Material Selection / 6061 Aluminum

How to Choose a 6061 Aluminum Temper for CNC Machining

The right 6061 temper depends on the part’s function, stock form, material-removal plan and final dimensional requirements—not on machinability alone.

6061-T6 is a common general-purpose choice. For stress-sensitive plate parts, 6061-T651 may be worth evaluating because stress-relieved plate can offer better dimensional stability during machining. Confirm the exact alloy, product form and material certification before substituting one temper for another.

  • Strength & service loads
  • Plate, bar or extrusion
  • Machining stability
Explore the CNC material selection guide
Aluminum stock and precision-machined components in a CNC workshop
Aluminum stock and precision-machined components for CNC manufacturing.

Use this guide to narrow the choice; the drawing, applicable material standard and required properties remain controlling.

Temper Comparison / Engineering Selection

6061-O vs T4 vs T6 vs T651: Which Temper Fits Your Part?

These tempers differ in strength, formability and processing history. For CNC machining, select the condition that matches the part's functional requirements and starting stock—not simply the hardest or most commonly available option.

Comparison of 6061-O, T4, T6 and T651 aluminum tempers for CNC machining.
Temper Condition CNC Selection Consideration Typical Fit and Caution
6061-O Annealed Soft condition with relatively low strength and high formability. Consider when forming or shaping is important before a subsequent processing step. Its lower strength may not suit the finished part. Check: final strength requirements and whether a later heat-treatment sequence is specified.
6061-T4 Naturally aged Solution heat-treated and naturally aged. Useful when a process plan needs more formability than a typical T6 condition, or when subsequent aging is part of the approved route. Check: whether the drawing requires T4 in the delivered part or permits a later aging operation.
6061-T6 Artificially aged Solution heat-treated and artificially aged for a high-strength engineering condition. A common choice for general-purpose CNC parts where the required strength and available stock form are compatible. Check: stock-removal volume, clamping, machining sequence and dimensional stability after release.
6061-T651 Stress-relieved plate condition T6-type condition with stress relief by stretching, commonly specified for plate products. Worth evaluating for precision plate parts with substantial material removal or demanding flatness and dimensional-stability requirements. Check: product form, thickness, applicable specification and material certification. It reduces residual-stress risk; it does not guarantee zero distortion.
Selection guide only. Actual mechanical properties and availability depend on product form, dimensions and the governing material specification.
QUICK RULE

O: prioritize forming. T4: consider formability and the planned aging sequence. T6: consider general-purpose strength. T651: evaluate when machining plate stability is critical. Always confirm the temper required by the drawing before ordering material.

Important: T651 and T6511 are not interchangeable labels for every stock form. T6511 is commonly associated with stress-relieved extruded products. Confirm the exact designation and supply condition with the material specification and mill certificate.

Machining Stability / Dimensional Control

How Temper and Material Removal Affect CNC Machining Distortion

Machining distortion is not determined by temper alone. Residual stresses in the starting stock can be redistributed as material is removed. Thin walls, deep pockets, asymmetric features and large stock-removal volumes can make the resulting movement more difficult to control.

For plate-based precision parts, 6061-T651 is worth evaluating because stress-relieved plate can reduce residual-stress-related movement compared with conventional plate stock. However, geometry, stock history, clamping and machining sequence still affect the final result. No temper guarantees zero distortion.

A practical approach to dimensional stability

  1. Match the stock to the part Confirm plate, bar or extrusion, specified temper, thickness and material documentation before purchasing.
  2. Plan balanced material removal Avoid unnecessarily removing most of the material from one side before addressing the opposite side.
  3. Control setup and finishing Use suitable workholding, a planned roughing sequence and a final finishing operation appropriate to the drawing.
  4. Verify after release Inspect critical dimensions and flatness after unclamping. If movement persists, review the stock, setup and machining route.

For related design considerations, see the CNC design guidelines for geometry, workholding and dimensional-risk planning.

CNC milling a pocketed aluminum plate secured in a machining fixture
CNC milling a pocketed aluminum plate secured in a machining fixture.

Engineering boundary: T651 may be advantageous for suitable plate applications, but dimensional stability must be evaluated against the part geometry, machining route, required tolerances and inspection results.

Downstream Processing / Material Selection

How Welding and Anodizing Affect 6061 Temper Selection

A part's final condition depends on more than CNC machining. If welding or anodizing follows machining, include these operations in the material and drawing review before production.

Process consideration 01

Welding: Account for Heat-Affected Strength

Welding can reduce the strength of the heat-affected zone in 6061-T6. The resulting properties depend on the welding procedure, heat input and any subsequent heat treatment. Do not assume the original T6 strength remains unchanged across a welded joint.

Confirm before release

Review joint loads, the required delivered condition, applicable welding requirements and whether post-weld treatment is specified.

Process consideration 02

Anodizing: Protect Dimensions and Appearance

Anodizing changes the surface layer and can affect final dimensions, particularly at precision bores, mating faces and threaded features. Appearance also depends on alloy condition, surface preparation and the specified anodizing process.

Confirm before finishing

Specify coating type, color and acceptance criteria. Identify critical dimensions, masking areas and surfaces that require consistent appearance.

See the aluminum anodizing guide for surface-treatment considerations.

Engineering decision: Select the temper against the part's functional requirements and complete process route. Welding and anodizing requirements should be agreed before material purchasing and final CNC finishing.

Selection Workflow / RFQ Preparation

How to Specify 6061 Temper Before Requesting a CNC Quote

Make the material decision part of the manufacturing plan. Confirm these five items before releasing the drawing or requesting a quotation.

  1. Define Part Requirements

    Identify loads, functional surfaces, critical dimensions and flatness requirements.

  2. Specify Stock Form

    Confirm plate, bar or extrusion, alloy, temper and applicable material specification.

  3. Plan CNC Machining

    Flag thin walls, deep pockets, substantial stock removal and distortion-sensitive features.

  4. Confirm Secondary Processes

    Specify welding, anodizing, masking and any post-process dimensional requirements.

  5. Agree on Verification

    Define material documentation, CTQ inspection and acceptance requirements before production.

Engineering Handoff

Need Help Reviewing a 6061 CNC Part?

Share the drawing, required temper, stock form, quantity, critical tolerances and finishing requirements. These details help establish a suitable manufacturing and inspection plan before quotation.

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Final check: The drawing and applicable material specification govern the required condition. Do not substitute a temper without customer approval when the material condition is specified.