ALUMINIUM ALLOY

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63/224,618, filed July 22, 2021, the contents of which are herein incorporated by reference in its entirety.

FIELD

Described herein are armour components for civil or military vehicles made of high- strength 7XXX-series aluminium alloys requiring ballistic protection. More particularly, described herein are armour components used for manufacturing armour hulls and add-on appliques, which are removable panels to be mounted on the external faces of the military vehicles.

BACKGROUND

Generally, an armour shield includes a metal panel, typically of steel, aluminium, titanium or alloys thereof. Such panels have an excellent ability to absorb kinetic energy of a penetrator during impact. However, particularly if they are made of a steel alloy, such panels are heavy and have a low effectiveness in terms of absorption of energy related to the weight carried by a vehicle. Because of their light weight, aluminium alloys have found wide use in military applications, including military vehicles such as personnel carriers. The light weight of aluminium allows for improved performance and ease of transporting equipment, including air transport of military vehicles. In some vehicles it is advisable to provide shielding or protection against assault, by providing armour plate to protect the occupants of the vehicle. Aluminium has enjoyed substantial use as armour plate, and there are a number of armour plate specifications for the use of different aluminium alloys.

The most relevant requirements for aluminium alloy armour plate are resistance to projectiles, good corrosion resistance and stress corrosion resistance in particular, and in some applications, good weldability. Ballistic tests are often conducted with armour piercing (“AP”) projectiles such as the 7.62 mm AP M2 and with fragment simulating projectiles (“PSp”) _suc h as the common 20 mm projectile. The first test is intended to characterize the resistance to perforation and the second test is intended to quantify to withstand the impacts which generate fragmented debris. During these tests, the armour panels are the target of projectiles of different shapes and sizes (spindle shape for the AP test, a more squat form for the FSP test). In each test type, several geometries are used in the projectile according to the thickness of the test panel and the nature of the threats that said armour panel is intended to protect. For example, according to US military specification MIL-DTL-46063H, plates made of 7039 alloy and thicker than 1.5 inches are submitted to AP tests with 0.5 inches calibre AP M2 bullets while plates thinner than 1.5 inches are submitted to AP tests with 0.3 inches calibre AP M2 bullets. However, in practice, 0.3 inches calibre AP M2 bullets are still used for AP tests on plates slightly thicker than 1.5 inches.

For both tests, the ability to stop bullets and absorb their kinetic energy is quantified by a parameter called “V50 ballistic limit” having a speed dimension. V50 is defined for example in MIL-STD-662 (1997) standard: it is the velocity at which the probability of penetration of an armour material is 50%. It is established by calculating the average of speeds attained by the projectiles on impact resulting from taking the same number of results having the highest speeds corresponding to a partial penetration and results having the lowest speeds corresponding to a complete penetration. A complete penetration occurs when the impacting projectile or any fragment (of the projectile or of the test specimen) perforates a thin witness plate located behind the test specimen.

Aluminium alloys which satisfy all the requirements for armour plate are desirable, and these desires have been met to varying degrees. For example, aluminium alloys AA5083, AA5456 and AA5059 are covered in the U.S. Military Specification for armour plate MIL- DTL-46027K (July 2007), and the alloy AA7039 is covered in the U.S. Military Specification MIL-DTL-46063H (September 1998). It is generally recognized that for many applications the alloy AA7039 armour plate is better than AA5083 and AA5456 armour plate, although the advantage is more for armour piercing ballistic performance and less so for fragment simulation performance, at least according to the military specifications. However, the alloy AA7039 can present corrosion or stress corrosion problems to a much greater degree than AA5083 and AA5456. The AA7039 alloy when used for armour plate applications is commonly in a T6 temper and the AA5083 and AA5456 alloys when used for armour plate applications is used in the HI 31 temper.

Patent document US-8,206,517-B1 discloses an armour component in the form of a plate having a thickness of 1-4 inches (25.4-101.6 mm) made of a 7xxx-series aluminium alloy, which contains essentially (in wt.%): 7.0-9.5% Zn, 1.3-1.68% Mg, 1.2-1.9% Cu, up to 0.4% of at least one grain structure element, the rest being aluminium and incidental elements and impurities. Said 7XXX-series aluminium alloy is over-aged via a three-step ageing process such that it should comply simultaneously with three conditions relating to yield strength of not greater than 68 ksi (455.6 MPa), FSP performance and spall resistance. The chemistry of the 7XXX-series alloy in US Patent No. 8,206,517 largely overlaps with that of alloy AA7085. US military specification MIL-DTL-32375 (MR) covers 7085 wrought aluminium armour plate for non-fusion welded applications in nominal thicknesses from 0.500 to 3.000 inches.

