Forming in T4
Bend or stretch-form while ductile, then age to T6. The forming window shortens as natural ageing proceeds over the first days after quenching.
Automotive structural extrusions
4B91 T4 / T6 Aluminium Profile for Automotive Structural Parts
4B91 is a grade name that appears on some automotive drawings and supplier lists for heat-treatable structural extrusions supplied in T4 and T6. It is not registered with the Aluminum Association or in EN 573-3, so the governing chemistry must come from the specification named on your drawing. Metline supplies structural automotive profiles to that specification, or in registered alloys such as 6061, 6082 and 6005A, to buyers worldwide.
- Grade
- 4B91 (to drawing specification)
- Tempers
- T4, T6
- Shapes
- Tube, angle, T, U and custom profiles
- Alternatives
- 6061 · 6082 · 6005A
What is 4B91, and how should buyers treat it?
In the Chinese GB numbering system a designation beginning with "4" indicates an alloy in which silicon is the main alloying element, and a letter in the second position marks a national variant rather than an international registration. 4B91 follows that pattern, but we have not been able to confirm it in any public edition of GB/T 3190 or in the published grade lists of the automotive extrusion standard GB/T 33910.
That matters for purchasing. A designation that is not in a public standard can mean an internal company grade, a draft or regional registration, or a transcription error for a nearby grade such as 4B01. Product sheets circulating for 4B91 show a chemistry (silicon up to 0.5 %, magnesium up to 1.2 %) that reads like a 6xxx alloy, not a silicon-led 4xxx alloy, and quote T4 strengths that are higher than any common Al-Mg-Si alloy reaches in T4. We therefore recommend confirming the source document before ordering.
The part itself is usually familiar: a T4 or T6 structural profile for frames, brackets, reinforcements and chassis members. Those requirements are fully covered by registered alloys with decades of automotive use, so a buyer who cannot obtain a reliable 4B91 specification is rarely stuck. The rest of this page explains how to verify the grade, what properties to expect from this class of material, and which registered alloys fit.
How to verify 4B91 chemistry and properties
- Find the defining documentThe drawing or material specification should name the standard that registers the grade. A grade without a document cannot be certified meaningfully.
- Compare chemistry against the alloy familyA "4" designation should show silicon as the largest addition. If the limits show magnesium greater than or comparable to silicon, the material is in effect a 6xxx alloy and should be compared with 6061 or 6082.
- Check the property tableT4 properties must be physically reasonable: for Al-Mg-Si alloys, T4 yield strength is typically well below T6, often around half of it.
- Agree the certificate wordingDecide whether the certificate will state 4B91 against the customer specification, or a registered alloy against ASTM B221, EN 755-2 or GB/T 33910.
- ValidateFor structural parts, run first-article tensile, bend and, where relevant, crush tests before series supply.
What strength to expect from T4 and T6 structural profiles
Since 4B91 values cannot be confirmed from a public standard, the table shows the specified minimums of the registered alloys normally used for the same parts. These are the numbers to compare against whatever 4B91 table your drawing supplies.
| Alloy and temper | Tensile strength (min) | Yield strength 0.2 % (min) | Elongation (min) | Standard |
|---|---|---|---|---|
| 6061-T4 extrusion | 180 MPa | 110 MPa | 16 % | ASTM B221 |
| 6061-T6 extrusion | 260 MPa | 240 MPa | 8 – 10 % | ASTM B221 |
| 6082-T6 profile (wall ≤ 5 mm) | 290 MPa | 250 MPa | 8 % | EN 755-2 |
| 6005A-T6 profile (wall ≤ 5 mm) | 270 MPa | 225 MPa | 8 % | EN 755-2 |
| 6063-T6 profile | 215 MPa | 170 MPa | 8 % | EN 755-2 |
EN 755-2 minimums fall slightly for thicker walls and differ between open and hollow profiles; the certificate states the values tested for your section.
Physical properties of Al-Mg-Si structural alloys (approximate)
- Density
- 2.70 g/cm³
- Modulus of elasticity
- about 69 – 70 GPa
- Coefficient of thermal expansion
- about 23 µm/m·°C
- Thermal conductivity
- about 150 – 200 W/m·K depending on alloy and temper
Profile shapes and sizes
| Shape | Typical sizes (mm) | Wall (mm) | Length |
|---|---|---|---|
| Square tube | 20 × 20, 25 × 25, 30 × 30 and larger | 1.0 – 2.0 for small sizes; thicker for larger | Cut to length |
| Round tube | 10, 12, 14 mm OD and larger | 1.0 – 2.0 | Cut to length |
| Angle | 20 × 20 to 30 × 30 and larger | 1.0 – 2.0 | Cut to length |
| T and U profiles | 20 × 20 × 2, 25 × 12 × 2 and similar | about 2 | Cut to length |
| Custom structural profiles | to drawing | to drawing | to drawing |
Typical supply range for structural profiles
- Temper
- T4 (for forming and later ageing), T6 (aged); T5 on request for registered alloys
- Section types
- Round, square and rectangular tube; angle, channel, T and I sections; flat bar; hollow and multi-cavity custom profiles to drawing
- Size envelope
- Small trim and tube sections up to large structural profiles; the feasible circumscribing circle and wall depend on the die and press, and are confirmed at quotation
- Wall thickness
- From about 1 mm on small sections; thicker walls on structural shapes
- Length
- Standard 6 m mill lengths; cut-to-length parts and longer lengths on request
- Finish
- Mill finish, anodised, anodised and electrophoretic lacquer, powder coated or wet painted
- Processing
- Precision sawing, punching, CNC machining, bending and deburring to drawing
Every custom profile is drawing-specific. Send the section drawing with critical dimensions and tolerances so feasibility can be confirmed.
