Aluminium Temper Guide: T4, T6 and T651 Explained

Two buyers send us the same drawing except for one character. The first calls out 6061-T6; the second calls out 6061-T651. The quotes come back different by 4%, the parts come back different by a lot more — one set arrives dead flat after pocketing, the other opens up like a banana across the 300 mm span. Nothing in the alloy changed. What changed is the temper: the thermal-mechanical history the mill gave the metal after it was cast.

Temper is the most misread callout on aluminium drawings, because it looks like a strength grade when it is actually a process recipe. This page is our shop's guide to reading it: what the letters and digits in the T and H systems actually mean, a full aluminium temper chart for the alloys we machine every week, what T4 versus T6 changes in forming and cutting, why the "51" suffix matters more for machined parts than any strength number, how to verify temper on the receiving dock, and a 30-second decision table for specifying temper on your next drawing. If you are still deciding between alloys, start with our 6061-T6 vs 6082-T6 head-to-head — this page assumes the alloy is chosen and covers the letter after the dash.

A Temper Is a Process, Not a Strength Number

The temper designation is defined by ANSI H35.1 / ASTM B221 and it always describes what was done to the metal, from which the properties follow. The basic letters:

Letter	Meaning	Typical use
F	As fabricated — no special control	Hot-rolled plate before finishing; rarely machined to print
O	Annealed — softest, most formable state	Deep drawing, heavy forming; 6061-O forms almost like copper
H	Strain hardened (cold worked) — non-heat-treatable alloys	5xxx and 3xxx sheet and plate: 5052-H32, 5083-H116
W	Solution heat treated only — unstable, age hardens at room temperature	Rarely seen on drawings; it is a transit state, usually with a time in parentheses (W ½ h)
T	Thermally treated — solution heat treated and aged, with or without cold work	2xxx, 6xxx and 7xxx alloys: 6061-T6, 7075-T651, 2024-T351

For the T tempers, the digit after the T is the post-solution route:

• T1 / T5 — cooled from an elevated-temperature working process (an extrusion press, usually), then for T5 artificially aged (furnace-aged). No separate solution treatment. 6063-T5 architectural extrusions are the everyday example: the press quench does the "solutioning", the furnace does the rest.
• T2 — cooled from hot working, then annealed (cast products).
• T3 — solution heat treated, then cold worked, then naturally aged. 2011-T3 free-machining bar is a common example.
• T4 — solution heat treated, then naturally aged at room temperature. 2024-T4 is the classic.
• T6 — solution heat treated, then artificially aged in a furnace. 6061-T6 is the everyday default of the machining world.
• T7 — solution heat treated, then overaged — held hotter/longer than the T6 peak. Strength drops a little; stress-corrosion resistance improves a lot. 7075-T73 is the reason this temper exists.
• T8 — solution heat treated, cold worked, then artificially aged. 2024-T851 plate is the aerospace example.
• T9 / T10 — rarer combinations (aged then cold worked; hot-worked, cold worked, aged). You will meet them mainly in free-machining and rivet alloys.

The second and third digits are suffixes, and this is where machined-part buyers should pay attention:

• -51 — stress relieved by stretching 1.5–3% permanent deformation after solution treatment, before aging. T651 = T6 + stretch. Applied to plate and rolled bar. T351 = T4 + stretch (2024 plate), T451 = T4 + stretch (6061 plate), T7351 = T73 + stretch (7075 plate).
• -511 — the same stretch route for extruded rod, bar and shapes (T6511, T3511). Same intent, slightly different tolerances on straightness.
• -52 — stress relieved by compressing 1–5% (forgings).
• -54 — stress relieved by combined stretching and compressing.

One consequence worth internalising: because temper is a process, the same alloy at the same temper from two mills can differ by 5–10% in actual tensile values while both are fully conformant. Conformance is judged against minimums, and the certificate reports actuals. That is why a substitution question like "is T651 stronger than T6?" has the answer it does — see the next two sections.

