7xxx alloy components for defense application with an improved spall resistance

BACKGROUND

Cross-Reference to Related Applications

This application is an international application which claims priority to US 62/215,842 (filed September 9, 2015), the contents of which are incorporated herein by reference in their entirety.

Technical Field

The present application relates to armor components made of high strength aluminum, such as 7xxx series aluminum alloys, which may be employed in civil or military ballistic protection systems. It relates more particularly to armor components such as armor hull walls and add-on armor appliques in vehicles.

Description of Related Art

Structures and vehicles are increasingly designed to protect their occupants from penetration of projectiles. These projectiles are typically metal, and may strike the structure at high speed, attempting to perforate the structure, and thereby inflicting damage to the occupants. The aim of an armor shield is to provide ballistic defeat of such projectiles or fragments.

The demand of ballistic grade aluminum has seen a large increase due to the ballistic advantages of this low density metal when utilized as primary armors on vehicles. Aluminum armors are enablers of lightweight shields, which fit with weight requirements of such vehicles.

An armor panel has a front face, exposed to impacts and shocks, and a rear face. Upon impact on such armor panel, an armor piercing projectile can be completely stopped in the panel. However, some fragments may be violently ejected from the rear face of the panel, towards the vehicle interior.

To characterize their shield effectiveness, armor panels performances must comply with different requirements. Key requirements includes ballistic grade aluminum alloy should achieve a high combination of armor piercing (AP) resistance, fragment simulated particle (FSP) resistance, as well as spall resistance.

AP resistance means achieving a predetermined armor piercing V50 ballistic limit using a piercing projectile, so called AP round. This characterizes the resistance to perforation. FSP resistance means achieving a predetermined V50 ballistic limit using another kind of projectile: FSP projectile, so-called FSP round, which is defined in M IL-DTL-46593B specification. FSP resistance characterizes the ability to withstand impacts that generate fragmented debris. V50 ballistic limit, which has a speed dimension, is defined in MIL-STD-662F (1997) standard. The V50 ballistic limits shall be calculated by taking the arithmetic mean of an equal number of the highest partial and the lowest complete penetration impact velocities. The specific V50 required at a given thickness, the amount of individual FSP tests to include in the calculation of a V50, and other material requirements can vary between different aluminum alloys. As an example M IL-DTL-46027K enumerates the requirements for 5083, 5456, and 5059 aluminum alloys.

Spall resistance as indicated within MIL-STD-662F (1997) means that, during ballistic tests conducted in accordance with this standard, no substantial detachment or delamination of a layer of material occurs in the area surrounding the location of the impact, either on the front or the rear surface of the armor. This pass-fail criterion for spalling and the related high-low impact velocity averaging used in a V50 evaluation facilitates the testing method but does not allow for further analysis into the amount of spall damage. Additionally such a testing method can require a sizable volume of metal which can hinder the ability to either rapidly prototype or sweep through a range of alloy chemistry, transformation routes, tempering options, etc... Consequently, in the present application, spall resistance is quantified using a spall resistance score. Spall resistance score is investigated with a FSP test according to M IL-STD-662F (1997) standard. Trials are performed using 20 mm FSP rounds which comply with M IL-DTL-46593B. The dimensions of the FSP rounds are the following :

diameter : 20 mm - 0. 787 inch ;

length : 23.0 mm - 0.9 inch ;

- flat : 9.3 mm - 0. 366 inch.

The spall resistance score is determined by the following method. As shown on Figure 1, on the rear surface, surrounding the site of impact b, is an area of material which was visibly affected by the ballistic strike and the partial or complete penetration by the FSP round into the plate. The boundary of this affected area is indicated on Figure 1 with the dotted line a. Along with any possible exit hole of the FSP round, any material plug and area of any delaminated material is included within the deformed area. The area also includes bulged or fractured material, which is still connected to the test plate. Radiating perpendicular from the direction of impact (i.e. along the rear surface) a length c is measured which represents the longest distance from the site of impact to the edge or boundary of the deformed area. This maximum length c is than doubled and from the new value 20 mm is subtracted, which corresponds to the diameter of the FSP round. The remaining total is divided by 20 mm, so as to get a dimensionless parameter, so- called Spall Resistance Score. The spall resistance score is 0 when only a plug is formed, and no significant lateral material is affected by the impact. Higher scores represent multiples of the impacting round diameter. A better score is as low as possible.

