The temperature needed to braze aluminum is typically between 580-620°C (1076-1148°F). This range is chosen because it is above the melting point of the filler metal used in brazing but below the melting point of the base aluminum alloy, ensuring that only the filler metal melts while the base metal remains solid.
Detailed Explanation:
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Filler Metal Melting Point: The filler metal used in aluminum brazing has a melting point between 580-620°C. This temperature is critical because it must be high enough to melt the filler metal, allowing it to flow and fill the gaps between the components being joined. However, it must not be so high that it melts the base aluminum alloy, which could lead to structural failure or distortion.
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Base Metal Stability: The base aluminum alloy does not melt during the brazing process. This is crucial for maintaining the integrity and shape of the components being joined. The brazing temperature is carefully selected to ensure that the base metal remains in its solid state throughout the process.
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Temperature Control and Duration: During the brazing process, the components are heated to the brazing temperature and then held at this temperature for a specific duration, typically between 5 and 10 minutes. This time is necessary to ensure that all parts of the assembly reach the desired temperature uniformly. Prolonged exposure to high temperatures can lead to unwanted reactions or diffusion, affecting the quality of the brazed joint.
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Cooling and Solidification: After the brazing temperature has been maintained for the required duration, the assembly is cooled. It is important that the cooling process is controlled to prevent rapid cooling that could lead to stress or cracks in the joint. The cooling should continue until the temperature is at least 25°C below the solidus temperature of the filler metal to ensure complete solidification of the braze alloy.
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Atmosphere Control: Brazing aluminum typically requires a controlled atmosphere, such as a neutral gas like nitrogen, with very low oxygen and humidity levels. This is to prevent oxidation and other chemical reactions that could degrade the quality of the brazed joint.
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Alloy Compatibility: Not all aluminum alloys can be brazed. The choice of alloy is crucial as the solidus temperature of the alloy must be higher than the minimum brazing temperature of the filler metal. For instance, alloys with a solidus temperature below 600°C (1112°F) are generally not suitable for brazing. Additionally, alloys with high magnesium content (above 2%) are problematic due to the stability of the oxide layer that forms on the surface, which can hinder the brazing process.
In summary, the brazing of aluminum requires careful control of temperature, duration, and atmosphere to ensure a strong and reliable joint. The temperature range of 580-620°C is optimal for melting the filler metal without compromising the base aluminum alloy.
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