Patent document US-8, 747, 580-B1 discloses a method of manufacturing a ballistic resistant aluminium alloy, comprising forging an aluminium alloy into an armour component having a thickness of 1-4 inches (25.4-101.6 mm) wherein the 7XXX-series aluminium alloy, which contains essentially (in wt.%) 7.0-9.5% Zn, 1.3-1.68% Mg, 1.2-1.9% Cu, up to 0.4% of at least one grain structure element, the rest being aluminium and incidental elements and impurities, solution heat treating the forged armour component followed and quenching and artificial over-ageing such that it should comply simultaneously with a longitudinal yield strength of not greater than 70 ksi (469 MPa) and a defined spall resistance. The chemistry of the 7XXX-series alloy in US Patent No. 8,747,580 largely overlaps with that of alloy AA7085.

Patent document US-8, 758, 530-B1 discloses a method of manufacturing an armour component made from 2XXX- or 7XXX-series aluminium alloys and wherein the aluminium alloy product is being underaged to produce a certain ballistic performance which is said to be better than that of a peak strength aged version of the aluminium alloy products. It is reported that in particular the FSP resistance is improved by the under-ageing treatment. AP resistance and FSP resistance are antagonist properties: when an armour material has a high FSP resistance, it has a reduced AP resistance. Under-ageing of 7XXX-series aluminium alloy leads to a reduced corrosion resistance compared to over-ageing.

Patent document US-10, 308, 998-B2 discloses an armour component produced from a 7XXX-series aluminium alloy, wherein the aluminium alloy consists essentially of (in wt.%): 8.4-10.5% Zn, 1.3-2% Mg, 1.2-2% Cu, at least 0.05-0.3% of a dispersoid forming element from the group consisting of (Zr, Sc, V, Hf, Ti, Cr, and Mn), the remainder substantially aluminium, incidental elements and impurities, wherein the 7XXX alloy is in the form of a plate having a thickness of 0.5 to 3 inches (12.7-76.2 mm), and wherein the 7XXX alloy is aged to achieve a defined lower-limit for both the AP and FSP resistance. The chemistry of the 7XXX-series alloy in US Patent No. 10,308,998 largely overlaps with that of alloy AA7056. US military specification MIL-DTL-32375 (MR) covers 7056 wrought aluminium armour plate for non-fusion welded applications in nominal thicknesses from 0.500 to 3.000 inches.

Patent document WO-2017/044471-Al discloses a method of producing an armour component, comprising casting a 7XXX-series aluminium alloy to obtain an ingot, wherein the 7XXX-series aluminium alloy comprises or consists essentially of (in wt.%), 8.4-10.5% Zn, 1.3-2% Mg, 1.2-2% Cu, at least 0.04-0.3% of a dispersoid forming element from the group consisting of (Zr, Sc, V, Hf, Ti, Cr, and Mn), the remainder substantially aluminium, incidental elements and impurities, homogenization; hot working the homogenised ingot to obtain a plate having a first thickness Tl; cold working the plate having the first thickness to obtain a plate having a second thickness T2, wherein T2 = Tl - (xl*T2)/100 and 0.5<xl<15); solution heat treating; quenching and ageing. The combination of the cold working operation, i.e., a combination of cold rolling and stretching, and using Zr as the dispersoid forming element in a range of 0.04-0.10%, preferably 0.05-0.08%, provides in particular an improved spall resistance. The chemistry of the 7XXX-series alloy in WO- 2017/044471-Al largely overlaps with that of alloy AA7056.

SUMMARY

Covered embodiments of the invention are defined by the claims, not this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below.

This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification, any or all drawings, and each claim.

