What Are Temper Designations?
When specifying aluminum extrusions for any project, the alloy number alone does not tell the full story. The temper designation that follows the alloy number is equally critical, as it describes the thermal and mechanical processing the material has undergone after initial forming. This processing directly determines the strength, hardness, ductility, and overall performance characteristics of the finished extrusion.
The Aluminum Association maintains a standardized temper designation system that uses letters and numbers to communicate the condition of the material. The most common designations include F (as-fabricated), O (annealed), H (strain-hardened, used primarily for wrought non-heat-treatable alloys), and T (thermally treated). Within the T category, a numeric suffix indicates the specific sequence of thermal treatments applied. For heat-treatable extrusion alloys in the 6xxx series, the T5 and T6 tempers are by far the most frequently specified.
Understanding the difference between T5 and T6 is essential for engineers, architects, and procurement professionals who need to balance mechanical performance with cost, surface quality, and manufacturability. The choice between these two tempers can affect everything from structural load capacity to anodizing response, and making the wrong selection can lead to over-engineering costs or, worse, under-performing components in the field.
In this article, we will examine both temper designations in detail, compare their properties side by side, and provide practical guidance for selecting the right temper for your aluminum extrusion project.
T5 Temper: Air-Cooled and Artificially Aged
The T5 temper designation indicates that an aluminum extrusion has been cooled from the elevated temperature of the hot working process (extrusion) and then artificially aged. In practice, this means the profile exits the extrusion press at temperatures typically between 480 and 530 degrees Celsius and is then cooled using forced air or standing fans at the run-out table. After cooling, the profiles are placed in an aging oven where they are held at a controlled temperature, usually between 170 and 200 degrees Celsius, for several hours to develop their final mechanical properties through precipitation hardening.
The air-cooling step is what distinguishes T5 from T6. Because air cooling produces a slower quench rate compared to water quenching, the supersaturated solid solution is less complete, and the subsequent aging treatment produces a somewhat lower level of strengthening precipitates. For the widely used 6063 alloy, the T5 temper typically yields a tensile strength of approximately 185 MPa (27 ksi) and a yield strength of around 145 MPa (21 ksi), with elongation values of 8 percent or more.
One of the significant advantages of the T5 temper is the superior surface finish quality it delivers. The gentler air-cooling process avoids the thermal shock and distortion that can sometimes accompany water quenching, resulting in profiles with smoother, more consistent surfaces. This makes T5 extrusions particularly well-suited for applications where appearance matters, such as architectural curtain wall mullions, window and door frames, decorative trim, furniture components, and consumer electronics enclosures.
T5 extrusions also respond exceptionally well to anodizing. The uniform microstructure produced by air cooling results in a more even oxide layer during the anodizing process, yielding a consistent, high-quality decorative finish. This is a key reason why the vast majority of architectural aluminum extrusions are specified in the T5 temper.
From a cost perspective, the T5 temper is generally more economical to produce. Air cooling requires less infrastructure than water quench systems, uses less energy, and generates fewer profile straightening issues downstream. These savings are typically passed on to the customer, making T5 the more budget-friendly option when the application does not demand the higher mechanical properties of T6.
T6 Temper: Solution Heat-Treated and Artificially Aged
The T6 temper designation indicates that an aluminum extrusion has been solution heat-treated and then artificially aged. Solution heat treatment involves heating the alloy to a temperature high enough to dissolve the alloying elements (primarily magnesium and silicon in 6xxx series alloys) into a uniform solid solution, typically between 510 and 550 degrees Celsius. The material is then rapidly quenched, most commonly using water spray or immersion, to lock these elements in a supersaturated solid solution. Following quenching, the extrusion undergoes artificial aging at controlled temperatures to promote the formation of fine, uniformly distributed strengthening precipitates throughout the aluminum matrix.
The rapid water quench is the critical differentiator. By cooling the material quickly enough to prevent premature precipitation, the T6 process creates conditions for a more complete and effective aging response. This results in significantly higher mechanical properties compared to T5. For 6063-T6, typical values include a tensile strength of approximately 240 MPa (35 ksi), a yield strength of around 215 MPa (31 ksi), and elongation values of 8 to 10 percent. For the stronger 6061-T6 alloy, tensile strength reaches approximately 310 MPa (45 ksi) with a yield strength of about 275 MPa (40 ksi).
The enhanced strength of T6 extrusions makes them the preferred choice for structural and load-bearing applications. Common uses include structural framing systems, bridge and walkway components, transportation equipment such as truck trailer bodies and rail car structures, machine frames, ladder rails, scaffolding components, and safety-critical assemblies where higher strength-to-weight ratios are required. In the automotive sector, T6 extrusions are increasingly used for crash management systems, battery tray enclosures for electric vehicles, and suspension components.
However, the T6 process does come with certain trade-offs. The rapid water quench can introduce thermal gradients across the profile cross-section, which may cause distortion, warping, or surface marking, particularly in profiles with varying wall thicknesses or asymmetric geometries. Additional straightening operations may be required after quenching, adding to production time and cost. The surface finish of T6 extrusions, while perfectly acceptable for most industrial applications, may not achieve the same level of cosmetic uniformity as T5, particularly on broad, visible surfaces intended for decorative anodizing.
