1. Introduction: The Temper Decision That Determines Facade Performance

I have been specifying aluminum extrusions for curtain wall and window systems for over a decade, and I can tell you that the most consequential decision in facade procurement is not the alloy grade. It is the temper.

Here is what that looks like in practice. A facade contractor specifies 6063-T5 mullions for a 40-story tower because it is the standard architectural temper, it extrudes beautifully, and it costs less than T6. The deflection calculation passes at the design wind load. The project moves forward. But then the structural engineer runs the check at the corner zone, where the wind suction load is 40% higher than the field-of-facade pressure. The T5 mullion deflects beyond the L/175 limit. The contractor has two options: increase the mullion depth, which changes the profile and the glass bite, or switch to T6, which provides 25% more yield strength in the same profile. The T6 switch is the simpler fix. But the T6 mullion costs 10–15% more, and the thermal break rolling process may be more challenging because the higher hardness increases the risk of cracking during crimping.

The problem is not that T5 is a bad temper. It is the architectural default for good reason — excellent extrudability, good surface finish, lower cost, and sufficient strength for most mid-rise and low-rise applications. The problem is that high-rise facades, particularly in corner zones and coastal exposure categories, push the wind load beyond what T5 can handle within the deflection limits. And the specification must account for the corner zone pressure, not just the average facade pressure.

The global aluminum extrusion market for architectural applications continues to expand, driven by high-rise construction in Asia, the Middle East, and North America. The selection between 6063-T5 and 6063-T6 is not a question of which temper is better. It is a question of which temper the wind load requires, and whether the formability and cost trade-offs are acceptable for the specific profile and fabrication process.

This guide breaks down the two tempers — the heat treatment, the mechanical properties, the wind load calculation, the deflection limits, the formability trade-offs, and the sourcing framework that matches the temper to the facade requirement.

2. What T5 and T6 Actually Mean in the Extrusion Plant

The difference between T5 and T6 is the heat treatment route, and that route determines the mechanical properties, the dimensional stability, and the cost.

6063-T5 is cooled from the extrusion temperature using air cooling — typically fans or forced air — and then artificially aged. The cooling rate is slower than water quenching, which means the magnesium silicide (Mg₂Si) precipitates partially during cooling rather than being fully dissolved and re-precipitated during aging. The result is a temper with moderate strength, good ductility, and lower residual stress. The production process is simpler and faster — 1 to 2 days from extrusion to finished profile — and the cost is lower because there is no solution furnace or water quenching system.

6063-T6 undergoes a full solution heat treatment at 520–540°C to dissolve the Mg₂Si, followed by water quenching to form a supersaturated solid solution, and then artificial aging to peak strength. The water quench is the critical difference. It cools the extrusion fast enough to retain the alloying elements in solution, which allows the subsequent aging treatment to produce a finer, more uniform precipitate distribution. The result is higher strength, higher hardness, and lower elongation. The trade-off is higher residual stress from the rapid quench and a more complex production process — 3 to 5 days — with a cost premium of 10–15% over T5.

3. Mechanical Properties: The Strength Gap in Numbers

The mechanical property difference between T5 and T6 is substantial. It is approximately a 20–25% increase in tensile strength and a corresponding increase in yield strength and hardness.

The published minimum properties for 6063 extrusions per ASTM B221 and EN 755 are:

Sources: ASTM B221, EN 755, beamdimensions.com, MakeItFrom

The modulus of elasticity is identical for both tempers. This is critical for deflection calculations. The stiffness of the mullion — its resistance to bending under wind load — depends on the modulus of elasticity and the moment of inertia, not on the yield strength. A T5 mullion and a T6 mullion of the same profile deflect exactly the same amount under the same load, as long as the stress remains below the yield point of T5.

