Custom Aluminum Round Tube & Pipe — Seamless, Welded, DOM. Structural, Fluid, Heat Exchanger — Specified to Project

A round tube is a round tube — until you have to specify it. Is it a tube or a pipe? Seamless, welded or DOM? 6061-T6 or 7075-T6? T6 or T9? Will it bend, will it carry pressure, will it be welded, will it sit in salt water? Each answer changes the alloy, the temper, the manufacturing route, the price, and the lead time. Get one wrong and the tube fails in service or the project overruns by weeks.

Linkedalu Metal Group Co., Ltd extrudes custom aluminum round tube and pipe for structural framing, fluid and gas transport, heat exchangers, handrails, marine hardware and machinery. Sections are produced as seamless extruded tube, welded (ERW) tube or drawn-over-mandrel (DOM) tube, with mill test certificates against ASTM B210, B221, ASTM B241, EN 754 and EN 10204 3.1. We work with 6061-T6, 6063-T5, 7075-T6, 5052-O and 3003-O depending on load, fluid pressure, fatigue and corrosion, and we ship to 50+ countries across Europe, the Americas, the Middle East, Africa, Asia and Oceania.

Tube vs pipe — the most common RFQ mistake

“Tube” and “pipe” both describe a hollow cylinder, but they don’t describe the same product. Confusing them costs a project weeks of fittings that don’t seat and calculations that don’t close.

The practical consequence: a tube order quotes against your OD; a pipe order quotes against the fitting it has to mate with. If you send an RFQ for “2-inch aluminum tube” expecting it to seat on a 2-inch pipe flange, you’ll get a part with a 2.000″ OD that won’t fit the 2.000″ NPS flange — because NPS 2″ is actually 2.375″ OD. Send the standard with the RFQ (ASTM B221 for tube, ASTM B241 for pipe) and the conversation gets short fast.

How aluminum round tube is made — and what that changes for you

Three manufacturing routes exist for aluminum round tube. Each gives a different product, and each has a use case it dominates.

Seamless extruded tube (highest pressure, cleanest bore).
A solid aluminum billet is heated and pushed through a die with a floating or fixed mandrel suspended inside the die opening. The aluminum flows around the mandrel and welds to itself as it exits, leaving no longitudinal seam. Result: a tube with uniform grain structure around the circumference, excellent concentricity, and the highest pressure capability. Specified for hydraulic lines, high-pressure pneumatic, heat-exchanger tubes and critical structural components.

Welded (ERW) tube (cost-effective for large OD).
A flat aluminum strip is roll-formed into a cylinder and welded along the longitudinal seam by electric resistance. The weld bead is usually scraped off (scarfed) inside and out for a smooth surface. Result: a cheaper tube, especially in larger diameters (above ~100 mm OD), but with a weld seam that is metallurgically distinct from the parent metal. Specified for handrails, furniture, structural tube, low-pressure fluid lines — anywhere bending is moderate and pressure is low.

Drawn-over-mandrel (DOM) tube (precision + strength).
DOM starts as welded tube, then is cold-drawn over a mandrel through a die. The drawing operation work-hardens the tube, refines the grain, and tightens dimensions substantially. Result: precision ID and OD, smoother surface, higher mechanical properties than the as-welded tube. Specified for hydraulic cylinders, pneumatic cylinders, precision shafts, and any application needing tight concentricity at moderate to high pressures.

Which one to specify — and where bending matters.
Bending is the moment of truth for welded tube. The weld seam is metallurgically distinct and, when bent, tends to crack or thin on the outside of the bend — especially at tight radii. If the tube will be bent, flared, or swaged, specify seamless or DOM. If you must use ERW, keep the weld seam on the neutral axis of the bend (the inner edge of the cross-section neither stretches nor compresses) to minimize crack risk. We confirm this at drawing review — it’s free, and it saves a lot of scrap on the shop floor.

Alloy selection for round tube — broader than square or angle

Round tube sees more alloys than any other profile because it’s used in more categories: structure, fluid, marine, fatigue, pressure. The wrong alloy isn’t a paperwork problem; it’s a service failure.

