6061-T651 aluminum
Stress-relieved 6061-T651 stock: temper meaning, scoped properties, machining flatness and fabrication boundaries.
Tags: materials, aluminum, plate, heat-treatment, machining
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Machining, residual stress and flatness
T651 specifies stress relief by controlled stretching. A thin wall can still distort when released from its clamps. Incoming plate flatness and finished-part flatness are separate acceptance conditions. Define the measurement state, support, span, temperature and datum scheme where relevant. Capalex temper definitions
KaiserSelect's general-engineering brochure associates its processing with lower residual stress, flatness and machining with minimal distortion. Those are claims about the named product; a T651 callout alone does not supply its complete dimensional or process guarantee. Agree measurable flatness, thickness, finish and inspection requirements before ordering. KaiserSelect enhanced characteristics
For a deep pocket or asymmetric stock removal, plan workholding, roughing allowance, operation sequence and inspection after unclamping. Use a representative trial to qualify the route.
Identify alloy and delivery condition
6061-T651 combines the wrought 6061 alloy with a heat-treatment and stress-relief condition. The Aluminum Association's Teal Sheets, revised December 2025, register 6061 with 0.40–0.80% silicon, 0.8–1.2% magnesium, 0.15–0.40% copper and 0.04–0.35% chromium by mass, plus limits on other elements. Composition identifies the alloy; it does not establish stock flatness, mechanical acceptance limits or the final part's performance. Aluminum Association 6061 registration
Capalex defines T651 as solution heat-treated, stress relieved by controlled stretching, then artificially aged, with no further straightening after stretching. Its T6 definition states solution treatment followed by artificial aging without the same explicit stretching requirement. Keep that distinction in the order and certificate. Capalex temper definitions
The extra digits describe processing, not an added universal strength increment. An engineer should not infer that every T651 product is stronger than every T6 product, or that the stress-relieved condition contains zero residual stress. Specify alloy/temper, product form, dimensions/tolerances, governing specification and edition, surface condition and required certification.
T651 plate and other product forms
Kaiser's Sheet Coil & Plate Alloy 6061 data groups T6 and T651 together for its typical-property comparison. Hydro's extrusion sheet instead discusses T6/T6511 among its rod, bar, shape and tube offerings. Preserve the product and complete temper rather than shortening every designation to “6061-T6.” Kaiser flat-rolled data, Hydro 6061 extrusion data
A supplier's availability range is also different from a universal material limit. Kaiser's General Engineering Plate brochure lists its 6061-T651 general-engineering product from 0.250 to 10.000 inches and a separate brushed offering from 0.250 to 4.000 inches. These are named supplier offerings; confirm the actual dimensions, finish and specification on the quotation. The undated brochure's web crawl date is not a verified revision. KaiserSelect General Engineering Plate, product table
Do not transfer plate properties to cast tooling plate, an extrusion or a machined wall based only on similar chemistry or the finished wall thickness. Keep the original stock thickness and test orientation with the material record.
Typical properties versus acceptance requirements
Kaiser's technical sheet is marked Rev. 05/06 and explicitly labels its mechanical data typical. Selected metric entries are shown below. The combined T6/T651 row does not distinguish a separate strength benefit from stress relief. Kaiser typical mechanical properties, p. 1
| Condition in source | Typical ultimate tensile strength | Typical yield strength | Typical modulus |
|---|---|---|---|
| T451 | 241 MPa | 145 MPa | 68.3 GPa |
| T6 / T651 | 310 MPa | 276 MPa | 68.3 GPa |
These comparison values are not guaranteed batch minima or structural design allowables. Determine acceptance from the applicable product specification, thickness band, direction and required test report. Preserve whether a number is a typical value, limit, range, measured result or allowable.
The same sheet lists typical thermal conductivity at 20°C as 154 W/(m·K) for T451 and 167 W/(m·K) for T6/T651. Retain condition and temperature when using these figures in a thermal model. They do not establish conductivity across every operating temperature or the contact conductance of a bolted assembly. Kaiser physical-property table, p. 2
Forming, welding and heat exposure
Kaiser's comparative table rates cold workability differently for T451 and T6/T651, while its description identifies softer O and T4 as useful forming conditions. A process qualified on softer stock therefore should not automatically be transferred to T651. Define bend geometry, thickness, rolling direction, tooling and final-condition requirements before forming. Kaiser formability description and comparative table
Hydro's Revision 2019/01 (2/2019) 6061 sheet warns that T6 strength can be reduced in the weld region and makes filler selection dependent on the desired weld characteristics. T651's prior stretching does not exempt an artificially aged weldment from local thermal effects. Assess weld metal, heat-affected zone and unaffected parent separately using the applicable joint requirements. Hydro 6061 joining guidance
Do not assume a simple postweld aging step restores the original delivered condition, dimensional stability or corrosion behavior. Any proposed thermal route needs its own controlled procedure and final inspection basis. The material label alone supplies no universal welding parameters, hot-forming recipe or continuous-service temperature rating.
Service and traceability
Hydro cautions that direct contact with dissimilar metals can cause galvanic corrosion. Evaluate actual contacts, moisture retention, coatings, fasteners and fluids rather than turning a favorable alloy corrosion description into chemical immunity. Cosmetic anodizing also needs a defined appearance requirement and sample, especially across welded or mixed-product assemblies. Hydro corrosion and joining guidance
Keep heat/lot identity, complete temper, stock dimensions, certificate and processing history linked through cutting and machining. Record substitutions and thermal operations explicitly. Release the final part against dimensional, surface and joint requirements as well as the incoming material specification; a parent-stock tensile certificate cannot prove all of those finished-part properties.