Brazing Process: Joint Design, Filler Metals, Steps and Applications

Brazing is a joining process in which a filler metal with a liquidus above 450 °C, but below the melting point of the parts, is melted and drawn into a tight joint by capillary action. The base metal never melts. Most brazing is done at roughly 580 to 1,200 °C with silver, copper-phosphorus, copper, aluminium-silicon or nickel filler metals, and it is the standard way to join copper refrigerant lines, aluminium heat exchangers and carbide tool tips.

This page covers the brazing process itself: how a joint is designed, prepared, heated and inspected, and which filler to pick. If you only want to know how brazing differs from soldering, read the side-by-side comparison on brazing vs soldering. In short, the dividing line is the 450 °C filler liquidus set by the American Welding Society, and brazing gives much stronger joints.

What is the brazing process?

Three things have to happen for a braze to work:

1. The joint is heated, not the filler. Both parts are brought up to brazing temperature, which is a little above the filler’s liquidus.
2. The filler wets the surface. Molten filler spreads over clean, oxide-free metal and forms a thin metallurgical bond with it by diffusion. On oxidised or greasy metal it balls up instead.
3. Capillary action pulls it in. Once it wets both faces, the filler is sucked into the narrow gap between them, even upwards or around a tube, and leaves a neat fillet at each edge.

Joint design and clearance

Capillary flow depends on the gap. Too tight and the filler cannot enter; too wide and capillary pull is lost, the gap fills with a thick layer of weaker filler, and voids form.

Flux-free furnace brazing tolerates tighter gaps because there is no flux that has to be pushed out of the joint ahead of the filler.

Lap joints, not butt joints

Brazing filler is weaker than steel, so the joint gets its strength from area. The usual choice is a lap joint (or a tube pushed into a socket) with an overlap of at least three times the thickness of the thinner member. A butt joint has only the cross-section of the part to bond on and is avoided wherever the load matters. A scarf joint, with the ends cut at a shallow angle, is a compromise when the thickness must not change.

Worked example: overlap. A copper tube with a 1.2 mm wall needs a minimum overlap of 3 × 1.2 = 3.6 mm. Standard fittings give far more socket depth than that, which is why an overloaded, well-brazed tube usually fails beside the joint rather than in it.

Watch the gap at brazing temperature

The clearance that matters is the one at brazing temperature, not the one you measure cold. When two different metals are joined, the one with the higher thermal expansion grows more.

Worked example: copper plug in a steel ring. Take a 20 mm diameter copper plug inside a steel ring and heat both through about 700 °C. Using approximate expansion coefficients of 17 × 10^-6 /°C for copper and 12 × 10^-6 /°C for steel:

• Copper plug grows by 20 × 17 × 10^-6 × 700 = 0.238 mm on diameter.
• Steel bore grows by 20 × 12 × 10^-6 × 700 = 0.168 mm on diameter.
• The gap closes by 0.238 – 0.168 = 0.070 mm on diameter, which is 0.035 mm per side.

A cold clearance of 0.05 mm per side shrinks to about 0.015 mm when hot, which is almost too tight for flux brazing. Put the copper on the outside instead and the gap opens up. The rule: the metal that expands more should be the outer part, or the cold gap must allow for it.

Surface preparation

Filler will not wet dirt, and flux is not a cleaner: it dissolves thin oxide but does nothing to oil, paint, heavy scale or rust. Degrease first with a solvent or alkaline cleaner, then remove oxide with emery cloth, a wire brush or acid pickling. Stainless steel and aluminium carry tough oxide films and need extra care. Braze soon after cleaning and handle the parts with clean gloves, because fresh oxide starts growing straight away.

Flux, self-fluxing filler or a protective atmosphere

Hot metal oxidises fast, and an oxide skin stops the filler from wetting. There are three ways to deal with it.

Flux. A paste or powder, usually based on borates and fluorides, is brushed onto the joint before heating. It melts below the filler, dissolves oxide and shields the surface until the filler flows. Higher-temperature, boron-modified fluxes are used for stainless steel and long heating cycles, and aluminium needs its own fluoride or chloride fluxes. Most residues are corrosive and must be washed off.

Self-fluxing filler. The phosphorus in copper-phosphorus (BCuP) fillers reduces copper oxide, so copper-to-copper joints need no flux. That is why AC technicians braze copper lines with just a rod and a torch. BCuP must not be used on steel or nickel alloys, because it forms brittle phosphide compounds at the interface.

Controlled atmosphere or vacuum. In furnace brazing the air is replaced by hydrogen, dissociated ammonia, a nitrogen-based gas or a vacuum, so oxides either never form or are reduced. Automotive aluminium radiators and condensers are brazed in nitrogen with a thin, non-corrosive potassium fluoroaluminate flux, a process called controlled atmosphere brazing (CAB).

