Aluminum Temper O, H, T4, T6 and T651: Meaning and Selection

Introduction

Aluminum temper O, H, T4, T6 and T651 describe the mechanical and thermal condition of aluminum after rolling, extrusion, solution heat treatment, cold working, aging or stress relief. The temper designation is part of the material specification and directly affects strength, hardness, formability, dimensional stability, machinability and corrosion performance.

Temper O means annealed and highly formable. H tempers are strain-hardened conditions mainly used for non-heat-treatable aluminum alloys. T4 means solution heat treated and naturally aged. T6 means solution heat treated and artificially aged for higher strength. T651 is based on T6 but includes stress relief by controlled stretching, making it suitable for machined plate and parts that require improved dimensional stability.

Aluminum Temper Product Data

Temper	Meaning	Main Property Direction	Typical Use
O	Annealed	Lowest strength and highest ductility for the alloy	Deep-drawn parts, formed sheet, tanks, cookware and complex fabrication.
H	Strain hardened, sometimes with partial annealing or stabilization	Higher strength than O with reduced ductility	Marine sheet, roofing, tanks, panels, packaging and structural fabrication.
T4	Solution heat treated and naturally aged	Moderate strength with better formability than T6	Formed structural parts, automotive panels and components aged after shaping.
T6	Solution heat treated and artificially aged	High strength and hardness with lower formability	Structural profiles, machined parts, transport equipment and general engineering.
T651	Solution heat treated, stress relieved by stretching and artificially aged	High strength with improved dimensional stability during machining	Machined plate, tooling, aerospace parts, fixtures and precision components.

What O Temper Means

O temper aluminum has been annealed to reduce hardness and relieve the effects of prior cold work. It normally provides the highest ductility and easiest forming condition available for that alloy. The material can be bent, rolled, spun, stamped or deep drawn with a lower risk of cracking than harder tempers.

O temper is common in sheet, strip, foil, tube and other products that require substantial deformation. Examples include 1050-O, 3003-O, 5052-O and 5083-O. The exact strength and elongation depend on the alloy, thickness and product standard.

Advantages of O Temper

• Excellent ductility and bendability.

• Suitable for deep drawing and complex forming.

• Reduced risk of cracking during fabrication.

• Useful when the part will be work hardened during forming.

Limitations of O Temper

The main limitation is low strength and hardness. O temper is not normally selected for highly loaded components unless the finished shape or subsequent processing provides adequate strength. Soft material may also scratch or distort more easily during handling and machining.

How H Tempers Work

H tempers are used mainly for non-heat-treatable aluminum alloys that gain strength through cold working. The designation normally includes two or more digits after the letter H. These digits describe the processing route and degree of strain hardening.

H Temper Group	General Meaning	Typical Example
H1x	Strain hardened only	H12, H14, H16 and H18.
H2x	Strain hardened and partially annealed	H22, H24, H26 and H28.
H3x	Strain hardened and stabilized	H32, H34 and H36.
Hxx4 / Hxx6 / Hxx8	Increasing degrees of strain hardening	H14 is softer than H16, while H18 is harder and stronger.

Common examples include 3003-H14 sheet, 5052-H32 sheet and 5083-H116 marine plate. The full temper must be stated because “H temper” alone does not define the actual strength or stabilization condition.

What T4 Temper Means

T4 temper applies to heat-treatable aluminum alloys that have been solution heat treated and naturally aged at approximately room temperature. The material develops moderate strength while retaining more formability than the T6 condition.

T4 is often selected when a component must be bent, shaped, stretched or formed before receiving a final artificial-aging treatment. It can also be used in the final T4 condition when the required strength is lower and ductility is more important.

Typical T4 Alloys

• 2024-T4 aerospace sheet and formed components.

• 6061-T4 sheet, tube and profiles requiring subsequent forming.

• 6082-T4 components formed before final aging.

• 6000-series automotive and structural parts requiring a forming-stage temper.

What T6 Temper Means

T6 aluminum has been solution heat treated and artificially aged to develop a high-strength precipitation-hardened condition. It is one of the most widely used tempers for 6000- and 7000-series aluminum products.

