Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the workpieces together.^[1] It is similar to soldering, except the temperatures used to melt the filler metal is above 450 °C (842 °F), or, as traditionally defined in the United States, above 800 °F (427 °C).

A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.

Braze alloy is generally available as rod, ribbon, powder, paste, cream, wire and preforms (such as stamped washers).^[5] Depending on the application, the filler material can be pre-placed at the desired location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used as they are the easiest to apply while heating. In the case of furnace brazing, alloy is usually placed beforehand since the process is usually highly automated.^[5] Some of the more common types of filler metals used are

• Aluminum-silicon
• Copper
• Copper-phosphorus
• Copper-zinc (brass)
• Gold-silver
• Nickel alloy
• Silver^[1][6]
• Amorphous brazing foil using nickel, iron, copper, silicon, boron, phosphorus, etc.

 Atmosphere

As the brazing work requires high temperatures, oxidation of the metal surface occurs in oxygen-containing atmosphere. This may necessitate use of other environments than air. The commonly used atmospheres are^[7][8]

• Air: Simple and economical. Many materials susceptible to oxidation and buildup of scale. Acid cleaning bath or mechanical cleaning can be used to remove the oxidation after work. Flux tends to be employed to counteract the oxidation, but it may weaken the joint.
• Combusted fuel gas (low hydrogen, AWS type 1, "exothermic generated atmospheres"): 87% N₂, 11–12% CO₂, 5-1% CO, 5-1% H₂. For silver, copper-phosphorus and copper-zinc filler metals. For brazing copper and brass.
• Combusted fuel gas (decarburizing, AWS type 2, "endothermic generated atmospheres"): 70–71% N₂, 5–6% CO₂, 9–10% CO, 14–15% H₂. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium carbon steels.
• Combusted fuel gas (dried, AWS type 3, "endothermic generated atmospheres"): 73–75% N₂, 10–11% CO, 15–16% H₂. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.
• Combusted fuel gas (dried, decarburizing, AWS type 4): 41–45% N₂, 17–19% CO, 38–40% H₂. For copper, silver, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.
• Ammonia (AWS type 5): Dissociated ammonia (75% hydrogen, 25% nitrogen) can be used for many types of brazing and annealing. Inexpensive. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys.
• Nitrogen+hydrogen, cryogenic or purified (AWS type 6A): 70–99% N₂, 1–30% H₂. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals.
• Nitrogen+hydrogen+carbon monoxide, cryogenic or purified (AWS type 6B): 70–99% N₂, 2–20% H₂, 1–10% CO. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, medium and high carbon steels.
• Nitrogen, cryogenic or purified (AWS type 6C): Non-oxidizing, economical. At high temperatures can react with some metals, e.g. certain steels, forming nitrides. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, low-nickel alloys, Monel, medium and high carbon steels.
• Hydrogen (AWS type 7): Strong deoxidizer, highly thermally conductive. Can be used for copper brazing and annealing steel. May cause hydrogen embrittlement to some alloys. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels and chromium alloys, cobalt alloys, tungsten alloys, and carbides.
• Inorganic vapors (various volatile fluorides, AWS type 8): Special purpose. Can be mixed with atmospheres AWS 1–5 to replace flux. Used for silver-brazing of brasses.
• Noble gas (usually argon, AWS type 9): Non-oxidizing, more expensive than nitrogen. Inert. Parts must be very clean, gas must be pure. For copper, silver, nickel, copper-phosphorus and copper-zinc filler metals. For brazing copper, brass, nickel alloys, Monel, medium and high carbon steels chromium alloys, titanium, zirconium, hafnium.
• Noble gas+hydrogen (AWS type 9A)
• Vacuum: Requires evacuating the work chamber. Expensive. Unsuitable (or requires special care) for metals with high vapor pressure, e.g. silver, zinc, phosphorus, cadmium, and manganese. Used for highest-quality joints, for e.g. aerospace applications.