Best Brazing Methods for Titanium and Titanium Alloys

Titanium and its alloys, which are composed of elements such as iron, aluminum, vanadium, and molybdenum, have excellent physical and mechanical properties such as high strength, high heat resistance, and good corrosion resistance. They are widely used in high-tech fields such as chemical engineering, marine engineering, transportation, medicine, construction, aerospace, and military industries and are important lightweight structural materials. Among them, aerospace is an important downstream application area.
Titanium and its alloys are reactive metals and are widely used in aerospace, petrochemical, and nuclear industries. The main problems in brazing of titanium and its alloys are as follows:
① The stable oxide film on the surface. Titanium and its alloys have a strong affinity for oxygen and are easy to generate a stable oxide film on the surface, which hinders the wetting and spreading of the brazing material. Therefore, it must be removed during brazing.
② Strongly absorb gases. Titanium and its alloys have a tendency to absorb hydrogen, oxygen, and nitrogen during the heating process, and the higher the temperature, the stronger the absorption, which leads to a sharp decrease in the plasticity and toughness of titanium. Therefore, brazing should be carried out in a vacuum or inert atmosphere.
③ Easy to form intermetallic compounds. Titanium and its alloys can react with most brazing materials to form brittle compounds, causing the joints to become brittle. Therefore, the brazing material used for brazing other materials is basically not suitable for brazing reactive metals.
④ The structure and properties are prone to change. Titanium and its alloys undergo phase transformation and grain coarsening during heating. The higher the temperature, the more serious the coarsening, so the temperature for high-temperature brazing should not be too high.
In summary, when brazing titanium and its alloys, attention must be paid to brazing heating temperature. Generally, the brazing temperature should not exceed 950-1000℃, and the lower the brazing temperature, the smaller the impact on the properties of the base material. For quenched and tempered alloys, brazing can also be carried out under the condition of not exceeding the aging temperature.
To prevent oxidation and oxygen and hydrogen absorption reactions in the brazed joint, titanium and titanium alloy brazing is performed in a vacuum and inert 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, which have fast heating speed and short holding time, resulting in a thinner layer of compounds in the interface zone and better joint performance. Therefore, the brazing temperature and holding time must be controlled to make the brazing material flow into the gap.
The reason why brazing titanium and its alloys is best carried out in vacuum and argon is that although titanium has a large affinity for oxygen, it can obtain a smooth surface in 13.3Pa vacuum due to the dissolution of the oxide film on the surface.
When brazing in an argon atmosphere and the brazing temperature range is 760-927℃, high-purity argon is required to prevent titanium from discoloration. Generally, liquid argon in refrigerant storage containers is used because it has high purity.
When brazing titanium and titanium alloys, brittle intermetallic compounds often form on the interface or in the brazing gap, thereby reducing the performance of the brazed joint. Diffusion bonding can be used to improve the performance of the brazed joint. During brazing, 50μm thick copper foil, nickel foil or silver foil is placed between the titanium alloys, which respectively form Cu-Ti, Ni-Ti, and Ag-Ti eutectics by relying on the contact reaction between titanium and these metals. Then these brittle intermetallic compounds are diffused off. The diffusion-bonded joint has relatively good performance under a certain temperature and time.
In addition, α+β-phase titanium alloys can be used in annealed, solution-treated or aged states. If annealing is required after brazing, three schemes are available: brazing at or below the annealing temperature after annealing; brazing at a temperature above the annealing temperature and adopting a segmented cooling process in the brazing cycle to obtain annealing structure; and brazing at a temperature above the annealing temperature and then annealing.