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What is the difference between aluminum alloy 6061-T6 and 7075-T651

I have a problem.

In mechanical design, we often use aluminum alloy.

For example, 6061-T6 and 7075-T651 are the two most used aluminum alloys.
Because they have a good weight strength ratio, that is, they are light in weight and good in strength, they are very popular, especially in places where weight is sensitive, such as high-speed motion platform, aircraft structure, bicycle frame, etc.
So the question is, what is the difference between 6061-T6 and 7075-T651? What do 6xxx and 7xxx mean? What do T6 and T651 mean?
Here, I have to talk about the classification and naming of aluminum alloys.

Classification of aluminum alloy

Wrought and cast aluminum alloys

We know that aluminum alloy is an alloy with metal properties based on aluminum and added one or two main alloy elements.
In most aluminum alloys, the aluminum content is 90% – 96%, and the alloy elements include copper, zinc, manganese, magnesium, silicon, etc.
According to the type of manufacturing process, aluminum alloy can be divided into forged aluminum alloy and cast aluminum alloy.
Forged aluminum alloy is produced in the form of ingot or billet, and then produced through a variety of processes, such as rolling, extrusion, deformation, drawing and so on. The end user can process it into an alloy of parts.
Casting aluminum alloy is the ingot alloy made by casting method.
The alloy composition of cast aluminum alloy is greater than 10%, while the alloy element of forged aluminum alloy is not more than 4%.
Because the more alloying elements, the lower the ductility, which is not conducive to later processing.
Therefore, in practical projects, forged aluminum alloys are used in most cases, such as 6061707550831100 and even al-li8090-t8771, which are forged aluminum alloys.

Heat treatable and non heat treatable aluminum alloys

According to whether it can be heat-treated, aluminum alloy can be divided into heat-treated aluminum alloy and non heat-treated aluminum alloy.
Heat treatable aluminum alloy is an alloy whose main alloy elements (and some secondary alloy elements) can provide significant solid solution and precipitation hardening during solution heat treatment and aging, so as to improve the strength and hardness.
This involves several concepts, such as solution heat treatment, aging.
As for the strengthening of alloy, some concepts will be involved in the future, such as cold working, strain hardening and so on. Let me talk about these concepts together here, so that we can find a place to save.
Cold working: plastic deformation of metal occurs at a certain temperature and rate to realize strain hardening.
For example, through rolling, drawing and other plastic deformation, to improve its strength.
The principle is that cold working can form dislocations and vacancies in the microstructure, inhibit the relative movement between atoms, and improve the strength of the alloy.
Strain hardening: modify the metal structure through cold working, so as to increase the strength and hardness and reduce the ductility.
Solution treatment: a heat treatment method, including heating the product to a suitable temperature and maintaining it at that temperature for a long enough time to allow the solute to enter the solid solution, and then cooling rapidly to maintain the solute in the solid solution.

For aluminum alloy, solution heat treatment is to heat the alloy to a high temperature of 440 ℃ – 530 ℃ (the specific temperature is related to the alloy elements) in order to make the alloy elements dissolve in aluminum, and the material will become soft. It is usually cooled (quenched) in water at a high speed to maintain the distribution of solute elements in the alloy.

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Solution heat treatment, quenching and aging treatment

Aging: after solution heat treatment, solute atoms combine (precipitate) together at room temperature, which is called natural aging. If it is in a low-temperature furnace, solute atoms combine faster, which is called artificial aging. Therefore, aging actually provides finer atomic precipitation, so it can improve the strength.
For aluminum alloys, aging refers to the precipitation of some alloying elements or compounds from over saturated solid solution to produce the required mechanical properties.
After solution heat treatment and quenching, the material is relatively soft. At this time, it is very suitable for stretching to strengthen the material.
If quenched and placed in air for natural aging, the material will become harder and harder, but this change is very slow. Some alloys will become the hardest state even after a few years.
If artificial aging is carried out immediately after quenching, that is, the material is heated to 100-200 ° C and maintained for a period of time, the material will become hard and the strength will be greatly improved due to the precipitation of hardened compounds.

