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Differences of Common Temper“T”Designations for 6061, 7075 Alloy Aluminum

In mechanical design and CNC machining, we often use aluminum alloy plate. For example, aluminum 6061-T6, 6061-T651, 7075 T6, 7075 T651, 7075 T6511, are the most wildly used aluminum alloys number. They are popular because they have a good weight to strength ratio, which means they are light in weight and good in strength. Especially for weight-sensitive places, such as high-speed sports platforms, aircraft structures, bicycle frames, car parts, etc. So, what are the differences between 6061 and 7075? What do T6, T651, T6511 mean? Speaking of this, we have to know the classification of aluminum alloy and its naming method.

 

Whats the meaning of Common Temper“T”?

“T” temper: Thermal Treated,Heat Treated

T is the most widely used symbol for heat-treatable alloys. It can be used for any heat treatable alloy. Heat-treated alloys are usually subjected to rapid quenching after solution heat treatment, supplemented by natural or artificial aging. “T” is always followed by one or more numbers that define the different treatments. Adding different numbers to the T will have different meanings.

 

T3: Solution heat treatment followed by cold working and finally natural aging to a steady-state. Used for products where strength can be improved by cold working, such as spreading or straightening.

T4: Solution heat treatment followed by natural aging to a stable state. Used for products that have not been cold worked after solution heat treatment, or where cold working does not improve strength.

T5: High temperature forming and cooling, followed by artificial aging. Applied to products that are molded at high temperature (e.g., cast or extruded) and cooled, then artificially aged to improve mechanical strength and dimensional stability.

T6: Solution heat treatment followed by artificial aging. Applied to products that are not cold worked after solution heat treatment, or where cold working does not improve strength.

T651: Applied to aluminum thick plates, rolled or cold-finished bars, and die-forged, forged, or rolled rings which are drawn without further straightening after solution heat treatment or after cooling from the high-temperature forming process by a specified amount. Pre-stretched sheet, a process added to remove stress from quenched aluminum sheet.

T6511: T6511 has an additional process of stretching to relieve stress than T6. These products are slightly straightened after stretching to meet standard tolerances. The permanent deformation is 1% ~ 3%; the permanent deformation of drawn tubes is 1.5% ~ 3%.

 

Summary: Six forms of aluminum 6061, 7075 for the same material, chemical composition is identical. T6, T651, T6510, T6511 for different heat treatment states, mechanical properties close, t651, etc. than t6 stress is less, the processing is not easy to deformation.

“T” temper process:

Step 1:  The aluminum alloy plate is placed at a constant temperature of about 527°C and kept for about 1 hour, with the purpose of dissolving the alloying elements in the aluminum alloy and distributing them evenly in the aluminum. Afterward, it is removed and quenched quickly in cold water. The purpose of quenching is to allow the alloying elements, such as magnesium and silicon, to remain in a fixed position. This is because if the part is cooled slowly, precipitation of the alloying elements usually occurs.

 

Step 2: Aging Treatment. This is to reheat the part to 177°C and hold it for 1-18 hours (the specific holding time will be different according to different sizes, shapes, applications, etc). The purpose of this step is to allow the precipitation of the hardening element Mg2Si to enhance the strength of the aluminum alloy.

 

What are the differences between aluminum sheet alloy 6061 T6 and 7075 T651?

As we know, aluminum alloy is an alloy with metallic characteristics based on aluminum with one or two major alloying elements added. In most aluminum alloys, the aluminum content is 90%-96%, and the alloying elements include copper, zinc, manganese, magnesium, silicon, etc.

 

6061-T6: The main alloying elements of 6061 aluminum alloy are magnesium and silicon, which will be synthesized into Mg2Si during solution heat treatment. It indicates that this aluminum alloy is a 6000 Series aluminum alloy. an aluminum-magnesium-silicon aluminum alloy with solution heat treatment and artificial aging treatment: T6, where T6 indicates that the aluminum alloy is tempered.

 

This heat treatment is a two-step process, with the first step heating the alloy and the second step aging treatment. These alloys can be heat treated to improve strength, although not as strong as 2xxx series aluminum sheet and 7xxx series aluminum sheet. But they combine good strength, workability, weldability, formability, and corrosion resistance. 6061 extruded aluminum is the first choice for mechanical and architectural construction. Aluminum alloy 6061, for example, is the most flexible of the heat treatable alloys, and it retains most of the excellent properties of aluminum, so it is also the aluminum alloy we use most frequently in our designs.

