WO2021003528A1

WO2021003528A1 - Aluminium alloys

Info

Publication number: WO2021003528A1
Application number: PCT/AU2020/050708
Authority: WO
Inventors: Thomas Henri DORIN; Alireza VAHID; Santu RANA; Sunil Gupta
Original assignee: Deakin University
Priority date: 2019-07-10
Filing date: 2020-07-06
Publication date: 2021-01-14

Abstract

A heat-treated aluminium based alloy comprising the elements: i. up to 1 wt.% lithium; ii. 0.02 to 0.3 wt.% scandium; iii. 0.4 to 3 wt.% copper; iv. 1.5 to 5 wt.% magnesium; v. 6 to 12 wt.% zinc; vi. 0.05 to 0.4 wt.% zirconium; vii. Up to 0.25 wt.% manganese; viii. Up to 0.25 wt.% chromium; ix. Up to 0.2 wt.% titanium; x. Up to 1 wt.% iron; xi. Up to 1 wt.% silicon; xii. remainder aluminium

Description

Aluminium alloys

Field of the Invention

[0001 ] The present invention relates generally to aluminium based alloys. The invention particularly relates to high strength heat-treated aluminium alloys where the properties have been optimized by controlling the composition and heat-treatment parameters. The invention further relates to processes for optimising the characteristics of the aluminium alloy through control of the manufacturing process. The invention particularly relates to process where the duration of heat treatment steps and temperature are controlled to allow for precipitation of particular dispersoids and precipitates to be formed.

Background of the Invention

[0002] Aluminium alloys have generally been developed as they demonstrate excellent mechanical and physical properties that make them useful for many applications. One particular advantage of aluminium alloys is that they are relatively low density, and as such they are useful for lightweight applications yet still demonstrating significant strength characteristics, The aircraft and automotive industries utilize aluminium alloys extensively to take advantage of the light weight and strength characteristics and many alloys have been specifically developed for those purposes.

[0003] Alloys may be categorized into 2 principle classifications, namely cast alloys and wrought alloys. Cast aluminium alloys generally have lower tensile strength than those of wrought alloys and generally have relatively high levels of silicon which contribute to good casting characteristics.

[0004] Wrought aluminium alloys vary in the alloying material used to provide different characteristics to the alloy. The alloys are generally classified by a numbering system which has been developed by the Aluminium Alloy Association of America. This numbering system may indicate the main alloying constituents of the alloy which leads to an indication of its properties.

[0005] The wrought alloy numbering system includes various series from the 1xxx series through to the 8xxx series. The 2xxx series for example have higher levels of copper and are precipitation hardened but have been susceptible to cracking. They have been increasingly replaced in the aircraft industry in particular by the 7xxx series which include magnesium and greater levels of zinc. The 8xxxx series for example is inclusive of aluminium-lithium alloys.

[0006] Wrought aluminium alloys are generally used in shaping processes, and may be shaped for example through rolling, forging, extrusion, pressing or stamping. There are two principle groups namely non-heat treated alloys and heat-treated alloys. The initial strength for non-heat treated alloys is achieved through a hardening process of the alloying elements in solid solution such as manganese, silicon and magnesium. Alloys such as the 1xxx, 4xxx and 5xxx series are examples of non-heat treated aluminium alloys.

[0007] Heat-treatable alloys on the other hand gain initial strength through the hardening effect of the alloying elements, for example, copper, silicon, magnesium and zinc. The solubility of these elements in solid aluminium depends on the temperature, so it is possible to harden the alloys from this group by heat treatment. The heated composition is allowed to cool and is aged which contributes to the hardening process through the formation of metastable precipitates. The process may also be called precipitation hardening or age hardening.

[0008] Precipitation hardening achieves strengthening by precipitation of fine particles of a super saturated solid solution.

[0009] One of the main applications of high strength heat-treated aluminium alloys such as the 2xxx, 6xxx or 7xxx series is in aerospace as it allows for a light weight yet a strong metallic component. Alloys having a yield strength of 800 MPa have been developed in the 7xxxx series. Alloys have been used in both the aerospace and the automotive industry.

[0010] Various elements are known to provide certain characteristics for aluminium alloys. For example, titanium forms a solidified grain structure that is finer than may be observed with other grain structures which may lead to improved fabricating capabilities.

[0011 ] Scandium is another element that has been used to strengthen aluminium alloys without increasing the density. Lithium may be included to decrease density, and maintain or improve strength.

[0012] Generally, the aluminium alloys may be developed to vary in relation to its tensile strength, density, ductility, formability, weldability and corrosion resistance amongst other properties. The selection of alloying elements is generally selected in order to maximize the particular property of the aluminium alloy that is desired, together with the consideration of ease of manufacture, cost and durability. The ratios between various properties are also a key factor as, for example, the ratio between tensile strength and density influences strength to weight capabilities.

[0013] The development of aluminium alloys is generally conducted by intense experimentation and trial and error approaches in order to develop the appropriate characteristics including yield strength, density, ductility and other characteristics. The development of alloys becomes extremely time-consuming particularly for heat-treated alloys given the difficulties with the multiple heat treatment steps involved, the high temperatures involved in forming such alloys and the variability in the properties achieved. A large number of experiments are required to identify the optimal solution to achieve the desired characteristics.

[0014] US patent 7060139 to Senkov et. al. aims to develop an aluminium alloy having improved strength and ductility. It discusses the benefit of the addition of scandium and high levels of zinc which the Inventors have found can improve the strength, particularly in alloys that are to be used at cryogenic temperatures, that is temperatures around -190 ºC.

