T4, T5 and T6 on a Machined Aluminium Drawing: What the Temper Actually Commits the Supplier To

Temper is a separate line item, not part of the alloy name

A drawing that says "6061" has not specified the part. Treatments such as T6, T651 and T7351 significantly alter the mechanical properties of the chosen base alloy, and the temper condition changes strength, corrosion, flatness and machining behaviour²³²⁴.

That is why RFQ guidance lists temper as its own field, alongside alloy, stock form and material standard: the standard is what prevents substitution and certificate disputes later²⁴.

The same guidance treats stock form as a separate field because plate, bar, extrusion, forging and cast tooling plate can behave differently even when their nominal alloy family looks similar²⁴. So the temper callout and the stock form it is bought in are two decisions, not one.

Less common alloys and sizes may need special ordering, which adds time before machining even starts²⁶. Practically, the buyer should price temper as its own line: state it on the drawing, state it in the RFQ, and expect the material certificate to name it.

Is the T6 callout justified, or is T5 enough?

T6 means the aluminium has been solution heat-treated and then artificially aged, a two-step thermal process that produces the highest standard strength condition for heat-treatable alloys and is the most commonly specified temper for structural work²⁷. T5 is described as less expensive and sufficient for many applications, particularly architectural extrusions²⁷.

Neither is universally better; the source frames the choice as depending on the structural requirements of the specific project²⁷. What the sources do not give is the size of the gap.

For 6061-T6 they report roughly 276 MPa yield and 310 MPa tensile, and for 7075-T6 roughly 503 MPa yield and 572 MPa tensile¹⁵¹⁶¹⁷. No source in this set reports T4 or T5 values for either alloy, so the strength difference between T4, T5 and T6 for one named alloy cannot be established here¹⁵¹⁶¹⁷.

One trap is worth naming: T tempers apply to heat-treatable alloys (2000, 6000 and 7000 series), while H tempers apply to non-heat-treatable alloys such as 5052²⁷. A T6 callout on a 5052 part is not a tight specification, it is an incorrect one²⁷.

Plate that will be precision machined: T6 versus T651

T6 and T651 are both solution heat-treated and artificially aged. T651 adds a controlled stretching step, a 1 to 3% permanent set, applied after quenching and before aging²⁷.

That stretch relieves residual stresses and improves dimensional stability, which the source states is important for plate stock that will be precision machined ²⁷. This is the clearest temper decision the evidence supports.

If the part is cut from plate and dimensional stability matters, the condition described as relieving residual stress is T651, not plain T6²⁷. The 1 to 3% figure is a vendor-tier source's description of the stretch, not a standard, so treat it as the mechanism rather than a tolerance to design to²⁷.

Before the quote is fixed, the buyer should confirm which condition the plate will actually be bought in, because the certificate will name it and the machining sequence depends on it.

Temper changes cutting behaviour, so the process route must be stated

Heat treatment is not only a property change. It alters microstructure, hardness and residual stress, which changes cutting forces, tool wear and finishing behaviour³.

The same source notes that after annealing a material may cut easily but smear or drag during finishing, because internal stresses were redistributed³. Higher hardness increases cutting forces and accelerates tool wear, while annealing reduces both³.

Distortion risk sits in the same chain. Internal stress in the stock, distortion after roughing and cutting heat are listed as material factors that affect achievable tolerance, and thin aluminium parts can distort if too much material is removed unevenly²⁰.

Chip behaviour, surface finish quality, tool wear patterns and stress-relief requirements are reported to vary significantly between 7075, 7050 and 6061 on the same machines²².

A stable route may be rough machine, then stress relieve, then finish machine, or machine soft, then heat treat, then grind critical surfaces; the supplier should explain the route before quoting capability²⁹.

No source in this set quantifies distortion or residual stress for T4, T5 or T6 aluminium specifically, and none gives a temper-specific stress-relief practice, so the route is a question to put to the supplier rather than a rule to assume³²⁰²⁹.

