WO2008120237A1

WO2008120237A1 - Alloy composition and preparation thereof

Info

Publication number: WO2008120237A1
Application number: PCT/IN2008/000196
Authority: WO
Inventors: Kumar Ashim Mukhopadhyay
Original assignee: Director General, Defence Research & Development Organisation
Priority date: 2007-03-30
Filing date: 2008-03-27
Publication date: 2008-10-09
Also published as: CN101835915A; EP2274454B1; KR20090127185A; CN101835915B; EP2274454A1

Abstract

The disclosure relates to a high strength Aluminum-Zinc-Magnesium-Copper-Zirconium alloy semi-products of composition Al-Zn(8-12.5)wt%-Mg (1.2-2.0)wt%-Cu(1.4-2.2)wt%-Zr(0.10-0.18)wt% and its method of preparation. The alloy semi-product could be further processed into high strength alloy extrusions or high strength alloy thin sheets.

Description

ALLOY COMPOSITION AND PREPARATION THEREOF TECHNICAL FIELD

This disclosure provides an Aluminum-Zinc-Magnesium-Copper-Zirconium alloy composition Al-(8- 12.5) wt% Zn-( 1.2-2.0) wt% Mg-( 1.4-2.2) wt% Cu-(0.10-0.18) wt%-Zr exhibiting significantly high strength. It also provides a process of preparing alloy semiproducts that are further processed into high strength alloy extrusions or alloy thin sheets. BACKGROUND

Aluminum-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloys are in great demand for various strength critical applications in areas of aerospace and defence. These alloys are commonly processed in the form of sheets, plates, extrusion, forgings, etc. for various structural applications. In aerospace applications, one of the requirements is the processing of these alloys in the form of thin sheets having thickness <0.3 mm, wherein the thin sheets of Al alloys are used in combination with fibres (preferably glass fibres) to form fibre-metal laminated (FML) composites. The strength properties of the thin sheets of Al alloys used in the FML composites play an important role in deciding the tensile yield strength of the FML composites [Ad Viot and J. W. Gunnink, Fibre Metal Laminates: An Introduction, Kluwer Academic Publishers, The Netherlands, 2001]. Further, Al alloy sheets having thickness of <0.3 mm are preferred for this purpose because it improves drapability. U.S. Pat. No. 4,477,292 and 4,832,758 discloses Aluminium-Zinc-Magnesium-

Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloys as a class of very high strength aluminium alloy that can be produced via ingot metallurgical route. The increased strength properties of this category of alloys improve specific properties (in terms of strength per density and strength per Young's modulus, etc.) and, therefore, permit significant weight savings, the major emphasis in the development efforts of these alloys has been on the improvement of strength through various metallurgical processes.

U.S. Pat. Nos. 4,699,673; 4,988,394 and U.S. Patent application No.20060191609 disclose the methods of preparation of thin sheets of Al-Zn-Mg-Cu-Zr alloys having thickness up to 1 .2 mm. But there is no information regarding the preparation of high strength Al-Zn-Mg-Cu-Zr sheets having thickness as low as <0.3 mm. The problem associated with the retention of high strength in such products is the onset of recrystallization resulting in cracking that becomes more pronounced as the sheets are progressively made thinner. However, due to the proprietary nature of the processing details of such thin sheets, there is no information on this topic in the literature.

One of the high strength Al-Zn-Mg-Cu-Zr alloy i.e. AA 7055, known in the art [Aluminium Company of America, (ALCOA) Technical Data available at www.millproducts-alcoa.com] has the composition of AI-(7.6-8.4) wt% Zn-(1.8-2.3) wt% Mg-(2-2.6) wt% Cu-(0.08-0.25) wt% Zr. The typical tensile properties of 7055 T7751 extrusions in the longitudinal direction (i.e. along the extrusion direction) are: 0.2 % proof stress = 655 MPa, UTS = 669 MPa, elongation = 1 1%. The T77 temper possesses strength properties that correspond to the peak aged, T6 temper, but stress corrosion cracking resistance similar to the T76 temper [ALCOA publication. Light Metal Age, October, 1991 , p. 14].

US Patent Application 20040099352 discloses another high strength Al-Zn-Mg- Cu-Zr alloy comprising (in wt%) of 8.2-10%Zn, 1.95-2.5%Cu, 1.9-2.5%Mg, 0.05-0.25 % Zr. The alloy when produced by extrusion processing and peak aged gives rise to 0.2 % P. S. of 703 MPa. Another high strength, Al-Zn-Mg-Cu-Zr base alloy containing scandium, processed in the form of extrusions and heat treated to the peak aged T6 temper, have the composition ofAl-8.6Zn-2.6Mg-2.4Cu-0.2 wt% Sc and an undisclosed amount of Zr [Metall. Mater. Trans. A, 3OA, 1017, 1999]. The typical tensile properties of this material are: 0.2% PS = 689 MPa, UTS = 715 MPa and elongation = 1 1.8%. Recent work has, however, shown that Al-Zn-Mg-Cu-Zr alloys containing Zn not exceeding 8.5wt%, and Mg and Cu contents each not exceeding 2wt% are capable of attaining similar or even higher 0.2%P.S. in the peak aged temper [Minerals & Metals Review, 10, 2005, p.65]. The implication is that neither Sc addition nor Zn content in excess of 8.5 wt% is required to attain 0.2% P. S. of about 700 MPa in peak aged extrusions of these alloys. US Patent Application 20050056353 discloses another high strength Al-Zn-Mg-

