A material used to hinder or prevent the formation of oxides and other undesirable substances in molten metal and on solid metal surfaces, and to dissolve or otherwise facilitate the removal of such substances.

Fluxes are used in fusion welding, brazing, and soldering to prevent the formation of oxides. They are used in brazing and soldering to dissolve or facilitate removal of oxides. See ACTIVE FLUX and NEUTRAL FLUX.

The oxides of all the commercial metals and alloys except steel have higher melting points than the metals themselves. Oxides are usually viscous (some are even insoluble) when the metal is fluid and at its proper welding temperature. An efficient flux combines with oxides to form fusible slag with a melting point lower than the metal. This slag forms a coating over the molten metal and thus serves as a protection against atmospheric oxidation. The chemical characteristics and melting points of the oxides of different metals vary greatly and therefore there is no one flux that will be satisfactory for all applications.

The melting point of a flux must be lower than that of either the metal or the oxides formed so that it will be liquid during the welding operation.

Fluxes are available packed in powder form in metal or plastic containers. Some lose their effectiveness if overexposed to atmosphere, and in such cases small containers are best. Some welders use a flux box, a short section of large pipe welded to a heavy plate about 150 mm (6 inches) square. This prevents the flux from tipping over during a job and holds only a small amount of flux so accidental losses are minimal.

Fluxes differ in their composition according to the metals with which they are used. In cast iron welding, a slag forms on the puddle and the flux serves to break up this slag. Equal parts of carbonate of soda and bicarbonate of soda make a good compound for this purpose. Also, for cast iron arc welding, various fluxes prevent oxidation and rapid cooling of the melt, and by combining with the excess carbon prevent the formation of hard compounds of iron and carbon.

Copper requires a filler rod containing phosphorus to produce weld metal without oxides. Powdered borax is often used as a flux with copper alloys.

Aluminum requires flux because there is a tendency for the heavy slag formed to mix with the melted aluminum and weaken the weld. For sheet aluminum welding it is customary to dissolve the flux in water and apply it to the rod. After welding aluminum, all traces of the flux must be removed.

Flux coatings often increase the speed of arc welding, although this is not universally true. They also concentrate the deposit, reduce spatter, and tend to prevent oxidation of the weld metal, as well as reduce the rate of cooling.