Aluminum brazing includes many different techniques for producing the bond and can be performed with or without flux. Flux comes in a number of different forms, such as liquid, paste or powder. Flux brazing processes comprise induction, torch brazing (automatic and manual), salt bath, and controlled atmosphere brazing (CAB). Some brazing rods have a flux core or are coated with flux so as to apply the required flux during the brazing process.
When brazing is carried out in a vacuum furnace, it is deemed fluxless brazing as it does not use flux to produce the joint. Such processes include ceramic to copper brazing, semi-conductor manufacturing, etc. Thanks to the cleanliness of the vacuum environment, flux is not required; however, magnesium is utilized as an additive in the vacuum aluminum brazing process.
Benefits of Vacuum Aluminum Brazing
When compared to other metal-joining processes, brazing offers a number of benefits. Since brazing does not melt the joint’s base metal, it provides more accurate control of tolerances and offers a clean joint without the need for further finishing. The filler metal forms the meniscus in the brazed joint, which is suitably shaped to reduce stress concentrations and enhance fatigue properties.
Vacuum aluminum brazing reduces distortion of parts and produces a continuous hermetically sealed bond, thanks to uniform heating and cooling. Components with numerous joints and large surface areas can be effectively brazed. In addition, hardening can be carried out in the same furnace cycle if a forced cooling system is incorporated into the furnace system and hardenable alloys are utilized. This can considerably reduce the cycle time. Furthermore, vacuum furnace brazing offers excellent repeatable results, thanks to the critical furnace parameters that are accomplished with every load, i.e. temperature uniformities and vacuum levels.
Applications of Vacuum Aluminum Brazing
Some of the ideal situations for vacuum aluminum brazing include parts of very thick or thin cross sections that need joining; dissimilar metals such as stainless steel and copper that need joining; compact components comprising many junctions to be sealed; and assemblies having a large number of joints.
Vacuum brazing is suitable for oxidation sensitive materials. It is considered a flux-free process that prevents corrosive flux residue. It is also non-polluting and eliminates post-braze cleaning procedure; post-brazed parts are clean and have a matte grey finish.
Vacuum Aluminum Brazed Parts
Figure 1. Vacuum aluminum brazed radiator
Figure 2. Vacuum brazed evaporator
Figure 3. Flat plate cooler
Some of the examples of vacuum aluminum brazed parts include condensers, heat exchangers and evaporators utilized in nuclear, energy, aerospace and automotive industries. Some of these components are illustrated in Figures 1, 2 and 3.
Types of Vacuum Aluminum Brazing Furnaces
Standard vacuum aluminum brazing furnace types include single-chamber or batch furnaces and multiple-chamber or semi-continuous furnaces. The former is generally loaded horizontally, but can be fabricated for a vertical loading operation, whilst the latter is horizontally loaded and automated using an external conveyor system and load carriers.
Figure 4. Batch vacuum aluminum brazing furnace
Figure 5. Semi-continuous vacuum aluminum brazing furnace layout
Figure 6. Semi-continuous furnace load carrier and conveyor system
Batch furnaces are less expensive, have a simpler design and are easier to maintain, while semi-continuous furnaces operate more efficiently and have higher production rates due to the multi-chamber design. Figures 4, 5 and 6 show examples of batch type and semi-continuous type furnaces.
Vacuum Aluminum Brazing Process
Figure 7. Typical vacuum aluminum brazing cycle
The vacuum aluminum brazing process is relatively short because of the superior temperature uniformity at soak temperatures, the rapid pumping and heating characteristics of the furnace and the high thermal conductivity of the aluminum components being brazed. A standard vacuum aluminum brazing cycle is shown in Figure 7.
The vacuum pumping capacity must be sufficiently sized so that the pumpdown time of a new load to a deep vacuum level is considerably reduced. A deep vacuum level ensures a relatively pure environment for brazing. Table 1 shows the difference in purity levels with respect to the various vacuum levels.
Table 1. Vacuum protection from undesirable gasses
|Pressure in mbar
Magnesium is used as an additive in the vacuum aluminum brazing process because, when it vaporizes at approximately 570 °C, it serves as a “getter” for water vapor and oxygen, and thereby enhancing the purity of the brazing vacuum. Magnesium also reduces the alumina oxide that exists on the aluminum’s surface to promote homogeneously accelerated wetting of the joint surfaces.
Temperature Uniformity and Heating Control
In addition to the deep vacuum level, accurate temperature control and uniformity are also critical process parameters in the vacuum aluminum brazing process. Preferred temperature uniformity during a brazing cycle is ±3 °C of set point.
Fundamentals of Braze Joints
Fundamentals of braze joints include types of braze joints, strength of braze joints, featuring of parts and cleaning of parts.
Figure 8. Lap joints
Figure 9. T joints
The differentiation between favorable and unfavorable types of joints is the degree of overlapping which results in a good braze joint. The strength of the braze joint depends on two mechanical characteristics: the size of the gap into which the filler metal flows and the joint wetted surface area. Typical braze joints used in aluminum component construction are shown in Figures 8 and 9.
Part assemblies must be appropriately fixtured for brazing so as to maintain joint alignment, joint gaps, flow passage alignment and overall assembly tolerances. Lastly, part assemblies must be cleaned properly before assembly so that contamination is not introduced prior to the brazing cycle.
In vacuum aluminum brazing, the important process parameters are deep vacuum levels, excellent temperature uniformity and accurate temperature control all rendered by optimum furnace design and controls. For effective part brazing, joint design with respect to joint gaps and joint surface areas, proper fixturing of the part assemblies and cleanliness of the parts are important. Additionally, a routine furnace maintenance program will enable repeatable and quality brazing results over time.
This information has been sourced, reviewed and adapted from materials provided by Ipsen.
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