Brazing Technology Special care must be taken before brazing to remove oxides on the surface of refractory metals. You can use mechanical grinding, sand blasting, ultrasonic cleaning or chemical cleaning. Immediately after the cleaning process is completed, braze. Due to the inherent brittleness of W, the W parts should be handled with care during assembly to avoid damage. To prevent the formation of brittle tungsten carbide, direct contact between tungsten and graphite should be avoided. Eliminate the prestress caused by pre-welding or welding before welding. It is easy to oxidize when the temperature rises, and the vacuum of W brazing must be high enough. When brazing in the temperature range of 1000 ~ 1400 ℃, the vacuum degree cannot be lower than 8 × 10-3Pa. In order to increase the remelting temperature and use temperature of the joint, the brazing process and post-weld diffusion treatment can be used in combination, for example;
B-Ni68Cr20Si10Fel solder brazes W at 1180 ° C. After welding, after three diffusion treatments of 1070 ° C / 4h, 1200 ° C / 3.5h and 1300 ° C / 2h, the working temperature of the brazed joint can reach above 2200 ° C. When using Mo in a brazed joint, a small thermal expansion coefficient should be considered, and the joint gap should be selected in the range of 0.05 to 0.13 mm. If a fixture is used, select a material with a small thermal expansion coefficient. Flame brazing, controlled atmosphere furnaces, vacuum furnaces, induction heating furnaces, and resistance heating above the recrystallization temperature or reduced recrystallization temperature due to the diffusion of solder elements will recrystallize Mo. Therefore, when the brazing temperature is close to the agglomeration temperature, the shorter the brazing time, the better. When the brazing temperature is higher than the recrystallization temperature of Mo, the brazing time and cooling rate must be controlled to avoid cracking due to excessive cooling. When oxyacetylene flame brazing is used, it is desirable to use a mixed flux, that is, an industrial borate or silver brazing flux and calcium fluoride containing a high temperature flux to obtain good protection. The method is to first coat a layer of silver soldering flux on the surface of Mo, and then apply a high temperature flux. The silver brazing flux is active in the lower temperature range, and the active temperature of the high temperature flux can reach 1427 ° C. The Ta or Nb component is preferably brazed under vacuum, and the degree of vacuum is not less than 1.33 × 10-2Pa. If brazing is performed under the protection of an inert gas, gas impurities such as carbon monoxide, ammonia, nitrogen and carbon dioxide must be strictly removed. When brazing or resistance brazing in air, special solders with suitable fluxes should be used.
In order to prevent Ta or Nb from contacting with oxygen at high temperature, a metal copper or nickel layer can be plated on the surface, and a corresponding diffusion annealing treatment can be performed. Nine, brazing of ceramics and metals 1. Brazing performance Ceramics and ceramics, brazing of ceramics and metal parts is more difficult, most of the brazing materials are in ceramics;
The surface is spherical with little or no wetting. The solder wets the ceramic and the joint interface easily forms various brittle compounds (such as carbides, silicides and ternary or multiple compounds) during the soldering process. The presence of these compounds can affect the mechanical properties of the joint. In addition, since the thermal expansion coefficients of ceramics, metals, and solders vary widely, after cooling the soldering temperature to room temperature, the joint will have residual stress and may cause the joint to crack. Adding active metal elements on the basis of ordinary solder to make active solder can improve the wettability of the solder on the ceramic surface; low-temperature and short-term soldering can reduce the effect of interfacial reactions; design a suitable joint form using single or multiple layers of metal as The intermediate layer reduces thermal stress on the joint. 2. The solder ceramic and metal are mostly connected in a vacuum furnace or a hydrogen or argon furnace. In addition to general characteristics, welding materials for vacuum electronic equipment should also have some special requirements. For example, solder should not contain elements that generate high vapor pressures to prevent leakage of the device dielectric and cathode poisoning. Generally, when the equipment is working, the vapor pressure of the solder does not exceed 10-3Pa, and the high vapor pressure impurities do not exceed 0.002% to 0.005%. The W (o) of the solder does not exceed 0.001% to avoid soldering in hydrogen. Water vapor can be generated, which can cause molten solder metal to splash. In addition, the solder must be clean and free of surface oxides. For brazing after ceramic metallization, alloy solders such as copper, alkali, silver copper and gold copper can be used.
For direct brazing of ceramics and metals, solders containing active elements Ti and Zr should be selected. Binary solders are mainly Ti-Cu and Ti-Ni, which can be used in the range of 1100 ° C. Among ternary solders, Ag-Cu-Ti (W) (Ti) is less than 5%) solder can be used for various ceramics;
Solder directly to metal. Ternary solder can be used for foil, powder or Ti-containing powder Ag-Cu eutectic solder. The corrosion resistance of B-Ti49Be2 solder is similar to that of stainless steel, and its vapor pressure is low. It is preferred for vacuum sealed joints that are resistant to oxidation and leakage. In Ti-V-Cr series solders, when W (V) is 30%, the melting temperature is the lowest (1620 ° C). The addition of Cr can effectively reduce the melting temperature range. Chromium-free B-Ti47.5Ta5 solder has been used for direct brazing of aluminum oxide and magnesium oxide, and its joints can work at an ambient temperature of 1000 ° C. Ceramic and metal active flux.
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