Achieving high AP resistance as well as high FSP resistance remains challenging. Provisional US application 61/948870, which is hereby incorporated by reference in its entirety, discloses an armor component produced from a 7XXX aluminum series alloy, having an improved combination of AP resistance and FSP resistance. The aluminum alloys disclosed therein consists essentially of :

- 8.4 wt.% < Zn < 10.5 wt.% ;

1.3 wt.% < Mg < 2 wt.% ;

1.2 wt.% < Cu < 2 wt.% ;

at least one dispersoid forming element, with a total dispersoid forming element content higher than 0.05 wt. % ;

- the remainder substantially being aluminum, incidental elements and impurities.

According to US61/948870, the armor component is in the form of a plate having a thickness ranging from about 0.5 to about 3 inches ; it is aged to achieve high AP and FSP V50 ballistic limits. Namely, the FSP V50 ballistic limit is such that :

V50 (FSP 20 mm) > 1633 T ² - 1479 T + 1290, where T is the plate thickness (inch) and the unit of V50 is feet/s. "FSP 20 mm" means FSP trials being conducted with 20 mm diameter rounds.

The AP V50 ballistic limit is such that :

V50 (0.3cal AP mm) > -282 T ² +1850 T + 610, where T is the plate thickness (inch) and the unit of V50 is feet/s. "0.3cal AP V50" means AP trials being conducted with 0. 30 caliber rounds.

The combined targeted ballistic properties can be obtained through a multiple steps aging during the manufacturing process. A typical aging treatment is about 4 - 8 hours at about 110°C - 130°C, followed by about 12 - 20 hours at about 140°C - 160 °C.

Besides FSP and AP ballistic resistance, a high spall resistance is also often required. The present invention provides armor components having high AP and FSP ballistic resistance as well as improved spall resistance. SUMMARY OF THE INVENTION

7xxx series aluminum alloy armor components produced in accordance with the present invention exhibit an improved spall resistance. Processing parameters, as well as the alloy composition, are controlled in order to increase spall resistance, and to have high AP and FSP ballistic resistance.

The composition of the 7xxx series aluminum alloy is preferably selected in order to comprise Zr as at least one dispersoid forming element, more preferably Zr from about 0.04 to about 0.15 wt. %, which results in a further improvement of the performances.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts an embodiment of the method used to determine the spall resistance score according to the present disclosure.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Unless otherwise stated, all compositional percentages are expressed in weight percent (wt.%), based on the total weight of the alloy. Alloy designation is in accordance with the regulations of The Aluminum Association known of those skilled in the art.

As used herein, the expression "up to" when referring to the amount of an element means that this elemental composition is optional, and includes a zero amount of this element. When the term "between" appears in the present application and claims, it is understood that the upper and lower numbers in the range are included. Similarly, "lower than" or "higher than" and such phrases include the numeral in the recited range.

The present invention relates to a method of producing an armor component, comprising : a) casting a 7XXX series aluminum alloy to obtain an ingot, said alloy consisting essentially of

- 8.4 wt.% < Zn < 10.5 wt.% ;

- 1.3 wt.% < Mg < 2 wt.% ;

- 1.2 wt.% < Cu < 2 wt.% ;

at least one dispersoid forming elements, with a total content of dispersoid forming elements of at least 0.04 wt. %; the remainder substantially including aluminum, incidental elements, and impurities;

b) homogenizing said ingot;

c) hot working said homogenized ingot to obtain a plate having a first thickness Tl ; d) cold working the plate having the first thickness to obtain a plate having a second thickness T2, wherein T2 = Tl - (xi*T2)/100 and 0.5 < Xi < 15;

e) solution heat treating ;

f) quenching ; and

g) aging.

The alloy product according to the invention can be prepared using a conventional melting process and by casting into an ingot form. During casting, the below described grain refiners may be added. After scalping and homogenizing at 460°C - 520 °C during 5 - 60 hours, the ingot is further hot worked, typically hot worked in several steps, to obtain a plate having a first thickness Tl. According to an embodiment, hot working is hot rolling or forging, and more preferably hot rolling.

It has been shown that performing a cold working step prior to solution heat treatment (SHT), and more specifically between hot working step and solution heat treatment, resulted in an improvement of the spall resistance. After the cold working step, the cold worked plate has a second thickness T2, wherein T2 = Tl - (xi*T2)/100 and 0.5 < Xi < 15. According to an embodiment, the cold worked plate has a second thickness T2, wherein T2 = Tl - (xi*T2)/100 and 2 < xi < 10, preferably 4 < xi < 8. According to an embodiment, said cold working step prior to solution heat treatment is a cold rolling step.

The cold worked plate having a second thickness T2 is then solution heat treated and quenched. In a preferred embodiment, the solution heat treatment is performed between 1 to 5 hours at 460°C - 480 °C and quenched typically to a temperature lower than 95°C.

