Oxidation during brazing is prevented through the control of the atmosphere within the brazing oven, which is crucial for ensuring the quality of the brazed joint. This is particularly important for materials like aluminum, which form a stable oxide layer that hinders the wetting of filler metals.
Controlled Atmosphere Brazing (CAB): In CAB, the atmosphere 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.
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.
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.
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.
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.
In summary, preventing oxidation during brazing involves meticulous control of the brazing environment, use of specific gases or vacuum conditions, and careful management of the brazing process to ensure that the filler metal can bond effectively with the base metal without interference from oxide layers.
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