[0015] US patent 5137606 looks at aluminium alloys that include lithium as one of the alloying minerals. The alloys of this disclosure demonstrate a decrease in density and an improvement in tensile and yield strengths. The alloys referred to in this patent include relatively low levels of zinc and higher levels of copper which are typical levels used in the 2xxx series of aluminium alloys. The presence of copper and zinc in those alloys at levels to achieve the desired characteristics of a 2xxx series of aluminium alloys can interfere with fracture toughness of the alloy.

[0016] The present invention has been developed in order to overcome or at least alleviate one or more of the difficulties associated with known practices and the prior art.

[0017] The present invention has been further developed to provide a method for optimizing the characteristics for heat-treated aluminium alloys.

[0018] A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[0019] Where any or all of the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims), they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components. Summary of the Invention

[0020] The present invention is particularly applicable to aluminium alloys that have been strengthened by heat treatment. In particular, the present invention resides in both a heat-treated aluminium alloy and a process for manufacturing that aluminium alloy.

[0021 ] The present invention resides in a heat-treated aluminium based alloy that is high strength and has low density characteristics that makes it suitable for use in areas where high strength but light-weight materials are needed. In a first embodiment, the present invention resides in a heat-treated aluminium alloy comprising: i. up to 1 wt.% lithium;

i i 8.02 to 0.3 wt.% scandium;

i ii. 0.4 to 3 wt.% copper;

v. 1 .5 to 5 wt.% magnesium;

v. 6 to 12 wt.% zinc;

vi. 0.05 to 0.4 wt.% zirconium;

vii. up to 0.25 wt.% manganese;

viii. up to 0.25 wt.% chromium;

ix. up to 0.2 wt.% titanium;

x. up to 1 wt.% iron;

xi. up to 1 wt.% silicon;

xii. remainder aluminium

[0022] In a preferred embodiment, the source of the aluminium may be from any suitable source. This may include fresh aluminium or recycled aluminium. A suitable source of recycled aluminium may for example be recycled 7xxx series aluminium, such as that which may be available from decommissioned aircraft. A preferred aluminium source would be a recycled 7050 or 7075 series aluminium which has the base elements of the alloy of the present application except for scandium, zirconium and lithium. A combination of recycled and fresh aluminium may also be used. [0023] The 7xxx series aluminium alloys have greater levels of zinc in comparison with the 2xxx series of aluminium alloys. The 2xxx series have greater levels of copper. Both alloys have been used extensively in the aeronautical and automotive industries. The alloys of the present application have greater levels of zinc and are therefore preferably aimed at recycling of 7xxx series alloys if recycled alloys are to be used, but if the final composition of elements is adjusted, the composition and process may be applied to the recycling of a range of aluminium alloys.

[0024] In a further embodiment of the invention the present invention resides in a process for the manufacture of the heat-treated aluminium alloy of the present invention. Since solubility of the elements, or precipitates formed from the elements used to form the aluminium alloy depend on the temperature, it is possible to harden the alloys from this group by heat treatment which is also known as precipitation hardening during an aging process after extrusion.

Precipitation hardening heat treatment processes generally involve the following stages:

[0025] Casting: The process of the present application generally includes a casting step where the elements are heated and melted at a temperature of at least 720 ºC in an induction furnace. This composition may be poured into a cylindrical mould or direct chill cast into cylindrical billets cooled and then subjected to a number of heat-treatment steps together with a forming step, a solution treatment step and aging step.

[0026] Homogenisation: Accordingly, in a further embodiment of the invention, there is provided an alloy wherein the alloy is homogenised during a heat-treatment process where Al₃Sc and Al₃Zr dispersoids are formed. A multi-step homogenisation treatment is preferred as it leads to the formation of an AI₃Sc/AI₃Zr core-shell dispersoids.

[0027] Forming: The developed alloy is primarily for extruded products but other forming processes can be used such as rolling and forging. The billet may be preheated to a given temperature and then extruded at a suitable strain rate and container temperature so that the temperature does not exceed the solidus temperature when exiting the extrusion press. The extruded product may be press-quenched via air/water spray or by exiting into a coolant bath.

[0028] Solution treatment: The alloy may be heated in one or more further heat- treatment steps to a temperature to dissolve the alloying element into a super saturated solid solution. The dissolved solid solution is held at a temperature which may vary from 1 hour to 24 hours to complete the dissolving. One or more heat-treatment steps may be applied with comparable varying duration. The temperature and the duration for which each heat-treatment step is held should be such as to not prevent excessive growth of the grains and of the dispersoids.

[0029] Quenching: Solution treatment is normally followed by quenching generally with water, although a water and air mixture or sometimes just air may be used. This leads to a super saturation of a solid solution at room temperature. The quenching phase is not necessary and results can be achieved without a quenching step, but quenching or cooling can lead to improving the hardness of the alloy and is generally employed.

[0030] Stretching: depending on the nature of the extruded profile, stretching might be conducted after quenching in order to straighten the extrusion. This step also provides work hardening and might generate dislocations that can help nucleating the strengthening precipitates.

[0031 ] Aging: Following the solution treatment and quenching, an aging process takes place. Desired precipitates form during the aging process. Aging may be either natural aging at essentially room temperature, or artificial aging where the temperature of the composition is elevated, or a combination of both. The aging process in the present application is controlled such that the present application includes heating the solid composition to a desired temperature to achieve precipitation of desired precipitates. A combination of the following precipitates, or corresponding precursors, is expected to form, AI₂CuU, Al₃Li, Al₂Cu , Al₂CuMg and Mg₂Zn. These precipitates might be enhanced by preferred nucleation on the Al₃Sc/Al₃Zr core-shell dispersoid. The aging step is optimised to provide a distribution of fine precipitates that provide most of the strengthening.

