AA2000-SERIES ALUMINIUM ALLOY PRODUCTS AND A METHOD OF MANUFACTURING THEREOF

FIELD OF THE INVENTION This invention relates to an AA2000-series alloy comprising 2 to 5.5% Cu ₁ 0.5 to 2% Mg, at most 1 % Mn ₁ Fe <0.25%, Si > 0.10 to 0.35%, and to a method of manufacturing these aluminium alloy products. More particularly, the invention relates to aluminium wrought products in relatively thick gauges, i.e. about 30 to 300 mm thick. While typically practiced on rolled plate product forms, this invention may also find use with manufacturing extrusions or forged product shapes. Representative structural component parts made from the alloy product include integral spar members and the like which are machined from thick wrought sections, including rolled plate. This invention is particularly suitable for manufacturing high strength extrusions and forged aircraft components. Such aircraft include commercial passenger jetliners, cargo planes and certain military planes. In addition, non- aerospace parts like various thick mould plates or tooling plates may be made according to this invention.

BACKGROUND TO THE INVENTION As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2006.

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

Different types of aluminium alloys have been used in the past for forming a variety of products for structural applications in the aerospace industry. Designers and manufacturers in the aerospace industry are constantly trying to improve fuel efficiency, product performance and constantly trying to reduce the manufacturing and service costs. The preferred method for achieving the improvements, together with the cost reduction, is the uni-alloy concept, i.e. one aluminium alloy that is capable of having improved property balance in the relevant product forms.

State of the art at this moment is high damage tolerant AA2x24 (i.e. AA2524) or AA6x13 or AA7x75 for fuselage sheet, AA2324 or AA7x75 for lower wing, AA7055 or AA7449 for upper wing and AA7050 or AA7010 or AA7040 or AA7140 for wing spars and ribs or other sections machined from thick plate. The main reason for using

different alloys for each different application is the difference in the property balance for optimum performance of the whole structural part.

For fuselage skin, damage tolerant properties under tensile loading are considered to be very important, that is a combination of fatigue crack growth rate ("FCGR"), plane stress fracture toughness and corrosion. Based on these property requirements, high damage tolerant AA2x24-T351 (see e.g. US-5,213,639 or EP- 1026270-A1) or Cu containing AA6xxx-T6 (see e.g. US-4,589,932, US-5,888,320, US-2002/0039664-A1 or EP-1143027-A1) would be the preferred choice of civilian aircraft manufactures. For lower wing skin a similar property balance is desired, but some toughness is allowably sacrificed for higher tensile strength. For this reason AA2x24 in the T39 or a T8x temper are considered to be logical choices (see e.g. US Patent No. 5,865,914, US Patent No. 5,593,516 or EP-1 114877-A1).

For upper wing, where compressive loading is more important than the tensile loading, the compressive strength, fatigue (SN-fatigue or life-time or FCGR) and fracture toughness are the most critical properties. Currently, the preferred choice would be AA7150, AA7055, AA7449 or AA7x75 (see e.g. US Patent No. 5,221 ,377, US Patent No. 5,865,911 , US Patent No. 5,560,789 or US Patent No. 5,312,498). These alloys have high compressive yield strength with at the moment acceptable corrosion resistance and fracture toughness, although aircraft designers would welcome improvements on these property combinations.

For thick sections having a thickness of more than 3 inch or parts machined from such thick sections, a uniform and reliable property balance through thickness is important. Currently, AA7050 or AA7010 or AA7040 (see US-6,027,582) or AA7085 (see e.g. US Patent Application Publication No. 2002/0121319-A1) are used for these types of applications. Reduced quench sensitivity, that is deterioration of properties through thickness with lower quenching speed or thicker products, is a major wish from the aircraft manufactures. Especially the properties in the ST- direction are a major concern of the designers and manufactures of structural parts. A better performance of the aircraft, i.e. reduced manufacturing cost and reduced operation cost, can be achieved by improving the property balance of the aluminium alloys used in the structural part and preferably using only one type of alloy to reduce the cost of the alloy and to reduce the cost in the recycling of aluminium scrap and waste. Accordingly, it is believed that there is a demand for an aluminium alloy capable of achieving the improved proper property balance in almost every relevant product form.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide AA2000-series alloys having improved property balance. It is another object of the present invention to provide a wrought aluminium alloy product of an AA2000-series alloy comprising 2 to 5.5% Cu, 0.5 to 2% Mg, at most 1% Mn, Fe <0.25%, Si >0.10 to 0.35%, having improved properties, in particular having improved fracture toughness.

