How about the welding effect of vacuum brazing furnace

How about the welding effect of vacuum brazing furnace

The brazing method 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 successfully carried out without applying flux. It is mainly used for brazing metals and alloys that are difficult to braze, such as aluminum alloys, titanium alloys, high-temperature alloys, 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.

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, and can be designed into side mounted furnace, bottom mounted furnace or top mounted furnace (Kang type) structure. The vacuum system can be used in general.

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. Mechanical pump alone can only get less than 1.35 × Vacuum degree of 10-1pa level. In order to obtain high vacuum, the oil diffusion pump must be used at the same time. At this time, it can reach 1.35 × Vacuum degree of 10-4Pa level. 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 with air extracted. It is especially suitable for brazing large and continuous joints, and also suitable for connecting some special metals, including titanium, zirconium, niobium, molybdenum and tantalum. It has a wide range of applications. However, vacuum brazing also has the following disadvantages:

① The metal is easy to volatilize under vacuum, so it is not suitable to use vacuum brazing for the base metal and filler metal of volatile elements. If it is necessary to use, corresponding complex process measures shall be adopted.

② Vacuum brazing is sensitive to the surface roughness, assembly quality and fit tolerance of brazed parts, and requires high theoretical level of working environment and operators.

③ Vacuum equipment is complex, with large one-time investment and high maintenance costs.

So, how to implement brazing process in vacuum furnace? When brazing is carried out in the vacuum furnace, the weldment to be welded shall be loaded into the furnace (or into the brazing vessel), the furnace door shall be closed (or the brazing vessel cover shall be closed), and pre vacuumized 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, rely on the mechanical pump and the diffusion pump to work within a limited time, pump the brazing furnace to the required vacuum degree, and then start the power on heating.

During the whole process of temperature rise and heating, the vacuum unit shall continue to work to maintain the vacuum degree in the furnace, offset the air leakage at various interfaces of the vacuum system and the brazing furnace, release of gas and steam adsorbed on the furnace wall, fixture and weldment, volatilization of metal and oxide, etc., so as to reduce the true air. There are two types 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 filler metal 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 vacuumizing will help to obtain better results in high vacuum brazing.