Titanium alloy is an active metal, which is widely used in aerospace, petrochemical and atomic energy industries. The main problems in titanium and titanium alloy brazing are as follows:
1 The affinity of titanium and its alloy with oxygen is large, and a very stable oxide film is easily formed on the surface, so that the solder can be wetted and spread, so it must be removed during brazing.
2 Titanium and its alloys have a tendency to absorb hydrogen, oxygen and nitrogen during heating, and the higher the temperature, the more severe the absorption, so that the plasticity and toughness of titanium metal are drastically reduced, so brazing should be carried out in a vacuum or inert atmosphere.
3 Titanium and its alloys can react chemically with most of the needles to form brittle compounds, causing the joints to become brittle. Therefore, solders used for brazing other materials are basically unsuitable for brazing active metals.
4 Titanium and its alloys undergo phase transformation and grain coarsening when heated. The higher the temperature, the more severe 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 ° C. The lower the brazing temperature is, the smaller the influence on the properties of the base metal is.
In order to prevent oxidation and oxygen absorption and hydrogen absorption reaction of the brazed joint, brazing of titanium and titanium alloy is carried out in a vacuum and an atmosphere, and flame brazing is generally not used. When brazing in vacuum or chlorine gas, high-frequency heating, furnace heating, etc. can be used, the heating speed is fast, the holding time is short, the compound in the interface region is thin, and the joint performance is good. Therefore, it is necessary to control the soldering temperature and the holding time so that the solder fills the gap.
The reason why titanium and titanium alloys are best brazed in vacuum and argon is that in vacuum brazing, although titanium has a large affinity for oxygen, titanium can obtain a smooth surface under a vacuum of 13.3 Pa. This is because the oxide film on the surface can be dissolved in titanium.
Brazing is under argon protection, and the temperature range is 760 ~ 927 °C. In order to prevent discoloration of titanium, high purity argon is required, and liquid argon in a refrigerating storage container is generally used because of its high purity.
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