1. I have a question
In mechanical design, we often use aluminum alloys.
For example, 6061-T6 and 7075-T651 are the two most used aluminum alloys.
Because they have a good weight-to-strength ratio, that is to say, they are light in weight and good in strength, so they are very popular, especially for weight-sensitive places, such as high-speed sports platforms, aircraft structures, bicycle racks, etc.
So here comes the question, what is the difference between 6061-T6 and 7075-T651? What is the meaning of 6xxx and 7xxx? T6 VS T651?
Having said that, I have to talk about the classification and naming of aluminum alloys.
2. Classification of aluminum alloy
(1) Forged and cast aluminum alloys
We know that aluminum alloy is an alloy with aluminum as the basic element and one or two main alloying elements added, which has metallic properties.
In most aluminum alloys, the aluminum content is 90%-96%, and the alloying elements include copper, zinc, manganese, magnesium, silicon, etc.
According to the type of manufacturing process, aluminum alloys can be divided into wrought aluminum alloys and cast aluminum alloys.
Wrought aluminum alloys are produced in the form of ingots or billets, and are subsequently produced through various processes, such as rolling, extrusion, deformation, drawing, etc., and can be processed into parts by end users.
Wrought and cast aluminum alloys
The alloy composition of cast aluminum alloy is greater than 10%, while the alloy element of wrought aluminum alloy does not exceed 4%.
Because the more alloying element content, the lower the ductility, which is not conducive to post-processing.
Therefore, in actual engineering, forged aluminum alloys are used in most cases, such as commonly used 6061, 7075, 5083, 1100, and even AL-Li8090-T8771, which are forged aluminum alloys.
(2) Heat-treatable aluminum alloys and non-heat-treatable aluminum alloys
According to whether it can be heat treated, aluminum alloy can be divided into heat treatable aluminum alloy and non heat treatable aluminum alloy.
Heat treatable aluminum alloy is an alloy whose main alloying elements (and some minor alloying elements) can provide significant solid solution and precipitation hardening during solution heat treatment and aging process, thereby improving strength and hardness.
Several concepts are involved here, such as solution heat treatment and aging.
Regarding the strengthening of alloys, some concepts will be involved in the follow-up, such as cold working, strain hardening, etc. I will talk about these concepts together here, so as to save you from looking everywhere.
Cold Working: Metal undergoes plastic deformation at a certain temperature and rate to achieve strain hardening.
For example, plastic deformation such as rolling and drawing can be used to improve its strength.
The principle is that cold working causes the formation of dislocations and vacancies in the microstructure, which inhibits the relative movement between atoms, thereby improving the strength of the alloy.
Strain Hardening: Modification of the metal structure by cold working, resulting in increased strength and hardness and decreased ductility. See Figure 4 in this article for an understanding of strain hardening.
Solution Treatment: A method of heat treatment that involves heating a product to a suitable temperature and holding it at that temperature long enough to allow the solute to go into solid solution, followed by rapid cooling to keep the solute in the solid in solution.
For aluminum alloys, solution heat treatment is to heat the alloy to a high temperature of 440°C-530°C (the specific temperature is related to the alloying elements), the purpose is to dissolve the alloying elements in the aluminum, and the material will become soft at this time. This is followed by rapid cooling (quenching), usually in water, to maintain the distribution of solute elements in the alloy.
Aging: After solution heat treatment, solute atoms are combined (precipitated) together at room temperature, which is called natural aging. If it is in a low-temperature furnace, the solute atoms combine faster, which is called artificial aging. Therefore, aging actually provides finer atomic precipitation, so the strength can be improved.
For aluminum alloys, aging refers to the precipitation of a part of alloying elements or compounds from a supersaturated solid solution to produce the desired 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 it is placed in the air for natural aging after quenching, the material will become harder and harder, but this change is very slow, and some alloys even become the hardest state after several years.
And if artificial aging is carried out immediately after quenching, that is, the material is heated to 100-200°C and kept for a period of time, then the material will become hard because of the precipitation of hardened compounds, and the strength will be greatly improved.
The effect of different aging temperatures on the strength and hardness of 6160 aluminum alloy: It can be seen from the figure that higher temperature can make the alloy reach its peak strength faster. But the higher the temperature, the final strength obtained is not as high as that obtained at low temperature.
In the aging process, it is very important to grasp the temperature and time. High temperature and too long aging time will form larger precipitated elements, and the effect of precipitation hardening will be greatly reduced.
On the contrary, too low aging temperature will consume too much precipitation time to produce a good strengthening effect, and a long time means low efficiency and high cost.
Annealing: heating and cooling slowly to eliminate internal stress and improve toughness.
