A Comprehensive Guide to Aluminum Alloy 6061 (UNS A96061)

What is aluminum alloy 6061?

Aluminum alloy 6061 (UNS A96061) is a kind of precipitation hardening aluminum alloy, which belongs to the 6XXX series aluminum alloy, and its main alloying elements are magnesium and silicon. Adding magnesium can improve the strength while adding silicon can reduce the melting temperature of the metal. It generally has good mechanical properties and can be heat treated and welded. Aluminum alloy 6061 is one of the most commonly used aluminum alloys.

Classification of aluminum alloy 6061

Common models include: 6061-O, 6061-T1, 6061-T4, 6061-T6, 6061-T42, 6061-T651, 6061-T42, etc.

6061-O
Annealed 6061 (6061-O tempered) has an ultimate ultimate tensile strength less than 150 MPa (22 ksi) and an ultimate yield strength less than 83 MPa (12 ksi) or 110 MPa (16 ksi). The elongation (elongation to ultimate failure) of this metal material is 10-18%. To obtain the annealed condition, the alloy is typically heat soaked at 415 °C for 2-3 hours.
6061-T1
6061-T1 aluminum alloy is 6061 aluminum in the T1 temper. To achieve this temper, the metal is naturally aged until it meets standard mechanical property requirements.
6061-T4
T4 tempered 6061 has an ultimate tensile strength of at least 180 MPa (26 ksi) or 210 MPa (30 ksi) and a yield strength of at least 110 MPa (16 ksi). The elongation rate is 10-16%.
6061-T6
T6 tempered 6061 is treated to provide maximum precipitation hardening (and therefore maximum yield strength) in 6061 aluminum alloy. Ultimate tensile strength is at least 290 MPa (42 ksi) and yield strength is at least 240 MPa (35 ksi). More common values are 310 MPa (45 ksi) and 270 MPa (39 ksi) respectively. This can exceed the yield strength of certain types of stainless steel. Elongation is greater than 8% for thicknesses less than 6.35 mm (0.250 inch). There is 10% elongation in thicker areas. T651 temper has similar mechanical properties. Typical thermal conductivity for 6061-T6 at 25 °C (77 °F) is approximately 152 W/m K. Using a standard RR Moore tester and specimen, the fatigue limit under cyclic loading is 97 MPa (14 ksi) at 500,000,000 full reversal cycles. Note that there is no clear “knee” in the S-N curve of aluminum, so there is some debate as to how many cycles equate to “infinite life”. Also note that the actual value of the fatigue limit for your application can be significantly influenced by traditional derating factors such as load, slope, and surface finish.
6061-T42
6061-T42 aluminum alloy is 6061 aluminum in the T42 temper. To achieve this temper, the metal is solution heat-treated and naturally aged. Unlike T4 temper, this is done by the receiver rather than the supplier.
6061-T451
6061-T451 aluminum alloy is 6061 aluminum in the T451 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then naturally aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (sheet, plate, bar, or forging).
6061-T4510
6061-T4510 aluminum alloy is 6061 aluminum in the T4510 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then naturally aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (extrusion or tube). The metal is not straightened after the stretching operation. This temper is closely related to T4511, which permits such straightening.
6061-T4511
6061-T4511 aluminum alloy is 6061 aluminum in the T4511 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then naturally aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (extrusion or tube). The metal is straightened after the stretching operation. This temper is closely related to T4510, which does not permit such straightening.
It has the second lowest strength compared to the other variants of 6061 aluminum.
6061-T51
6061-T51 aluminum alloy is 6061 aluminum in the T51 temper. To achieve this temper, the metal is artificially underaged. The degree of underaging is different from T53 and T54. It has the second lowest ductility compared to the other variants of 6061 aluminum.
6061-T62
6061-T62 aluminum alloy is 6061 aluminum in the T62 temper. To achieve this temper, the metal is solution heat-treated and artificially aged until it meets standard mechanical property requirements. Unlike T6 temper, this is done by the receiver rather than the supplier.
6061-T651
6061-T651 aluminum alloy is 6061 aluminum in the T651 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then artificially aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (sheet, plate, bar, or forging).
6061-T6510
6061-T6510 aluminum alloy is 6061 aluminum in the T6510 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then artificially aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (extrusion or tube). The metal is not straightened after the stretching operation. This temper is closely related to T6511, which permits such straightening.
6061-T6511
6061-T6511 aluminum alloy is 6061 aluminum in the T6511 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then artificially aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (extrusion or tube). The metal is straightened after the stretching operation. This temper is closely related to T6510, which does not permit such straightening.
6061-T652
6061-T652 aluminum alloy is 6061 aluminum in the T652 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then artificially aged. The stress relief is accomplished by compressing the metal by 1-5%.
6061-T89
6061-T89 aluminum alloy is 6061 aluminum in the T89 temper. 
6061-T94
6061-T94 aluminum alloy is 6061 aluminum in the T94 temper. To achieve this temper, the metal is solution heat-treated, artificially aged, then strain hardened to a higher degree than T9 temper.

Equivalents of Grade 6061

US				European  Union		ISO		Japan		China
Standard	Grade (UNS)	SAE AMS Standard	Grade	Standard	Numerical (Chemical Symbols)	Standard	Grade	Standard	Grade	Standard	Grade
AA;	6061  (UNS  A96061)	SAE  AMS  4025;	6061	EN  573-3	EN  AW-6061	ISO  209	AW-6061	JIS  H4000;	6061	GB/T  3880.2	6061
ASTM  B209;		SAE  AMS  4026;			(EN  AW-AlMg1SiCu)			JIS  H4040		GB/T  3190
ASTM  B211;		SAE  AMS  4027;
ASTM  B221;		SAE  AMS  4117
ASTM  B210;
ASTM  B308/ B308M;
ASTM B241 /B241M

Standards of Aluminum Alloy 6061

The different forms and temperatures of 6061 aluminum alloy are discussed in the following standards:

ASTM B209	Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate.
ASTM B210	Standard Specification for Aluminum and Aluminum-Alloy Drawn Seamless Tubes.
ASTM B211	Standard Specification for Aluminum and Aluminum-Alloy Bar, Rod, and Wire.
ASTM B221	Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes.
ASTM B308/308M	Standard Specification for Aluminum-Alloy 6061-T6 Standard Structural Profiles.
ASTM B483	Standard Specification for Aluminum and Aluminum-Alloy Drawn Tube and Pipe for General Purpose Applications.
ASTM B547	Standard Specification for Aluminum and Aluminum-Alloy Formed and Arc-Welded Round Tube.
ISO 6361	Wrought Aluminium and Aluminium Alloy Sheets, Strips and Plates.

Characteristics of aluminum alloy 6061

Aluminum alloy 6061 has excellent processing performance, good corrosion resistance, high toughness, no deformation after processing, easy coloring film, and excellent oxidation effect.

Chemical composition of aluminum alloy 6061

The main alloying elements of 6061 aluminum alloy are magnesium and silicon, and the Mg₂Si phase is formed. If it contains a certain amount of manganese and chromium, it can neutralize the bad effect of iron; sometimes, a small amount of copper or zinc is added to improve the strength of the alloy without significantly reducing its corrosion resistance.
There is also a small amount of copper in the conductive material to offset the adverse effects of titanium and iron on the conductivity; zirconium or titanium can refine grains and control the recrystallization structure. In order to improve the machinability, lead and bismuth can be added. The solid solution of Mg₂Si in aluminum makes the alloy have the function of artificial aging hardening.

Chemical Elements	Content (%)
Aluminum, Al	≤ 97.5 %
Chromium, Cr	≤ 0.10 %
Copper, Cu	≤ 0.10 %
Iron, Fe	≤ 0.35 %
Magnesium, Mg	0.45 – 0.90 %
Manganese, Mn	≤ 0.10 %
Other, each	≤ 0.05 %
Other, total	≤ 0.15 %
Silicon, Si	0.20 – 0.60 %
Titanium, Ti	≤ 0.10 %
Zinc, Zn	≤ 0.10 %

Mechanical properties of 6061

Properties	Metric	Imperial
Tensile strength	310 MPa	45000 psi
Yield strength	276 MPa	40000 psi
Shear strength	207 MPa	30000 psi
Fatigue strength	96.5 MPa	14000 psi
Elastic modulus	68.9 GPa	10000 ksi
Poisson’s ratio	0.33	0.33
Elongation	12-17%	12-17%
Hardness, Brinell	95	95

Physical properties of aluminum alloy 6061

Properties	Metric	Imperial
Density	2.7 g/cm3	0.0975 lb/in3
Melting point	588°C	1090°F

Thermal properties of aluminum alloy 6061

Properties		Conditions
		T (ºC)	Treatment
Thermal expansion co-efficient	23.2 (10-6/ºC)	20-100	–
Thermal conductivity	167 W/mK	25	–

Preparation and Heat Treatment of Aluminum Alloy 6061

Machinability of aluminum alloy 6061

Aluminum alloy 6061 has good machinability in the harder T4 and T6 states. It can be machined in the annealed state.