Tolerances and inspection
Unless the drawing states tighter values, profile dimensions follow the general tolerance tables of the order standard. Automotive programmes often add their own limits on critical features such as mounting faces, wall thickness at crush zones and twist over the part length.
Tolerance and inspection references
- Cross-section tolerances
- EN 755-9 (profiles), ASTM B221 with ANSI H35.2, or GB/T 14846 for orders to Chinese standards
- Straightness and twist
- Per the same tables, or as agreed for long automotive parts
- Surface quality
- Visible faces free of die lines, pick-up and blisters beyond the agreed limit sample
- Mechanical testing
- Tensile test per lot to ISO 6892-1 or ASTM B557; hardness checks on request
- Certification
- EN 10204 3.1 mill test certificate with chemistry and mechanical results
Forming, joining and finishing T4 / T6 profiles
Welding
MIG and TIG with ER4043 or ER5356 filler. Allow for a softened heat-affected zone in the strength calculation.
Riveting and bonding
Self-piercing rivets, flow-drill screws and structural adhesives are common in mixed-material bodies.
Machining
T6 machines cleanly; T4 is gummier. Use sharp carbide tools with coolant.
Surface treatment
Anodising, e-coat, powder coat and wet paint are all suitable; anodised colour depends on silicon and iron content.
Corrosion
Good general corrosion resistance; isolate from steel fasteners in wet areas to avoid galvanic attack.
Typical automotive uses of T4 / T6 structural profiles
- Body structure: door frames, roof bows, pillar reinforcements and front, middle and rear frame members
- Chassis and suspension: subframe members and brackets; forged parts are usually chosen for arms and knuckles
- Powertrain mounts: radiator supports, brackets and lifting lugs
- EV battery packs: tray frames, cross members and module rails
- Interior structure: seat frames, pedal brackets and instrument-panel carriers
How to order
- SpecifyGrade as on the drawing (4B91) plus the defining specification, temper, section drawing, length and annual volume; state whether a registered alternative is acceptable.
- Confirm the standardThe customer specification for 4B91, or GB/T 33910, ASTM B221 or EN 755-2 for a registered alloy.
- QuoteWe confirm availability, tolerances, certification and delivery terms (EXW, FOB, CIF or DAP) for your plant or port.
- Process and inspectProfiles are cut, inspected and tested per lot, then packed with separators to protect surfaces.
- Document and shipEach lot ships with an EN 10204 3.1 mill test certificate, lot traceability and a packing list, in seaworthy export packing. Third-party inspection can be arranged on request.
References and standards
- GB/T 33910-2017 – Aluminium and aluminium alloy extruded profiles for automobiles
- GB/T 3190-2020 – Chemical composition of wrought aluminium and aluminium alloys
- ASTM B221 – Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes
- EN 755-2 – Extruded rod/bar, tube and profiles: mechanical properties
- The Aluminum Association – International Alloy Designations and Chemical Composition Limits (Teal Sheets)
4B91 automotive aluminium (aluminum) profile: frequently asked questions
Is 4B91 a standard aluminium alloy?
We could not confirm it in public editions of GB/T 3190 or GB/T 33910, and it is not an Aluminum Association or EN grade. Ask the drawing owner for the document that defines it.
What can I use instead of 4B91?
For T4 / T6 structural profiles, 6005A, 6061 and 6082 are the usual registered alloys. The choice depends on the strength, extrudability and joining method required, and the designer should approve it.
Could 4B91 be a typo for 4B01?
It is possible. 4B01 is listed in GB/T 33910 for automotive extrusions. Check with the drawing owner before ordering either grade.
What do T4 and T6 mean?
T4 is solution heat-treated and naturally aged, giving a ductile condition for forming. T6 is solution heat-treated and artificially aged, giving near-peak strength.
Can you certify profiles to a customer specification?
Yes. If you provide the specification, we certify chemistry and properties against it on an EN 10204 3.1 certificate.
Do you supply worldwide?
Yes. Metline ships automotive profiles worldwide from Mumbai on EXW, FOB, CIF or DAP terms with export packing.
Have a drawing that calls for 4B91?
Send it with any specification you hold. We will confirm the right material route and reply with certification and a delivered price.
