Research on aluminum alloy brazing technology

1. Aluminum alloy brazing method

1. Dip brazing

◇ Principle: Immerse the workpiece in molten salt or brazing material for heating, relying on liquid medium for heat conduction.

◇ Features: Large heat capacity, uniform heating, small deformation, but large environmental pollution (containing chloride brazing flux), need to be cleaned after welding. Due to environmental impact assessment requirements, there are only a few manufacturers in the country.

◇ Application: Suitable for scenes requiring high strength and mass production, but the overall evaluation is poor.

2. Low dew point brazing

◎ Principle: Dry air is used as the protective atmosphere, and the fluoride-containing brazing flux destroys the oxide film.

◎ Disadvantages: The residual agent needs chemical cleaning, the process is complicated, and it is mainly used in automotive heat exchangers.

3. Nocolok method

◎ Principle: Use nitrogen to protect the environment and non-corrosive fluoride brazing flux.

◎ Advantages: No need for post-weld cleaning, low equipment cost, and large joint gap.

◎Limitation: The magnesium content of the base material must be less than 0.5%, and the operating temperature is high (≥570℃), which may cause the base material to burn.

4. Vacuum brazing (VB)

◎Principle: The magnesium in the brazing material is evaporated to destroy the oxide film, and no brazing agent is required. Sometimes, magnesium is also placed in the furnace.

◎Advantages: No pollutants are left, the joint has good corrosion resistance, and the surface is smooth.

◎Disadvantages: The equipment cost is high, and the magnesium deposits in the furnace need to be cleaned regularly.

5. VAW method

◎Features: Chemical method is used to remove the oxide film, and nitrogen protection (dew point < -65℃).

◎Limitations: Alkaline/acid washing pretreatment is required, and the processing cost is high.

6. Other technologies

◎Ultrasonic vibration assisted brazing: Remove the oxide film through vibration and improve wettability.

◎Laser/induction heating: Precise temperature control, suitable for complex structures.

 

2. Development of low-temperature solder

1. Demand background

◎ Traditional Al-Si eutectic solder (577℃) is suitable for 3003 alloy.

However, the solidus temperature of 6061 and 7000 series alloys is low, and high temperature easily leads to grain coarsening and dissolution.

Developing low-temperature solder can avoid softening of the base material and interface reaction of dissimilar metals.

2. Main alloy systems

◎ Al-Si-Cu-Zn system: adjust the performance through Cu (↓ melting point) and Zn (↑ fluidity), such as Al-10Si-20Cu-5Zn (liquidus 523℃).

◎ Al-Ge-Si system: Ge greatly reduces the melting point (Al-35Ge-12Si eutectic point 480℃), but the cost is high and the brittleness is large.

◎ Zn-Al-X system: the liquidus is as low as 380℃, but Zn is volatile and the corrosion resistance of the joint is poor.

◎ Microalloying: Add Bi, Sr, Sr, RE (rare earth) and other elements to optimize spreadability and corrosion resistance.

3. Key progress

◎ Al-6.5Si-42Zn-0.12Sr: Wetting area 285mm², strength 136MPa (6061 base material).

◎ Al-10Si-15Cu-4Ni: Liquidus dropped to 515℃, strength reached 144MPa.

 

3. Fluxless brazing technology for aluminum alloys

1. Surface modification method

◎ By plating Ni or Ni alloy on the surface of the brazing material, the oxide film is destroyed (Al3Ni eutectic formation), which is costly and has not yet been widely used.

2. Microalloying method

◎ Adding elements such as Mg and Bi to the brazing material (acting as a vapor brazing agent), high-purity nitrogen protection is required (O₂ <5ppm).

3. Flux composite

◎ Premixing solid brazing agents such as K₂SiF₆, or self-brazing soft brazing agents (the brazing agent contains a high proportion of brazing alloy), simplifies the process but is difficult to prepare.

 

4. Future trends

1. Continuous vacuum brazing: Develop efficient equipment and reduce the maintenance cost of vacuum furnaces.

2. Low-temperature brazing optimization: Reduce temperature sensitivity and balance strength and corrosion resistance.

3. Environmentally friendly fluxless technology: Promote the industrialization of brazing-flux integrated materials.