Forming of Aluminum Alloy 6061

Aluminum Alloy 6061 is easy to form and machine in the annealed state. Bending, stamping, deep drawing, spinning, and other operations are carried out using standard methods.

Welding of Aluminum Alloy 6061

Aluminum Alloy 6061 has excellent weldability. Thinner sections can be welded using tungsten argon arc welding technology. The thicker section can be welded by melting electrode gas shielded welding technology. Using 4043 alloy wire can achieve better results but will affect the T6 performance.

Heat treatment of aluminum alloy 6061

Heat treat aluminum alloy 6061 at 533°C (990°F) for a sufficient period and then quench in water. The precipitation hardening process may be held at 160°C (320°F) for 18h, and then air cooled. The process is repeated at 177°C (350°F) for 8 hours and then cooled in air.

Annealing process 

• Rapid annealing: heating temperature 350-410 °C; with the different effective thicknesses of the material, the holding time is between 30-120min; air or water cooling. 
• High-temperature annealing: heating temperature 350-500 °C; when the thickness of the finished product is ≥ 6mm, the holding time is 10-30min, and when it is less than 6mm, it is hot through, air is cold. 
• Low temperature annealing: heating temperature 150-250 °C; the holding time is 2-3h; air or water cooling. Homogenization: 570 degrees Celsius, holding for 7 hours, air cooling.

Manufacturing job shops typically interpret a metal material specification of “ALUMINUM 6061-T6” as allowing 6061-T6, 6061-T651, or 6061-T6511. All of these are acceptable tempers and are permitted for most applications. But, from years of experience, this is not necessarily the best interpretation.
To explain, we will give a basic background in the metallurgical process differences for Aluminum 6061 – specifically in thermally treated temper designations 6061-T6, 6061-T651, and 6061-T6511. There are other designations, such as F (As Fabricated), O (Annealed), W (Solution Heat Treated, naturally), and H (Strain Hardened), but they are not discussed here.

Different industries, different specs

Various industries have different material specification methods. On many prints, raw materials that specify aluminum are not very specific and simply state use “Aluminum Alloy 6061-T6” with no other designations. For the most part, this generic raw material requirement is used because a part may be fabricated from plate, extruded, or rolled material. This gives flexibility to the job shop, and the engineer does not want to tie down the job shop due to an undefined manufacturing process method or material availability.
Once a process is defined, the engineer or quality group should lock down the specific type of aluminum that is best for the application. Company quality groups can identify the material from the prototype inspection sample warrant and material certification. Engineering can then update the print material designation for the production run.
You must understand the differences in the “T” temper designations to understand some of the thinking.

• T6 Temper – “Solution Heat-Treated and Artificially Aged”.
• To get to the -T6 temper, the 6061-O aluminum billet is heated to about 990˚ F, quenched in water, and aged at about 350˚ F for around 8 hours. That changes the typical yield strength from 8 ksi to about 35 ksi (ksi is a stress unit, equivalent to 1000 lbf/in²).
• Plate T6 is called T651 – “Solution Heat-Treated, Artificially Aged and Permanent Set”.
• Quenching in water puts residual stresses in the aluminum since there is a surface-to-center cooling gradient. The -T651 designation means the aluminum mill took that extrusion and gave it a 1% to 3% stretching, or permanent set, to eliminate some residual stresses. Now we can machine it, and it shouldn’t distort as much. Common specifications are ASTM-B209 and AMS-QQ-A-250/11 for plate.
• Extruded T6 is called T6511 – “Solution Heat-Treated, Artificially Aged, Permanent Set and Straighten”
• The final digit in the -T6511 designation (and how I know it was an extrusion since this only applies to extruded stock) means that the aluminum mill is allowed to straighten the extruded bars, like in a press, to get the material to meet the straightness tolerances. Common specifications are ASTM-B221 and AMS-QQ-A-200/8 for rounds.
• Cold Finished/Rolled T6 is also called T651 – “Solution Heat-Treated, Artificially Aged, Permanent Set”.
• Cold Finished, also called rolled material, is designated as -T651; the extra “one” designation is missing. This applies to plate and cold finished rounds. The extra “one” is missing because the process is similar to the plate. This material is commonly known as “screw machine stock.” This material has better manufacturing tolerances as compared to extruded. For example, a 1.000″ diameter round bar in a “close tolerance extruded rod” has a diametrical tolerance of 0.005’’, and the “cold finished” is much more consistent, with a tolerance of 0.002’’. Additionally, a comparison of mechanical properties is very similar – both have a 35 ksi yield strength. Still, extrusion stock is 38 ksi ultimate tensile strength, and cold finished is slightly more, at 42 ksi. Common cold finished/rolled specifications are ASTM-B211 and AMS-QQ-A-225/8 for rounds.

Chemically and physically, all tempers are the same, although the grain structure and additional stresses from straightening may be present (if straightened, and most bars are straightened) in the 6061-T6511.

Forging of Aluminum Alloy 6061

Aluminum alloy 6061 is forged at 233-483°C.

Hot working of Aluminum Alloy 6061

Aluminum Alloy 6061 is hot worked at 260-372°C (500-700°F).

Cold working of Aluminum Alloy 6061

Aluminum alloy 6061 is cold-worked in the O state. It can also be cold-formed in the T4 and T6 states.

Annealing of Aluminum Alloy 6061

Aluminum Alloy 6061 can be annealed at 775°F for 2-3h, followed by controlled cooling at 10-260°C (50-500°F)/h in air.

Aging of Aluminum Alloy 6061

Aluminum alloy 6061 may be aged at 177°C (350°F) for 8h. then cooled in the air.

Varieties, states, and typical uses of 6XXX series aluminum alloys

Aluminum/Aluminum alloy	Main varieties	State	Typical use
6005	Extruded tubes, shapes, rods, and wires	T1, T5	Extruded profiles and pipes are used for structural components requiring strength greater than 6063 alloy, such as ladders, television antennas, etc.
6009	Plates	T4, T6	Car body panels
6010	Plates	T4, T6	Car body panels
6061	Plates	O, T4, T6	Various industrial structural components with certain strength, high weldability, and corrosion resistance are required, such as pipes, rods, and profiles used in the manufacturing of trucks, tower buildings, ships, trams, railway vehicles, furniture, etc.
	Thick plate	O, T451, T651
	Stretching tube	O, T4, T6
		O, T1, T4, T4510, T4511,
	Extruded tubes, shapes, rods, and wires	T51, T6, T6510, T6511
	Catheter	T6
	Rolled or extruded structural profiles	T6
	Cold worked bars	O, H13, T4, T541, T6, T651
	Cold worked wire	O, H13, T4, T6, T89, T913, T94
	Rivet wire	T6
	Forging	F, T6, T652
6063	Stretching tube	O, T4, T6, T83, T831, T832	Architectural profiles, irrigation pipes, extruded materials for vehicles, racks, furniture, elevators, fences, etc., as well as decorative components of different colors used in aircraft, ships, light industry departments, buildings, etc.
	Extruded tubes, shapes, rods, and wires	O, T1, T4, T5, T52, T6
	Catheter	T6
6066	Stretching tube	O, T4, T42, T6, T62	Forgings and extruded materials for welded structures
	Extruded tubes, shapes, rods, and wires	O, T4, T4510, T4511, T42,
		T6, T6510, T6511, T62
	Forging	F, T6
6070	Extruded tubes, shapes, rods, and wires	O, T4, T4511, T6, T6511, T62	Heavy duty welded structures and extruded materials and pipes used in the automotive industry, bridges, cable towers, navigation components, machine parts conduits, etc.
	Forging	F, T6
6101	Extruded tubes, shapes, rods, and wires	T6, T61, T63, T64, T65, H111	High strength bars, high strength busbars, conductors, and heat dissipation devices for buses.
	Catheter	T6, T61, T63, T64, T65, H111
	Rolled or extruded structural profiles	T6, T61, T63, T64, T65, H111
6151	Forging	F, T6, T652	Used for forging crankshaft parts, machine parts, and production of rolling ring guide mines and machine parts, for use that requires both good malleability, high strength, and good corrosion resistance.
6201	Cold worked wire	T81	High strength conductive bars and wires
6205	Plates	T1, T5	Thick plates, pedals, and high impact extruded parts
	Extruded material	T1, T5
6262	Stretching tube	T2, T6, T62, T9	The corrosion resistance of threaded high stress mechanical parts with 2011 and 2017 alloys is required (with good cutting performance).
	Extruded tubes, shapes, rods, and wires	T6, T6510, T6511, T62
	Cold worked bars	T6, T651, T62, T9
	Cold worked wire	T6, T9
6351	Extruded tubes, shapes, rods, and wires	T1, T4, T5, T51, T54, T6	Extruded structural components of vehicles, transportation pipelines for water, oil, etc., and pressure controlled profiles.
6463	Extrusion rod, shape, wire	T1, T5, T6, T62	Architectural and various instrument profiles, as well as automotive decorative parts with bright surfaces after anodizing treatment.
6A02	Plates	O, T4, T6	Aircraft engine parts, complex shaped forgings and die forgings, require mechanical parts with high plasticity and high corrosion resistance.
	Thick plate	O, T4, T451, T6, T651
	Pipes, rods, profiles	O, T4, T4511, T6, T6511
	Forging	F, T6