It is another object of the present invention to provide an AA2x24-series alloy having an improved property balance.

It is another object of the present invention to provide a method of manufacturing such AA2000-sehes alloy products.

These and other objects and further advantages are met or exceeded by the present invention method of manufacturing a wrought aluminium alloy product of an AA2000-series alloy, the method comprising the steps of: a. casting stock of an ingot of an AA2000-series aluminium alloy comprising 2 to 5.5% Cu ₁ 0.5 to 2% Mg, at most 1% Mn, Fe <0.25%, Si >0.10 to 0.35%, b. preheating and/or homogenizing the cast stock; c. hot working the stock by one or more methods selected from the group consisting of rolling, extrusion, and forging; d. optionally cold working the hot worked stock; e. solution heat treating (SHT) of the hot worked and optionally cold worked stock at a temperature and time sufficient to place into solid solution the soluble constituents in the aluminium alloy; f. cooling the SHT stock, preferably by one of spray quenching or immersion quenching in water or other quenching media; g. optionally stretching or compressing the cooled SHT stock or otherwise cold working the cooled SHT stock to relieve stresses, for example levelling or drawing or cold rolling of the cooled SHT stock; h. ageing of the cooled and optionally stretched or compressed or otherwise cold worked SHT stock to achieve a desired temper.

According to this invention there is at least one heat treatment carried out at a temperature in a range of more than 505 ⁰ C but lower than the solidus temperature of the subject aluminium alloy, and wherein this heat treatment is carried out either: (i) after the homogenisation heat treatment but prior to hot working, or (ii) after the solution heat treatment of step e.), or (iii) both after the homogenisation heat

treatment but prior to hot working and also after the solution heat treatment of step e.).

The aluminium alloy can be provided as an ingot or slab or billet for fabrication into a suitable wrought product by casting techniques regular 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 casters, also may be used, which in particular may be advantageous when producing thinner gauge end products. Grain refiners such as those containing titanium and boron, or titanium and carbon, may also be used as is well-known in the art. After casting the alloy stock, the ingot is commonly scalped to remove segregation zones near the cast surface of the ingot.

It is known in the art that the purpose of a homogenisation heat treatment has the following objectives: (i) to dissolve as much as possible coarse soluble phases formed during solidification, and (ii) to reduce concentration gradients to facilitate the dissolution step. A preheat treatment achieves also some of these objectives. A typical preheat treatment for AA2x24-series alloys would be a temperature of 420 to 500 ⁰ C with a soaking time in the range of 3 to 50 hours, more typically for 3 to 20 hours.

Firstly, the soluble eutectic phases such as the S-phase in the alloy stock are dissolved using regular industry practice. This is typically carried out by heating the stock to a temperature of less than 500°C as the S-phase eutectic phase (AI ₂ MgCu- phase) has a melting temperature of about 507°C in AA2x24-series alloys. In AA2x24-series alloys there is also a ©-phase having a melting point of about 51O ⁰ C. As is known in the art this can be achieved by a homogenisation treatment in said temperature range and allowing the stock to cool to the hot working temperature, or after homogenisation the stock is subsequently cooled and reheated to hot working temperature. The regular homogenisation process can also be done in a two or more steps if desired, and which are typically carried out in a temperature range of 430 to 500 ⁰ C for AA2x24-series alloys. For example in a two step process, there is a first step between 457 and 463 ⁰ C ₁ and a second step between 470 and 493 ⁰ C, to optimise the dissolving process of the various phases depending on the exact alloy composition.

The soaking time at the homogenisation temperature according to industry practice is alloy dependent as is well known to the skilled person, and is commonly in the range of about 1 to 50 hours. The heat-up rates that can be applied are those which are regular in the art.

