Why Aluminium Foil Dominates Radiator Construction
Radiators and condensers are built from thin aluminium stock because the metal combines high thermal conductivity with low density. Alloy 3003 conducts heat at approximately 193 W/m.K, roughly half that of copper by volume but at about one third of the density, of 2.73 g per cubic centimetre, so a brazed aluminium core delivers the same heat rejection as a copper-brass core at substantially lower mass and cost. Aluminium foil also lends itself to the louvred and lanced fin geometries that break up the air-side boundary layer, and it is compatible with the brazing processes used for high-volume production.
Fin, Tube and Clad Alloys
| Function | Alloy | Key characteristics |
|---|---|---|
| Fin and tube core | 3003 | Mn 1.0-1.5; moderate strength, good formability, brazable |
| Brazing clad layer | 4343 | Si 6.8-8.2; melts below the core and forms the joint |
| Alternative clad layer | 4045 | Si 9.0-11.0; lower melting point, used for thin clad ratios |
| Sacrificial fin layer | 7072-type clad | Zn-bearing surface layer, corrodes preferentially |
Manganese in 3003 raises strength and resistance to thermal fatigue; the silicon-rich clad layer on brazing sheet is formulated so that it melts before the core during the brazing cycle, wets the joint by capillary action and forms a fillet without collapsing the fin. Sheet and foil dimensions and mechanical properties for the core alloy are ordered to ASTM B209 and, in metric markets, to EN 573-3 and EN 485-2 for composition and properties respectively.
Gauge Selection and Heat Transfer
Fin stock is normally 0.08-0.12 mm thick. Reducing gauge lowers the thermal resistance through the fin and reduces material cost, but thin fins are more easily distorted by vibration, by cleaning jets and by handling during core assembly. Heavier sections, from about 0.15 mm to 0.2 mm, are selected for tubes, plates and headers, and for applications exposed to pressure cycling or vibration. The optimum thickness depends on fin density, coolant flow and air velocity, and is normally fixed by thermal simulation validated on a test core rather than by rule of thumb.
Brazing Routes
Two processes dominate. Vacuum brazing relies on magnesium in the clad or on a magnesium-bearing core surface to disrupt the oxide film, and needs a controlled vacuum atmosphere and careful flux-free furnace practice. Controlled atmosphere brazing uses a non-corrosive potassium fluoroaluminate flux applied to the assembled core and a nitrogen atmosphere with a controlled dew point; it is more tolerant of alloy variation and is the more common route for high-volume radiator production. Both routes demand clean, oil-free foil: residual rolling lubricant and excessive oxide both produce skip and non-wetting along the fin-tube joint.
Coatings for Air-Side and Coolant-Side Durability
Hydrophilic coatings are applied to the fin surface so that condensed water spreads into a film rather than forming droplets that bridge fin gaps and block airflow. Anti-corrosion coatings, including chromate-free conversion layers and silica or resin-based systems, protect the air side against road salt and the coolant side against the mildly alkaline glycol environment. Coating adhesion and corrosion performance are assessed by cyclic humidity and salt-spray testing, for example to ASTM B117 and to ISO 9227, so that the coated core meets the durability requirement of the vehicle platform.
Recycling
Brazed aluminium radiators are fully recyclable. Scrap cores are shredded to separate steel and plastic, and the aluminium is melted and refined back to foundry or wrought ingots; remelting aluminium requires only a small fraction of the energy needed to produce primary metal from ore, with savings commonly quoted at up to 95 percent. In practice the recycling rate depends on collection and on the efficiency of separation from the copper, brass and plastic content of the dismantled vehicle.
Frequently Asked Questions
Q: Which alloy is used for radiator fins?
A: Alloy 3003 is the standard fin and tube core alloy, with manganese in the range of 1.0-1.5 percent for strength and thermal fatigue resistance, and 4343 or 4045 as the brazing clad layer.
Q: What fin thickness is typical?
A: Most fin stock is 0.08-0.12 mm. Sections of about 0.15-0.2 mm are used where greater stiffness or vibration resistance is needed, such as tubes, plates and headers.
Q: Why are hydrophilic coatings used?
A: They stop condensate from forming droplets that bridge adjacent fins and block the air path, keeping air-side heat transfer and pressure drop stable in humid operation.
Q: What causes non-wetting during brazing?
A: Residual rolling oil, excessive aluminium oxide or an incorrect clad ratio. Foil for brazing is supplied clean and is usually degreased immediately before core assembly.
Q: Can end-of-life radiator foil be recycled?
A: Yes. Shredded and separated radiator aluminium is remelted into new ingots, and remelting consumes a small fraction of the energy required for primary aluminium production.
Q: How is corrosion performance verified?
A: By salt-spray and cyclic corrosion tests such as ASTM B117 and ISO 9227, together with long-term electrochemical testing of the fin and clad layer combinations used in the core.