Aluminium brazing sheet is a roll-bonded composite: a core alloy, usually 3003 or another Al-Mn alloy, clad on one or both sides with a thin layer of low-melting Al-Si filler alloy such as 4343 or 4045. In the brazing furnace the cladding melts and flows into every joint, so a whole radiator or cold plate is joined in one pass. Metline supplies brazing sheet, strip and fin coil with the layer stack, clad ratio and temper you specify, to customers worldwide.
Brazing joins metals with a filler that melts below the parent metal. Aluminium alloys melt only a little above the usual aluminium-silicon fillers, so the filler must be placed exactly where it is needed and heated precisely. Brazing sheet solves this by carrying the filler as a cladding layer bonded to the sheet during hot rolling.
The core alloy (for example 3003, solidus about 640 °C) stays solid while the Al-Si cladding (4343 or 4045, which start to melt at about 577 °C) becomes liquid at the brazing temperature of roughly 590 – 610 °C. Capillary action draws the molten filler into the gaps between tubes, fins, plates and headers, forming fillets at every contact point. On cooling, the assembly is a single metallurgically bonded unit.
Brazing sheet is the basis of almost every modern aluminium heat exchanger: car radiators, condensers, evaporators and heater cores, charge-air coolers, HVAC coils, oil coolers, plate-fin exchangers for industrial gases, and the liquid cold plates that cool electric-vehicle batteries.
| Layer | Alloy | Approx. Si / key element | Role |
|---|---|---|---|
| Core | 3003 | Mn 1.0 – 1.5 % | Standard core: good strength after brazing and corrosion resistance |
| Core | 3005 / long-life 3xxx | Mn plus Mg, Cu or Ti additions | Higher post-braze strength and better internal corrosion life |
| Core | 6951 | Al-Mg-Si | Heat-treatable core for higher strength after brazing (e.g. evaporator plates) |
| Filler clad | 4343 | Si 6.8 – 8.2 % | Common CAB cladding with moderate fluidity |
| Filler clad | 4045 | Si 9.0 – 11.0 % | More fluid filler; widely used for CAB sheet and fin stock |
| Filler clad | 4047 | Si 11.0 – 13.0 % | Near-eutectic; lowest melting, most fluid |
| Filler clad | 4004 / 4104 | Al-Si with Mg (4104 also Bi) | Vacuum brazing: magnesium disrupts the oxide film without flux |
| Liner | 7072 | Zn about 1 % | Sacrificial waterside layer that corrodes in preference to the core |
Composition ranges follow the Aluminum Association / EN 573-3 registrations. AWS A5.8 classifies the same filler alloys for brazing, for example BAlSi-2 (4343), BAlSi-4 (4047) and BAlSi-5 (4045).
Parts are coated with a non-corrosive potassium fluoroaluminate flux (commonly known by the trade name Nocolok) and brazed in a nitrogen atmosphere. Uses 4343 or 4045 cladding. The dominant process for automotive and HVAC exchangers.
No flux: magnesium in 4004 or 4104 cladding evaporates and breaks up the oxide film in a high-vacuum furnace. Preferred for complex plate-fin exchangers and parts that must be free of flux residue.
Used for repairs and joining tubes to fittings with separate filler rod, wire or rings rather than brazing sheet.
Typical brazing cycles reach about 590 – 610 °C and hold for a few minutes. The window between filler melting and core melting is narrow, so furnace uniformity matters. Too hot or too long, and silicon diffuses from the cladding into the core ("erosion"), weakening thin fins and tubes; too cool, and joints are incomplete.
Final layer stack and clad ratio are confirmed against your process and drawing.
Post-braze properties are what count. The brazing cycle anneals the sheet, so a strain-hardened H14 temper loses most of its extra strength and finishes close to the O-temper values of the core alloy. Long-life cores are designed to keep more strength and resist the internal corrosion caused by coolant, which is why they have largely replaced plain 3003 in automotive radiators.
For tube and fin designs, a common arrangement is clad tube stock with bare fins, or clad fin stock with bare extruded tubes. Fins are often made from slightly zinc-rich alloys so that they are sacrificial to the tube, protecting the refrigerant or coolant passage from pitting.
4045 contains more silicon (9 – 11 %) than 4343 (6.8 – 8.2 %), so it melts more completely at the brazing temperature and flows more freely. 4343 gives less erosion of thin cores; 4045 fills larger gaps.
Typically 5 – 15 % of the total thickness per side. Thin fins and tubes use lower ratios to limit core erosion; plates and headers that must fill larger joints use higher ratios.
The brazing cycle at about 600 °C anneals the sheet. Strain-hardened tempers revert to near O-temper strength, so designs should use post-braze properties.
Yes, with magnesium-containing cladding such as 4004 or 4104. Standard 4343 and 4045 clad sheet is intended for flux brazing (CAB).
A thin layer, usually 7072 (Al-1 % Zn), on the coolant side of the tube. It is anodic to the core and corrodes sideways instead of letting pits penetrate the tube wall.
Yes. We supply bare and clad fin stock in narrow slit coils, as well as tube stock and header plate.
Send the layer stack, clad ratio, thickness, width and quantity. Our team will confirm availability and quote.
