Brazing Glossary

Contract brazing involves a lot of specific terminology. This glossary covers the core terms and definitions behind our brazing services — useful background whether you’re specifying a joint, reading one of our quotes, or just curious how the process works.

Active Temperature Range

Flux only becomes active above a defined temperature, and loses its effectiveness above a defined upper temperature. Within this active temperature range, the flux is effective and enables or promotes wetting of the workpiece surface by molten filler metal.

Alloy

A mixture of two or more metals is called an alloy.

Aluminium and Aluminium Alloys

Most commonly brazed with AlSi filler metals at a working temperature of around 600°C, which limits the process to aluminium alloys with a solidus temperature above 630°C. Well-suited grades include pure aluminium and Al-Mn, Al-Mg and Al-Mg-Si alloys containing less than 2% Mg and Si.

Assembly Gap

The gap between the parts being joined, measured at room temperature before heating.

Brazing (Hard Soldering)

Joining with filler metals whose liquidus temperature is above 450°C is called brazing. Braze alloys are typically silver-copper-zinc alloys, which may also contain tin or cadmium. See our brazing services for the processes we offer.

Brazing Atmosphere

The atmosphere present during brazing. Common options include open air with flux, protective gases such as argon, nitrogen or helium, reducing atmospheres such as hydrogen or carbon monoxide, and vacuum. We offer induction brazing under protective atmosphere for joints that need to stay oxide-free and flux-free.

Brazing Time

The time from the start of heating to the complete solidification of the filler metal. Under open-air conditions this should be no more than around 5 minutes, so the flux retains its oxide-dissolving effect; beyond that, the flux becomes saturated with dissolved oxides and can no longer support a sound joint.

Cadmium-Containing Silver Braze Alloys

Valued for their lower working temperature and good wetting behaviour, cadmium-containing silver braze alloys are still used in some industries, though we generally recommend switching to a cadmium-free alternative wherever the application allows it.

Cadmium-Free Silver Braze Alloys

For the same silver content, cadmium-free silver braze alloys have a higher working temperature than their cadmium-containing equivalents. Cadmium-containing alloys are increasingly being replaced with cadmium-free versions because of the health risks associated with cadmium.

Capillary Action

In a joint gap of up to around 0.2 mm, molten filler metal is drawn into the gap by capillary action. This effect pulls the filler metal deep into the gap and can even draw it upward through a vertical gap.

Capillary Fill Pressure

The pressure that draws molten filler metal into the joint gap, even against gravity. It depends on the gap width and joint geometry — as a rule, the narrower the gap, the higher the capillary fill pressure.

Diffusion

A physical process that leads to the even distribution of particles between two materials, resulting in their complete intermixing at the joint interface.

Eutectic Alloys

Eutectic alloys have no melting range at all — only a single melting point, at which the alloy passes directly from solid to fully liquid.

Flame Brazing

Both brazing and soldering joints can be made by flame brazing (torch brazing), most commonly using an acetylene-oxygen flame, though other gas-oxygen mixtures can also be used. Flame brazing generally requires flux, with one exception: copper-to-copper joints made with phosphorus-containing copper filler metals, which can be brazed flux-free.

Flux

Flux keeps the surfaces of the parts being joined metallically clean: it dissolves existing oxides on the joint face and prevents new oxides from forming during heating. The parts are coated with a thin layer of flux, and once the joint reaches the flux’s active temperature range, it breaks down the oxide layer and keeps it from re-forming. We supply braze alloys, solders and matching flux alongside our brazing services.

Flux Fumes

Flux fumes are irritating to hazardous depending on composition. Extraction is advisable, and mandatory for certain flux types — always check the relevant safety data sheet.

Furnace Brazing

The parts, filler metal and any required flux are heated together in a furnace to brazing temperature, typically around 50–100°C above the filler metal’s working temperature. Furnace brazing is usually carried out flux-free, under a protective-gas atmosphere or in vacuum.

