Gauge reduction
Fin gauges have fallen steadily to save weight. Thinner fins need cores with higher sag resistance and tighter clad control, or they collapse or erode in the furnace.
Fin stock for brazed heat exchangers
3003 / 4343 H14 and H16 Automotive Brazing Clad Foil
Brazing clad foil is thin fin stock, typically 0.05 – 0.20 mm, made from a 3xxx core with an aluminium-silicon filler layer on one or both faces. Formed into louvred fins, it supplies the braze metal that joins fins to tubes in radiators, condensers, heater cores and battery coolers. Metline supplies 4343 / 3003 / 4343 and 4045-clad foil in H14 and H16 to automotive heat-exchanger makers worldwide.
- Structure
- 4343 / 3003 / 4343 and others
- Tempers
- H14, H16
- Thickness
- 0.05 – 0.20 mm
- Cladding
- about 5 – 20 % per side
What is brazing clad fin foil?
Fins account for most of the air-side heat-transfer area of a vehicle radiator or condenser, and in a brazed core they also supply most of the filler metal. Each fin is clad on both faces, so wherever a fin crest touches a tube the molten clad forms a small fillet in the furnace. A good fin joint is continuous along every crest; a poor one leaves gaps that cut heat transfer.
The core is usually 3003 or a modified 3xxx alloy. Many fin cores add zinc (often in the region of 1 – 2.5 %) to make the fin electrochemically more active than the tube, so the fins corrode sacrificially and protect the tubes against pitting. Others add zirconium or adjust manganese and silicon to raise strength and sag resistance at brazing temperature.
Fin foil is rolled to very tight gauge control, slit to narrow widths and wound onto coils for high-speed fin mills, which stamp louvres and fold the strip in one pass. Any variation in thickness or temper changes fin height, louvre angle and finally the cooling capacity.
Clad and core alloys for fin foil
Common fin structures are 4343 / 3003 / 4343 and 4045 / 3003 / 4045 for CAB brazing, and 4004 or 4104 on 3003 for vacuum brazing. Clad on one side only is used for some plate-type fins.
| Filler (clad) alloy | Silicon, wt % | Other key elements | Melting range (approx.) | Process | AWS class |
|---|---|---|---|---|---|
| 4343 | 6.8 – 8.2 | — | 577 – 613 °C | CAB (flux) | BAlSi-2 |
| 4045 | 9.0 – 11.0 | — | 577 – 591 °C | CAB (flux) | BAlSi-5 |
| 4047 | 11.0 – 13.0 | — | 577 – 582 °C | CAB, flame, induction | BAlSi-4 |
| 4004 | 9.0 – 10.5 | Mg 1.0 – 2.0 | about 559 – 591 °C | Vacuum (fluxless) | BAlSi-7 |
| 4104 | 9.0 – 10.5 | Mg 1.0 – 2.0, Bi 0.02 – 0.20 | about 559 – 591 °C | Vacuum (fluxless) | BAlSi-11 |
| Fin core | Feature | Typical use |
|---|---|---|
| 3003 | Standard formability and strength | General radiators and heater cores |
| 3003 + Zn | Lower corrosion potential; sacrificial fin | Condensers and radiators exposed to road salt |
| High-strength long-life 3xxx | Better sag resistance and post-braze strength | Thin-gauge fins (below about 0.08 mm) |
Mechanical properties of clad fin foil
Typical specified values for 4xxx / 3003 / 4xxx fin foil in the as-rolled tempers. H14 gives better formability for tall fins; H16 gives stiffer foil for thin gauges and fine louvres.
| Temper | Thickness (mm) | Tensile strength Rm | Yield Rp0.2 (min.) | Elongation A50 (min.) |
|---|---|---|---|---|
| H14 | 0.050 – 0.100 | 150 – 210 MPa | 135 MPa | 0.5 % |
| H14 | > 0.100 – 0.200 | 150 – 210 MPa | 135 MPa | 1.0 % |
| H16 | 0.050 – 0.100 | 170 – 230 MPa | 160 MPa | 0.5 % |
| H16 | > 0.100 – 0.200 | 170 – 230 MPa | 160 MPa | 1.0 % |
Brazing anneals the fin. Post-braze strength of a plain 3003 fin is roughly that of 3003-O; long-life cores retain more strength thanks to fine dispersoids that resist recrystallisation.
