EP2032729A1 - High damage tolerant aa6xxx-series alloy for aerospace application - Google Patents

Abstract

This invention relates to a weldable aluminium alloy wrought product having high strength and improved resistance to intergranular corrosion, the alloy consisting essentially of, in weight percent: Si 0.2 - 1.3 Mg 0.4 - 1.5 Cu 0.1 - 1.1 Mn up to 0.7 Fe 0.02 - 0.3 Zn up to 0.9 Cr up to 0.25 Ti 0.06 - 0.19 Zr up to 0.2 Ag up to 0.5, and wherein 0.1 < Ti + Cr < 0.35, other elements and unavoidable impurities each <0.05, total <0.20, balance aluminium.

Description

High damage tolerant AA6xxx-series alloy for aerospace application.

FIELD OF THE INVENTION

This invention pertains to aluminium aerospace alloys. More particularly, this invention pertains to aluminium alloys of the AAΘxxx-series (or AA6000-series) that are suitable for welding, yet have improved performance properties, particularly corrosion resistance and damage tolerance properties.

BACKGROUND TO THE INVENTION As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminium Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association.

For any description of alloy composition or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. Aluminium alloys 6061 and 6063 are well known heat treatable aluminium alloys. These alloys have useful strength and toughness properties in both T4 and T6 tempers. As is known, the T4 condition refers to a solution heat treated and quenched condition naturally aged to a substantially stable property level, whereas T6 tempers refer to a stronger condition produced by artificially ageing. These known alloys lack, however, sufficient strength for most structural aerospace applications. Several other Aluminium Association ("AA") 6000 series alloys are generally unsuitable for the design of commercial aircraft which require different sets of properties for different types of structures. Depending on the design criteria for a particular aircraft component, improvements in strength, fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety. To meet these demands several 6000 series alloys have been developed. US Patent No. 4,589,932 (Alcoa) discloses an aluminium alloy for automobile, rail, naval or aeronautical construction, with the following composition, in weight percent:

Si 0.4 - 1. 2

Mg 0.5 - 1. 3

Cu 0.6 - 1. 1

Mn 0.1 - 1

Fe max. 0 .6 balance aluminium and incidential elements and impurities. This aluminium alloy was subsequently registered in March 1983 by the Aluminum Association under the designation AA6013. The registered compositional ranges for AA6013 are, in weight percent:

Si 0.6 - 1.0 Fe max. 0.50

Cu 0.6 - 1.1

Mn 0.2 - 0.8 Mg 0.8 - 1.2 Cr max. 0.10 Zn max. 0.25

Ti max. 0.10, others each 0.05 max., total 0.15 balance aluminium.

The AA6013 alloy has attractive mechanical properties for use amongst others as a fuselage skin and furthermore this alloy is also weldable. However, there are at least two factors limiting the application of this AA6013 alloy. The first one is that the AA6013 alloy is susceptible to intergranular corrosion (IGC) attack, which can increase local stress concentrations when an alloy product is subjected to stress conditions such as repeated pressurization and depressurization of an aircraft fuselage in use, see for example the paper of T.D. Burleigh, "Microscopic Investigation of the Intergranular Corrosion of 6013-T&',

ICAA3, Trondheim, 1992, p. 435. And the second drawback is that the AA6013 alloy has significant lower damage tolerant properties compared to its AA2x24 counterpart.

Another AAβxxx-series alloy suitable for aerospace application is the AA6056-series alloy. The registered compositional ranges for AA6056 are, in weight percent: Si 0.7 - 1.3

Fe max. 0.50

Cu 0.5 - 1.1

Mn 0.4 - 1.0 Mg 0.6 - 1.2 Cr max. 0.25

Zn 0.1 - 0.7

Ti+Zr max. 0.20, others each 0.05 max., total 0.15 balance aluminium. However, it has been reported that also this AA6056 alloy is susceptible to intergranular corrosion. The resistance to intergranular corrosion of the AA6056 has been improved by overageing, (i.e. artificially ageing by a practice that causes the metal to go past peak strength to a lower strength condition). In order to obtain the improved corrosion resistance it is also essential for the disclosed overageing process that in the aluminium alloy the Mg/Si ratio is less than 1. This specific overaging practice has been disclosed in US-5,858,134, but has amongst others the drawback of a significant decrease in strength compared to peak aged tempers.

Another method of controlling the resistance to intergranular corrosion of the AA6056 alloy is by providing it with a dilute AA7072 cladding having 0.25-0.7 wt.% Zn as disclosed in EP-1170118.

