High-Frequency Welded Aluminium Pipe and Tube

What is HFW aluminium pipe?

An HFW tube begins as a slit coil of aluminium strip. Forming rolls gradually curl the strip into a tube, and a high-frequency current, induced by a coil or fed through sliding contacts, heats the two abutting edges to forging temperature in a fraction of a second. Squeeze rolls then press the edges together and the joint forms in the solid state, with molten or oxidised metal expelled as a small bead that is trimmed off.

Because the heat is concentrated in a very narrow band at the seam, the rest of the strip keeps its original temper and the heat-affected zone is narrow. Peer-reviewed metallographic studies of HF-welded aluminium tube report a sound joint with a deformed zone only a fraction of a millimetre wide. The process runs continuously, so long lengths and high volumes are economical, and the wall thickness is as uniform as the rolled strip it came from.

The most important advantage for thermal engineers is that the strip can be clad before forming. A thin layer of aluminium-silicon filler on the outside lets the tube be brazed to fins and headers in one furnace pass, and a zinc-bearing layer can protect the core sacrificially. Seamless extruded tube cannot carry such a layer in the same way.

HFW tube shapes and where they are used

Size ranges describe what HFW lines commonly produce; final dimensions and tolerances are agreed per drawing. For multi-port extruded tube see microchannel tube.

Single-alloy versus clad strip: choosing the material

Choose a single alloy when the tube will be mechanically joined, welded or used as a simple conduit. Choose clad strip when the tube will be brazed into an assembly or must survive a corrosive environment. The core provides strength; the cladding, usually 5 – 15 % of the thickness on one or both sides, provides the function.

A common mistake is to choose a brazing cladding by habit rather than by furnace. Flux (controlled-atmosphere) brazing uses 4343, 4045 or 4047 claddings; vacuum furnaces need a magnesium-bearing cladding such as 4004 or 4104, because magnesium breaks up the oxide film without flux. Magnesium in the core or cladding, on the other hand, poisons fluoride fluxes, which is why flux-brazed tubes use low-Mg cores.

How HFW aluminium tube is made

1. Strip preparationClad or bare coil is slit to the exact developed width with clean, burr-free edges; edge quality directly controls weld quality.
2. Roll formingA series of forming stands curls the strip progressively into a round or shaped open tube, keeping any cladding on the specified face.
3. High-frequency weldingInduction or contact heating brings the edges to welding temperature and squeeze rolls forge them together. Frequency, power and squeeze pressure are set by wall thickness and line speed.
4. Bead removal and sizingThe external weld bead is scarfed flush (internal bead control on larger tubes) and sizing rolls bring the tube to final shape and dimension.
5. Inline testingEddy-current testing checks the seam continuously; defective sections are marked and rejected.
6. Cutting and finishingFlying cut-off to length, end forming, piercing of slots in headers, cleaning, and packing in protective layers.

Applications by system

Testing welded heat-exchanger tube

The acceptance level for each test should be written on the order. Battery and refrigerant circuits normally call for helium leak testing at a stated leak rate; radiator tubes are usually accepted on eddy-current results and sampled pressure tests. A mill test certificate to EN 10204 3.1 accompanies each lot.

Internal cleanliness is often overlooked. Residual rolling oil, scarfing chips or fine particles inside a tube can contaminate refrigerant circuits or block narrow coolant channels, so specify a cleanliness requirement (for example a maximum residue per square metre of internal surface) together with sealed end caps for battery and refrigerant tubes.

HFW, seamless extruded and folded tube compared

For high-pressure refrigerant circuits, such as CO₂ systems, multi-port extruded tube is usually preferred. For brazed cores at normal coolant pressures, HFW and folded tubes give the lowest cost and weight. See the folded microchannel tube guide and our heat-exchanger tube overview to compare structures.

Common problems and how material choice helps

• Weak or brittle seam: usually poor edge condition, oxide on the strip edges, or wrong heat input and squeeze. Specify slit-edge quality and clean, dry strip; qualify the seam with flattening tests.
• Thin-wall burn-through: walls below about 0.3 mm need lower heat input and very stable strip gauge; order strip with tight thickness tolerance.
• Poor braze fillets: check that the clad ratio and filler alloy suit the furnace; switch from 4343 to 4045 or 4047 when more filler flow is needed.
• Early pitting in salt spray or field service: add a 7072 sacrificial layer on the exposed side, use a long-life core, and design to avoid standing water at joints.
• Tube collapse in brazing: 3003 softens at brazing temperature; move to a stronger 3005 or long-life core for thin tubes under load.

How to order HFW aluminium tube or strip

1. Define the tubeSection shape and dimensions with tolerances, wall thickness, length, and straightness or twist limits.
2. Choose the materialCore alloy and temper, clad alloy, clad side and clad ratio, or a single alloy for non-brazed tube.
3. Set the testsEddy-current acceptance, pressure or burst test, leak-test method and rate, corrosion test if required.
4. PackingCleanliness level, end caps, protective layers and export cases to prevent crushing.
5. ShipEN 10204 3.1 certificate with each lot; delivery worldwide on EXW, FOB, CIF or DAP terms.

Metline can also supply the clad brazing strip itself to tube makers who run their own HFW mills.

Related tubes, strip and thermal-management guides

HFW aluminium (aluminum) pipe: frequently asked questions