T4, T5 or T6 on a Machined Aluminium Part: What the Temper Callout Actually Changes

The temper callout is a material specification, not a label

A drawing that says "6061" and stops is underspecified. Temper condition significantly alters the mechanical properties of the chosen base alloy.

And the same source lists it alongside alloy selection as one of the decisions that determine whether an aluminium part performs reliably and stays cost-effective²⁴. A second source is more specific about what moves: temper changes strength, corrosion, flatness and machining behaviour²⁵.

That list matters because it crosses the boundaries a buyer usually treats as separate. Strength is a design question, flatness is a tolerance question, and machining behaviour is a cost and cycle-time question — and the temper callout touches all three at once²⁵.

Two parts cut from the same alloy in two tempers are therefore not the same stock, even when the alloy line on the drawing is identical^24,25. What the sources do not do is quantify any of it.

Treat the callout as a real requirement you are entitled to have certified, and treat every number behind it as something to obtain rather than assume^24,25.

Which direction the strength difference runs

On strength, the sources agree on direction. T6 means the alloy has been solution heat-treated and artificially aged, a two-step thermal process described as producing the highest standard strength condition for heat-treatable aluminium alloys and the most commonly specified temper for structural work²⁹.

T5 sits below it: the same source calls T6 the higher-strength option and T5 the less expensive one, adequate for many applications such as architectural extrusions²⁹. So the trade is legible even without numbers.

If your part carries structural load, the callout is doing real work and you should be able to say what strength you need. If it does not, T5 may be the cheaper route to the same function.

The source's own framing is that neither temper is universally better and the choice depends on the structural requirements of the project²⁹. What you cannot do is read a strength delta off this article.

No source here gives T4, T5 and T6 yield or tensile values for the same alloy. So the size of the gap on your specific grade is unknown until the mill certificate or a test report states it²⁹.

Ask for the certified mechanical properties of the temper you specified, on the alloy you specified, before you sign off the drawing.

Flatness-critical plate: why plain T6 may not hold the tolerance

Distortion after machining is a stock problem before it is a cutting problem. Residual stress in the material can cause a part to move once metal is removed, and thin aluminium sections can distort when material comes off unevenly^19,20.

A supplier facing a distortion-prone feature may need a route that includes stress relief or a final sizing step rather than a single pass to finish³¹. Temper is one of the levers on that stress state.

T651 is T6 plus a controlled stretch applied after quenching and before aging, which relieves residual stress and improves dimensional stability. The source ties this specifically to plate stock that will be precision machined²⁹.

Plain T6 is not described as stress-relieved in that source²⁹. If your part is cut from plate and carries a flatness or datum callout that a warp would break, the evidence points to specifying the stress-relieved temper rather than plain T6²⁹.

If it is a compact, rigid part where a few tenths of movement are absorbed by clearance, that upgrade buys you less. The sources give no quantified distortion figure for either temper.

So how much movement to expect on your geometry is a question for the supplier's process plan, not a number you can look up here^19,20,29.

Machinability: a real effect you cannot rate per temper

Temper does reach the cutting edge. Heat treatment alters a metal's microstructure, and with it hardness and residual stress, which changes cutting forces, tool wear and finishing behaviour³.

For aluminium specifically, machinability depends largely on silicon content and varies with alloying elements, heat treatments and manufacturing processes^2,6. That establishes the mechanism and stops there.

No source in this set gives a machinability rating or a cutting-speed recommendation for T4 versus T5 versus T6, so you cannot budget cycle time from the temper letter on the drawing^2,3,6.

The practical consequence for a buyer is that the temper callout should travel to the shop floor with the part: ask the supplier to state the cutting parameters and tooling they will run for the specific alloy and temper you specified.

And to flag if that temper changes their cycle time against a T6 baseline^3,6. A quote that prices a T5 part at T6 machining assumptions is not necessarily wrong — it is simply unverified until the supplier says which temper they priced.

Finishing follows the alloy, not the temper

Aluminium pairs with anodizing, hard coat, powder coating and painting, and finish selection is normally discussed at the level of the alloy and its series rather than the temper^24–26. Where exposure matters, the alloy drives the answer: 2xxx and 7xxx parts in corrosive service need anodizing or a chemical film³⁵.

No source here distinguishes how anodizing or painting behaves on T4 versus T5 versus T6, so do not expect the temper callout to change your finish specification^24,25,35.

What the temper can change is the dimension the finish lands on — anodizing adds measurable thickness, which has to be accounted for where the drawing carries tight-tolerance features²⁴.

For a machined part, that makes the finish decision a sequencing question: confirm with the supplier whether critical features are masked or left with finish stock. And keep the temper callout out of that conversation unless the alloy itself is in question^24,35.

What to put in the RFQ before you release the drawing

The sources establish that temper matters but publish no temper-specific cost, availability, lead-time or verification data. So each of these has to be a written answer from the supplier rather than a figure you can look up^24,25,29.

• ✓State alloy and temper together on the drawing and in the RFQ — a temper alone does not define the stock.
• ✓Ask for the mill certificate showing the temper actually supplied, and check it names the alloy and temper you specified.
• ✓Ask whether the quoted price assumes T5 or T6, since T5 is described as the less expensive option.
• ✓For flatness-critical plate, ask whether the stock will be supplied in the stress-relieved temper rather than plain T6.
• ✓Ask the supplier to state cutting parameters and any cycle-time difference for the temper you specified.
• ✓Ask how the supplier will verify temper on the delivered part, and what evidence they will provide.
• ✓Confirm the finish route and whether anodizing thickness is accounted for on tight-tolerance features.

Where the evidence supports a choice, and where it only supports a direction

The sources put T5 and T6 side by side on strength and cost, and T6 against T651 on stress relief; T4 is not compared on any of these axes in this set²⁹. Read the table as a map of what you can decide now and what still needs a certificate.

Before you request a quote

• Request certificate numbers, issuing bodies, scope and validity for the exact model and destination market.
• Public sources do not settle this — put it to the supplier: no machinability ratings or cutting speeds per temper.
• Public sources do not settle this — put it to the supplier: no quantified distortion or residual-stress figures per temper, and no stress-relief practice recommendations.
• Public sources do not settle this — put it to the supplier: no data on whether anodizing or painting behaves differently on T4, T5 or T6.
• Public sources do not settle this — put it to the supplier: no temper-specific cost, minimum order quantity or lead-time figures.
• Public sources do not settle this — put it to the supplier: no information on supplier heat-treatment capability, in-house versus subcontracted.
• Public sources do not settle this — put it to the supplier: no inspection or testing methods or standards for verifying temper, such as hardness or tensile testing.
• Public sources do not settle this — put it to the supplier: no application guidance for T4 at all.