Coefficient of Thermal Expansion of 6061 Aluminum: T4 vs T6 Values
For preliminary dimensional calculations from about 20 to 100°C, 6061-T4 and 6061-T6 are commonly assigned the same mean linear coefficient of thermal expansion: about 23.6 µm/(m·K), or 13.1 µin/(in·°F).
Same nominal CTE
T4 and T6 normally use the same first-pass 20–100°C value. Do not choose a temper because you expect T6 to expand less.
Different strength
T6 is stronger and harder. T4 is more formable. Those differences matter when thermal growth is restrained.
Interval matters
A mean CTE over 20–100°C is not a universal constant for cryogenic or sustained high-temperature service.
Restraint changes the result
Fasteners, friction, mixed materials, welds and mounting geometry can turn free movement into stress or distortion.
What CTE should you use for 6061-T4 and 6061-T6?
Use about 23.6 µm/(m·K) as a practical mean linear CTE for either 6061-T4 or 6061-T6 in preliminary calculations over roughly 20–100°C. In imperial units, that is about 13.1 µin/(in·°F).
This does not mean every specimen expands by exactly the same amount. Published values near 23.0–23.6 can reflect a different temperature interval, source dataset, product form, specimen condition and rounding. For tight tolerances, cryogenic service, high temperatures or contract acceptance, use the value and test method required by the drawing or material specification.
The temper decision should normally be based on strength, hardness, formability, machining response, residual-stress condition and joining requirements. A stronger T6 part does not automatically have a lower reversible thermal expansion rate.
Same nominal CTE for preliminary calculations. Different mechanical response.
The heat treatment changes precipitation state and strength much more than it changes the nominal CTE used for routine dimensional planning.
Naturally aged
Solution heat-treated and naturally aged to a substantially stable condition. It is generally selected when forming or later processing is important.
- Common preliminary CTE: 23.6 µm/(m·K), 20–100°C
- Lower yield strength than T6
- Greater forming latitude in many operations
- Later artificial aging changes the temper designation and properties
Artificially aged
Solution heat-treated and artificially aged. It is commonly selected for higher strength and hardness in finished structural or machined parts.
- Common preliminary CTE: 23.6 µm/(m·K), 20–100°C
- Higher yield strength than T4
- Less forgiving of some forming operations
- Welding can locally soften the heat-affected zone
Stress-relieved suffixes can reduce machining distortion caused by residual stress, but the material still undergoes normal reversible thermal expansion. “Moves less after machining” and “has a lower CTE” are different claims.
6061 Thermal Expansion Planner
Estimate free thermal growth for one 6061 part, or compare it with a mating material. The tool is a planning aid, not a joint-stress, weld-distortion or code-compliance calculation.
Describe the part
Enter the original dimension and temperature range. Keep the default CTE for a first pass, or replace it with a project-specific value.
6061-T6 grows about 1.888 mm
A 1000 mm part heated from 20°C to 100°C reaches about 1001.888 mm if it can expand freely.
Use this result for early dimensional planning. Add joint geometry, restraint, gradients and material-strength checks before releasing the design.
The default 23.6 µm/(m·K) value represents about 20–100°C (13.1 µin/(in·°F) over 68–212°F). Use an integrated low-temperature curve for cryogenic work or an interval-specific, temperature-dependent value for elevated-temperature precision work.
It tells you how much length changes for each degree of temperature change.
A value of 23.6 µm/(m·K) means that a one-meter length grows about 23.6 micrometers for every 1°C or 1 K rise, within the stated averaging interval.
Start with the free-expansion equation
The part needs three inputs: its original size, the temperature change and the mean CTE for that interval. The equation works for a length, rail, bore diameter or any other linear dimension when expansion is approximately uniform and unconstrained.
Example: for a 100 mm bore heated by 80°C, the diameter increase is 23.6 × 10⁻⁶ × 100 × 80 = 0.1888 mm. The hot bore is therefore about 100.1888 mm before other effects are considered.
Cooling gives a negative temperature change and therefore a negative dimensional change. The same formula handles contraction as long as the selected CTE is valid for the temperature interval.
