A machined aluminum plate arrives flat and square, passes incoming inspection, and bows 0.15 mm after the first machining pass removes material from one face. The material certificate says 6061-T6, exactly as ordered — and that is the problem. T6 describes strength after aging, not residual stress after quenching, and a plate that was not stress-relieved can move as machined layers are removed. The temper code on a drawing controls strength, formability, machined stability, and finish behavior, and “6061” without a temper leaves all of those choices to the supplier. Reading temper designations correctly is how engineers specify the material they actually need.
Temper codes describe processing history, not quality
Temper codes describe the mechanical and thermal history of the metal. The letter families cover the processing route: F means as-fabricated, O means annealed, H means strain-hardened, T means heat-treated, and W means solution-treated. For heat-treatable alloys such as 6061, the T designations matter most: T4 is solution heat-treated and naturally aged, T6 is solution heat-treated and artificially aged to peak strength, and T651 adds a controlled stress-relief stretch between solution treatment and aging. The digits describe a reproducible sequence, not a vague quality grade.
The alloy and the temper are inseparable in practice. “6061” names the chemistry; “T6” names the heat treatment that develops its strength. Two bars of 6061 in different tempers can differ in yield strength by a wide margin, and one may crack in a bend that the other forms easily. A drawing that says only the alloy is delegating a material decision to the stock supplier.
T6 and T651 differ in machined stability
T6 and T651 share the same aging treatment, but T651 includes a controlled stretch that relieves residual stress left by quenching. On machined parts, the difference shows up as stability: T651 plate and bar are less likely to move or distort when material is removed, because the internal stress has been reduced. If the part is machined thin, has tight flatness, or removes a large share of the original stock, T651 is usually the safer specification.
For simple shapes with light machining, T6 is fine and more commonly stocked, and specifying T651 where it is not needed adds cost and lead time. The practical rule is to ask where the residual stress will matter: if the part keeps most of its stock and the machined features are shallow, T6 works; if the part is a thin plate or a frame machined from a thick block, T651 earns its extra cost.
Forming in T4 before aging keeps bends and strength together
Choose T4 when the part must be formed or bent before aging. In T4, 6061 has not been artificially aged, so it is softer and more formable; parts can be bent or formed and then aged to bring the strength up. The trade-off is logistics: forming in T4 and aging afterward requires coordination between the former and the heat treater, while forming in T6 requires larger bend radii, more force, and a higher risk of cracking.
If the design calls for tight bends in a 6xxx part, state both conditions on the drawing: the temper at forming and the final temper after aging. A drawing that says “6061-T6, bend R6” without noting the forming sequence leaves the fabricator choosing between cracking the bend and silently changing the process. The same logic applies to 7075 and other heat-treatable alloys, where the formability window is even narrower.
Temper changes machining and finishing response
Temper influences more than strength. Harder tempers machine with better surface finish but wear tools faster and may need different speeds and feeds; softer tempers are easier on tools but can smear or leave a rougher edge. Anodizing response shifts slightly with alloy and heat treatment, and over-aging can change corrosion behavior. When a part is both machined and anodized, the alloy-and-temper callout should be fixed before finish samples are approved, because changing it later changes the finish as well as the strength.
| Temper | Key property | Typical design role |
|---|---|---|
| T4 | Formable before aging | Bent or formed parts later aged to strength |
| T6 | Peak aged strength | General machined and structural parts |
| T651 | Stress-relieved by stretch | Thin or heavily machined parts needing stability |
| O | Annealed, softest | Severe forming before re-treatment |
The table is a selection aid, not a property chart; verify the actual mechanical values with the material supplier or standard. The point of the table is that each temper exists for a reason, and the drawing should say which reason applies.
Write alloy, temper, and form together on every callout
Write the full designation: alloy, temper, and form, such as “6061-T651 plate” or “6061-T6 bar,” plus any special requirement such as low residual stress, grain direction, or certified mechanical properties. If the part will be formed, state both the as-received and final conditions. If you need a material certificate, ask for one that shows the temper, not just the alloy, because the certificate is the record of what was delivered and inspected.
The materials section of this site lists the aluminum grades available for CNC work, and the machining team can confirm stock conditions and machined-stability behavior before the first part is cut. If flatness after machining is critical, say so in the RFQ; the difference between T6 and T651 stock is one of the first things a shop will check when the part must stay stable.
The form of the material — plate, bar, sheet, or extrusion — interacts with the temper designation in ways that matter on the shop floor. Plate and bar in T651 are stress-relieved by stretching, but the stretch direction leaves slight directional properties, and a part machined across the width of a wide plate can behave differently from one machined along its length. Extruded profiles are quenched and aged in their own sequence, and the residual stress pattern follows the profile shape, so a machined extrusion can move differently than a machined plate of the same alloy and temper. When a part is machined from multiple forms — a plate base with a bar feature — the stability of the assembly depends on each piece’s form and temper, not just the shared alloy name. The drawing should state the form alongside the temper, and the shop should know which form the critical features come from. If flatness after machining is a recurring problem, the solution is often not a tighter drawing but a change in stock form or temper — moving from T6 plate to T651, or from a rolled bar to a stress-relieved forging for the most sensitive parts.
Cast and wrought aluminum use different temper notations, and mixing them up produces meaningless callouts. Wrought alloys such as 6061 use the T system described here, while cast alloys use a different designation that includes the product form and the heat-treatment code; writing “T6” on a casting drawing is incomplete because the casting process and the solution treatment differ. If the part is machined from cast stock or from a casting that will be machined, the material callout should name the casting alloy and its heat treatment, not a wrought temper. The distinction matters on the shop floor because machinability, porosity response, and finishing behavior differ between cast and wrought forms of similar alloy families. When a design changes from a machined-from-bar part to a casting, the temper language, the inspection, and the finishing expectations all change, and the drawing should be updated as a complete material specification rather than a quick copy of the old callout. Keep the form and the temper together in every document, from the drawing to the certificate, and the material history stays readable for the life of the part.
Frequently asked questions
Can a T6 part be re-aged after machining?
Aging after machining can restore strength lost in forming or welding, but re-aging a fully machined part risks distortion and dimensional change, and it does nothing for residual stress that was already relieved or not relieved. If the part needs stability, choose T651 stock and machine in the correct sequence rather than planning to re-age after machining.
Does T651 affect anodizing color?
The stress-relief stretch does not directly change the oxide color, but the alloy and the heat treatment do affect anodizing response slightly. What changes color in practice is mixing tempers or alloys in one assembly or one batch. Keep the alloy and temper consistent across visible parts and confirm color on samples with the finisher.
How do I know which temper a supplier actually shipped?
Ask for the material certificate and read the temper line; the certificate should state the full designation including the temper. If the certificate says only “6061,” treat it as incomplete and request the temper before accepting the material. For critical parts, specify the temper in the purchase order so the certificate and the PO agree.
Conclusion
Temper designations are the part of the material spec that controls what the metal does after it arrives: how it forms, how it machines, how it stays stable, and how it finishes. Specify the full designation, choose T651 when machined stability matters, and state the forming sequence when bends are involved. The extra characters on the drawing are the cheapest insurance against a part that moves after machining.
If you are specifying aluminum for a machined part with flatness or stability requirements, send the drawing and the stock size to the 6CProto CNC team. Confirming the temper and the machining sequence before quoting is faster than discovering the bow after the first part.