Tempering (Temper): Heating again after quenching. Tempering is Temper in English. Temper has the meaning of losing one’s temper. Usually, the anger is not big, but when one loses one’s temper, the fire becomes big. It can be understood that when one loses one’s temper, the fire returns, so Temper is called tempering (only for convenient memory).
Well, a bunch of concepts have been explained, now we can move on.
Non-heat-treatable aluminum alloy, in the process of solution heat treatment and aging, its main alloying elements cannot provide significant solid solution and precipitation hardening, so it can only be strain hardened, such as cold rolling or wire drawing, to improve its strength .
For cast aluminum alloys, types 1, 4, and 5 are not heat treatable, while types 2, 3, 7, and 8 are heat treatable.
Aluminum alloys cannot be heat treated, and their strength can only be increased by work hardening, such as rolling, drawing, etc.
Because cold working causes dislocations and vacancies to form in the structure and inhibits the relative motion between atoms, the strength of the alloy is increased.
Heat-treatable aluminum alloys can be heat-treated or work-hardened to increase their strength.
That is to say, whether it can be heat treated or not determines the strengthening method of aluminum alloy.
3. Representation method of aluminum alloy
Aluminum alloys are represented by four digits plus some symbols, such as 5083-H112, 7075-T73, etc.
In terms of representation, the difference between wrought aluminum alloy and cast aluminum alloy is also obvious.
Cast aluminum alloys have a decimal point in the first 4 digits, while wrought aluminum alloys do not have a decimal point.
For example, 1xxx, 3xxx, 5xxx, 7xxx, etc. indicate wrought aluminum alloys, while 1xx.x, 3xx.x, 5xx.x, 7xx.x, etc. indicate cast aluminum alloys.
Because forged aluminum alloys are mostly used in actual engineering, so next, I will mainly focus on forged aluminum alloys.
The first digit indicates the type of aluminum alloy, consisting of 1-9, and different numbers indicate different alloy compositions.
The second number indicates the modification of the alloy composition, 0 indicates the original composition, 1 indicates the first modification, 2 indicates the second modification, and so on, indicating the difference in the content of different alloying elements. For example, 7075 represents the original aluminum-zinc alloy, 7175 and 7475 represent the modified aluminum-zinc alloy, and 7175 and 7475 are modified grades of 7075.
The 3rd and 4th digits indicate the specific alloy within the alloy family. The values of these numbers have no special meaning.
Representation method of wrought aluminum alloy
Class 1 aluminum alloy is not actually a real aluminum alloy, because its aluminum content is 99%, which is commercially pure aluminum.
In terms of machinery, this type of alloy has good ductility. For example, 1100 is often used to make sheet metal, and common aluminum foil packaging for medicines and foods is also a type 1 alloy.
In addition, Class 1 alloys have good corrosion resistance, machinability, and can be work hardened to increase their strength.
This type of alloy is widely used in the field of power transmission because of its excellent electrical and thermal conductivity.
The main alloying element of class 2 alloys is copper, and also contains a small amount of magnesium.
Because copper dissolves in aluminum at high temperatures, such alloys respond to solid solution strengthening and are called heat treatable aluminum alloys.
After heat treatment, it can have very good strength, comparable to mild steel.
Of course, because it contains copper, it is also more prone to corrosion.
2024 is typical and the most widely used type 2 aluminum alloy.
The main alloying element of Class 3 aluminum alloys is manganese.
These alloys have medium strength and have excellent machining properties.
For example, the 3003 aluminum alloy in this grade is often used in heat sinks because of its good machinability.
Another example is 3004 aluminum alloy, which has good ductility and processability, and is often used to make cans.
The main alloying element of Type 4 aluminum alloys is silicon.
The addition of silicon can lower the melting point without affecting ductility, so this type of alloy is often used as a welding wire to join other aluminum.
In addition, 4 series alloys are commonly used in construction because of the aesthetically pleasing oxide layer. The most representative is 4047, which has good thermal conductivity, electrical conductivity and corrosion resistance.
Such alloys are generally not heat treatable, but some can be heat treated to a certain extent, depending on the silicon content and composition of other alloying elements.
The main element of the 5xxx series alloys is magnesium and, in certain alloys, small amounts of manganese.
These alloys are strain hardenable, easy to weld, and have excellent corrosion resistance, so they can be used in marine environments, such as hulls, gangways and other marine equipment.
For example, 5052 alloy has good seawater corrosion resistance and excellent machinability. It is often used in marine vessels, while 5083 can be used in tanks and fighters, and 5005 is often used in building structures.
The main alloying elements of the 6 series aluminum alloys are magnesium and silicon, which will synthesize Mg2Si during solution heat treatment.
