The best method for brazing titanium and titanium alloys

Titanium and titanium alloy are active metals, which are widely used in aerospace, petrochemical and atomic energy industries. The main problems in titanium and titanium alloy brazing are as follows:

(1) The oxide film on the surface is stable, titanium and its alloy have a large affinity with oxygen, and the surface is easy to generate a layer of very stable oxide film, thus preventing the wetting and spreading of the solder, so it must be removed when brazing.

(2) Has a strong suction tendency, titanium and its alloy in the heating process of hydrogen, oxygen and nitrogen absorption tendency, and the higher the temperature, the more serious absorption, so that the plastic and toughness of titanium metal sharply reduced, so brazing should be carried out in vacuum or inert atmosphere.

(3) It is easy to form intermetallic compounds. Titanium and its alloys can react with most needle materials to generate brittle compounds, resulting in brittle joints. Therefore, the solder used for brazing other materials is basically not suitable for brazing active metals.

(4) The structure and performance are easy to change. Titanium and its alloys will undergo phase transformation and grain coarsening when heated. The higher the temperature, the more serious the coarsening, so the temperature of high temperature brazing should not be too high.

In conclusion, attention must be paid to the brazing heating temperature when brazing titanium and its alloys. Generally speaking, the brazing temperature should not exceed 950 ~ 1000℃, the lower the brazing temperature, the less impact on the performance of the base metal. For quenched aged alloys, brazing can also be done under the condition of not exceeding the aging temperature.

In order to prevent the oxidation and oxygen absorption, hydrogen absorption reaction of brazing joint, titanium and titanium alloy brazing is carried out in vacuum and emotional atmosphere, generally do not use flame brazing. When brazing in vacuum or chlorine gas, high frequency heating and furnace heating 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 needle welding temperature and holding time must be controlled to make the solder flow full gap.

The reason why titanium and titanium alloy is best in vacuum and argon brazing, because in vacuum brazing, although titanium has a great affinity for oxygen, but titanium in 13.3Pa vacuum degree can get a smooth surface, this is because the oxide film on the surface can be dissolved in titanium.

In the soldering temperature range of 760 ~ 927℃, in order to prevent titanium discoloration, high purity argon is required. Liquid argon in refrigerated storage containers is generally used because of its high purity.

When brazing titanium and titanium alloys, brittle compounds are often formed on the interface or in the brazing joint, which reduces the performance of the brazing joint. Therefore, diffusion welding method can be used to improve the performance of brazing joint. During brazing, copper foil, nickel foil or silver foil 50μm thick are placed between titanium alloys respectively. Cu-Ti, Ni-Ti and Ag-Ti eutectic are formed by the contact reaction between titanium and these metals respectively. The brittle intermetallic compounds are then diffused away, and the joints have good properties under certain temperature and time.

In addition, a+B phase titanium alloys can be used in annealing, solution treatment or aging state. If annealing is required after brazing, three options are available: brazing at or below annealing temperature after annealing; Temperature brazing above annealing temperature, and in the brazing cycle to adopt a piecewise cooling process, so as to obtain annealing microstructure; Brazing at temperatures above annealing temperature is followed by annealing treatment.