Brazing properties of aluminium and aluminium alloys
The brazing properties of aluminum and aluminum alloys are poor, mainly because the oxide film on the surface is difficult to remove. Aluminum has a high affinity for oxygen, and it is easy to form a dense and stable oxide film Al2O2 on the surface with a high melting point. At the same time, the aluminum alloy containing magnesium will also form a very stable oxide film MgO. They can seriously hinder the wetting and spreading of the solder. And it's hard to remove. In brazing, the brazing process can only be carried out with a suitable flux. Secondly, the operation of aluminum and aluminum alloy brazing is difficult. The melting point of aluminum and aluminum alloys is not much different from the melting point of the brazing material used. The temperature range that can be selected during brazing is very narrow. A little improper temperature control will easily cause the base metal to overheat or even melt, making the brazing process difficult to carry out. . Some heat-treated aluminum alloys can also cause softening such as over-aging or annealing due to brazing heating, resulting in reduced performance of brazed joints.
Brazing of aluminum and aluminum alloys has high requirements on the cleaning of the workpiece surface. To obtain good quality, oil and oxide film on the surface must be removed before brazing. To remove the surface oil, it can be washed with Na2CO3 aqueous solution at a temperature of 60-70 °C for 5-10 minutes, and then rinsed with clean water. To remove the oxide film on the surface, it can be etched with NaOH aqueous solution at a temperature of 20 to 40 ° C for 2 to 4 minutes, and then heated
Wash with water. After removing the surface oil and oxide film, the workpiece is glossy treated with HNO3 aqueous solution for 2 to 5 minutes, then washed in running water and finally air-dried. The workpieces treated by these methods should not be touched by hand or contaminated with other contaminants, and should be brazed within 6 to 8 hours. It is better to braze immediately if possible.
Requirements for vacuum brazing of aluminium and aluminium alloys
Vacuum brazing of aluminum and aluminum alloys often uses metal operating activators to deteriorate the oxide film on the surface of aluminum and ensure the wetting and spreading of the solder. Magnesium can be used directly on the workpiece in the form of particles, or introduced into the brazing zone in the form of vapor, or it can be added as an alloying element to the aluminum-silicon brazing filler metal. For workpieces with complex structures, in order to ensure the sufficient effect of magnesium vapor on the base metal to improve the quality of brazing, local shielding process measures are often adopted. That is, the workpiece is first put into a stainless steel box (known as a process box), and then placed in a vacuum furnace for heating and brazing. Vacuum brazed aluminum and aluminum alloy joints have smooth surface and dense brazing seam, and do not need to be cleaned after brazing; however, the cost of vacuum brazing equipment is high, and magnesium vapor pollutes the furnace seriously, requiring frequent cleaning and maintenance.
For aluminum and aluminum alloys brazed with fluxes other than fluoride fluxes, flux residues must be thoroughly removed after brazing. The residue of organic flux for aluminum can be washed with organic solutions such as methanol and trichloroethylene, then neutralized with sodium hydroxide aqueous solution, and finally washed with hot and cold water. Chloride is aluminum brazing flux residue can be removed according to the following method: first soak in hot water of 60 ~ 80 ℃ for 10 minutes, carefully clean the residue on the brazing seam with a brush, and wash with cold water. Then soak in 15% nitric acid aqueous solution for 30min, and finally rinse with cold water.
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