Porosity in brazing is primarily caused by the presence of oxidants, moisture, and certain volatile elements in the brazing environment. Here's a detailed explanation of each factor:
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Presence of Oxidants and Moisture: Brazing requires a controlled atmosphere free from oxidants like oxygen and water. The presence of these elements can lead to the formation of oxide layers on the metal surfaces. These oxide layers prevent the brazing filler metal from bonding effectively with the base metal, leading to weak joints and potential porosity. The atmosphere for brazing should ideally contain neutral gas (like pure nitrogen), with an oxygen content below 100 ppm and a humidity level equivalent to -40°C dew point to minimize the risk of oxide formation.
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Volatile Elements in Base Metals and Brazing Filler Metals (BFM): The chemical composition of the metals used in brazing can also contribute to porosity. Metals containing elements such as cadmium, zinc, or lead can off-gas when heated to brazing temperatures. These gases can contaminate the brazing furnace and form bubbles in the liquid brazing filler metal, leading to voids in the brazed joint. This is particularly problematic in vacuum brazing, where the gases can affect the entire extraction system and the quality of the brazed joints.
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Inadequate Brazing Cycle and Atmosphere Control: The brazing process involves precise control over temperature and atmosphere. Inadequate control during the initial pump-down phase, where water vapor and other gases are removed from the furnace, can lead to residual moisture or gases that contribute to porosity. Similarly, improper heating rates can cause issues like spalling of the braze alloy or distortion of the assembly, which indirectly affect the formation of porosity.
To prevent porosity, it is essential to maintain a clean, controlled environment during brazing, use appropriate base and filler metals that do not off-gas harmful elements, and ensure precise control over the brazing cycle parameters. This includes careful management of the furnace atmosphere, heating rates, and soak times to ensure uniform heating and effective bonding of the filler metal to the base materials.
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