This is where the homogenisation practice according to the prior art stops. However, it is an important aspect of the present invention that after the regular

homogenisation practice where the alloy composition allows complete dissolution of soluble phases (eutectics) present from solidification at least one further heat treatment can be carried out at a temperature in a range of more than 500 ⁰ C but at a temperature lower than the solidus temperature of the subject alloy. For the AA2000-series alloys processed according to the invention the preferred temperature is in a range of >505 to 55O ⁰ C, preferably 505 to 54O ⁰ C, and more preferably 510 to 535 ⁰ C, and more preferably at least 515°C.

For the system the soaking time at this further heat treatment is from about 1 to up about 50 hours. A more practical soaking time would not be more than about 30 hours, and preferably not more than about 15 hours. A too long soaking time at too high a temperature may lead to an undesired coarsening of dispersoids adversely affecting the mechanical properties of the final alloy product.

The skilled person will immediately recognise that at least the following alternative homogenisation practices can be used, while achieving the same technical effect:

(a) regular homogenisation according to industry practice, wherein afterwards the temperature is further raised to carry out the additional step according to this invention, followed by cooling to hot working temperature, such as, for example, 47O ⁰ C. (b) as alternative (a), but wherein after the additional step according to this invention the stock is cooled, for example to ambient temperature, and subsequently reheated to hot working temperature.

(c) as alternative (a), but wherein between the heat treatment according to regular industry practice and the further heat treatment according to this invention the stock is being cooled, for example to below 15O ⁰ C or to ambient temperature,

(d) a practice wherein between the various steps (regular practice, heat treatment according to invention, and heating to hot working temperature) the stock is cooled, for example to below 15O ⁰ C or to ambient temperature, where after it is reheated to the relevant temperature. In the alternatives wherein following the heat treatment according to this invention the stock is firstly cooled to, for example, ambient temperature prior to reheating for hot working, preferably a fast cooling rate is used to prevent or at least minimise uncontrolled precipitation of various secondary phases, e.g. AI ₂ CuMg or AI ₂ Cu. Following the preheat and/or homogenisation practice according to this invention the stock can be hot worked by one or more methods selected from the

group consisting of rolling, extrusion, and forging, preferably using regular industry practice. The method of hot rolling is preferred for the present invention.

The hot working, and hot rolling in particular, may be performed to a final gauge, e.g. 3 mm or less or alternatively thick gauge products. Alternatively, the hot working step can be performed to provide stock at intermediate gauge, typical sheet or thin plate. Thereafter, this stock at intermediate gauge can be cold worked, e.g. by means of rolling, to a final gauge. Depending on the alloy composition and the amount of cold work an intermediate anneal may be used before or during the cold working operation. In an embodiment of the method according to this invention following the regular practice of SHT and fast cooling for the subject aluminium alloy product, the stock is subjected to the further heat treatment according to this invention, one may designate this as a second SHT, at a higher temperature than the first regular SHT, wherein afterwards the stock is rapidly cooled to avoid undesirable precipitation out of various phases. Between the first and second SHT the stock can be rapidly cooled according to regular practice, or alternatively the stock is ramped up in temperature from the first to the second SHT and after a sufficient soaking time it is subsequently rapidly cooled. This second SHT is to further enhance the properties in the alloy products and is preferably carried out in the same temperature range and time range as the homogenisation treatment according to this invention as set out in this description, together with the preferred narrower ranges. However, it is believed that also shorter soaking times can still be very useful, for example in the range of about 2 to 180 minutes. This further heat treatment may dissolve as much as practically possible any of the Mg ₂ Si phases which may have precipitated out during cooling for the homogenisation treatment or the during a hot working operation or any other intermediate thermal treatment. The solution heat treatment is typically carried out in a batch furnace, but can also be carried out in a continuous fashion. After solution heat treatment, it is important that the aluminium alloy be cooled to a temperature of 175°C or lower, preferably to ambient temperature, to prevent or minimise the uncontrolled precipitation of secondary phases, e.g. AI ₂ CuMg and AI ₂ Cu. On the other hand cooling rates should preferably not be too high in order to allow for a sufficient flatness and low level of residual stresses in the product. Suitable cooling rates can be achieved with the use of water, e.g. water immersion or water jets.

