Oxidation during brazing is a critical issue that can affect the quality of the brazed joint.
This is especially true for materials like aluminum, which form a stable oxide layer that can hinder the wetting of filler metals.
Preventing oxidation is essential for ensuring a clean and high-quality joint.
Here are seven key methods to prevent oxidation during brazing:
How is Oxidation Prevented When Brazing? 7 Key Methods Explained
1. Controlled Atmosphere Brazing (CAB)
In CAB, the atmosphere within the brazing oven is modified by removing oxygen and introducing a mixture of hydrogen and nitrogen.
This environment lacks oxygen molecules, which are the primary agents of oxidation.
The absence of oxygen allows the filler metal to flow smoothly and adhere properly to the base metal, forming a clean and high-quality joint.
2. Vacuum Brazing
For materials like aluminum, which are highly susceptible to oxidation, vacuum brazing is employed.
This method involves creating a vacuum environment with a very low pressure (10^-4 mbar or better) to completely eliminate oxygen.
Additionally, getter materials are used to absorb any residual oxygen atoms.
This stringent control over the atmosphere ensures that the aluminum oxide layer does not form or is mechanically detached, allowing for successful brazing.
3. Suppression of Aluminum Oxide
To prevent the formation of aluminum oxide during brazing, various methods are used.
These include chemical actions such as the use of corrosive fluxes, base or acid attacks, or the incorporation of magnesium.
Mechanical methods like sanding can also be used to prepare the surface prior to brazing.
These techniques ensure that the oxide layer is suppressed before the filler metal melts and resolidifies.
4. Atmospheric Conditions for Brazing
For all brazing processes, especially those involving aluminum alloys, the atmosphere must be strictly controlled.
Conditions typically include the use of neutral gases like pure nitrogen, with an oxygen content below 100 ppm and a humidity level very low (less than -40°C dew point).
These conditions prevent the reformation of oxide layers and ensure the integrity of the brazed assembly.
5. Phases of Brazing Process
The brazing process typically involves several phases, starting with the cracking of the aluminum oxide layer at around 400°C due to differential expansion.
Subsequent phases involve the heating of the base metal, flux, and filler metal, with careful monitoring of temperatures to ensure they do not exceed 565°C until the filler metal is in a solid form.
6. Use of Getter Materials
Getter materials are used in vacuum brazing to absorb any residual oxygen atoms.
This helps in maintaining a pure environment free from oxidation.
7. Mechanical Surface Preparation
Mechanical methods like sanding are used to prepare the surface prior to brazing.
This helps in removing any existing oxide layers and ensures a clean surface for the filler metal to bond with.
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