State codes and representation methods for Chinese deformed aluminum alloys

According to the GB/T 16475-1996 standard, the basic state code is represented by a capital letter in English. The subdivision status code is represented by the basic status code followed by one, two, or more Arabic numerals.

Basic status code

Code	Name	Explanation and Application
F	Free processing status	Applicable to products without special requirements for work hardening and heat treatment conditions during the forming process, and the mechanical properties of products in this state are not specified.
O	Annealing state	Suitable for processed products that have undergone complete annealing to obtain the lowest strength.
H	Work hardening state	Suitable for products that increase strength through work hardening, the product can undergo (or may not undergo) additional heat treatment to reduce strength after work hardening. The H code must be followed by two or three Arabic numerals.
T	Heat treatment state (different from F, O, H state)	Suitable for products that have reached a stable state through (or without) work hardening after heat treatment. The T code must be followed by one or more Arabic numerals.

Subdivision status code

HXX status
The first digit after H represents the basic processing program that obtains this state.
H1 – Simple work hardening state
Suitable for states where the required strength can be obtained through work hardening without additional heating treatment.
H2 – state of work hardening and incomplete annealing
Suitable for products that have undergone incomplete annealing after the degree of work hardening exceeds the specified requirements of the finished product, resulting in a decrease in strength to the specified specifications.
H3 – State of work hardening and stabilization treatment
Suitable for products that have undergone low-temperature heat treatment after work hardening or whose mechanical properties have reached stability due to the heating effect during the processing.
H4 – Status of work hardening and painting treatment
Suitable for products that have undergone incomplete annealing due to painting treatment after work hardening.
The second digit after H represents the degree of work hardening of the product. Numbers from 1 to 9 represent different degrees of hardening, with 8 indicating the hard state and 9 indicating the superhard state.
HXXX status
H111 – Suitable for products that have undergone an appropriate amount of work hardening after final annealing, but the degree of work hardening is not as high as the H11 state.
H112 – Suitable for products formed by hot working, and the mechanical properties of products in this state have specified requirements.
H116 – Suitable for products made of 5XXX series alloys with magnesium content ≥ 4.0%. These products have specified mechanical properties and anti stripping corrosion performance requirements.
TX status
Add Arabic numerals from 0 to 10 after T to represent the sub state called TX state, as shown in the table below. The number after T represents the basic processing program for the product.

Status code	Explanation and Application
T0	After solution heat treatment and natural aging, it is suitable for products that have been cold worked to improve their strength.
T1	The product is cooled by the high-temperature forming process and then naturally aged to a basically stable state. It is suitable for products that are cooled by the high-temperature forming process and no longer undergo cold processing (can be straightened or flattened, but do not affect the mechanical performance limit).
T2	Cooled by the high-temperature forming process and naturally aged to a basically stable state after cold processing, it is suitable for products that are cooled by the high-temperature forming process and undergo cold processing, straightening, and leveling to improve strength.
T3	After solution heat treatment, it is subjected to cold processing and then naturally aged to a basically stable state. It is suitable for products that undergo cold processing or straightening or leveling to improve strength after solution heat treatment.
T4	After solution heat treatment, natural aging to a basically stable state is suitable for products that no longer undergo cold processing (can be straightened or flattened, but do not affect the mechanical property limit) after solution heat treatment.
T5	The product that is cooled by the high-temperature forming process and then subjected to artificial aging is suitable for products that are cooled by the high-temperature forming process and not subjected to cold processing (can be straightened or flattened, but does not affect the mechanical property limit), but are subjected to artificial aging.
T6	The state of artificial aging after solution heat treatment is applicable to products that are no longer subjected to cold processing (can be straightened or flattened, but do not affect the mechanical property limit) after solution heat treatment.
T7	The state of aging after solution heat treatment is suitable for products that, in order to obtain certain important characteristics, exceed the peak point of strength on the aging curve during artificial aging.
T8	The state of cold processing followed by artificial aging after solid solution heat treatment is suitable for products that have undergone cold processing, straightening, and leveling to improve strength.
T9	The state of artificial aging after solution heat treatment followed by cold working is suitable for products that have been cold worked to improve their strength.
T10	The state of cooling by the high-temperature forming process, followed by cold processing, and then artificial aging is suitable for products that have been straightened and leveled by cold processing to improve strength.

Note: Some 6XXX series alloys can achieve the same solution heat treatment effect, whether in the furnace solution heat treatment or rapid cooling from the high-temperature forming process to retain soluble components in the solid solution. These alloys’ T0, T3, T4, T6, T7, T8, and T9 states can use the above two methods.

TXX status, TXXX status

Add another Arabic numeral after the TX status code to refer to the TXX status, or add two Arabic numerals to refer to the TXXX status, indicating the status after a specific process treatment that significantly changes the product characteristics (such as mechanical properties, corrosion resistance, etc.), as shown in the table below.

Status code	Explanation and Application
T42	Suitable for products that have been naturally aged to a fully stable state after solid solution heat treatment in the O or F state, as well as products that have achieved mechanical properties in T42 state after heat treatment in any state of the purchaser.
T62	Suitable for products that undergo artificial aging after solid solution heat treatment in the O or F state, and also suitable for products that have mechanical properties reaching T62 state after heat treatment of processed products in any state by the purchaser.
T73	Suitable for products that undergo solution heat treatment and aging to achieve the specified mechanical properties and stress corrosion resistance indicators.
T74	Same as T73 state definition. The tensile strength of this state is greater than that of T73 state, but less than that of T76 state.
T76	Same as T73 state definition. The tensile strength of this state is higher than that of T73 and T74 states, and the resistance to stress corrosion fracture is lower than that of T73 and T74 states, respectively. However, its resistance to peeling corrosion is still good.
T7X2	Suitable for products that undergo artificial aging treatment after solid solution heat treatment in the O or F state, with mechanical properties and corrosion resistance reaching the T7X state.
T81	Suitable for products that undergo solution heat treatment, undergo about 1% cold working deformation to increase strength, and then undergo artificial aging.
T87	Suitable for products that undergo solid solution heat treatment, undergo about 7% cold working deformation to increase strength, and then undergo artificial aging.

Stress relieving state
Add “51”, “510”, “511”, “52”, and “54” after the TX, TXX, or T XXX status code mentioned above to indicate the product status code that has undergone stress relief treatment, as shown in the table below:

Status code	Explanation and Application
TX51, TXX51, TXXX51	Suitable for thick plates, rolled or cold finished bars, as well as die forgings, forged rings or rolled rings that are stretched according to the specified amount after solid solution heat treatment or self high-temperature forming process cooling. The permanent deformation of these products without straightening the thick plates after stretching is 1.5% -3%; The permanent deformation of rolled or cold finished bars is 1% -3%; The permanent deformation of die forgings, forged rings, or rolled rings is 1% -5%.
TX510, TXX510, TXXX510	Suitable for extruded rods, shapes, and pipes that are stretched according to the specified amount after solid solution heat treatment or self high-temperature forming process cooling, as well as drawn pipes. These products are no longer straightened after stretching, and the permanent deformation of extruded rods, shapes, and pipes is 1% -3%; The permanent deformation of drawn pipes is 1.5% -3%.
TX511, TXX511, TXXX511	Suitable for extruded rods, shapes, and pipes that are stretched according to the specified amount after solid solution heat treatment or self high-temperature forming process cooling, as well as drawn pipes. These products are slightly straightened after stretching to meet the standard tolerance of permanent deformation of extruded rods, shapes, and pipes by 1% -3%; The permanent deformation of drawn pipes is 1.5% -3%.
TX52, TXX52, TXXX52	Suitable for products with a permanent deformation of 1% -5% after solid solution heat treatment or high-temperature forming process cooling and stress relief through compression.
TX54, TXX54, TXXX54	Suitable for die forgings that eliminate stress through cold shaping within the final forging die.

