Brazing is the process of joining metal by heating the base metal to a temperature above 800°F and adding a nonferrous filler metal that melts below the base metal. Brazing should not be confused with braze welding, even though these two terms are often interchanged. In brazing, the filler metal is drawn into the joint by capillary action and in braze welding it is distributed by tinning. Brazing is sometimes called hard soldering or silver soldering because the filler metals are either hard solders or silver-based alloys. Both processes require distinct joint designs.
Brazing offers important advantages over other metal joining processes. It does not affect the heat treatment of the original metal as much as welding does, nor does it warp the metal as much. The primary advantage of brazing is that it allows you to join dissimilar metals.
Brazing requires three basic items. You need a source of heat, filler metals, and flux. In the following paragraphs these items are discussed.
The source of heat depends on the type and amount of brazing required. If you are doing production work and the pieces are small enough, they can be put into a furnace and brazed all at once. Individual torches can be mounted in groups for assembly line work, or you can use individual oxyacetylene or Mapp-oxygen torches to braze individual items.
Filler metals used in brazing are nonferrous metals or alloys that have a melting temperature below the adjoining base metal, but above 800°F. Filler metals must have the ability to wet and bond with the base metal, have stability, and not be excessively volatile. The most commonly used filler metals are the silverbased alloys. Brazing filler metal is available in rod, wire, preformed, and powder form.
Brazing filler metals include the following eight groups:
1. Silver-base alloys
2. Aluminum-silicon alloys
4. Copper-zinc (brass) alloys
5. Copper-phosphorus alloys
6. Gold alloys
7. Nickel alloys
8. Magnesium alloys
Brazing processes require the use of a flux. Flux is the substance added to the metal surface to stop the formation of any oxides or similar contaminants that are formed during the brazing process. The flux increases both the flow of the brazing filler metal and its ability to stick to the base metal. It forms a strong joint by bringing the brazing filler metal into immediate contact with the adjoining base metals and permits the filler to penetrate the pores of the metal.
You should carefully select the flux for each brazing operation. Usually the manufacturer's label specifies the type of metal to be brazed with the flux. The following factors must be considered when you are using a flux:
• Base metal or metals used Brazing filler metal used
• Source of heat used
Flux is available in powder, liquid, and paste form. One method of applying the flux in powdered form is to dip the heated end of a brazing rod into the container of the powdered flux, allowing the flux to stick to the brazing rod. Another method is to heat the base metal slightly and sprinkle the powdered flux over the joint, allowing the flux to partly melt and stick to the base metal. Sometimes, it is desirable to mix powdered flux with clean water (distilled water) to form a paste.
Flux in either the paste or liquid form can be applied with a brush to the joint. Better results occur when the filler metal is also given a coat.
The most common type of flux used is borax or a mixture of borax with other chemicals. Some of the commercial fluxes contain small amounts of phosphorus and halogen salts of either iodine, bromine, fluorine, chlorine, or astatine. When a prepared flux is not available, a mixture of 12 parts of borax and 1 part boric acid may be used.
Nearly all fluxes give off fumes that may be toxic. Use them only in WELL-VENTILATED spaces.