This type of alloy can be improved by heat treatment. Although it is not as strong as the 2xxx and 7xxx series aluminum alloys, it combines good strength, machinability, weldability, formability and corrosion resistance.
The 6 series alloys formed by extrusion are the first choice in the field of mechanical and architectural structures.
For example, 6061 aluminum alloy is the most flexible heat-treatable aluminum alloy, and it retains most of the excellent characteristics of aluminum, so it is also the most frequently used aluminum alloy in our design. The grade has a wide range of mechanical properties and corrosion resistance, has excellent machinability in the annealed condition, can be processed using conventional methods, and it can also be welded.
7xxx aluminum alloys
The main alloying element of Class 7 aluminum alloys is zinc, usually with certain amounts of copper and magnesium.
Because of the use of zinc, these alloys are the strongest of all wrought alloys, surpassing even some steels in strength.
Because of this, Group 7 alloys are commonly used in the aircraft industry. Although the addition of zinc also reduces its machinability, its excellent strength makes up for these shortcomings.
7075 aluminum alloy, for example, is ideal for highly stressed parts due to its excellent weight-to-strength ratio. And it can carry out molding processing, heat treatment and other operations according to the needs.
8xxx series aluminum alloy
Type 8 aluminum alloys use less commonly used elements as their alloying elements, such as lithium, tin, or iron.
Such alloys are generally used in occasions with specific requirements, such as high temperature performance, lower density, higher stiffness and other requirements.
For example, aluminum-lithium alloy 8090-T8771 is used for large turntables with high-speed rotation, low moment of inertia and high rigidity.
Group 8 alloys are also commonly used in helicopter components, and other aerospace applications.
4. Tempering treatment of aluminum alloy
The aluminum alloy group is represented by four digits, and different numbers represent different alloy compositions.
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 alloys is represented by capital letters and numbers.
Capital letters, such as F, O, H, W, T, etc., indicate different types of heat treatment.
For example, 6061-T6: indicates that the aluminum alloy belongs to the sixth type of aluminum alloy, that is, aluminum-magnesium-silicon aluminum alloy, and has undergone solution heat treatment and artificial aging: T6.
For another example, 7075-T651, the basic tempering is T6, which means solution heat treatment, quenching, and then artificial aging, 5 means that it has undergone stress release, and 1 means that the stretching amount of stress release is 0.5-2%.
The specific meanings of different letters are as follows:
F=As Fabricated, which means products made by forming process.
For alloys with no special requirements for strain hardening and heat treatment, only some tempering is obtained during forming, and the mechanical properties are not limited.
For example, wrought or cast alloy products manufactured by processes such as rolling, extrusion, forging, drawing or casting, which do not have special control over the thermal conditions during working or strain hardening.
For example, 2014-F indicates the processed product form of 2014 aluminum alloy, and the processing technology can be rolling, extrusion, forging, or a combination of these processes.
The main purpose of annealing is to improve machinability, toughness, and ductility, and to bring the aluminum alloy to its lowest strength state.
For example, 5083-O means any product form of 5083, the most recent treatment of which is heating to a high temperature of 345°C, and then naturally cooling to room temperature.
H: Strain Hardened
For aluminum alloys that cannot be heat treated, their strength is usually increased by strain hardening at room temperature.
There are usually 2 or 3 symbols after H, which are used to indicate the amount of cold processing and subsequent heat treatment.
For example, the first number after H, H1 means only strain hardening, H2 means strain hardening and partial annealing. H3 means strain hardening followed by heat stabilization treatment, H4 means strain hardening and painting.
The specific meanings of H1-H4 are as follows.
H1: There is no heat treatment process, and the strength is only increased by strain hardening. The value behind this code indicates the degree of hardening.
H2: Strain hardened and partially annealed. Used for: Products that have undergone excess strain hardening followed by partial annealing to reduce strength to the required level. The number after H2 indicates the amount of strain hardening remaining after annealing.
H3: Strain hardened and stabilized at low temperature. Used for products that reduce strength and increase ductility by strain hardening followed by low temperature stabilization. The number after this symbol indicates the remaining amount of hardening after strain hardening and low temperature stabilization.
The second number after the H, such as the X in H1X, indicates the actual strain hardening of the alloy.
Another example is the X in H2X, which indicates the remaining effective cold working amount after the metal exceeds the required cold working amount and undergoes partial annealing.
And X in H3X, after cold working and temperature stabilization treatment, the remaining effective cold working.
The X in H4X denotes effective cold work remaining after cold work and subsequent heat exposure involved in forming and painting processes.
