Wrought aluminium alloys

6061 aluminium alloy

Aluminium alloy 6061:

  • A medium to high strength heat treatable wrought aluminium alloy.
  • It has very good corrosion resistance and very good weldability, although it has reduced strength in the weld zone.
  • It has medium fatigue strength.
  • It has good cold workability in the T4 temper, but limited formability in the T6 temper.
  • Not suitable for very complex sections.

Chemical composition of aluminium alloy 6061

Nominal chemical composition [1]

Al-1,0Mg-0,6Si-0,30Cu-0,20Сr

Chemical composition according to EN 573-3

  • In the current European standard EN 573-3:2019, there is only one alloy EN AW-6061A. Alloy 6061 without the letter “A” is missing (Table 1).
  • In early editions of the EN 573-3 standard, there were two alloys: 6061 and its modification 6061A (Table 2)
  • Notes “e” and “15” indicate that the lead content in the 6061A alloy should be no more than 0.003%
  • In the EN 755-2 standard, which specifies mechanical properties, only alloy 6061 is present, without the letter A.

Table 1 – Chemical composition of 6061 aluminium alloy according to EN 573-3:2019
Chemical composition of 6061 aluminium alloy according to EN 573-3:2019

Таблица 2 – Chemical composition of 6061 aluminium alloy according to EN 573-3:2001

Chemical composition of 6061 aluminium alloy according to EN 573-3:2001

Designations

  • EN 573-3: EN AW-6061A and EN AW-Al Mg0.7Si(A)
  • International Registration (Teal Sheats) – Aluminum Association: 6061
  • Unified Numbering System (UNS): A96061
  • ISO 209:2007: 6061

Metallurgical characteristics of aluminium alloy 6061

  • Wrought
  • Thermally hardenable
  • Refers to alloys with a strength level from medium to high
  • Increased strength is achieved through thermal hardening (ageing)
  • Achieves strength higher than alloy 6005A
  • Good corrosion resistance
  • It welds well, but has reduced strength in the weld area
  • Has average fatigue strength
  • It can be cold formed well in the T4 condition, but has limited formability in the T6 condition
  • Not suitable for extruded profiles with complex cross-sections
  • The place of alloy 6061 among other alloys of the 6xxx series – see the figure below.

The place of alloy 6061 among other alloys of the 6xxx series

Typical application of aluminium alloy 6061

  • Bicycle frames
  • Trucks
  • Passenger carriages
  • Sports equipment
  • Structural pipes
  • Structural parts that require increased strength, good weldability and high corrosion resistance
  • Frame parts, seat rails, passenger car bumpers
  • Truck frames
  • Shipbuilding
  • Bridges and mobile military bridges
  • Aerospace products
  • Pylons and Towers
  • Transport
  • Boiler making
  • Motorboats
  • Rubber-Pad Forming

Standardized production from aluminium alloy 6061 [1]

Standardized production from aluminium alloy 6061

Mechanical properties

Standardized mechanical properties (EN 755-2)

Table 2 – Requirements for mechanical properties of extruded products from aluminum alloy 6061 according to EN 755-2:2016

Requirements for mechanical properties of extruded products from aluminum alloy 6061 according to EN 755-2:2016

Typical mechanical properties

Table 3 – Typical mechanical properties of 6061 alloy [1]
Typical mechanical properties of 6061 alloy

Physical properties

Elastic modulus

  • in tension – 68900 MPa
  • under compression – 69700 MPa.

Density

  • 2,70 g/cm3 at 20 °C

Thermal properties

  • Melting temperature range: 575 – 650 °C
  • Linera coefficient of thermal expansion: 23.6 µm/(m °C) in the range from 20 to 100 °C

Weldability

Methods

It is well welded by inert gas arc welding, in particular, by argon-arc welding, both with:

  • a non-consumable electrode (GTAW-TIG) and
  • a consumable electrode (GMWA-MIG).

Common welding alloy:

  • 4043

Welded joint strength

Weld strength is an important factor when selecting the appropriate filler alloy. The heat of the weld softens the aluminum alloys adjacent to the weld if they are in any condition other than annealed. In most groove welds, the HAZ of the base alloy will control the strength of the joint after welding (Fig. 1)

Heat treatable alloys, including alloy 6061, require 2 to 3 hours at annealing temperature combined with slow cooling to fully anneal. In welding this does not occur and the HAZ will consist of several stages of dissolution and varying degrees of precipitation depending on the thermal conditions, as shown in Fig. 2. The degree of HAZ softening is very sensitive to the maximum temperature reached at a particular location, as well as the time at this temperature [1].

The influence of welding conditions on the strength of the 6061 alloy weld is shown in Table 4.

Heat-Affected Zone in Welded Aluminium JointsFig. 1 – Heat-Affected Zone in Welded Aluminium Joints [2]

Photomicrographs showing HAZ micrcstructurs of a single-pass gas-metal arc weld on 6.35 mm (0.25 in.) thick 6061-T6 alloy

Fig. 2 – Photomicrographs showing HAZ micrcstructurs of
a single-pass gas-metal arc weld on 6.35 mm (0.25 in.) thick 6061-T6 alloy.
Keller’s reagent used as etchant [1]

Table 4 – Effect of welding conditions on 6061 alloy weld strength [1]
Effect of welding conditions on 6061 alloy weld strength

Thermal hardening

Solution treatment temperature [1]:

  • 530 °C

Artificial aging [1]:

  • Rolled and drawn products: 160 °C for 18 hours
  • Pressed or forged products: 175 °C for 8 hours

Press quenching (cooling)

The cooling rate after extrusion must be fast enough to maintain Mg and Si in solid solution. In this case, the mechanical properties will be maximum due to the release of Mg2Si particles during subsequent aging (Figure 3).