It is worth noting that for some alloys and press configurations, the solution heat treatment step can be integrated into the extrusion process itself, a practice known as press quenching. In this approach, the billet temperature and extrusion speed are controlled so that the profile exits the press at the correct solution temperature, and it is immediately water-quenched at the press exit. This is followed by standard artificial aging. Press quenching achieves T6 properties without requiring a separate solution heat treatment furnace, improving efficiency and reducing lead times.
T5 vs T6: Key Differences
The fundamental difference between T5 and T6 lies in the quenching method and its downstream effects on mechanical performance. While both tempers involve artificial aging as the final step, the cooling rate after extrusion or solution treatment determines how much strengthening potential the alloy can realize. Here is a breakdown of the key differences across several important criteria.
Mechanical Strength: T6 extrusions are approximately 30 to 40 percent stronger than their T5 counterparts in the same alloy. For 6063, T6 delivers a yield strength of roughly 215 MPa compared to 145 MPa for T5. This strength advantage is even more pronounced in alloys like 6061, where the T6 temper can achieve yield strengths exceeding 275 MPa. If your design is governed by structural calculations or load-bearing requirements, T6 will allow the use of thinner walls and lighter profiles while still meeting the required strength criteria.
Surface Finish Quality: T5 extrusions generally exhibit superior surface quality, with fewer quench marks, less distortion, and a more uniform appearance. This advantage is especially important for profiles that will be anodized for architectural or decorative purposes. T6 extrusions may show water staining, quench lines, or slight surface irregularities, though these are typically acceptable for industrial and structural applications where the profile will be painted, powder-coated, or hidden from view.
Cost: T5 temper is generally less expensive to produce because it eliminates the need for water quench systems and reduces the risk of post-quench distortion that requires corrective straightening. The simpler cooling process also allows for higher extrusion speeds in some cases, increasing throughput and lowering per-unit cost. T6 temper adds cost through the quench system, potential straightening labor, higher scrap rates from quench-related defects, and in some cases, a separate solution heat treatment step.
Formability: T5 extrusions are somewhat easier to bend and form after extrusion due to their lower hardness and higher ductility relative to T6. For applications requiring post-extrusion bending, roll forming, or stretch forming, T5 can be more forgiving and produce better results with less risk of cracking. T6 extrusions, while still formable, require more careful consideration of bend radii and may benefit from bending in the T4 condition (solution treated but not yet aged) followed by aging to T6 properties.
Machinability: Both tempers machine well, as is characteristic of the 6xxx series alloys. However, T6 extrusions generally produce cleaner chips and tighter tolerances during CNC machining operations due to their higher hardness. T5 material, being softer, may produce longer, stringier chips and require adjusted tooling parameters. For precision-machined components, T6 is often the preferred starting material.
Choosing the Right Temper for Your Project
Selecting between T5 and T6 should be driven by the specific demands of your application rather than defaulting to the highest available strength. Over-specifying temper is a common and costly mistake in the extrusion industry. Many projects that specify T6 could achieve the same functional performance with T5 at a lower cost and with better surface quality.
Choose T5 when your application is primarily architectural or decorative, when surface appearance and anodizing quality are critical, when the extrusion is not subject to significant structural loads, when cost optimization is a priority, or when post-extrusion forming operations are required. Typical T5 applications include window and door systems, curtain wall mullions, display fixtures, furniture components, lighting housings, trim profiles, and interior architectural elements.
Choose T6 when your application demands higher mechanical strength, when the extrusion must meet specific structural load or safety requirements, when the profile will be used in a fatigue-critical environment, when precision machining is required, or when weight reduction through thinner walls is a design objective. Typical T6 applications include structural framing, transportation and automotive components, machine frames, safety railings, ladder rails, heat sinks subjected to clamping forces, and any application governed by structural engineering codes.
It is also worth considering the alloy selection in conjunction with the temper. In some cases, selecting a stronger alloy in T5 temper can deliver equivalent or better performance than a weaker alloy in T6, while retaining the surface quality advantages of air cooling. For example, 6005A-T5 offers higher mechanical properties than 6063-T6, making it an excellent alternative for applications that need both strength and surface quality. Your extrusion supplier can help you navigate these alloy-temper combinations to find the optimal solution for your project.
Additionally, consider the full lifecycle of your product. If the extrusion will undergo secondary operations such as welding, the temper in the heat-affected zone will revert to a softer condition regardless of whether T5 or T6 was specified. In welded assemblies, the design must account for the reduced properties in the weld zone, which may diminish the practical advantage of specifying T6 over T5 for the base material.
How Yogi Extrusions Can Help
At Yogi Extrusions, we produce aluminum extrusions in both T5 and T6 tempers across a wide range of alloys in the 6xxx series. Our extrusion presses are equipped with advanced cooling systems that give us precise control over the quenching process, enabling us to consistently meet the mechanical property requirements for both temper designations. Whether your project calls for a decorative architectural profile in 6063-T5 or a high-strength structural section in 6061-T6, our engineering team can guide you to the right alloy-temper combination.
We encourage customers to engage with our team early in the design process, when alloy and temper selections can be optimized without impacting project timelines. Our metallurgical expertise and in-house testing capabilities ensure that every extrusion we deliver meets the specified mechanical properties and dimensional tolerances. Explore our full alloy portfolio to see the range of materials we offer, or contact our engineering team directly to discuss your project requirements.