Where the tempers differ is in the strength limit — the point at which the material yields and the deflection becomes permanent. A T5 mullion yields at 110–145 MPa. A T6 mullion yields at 170–214 MPa. The T6 mullion can carry approximately 30–45% more load before yielding. That additional strength margin is what matters in high-wind zones where the stress approaches the yield limit.

The hardness difference — approximately 33% — affects the surface durability and the machinability. T6 machines more cleanly with better dimensional stability, while T5 is more forgiving for bending and forming.

4. Wind Load Fundamentals for High-Rise Facades

The wind load on a high-rise facade is not a single number. It varies with height, exposure category, building geometry, and — critically — location on the facade.

The design wind pressure is calculated per the applicable code: ASCE 7 in the United States, EN 1991-1-4 in Europe, or the local building code in other regions. The calculation produces a velocity pressure that increases with height above grade, and pressure coefficients that vary with the building's shape and the location on the facade.

The key variable that most specifiers underestimate is the corner zone pressure. Wind flowing around a building corner accelerates, creating a suction pressure that is significantly higher than the field-of-facade pressure. In ASCE 7, the corner zone pressure coefficient can be 1.5 to 2.0 times the field-of-wall coefficient. In the corner zones of a high-rise tower, the design wind pressure can reach 3.5 to 4.0 kPa or more, compared to 1.5 to 2.5 kPa in the field of the facade.

The mullion must be designed for the worst-case pressure at its location, not the average. A mullion in the corner zone sees a higher load than a mullion in the center of the facade. The specification must either use a conservative average pressure for the entire facade or design the corner mullions separately with a higher pressure.

The mullion's flexural capacity is governed by the Aluminum Design Manual (ADM) in the U.S. or Eurocode 9 in Europe. The ADM specifies four flexural limit states: yielding, rupture, lateral-torsional buckling, and local buckling. The available strength is the least of these. For 6063-T6, the ADM gives a minimum tensile yield strength (Fty) of 25 ksi (172 MPa) and a minimum tensile ultimate strength (Ftu) of 30 ksi (207 MPa) . For 6063-T5, the values are lower — approximately 16 ksi (110 MPa) yield and 22 ksi (150 MPa) ultimate.

5. Why Deflection, Not Strength, Often Governs the Selection

In most curtain wall mullion designs, the deflection limit governs the selection, not the strength limit. The deflection limit is typically L/175 for mullions supporting glass, where L is the span between supports. This limit exists to prevent excessive glass edge movement, water infiltration, and visual distortion.

Deflection is calculated using the modulus of elasticity (E) and the moment of inertia (I) of the mullion profile:

δ = (5 × w × L⁴) / (384 × E × I)

Where δ is the deflection, w is the distributed wind load, L is the span, E is the modulus of elasticity, and I is the moment of inertia. Since E is the same for T5 and T6, the deflection depends on the profile geometry (I) and the load (w). Switching from T5 to T6 does not reduce the deflection — it only increases the load at which the mullion yields.

This is the critical distinction. If the mullion deflects beyond L/175 at the design wind load, switching to T6 will not fix the deflection problem. The profile must be deepened, the moment of inertia must be increased, or the span must be reduced. The T6 switch only helps if the mullion is strong enough to carry the load but the stress exceeds the yield strength of T5.

The typical scenario where T6 is the correct fix is this: the deflection check passes, but the stress check fails — the bending stress exceeds the yield strength of T5. The stress in a mullion is calculated as:

σ = M / S

Where σ is the bending stress, M is the moment, and S is the section modulus. If the stress exceeds 110–145 MPa (the T5 yield range), the mullion yields. Switching to T6 raises the yield limit to 170–214 MPa, which may resolve the stress failure without changing the profile.

6. The Formability Trade-Off: Bending, Crimping, and Thermal Break Rolling

The higher hardness and lower ductility of T6 create fabrication challenges that T5 avoids. These challenges are not academic — they can determine whether the profile can be produced at all.