Yet, in a further embodiment of this invention the defined AA2000-series alloy products are processed using regular homogenisation and/or preheat practice, and wherein afterwards the products are processed using the preferred SHT as set out

above, thus regular SHT followed by the second solution heat treatment in the defined temperature and time range, together with the preferred narrower ranges This will result in the same advantages in product properties It is possible to carry out the first regular SHT followed by rapid cooling and reheating to the soaking temperature of the second SHT, alternatively the temperature is ramped up from the first to the second SHT and after a sufficient soaking time it is subsequently rapidly cooled

The stock may be further cold worked, for example, by stretching in the range of about 0 5 to 10 % of its original length to relieve residual stresses therein and to improve the flatness of the product Preferably the stretching is in the range of about 0 5 to 6%, more preferably of about 0 5 to 5% The stock can for example also be cold rolled with a rolling degree of for example 8 to 13%

After cooling the stock is aged, typically at ambient temperatures, and/or alternatively the stock can be artificially aged The artificial ageing can be of particular use for higher gauge products Depending on the alloy system this ageing can de done by natural ageing, typically at ambient temperatures, or alternatively by means of artificially ageing All ageing practices known in the art and those which may be subsequently developed can be applied to the AA2000-seπes alloy products obtained by the method according to this invention to develop the required strength and other engineering properties Typical tempers would be for example T4, T3, T351 , T39, T6, T651 , T8, T851 , and T89

A desired structural shape is then machined from these heat treated plate sections, more often generally after artificial ageing, for example, an integral wing spar SHT, quench, optional stress relief operations and artificial ageing are also followed in the manufacture of thick sections made by extrusion and/or forged processing steps

The effect of the heat treatment according to this invention is that the damage tolerance properties are improved of the alloy product compared to the same aluminium alloy having also high Si content but processed without this practice according to the present invention In particular an improvement can be found in one or more of the following properties the fracture toughness, the fracture toughness in S-L orientation, the fracture toughness in S-T orientation, the elongation at fracture, the elongation at fracture in ST orientation, the fatigue properties, in particular FCGR, S-N fatigue or axial fatigue, the corrosion resistance, in particular exfoliation corrosion resistance, or SCC or IGC It has been shown that there is a significant enhancement in mechanical properties of as much as 15%

In addition, similar enhanced properties are achieved, or at least not adversely affected, with the aluminium alloy products according to this invention and preferably processed according to this invention compared to the same alloy composition but having the regular low Si content and processed according to regular industry practice. This would allow the manufacturing of aluminium alloy product having similar or equivalent properties compared to the low Si alloys, but in a more cost effective manner as source material having a low Si-content is more expensive.

The following explanation for the surprisingly improved properties of the wrought product of this invention is put forward, with the caveat that it is merely an expression of belief and does not presently have complete experimental support.

The prior art refers to the Mg ₂ Si constituent phase as being insoluble in AA2000-series aluminium alloys and these particles are known fatigue initiation sites. In particular for aerospace applications, the prior art indicates that the Fe and Si content need to be controlled to very low levels to provide products with improved damage tolerant properties such as Fatigue Crack Growth Rate resistance ("FCGR") and fracture toughness. From various prior art documents it clear that the Si content is treated as an impurity and should be kept at a level a low as reasonably possible. For example US-2002/0121319-A1 , incorporated herein by reference, discusses for an AA7000-series alloy the impact of these impurities on the alloying additions and states that Si will tie up some Mg thereby leaving an "Effective Mg" content available for solution, it is suggested that this be remedied by additional additions of Mg to compensate for the Mg tied up with the Mg ₂ Si, see section [0030] of US- 2002/0121319-A1. However, at no point it is suggested that the Mg ₂ Si could be reintroduced into solution by a controlled heat treatment practice. With regard to the homogenisation practice it is mentioned that homogenisation may be conducted in a number of controlled steps but ultimately state that a preferred combined total volume fraction of soluble and insoluble constituents be kept low, preferably below 1% volume, see section [0102] of US-2002/0121319-A1. Within the examples, times and temperatures of heat treatments are given but at no point are the temperatures or times disclosed adequate in attempting the dissolution of Mg ₂ Si constituent particles, i.e. homogenisation temperature of up to 900 ⁰ F (482 ⁰ C) and solution treatment temperature of up to 900°F (482°C).