Described herein is an armour component produced from a 7XXX-series aluminium alloy, wherein the aluminium alloy comprises:

Zn 7.1% to 7.5%,

Mg 1.90% to 2.25%,

Cu 1.3% to 1.8%, at least 0.05-0.4% of a dispersoid forming element selected from the group consisting of Zr, Sc, V, Hf, Ti, Cr, and Mn,

Ti 0.01% to 0.06%,

Si up to 0.15%,

Fe up to 0.15%, balance unavoidable impurities and aluminium, wherein the 7XXX-series aluminium alloy is in the form of a plate having a thickness of 12.7 mm to 76.2 mm; wherein the 7XXX-series aluminium alloy is over-aged to achieve a combination of:

(i) tensile yield strength in LT-direction > 497 MPa,

(ii) ultimate tensile strength in LT-direction > 538 MPa,

(iii) elongation in LT-direction > 9%, and

(iv) an armour piercing V50 ballistic limit such that meets the requirements of

US military spec MIL DTL-32375B (MR) (2021).

Optionally, the Zn content is in a range of 7.20% to 7.5% or 7.30% to 7.5%. In some cases, the Zn/Mg ratio is less than 4 (e.g., less than 3.9). Optionally, the dispersoid forming element comprises Zr in a range of 0.06% to 0.15% (e.g., 0.08% to 0.14% or 0.09% to 0.13%). In some cases, the Mg content is in a range of 1.9% to 2.25% (e.g., 1.95% to 2.20%).

In some cases, the over-ageing treatment comprises the following 2-step treatment: 4- 12 hours at 110°C-130°C followed by 4-20 hours at 140°C-160°C. Optionally, the over ageing treatment comprises the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C. Optionally, the over-ageing treatment comprises the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-18 hours at 140°C-160°C.

The 7XXX-series aluminium alloy can optionally be manufactured with the following steps: a. casting said alloy into an ingot form; b. homogenizing said ingot; c. hot working said ingot to obtain a plate; d. solution heat treating; e. quenching; f. stretching to obtain a permanent elongation from 1% to 3%; and g. over-ageing at least in two steps, the over-ageing treatment corresponding to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C.

Optionally, the 7XXX-series alloy comprises or consists of, in wt.%:

Zn 7.1% to 7.5%, preferably 7.20% to 7.5%,

Mg 1.90% to 2.25%, preferably 1.95% to 2.25%, Cu 1.3% to 1.8%,

Zr 0.06% to 0.15%, preferably 0.08% to 0.13%,

Ti 0.01% to 0.06%,

Si up to 0.15%

Fe up to 0.15%, balance unavoidable impurities and aluminium.

The Si content or the Fe content can optionally be up to 0.10 wt. %. In some cases, the elongation in LT-direction is > 9 %.

Further described herein is a method of producing an armour component as described herein, comprising: a. casting said alloy into an ingot form; b. homogenizing said ingot; c. hot working said ingot to obtain a plate; d. solution heat treating; e. quenching; f. stretching to obtain a permanent elongation from 1% to 3%; and g. over-ageing at least in two steps, the over-ageing heat treatment corresponding to the following two-step heat treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C.

Optionally, the over-ageing treatment corresponds to the following 2-step treatment: 4- 12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C. Optionally, the over ageing treatment corresponds to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-18 hours at 140°C-160°C.

Also described herein is an armour component produced from a 7XXX-series aluminium alloy, wherein the aluminium alloy consists essentially of:

Zn 7.1% to 7.5%,

Mg 1.90% to 2.25%,

Cu 1.3% to 1.8%, at least 0.05-0.4% of a dispersoid forming element selected from the group consisting of Zr, Sc, V, Hf, Ti, Cr, and Mn,

Ti 0.01% to 0.06%,

Si up to 0.15%,

Fe up to 0.15%, balance unavoidable impurities and aluminium, wherein the 7XXX-series aluminium alloy is in the form of a plate having a thickness of 12.7 mm to 76.2 mm; wherein the 7XXX-series aluminium alloy is over-aged to achieve a combination of:

(i) tensile yield strength in LT-direction > 497 MPa,

(ii) ultimate tensile strength in LT-direction > 538 MPa,

(iii) elongation in LT-direction > 9%, and

(iv) an armour piercing V50 ballistic limit such that meets the requirements of

US military spec MIL DTL-32375B (MR) (2021).

