How to Select the Right Aluminum Temper for Extrusions

Understand Your Structural Requirements

Start by mapping the load path for the extrusion. Identify every force acting on the part, including dead load, live load, wind load, and seismic forces. Calculate the maximum bending moment, shear force, and compressive load the member will experience over its service life. These numbers drive the required yield strength and ultimate tensile strength. A handrail and a structural beam both use aluminum, but the beam demands a much higher yield strength to prevent permanent deformation under load. If the member acts as a column, buckling must be checked before strength. If it acts as a tie or brace, elongation becomes the limiting factor rather than compressive stress.

Write down the safety factor your project requires. Industry standards for structural aluminum often call for a specific safety margin, but your internal engineering rules may be stricter. If the part is subject to cyclic loading or impact, note that as well. Fatigue behavior differs between tempers, and a high-strength condition may perform poorly under repeated stress cycles compared to a moderate-strength condition that has better ductility. For example, a sign bracket subjected to wind-induced vibration may fail in a high-strength temper due to fatigue cracking, even if the static strength is more than sufficient. In such cases, a temper with higher ductility and lower hardness often provides a longer service life, despite lower peak strength.

Check Formability and Processing Constraints

Extrusion profiles are rarely used as raw stock. They are often cut, drilled, bent, or welded. The temper you select must survive these secondary operations without cracking or losing its shape.

Bending is the most common constraint. If your profile requires a tight radius bend, you likely need the O temper, which is the annealed condition. The O temper offers high ductility and workability, allowing the profile to deform without fracturing. However, it has the lowest mechanical strength of the common tempers. If the part does not need bending, or if it will be formed after final heat treatment, you can choose a stronger tempered condition. Consider the bend radius relative to the wall thickness. A rule of thumb in extrusion fabrication is that the inner radius should be at least two to three times the wall thickness for standard tempers, but this margin tightens significantly as the temper strength increases. For T6 material, a tight bend often requires pre-heating the profile to a low temperature to restore some ductility, a practice that requires careful control to avoid unwanted aging.

Drilling and tapping also affect temper selection. Harder tempers like T6 are easier to drill because the material is less prone to work hardening and chipping. However, they are more difficult to machine in general. If you plan to do extensive CNC machining, consider whether the material hardness will increase tool wear or affect surface finish. Carbide tools handle T6 well, but high-speed steel tools will wear quickly. For thin-walled profiles, the stiffness of the material matters as much as hardness. A softer temper flexes more under tool pressure, which can cause chattering and poor hole quality. A harder temper holds its shape better but may require higher feed rates to prevent rubbing.

Evaluate Corrosion and Environmental Exposure

Aluminum does not rust, but it does corrode. The rate and type of corrosion depend on the alloy, the temper, and the environment. In general, tempering does not drastically change the corrosion resistance of the base alloy, but it can affect surface properties and the effectiveness of coatings. If the extrusion will be exposed to salt air, chemical processing, or high humidity, specify the corrosion conditions clearly. For marine or coastal applications, you may need a higher alloy number with better corrosion resistance, regardless of temper. The temper choice here is secondary to the alloy choice, but you should confirm that your chosen temper is stable in the intended environment. For example, some tempers may be more susceptible to stress corrosion cracking than others if the part is under tension. Stress corrosion cracking is most likely in high-strength tempers when the part is under residual stress from forming or welding. In these cases, a stress-relieved temper or a lower-strength alloy may be more appropriate, even if it sacrifices some static strength.

Match the Temper to the Alloy System

The 6xxx series is the workhorse of structural aluminum extrusions. It is aluminum-copper-magnesium based and responds well to heat treatment. The 7xxx series is aluminum-zinc-magnesium-copper and offers higher strength, but it is more expensive and more difficult to work with.

The following table compares common tempers in the 6xxx series and their typical behavior. This is a general guide. Always check the specific alloy and temper from the mill test report for your project.

Temper	Description	Typical Use Case
O	Annealed, soft, high ductility	Profiles that must be bent or formed after extrusion
T4	Solution treated, medium strength	General purpose, good corrosion resistance
T5	Stress relieved, medium strength	Parts that need dimensional stability after extrusion
T6	Solution treated and artificially aged, high strength	Structural beams, frames, load-bearing applications
T651	Solution treated and artificially aged, high strength	Profiles that need good bending ability after heat treatment

The T6 temper is the most common choice for structural extrusions. It provides a good balance of strength, machinability, and weldability. However, T6 is relatively stiff. If you need to bend a T6 profile, you may find it difficult or impossible without cracking. The T651 temper is designed for this exact problem. It is solution treated and artificially aged, but the aging process is controlled to leave the material in a condition that allows for bending without losing all of its strength. T651 is often used for architectural columns and frames where the profile must be bent into shape after extrusion and then heat treated to achieve final strength. The difference between T6 and T651 is subtle but significant. T651 retains more ductility after aging, which reduces the risk of cracking during forming. It also tends to have slightly lower yield strength than T6, but the difference is usually small enough to be ignored in most structural calculations.