Bending and forming. T5 handles larger bending angles without cracking. T6 requires larger bend radii due to its higher hardness and lower elongation. For profiles that require post-extrusion bending — curved mullions, archtop windows, radiused corners — T5 is the more forgiving temper. A bend radius that works in T5 may crack in T6.

Thermal break rolling. The thermal break in a curtain wall mullion is created by crimping the aluminum around a polyamide or polyurethane isolator. The crimping process requires the aluminum to deform plastically without cracking. T6, with its lower elongation and higher hardness, is more susceptible to cracking during crimping. Industry sources note that T6 is suitable for large curtain walls and high-strength applications, but because of its high hardness, it can crack during thermal break rolling. This is a production constraint that must be evaluated before specifying T6 for a thermally broken profile.

Welding. T5 welds more stably with smaller strength loss. T6 requires tighter control to prevent over-aging in the heat-affected zone.

Machining. T6 performs better in precision machining with smoother surfaces and better dimensional stability. T5 may cause tool adhesion during cutting. For profiles that will be CNC-machined after extrusion — for hardware mounting, drainage slots, or connection details — T6 provides a better surface finish and tighter tolerances.

7. Corrosion and Stress Corrosion Considerations

Both T5 and T6 belong to the Al-Mg-Si system and have good natural corrosion resistance. The oxide layer that forms on the surface protects the material in most atmospheric environments, including humid and coastal conditions.

The difference lies in residual stress. T6 is water-quenched, which introduces higher internal stress than the air-cooled T5 process. That residual stress increases the risk of stress corrosion cracking — a failure mode where a sustained tensile stress combined with a corrosive environment causes crack propagation. The risk is higher in coastal environments with chloride exposure and in profiles that carry sustained tensile stress in service.

The mitigation for T6 is stress relief or anodizing. Stress relief can be achieved by stretching the extrusion after quenching (T651 temper) or by a low-temperature thermal treatment. Anodizing creates a thicker, more protective oxide layer that resists chloride attack. For coastal high-rise projects, the specification should require either T651 temper (stress-relieved T6) or a minimum anodizing thickness of 15 μm for exterior exposure.

T5, with its lower residual stress, is less susceptible to stress corrosion cracking. For coastal projects where the stress levels are within the T5 yield limit, T5 may be the more corrosion-resistant choice despite its lower strength.

8. Cost and Lead Time: The Real Premium for T6

The cost difference between T5 and T6 is real, and it affects both the material price and the project schedule.

Material cost. T6 costs approximately 10–15% more per kilogram than T5. The premium covers the additional solution furnace time, the water quenching system, the higher energy consumption, and the slower production cycle. For a 45-story tower with 80,000 kg of mullion extrusions, a 12% premium is approximately $24,000 to $36,000 depending on the base extrusion price.

Production lead time. T5 production takes 1–2 days from extrusion to finished profile. T6 production takes 3–5 days because of the additional solution treatment and quenching steps. For a project on a tight schedule, the additional 2–3 days per batch may not be critical. But for projects with multiple extrusion batches and a compressed facade installation programme, the cumulative lead time difference can affect the critical path.

The breakeven question. The T6 premium is justified when the stress check fails in T5 and the alternative is a deeper profile. A deeper profile increases the material weight, the glass bite, the hardware size, and the thermal break complexity. The T6 premium of 10–15% is often cheaper than the cost of redesigning the profile, retooling the die, and re-qualifying the system.

9. Head-to-Head: Complete Temper Comparison Table

10. The Selection Decision Matrix

How to read this matrix. The wind pressure is the first gate. Below 2.5 kPa, T5 is almost always sufficient. Above 3.5 kPa, T6 is usually required — or a deeper T5 profile. The deflection check is the second gate. If the deflection fails, neither temper helps; the profile must change. The stress check is the third gate. If the stress exceeds the T5 yield limit but the deflection passes, T6 is the correct fix. The fabrication process is the fourth gate. If the profile requires tight-radius bending or thermal break crimping, T5 may be the only viable option regardless of the wind load.

