6061-T651 vs T6 Aluminum Sheet: Stress Relief and Machining

The main distinction between 6061-T6 and 6061-T651 is specified stress relief—not a different base alloy. T6 identifies solution heat treatment and artificial ageing. T651 additionally identifies stress relief by stretching before artificial ageing. For a part requiring substantial machining, that distinction matters, but T651 is not a promise of zero distortion or a substitute for a finished-part tolerance.

Read the Temper in the Correct Processing Order

Temper	Processing meaning
6061-T6	Solution heat-treated, then artificially aged.
6061-T651	Solution heat-treated, stress-relieved by stretching, then artificially aged.

The stretching step should not be described as a general operation performed after completing the T6 ageing cycle. A temper designation describes a controlled material condition, not a workshop instruction to stretch an already purchased sheet. Any reprocessing must follow an approved procedure and the governing specification.

Why This Matters During Machining

Machining removes material and changes the balance of stress in the remaining section. The resulting movement depends on the incoming stock, geometry, amount and sequence of material removal, fixturing and thermal conditions. Specified stress relief helps address the material side of that problem, but machining practice remains important.

Parts with substantial material removal

For pockets, asymmetric sections or tight final flatness requirements, identify the desired stock condition in the drawing or purchase specification. Do not rely on a verbal description such as “precision aluminium.” Discuss whether the incoming dimensions, machining allowance and inspection plan are appropriate for the finished part.

Simple cut or fabricated components

A part that requires little machining may not obtain the same practical benefit from a stress-relieved condition. The material must nevertheless meet its design and fabrication requirements. The decision is not simply “cheap T6” versus “better T651”; it is the approved temper that meets the application and is available in the required product form.

Strength Values Need a Product and Thickness Basis

Both designations belong to the same alloy family and ageing category, but a general article cannot assign one universal tensile strength, yield strength or elongation to every thickness. Use the governing product standard and its applicable thickness range. Distinguish typical reference values from guaranteed minimum values and from test results for a supplied lot.

Sheet, Plate and Extrusions Must Not Be Confused

Confirm the actual product form and thickness before comparing offers. ASTM B209/B209M is a sheet-and-plate specification; other product forms have their own requirements. A temper used for an extrusion, such as a longer T6511 designation, must not be shortened or substituted without checking the drawing and standard.

For aerospace work, quote the exact approved material specification and revision. Similar-looking specification numbers are not interchangeable, and not every aerospace aluminium document applies to 6061. The purchase order should identify the required alloy, temper, form, dimensions and certification scope together.

Compare Offers on a Like-for-Like Basis

• Same alloy, full temper and product standard.
• Same thickness, width, length and permitted dimensional variation.
• Same surface condition, protective film and machining allowance.
• Same inspection, traceability and documentation requirements.
• Same cut-piece or full-plate scope, quantity and destination.

A cost difference can arise from stock form, processing, cutting yield and inspection as well as temper. Compare the complete supplied item rather than assuming a fixed premium for T651. If an alternative condition is proposed, obtain engineering acceptance before the material is ordered.

Specify the Requirement Before Requesting a Price

Use the aluminium sheet and plate range as a starting point and send your drawing, required condition, dimensions and quantity. For machining-sensitive parts, add the final flatness or positional tolerance and the planned material-removal pattern so that the supply review addresses the actual concern.

Discuss 6061 Sheet or Plate Requirements

Temper reference: Hydro—6061 Aluminium. Product specifications: ASTM B07.03.