Surface yellowing of radiators after vacuum brazing not only affects product appearance but may also indicate potential internal quality issues. Below, I will provide a detailed analysis of the causes of yellowing in aluminum plate-fin radiators after vacuum brazing and propose corresponding solutions and preventive measures.
I. Cause Analysis
In theory, vacuum brazing occurs in an oxygen-free environment and should not result in oxidation or discoloration. The occurrence of yellowing is fundamentally due to trace amounts of oxygen or water vapor infiltrating during the brazing process or cooling stage, reacting with the high-temperature aluminum surface to form an extremely thin oxide film. The thickness of this oxide film causes interference with specific wavelengths of visible light, resulting in yellow, blue, or iridescent colors.
The specific causes can be categorized into the following main groups:
1. Insufficient Purity of Furnace Atmosphere (Root Cause)
This is the primary and most common cause of yellowing.
(1)Inadequate Vacuum Level:
The vacuum level required for brazing is typically in the range of 3×10−3 Pa3×10−3Pa to 10−4 Pa10−4Pa. If the vacuum pump performance declines, there are minor leaks in the vacuum system, or the furnace sealing rings age, the actual vacuum level may fail to meet requirements, leading to excessive residual oxygen.
(2)Contaminants in the Furnace:
Water Vapor: The most significant culprit. Water vapor may originate from: incomplete drying of workpieces after cleaning, moisture adsorbed by fixtures, internal furnace leaks (especially in water-cooling jackets), or moisture adsorbed on furnace walls released during heating.
Oil Vapor: If an oil diffusion pump is used, backstreaming can introduce oil vapor into the furnace. Under high temperatures, oil vapor decomposes into compounds containing carbon and hydrogen, contaminating the workpieces and potentially causing discoloration.
Other Volatile Substances: Residual cleaning agents, oils, fingerprints, etc., can volatilize under high temperature and vacuum, polluting the furnace atmosphere.
2. Brazing Material and Process Issues
(1)Brazing Material Problems:
Magnesium (Mg) in aluminum-silicon brazing filler metal acts as a key "getter," preferentially reacting with trace oxygen and water vapor in the furnace to protect the aluminum substrate from oxidation. Insufficient Mg content in the filler metal or premature volatilization of Mg during heating can compromise this protective effect.
(2)Inappropriate Process Parameters:
Improper Heating Rate: During the critical "degassing" stage (typically 300–500°C), excessively rapid heating may prevent adsorbed moisture and gases from being fully evacuated, releasing them later at high temperatures and disrupting the atmosphere.
Inadequate Brazing Temperature/Time: Excessively high temperatures or prolonged holding times can accelerate Mg volatilization and increase opportunities for reactions between the aluminum surface and residual gases.
Poor Cooling Process Control: During cooling, if the purity of the nitrogen or argon introduced is insufficient, or if gas is introduced too early causing pressure fluctuations that allow air ingress, oxidation and yellowing may occur while the workpiece is still at elevated temperatures (e.g., above 200°C).
3. Improper Pre-Treatment and Cleaning
(1)Incomplete Cleaning:
Residual organic contaminants such as stamping oils, cutting fluids, or fingerprints on the workpiece surface decompose into hydrocarbons and water vapor during brazing, causing localized contamination and oxidation.
(2)Insufficient Drying After Cleaning:
Residual moisture in workpiece crevices or fixtures is a major source of water vapor in the furnace.
(3)Prolonged Storage After Cleaning:
Cleaned aluminum parts quickly form a natural oxide film in humid air, which can impede brazing filler metal flow and exacerbate discoloration during brazing.
4. Material and Fixture Issues
(1)Composite Material Problems:
Non-uniform thickness or substandard composition of the cladding layer (brazing filler layer) on aluminum brazing sheets/strips.
(2)Fixtures and Tooling:
Fixtures that are not thoroughly cleaned, adsorb significant amounts of gas, or have coatings/paints that volatilize at high temperatures can become sources of contamination.
II. Solutions and Preventive Measures
Systematic measures must be implemented to address and prevent the above causes.
1. Ensure Furnace Atmosphere Purity (Most Critical Measure)
(1)Regular Maintenance and Leak Detection:
Establish a strict periodic maintenance schedule for the vacuum furnace, inspecting and replacing aging sealing rings.
Perform regular helium mass spectrometer leak detection to ensure the integrity of the furnace vacuum system.
Regularly inspect and maintain vacuum pump groups (mechanical pumps, roots pumps, diffusion pumps/turbomolecular pumps) to ensure their ultimate vacuum levels and pumping speeds.
(2)Optimize Process Degassing Curve:
Incorporate sufficient holding stages in the 300–500°C range to allow adequate time for adsorbed moisture and gases to be evacuated from the workpiece and furnace. This is the most effective stage for moisture removal.
Ensure the furnace vacuum reaches and stabilizes at the required high vacuum level (e.g., ≤ 5×10−3 Pa5×10−3Pa) before entering the brazing temperature.
(3)Use High-Purity Protective Gases:
The nitrogen or argon introduced during cooling must have a purity of at least 99.999%.
(4)Maintain Furnace Cleanliness:
Regularly perform high-temperature baking and cleaning of the furnace chamber to remove adsorbed contaminants.
Avoid introducing contaminants into the furnace.
2. Optimize Brazing Process
(1)Select Appropriate Brazing Material:
Use brazing filler metals with suitable Mg content and stable quality. For extremely demanding applications, consider filler metals containing special elements like Bi to suppress premature Mg volatilization.
(2)Refine Process Parameters:
Determine the optimal heating rate, brazing temperature, holding time, and cooling rate through experimentation.
Avoid excessively high brazing temperatures and prolonged holding times.
Ensure the furnace temperature cools below 450°C (lower is better) before breaking the vacuum and removing the workpieces.
3. Strengthen Pre-Treatment and Process Control
(1)Strict Cleaning Procedures:
Use ultrasonic cleaning with alkaline (or neutral) degreasers to thoroughly remove oils.
Rinse with deionized water to avoid water spots.
Immediately after cleaning, dry thoroughly using clean hot air or an oven.
(2)Control Turnaround Time:
Cleaned and dried parts should be assembled and loaded into the furnace for brazing as soon as possible (e.g., within 4 hours) to minimize exposure to air.
(3)Cleanroom Practices:
Operators should wear clean gloves during assembly and handling to prevent contamination from sweat or fingerprints.
4. Material and Fixture Management
(1)Inspect Incoming Materials:
Verify the composition and cladding layer thickness of raw materials such as aluminum brazing sheets/strips.
(2)Clean Fixtures:
Fixtures must undergo the same rigorous cleaning and drying procedures as the workpieces before each use. It is advisable to designate fixtures exclusively for brazing to avoid cross-contamination. Fixtures and brazing furnaces that have been idle for more than 72 hours must be dried and pre-baked before reuse.
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