[0032] In a further embodiment, the invention resides in a process for producing a heat-treated aluminium alloy, said process including the steps of: i) melting the elements of the preferred composition as outlined above and cast into billets.

ii) raising the temperature of the composition in a first heat-treatment step to between 250ºC to 350ºC, preferably between 275ºC to 300ºC to form Al₂Sc precipitates;

iii) raising the temperature in a second heat-treatment step to between 420ºC to 470ºC; preferably between 430ºC to 450ºC to form Al₃Zr precipitates which surrounds the Al₃Sc precipitates to form Al₃Sc/Al₃Zr core-shell dispersoids.

[0033] The elevated temperature for the first heat-treatment steps are preferably maintained for a period of greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 20.5 hours.

[0034] The elevated temperature for the second heat-treatment steps are preferably maintained for a period of greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 23.5 hours.

[0035] The process preferably includes a number of heat-treatment steps. In a further embodiment, the composition is subject to a third heat treatment step wherein the temperature is raised to between 440ºC to 490ºC , preferably from 445ºC to 480ºC to assist homogenisation and removal of any iron and silicon impurities.

[0036] Preferably, the elevated temperature of the third heat-treatment step is maintained for a period of greater than 6 hours, preferably from 6 to 25 hours, and more preferably 10 to 18.5 hours at the elevated temperature. [0037] The process may include an extrusion step where the composition is allowed to cool to room temperature and the billets are extruded at a temperature of from 430ºC to 455ºC. The exit temperature should n ot exceed 490 ºC.

[0038] The extruded billets are preferably subjected to a fourth heat-treatment step (solution treatment) wherein the temperature is then raised to between 460ºC to 490ºC, preferably between 470ºC to 480ºC to dissolve the m agnesium, manganese, zinc, copper and any lithium.

[0039] Preferably the elevated temperature of the fourth heat-treatment step is maintained for a period of greater than 6 hours, preferably from 6 to 25 hours, and more preferably from 13.5 to 22 hours.

[0040] The composition is then allowed to cool following the further heat-treatment step and undergo an aging process. The aging process preferably includes a natural aging step where the composition is left to age for a period of from 3 to 7 days, preferably 3 to 5 days. This is then followed by an artificial aging step where the temperature is raised to between 110 ºC and 150 ºC for a period of from 3 to 30 hours. The combination of both the natural and artificial aging steps allows for the formation of metastable precipitates, and corresponding precursors, such as Al₂CuLi, Al₃Li, Al₂Cu, and/or Al₂CuMg and Mg2Zn precipitates. The process may be carried out without the natural or artificial aging steps and may for example include only artificial aging.

[0041 ] The artificial aging process may be carried out as two separate steps where the composition is heated to a temperature of 110 °C to 150 ºC in a first artificial aging step, followed by cooling and then raising the temperature to 120 ºC to 180 ºC, more preferably 140 ºC to 160 ºC in a second artificial aging step to further assist the precipitation of metastable precipitates. Two steps can become favourable depending on the thermodynamic and kinetics of formation of the desired precipitates but generally only one artificial aging step may be used. [0042] Precipitation hardening or aging is the strengthening part of precipitation of fine particles of a second phase from a super saturated solid solution.

[0043] Aging may however be carried out either exclusively artificially, that is by heating to a temperature to assist the aging process, or naturally at room temperature which can take from several days to even several weeks. It is however preferred to have both a natural and artificial aging step.

[0044] Quenching also forms part of the process in achieving the desired properties for the alloy, and a quenching step is preferably included between one or more of the heat treatment steps, and the extrusion and aging phases, but preferably between each of the heat-treatment steps and phases. Quenching is not preferred in the heat treatment step preceding extrusion.

Detailed Description of the Preferred Embodiments

[0045] The present application relates to a heat-treated aluminium alloy composition and a process for producing that alloy. Aluminium alloys maybe classified in 8 main series depending upon the main alloying elements. Typical alloying elements for an aluminium alloy include copper, magnesium, zinc, zirconium and scandium, each of which can provide various characteristics to the aluminium alloy.

[0046] Most of the mechanical properties of aluminium alloys are dictated by the presence of nanometer sized precipitates. The aluminium alloys contain a number of refining elements which help nucleate grains during casting and allow the aluminium to form dispersoids. The dispersoids that may be formed are dependent upon the main alloying elements used.

[0047] The composition of the present application has been developed by the control of the formation of preferably three main precipitates and dispersoids providing the desired characteristics for the alloy. The applicants have found that the desired characteristics have been achieved with the formation of desired dispersoids through control of the homogenisation process and the precipitation of desired precipitates during the aging treatment. In a preferred embodiment, one of those precipitates is inclusive of lithium however an alloy having the desired strength and properties may be achieved without lithium being present. The applicants have found however that the inclusion of lithium provides for an alloy with improved strength and a decrease in the density of the alloy allowing for improved yield strength to weight ratio.

[0048] The 7 xxx series of aluminium alloys have been developed as strong and light-weight materials. They acquire their high strength through the formation of nanometer-size particles, known as precipitates and are generally rich in zinc and magnesium. The strength contribution from precipitates is strongly influenced by the heat-treatment process applied in developing the alloy. The process of producing such alloys is a heat treatment process where the precipitates are formed during heat- treatment sequences. They acquire strength through an age-hardening process.

[0049] The present application has been developed such that various precipitates are formed by control and optimization of the heat treatment process. The composition of the present application has been developed by optimizing the precipitates that are formed though the heat treatment process. Preferably, an AI₃Sc/AI₃Zr core-shell dispersoid is formed through the combination of Al₃Sc and Al₃Z particles. The scandium is able to provide strength to the aluminium alloy. This is enhanced with the added combination of zirconium in the spherical dispersoids formed. The AI₃Sc/AI₃Zr spherical dispersoids form with core-shell morphology with a scandium enriched core and a zirconium enriched shell. The scandium controls the nucleation process while zirconium creates a strong shield against coarsening which leads to high dispersoid density and great thermal stability of the dispersoids.