Cu-Zr-Sc alloy comprising (in wt%) of 8.5-1 1 %Zn, 1.8-2.4%Mg, 1.8-2.6%Cu, 0.05- 0.30%Sc and at least one element from the group Zr, V and Hf not exceeding 0.5wt%. The alloy when produced by extrusion processing and peak aged gives rise to 0.2% P. S. values that range from 670 to 715 MPa. US Patent Application 20050072497 discloses yet another high strength Al-Zn-Mg-Cu-Zr alloy comprising (in wt%) of 8.3-14%Zn, 0.3- 2%Cu, 0.5-4.5%Mg, 0.03-0.15%Zr and at least one element from the group Sc, Hf, La, Ce, Nd [the amount of the selected element ranging between 0.02 and 0.7wt%]. The alloy when produced by extrusion processing and peak aged showed 0.2%P.S. values ranging from 670 to 783 MPa.

A major drawback of the Sc-bearing alloys is that scandium metal is expensive.

Yet another limitation of the abovementioned alloys is that scandium ores are not available in many countries including India. Recent revelations are that the fatigue properties are deteriorated by the presence of Sc in Al-Zn-Mg-Cu-Zr alloys [Scripta Materialia, Vol.52, p. 645, 2005].

There is, therefore, a tremendous scope for the further development of Aluminium- Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy having significantly high strength that have a variety of applications especially in the field of aerospace applications, wherein sheets of thickness even less than 0.30 mm are needed, and in the filed of defence applications, extrusions of various dimensions are utilized for a number of applications. SUMMARY This disclosure provides an Aluminum-Zinc-Magnesium-Copper-Zirconium alloy of composition Al-(8-12.5)wt%Zn-(1.2-2.0)wt%Mg-( 1.4-2.2)wt%Cu-(0.10-0.18)wt%Zr and method of its preparation. The disclosure also provides a method for preparing alloy semi-products of this composition that can be further processed into significantly high strength alloy extrusions or alloy thin sheets. One aspect of the disclosure provides an Aluminium-Zinc-Magnesium-Copper-

Zirconium (Al-Zn-Mg-Cu-Zr) alloy extrusion of composition AI-(1 1.5-12.5)wt%Zn-(l .3- 2.0)wt%Mg-( l .5-2.2) wt%Cu-(0.12-0.18) wt% Zr.

Another aspect of the disclosure provides an Aluminium-Zinc-Magnesium- Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy thin sheet of composition AI-(8-10)wt%Zn- ( 1.2-2.0)wt%Mg-( 1.4-2.2) wt%Cu-(0.12-0.18)Zr.

One aspect of the present disclosure provides a process for the preparation of an Al-Zn-Mg-Cu-Zr alloy using two-step homogenization process so as to cause more effective dissolution of the solidification products, thereby enabling the alloy to retain increased solute supersaturation during subsequent thermal and mechanical processing and to obtain high strength upon aging.

Another aspect of the present disclosure provides a process of preparation of Al- Zn-Mg-Cu-Zr alloy extrusions using optimized alloy composition and extrusion processing parameters so as to enable the alloy to obtain high strength through retaining essentially unrecrystallized grain structure and imparting solutionising effect during extrusion processing.

Another aspect of the present disclosure provides a process of preparation of an Al-

Zn-Mg-Cu-Zr alloy thin sheets using optimized alloy composition, homogenization treatment, rolling parameters and intermediate annealing treatments so as to enable the alloy to greatly minimize the extent of edge cracking and to obtain high strength through the retention of essentially unrecrystallized grain structure in most parts of the sheets.

Yet another aspect of the present disclosure provides a process of preparation of an Al-Zn-Mg-Cu-Zr alloy using two-step artificial aging treatment so as to enable the alloy to obtain reproducible strength properties.

Yet another aspect of the present disclosure provides a process for preparation of less than 0.30 mm thick Al-Zn-Mg-Cu-Zr alloy sheets having significantly high strength. BRIEF DESCRIPTION OF DRAWINGS

The above and other features, aspects, and advantages of the subject matter will become better understood with regard to the following description, appended claims, and accompanying figures where:

FIG 1. Optical micrograph showing partially recrystallized grain structure in the peak aged alloy extrusions of present disclosure.

