Vacuum brazing

The use of vacuum furnaces for brazing is very well established, especially for brazing complex assemblies in stainless steels or nickel alloys. The method allows flux free brazing and produces ultra clean assemblies which require no post braze cleaning. A variety of braze alloys are used, including copper based, gold based and nickel based alloys. These enable a range of the higher temperature materials to be brazed, using brazing temperatures between 1000°C and 1200°C. The vacuum environment provides ideal conditions for the braze alloy to wet the surfaces of the joint and allow capillary action to draw the braze into filling the whole joint. Care and expertise is required to calculate the effect of thermal expansion of the mating parts upon the joint gap. Each braze alloy has an optimum gap filling capability. If the gap is too wide it encourages the formation of shrinkage voids and the precipitation of inter-metallic compounds at the centre of the cooling joint, thus weakening it. If the gap is too narrow, capillary action will not be able to fill the joint, resulting in a dry joint and, again, a weak result.

The close control of the heating cycle and temperature uniformity, which is provided by the radiation heating under vacuum conditions, ensures that all the assembly reaches braze temperature at the same time and thus prevents uneven stress distribution and hence results in a high integrity joint with minimal internal stress. This temperature uniformity, which can be as close as +/- 2°C throughout the furnace chamber, also allows batches of similar assemblies to be brazed together, thus capitalising upon the economic benefits of using large vacuum furnaces. Hence, this high capital cost method has become cost effective for a wide variety of parts.

As with other methods of brazing, the fixturing of the assemblies prior to brazing is important and in some cases a precision engineered jig is used to hold the assembly throughout the braze cycle. Such jigs may be made from ceramics, graphite or heat resisting alloys. Positional TIG welding is also routinely employed for positioning the elements of the assembly to be brazed. The braze alloy may be applied as a paste, powder, foil or wire, depending upon the joint design used.