6061-T6 — structural workhorse.
Yield ≥ 240 MPa, tensile ≥ 260 MPa. Welds well. Machines well. Holds a load. The default for structural framing, handrails, machinery frames, transport equipment, roll cages. Limitation: HAZ in welds drops to ~60–70 % of T6 strength, so welded assemblies need design allowance or post-weld heat treatment for critical service.

6063-T5 — visible and fluid transport.
Yield ≥ 110 MPa, tensile ≥ 160 MPa. Extrudes cleanly, takes the brightest anodized finish. Lower strength, but better surface. Default for visible architectural tube (handrails where anodizing matters), irrigation, low-pressure fluid lines, HVAC condensate lines, decorative trim.

7075-T6 — high strength at the cost of corrosion.
Yield ~ 503 MPa, tensile ~ 572 MPa. Strongest aluminum alloy in common commercial use. Used where specific strength rules — motorsport, aerospace, military, competitive cycling. Limitation: poor corrosion resistance and not weldable in the conventional sense. If your tube lives near salt water or has to be welded into an assembly, 7075 isn’t the right answer. For one-off high-stress components it’s unmatched.

5052 / 5083 — marine and chemical.
Non-heat-treatable. Excellent corrosion resistance in salt water and many chemicals. 5083 is the workhorse for hull plating and boat structural tube; 5052 for formed sheet and lighter tube. Both weld cleanly. Yield ~ 215 MPa (5083-H116), lower than 6061-T6, but they survive where 6061 won’t.

3003-O — formed fluid lines and HVAC.
The classic drawn aluminum tubing for refrigeration, AC and low-pressure fluid transport. Low strength (yield ~ 35 MPa in O temper) but excellent formability and corrosion resistance. Brazed into headers, bent into coils, drawn into thin-wall tube. If your tube carries refrigerant, runs through a chiller, or lives inside an HVAC cabinet, 3003 is the answer.

6061-T9 — the underrated high-fatigue version.
6061-T9 starts as T6, then gets cold-worked (drawn or rolled) after aging. The cold work raises fatigue strength and stress tolerance significantly — at the cost of ductility. For tube in cyclic-load environments (suspension components, vibrating machinery, robotic arms, off-road chassis), 6061-T9 outlasts T6 by a wide margin. The trade-off: it’s harder to bend post-cold-working, so bend first and age to T9 if you can.

Pressure, fatigue and bending — the three engineering checks

Pressure: P = 2 × S × t / D, with safety factor.
For a thin-wall tube under internal pressure, the hoop stress formula is:

P = (2 × S × t) / D

• Where:
  • P= Working/Burst Pressure (MPa or PSI)
  • S= Allowable Design Stress (Yield Strength / Safety Factor)
  • t= Nominal Wall Thickness (mm})
  • D= Outside Diameter (mm)

Example: 6061-T6 tube at 50 mm OD, 3 mm wall, yield 240 MPa, safety factor 4 → allowable P ≈ (2 × 240/4 × 3) / 50 = 7.2 MPa working pressure. Burst pressure (at yield) ≈ 28.8 MPa. For pressure piping, also check that the tube meets ASTM B241 (which covers alloy and pressure-rated delivery), not just B221.

Fatigue: T6 vs T9, when cyclic loading matters.
Static load + 6061-T6 = fine. Cyclic load (suspension, vibrating screen, robotic arm) = consider 6061-T9, or step the section to increase wall thickness and reduce stress amplitude. The fatigue limit of T6 in the HAZ after welding is roughly 60 % of the parent-metal value; post-weld aging can recover some, but not all.

Bending: minimum radius, mandrel, and which alloys crack.
A common rule: minimum bend radius ≈ 2 × OD for cold bending on standard rotary draw benders. Tight radii (down to ½ × OD) require mandrel bending to prevent the wall from collapsing or wrinkling on the inside of the bend. Alloy and temper matter:

• O temper (annealed)​ — best formability, bends to ~ 1 × OD; can be aged to T6 after forming.
• T4 temper — moderate formability, bends to ~ 2 × OD.
• T6 temper — limited formability, cracks at tight radii; don’t try to bend T6 below 3 × OD without mandrel and likely pre-heating.

