Brazing in vacuum furnace is a relatively new brazing method without flux under vacuum conditions. Because the brazing is in a vacuum environment, the harmful effect of air on the workpiece can be effectively eliminated, so the brazing can be carried out successfully without applying flux. It is mainly used for brazing metals and alloys that are difficult to braze, such as aluminum alloy, titanium alloy, superalloy, refractory alloy and ceramics. The brazed joint is bright and dense, with good mechanical properties and corrosion resistance. Vacuum brazing equipment is generally not used for needle welding of carbon steel and low alloy steel.
The brazing equipment in vacuum furnace is mainly composed of vacuum brazing furnace and vacuum system. There are two types of vacuum brazing Furnaces: Hot fireplace and cold fireplace. The two types of furnaces can be heated by natural gas or electric heating. They can be designed into side mounted furnace, bottom mounted furnace or top mounted furnace (Kang type) structure, and the vacuum system can be universal.
Vacuum system mainly includes vacuum unit, vacuum pipeline, vacuum valve, etc. The vacuum unit is usually composed of rotary vane mechanical pump and oil diffusion pump. Single use mechanical pump can only get less than 1.35 × Vacuum degree of 10-1pa level. To obtain high vacuum, the oil diffusion pump must be used at the same time, which can reach 1.35 at this time × Vacuum degree of 10-4Pa level. The gas pressure in the system is measured with a vacuum gauge.
Brazing in vacuum furnace is brazing in the furnace or brazing chamber with air extracted. It is especially suitable for brazing large and continuous joints. It is also suitable for connecting some special metals, including titanium, zirconium, niobium, molybdenum and tantalum. However, vacuum brazing also has the following disadvantages:
① Under vacuum conditions, metal is easy to volatilize, so vacuum brazing should not be used for the base metal and solder welding volatile elements. If necessary, corresponding complex process measures should be adopted.
② Vacuum brazing is sensitive to the surface roughness, assembly quality and fit tolerance of brazed parts, and has high requirements for the working environment and the theoretical level of operators.
③ Vacuum equipment is complex, with large one-time investment and high maintenance cost.
So, how to implement the brazing process in vacuum furnace? When brazing is carried out in the vacuum furnace, put the weldment with welding into the furnace (or into the brazing container), close the furnace door (or close the brazing container cover), and pre vacuumize before heating. Start the mechanical pump first, turn the steering valve after the vacuum degree reaches 1.35pa, close the direct path between the mechanical pump and the brazing furnace, make the pipeline connected with the brazing furnace through the diffusion pump, work within a limited time by relying on the mechanical pump and diffusion pump, pump the brazing furnace to the required vacuum degree, and then start electric heating.
During the whole process of temperature rise and heating, the vacuum unit shall work continuously to maintain the vacuum degree in the furnace, offset the air leakage at various interfaces of the vacuum system and brazing furnace, the release of gas and water vapor adsorbed by furnace wall, fixture and weldment, and the volatilization of metal and oxide, so as to reduce the true air drop. There are two kinds of vacuum brazing: high vacuum brazing and partial vacuum (medium vacuum) brazing. High vacuum brazing is very suitable for brazing the base metal whose oxide is difficult to decompose (such as nickel base superalloy). Partial vacuum brazing is used for the occasions where the base metal or solder volatilizes under the brazing temperature and high vacuum conditions.
When special precautions must be taken to ensure high purity, vacuum purification method shall be adopted before dry hydrogen brazing. Similarly, using dry hydrogen or inert gas purification method before vacuum pumping will help to obtain better results in high vacuum brazing.
Post time: May-07-2022