ΔT = Tend − TstartΔL = α × L₀ × ΔTLhot = L₀ + ΔLΔLdiff = (α₁ − α₂) × L₀ × ΔTUse µm/(m·K) with metric dimensions and Celsius/Kelvin temperature differences. Use µin/(in·°F) with inch dimensions and Fahrenheit temperature differences. A temperature difference of 1°C equals 1 K; the Fahrenheit interval needs the imperial CTE.
Mean CTE, instantaneous CTE and total expansion answer different questions.
Two trustworthy sources may publish different-looking numbers because they report a different property definition or temperature range.
| Term | What it describes | Use it when | Common mistake |
|---|---|---|---|
| Mean linear CTE | Average expansion rate between two temperatures. | Routine dimensional growth over the stated interval. | Calling it an exact value at one temperature. |
| Instantaneous CTE | Local slope of the expansion curve at a particular temperature. | Detailed temperature-dependent modeling. | Using it across a wide temperature range without integration. |
| Relative linear expansion | Total length change relative to a reference temperature. | Cryogenic curves and high-accuracy thermal models. | Multiplying one room-temperature CTE across the entire curve. |
| Thermal conductivity | How quickly heat moves through the material. | Temperature gradients, heating rate and heat-sink analysis. | Assuming a stronger conductor expands more or less. |
A quoted number is incomplete without its units, temperature interval, material condition and whether it is typical or guaranteed.
Aging changes precipitation strengthening—not the aluminum matrix into a new material.
Both tempers begin with solution heat treatment and quenching. Natural or artificial aging changes the distribution of strengthening precipitates, which has a large effect on yield behavior but a small effect on the handbook CTE used for preliminary work.
Solution heat treat → quench → natural age
The alloy develops useful strength at room temperature while retaining more forming latitude than T6.
- Good starting point when forming is required
- Lower yield strength under restraint
- Same nominal CTE input for routine calculations
Solution heat treat → quench → artificial age
Controlled elevated-temperature aging produces higher strength and hardness for many finished applications.
- Common for machined and structural parts
- Higher resistance to yielding under the same idealized stress
- Same nominal CTE input for routine calculations
The values are close, but the definitions are not always identical.
For a one-meter member and an 80°C rise, using 23.2 instead of 23.6 changes the calculated growth by only 0.032 mm. That can be negligible for a loose structure and important for precision alignment.
| Published context | Representative value | Temperature interval | How to use it |
|---|---|---|---|
| Kaiser Aluminum 6061 data | 23.6 µm/(m·K) | 20–100°C mean | Strong first-pass value for the stated interval; still typical, not a lot guarantee. |
| NIST-hosted wrought-aluminum table | 23.2 µm/(m·K) | 20–100°C mean | Useful cross-check and reminder that the source dataset matters. |
| Rounded mean design value | About 23 µm/(m·K) | Approximately 20–100°C | Suitable only when the rounding error is small relative to the design allowance. |
| Project-specific measured data | Test result | Defined by the test plan | Use for acceptance only when method, uncertainty, specimen and interval are controlled. |
If thermal expansion controls fit, optical alignment, sealing or calibration, write the requirement into the drawing or RFQ. A purchasing description that says only “6061-T6” may not guarantee one exact CTE value.
Use 23.6 for its intended interval—not as a lifetime constant.
The reported mean CTE generally rises as the upper temperature increases. At cryogenic temperatures, use an integrated expansion curve referenced to a stated temperature.
Representative mean CTE from −50 to 20°C, in µm/(m·K).
Representative mean values from 20 to 100°C.
Representative mean CTE from 20 to 200°C.
Representative mean CTE from 20 to 300°C.
Above roughly 100–120°C, a precision design should usually move to interval-dependent data and separately review strength retention, creep, aging and joining effects. For cryogenic service, NIST provides integrated 6061-T6 expansion data from about 4 to 300 K referenced near 293 K.
Mixed-material movement can be larger than the tolerance.
For a one-meter assembly heated from 20 to 100°C, 6061 at 23.6 µm/(m·K) grows about 1.888 mm. A steel member at 11.6 µm/(m·K) grows about 0.928 mm. Their free differential movement is about 0.960 mm.
Design the path for movement
Long rails, frames and enclosures often need a fixed datum at one location and a sliding or compliant connection elsewhere. If both ends are locked, the assembly cannot follow the free-expansion calculation.