Optionally, the Zn content is in a range of 7.20% to 7.5% or 7.30% to 7.5%. In some cases, the Zn/Mg ratio is less than 4 (e.g., less than 3.9). Optionally, the dispersoid forming element is essentially Zr in a range of 0.06% to 0.15% (e.g., 0.08% to 0.14% or 0.09% to 0.13%). In some cases, the Mg content is in a range of 1.9% to 2.25% (e.g., 1.95% to 2.20%).

In some cases, the over-ageing treatment corresponds to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 4-20 hours at 140°C-160°C. Optionally, the over-ageing treatment corresponds to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C. Optionally, the over-ageing treatment corresponds to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-18 hours at 140°C-160°C.

The 7XXX-series aluminium alloy can optionally be manufactured with the following steps: a. casting said alloy into an ingot form; b. homogenizing said ingot; c. hot working said ingot to obtain a plate; d. solution heat treating; e. quenching; f. stretching to obtain a permanent elongation from 1% to 3%; and g. over-ageing at least in two steps, the over-ageing treatment corresponding to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C.

Optionally, the 7XXX-series alloy consists of, in wt.%:

Zn 7.1% to 7.5%, preferably 7.20% to 7.5%,

Mg 1.90% to 2.25%, preferably 1.95% to 2.25%,

Cu 1.3% to 1.8%, Zr 0.06% to 0.15%, preferably 0.08% to 0.13%,

Ti 0.01% to 0.06%,

Si up to 0.15%

Fe up to 0.15%, balance unavoidable impurities and aluminium.

The Si content or the Fe content can optionally be up to 0.10 wt. %. In some cases, the elongation in LT-direction is > 9 %.

Further described herein is a method of producing an armour component as described herein, comprising: a. casting said alloy into an ingot form; b. homogenizing said ingot; c. hot working said ingot to obtain a plate; d. solution heat treating; e. quenching; f. stretching to obtain a permanent elongation from 1% to 3%; and g. over-ageing at least in two steps, the over-ageing heat treatment corresponding to the following two-step heat treatment: 4-12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C.

Optionally, the over-ageing treatment corresponds to the following 2-step treatment: 4- 12 hours at 110°C-130°C followed by 12-20 hours at 140°C-160°C. Optionally, the over ageing treatment corresponds to the following 2-step treatment: 4-12 hours at 110°C-130°C followed by 12-18 hours at 140°C-160°C.

Other objects and advantages of the invention will be apparent from the following detailed description of non-limiting examples.

DETAILED DESCRIPTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2019, and frequently updated, and are well known to the person skilled in the art. The temper designations are laid down in European standard EN515. Unless mentioned otherwise, static mechanical characteristics, in other words the ultimate tensile strength UTS, the tensile yield stress TYS and the elongation at fracture A, are determined by a tensile test according to standard ASTM B557.

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

The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying element to which it refers. For example, up to 0.03% Mn may include an aluminium alloy having no Mn.

Provided herein is an armour component product from a 7XXX-series aluminium alloy in the form of a plate and having been over-aged to achieve a combination of a high strength, high elongation at fraction and good AP resistance.

Also provided herein is a method for manufacturing an armour component product from a 7XXX-series aluminium alloy in the form of a plate and having been over-aged to achieve a combination of a high strength, high elongation at fraction and good AP resistance.

These and other objects and further advantages are met or exceeded by the present disclosure providing an armour component produced from a 7XXX-series aluminium alloy, wherein the aluminium alloy comprises or consists essentially of:

Zn 7.1% to 7.5%,

Mg 1.90% to 2.25%,

Cu 1.3% to 1.8%, at least 0.05-0.4% of a dispersoid forming element selected from the group consisting of Zr, Sc, V, Hf, Ti, Cr, and Mn,

Ti 0.01% to 0.06%,

Si up to 0.15%,

Fe up to 0.15%, balance unavoidable impurities and aluminium. Typically, inevitable impurities are each up to 0.05% maximum and in total 0.15% maximum. The 7XXX-series aluminium alloy is in the form of a plate having a thickness of 12.7 mm to 76.2 mm (0.5 to 3 inches); and the 7XXX-series aluminium alloy is over-aged to achieve a combination of target properties:

(i) tensile yield strength in long transverse-direction (LT-direction) > 504 MPa for thicknesses of 1.501 to 2.000 inch inch and > 497 MPa for thicknesses of 2.001 to 3.000 inch inch

(ii) ultimate tensile strength in LT-direction > 545 MPa for thicknesses of 1.501 to 2.000 inch inch and > 538 MPa for thicknesses of 2.001 to 3.000 inch inch

(iii) elongation in LT-direction >10% for thicknesses of 1.501 to 2.000 inch inch and > 9% for thicknesses of 2.001 to 3.000 inch inch

(iv) an armour piercing V50 ballistic limit such that: the minimum requirements of MIL-DTL-32375B Class I Type A are met.

