The metal used to braze is typically a brazing filler metal, which can be based on various systems such as Al-Si for aluminum alloys, nickel-based alloys for high-temperature applications, and copper or silver-based alloys for other materials. The choice of brazing alloy depends on the base metal and the specific requirements of the joint, including temperature resistance, strength, and corrosion resistance.
Aluminum Alloys: For aluminum alloys, the most common brazing alloys are based on the Al-Si system, with silicon content generally ranging from 7% to 12%. The Al-Si system with 11.7% silicon is a eutectic system with a eutectic temperature of 577°C, making it a standard brazing material suitable for a variety of aluminum alloys. The addition of elements like magnesium can enhance the properties of these brazing alloys, improving their strength and corrosion resistance.
Nickel-Based Alloys: Nickel-based alloys are used for brazing in high-temperature environments, such as in aerospace applications. These alloys are typically brazed in vacuum conditions at levels ranging from 10-3 to 10-5 mbar. A preheat soak at 920-980°C ensures uniform heating of large components. After brazing, the furnace temperature can be adjusted for additional heat treatments to enhance the properties of the joint.
Copper and Silver-Based Alloys: Copper and silver-based brazing alloys are used for a wide range of applications due to their good thermal and electrical conductivity. These alloys often contain elements like zinc, cadmium, and manganese to lower the melting point. However, these volatile elements may require brazing under a partial pressure of gases like hydrogen or argon to prevent vaporization.
Compatibility and Considerations: The choice of brazing alloy must consider its compatibility with the base metal. Interaction between the braze alloy and base metal can lead to issues like erosion or embrittlement, especially if the base metal is thin. Therefore, brazing should be done at the lowest possible temperature and for the shortest duration to minimize these effects. Additionally, the presence of oxides on the base metal can impede the flow of the brazing filler metal, necessitating techniques like brush nickel plating or chemical etching to improve brazeability.
In summary, the metal used for brazing is not a single material but a selection of alloys tailored to the specific requirements of the base metal and the application. These alloys are chosen based on their ability to form strong, durable joints while minimizing adverse interactions with the base metal.
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