HVAC and refrigeration
Aluminium tube-to-fitting joints, Cu-Al transition joints, evaporator and condenser repairs.
Filler and flux in one rod
Flux Core Aluminium Brazing Rod
A flux core aluminium brazing rod is a hollow filler-metal rod or wire filled with brazing flux, so the flux is delivered exactly where and when the filler melts. It joins aluminium to aluminium, and with zinc-aluminium grades also aluminium to copper, without brushing on a separate flux. Metline supplies Al-Si (4047-type) and Zn-Al flux-cored rods, wire and rings with standard and caesium-modified fluxes, worldwide.
- Filler alloys
- Al-12Si (4047), Zn-Al
- Flux
- K-Al-F, Cs-modified
- Joints
- Al-Al, Al-Cu
- Forms
- Rods, wire, rings
How a flux-cored aluminium rod works
Aluminium is covered by a tough oxide film that melts at over 2000 °C, far above any brazing temperature. Without a flux to break that film, molten filler will not wet the parent metal. A flux-cored rod carries the flux inside the filler, so the two arrive at the joint together.
As the joint heats, the flux in the core melts first, flows onto the surface and dissolves the oxide. A few tens of degrees higher, the filler sheath melts and is drawn into the joint by capillary action. Because the flux is sealed inside the rod, it is not knocked off during handling, it does not absorb moisture as quickly as an open paste, and the quantity applied is the same every time.
These rods braze rather than weld: the parent aluminium never melts. That distinction matters for repairs on thin sheet, tube and castings, where a TIG weld can burn through or distort the part. In maintenance shops the products are often called "aluminium welding rods", but the joint is a brazed one and must be designed with a lap or socket to give the filler a capillary gap to fill.
Types of flux-cored aluminium brazing rod
| Type | Filler alloy | Approx. melting range | Flux | Main use |
|---|---|---|---|---|
| Al-Si, standard flux | Al-12Si (4047 type) | 577 – 582 °C | Potassium fluoroaluminate (NOCOLOK-type), non-corrosive | Al-Al joints on 1xxx and 3xxx alloys: tube, fittings, heat exchangers |
| Al-Si, caesium-modified flux | Al-12Si (4047 type) | 577 – 582 °C | K-Al-F with caesium fluoroaluminate | Al-Al joints on alloys with higher magnesium, e.g. some 6xxx tube and castings |
| Zn-Al, caesium flux | Zn with about 2 – 22 % Al | About 380 – 490 °C, depending on Al content | Caesium-based fluoroaluminate, active at low temperature | Aluminium to copper, and Al-Al where low heat is needed (e.g. 6061, castings) |
| Zn-Al, chloride-containing flux | Zn-Al | About 380 – 490 °C | Corrosive chloride flux | Repair work where residues can be washed off thoroughly |
Melting ranges are nominal for the filler alloy. The standard K-Al-F flux melts at roughly 565 – 572 °C; caesium additions lower the flux melting point and improve tolerance of magnesium. Confirm the exact range on the product data sheet for each lot.
4047 (BAlSi-4 in AWS A5.8) is near the aluminium-silicon eutectic, so it melts over a very narrow range and flows freely. 4045 (BAlSi-5, about 10 % Si) melts over a slightly wider range and is sometimes preferred where a little more control is wanted. Zinc-aluminium fillers melt about 100 – 200 °C lower, which gives a much wider safety margin below the melting point of the parent metal.
Which aluminium alloys can be brazed with flux-cored rods?
Brazeability depends on two things: the solidus of the parent alloy, which must be well above the filler's working temperature, and its magnesium content, which interferes with standard fluoride fluxes.
| Parent alloy | Solidus (approx.) | Al-Si (4047) rod | Zn-Al rod |
|---|---|---|---|
| 1050, 1100, 1350 | 640 – 646 °C | Very good | Very good |
| 3003, 3103 | 640 °C | Very good | Very good |
| 6063 | 615 °C | Good with care | Very good |
| 6061 | 582 °C | Difficult: solidus is close to the filler melting point | Good; the usual choice |
| 5052 (2.5 % Mg) | 607 °C | Difficult with standard flux; Cs flux helps | Possible with Cs flux |
| Cast A356 / LM25 | About 555 °C | Not suitable | Possible on small repairs |
| 2xxx and 7xxx (2024, 7075) | 500 °C or lower | Not suitable | Not recommended |
Solidus temperatures are typical published values (ASM). As a working rule, standard K-Al-F flux performs well on alloys with up to roughly 0.3 – 0.5 % Mg; above that, caesium-modified flux or a zinc-aluminium rod is needed.
How to braze aluminium with a flux-cored rod
- Design the jointUse a lap, socket or sleeve joint with a gap of about 0.1 – 0.25 mm. A butt joint relies on the fillet alone and is much weaker.
- CleanDegrease, then brush with a stainless-steel brush used only on aluminium. Oil and heavy oxide stop the flux working.
- Heat the parent metal broadlyUse a soft, neutral flame and keep it moving. Heat the thicker part first. Aluminium does not glow before it melts, so watch the flux: when the rod tip touched to the joint shows flux melting and spreading, the joint is nearly hot enough.