The Aluminium Temper Chart

This is the chart we keep laminated in the purchasing office: the tempers that actually arrive on our floor, with typical properties for common product forms. Values are typical/minimum-range figures for the standard mill forms (per ASTM B209/B221 nominal values); always design against the certificate minimums for the exact product form and thickness, because properties drop with thickness.

Alloy-Temper	UTS (MPa)	Yield 0.2% (MPa)	Elong. (%)	Hardness (HB)	What it is for
6061-O	~125	~55	25–30	~30	Deep forming, spinning
6061-T4	~240	~145	~22	~65	Form-then-age route, weld-then-age
6061-T6	~310	~276	~12	95 min	The machining default
6061-T651	~310	~276	~12	95 min	Plate for hogged-out parts, low distortion
6082-T6 / T651	~310	~260	8–10	~95	European 6061 equivalent, structural profile
6063-T5	~185	~145	~12	~55	Extrusions, architectural, light duty
6063-T6	~240	~215	~12	~73	Extrusions needing more strength
2011-T3	~380	~295	~15	~95	Free-machining screw-machine parts
2024-T351	~470	~325	~15–19	~120	Airframe structure, fatigue-critical parts
7075-T6 / T651	~570	~500	~11	150	Maximum-strength static parts
7075-T73 / T7351	~505	~435	~13	~135	Same alloy, stress-corrosion-resistant temper
5052-H32	~230	~195	~12	~60	Sheet fabrications, enclosures, marine
5083-H116	~315	~230	~16	—	Marine plate, welded hulls and tanks
5083-H321	~305	~215	~12	—	Marine/pressure plate, stabilized

Three reading notes. First, the T651 line shows the same numbers as T6 because the stretch adds no strength — the datasheet cannot see it, the machinist can. Second, the T73 line shows the cost of overaging: roughly 10–13% of yield traded for a different failure mode. Third, note where the O and T4 lines sit: a 6061 part that must be formed first is bought at ~55 MPa yield and then strengthened, which is a sequencing decision, not a material substitution.

T4 vs T6: Natural Versus Artificial Ageing

Solution treatment dissolves the hardening phases (for 6xxx, Mg₂Si) into the matrix at around 530 °C, then quenches them into a supersaturated solid solution. What happens next is the whole difference between the two tempers:

• T4 — natural ageing. The part sits at room temperature for 4–5 days (2024 needs ~96 h minimum) while fine GP zones precipitate. Properties are respectable and stable enough for use, forming is still easy, and the metal stays soft relative to T6. If you wait long enough, some alloys keep creeping up in strength for months — 2011-T3 bar often machines "nicer" a season after delivery.
• T6 — artificial ageing. The part goes into a furnace at roughly 160–180 °C for 8–18 h (7075 uses a two-step 120 °C + 160 °C cycle). The precipitates grow into a different, more coherent population that pins dislocations far more effectively. Yield nearly doubles versus T4 in 6061 (145 → 276 MPa) and elongation drops from ~22% to ~12%.

Practical consequences, in the order they bite:

1. Formability. T6 cracks where T4 bends. A 6061-T6 bracket that needs a 90° form wants an inside radius of 4–6× thickness across the grain and will still gray-crack on tight radii; the same form in T4 or O is routine. The standard production route for formed-and-strong parts: buy T4 (or O), form, then artificially age to T6 — the precipitation treatment does not care that the metal has been bent in between, and it can also be used to straighten distortion from forming.
2. Machining. T4 is gummier: longer chips, built-up edge on the rake face, slightly worse finish at the same speeds. T6 cuts crisply with a sharper, more stable shear zone but demands ~10–15% more spindle power. Numbers in the next section.
3. Welding. A T6 weld heat-affected zone locally re-dissolves and re-ages in an uncontrolled cycle and typically lands near 50% of base strength; a T4 weldment can be artificially aged after welding to recover most of it. This is why weld-then-age sequences exist on 6xxx frames.
4. Stability. Natural ageing (T4) continues after delivery — hardness and yield drift upward over weeks. Any part that is final-machined at T4 and then heated in service will move once more. Precision parts get their final machining after the thermal cycle, whatever the temper.