Compared to AA7085, the alloys as described herein have a higher tensile strength of at least 1 Ksi for each thickness range. Compared to AA7056, the alloys as described herein generally have an elongation of at least a minimum 2-3% or higher. Compared to 7081 and 7056, the alloys as described herein have a lower density, which is beneficial for lightweighting, as further described herein.

Another advantage is also the improved Mass Efficiency compared to AA7056-T711 counterparts. Due to amongst others a significantly lower Zn content, the 7XXX-series aluminium alloy product described herein has a lower specific density measured at 20 ^° C compared to AA7056 alloys resulting in a favourable strength-to-weight ratio or specific strength (tensile strength divided by specific density). The Mass Efficiency is a measure for the AP performance and relates also to the specific density and allows for a fair comparison of various armour plate materials of similar gauge against each other. Mass Efficiency or “Em” is defined as the weight per unit area of a reference material, for example an AA7056- T761 counterpart alloy, required for defeating a given ballistic threat divided by the weight per unit area of the subject material. The improved mass efficiency of the armour plate component allows for the construction of a lighter vehicle while offering the same resistance against incoming projectiles. Weight saving in an armoured vehicle can translate amongst other advantages, into vehicle mobility. Alternatively, when constructing an armoured vehicle an unchanged plate thickness can be used while offering a significantly improved resistance against incoming projectiles and thereby an increased survivability.

Zinc (Zn), magnesium (Mg), and copper (Cu) are the major alloying elements of the 7XXX-series aluminium alloy armour component. Zn combined with Mg and Cu within the defined compositional ranges provide simultaneously high static mechanical properties in combination with good AP ballistic test results.

The Zn content should be in a range of 7.1% to 7.5%. In a preferred embodiment the Zn content is at least 7.20%, and more preferably at least 7.30%.

The Mg content should be in a range of 1.90% to 2.25%. In a preferred embodiment the Mg content is at least 2.0%. In an embodiment the Mg content is not more than about 2.20%.

In an embodiment the 7XXX-series aluminium alloy has a Zn/Mg ratio (in wt.%) of less than 4. In an embodiment the Zn/Mg ratio is less than 3.9, and preferably less than 3.8.

Exfoliation corrosion tests according to ASTM G34 exhibit a rating of EB or EA regardless if the test is performed on the as-rolled surface or if the surface has been milled down to 90% of its thickness (s/10). A rating of EA indicates superficial corrosion, while a rating of EB indicates moderate corrosion.

Furthermore, it has been found that by controlling the Zn/Mg ratio the resistance to hydrogen embrittlement is improved. Hydrogen embrittlement is where brittle cracking of an aluminium alloy can occur when a susceptible aluminium alloy is subjected to a sustained stress in particular in the short transverse (ST) direction for longer periods of time in a humid atmosphere. This phenomenon, also known as environmentally assisted cracking (“EAC”), can be a challenge for component manufacturers since under certain conditions the structural integrity can be affected. Sensitivity to this form of EAC has been observed especially in high Zn containing high strength aluminium alloys.

In an embodiment the 7XXX-series aluminium alloy product has a Cu-content of maximum about 1.8%, and preferably of maximum about 1.75%, and more preferably of maximum about 1.70%. The minimum Cu-content is about 1.3%, and more preferably 1.35% to provide sufficient strength and elongation at fracture, AP resistance, good stress corrosion cracking (SCC) resistance in combination with a low sensitivity to EAC under conditions of high stress and humid environment.

The aluminium alloy comprises at least about 0.05%-0.4% of a dispersoid forming element selected from the group consisting of Zr, Sc, V, Hf, Ti, Cr, and Mn to control the grain structure during thermo-mechanical processing. Two or more dispersoid forming elements can be added but the sum should not exceed 0.4%. In a preferred embodiment the dispersoid forming element is Zr. Preferably the Zr content is maximum about 0.15%, and more preferably about 0.14%. In an embodiment the Zr content is at least 0.06%, and more preferably about 0.08%. The iron (Fe) and silicon (Si) contents should be kept low, for example not exceeding about 0.15% Fe, and preferably less than about 0.10% Fe, and not exceeding about 0.15% Si and preferably about 0.10% Si or less.