And for example US-6,444,058, incorporated herein by reference, discusses for an AA2x24-series alloy that in order to improve on plane strain and plane stress toughness, fatigue resistance, or fatigue crack growth resistance that the second phase particles derived from Fe and Si and those derived from Cu and/or Mg are substantially eliminated by composition control and heat treatment. To that effect the

Si content should be no greater than 0 05%, and the heat treating temperature should be controlled at as high a temperature as possible while still being safely below the lowest incipient melting temperature of the alloy, which is about 935°F (502 ⁰ C), see e g column 2, lines 35 to 52 However, it has been found in accordance with the invention that for various

AA2000-seπes aluminium alloys, the generally perceived constituent phase Mg ₂ Si is soluble via carefully controlled heat treatment and if they cannot be taken in complete solution then their morphology can be spherodised in such a way that fatigue and/or fracture toughness properties are improved Once in solid solution, most of the Si and/or Mg will be available for subsequent ageing that may further enhance mechanical and corrosion properties By deliberately increasing the Si content in the alloys according to this invention more of this Si is available for subsequent ageing practices but without having the detrimental coarse Mg ₂ Si phases in the final product The gained improvements by the purposive addition of Si could also be sacrificed to some extent by making the alloy composition leaner in Mg and/or Cu thus improving the toughness of the alloy product Thus the generally perceived detrimental impurity element Si is now being converted into a purposive alloying element having various advantageous technical effects

For the AA2000-serιes alloys the upper limit for the Si content is about 0 35%, and preferably of about 0 25%, as a too high Si content may result in the formation of too coarse Mg ₂ Si phases which cannot be taken in complete solid solution and thereby adversely affecting the property improvements gained The lower limit for the Si-content is >0 10% A more preferred lower limit for the Si-content is about 0 15%, and more preferably about 0 17% A wrought AA2000-serιes aluminium alloy that can be processes favourably according to this invention, comprises, in wt % Cu about 2 to 5 5%

Mg about 0 5 to 2% Mn at most 1 % Zn < 1 3%

Fe < 0 25%, preferably <0 15%

Si >0 10 to 0 35%, preferably >0 10 to 0 25%, more preferably about

0 15 to 0 25%, and optionally one or more elements selected from the group consisting of Zr about 0 02 to 0 4%, preferably 0 04 to 0 25%

Ti about 0 01 to 0 2% V about 0 01 to 0 5%

Hf about 0.01 to 0.4%

Cr about 0. 01 to 0. 25%

Ag at most 1%

Sc about 0. 01 to 0. 5%, balance being Al, incidental elements and impurities. Typically such impurities are present each <0.05%, total <0.15%.

Compared with the prior art, the alloy according to this invention has a high silicon content in the alloy composition, wherein the Si content is more than 0.10% and having a maximum of 0.35%. The rise in Si content has amongst others the advantage of improving the castability of the alloy.

In an embodiment of the AA2000-sehes alloy processed according to the invention the Cu content has a preferred lower limit of about 3.6%, and more preferably of about 3.8%. A preferred upper limit is of about 4.5%, and more preferably of 4%. In an embodiment of the AA2000-series alloy processed according to the invention the Mg content has a preferred upper limit of 1.5%. In a more preferred embodiment the Mg is in a range of 1.1 to 1.3%.

The Mn content in the alloy according to the invention is preferably in a range of 0.1 to 0.9%, and more preferably in a range of 0.2 to 0.8%. In an embodiment of the AA2000-sehes alloy processed according to this invention the Zn is present as an impurity element which can be tolerated to a level of at most about 0.3%, and preferably at most about 0.20%.

In another embodiment of the AA2000-series alloy processed according to this invention the Zn is purposively added to improve the damage tolerance properties of the alloy product. In this embodiment the Zn is typically present in a range of about 0.3 to 1.3%, and more preferably in a range of 0.45 to 1.1 %.

If added, the Ag addition should not exceed 1.0%, and a preferred lower limit is

0.05%, more preferably about 0.1%. A preferred range for the Ag addition is about

0.20-0.8%. A more suitable range for the Ag addition is in the range of about 0.20 to 0.60%, and more preferably of about 0.25 to 0.50%, and most preferably in a range of about 0.3 to 0.48%.

In the embodiment where Ag it is not purposively added it is preferably kept at a low level of preferably <0.02%, more preferably <0.01%.