Heating methods

Brazing filler metals (AWS A5.8 classes)

Brazing filler metals are classified in AWS A5.8, “Specification for Filler Metals for Brazing and Braze Welding”. The prefix B means brazing filler and the letters after it are the main elements; an R in front, as in RBCuZn, marks a filler also sold as rod for braze welding, and a V, as in BVAg, marks a grade made for vacuum service. The main families:

A5.8 also covers gold (BAu), magnesium (BMg), cobalt (BCo), titanium and palladium-bearing fillers, used in aerospace, electronics and vacuum devices.

Two practical points on silver fillers. First, more silver usually means a lower flow temperature, better fluidity and a higher price. Second, some older grades such as BAg-1 contain about 24% cadmium, whose fumes are highly toxic. Cadmium-free grades such as BAg-7 or BAg-24 are the normal choice now, and are the only sensible choice for food, drinking water and medical equipment.

To choose a filler, start from the base metals (the table above), then check that the service temperature stays well below the filler’s solidus, that the parts can take the brazing heat, and that the filler resists the fluid in service. Silver content drives the price, so use the least silver that does the job.

Step-by-step brazing procedure

For a torch-brazed joint with flux:

1. Fit the parts to the right clearance and check the overlap.
2. Clean both surfaces: degrease, then remove oxide.
3. Apply flux to both faces and to the filler rod, unless you are using BCuP on copper or a protective atmosphere.
4. Assemble and fixture so the parts cannot move while the filler is liquid.
5. Heat the joint evenly, with a broad flame and more heat on the heavier part. The flux goes clear and fluid as the metal nears brazing temperature.
6. Feed the filler to the edge of the joint. The hot parts melt it and capillary action draws it through, towards the hotter side. Do not melt the rod in the flame and drip it on.
7. Cool without disturbing the joint until the filler has solidified.
8. Remove flux residue, usually by soaking or scrubbing in hot water once the part has cooled.
9. Inspect the joint.

Student tip: in a lab or viva, “heat the work, not the rod” is the one sentence examiners want to hear about brazing technique.

Brazing defects and inspection

Inspection starts with a visual check for a continuous fillet on both sides. Pressure joints get a leak test (soap bubble, pressure decay, or helium for refrigeration and vacuum work), and ultrasonic or radiographic testing can reveal internal voids. To qualify a procedure, sample joints are sectioned or peeled apart to see how much of the area actually bonded.

Advantages and limitations of brazing

Compared with welding, brazing gives up some strength and high-temperature capability in return for low distortion and the ability to join unlike metals. TIG welding is the better choice for thick, load-bearing butt joints.

Applications of brazing

• Air conditioning and refrigeration: copper suction and discharge lines, evaporator and condenser coils, and compressor connections in HVAC systems. BCuP is used for copper to copper, and silver filler where copper meets steel or brass. These joints carry refrigerant under pressure round the vapour compression cycle.
• Heat exchangers: aluminium car radiators and condensers (CAB furnace); stainless plate heat exchangers brazed with copper or nickel.
• Cutting tools: carbide tips on lathe tools, saw teeth and mining bits, silver-brazed to steel shanks, often with a copper shim to absorb thermal stress.
• Aerospace and power: nickel superalloy turbine parts and seals, vacuum brazed with BNi.
• Plumbing, gas and electrical: copper gas and medical gas pipework, busbars, motor windings and switchgear contacts.
• Bicycles and jewellery: lugged steel frames, gold and silver jewellery.

A related process, braze welding, uses a brass (RBCuZn) filler deposited in a groove or fillet like a weld, without relying on capillary flow. It is used to repair cast iron and to join galvanised steel.

References

• AWS A5.8M/A5.8, Specification for Filler Metals for Brazing and Braze Welding, American Welding Society (AWS).
• M. Way, J. Willingham and R. Goodall, “Brazing filler metals”, International Materials Reviews, 2020: review of brazing filler metals.
• TWI – Welding Job Knowledge, articles on brazing practice.

FAQs

What is brazing in engineering?

Brazing is a thermal joining process in which a filler metal with a liquidus above 450 °C, but below the melting point of the base metals, is melted and drawn into a close-fitting joint by capillary action.

What filler metal is used for brazing?

It depends on the base metals: copper-phosphorus (BCuP) for copper to copper without flux, silver alloys (BAg) for steel, stainless, brass and carbide, pure copper (BCu) for furnace brazing of steel, aluminium-silicon (BAlSi) for aluminium, and nickel alloys (BNi) for hot, corrosive service.

What is the ideal joint clearance for brazing?

For flux brazing with silver filler, about 0.05 to 0.13 mm per side, with the strongest joints near 0.04 mm. Flux-free furnace brazing can use tighter gaps. With dissimilar metals, check the gap at brazing temperature.

Why is flux used in brazing?

Flux dissolves the thin oxide film on the hot metal and stops new oxide forming, so the molten filler can wet the surface and flow into the joint. It is not needed with copper-phosphorus filler on copper, or in vacuum and protective-atmosphere furnaces.

What are the main applications of brazing?

AC and refrigeration copper lines, aluminium radiators and heat exchangers, carbide-tipped tools, turbine parts, gas pipework, electrical busbars and windings, bicycle frames and jewellery.

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