Compared with T4, T6 normally provides higher tensile strength, yield strength and hardness but lower ductility and reduced formability. Severe bending after T6 aging can increase the risk of cracking, especially with small bend radii or thick sections.

Typical T6 Alloys and Uses

• 6061-T6 for structural parts, frames, machine components and transport equipment.

• 6063-T6 for stronger architectural and industrial profiles.

• 6082-T6 for cranes, platforms, bridges and structural fabrication.

• 7075-T6 for aerospace, tooling and highly loaded precision parts.

What T651 Temper Means

T651 is a stress-relieved version of the T6 condition. After solution heat treatment, the material is stretched by a controlled amount to reduce residual stress and is then artificially aged. This process improves dimensional stability during machining.

Residual stress can cause thick plate to move, twist or bow as material is removed from one side. T651 does not eliminate every machining distortion, but it normally provides better stability than standard T6 plate for precision components.

Typical T651 Applications

• CNC-machined tooling plates and fixtures.

• Aerospace structural components.

• Precision machine bases and molds.

• Thick 6061, 6082, 2024 and 7075 aluminum plate.

• Components requiring controlled flatness after machining.

O vs H vs T4 vs T6 vs T651

Comparison	O	H	T4	T6	T651
Strength	Lowest	Low to high depending on number	Moderate	High	High
Formability	Excellent	Varies with work-hardening level	Good	Lower	Lower
Machining Stability	Low hardness may reduce cutting stability	Depends on temper	Moderate	Good	Best for stress-relieved plate
Primary Selection Reason	Forming	Controlled cold-work strength	Balance of formability and strength	High strength	High strength plus dimensional stability

How Temper Affects Corrosion Resistance

Temper selection primarily controls mechanical condition, but it can also influence corrosion behavior. Heat treatment, cold work, residual stress and precipitate distribution can affect resistance to stress-corrosion cracking, exfoliation and intergranular corrosion in some aluminum alloys.

For example, 7075-T6 provides very high strength but may be less resistant to stress-corrosion cracking than overaged tempers such as T73. Marine 5xxx alloys use stabilized or marine-service H tempers because improper thermal exposure or unsuitable work-hardening conditions can affect long-term corrosion performance.

The buyer should therefore select the alloy and temper together rather than choosing the alloy first and treating temper as a secondary detail.

Recommended Temper by Application

Application	Recommended Starting Temper	Reason
Deep-Drawn Tank or Cover	O	Maximum ductility and lowest forming resistance.
Marine Plate and Hull Structure	Approved H temper such as H116 or H321	Controlled strength and marine corrosion performance.
Part Formed Before Final Aging	T4	Better forming capability than T6.
Structural Extrusion	T6	High yield strength and reliable engineering performance.
CNC-Machined Thick Plate	T651	Reduced residual stress and improved dimensional stability.
Architectural Profile	T5 or T6	Selection depends on required strength, surface and extrusion process.

Standards and Certificate Requirements

The alloy and temper should be stated in the complete material designation, such as 5052-H32, 5083-H116, 6061-T6, 6061-T651 or 7075-T651. The applicable product standard may include ASTM B209 for sheet and plate, ASTM B221 for extruded bars and profiles, ASTM B211 for rolled or cold-finished bars, or another product-specific specification.

Certificate Item	What to Verify
Alloy and Temper	Complete designation, not alloy number alone.
Product Standard	Correct standard for plate, sheet, bar, extrusion, tube or forging.
Chemical Composition	Actual controlled element results for the production lot.
Mechanical Properties	Tensile strength, yield strength and elongation for the ordered temper and thickness.
Heat Treatment	Solution treatment, aging or stabilization where applicable.
Traceability	Lot, batch or heat-treatment identification matching labels and packing documents.

An EN 10204 3.1 MTC may be requested to confirm the material results and production-lot traceability. Additional hardness, conductivity, residual-stress, corrosion or third-party tests should be specified separately when required.

How to Specify Aluminum Temper

A complete aluminum RFQ or purchase order should include:

✅ Alloy and complete temper, such as 6061-T651 or 5052-H32.