Calculation method for aluminum alloy 6061 products

• Weight of aluminum alloy plate (kg)=0.00000275 × thick × wide × Length unit (mm);
• Weight of aluminum alloy tube (kg)=0.00000275 × (Outer diameter × Outer diameter – inner diameter × Inner diameter) x 3.14/4 × Length unit (mm);
• Weight of aluminum alloy rod (kg)=0.00000275 × three point one four × radius × radius × Length unit (mm).

6063 Aluminium Round Bar Weight Chart

Theorical Weights Table
The values presented on this table are theorical and they should be confirmed if you make an order.

Product	Diameter	Weight
6063 Aluminium Round Bar	6 mm	0,076 Kg/m
6063 Aluminium Round Bar	7 mm	0,104 Kg/m
6063 Aluminium Round Bar	8 mm	0,136 Kg/m
6063 Aluminium Round Bar	9 mm	0,172 Kg/m
6063 Aluminium Round Bar	9,5 mm	0,191 Kg/m
6063 Aluminium Round Bar	10 mm	0,212 Kg/m
6063 Aluminium Round Bar	11 mm	0,257 Kg/m
6063 Aluminium Round Bar	12 mm	0,305 Kg/m
6063 Aluminium Round Bar	12,5 mm	0,331 Kg/m
6063 Aluminium Round Bar	13 mm	0,358 Kg/m
6063 Aluminium Round Bar	14 mm	0,416 Kg/m
6063 Aluminium Round Bar	16 mm	0,543 Kg/m
6063 Aluminium Round Bar	16,5 mm	0,577 Kg/m
6063 Aluminium Round Bar	18 mm	0,687 Kg/m
6063 Aluminium Round Bar	19 mm	0,766 Kg/m
6063 Aluminium Round Bar	20 mm	0,848 Kg/m
6063 Aluminium Round Bar	22 mm	1,026 Kg/m
6063 Aluminium Round Bar	25 mm	1,325 Kg/m
6063 Aluminium Round Bar	28 mm	1,663 Kg/m
6063 Aluminium Round Bar	30 mm	1,909 Kg/m
6063 Aluminium Round Bar	31 mm	2,038 Kg/m
6063 Aluminium Round Bar	32 mm	2,171 Kg/m
6063 Aluminium Round Bar	35 mm	2,598 Kg/m
6063 Aluminium Round Bar	38 mm	3,062 Kg/m
6063 Aluminium Round Bar	40 mm	3,393 Kg/m
6063 Aluminium Round Bar	45 mm	4,294 Kg/m
6063 Aluminium Round Bar	50 mm	5,301 Kg/m
6063 Aluminium Round Bar	55 mm	6,415 Kg/m
6063 Aluminium Round Bar	60 mm	7,634 Kg/m
6063 Aluminium Round Bar	65 mm	8,959 Kg/m
6063 Aluminium Round Bar	70 mm	10,391 Kg/m
6063 Aluminium Round Bar	75 mm	11,928 Kg/m
6063 Aluminium Round Bar	80 mm	13,572 Kg/m
6063 Aluminium Round Bar	90 mm	17,177 Kg/m
6063 Aluminium Round Bar	100 mm	21,206 Kg/m
6063 Aluminium Round Bar	105 mm	23,379 Kg/m
6063 Aluminium Round Bar	110 mm	25,659 Kg/m
6063 Aluminium Round Bar	120 mm	30,536 Kg/m
6063 Aluminium Round Bar	127 mm	34,203 Kg/m
6063 Aluminium Round Bar	130 mm	35,838 Kg/m
6063 Aluminium Round Bar	140 mm	41,563 Kg/m
6063 Aluminium Round Bar	150 mm	47,713 Kg/m
6063 Aluminium Round Bar	180 mm	68,707 Kg/m
6063 Aluminium Round Bar	200 mm	84,823 Kg/m

6063 Aluminium Square Bar Weight Chart

Product	Side	Weight
Aluminium Square Bar	10 mm	0,270 Kg/m
Aluminium Square Bar	15 mm	0,608 Kg/m
Aluminium Square Bar	16 mm	0,691 Kg/m
Aluminium Square Bar	20 mm	1,080 Kg/m
Aluminium Square Bar	25 mm	1,688 Kg/m
Aluminium Square Bar	35 mm	3,308 Kg/m
Aluminium Square Bar	45 mm	5,468 Kg/m
Aluminium Square Bar	60 mm	9,720 Kg/m
Aluminium Square Bar	100 mm	27,000 Kg/m

6063 Aluminium Rectangular Bar Weight Chart

Comparison between 6061 aluminum alloy and other aluminum alloys

6061 aluminum alloy is a commonly used aluminum alloy for extrusion, but you may consider other options for your specific application. The following are some other common aluminum alloys that are commonly considered substitutes.
Comparison between 6061 and 7075
One of the key reasons for choosing 7075 aluminum alloy is its high strength. It forms an aluminum alloy with zinc, which is stronger than 6061. However, its corrosion resistance is lower than 6061, and welding is more difficult. 7075 is used in ship, automotive, and aerospace applications, where strength is crucial.
Comparison between 6061 and 6063
6061 and 6063 are two very popular extruded aluminum alloys. The 6000 series of aluminum alloys use magnesium and silicon as the main aluminum alloy elements. Therefore, they have many similar characteristics. 6061 has higher strength. Therefore, it is typically used for more structured applications. On the other hand, 6063 is used in lower-strength applications, such as railings or decorations, windows, and doors.
Comparison between 6061 and 5052
The main aluminum alloy elements of 6061 aluminum are magnesium and silicon, while the main aluminum alloy elements of 5052 aluminum are magnesium. One of the main advantages of 5052 aluminum alloy is its high weldability compared to other aluminum alloys. For projects with weldability as the key, it is worth considering. However, one drawback of 5052 is that it cannot undergo heat treatment, as it is suitable for various welding applications and performs well in maritime applications due to its high corrosion resistance.
Comparison between 6061 and 2024
2024 aluminum alloy is mainly used in aerospace applications and is known for its high strength. Its main aluminum alloy element is copper. Although sturdy and has good fatigue resistance, its machinability and weldability are worse than 6061. It also has poor corrosion resistance and is commonly used in wing and fuselage structures that withstand high tension.
Comparison between 6060 and 6061
The silicon content of 6060 aluminum alloy is lower than 6061, and the content of copper and magnesium is also relatively low. This makes 6060 aluminum alloy have high plasticity and good corrosion resistance, making it suitable for making products with complex shapes.
These two aluminum alloys have differences in chemical composition, physical properties, and applications.
Comparison between 6082 and 6061
6082 aluminum alloy is also a high-strength and corrosion-resistant aluminum alloy, which has better weldability and machinability than 6061 aluminum alloy and is suitable for making complex-shaped mechanical parts.
Of course, the final choice also needs to consider the specific usage environment and requirements of mechanical parts, as well as factors such as manufacturing process and cost. You should consult a professional aluminum alloy material supplier or mechanical processing manufacturer to choose the most suitable aluminum alloy material.

Influence of different heat treatment processes on plasticity and hardness of 6061 aluminum alloy

Taking 6061-T6 aluminum alloy plate as the research object, the effects of different heat treatment process parameters such as heat treatment temperature, holding time, and cooling method on the plastic properties and hardness of 6061 aluminum alloy were analyzed by one-way tensile test, Vickers microhardness test and metallographic test.