11. Case Study: A 45-Story Coastal Tower Mullion Specification

The most instructive scenario for the T5 versus T6 decision comes from a 45-story mixed-use tower on a coastal site in Southeast Asia.

The project. A 45-story tower with a unitized curtain wall system. The mullion spans were 3.6 meters between floor slabs. The design wind pressure was 2.8 kPa in the field of the facade and 4.2 kPa in the corner zones, based on ASCE 7 exposure category C and the building's height and geometry.

The original specification. The facade contractor specified 6063-T5 mullions for the entire facade. The deflection check at 2.8 kPa passed with a T5 profile with a moment of inertia of 1,850 cm⁴. The stress check at 2.8 kPa produced a bending stress of 118 MPa — within the T5 yield range of 110–145 MPa.

The corner zone problem. The structural engineer recalculated the corner zone mullions at 4.2 kPa. The bending stress rose to 177 MPa — above the T5 yield limit of 145 MPa and above the T6 minimum yield of 170 MPa. The deflection at 4.2 kPa was L/210 — within the L/175 limit, but with less margin.

The options evaluated. Option 1: Switch the corner mullions to 6063-T6. The stress dropped to 177 MPa against a T6 yield of 170–214 MPa — marginal but acceptable with the actual T6 properties from the supplier. Option 2: Deepen the T5 mullion from 150mm to 175mm to increase the moment of inertia. This would resolve both the deflection and the stress, but it would change the glass bite, the thermal break, and the hardware interface. The tooling cost for the new die was $8,000, and the lead time was 6 weeks.

The decision. The contractor selected 6063-T6 for the corner zone mullions and retained 6063-T5 for the field of the facade. The T6 corner mullions were produced with T651 stress-relieved temper to mitigate the stress corrosion risk in the coastal environment. The cost premium for the T6 corner mullions was approximately $4,200 — significantly less than the $8,000 tooling cost and 6-week delay of the deeper profile option.

The lesson. The T5 mullion was adequate for the field of the facade but failed the stress check in the corner zone. The T6 switch resolved the stress failure without changing the profile. The hybrid approach — T5 in the field, T6 in the corners — optimized the cost while meeting the structural requirement. The stress-relieved temper and the coastal exposure category drove the decision to specify T651 rather than standard T6.

12. Practical Sourcing Framework for B2B Buyers

If you are specifying aluminum extrusions for a high-rise facade, here is the framework I recommend.

Step 1: Calculate the wind load at every zone, not just the field of facade. The corner zone pressure can be 1.5 to 2.0 times the field pressure. The mullion specification must account for the worst-case pressure at each location. If the corner zones require a higher temper, specify them separately.

Step 2: Run the deflection check first. Deflection is governed by the modulus of elasticity and the moment of inertia. Since E is the same for T5 and T6, switching temper does not reduce deflection. If the deflection fails, the profile must be deepened or the span reduced.

Step 3: Run the stress check against both tempers. Calculate the bending stress at the design wind load. Compare it to the T5 yield range (110–145 MPa) and the T6 yield range (170–214 MPa). If the stress exceeds 145 MPa but is below 170 MPa, T6 may be the correct specification. If the stress exceeds 214 MPa, neither temper is sufficient — the profile must change.

Step 4: Evaluate the formability requirements. Does the profile require tight-radius bending? Does it require thermal break crimping? If yes, T6 may be difficult or impossible to produce. The fabrication process must be evaluated before the temper is specified.

Step 5: Specify stress-relieved T6 for coastal or high-chloride environments. Standard T6 has higher residual stress than T5, which increases the stress corrosion cracking risk. Specify T651 or T652 temper (stress-relieved by stretching) for coastal projects, or require a minimum anodizing thickness of 15 μm.

Step 6: Calculate the cost premium against the alternative. The T6 premium is 10–15% over T5. The alternative — deepening the T5 profile — costs tooling, increases weight, and changes the system geometry. For a corner zone application, T6 is often the lower-cost solution.