Yet another method to improve the properties of the AA6056 alloy for its application as aircraft structural component is disclosed in US-2002/0014290-A1. This document discloses an ageing practice to improve the static mechanical characteristics and the tolerance to damage.

In order to take full advantage of the potential cost savings offered by fuselage skin panel welding as a low cost alternative to fastening them with rivets, therefore, it would be desirable to develop a weldable aluminium alloy suitable for aerospace application having sufficient strength combined with improved damage tolerance properties and improved resistance to intergranular corrosion.

SUMMARY OF THE INVENTION A principal objective of the present invention is to provide an improved AAβxxx series alloy that is weldable, yet exhibits improved corrosion resistance properties.

An object of the present invention is to provide a weldable AA6xxx-type series alloy product having improved resistance to intergranular corrosion compared to its AA6013 counterpart. A further object is to provide a weldable AA6xxx-type series alloy product having improved damage tolerance properties compared to its AA6013 counterpart.

Another object is to provide a weldable AA6xxx-type series alloy product having an improved balance of intergranular corrosion resistance and damage tolerance properties compared to its AA6013 counterpart. These and other objects and further advantages are met or exceeded by the present invention concerning an aluminium alloy wrought product consisting essentially of, in weight percent:

Si about 0.2 to 1.15

Mg about 0.4 to 1.5 Cu about 0.1 to 1.3

Mn up to 0.7 Fe about 0.02 to 0.3 Zn up to about 0.9

Cr up to about 0.25

Ti about 0.06 to 0.19

Zr up to about 0.2 Ag up to about 0.5, and wherein 0.1 < Ti + Cr < 0.35, other elements and unavoidable impurities each <0.05, total <0.20, balance aluminium.

In the peak aged condition (i.e. T6 type condition), the aluminium alloy of this invention offers greater resistance to intergranular corrosion resistance compared to its AA6013 aluminium alloy counterpart. Furthermore, in the peak aged condition, the aluminium alloy of this invention offers an improved ratio of UPE versus tensile strength.

By an AA6013 counterpart it is meant an aluminium alloy wrought product having a composition as defined above for AA6013 and processed and heat treated and having the same dimensions of length, width and thickness as the wrought product of the present invention to which it is compared.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows schematically the UPE (vertical axis) against the yield strength (horizontal axis) of the five alloys tested.

Fig. 2 shows schematically the maximum IGC depth of the five alloys tested.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a weldable aluminium alloy wrought product having high strength and improved resistance to intergranular corrosion, the alloy consisting essentially of, in weight percent:

Si about 0.2 to 1.3, preferably about 0.6 to 1.15, and more preferably about

0.65 to 1.10

Mg about 0.4 to 1.5, preferably about 0.7 to 1.25, and more preferably about 0.7 to 1.05

Cu about 0.1 to 1.1 , preferably about 0.5 to 1.1 , and more preferably about

0.6 to 1.0 Mn up to about 0.7, preferably about 0.15 to 0.7, and more preferably about

0. 2 to 0.6 Fe about 0.02 to 0.3, preferably about 0.02 to 0.2, and more preferably about 0.02 to 0.15 Zn up to about 0.9 Cr up to about 0.25

Ti about 0.06 to 0.19

Zr up to about 0 .2

Ag up to about 0 .5, preferably up to about 0.2%, and wherein 0.1 < Ti + Cr < 0.35, other elements and unavoidable impurities each 0.05, total <0.20, balance aluminium.

In referring to an element, "up to" includes zero except that, when an element is stated to be present, such excludes zero since the element is stated to be present. In the alloy product according to the invention it is preferred to control the Mg and Si contents such that Mg+1.1Si<2.0, and more preferably Mg+1.1Si<1.85. This is to optimize the required strength levels by ensuring that second phase particles of Mg₂Si can be fully dissolved during the homogenization and/or preheat prior to the hot working operation. It has been found that such a compositional control can increase both the Unit Propagation Engergy ("UPE") and the ratio of TS/Rp ("Tear Strength/Yield Strength") significantly.

Apart from the Si, Mg, Cu and Mn in the defined ranges, and preferred narrower ranges, an important alloying element in the alloy according to this invention is titanium. The addition of Ti at levels of more than 0.06% to the alloy according to this invention has the effect of increasing the corrosion resistance, and the resistance against intergranular corrosion in particular. Ti levels at significant lower levels (e.g. at about 0.03% or less) can be found also in aluminium alloys, but at such low levels it is commonly purposively added to obtain a grain refining effect during industrial scale casting of ingots or stock for rolling, extrusion or forging. No effect on the corrosion resistance could be found at such low levels.