- Let the work melt the rodTouch the rod to the joint, not the flame. When the parent metal is at temperature the filler melts and runs into the gap. Feed until a continuous fillet forms.
- Stop heating promptlyOverheating erodes the parent metal (the silicon in 4047 dissolves aluminium) and can melt thin walls. With Zn-Al rods, prolonged heat on Cu-Al joints grows brittle intermetallic layers.
- Cool and cleanCool in still air. Non-corrosive K-Al-F residue can stay on most parts; chloride flux residue must be washed off with hot water.
Flux-cored rods for copper-to-aluminium joints
Air-conditioner and refrigerator makers increasingly join aluminium tube or coils to copper pipework. A Zn-Al flux-cored filler is the usual method: it melts well below the aluminium tube, wets copper and aluminium with a caesium flux, and keeps the time at temperature short enough to limit brittle copper-aluminium intermetallic layers.
- Design the joint so the aluminium tube fits over or inside the copper with a uniform capillary gap
- Heat mainly the copper side, which conducts heat well and tolerates higher temperature
- Keep brazing time short; a thick, grey interface layer is a sign of overheating
- Protect the finished joint from moisture with a sleeve or coating, because copper and aluminium form a galvanic couple
Flux-cored rings in Zn-Al are widely used on production lines for these joints. Read more in our guide to brazing aluminium to copper.
Typical applications
Automotive
Radiator, charge-air cooler and A/C line repairs; aftermarket heat-exchanger service.
Heat-exchanger production
Return bends and header connections on aluminium coils, often with rings on induction lines.
Maintenance and repair
Cracked tube, thin sheet and extrusions where a TIG weld would burn through.
Fabrication
Railings, frames, furniture and enclosures in 6063 and 3003 where a neat, low-distortion joint is needed.
Electrical
Aluminium conductor and busbar connections where a brazed joint is specified.
Flux-cored rod compared with other aluminium joining methods
| Factor | Flux-cored brazing rod | Bare rod + paste flux | TIG / MIG welding | Aluminium stick electrode |
|---|---|---|---|---|
| Parent metal melts | No | No | Yes | Yes |
| Equipment | Torch or induction | Torch or induction | Power source, gas | DC welding set |
| Thin wall (under 1 mm) | Good | Good | Skilled work | Poor |
| Flux handling | None | Separate step | None (shielding gas) | Coating on electrode |
| Joint type | Lap, socket, sleeve | Lap, socket, sleeve | Butt, fillet | Butt, fillet on thick sections |
| Strength | Limited by filler and lap length | Same | Highest | Moderate |
For thick structural parts, see TIG filler rods or the aluminium stick welding rod.
Sizes, forms and packing
Supply range
- Rod diameter
- 1.6, 2.0, 2.4 and 3.2 mm; other sizes on request
- Lengths
- 330 mm and 500 mm straight rods
- Wire
- Flux-cored wire on spools for automatic feeding
- Rings
- Flux-cored rings sized to tube OD for production lines
- Packing
- Sealed tubes or bags against moisture, trade packs and bulk export cartons
- Documents
- Product data sheet, safety data sheet for the flux and certificate of analysis per lot
References and standards
- AWS A5.8M/A5.8 – Specification for Filler Metals for Brazing and Braze Welding (BAlSi classifications)
- ISO 17672 – Brazing: Filler metals
- Solvay – NOCOLOK flux powders and caesium additives (technical information)
- ASM Handbook, Volume 6A – Brazing of aluminium alloys
Flux core aluminium (aluminum) brazing rods: FAQ
Do I need to add flux when using a flux-cored rod?
No. The flux inside the rod is enough for normal joints. On heavily oxidised or contaminated parts, clean better rather than adding more flux.
What temperature does a flux-cored aluminium rod work at?
Al-Si (4047-type) rods melt at about 577 – 582 °C and are used with the joint at roughly 585 – 600 °C. Zinc-aluminium rods work much lower, at roughly 400 – 500 °C depending on aluminium content.
Can flux-cored rods join aluminium to copper?
Yes, with a zinc-aluminium rod and caesium-type flux. Al-Si rods are not normally used for Cu-Al joints because their temperature is too close to the melting point of the aluminium tube.
Can I braze 6061 with a 4047 flux-cored rod?
It is difficult: 6061 starts to melt at about 582 °C, almost the same as 4047. Use a zinc-aluminium rod for 6061, or design the joint so heat can be controlled very closely.
Is flux residue corrosive?
Residue from potassium or caesium fluoroaluminate fluxes is non-corrosive and can usually be left in place. Chloride-containing fluxes are corrosive and their residue must be washed off thoroughly.
Can these rods be used on 2024 or 7075?
No. High-strength 2xxx and 7xxx alloys start to melt at or below the working temperature of the filler, and their magnesium and zinc content interferes with the flux. Join them mechanically or by special welding processes.
Need flux-cored aluminium brazing rods?
Send the parent alloys, joint type, rod size and quantity. We will recommend the filler and flux and quote delivery to your site.