Zr can be added as dispersoid forming element, and is preferably added in a range of 0.02 to 0.4%, and more preferably in a range of 0.04 to 0.25%.

In another preferred embodiment of the invention the alloy has no deliberate addition of Cr and Zr as dispersoid forming elements. In practical terms this would

mean that each of the Cr and Zr are at regular impurity levels of <0.05%, and preferably <0.03%, and more preferably the alloy is essentially free or substantially free from Cr and Zr. 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. In particular for thicker gauge products (e.g. more than 3 mm) the Cr ties up some of the Mg to form AI ₁₂ Mg ₂ Cr particles which adversely affect quench sensitivity of the wrought alloy product, and may form coarse particles at the grain boundaries thereby adversely affecting the damage tolerance properties. As dispersoid forming element it has been found that Zr is not as potent as Mn is in AA2x24-type aluminium alloys.

The Fe content for the alloy should be less than 0.25%. When the alloy product processed according to the invention is used for aerospace application preferably the lower-end of this range is preferred, e.g. less than about 0.10%, and more preferably less than about 0.08% to maintain in particular the toughness at a sufficiently high level. Where the alloy product is used for tooling plate application, a higher Fe content can be tolerated. However, it is believed that also for aerospace application a moderate Fe content, for example about 0.09 to 0.13%, or even about 0.10 to 0.15%, can be used. Although the skilled person would believe that this has an adverse effect on the toughness of the product, some of this loss in properties, if not all, is gained back when using the method according to this invention. The resultant would be an alloy product, although having moderate Fe levels, but when processed according to this invention it has properties equivalent to the same alloy product safe to a lower Fe content, e.g. 0.05 or 0.07%, when processed using regular practice.

Thus similar properties are achieved at higher Fe-levels, which has a significant cost advantage as source material having very low Fe-contents is expensive.

In another preferred embodiment of the invention the AA2000-series alloy that can be processed favourably according to this invention has a composition, consisting of, in wt.%:

Cu 3.6 to 4.4, preferably 3.8 to 4.4

Mg 1.2 to 1.8

Mn 0.3 to 0.8

Cr max. 0.10, preferably max. 0.05 Zr max. 0.05, preferably max. 0.03

Zn max. 0.25

Fe max. 0.12, preferably max. 0.08

Si > 0.10 to 0.35, and preferably >0.10 to 0.25,

Ti max. 0.15, preferably max. 0.10 balance aluminium and incidental elements and impurities. Typically such impurities are present each <0.05%, total <0.15%. This alloy composition embraces the AA2324 alloy (registered in 1978).

In another preferred embodiment of the invention the AA2000-series alloy that can be processed favourably according to this invention has a composition consisting of the AA2524 alloy (registered in 1995), but with the proviso that the Si is in the range of >0.10 to 0.35%, or an above-described preferred narrower range of the present invention. The composition ranges for the AA2524 alloy is, in wt.%:

Cu 4.0 - 4.5

Mn 0.45 - 0.7

Mg 1.2 - 1.6

Cr max. 0.05 Zn max. 0.15

Ti max. 0.1

Si max. 0.06

Fe max. 0.12, incidental elements and impurities each <0.05, total <0.15, balance aluminium. The AA2000-series alloy products manufactured according to this invention may be provided with a cladding. Such clad products utilise a core of the aluminium base alloy of the invention and a cladding of usually higher purity which in particular corrosion protects the core. The cladding includes, but is not limited to, essentially unalloyed aluminium or aluminium containing not more than 0.1 or 1 % of all other elements. Aluminium alloys herein designated AA1xxx-type series include all

Aluminium Association (AA) alloys, including the sub-classes of the 1000-type, 1100- type, 1200-type and 1300-type. Thus, the cladding on the core may be selected from various Aluminium Association alloys such as 1060, 1045, 1050, 1100, 1200, 1230,

1 135, 1235, 1435, 1145, 1345, 1250, 1350, 1170, 1175, 1 180, 1 185, 1285, 1188, or 1 199. In addition, alloys of the AA7000-series alloys, such as 7072 containing zinc

(0.8 to 1.3%) or having about 0.3 to 0.7% Zn, can serve as the cladding and alloys of the AA6000-series alloys, such as 6003 or 6253, which contain typically more than

1 % of alloying additions, can serve as cladding. Other alloys could also be useful as cladding as long as they provide in particular sufficient overall corrosion protection to the core alloy. The clad layer or layers are usually much thinner than the core, each constituting about 1 to 15 or 20 or possibly about 25% of the total composite

thickness. A cladding layer more typically constitutes around 1 to 12% of the total composite thickness.

