6061-T5 vs 6063-T5: What the Temper Label Really Decides

Alloy and temper · what changes

6061-T5 vs 6063-T5: What the Temper Label Really Decides

Two things need separating here. One is the alloy, which changes how the profile extrudes, machines and anodises. The other is the temper, which changes the yield point. Neither of them changes stiffness by a single per cent — and that single fact invalidates a lot of specification decisions. There is also a third problem: 6061-T5 is not a mainstream extrusion temper, and a quotation that offers it should be checked.

CNC machined aluminum parts and extruded profile assemblies
CNC machined aluminum parts and profiles

Quick answer

  1. 6061-T5 is an unusual specification. For extrusion, 6061 is normally supplied as T6 or T651. T5 is not a mainstream extrusion temper for this alloy, so a quotation offering “6061-T5” with a tensile strength around 240 MPa should be checked against the applicable standard before it goes on a drawing.
  2. Alloy and temper do not change stiffness. Both 6061 and 6063 have a modulus of elasticity of about 68 to 70 GPa, and the temper does not move it. A section in 6063-T5 and the same section in 6061-T6 deflect by exactly the same amount under the same load.
  3. Temper changes the yield point, and that is all it changes. 6063-T5 yields at 110 to 130 MPa depending on wall thickness; 6063-T6 at 160 to 170 MPa; 6061-T6 at 276 MPa. Stiffness is identical across all three.
  4. Alloy changes extrudability, finish and machining. 6063 extrudes complex hollow sections and anodises cleaner because of its lower iron content. 6061 machines better because it forms short broken chips rather than long stringy ones.
  5. The alloy upgrade only pays in the short-span regime. Below the span at which yield starts to govern, a stronger alloy raises the load ceiling. Above it, deflection governs and the stronger alloy changes nothing.
  6. What to write: 6063-T5 for general framing, 6061-T6 when the drawing sets a mechanical property requirement or the part will be machined. Never write “6061 aluminium” without a temper — annealed 6061 has roughly a third of the yield strength of T6.

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Start with the specification problem

Most comparison articles on this topic compare 6061-T5 with 6063-T5 as though both were routine extrusion products. They are not. 6063 is the extrusion alloy; 6061 is the machining and structural alloy that can be extruded with difficulty. And within 6061, the temper you actually get for extrusion is T6 or T651, not T5.

The practical consequence is that a quotation for 6061-T5 should prompt a question. Either the supplier means 6061-T6 and has written the wrong temper, or they are offering a non-standard condition. Both are worth catching before the material is on a drawing, because the mechanical properties you design to depend on which one it is.

Check before you specify

Ask for the mechanical property values the supplier will certify against, and the standard they come from. If the answer is a tensile strength figure with no standard behind it, you have no basis for a design calculation and no basis for a rejection at goods-in.

For the rest of this article, the comparison is between the conditions that actually exist in the market: 6063-T5, which is what almost every T-slot framing profile is made from, and 6061-T6, which is what you specify when you need more strength and are willing to pay for the extrusion difficulty.

The properties, by standard and by wall thickness

Mechanical property limits for extrusions are minimum values, and they depend on the wall thickness of the section. That dependency is the part most comparison tables leave out, and it is the part that matters when you are checking a mill certificate.

Alloy and temperSectionTensile Rm (min)Yield Rp0.2 (min)Elongation A
6063-T5wall ≤ 3 mm175 MPa130 MPa≥ 8%
6063-T53 < wall ≤ 25 mm160 MPa110 MPa≥ 7%
6063-T6wall ≤ 10 mm215 MPa170 MPa≥ 8%
6063-T610 < wall ≤ 25 mm195 MPa160 MPa≥ 8%
6063-T66wall ≤ 10 mm245 MPa200 MPa≥ 8%
6061-T6typical extrusion310 MPa276 MPa≥ 8%

All figures are minimum values for extruded profile in the relevant standard, and all are in newtons per square millimetre. The 6063 values follow the European standard for extruded profiles; the 6061-T6 figures are the familiar North American minimums.

Two things in that table deserve attention.

The wall thickness dependency is real and it is significant. A 6063-T5 profile with a 2 mm wall must certify at 175 MPa tensile and 130 MPa yield. The same alloy in a section with a 12 mm wall only has to certify at 160 MPa and 110 MPa. The material is the same; the cooling rate through a thicker section is slower, so the precipitation hardening is less complete. If you are checking a certificate and you do not know the wall thickness of the section it covers, you cannot say whether the figures are correct.

The 6061 jump is large but it is a jump in yield, not in stiffness. Yield strength nearly doubles from 6063-T5 to 6061-T6. Modulus of elasticity does not move at all. That asymmetry is the whole subject of the next section.

Extruded aluminium profiles and CNC machined aluminium components made from different alloys
Extrusion and machining pull in different directions. The profiles are 6063, chosen for extrudability and finish. The machined parts alongside them are 6061, chosen for strength and chip formation. A single part that has to do both jobs is a compromise.

