Aluminium flanges join aluminium pipes, valves, pumps, ducts and structures where low weight, corrosion resistance or cryogenic toughness matter more than the high pressure ratings of steel. Most are forged or machined from 6061-T6, with 5083 and 5086 for marine and cryogenic service. This guide covers flange types, alloys, standards, face finishes, design precautions and what to include in an enquiry.
A flange is a ring or disc bolted to a matching flange, with a gasket between them, to make a joint that can be taken apart. In aluminium systems flanges connect pipe spools to valves, pumps, tanks and instruments, close off nozzles, and join sections of ductwork and structures. Aluminium flanges weigh about a third as much as steel flanges of the same size, do not rust, and keep their toughness down to cryogenic temperatures.
There are three broad groups. Pipe flanges follow pipe-flange standards for bore, bolt circle and drilling, and carry a pressure rating. Duct flanges are lighter profiles and rings that join ventilation, exhaust and clean-room ducting at low pressure. Base or structural flanges anchor posts, railings, masts and frames to floors and walls, where the load is mechanical rather than internal pressure.
Aluminium is chosen for flanges for a specific reason in each industry: corrosion resistance and weight on ships and offshore platforms, freedom from contamination in food, pharmaceutical and semiconductor gas lines, toughness without a ductile-to-brittle transition in LNG and air-separation plants, and compatibility with the rest of an all-aluminium pipe system so that no galvanic couple is created. Mixing a steel flange into an aluminium line, by contrast, sets up exactly that couple in wet service.
| Type | How it attaches | Typical use in aluminium systems |
|---|---|---|
| Weld neck | Long tapered hub butt-welded to the pipe | Higher-pressure, cyclic or cryogenic lines; best stress distribution and full-penetration weld that can be radiographed |
| Slip-on | Slides over the pipe and is fillet-welded inside and outside | Low-pressure water, air and oil lines; easy alignment and lower cost |
| Lap joint (loose) | Free-rotating ring behind a welded stub end or flared pipe end | Lines dismantled often; the ring can be aluminium or a cheaper coated steel because it never touches the fluid |
| Blind | Solid disc, no bore | Closing pipe ends, nozzles and vessel openings; pressure testing |
| Socket weld | Pipe inserted into a socket and fillet-welded | Small-bore lines, usually NPS 2 and below |
| Threaded | Internal pipe thread, no welding | Small, low-pressure lines and where welding is not allowed; aluminium threads gall easily, so use thread compound |
| Plate flange | Flat plate cut from sheet or plate, welded to the pipe | Low-pressure services and custom drillings; often machined from 5083 or 6061 plate |
| Special flanges | Reducing, spectacle blind, orifice, long weld neck, swivel ring | Made to drawing for specific process needs |
In aluminium piping, lap-joint (loose) flanges are particularly popular. The welded stub end is in the same alloy as the pipe, so the weld is simple and corrosion-matched, while the loose backing ring can be rotated to line up bolt holes. Because the ring carries only bolt load, it can be made thicker or from a stronger temper without affecting the welded joint.
| Alloy and temper | Typical product form | Strengths | Watch-outs |
|---|---|---|---|
| 6061-T6 / T651 | Forgings to ASTM B247, machined plate or bar | Most common flange alloy: good strength, machinability and corrosion resistance; anodises well | Heat-affected zone softens when welded; design to as-welded strength |
| 6082-T6 | Forgings and plate (European practice) | European counterpart of 6061 with similar or slightly higher strength | Same weld softening as 6061 |
| 5083-O / H111 / H112 | Plate, forgings, rolled rings | Excellent seawater resistance and cryogenic toughness; welds without large strength loss | Lower yield strength; not for service above about 65 °C |
| 5086 | Plate and forgings | Marine piping and seawater systems | Same temperature limit as 5083 |
| 3003 / 1100 | Plate, forgings, spun or pressed rings | Excellent chemical and food compatibility, very formable | Low strength: low-pressure and duct work only |
| 7075-T6 / T73 | Machined bar or forgings | Very high strength for bolted structural and aerospace flanges | Not fusion weldable and less corrosion resistant; not used for process pipe |
Minimum values depend on product form, size and temper; the mill certificate for each lot gives the actual results.