[0050] A further precipitate is an AI₂CuU or Al₃Li particle which is preferably formed in order to incorporate lithium into the alloy. With the inclusion of this precipitate, it has been found the strength of the alloy is increased and the density is decreased which significantly improves the strength to weight ratio. Alternatively an Al₂Cu and/or Al₂CuMg particle may be formed without the presence of lithium. [0051 ] A further precipitate is an Mg₂Zn particle which the applicants have found is the biggest contributor to the strength of the alloy.

[0052] The applicants have found that a high strength aluminium alloy having desirable characteristics may be produced by careful and appropriate selection of the process parameter in the production of the alloy. A preferred method is to use the Bayesian optimisation technique which together with ThermoCalc enables the inventor to optimize the processing conditions including the selection of the elements and temperature and duration of each heat-treatment step to achieve a high strength alloy having the desired characteristics. This technique reduces the need for trial and experimentation based on the multitude of options that are available to the inventors. Some trial and experimentation is still necessary but appropriate utilisation of the Bayesian optimisation technique has the benefit of reducing substantially the trial and experimentation requirements.

[0053] The first step in the process is to select the alloying elements which will be used to form the alloy. This will vary depending upon the base aluminium element to be used, for example, selection of the elements to be added is dependent upon the base aluminium product. In a preferred embodiment, the process of this application may use a recycled aluminium product as the base aluminium such as a recycled aluminium alloy. The 7xxx series aluminium alloys have appropriate zinc and magnesium levels for use in the alloy of this application. Fresh aluminium may also be used either on its own, or together with recycled aluminium alloy. Recycled 2xxx series aluminium which is rich in copper as an alloying element may also be used. Preferably, the applicants aim to achieve a modified 7xxx series aluminium alloy.

[0054] The elements however are selected based upon the properties of the final product to be achieved. The inventors in the present application aim to produce a high strength aluminium alloy with reduced density so as to produce a strong lightweight product that maybe suitable for such industries as the aeronautical or automotive industries. The preferred properties that are to be achieved include a yield strength of between 600 to 900 MPa, preferably between 630 to 800 MPa for the final extruded product at room temperature. Preferably the alloy also has total elongation ranges from greater than 2% at room temperature, preferably from 2% to 10% at room temperature, and more preferably from 4% to 10% at room temperature.

[0055] It is preferred that the composition of the alloy includes lithium as it has been found that lithium may contribute to reducing the density of the alloy product. It is however possible that lithium is not included yet still achieve the desired characteristics of the final aluminium alloy product.

[0056] In a first embodiment of the invention, the Applicants have found that a heat- treated aluminium alloy having the desired properties may be achieved with the selection of elements in the desired proportions.

[0057] In a first embodiment the invention resides in a heat-treated aluminium alloy comprising the following elements: i. up to 1 wt.% lithium;

i i. 0.02 - 0.3 wt.% scandium;

i ii. 0.4 - 3 wt.% copper;

i v. 1 .5 - 5 wt.% magnesium;

v. 6 - 12 wt.% zinc;

Vi. 0.05 - 0.4 wt.% zirconium;

Vii. up to 0.25 wt.% manganese;

Viii. up to 0.25 wt. % chromium;

ix. up to 0.2 wt. % titanium

x. up to 1 wt. % iron

xi. up to 1 wt. % silicon

xii. remainder is aluminium

[0058] The aluminium, which forms the base of the alloy, may be sourced from a recycled aluminium alloy, preferably a 7xxx series aluminium alloy, and/or fresh aluminium and/or additional alloying elements in the form of Master alloys. [0059] The heat-treated aluminium alloy preferably includes lithium as it reduces density and improves the strength of the alloy. However, it has also been found that alloys that include lithium may be difficult to cast. Therefore, in a number of applications, such as for example fasteners, the alloy may not include any lithium, but may include up to 1 wt.% lithium. In a preferred embodiment, the alloy may include from 0.03 to 0.5 wt.% lithium, or alternatively from 0,05 to 0.44 wt.% lithium, or alternatively from 0.05 to 0.3 wt.% lithium.

[0060] The alloy also includes zirconium and scandium which form nanometer sized particles that remain stable at high processing temperatures. The zirconium and scandium particles also add significant strength to the alloy with limited impact on ductility. The particles may also act as recrystallisation inhibitors. Surface recrystallisation may be an issue with high strength wrought aluminium products and the zirconium and scandium particles may assist in mitigating this issue.

[0061 ] The alloy includes from 0.05 to 0.4 wt.% zirconium, preferably from 0.1 to 0.3 wt.% zirconium, or alternatively 0.1 to 0.29 wt.%, or alternatively 0.25 to 0.29 wt.% zirconium.

[0062] The alloy also includes 0.02 to 0.3 wt.% scandium, preferably 0.03 to 0.2 wt.% scandium, or alternatively 0.05 to 0.15 wt.%, or alternatively 0.11 to 0.15 wt.% scandium.

[0063] The invention preferably resides in a heat-treated aluminium alloy comprising the following elements: i) 0.03 to 0.6 wt. % lithium, preferably 0.05 to 0.3 wt % lithium

ii) 0.03 to 0.2 wt. % scandium, preferably 0.05 to 0.15 wt %

iii) 0.5 to 2.0 wt. % copper, preferably 0.7 to 1.1 wt %

iv) 1 .7 to 3.5 wt. % magnesium preferably 2.5 to 3.0 wt. %

v) 8 to 10 wt. % zinc, preferably 8.6 to 10 wt. %

vi) 0.1 to 0.3 wt. % zirconium; preferably 0.25 to 0.29 wt. %

vii) 0.01 to 0.2 wt. % manganese, preferably 0.04 to 0.18 wt. % viii) 0.01 to 0.1 wt. % chromium, preferably 0.01 to 0.05 wt. %

ix) 0.02 to 0.2 wt. % titanium, preferably 0.03 to 0.14 wt. %

X) up to 0.5 wt. % iron, preferably up to 0.3 wt. % iron

xi) up to 0.5 wt. % silicon, preferably up to 0.3 wt. % silicon

xii) remainder aluminium

[0064] If recycled aluminium is used, the elements are balanced out to achieve the desired element balance.