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Effect of different aging temperature on strength and hardness of 6160 aluminum alloy: it can be seen from the figure that higher temperature can make the alloy reach its maximum strength faster. However, the higher the temperature, the higher the final strength is than that obtained at low temperature.
In the aging process, it is very important to grasp the temperature and time. High temperature and long aging time will form larger precipitation elements, and the precipitation hardening effect will be greatly reduced.
On the contrary, too low aging temperature will consume too much precipitation time to produce good strengthening effect. Long time means low efficiency and high cost.
Annealing: heating and cooling slowly to eliminate internal stress and improve toughness.
Tempering: heat again after quenching. Tempering English is temperature. Temperature means to lose your temper. Usually, you don’t get angry. When you lose your temper, you get angry. It can be understood that when you lose your temper, the fire comes back, so temperature is called tempering (just for convenience of memory).
Well, we’ve explained a bunch of concepts. Now we can go on.
For non heat treatable aluminum alloy, its main alloy elements cannot provide significant solid solution and precipitation hardening during solution heat treatment and aging, so its strength can only be improved by strain hardening, such as cold rolling or wire drawing.
For example, class 1, 3 and 5 forged aluminum alloys are non heat treatable aluminum alloys, while class 2, 6 and 7 can be heat treated.

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Heat treatable and non heat treatable wrought aluminum alloy
For cast aluminum alloys, classes 1, 4 and 5 cannot be heat treated, while classes 2, 3, 7 and 8 can be heat treated.
The strength of non heat treatable aluminum alloy can only be improved by work hardening, such as rolling, drawing, etc.
Because cold working causes the formation of dislocations and vacancies in the structure and inhibits the relative movement between atoms, the strength of the alloy is improved.
Heat treatable aluminum alloy can improve its strength through heat treatment or work hardening.
In other words, whether the aluminum alloy can be heat treated or not determines the strengthening mode of the aluminum alloy.

Expression method of aluminum alloy

Aluminum alloy is represented by four digits and some symbols, such as 5083-H1127075-T73, etc.
In terms of representation, the difference between forged aluminum alloy and cast aluminum alloy is also obvious.
There is a decimal point in the first four digits of cast aluminum alloy, but there is no decimal point in forged aluminum alloy.
For example, 1xxx, 3xxx, 5xxx, 7xxx, etc. represent forged aluminum alloy, while 1XX. X, 3xx. X, 5xx. X, 7xx. X, etc. represent cast aluminum alloy.
Because in practical engineering, forging aluminum alloy is the majority, so next, I mainly focus on Forging Aluminum alloy.
The first digit represents the type of aluminum alloy, which is composed of 1-9, and different numbers represent different alloy components.
The second number represents the modification of alloy composition, 0 represents the original composition, 1 represents the first modification, 2 represents the second modification, and so on, indicating the difference of different alloy element contents. For example, 7075 represents the aluminum zinc original alloy, 7175 and 7475 represent the aluminum zinc modified alloy, and 7175 and 7475 are the modified grades of 7075.
The 3rd and 4th digits indicate the specific alloy in the alloy series. The values of these numbers have no special significance.

1xxx series

Class 1 aluminum alloy is not a real aluminum alloy, because its aluminum content is 99%, which belongs to commercial pure aluminum.
In terms of machinery, this kind of alloy has good ductility. For example, 1100 is commonly used to make sheet metal, as well as common aluminum foil packaging of drugs and food. It also uses class 1 alloy.
In addition, class 1 alloy has good corrosion resistance and workability, and its strength can be improved by work hardening.
This kind of alloy is widely used in the field of power transmission because of its excellent conductivity and thermal conductivity.

2XXX series

The main alloying element of class 2 alloy is copper and also contains a small amount of magnesium.
Because copper can be dissolved in aluminum at high temperature, this kind of alloy will react to solid solution strengthening, which is called heat treatable aluminum alloy.
After heat treatment, it can have good strength, comparable to low carbon steel.
Of course, because it contains copper, it is also more prone to corrosion.
2024 is a typical and most widely used class 2 aluminum alloy.

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Relationships among commonly used alloys in the 2xxx series (Al-Cu)

3xxx series

The main alloying element of class 3 aluminum alloy is manganese.
This kind of alloy has medium strength and excellent processability.
For example, 3003 aluminum alloy in this grade is commonly used in heat dissipation devices because of its good machinability.
Another example is 3004 aluminum alloy, which has good ductility and processability and is often used to make cans.

4xxx series

The main alloying element of class 4 aluminum alloys is silicon.
The addition of silicon can reduce the melting point, but does not affect the ductility, so this kind of alloy is usually used as welding wire to connect other aluminum.
In addition, because the oxide layer of 4 series alloys is aesthetically pleasing, it is also commonly used in architecture. The most representative is 4047, which has good heat conduction, conductivity and corrosion resistance.
This kind of alloy usually cannot be heat treated, but some can be heat treated to a certain extent according to the content of silicon and the composition of other alloy elements.