Confirm the supplier can produce and certify the temper

The evidence chain for a temper claim is material and performance test results plus traceability documentation, not a verbal assurance. PPAP documentation includes material and performance test results, dimensional reports, initial process studies and laboratory documentation verifying that the test laboratory complies with industry regulatory standards¹⁴.

In aerospace programmes, AS9100 Rev D requires complete material traceability with 100% documentation retention from raw material to finished part for a minimum of 10 years, and certifications should be current and issued by an IAQG-recognised registrar²⁸.

No source in this set names a hardness or tensile test standard for verifying T4, T5 or T6 in aluminium, so the checks below are about demanding evidence, not about prescribing a test method¹⁴²⁸³¹.

• ✓Ask which temper the material certificate will name, and whether it matches the drawing callout exactly.
• ✓Ask whether the temper is produced in-house or by a subcontractor, and who certifies it — no source here evidences any supplier's heat-treat capability.
• ✓Require material and performance test results with the shipment, not on request.
• ✓Require the material standard (ASTM, AMS, EN or customer spec) on the certificate to prevent substitution disputes.
• ✓For aerospace or defence work, confirm current certification from an IAQG-recognised registrar and 10-year documentation retention.
• ✓Ask the supplier to state the machining route — including any stress relief between roughing and finishing — before they quote capability.
• ✓Ask what evidence they will provide that the delivered parts still meet the specified condition after machining.

Does the function justify the alloy and temper premium?

Temper and alloy both feed into cost, but the sources give them at different resolutions. T5 is stated to be less expensive than T6²⁷.

At alloy level, 7075-T6 carries a typical price premium of +30-40% over 6061¹⁶. Quantity moves unit price and lead time in opposite directions.

For one large simple as-machined aluminium part, the cited figures run from £1,209.60 at 8 days for a single piece to £662.40 at 60 days at 1000 off¹². The buyer should read that as a trade, not a discount: the unit price falls while the lead time rises¹².

What is missing is the temper-specific number. No source gives a T4/T5/T6 price delta, MOQ or lead-time figure for the same alloy and part, so the premium for stepping from T5 to T6 has to be quoted rather than estimated^12162627.

Finishing: flag it on the RFQ, but the temper does not rule it out

Aluminium pairs with a broad range of surface finishes, including anodizing, hard coat and powder coating²³. RFQ guidance lists anodizing, conversion coating, bead blasting, polishing, painting or as-machined as finish options that affect alloy choice, masking, dimensions and appearance²⁴.

The sequencing point matters more than the temper here: anodizing adds measurable thickness to the part, which has to be accounted for if the design has tight-tolerance features²³. That is a dimensioning decision, and it belongs on the RFQ so masking and thickness allowance are quoted rather than discovered at inspection²³²⁴.

What the sources do not do is link a specific temper to a specific finish outcome. No source in this set states whether T4, T5 or T6 differs in anodizing, painting or other finishing compatibility, so finish selection does not by itself rule a temper in or out²³²⁴.

Where the sources disagree or fall silent on temper figures

The table below separates what is corroborated from what is missing. The T6 strength values are reported consistently by three publishers, which is what makes them usable; the T4 and T5 columns are empty because no source in this set fills them.

What the sources do not establish

• No source gives yield or tensile values for T4 or T5 for any specific alloy, so the T4 vs T5 vs T6 strength difference for one alloy is missing.
• No machinability rating or cutting-speed recommendation per temper for aluminium.
• No quantified distortion or residual-stress data per temper, and no temper-specific stress-relief recipe.
• No statement of which tempers are compatible or incompatible with anodizing, painting or other finishes.
• No T4/T5/T6-specific cost, MOQ or lead-time data.
• No evidence of any supplier's in-house heat-treatment or certified-subcontractor capability for a specific temper.
• No named inspection or test method or standard for verifying T4/T5/T6 temper, such as a hardness or tensile testing standard.
• No application guidance for T4 specifically; T4 is named in the question but not covered by any source.
• No source addresses T4/T5/T6 for a named alloy such as 6061 or 7075 in a single comparative table.