Expansion-joint image: Daniel Schwen, Wikimedia Commons, CC BY-SA 3.0. The image illustrates movement allowance; it is not a 6061-specific assembly.
Free growth is easy. Restrained behavior is a system problem.
The expansion equation predicts how a uniform part would move if nothing stopped it. It does not calculate bolt loads, bearing pressure, weld distortion, buckling, local yielding or stress around a brittle insert.
Free expansion
The part can slide or grow without meaningful resistance. Use ΔL = αL₀ΔT as a direct dimensional estimate.
Partial restraint
Fastener compliance, friction, slots, seals and contact geometry share the movement. The real load depends on the joint.
Full or complex restraint
Temperature gradients, multiple materials and rigid boundaries may require structural analysis, test evidence or finite-element modeling.
With E = 68.3 GPa, α = 23.6 × 10⁻⁶/K and ΔT = 80 K, the ideal elastic stress magnitude is about 129 MPa; fully restrained heating creates compression. This is not a real-joint formula. It assumes uniform temperature, complete restraint, one-dimensional stress and no yielding, slip, relaxation or geometric compliance. Depending on product form and actual strength, T4 may yield or relax before the ideal value develops.
Translate the CTE number into the failure mode you need to prevent.
A useful thermal-growth review starts with the dimension that matters, the actual temperature at that location and the way the part is attached.
For every meter of length, a 1 µm/(m·K) data difference creates 0.08 mm of result difference over an 80°C change.
Photo: Mike1024, Wikimedia Commons, public domain. Alloy and temper are not identified.
Bulk CTE remains useful, but the thermal cycle, weld sequence, restraint and local loss of temper control the final shape and strength.
6061 weld photo: W. S. Yerazunis / Dr. Crash, Wikimedia Commons, public domain.First decide whether the change is reversible.
ISO 1:2022 sets 20°C as the standard reference temperature for specifying geometrical and dimensional properties. A 500 mm 6061 feature measured at 25°C can be about 0.059 mm larger than its 20°C reference size.
Test the expansion curve when the tolerance justifies it.
Handbook data is usually enough for early design. A controlled test becomes valuable when a small error can affect alignment, interference, sealing, calibration or contract acceptance.
Choose the method around the specimen and uncertainty
ASTM E228-22 covers linear thermal expansion using a push-rod dilatometer. ASTM E831-25 covers thermomechanical analysis, and ASTM E289-25 covers interferometric measurement. The correct method depends on specimen geometry, temperature range, required accuracy and laboratory capability.
- Define the reference and operating temperature interval.
- Identify the exact 6061 temper, suffix, product form and material lot.
- State whether you need mean CTE, instantaneous CTE or relative expansion.
- Control specimen direction, dimensions, thermal soak and heating rate.
- Report method, calibration, uncertainty, cycles and the fitted temperature range.
- Compare the result with the actual assembly tolerance—not only a handbook table.
Turn “6061-T6” into a complete dimensional requirement.
The alloy name alone does not tell a supplier which thermal condition, dimension, temperature range or verification rule controls acceptance.
Put these eight items in the inquiry
- Exact alloy, temper and suffix: T4, T6, T651, T6511 or another specified condition.
- Product form and governing material standard, such as plate, sheet, bar or extrusion.
- Critical dimension, datum system, tolerance and direction of measurement.
- Reference temperature plus minimum, maximum and steady operating temperatures.
- Required CTE definition, units and averaging interval.
- Whether the value is for engineering guidance or must be guaranteed.
- Required test method, specimen plan, uncertainty and report format.
- Substitution control for temper, suffix, product form and material source.
Also send the assembly context
A supplier or process engineer can give a better answer when the thermal path and constraint are visible.
- Drawing or marked-up critical dimensions
- Mating material and joint geometry
- Fixed, sliding or compliant mounting points
- Heat source and expected temperature gradient
- Machining, forming, welding or heat-treatment sequence
- Inspection temperature and soak procedure
- Failure mode the design must avoid
6061 thermal expansion, answered.
Short answers for engineers, buyers, machinists and quality teams.
What is the coefficient of thermal expansion of 6061-T6 aluminum?