The fact that invalidates most alloy upgrades

Here is the number that matters more than any other in this comparison. The modulus of elasticity of both alloys is approximately 68 to 70 GPa, and the temper does not change it. 6063-T5, 6063-T6, 6061-T6 — all the same.

Stiffness is not a material property in the way most people assume. The deflection of a beam depends on the modulus and the geometry, not on the yield strength. Doubling the yield strength of the material a beam is made from changes the load at which it permanently deforms. It does not change the load at which it sags.

Why the alloy does not appear in the deflection equation

δ = F × L3 ÷ (48 × E × I)

F is the load, L the span, E the modulus of elasticity and I the second moment of area. There is no yield strength term. Change the alloy from 6063-T5 to 6061-T6 and E stays at 70,000 N/mm2, so the deflection is identical.

Put a number on it. A 40 × 40 standard section with a 9.2 cm4 second moment of area, spanning 1,000 mm with a 50 kg load at mid-span:

Check6063-T56061-T6Difference
Deflection at 1,000 mm, 50 kg1.59 mm1.59 mmnone
Bending stress at that load26.6 MPa26.6 MPanone
Allowable stress at a safety factor of 255 MPa138 MPa2.5×
Yield-limited load, 1,000 mm span103 kg258 kg2.5×
Deflection-limited load at L/300105 kg105 kgnone
Governing limitdeflection, 105 kgdeflection, 105 kgnone

Read the last two rows. At a 1,000 mm span, the 6063-T5 section is already deflection-limited: it reaches its allowable deflection at 105 kg, before it reaches its yield limit at 103 kg — they are effectively level, and both are below any concern. Upgrading to 6061-T6 raises the yield-limited load to 258 kg, which sounds like a large gain, but the deflection limit is still 105 kg. You would pay a substantial premium for a load increase you cannot use.

Now shorten the span to 500 mm. Deflection drops with the cube of span, so the deflection-limited load rises to 420 kg, while the yield-limited load only doubles to 206 kg for 6063-T5 and 518 kg for 6061-T6. At 500 mm, the 6063-T5 section is yield-limited at 206 kg. Upgrading the alloy takes it to 518 kg, above the deflection limit of 420 kg, so the upgrade now buys a real increase from 206 to 420 kg.

The rule that falls out of this

The alloy upgrade is worth paying for only in the span range where yield governs rather than deflection. For a 40 × 40 section that is roughly below a metre. Above a metre, the frame is deflection-limited, and the money spent on a stronger alloy would buy more stiffness if it were spent on a larger section instead.

Changes with alloy and temper

Yield strength 110 → 276 MPaHardness 60 → 95 HBExtrudabilityAnodised appearanceChip formationBendability

Does not change

Modulus of elasticity, 68 – 70 GPaDeflection at a given loadDensity, 2.70 g/cm3Thermal expansionSecond moment of area

Everything on the left can be bought by changing the specification. Nothing on the right can be bought at all, because it is fixed by the metal itself. When a design problem is on the right-hand list, the answer is geometry — a bigger section, a shorter span, an extra support — and no amount of specification change will substitute for it.

What the alloy does change

Once you accept that alloy selection is not a stiffness decision, the things it does decide become clearer, and they are mostly about manufacture rather than about service performance.

6063

Built for extrusion and finish

  • Extrudes complex hollow sections and thin walls at low pressure
  • Lower iron content gives a cleaner, brighter anodised finish
  • Fewer die lines and less streaking after anodising
  • Bends more easily at tight radii without cracking
  • Gummy in machining — long stringy chips that wrap around tools
  • Yield 110 to 130 MPa depending on wall thickness
6061

Built for strength and machining

  • Extrudes with difficulty — higher pressure, slower, limits section complexity
  • Higher iron and copper content can leave the anodised finish slightly cloudy
  • Machines well: short broken chips, higher feeds and speeds
  • Better for hard anodising and for welded assemblies
  • Yield 276 MPa in T6 — roughly 2.5 times 6063-T5
  • Roughly 15 to 25 per cent cheaper to machine, part for part

The practical trade in a framing project

Almost every T-slot framing project should use 6063-T5, and the reason is not that it is cheaper. It is that the accessory system — the brackets, the nuts, the hinges, the feet — is designed and priced around 6063 extrusions, and the anodised finish that makes the system look like a system depends on the cleaner surface 6063 produces. Specifying 6061 for a standard frame buys strength the frame cannot use and costs you finish quality and extrusion flexibility.

There are three cases where 6061 is the right call:

  1. The drawing sets a mechanical property requirement. If the customer specification, the applicable code or the end-user’s technical file names a minimum yield strength above what 6063-T6 can deliver, the alloy is not a choice. Write the temper as T6 or T651 and get the certificate.
  2. The part is machined rather than used as extruded. A bracket, a plate, a mount or a machined block in 6061 machines faster, finishes better and holds a thread better than the same part in 6063. When the extrusion is only the blank, choose the alloy for the machining.
  3. The span is short and the load is high. This is the yield-governed regime. Below about a metre on a 40 series section, the alloy upgrade raises the usable load. Above it, it does not.

