Flux-cored rod
A tube or folded rod of Zn-Al alloy with cesium flux inside. Convenient for torch repair and field work: no separate flux, controlled flux-to-filler ratio, and little residue.
Joining materials guide
Aluminium to Copper Brazing Rods
Aluminium-to-copper joints are normally brazed with low-melting zinc-aluminium filler rods, often with a non-corrosive cesium fluoroaluminate flux built into the core. A typical Zn-22Al rod melts at roughly 440 – 470 °C and is applied at about 510 – 540 °C, well below the 548 °C at which aluminium and copper form a liquid eutectic. This guide explains which filler to choose, why temperature control matters so much, and how to make reliable joints in HVAC, refrigeration and electrical work.
- Main filler type
- Zn-Al (about 2 – 22 % Al)
- Typical work temperature
- About 510 – 540 °C
- Flux
- Cesium fluoroaluminate (non-corrosive)
- Forms
- Flux-cored rod, bare rod, wire, rings
Why brazing aluminium to copper is different
Brazing aluminium to aluminium is routine with Al-Si fillers such as 4047 at about 580 – 610 °C. Brazing aluminium to copper is harder because the two metals react. At 548 °C aluminium and copper form a eutectic: where they touch, a liquid appears below the melting point of either metal, and copper dissolves rapidly into the aluminium. The resulting Al-Cu intermetallic compounds are hard and brittle.
Al-Si fillers work at temperatures above that eutectic, so with copper they can erode the joint and leave a thick, brittle intermetallic layer. That is why the standard approach for Al-Cu is a zinc-aluminium filler that melts well below 548 °C. Zinc wets both copper and aluminium, and the low working temperature keeps intermetallic growth thin.
The second difficulty is the oxide on aluminium. It forms instantly, melts at over 2,000 °C and must be removed by flux during brazing. Standard potassium fluoroaluminate fluxes (as used in furnace brazing) become active only at about 565 °C, too hot for Zn-Al fillers. Cesium-containing fluoroaluminate fluxes are active at lower temperatures and are therefore used with Zn-Al rods, either applied as a paste or contained in the core of the rod.
Filler alloys for aluminium-to-copper brazing
| Filler family | Approximate melting range | Typical working temperature | Use |
|---|---|---|---|
| Zn-22Al (Zn78Al22) | about 440 – 470 °C (vendor data vary) | about 510 – 540 °C | General Al-Cu and Al-Al joints; stronger joints with a wider gap tolerance |
| Zn-15Al (Zn85Al15) | starts melting near 380 °C; liquid by about 450 °C | about 450 – 500 °C | Al-Cu HVAC joints with a lower heat input |
| Zn-2Al to Zn-5Al | close to the Zn-Al eutectic, about 380 – 400 °C | about 420 – 460 °C | Low-temperature joints, thin aluminium, repair work |
| Al-Si 4047 (Al-12Si, AWS BAlSi-4) | about 577 – 582 °C | about 585 – 605 °C | Al-Al joints; Al-Cu only with very short cycles and expert control |
| Al-Si 4045 (Al-10Si, AWS BAlSi-5) | about 577 – 590 °C | about 590 – 605 °C | Al-Al joints and clad brazing sheet; not recommended for Al-Cu |
| Al-Si-Cu (e.g. AlSi10Cu4) | lower than plain Al-Si | about 560 – 590 °C | Lower-temperature Al-Al brazing of heat-sensitive parts |
Melting ranges for Zn-Al rods differ by maker and exact composition. Use the solidus, liquidus and recommended working range on the data sheet for the specific rod.
As the aluminium content of a Zn-Al filler rises, its melting range moves up, its strength and creep resistance improve, and its density falls. Zn-22Al is the most common general-purpose choice for HVAC copper-to-aluminium joints. Phosphorus-copper rods (BCuP) and silver-brazing alloys used for copper-to-copper joints are not suitable for aluminium: they melt far above aluminium's melting point and the flux does not remove aluminium oxide.
Flux-cored rods, bare rods, wire and preforms
Bare rod with flux paste
Used where the operator wants to control flux amount, or for alloys not available cored. Flux is brushed onto the cleaned joint before heating.
Coiled wire
For automatic torch or induction brazing machines, fed at a controlled rate.
Rings and preforms
Pre-placed rings on tube joints for induction or furnace brazing in volume production of evaporators and condensers.
Paste
Filler powder mixed with flux and binder, dispensed onto joints in automated lines.
Rod sizes
Commonly about 1.6 – 3.2 mm diameter in short lengths for torch work; wire and rings sized to the joint.
How to braze copper to aluminium step by step
- Design the jointUse a lap or socket joint. Keep the clearance small and even, typically about 0.08 – 0.15 mm per side, so capillary action draws the filler in. Allow overlap of at least three times the thinner wall.
- CleanRemove oil and grease with a solvent, then abrade the aluminium with a stainless brush or abrasive pad to break the oxide. Clean the copper to bright metal.
- FluxIf the rod is not flux-cored, apply cesium-based flux to both surfaces.
- Heat the copper firstCopper conducts heat faster and needs more energy. Direct the flame mostly onto the copper and let heat flow into the aluminium. Use a soft, neutral to slightly reducing flame.