FIG 2. Optical micrograph showing essentially unrecrystallized grain structure in the peak aged alloy sheets of thickness 0.28 mm of present disclosure.

FIG 3. Transmission electron micrograph showing subgrain structure present in the unrecrystallized portion of the peak aged alloy extrusions of present disclosure.

FIG 4. Transmission electron micrograph showing a uniform and fine distribution of strengthening η' precipitates in the peak aged alloy extrusions of present disclosure. FIG 5. Transmission electron micrograph showing a uniform and fine distribution of strengthening η' precipitates in the peak aged alloy sheets of thickness 0.28 mm of present disclosure. DETAILED DESCRIPTION

The present disclosure provides an Aluminium-Zinc-Magnesium-Copper- Zirconium (Al-Zn-Mg-Cu-Zr) alloy of composition (in weight %): Zn 8- 12.5; Mg 1.2-2.0; Cu 1.4-2.2; and Zr 0.10-0.18.

The present disclosure also provides an Aluminium-Zinc-Magnesium-Copper- Zirconium (Al-Zn-Mg-Cu-Zr) alloy semi-product of composition (in weight %):

Zn 8-12.5; Mg 1.2-2.0;

Cu 1.4-2.2; and Zr 0.10-0.18.

One aspect of the present disclosure provides an Aluminium-Zinc-Magnesium- Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy extrusion of composition (in weight %): Zn 1 1.5-12.5;

Mg 1 .3-2.0; Cu 1 .5-2.2; and Zr 0.12-0.18.

Another aspect of the present disclosure provides an Aluminium-Zinc-Magnesium- Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy thin sheet of composition (in weight %): Zn 8- 10; Mg 1 .2-2.0; Cu 1 .4-2.2; and Zr 0.12-0.18. Further aspect of the present disclosure provides a process for preparing

Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy semi-products of composition AI-(8-12.5)wt%Zn-(1.2-2.0)wt%Mg-(1.4-2.2)wt%Cu-(0.10-0.18)wt%Zr. The process comprising: a. melting a charge mixture of primary aluminium and Al-33wt%Cu master alloy; b. adding elemental pure Zn, followed by raising the temperature of the molten charge; c. adding AI-50wt%Mg master alloy and Mg-28wt%Zr master alloy to the molten charge followed by super heating; d. adding grain refining nucleant pellets at a reduced temperature; e. adding degasser pellets; f. pouring the molten melt under argon atmosphere into a preheated metallic mould; and g. solidifying to obtain an as-cast billet or slab of alloy.

One aspect of the present disclosure provides a process for preparing Aluminium- Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy semi-products wherein the primary aluminium has a minimum purity of 99.80 wt%. Another aspect of the present disclosure provides a process for preparing Al-Zn-

Mg-Cu-Zr alloy semi-product wherein a charge mixture comprising of 76.38 to 84.81% by weight of primary aluminium and 4.54 to 6.97% by weight of the master alloy Al- 33wt%Cu is melted in an induction furnace by melting at a temperature in the range of 720 to 730⁰C. Another aspect of the present disclosure provides a process for preparing Al-Zn-

Mg-Cu-Zr alloy semi-product wherein elemental pure Zn in the range of 8.15 to 12.65% by weight is added to the molten charge and the temperature of the charge is raised in the range of 735 to745°C. Subsequently, 2.04 to 3.32% by weight of AI-50wt%Mg master alloy and 0.46 to 0.68% by weight of Mg-28wt%Zr master alloy are added and the molten alloy is superheated to a temperature in the range of 755 to 765°C for 10 to 15 minutes.

Yet another aspect of the present disclosure provides a process for preparing Al- Zn-Mg-Cu-Zr alloy semi-product wherein the temperature of the superheated molten alloy is reduced to a temperature in the range of 735 to 74O⁰C and nucleant pellets in a quantity of 0.01 to 0.1 kg by weight are added for grain refinement. The nucleant pellet used is either Di-Potassium Titanium Hexafluoride (K₂TiF₆) or Potassium tetrafluoroborate (KBF₄).

Further aspect of the present disclosure provides a process for preparing Al-Zn- Mg-Cu-Zr alloy semi-product wherein dissolved gases like hydrogen are removed from the melt by degassing that is effected using degasser pellets such as Hexachloroethane (C₂CIo) taken in a quantity of 0.01 to 0.5 kg by weight.

An aspect of the present disclosure provides a process for preparing Al-Zn-Mg-Cu- Zr alloy semi-product wherein the melt, at a reduced temperature in the range of 710 to720°C, is poured under argon atmosphere into a metallic mould of suitable size preheated to a temperature in the range of 145 to 155⁰C. Another aspect of the present disclosure provides a process for preparing Al-Zn-

Mg-Cu-Zr alloy semi-product wherein the melt is solidified to obtain an as-cast billet or slab of alloy.

