Titanium and iron, aluminum, vanadium, molybdenum and other metal elements to create alloys with high strength, high heat resistance, good corrosion resistance and other excellent physical and mechanical properties, is widely used in the chemical industry, marine engineering, transportation, medical care, construction, and aerospace, military and other high-tech fields, is extremely important lightweight structural materials, of which the aerospace industry is an important downstream application areas.
Titanium and titanium alloy for the active metal, in the aerospace, petrochemical and atomic energy industry has been widely used, titanium and titanium alloy brazing in the main problems in the following aspects:
① the stability of the surface of the oxide film, titanium and its alloys and oxygen affinity, the surface is easy to generate a layer of very stable oxide film, thus blocking the brazing material wetting and spreading, so brazing must be removed.
② has a strong tendency to absorb, titanium and its alloys in the heating process of hydrogen, oxygen and nitrogen have a tendency to absorb, and the higher the temperature, the more serious the absorption, so that the plasticity and toughness of the titanium metal is sharply reduced, so brazing should be carried out in a vacuum or inert atmosphere.
③ easy to form intermetallic compounds, titanium and its alloys can be chemically reacted with most of the needle material, generating brittle compounds, resulting in joints become brittle. Therefore, the brazing material used for brazing other materials is basically not suitable for brazing active metals.
④Organization and properties are easy to change. Titanium and its alloys in the heating will occur phase change and grain coarsening, the higher the temperature, the more serious coarsening, so the temperature of high temperature brazing should not be too high.
In short, brazing titanium and its alloys must pay attention to the brazing heating temperature. Generally speaking, brazing temperature should not exceed 950 ~ 1000 ℃, the lower the brazing temperature, the lower the impact on the performance of the base material. For quenched aging alloy, can also be brazed under the condition of not exceeding the aging temperature.
In order to prevent the oxidation of brazed joints and oxygen absorption, hydrogen absorption reaction, titanium and titanium alloy brazing is carried out in a vacuum and emotional atmosphere, generally do not use flame brazing. In a vacuum or chlorine brazing, you can use high-frequency heating, furnace heating and other methods, the heating rate is fast heat preservation time is short, the interface area of the compound is thinner, joint performance is better. Therefore, it is necessary to control the needle welding temperature and heat preservation time, so that the brazing material can flow full gap.
The reason why titanium and titanium alloy is best in a vacuum and argon brazing, because in vacuum brazing, although the affinity of titanium for oxygen is very large, but titanium in a vacuum of 13.3Pa will be able to get a smooth surface, which is due to the surface of the oxide film can be dissolved in titanium.
Brazing under the protection of argon, brazing temperature range of 760 ~ 927 ℃, in order to prevent titanium discoloration, the requirement of high purity argon, generally use the liquid argon in the refrigeration storage containers because it has a very high purity.
Brazing of titanium and titanium alloys, often at the interface or brazing seam in the formation of brittle compounds, reducing the performance of brazed joints. For this reason, diffusion welding can be used to improve the performance of brazed joints. Brazing, in the titanium alloy were put between 50μm thick copper foil, nickel foil or silver foil, relying on titanium and these metal contact reaction, respectively, the formation of Cu-Ti, Ni-Ti and Ag-Ti eutectic. Then these brittle intermetallic compounds diffusion off, at a certain temperature and a time under the diffusion brazing joints have quite good performance.
In addition, a + B phase titanium alloys can be used in the annealed, solution-treated or aging state. If the brazing requirements after annealing, there are three options: after annealing in the annealing temperature or below the annealing temperature brazing; in the annealing temperature above the temperature brazing, and in the brazing cycle to take a section of the cooling process, so as to also obtain the annealed organization; in the annealing temperature above the temperature brazing, and then annealing treatment.
