Heat-treatable aluminium in Eurocode 9

The European standard EN 1999-1-1, which is part of a series of standards EUROCODE 9 or Eurocodes 9, defines the general rules and regulations of the use of aluminum alloys in structures. Eurocodes 9 is an, in turn, part of a series of European building standards under the general name EUROCODE.

Aluminum alloys in Eurocode 9

Eurocodes 9 includes recommendations for the use in construction of the following types of aluminum alloys:

• wrought, heat-treatable
• wrought, non-heat-treatable
• foundry, as thermally hardened, and thermally non-hardened.

Below are the recommendations of the Eurocode 9 Usage deformable aluminum alloys, which are hardened by heat treatment [1]

Heat-treatable aluminium

Heat hardening

In heat-hardened alloys, one (Cu, Zn) or several elements (Mg and Si) can form intermetallic compounds, which are associated with an aluminum matrix (GP Zones) (Fig.. 1). These GP Zones form atomic lattice defects. This gives an increase in the strength of the aluminum alloy due to an increase in resistance to the movement of dislocations in the atomic lattice (Fig.. 2).

The thermal hardening process begins with, that the alloy is heated above a certain control temperature and maintained until, until a homogeneous (solid) solution is formed. Then hardening is necessary to obtain an even distribution of all elements at ambient temperature.. After that, aging leads to, that the participating elements begin to diffuse into the aluminum matrix, numerous nucleation centers appear and reinforcing intermetallic compounds are formed.

Fig. 1 – GP zones in Al-Cu, Al-Zn and Al-Mg-Si alloys [3]

Fig. 2 – GP zones rezisting dislocation movement [3]

Artificial aging

Natural aging begins immediately after quenching at a relatively high rate, but gradually, and then asymptotically approaches the upper limit (T4 in Fig.. x). Depending on the alloy, the natural aging process can take several weeks., but for most alloys it can be considered completed in a week.

Artificial aging can begin for several hours (but also days, depending on production needs) later. Material, aged, put in the oven, which allows aging in various temperature conditions. Typical for all temperatures is rapid hardening, gradually reaching a maximum (T6). If the material is exposed to high temperatures for a longer time, the effect of hardening precipitates on strength is reduced, and we get the state of overaging (T7).

Usually, aged alloys are characterized by better ductility, corrosion resistance (some alloys, containing copper and/or zinc) and better electrical conductivity. Heat-treated aluminum alloys dominate in many areas, for example, extruded profile.

Fig. 3 – Typical ageing curves [3]

Fig. 4 – Strength in function of ageing time at ambient and elevated temperature [2]

Influence of welding heat

Welding causes significant loss in material strength. In case of welding, the temperature is so high, that the effects of strength reduction in the vicinity of the weld (the so-called HAZ (Fig.. 5)) must be taken into account Often this is an important aspect of design verification of a structure.. Heat-treatable alloys in the T6 temper have a loss of approximately 40 % its strength with the sole exception of the alloy 7020, which loses only 20 % its original strength. To facilitate design calculations, the area of ​​strength loss is replaced by a rectangular area with a width Bhaz, which is standardized and can depend on the thickness of the material within a few tens of millimeters. The strength value in this zone is also normalized and depends on the alloy and state [2].

Fig. 5 – Reduction of strength in the heat affected zone (HAZ) (typical for EN AW-6082-T6) [2]

Heat-treatable alloys in Eurocode 9

Eurocodes 9 recommends the use in building structures of thermally hardened deformable aluminum alloys from two series:

• 6xxx
• 7xxx.

Of aluminum alloys of the 6xxx series, the following alloys are recommended for use in construction (we omit hereinafter, when designating the bulky "prefix" EN AW):

• 6060
• 66061
• 6063
• 6005A
• 6082
• 6106

Of the aluminum alloys of the 7xxx series, only the alloy is used in building structures:

• 7020.

Types of building products

Fig. 6 – Fragment of table 3.1a from the EN standard 1999-1-1:
building products from heat-strengthened wrought aluminum alloys [1]

Chemical composition of the aluminum alloys

The figure 7 table fragment shown 6 from EN 573-3 for heat-strengthened wrought aluminum alloys, included in Eurocode 9 [4].

Fig. 7 – Table fragment 6 from EN 573-3
for heat-strengthened wrought aluminum alloys in Eurocode 9 [4]

Mechanical properties of profiles, sheets and strips

On pic. 8 and 9 the characteristic strength properties of aluminum alloy products are shown 6082. 6061, 6005A, 6106, 6063, 6060 and 7020.
Fig. 8 – Fragment of table 3.2a from EN 1999-1-1
for heat-hardened wrought aluminum alloys [1]

Fig. 9 – Extract from table 3.2b from EN 1999-1-1
for heat-hardened wrought aluminum alloys [1]

Selection of heat-treatable wrought aluminum alloy

Choosing the right aluminum or aluminum alloy material for any structural application is determined by a combination of factors:

• strength
• durability
• physical properties
• weldability
• formability
• availability as in alloy, and in the specific form required.

Comparative properties and characteristics of these alloys are presented in Table C.I of the EN standard. 1999-1-1 (Fig. 5). Properties and characteristics may vary depending on the state of the alloy.