Gas Flux

A gaseous flux, typically generated from a volatile liquid mixture of boric acid esters and an easily vaporised carrier solvent, used only in flame brazing. The fuel gas stream is passed through the liquid mixture, picking up flux, which is then carried by the flame onto the joint, where it removes surface oxides. The drawback of gas flux is that it only becomes active above roughly 750°C and doesn’t penetrate narrow gaps well, which can prevent full capillary fill.

Induction Brazing

In induction brazing, the workpiece is heated electrically: it’s placed, without contact, inside a coil carrying an alternating current. Brazing is usually carried out in open atmosphere with flux, though it can also be done under a protective atmosphere.

Joint Gap

The gap between the parts being joined. Because of thermal expansion, the joint gap at brazing temperature can differ from the assembly gap measured at room temperature.

Knife-Line Corrosion

A form of interfacial corrosion that can occur on stainless steel components brazed with zinc-containing filler metals when they come into contact with chloride-containing aqueous media. It attacks the steel surface mainly at the edge of, and beneath, the joint. Using zinc-free filler metals reduces this risk.

Layered Filler Metals

Layers of copper or nickel mesh clad on both sides with filler metal, used for brazing carbide and PCD/CBN materials, where they help absorb the internal stresses that arise from the differing thermal expansion coefficients of the materials being joined.

Liquidus Temperature

The upper temperature limit of the melting range (or melting interval). Above this temperature, the filler metal is completely liquid.

Melting Range

All non-eutectic filler alloys have a melting range, calculated as the difference between the liquidus and solidus temperatures.

Non-Eutectic Alloys

Non-eutectic alloys have no single melting point — only a melting range, between a solidus and a liquidus temperature.

Phosphorus-Containing Filler Metals

Allow copper-to-copper joints to be brazed without flux. As the filler metal melts, its phosphorus content reacts with atmospheric oxygen to form phosphorus pentoxide, which combines with the copper oxide on the joint surface to form a low-melting copper metaphosphate that itself acts as a flux. Because copper metaphosphate is not corrosive, the joint requires no further post-treatment. In open atmosphere, these filler metals can be used flux-free on copper, silver and copper-tin bronze; copper-zinc alloys, however, still require flux. They are not suitable for sulphur-containing media, and are not recommended for steels or iron- and nickel-based alloys, since they tend to form brittle intermetallic layers.

Service Temperature

Elevated service temperatures inevitably reduce the strength of a brazed joint over time. The filler metal’s technical datasheet should always be checked to make sure the specified continuous service temperature isn’t exceeded in operation.

Soft Solders

Used mainly in plumbing/installation work and electronics, where tin-copper and tin-silver alloys are the most common choice.

Soldering (Soft Soldering)

Joining with filler metals whose liquidus temperature is below 450°C is called soldering. Typical lead-free soft solders are tin-copper (SnCu) and tin-silver (SnAg) alloys.

Solidus Temperature

The lower temperature limit of the melting range (or melting interval). Below this temperature, the filler metal is fully solid.

Vacuum Brazing

Vacuum brazing requires no flux. Filler metals with a high vapour pressure at brazing temperature — typically because they’re alloyed with volatile elements like cadmium or zinc — aren’t suitable for vacuum brazing. Unlike open-atmosphere brazing, where gases and flux residue can become trapped in the joint gap, vacuum-brazed joints come out clean and need no further post-treatment. Vacuum-brazed joints typically show excellent gap fill and high strength, which is why highly loaded components such as turbine blades, heat exchangers, honeycomb structures and engine parts are frequently brazed under vacuum.

Wetting

The spreading of molten filler metal across the workpiece surface. Filler metal only wets a base material once both the joint faces and the filler metal itself are metallically clean, the working temperature has been reached at the joint, and at least one base material has begun to alloy with the filler metal.

Working Temperature

The lowest surface temperature at the joint at which the materials being joined wet, or form a liquid phase through interfacial diffusion. Working temperature is always higher than the filler metal’s solidus temperature, and can fall below, above, or exactly at its liquidus temperature.

Have a term or process we haven’t covered, or a question about a specific application? Get in touch — we’re happy to talk it through.