As a rule, choose 6061-T6 forgings for general process and utility piping, 5083 or 5086 for anything in or near seawater and for cryogenic duty, and 3003 or 1100 where the fluid is a food product, acid or high-purity gas at low pressure. If the flange will be welded to a 6061 pipe, remember that the weld zone will end up close to T4 strength regardless of the flange temper.
Flange standards cover two separate things: the dimensions (bore, outside diameter, thickness, bolt circle, number and size of holes) and the material with its pressure-temperature rating. For aluminium, the two often come from different documents.
| Standard | What it covers |
|---|---|
| EN 1092-4 | European standard specifically for aluminium-alloy circular flanges, PN-designated, DN 15 to DN 600 and PN 10 to PN 63, including types, facings, dimensions, materials and pressure-temperature ratings |
| ASME B16.5 | Dimensions for NPS 1/2 to NPS 24 flanges in Classes 150 – 2500. Its material tables list steels and nickel alloys, not aluminium, so aluminium flanges made to B16.5 dimensions need their rating set by the piping designer |
| ASME B16.47 | Large-diameter flanges NPS 26 to NPS 60 (Series A and B), used as a dimensional basis for big aluminium flanges |
| ASME B31.3 | Process piping code: gives allowable stresses for aluminium alloys used to qualify flange designs |
| ASTM B247 | Aluminium-alloy die forgings, hand forgings and rolled ring forgings: the usual material specification for forged flanges |
| ASTM B209 / EN 485 | Aluminium plate, used for plate flanges and blind flanges machined from plate |
| JIS B2220 | Japanese pipe-flange dimensions (5K, 10K, 16K, 20K) |
| EN 10204 3.1 | Inspection certificate with chemistry and mechanical properties of the flange material |
An older ANSI standard for non-ferrous pipe flanges (B16.31), which included aluminium, has been withdrawn. Projects that quote it should agree a current equivalent, usually B16.5 dimensions with ratings from B31.3 or EN 1092-4.
A billet is upset and pierced, then die-forged or ring-rolled before heat treatment and machining. Forging closes porosity and aligns the grain around the ring, giving the best strength, fatigue and leak-tightness. Preferred for weld necks and any pressure service.
Flanges cut by waterjet or saw from plate, or turned from round bar, then CNC-machined and drilled. Fast for prototypes, small batches and odd drillings; plate flanges suit low and medium pressure.
Plate or bar is rolled into a ring and the seam welded, then the ring is machined. This is the practical route for very large duct, tank and structural flanges several metres across.
Sand or gravity-cast flanges allow complex shapes at low cost, but have lower strength and ductility. Suited to ducting, low-pressure drainage and structural bases, not to pressure piping.
Duct flange frames are made from extruded aluminium sections cut and joined with corner pieces, giving a rigid, sealed connection for rectangular ducting.
Thin rings and collars formed from 1100 or 3003 sheet for light ducting and vessel connections.
The whole face is flat and a full-face gasket covers it out to the outside diameter. Recommended for aluminium mating with aluminium, cast or plastic flanges, because it avoids bending the flange when bolts are tightened.
A raised ring around the bore concentrates gasket stress. Common on forged 6061 flanges; mate RF with RF only.
Interlocking faces that locate and confine the gasket, useful in vacuum, toxic or high-purity service.
Groove for a metal ring gasket. Rare in aluminium because the soft groove deforms; used only with matching soft ring gaskets and designer approval.
Spiral (phonographic) or concentric grooves on the face grip spiral-wound and sheet gaskets. A typical serrated finish is around 3.2 – 6.3 µm Ra.
Machined smooth for metal or thin gaskets and for O-ring grooves in vacuum and gas systems; flanges can also be anodised for wear and corrosion protection.
Following these precautions, aluminium flanged joints are reliable for decades in marine piping, LNG plant and food processing. Most service problems trace back to one of three causes: steel fasteners without isolation, steel-class pressure ratings applied to aluminium, or 5xxx alloys used in hot service.
