Tue, 17 Jul 2018 02:05:43 +0000
Ceramic direct brazing process
Ceramic metallization of all sorts of braze welding head, because there is a coating or coating, make ceramic components used in high temperature is restricted, difficult to exploit the advantages of ceramic high temperature stability, asking people to find out the ways to direct soldering.
Direct brazing ceramics can be solved by blind hole or geometry factors cannot be fully metallization pretreatment parts connection issue, can greatly simplify the brazing process and meet the requirements of high temperature ceramic components to use. In this method, the solder containing Ti or Zr active elements is used to directly weld the brazing of ceramics and metals, ceramics and ceramics, ceramics and graphite. The solder used for direct brazing has a high melting temperature, which can meet the requirements of high temperature application of ceramic components. In order to avoid cracking of component materials due to different thermal expansion coefficient, a buffer layer can be sandwiched between them. According to conditions of use to solder should as far as possible to solder between set by brazing in two parts or placed on the use of the position of the solder fill the gaps, then like common vacuum brazing brazing.
For metal ceramics, such as containing 10% free silica and less than 500 PPM iron and aluminum silicon carbide ceramics, can choose germanium powder as the direct brazing filler metals, the germanium powder sandwiched between ceramic and germanium powder for 20 to 200 microns thick, background vacuum pump to 10-2 pa, vacuum degree of work for 8 x 10-2 pa, brazing temperature 1180 ℃, heat preservation for 10 min, the brazing seam organization for Ge Si solid solution, remelting temperature as high as 1200 ℃, can be used in high temperature environment. In heat exchanger and infrared radiation source type of production, often the alumina ceramics with alkaline corrosion resistant alloy brazing metal Ta, Nb, together, brazing process using plasma spraying equipment, the alumina ceramic surface spraying a layer of tungsten and molybdenum, and then make good mixed solder (Ni powder + Fe powder 17% or 15% Nb powder + Ni powder, binder, mix into the roller grinding within a few hours, to promote solder powder suspending in the binder) besmear in the combination of ceramic and metal surface, thickness of 0.125 ~ 0.125 mm, dry after a few hours, Background in vacuum furnace brazing, cold vacuum pump to 8 x 10-3 Pa, vacuum degree of work not less than 3 x 10-2 Pa, brazing temperature 1500 ℃ ~ 1675 ℃, insulation 5 ~ 10 min, so the brazing seam density, tensile strength of 140 mpa, joint corrosion resistant performance is good, the brazing seam can bear high temperature above 1000 ℃.
Due to the poor thermal conductivity of ceramics, the heating rate of brazing cannot be too fast. It takes a long time for the brazing to achieve the uniformity of temperature. In addition, poor plasticity, improper heating or cooling rate will lead to deformation and ceramic cracking. Therefore, the correct selection of brazing process parameters is more important for ceramic brazing.