Brazing in vacuum furnace is a relatively new brazing method without flux under vacuum condition. Because the brazing is in a vacuum environment, which can effectively eliminate the harmful effects of air on the workpiece, it can be successfully brazed without applying flux. It is mainly used to braze those metals and alloys that are difficult to braze, such as aluminum alloys, titanium alloys, superalloys, refractory alloys and ceramics. The brazed joint is bright and compact, 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 furnace: hot wall furnace and cold wall furnace. The two types of furnaces can be heated by natural gas or electric heat, and can be designed as side loading furnace, bottom loading furnace or top loading furnace (kang type) structure. The vacuum system can be used universally.
The vacuum system mainly includes vacuum unit, vacuum pipeline, vacuum valve, etc. Vacuum unit is usually composed of rotary vane mechanical pump and oil diffusion pump. Single use mechanical pump can only get lower 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 × 10-4Pa vacuum degree. The gas pressure in the system is measured with a vacuum gauge.
Brazing in vacuum furnace is to braze in the furnace or brazing chamber where air is extracted, which is especially suitable for joints with large and continuous brazing area, and also for connecting some special metals, including titanium, zirconium, niobium, molybdenum and tantalum, with a wide range of applications. However, vacuum brazing also has the following disadvantages:
① The metal is easy to volatilize under vacuum conditions, so vacuum brazing should not be used for the base metal and filler metal for welding volatile elements. If it is necessary to use it, corresponding complex process measures should be taken.
② Vacuum brazing is sensitive to the influence of surface roughness, assembly quality and fit tolerance of brazed parts, and requires high theoretical level of working environment and operators.
③ The vacuum equipment is complex, with large one-time investment and high maintenance costs.
So, how to implement the brazing process in vacuum furnace? When brazing in the vacuum furnace, put the welded weldment into the furnace (or into the brazing container), close the furnace door (or close the cover of the brazing container), and pre vacuum before heating. Start the mechanical pump first, turn the steering valve after the vacuum degree reaches 1.35Pa, turn off the direct path between the mechanical pump and the brazing furnace, make the pipeline connect with the brazing furnace through the diffusion pump, rely on the mechanical pump and the diffusion pump to work in a limited time, pump the brazing furnace to the required vacuum degree, and then start the 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 the brazing furnace, release the adsorbed gas and water vapor from the furnace wall, clamps and weldments, and volatilize the metal and oxides to lower the true air. 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), and some vacuum brazing is used for the occasions where the base metal or filler metal volatilizes under brazing temperature and high vacuum conditions.
When special precautions must be taken to ensure high purity, vacuum cleaning method shall be adopted before dry hydrogen brazing. Similarly, using dry hydrogen or inert gas purification method before vacuumizing will help to obtain better results in high vacuum brazing.
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