Why are titanium and titanium alloys brazed in vacuum and argon?

In order to prevent the oxidation of the brazed joint and the reaction of oxygen absorption and hydrogen absorption, the brazing of titanium and titanium alloys is carried out in a vacuum and an emotional atmosphere, and flame brazing is generally not used. When brazing in vacuum or chlorine, high-frequency heating, furnace heating and other methods can be used. The heating speed is fast and the holding time is short. The compound in the interface area is thinner and the joint performance is better. Therefore, the brazing temperature and holding time must be controlled so that the filler metal can flow to the full gap.

The reason why titanium and titanium alloys are better brazed in vacuum and argon is because during vacuum brazing, although titanium has a great affinity for oxygen, titanium can get a smooth surface under a vacuum of 13.3Pa. This is because the oxide film on the surface can be dissolved into titanium.

Brazing under argon protection, when the brazing temperature range is 760～927℃, in order to prevent titanium from discoloration, high purity argon is required. Generally, liquid argon in a refrigeration storage container is used because of its high purity.

When brazing titanium and titanium alloys, brittle compounds are often formed on the interface or in the brazing seam, which reduces the performance of the brazed joint. For this reason, diffusion welding can be used to improve the performance of brazed joints. During brazing, a 50μm thick copper foil, nickel foil or silver foil is placed between the titanium alloys. Depending on the contact reaction between titanium and these metals, Cu-Ti, Ni-Ti and Ag-Ti eutectic are formed respectively. Then these brittle intermetallic compounds are diffused away, and the joints brazed by diffusion brazing at a certain temperature and a period of time have quite good performance.

In addition, the a+B phase titanium alloy can be used in an annealed, solution treated or aging state. If annealing is required after brazing, there are three options to choose from: brazing at or below the annealing temperature after annealing; brazing at a temperature above the annealing temperature, and adopting a staged cooling process in the brazing cycle. Thereby, an annealed structure is also obtained; brazing at a temperature above the annealing temperature, and then annealing treatment.
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