In every other case, the money is better spent on a larger section, a heavier wall, or an extra support — all three of which change the geometry, and geometry is what the deflection equation responds to.

What to write, and what to check on delivery

The specification on a drawing is short. Getting it complete is what prevents the argument at goods-in.

ApplicationAlloy and temperWhy
General T-slot framing, guards, workstations6063-T5Extrudes cleanly, anodises well, matches the accessory system
Frames needing higher specified strength6063-T6170 MPa yield instead of 110 to 130, same stiffness, same finish
Structural members with a property requirement6061-T6 or T651276 MPa yield; check that the section can be extruded in this alloy
Machined brackets, plates and mounts6061-T6Better chip formation, faster cycles, better thread holding
Parts to be bent after ageing6063-T5Lower yield means less cracking on the outer radius
Architectural and decorative anodised profiles6063-T5Lower iron gives a cleaner, more uniform anodised surface

On delivery, three things on the mill certificate are worth checking, and they take a minute each.

  • The temper is stated, not implied. “6063” alone is not a specification. Look for T5, T6 or another designation from the applicable temper standard.
  • The property values match the wall thickness. A 6063-T5 certificate showing 160 MPa tensile is correct for a 12 mm wall and wrong for a 2 mm wall, where the minimum is 175 MPa. Compare the figure with the section it covers.
  • The standard is named. A certificate that quotes tensile and yield figures without naming the standard they were measured against cannot be used to accept or reject a lot.

The most common specification error

Writing “6061 aluminium” with no temper. Annealed 6061 has a yield strength of roughly 55 to 70 MPa. 6061-T6 has 276 MPa. That is a factor of four or five, and it is invisible in the material name. The temper designation is not optional detail; it is the specification.

Frequently asked questions

Is 6061-T5 a standard extrusion temper?
It is not a mainstream one. For extrusion, 6061 is normally supplied as T6 or T651, and those are the conditions the mechanical property tables are built around. T5 exists as a temper designation and can apply to some products, but a quotation offering 6061-T5 for an extruded profile should be checked — either the temper is a typographical error for T6 or the supplier is offering a non-standard condition. Ask which standard the certified values come from before the material goes on a drawing.
Does 6061-T6 deflect less than 6063-T5?
No. Deflection depends on the modulus of elasticity and the geometry, and both alloys have a modulus of about 68 to 70 GPa, unchanged by temper. A 40 × 40 section at 1,000 mm under a 50 kg mid-span load deflects 1.59 mm whether it is 6063-T5 or 6061-T6. What changes is the load at which it permanently deforms: 103 kg for 6063-T5 against 258 kg for 6061-T6 at that span, which is above the deflection limit and therefore unusable.
Why does the wall thickness change the mechanical properties?
Because cooling rate depends on section thickness. A thin wall cools quickly as it leaves the die, which leaves more magnesium and silicon in solution to precipitate during artificial ageing. A thick wall cools more slowly and the precipitation hardening is less complete. That is why the standard gives a higher minimum tensile and yield for a 3 mm wall than for a 25 mm wall in the same alloy and temper. When you check a mill certificate, compare the values with the wall thickness of the section they cover.
Which alloy is better for anodising?
6063, and the reason is iron content. 6063 is limited to about 0.35 per cent iron against about 0.7 per cent for 6061. Iron forms intermetallic particles that show up as dark streaks or a cloudy appearance after anodising. 6063 also contains almost no copper, which avoids smutting reactions in the acid bath. For clear or dyed decorative finishes, 6063 is the standard choice. For hard anodising where wear resistance matters more than appearance, 6061 is preferred.
Which alloy machines better?
6061, despite being harder. The reason is chip formation. 6061 produces short broken chips that clear from the cutting edge, which allows higher feeds and speeds and gives a better surface finish. 6063 is softer and gummier, so chips come off as long ribbons that wrap around the tool, cause built-up edge and force more frequent stops. If the part is machined rather than used as extruded, choose the alloy for the machining operation, not for the extrusion.
Can I get a stronger frame by changing the alloy?
Only if the frame is yield-limited rather than deflection-limited. For a 40 × 40 section, yield governs below roughly a metre of span and deflection governs above it. In the deflection-governed range, which covers most machine frames, changing the alloy produces no measurable improvement because the modulus is unchanged. In that range the money is better spent on a larger section, a heavier wall or an extra support — all of which change the geometry that the deflection equation responds to.

Tell us the application and we will name the alloy

We extrude 6063 in T5 and T6 and machine 6061 in T6 and T651, and we supply the mill certificate with the delivery. If a stronger alloy will not change your deflection, we will say so rather than sell it to you.

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