6061 Aluminum Thermal Conductivity: T4 vs T6
At about 20°C (68°F), Kaiser lists typical thermal conductivity of 154 W/(m·K) for 6061-T4/T451 and 167 W/(m·K) for 6061-T6/T651. In that dataset, T6 is about 8.4% higher.
The 60-second answer
Most visitors only need four decisions before using the data. Start here, then move into the formulas, test methods and purchase specification only if the thermal result is important to the design or contract.
Use 154 W/(m·K)
Use it near room temperature when your source and product condition match 6061-T4/T451. Label it as a typical reference value.
Use 167 W/(m·K)
T6 is about 8.4% higher in the same table. That does not mean the final device temperature improves by 8.4%.
Choose by process first
T4 often helps when forming or later aging is planned. T6 is a common final condition when strength and machining matter.
Typical is not guaranteed
ASTM product compliance does not automatically make 154 or 167 W/(m·K) a lot acceptance limit.
6061-T4 vs T6 thermal conductivity at a glance
| 6061 condition | Typical k at 20°C | BtuIT/(h·ft·°F) | BtuIT·in/(h·ft²·°F) | How to use it |
|---|---|---|---|---|
| O temper | 180 W/(m·K) | 104 | About 1,250 | Reference for annealed material; not the normal strength choice for a finished structural part. |
| T4 / T451 | 154 W/(m·K) | 89.0 | About 1,070 | Preliminary room-temperature input when naturally aged material and the source conditions match. |
| T6 / T651 | 167 W/(m·K) | 96.5, often rounded to 97 | About 1,160 | Preliminary room-temperature input for artificially aged material under matching conditions. |
Primary values: Kaiser 6061 sheet, coil and plate and Kaiser 6061 rod and bar. Both label the conductivity values as typical.
The percentage depends on the denominator. Saying “T6 is 8.44% higher than T4” uses 154 as the baseline. Saying “T4 is 7.78% lower than T6” uses 167. Both statements describe the same pair, but reports should state the baseline so readers do not compare unlike percentages.
The O-temper value also prevents a common mistake: strength and conductivity do not follow one simple ranking. Annealed 6061 is softer but has the highest listed conductivity of the three conditions. Heat treatment changes both strength and how electrons move through the alloy.
What does thermal conductivity actually mean?
Thermal conductivity, written as k or λ, tells you how easily heat moves through the bulk material. A higher value means the same shape can carry more heat for the same temperature difference.
It does not describe the whole component. A thin bridge, a rough joint or a poor air path can still make a high-conductivity material run hot.
Q̇ = kAΔT / LHeat flow rises with conductivity and area, and falls as the path gets longer.
R = L / (kA)Resistance falls when k or area increases. Units are kelvin per watt, K/W.
Three terms that are often mixed up
- Conductivity k is a material property in W/(m·K).
- Conductance G = kA/L includes the actual geometry and is reported in W/K.
- Thermal diffusivity α = k/(ρcp) describes how quickly a temperature change spreads. It also depends on density and heat capacity.
Heat-conduction diagram: Chcastan / Wikimedia Commons, public domain.
Why can 6061-T6 conduct heat better than T4?
The answer is not “T6 is purer.” The alloy chemistry remains 6061. Heat treatment changes where magnesium, silicon and other solute atoms sit inside the aluminum.
Create a supersaturated matrix
Heating dissolves strengthening elements into the aluminum matrix. A rapid quench keeps more solute in solution than would remain at equilibrium.
T4 develops at room temperature
Small clusters and early zones form over time. Strength rises from the freshly quenched state, but dissolved solute still scatters the electrons that carry much of the heat.
T6 forms fine precipitates
Controlled furnace aging creates a stronger precipitate structure and removes some solute from the matrix. In the cited data, electrical and thermal conductivity both rise from T4 to T6.
This diagram shows the main path to T6. Real soak, quench and aging practice depends on the product and qualified procedure.
Diagram: Bob Clemintime / Wikimedia Commons, CC BY-SA 4.0.
An etched 6061-T6 microstructure shows grain boundaries and secondary phases. It is not a direct heat-flow map or a universal structure for every 6061 product.
Image: Bob Clemintime / Wikimedia Commons, CC BY-SA 4.0.
Do not turn a trend into a universal rule
Composition within 6061 limits, quench rate, aging history, product form, defects and test temperature can shift the result.
- T6 is not always exactly 167 W/(m·K).
- T4 is not always exactly 154 W/(m·K).
