Brazing is an ancient metalworking process that is used to join two pieces of metal together using a third, molten filler metal. The earliest uses of brazing date back to the Bronze Age, nearly 6,000 years ago.

In brazing, a heat source, either a hand-held torch, an induction heating source or a furnace, are used to heat the filler metal above its point of melting. However, the heat added to the process is below the melting point of the metals that are being joined. The filler metal flows into the gap between the other two metal pieces and forms a metallurgical bond with those metals as it cools.

Brazing is similar to welding and soldering, but brazing is done at temperatures beginning at 450 degrees C., while soldering, which produces weaker joints, typically is done at temperatures below that and welding is done at temperatures far higher. In welding, high temperatures are needed to melt the metals that are being joined together while a filler metal is added, and the bond forms as the metals cool and solidify.

Brazing is considered more versatile than welding and soldering because it can be used for a wide number of metals and because it provides great tensile strength at the joints where the bond is placed.

Brazed joints often are stronger than the metals that are being joined, and they tend to resist gases and liquids, while being able to bear up under shock and vibration. Also, because the brazing process does not involve the melting of the metals that are being bonded, brazing does not warp the joined metals or distort their metallurgical properties.

Brazing is a useful, economic process for industrial applications that require strength, permanence, good aesthetic characteristics, and close tolerances for shape and the fit of joined parts.

Brazing is used in the production of tools, heavy construction equipment, plumbing fixtures and consumer products, such as appliances.

Brazing is used with dissimilar metals and when metals with different melting points must be joined, and to make linear bonds between two pieces of metal. It is a process that can be automated, and is the preferred process for parts that are thin and that may warp from the high heat involved in wielding.

Brazing is easier to automate than welding because parts can be positioned and flux can be applied to them prior to the start of the process. The molten filler metal used in the brazing process flows via capillary action into the gap between the metals that are being joined, so if products are pre-positioned and pre-fluxed, the brazing process can proceed quickly and economically by means of induction heating or in a furnace without the need for a person to be involved.

Brazing can be done by electrical, gas-or oil-fueled furnaces, or by induction coils that are used to surround and heat joint sites. The American Welding Society's Brazing Handbook identifies 12 atmospheres that are used in furnace brazing.

Brazing is done with the same handheld torches that are used in gas cutting or welding operations, but those torches are equipped with brazing tips for the process. Typical commercial gas mixtures can be used in brazing, and oxyacetylene and oxygen-natural gas are the mixtures that are most commonly used.

Brazing filler materials are available as wire, foil, rod, preforms or paste, and are covered by the specifications-from the American Welding Society. Filler metals include bag fillers (for ferrous parts); or BCuP, BCu, or RBCuZn fillers for brazing copper parts. Baths for brazing carbon and low-alloy contain fluxing agents such as borax or cryolite.

Materials Suitable for Brazing Most steels, brass, copper, iron, bronze

Types of Brazed Joints Although brazing can be done in a number of ways, there are two basic types of brazed joints: the butt joint and the lap joint. These are used to suit varying part and assembly geometries and functions, and other brazed joints are variations of these two basic types. A butt joint is relatively simple. It is formed when two pieces of metal are placed in an edge-to-edge position. The brazed bond is put between the two pieces of metal, forming a strong joint that is used in many applications. A lap joint is slightly more complex, and typically forms a stronger bond because a larger bonding surface is joined in the brazing process. A lap joint is formed by laying one piece of metal over another. The brazing metal flows into the gap between the overlaying pieces to form the bond. This type of joint is often used in plumbing applications and for other tubular products.

Advantages: Brazing is an easy process to learn. Brazing can join many dissimilar metals. Brazed bond lines can be very neat in appearance. Brazed joints are strong enough for most applications that are not heavy duty. Disadvantages: A poorly brazed joint looks similar to a good joint, and can have low strength. The metal used to bond the two parts may be different in color than the parts being bonded, detracting from the look of the brazed joint. Brazed parts cannot be used in applications where they are subjected to high heat because filler metals typically melt at relatively low temperatures. Dissimilar metals that are brazed may react to each other over a long period of time.

Sources for this report include the AWS specifications and books, including the Brazing Handbook, and Welding Design & Fabrication articles.