A similar effect on the corrosion resistance has been found for the purposive addition of Cr up to about 0.25%.

However, an even significant further improvement in the corrosion resistance, and intergranular corrosion in particular, has been found according to this invention in the case of the combined addition of Ti with Cr, and optionally with a further addition of Zr. To achieve the best improvements the Ti content is in the range of about 0.06 to 0.19%, and preferably about 0.09 to 0.19%. Preferably the Cr content should be in the range of up to about 0.25%, and preferably about 0.05 to 0.25%, more preferably about 0.08 to 0.19%. The combined addition of Ti plus Cr should be in the range of about 0.12 to 0.3%, and preferably about 0.15 to 0.28%. The combined addition of Ti and Cr has also a very favourable effect of the strength levels and the Unit Propagation Energy ("UPE") making the alloy product a very attractive candidate for aerospace applications. The ranges for the Ti and Cr contents are very critical. For example it has been found that the addition of more than 0.2% Ti may result in the formation of large primary phases which significantly reduce amongst others the tear strength ("TS") and the UPE.

Zr may be added to the aluminium alloy according to this invention up to 0.2%. If purposively added to the alloy it is preferably in the range of about 0.06 to 0.18%. Adding Zr to the alloy has the effect of maintaining favourable UPE levels while offering an increased yield strength. The intergranular corrosion resistance is slightly decreased compared to the alloy variant with solely the combined addition of Ti plus Cr. However, the overall balance of strength, damage tolerance and corrosion resistance is still favourable compared to its AA6013 counterpart.

In another embodiment the Zr content is less than 0.05%, and more preferably the aluminium alloy is substantially free from Zr to obtain a fully recrystallised microstructure.

In an embodiment of the aluminium alloy according to this invention there is no purposive addition of Zn, but it may be tolerated as an impurity. In this embodiment the Zn content is in a range of less than about 0.25%, preferably less than about 0.05%, and more preferably less than about 0.02%.

In another embodiment of the aluminium alloy according to this invention there is a purposive addition of Zn to further improve the strength, wherein the Zn is preferably present in a range of about 0.5 to 0.9%, and preferably in a range of about 0.6 to 0.85%. A too high Zn content may have an adverse effect on the intergranular corrosion performance.

In a preferred embodiment the aluminium alloy according to the invention is substantially free from each of V, Sr, and Be.

For this invention, with "substantially free" and "essentially free" we mean that no purposeful addition of this alloying element was made to the composition, but that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of this element may, nevertheless, find their way into the final alloy product.

The best results are achieved when the alloy rolled products have a recrystallised microstructure, meaning that 80% or more, and preferably 90% or more of the grains in a T4 or artificially aged condition are recrystallised. Increased intergranular corrosion resistance is particularly useful for applications that expose the metal to corrosive environments, such as the lower portion of an aircraft fuselage. Moisture and corrosive chemical species tend to accumulate in these areas of an aircraft as solutions drain to the bottom of the fuselage compartment. In a preferred embodiment the alloy product according to this invention has in a T6 temper an intergranular corrosion depth of attack of less than 100 micron when measured according to the MIL-H-6088 test, and preferably less than 90 micron, and in the best examples less than 50 micron. The aluminium alloy wrought product according to this invention is preferably provided as a rolled product such as a sheet or plate. However, the advantages in improved corrosion resistance and damage tolerance properties can be obtained also when the wrought product is in the form of an extruded product, and less preferentially in the form of forged product shapes using regular product manufacturing processes.

It is important to note that the alloy composition of this invention works well at resisting intergranular corrosion in both its clad and unclad varieties. For some clad versions, the alloy layer applied overtop the invention is a AA7xxx-series alloy cladding, more preferably an AA7072 series alloy or the AlZn-cladding as disclosed in EP-1170118 (incorporated herein by reference), or the more commonly known cladding of the AAIxxx-series, such as the AA1145 aluminium.

