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The best welding method for titanium and titanium alloys

2020-11-14
The alloy made of titanium and iron, aluminum, vanadium, molybdenum and other metal elements has excellent physical and mechanical properties such as high strength, high heat resistance, and good corrosion resistance. It is widely used in chemical engineering, marine engineering, transportation, medical treatment, and construction. As well as high-tech fields such as aerospace and military industry, they are extremely important lightweight structural materials. Among them, aerospace is an important downstream application field.

Titanium and titanium alloys are active metals, which are widely used in aerospace, petrochemical and atomic energy industries. The main problems in brazing titanium and titanium alloys are shown in the following aspects:
①The oxide film on the surface is stable. Titanium and its alloys have a high affinity for oxygen, and a very stable oxide film is easily formed on the surface, which prevents the wetting and spread of the solder. Therefore, it must be removed during brazing.
②It has a strong tendency to get gas. Titanium and its alloys have a tendency to absorb hydrogen, oxygen and nitrogen during the heating process. The higher the temperature, the more serious the absorption, so that the plasticity and toughness of 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 and make joints brittle. Therefore, the brazing filler metal used for brazing other materials is basically not suitable for brazing active metals.
④ 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 high temperature brazing temperature 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 effect on the properties of the base metal. For quenching and aging alloys, brazing can also be carried out under the condition of not 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 emotional atmosphere, and flame brazing is generally not used. When brazing in vacuum or chlorine, methods such as high-frequency heating and furnace heating can be used. The heating speed is fast and the holding time is short. The compound in the interface zone is thinner and the joint performance is better. Therefore, it is necessary to control the brazing temperature and holding time so that the solder can flow to the full 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 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 ranges from 760 to 927°C, high purity argon is required to prevent titanium from discoloration. Liquid argon in a refrigerated storage container 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. For this reason, diffusion welding can be used to improve the performance of brazed joints. When brazing, a 50μm thick copper foil, nickel foil or silver foil is placed between the titanium alloys. Depending on the contact reaction between titanium and these metals, Cu-Ti, Ni-Ti and Ag-Ti eutectic are formed respectively. Then these brittle intermetallic compounds are diffused away, and the joints brazed by diffusion at a certain temperature and a period of time have quite good performance.
In addition, the a+B phase titanium alloy can be used in an annealed, solution treated or aging 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, and adopting a staged cooling process in the brazing cycle. Thereby an annealed structure is also obtained; brazing at a temperature above the annealing temperature, and then annealing treatment.
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