Where aluminum round tube shows up in real projects

Structural framing, trusses and bracing.
6061-T6 round tube is the default for lightweight trusses, exhibition structures, machine guards and equipment bracing. Round beats square for torsion resistance; thin for a given cross-section, a round tube resists torsional buckling better than a square tube of equal OD/width. The trade-off is that round tubes don’t stack flat.

Handrails, balustrades and safety barriers.
6063-T5 (visible, anodized) is the default for architectural handrails and balustrades. 6061-T6 for industrial barriers and safety rails. Both hold a bend through standard rotary draw equipment. We supply tube pre-cut to length, pre-bent to your radii, with drilled or punched mounting holes.

Marine hardware — boat rails, ladders, dock frames.
6061-T6 for structural boat tubing (T-top frames, leaning posts, T-tops). 5083 or 5052 for hull-adjacent structural members (better corrosion under salt-water exposure). Tube joins welded with 5356 filler, then anodized for finish. We supply cut and bent to drawing with MTCs that pass marine surveyors.

Heat exchangers and condenser coils.
3003-O and 6063-O are the standards. Thin-wall tube (0.5–1.5 mm), small OD (6–25 mm), high length per coil. Tight concentricity matters for header fitting. We supply drawn-over-mandrel tube in long coils or straight lengths to heat-exchanger OEMs.

Hydraulic, pneumatic and instrument lines.
DOM 6061-T6 is the workhorse for hydraulic and pneumatic cylinder tubing. Tight ID tolerance, high surface finish, predictable pressure rating. We supply cut-to-length, deburred, washed and capped for clean assembly.

Roll cages, chassis and off-road frames.
6061-T6 (weldable) for tube-frame chassis and roll cages; 6061-T9 for high-fatigue members; 7075-T6 only for one-off, non-welded, non-marine components. We supply bent, notched, fish-mouthed and labeled to your chassis drawings.

Manufacturing capability, tolerances and standards

Standard tolerances per ASTM B221 / EN 754-7.

Concentricity and roundness — what extrusion actually holds.
Concentricity (wall thickness variation around the circumference) is where tube and pipe differ most from angle or flat bar. A seamless extruded tube holds ~ ±5 % wall variation around the circumference; a DOM tube can be held to ±2 %. For a tube that has to press-fit into a header or mate with a precision boss, specify DOM and call out the concentricity you need; don’t assume standard tolerance is tight enough.

Surface finishes and welding.

• Mill finish — standard supply.
• Anodized — Class AA10 / AA15 / AA20 for visible handrails and architectural tube.
• Powder coated — architectural-grade, ≥ 60 μm, Qualicoat or GSB certified.
• PVDF (fluorocarbon) — for high-UV or coastal environments.
• Welding — 6061-T6 welds cleanly with TIG/MIG (4043/5356 filler); HAZ drops to ~60–70 % of T6 strength, design accordingly. 7075-T6 is not recommended for welded assemblies.

How a round tube order moves through the mill

1. You send the drawing — OD × wall, alloy, temper, manufacturing method (seamless / ERW / DOM), finish, fabrication list, bend radii if any.
2. We review extrudability, check OD / wall / concentricity targets, and confirm alloy and method against the application.
3. New die or reused die; locked to a single billet batch (seamless route) or coil lot (DOM route).
4. Extrusion → straightening → cut-to-length → CNC bending (if ordered) → deburring / end caps → finish → labeling.
5. MTC (EN 10204 3.1) issued to ASTM B210 / B221 / B241, EN 754 with measured mechanical properties, chemical composition and dimensional records.
6. Packed in bundles, end-capped, on pallets or in crates. Shipped to 50+ countries with CIQ, C/O, Form A, CE / RoHS / REACH where destination requires.