T651, T351, T7351: Why the Stretch Matters More Than Any Strength Number

Here is the mechanism behind the banana. Thick 6xxx and 7xxx plate is quenched after solution treatment, and the quench is not uniform: the faces cool first and freeze while the core is still hot and wants to contract. The result is a plate with locked-in residual stress — compression near the surfaces, tension in the core, sometimes exceeding 100 MPa. Every layer you machine away unbalances the equation, and the plate moves to find a new equilibrium. A deep pocket milled into T6 plate is a stress-relief operation whether you wanted one or not.

The mill fixes most of this by stretching the quenched plate 1.5–3% before final sawing. The permanent plastic stretch equalises the surface and core stress fields, cutting residual stress by roughly an order of magnitude. That is the entire content of the "-51" suffix — and it is the single highest-leverage word on an aluminium drawing for a machined part.

Our rule of thumb, developed across thousands of hogged-out housings and fixture plates:

• Material removal below ~50% and minimum web thickness above ~5 mm: T6 plate behaves. The movement stays inside typical 0.1–0.25 mm general tolerances.
• Material removal above ~50%, webs under 3 mm, or flatness callouts under 0.1 mm over 200 mm: insist on T651/T6511 (or T351 for 2024). On thick 7075 and 6082 plate this is standard anyway; on 6061 plate it costs nothing extra on most stock sizes because T651 is the mill default above roughly 6 mm.
• Truly stress-critical geometry — thin diaphragms, long slot windows, split-line housings: even T651 leaves enough residual stress to matter. The remaining options are rough-machine → stress-relieve anneal (for 2xxx/7xxx, a dedicated precipitation cycle) → finish machine, or buy pre-stretched plate and rough both sides symmetrically before finishing. This is a conversation to have at quotation, not after the first part bows.

Note what the stretch does not do: it does not change the certificate numbers, it does not change machinability ratings, and it does not show up in any datasheet. It shows up when the part measures flat two weeks later. When a supplier is asked to quote "6061-T6" plate work cheaply, one of the quiet ways to hit the price is to buy unstretched plate — the difference only appears on your CMM.

What Temper Does on the Shop Floor

Typical carbide face-milling and slotting windows we run per temper (10–20 mm cutters, MQL, coated carbide; adjust for your tooling):

Alloy-Temper	Cutting speed (m/min)	Feed per tooth (mm)	Behaviour notes
6061-T4	250–400	0.08–0.15	Gummy, built-up edge, long chips; polished flutes help
6061-T6 / T651	300–500	0.10–0.20	Crisp chips, excellent finish, forgiving
6082-T6	300–450	0.10–0.18	Nearly identical to 6061-T6; slightly more abrasive
2024-T351	200–350	0.08–0.12	Good finish; copper phase dulls edges faster
7075-T651	250–400	0.08–0.15	Clean breaks, brittle chips, notch wear at depth-of-cut lines
5083-H116	300–600	0.08–0.15	Very gummy at high speed; sharp geometry and high rake mandatory

The distortion story is worth a number, because it is the one buyers never get from a datasheet: on 100 × 100 × 25 mm blocks cut from 50 mm unstretched 6061-T6 plate, pocketing to leave a 2 mm floor typically moves the surface 0.15–0.4 mm within minutes of the roughing pass. The same block from T651 plate moves 0.02–0.05 mm. At anodising thicknesses, the second number is the only one that survives a tolerance block with flatness on it. Related reading on the geometry side: our aluminum tolerances guide covers what a shop can hold and which callouts drive cost.

