What Is Brazing?

Brazing is a metal-joining process in which a filler metal is melted and drawn by capillary action into the gap between two base metals held in close proximity, bonding them as the filler solidifies. The defining characteristic of brazing is that the filler metal has a liquidus above 450°C (840°F) while the base metals remain solid throughout the process. This distinguishes brazing from soldering, which uses lower-temperature filler alloys, and from welding, which involves melting the base metals themselves. The resulting joints are metallurgically bonded, typically strong enough to meet or exceed the strength of the filler metal, and are leak-tight under pressure.

Brazing draws from metallurgy, heat transfer, and surface science. Capillary flow of the filler into the joint depends on joint clearance, surface cleanliness, and the wetting characteristics of the filler-base metal system. A flux or controlled atmosphere is normally applied to prevent oxide formation on the base metal surfaces during heating, since oxides inhibit wetting and produce defective joints. Copper.org's technical resource on soldering and brazing in plumbing provides a useful overview of how flux chemistry and joint preparation interact in copper-based systems.

Filler Metal Selection

Brazing filler metals are classified according to their base alloy system, each suited to different combinations of base materials, service temperatures, and joint requirements. The American Welding Society standard AWS A5.8/A5.8M, Specification for Filler Metals for Brazing and Braze Welding, classifies more than 75 filler metals across alloy families including silver, copper, aluminum, nickel, gold, and cobalt. Silver-based fillers are the most widely used because they combine relatively low brazing temperatures (typically 600°C to 850°C) with good flow characteristics and broad compatibility with ferrous and non-ferrous base metals. Nickel-based fillers are selected for joints in stainless steels and superalloys intended for elevated service temperatures, while aluminum-silicon alloys are standard in HVAC heat exchanger manufacturing.

Brazing Methods

Heat is delivered to the joint by several methods, each suited to specific production contexts. Torch brazing, using an oxyacetylene or oxyhydrogen flame, is the most flexible method and is common in plumbing, HVAC, and repair work. Furnace brazing processes parts in a controlled atmosphere or vacuum, eliminating the need for flux and producing clean, scale-free joints at high throughput; it is widely used for aerospace components, heat exchangers, and carbide tool manufacture. Induction brazing uses an electromagnetic coil to heat the joint rapidly and selectively, which is advantageous for production lines where cycle time and precise heat input are critical. Dip brazing immerses assembled parts in a molten salt or filler bath, delivering uniform heat to complex assemblies.

Joint Design

Effective brazing joints rely on controlled clearance between mating surfaces, typically in the range of 0.025 mm to 0.125 mm for common filler systems. Too wide a gap reduces capillary pressure and produces porous, weak joints; too narrow a gap restricts filler flow. Overlap length and joint geometry are designed to carry load in shear rather than tension, since brazed joints are generally stronger in shear. Detailed guidance on joint geometry and clearance tolerances for different alloy systems is covered in the AWS Brazing Handbook, Fifth Edition, the principal reference for the field. Fluxes are selected to match the brazing temperature range of the filler and the oxide tenacity of the base metal; residual flux must be removed after joining to prevent corrosion.

Applications

Brazing has applications across a wide range of manufacturing and engineering sectors, including:

• HVAC systems, where copper tubing and aluminum heat exchangers are joined in high volumes
• Aerospace structures and turbine components, using nickel and gold-based fillers for high-temperature service
• Electrical and electronic assemblies, including hermetic package sealing and waveguide joining
• Carbide cutting tool manufacture, where carbide tips are brazed to steel shanks
• Plumbing and refrigeration systems, using silver-based fillers for pressure-tight copper joints