Frequently Asked Questions

Q1: Is an aluminum tube and an aluminum pipe the same thing?​
No, and confusing them costs projects fittings that won’t seat. Aluminum tube is specified by exact outside diameter (OD) and wall thickness, per ASTM B210 / B221 / B345, and is used for structural, heat-exchanger, furniture and precision frames. Aluminum pipe is specified by nominal pipe size (NPS) and Schedule (wall class), per ASTM B241 / B429, and is used for fluid and gas transport. The practical trap: NPS 2″ pipe is actually 2.375″ OD — a 2″ tube won’t fit a 2″ pipe flange. Send the standard with your RFQ (B221 for tube, B241 for pipe) and the conversation gets short fast.

Q2: Seamless, welded or DOM tube — which should I specify?​
Depends on what the tube has to do. Seamless has the highest pressure rating and cleanest bore — use for hydraulic lines, high-pressure pneumatic, critical structural tube. Welded (ERW) is the cheapest, especially above ~100 mm OD — use for handrails, furniture, structural tube, low-pressure fluid lines. DOM (drawn-over-mandrel) gives the tightest dimensional tolerance and the highest mechanical properties — use for hydraulic cylinders, pneumatic cylinders, precision shafts. The single most important rule: if the tube will be bent, flared or swaged, specify seamless or DOM. ERW weld seams crack on tight bends — keep the seam on the neutral axis of the bend if you must use welded tube.

Q3: Can 6061-T6 aluminum round tube be bent without cracking?​
It depends on the bend radius and the wall. The general rule: minimum cold bend radius is2 × OD for standard rotary draw bending. Tighter than that needs mandrel bending to stop the inside wall from wrinkling or collapsing. T6 temper has limited formability; bending T6 below 3 × OD is risky — it can crack on the outside of the bend. For tight radii, use O temper (annealed) and age to T6 after forming, or specify T4 for moderate bends. T9 is even harder to bend because of the cold work, so bend first then age.

Q4: What’s the burst pressure rating for 6061-T6 round tube?​
Use the thin-wall hoop stress formula P = (2 × S × t) / D, where S is allowable stress (yield ÷ safety factor), t is wall thickness, D is outside diameter. Worked example: 6061-T6 tube at 50 mm OD × 3 mm wall, yield 240 MPa, safety factor 4 → working pressure ≈ 7.2 MPa, burst (at yield) ≈ 28.8 MPa. For pressure-piping service, also verify the tube is delivered against ASTM B241 (pressure-rated) rather than just B221 (extruded). Specify Safety factor 3–4 for cyclic or uncertain service, factor 2 only for static, well-understood service.

Q5: When do I need 6061-T9 instead of 6061-T6?​
When the tube lives in a cyclic load environment. 6061-T9 is T6 that has been cold-worked after aging — the cold work raises fatigue strength substantially at the cost of ductility. For suspension components, vibrating machinery, robotic arms, off-road chassis, and other high-cycle applications, T9 outlasts T6 by a wide margin. Two caveats: T9 is harder to bend (bend first, age to T9 if you can); and like T6 the weld HAZ loses strength, so welded T9 assemblies need careful joint design.

Q6: What tolerances can I expect on aluminum round tube, and what documents ship with the order?​
Per ASTM B221 / EN 754-7: OD ± 0.10–0.25 mm depending on size; wall thickness ± 10 % of nominal; roundness ≤ 1 % of OD; straightness ≤ 0.8 mm per 1000 mm; standard cut ± 1–5 mm, precision cut ± 0.1–0.5 mm. Concentricity (wall variation around the circumference) is where tube is most distinct from angle — seamless holds ~ ±5 % wall variation, DOM holds ~ ±2 %. Specify the concentricity you need; don’t assume standard tolerance is tight enough for press-fit applications. Documents shipped include a Mill Test Certificate (EN 10204 3.1) with measured mechanical properties, chemical composition and dimensional records, issued to ASTM B210 / B221 / B241 or EN 754 as the project requires. We currently ship to 50+ countries across six continents. ISO 9001 and ISO 14001 certified.