The key to success or failure of vacuum brazing: the core influence of vacuum level

Vacuum degree, or the pressure level of the vacuum environment, is the most critical control parameter in vacuum brazing. Vacuum degree directly determines the cleanliness of the brazing environment, which in turn affects the removal of oxide films from the workpiece, the wetting and filling of the brazing filler metal, and the quality and mechanical properties of the weld. Vacuum brazing is a technique that uses heating to melt a low-melting-point brazing filler metal in a vacuum environment below 10⁻² Pa to join workpieces. Its advantages include the elimination of flux, avoidance of residual corrosion, and the production of clean, dense welds. The vacuum degree is fundamental to maintaining this environment.

The Core Functions of Vacuum:

1. Anti-oxidation and Oxide Film Removal: At high temperatures, metals readily react with oxygen and water vapor to form oxide films. High vacuum reduces the partial pressure of harmful gases, inhibiting secondary oxidation. Oxide films on copper, iron, and nickel alloys (such as Cu₂O and FeO) can decompose under high temperature and high vacuum, with the products volatilizing in the vacuum environment.
2. Clean Surface: High vacuum maintains “atomic-level” cleanliness on the workpiece and brazing filler metal surfaces, providing a prerequisite for good wetting and spreading of the filler metal.
3. Gas Exhaust and Dense Compaction: Gases such as H₂ and N₂ adsorbed in the base material and brazing filler metal escape during heating, preventing the weld from solidifying and forming pores.

The Specific Effects of Vacuum Degree on Vacuum Brazing

1. Oxide Film Removal and Surface Activity: Low vacuum (high pressure) results in more residual gas, leading to severe oxidation of the workpiece and brazing filler metal. The oxide film hinders the connection between the filler metal and the base metal, causing the filler metal to “not stick” or “spheroidize,” resulting in brazing failure. A suitable vacuum degree effectively removes the oxide film, exposing the active metal surface.

2. Low Wetting and Flow of the Filler Metal: Under vacuum, the oxide film increases the surface tension of the filler metal, widening the wetting angle and reducing flowability, easily leading to incomplete penetration and weak welds. A suitable vacuum degree reduces the surface tension of the filler metal, resulting in a wetting angle <20°, allowing the filler metal to fill the weld seam evenly, like water flowing through clean glass.

3. Weld Quality: At low vacuum, the oxide film easily gets trapped in the weld, forming slag inclusions. Unable to expel gas, porosity is generated, reducing weld density and mechanical properties, and causing the weld to be discontinuous and dark. Under high vacuum, the filler metal flows smoothly, resulting in a smooth, continuous weld with full rounded corners. 4. Base Material Properties High vacuum can remove harmful gases such as hydrogen from base materials like high-temperature alloys and stainless steel, improving their properties. However, excessively high vacuum combined with prolonged heat treatment can lead to excessive volatilization of high vapor pressure elements such as zinc and cadmium, altering the brazing filler metal composition, contaminating the furnace, and forming pits on the workpiece surface.

Vacuum degree is the soul of the vacuum brazing process. Its influence permeates the entire process. In actual production, vacuum degree must be precisely controlled as a dynamic core parameter closely coupled with temperature and time to stably produce high-quality, high-performance vacuum brazed products.