✅ Applicable ASTM, EN, AMS or customer standard.

✅ Plate, sheet, coil, bar, profile, tube or forging.

✅ Thickness, width, length, OD, wall or profile drawing.

✅ Required tensile strength, yield strength and elongation.

✅ Forming, welding or machining application.

✅ Flatness, straightness and dimensional tolerances.

✅ Mill, polished, anodized or coated surface.

✅ EN 10204 3.1 MTC and supplementary test reports.

✅ Quantity, packaging, destination port and delivery date.

Common Buyer Mistakes

Ordering alloy without temper: 6061-O, 6061-T4 and 6061-T6 have very different strength and forming behavior.

Writing only “H temper”: The full designation, such as H14, H32 or H116, is required to define processing and mechanical properties.

Using T6 and T651 as exact equivalents: Both are high-strength conditions, but T651 includes stress relief by stretching and is preferred for many machined plates.

Trying to perform severe bending in T6: T6 has lower ductility than T4 or O and may crack at tight bend radii.

Assuming temper does not affect corrosion: Some alloy-temper combinations have different stress-corrosion or exfoliation resistance.

Comparing mechanical data from different thicknesses: Required properties may vary by product form, thickness and test direction.

Accepting a certificate that lists only the alloy: The MTC should identify the complete temper and corresponding mechanical results.

FAQ

What does O temper mean in aluminum?

O temper means the aluminum has been annealed. It normally provides the lowest strength and highest ductility for that alloy, making it suitable for bending, drawing and complex forming.

What does H temper mean?

H temper means the aluminum has been strain hardened. Additional digits identify whether it was only cold worked, partially annealed or stabilized and indicate the degree of hardening.

What is the difference between T4 and T6 aluminum?

T4 is solution heat treated and naturally aged, providing moderate strength and better formability. T6 is solution heat treated and artificially aged, providing higher strength and hardness with reduced ductility.

What is the difference between T6 and T651?

T651 is based on the T6 condition but includes stress relief by controlled stretching. This reduces residual stress and normally improves dimensional stability during machining, especially in thick plate.

Which aluminum temper is best for bending?

O temper is generally the easiest to bend. T4 may also be selected for heat-treatable alloys that require forming before final aging. The minimum bend radius still depends on alloy, thickness and grain direction.

Which aluminum temper is best for CNC machining?

T6 is widely used for general machining, while T651 is normally preferred for thick plate and precision parts requiring improved dimensional stability after substantial material removal.

Can aluminum temper be confirmed by hardness alone?

Hardness can support temper verification, but it should not replace the material certificate, tensile testing or heat-treatment records when these are required. Different alloy and processing combinations can produce overlapping hardness values.

Related Aluminum Products and Guides

Related Resource	Procurement Relevance
Aluminum Plate	Aluminum plate in O, H, T4, T6, T651 and other product-specific tempers.
Aluminum Sheet	Sheet and coil for forming, fabrication, marine, architectural and industrial applications.
6061-T6 Aluminum Round Bar	High-strength bar for machining, structural parts, fixtures and engineering components.
ASTM B209 Aluminum Sheet and Plate Guide	Guidance on sheet and plate standards, alloy-temper designations, tolerances and certificates.
How to Read an Aluminum Mill Test Certificate	Explains alloy, temper, chemistry, mechanical properties and production-lot traceability.
ASTM B221 Aluminum Bar and Profile Standards	Standards and temper guidance for aluminum extruded bars, profiles and tubes.

Conclusion

Aluminum temper O, H, T4, T6 and T651 describe how the material has been annealed, strain hardened, solution heat treated, aged or stress relieved. O provides the highest formability, H tempers provide controlled strain-hardened strength, T4 balances ductility and strength, T6 provides higher strength, and T651 adds stress relief for improved machining stability.

The temper should always be specified together with the alloy and product standard. A complete designation such as 5052-H32, 5083-H116, 6061-T6 or 7075-T651 gives the supplier and buyer a clear basis for chemistry, mechanical properties, fabrication and inspection.