In the aluminium alloy the balance is made by aluminium and unavoidable impurities, typically each up to 0.05% maximum and in total 0.15% maximum.

In an embodiment the aluminium alloy includes or consists of: Zn 7.1% to 7.5%, Mg 1.90% to 2.25%, Cu 1.3% to 1.8%, Zr 0.06% to 0.15%, Ti 0.01% to 0.06%, Si up to 0.15%, Fe up to 0.15%, balance unavoidable impurities and aluminium, and with preferred narrower ranges as herein described and claimed.

The 7XXX-series aluminium alloy of the armour component has been over-aged in at least two steps to achieve the combination of properties.

In an embodiment, the over-ageing heat treatment corresponds to the following 2-step treatment: about 4 to 12 hours at 110°C to 130°C followed by 1 to about 2 to 20 hours at 140°C to 160°C, and preferably about 12 to 18 hours at 140°C to 160°C.

In an embodiment the armour component from the 7XXX-series aluminium alloy plate achieves an armour piercing V50 ballistic limits described by MIL-DTL-32375B Class 1 Type A.

In an embodiment the armour component from the 7XXX-series aluminium alloy achieves a tensile yield strength in LT-direction > 497 MPa, and preferably > 504 MPa.

In an embodiment the armour component from the 7XXX-series aluminium alloy achieves an ultimate tensile strength in LT-direction > 538 MPa, and preferably > 545 MPa.

In an embodiment the armour component from the 7XXX-series aluminium alloy achieves an elongation in the LT-direction > 10 %.

According to the present description, the 7XXX-series aluminium alloy plate of the armour component has a thickness in the range of about 12.7 mm to about 76.2 mm (about 0.5 to about 3 inches), and preferably of about 25.4 mm to about 76.2 mm (about 1 to about 3 inches).

The 7XXX-series plate product forming part of the armour component is manufactured in the conventional method, the method comprising the steps, in that order, of: a. casting stock of a rolling ingot of the AA7XXX-series aluminium alloy according to the present disclosure; b. homogenizing the rolling ingot; c. hot working by means of rolling the ingot to obtain a plate; d. solution heat treating (“SHT”) of the plate; e. cooling the SHT plate, preferably by one of spray quenching or immersion quenching in water or other quenching media; f. stretching or compressing of the cooled SHT plate or otherwise cold working of the cooled SHT plate to relieve stresses, for example levelling or drawing or cold rolling of the cooled SHT stock; g. artificial over-ageing at least in two steps to obtain the combination of target properties.

The 7XXX-series aluminium alloy can be provided as an ingot or slab or billet for fabrication into a suitable wrought product by casting techniques regular in the art for rolling ingots, e.g., Direct-Chill (DC)-casting, Electro-Magnetic-Casting (EMC)-casting, Electro- Magnetic-Stirring (EMS)-casting. Slabs resulting from continuous casting, e.g., belt casters or roll casters, also may be used, which in particular may be advantageous when producing thinner gauge plate products. Grain refiners such as those containing titanium (Ti) and boron (B), or titanium and carbon (C), can be used. The Ti-content in the aluminium alloy is up to 0.15%, and preferably in a range of about 0.01% to 0.06%. Optionally a cast ingot can be stress relieved, for example by holding it at a temperature in a range of about 350°C to 450°C followed by slow cooling to ambient temperature. After casting the ingot can be scalped to remove segregation zones near the as-cast surface of the ingot and to improve product flatness.

The purpose of a homogenization heat treatment has at least the following objectives: (i) to dissolve as much as possible coarse soluble phases formed during solidification, and (ii) to reduce concentration gradients to facilitate the dissolution step. A preheat treatment achieves also some of these objectives.