The Tempers That Bite: 7075-T6 vs T73, 2024-T351 and 5083-H116

Three tempers exist for reasons that never appear on the datasheet line:

• 7075-T6 vs T73/T7351 — stress-corrosion cracking. 7075-T6 is a superb alloy with a nasty failure mode: under sustained tensile stress in the short-transverse direction (through the plate thickness), especially in humid or chloride environments, it can crack at stresses far below its tensile strength — at a fraction of the load a static test predicts. T73 overages the alloy to shift the SCC threshold dramatically upward, at the cost of ~13% yield (503 → 435 MPa). Aerospace and defence specifications routinely prohibit T6 for parts carrying sustained ST-direction tension. If your 7075 part is press-fitted, bolted preload-loaded, or shrunk onto a steel shaft, raise T73 with your supplier before the drawing freezes. Our dedicated 7075 deep-dive covers the full temper decision.
• 2024-T351 — fatigue, not strength. 2024 is weaker than 7075 but far more damage-tolerant: it spreads fatigue cracks slowly and visibly instead of failing abruptly. That is why airframe skins and fatigue-critical fittings specify 2024-T351 (naturally aged after stretching, ~96 h) even though 7075-T651 out-tests it on every static number. The -351 stretch matters double here, because airframe parts are exactly the thin-web, high-removal geometries where residual stress ruins flatness. Cost angle in our 2024 vs 6061 comparison.
• 5083-H116/H321 — the H-system rules. Non-heat-treatable 5xxx alloys take their temper from cold work, not from precipitation. H1 = strain hardened; H2 = strain hardened then partially annealed; H3 = strain hardened then stabilized (heated to relieve the cold work). The second digit is the amount: H32 is quarter-hard, H34 half-hard. 5083-H116 and H321 are the marine plate tempers, both carrying an intergranular/exfoliation-corrosion test attached to the designation — for a welded hull, that test is the spec, not the hardness.

How to Verify Temper on the Dock

Temper is a process, so it cannot be read off the surface — but two cheap instruments bracket it. A receiving inspector with a hardness tester and a conductivity meter (%IACS) can separate the common tempers in minutes; conductivity rises with overaging and falls with cold work, which is exactly why it separates T4 from T6 and T6 from T73:

Alloy-Temper	Typical hardness	Typical conductivity (%IACS)	What a mismatch usually means
6061-T4	~65 HB	~45–50	Under-aged or mislabelled F/O stock
6061-T6	95 HB min	~40–45	Hardness short = under-age; conductivity high = T4
2024-T351	~120 HB	~28–32	Low conductivity is normal — do not fail it against a 6xxx expectation
7075-T6	150 HB	~32–36	Hardness short + conductivity high → T73 substitution
7075-T73	~135 HB	~38–41	Often a deliberate spec, not a defect — check the drawing

Two rules for using the table. First, never verify by hardness alone across alloys — 95 HB describes both 6061-T6 and a mislabelled 2024-T3; conductivity is the alloy fingerprint and hardness is the temper fingerprint, and the pair together is the check. Second, treat the numbers as bands, not pass/fail limits: instrument type, plate thickness and heat-to-heat chemistry move them a few points. When the numbers disagree with the mill certificate, the certificate's heat number — stamped or ink-jetted on the material — is the evidence chain, which is how we ship every aluminium batch.

Choosing a Temper in 30 Seconds

If the part…	Specify	Why
Is hogged from plate, thin webs, flatness ≤0.1 mm	6061-T651 (or 7075-T651)	Stretch kills quench residual stress; T6 will bow
Must be formed, then carry load	6061-T4 (form) → artificially age to T6	T6 cracks on tight radii; T4 forms, ageing restores strength
General machined bracket/housing, moderate loads	6061-T6 / T651	The default; cheapest to buy and cut
European sourcing, structural profile	6082-T6 / T651	EN-standard equivalent of 6061
Maximum static strength, thick section, benign environment	7075-T651	Highest strength of the stock alloys
Carries sustained tensile stress through thickness	7075-T73 / T7351	T6 SCC risk in the ST direction
Fatigue-critical, aerospace structure	2024-T351	Damage tolerance beats peak strength
Welded marine plate or tank	5083-H116 / H321	Weldable, corrosion-tested temper
High-volume small turned parts	2011-T3 or 6262-T9	Free-machining additives; accept lower corrosion resistance
Serves above ~120–150 °C continuously	Re-think the alloy family	2xxx/7xxx overage and lose strength; 2xxx/6xxx each have ceilings

