Titanium and iron, aluminum, vanadium, molybdenum and other metal elements made of alloy, with high strength, high heat resistance, good corrosion resistance and other excellent physical and mechanical properties, is widely used in chemical industry, marine engineering, transportation, medical, construction, aerospace, military and other high-tech fields, is an extremely important lightweight structural materials, of which aerospace is an important downstream application field.
Titanium and titanium machining are active metals, which are widely used in aerospace, petrochemical and atomic energy industries. The main problems in brazing titanium and titanium alloys are as follows:
- ① The oxide film on the surface is stable. Titanium and its alloy have a large affinity with oxygen, and a very stable oxide film is easily formed on the surface, which can wet and spread the solder. Therefore, it must be removed during brazing.
- ② Titanium and its alloys have a strong tendency to absorb hydrogen, oxygen and nitrogen during heating. The higher the temperature is, the more serious the absorption will be. As a result, the plasticity and toughness of titanium metal will decrease sharply. Therefore, 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 form brittle compounds, resulting in brittle joints. Therefore, the filler metals used for brazing other materials are basically not suitable for brazing active metals.
- ④ The microstructure and properties are easy to change. The higher the temperature is, the more serious the coarsening is. So the temperature of high temperature brazing should not be too high.
In a word, attention must be paid to the heating temperature when brazing titanium and its alloys. Generally speaking, the brazing temperature should not exceed 950 ~ 1000 ℃, and the lower the brazing temperature, the less influence on the properties of the base metal. For Quenched and aged alloys, brazing can also be carried out under the condition that the aging temperature is not exceeded.
In order to prevent oxidation, oxygen absorption and hydrogen absorption reaction of brazed joint, titanium and titanium alloy brazing and titanium casting is carried out in vacuum and emotional atmosphere, and flame brazing is not used. When brazing in vacuum or chlorine gas, high frequency heating and furnace heating can be used. The heating speed is fast, 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 welding temperature and holding time to make the solder flow full of the gap.
The reason why titanium and titanium alloys are brazed in vacuum and argon is that although titanium has a great affinity for oxygen during vacuum brazing, titanium can get a smooth surface under vacuum of 13.3pa. This is due to the fact that the oxide film on the surface can dissolve into titanium.
Under the protection of argon, when the brazing temperature range is 760 ~ 927 ℃, in order to prevent titanium discoloration, high purity argon is required. Generally, liquid argon in refrigeration storage container is used because of its high purity.
When brazing titanium and titanium cnc machining, brittle compounds are often formed on the interface or in the brazing seam, which reduces the properties of brazed joints. Therefore, diffusion welding can be used to improve the properties of brazed joints. During brazing, copper foil, nickel foil or silver foil with a thickness of 50 μ m are placed between the titanium alloys to form Cu Ti, Ni Ti and Ag Ti eutectic respectively by the contact reaction between titanium and these metals. Then these brittle intermetallic compounds are diffused out, and the diffusion brazing joint has good properties at a certain temperature and time.
In addition, a + B phase titanium alloy can be used in annealing, solution treatment or aging state. If annealing is required after brazing, there are three options: brazing at or below the annealing temperature after annealing; brazing at a temperature above the annealing temperature and adopting sectional cooling process in the brazing cycle to obtain annealing structure; brazing at a temperature above the annealing temperature and then annealing.