Vacuum brazing is under vacuum condition, heating and heat preservation was carried out on the structure, make the solder melted in the appropriate temperature and time scale, under the effect of capillary force and solid metal infiltration, dissolution, diffusion, welding, an advanced welding method to reach the purpose of welding. Prominent advantage of vacuum brazing is can connect different metals, complex structure and welding, after welding of the welding head is bright and clean density, small deformation and good mechanical properties and corrosion resistance.
Thickness of solder layer
When the filler metal thickness is too thin, it is easy to cause welding defects such as low welding strength, weak welding and substandard pressure. When the core layer is too thick, the thickness of the core layer alloy will be too thin, the pressure will not meet the standard, or even the corrosion phenomenon will lead to leakage. Therefore, the thickness and uniformity of solder layer are important indexes to measure the quality of solder layer and one of the important factors to influence the quality of brazing.
Other quality requirements
Internal defects such as porosity of core layer alloy, slag inclusion, poor bonding with filler metal layer, etc. External defects in addition to the surface treatment shall be unclean, and knock against injury, damage, in the process when the depth more than solder layer thickness, will directly destroy the continuity of the metal, led to the decrease of the bearing capacity.
Vacuum brazing process system
In vacuum brazing furnace, the workpiece is heated mainly by thermal radiation.
(1) vacuum brazing temperature: (1) at low temperature solder has yet to reach the required temperature, poor liquidity of solder, invasive, easy brazing seam internal porosity, brazing seam discontinuity, virtual welding defects, such as the decrease of strength of the braze welding head, confined ability is not up to standard, resulting in leakage, serious when even tear; (2) at high temperature, the solder melts completely and liquidity is too large, easy to produce solder oxide formation porosity and capillary force variation of weld, resulting in the loss of solder, the corrosion, the product defects such as bending. Suitable for constant temperature should pay attention to the flow of solder point, usually the low melting point solder melting point should be welded metal than 60 ℃ or so. At this point, the liquid solder on the weld metal has good invasive and viscosity, can under the effect of capillary force to fill the brazing gap, and can produce good alloying effect, and the weld metal forming high strength connectors.
(2) vacuum holding time: when the brazing solder wetting and joint formation needs around 1 s ~ 2 s, so the heat preservation time is mainly composed of stay brazing temperature of products, the core to the time needed for brazing temperature and oxidation film layer dissipate time needed for the decision. If the thermal insulation time is too short, the center temperature of the product to be brazed does not reach the brazing temperature. Over time, the liquid solder will corrode the welded metal easily.
2.4 vacuum degree of vacuum brazing furnace
When the vacuum is low at high temperature, the residual oxidizing gases such as O2 and H2O in the furnace are easy to react with the product metal to form a hard oxide film. The oxide structure is dense, stable and high melting point. It is not easy to decompose at the normal vacuum brazing temperature. When the welded metal is oxidized, it becomes difficult to infiltrate, which leads to the deterioration of the welding performance between the solder and the substrate. Therefore, it is necessary to improve the vacuum degree of vacuum brazing as much as possible, reduce the content of oxidizing gases, such as O2 and H2O, and control the production of metal oxides. General requirements, the temperature range of vacuum brazing furnace adopts many temperature control, furnace temperature uniformity for plus or minus 3 ℃, the vacuum degree of work shall ensure that no greater than 2.0 x 10-3 Pa, limiting vacuum of vacuum must beforehand in 10-4 orders of magnitude.
2.5 environmental conditions during vacuum brazing
Humidity in the environment will affect the quality of radiator brazing. The metal products assembled under high humidity are put into the vacuum furnace for brazing, and the water will evaporate and release more gas, and the water evaporation and gas release inside the radiator are a slow process. Water needs a lot of evaporation heat, which affects the temperature inside the radiator. Water also affects vacuum. The moisture will aggravate the oxidation of metal and affect the quality of brazing. Surface treatment, so on the radiator components before assembly and soldering should keep certain environment humidity, or take the oven to bake and dry processing, control of the radiator components due to environmental humidity surface moisture content.