[0065] In a further embodiment, the invention resides in a method for producing a heat-treated aluminium alloy by carefully controlling both the temperature and duration of each heat-treatment step depending on the element selected for the alloy.

[0066] The elements, including any recycled aluminium product are melted at a temperature preferably of 720ºC or above so as to I iquefy and homogenise the elements. This may be conducted in an induction furnace optionally under a flux of argon. Stirring takes place utilising a magnetic field to assist homogenisation. The homogenised liquid is then poured into moulds to form billets and allowed to cool back to room temperature. A preferred cooling rate may be for example 0.1 -10ºC per second. Cooling may be achieved through water quenching, or natural cooling with air, or a combination of both.

[0067] The cooled billets are then subjected to a first heat-treatment step where the temperature is raised preferably to about 250ºC to 350ºC and maintained for a period of about 6 hours or more. Preferably the temperature range is between 275ºC to 300ºC and for a duration of greater than 6 hours, more preferably between 6 to 25 hours, and more preferably 8 to 20.5 hours. At this temperature, Al₃Sc particles are formed. The Al₃Sc particles will precipitate as small particles in a size of from 5 to 10 nm. These particles have a very fine distribution combined with excellent stability during conventional thermo-mechanical treatment steps and provide strength for the alloy. [0068] Optionally, the composition is allowed to cool following this period of Al₃Sc precipitation to form billets. Again, the cooling may be carried out by quenching with water, or air-water mix or air alone. Quenching is not however essential after this step.

[0069] The temperature of the billets may then be raised in a second heat-treatment step to a temperature of preferably about 420° to 470ºC, preferably 430ºC to 450ºC and maintained at that temperature for a duration of preferably greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 23.5 hours. At this temperature, Al₃Zr particles form and precipitate around the scandium particles to form Al₃Sc/Al₃Zr dispersoids. The heavier zirconium particles surround the lighter scandium particles to form a core-shell structure of a size of approximately 5-25 nm. The benefit of this is that zirconium and scandium particles are very stable particularly at higher processing temperatures. The Al₃Zr shell makes the dispersoids extremely stable and prevents them from coarsening during the following thermo-mechanical steps.

[0070] In a third heat-treatment step, the temperature of the billets is then raised to preferably around 440ºC to 490ºC, more preferably 4 45ºC to 480ºC and maintained preferably for a duration of greater than 6 hours, more preferably 6 to 25 hours, and more preferably 8 to 23.5 hours. The raising of the temperature may act as a continuum rather than quenching the composition between each heat-treatment step.

[0071 ] At the higher temperature of 440ºC to 490ºC, the zirconium / scandium particles remain unchanged. All the other alloying elements are dissolved at this temperature without changing the AI₃Sc/AI₃Zr dispersoid structure and size.

[0072] The composition is inclusive of up to 0.25 wt.% manganese. At these temperatures in the third heat-treatment step, the manganese is able to trap impurities such as iron and silicon which assists in the ability to remove these impurities from the subsequently formed alloy.

[0073] The composition is then preferably allowed to slowly cool to room temperature without quenching. Slow cooling is preferable for this step to allow for a homogenised composition free from residual stress, that will be easier to extrude. [0074] The solid composition then undergoes an extrusion phase where the billet is heated to a temperature of preferably around 430° to 455ºC. At this temperature, the scandium and zirconium dispersoids are not subjected to change. The composition may then be extruded at a rate preferably of between 0.5 to 2 mm per second, preferably about 1 mm per second to form profiles with the AI₃Sc/AI₃Zr dispersoids evenly spread. These extruded profiles are cooled down to room temperature and can be quenched at the exit of the extrusion press. However, quenching is not a requirement.

[0075] Depending on the nature of the extruded profiles, the profile may be stretched in order to straighten the extrusion. This may be conducted after quenching if quenching has taken place. Stretching provides work hardening and may generate dislocations that can assist in nucleating the strengthening precipitates.

[0076] The extruded profiles then undergo a fourth heat-treatment step where they are treated to a temperature of preferably around 460° to 490ºC, more preferably 470 ºC to 480 ºC. Preferably, the duration of the four th heat-treatment step is greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 22 hours. At these temperatures, magnesium, manganese, zinc, copper and any lithium elements will dissolve and undergo continued solutionisation. The composition is then preferably allowed to cool rapidly back to room temperature through a water quenching process. Preferably, recrystallisation should be avoided during this step and the texture should remain fibrous. The applicants have found that the presence of the Al₃Sc/Al₃Zr dispersoids helps to prevent the development of peripheral coarse grains at the profiles surfaces, which has the potential to increase the fracture strength, even more in thin walled extrusions.

[0077] The profiles then undergo an aging process. The aging process may be a natural aging process or an artificial aging process or a combination of both. The aging process itself assists in strengthening the aluminium alloy and allows for formation of desired precipitates with appropriate control of the aging process. Maximum strength may be achieved when the precipitates reach a high number density and volume fraction and remain small and semi-coherent. The semi-coherent particles will assist in preventing the dislocation movement and hence strengthen the alloy. Highest strengthening is obtained when the precipitates are small and closely spaced.