First, the soluble eutectic phases and/or intermetallic phases such as the S-phase, T- phase, and M-phase in the aluminium alloy ingot are dissolved using regular industry practice. This is typically carried out by heating the stock to a temperature of less than 500°C, typically in a range of 450°C to 485°C, as S-phase (AkMgCu-phase) has a dissolution temperature of about 489°C in AA7XXX-series alloys and the M-phase (MgZm-phase) has a dissolution temperature of about 478°C. This can be achieved by a homogenization treatment in said temperature range and allowed to cool to the hot rolling temperature, or after homogenization the stock is subsequently cooled and reheated before hot rolling. The homogenization process can also be done in two or more steps if desired, and which are typically carried out in a temperature range of 440°C to 490°C for the AA7XXX-series alloy. In a particular favourable embodiment a two-step homogenization process is applied. There is a first step between 455°C and 470°C, and a second step between 470°C and 485°C, to optimise the dissolving process of the various phases depending on the exact aluminium alloy composition.

The soaking time at the homogenization temperature is in the range of 1 to 50 hours, and more typically for 2 to 20 hours. The heat-up rates that can be applied are those which are regular in the art.

The hot working, and hot rolling in particular, may be performed to a near final gauge, which is between 12.7 mm and 76.2 mm.

In an embodiment the plate material is hot rolled in a first hot rolling step to an intermediate hot rolled gauge, followed by an intermediate annealing step and then hot rolled in a second hot rolling step to near final hot rolled gauge.

In another embodiment the plate material is hot rolled in a first hot rolling step to an intermediate hot rolled gauge, followed by a recrystallization annealing treatment at a temperature up to the SHT temperature range and then hot rolled in a second hot rolling step to near final hot rolled gauge.

A next process step is solution heat treating (“SHT”) of the hot rolled plate. The product should be heated to bring as much as possible, all or substantially all portions of the soluble zinc, magnesium and copper into solution. The SHT is preferably carried out in the same temperature range and time range as the homogenization treatment according as set out in this description, i.e. about 460-490°C. However, it is believed that also shorter soaking times can still be very useful, for example in the range of about 2 to 180 minutes. The SHT is typically carried out in a batch or a continuous furnace. After SHT, it is important that the aluminium alloy be cooled with a high cooling rate to a temperature of 90°C or lower, preferably to ambient temperature, to prevent or minimise the uncontrolled precipitation of secondary phases, e.g., AbCuMg and AbCu, and/or MgZm. On the other hand cooling rates should preferably not be too high to allow for a sufficient flatness and low level of residual stresses in the product. Suitable cooling rates can be achieved with the use of water, e.g. water immersion or water jets. The cooling rate is preferably in a range of about l°C/sec to 9°C/sec, and preferably about 2°C/sec to 5°C/sec when measured at mid-thickness of the product.

The plate is further cold worked, for example, by stretching in the range of about 1% to 6% to relieve residual stresses therein and to improve the flatness of the plate product. Preferably the stretching is in the range of about 1% to 3%. After cooling the plate is artificially over-aged, in at least two steps to achieve the combination of properties. In an embodiment the over-ageing treatment corresponds to the following two-step ageing heat treatment: about 4 to 12 hours at 110°C to 130°C followed by about 12 to 20 hours, and about preferably 12-18 hours, at 140°C to 160°C.

In another embodiment, the 7XXX-series plate product forming part of the armour component can be manufactured by: a. casting stock of a rolling ingot of the AA7XXX-series aluminium alloy according to this disclosure; b. homogenizing the rolling ingot; c. sawing or machining the ingot; d. preheating the ingot; e. rolling the ingot to form a plate; f. sawing the plate; g. solution heat treating (or SHT) the plate at approximately 470°C; h. stretching the plate approximately 2% to 3% with respect to length; i. heat treatment of the plate at 120°C for a period (e.g., up to eight hours) and then at 155°C for a period (e.g., up to six hours); j . marking or sawing the plate; k. testing properties and performance of the plate; and l. inspection, and packaging, of the plate.

The 7XXX-series aluminium alloy can be provided and cast as described above. The homogenization can be performed on the ingot as described above. The ingot can be sawed or machined as described above or using other suitable sawing or machining techniques. The ingot can be preheated using a pit furnace or by other suitable techniques, for heating the ingot to a suitable temperature for rolling. The ingot can be rolled to form a plate as described above or using other suitable rolling techniques. The plate can be solution heat treated (or SHT) at approximately 470°C and can be stretched to between 2% and 3%. The plate can be heat treated using the techniques described herein, or using other suitable techniques. The plate can be marked, or sawed, using suitable techniques. The plate can be tested for ballistic properties or performance, static properties or performance, or for other suitable properties or performance. The plate can be inspected to verify the plate is free from defects and can be packaged.

