Aluminum Tempering Guide: T4, T5, T6, T7 and T8 Tempers Explained

Understanding Aluminum Temper Designations

Aluminum temper designations, defined by ANSI H35.1 and ISO 2107, describe the thermal and mechanical processing history of heat-treatable aluminum alloys. The basic system uses a letter (F, O, H, T) followed by one or more digits. The T temper series applies to heat-treatable alloys (2xxx, 6xxx, 7xxx) that are solution heat-treated and optionally aged to achieve precipitation hardening. Understanding the T-temper system is essential for specifying the correct mechanical properties for aluminum components in aerospace, automotive, and structural applications. The key distinction is between natural aging (T4) — precipitation at room temperature — and artificial aging (T5, T6, T7, T8) — precipitation at elevated temperature (150-200°C) to accelerate and control the process.

T4: Solution Heat-Treated and Naturally Aged

T4 temper involves solution heat treatment (typically 500-540°C depending on alloy) followed by quenching (usually in water) and natural aging at room temperature for a minimum of 4-5 days. Natural aging in 6061 produces Guinier-Preston (GP) zones — coherent precipitates only a few atomic layers thick — that increase yield strength from approximately 145 MPa (as-quenched) to 275 MPa (fully naturally aged). T4 provides good formability and moderate strength, making it suitable for applications requiring subsequent forming operations. T4 material will continue to age at room temperature — strength increases slowly for years, though 90% of the age-hardening response occurs within the first 2 weeks. 2024-T4 is the classic aerospace aluminum alloy combination, offering 345 MPa yield with excellent fatigue crack growth resistance.

T5: Artificially Aged from Elevated-Temperature Forming

T5 temper is applied to products that are cooled from an elevated-temperature forming process (extrusion, forging) and then artificially aged without separate solution heat treatment. The forming temperature is sufficient to put the alloying elements into solution, eliminating the need for a separate solution treatment step. T5 is commonly used for 6063 extrusions (window frames, architectural sections) and 4032 forging alloys. The mechanical properties of T5 are generally lower than T6 because the solution treatment temperature and time are not independently optimized. 6063-T5 achieves 145 MPa yield versus 6063-T6's 215 MPa yield — but the cost savings from eliminating the separate solution treatment step make T5 economical for non-critical applications.

T6: Solution Heat-Treated and Artificially Aged

T6 is the most common temper for heat-treatable aluminum alloys, combining solution heat treatment, quenching, and artificial aging to peak hardness. The aging temperature is typically 170-190°C for 6-12 hours, depending on the alloy and desired properties. T6 produces the highest strength in the T-temper series for most alloys. 6061-T6 achieves 276 MPa yield (12% elongation), 7075-T6 achieves 503 MPa yield (11% elongation), and 2024-T6 achieves 400 MPa yield (5% elongation). The T6 condition is the standard for structural aluminum applications where maximum strength is required. However, T6 may have reduced corrosion resistance in some alloys (particularly 7075-T6, which is susceptible to exfoliation corrosion and stress corrosion cracking) — T73 temper was developed to address this limitation.

T7: Solution Heat-Treated and Overaged

T7 temper involves artificial aging beyond peak hardness (overaging), sacrificing some strength for improved corrosion resistance, dimensional stability, or elevated-temperature performance. The most important T7 variant is T73 — developed specifically for 7075 to eliminate exfoliation corrosion and stress corrosion cracking susceptibility. 7075-T73 achieves 435 MPa yield (vs 503 MPa for T6) but with guaranteed immunity to SCC in standard test environments. T76 and T74 provide intermediate combinations of strength and corrosion resistance. T7 tempers are also used for high-temperature applications where overaging stabilizes the precipitate structure — 2618-T61 (a T6-type) is used for elevated-temperature service up to 200°C in piston and compressor applications.

T8: Solution Heat-Treated, Cold Worked, and Artificially Aged

T8 temper introduces cold work (stretching or compression) between solution treatment and artificial aging. The cold work introduces dislocations that act as nucleation sites for precipitation, producing a finer, more uniform precipitate distribution and higher strength than T6. T8 is commonly used for 2024 (2024-T81 achieves 425 MPa yield vs 345 MPa for 2024-T4) and 2219 (2219-T87 for aerospace fuel tanks). The cold work also improves stress corrosion resistance and dimensional stability. The disadvantage is that the cold work operation adds cost and limits the product forms that can be produced in T8 temper — typically sheet, plate, and extruded profiles that can be stretched after quenching.

Temper Selection Guide

Choose T4 for maximum formability and moderate strength where post-forming natural aging is acceptable. Choose T5 for cost-sensitive extruded products where the forming heat provides solution treatment. Choose T6 for maximum strength in structural applications. Choose T7 (T73) for 7075 and 7xxx alloys where stress corrosion cracking resistance is critical. Choose T8 for maximum strength in sheet and plate requiring improved SCC resistance. For alloys that are not heat-treatable (1xxx, 3xxx, 5xxx), use the H temper series (strain-hardened) instead — H32, H34, H36 provide graduated strength levels through controlled cold work.

References & Standards

• ASTM International. Steel & Alloy Standards. astm.org
• International Organization for Standardization (ISO). iso.org
• National Institute of Standards and Technology (NIST). Materials Data. nist.gov
• ASM International. Materials Information Society. asminternational.org
• World Steel Association. Steel Statistical Yearbook. worldsteel.org