Frequently Asked Questions

Is T651 stronger than T6?

No — the certificate numbers are identical, because the 1.5–3% stretch adds no strength; it removes residual stress. A 6061-T651 part is materially as strong as a T6 one but will machine 5–10 times flatter, which for anything with a flatness callout is worth more than the 0–5% plate premium. If a datasheet shows a T651 line slightly below T6, it reflects product-form differences (plate vs sheet), not the stretch itself.

Can I machine 6061-T4 instead of T6 to save money?

Sometimes, and not for the reason people expect. T4 stock itself is not much cheaper; the saving is that T4 tolerates forming and welding-then-ageing routes. But T4 parts are gummier to machine (built-up edge, worse finish), their properties keep drifting for weeks after solution treatment, and the minimum yield of 145 MPa is half of T6's 276 MPa. For a straight machined part with no thermal processing afterwards, T6/T651 is both cheaper to cut and predictably dimensioned.

What does the third digit in H32 or H116 mean?

It is a variation code within the H-family. The first digit is the route (H1 strain hardened, H2 strain hardened + partially annealed, H3 strain hardened + stabilized), the second is the degree of cold work (H32 = quarter hard, H34 = half hard, H18 = full hard), and the third digit refines the route — as in H111 (slightly strain hardened after annealing, for straightness) or H116, a 5xxx plate temper with a mandatory intergranular-corrosion/exfoliation test for marine service. You cannot translate H tempers into T-strength equivalents; they are different systems for different alloy families.

Why did my aluminium part move after machining?

Almost always quench-induced residual stress in unstretched plate. Deep pocketing unbalances the through-thickness stress profile and the part distorts to a new equilibrium — typically 0.15–0.4 mm on a 100 mm pocket in 50 mm T6 plate. The fixes, in cost order: specify T651/T351 plate, rough both sides symmetrically before finishing, or insert a stress-relief anneal between roughing and finishing. Our heat-treatment sequencing guide covers where that anneal sits in the process chain.

Does temper affect anodising and colour?

Slightly, and mostly through alloy chemistry rather than the temper itself. T6 versus T651 anodises identically. What does change appearance: 2xxx and 7xxx alloys anodise darker and less uniformly because of their copper and zinc content, 5xxx stays silvery, and overaged tempers (T73) can take colour marginally differently than T6 of the same alloy. For cosmetic parts, approve a sample coupon from the actual heat rather than a datasheet colour chip — the full alloy trade-offs are in our anodizing vs powder coating comparison.

How do I write the temper on my drawing correctly?

Full callout, product form included: "6061-T651 plate per ASTM B209", or "6082-T6511 extruded bar per EN 755-2". Avoid bare "6061" (temper undefined — the supplier chooses, and F or T4 may arrive), avoid "T6 or T651" (for stressed, flat parts that "or" is where distortion enters), and add the certificate requirement explicitly: "mill test certificate with heat number traceable to parts". If flatness is critical, one line — "stress-relieved plate required" — prevents the cheapest-stock substitution before it happens.

For the surrounding decisions — alloy family trade-offs, surface finish targets and where material sits in the piece price — see our four-alloy overview, the 6061 vs 7075 comparison and our material-vs-machining cost breakdown. And if your part lives or dies by its surface finish, the Ra chart pairs well with the temper choice, since T6 and T4 machine to visibly different Ra at the same feed.