The  vacuum degree directly determines the cleanliness of the brazing  environment, which in turn affects the removal of the oxide film of the  workpiece, the wetting and filling of the brazing material, and the  quality and mechanical properties of the weld. Vacuum brazing is a  technology that melts a low-melting point brazing material to connect  the workpiece by heating it in a vacuum environment of less than 10⁻²  Pa, with the advantage of eliminating the need for brazing agent and  avoiding residue corrosion, and the vacuum level is the basis for  maintaining this environment.

At  high temperatures, metals are easy to react with oxygen and water vapor  to form oxide films, and high vacuum reduces the partial pressure of  harmful gases and inhibits secondary oxidation. Oxide films of copper,  iron, nickel alloys, etc. (such as Cu₂O, FeO) can be decomposed under  high temperature and high vacuum, and the products are volatilized with  the vacuum environment.

The  high vacuum keeps the workpiece and the surface of the brazing material  clean at the "atomic level", providing the premise for good wetting and  spreading of the brazing material.

It  helps the gases such as H₂ and N₂ adsorbed in the base metal and  brazing material to escape during heating to prevent the weld from  solidifying and forming pores.

The  vacuum is too low (high pressure), there is a lot of residual gas, the  workpiece and brazing material are prone to serious oxidation, and the  oxide film blocks the brazing material and the base metal, resulting in  the brazing material "non-stick" or "spheroidization", and the brazing  fails; The appropriate vacuum level can effectively remove the film and  expose the active metal surface.

Under  low vacuum, the oxide film increases the surface tension of the brazing  material, the wetting angle becomes larger, the fluidity is poor, and  it is easy to appear unpenetrated and false welding. The appropriate  vacuum degree reduces the surface tension of the brazing material, and  the wetting angle < 20°, which evenly fills the brazing seam like  water flowing through clean glass.

When  the vacuum is low, the oxide film is easy to get involved in the weld  to form slag inclusions, and if the gas cannot be removed, pores will be  generated, which will reduce the density and mechanical properties of  the weld, and the weld will be intermittent and darkened. Under high  vacuum, the brazing material flows smoothly, the weld seam is smooth and  continuous, and the rounded corners are full.

High  vacuum can remove hydrogen and other harmful gases from superalloys,  stainless steel and other base metals, and improve the performance of  the base metal. However, too high vacuum + too long heat preservation  will lead to excessive volatilization of high vapor pressure elements  such as zinc and cadmium, change the composition of the brazing  material, pollute the furnace body, and form a "pit" on the surface of  the workpiece.