The term alloy semi-product as used herein refers to as cast billets and as-cast slabs of the alloy.

One aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy extrusions, said process comprising of: a. homogenizing, scalping and extruding the as-cast billets of the alloy; b. solution treating, quenching; stretching the alloy extrusions to obtain 1 to

1.5% permanent set; and artificial aging to obtain peak aged, high strength alloy extrusions.

Another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy extrusions of composition AI-(1 1.5-12.5)wt%Zn-(l .3- 2.0)wt%Mg-( 1 .5-2.2) wt%Cu-(0.12-0.18) wt%, said process comprising of: a. melting a charge mixture of primary aluminium and AI-33wt%Cu master alloy; b. adding elemental pure Zn, followed by raising the temperature of the molten charge; c. adding AI-50wt%Mg master alloy and Mg-28wt%Zr master alloy to the molten charge followed by super heating; d. adding grain refining nucleant pellets at a reduced temperature; e. adding degasser pellets; f. pouring the molten melt under argon atmosphere into a preheated metallic mould; g. solidifying to obtain an as-cast billet of the alloy; h. homogenizing, scalping and extruding the as-cast billet of the alloy; i. solution treating, quenching, stretching the alloy extrusions to obtain 1 to

1.5% permanent set; and j. artificial aging to obtain peak aged, high strength alloy extrusions.

Another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy extrusions wherein homogenization is carried out in two steps. The first step is carried out at a temperature in the range of 440 to 450⁰C for 25 to 35 h followed by a second step homogenization at a temperature in the range of 450 to 460⁰C for 20 to 30 h followed by cooling in air. The homogenization treatment eliminates dendritic segregation in the cast microstructure and causes more effective dissolution of the solidification products, thereby enabling the alloy to retain increased solute supersaturation during subsequent thermal and mechanical processing so as to obtain high strength upon aging.

The homogenized billets are scalped to remove the oxide layers formed on the surfaces. The scalped billets are subjected to non-destructive testing to detect casting defects.

5 Further aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy extrusions wherein the extrusion processing of the billets is carried out at an initial billet temperature in the range of 400 to 430⁰C; extrusion ratio in the range of 15: 1 to 25: 1 and a ram speed of 2-5 mm/s.

One aspect of the present disclosure provides a process for preparing high strength

I O Al-Zn-Mg-Cu-Zr alloy extrusions wherein solution treatment is carried out at a temperature in the range of 450 to 46O⁰C for 1 to 2 h followed by water quenching at room temperature. The extrusions are subjected to stretching to obtain 1 to 1.5% permanent set for stress relieving purpose.

Another aspect of the present disclosure provides a process for preparing high

15 strength Al-Zn-Mg-Cu-Zr alloy extrusions wherein the stretched alloy extrusions are subjected to two-step artificial aging at 90 to 100⁰C for 6 to 8 h in the first stage followed by a second stage aging in the temperature range of 120 to 125°C for 20 to 25 h. This treatment produces peak strength in the alloy extrusion in a reproducible manner.

Yet another aspect of the present disclosure provides a process for preparing high 0 strength Al-Zn-Mg-Cu-Zr alloy extrusions using optimized extrusion processing parameters so as to enable the alloy to obtain high strength the alloy through retaining essentially unrecrystallized grain structure and imparting solutionising effect during extrusion processing.

Further aspect of the present disclosure provides a process for preparing high 5 strength Al-Zn-Mg-Cu-Zr alloy extrusions having minimum 0.2% tensile P. S. of 750 MPa.

An aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets, said process comprising of: a. homogenizing, scalping, hot rolling the as-cast slabs of the alloy into sheets and subsequent cold rolling of the sheets with intermediate annealing to

30 obtain alloy sheets of required thickness; and b. solution treating, quenching; and artificial aging to obtain peak aged, high strength alloy sheets.

Another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets of composition AI-(8- 10)wt%Zn-( 1.2- 2.0)wt%Mg-( 1.4-2.2) wt%Cu-(0.12-0.18)Zr, said process comprising of: a. melting a charge mixture of primary aluminium and AI-33wt%Cu master alloy; b. adding elemental pure Zn, followed by raising the temperature of the molten charge; c. adding AI-50wt%Mg master alloy and Mg-28wt%Zr master alloy to the molten charge followed by super heating; d. adding grain refining nucleant pellets at a reduced temperature; e. adding degasser pellets; f. pouring the molten melt under argon atmosphere into a preheated metallic mould; g. solidifying to obtain an as-cast slab of the alloy;. h. homogenizing, scalping, hot rolling the as-cast slabs of the alloy into sheets and subsequent cold rolling of the sheets with intermediate annealing to obtain alloy sheets of required thickness; i. solution treating, quenching; and j. artificial aging to obtain peak aged, high strength alloy thin sheets. Another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets wherein the homogenization treatment of the as- cast slabs of the alloy is carried out at a temperature in the range of 445 to 455°C for 25 to35 h in the first step followed by a second step homogenization at a temperature in the t range of 455 to 465⁰C for 10 to 20 h followed by cooling in air. The homogenization treatment eliminates dendritic segregation in the cast structure. The homogenized slabs are scalped to remove the oxide layers formed on the surfaces. The scalped slabs are then subjected to the non-destructive testing to detect casting defects.

