. Flow barrier mainly consists of oxides, such as stable oxides such as aluminum oxide, titanium oxide or magnesium oxide and appropriate adhesives. Apply paste flow barrier on the surface of adjacent parts before brazing. Since the brazing material cannot wet these materials, it is prevented from flowing. It is removed after brazing. Flow inhibitors are widely used in brazing in protective atmosphere and brazing in vacuum furnace.
Vacuum brazing process parameters: The main parameters are brazing temperature and holding time. The brazing temperature is usually selected to be 25-60℃ higher than the liquidus line of the brazing material to ensure that the brazing material can fill the gap. The brazing holding time depends on the size of the workpiece and the intensity of the interaction between the brazing material and the base material.
Post-brazing cleaning: Most of the brazing flux residues corrode the brazed joints and also hinder the inspection of the brazing seam, so they often need to be cleaned.

Vacuum Brazing Furnace