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Brazing of Active Metals
1. Brazing Material
Titanium and titanium alloys are rarely brazed with soft solder. The solder used for brazing mainly includes silver base, aluminum base, titanium base or titanium zirconium base.
Silver based solder is mainly used for components with working temperature less than 540 ℃. The joint with pure silver solder has low strength, easy to crack, and poor corrosion resistance and oxidation resistance. The brazing temperature of Ag Cu solder is lower than that of silver, but the wettability decreases with the increase of Cu content. Ag Cu solder containing a small amount of Li can improve the wettability and the alloying degree between solder and base metal. AG Li solder has the characteristics of low melting point and strong reducibility. It is suitable for brazing titanium and titanium alloys in protective atmosphere, but vacuum brazing will pollute the furnace due to the evaporation of Li. Ag-5a1 – (0.5 ~ 1.0) Mn solder is the preferred solder for titanium alloy thin-walled components. The brazed joint has good oxidation and corrosion resistance. The titanium and titanium alloy joints brazed with silver base solder are shown in Table 1.
Brazing process parameters
Brazing temperature/℃
Holding time/Min
Atmosphere
Table 1 Brazing Process Parameters of Titanium and Titanium Alloys
The brazing temperature of aluminum base solder is low, which will not cause the occurrence of titanium alloy β Phase transformation reduces the selection requirements of brazing fixture materials and structures. The interaction between the solder and the base metal is low, and the dissolution and diffusion are not obvious, but the solder has good plasticity and is easy to roll the solder and the base metal together, so it is very suitable for brazing titanium alloy radiator, honeycomb structure and laminate structure.
(1) Titanium based or titanium zirconium based fluxes generally contain elements such as Cu and Ni, which can quickly diffuse into the matrix and react with titanium during brazing, resulting in the dissolution of the matrix and the formation of brittle layer. Therefore, the brazing temperature and holding time should be strictly controlled during brazing, and should not be used for the brazing of thin-walled structures as far as possible. B-ti48zr48be is a typical Ti Zr solder. It has good wettability to titanium, and the base metal has no tendency of grain growth during brazing.
(2) Brazing filler metals for zirconium and base alloys brazing of zirconium and base alloys mainly include b-zr50ag50, B-zr76sn24, B-zr95be5, etc., which are widely used in the brazing of zirconium alloy pipes of nuclear power reactors.
(3) Satisfactory results can be obtained in vacuum and inert atmosphere (helium and argon). High purity argon shall be used for argon shielded brazing, and the dew point must be – 54 ℃ or lower. Special flux containing fluoride and chloride of metal Na, K and Li must be used for flame brazing.
2. Brazing Technology
Before brazing, the surface must be thoroughly cleaned, degreased and oxide film removed. Thick oxide film shall be removed by mechanical method, sand blasting method or molten salt bath method. The thin oxide film can be eliminated in a solution containing 20% – 40% nitric acid and 2% hydrofluoric acid.
Ti, Zr and their alloys are not allowed to contact the joint surface with air during brazing heating. Brazing can be carried out under the protection of vacuum or inert gas. High frequency induction heating or heating in protection can be used. Induction heating is the best method for small symmetrical parts, but for large and complex parts
Components, it is more advantageous to braze in the furnace.
Ni Cr, W, Mo, Ta and other materials should be selected as heating elements for brazing Ti, Zr and their alloys. Equipment with bare graphite as heating elements should not be used to avoid carbon pollution. Brazing fixture shall be made of materials with good high temperature strength, thermal expansion coefficient close to Ti or Zr and low reactivity with base metal.
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