The cooling rate is determined by the cross-sectional size of the profile and the cooling medium, such as:

  • calm air
  • fans
  • water-air mixture
  • water.

Figure 4 shows sufficient cooling for 6060 type alloys, which is usually achieved by using still air cooling or forced air cooling, although for higher strength alloys, such as 6061 alloy, air-water or water cooling is preferred. Table 5 shows the typical cooling rates required for effective hardening of profiles made from 6000 series alloys, including alloy 6061 [5]

 

Diagrammatic sketch of press heat treatment cycle for 6061 alloy

Fig. 3 – Diagrammatic sketch of press heat treatment cycle for 6061 alloy [4]

Schematic showing various cooling rates and areas for precipitation of coarse Mg2Si

Fig. 4- Schematic showing various cooling rates and areas for precipitation of coarse Mg2Si [5]

Table 5 – Recommended Quench Rates and Methods for Various 6000 Series Alloys [5]
Recommended Quench Rates and Methods for Various 6000 Series Alloys

Microstructures of aluminium alloy 6061

6xxx or aluminum-magnesium-silicon alloys are designed taking into account the solubility of Mg2Si and thus precipitation strengthening. In the absence of manganese or chromium, the iron-rich phases are Fe3SiAl12, Fe2Si2Al9, or a mixture of both, depending on the proportions of magnesium, silicon and iron (Fig. 4–6). Manganese and chromium stabilize (Fe,Mn,Cr)3SiAl12.

  • In dilute alloys such as 6063, heating the cast structure to moderate temperatures dissolves all the Mg2Si.
  • The higher aluminium alloy 6061 usually has excess Mg2Si at solution temperature and will precipitate as Widdmanstatten when cooled slowly.
  • Some of the 6xxx alloys used for electrical conductivity are over-aged and, when etched, have a light stripe along the grain boundaries. It is caused by the presence of a zone without precipitates [1].

 

Alloy 6061-F plate, 38 mm (1.5 in.) thick, as hot rolled (91% reduction).

Fig. 4 – Alloy 6061-F plate, 38 mm (1.5 in.) thick, as hot rolled (91% reduction).
Longitudinal section from center of plate thickness.
Particles are Fe3SiAll2 (groy, scriptlike) and MgzSi (block).
See also Fig. 5 and 6.
0.5% HF [1]

Particles of Fe3SiAl12 and Mg2Si are more broken
Fig. 5 – Some alloy and condition as Fig. 4,
but a longitudinal section from neat plate surface.
Particles of Fe3SiAl12 and Mg2Si are more broken
up and uniformly distributed than in Fig. 5 (midthickness)
See also Fig. 6.
0.5% HF [1]

Alloy 6061-F 6.4-mm (0.25-in.) sheet, hot rolled (reduced 98%); midthickness longitudinal section
Fig. 6 – Alloy 6061-F 6.4-mm (0.25-in.) sheet, hot rolled (reduced 98%);
midthickness longitudinal section.
Fe3SiAl12 and MgzSi particles more broken and dispersed than in Fig. 5.
Most Mg2Si will dissolve during solution treating.
0.5% HF [1]

 

Aluminium alloy 6061 in Eurocode 9 (EN 1999-1-1)

Table 5 – Условные обозначения [3]

 

  • The material properties given in this section are specified as characteristic values. They are based on the minimum values given in the relevant product standard (Table 6) [3]
  • Characteristic values of the proof strength f0 and the ultimate tensile strength fu for aluminium alloy 6061 for a range of tempers and thicknesses are given in Table 7 for sheet, strip and plate products; Table 8 for extruded rod/bar, extruded tube and extruded profiles and drawn tube [3].

Таблица 6 – Фрагмент таблицы 3.1а из EN 1999-1-1 для сплавов серии 6000 [3]

Fragment of table 3.1a from EN 1999-1-1 for series alloys 6000

Table 7 – The fragment of Table 3.2а from EN 1999-1-1 for aluminium alloy 6061 [3]

The fragment of Table 3.2а from EN 1999-1-1 for aluminium alloy 6061

Table 8 – The fragment of Table 3.2.b from EN 1999-1-1 for aluminium alloy 6061 [3]
The fragment of Table 3.2.b from EN 1999-1-1 for aluminium alloy 6061

6061 Alloy modifications

6061A:

  • Lead content no more than 0.003%

6261:

  • 0.2-0.35% Mn
  • It is 15-20 MPa stronger than alloy 6061 in T6 condition.

Источники:

  1. Aluminum and Aluminum Alloys, ed. J. R. Davis – 1996
  2. TALAT Lecture 4204 – Design Aspects
  3. EN 1999-1-1:2007 Eurocode 9: Design of aluminium structures – Part 1-1 : General structural rules
  4. Carl V. Lynch, Aluminum Extrusion Technology Seminar, Chicago, 1969
  5. BASIC METALLURGY: 6000 SERIES EXTRUSION ALLOYS