In another embodiment the alloy product according to the invention is being provided with a cladding thereon on one side of the AA1000-series and on the other side thereon of the AA4000-series. In this embodiment corrosion protection and welding capability are being combined. In this embodiment the product may be used successfully for example for pre- curved panels. In case the rolling practice of an asymmetric sandwich product (1000-series alloy + core + 4000-series alloy) causes some problems such as banaring, there is also the possibility of first rolling a symmetrical sandwich product having the following subsequent layers 1000-series alloy + 4000-series alloy + core alloy according to this invention + 4000- series alloy + 1000-series alloy, where after one or more of the outer layer(s) are being removed, for example by means of chemical milling.

Aerospace applications of this invention may combine numerous alloy product forms, including, but not limited to, TIG welding, laser and/or mechanically welding (i.e. friction stir welding): sheet to a sheet or plate base product; plate to a sheet or plate base product; or one or more extrusions to such sheet or plate base products. One particular embodiment envisions replacing the manufacture of today's airplane fuselage parts from large sections of material from which significant portions are machined away. Using the alloy composition set forth above, panels can be machined or chemically milled to remove metal and reduce thickness at selective strip areas to leave upstanding ribs between the machined or chemically milled areas. These upstanding ribs provide good sites for welding stringers thereto for reinforcement purposes. Such stringers can be made of the same or similar composition, or of another AAβxxx-series alloy composition, so long as the combined components still exhibit good resistance to intergranular corrosion attack.

In a further aspect of the invention there is provided a method of manufacturing the alloy product according to this invention, the method comprising the steps of: a. casting an ingot having a chemical composition of, in wt.%: Si about 0.2 to 1.15 Mg about 0.4 to 1.5 Cu about 0.1 to 1.3 Mn up to 0.7

Fe about 0.02 to 0.3 Zn up to about 0.9

Cr up to about 0.25

Ti up to about 0.19, and preferably about 0.06 to 0.19

Zr up to about 0.2

Ag up to about 0.5, and preferably wherein 0.1 < Ti + Cr < 0.35, other elements and unavoidable impurities each <0.05, total <0.20, balance aluminium, and whereby preferred embodiments of the alloy composition are set forth above and in the examples; b. homogenising and/or pre-heating the ingot after casting, at a temperature of 54O⁰C or higher; c. hot working the ingot into a pre-worked product by one or more methods selected from the group consisting of rolling, extrusion, and forging; d. optionally reheating the pre-worked product; and e. hot working and/or cold working to a desired work piece form; f. solution heat treating said work piece; g. quenching the solution heat treated work piece to minimise uncontrolled precipitation of secondary phases; h. optionally stretching or compressing of the quenched work piece; i. ageing the quenched and optional stretched or compressed work piece to achieve a desired temper. The alloy product is ideally provided in a T4 temper by allowing the product to naturally age to produce an improved alloy product having good formability, or in a T6 temper by artificial ageing. To artificial age, the product in subjected to an ageing cycle comprising exposure to a temperature of between

150 and 21O⁰C for a period between 0.5 and 30 hours. However, under-ageing or over-ageing would still be possible for the alloy product according to this invention.

The aluminium alloy as described herein can be provided in process step (a) as an ingot or slab for fabrication into a suitable wrought product by casting techniques currently employed in the art for cast products, e.g. DC-casting, EMC-casting, EMS-casting. Slabs resulting from continuous casting, e.g. belt casters or roll caster, may be used also.

Typically, prior to hot rolling the rolling faces of both the clad and the non-clad products are scalped in order to remove segregation zones near the cast surface of the ingot. The cast ingot or slab may be homogenised prior to hot working, preferably by means of rolling and/or it may be preheated followed directly by hot working. The homogenisation and/or preheating of the alloy prior to hot working should be carried out at a temperature in the range of 490 to 58O⁰C in single or in multiple steps. In either case, the segregation of alloying elements in the material as-cast is reduced and soluble elements are dissolved. If the treatment is carried out below 49O⁰C, the resultant homogenisation effect is inadequate. If the temperature is above 580⁰C, eutectic melting might occur resulting in undesirable pore formation. The preferred time of the above heat treatment is between 2 and 30 hours. Longer times are not normally detrimental. Homogenisation is usually performed at a temperature above 540⁰C. A typical preheat temperature is in the range of 540 to 570⁰C with a soaking time in a range of 4 to 16 hours.

After the alloy product is cold worked, preferably after being cold rolled, or if the product is not cold worked then after hot working, the alloy product is solution heat treated at a temperature in the range of 480 to 590⁰C, preferably 530 to 57O⁰C, for a time sufficient for solution effects to approach equilibrium, with typical soaking times in the rang of 10 sec. to 120 minutes. With clad products, care should be taken against too long soaking times to prevent diffusion of alloying element from the core into the cladding detrimentally affecting the corrosion protection afforded by the cladding.

