Vacuum brazing is attractive as it produces higher quality joints than any other known
brazing technique, the process is non-polluting, the as-brazed-surface is free of any
contaminants that are considered corrosive to aluminium and the processing costs are
commercially attractive (no flux, post-cleaning). The preparation of the assemblies to be brazed are generally the same as discussed before. However, more emphasis has to be put on surface control and pre-cleaning (e.g. aqueous type chemical cleaning of clad vacuum brazing sheet must be avoided). In principle, the process works as follows: the thin oxide layer cracks during heating as the coefficient of thermal expansion of aluminium is three times greater than the coefficient of the oxide. Due to a vacuum of between 10-4 and 10-6 Torr the nascent aluminium surface will not oxidise again. The process is supported by magnesium containing filler metals, since magnesium vaporizes and reacts with traces of oxygen ("getter effect").
VACUUM BRAZING FURNACE: THE KEY FEATURES
The vacuum furnace, given its high uniformity within the hot zone of the entire temperature curve, is the ideal system to carry out an identical heating cycle on a high number of loads as well as continuous brazing on larger pieces. It is furthermore capable of repeating the same cycle accurately. The work pieces can be recognized as they are loaded via a bar code linked to the cycle programme. Each piece can thus be associated with its own heating cycle.
The vacuum furnace operating at the melting temperature of the filler is generally a higher temperature than the temperature at which oxides form. As a result even oxidized pieces lose the oxide layer in the vacuum, before reaching the filler melting temperature.
The vacuum furnace must be cleaned. This means leaving no contaminants on the load.
The vacuum furnace for applications in aerospace, particle physics and metallurgical research in general, is a complex and sophisticated system, manufactured with a metallic thermal chamber, dry pumping and the vessel construction criteria characteristic of an ultra-high vacuum furnace. The furnace conforms to the NADCAP requirements, while the cycle programming software allows automatic cycles with great precision.
The metal chamber, containing the hot zone, is manufactured with multiple layers of metal reflecting shielding in molybdenum/lanthanum for the internal shields, the intermediate ones in molybdenum and in SS for the external shields. The level of cleaning in the vacuum environment required by the specification is obtained by avoiding the use of panels of insulating materials in the construction of the thermal chamber, such as aluminium fibre or graphite sponge, materials capable of retaining humidity or worse oil vapors.
The vacuum seal vessel, created entirely in stainless steel, is ultrasonically cleaned internally before assembly and subsequently subjected to a seal test with a helium leak detector. The components are entirely ultra-high vacuum, metallic seals or Viton.
The pumping system does not provide for the use of lubricated pumps, but a dry primary pump instead. The roots pump (lobe booster pump) is a typical pump in which the seal is entrusted to the tolerances between the lobes and stator. The ultra-high vacuum pump can be turbomolecular or a cryogenic pump. However, oil vapor diffusion pumps can also be used, if a cryogenic trap with supplied liquid nitrogen or a double stage Polycold system is fitted. Both these systems are capable of drawing condensable vapors and impeding the backscatter of oil vapors.
As we have seen, vacuum furnace brazing is simple and easy. You can obtain perfect brazed joints and no secondary fabrication or finishing is required. Do you want more details to solve your braze problems? Just ask and I’ll be happy to provide you with more information for a perfect brazing.