Fig. 10 – Excerpt from Table C.1 from EN 1999-1-1
for heat-hardened wrought aluminum alloys [1]

Aluminum alloys 6082 and 6061

Construction products

Aluminium alloy 6082 the most widely used in construction industry as heat-hardenable alloy. Usually, this alloy is the main building for the aluminum alloy as the weldments, and constructions without welding. Alloy 6082 - a high-strength alloy, which is used in various types of rolled aluminum:

• solid and hollow extruded profiles;
• plates (thick sheets);
• sheets;
• forgings.

Alloy 6082 also increasingly used in constructions, who work in the marine atmosphere.

Alloy 6061 It is also commonly used heat-hardenable aluminum alloy for both structures using welding, so without. This alloy is used in the form of:

• bars,
• solid and hollow profiles, and
• pipe.

Both alloys - 6082 and 6061 - usually used in a fully heat-strengthened state T6: 6082-T6 and 6061-T6.

Properties

Selection of alloys 6082 and 6061 as structural materials, it provides a favorable combination of their properties:

• high strength after heat hardening;
• good corrosion resistance;
• good weldability, whether by MIG, and by the TIG method;
• good formability (e.g., bending) in the T4 state (natural aging);
• good machinability.

The use of alloys 6082 and 6061 in extruded profiles is limited to less complex cross-sectional shapes, than for other 6xxx series alloys.

Aluminium alloy 6082 can be joined by rivets made of alloys 6082, 5754 or 5019 in the annealed condition O or a solid state.

Welding

For these alloys, it is necessary to take into account the loss of strength in the heat-affected zone of welding of welded joints (see. tables 2 and 3). The level of strength of welded joints can be restored to a certain extent due to the natural aging of the material in the heat affected zone of the weld. Loss of strength for the T6 condition is usually about 40 % (cm. tables 2) [1].

Aluminum Alloy 6005A

Direct domestic analogue alloy 6005A absent.

Construction products

alloy 6005A, which is also suitable for building structures, It is used only in the form of extruded sections.

Properties

• This alloy has a relatively high strength and, in the same time, It can be compressed into a more complex profiles, than alloys 6082 and 6061. This applies particularly to thin-walled hollow profiles.
• The corrosion resistance of welded and non-welded structural members of similar resistance alloy 6005A alloy 6082.
• The other properties of the alloy similar to alloy properties 6005A 6082 [1].

Welding

• Similarly alloys 6082 and 6061 Alloy 6005A lends itself well to TIG and MIG welding.
• Has a similar alloy 6082 loss of strength in the heat-affected zone of the weld: for condition T6 - about 40 % (cm. tables 2 and 3) [1].

Aluminum alloys 6060, 6063 and 6106

Construction products

alloys 6060, 6063 and 6106 recommended for building constructions. These alloys are used only in the form of cold-pressed and articles. they are used, if the strength is not of paramount importance, but it takes a good product appearance. These alloys provide good resistance to environmental effects, well to application of protective and decorative coatings and, the most important thing, They have the ability to be pressed into profiles with thin walls and complex cross-section.

These alloys are particularly well suited for anodizing [1].

Welding

Like all alloys 6xxx series alloys 6060, 6063 and 6106 well as a welded with MIG, and a method of TIG and also lose strength in the heat affected zone of welded joints. Loss of strength for the T6 condition is about 40 % (cm. tables 2 and 3).

Aluminium alloy 7020

Alloy 7020 It recommended for use in welded and non-welded construction structures.

Construction products

From this high-strength alloy produce solid and hollow extruded sections, thick and thin sheets, and pipe. Of the alloy is not easy to extrude complex profiles, as the 6xxx series alloys. Therefore, the product of an alloy 7020 usually only made to order, and they may have a longer delivery time.

Alloy 7020, usually, used in a fully thermally hardened state - 7020-T6. This alloy has a high strength in the heat affected zone of the weld, than the 6xxx series alloys, thanks to its high strength properties after natural aging.

Properties

this alloy, as well as other 7xxx series alloys, It is highly sensitive to climatic influences. Therefore, its normal operation depends on the correct processing and manufacturing methods, and control of the chemical composition. Due to the exposure of the subsurface corrosion in the step of manufacturing the alloy product is used only in the state T4, and after the complete manufacture of the entire structure, possibly, subjected to artificial aging.

If the welding heat-affected zone is not subjected to heat treatment after welding, it may be necessary to protect it from the impact of climatic factors.

If the product of the alloy 7020 T6 state is subjected to any processing steps, which can cause hardening, for example, such as, flexible, cutting or punching, It increases the danger of cracking due to stress corrosion. It is very important and therefore should be direct collaboration of designers and manufacturers for the intended application and the possible operating conditions of the alloy product 7020.

Welding

Alloy 7020 It lends itself well to TIG welding and MIG. Loss of strength in the HAZ to T6 state is approximately 20 % (cm. tables 2 and 3).

 

Sources:

1. EN 1999-1-1:2014
2. Design of aluminium structures Introduction to Eurocode 9 with worked examples – European Aluminium – 2020
3. TALAT 1204
4. EN 573-3