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Vacuum Aluminum Brazing

Common vacuum aluminum brazed components include heat exchangers, condensers and evaporators used in the automotive, aerospace, energy and other industries.

Vacuum aluminum brazing process

The vacuum aluminum brazing process usually takes a short time because the vacuum furnace evacuates and heats up very quickly, the temperature uniformity during insulation is very good, and the thermal conductivity of the aluminum parts being brazed is very high.

Vacuum

The vacuuming capacity must be sufficient to minimize the time for a new batch of charge to reach high vacuum, start heating as soon as possible, and have sufficient pumping capacity to compensate for the obvious outgassing caused by the evaporation of magnesium during the heating process. High vacuum is an important process parameter because it ensures a relatively pure brazing environment (extremely low oxygen content).

The use of magnesium as an additive to the filler metal and/or the base metal of the parts to be brazed is an important measure in vacuum aluminum brazing. This fluxless brazing process is necessary for the following reasons:

When magnesium begins to evaporate around 1,058°F (570°C), it acts to “absorb” oxygen and water vapor, thereby improving the purity of the brazing vacuum environment. Magnesium also reduces the oxide (aluminum oxide) present on the aluminum surface, helping to wet the joint surfaces quickly and evenly.

The evaporation of magnesium (also known as “magnesium deflagration”) produces large amounts of outgassing in a short period of time. The slower the heating rate, the slower the evaporation rate of magnesium. Due to the presence of these gas loads, the vacuum pump must have sufficient capacity to maintain good operating vacuum conditions (10^-4–10^-5 Torr).

Heating control and temperature uniformity

In addition to high vacuum levels, precise temperature control and uniformity are also important process parameters. An acceptable temperature uniformity specification during the brazing process is +/-5°F (3°C) from the set point.

The acceptable brazing temperature range for aluminum brazing is narrow. The controlling principle of aluminum brazing is that the filler metal must melt before the base metal reaches the solidus temperature. This temperature difference may be as small as 10°F–18°F (5°C– 10°C).

For example, the 6061 alloy base metal has a solidus temperature of 1,099°F (593°C) and a liquidus temperature of 1,206°F (652°C). Depending on the brazing material used, the brazing temperature can only be maintained within the range of 1,049°F – 1,085°F (565°C– 585°C).

The heating system used must control the soaking temperature to a level slightly lower than the melting point of the solder to ensure that all the parts and joints being soldered reach the correct temperature basically at the same time. Then, start to increase the soldering temperature to melt the solder and start to wet the soldered joint under capillary action.

The time held at brazing temperature must be kept as short as possible because the braze metal evaporates under high vacuum while wetting the brazed joint. If the evaporation loss of the solder is too much, it will lead to poor wetting of the joint parts, thus reducing the joint strength and tightness.

After the holding time at the brazing temperature is reached, the vacuum cooling process starts immediately to solidify the solder at the joint and stop evaporation.

In order to achieve the precise temperature control and uniformity required for vacuum aluminum brazing, on the one hand, multiple heating control zones are set up around the component, and on the other hand, the surface temperature of the heating element is kept as close as possible to the component temperature. If the temperature difference between the heating element and the component is too large, the surface of the component will be overheated, causing the component temperature when the solder begins to melt to exceed the solidus temperature of the base metal.