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, and are widely used in chemical industry, marine engineering, transportation, medical care, construction As well as aerospace, military and other high-tech fields, it is an extremely important lightweight structural material, of which aerospace is an important downstream application field.
Titanium and titanium alloys are active metals and are widely used in aerospace, petrochemical and atomic energy industries. The main problems in brazing of titanium and titanium alloys are as follows:
①The oxide film on the surface is stable, titanium and its alloys have a high affinity with oxygen, and a very stable oxide film is easily formed on the surface, which prevents the wetting and spreading of the solder, so it must be removed during brazing.
②It has a strong tendency to inhale. Titanium and its alloys have a tendency to absorb hydrogen, oxygen and nitrogen during the heating process, and the higher the temperature, the more serious the absorption, so that the plasticity and toughness of the titanium metal are sharply reduced, so brazing should be Carry out in vacuum or inert atmosphere.
③It is easy to form intermetallic compounds. Titanium and its alloys can chemically react with most needle materials to form brittle compounds, resulting in brittle joints. Therefore, brazing filler metals for brazing other materials are basically not suitable for brazing active metals.
④ The 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, the brazing heating temperature must be paid attention to when brazing titanium and its alloys. 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 metal. For quenched aged alloys, brazing can also be carried out without exceeding the aging temperature.
In order to prevent the oxidation of the brazed joint and the reaction of oxygen absorption and hydrogen absorption, the brazing of titanium and titanium alloys is carried out in a vacuum and an inert atmosphere, and flame brazing is generally not used. When brazing in vacuum or chlorine gas, high-frequency heating, furnace heating and other methods 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 better. Therefore, it is necessary to control the needle welding temperature and holding time so that the brazing material can fill the gap.
The reason why titanium and titanium alloys are best brazed in vacuum and argon is because during vacuum brazing, although titanium has a great affinity for oxygen, titanium can get a smooth surface under 13.3Pa vacuum. This is because the oxide film on the surface can dissolve into titanium.
When brazing under argon protection, and the brazing temperature range is 760-927 °C, in order to prevent the discoloration of titanium, high-purity argon is required, and liquid argon in a refrigerated storage container is generally used because it has high purity.
When brazing titanium and titanium alloys, brittle compounds are often formed on the interface or in the brazing seam, reducing the performance of the brazed joint. For this reason, diffusion welding methods can be used to improve the performance of brazed joints. During brazing, copper foil, nickel foil or silver foil with a thickness of 50 μm are respectively placed between titanium alloys, and Cu-Ti, Ni-Ti and Ag-Ti eutectics are formed respectively depending on the contact reaction between titanium and these metals. Then these brittle intermetallic compounds are diffused away, and the joints of diffusion brazing have quite good performance under certain temperature and time.
In addition, the a+B phase titanium alloy can be used in the annealed, solution treated or aged state. If annealing is required after brazing, there are three options to choose from: brazing at or below the annealing temperature after annealing; brazing at a temperature above the annealing temperature with a staged cooling process in the brazing cycle, An annealed structure is also obtained; brazing at a temperature above the annealing temperature, followed by an annealing treatment.
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