As mentioned above, the second number after H indicates the degree of strain hardening. If HX (X=1, 2, 3, 4) is followed by the following numbers, then its specific meaning is as follows:
- 2: 1/4 hardening amount.
- 4: 1/2 hardening amount.
- 6: 3/4 hardening amount.
- 8: Full hardening amount.
- 9: Excess hardening amount.
In short, the second number after H indicates the remaining amount of cold work.
The third digit after the H, such as HXX1, is a variant of the two-digit temper, which is used to indicate mechanical properties or finishing control, but the difference is usually not large.
For example, H111 indicates slight strain hardening in tension after annealing, and is usually applied to extruded profiles that must be straightened after annealing to achieve straightness tolerances.
H112 is used for products that have obtained a little tempering through high temperature forming process, and have no special control on the amount of strain hardening and heat treatment, but have certain requirements on mechanical properties.
H111, H311 and H321 are used for alloys with less hardening than H11, H31 and H32 respectively.
W: Solution Heat Treated
This is an unstable temper and should only be applied to alloys that have undergone natural aging at room temperature after solution heat treatment. Use this notation only when specifying a natural aging period.
T: Heat Treatment (Thermal Treated, Heat Treated)
T indicates heat treatment, which is heat treated to produce a stable temper other than F, O or H.
T is the most widely used symbol for heat treatable alloys, and it can be used for any heat treatable alloy.
Heat treatable alloys are usually solution heat treated followed by rapid quenching, supplemented by natural or artificial aging.
T is always followed by one or more numbers, which are used to define different subsequent processing.
- T1: After forming at high temperature and cooling, it is naturally aged to a basically stable state.After being applied in a high temperature forming process (such as casting or extrusion), the product is subjected to room temperature aging treatment at a cooling rate sufficient to increase strength.It is suitable for products that have not been cold-worked after forming and cooling at high temperature, or products that have no obvious effect on mechanical properties due to cold work, such as flattening or straightening.
- T2: High temperature forming and cooling, followed by cold working and natural aging to steady state.
- T3: Solution heat treatment, followed by cold working, and finally natural aging to steady state. Applied to products where strength can be increased by cold working, such as flattening or straightening.
- T4: Solution heat treated, then naturally aged to steady state. Applied to products that have not been cold-worked after solution heat treatment, or products that cannot be cold-worked to increase strength.
- T5: Formed at high temperature and cooled, then artificially aged. A product that is applied to high-temperature molding (such as casting or extrusion) and cooled to artificially age to improve mechanical strength and dimensional stability.
- T6: Solution heat treatment followed by artificial aging. It is applied to products that do not undergo cold working after solution heat treatment, or that cold working cannot improve the strength.
- T7: Solution heat treatment followed by furnace aging to steady state. The purpose of stabilization is to increase its tensile strength.
- T8: Solution heat treatment, then cold work hardening, and finally artificial aging. Applied to products where strength can be increased by cold working, such as flattening or straightening.
- T9: Solution heat treatment, then artificial age hardening, and finally cold working to increase strength.
- T10: Cold working after high temperature forming and cooling, and then artificial aging to achieve precipitation hardening.
5. The difference between 6061-T6 and 7075-T651
OK, here we have a general understanding of the aluminum alloy system.
So now, let’s talk about 6061 and 7075, it should be easier to understand.
Performance comparison of aluminum alloy 6061 and 7075
6061-T6: Indicates that the aluminum alloy belongs to the sixth type of aluminum alloy, that is, aluminum-magnesium-silicon aluminum alloy, and has undergone solution heat treatment and artificial aging treatment: T6.
Among them, T6 indicates that the aluminum alloy has been tempered and heat treated.
The heat treatment is divided into two steps, the first step is to heat the alloy, and the second step is to effectively treat it.
In the first step, the aluminum alloy is placed at a constant temperature of about 527°C for about 1 hour, in order to dissolve the alloying elements in the aluminum alloy and distribute them evenly in the aluminum.
Then take it out and quickly quench it in cold water. The purpose of quenching is to keep the alloying elements magnesium and silicon in a fixed position.
Because if the part is cooled slowly, precipitation of alloying elements usually occurs.
The second step, aging treatment, is to reheat the workpiece to 177°C and keep it warm for 1-18 hours (the specific holding time is determined according to the size, shape, application and other factors of the workpiece).
The purpose of this step is to precipitate the hardening element Mg2Si to enhance the strength of the aluminum alloy.
7075-T651: This is a typical 7 series alloy, which is an 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, then quenching, and finally artificial aging. The strengthening elements of aging are Mg and ZnAlCu2.
Another difference is that 5 means that it has been stretched to release the stress, and 1 means that the amount of stretching for stress release is 0.5-2%.