Yet another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets wherein the scalped slabs (having an initial thickness of 100 mm) are subjected to hot rolling at an initial billet temperature in the range of 425 to 435°C and at a linear speed of 20 m/minute to produce plates having thickness of around 22 mm. These plates are then cross-rolled using the same initial billet temperature and rolling speed to finally produce sheets having thickness of about 5 mm. During hot rolling, 4 to 8% reduction in the thickness of the billet in three passes followed by intermediate annealing at the temperature in the range of 415 to 435°C for 15 to 25 minutes. This cycle is continued till such time the targeted plate thickness of 22 mm and subsequently the sheet thickness of 5 mm is obtained. The intermediate annealing treatment is given for stress relief purposes.

5 Further aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets wherein the hot rolled sheets are subjected to cold rolling at room temperature in steps such that each step involves 15 to 25% reduction in thickness in three passes and subsequent intermediate annealing at the temperature in the range of 415 to 425⁰C for 15 to 25 minutes followed by cooling in air. The

I O intermediate annealing treatment is given for stress relief purpose. The process of cold rolling and subsequent intermediate annealing is repeated till the thickness of the sheet is reduced to 0.28 mm i.e. below 0.3 mm.

An aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets wherein solution treatment of rolled sheets is carried out

15 at a temperature range in the range of 450 to 46O⁰C for 1 to 1 .5 h followed by water quenching at room temperature.

Another aspect of the present disclosure provides a process for preparing high strength Al-Zn-Mg-Cu-Zr alloy thin sheets wherein artificial aging is a two-step process carried out at a temperature in the range of 90 to 100⁰C for 6 to 8 h followed by aging at a

20 temperature in the range of 1 15 to 125°C for 20 to 30 h. This treatment produces peak strength in the alloy.

One aspect of the present disclosure is to provide a process for the preparation of high strength Al-Zn-Mg-Cu-Zr alloy in the form of sheets of thickness less than 0.3 mm having high, reproducible strength properties i.e. a minimum 0.2% tensile PS of 623 MPa

25 in the peak aged temper.

These, as well as other aspects of the present disclosure are explained in more detail with working examples, which are intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of present disclosure.

30 EXAMPLES

Example 1 : Preparation of Al-11.8 wt% Zn-1.4 wt% Mg-1.8 wt% Cu-0.16 wt% Zr

For a 15 kg melt of the alloy of present disclosure, a mixture of 1 1.98 kg of primary aluminium (purity 99.80 wt% Al and the balance being 0.15 wt% Fe and 0.05 vvt% Si impurities) and 0.864 kg of Al-33 wt% Cu master alloy was charged into the induction furnace. The above charge mixture was melted at 725°C. To this molten charge, 1.7 kg of pure Zn in the ingot form was added. When the charge had melted, the temperature of the molten charge was raised to 740⁰C and 0.364 kg of AI-50 wt% Mg master alloy and 0.092 kg of Mg-28 wt% Zr master alloy were added in the above sequence. The charge was superheated to 760⁰C and the whole material was held at this temperature for 10 minutes. The temperature was then reduced to 740⁰C and 0.03 kg of nucleant pellets were added for grain refinement purpose. After 5 minutes, 0.04 kg of degasser pellets were added for degassing purpose. The molten alloy, at a temperature of 720⁰C, was then poured under argon atmosphere into a preheated (to the temperature of 150⁰C) metallic mould of suitable size.

The melt was solidified and the ingot was cleared of the portions having casting defects. A cylindrical as-cast billet of 95 mm diameter and 250 mm height was obtained. The billet was subjected to two-step homogenization treatment comprising of 24 h at 445°C followed by 24 h at 455°C followed by cooling in air. The billet was scalped, machined to produce 74 mm diameter cylindrical billets and cut into two halves along the length. Cylindrical billets of 74 mm diameter and 125 mm height were ready for subsequent extrusion processing. Billets were extruded at an initial billet temperature of 420⁰C, extrusion ram speed of 3 mm/sec and extrusion ratio of 20: 1. The extrusions were solution treated at 460⁰C for 1.5 h, quenched in water at ambient temperature and peak aged using a two-step aging treatment involving 8 h at 100⁰C in the first step followed by aging at 120⁰C for 24 h in the second step. This heat treatment produced peak strength in the alloy. These peak aged extrusions were then utilized for evaluation of the tensile properties (see Table 1 ). Example 2: Preparation of AI-12.1 wt% Zn-1.5 wt% Mg-1.8 wt% Cu-0.16 wt% Zr