Cladding and width tolerances
| Cladding ratio per side | Normal tolerance | High-precision tolerance |
|---|---|---|
| 5 – 6 % | ±2.0 points | ±1.0 point |
| > 6 – 8 % | ±2.5 points | ±1.5 points |
| > 8 – 13 % | ±3.0 points | ±2.0 points |
| > 13 – 20 % | ±4.0 points | ±3.0 points |
| Slit width (mm) | Width tolerance |
|---|---|
| 7 – 100 | ±0.1 mm |
| > 100 – 300 | ±0.3 mm |
| > 300 – 1320 | ±1.0 mm |
Thickness tolerance on fin foil is usually agreed as a percentage of gauge, often within a few per cent, because fin height and pitch depend on it. Coils are wound with consistent tension, with no joins or limited, marked joins, to suit fin-mill speeds.
Fin design and brazing considerations
Erosion
Too much clad, too high a peak temperature or too long a time at temperature lets silicon penetrate the core and thin the fin. Match cladding ratio and braze cycle.
Galvanic design
Design the system so that corrosion potentials step down from tube to fin: the fin should be the most anodic part. Zinc in the fin core is the usual tool.
Bare fin alternative
In some designs, bare fins are used with clad tubes or with flux pre-loaded with silicon. This changes which part carries the filler; agree the concept before ordering.
Where brazing clad foil is used
- Engine radiators: louvred fins between flat tubes
- Air-conditioning condensers and evaporators: fins on microchannel tube
- Heater cores: compact cores in the cabin HVAC unit
- Charge-air and oil coolers: turbulators and fins in plate-fin cores
- EV thermal management: battery chillers, inverter and motor coolers, heat-pump exchangers
Brazing fin foil is not used in exhaust or catalytic-converter parts, which run far above the melting range of aluminium filler alloys.
How to order brazing clad foil
- SpecifyStructure (e.g. 4343 / 3003 / 4343), core chemistry (with or without Zn), gauge, cladding ratio, temper, slit width, coil ID/OD and quantity.
- Confirm the standardYour heat-exchanger or OEM specification, including any post-braze strength, sag or corrosion test.
- QuoteWe confirm availability, tolerances, certification and delivery terms (EXW, FOB, CIF or DAP) for your plant or port.
- Process and inspectFoil is slit, checked for gauge, width and cladding ratio, and packed on pallets with moisture protection.
- Document and shipEach lot ships with an EN 10204 3.1 mill test certificate and packing list, in seaworthy export packing by sea or air; third-party inspection on request.
References and standards
- AWS A5.8 – Specification for Filler Metals for Brazing and Braze Welding
- The Aluminum Association – International Alloy Designations and Chemical Composition Limits (Teal Sheets)
- EN 573-3 – Aluminium and aluminium alloys: chemical composition and form of wrought products
- ASTM G85 Annex A3 – Acidified synthetic sea water (SWAAT) fog testing
- ASTM B117 – Standard Practice for Operating Salt Spray (Fog) Apparatus
Brazing clad aluminium (aluminum) foil: FAQ
What does 3003 / 4343 mean?
A 3003 core with a 4343 aluminium-silicon filler layer. Fin foil is normally clad on both faces, written 4343 / 3003 / 4343.
Should I choose H14 or H16 foil?
H14 forms more easily and suits taller fins; H16 is stiffer and suits very thin gauges and fine louvres. Both anneal during brazing.
Why add zinc to the fin core?
Zinc lowers the corrosion potential of the fin, so it corrodes in preference to the tube and protects the tube from perforation.
What is sag resistance?
The ability of a fin to keep its shape while soft at brazing temperature. Poor sag resistance lets fins collapse, which reduces air flow and heat transfer.
What cladding ratio is typical for fin foil?
Commonly about 5 – 15 % per side. Too little gives weak joints; too much increases erosion of the thin core.
Can you supply narrow slit coils for fin mills?
Yes. Foil can be slit from about 7 mm wide with tight width tolerances and shipped worldwide.
Sourcing fin foil for brazed cores?
Send your structure, gauge, width and volume. We will reply with availability, certification and a delivered price.
