Aluminum brazing: surface preparation

 

Before brazing aluminum parts, their surface requires some preparation. At least degrease soldered surfaces, that is, remove all oils from it, fats and other contaminants. This applies to any type of soldering – low temperature brazing and high temperature brazing.

Mechanical surface treatment

For non-thermally hardened aluminum alloys, steam or solvent washing is usually sufficient.. For thermally hardened aluminum alloys, chemical surface preparation is usually carried out to reduce the thickness of the oxide film.. Chemical preparation is not recommended for vacuum brazing without the use of flux. Cleaning with steel wool is also used, abrasive cloth or mechanical wire brushes. Burrs on aluminum parts must be removed before soldering..

Chemical surface preparation

chemicals

Before brazing aluminum parts, they are often immersed in 30 seconds into solution, composed of equal parts nitric acid and water. После такой обработки детали тщательно промывают в чистой воде (желательно горячей) и сушат горячим воздухом.

Aluminum-silicon alloys require special etchants, so their silicon components are not etched by many alkaline and acidic solutions. However, these special solutions include hydrofluoric acid, which is very dangerous to health, and also requires a special bath lining. Therefore, it is rarely used and with great care..

More often, for chemical surface preparation of these aluminum alloys, treatment is used for 30 секунд в теплом (65 ºС) водном растворе гидроксида натрия. To remove dark plaque, which is formed after this treatment is applied rinsing in cold water, treatment in nitric acid solution, промывку в воде (лучше горячей) и сушку горячим воздухом.

Chemical cleaning quality

Chemical cleaning of aluminum provides very good conditions for brazing. Cleaning in caustic soda solution is especially effective, although alkaline residues can adversely affect the quality of the solder. Remains of nitrogen, sulfuric and phosphoric acids can make soldering completely impossible. Therefore, after chemical treatment - alkaline or acidic - thorough rinsing with clean water is very important..

For the best soldering quality, it should be carried out immediately after surface preparation and no later than 48 hours after her. If proper precautions are taken to prevent contamination of the prepared surface, then the prepared parts can be soldered even after a few weeks.

Thermal degreasing

In production, for example, heat exchangers, применяют специальную печь термического обезжиривания (рисунок). It usually works at temperatures between 250 by 300 oC. Vapors from easily evaporating oils are oxidized in special combustion chambers. For heavier oils, a special flue gas cleaning system may be required. After processing, low-fat products are cooled to room temperature and only then proceed to the next operation, for example, applying fluxes.


Figure – Печь термического обезжиривания [2]

Sources:

  1. Aluminum and Aluminum Alloys, ASM International, 1996
  2. EEA Aluminium Automotive manual – Joining – Brazing, EEE, 2015