[0078] The process of the present application preferably includes a natural aging phase where the composition is allowed to age for a period of greater than 3 days, preferably from 3 to 7 days, more preferably 3 to 5 days. The process then preferably includes an artificial aging phase where the temperature of the composition is raised in a fifth heat-treatment step. The temperature of the artificial aging step is raised to a temperature of for example, 110ºC to 150 ºC, more p referably 120ºC to 140 ºC. The composition may be allowed to age for greater than 3 hours, preferably for 3 to 15 hours, up to a day. Higher temperatures through this aging process may decrease the strength of the alloys and the temperature should be selected to provide a compromise between high strength and the shorter aging time.

[0079] In one embodiment, the artificial aging process may be conducted instantaneously following quenching from the previous heat-treatment step.

[0080] Through both the natural and artificial aging process, the following precipitates, or precursors, may form, Al₂CuLi, Al₃Li or Al₂Cu and/or Al₂CuMg in the absence of lithium, together with Mg₂Zn.

[0081 ] It is preferred to control and optimize the heat-treatment and aging phases to maximise Al₂CuLi and MgZn precipitates and their precursors, but Al₃Li, Al₂Cu and/or Al₂CuMg precipitates may or will also form.

[0082] These precipitates may nucleate on the zirconium / scandium core-shell dispersoids resulting in smaller and higher number density of precipitates. A preferred natural aging time is for greater than 3 days, preferably from 3 to 7 days or longer, and more preferably 3 to 5 days, while the artificial aging step may be from greater than 3 hours, more preferably from 3 to 30 hours and more preferably 8 to 21 hours. The composition may be allowed to cool following the artificial aging step, and then additionally subjected to a second artificial aging step or a sixth heat-treatment step where the temperature is again raised to about 120 ºC to 180 ºC, more preferably 140ºC to 160 ºC having comparable duration conditions to the first artificial aging step. This sixth step may assist in further Al₂CuLi, Al₃Li, Al₂Cu, Al₂CuMg and/or Mg₂Zn precipitation.

[0083] The composition is preferably cooled or quenched between each heat- treatment step and the extrusion and aging phases, although it is preferred to allow the composition to naturally cool prior to the extrusion step. The cooling or quenching may be achieved with the application of water, air or a combination of water and air. Similar results may however be achieved by a continuum in elevating the temperature conditions, or not allowing complete cooling back to room temperature, but the applicants have found that if the composition is allowed to cool to room temperature between each heat-treatment step and/or the extrusion and aging phases, that greater stability is achieved with the formation of the dispersoids and particles leading to greater stability of the alloy.

[0084] A preferred method for assisting in the calculation of the preferred compositional elements, the desired temperature and duration of each of the heat- treatment phases, the rate of cooling and aging profile may be calculated using Bayesian Optimisation together with ThermoCalc. The Applicants have found that this method allows for calculation of the desired characteristics based on assessing anticipated characteristics of the desired alloy For example, utilising this method allowed for calculations to be performed to maximize the different heat-treatment and hardening phases of the preferred AI₃Sc/AI₃Zr, AI₂CuLi, Al₃Li or Al₂Cu and/or Al₂CuMg and Mg₂Zn precipitates with careful control of the temperature and duration during the heat-treatment phases and the extrusion and hardening phases.

[0085] Utilising the Bayesian Optimisation technique the Applicants were able to determine if the composition would meet the desired characteristics while minimising trial and experimentation techniques. Experimentation work is still necessary for the Bayesian Optimisation technique together with ThermoCalc may be used with appropriate application to reduce the need for trial and experimentation to optimize characteristics. Examples

[0086] The following examples were conducted with a study of extrusion and natural and artificial aging conditions in order to create a heat-treated extruded aluminium alloy. In the following examples, the alloying elements were a combination of some of the following elements namely Zn, Mg, Cu, Cr, Ti, Sc, Zr, Mn and Li together with four or five heat-treatment steps having varying temperatures and duration. The balance of each alloy was aluminium, some of which included a recycled 7075 aluminium alloy.

The combination of elements tested are shown in Figure 1 (Table 1).

[0087] There were 8 different combinations tested ranging from a base 7075 aluminium alloy, a modified 7075 aluminium alloy where the zinc content has been increased and the copper content decreased based upon the base 7075 alloy. These combinations are included for comparative purposes as they do not include scandium or zirconium. There was a further modified 7075 alloy tested with scandium and zirconium added.

[0088] Two further alloys were tested that fell within the scope of the invention. These combinations do not include chromium, manganese or titanium but do include scandium, zirconium, and lithium.

[0089] Three further alloys were tested falling in the scope of the present invention, having the elements present including zinc, magnesium, copper, chromium, manganese, titanium, scandium, zirconium and lithium with a balance being aluminium. The heat treatment for these alloys was optimised by Bayesian optimisation based on the results from the other examples.

Comparative Examples 1 and 2

[0090] These Examples are provided for comparative purposes. The results for Example 1 (Base 7075 alloy) and Example 2 (Modified 7075 alloy) are shown in Figures 2, 3, 4 and 5 (Tables 2, 2a 3a and 3b ) respectively and provide the results from altering the temperature and duration of each of the heat-treatment steps. [0091 ] Examples 1 and 2 did not include the first heat-treatment phase as do the other Examples, as there is no scandium to precipitate.

[0092] Example 1 utilises a recycled base 7075 alloy, while Example 2 utilises a modified 7075 alloy where the zinc content has been increased and the copper content decreased based on the base 7075 alloy. The yield strength, ultimate tensile strength and total elongation were measured and shown in Figures 14 to 16. The base 7075 aluminium had weaker yield strength and Ultimate Tensile strength but greater total elongation.

