The first step in soldering aluminum is to clean the surfaces and remove the layer of oxide. If a thick layer of oxide is present, you should remove the main part of it mechanically by filing, scraping, sanding, or wire brushing. A thin layer of oxide can often be removed by using a corrosive flux. Remember, remove any residual flux from the joint after the soldering is finished.

After cleaning and fluxing the surfaces, you should tin the surfaces with aluminum solder. Apply flux to the work surfaces and to the solder. You can tin the surfaces with a soldering copper or with a torch. If you use a torch, do not apply heat directly to the work surfaces, to the solder, or to the flux. Instead, play the torch on a nearby part of the work and let the heat conduct through the metal to the work area. Do not use more heat than is necessary to melt the solder and tin the surfaces. Work the aluminum solder well into the surfaces. After tinning the surfaces, the parts may be sweated together.

Another procedure you can use for soldering aluminum alloys is to tin the surfaces with an aluminum solder and then use a regular tin-lead solder to join the tinned surfaces. This procedure can be used when the shape of the parts prevents the use of the sweating method or demands a large amount of solder. When using tin-lead solder with aluminum solder, you do not have to use flux.

After soldering is complete, you should clean the joints with a wire brush, soap and water, or emery cloth. Ensure that you remove all the flux from the joint since any flux left will cause corrosion.

BRAZING

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.

EQUIPMENT

Brazing requires three basic items. You need a source of heat, filler metals, and flux. In the following paragraphs these items are discussed.

Heating Devices

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

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 silver- based 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

3. Copper

4. Copper-zinc (brass) alloys

5. Copper-phosphorus alloys

6. Gold alloys

7. Nickel alloys

8. Magnesium alloys

Fluxes

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