Consider Surface Finish and Coating Requirements

The temper you choose can affect how the extrusion looks and how it is coated. The O temper has a softer surface that is easier to polish or anodize to a uniform finish. The T6 temper is harder, which can make it more resistant to scratching but also more difficult to polish to a mirror finish.

If the extrusion will be painted, the surface must be properly prepared. The temper does not change the need for cleaning and priming, but a softer temper may be more forgiving of handling damage during fabrication. Scratches in T6 are harder to grind out without removing a significant amount of material. Scratches in O temper can often be buffed out easily. If the extrusion will be anodized, the mill may specify a temper that works best with the anodizing process. Some tempers produce a more even oxide layer than others. T6 generally produces a consistent anodized finish, but the hardness of the underlying material can affect the appearance of the anodize layer. In some cases, a pre-anodizing anneal is required to ensure uniformity. This is a process detail that should be confirmed with the coating supplier before finalizing the temper specification.

Verify with Mill Test Reports and Certification

Do not rely solely on the mill’s general datasheet. Request a mill test report for the specific batch of material you are using. This document lists the actual chemical composition, heat treatment cycle, and mechanical test results. The mechanical test report will show the yield strength, tensile strength, and elongation for that specific batch.

Compare these numbers to your design requirements. If the design requires a yield strength of a certain value, the mill test report must confirm that the material meets or exceeds that value. A standard like T6 has a minimum specification, but individual batches can vary. In high-stakes applications, this verification step is non-negotiable. For example, if a bridge truss member requires a minimum yield strength of 240 MPa, the mill test report must show that the specific batch of 6061-T6 used in that member meets or exceeds that value. A batch that tests low might be acceptable for a non-critical architectural element but unacceptable for a load-bearing structural component. Always retain the mill test reports with your project files. They are essential for warranty claims, insurance documentation, and future maintenance records.

Common Mistakes in Temper Selection

The most frequent error is choosing the strongest temper available without considering formability. Engineers often select T6 for everything because it is the default structural temper. This works for straight beams. It fails for bent brackets. If the profile needs to be bent, the T6 temper will crack at the outside of the bend. The solution is to use the O temper for bending, then age the part to T6 or T651 afterward if strength is required. This is called post-extrusion heat treatment. It adds a step to the manufacturing process and requires precise control of the aging cycle. If the part is too large for a conventional heat treat furnace, post-extrusion heat treatment may not be practical. In that case, you must select a temper that can be bent in the required condition, such as T651, or redesign the part to avoid bending.

Another mistake is ignoring the welding requirements. Some tempers are difficult to weld. The T6 temper loses much of its strength in the heat affected zone near the weld. If the joint is load-bearing, you may need to choose a T73 temper or a specific weldable alloy. If the weld is not critical, T6 is usually fine. For example, a welded frame for a machine base may not require full strength retention at the weld, but a welded joint in a crane hook or a structural column must maintain a minimum strength. In these cases, a weldable alloy such as 6063 or 6061 in a stress-relieved temper may be more appropriate, or the joint must be designed to avoid relying on the weld for primary load transfer.

Finally, do not assume that a higher temper number always means better. T73 is stronger than T6, but it is more expensive and harder to machine. If T6 meets your strength requirements, use T6. Paying for extra strength you do not need increases cost and complexity. It also increases the risk of fabrication defects. Higher strength tempers are less forgiving of errors in machining, bending, or welding. A small mistake that would be easily corrected in T6 might result in a cracked part in T73.

Final Verification Step

Before cutting the first piece, perform a trial bend or a small test coupon. Take a sample of the extrusion in the selected temper and apply the maximum expected bending load or form it to the tightest radius required. Observe the behavior. If it cracks, change the temper. If it deforms permanently when it should not, change the temper. This physical test catches problems that calculations can miss.

Once the trial is successful, proceed with the full production run. Keep a sample of the material from the final heat treatment cycle. This sample serves as a reference for quality control. If future batches do not match, you have a baseline to compare against. This simple step prevents costly rework and ensures that the material performs as designed. For large production runs, test multiple samples from different positions in the heat treat furnace. Temperature gradients within the furnace can cause variations in the final temper, especially for large or complex profiles. A sample from the center of the furnace may not match a sample from the edge. By testing multiple points, you ensure that every part in the production run meets the required specifications.