0. Introduction

Aluminum alloy has many excellent characteristics, such as high specific strength, strong corrosion resistance, excellent welding performance, good fatigue resistance, and recyclability, which is widely used in rail transportation, automobile, ship, aerospace, and other industrial manufacturing fields. However, due to the non-special treatment of aluminum alloy materials at room temperature environments, there are poor plasticity, low hardness, and other problems, restricting its application in the industry. Currently, aluminum alloys are mainly treated by heat treatment processes such as solid solution, aging, and homogenized annealing to improve their comprehensive mechanical properties to meet industrial demands. Liu Qulin and other research results show that the spray forming 6061 aluminum alloy solid solution aging, you can get the distribution density, the number of spherical GP area and a small number of needle-like β″ phase, and 530 ℃ solid solution 1 h + 175 ℃ aging 8h when the alloy to obtain the comprehensive mechanical properties of higher. The results of Feng Yincheng et al. show that after direct artificial aging (180 ℃,6h) of 6061 aluminum alloy, a large number of slaty β-Mg₂Si precipitation phases and GP zones are diffusely distributed in the matrix, and at this time, the strength of 6061 aluminum alloy is higher and the elongation is lower. The results of Liu Xinguang et al. showed that the main precipitation phases of as-cast 6061 aluminum alloy consisted of Mg₂Si, β-Al₅FeSi, β-Al₅(FeMn)Si, and a small amount of α-Al₁₂-(FeMn)₃Si₂. Xu Kaisheng of Jilin University studied the effect of the uniform annealing process on the microstructure of 6061 aluminum alloy, and the results showed that the cooling mode of uniformized annealing affected the microstructure of 6061 aluminum alloy mainly in the amount of precipitation of Mg₂Si and the precipitation size. Jin et al. found that with the increase of solid solution temperature from 535 ℃ to 555 ℃, the hardening of the solid solution improved the hardness of casting aluminum-magnesium-silicon alloy and the tensile strength; when the solid solution temperature increased from 535 to 555 ℃, the hardness of casting aluminum-magnesium-silicon alloy increased. Jin et al. found that as the solid solution temperature increased from 535 °C to 555 °C, solid solution hardening improved the hardness and tensile strength of cast Al-Mg-Si alloys; when the solid solution temperature increased to 565 °C and the solid solution time was prolonged, the hardness and tensile strength of the alloys decreased due to the softening of the grains due to grain growth. Zhang et al. investigated the effect of the test temperature on the tensile properties of cast aluminum alloys Al-10Si-1.2Cu-0.7Mn. They found that different tensile properties of the alloys were mainly attributable to the different matrix strengths, phase particle strengths, and damage rates. Xu et al. heat-treated Al-Zn-Mg-Cu extruded aluminum alloys using a multistage solid solution aging process, and the experimental results showed that the multistage solid solution aging process can increase the dislocation density and improve the intergranular corrosion resistance of the alloy, and in the solid solution treatment system (460 ℃×2 h + 470 ℃×2 h + 480 ℃×2 h) and aging system (121 ℃×5 h + 153 ℃×16h) heat treatment state to obtain alloys with excellent comprehensive performance. Liu Lei et al. investigated the effect of aging temperature and time on the organization and properties of the new Al-6Zn-1.1Mg alloy, and the results showed that both the GP zone and the η′ phase can hinder the dislocation motion and play a good strengthening effect on the alloy.Duan et al. focused on the mechanical properties and precipitation characteristics of Al-Mg-Si alloys concerning the second-stage treatment temperature in the two-stage pre-age treatment (T4P2) process, and the results showed that the second-stage treatment temperature can prevent the dislocation movement and play a good strengthening effect on the alloy. The results showed that the bake-hardening properties of Al-Mg-Si alloys peaked at 180 °C for the secondary pre-ageing temperature. Li Shenlan and other research results show that 6061 aluminum alloy hot extrusion plate in the solid solution treatment process occurs in the dissolution of the precipitation phase particles are mainly Mg₂Si, and the residual coarse precipitation phase is mainly Fe-rich compounds; when the solid solution temperature of 565 ℃, holding temperature of 40min, aluminum alloy can obtain good strength, hardness, and plastic properties. Li and other research on solid solution treatment of the iso-channel angle of the extruded Li et al. investigated the effect of solid solution treatment on Al-Zn-Mg-Cu alloy’s organization and mechanical properties after equal-channel angle extrusion. The results showed that the solid solution treatment induced the precipitation of skeleton-like and acicular second phases at the grain boundaries and significantly increased the proportion of large-angle grain boundaries and recrystallization, improving the alloy’s plasticity. The results of the study by Froeck et al. indicated that the solid solution temperature was too high or too low, and the holding time was too long or too short, which would result in the incomplete solid solution of the second-phase particles, thus increasing the EN-Al alloy strength, hardness, and plasticity. completely, thus increasing the quenching sensitivity of EN-AW-6082 aluminum alloy. Li Caiwen et al. studied the effect of three quenching cooling methods (air cooling, air cooling, water cooling) on the organization and properties of 6061 aluminum alloy profiles, and the results show that with the increase of quenching cooling rate, the strength of the profiles is significantly increased, while its elongation slightly decreased. Xu et al.’s study shows that 6061 aluminum alloy precipitation grains in the peak aging condition are needle-like β″ phase; the higher the quenching temperature is, the more quenching vacancies are present, and more solid solution atoms participate in the aging process. The higher the quenching temperature, the more quenching vacancies, the more solid solution atoms are involved in the aging process, resulting in finer size and increased number of precipitated phases, which makes the hardness of the aluminum alloy increase significantly; when the quenching temperature exceeds 550 ℃, the quenching temperature does not have a significant effect on the enhancement of the alloy’s hardness and reinforcement of the β″ precipitation rate of phase β. Kemsies et al. investigated the effect of the homogenization treatment temperature and holding time on the hardness of forged Al-Mn-Fe-Si (-Mg) aluminum alloys. The results showed that cast aluminum alloys containing β″ phase are in the peak aging condition. Kemsies et al. investigated the effect of temperature and holding time on the hardness of forged Al-Mn-Fe-Si(-Mg) aluminum alloys, and the results showed that the as-cast Mg-containing alloys were able to obtain a large number of diffusely reinforced phases after homogenization treatment (400 ℃, 3 – 10h), which significantly improved the hardness of the alloy. Li Yan and other research results show that the semi-continuous casting 6061 aluminum alloy crystal and grain boundaries of the second phase mainly include a very small number of fine Mg₂Si phase, pure Si phase, and AlFeSi phase by homogenization treatment (550 ℃, 12h), aluminum alloy organization in the diffuse distribution of a large number of the fine second phase, the alloy at this time the hardness of the alloy to reach its maximum; continue to extend the holding time and improve the uniformity, the grain coarsening in the aluminum alloy, resulting in a decrease in its hardness. Utsunomiya et al. studied the precipitation behavior of die-quenched 6061 aluminum alloy billet in the process of artificial aging, and the results showed that after peak aging, the needle-like or granular precipitation phases with sizes less than 30 nm were uniformly distributed in the water-cooled and die quenched aluminum alloy tissues (Mg₂Si).Ma et al. carried out solution treatment of AlN_P/Al-0.4Cu Ma et al. carried out solid solution treatment (530 ℃/1h) on AlN_P/Al-0.4Cu composites. The results showed that compared with water cooling, cooling in liquid nitrogen produces a high proportion of high-angle grain boundaries, high dislocation density, and low point defect concentration in the alloys, which contribute to the stability of the microstructures. Cavazos et al. investigated the effect of aging on the susceptibility of the Aluminum alloy 6063 to annealing, and the results showed that the quenching susceptibility of the alloy is caused by the initial precipitation that occurs during the cooling process. The results show that the quenching sensitivity of aluminum alloy 6063 is caused by the initial precipitation that occurs during the cooling process, and the peak aging hardness is related to the cooling rate. Fan et al. investigated the effect of cooling rate and aging conditions on the microstructure of AA6016 aluminum alloy plate, and the results show that in the state of natural aging and artificial aging, the comprehensive performance of AA6016 aluminum alloy with a faster cooling rate is better than that of the slow-cooling mode, which is mainly due to the slow-cooling method with a slower cooling rate, the formation of a crystal within the crystal, the cooling rate is slower, and the cooling rate is lower. This is mainly due to the slower cooling rate in the slow cooling mode and a large amount of coarse Si and Mg₂Si precipitated along the grain boundaries in the crystal. The results of Liu Haiquan et al. show that different cooling methods affect the final organization of 2524 aluminum alloy, with fine diffuse phases precipitating in the crystal when the cooling rate is faster and coarse, striated S phases precipitating at the grain boundaries and in the crystal when the cooling rate is slower.
In summary, 6061 aluminum alloy has a high sensitivity to heat treatment, and a reasonable heat treatment process can improve the comprehensive mechanical properties of the alloy. Still, the research on the heat treatment process for 6061-T6 state aluminum alloy is rarely reported. Considering the poor plastic properties of 6061-T6 aluminum alloy at room temperature, which is not conducive to plastic forming at room temperature, to study the comprehensive effects of different heat treatment process parameters (heating temperature, holding time, and cooling mode) on the plastic properties and hardness of 6061 aluminum alloy, the test was conducted with commercial 6061-T6 aluminum alloy plate as the research object, with the help of metallographic test, one-way tensile test and Vickers microhardness test. With the help of a metallographic test, one-way tensile test, and Vickers microhardness test, three kinds of technical means are used to study the influence of different heat treatment process parameters on the plastic properties and hardness of 6061 alloy.

