Superalloy is usually divided into: nickel based, cobalt based, iron based three categories, at high temperature has good mechanical properties, oxidation resistance, corrosion resistance and so on.Among them, nickel-based alloy is used most, and the difficulties in brazing are mainly as follows:

 

Difficult removal of oxide film

 

How to reduce and remove the oxide film of superalloy is the key to the brazing of superalloy.Because the superalloy contains more Cr, the surface is easy to form Cr2O3 oxide film when heating.Moreover, superalloys contain aluminum and titanium, which are easily oxidized when heated, forming more stable Al2O3 and TiO2 oxide films.They can not be removed under the protection of hydrogen or argon, other measures must be taken, such as vacuum environment, and to ensure that the vacuum degree in the hot state is greater than 0.001 ~ 0.01Pa, in order to prevent oxidation of the alloy surface during reheating.It is worth noting that: in the nickel-based superalloy brazing do not use the flux to remove the surface oxide film, this is due to the borax or boric acid in the flux at the temperature of the precursor and the base metal corrosion, and the reaction produced by boron infiltration of the base metal caused by penetration between the fine.Therefore, vacuum brazing of nickel-based superalloys is commonly used to prevent the formation of oxide film.

 

Brazing parameters have great influence on the properties of base metal

 

The high temperature properties of nickel-based superalloys can be obtained only when the alloying elements and their compounds are fully dissolved in the body, no matter in solution strengthening or precipitation strengthening.Therefore, the brazing parameters should match the heat treatment temperature of the alloy as much as possible.That is to say, the brazing temperature should be as consistent as possible with the heating temperature of the solid solution treatment of the base metal to ensure the full dissolution of the alloying elements.However, if the brazing temperature is too high and the base metal grains grow up, the properties of the alloy cannot be recovered even if the heat treatment is carried out after welding.Of course, the solid solution temperature of the cast nickel-based alloy is high, and the grain is not easy to grow up, and generally will not affect its performance due to high brazing temperature.

 

Stress cracking tendency is greater

 

Some superalloys, especially precipitation - strengthened alloys, have a tendency to stress cracking.Therefore, the stress formed in the process must be fully removed before brazing, and the thermal stress should be reduced as much as possible in the process of brazing to reduce the cracking tendency.