Duct flanges. Rectangular ducts are joined with flange frames made from extruded or roll-formed aluminium profiles, assembled with corner pieces, sealed with a gasket strip and clamped or bolted. Round and spiral ducts use angle rings or companion flanges. Aluminium is used where the air is humid or corrosive, where weight matters (ships, rail vehicles, offshore modules), and in clean rooms and kitchens where rust-free surfaces are needed. Metline supplies the aluminium profiles, angle and sheet that duct fabricators turn into these systems.
Base flanges. Square, rectangular and round base plates anchor railing posts, fence posts, handrails, signs and light frames to concrete or steel. They are machined from plate, cast, or cut from thick extrusions, often in 6061 or 6063, and are frequently powder-coated or anodised to match the posts. For railing and fence work, Metline supplies the aluminium base-flange material, post tube and profiles to fabricators and system companies, not finished installed systems.
Duct and base flanges are usually made to the fabricator's own drawing rather than a pipe standard. The order then needs the profile or plate thickness, hole pattern, finish and quantity.
Flanges are machined from forged or cut discs and rings. The table gives the weight of solid aluminium disc blanks at three thicknesses, calculated at 2.70 g/cm³, with the weight of the same 30 mm disc in carbon steel for comparison. Subtract the bore and bolt holes to estimate a finished flange.
| Disc diameter (mm) | 12 mm thick (kg) | 20 mm thick (kg) | 30 mm thick (kg) | 30 mm in carbon steel (kg) |
|---|---|---|---|---|
| 150 | 0.6 | 1.0 | 1.4 | 4.2 |
| 200 | 1.0 | 1.7 | 2.5 | 7.4 |
| 250 | 1.6 | 2.7 | 4.0 | 11.6 |
| 300 | 2.3 | 3.8 | 5.7 | 16.6 |
| 400 | 4.1 | 6.8 | 10.2 | 29.6 |
| 500 | 6.4 | 10.6 | 15.9 | 46.2 |
| 600 | 9.2 | 15.3 | 22.9 | 66.6 |
The comparison shows why aluminium flanges are popular on ships, aircraft ground equipment and portable systems: a flange of the same dimensions weighs about 34 % of its steel counterpart. Even when the aluminium flange is made thicker to make up for lower strength, the total joint weight is usually less than half that of steel.
6061-T6, usually forged to ASTM B247 or machined from 6061-T651 plate or bar. It combines good strength, machinability and corrosion resistance. 5083 and 5086 are used for seawater and cryogenic service, and 3003 or 1100 for low-pressure food and chemical lines.
They can be made to ASME B16.5 dimensions and drilling so they bolt to standard flanges, but B16.5 does not list aluminium in its pressure-temperature tables. The rating must be set by the piping designer, for example from ASME B31.3 allowable stresses, or the flange can be specified to EN 1092-4, which is written for aluminium.
Yes, but isolate the joint. Use an insulating gasket kit with sleeves and washers so the metals are not in electrical contact, and stainless or coated bolts. Without isolation, the aluminium corrodes galvanically in wet or marine conditions.
Yes. Aluminium has no ductile-to-brittle transition and actually gains strength as temperature falls, so 5083 and 6061 flanges are widely used in LNG, liquid nitrogen and oxygen systems.
A forged flange is shaped from a hot-worked billet, giving dense metal with grain flow around the ring and the best strength and leak-tightness. A plate flange is cut and machined from rolled plate; it is quicker and cheaper for low-pressure duty and custom drillings, but is not preferred for high-pressure or cyclic service.
Flat face with a full-face gasket is safest when mating aluminium to aluminium, cast or plastic flanges, because it avoids bending the flange under bolt load. Raised face is normal on forged weld-neck and slip-on flanges paired with raised-face flanges.
Yes. Metline supplies aluminium flanges, flange blanks, forged rings and the related pipe and plate worldwide, with EN 10204 3.1 certificates and export packing.
Send the flange type, standard, size, class, alloy and quantity, or a drawing. We will reply with material options, certification and a delivered price.
