When it comes to brazing aluminum, several factors influence the strength of the resulting joint.
1. Brazing Alloys and Their Impact on Strength
The choice of brazing alloy is crucial.
Al-Si brazing alloys, especially those with a silicon content of 7% to 12%, are known for their excellent brazeability, strength, and corrosion resistance.
These alloys can be densified to enhance the toughness and bending strength of the brazed joint.
The Al-Si system with a silicon content of 11.7% (eutectic composition) is commonly used due to its low eutectic temperature of 577°C.
This makes it suitable for brazing a variety of aluminum alloys.
2. Brazing Process and Its Role in Strength
Aluminum brazing typically occurs at temperatures between 580-620°C.
This process requires precise temperature control to ensure that the filler metal adequately wets and bonds with the base metal without causing damage.
The use of controlled atmospheres, such as nitrogen in CAB (Controlled Atmosphere Brazing), helps prevent oxidation and ensures the quality of the brazed joints.
3. Compatibility of Aluminum Alloys with Brazing
Not all aluminum alloys can be brazed.
The solidus temperature of the aluminum alloy must be higher than the minimum brazing temperature of the filler metal, typically above 600°C.
Alloys with high magnesium content (over 2%) are difficult to braze due to the stability of the oxide layer that forms on the surface.
Commonly brazable alloys include those from the 1xxx, 3xxx, and some from the 5xxx series, provided they have a low magnesium content.
4. Challenges in Aluminum Brazing
Aluminum's high oxidation rate results in a stable aluminum oxide layer that prevents wetting by the filler metal.
This layer must be removed or suppressed before brazing, either chemically or mechanically.
The close melting ranges of some aluminum alloys and filler metals require precise temperature control and uniform heat distribution during brazing to ensure successful joint formation.
5. Effective Suppression of the Aluminum Oxide Layer
Effective suppression of the aluminum oxide layer is crucial for achieving strong, durable brazed joints in aluminum.
Careful selection of brazable alloys is also essential.
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