For a 1 5 kg melt of the alloy of present disclosure, a mixture of 1 1.85 kg of primary aluminium (minimum purity 99.80 wt% Al and the balance being 0.15 wt% Fe and 0.05 wt% Si impurities) and 0.86 kg of Al-33 wt% Cu master alloy was charged into the induction furnace. The above charge mixture was melted at 725°C. To this molten charge, 1.84 kg of pure Zn in the ingot form was added. When the charge had melted, the temperature of the molten charge was raised to 740⁰C and 0.36 kg of AI-50 wt% Mg master alloy and 0.09 kg of Mg-28 wt% Zr master alloy are added in the above sequence. The charge was superheated to 760⁰C and the whole material was held at this temperature for 10 minutes. The temperature was then reduced to 740⁰C and 0.03 kg of nucleant pellets were added for grain refinement purpose. After 5 minutes, 0.04 kg of degasser pellets were added for degassing purpose. The molten alloy, at a temperature of 720⁰C, was then poured under argon atmosphere into a preheated (to the temperature of 150⁰C) metallic mould of suitable size.

When the melt was solidified, the ingot was cleared of the portions having casting defects. A cylindrical as-cast billet of 95 mm diameter and 250 mm height was obtained. The billet was subjected to two-step homogenization treatment comprising of 24 h at 445°C followed by 24 h at 455⁰C followed by cooling in air. The billet was scalped, machined to produce 74 mm diameter cylindrical billets and cut into two halves along the length. Cylindrical billets of 74 mm diameter and 125 mm height were ready for subsequent extrusion processing. Billets were extruded at an initial billet temperature of 420⁰C, extrusion ram speed of 3 mm/sec and extrusion ratio of 20: 1. The extrusions were solution treated at 460⁰C for 1.5 h, quenched in water at ambient temperature and peak aged using a two-step aging treatment involving 8 h at 100⁰C in the first step followed by aging at 120⁰C for 24 h in the second step. This heat treatment produced peak strength in the alloy. These peak aged extrusions were then utilized for evaluation of the tensile properties (see Table 2). Example 3: Preparation of AI-8.3wt%Zn-1.9wtMg-1.7wt%Cu-0.18wt%Zr For a 50 kg melt of the present disclosure, a mixture of 41.154 kg of primary aluminium (purity 99.85 wt% Al and the balance being a maximum of 0.09 wt% Fe and 0.06 wt% Si impurities) and 2.72 kg of AI-33 wt% Cu master alloy was charged into the induction furnace. The above charge mixture was melted at 725°C. To this molten charge, 4.225 kg of pure Zn in the ingot form was added. When the charge had melted, the temperature of the molten charge was raised to 74O⁰C and 1.562 kg of AI-50 wt% Mg master alloy and 0.339 kg of Mg-28 wt% Zr master alloy were added in the above sequence. The charge was superheated to 76O⁰C and the whole material was held at this temperature for 10 minutes. The temperature was then reduced to 740⁰C and 0.10 kg of nucleant pellets were added for grain refinement purpose. After 5 minutes, 0.25 kg of degasser pellets were added for degassing purpose. The molten alloy at the temperature of 720⁰C, was then poured- under argon atmosphere into a preheated (to the temperature of 150⁰C) metallic mould of suitable size. When the melt was solidified, the ingot was cleared of the portions having casting defects. A rectangular as-cast billet of 340 mm (length) x 300 mm (width) x 100 mm (thickness) was then obtained. The billet was subjected to the homogenization annealing in two steps annealing at 450⁰C for 25 h in the first step and annealing at 460⁰C for 15 h in the second step followed by cooling in air. The homogenized billet was scalped in order to remove the oxidized layers on the surfaces of the billet. The billet was subjected to hot rolling. Hot rolling was carried out at an initial billet temperature of 425⁰C and at a linear speed of 20 m per minute. Following 6% reduction in thickness of the billet in three passes, and subsequent intermediate annealing at 430⁰C for 20 minutes for stress relief purposes, the same hot rolling cycle was continued till such time the targeted plate thickness of 22 mm was achieved and subsequent cross rolling was carried out to obtain the sheet thickness of 5 mm. The hot rolled sheets were then subjected to cold rolling. Following 20% reduction in thickness in three passes and subsequent intermediate annealing at 420⁰C for 15 minutes for stress relief, the sheets were subjected to the same cycle till such time the thickness of 0.28 mm i.e. the targeted thickness of below 0.30 mm was achieved. The sheets were then subjected to solution treatment at 46O⁰C for 1 h followed by water quenching at room temperature. The sheets were then subjected to artificial aging at 100⁰C for 8 h followed by artificial aging at 120⁰C for 24 h. This heat treatment produced peak strength in the alloy. These peak aged materials were then utilized for evaluation of the tensile properties (see Table 3).