- Feed the fillerWhen the joint reaches working temperature (the flux becomes clear and liquid), touch the rod to the joint on the side opposite the flame and let it flow through the gap. Do not melt the rod in the flame.
- Cool and inspectLet the joint cool naturally without quenching. Check for a continuous fillet around the joint. Cesium flux residue is generally non-corrosive; remove it if the joint will be painted or in humid service.
Aluminium does not change colour as it approaches its melting point, so overheating can collapse thin tubes without warning. Practise on scrap, use a temperature-indicating crayon or the behaviour of the flux as a guide, and keep the flame moving.
Common problems and how to avoid them
| Problem | Likely cause | Remedy |
|---|---|---|
| Filler balls up and will not flow | Joint too cold or oxide not removed | Heat the base metals, not the rod; clean and abrade the aluminium; check the flux |
| Aluminium tube melts or sags | Overheating, flame on the aluminium | Heat the copper side, use a smaller flame tip, work faster |
| Brittle joint that cracks | Thick Al-Cu intermetallic from excess temperature or time, or Al-Si filler on copper | Use Zn-Al filler, keep below about 540 °C, short heating time |
| Leak at the joint | Uneven clearance, incomplete penetration, porosity | Improve fit-up, feed until filler shows on the far side |
| Corrosion after service | Moisture at the bimetallic joint, flux residue | Seal or coat the joint, remove residue in humid locations |
Where aluminium-to-copper brazing is used
- Air conditioning and heat pumps: copper stub tubes or line sets joined to aluminium coils, distributors and headers, both in production and in field repair
- Refrigeration: aluminium evaporators in domestic and commercial refrigerators connected to copper capillary and suction lines
- Automotive thermal systems: transition joints in older or aftermarket A/C systems and oil coolers
- Electrical: bimetallic connections between copper and aluminium conductors, busbars and terminals where mechanical joints are unsuitable
- Marine and offshore HVAC: joints that are subsequently sealed against the salt atmosphere
Where production volumes are high, many manufacturers avoid in-place Al-Cu brazing by using pre-made copper-aluminium transition tubes (friction-welded or flash-welded joints) and then brazing copper to copper and aluminium to aluminium on either side. Brazing remains the practical method for repair and lower volumes.
Standards, documentation and safety
Aluminium-silicon brazing fillers are classified in AWS A5.8 (for example BAlSi-4 for 4047, BAlSi-5 for 4045) and in EN ISO 17672 (for example Al 112 for Al-12Si). Zinc-aluminium fillers are mostly supplied to manufacturers' specifications, so buyers should ask for a data sheet giving composition, solidus, liquidus, recommended working range and flux type, together with a certificate of analysis for each lot and the safety data sheet.
Brazing fumes from zinc and fluoride fluxes must be extracted. Work in a well-ventilated area, follow ANSI Z49.1 or the local equivalent, and wear eye protection and gloves. Store flux-cored rods in dry conditions in their original packaging, since moisture degrades flux performance.
References and standards
- AWS A5.8/A5.8M – Specification for Filler Metals for Brazing and Braze Welding
- EN ISO 17672 – Brazing: filler metals
- ANSI Z49.1 – Safety in Welding, Cutting, and Allied Processes (American Welding Society)
- ASM Handbook, Volume 6A – Welding Fundamentals and Processes (brazing of aluminium alloys)
- ASM Handbook, Volume 3 – Alloy Phase Diagrams (Al-Cu and Al-Zn systems)
Brazing aluminium (aluminum) to copper: FAQ
What rod do you use to braze aluminium to copper?
A zinc-aluminium filler, most commonly about Zn-22Al, used with a cesium fluoroaluminate flux either in the rod core or applied as paste. It works at roughly 510 – 540 °C, below the temperature at which aluminium and copper form a liquid eutectic.
Can I use 4047 or 4045 rod to join copper to aluminium?
It is not recommended. Al-Si fillers work at about 585 – 605 °C, above the 548 °C Al-Cu eutectic, so copper dissolves quickly and a brittle intermetallic layer forms. They are the right choice for aluminium-to-aluminium joints.
Can I use silver solder or phos-copper rod on aluminium?
No. Silver brazing alloys and BCuP rods melt at temperatures above or close to aluminium's melting point, and their fluxes do not remove aluminium oxide.
Do flux-cored aluminum-to-copper rods need cleaning after brazing?
Cesium-based flux residues are generally non-corrosive and non-hygroscopic, so many joints need no cleaning. Remove residue if the joint will be painted, sealed or exposed to moisture.
How strong is a Zn-Al brazed joint?
A well-designed lap or socket joint is normally stronger than the annealed aluminium tube beside it. Joint strength depends on clearance, overlap and avoiding overheating.
Will the joint corrode?
Copper and aluminium form a galvanic couple, so moisture at the joint can cause corrosion of the aluminium side. Indoors this is rarely a problem; outdoors or in humid locations, seal or coat the joint.
Need brazing rods for aluminium-to-copper joints?
Send the filler type, form, size and volume. We will supply data sheets, lot certificates and delivery worldwide.
