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T6 VS T651 – Aluminum Tempers Designation System Explain


​The main alloying elements in 6061 aluminum plate are magnesium and silicon, with moderate strength, good corrosion resistance, weldability and oxidation effect. Widely used in various industrial structures requiring certain strength and high antibiotic corrosion resistance. In addition to alloying elements, tempering also affects the properties of the metal. T6 and T651 are both temper designations for aluminum alloys. These designations indicate different levels of heat treatment and resulting material properties.So what is the difference between aluminum 6061 T6 and 6061 T6511? In this article, we discuss the state designation system for aluminum.

  • T6: The T6 temper is a solution heat-treated and artificially aged condition for aluminum alloys. In this process, the material is heated to a specific temperature range (around 495°C or 925°F) and then rapidly cooled, known as quenching. This step helps to dissolve alloying elements and create a supersaturated solid solution. After quenching, the material is artificially aged at a lower temperature (around 120-180°C or 250-360°F) to precipitate the alloying elements, enhancing the strength and hardness of the aluminum alloy. T6 temper provides good strength, but it may have some residual stress.
  • T651: The T651 temper is a stretched and artificially aged condition for aluminum alloys. It involves the same steps as the T6 temper, including solution heat treatment and artificial aging. However, after the aging process, the material is further stretched or straightened while still at room temperature. This stretching process helps to relieve internal stresses and improve dimensional stability. T651 temper offers improved flatness and dimensional control compared to T6, making it suitable for applications that require tighter tolerances.

The main difference between T6 and T651 lies in the additional stretching process involved in the T651 temper. T651 offers improved dimensional stability and flatness compared to T6, making it preferred for applications where precise dimensions are crucial. However, both temper conditions provide enhanced strength and hardness compared to the base alloy. The choice between T6 and T651 depends on the specific requirements of the application and the desired material properties.

Aluminum Temper Designation System


The aluminum temper designation system is a standardized method used to indicate the heat treatment and resulting mechanical properties of aluminum alloys.

The temper designation is typically represented as a combination of letters and numbers. Here are the key components of the aluminum temper designation system:

1.Letter Prefixes

The temper designation begins with a letter prefix that indicates the general tempering process applied to the aluminum alloy. The most commonly used letter prefixes are:

  • F: As-fabricated condition (No specific control over temper)
  • O: Annealed condition (Fully annealed for maximum softness)
  • H: Strain-hardened condition
  • W: Solution heat-treated condition (Water quenched)
  • T: Thermally treated condition (Other than F, O, H, or W)

2.Numeric Digits

The numeric digits that follow the letter prefix provide further details about the specific temper condition. The specific meaning of these digits can vary depending on the aluminum alloy series and specifications. However, there are some general guidelines:

  • H Temper: The H temper is followed by two or more digits. The first digit indicates the basic degree of strain hardening, with higher numbers representing higher levels of hardening. The subsequent digits indicate modifications or variations of the basic temper.
  • T Temper: The T temper is followed by one or more digits, which usually represent the heat treatment process and its specific details. Different alloys may have different meanings for the numeric digits following the T.

3.Subscript Letters

Subscript letters may follow the numeric digits to indicate special characteristics or variations of the temper condition. Common subscript letters include:

  • F: Indicates as-fabricated condition after a specific temper treatment
  • H: Indicates strain-hardening only without subsequent thermal treatment
  • W: Indicates artificial aging after solution heat treatment
  • X: Indicates special control over thermal treatment parameters

Examples of aluminum temper designations include:

  • O: Annealed condition
  • H32: Strain-hardened and stabilized condition, 1/4 hard
  • T6: Solution heat-treated and artificially aged condition
  • T651: Solution heat-treated, stretched, and artificially aged condition
  • H112: Strain-hardened and stabilized condition, with specific control over properties for use in marine applications

It’s important to refer to the appropriate specifications and documentation for the specific aluminum alloy being used to ensure accurate interpretation of the temper designation.

4.Subdivisions of T Temper

The T temper designation in aluminum alloys represents a thermally treated condition. It indicates that the aluminum alloy has undergone specific heat treatment processes to achieve desired mechanical properties. The T temper can be further subdivided to provide more detailed information about the heat treatment applied. Here are the common subdivisions of the T temper:

  • T1: Solution heat-treated only. This temper involves heating the aluminum alloy to a specific temperature and then quenching it to achieve a solutionized state. No subsequent aging treatment is performed.
  • T2: Annealed and then naturally aged. The alloy is first fully annealed for maximum softness, and then it is allowed to naturally age over time at room temperature. This temper is generally used for alloys that age-harden at a slow rate.
  • T3: Solution heat-treated and then cold worked. The alloy is heated to a specific temperature, quenched, and then cold worked (strain-hardened) to increase its strength. No subsequent aging treatment is performed.
  • T4: Solution heat-treated and then naturally aged. After solution heat treatment, the alloy is naturally aged at room temperature to allow precipitation hardening to occur. This temper provides a balance between strength and formability.
  • T5: Artificially aged. The alloy is solution heat-treated and then artificially aged at an elevated temperature. This temper provides improved strength and hardness compared to T4 and is commonly used in structural applications.
  • T6: Solution heat-treated and then artificially aged. The alloy is heated to a specific temperature, quenched, and then artificially aged at an elevated temperature. This temper provides the highest strength and hardness among the T tempers and is widely used in various applications.
  • T7: Solution heat-treated and then overaged. The alloy is heated to a specific temperature, quenched, and then aged at a higher temperature for an extended period. This temper is used to maximize the toughness and fracture resistance of the alloy, sacrificing some strength.

These are some of the common subdivisions of the T temper for aluminum alloys. The specific subdivision and its associated heat treatment process depend on the desired properties and requirements of the alloy and the application it is intended for.

Difference Between T6 and T651 – T651 vs T6 Temper


T6 and T651 are two specific subdivisions of the T temper designation used in aluminum alloys. While they are similar in some aspects, there is a key difference between T6 and T651.

T6 Temper

The T6 temper is a solution heat-treated and artificially aged condition for aluminum alloys. In this process, the alloy is heated to a specific temperature (around 495°C or 925°F) and then rapidly quenched to achieve a solutionized state. This step helps dissolve alloying elements and create a supersaturated solid solution. After quenching, the material is artificially aged at a lower temperature (around 120-180°C or 250-360°F) to precipitate the alloying elements, which contributes to the increase in strength and hardness of the alloy. T6 temper provides high strength and good formability.

T651 Temper

The T651 temper is a stretched and artificially aged condition for cnc machining aluminum alloys. It involves the same steps as the T6 temper, including solution heat treatment and artificial aging. However, after the artificial aging process, the material is stretched or straightened while still at room temperature. This stretching process helps to relieve internal stresses and improve dimensional stability. The T651 temper offers improved flatness and dimensional control compared to the T6 temper. It is particularly suitable for applications where precise dimensions and dimensional stability are important.

Difference Between T6 And T651

The key difference between T6 and T651 is the additional stretching process involved in the T651 temper. This stretching step is performed after the artificial aging process and aims to reduce residual stresses and improve dimensional stability. The T651 temper offers enhanced flatness, tighter dimensional control, and reduced internal stresses compared to T6.

When choosing between T6 and T651, it is important to consider the specific requirements of the application. If precise dimensions and dimensional stability are critical, T651 may be preferred. However, if high strength and good formability are the primary considerations, T6 can be a suitable choice.

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