Example 4: Preparation of AI-9.5wt%Zn-1.6wtMg-2.0wt%Cu-0.17wt%Zr

For a 50 kg melt of the present disclosure, a mixture of 40.536 kg of primary aluminium (purity 99.85 wt% Al and the balance being a maximum of 0.09 wt% Fe and 0.06 wt% Si impurities) and 3.03 kg of AI-33 wt% Cu master alloy was charged into the induction furnace. The above charge mixture was melted at around 725°C. To this molten charge, 4.825 kg of pure Zn in the ingot form was added. When the charge had melted, the temperature of the molten charge was raised to 740⁰C and 1.288 kg of AI-50 wt% Mg master alloy and 0.321 kg of Mg-28 wt% Zr master alloy were added in the above sequence. The charge was superheated to 760⁰C and the whole material was held at this temperature for 10 minutes. The temperature was then reduced to 740⁰C and 0.10 kg of nucleant pellets was added for grain refinement purpose. After 5 minutes, 0.25 kg of degasser pellets was added for degassing purpose. The molten alloy at the temperature of 720⁰C was then poured under argon atmosphere into a preheated (to the temperature of 150⁰C) metallic mould of suitable size.

When the melt was solidified, the ingot was cleared of the portions having casting defects. A rectangular as-cast billet of 340 mm (length) x 300 mm (width) * 100 mm (thickness) was then obtained. The billet was subjected to the homogenization annealing in two steps annealing at 450⁰C for 25 h in the first step and annealing at 460⁰C for 15 h in the second step followed by cooling in air. The homogenized billet was scalped in order to remove the oxidized layers on the surfaces of the billet. The billet was subjected to hot rolling. Hot rolling was carried out at an initial billet temperature of 425°C and at a linear speed of 20 m per minute. Following 6% reduction in thickness of the billet in three passes, and subsequent intermediate annealing at 430⁰C for 20 minutes for stress relief purposes, the same hot rolling cycle was continued till such time the targeted plate thickness of 22 mm was achieved and subsequent cross rolling was carried out to obtain the sheet thickness of 5 mm. The hot rolled sheets were then subjected to cold rolling. Following 20% reduction in thickness in three passes and subsequent intermediate annealing at 420⁰C for 15 minutes for stress relief, the sheets were subjected to the same cycle till such time the thickness of 0.28 mm i.e. the targeted thickness of below 0.30 mm was achieved. The sheets were then subjected to solution treatment at 460⁰C for 1 h followed by water quenching at room temperature. The sheets were then subjected to artificial aging at 100⁰C for 8 h followed by artificial aging at 120⁰C for 24 h. This heat treatment produced peak strength in the alloy. These peak aged materials were then utilized for evaluation of the tensile properties (See Table 4). Example S: Evaluation of the Alloy

The tensile properties of the alloys of examples 1-4 of present disclosure were examined using tensile tests carried out at ambient temperature on tensile specimens (25 mm gauge length). The alloys of example 1 and 2 were tested on round bar tensile specimens (25 mm gauge length) while alloy of example 3 and 4 were tested on flat tensile specimens. Tables 1 to 4 represent the tensile test results for examples 1 to 4, respectively.

The most noticeable feature of these results is the significantly high 0.2% P.S. values i.e. a minimum 0.2% P.S. of 755 MPa for the high strength alloys of example 1 and 2 and a minimum 0.2% P.S. of 623 MPa for the high strength thin sheets of alloys of example 3 and 4. Figure 1 shows the partially recrystallized grain structure in the peak aged alloy extrusions of present disclosure. The obtained data of 0.2% P.S. values is consistent with the retention of subgrain structure (Figure 3) in the predominantly unrecrystallized grain structure in the peak aged alloy extrusions and essentially unrecrystallized grain structure in the peak aged sheets of thickness 0.28 mm (Figure 2) of present disclosure. It also characterizes the presence of a uniform and fine distribution of strengthening ή precipitates in the peak aged alloy extrusions (Figure 4) and alloy thin sheets (Figure 5).

It may be noted that the alloys of present disclosure was prepared using <99.85 wt% purity primary Al. Therefore, it is understood that the ductility (i.e. % elongation) of the alloys when prepared using higher purity e.g. 99.9 wt% Al would be considerably higher.

Table 1. Tensile Properties of Peak Aged Alloy Extrusions of Example 1 of Present Disclosure

Table 2. Tensile Properties of Peak Aged Alloy Extrusions of Example 2 of Present Disclosure

Table 3. Tensile Properties of 0.28 mm Thick Peak Aged Alloy Sheets of Example 3 of present disclosure

Table 4. Tensile Properties of 0.28 mm Thick, Peak Aged Alloy Sheets of Example 4 of Present disclosure

Claims

What is claimed is:

1 . An Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy of composition (in weight %): Zn 8-12.5;

Mg 1.2-2.0; Cu 1.4-2.2; and Zr 0.10-0.18.