- IACS is useful process evidence, not an automatic W/(m·K) guarantee.
- Hot service or welding can change the local temper.
Convert W/(m·K) to BTU units correctly
Thermal conductivity contains power, length and temperature-interval units. Multiplying watts by 3.412 alone does not convert the complete quantity.
BtuIT/(h·ft·°F) = W/(m·K) × 0.577789154 becomes 88.98, normally shown as 89.0. 167 becomes 96.49, often rounded to 97.
W/(m·K) = BtuIT/(h·ft·°F) × 1.730735Use this inverse only for the complete foot-based conductivity unit.
BtuIT·in/(h·ft²·°F) = W/(m·K) × 6.93347This result is 12 times the foot-based number because the length numerator changes from feet to inches.
W/(in·K) = W/(m·K) × 0.0254T4 becomes 3.91 W/(in·K), while T6 becomes 4.24 W/(in·K).
| Starting value | BtuIT/(h·ft·°F) | BtuIT·in/(h·ft²·°F) | W/(in·K) | W/(m·°C) |
|---|---|---|---|---|
| 6061-T4/T451: 154 W/(m·K) | 89.0 | About 1,070 | 3.91 | 154 |
| 6061-T6/T651: 167 W/(m·K) | 96.5 | About 1,160 | 4.24 | 167 |
Conversion factors follow the NIST Guide to the SI, Appendix B.9. The table uses the international-table Btu, BtuIT.
1 K equals 1°C
W/(m·K) and W/(m·°C) have the same numerical conductivity because kelvin and Celsius intervals are equal in size.
20°C = 293.15 K
Add 273.15 only when converting an absolute Celsius reading to kelvin. Do not add it to a temperature difference.
Δ°F = 1.8 × Δ°C
The 32-degree offset belongs to absolute temperature readings, not temperature differences used in conduction formulas.
What does the 8.4% difference change in a part?
Consider an ideal solid path that is 50 mm long with a 20 mm × 20 mm cross section. In SI units, L = 0.050 m and A = 0.000400 m².
R = L/(kA)This example assumes steady, one-dimensional conduction, constant k, no contact resistance and no internal heat generation.
| Result | T4 at 154 W/(m·K) | T6 at 167 W/(m·K) |
|---|---|---|
| Bulk resistance | 0.812 K/W | 0.749 K/W |
| Temperature drop at 100 W | 81.2 K | 74.9 K |
| Difference | T6 reduces this idealized drop by about 6.3 K. | |
The example shows a visible difference because the metal path is the only resistance. A real assembly may behave very differently.
A real heat path contains several resistances, not only the aluminum. Diagram: Dtc5341 / Wikimedia Commons, public domain.
Five conditions that can move the real thermal result
The catalog number is a starting point. Use a more specific value or a sensitivity range when one of these factors is important.
k changes with temperature
The Kaiser figures are at 20°C. A room-temperature value should not be carried across cryogenic or high-temperature service without a matching k(T) dataset.
Plate is not automatically extrusion
Plate, bar, extrusion, tube and forging can have different processing histories. Match the source to the purchased form and size.
Keep the stress-relief suffix
T451, T651, T4511 and T6511 record different processing routes. Do not shorten the ordered condition because the conductivity table groups some values.
Critical designs need matching specimens
Texture, porosity, coatings, specimen direction and local defects can affect measured or effective conductivity.
Welding and hot exposure matter
A weld, brazing cycle, solution treatment, aging cycle or long hot bake can change the local precipitation state and the final part.
Use a value ladder
Move from handbook value → supplier-specific value → temperature curve → lot test → assembly validation as the risk and thermal sensitivity increase.
Fin height, spacing, base thickness, source footprint and airflow often control more of the total result than 13 W/(m·K).
Aluminum heat-sink profiles; alloy and temper are not certified by the image. Ulfbastel / Wikimedia Commons, public domain.
Should you choose T4 or T6?
Do not choose the temper from conductivity alone. The manufacturing route and final mechanical requirements normally come first.
Choose T4 or T451 when
- The part needs meaningful forming before its final age.
- The producer or fabricator will artificially age it after forming.
- Lower strength in the supplied state is acceptable.
- Your preliminary model can tolerate the lower typical k.
Choose T6 or T651 when
- The delivered part needs the common higher-strength final condition.
- Machining and dimensional stability favor a stress-relieved product.
- The modest conductivity increase helps a bulk conduction path.
- No later thermal cycle will quietly change the qualified state.
