Vacuum Furnace Brazing

Brazing is a joining process where two or more parts of similar or dissimilar materials (traditionally metals, but can be used with ceramics) are joined by melting and flowing a filler material into the gaps between the parts to be joined by capillary action. The braze filler metal or ceramic must have a lower melting point than the adjoining parts and by heating the joint to a temperature above the liquidus temperature of the filler, but below the solidus temperature of the parent materials, a strong joint can be achieved.

Watch our short video below to see the process in action.

The popularity of the furnace brazing process stems from the clean atmosphere used during the brazing cycle, which mostly eliminates use of fluxes and also eliminates post-braze cleaning. Various types of furnace are used for brazing, mostly employing either a gaseous atmosphere or typical vacuum environment, created with a vacuum pump to achieve the desired vacuum level. The overall furnace construction is based on either batch type or continuous operation. Batch operation includes retort type furnaces used for hydrogen brazing and vacuum chambers for vacuum brazing.

Vacuum furnaces are widely used, and often companies will utilise them for the heat treatment process, using heat exchangers, as well as brazing. Furnaces today are based on a cold wall construction, with internal heating elements, usually carbon or molybdenum. The cold wall refers to the water-cooled double skin construction used to keep external temperatures down to room temperature (or less). Braze furnaces can be either horizontal or vertical and can be either top or bottom loading. High temperatures of up to 1300°C and vacuum levels < 1x10^-5 mbar can be achieved.

Most materials can be brazed in a furnace, although high vapour pressure elements should be avoided in vacuum brazing (zinc, cadmium, lead, etc). All materials need to be cleaned prior to insertion in the furnace to remove surface scale, grease and other contaminants (high quality in, high quality out). The most widely used fillers for furnace brazing are based on silver, copper, nickel and gold, the latter two being most applicable to stainless steels and heat and corrosion resistant alloys.

When compared to the other heating methods, vacuum brazing offers significant advantages:

clean, flux free, quality braze joints with joint surfaces free of oxides
forming multiple joints at once

joining of dissimilar materials e.g. ceramic to metal
ability to control accurately every stage of the heating cycle and cooling cycle, resulting in reduction in residual stress in the brazed parts
temperature uniformity leads to minimal distortion or movement, hence ideal for high precision assemblies
step brazing using multiple braze alloys
process repeatability
heat-treatments, g. hardening or annealing, of the parts in a single furnace thermal cycle while performing a metal-joining process, saves time and cost.

TWI has four different vacuum furnaces capable of brazing a range of materials under different conditions. The largest hot zone measures 750mm x 750mm x 1000mm and is operated in accordance with AMS2750E requirements.

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