Oxidation in brazing refers to the process where metal atoms lose electrons to oxygen atoms in the air, forming metal oxides.
This process is detrimental to the brazing process as it hinders the flow of the molten filler metal, preventing the formation of a strong brazed joint.
5 Key Points to Understand
1. Mechanism of Oxidation
Oxidation occurs when metal atoms, under the influence of heat, transfer electrons to oxygen atoms present in the air.
This results in the formation of metal oxides.
For instance, iron rusts when it oxidizes, forming iron oxide.
This process is not limited to iron; most metals undergo oxidation when heated, which can significantly impair their properties and usability.
2. Impact on Brazing
During brazing, the goal is to create a strong bond between metal parts using a filler metal that melts at a lower temperature than the base metals.
Oxidation becomes a significant issue at the high temperatures used in brazing because it forms a layer of oxides on the metal surfaces.
This oxide layer prevents the molten filler metal from wetting and bonding with the base metal effectively, leading to weak joints.
3. Prevention of Oxidation in Brazing
To prevent oxidation, the brazing environment is carefully controlled.
In controlled atmosphere brazing (CAB), oxygen is removed from the brazing oven and replaced with a mixture of hydrogen and nitrogen.
This environment lacks oxygen molecules, thus inhibiting the oxidation process.
Similarly, in furnace brazing, maintaining the right atmosphere is crucial to prevent oxidation.
For materials like aluminum, which forms a stable oxide layer (aluminum oxide), special measures such as chemical suppression or mechanical removal of the oxide layer are necessary before brazing.
4. Specific Challenges with Aluminum Brazing
Aluminum is highly prone to oxidation, forming a stable aluminum oxide layer that is difficult to wet with filler metals.
This necessitates the use of fluxes or special atmospheres to suppress or remove the oxide layer before brazing.
The close melting ranges of some aluminum alloys also pose challenges in achieving accurate brazing temperatures and uniform heat distribution.
5. Atmosphere Requirements for Brazing
The brazing atmosphere must be free from oxidants, with very low oxygen content (less than 100 ppm) and low humidity levels.
This is typically achieved using pure nitrogen or other inert gases, ensuring that the metal surfaces remain oxide-free during the brazing process.
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