Keep the full suffix
T651 and T6511 are not decorative names. The Aluminum Association notes that temper designations represent different processing parameters and should not be treated as automatic substitutions.
Where does 6061 thermal conductivity matter most?
The same alloy value can be important in one product and almost irrelevant in another. Identify which part of the heat path controls temperature.
Bulk k can matter
The benefit grows when heat travels a long in-plane distance through the aluminum. Also check channel walls, flow distribution, sealing and the source interface.
Interfaces often dominate
Mounting pads, flatness, thermal interface material and external convection can matter more than the T4-to-T6 difference.
Fin design shares control
Base spreading, fin efficiency, profile tolerance and airflow decide the final temperature together with conductivity.
Long paths increase sensitivity
Conductivity can influence temperature uniformity, but joints, cell contact, cooling flow, crash loads and leak integrity still govern the design.
Couple solid and fluid resistance
Wall conduction may be smaller than the fluid-film resistance. Pressure, fatigue, corrosion and sealing remain central.
Uniformity may be the goal
Conductivity can help warm-up and temperature balance, while residual stress, heater placement, control sensors and cycling control accuracy.
How should critical conductivity be tested?
Choose a method that fits the temperature, specimen and required uncertainty. A flash result and a steady-state conductivity result are not the same measurement route.
Steady-state comparative route
The method determines conductivity of homogeneous opaque solids using a guarded comparative longitudinal heat-flow approach. Review specimen mean temperature, orientation, contacts, reference materials and uncertainty.
Flash diffusivity route
The flash method primarily measures thermal diffusivity. Conductivity is then derived from k = αρcp, so density and heat-capacity inputs must match the condition and temperature.
Test what the customer experiences
A finished-part test can include joints, coatings, interfaces and convection. It is often more useful for product temperature, but it does not isolate bulk k without a suitable model.
What should the RFQ or PO actually say?
A strong order separates a reference model input from a property that the supplier must test and guarantee.
Alloy and complete temper
State AA 6061 / UNS A96061 and the exact T4, T451, T6, T651, T6511 or other required condition.
Form, size and standard
Name plate, sheet, extrusion, bar, tube or forging; dimensions; and the applicable current product specification.
Reference or acceptance?
Say whether k is only a design input or a contractual minimum/range. Include temperature and heat-flow direction.
Method and sampling
Define the method, edition, specimen locations, quantity per lot, temperatures and any density or cp inputs.
Uncertainty and conformity
State how reported uncertainty and guard bands affect acceptance near a limit.
Reports and traceability
Require the inspection certificate, heat-treatment record, thermal report, calibration traceability and change notification needed by the contract.
Who forms, welds or ages it?
List machining, forming, welding, brazing, aging, anodizing, coating and adhesive cure that can change the final heat path.
Define the actual service
Provide metal temperature range, heat load, interfaces, environment and prototype acceptance temperatures.
Common product standards include ASTM B209/B209M for sheet and plate, ASTM B221 for extruded products and ASTM B211/B211M for rolled or cold-finished bar, rod and wire. Compliance with these product standards does not by itself guarantee 154 or 167 W/(m·K).
What happens after machining, welding or anodizing?
Machining changes geometry, not normally the named temper
Removing metal changes area and path length, so conductance changes even if bulk k does not. Heavy cutting can also reveal residual-stress problems, making the correct stress-relieved suffix valuable.
Welding creates a local thermal history
The weld metal and heat-affected zone do not automatically keep the parent T6 condition. Joint geometry, porosity, oxide and contact area also create a nonuniform heat path.
Anodizing is a separate layer
An oxide or coating should normally be modeled as its own layer and interface. Do not lower the conductivity of the entire aluminum part just because one surface is anodized.
Thermal interface material does not change aluminum k
It changes joint resistance. Bond-line thickness, pressure, voids and aging can easily outweigh the 13 W/(m·K) T4-to-T6 difference.
Use a qualified process and a model or test that represents the local joint when heat flow crosses the weld.
Experimental TIG weld on 6061 plate; it does not establish a production post-weld temper. W.S. Yerazunis / Wikimedia Commons, public domain.
Eight common 6061 conductivity mistakes
Calling 167 a universal minimum
The cited data label it typical. Turn it into an acceptance limit only through an agreed specification and evidence plan.
Mixing temperatures
Comparing T4 at 20°C with T6 at another temperature mixes temper and temperature effects.
Dropping the temper suffix
T6, T651 and T6511 are not automatic substitutes for procurement or dimensional behavior.