Step 7: Require mill certificates and mechanical test data. The supplier should provide the actual tensile and yield strength from the production batch, not just the minimum specification values. The actual T6 properties may be higher than the minimum, providing additional margin.

13. Frequently Asked Questions

Q: What is the main difference between 6063-T5 and 6063-T6?

The main difference is the heat treatment. T5 is air-cooled after extrusion and artificially aged, resulting in moderate strength and good formability. T6 is solution heat-treated at 520–540°C, water-quenched, and artificially aged, resulting in higher strength (20–25% higher tensile strength) and higher hardness, but lower ductility and higher residual stress.

Q: Which temper is better for high-rise curtain wall mullions?

It depends on the wind load and the profile geometry. T5 is adequate for most mid-rise and low-rise facades. T6 is required for high-rise corner zones where the wind pressure exceeds the T5 yield limit. The stress check determines which temper is needed.

Q: Does switching from T5 to T6 reduce deflection?

No. The modulus of elasticity is identical for both tempers. Deflection depends on the modulus and the moment of inertia, not the yield strength. Switching to T6 only helps if the failure mode is yielding, not deflection.

Q: Can 6063-T6 be used for thermally broken profiles?

It can be used, but the thermal break crimping process is more challenging because the higher hardness and lower ductility of T6 increase the risk of cracking during crimping. The profile design and the crimping parameters must be evaluated for T6 compatibility. T5 is more forgiving for this process.

Q: Is T6 more expensive than T5?

Yes. T6 costs approximately 10–15% more per kilogram than T5. The premium covers the additional solution treatment, water quenching, and longer production cycle.

Q: What is stress corrosion cracking and why does it matter for T6?

Stress corrosion cracking is a failure mode where a sustained tensile stress combined with a corrosive environment causes crack propagation. T6 has higher residual stress from water quenching, which increases the risk. For coastal projects, specify stress-relieved T6 (T651) or use a thicker anodizing layer.

Q: Can I use T5 for the field of the facade and T6 for the corners?

Yes. This hybrid approach is common in high-rise projects. The field mullions use T5 for cost efficiency and formability. The corner mullions use T6 for the higher wind load. The two tempers are specified separately by zone.

Q: What does the Aluminum Design Manual say about 6063-T6?

The ADM gives a minimum tensile yield strength (Fty) of 25 ksi (172 MPa) and a minimum tensile ultimate strength (Ftu) of 30 ksi (207 MPa) for 6063-T6. The available flexural strength is governed by yielding, rupture, lateral-torsional buckling, or local buckling — whichever is the least.

14. Final Thoughts

Let me leave you with this.

The choice between 6063-T5 and 6063-T6 is not a choice between a standard temper and a premium temper. It is a choice between two heat treatment routes that produce different mechanical properties and different fabrication behaviors. T5 is the architectural default — excellent extrudability, good formability, lower cost, and sufficient strength for most applications. T6 is the high-wind temper — 20–25% stronger, harder, and more expensive, with formability limitations that must be evaluated before specification.

The data is clear. T5 delivers 150–186 MPa tensile strength and 110–145 MPa yield strength. T6 delivers 205–241 MPa tensile strength and 170–214 MPa yield strength. The modulus of elasticity is the same — 69–70 GPa — so switching to T6 does not reduce deflection. The T6 premium is 10–15%, and the production cycle is 2–3 days longer. The thermal break crimping process is more challenging in T6 because of the higher hardness.

The 45-story coastal tower case study demonstrates the principle. The T5 mullion was adequate for the field of the facade at 2.8 kPa. The corner zone at 4.2 kPa produced a stress of 177 MPa — above the T5 yield limit but within the T6 range. The T6 switch resolved the stress failure without changing the profile. The hybrid approach — T5 in the field, T6 in the corners — optimized the cost.