A commonly used mean linear CTE is about 23.6 µm/(m·K), or 13.1 µin/(in·°F), from about 20 to 100°C. Treat it as a typical planning value for that interval, not a guaranteed constant for every temperature and product.
Is the CTE of 6061-T4 different from 6061-T6?
For preliminary engineering work, T4 and T6 are normally assigned the same nominal CTE. The tempers differ much more in yield strength, hardness, formability and response to restraint than in free thermal expansion.
What does 23.6 µm/(m·K) mean?
It means a one-meter dimension changes by about 23.6 micrometers for each 1°C or 1 K temperature change, when the material is within the stated averaging range and free to move.
How much does one meter of 6061 grow from 20 to 100°C?
Using 23.6 µm/(m·K), the temperature change is 80 K and the free growth is 1.888 mm. The final length is about 1001.888 mm.
Does 6061-T651 expand less than 6061-T6?
Not in the usual nominal CTE calculation. T651 is stress relieved and can show better dimensional stability during machining, but that does not remove normal reversible thermal expansion.
Why do published 6061 CTE values range from about 23.0 to 23.6?
Sources may use different temperature intervals, datasets, specimen conditions, product forms and rounding. Compare the definition and interval before treating two values as a material disagreement.
Can I use 23.6 µm/(m·K) at cryogenic temperatures?
Not across the full cryogenic range. Use an integrated low-temperature expansion curve, such as NIST 6061-T6 data, referenced to a stated temperature.
How do I calculate the thermal change of a bore?
Use the bore diameter as the original linear dimension in ΔD = αD₀ΔT. For a 100 mm bore and an 80°C rise, the increase is about 0.1888 mm using 23.6 µm/(m·K).
Does 6061 aluminum expand more than steel?
Yes, in most common comparisons. Using 23.6 for 6061 and 11.6 µm/(m·K) for a representative steel, a one-meter assembly changes by about 0.960 mm differentially over an 80°C rise.
What happens if the 6061 part is fully restrained?
Thermal strain becomes stress instead of free movement. An idealized EαΔT calculation can screen the upper-bound risk, but real loads require joint stiffness, slip, gradients, plasticity and geometry.
Why is 20°C important when inspecting aluminum parts?
ISO 1:2022 defines 20°C as the standard reference temperature for geometrical and dimensional properties. A warm aluminum part can measure larger even when it was machined correctly.
When should I test the exact material lot?
Consider testing when thermal growth controls a tight fit, optical alignment, sealing, calibration or contractual acceptance, especially over unusual temperature ranges. State the method, interval, specimen direction and uncertainty in advance.
Send the drawing and temperature range—not only the alloy name.
For an aluminum laser-welding or surface-preparation project, share the 6061 temper and product form, critical dimension, operating temperatures, mating material, joint restraint and required inspection condition. Oceanplayer can help review process fit and sample-test planning around the real part.
- Drawing and critical tolerance
- 6061 temper, suffix and product form
- Minimum, maximum and inspection temperatures
- Mating material and mounting method
- Weld, cleaning or marking requirement
- Photos or samples for process validation
Primary data, standards and engineering sources.
Use the revision invoked by the drawing, contract or laboratory quality system. Standards pages describe scope; access to the full standard may require purchase.
- Kaiser Aluminum — 6061 Sheet, Coil and Plate: typical 20–100°C mean CTE and temper properties.
- NIST-hosted ASM data — Properties of Wrought Aluminum and Aluminum Alloys: temperature-interval CTE values.
- NIST — 6061-T6 cryogenic material properties: integrated low-temperature expansion data.
- NIST — Cryogenic Material Properties Database paper: method and equation context.
- NASA CR-195381: reported high-temperature expansion behavior for 6061-T4.
- ISO 1:2022 — Geometrical product specifications: standard reference temperature.
- ASTM E228-22: linear thermal expansion by dilatometry.
- ASTM E831-25: linear thermal expansion by thermomechanical analysis.
- ASTM E289-25: linear thermal expansion by interferometry.
- ASTM B209/B209M-21: aluminum-alloy sheet and plate product scope.
- ASTM B221-21 and ASTM B221M-21: companion standards for aluminum-alloy extruded bars, rods, wire, profiles and tubes.
- The Aluminum Association — ANSI H35.1/H35.1M: alloy and temper designation system.