 

This grade has a wide range of mechanical properties and corrosion resistance, excellent machinability under annealed conditions which can be machined by conventional methods and welded.

 

 

7075 T651: This is a typical 7000Series alloy. The main alloying element of 7075 aluminum alloy is zinc, usually with some amount of copper and magnesium. It has the same type of heat treatment as 6061-T6 aluminum sheet, with a basic temper of T6, indicating solution heat treatment, followed by quenching and finally artificial aging, with the strengthening elements of Mg and ZnAlCu2. Another difference is that figure 5 indicates that it has undergone stretching to release stress, and 1 indicates that the amount of stretching for stress release is 0.5-2%.

 

Because of the use of zinc, these alloys are the strongest of all forged alloys and can even exceed the hardness of some steels. Because of this, 7000 series aluminum alloys are commonly used in the aircraft industry. Although the addition of zinc reduces its machinability, its excellent strength makes up for these disadvantages. 7075 aluminum alloy, because of its excellent weight-to-strength ratio, is ideal for highly stressed parts. And it can be molded and processed, heat-treated, and other operations, as needed.

 

Besides the T temper, what are the other tempers of the aluminum sheet?

The specific meanings of the different letters are as follows.

F = As Fabricated. indicating products made by the forming process method.

It is used for alloys that have no special requirements for strain hardening and heat treatment, and only obtain some tempering during the forming process, with no limitation on the mechanical properties. For example, 2014-F indicates the processed product form of 2014 aluminum alloy, the processing can be rolling, extrusion, forging, or a combination of these processes.

O: Annealing (Annealed)

The main purpose of annealing is to improve machinability, toughness, and ductility, and to give the aluminum alloy a minimum strength condition.

For example, 5083-O indicates any product form of 5083, which has been recently treated by heating to a high temperature of 345°C and then naturally cooling to room temperature.

H: Strain Hardened

For aluminum alloys that are not heat treatable, the strength is usually increased by strain hardening at room temperature.

H is usually followed by 2 or 3 symbols to indicate the amount of cold working, and subsequent heat treatment.

For example, the first number after H is H1 for strain-hardening only, H2 for strain-hardening and partial annealing, H3 for strain-hardening followed by heat stabilization, and H4 for strain-hardening and painting.

W: Solution Heat Treatment (Solution Heat Treated)

This is an unstable tempering and should only be used for alloys that have undergone natural aging at room temperature after using solution heat treatment. This symbol is used only when a natural aging cycle is specified.

 

The common problems of 6061 and 7075 during the process procedure are as followed:

  1. What makes us concern greatly during molding processing is sand holes or unknown grains are found on the final polished surface in the completion of most of the processing procedures and the workpiece is scrapped.

we in the mold manufacturing process is a deep headache is in the completion of most of the processing procedures, suddenly found 6061 T6 aluminum plate final polishing surface sand holes or unknown grains appear, and make the workpiece scrap.

A: Our unique grain refinement process ensures that all of our aerospace products pass the ultrasonic flaw detection process for aerospace aluminum sheets without sand holes, cross lines, bubbles, and impurities. 

 

  1. Our products are electronic robots and electronic assembly boards, so we require 7075 aluminum parts with no deformation after processing and forming. But why buy the European plate or domestic aluminum plate suddenly deformed in a day or two days after processing?

A: 7075 T651 aluminum plate, perfectly pre-stretching elimination of internal stress processed, can completely eliminate internal stress and it will not warp, crack or deform when processing and being stressed.

 

  1. We often see AA6061, AA7075, AA2024, AA5052 in the engineering data or drawings. What does AA represent?

A: AA is the abbreviation of ALUMINIUM ASSOCIATION, which is the abbreviation of the American Aluminum Association. AA is only in front of the deforming aluminum alloy grade. It represents the grade is produced and inspected according to Alcoa standards.  

 

  1. Super hard aluminum must be harder than hard aluminum, right?

A: Not exactly. Some technical performance indexes of aluminum alloy (such as strength, hardness, etc.) can be compared only when they belong to the same standard, a grade, and the same treatment state. For example, the hardness of Alcoa AA7075-T651 (HB150) is hard, but the hardness of AA7075-O (HB60) is much lower than that of AA6061-T651. It is often seen in the market that certain grades are misaligned (such as the hardness of domestic grade hard aluminum LY12 is lower than 6061-T6) for this reason.

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