The AA2000-series alloy product processed according to this invention can be used amongst others in the thickness range of at most 0.5 inch (12.5 mm), the properties will be excellent for fuselage sheet. In the thin plate thickness range of 0.7 to 3 inch (17.7 to 76 mm) the properties will be excellent for wing plate, e.g. lower wing plate. The thin plate thickness range can be used also for stringers or to form an integral wing panel and stringer for use in an aircraft wing structure. When processed to thicker gauges of more than 2.5 inch (63 mm) to about 11 inch (280 mm) excellent properties have been obtained for integral part machined from plates, or to form an integral spar for use in an aircraft wing structure, or in the form of a rib for use in an aircraft wing structure. The thicker gauge products can be used also as tooling plate, e.g. moulds for manufacturing formed plastic products, for example via die-casting or injection moulding. The alloy products processed according to the invention can also be provided in the form of a stepped extrusion or extruded spar for use in an aircraft structure, or in the form of a forged spar for use in an aircraft wing structure.

In the following, the invention will be explained by the following, non-limitative example.

Example

On a pilot scale of testing a billet have been DC-cast having a diameter of 250 mm and a length of over 850 mm. The alloy composition is listed in Table 1 , and whereby it is noticed that alloy 3 has an Fe content slightly higher than what is currently customary for aerospace grade rolled products. Alloy 3 would be a typical example of the AA2324 series alloy, save to the higher Si and Fe contents. The alloy composition would also fall within the known compositional ranges of AA2524, save for the higher Si content. From the billet two rolling blocks have been machined having dimensions of 150x150x300 mm. By following this route blocks with an identical chemistry were obtained making it easier to fairly assess the influence of the heat treatments at a later stage on the properties. The blocks were all homogenised using the same cycles of 25 hours at 490 ⁰ C whereby industrial heat up rates and cooling rates were applied. Depending on the block a further homogenisation treatment according to the invention was applied whereby the furnace temperature is further increased and where after a second heat treatment or homogenisation treatment of 5 hours at 515°C was applied. Following the homogenisation the blocks were cooled to room temperature. Thereafter all the blocks were preheated for 5 hours at 460 ⁰ C in one batch and hot rolled from 150 to 40 mm. The entrance

temperatures (surface measurements) were in the range of 450 to 460°C and mill exit temperatures varied in the range of 390 to 400 ^c C. After hot rolling the plates received a one or two step solution heat treatment followed by a cold water quench. One further comparative sample (Sample 1A3) was processed using a more common SHT practice of 4 hours at 495°C. All the plates were naturally aged for 5 days to T4 temper. The plates were not stretched prior to ageing. All heat treatments are summarised in Table 2 .

The average mechanical properties according to ASTM-B557 standard over 2 samples of the 40 mm plates produced with the various heat treatments are listed in Table 3 and wherein "TYS" stands for Tensile Yield Strength in MPa, UTS for Ultimate Tensile Strength in MPa, and "Kq" for the qualitative fracture toughness in MPa. Vm. The fracture toughness has been measured in accordance with ASTM B645. All testing was done at 1/2T.

From the results of Table 3 with respect to the mechanical properties the following can be seen:

The plate produces via a standard processing (Sample 1A3) has generally the lowest set of properties. The other samples exhibit better properties when using higher processing temperatures, especially the toughness is improved with on average 10%. Further improvements, in particular in toughness, can be made by lowering the Fe content to standard aerospace levels of <0.05%.

The current set of obtained properties despite the high Si and relatively high Fe levels, and especially sample 1A2 and 1 B2, meet the Airbus specification AIMS 03-

02-020, Issue 3, February 2002, for 2024/2xxx T351 plate (incorporated herein by reference) even though the plates processed according the invention have a relatively high Fe levels and are in a T4 temper.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.