After solution heat treatment, it is important that the alloy product be cooled to a temperature of 175°C or lower, preferably to room temperature, to prevent or minimise the uncontrolled precipitation of secondary phases, e.g. Mg₂Si. On the other hand cooling rates should not be too high in order to allow for a sufficient flatness and low level of residual stresses in the alloy product. Suitable cooling rates can be achieved with the use of water, e.g. water immersion or water jets. While the invention is particularly suited to fuselage skins, it also may find other applications such as automotive sheet, railroad car sheet, and other uses.

The invention will now be illustrated with reference to non-limiting embodiments according to the invention.

EXAMPLE

Five different alloys have been DC-cast into ingots, then subsequently scalped, preheated for about 6 hours at 56O⁰C (heating-up speed about 30°C/h), hot rolled to a gauge of 8 mm whereby the hot-mill entry temperature was about 48O⁰C, cold rolled to a final gauge of 2 mm, solution heat treated for 10 min. at 56O⁰C, water quenched, 2% stretch, aged to a T6- temper by holding for 4 hours at 19O⁰C and followed by air cooling to room temperature. Table 1 gives the chemical composition of the five alloys cast. The alloy composition of alloy A is a regular 6013 alloy for reference purposes. Alloy no. B is the 6013 alloy with increased Cr content, and is also a reference alloy. Alloys No. C to E are according to this invention.

Table 1. The chemical compositions of the ingot cast alloys. All percentages are by weight, balance aluminium and unavoidable impurities.

The tensile testing has been carried out on the bare sheet material in the T6-temper and having a fully recystallised microstructure. For the tensile testing in the L-T direction small euro-norm specimens were used, average results of 3 specimens are given, and "Rp" stands for yield strength, "Rm" for ultimate tensile strength, and El for elongation (A50). The results of the tensile tests have been listed in Table 2. The UPE versus the yield strength is also schematically shown in Fig. 1. In the same Table 2 the "TS" stands for tear strength, and has been measured in the L-T direction in accordance with ASTM-B871-96. "UPE" stands for Unit Propagation Energy, and has been measured in accordance with ASTM-B871-96, and is a measure for toughness, in particular for the crack growth, whereas TS is in particular a measure for crack initiation. The higher the UPE the lower the fatigue crack growth rate, lntergranular corrosion resistance ("ICG") was tested on two specimens of 50x60 mm in accordance ASTM G110, MIL-H-6088 (AMS-H6088) and QVA-Z-59-3. The maximum depth in microns has been reported in Table 3 and schematically shown in Fig. 2, wherein in Table 3 "Type 1" represents only pitting corrosion, "Type 2" pitting and slight IGC, and "Type 3" local IGC.

Table 2. The mechanical properties measured in the LT directions.

Table 3. IGC corrosion results in the T6-temper.

From the results of Table 2 and Fig. 1 it can be seen that the addition of Ti or Ti with Cr or Ti with Cr and Zr results in a favourable increase of the UPE in combination with an increase in the yield strength. The addition of only Ti results in a significant increase of the Tear Strength. The combined addition of Ti plus Cr or Ti plus Cr and Zr results in a Tear Strength compared to regular 6013, but this is balanced by a significant increase in the IGC performance.

From the results of Table 3 and Fig. 2 it can be seen that the addition of Ti results in a significant improvement of the corrosion resistance, and the intergranular corrosion resistance in particular compared to its AA6013 counterpart, whereas the single addition of Cr only has a marginal influence on the IGC performance. Only pitting has been found with Ti addition, and laminar corrosion takes place instead of IGC. And the combined addition of Ti and Cr improves the IGC properties even further. The combined addition of Ti, Cr and Zr results still in improved IGC performance compared to the 6013 counterpart, while balancing this with a small further increase in strength. As can be seen from Fig. 1 , the alloy with the combined addition of Ti, Cr and Zr still has a favourable ratio of UPE and Rp compared to the regular AA6013.

In a further experiment the effect of homogenisation and pre-heat temperature prior to hot working has been investigated on the AA6013 alloy of Table 1 above. After casting the ingots were scalped, homogenised for about 6 hours at different temperatures, hot rolled to a gauge of 8 mm whereby the hot-mill entry temperature was about 480⁰C, cold rolled to a final gauge of 2 mm, solution heat treated for 15 min. at 565⁰C, water quenched, 2% stretch, aged to a T6-temper by holding for 4 hours at 190⁰C and followed by air cooling to room temperature. The results on the mechanical properties are listed in Table 4.