5xxx series

The main element of 5xxx series alloys is magnesium, and a small amount of manganese is also contained in certain alloys.
These alloys are strain hardening, easy to weld, and have excellent corrosion resistance, so they can be used in marine environment, such as hull, gangway and other marine equipment.
For example, 5052 alloy has good seawater corrosion resistance and excellent machinability. It is commonly used in marine ships, 5083 can be used in tanks and fighters, 5005 can be used in building structures, etc.

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6xxx series

The main alloying elements of 6 series aluminum alloys are magnesium and silicon. They will synthesize Mg2Si during solution heat treatment.
This kind of alloy can improve its strength through heat treatment. Although it is not as strong as 2XXX and 7xxx series aluminum alloys, it integrates good strength, machinability, weldability, formability and corrosion resistance.
The 6 series alloy formed by extrusion is the first choice in the field of machinery and building structure.
For example, 6061 aluminum alloy is the most flexible one in the heat treatable aluminum alloy, which retains the excellent characteristics of most aluminum at the same time, so it is also the aluminum alloy that we use most frequently in our design. This brand has a wide range of mechanical properties and corrosion resistance. It has excellent machinability under annealing conditions. It can be processed by conventional methods, and it can also be welded.

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7xxx aluminum alloy

The main alloying element of class 7 aluminum alloys is zinc, usually with a certain amount of copper and magnesium.
Because of the use of zinc, this alloy is the strongest of all forged alloys, and its strength can even exceed that of some steels.
Because of this, class 7 alloys are commonly used in the aircraft industry. Although the addition of zinc will also reduce its processability, its excellent strength makes up for these shortcomings.
For example, 7075 aluminum alloy is an ideal choice for high stress parts because of its excellent weight strength ratio. And can carry out forming processing, heat treatment and other operations as required.

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8xxx series aluminum alloy
Class 8 aluminum alloys use less commonly used elements as their alloy elements, such as lithium, tin, or iron.
This kind of alloy is generally used in occasions with specific requirements, such as high temperature performance, low density, high stiffness and so on.
For example, aluminum lithium alloy 8090-T8771 is used for large turntable with high speed, low moment of inertia and high rigidity.
Class 8 alloys are also commonly used in helicopter components and other aerospace applications.

Tempering treatment of aluminum alloy

Aluminum alloy group is represented by four digits, and different numbers represent different alloy components.
For example, the main alloying elements of class 2 alloys are copper, the main alloying elements of class 6 aluminum alloys are magnesium and silicon, and the main elements of class 7 aluminum alloys are zinc.
The heat treatment of aluminum alloy is represented by capital letters and numbers.
Capital letters, such as F, O, H, W, T, etc., indicate different heat treatment types.
For example, 6061-T6: indicates that the aluminum alloy belongs to class 6 aluminum alloy, i.e. aluminum magnesium aluminum silicon alloy, which has undergone solution heat treatment and artificial aging: T6.
For another example, 7075-T651 is basically tempered to T6, which means solution heat treatment, quenching and then artificial aging, 5 means stress release, and 1 means the tensile amount of stress release is 0.5-2%.
Here, we have a global understanding of the aluminum alloy system.

6061-T6: indicates that the aluminum alloy belongs to class 6 aluminum alloy, i.e. aluminum magnesium silicon aluminum alloy, which has been subjected to solution heat treatment and artificial aging treatment: T6.
T6 indicates that the aluminum alloy has been annealed.
The heat treatment is divided into two steps, the first step is heating the alloy, the second step is heat treatment.
In the first step, the aluminum alloy is placed at a constant temperature of about 527 ℃ and maintained for about 1 hour in order to dissolve the alloy elements in the aluminum alloy and distribute them evenly in the aluminum.
Then take it out and quench it quickly in cold water. The purpose of quenching is to keep the alloy elements magnesium and silicon in a fixed position.
Because if the parts are cooled slowly, alloying elements usually precipitate.
The second step, aging treatment, is to reheat the workpiece to 177 ℃ and keep it warm for 1-18 hours (the specific holding time is determined according to the workpiece size, shape, application and other factors).
The purpose of this step is to precipitate the hardening element Mg2Si to enhance the strength of aluminum alloy.
7075-T651: This is a typical 7 series alloy, that is, aluminum alloy with zinc as the main alloying element.
Its heat treatment type is the same as 6061-T6, and the basic tempering is T6, which means solution heat treatment, quenching and artificial aging. The strengthening elements of aging are mg and znalcu2.
Another difference is that 5 indicates that the stress is released after stretching, and 1 indicates that the tensile amount of stress release is 0.5-2%.

Source: China Aluminum Flange Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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