Using 3.412 alone
That converts watts to Btu/h, not the full conductivity unit containing length and temperature interval.
Adding 32 to ΔT
The offset belongs to absolute Fahrenheit and Celsius readings. Temperature differences use a scale factor only.
Confusing the two BTU units
Btu/(h·ft·°F) and Btu·in/(h·ft²·°F) differ numerically by a factor of 12.
Expecting an 8.4% device gain
Only the bulk aluminum term changes. Interfaces, convection and spreading may control most of the system.
Ignoring final processing
Welding, aging, coating and adhesive cure can change the condition or add a new resistance after the material arrives.
Label every model input
Record source, product form, temper, temperature, direction and whether the value is typical, conservative, measured or guaranteed.
Send the 6061 form, temper, temperature and heat path.
For a useful review, include product form, exact temper, section size, operating temperature, heat load, heat-flow direction, interfaces, coating, joining method and whether conductivity is a design reference or a contractual requirement.
Related Oceanplayer engineering guides
These published pages add alloy selection, fabrication and thermal-property context.
6061 aluminum thermal conductivity FAQ
Short answers to the questions most often asked during design, conversion and sourcing.
What is the thermal conductivity of 6061-T6 aluminum?
Kaiser lists 167 W/(m·K) as a typical value for 6061-T6/T651 at 20°C. Use it as a documented preliminary value unless the actual supplier or test report guarantees something else.
What is the thermal conductivity of 6061-T4?
Kaiser lists 154 W/(m·K) as a typical room-temperature value for 6061-T4/T451 in its rod-and-bar data. The sheet-and-plate table lists 154 W/(m·K) for T451.
Is 6061-T6 always 8.4% more conductive than T4?
No. The 8.44% result compares 167 with 154 in one 20°C producer dataset. Chemistry, heat treatment, product form, temperature and measurement can change the actual difference.
Why can T6 conduct heat better than T4?
Artificial aging changes the precipitate and solute distribution. Less solute in the aluminum matrix can reduce electron scattering, so conductivity can rise while fine precipitates increase strength.
How do I convert W/(m·K) to Btu/(h·ft·°F)?
Multiply the W/(m·K) value by 0.577789 for the IT-Btu form. Therefore, 154 becomes about 89.0 and 167 becomes about 96.5 Btu/(h·ft·°F).
Why do some tables show about 1,160 instead of 97?
About 1,160 Btu·in/(h·ft²·°F) is the inch-based form. About 97 Btu/(h·ft·°F) is the foot-based form. They describe the same conductivity and differ by a factor of 12.
Are W/(m·K) and W/(m·°C) numerically the same?
Yes, for thermal conductivity they have the same numerical value because a temperature interval of 1 K equals an interval of 1°C.
Does ASTM B209 or B221 guarantee thermal conductivity?
Not automatically. Product standards control stated requirements such as composition and mechanical properties. A conductivity guarantee must be added clearly to the purchase contract with a test and acceptance plan.
Does welding change 6061-T6 conductivity?
Welding changes the local thermal history and can alter the parent T6 condition in the heat-affected zone. For a critical path, model or test the weld region rather than assigning the parent value everywhere.
Is T6 always the best 6061 temper for heat sinks?
No. T6 offers a modest bulk-conductivity advantage in the cited dataset and higher final strength, but extrusion geometry, interfaces, airflow, forming and cost can matter more.
Sources used for this guide
The page separates manufacturer typical values, official unit factors, product standards and thermal-test methods so the reader can see what each source does—and does not—prove.
- Kaiser Aluminum — 6061 Sheet, Coil and Plate Technical Data: typical 20°C physical-property values for O, T451, T6 and T651.
- Kaiser Aluminum — 6061 Rod and Bar Technical Data: typical 20°C values for T4/T451 and T6/T651.
- Hydro — Alloy 6061 Datasheet: product, fabrication and temper context for extruded 6061.
- NIST Guide to the SI, Appendix B.9: conductivity and thermal-resistance conversion factors.
- NIST Cryogenic Materials — 6061-T6 Aluminum: temperature-dependent correlation within its stated range.
- Aluminum Association — ANSI H35.1/H35.1M: current aluminum alloy and temper designation system.
- ASTM E1225-25a: guarded comparative longitudinal heat-flow conductivity method.
- ASTM E1461: flash-method thermal diffusivity and derived conductivity context.
- MIT Unified Engineering — Heat Conduction: Fourier’s law and the role of temperature-dependent conductivity.