The vacuum level for brazing is typically maintained between 10^-4 and 10^-6 mbar. This range ensures a clean environment free from oxygen and moisture, which are detrimental to the brazing process. The use of magnesium as a "getter" further enhances the vacuum environment by sublimating at the operating temperature and reacting with any residual oxygen or water, thereby purifying the atmosphere within the furnace.
Detailed Explanation:
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Vacuum Levels in Brazing:
- The vacuum level is critical in brazing processes, particularly for materials like aluminum and copper. For aluminum, the vacuum is maintained at 10^-5 mbar or better, ensuring that the oxide film on aluminum is effectively broken down, facilitating better wetting and brazing. For copper, the initial vacuum is set between 10^-2 to 10^-4 mbar to remove residual air before raising the temperature for outgassing and surface cleaning.
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Role of Magnesium:
- Magnesium plays a crucial role in vacuum brazing, especially for aluminum alloys. Its higher affinity for oxygen compared to aluminum allows it to act as a getter, removing oxygen and water vapor from the furnace atmosphere. This process is vital as it not only cleans the atmosphere but also helps in breaking the oxide film on aluminum, which is essential for the brazing filler metal to wet the base metal effectively.
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Temperature Control and Uniformity:
- Temperature control is another critical factor in vacuum brazing. For aluminum alloys, parts are heated to 575-590°C, with a temperature uniformity of ±5.5°C. This precise control ensures that the brazing filler metal melts uniformly and flows properly into the joint, leading to a strong bond. For copper alloys, the temperature is raised to 1100–1120°C under an inert-gas partial pressure to inhibit copper evaporation.
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Process Control in Vacuum Brazing Furnace:
- The vacuum brazing process involves placing the assembled workpiece in a vacuum chamber, evacuating the chamber to the required vacuum level, and then heating the parts. Continuous pumping is maintained during the heating process to sustain the vacuum level. The heating rate is controlled to prevent a sharp drop in vacuum, and cooling is carefully managed to avoid oxidation of the workpiece.
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Quality and Applications:
- Vacuum brazing is favored for its ability to accurately control temperature, leading to uniform heating and minimal workpiece deformation. It is particularly useful for brazing high-temperature alloys and critical components where brazing quality is paramount. The absence of flux in vacuum brazing eliminates the risk of flux-related issues such as corrosion and interference with joint inspection.
In summary, maintaining a vacuum level between 10^-4 and 10^-6 mbar during brazing is essential for creating an environment that supports high-quality brazing, particularly for sensitive materials like aluminum and copper. The use of magnesium as a getter and precise temperature control further enhance the effectiveness and reliability of the brazing process.
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