Alloys made of titanium and iron, aluminum, vanadium, molybdenum and other metal elements have excellent physical and mechanical properties such as high strength, high heat resistance and good corrosion resistance. As well as high-tech fields such as aerospace and military, they are extremely important lightweight structural materials. Among them, aerospace is an important downstream application field.
Titanium and titanium alloys are active metals and are widely used in the aerospace, petrochemical and atomic energy industries. The main problems in the brazing of titanium and titanium alloys are shown in the following aspects:
- ①The oxide film on the surface is stable. Titanium and its alloys have a large affinity for oxygen. It is easy to form a very stable oxide film on the surface, which can prevent the solder from wetting and spreading.
- ②It has a strong tendency to inhale. Titanium and its alloys have a tendency to absorb hydrogen, oxygen and nitrogen during heating, and the higher the temperature, the more serious the absorption, so that the plasticity and toughness of titanium metal sharply decrease, so brazing Carried out in a vacuum or inert atmosphere.
- ③ It is easy to form intermetallic compounds. Titanium and its alloys can chemically react with most needle materials to form brittle compounds, causing joints to become brittle. Therefore, the brazing material used for brazing other materials is basically not suitable for brazing active metals.
- ④The organization 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 short, when brazing titanium and its alloys, attention must be paid to the brazing heating temperature. Generally speaking, the brazing temperature should not exceed 950 ~ 1000 ℃, the lower the brazing temperature, the smaller the impact on the performance of the base material. For quenched aging alloys, brazing can also be carried out without exceeding the aging temperature.
In order to prevent the oxidation and oxygen absorption and hydrogen absorption reaction of the brazed joint, titanium and titanium alloy brazing is carried out in a vacuum and emotional atmosphere, flame brazing is generally not used. When brazing in vacuum or chlorine gas, you can use high-frequency heating, furnace heating, etc., the heating speed is fast and the holding time is short, the compound in the interface area is thin, and the joint performance is good. Therefore, it is necessary to control the temperature and the holding time of the needle welding to make the solder flow to the gap.
The reason why titanium and titanium alloys are best to be brazed in vacuum and argon is because during vacuum brazing, although titanium has a great affinity for oxygen, titanium can obtain 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 the protection of argon. When the brazing temperature range is 760 ~ 927 ℃, in order to prevent titanium discoloration, high purity argon is required. Liquid argon in refrigeration 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 seam, which reduces the performance of the brazed joint. To this end, diffusion welding can be used to improve the performance of the brazed joint. During brazing, copper foil, nickel foil or silver foil with a thickness of 50 μm were placed between the titanium alloys, and Cu-Ti, Ni-Ti and Ag-Ti eutectic were formed by the contact reaction between titanium and these metals. Then, these brittle intermetallic compounds are diffused away, and the diffusion-brazed joints at a certain temperature and time have quite good performance.
In addition, the a + B phase titanium alloy can be used under annealing, solution treatment, or aging. If annealing is required after brazing, there are three options to choose from: brazing at the annealing temperature or below the annealing temperature after annealing; brazing at a temperature above the annealing temperature and taking a staged cooling process in the brazing cycle Thereby an annealing structure is obtained; brazing at a temperature above the annealing temperature, followed by annealing treatment.