From the results of Table 4 it can be seen that the UPE and Tear Strength systematically increase with increasing homogenization temperature whereas the yield strength does not change. Although illustrated on the AA6013 alloy, the same trend can be found on the alloy product according to this invention. Furthermore, contrary to prior research (see for example the paper by V.G. Davydov et al., "Influence of SSTT, Ageing Regime and Stretching on IGC, Complex of Properties and Precipitation Behavior of 6013 alloy.", Materials Science Forum VoIs. 331-337, (2000), pp. 1315-1320) increasing the preheat or homogenization temperature does not have an adverse effect on the IGC resistance when producing the alloy product according to this invention.

Table 4. The mechanical properties as function of the homogenization temperature in AA6013 alloy.

Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims

1. A weldable aluminium alloy wrought product having high strength and improved resistance to intergranular corrosion, the alloy consisting essentially of, in weight percent:

Si 0.2 - 1.3

Mg 0.4 - 1.5

Cu 0.1 - 1.1

Mn up to 0.7 Fe 0.02 - 0.3

Zn up to 0.9

Cr up to 0.25

Ti 0.06 - 0.19

Zr up to 0.2 Ag up to 0.5, and wherein 0.1 < Ti + Cr < 0.35, other elements and unavoidable impurities each <0.05, total <0.20, balance aluminium.

2. Aluminium alloy product according to claim 1, wherein the Cr content is in a range of 0.05 to 0.25%, and preferably in a range of 0.08 to 0.19%.

3. Aluminium alloy product according to claim 1 or 2, wherein the Ti content is in a range of 0.09 to 0.19%.

4. Aluminium alloy product according to any one of claims 1 to 3, wherein 0.12 < Ti + Cr < 0.3, and preferably 0.15 < Ti + Cr < 0.28.

5. Aluminium alloy product according to any one of claims 1 to 4, wherein the Zr content is a range of 0.06 to 0.18%.

6. Aluminium alloy product according to any one of claims 1 to 4, wherein the Zr content is < 0.05%, and preferably the alloy is substantially free from Zr.

7. Aluminium alloy product according to any one of claims 1 to 6, wherein the Zn content is in a range of 0.5 to 0.85%, and preferably 0.6 to 0.85%.

8. Aluminium alloy product according to any one of claims 1 to 6, wherein the Zn content is in a range of <0.2%, and preferably <0.05%.

9. Aluminium alloy product according to any one of claims 1 to 8, wherein the Si content is in a range of 0.6 to 1.15%, and preferably 0.65 to 1.10%.

10. Aluminium alloy product according to any one of claims 1 to 9, wherein the Mg content is in a range of 0.7 to 1.25%, and preferably 0.7 to 1.05%.

11. Aluminium alloy product according to any one of claims 1 to 10, wherein the Cu content is in a range of 0.5 to 1.1%, and preferably 0.6 to 1.0%.

12. Aluminium alloy product according to any one of claims 1 to 11, wherein the Mn content is in a range of 0.15 to 0.7%, and preferably 0.2 to 0.6%.

13. Aluminium alloy product according to any one of claims 1 to 12, wherein the Fe content is in a range of 0.02 to 0.2%.

14. Aluminium alloy product according to any one of claims 1 to 13, wherein the Mg+1.1 Si<2.0%, and preferably Mg+1.1Si<1.85%.

15. Aluminium alloy product according to any one of claims 1 to 14, wherein the alloy product has in the T6 temper an intergranular corrosion depth of attack of less than 100 micron when measured according to the MIL-H-6088 test, and preferably less than 90 micron.

16. Aluminium alloy product according to any one of claims 1 to 15, wherein the wrought product is in the form of a sheet or plate.

17. Aluminium alloy product according to any one of claims 1 to 16, wherein the wrought product is provided with a clad layer selected from the group consisting of AA7xxx- and AAIxxx-series alloys.

18. Aluminium alloy product according to any one of claims 1 to 15, wherein the wrought product is in an extruded form.

19. Aluminium alloy product according to any one of claims 1 to 18, wherein the alloy product has been tempered to a T6-type condition.

20. Aluminium alloy product according to any one of claims 1 to 19, wherein the alloy product is an airplane fuselage part selected from the group consisting of fuselage skin, extruded stringers and combinations thereof welded together by laser and/or mechanical welding.