Optionally, another cold working step is performed after SHT and quench to obtain a plate having a third thickness T3. This other cold working step can be a stretching. Said other cold working step gives rise to a plastic deformation for example up to 3%, typically between 1 and 3 %. When no second cold working step is applied, the second thickness T2 is the final thickness of the armor component. When the method according to the invention involves two cold working steps, the final thickness is the third thickness T3 of the plate obtained after the second cold working step and the final thickness of the armor component is lower than the second thickness T2.

According to an embodiment, step g) includes at least a two step aging. Preferably, this two-step aging includes :

- a first aging step of about 4-8 hours, and more preferably of about 5-7 hours, at about

110°C to 130°C, and more preferably at about 115°C to 125 °C ; and

a second aging step of about 12-20 hours, and more preferably about 14-18 hours, at about 130°C to 180 °C and more preferably at about 145°C to 165 °C.

According to another embodiment, aging is performed so that the total equivalent time at 150°C ranges between about 5h to about 50h, and more preferably from about lOh to about 40h. The total equivalent time t(eq) at 150°C is given by the expression :

-15683

/ e T dt

f ( ^ei ? = -15683

e ^Tre f

where :

T is the instantaneous temperature in Kelvin of treatment ;

- t denotes the time, including heating and cooling steps ;

T _re f is a reference temperature, whose value is 423 K (i-e 150°C) ;

t(eq) is expressed in hours ;

15683 is a temperature derived from the diffusion activation energy of Mg, Q = 130400 J/mol.

Aging step, particularly according to the preceding embodiments, is assumed to improve both AP and FSP resistance.

According to a preferred embodiment, the armor component provided is a plate having a final thickness ranging between 0.5 inch and 3 inches.

The alloy generally comprises, and in some instances consists essentially of, zinc (Zn), copper (Cu) and magnesium ( Mg) as main alloying ingredients, with at least one dispersoid forming elements, the total content of dispersoid forming elements being at least 0.04 wt. %, and the remainder substantially including aluminum, incidental elements, and impurities.

The Zn content ranges between about 8.4 wt.% and about 10.5 wt.%, preferably between 8.5 wt.% and 9.5 wt.% and more preferably between 8.5 wt.% to 9 wt.% Trials have shown that such content resulted in the highest results in both AP and FSP ballistic tests and/or in an improvement of spall resistance score.

The magnesium content ranges between about 1.3 wt.% and about 2 wt.%, preferably between 1.5 wt.% to about 2 wt.%, and more preferably from about 1.6 wt.% to 1.9 wt.%.

In a preferred embodiment, the Mg/Zn ratio is lower than or equal to 0.25, and preferably lower than or equal to 0.20, where Mg and Zn denote weight percentages of magnesium and zinc respectively.

The copper content ranges between about 1.2 wt.% and about 2 wt.%. Preferably, Cu content lies between 1.5 wt.% and 1.9 wt.%. High AP and FSP resistances were obtained when copper and magnesium contents were approximately the same, i-e typically when 0.9 < Cu/Mg < 1.1.

At least one dispersoid forming element such as Zr, Sc, V, Hf, Ti, Cr and Mn is added. As used herein, dispersoid forming elements means elements that are deliberately added so as to control the grain structure. The total content of dispersoid forming elements is at least 0.04 wt.%, preferably from about 0.04 wt. % to about 0.3 wt. %. The optimum levels of dispersoid forming elements depend on the processing. When scandium is added, its content is preferably lower than about 0.3 wt.%, and more preferably lower than 0.17 wt.%. When combined with Zr, the sum of Sc and Zr is preferably less than about 0.17 wt.%. The alloy may also include Cr, Hf and V, with content lower than 0.3 wt.%, and preferably lower than 0.15 wt.%. When Mn is added, its content is preferably less than 0.3 wt.%.

In a preferred embodiment, the dispersoid forming element is essentially zirconium. Preferably, Zr content is less than about 0.15 wt.%, more preferably from 0.04 to 0.1 wt. %, and even more preferably from 0.05 to 0.08 wt.%. The best AP and FSP resistance compromise and/or the highest spall resistance score is observed when 0.05 wt.% < Zr < 0.08 wt.%. The inventors have shown that, in comparison with the 7XXX alloy series described in US61/948870, lowering the Zr content as well as introducing a cold working before SHT improved spall resistance and maintained a high AP and FSP compromise so that

(i) V50 (FSP 20mm) > 1633 T ² - 1479 T + 1290 where T is the thickness plate (unit: inch) and the unit of V50 is feet/s; and

(ii) V50 (0.30cal AP M2) > -282 T ² + 1850 T +610 where T is the thickness plate (unit: inch) and the unit of V50 is feet/s.