Example 3

[0093] Further tests were conducted using the modified 7075 base alloy together with the additional scandium and zirconium. The results for 15 tests where the temperature and duration of each heat-treatment phase was varied is shown in Figure 6 and 7 (Table 4). A graph of the relative timing and temperature variations are shown in Figure 7.

[0094] The temperature in zone 1 varied from between 275ºC to 330ºC for a duration of from 6 to 23 hours. In this step, Al₃Sc particles are formed and precipitate. The composition was allowed to cool to room temperature between heat-treatment phases.

[0095] In phase 2, the temperature varied between 430 ºC to 450 ºC for a duration of from 6 to 23.5 hours. In this phase, Al₃Zr particles precipitate and surround the Al₃Sc particles to form Al₃Sc/Al₃Zr dispersoids. The composition is then again allowed to cool to room temperature.

[0096] In phase 3, the composition is again heated to a temperature of between 460ºC to 480ºC for a duration of from 6 to 18 hours . At these temperatures, all the elements are dissolved without changing the Al₃Sc/Al₃Zr dispersoid structures while allowing for continued homogenisation. The composition is slowly allowed to cool back to room temperature. [0097] The composition then underwent an extrusion step where the extrusion rate was 1 mm per second to form profiles where each of the elements is substantially homogenised. The profiles were allowed to cool back to room temperature.

[0098] The composition then underwent a further heat-treatment step 4 at a temperature of between 470ºC and 480ºC. During thi s heat-treatment step, all the elements were dissolved and homogenised, apart from the dispersoids and intermetallic forming elements. The composition was then allowed to rapidly cool back to room temperature via a water quenching step.

[0099] The profiles then underwent a natural aging process for between 3 to 4 days.

[0100] The composition then undergoes an artificial aging step at step 5 where the temperature is raised between 120ºC and 140ºC for a duration of from 6 to 19 hours. Under these conditions and the natural aging step, the following precipitates or precursors may form, Al₂CuLi or Al₃Li and Mg₂Zn and may nucleate on the edge of the already existing Al₃Sc/Al₃Zr core-shell dispersoids and thicken in both directions embedding partially in the dispersoids.

[0101 ] The results for the modified 7075 base aluminium that included scandium and Zirconium showed improved yield strength and Ultimate Tensile strength but reduced Total Elongation compared to the modified 7075 aluminium without scandium or zirconium. (Example 2).

Examples 4 and 5

[0102] Various tests were conducted using the T -alloy 1 and T -alloy 2 base alloys, which did not include chromium, magnesium or titanium, but were inclusive of scandium, zirconium and lithium. The results of these 2 tests are in Figures 5 and 6 (Tables 5 and 6) respectively. A graph of the relative timing and temperature variations are also shown in Figure 12.

[0103] Similar temperature conditions and duration conditions for the five heat- treatment stages including both a natural and artificial aging step as for Example 3 were used. A further second artificial aging step zone 6 under similar temperature and duration conditions was conducted in Example 5 and a graph of the duration and temperature variation is shown in Figure 13, in order to complete the Al₂CuLi, Al 3 Li and the Mg₂Zn precipitation for the test with T-alloy 2.

[0104] The yield strength, ultimate tensile strength and total elongation for each of the 5 Examples are shown in figures 14 to 16. The yield strength and ultimate tensile strength highest for in T-alloy 1 (Example 4) and significantly more than the base 7075 alloy which has the lowest demonstrated yield strength and ultimate tensile strength. The base 7075 alloy had the higher percentage total elongation while T-alloy 1 had the lowest.

[0105] Each of the Examples that included scandium, zirconium and lithium demonstrated very good yield strength and ultimate tensile strength, when compared to the comparative Examples 1 and 2.

Examples 6, 7 and 8

[0106] The heat treatment conducted for Opt 1 , Opt 2 and Opt 3 was based on the best results from the previous alloy and heat treatments. The same heat treatment was conducted for all three compositions as per Table 7.

Table 7

[0107] The mechanical properties for the three alloys Opt 1 , Opt 2 and Opt 3 are presented in Table 8. For these alloys, artificial aging was also conducted instantaneously after quenching with no natural aging for comparison. No significant changes in properties was observed with or without natural aging for these alloys. The tensile test curves are reported for Opt 1 , Opt 2 and Opt 3 in Figure17.

Table 8

Claims

1 . A heat-treated aluminium based alloy comprising the elements: i. up to 1 wt.% lithium;

i i. 0.02 to 0.3 wt.% scandium;

iii. 0.4 to 3 wt.% copper;

i v. 1 .5 to 5 wt.% magnesium;

v. 6 to 12 wt.% zinc;

vi. 0.05 to 0.4 wt.% zirconium;

vii. Up to 0.25 wt.% manganese;

viii. Up to 0.25 wt.% chromium;

ix. Up to 0.2 wt.% titanium;

x. Up to 1 wt.% iron;

xi. Up to 1 wt.% silicon;

xii. remainder aluminium

2. A heat-treated aluminium based alloy according to claims 1 , comprising the elements:

i) 0.03 to 0.6 wt. % lithium, preferably 0.05 to 0.3 wt %;

ii) 0.03 to 0.2 wt. % scandium, preferably 0.05 to 0.15 wt %;

iii) 0.5 to 2.0 wt. % copper, preferably 0.7 to 1.1 wt %;

iv) 1.7 to 3.5 wt. % magnesium preferably 2.5 to 3.0 wt. %;

v) 8 to 10 wt. % zinc, preferably 8.6 to 10 wt. %;

vi) 0.1 to 0.3 wt. % zirconium; preferably 0.25 to 0.29 wt. %;

vii) 0.01 to 0.2 wt. % manganese, preferably 0.04 to 0.18 wt. %;

viii) 0.01 to 0.1 wt. % chromium, preferably 0.01 to 0.05 wt. %;

ix) 0.02 to 0.2 wt. % titanium, preferably 0.03 to 0.14 wt. %;

x) up to 0.5 wt. % iron, preferably up to 0.3 wt. %;

xi) up to 0.5 wt. % silicon, preferably up to 0.3 wt. %;

xii) remainder aluminium.