2. An Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy extrusion of composition (in weight %):

Zn 1 1.5-12.5;

Mg 1.3-2.0;

Cu 1.5-2.2; and

Zr 0.12-0.18.

3. An Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg-Cu-Zr) alloy thin sheet of composition (in weight %):

Zn 8- 10;

Mg 1.2-2.0;

Cu 1.4-2.2; and Zr 0.12-0.18.

4. A process for preparing Aluminium-Zinc-Magnesium-Copper-Zirconium (Al-Zn-Mg- Cu-Zr) alloy semi-product of composition claimed in claim 1 , said process comprising: a. melting a charge mixture of primary aluminium and AI-33wt%Cu master alloy; b. adding elemental pure Zn, followed by raising the temperature of the molten charge; c. adding AI-50wt%Mg master alloy and Mg-28wt%Zr master alloy to the molten charge followed by super heating; d. adding grain refining nucleant pellets at a reduced temperature; e. adding degasser pellets; f. pouring the molten melt under argon atmosphere into a preheated metallic mould; and g. solidifying to obtain an as-cast billet or slab of the alloy.

5. The process as claimed in claim 4 wherein the primary aluminium has a minimum purity of about 99.80 wt%.

6. The process as claimed in claim 4 wherein the charge mixture comprises of 76.38 to

84.81 % by weight of primary aluminium and 4.54 to 6.97% by weight of the master alloy Al-33wt%Cu.

7. The process as claimed in claim 4, wherein elemental pure Zn is added in the range of

8.15 to 12.65% by weight. 8. The process as claimed in claim 4, wherein the AI-50wt%Mg master alloy added to the charge mixture is in the range of 2.04 to 3.32% by weight and the Mg-28wt%Zr master alloy is in the range of 0.46 to 0.68% by weight.

9. The process as claimed in claim 4, wherein the grain refinement is effected using nucleant pellets taken in quantity of about 0.01 to 0.1 kg by weight.

10. The process as claimed in claim 4, wherein degassing is effected using degasser pellets taken in quantity of about 0.01 to 0.5 kg by weight.

1 1 . The process as claimed in claim 4 further comprising of a. homogenizing, scalping and extruding the as-cast billets of the alloy; b. solution treating, quenching; stretching the alloy extrusions to obtain 1 to 1.5% permanent set; and artificial aging to obtain peak aged, high strength alloy extrusions.

12. The process as claimed in claim 1 1 wherein the homogenization is carried out at a temperature in the range of 440 to 45O⁰C for 25 to 35 h in the first step followed by a second step homogenization at a temperature in the range of 450 to 460⁰C for 20 to 30 h.

13. The process as claimed in claim 1 1 wherein the extrusion processing is carried out at initial billet temperatures in the range of 400 to 430⁰C, extrusion ratio in the range of 15: 1 to 25: 1 and a ram speed of 2-5 mm/s.

14. The process as claimed in claim 1 1 wherein solution treatment is carried out at a temperature in the range of 450 to 460⁰C for 1 to 2 h followed by water quenching at room temperature.

15. The process as claimed in claim 1 1 , wherein artificial aging is a two-step process carried out at a temperature in the range of 90 to 100⁰C for 6 to 8 h followed by 120 to

125^oC for 20 to 25 h.

16. The process as claimed in claim 4 further comprising of: a. homogenizing, scalping, hot rolling the as-cast slabs of the alloy into sheets and subsequent cold rolling of sheets with intermediate annealing to obtain alloy sheets of required thickness; and b. solution treating, quenching; and artificial aging to obtain peak aged, high strength alloy sheets.

5 15. The process as claimed in claim 16 wherein the homogenization is carried out at a temperature in the range of 445 to 455°C for 25 to 35 h in the first step followed by a second step homogenization at a temperature in the range of 455 to 465°C for 10 to 2O h.

16. The process as claimed in claim 16 wherein the hot rolling is carried out at an initial I O billet temperature in the range of 425 to 435°C and at a linear speed of 20 m/minute and subsequent intermediate annealing at a temperature in the range of 415 to 425°C for 20 minutes.

17. The process as claimed in claim 16 wherein the cold rolling is carried out at room temperature in steps such that each step involves 15 to 25% reduction in thickness in

15 3 passes and subsequent intermediate annealing at a temperature in the range of 415 to 425°C for 1 5 to 20 minutes, followed by air cooling.

1 8. The process as claimed in claim 16 wherein solution treatment is carried out at a temperature in the range of 450 to 460⁰C for 1 to 1 .5 h followed by water quenching at room temperature.

20 19. The process as claimed in claim 16 wherein artificial aging is a two-step process carried out at a temperature in the range of 90 to 100⁰C for 6 to 8 h followed by aging at a temperature in the range of 1 1 5 to 125°C for 20to 25 h.