It is assumed that such Zr content, as well as performing a cold working before SHT and quench, results in an increase of the grain size, as well as more equiaxed grain shapes. As used herein, impurities mean elements that may be found in the alloy in minor amounts, but that are not intentionally added to the alloy. Those elements result from natural impurities in the individual alloy elements or from the manufacturing process. Fe and Si are the main impurities generally present in aluminum alloys. Fe content is preferably lower than 0.3 wt.%, and more preferably lower than 0.1 wt.%. Si content is preferably lower than 0.2 wt.%, and more preferably lower than 0.1 wt.%. Preferably, each other impurity element is present at up to about 0.05 wt.% with a total impurity (other than Fe or Si) content up to about 0.15 wt.%, based upon the total weight of the alloy taken as wt.%.

As used herein, incidental elements mean elements that may be optionally added to the alloy during the manufacturing process. Addition of such elements results from casting aids and deoxidizers. Titanium, Titanium boride (T1B2) or titanium carbide (TiC) are usual grain refiners. Deoxidizers may include Ca, Sr and Be. Preferably, the amount of the incidental elements Ca, Sr, Be, Br and C lies below 0.005 wt.%, that of Ti below 0.05 wt. %.

In a preferred embodiment, the alloy chemistry is :

- 8.5 wt.% < Zn < 9.5 wt.% ;

1.5 wt.% < Mg < 2 wt.% ;

1.5 wt.% < Cu < 1.9 wt.% ;

- 0.04 wt.% < Zr < 0.10 wt.% , and preferably 0.05 wt.% < Zr < 0.08 wt.%;

up to 0.15 wt.% dispersoid forming element other than Zr ;

- the remainder substantially including aluminum, incidental elements, and impurities.

Another aspect of the present invention is an armor component produced from a 7XXX series aluminum alloy consisting essentially of:

- 8.4 wt.% < Zn < 10.5 wt.% ;

- 1.3 wt.% < Mg < 2 wt.% ;

1.2 wt.% < Cu < 2 wt.% ;

at least one dispersoid forming elements, with a total content of dispersoid forming elements of at least 0.04 wt. %;

the remainder substantially including aluminum, incidental elements, and impurities ;

wherein said 7xxx series aluminum alloy is cold worked with a thickness reduction from 0.5 to 15% (as defined previously, 0.5 < Xi < 15) before a solution heat treatment and is in the form of a plate having a final thickness of about 0.5 to about 3 inches so as to achieve an improved spall resistance compared to an armor component obtained with the same manufacturing process except that said manufacturing process does not comprise a cold working step before the solution heat treatment.

Examples

Example 1 : Spall resistance

Alloy plate products were made from alloys having the chemical compositions, in weight percent, shown in Table 1.

Table 1 : composition of plate products (wt.%)

Plate products were made using the following process :

casting an ingot of an alloy whose composition is indicated in table 1;

homogenizing the ingot;

hot working the homogenized ingot to arrive at an intermediate gauge Ti;

- for plate products B-3 and B-4 solely, cold working the ingot at said intermediate gauge to arrive at a final gauge T ₂ , thereby reducing the thickness of the ingot at said intermediate gauge Ti by respectively 2.9 % and 6.3%;

solution heat treating the plate ;

quenching ;

- for plate product A solely, stretching the plate to obtain a plastic deformation of about

2 % ;

artificially aging the plate.

All sample plates were aged during a two-stage process. Sample A was aged during 24 hours at 120°C (1 ^st step) followed by 35 hours at 150 °C (second step). Samples B-l, B-2, B-3 and B-4 were aged during 6 hours at 120°C (1 ^st step), followed by 16 hours at 160 °C (second step). Plate product Final gauge Cold Working Spall Resistance Velocity (FSP 20 (inch) mm)

Score

ft/s

A 1.37 No 5.35 3608

B-l 1.35 No 3 2318

B-2 1.35 No 3 2346

B-3 1.3 Yes 2.5 2299

B-4 1.25 Yes 2 2279

Xi = 6.3

Table 2 : experimental results

The final thickness of each plate product ranges between 1.25 and 1.37 inches. As can be noticed on the third column of Table 2, samples A, B-l and B-2 were not submitted to cold working before SHT, whereas cold rolling before SHT was performed on samples B-3 and B-4, thereby reducing the thickness of the so called intermediate sample plate respectively by 2.9 % and 6.3 %. Values given with respect to samples B-3 and B-4, in the third column of table 2, represent the value Xi such that T ₂ = Ti - (xi * T ₂ )/100.

