Brazing is a process used to adjoin metal materials together by the use of a consumable metal filler. This process is similar to soldering, except temperatures required to melt the filler metal are higher (above 840° F). The molten metal filler is wicked across the seam of the metal joint, and when cooled it produces a solid bond between the workpieces.

Brazing Operation

Brazing is an efficient and easy means of adhering components, and workpieces are able to maintain their structural integrity. Items of dissimilar composition can be brazed with the correct alloys, and brazing is particularly useful when base metals may contain toxic components such as lead or cadmium. Brazing is also preferable if the assembly contains brittle compounds, extensive liquation and segregation, or residual stress and porosity. The brazing process is not limited to metal components, as ceramic components can be brazed. When electrical contacts need to be connected, brazing does not result in the increased resistivity between contacts that welding creates.

Brazing's capillary action. Image credit: Abbott Furnace

To begin, workpieces need to be clean, free of oxides, and relatively close fitting. For the best brazed joints, it is recommended to keep the expanse between workpieces less than 1 mm, though many gap sizes can be accommodated. Once components are in place, the filler metal is applied to the seam of the workpieces. The filler metal is brought to its melting point, at which time it is wicked into the cavity by capillary action. A flux is often used to prevent oxides or other contaminants from compromising the braze, but it can also be accomplished in an inert or reduced environment. The flux may even be provided within the filler material for convenience. Flux should be cleaned from the base metal after the brazing process.

Video credit: Panofish / CC BY-SA 4.0

Brazing Advantages and Disadvantages

Filler Materials

Braze fillers come in a variety of forms, such as:

Image credit: Sulzer

Fillers are chosen based upon their ability to flow across the base metal, ability to withhold a bond while in service, and the temperature at which they melt (below that of the base metal).

The most versatile brazing alloys are made up of three or more metals, called a trifoil. They consist of a copper base layer that is coated on each side with a brazing alloy. The copper absorbs mechanical stresses and acts as a diffusion barrier. The resulting joint minimizes the amount of cracking during cooling. Trifoils are useful for conjoining metal-to-cermet, metal-to-carbide, and metal-to-ceramic components.

Trifoil brazing alloy. Image credit: Aufhauser Corp.

Some of the most common types of filler materials, and their suitability determined upon the brazing method and application, are as follows.

Image credit: Sulzer

Ceramics are best adjoined by using filler materials of titanium, zirconium, hafnium, or vanadium. When making electrical connections, silver and copper-based brazes provide the best thermal and electrical conductivity.

Obviously, different alloys exhibit different characteristics when being heated. When bonding base metals with a relatively large gap, it is helpful to use an alloy where only a small portion of the filler is molten at a lower melting temperature. On the contrary, alloys with a tendency to liquefy easily are best used on workpieces that have narrow tolerances. The viscosity of the filler material as it is heated through its melting range must be carefully monitored to ensure an adequate union between workpieces. However, eutectic alloys melt at a single temperature and are therefore very ductile. They are useful for strong bonds between items with very little clearance for the filler.

The following table lists the best filler materials based upon the composition of the base metal.

Table credit: Sulzer

Technique

Many methods exist to accomplish brazing, but the most important consideration is the efficient transfer of heat to filler materials without damaging the base metal. Depending upon the application, the aesthetic and volume of brazing may be considered as well.

Brazing Safety

Brazing can pose an extreme danger to employees within the vicinity of the brazing process. Notably, fumes and smoke emanated by the materials, material coatings (such as cadmium and zinc), shielding gases, fluxes (which release fluoride), and contaminants can be diluted by the use of proper ventilation equipment. The extreme heat and light from brazing can cause burns and damage retinas, respectively, so proper PPE such as gloves, helmets and goggles, and clothing should be worn.

Brazing Standards

The American Welding Society has a collection of documents regarding the specifications for brazing.  Other selected standards are linked below.

ASD-STAN PREN 3878 - Aerospace brazing

ISO 18279 - Imperfections in brazed joints

AN 785 - American military specifications on brazing

Resources

Wikipedia - Brazing; List of brazing alloys

Brazco Manufacturing - Products

Torch Brazing - Heat Sources of Brazing

Aufhauser - Brazing