In yet another embodiment, the 7XXX-series plate product forming part of the armour component can be manufactured by: a. casting stock of a rolling ingot of the AA7XXX-series aluminium alloy as described herein; b. homogenizing the rolling ingot; c. sawing or machining the ingot; d. preheating the ingot; e. rolling the ingot to form a plate; f. sawing the plate; g. solution heat treating (or SHT) the plate at approximately 470°C; h. stretching the plate approximately 2% to 3% with respect to length; i. heat treatment of the plate at 120°C for a period (e.g., up to eight hours) and then at 155°C for a period (e.g., 14, 16, or 18 hours); j . marking or sawing the plate; k. testing properties and performance of the plate; l. inspection, and packaging, of the plate.

The 7XXX-series aluminium alloy can be provided and cast as described above. The homogenization can be performed on the ingot as described above. The ingot can be sawed or machined as described above or using other suitable sawing or machining techniques. The ingot can be preheated using a pit furnace, or by other suitable techniques, for heating the ingot to a suitable temperature for rolling. The ingot can be rolled to form a plate as described above or using other suitable rolling techniques. The plate can be solution heat treated (or SHT) at approximately 470°C and can be stretched to between 2% and 3%. The plate can be heat treated using the techniques described herein, or using other suitable techniques, at 120°C for a period (e.g., eight hours) and then at 155°C for a period (e.g., 14, 16, or 18 hours). The plate can be marked, or sawed, using suitable techniques. The plate can be tested for ballistic properties or performance, static properties or performance, or for other suitable properties or performance. The plate can be inspected to verify the plate is free from defects and can be packaged.

In some examples, the plate can be manufactured by any of the above-described processes and can be tempered to a T76 temper or a T79 temper. The T76 temper can include solution heat treating the plate and having the plate artificially overaged for achieving a high degree of exfoliation corrosion resistance. The T79 temper can include solution heat treating the plate and having the plate artificially overaged, to a lesser degree than the T76 temper, for achieving a high degree of exfoliation corrosion resistance.

The invention will now be illustrated with reference to non-limiting examples according to the invention. Example 1.

Aluminium alloy plates have been produced having different thicknesses (38.1, 50.8 and 76.2 mm) and subjected to varying over-ageing heat treatment followed by testing for the various properties. All aluminium alloy plates had of nominal composition (in wt. %) of 7.38% Zn, 2.03%

Mg, 1.55% Cu, 0.12% Zr, 0.03% Ti, 0.07% Fe, 0.04% Si, <0.001% Mn, balance aluminium and unavoidable impurities. The alloy composition of the 7XXX-series aluminium alloy plate is according to this disclosure. The alloy has a Zn/Mg-ratio of 3.63.

The industrial manufacturing process includes DC casting of rolling ingots, homogenizing the ingot, hot rolling the homogenized ingot to arrive at an intermediate thickness, solution heat treating the plates at about 470°C, quenching, stretching of the plates for about 2.5% to arrive at final thickness, artificial over-ageing of the stretched plate as indicated in Tables 1, 2, 3, and 4.

The plate products have been tested for their ballistic properties, in accordance with US military standard MU. DTL-32375B (MR) (2021). In accordance to this standard Cal. 50 AP M2 projectiles were used for ballistic testing of the 38.1 and 50.8 mm plates and 14.5-mm BS-41 projectiles were used for ballistic testing of the 76.1 mm plates respectively. The results are listed in Tables 1 and 4.

The plate products have been tested in their static mechanical properties (total yield strength (TYS), ultimate tensile strength (UTS), and percent elongation (A)) in both the LT- and longitudinal (L)-direction. The results (average over [three] tests) are listed in respectively Table 2 and Table 3.

Table 1. Ballistic properties of plates with respect to various thicknesses and heat treatments.

Table 2. Mechanical properties in LT-direction as function of plate thickness and over ageing practice. Table 3. Mechanical properties in L-direction as function of plate thickness and over-ageing practice.

Table 4: Ballistic properties of various lots of plates with varying thicknesses.

While various embodiments of the technology described herein have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the presently disclosed technology.