Samples B-3 and B-4 combine a low Zr content within a preferred range as well as a cold working step before solution heat treatment and quench.

For all plate products, spall resistance was investigated with a FSP test according to M IL-STD- 662F (1997) standard. Trials were performed using 20 mm FSP rounds which comply with MIL- DTL-46593B. The dimensions of the FSP rounds are the following :

diameter : 20 mm - 0. 787 inch ;

length : 23.0 mm - 0.9 inch ;

- flat : 9.3 mm - 0. 366 inch.

The fourth column of table 2 shows the spall resistance score. This score is determined by the following method. As shown on Figure 1, on the rear surface, surrounding the site of impact b, is an area of material which was visibly affected by the ballistic strike and the partial or complete penetration by the FSP round into the plate. The boundary of this affected area is indicated on Figure 1 with the dotted line a. Along with any possible exit hole of the FSP round, any material plug and area of any delaminated material is included within the deformed area. The area also includes bulged or fractured material, which is still connected to the test plate. Radiating perpendicular from the direction of impact (i.e. along the rear surface) a length c is measured which represents the longest distance from the site of impact to the edge or boundary of the deformed area. This maximum length c is than doubled and from the new value 20 mm is subtracted, which corresponds to the diameter of the FSP round. The remaining total is divided by 20 mm, so as to get a dimensionless parameter, so-called Spall Resistance Score. The spall resistance score is 0 when only a plug is formed, and no significant lateral material is affected by the impact. Higher scores represent multiples of the impacting round diameter. A better score is as low as possible.

The fifth column of Table 2 shows the velocities of the projectile that were measured when performing FSP tests.

From these results, it can be concluded that

Plate products B-3 and B-4 exhibit a better spall resistance than Plate products B-l and B-2;

Plate product B-4 exhibits a better spall resistance than Plate product B-3.

This tends to show that a cold working step before SHT results in an improvement of the spall resistance. Moreover, combining such cold working together with a low amount of Zr might significantly improve the spall resistance score, while maintaining a high resistance with respect to both AP and FSP rounds, as will be assessed in example 2.

Example 2: AP and FSP properties

Alloy plate products were made from alloys having the chemical compositions, in weight percent, shown in Table 3.

Table 3 : composition of sample plates (wt.%) in example 2

Alloys C, D and E have a chemistry according to the invention. Plate products were made using the following process according to the invention:

casting an ingot of an alloy whose composition is indicated in table 3;

homogenizing the ingot;

hot working the homogenized ingot to arrive at an intermediate gauge Ti;

- cold working the ingot at said intermediate gauge Ti to arrive at a final gauge T ₂ , thereby reducing the thickness of the ingot, at said intermediate gauge Ti by 2.9 % (xi = 2,9); solution heat treating the plate ;

quenching ;

stretching the plate to obtain a plastic deformation between 2 % to 3% ;

- artificially aging the stretched plate during 6 hours at 120°C, followed by 16 hours at

160°C.

Plate products had different thicknesses varying from 1.0" to 1.6" and were tested for their ballistic properties. Two ballistic tests have been carried out pursuant to U.S. military standard MIL-STD-662F (1997), namely the armor piercing test using 0.3 inch (7.62 mm) projectiles and the FSP test using 20 mm fragment simulating projectiles. The AP and FSP results are listed in Table 4 and compared to the minimum required ballistic limits defined in US Military specification MIL-DTL 32375 ( respectively, V50 (0.3Cal AP M2) 7085 requirement: MIL-DTL- 32375 (M R) Appendix A, page 19-20, column "Required BL(P) - Type B" and V50 (FSP 20 mm) 7085 requirement: M IL-DTL-32375 (MR) Appendix A, page 18, column "Required BL(P) - Type B"). This specification covers 7085 wrought aluminum alloy armor plate for non-fusion welded applications in nominal thickness from 0.500 to 3.000 inch and defines minimum required ballistic limits with respect to different kinds of projectiles as well as plate thicknesses.

Table 4 : experimental results of example 2

Plate products C, D and E have combined high AP and FSP performances. In particular, all the preceding plate products meet the minimum required ballistic limits defined in US Military specification MIL-DTL 32375 for alloy 7085. Moreover, regarding examples 1 and 2 combining cold working before SHT together with a low amount of Zr, namely Zr<0.08 wt. %, seems to significantly improve the spall resistance score, while maintaining a high resistance with respect to both AP and FSP rounds.