Aluminum brazing in a vacuum furnace is considered fluxless brazing because flux is not used to remove oxides. Vacuum pumps evacuate the furnace, thereby reducing the PPM oxygen level in the brazing chamber thus inhibiting oxide formation. Magnesium is used as a getter in vacuum aluminum brazing to enhance the purity of the braze environment and facilitate oxide removal.
Brazing has advantages in comparison to other metal-joining technologies in that brazing does not melt the parent metal of the joint, thus brazing allows for more precise control of tolerances.
Industry utilization of aluminum brazing ranges from semiconductor to aerospace and beyond.
Applications well suited for brazing include:
- Parts with thin cross sections as well as thick cross sections
- Components with joints located below the surface or with
- Assemblies with a large number of joints
7 Advantages of Vacuum Aluminum Brazing
VAB offers some advantages not seen in other processes. Including:
- Provides a clean joint without the need for additional finishing and cleaning
- Minimizes part distortion because the complete assembly is uniformly heated and cooled compared to localized joining processes
- Assemblies with large surface areas and numerous joints can be successfully brazed in vacuum
- Offers highly repeatable results because critical furnace parameters are controlled with every load, i.e.: vacuum levels and temperature
- Creates a continuous hermetically sealed bond
- Ideal for corrosion or oxide-sensitive materials, because corrosive flux is eliminated
- Long joint paths are effectively purged of entrapped gas during initial evacuation of the furnace chamber resulting in more complete joint wetting
Vacuum Aluminum Brazing Furnaces
Commonly, VAB furnaces are single-chamber batch type and are usually loaded horizontally but can be vertical loading type. Horizontal furnaces tend to be simpler in design (one loading/unloading door), less expensive, and easier to maintain.
The VAB cycle is typically relatively short due to the pumping and heating characteristics of the furnace, excellent temperature uniformity, and high thermal conductivity of the aluminum assemblies being brazed.
Vacuum Pumping Systems
Low pressure (high vacuum) is an important process parameter because it ensures a relatively pure environment (less PPM of oxygen) for brazing. The pumping capacity required for an aluminum brazing furnace depends on the load surface area being brazed.
Larger load surface area requires lager pumping capacity. Pumping systems must have adequate capability to minimize pump-down time to a low pressure.
Vaporization & Water Vapor
The pumping system must also have adequate throughput to keep up with outgassing that takes place during the heating cycle. Magnesium vaporization occurs in the 10-4 to 10-5 Torr range prior to filler metal melting causing what is called outgassing.
A key constituent of VAB is the use of magnesium as an additive to the filler metal and/or base metal of the parts to be brazed. It is required in this fluxless brazing environment because of the following:
- Magnesium vaporizes at approximately 1050-1060oF and acts as a “getter” for oxygen and water vapor, thus improving the quality of the vacuum in the brazing environment
- Magnesium reduces the aluminum oxide on the components surfaces, promoting uniform accelerated wetting of joint surfaces
Magnesium vaporization produces heavy outgassing for a short period. To maintain a good working vacuum (10-4 to 10-5 torr range), vacuum pumps must be sized to accommodate this heavy outgassing.
Water vapor is an adversary to vacuum pumping. Water vapor slows pumping speed because it reduces the diffusion pumps capacity to remove real gasses from the furnace. Water also breaks down when heated, releasing oxygen into the furnace.
To combat this vacuum pumping adversary, the water in the cooling jacket of the furnace vessel is kept at a higher temperature than the ambient temperature. This warmer water keeps the vessel wall warmer, thus helping to prevent water vapor condensation when the door is open.
Accurate temperature control and uniformity are also very important process parameters. Temperature uniformity during a brazing cycle of ± 5oF (3oC) of set point is the accepted standard.
Holding (soaking) at a temperature just below the solidus point of the filler metal helps to ensure that all the components and joints to be brazed reach the same temperature at approximately the same time. After this hold (soak), the temperature is raised (ramped) to the brazing temperature, filler metal begins to melt, and capillary wetting of the braze joints occurs.
The dwell time (soak) at braze temperature must be minimized as melted filler metal is vaporizing in the low pressure (high vacuum) environment. Too much filler metal vaporization can result in poor joint wetting and subsequent loss of joint strength and sealing ability. After the final brazing soak is complete, a vacuum cooling cycle follows, which stops material vaporization and solidifies the filler metal.
How Vacuum Aluminum Brazing Is Done
Temperature control and uniformity for VAB is achieved through the use of multiple heating control zones and heating element design. Maintaining the surface temperature of the heating elements as close to the required part (load) temperature as possible is critical.
A large temperature difference between the heating elements and the parts would result in overheating the parts’ surface.
Heating element design is very important in maintaining this close temperature relationship between element and load. Low watt density (watts divided by square inches) and a close ratio of heating element surface area to load surface area provide for element temperatures only slightly above load or setpoint temperatures during brazing.
Other VAB Considerations
Although a properly designed vacuum furnace is important in successful vacuum aluminum brazing, there are many other factors such as joint design, part cleanliness, fixturing and filler metal selection that are also critical.
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