1. Experiment

Test raw materials for commercial 6061-T6 aluminum alloy cold rolled plate, the main components and content of the alloy in Table 1. according to GB/T 228.1-2010 standard, along the 6061-T6 aluminum alloy plate rolling direction to intercept the tensile specimen.
Table.1 6061-T6 aluminum alloy plate main components and content (mass fraction, %)

Si	Mg	Fe	Cu	Mn	Cr	Zn	Ti	Al
1.3	0.26	0.5	0.5	0.2	0.1	0.2	0.15	Bal.

By the heat treatment process plan formulated in Table 2, the first SX2-4-10 box-type resistance furnace heating temperature was set to 410 ℃, 440 ℃, 470 ℃, 500 ℃, 530 ℃, 560 ℃, and 590 ℃, to reach the preset temperature, respectively, into the intercepted 6061-T6 aluminum alloy specimens, to be stable temperature readings in the furnace began to time, holding time and cooling mode as Table 2 shows.

Table.2 6061-T6 aluminum alloy heat treatment process

Test group number	Heating temperature	Holding time	Cooling number
1	410 ℃, 440℃, 470℃, 500 ℃, 530 ℃, 560 ℃, 590 ℃	2 h	AC
2	The optimal heating temperature in test one	1 h, 3 h, 4 h, 5 h	AC
3	The optimal heating temperature in test one	The optimal holding time in test two	AC, WQ, FC

For the 6061 aluminum alloy specimens treated with different heating temperatures, the CMT5105 electronic universal testing machine and 4P2MVA Vickers hardness tester were used to measure the strength, elongation hardness, and other performance parameters. The microstructure of 6061 aluminum alloy under different heat treatment processes was observed using an Axiovert 200 MAT Zeiss metallurgical microscope. The microstructure was obtained by chemical corrosion with Keller’s reagent (1 mL HF + 1.5 mL HCl + 2.5 mL HNO₃ + 95 mL H₂O).

2. Results and analysis

2.1 Effect of heat treatment temperature on plastic properties and hardness of 6061 aluminum alloy

2.1.1 Real stress-strain curves of 6061 aluminum alloy plates at different heat treatment temperatures
The real stress-strain curve of a 6061 aluminum alloy plate heat-treated with different heating temperatures under the same holding time and cooling conditions (2h, air-cooled) is obtained through the unidirectional tensile test, as shown in Figure 1. As can be seen from Fig. 1, the yield strength of the 6061-T6 aluminum alloy plate, which is no longer subjected to secondary heat treatment, is 250MPa, and the tensile strength is 350MPa, which is higher than the yield strength and tensile strength of 6061 aluminum alloy obtained after heat treatment again. Comparing the curves in Fig. 1a and b, it can be seen that when the heat treatment heating temperature is 410 ℃, the yield strength of 6061 aluminum alloy after re-heat treatment is reduced compared with the yield strength and tensile strength of the original 6061-T6 plate. Comparing with the curves in Fig. 1b-f, it can be seen that the yield strength of 6061 aluminum alloy decreases first and then increases when the heat treatment heating temperature increases from 410 ℃ to 530 ℃ and reaches the minimum value (14.19MPa) at 500 ℃, while the tensile strength increases gradually. A comparison of the curves in Figure 1f-h shows that when the heat treatment heating temperature increases from 530 ℃ to 590 ℃, the yield strength and tensile strength of the 6061 aluminum alloy plate show an increasing trend. With the heat treatment temperature increased from 410 ℃ to 590 ℃, the yield strength of 6061 aluminum alloy first decreased and then increased, while the tensile strength gradually increased.

Figure.1 Real stress-strain relationship curve of 6061 aluminum alloy under different heat treatment heating temperature
2.1.2 Influence of different heat treatment temperatures on plastic properties of 6061 aluminum alloy
Based on the obtained true stress-strain, the effect of different heat treatment temperatures on the uniform elongation of the 6061 aluminum alloy plate under the same holding time and cooling mode is shown in Figure 2. As shown in Fig. 2, the uniform elongation of the 6061-T6 aluminum alloy sheet without secondary heat treatment is 10%. According to the heat treatment parameters shown in Table 2, the uniform elongation of 6061 aluminum alloy after re-heat treatment increases under the conditions of the same holding time and cooling mode, and the heat treatment temperature increases from 410 ℃ to 590 ℃, the uniform elongation of 6061 aluminum alloy shows a fluctuating increase. When the heat treatment degree is 560 ℃, and the strain rate is 0.0001s^-1, the uniform elongation of the 6061 plate reaches the maximum value of 22.92%.
2.1.3 The effect of different heat treatment temperatures on the hardness of 6061 aluminum alloy
To further investigate the influence of different heat treatment temperatures on the comprehensive mechanical properties of 6061 aluminum alloy plate, the Vickers hardness measuring instrument was used to obtain the curves of the influence of different heat treatment temperatures on the hardness of 6061 aluminum alloy under the conditions of the same holding time and cooling mode, as shown in Figure 3. As shown in Fig. 3, the initial hardness of the 6061-T6 aluminum alloy sheet without secondary heat treatment is 132.5 HV. The hardness value of 6061 aluminum alloy decreases from 90.3 HV to 57.5 HV when the heat treatment temperature increases from 410 ℃ to 500 ℃ under the same heat preservation time and cooling mode. The hardness value of 6061 aluminum alloy decreases from 90.3 HV to 57.5 HV when the heat treatment temperature exceeds 500 ℃. When the heat treatment temperature is higher than 500 ℃, the hardness of the 6061 aluminum alloy plate shows an obvious increasing trend with the increase of heat treatment temperature, and the higher the heating temperature, the more obvious the phenomenon of increasing hardness. When the heating temperature is 590 ℃, the hardness of the aluminum alloy reaches 93.7 HV. It can be seen from the above study that the hardness of 6061 aluminum alloy is greatly affected by the heating temperature of the heat treatment, and the hardness of the alloy shows a trend of decreasing and then increasing with the increase of the heating temperature. However, the hardness values of the 6061 aluminum alloys obtained after re-heat treatment are all lower than the hardness of the initial 6061-T6 aluminum alloy sheet.

Fig.2 Uniform elongation of 6061 aluminum alloy plates under different heat treatment temperature conditions
Through the above study, it is found that under the condition of the same holding time and cooling mode, the heating temperature during heat treatment has a great influence on 6061 aluminum alloy; with the increase of heat treatment heating temperature, the yield strength of aluminum alloy decreases and then increases, the tensile strength gradually increases with the increase of heating temperature, the hardness shows a decrease. Then it rises, and the uniform elongation shows a fluctuating increase. Therefore, it can be seen that when 6061-T6 aluminum alloy is heat-treated again, a reasonable heat treatment heating temperature can significantly improve its plasticity performance index.

Figure.3 Microhardness of 6061 aluminum alloy under different heat treatment temperatures

2.2 The effect of holding time on the plastic properties and hardness of 6061 aluminum alloy plate

2.2.1 Real stress-strain curve of 6061 aluminum alloy plate under different holding time
Based on the research results of 6061 aluminum alloy under different heat treatment temperatures, to study the influence of heat treatment holding time on the comprehensive mechanical properties of 6061 aluminum alloy sheet, the basic performance parameters of 6061 aluminum alloy obtained under different holding time conditions are studied with the help of one-way tensile test and Vickers hardness test respectively.
Through the unidirectional tensile test to obtain in the heat treatment temperature of 560 ℃, air-cooled conditions, after different holding times after the 6061 aluminum alloy of the real stress-strain curve shown in Figure 4. As shown in Figure 4, when the holding time is extended from 1h to 2h, the yield strength of 6061 aluminum alloy gradually decreases while the tensile strength gradually increases. When the holding time is extended from 2h to 3h, the yield strength of 6061 aluminum alloy does not change significantly, while the tensile strength gradually decreases. When the heat preservation time is extended from 3h to 4h, the yield strength and tensile strength of 6061 aluminum alloy gradually increase, but the increase is small. Continue to extend the holding time to 5h. 6061 aluminum alloy yield strength change is not obvious, while the tensile strength gradually decreased. It can be seen that, with the extension of the holding time from 1h to 5h, the yield strength of 6061 aluminum alloy first decreases and then increases. At the same time, the tensile strength first increases and then decreases, and in the holding time of more than 3h, the strength of the heat-treated aluminum alloy plate after the change trend becomes slower.

