Vacuum Brazing of Ceramics and Metals

The general process of ceramic and metal brazing can be divided into seven processes: surface cleaning, paste coating, ceramic surface metallization, nickel plating, brazing and post-weld inspection.

Surface cleaning is to remove oil, sweat and oxide film on the surface of the base material. Metal parts and brazing materials are first de-oiled, then pickled or alkali washed to remove the oxide film, rinsed with running water and dried. Parts with high requirements should be heat treated in a vacuum furnace or hydrogen furnace (also available by ion bombardment) with appropriate temperature and time to purify the surface of the parts. The cleaned parts should not come into contact with oily objects or bare hands, and should immediately enter the next process or put them in the dryer, and should not be exposed to the air for a long time. The ceramic parts should be cleaned with acetone and ultrasonic, then rinsed with running water, and finally boiled twice with deionized water for 15min each time.

Direct brazing In direct (active metal method) brazing, the surface of the ceramic and metal parts to be welded is first cleaned, and then assembled. In order to avoid cracks due to different thermal expansion coefficients of component materials, the buffer layer (one or more layers of metal sheets) can be rotated between the weldments, and the brazing filler metal should be sandwiched between the two weldments as much as possible or placed in the The brazing filler metal fills the gap, and then brazes like ordinary vacuum brazing.

For direct brazing with Ag-Cu-Ti solder, vacuum brazing should be used. When the vacuum degree in the furnace reaches 2.7×10-3Pa, the heating starts, and the temperature can be raised rapidly at this time; when the temperature is close to the melting point of the solder, the temperature should be raised slowly, so that the temperature of each part of the weldment tends to be consistent; when the solder is melted , Rapidly heat up to the brazing temperature, and the holding time is 3 to 5 minutes; when cooling, it should be cooled slowly before 700 °C, and it can be cooled naturally with the furnace after 700 °C.

When Ti-Cu active solder is directly brazed, the solder can be in the form of Cu foil plus Ti powder or Cu parts plus Ti foil, or Ti powder plus Cu foil can be coated on the ceramic surface. Before brazing, all metal parts should be degassed by vacuum. The degassing temperature of oxygen-free copper is 750~800℃. Ti, Nb, Ta, etc. are required to be degassed at 900℃ for 15min. At this time, the vacuum degree should not be lower than 6.7× 10-3Pa. When brazing, the components to be welded are assembled in a fixture, heated in a vacuum furnace to between 900 and 1120° C., and the holding time is 2 to 5 minutes. During the whole brazing process, the degree of vacuum shall not be lower than 6.7×10-3Pa.

The brazing process of Ti-Ni method is similar to that of Ti-Cu method, and the brazing temperature is 900±10℃.

In addition to surface quality inspection, the welded parts after brazing should also be inspected for thermal shock and mechanical properties. The sealing parts used for vacuum devices must also be subjected to leak detection tests according to relevant regulations.