The success of brazing and the joint integrity will be dependent on the quality of prior surface preparation before brazing is completed. The surface cleanliness can make the difference between a good metallurgical joint and a bad one.
There are many criteria that will affect the joint quality, but it is felt that pre-clean in preparation for the braze operation is a most critical one. The cleaning of both joint faces will remove the obstructions to the joint-face wetting when the filler metal turns liquidus. The joint-face cleanliness will also assist in the filler-metal flow to fill the appropriate joint cavity volume.
Common surface contaminants are: oil, grease, paint, pigment markings, crayon, die lubricants, casting fluids and mill scale. These contaminants can be easily washed clean by the hot-water wash and rinse method. When (for example) refractory metals are to be brazed, however, the pre-cleaning is somewhat different. Generally, the precision aerospace and precision electrical components are prepared under clean-room conditions.
The use of a fluxing agent is purely to kill any surface oxides that might be present on the base metal at the time of the braze operation. It is mandatory that the base-metal surface be free of oxides at the time that the filler metal liquefies.
Remember that most metals (especially aluminum and stainless steels) will have a slight surface oxide film adhering to the joint surfaces to be brazed. The surface cleaning can be by solvent wiping, some form of degreasing, abrasive cleaning or hot alkaline cleaning. Another two factors that will affect the effectiveness of the pre-cleaning is the cleaning-medium temperature and the length of residence time of the component at the selected cleaning-medium temperature.
The methods of cleaning can be categorized into two groups (Figure 1.).
The chemical methods are the pre-cleaning methods that are most commonly practiced today. The chemical cleaning methods are shown in Figure 2.
The mechanical methods of pre-cleaning are shown in Figure 3.
Methods of the alkaline cleaning of components to be brazed are generally used in conjunction with a soaking, spray wash or barrel cleaning to remove solids and semi-solids from the metal surface.
Vapor degrease is also a common method of surface preparation prior to the braze operation. The previously preferred method was with the use of Trichloroethylene or Perchloroethylene.
Emulsions can also be used to clean and prepare the surface for brazing. Some oxides are removed by immersion, rinse and dry in:
- Phosphate-acid cleaners
- Acid pickling (nitric, hydrochloric, sulfuric)
- Electrolytic cleaning and non electrolytics
The surface oxides will form on steel very easily and readily in the presence of moisture, oxygen and other oxygen-bearing gases. It is necessary to decompose the surface oxides in order that wetting and filler-metal flow can occur.
The next article will continue with the pre-cleaning methods currently in use with modern brazing techniques.
By: Dan Kay
Posted: March 30, 2010 3:46 PM
It would be a good idea, too, to note that surface contaminants fall into two (2) different categories, and have to be treated separately, or else problems with brazing filler metal wetting of the faying-surfaces can occur. Oils, greases, etc., should always be removed first by appropriate degreasing techniques in a completely separate operation from the subsequent removal of surface oxides.
Readers need to be made aware that mechanical methods are not appropriate under any circumstances for the removal of oils, greases, etc., and that mechanical methods of cleaning are only to be used for removal of surface oxides after the parts have been completely degreased first by other methods.
It might also be wise to list the actual chemistries of the vapor-degreasing baths that are still suitable for use in vapor degreasers, since trichloroethylene has long been out use, and any current use of 1,1,1-trichloroethane is being priced out of the market.
Dan Kay
Kay & Associates Brazing
860-651-5595