Figure.4 Real stress-strain relationship curve of 6061 aluminum alloy under different holding times
2.2.2 The effect of different holding times on the plastic properties of 6061 aluminum alloy plate
Based on the influence of heat treatment holding time on the strength index of 6061 aluminum alloy, the influence curve of different holding time on the uniform elongation of 6061 aluminum alloy is obtained under the same conditions of heat treatment heating temperature and cooling mode (560 ℃, air-cooled), as shown in Fig. 5. As can be seen from Figure 5, under the same heat treatment heating temperature and cooling mode conditions, the effect of holding time on the plastic properties of 6061 aluminum alloy plate is more significant. Under the condition of a strain rate of 0.0001s^-1, the uniform elongation of aluminum alloy increases gradually when the holding time is extended from 1h to 2h; when the holding time is extended from 2h to 5h, the uniform elongation of aluminum alloy decreases first and then increases; when the holding time is 2h, the uniform elongation of aluminum alloy reaches the maximum value (22.92%). At a strain rate of 0.0005s^-1, the uniform elongation of the aluminum alloy shows a bimodal change when the holding time is extended from 1h to 5h. The uniform elongation of the aluminum alloy reaches the maximum value of 25% when the holding time is 4h. Under the condition of a strain rate of 0.001s^-1, the uniform elongation of the aluminum alloy decreases and then increases when the holding time is extended from 1h to 4h; the uniform elongation of the aluminum alloy decreases when the holding time is continued to be extended to 5h; the uniform elongation of aluminum alloy reaches the maximum value of 23.33% when the holding time is 4h. In summary, with the prolongation of the holding time, the uniform elongation of 6061 aluminum alloy is generally on the rise, and the plastic properties of 6061 aluminum alloy obtained under the heating temperature of 560 ℃, holding time of 4h, and air-cooling mode are the best.

Figure.5 Uniform elongation of 6061 aluminum alloy under different holding times
2.2.3 The effect of different holding times on the hardness of 6061 aluminum alloy plate
Under the condition of the same heat treatment temperature and cooling mode (560 ℃, air cooling), the hardness change curve of 6061 aluminum alloy with different heat treatment holding time is shown in Fig.6. As can be seen from Figure 6, when the holding time is extended from 1h to 4h, the hardness of 6061 aluminum alloy decreases from 85.7HV to 73.9HV. Continue to extend the holding time to 5h, and the hardness value of the aluminum alloy rises from 73.9HV to 81.4HV. With the prolongation of the holding time, the hardness of the 6061 aluminum alloy plate shows a trend of change of the first decline and then rise. When the holding time is 1h, the microhardness value of 6061 aluminum alloy is the largest, 85.7HV; when the holding time is 4h, the microhardness value is the smallest, 73.9HV.

Figure.6 Hardness variation curve of 6061 aluminum alloy under different holding times
Through the above study, it is found that under the condition of the same heat treatment heating temperature and cooling mode, the influence of different holding times on the comprehensive mechanical properties of 6061-T6 aluminum alloy after heat treatment again is relatively small, with the prolongation of heat treatment holding time, the yield strength of aluminum alloy firstly decreases and then increases. In contrast, the tensile strength first increases and then decreases, the hardness shows firstly decreases and then rises, and the uniform elongation rate generally shows an upward trend. Therefore, it can be seen that when 6061-T6 aluminum alloy is heat-treated again, reasonable heat treatment holding time can effectively improve its plastic performance index. Compared with the influence of heat treatment holding time on the comprehensive mechanical properties of aluminum alloy, the heating temperature of heat treatment has a greater influence.

2.3 Influence of cooling mode on the plastic properties and hardness of 6061 aluminum alloy plate

2.3.1 Real stress-strain curve of 6061 aluminum alloy plate under different cooling methods
In the heat treatment process, the heating temperature, holding time, and cooling method are three essential links, and the parameters of each link will impact the properties and organization of the aluminum alloy finally obtained. Therefore, after determining the influence of different heat treatment temperatures and holding times on the comprehensive mechanical properties of 6061 aluminum alloy, we continue to use the one-way tensile test and Vickers hardness test to study the basic mechanical property parameters of 6061 aluminum alloy under the conditions of different cooling methods.
Under the condition of the same heat treatment temperature and holding time (560 ℃, 4h), the real stress-strain curves of 6061 aluminum alloy under different cooling modes (furnace cooling, water cooling, air cooling) were obtained by unidirectional tensile test as shown in Fig. 7. Comparing the curves in Fig. 7a and c, it can be seen that the yield strength and tensile strength of 6061 aluminum alloy plate under the furnace cooling method are reduced. The curves in Fig. 7b and c show that the yield strength and tensile strength of the 6061-aluminum alloy plate under water-cooling mode are improved.
2.3.2 Influence of different cooling methods on the plastic properties of 6061 aluminum alloy plate
Based on the influence of heat treatment cooling mode on the strength index of 6061 aluminum alloy, the influence of cooling mode on the uniform elongation of 6061 aluminum alloy is obtained under the same conditions of heat treatment heating temperature and holding time (560 ℃, 4h), as shown in Figure 8. As shown in Fig. 8, when the strain rate is 0.0001s^-1, the uniform elongation of 6061 aluminum alloy plates under air cooling, water cooling, and furnace cooling are 21.25%, 22.50%, and 23.75%, respectively. When the strain rate is 0.0005s^-1, the uniform elongation of 6061 aluminum alloy plates in air-cooled, water-cooled, and furnace-cooled modes are 25%, 23.33%, and 25.83%, respectively. The uniform elongation of 6061 aluminum alloy plates under all three cooling methods is 23.33% when the strain rate is 0.001s^-1. From the elongation point of view, 6061 aluminum alloy has the best plasticity under the furnace cooling method, air cooling is the second best, and the plasticity under the water-cooling method is the worst, but the difference is small. In addition, the appropriate increase in strain rate is conducive to improving the elongation of the material, but the two are not monotonically increasing relationships.

Figure.7 The real stress-strain relationship curve of 6061 aluminum alloy under different cooling methods: (a) furnace cooling; (b) water cooling; (c) air-cooling

Fig.8 Uniform elongation of 6061 aluminum alloy under different cooling methods
2.3.3 The effect of different cooling methods on the hardness of 6061 aluminum alloy plate
Based on obtaining the influence of different cooling methods on the strength and uniform elongation of 6061 aluminum alloy, the hardness change curve of 6061 aluminum alloy under different heat treatment cooling methods is obtained through hardness measurement, as shown in Figure 9. As can be seen from Fig. 9, the hardness of 6061 aluminum alloy is 73.9 HV under air cooling mode, the hardness of 6061 aluminum alloy is 86.7 HV under water cooling mode, and the hardness of 6061 aluminum alloy is 69.7 HV under furnace cooling mode, in which the difference between the maximum hardness value and the minimum hardness value is 7 HV. There is a correlation between the hardness of 6061 aluminum alloy and the cooling speed. The selection of a suitable cooling method greatly influences the hardness of 6061 aluminum alloy.

Figure.9 Microhardness of 6061 aluminum alloy under different cooling methods
To sum up, under the same heat treatment heating temperature and holding time, different cooling methods (air cooling, water cooling, and furnace cooling) have a greater influence on the 6061-T6 aluminum alloy after re-heat treatment. Furnace cooling mode 6061 aluminum alloy plate yield strength, tensile strength, and hardness value is the lowest, while the plastic properties are stronger; water-cooled mode 6061 aluminum alloy plate yield strength, tensile strength, and hardness value is the highest, the plastic properties of the lowest; air-cooled mode 6061 aluminum alloy plate mechanical properties between the two. Therefore, it can be seen that when 6061-T6 aluminum alloy is heat-treated again, a reasonable heat-treatment cooling method can effectively improve its plastic performance index. Compared with the influence of heating temperature and holding time on the comprehensive performance of 6061 aluminum alloy during heat treatment, the heating temperature of heat treatment plays a major role, the cooling method plays a secondary role, and the holding time plays a minimal role.