3. An alloy according to claim 1 having a yield strength of between 600 to 900 MPa for the final extruded product at room temperature; preferably between 630 to 800 MPa.

4. An alloy according to claim 1 having an ultimate tensile strength of between 600 to 900 MPa for the final extruded product at room temperature; preferably between 630 to 800 MPa.

5. An alloy according to claim 1 when the total elongation ranges from greater than 2.0 % at room temperature; preferably 2.0 to 10.0 % at room temperature, more preferably 4.0 to 10.0 % at room temperature.

6. An alloy composition according to any one of the preceding claims wherein the aluminium used is selected from recycled 7xxx aluminium alloy and/or fresh aluminium as a base metal.

7. An alloy according to claim 6 wherein the 7xxxx aluminium alloy is a 7075 aluminium alloy.

8. An alloy according to any one of the preceding claims wherein the alloy includes an AI₃Sc/AI₃Zr core-shell dispersoid and one or more of AI₂CuLi, Al₃Li, Al₂Cu, Al₂CuMg and/or Mg₂Zn precipitates or their corresponding precursors.

9. A process for producing a heat-treated aluminium alloy, said process including the steps of:

i) melting the elements of claim 1 to form a molten homogenised composition and casting into a cylindrical billet;

ii) raising the temperature of the composition in a first heat-treatment step to between 250ºC to 350ºC, preferably between 275ºC to 300ºC to form Al₃Sc precipitates;

iii) raising the temperature in a second heat-treatment step to between 420ºC to 470ºC; preferably between 430ºC to 450ºC t o form Al₃Zr precipitates which surround the Al₃Sc precipitates to form Al₃Sc/Al₃Zr core-shell dispersoids.

10. A process according to claim 9 wherein the elevated temperature of the first heat-treatment step is maintained for a period of from 6 to 25 hours, preferably from 8 to 20.5 hours.

11 . A process according to claim 9, where the elevated temperature of the second heat treatment step is maintained for a period of from 6 to 25 hours, preferably from 8 to 23.5 hours.

12. A process according to any one of claims 9 to 11 wherein the composition is subject to a third heat treatment step wherein the temperature is raised to between 440ºC to 490ºC, preferably from 450ºC to 475ºC to a ssist homogenisation and removal of any iron and silicon impurities.

13. A process according to any one of claims 9 to 12 wherein the elevated temperature of the third heat-treatment step is maintained for a period of from 6 to 25 hours, preferably 8 to 23.5 hours at the elevated temperature.

14. A process according to any one of claims 9 to 13 wherein the composition is allowed to cool to room temperature to form billets, and then heated to a temperature of from 430ºC to 455ºC and maintained at that tempe rature for a period from 6 to 25 hours, to allow for the billets to be extruded.

15. A process according to claim 14 wherein the billets are extruded at a rate of about 0.5 to 2mm per second, preferably about 1 mm per second to form profiles.

16. A process according to claim 15 wherein the extruded billets are subjected to stretching prior to further heat treatment.

17. A process according to any one of claims 14 to 16 wherein the extruded profiles are subjected to a fourth heat-treatment step wherein the temperature is then raised to between 460ºC to 490ºC, preferably between 470ºC to 480ºC to dissolve the magnesium, manganese, zinc, copper and any lithium.

18. A process of claim 17 wherein the elevated temperature of the fourth heat- treatment step is maintained for a period of from 6 to 25 hours, preferably from 8 to 22 hours.

19. A process according to claim 18 wherein the composition is allowed to rapidly cool through water quenching followed by naturally aging for a period of from 3 to 7 days, preferably 3 to 4 days, and/or artificially aging at low temperatures for a period of from 5 to 15 hours.

20. A process according to claim 18 or 19 wherein the composition is subjected to an artificial aging step, either replacing or together with the natural aging step where the temperature is raised to between 110ºC to 150ºC , preferably 120 ºC to 140 ºC and maintained at that temperature for a period of from 3 to 30 hours, preferably 8 to 21 hours.

21 . A process according to claim 19 or 20 wherein Al₂CuLi; Al₃Li or Al₂Cu and/or Al₂CuMg and Mg2Zn precipitates are formed through the natural and/or artificial aging process.

22. A process according to claim 20 where the composition is subjected to a second artificial aging process where the temperature is raised to about 120ºC to 180ºC, more preferably 140 ºC to 160 ºC for a period of from 3 to 30 hours, preferably 8 to 21 hours.

23. A process according to any one of claims 9 to 22 where the composition is allowed to cool to room temperature between each heat-treatment step, or the extrusion and aging phases.

24. A process according to claim 9 wherein the Al₃Sc precipitates have a particle size of 5 to 10 nm

25. A process according to claim 9 wherein the Al₃Sc/Al₃Zr core shell dispersoid has a particle size of from 5 to 25 nm.

26. A process according to claim 20 wherein one or more of the Al₂CuLi, Al₃Li or Al₂Cu and/or Al₂CuMg and Mg₂Zn precipitates, or corresponding precursors, nucleate on the Al₃Sc/Al₃Zr core-shell dispersoid.

27. A process according to any one of claims 9 to 26 wherein the compositional make up, temperature of each heat-treatment step, duration of cooling, and length of aging is calculated in order to optimize the yield strength, ultimate tensile strength and total elongation property of the alloy.

28. A process according to any one of claims 9 to 27 wherein the total duration of the combined heat treatment steps, extrusion and aging phases vary from 40 hours to 100 hours, preferably 60 to 90 hours.