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Vacuum brazing is a metal-joining process used to join two pieces of base metal using a filler metal in a special vacuum chamber. Brazing is similar to the process of soldering, but it requires higher temperatures to melt the filler metal. In the process of brazing, the heated liquid filler metal flows between the two pieces of metal to be joined. This flow of molten metal is called capillary action, meaning that the metal flows through the open channel created by the small gap between the two pieces of base metal. The filler metal is then cooled, causing it to solidify and act as an adhesive between the two pieces of metal.
During the process of vacuum brazing, the filler metal must have a lower melting point than the base metals that are to be joined together. The base metals are not melted during the brazing process but instead are joined by the filler metal. Although the process results in a weaker bond than the traditional welding process, which melts the base metals, vacuum brazing produces a stronger bond than traditional brazing.
This metal-joining process differs from traditional brazing because the whole process occurs in a type of vacuum chamber that is commonly called a vacuum brazing furnace. The use of a vacuum chamber to create welds ensures a cleaner weld with a stronger bond. Corrosion does not form within a vacuum, so the process does not require flux to protect the metal.
Products assembled using vacuum brazing come out of the furnace looking clean and bright because the low levels of oxygen in the vacuum brazing furnace prevent oxidation and corrosion. Vacuum brazing is commonly used in the case of metals that might respond poorly to flux or to gases present in the atmosphere outside of the vacuum chamber. These metals include stainless steel, carbon steel and other alloys.
Besides eliminating the need for flux, this process has several additional benefits. The use of a vacuum chamber allows careful control of the temperature inside the chamber. This allows for the heat-treating or age-hardening of the metal project during the vacuum brazing process, thus reducing the amount of time necessary for project completion. The controlled temperature inside the vacuum brazing furnace heats all of the metal in the chamber to the brazing temperature, so multiple joints can be made at one time. This reduces the stress placed on the joint and base metals from thermal expansion and contraction, because the whole project is heated and cooled at the same rate.