2.4 Metallographic analysis

To further reveal the influence mechanism of heating temperature, holding time, and cooling mode on the mechanical properties of 6061 aluminum alloy from a microscopic point of view, the microstructure of 6061 aluminum alloy is analyzed with the help of a Zeiss microscope under different heat treatment parameters.
Under the same holding time and cooling mode, the microstructure of 6061 aluminum alloy under different heat treatment heating temperatures was obtained, as shown in Figure 10. As shown in Fig. 10, the microstructure of 6061 aluminum alloy has obvious differences with the increase in heating temperature. From Fig. 10a, it can be seen that the 6061-T6 state aluminum alloy has an obvious recrystallization process. The fibrous tissue produced by plate rolling deformation is eliminated, forming isometric crystals with a large difference in size, and a large number of fine second phases exist inside the grains, with a high degree of dispersion. This dispersed distribution of the second phase will increase the resistance to dislocation motion of the grains, increasing the strength, hardness, and plasticity of the 6061 aluminum alloy. The strength and hardness of 6061 aluminum alloy will increase, and the plastic properties will deteriorate. The secondary heating of 6061-T6 aluminum alloy, compared with Figure 10b-e, can be seen after the secondary heating of 6061 aluminum alloy grain recrystallization, grain boundary dissolution, and the emergence of dendritic organization. As the heating temperature increases, the driving force of recrystallization of 6061 aluminum alloy gradually increases, the degree of connectivity of the grain boundary increases, the precipitated phase gradually dissolves, the hindering ability of dislocations decreases, and the diffusion strengthening gradually disappears. When the heating temperature is 500 ℃, the recrystallization process of aluminum alloy has been completed, and the recrystallized grains grow sufficiently and evenly distributed. Therefore, when heating 6061-T6 aluminum alloy, when the heating temperature rises from 410 ℃ to 500 ℃, the plastic properties of the aluminum alloy gradually increase while the hardness gradually decreases. When the heating temperature continues to rise, compared with Fig. 10f, g can be seen when the heating temperature of 530 ℃, the aluminum alloy in the grain, through the merger of grain boundaries, gradually grows, the second phase in the crystal and began to re-precipitate, the aluminum alloy’s plastic properties have been reduced, and the hardness gradually increased. Comparing the metallographic organization shown in Fig. 10g and h, it can be seen that when the heating temperature is 560 ℃, the grains in the aluminum alloy gradually grow up to form a mesh by merging, the grain boundaries are clear, the degree of connectivity is high. The number of precipitated phases in the grain gradually increases. At this time, the plastic properties of the aluminum alloy are significantly improved, and the hardness and strength are also increased. When the heating temperature rises to 590 ℃, a few re-melting balls appear in the aluminum alloy crystal; the grains are polygonal, and the grain boundaries are coarsened and zigzagged. Due to the re-precipitation of precipitated phases and re-melting balls, the strength and hardness of the aluminum alloy increase, and the plastic properties decrease when the heating temperature is increased from 410 ℃ to 590 ℃, the plasticity of heat-treated 6061 aluminum alloy fluctuating increase, while the hardness increases gradually with the increase of heating temperature.

Figure.10 Microstructure of 6061 aluminum alloy at different heating temperatures
Under the condition of the same heat treatment heating temperature and cooling mode, the microstructure of 6061 aluminum alloy under different heat treatment holding time conditions was obtained, as shown in Fig.11. As can be seen from Figure 11, under the same heating temperature and cooling mode (560 ℃, air cooling) conditions, with the holding time from 1h to 5h, the organization of 6061 aluminum alloy morphology changes, reticulation dendrites through the merger of gradually become larger, the grain boundary contour is clear as an irregular polygonal, the size of the precipitation phase gradually become larger. When the holding time is 1h (Figure 11a), the grain morphology of aluminum alloy is mostly characterized by irregular grain boundaries, large blocky grains, a high degree of connectivity of grain boundaries, and small-sized grains in the intergranular lap. Due to the gradual dissolution of solute atoms into the matrix, the α(Al) matrix gradually becomes supersaturated, resulting in precipitation hardening, increasing the strength and hardness of the aluminum alloy. Comparison of the metallographic organization in Figure 11a, b can be seen: the insulation time of 2h, 6061 aluminum alloy in the grain boundary connectivity is high, grain size is not the same, the number of precipitated phases increased, the size of the larger, at this time, 6061 aluminum alloy of plastic properties increased, the hardness gradually decreased. Comparison of Figure 11b, c in the metallographic organization can be seen, insulation time of 3h, 6061 aluminum alloy organization of small angle grain boundaries enhancement, part of the small angle grain boundaries appeared in the internal large grains, the size of the grain size distribution is not uniform, the grain boundaries are clear, the degree of connectivity is high. Comparison of Figure 11c, d can be seen, insulation time of 4h, 6061 aluminum alloy organization precipitation phases dissolved a lot, making the pinning effect on the crystallization reduced, the grain grows rapidly, distribution is uniform, at this time the plastic properties of aluminum alloy have been improved, while the hardness is gradually reduced. Holding time for 5h (Figure 11e), aluminum alloy organization in the small angle grain boundaries increased, clear grain boundaries, crystal and grain boundaries in the precipitation of reinforced phases, this time the plasticity of the aluminum alloy has been reduced, and the gradual increase in hardness. When the holding time is extended from 1h to 5h, the plastic properties of 6061 aluminum alloy increase and decrease, and the hardness decreases and then increases.

Figure.11 Microstructure of 6061 aluminum alloy under different holding times
Under the same heat treatment temperature and holding time, the microstructure of 6061 aluminum alloy was obtained under different heat treatment cooling methods, as shown in Figure 12. As can be seen from Figure 12, the use of different cooling methods (air cooling, water cooling, furnace cooling) on the 6061-aluminum alloy organization in the precipitation phase number, size, and distribution of certain differences. Figure 12a for the air-cooling mode under the organization of 6061 aluminum alloy morphology, aluminum alloy in the grain size is larger, the grain boundary broadening clear, the grain observed a small amount of granular precipitation phase precipitation along the grain surface. Due to the relatively slow cooling rate in air cooling, the reinforced phase in the grain of 6061 aluminum alloy is easy to precipitate and grow. Still, the precipitation of the reinforced phase needs to be increased, which reduces the strength and hardness of the 6061 aluminum alloy plate, and the plasticity performance is slightly reduced. Figure 12b for the microstructure of 6061 aluminum alloy in water-cooled mode, aluminum alloy organization in the grain size is relatively small; there are a large number of fine granular precipitation phases in the crystal, distribution is uniform, the grain boundary broadening is irregular polygonal. Due to the large cooling rate in water cooling, the aluminum alloy plate maintains its organizational state at high temperatures. At this time, the strength and hardness of the aluminum alloy are the highest, and the plastic properties are reduced. Figure 12c for the furnace cooling mode under the 6061 aluminum alloy organization morphology, aluminum alloy organization in the grain boundary coarsening, in the grain boundary observed in a large number of coarse strip precipitation phase, the size of the coarser. Due to the very slow cooling rate during furnace cooling, the supersaturated solute atoms have enough time to absorb enough energy to grow up and precipitate during the cooling process, which leads to lower strength and hardness values and stronger plastic properties of the furnace-cooled aluminum alloy.

Figure.12 Microstructure of 6061 aluminum alloy under different cooling modes: (a) air-cooled; (b) water-cooled; (c) furnace-cooled

3. Conclusion

• (1) Heat treatment heating temperature from 410 ℃ to 590 ℃, 6061 aluminum alloy yield strength first decreased and then increased, tensile strength gradually increased; plastic properties fluctuating increase in the heating temperature of 560 ℃ to reach the maximum value (22.92%); while the hardness of the aluminum alloy is shown to be the first decline and then rise in the trend of change.
• (2) Prolonging the holding time, the plastic property of 6061 aluminum alloy increases first and then decreases, with a rising trend. In contrast, the hardness of aluminum alloy shows a decreasing trend first and then rises.
• (3) The cooling methods of air, water, and furnace cooling have little effect on the plastic properties of 6061 aluminum alloy but have a greater effect on the hardness of aluminum alloy. This is mainly due to the different cooling speeds leading to the number, size, and distribution of precipitated phases of 6061 aluminum alloy being different. Under the selected test conditions, taking into account the performance indexes of 6061 aluminum alloy, 6061 aluminum alloy with the ideal strength, hardness, and plastic properties can be obtained at the heat treatment heating temperature of 560 ℃, the holding time of 4h, and water-cooling mode.

Author: Ding Fengjuan