Advantages and applications of vacuum brazing

Advantages and Applications of Vacuum Brazing

1. The Protective Effect of Vacuum Brazing

One of the most significant features of vacuum brazing is that during the brazing process, only the protective effect of the vacuum is utilized to expel harmful gases from the surrounding space, resulting in high-quality brazed joints. While a certain amount of residual gas remains in the vacuum, its impurity content is much lower than that of the most commonly used pure inert gases, such as argon or nitrogen. Therefore, its protective effect is superior to any other shielding gas. Under normal high vacuum conditions, excellent protection can be achieved when brazing aluminum and its alloys, titanium and its alloys, high-temperature alloys, and other reactive metals.

2. Degassing Effect of Vacuum Brazing

When brazing parts in a vacuum, gases (such as H₂O₂, N₂, and CO) present within the metal and gases adsorbed on the surface will be released as the vacuum level, brazing temperature, and holding time increase or increase. Outgassing occurs through diffusion of gases from the inside out, and lower ambient pressure facilitates this diffusion. Therefore, the vacuum level selected during brazing directly affects the degassing rate and effectiveness. The higher the temperature, the better the degassing effect; the longer the holding time, the more thorough the degassing. While vacuum brazing primarily degasses the base metal, the vacuum also has a strong degassing effect on the brazing filler metal, especially when using a filler metal containing a binder.

3. Purification Effect of Vacuum Brazing

If a piece of rusted steel is placed in a furnace with a vacuum of 1.33 × 10-² Pa, heated to 1050°C, held for 10 minutes, and then removed from the furnace, the rust on the surface will naturally disappear. This practical experience demonstrates that vacuum can effectively remove oxides from the workpiece surface. This phenomenon is achieved through a complex combination of reduction, decomposition, volatilization, and dissolution. This purifying effect of vacuum creates optimal conditions for the brazing process.
It enhances the metal’s surface activity, facilitating the absorption of atoms such as carbon, nitrogen, chromium, and silicon, and promoting wetting by the brazing filler metal. In crack repair brazing, a dedicated vacuum purification step is implemented to remove oxides and other foreign matter from within the cracks, a typical example of utilizing this effect.

4. Evaporation in Vacuum Brazing

Various elements evaporate to varying degrees under different temperatures and vacuum conditions. The higher the temperature, the more severe the evaporation; the higher the vacuum, the more severe the evaporation.
During vacuum brazing, as the temperature increases, the evaporation temperature of the metal decreases. In higher vacuum conditions (approximately 10-2 Pa), at temperatures of 1000°C, most metals experience significant evaporation. This evaporation removes oxides and impurities from the metal surface, achieving the goal of oxide removal. If necessary, the vacuum level selected at the brazing temperature should be sufficient to allow some of the metal elements to evaporate. This limited evaporation effectively breaks down the oxide film on the metal surface, transforming it into a mixture or low-valent oxide that can be pumped away. This is particularly useful for brazing metal surfaces that require clean surfaces. However, the volatilization of large amounts of alloying elements from the base metal or brazing filler metal can alter the base metal’s properties, increase the brazing filler metal’s melting temperature, or expand its melting temperature range. In severe cases, this can lead to brazing failure. This can also lead to other adverse effects, such as causing parts to stick together, short-circuiting between the heating element and the furnace, increasing the surface roughness of parts, and even damaging the parts. Therefore, it is crucial to choose the right vacuum level during brazing. The vapor pressure of the metal element provides one basis for selecting the furnace vacuum level during brazing.

Vacuum Brazing Furnace