CN113166859A

CN113166859A - Al-Zn-Cu-Mg alloy and preparation method thereof

Info

Publication number: CN113166859A
Application number: CN201980083452.1A
Authority: CN
Inventors: E·尼泽里; R·惠切尔; J-C·埃斯特罗姆
Original assignee: Constellium Issoire SAS
Current assignee: Constellium Issoire SAS
Priority date: 2018-12-20
Filing date: 2019-12-18
Publication date: 2021-07-23
Also published as: EP3899075A1; BR112021010783A2; US20220081740A1; EP3899075B1; WO2020127592A1; EP3670690A1; CA3121837A1

Abstract

The invention relates to a rolled aluminium-based alloy product having a thickness of at least 80mm, comprising (in wt%): zn 6.85-7.25, Mg 1.55-1.95, Cu 1.90-2.30, Zr 0.04-0.10, Ti 0-0.15, Fe 0-0.15, Si 0-0.15, other elements each 0.05 or less and total amount 0.15 or less, and the balance Al, wherein more than 75% of the grains in the middle of the thickness are recrystallized or 30 to 75% of the grains in the middle of the thickness are recrystallized and the aspect ratio of the non-recrystallized grains in the L/ST cross section is less than 3. The method of preparing a rolled aluminium-based alloy product comprises the steps of: (a) casting an ingot made of the alloy of the present invention, (b) homogenizing the ingot, (c) hot rolling the homogenized ingot in one or more stages of rolling, (d) solution heat treating and quenching, (e) stress relieving, and (f) artificially aging. The product of the invention is suitable for aircraft construction and has advantageous fatigue crack propagation properties.

Description

Al-Zn-Cu-Mg alloy and preparation method thereof

Technical Field

The present invention relates generally to aluminum-based alloys, and more particularly to Al-Zn-Cu-Mg aluminum-based alloys, particularly such alloys for aerospace applications.

Background

For many years, Al-Zn-Cu-Mg aluminum-based alloys have been widely used in the aerospace industry. With the development of aircraft structures and efforts aimed at reducing weight and cost, optimum compromises between strength, toughness and corrosion resistance are continually sought. In addition, process improvements of casting, rolling and heat treatment may advantageously provide further control and performance tradeoffs in alloy composition maps.

Thick rolled, forged or extruded products made of Al-Zn-Cu-Mg aluminium-based alloys are particularly used for producing integrally machined high strength structural components for the aerospace industry, such as wing elements like ribs (wing ribs), spars (spar), bulkheads (frame), etc., which are usually machined from thick forged profiles.

The performance values obtained for various properties (such as static mechanical strength, fracture toughness, corrosion resistance, quench sensitivity, fatigue resistance and residual stress level) will determine the overall performance of the product, the ability of the structural designer to advantageously use it, and the ease with which it can be used for further processing steps, such as machining.

Of the properties listed above, some are often inherently contradictory and often a compromise must be found. Contradictory properties are, for example, static mechanical strength and toughness, and strength and corrosion resistance. In corrosion or environmental Cracking (EAC) performance, EAC under high stress and humid environmental conditions can be distinguished from EAC under standard Stress Corrosion Cracking (SCC) test conditions, such as ASTM G47, in which samples are tested using alternating soak and dry cycles of NaCl solution (ASTM G44) and typically using lower stresses.

Crack deviation, crack turning, or crack branching is a term used to express the propensity of a crack to propagate away from an expected fracture plane perpendicular to the direction of loading during fatigue or toughness testing. Crack deviation occurs on the micro scale (<100 μm), on the micro scale (100-. This phenomenon is a particular concern for fatigue testing in the L-S direction. The term crack branching is used herein to denote the macroscopic deviation of a crack from the S direction towards the L direction in an L-S fatigue or toughness test, which deviation occurs on rolled products having a thickness of 30 mm or more. The occurrence of crack branching may be related to the composition and microstructure of the rolled product and the test conditions.

Crack deviation has been recognized as a significant problem by aircraft manufacturers because of the difficulty in taking part design considerations using conventional design methods. This is because crack deviation invalidates traditional mode I-based material testing procedures and design models. The crack deviation problem has proven difficult to solve. Recently, it has been considered that in the absence of solutions for avoiding crack deviation, efforts should be made to predict crack deviation behavior. (M.J.Crill, D.J.Chellman, E.S.Balmuth, M.Philbrook, K.P.Smith, A.Cho, M.Niedzinski, R.Muzzolini and J.Feiger, Evaluation of AA 2050-T87 Al-Li Alloy Crack Turning Behavior, Materials Science Forum, Vol.519-521 (2006, 7 months) 1323-1328).

Patent US 8,323,426 proposes a solution to improve crack branching in certain Al-Cu-Li alloys.

However, crack deviation improvement is generally associated with a higher fatigue crack propagation rate in the original crack plane prior to deviation.

Us patent 5,560,789 describes AA 7000-series alloys having high mechanical strength and a method for obtaining them. The alloy contains, by weight, 7 to 13.5% Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others each not exceeding 0.05% and the total not exceeding 0.15%, the balance being Al, however, no mention is made of corrosion performance.

U.S. Pat. No. 5,865,911 describes an aluminum alloy consisting essentially of, in weight percent, about 5.9 to 6.7% zinc, 1.8 to 2.4% copper, 1.6 to 1.86% magnesium, 0.08 to 0.15% zirconium, the balance being aluminum and incidental elements and impurities. The' 911 patent specifically mentions a compromise between static mechanical strength and toughness.

US patent application US20050167016a1 discloses inter alia an Al-Zn-Cu-Mg product comprising (in weight%): 5.8-6.8% Zn, 1.5-2.5% Cu, 1.5-2.5% Mg, 0.04-0.09% Zr, the balance being aluminum and incidental impurities, wherein said product has a recrystallization rate greater than about 35% at one-quarter of the thickness and has improved fatigue crack growth resistance.

Us patent No 6,027,582 describes a rolled, forged or extruded Al-Zn-Mg-Cu aluminium-based alloy product having a thickness greater than 60 mm and a composition (in wt.%), Zn: 5.7-8.7, Mg: 1.7-2.5, Cu: 1.2-2.2, Fe: 0.07-0.14, Zr: 0.05-0.15, Cu + Mg <4.1 and Mg > Cu. The' 582 patent also describes an improvement in quench sensitivity.

U.S. Pat. No. 6,972,110 teaches an alloy preferably containing (in weight%) Zn: 7-9.5, Mg: 1.3-1.68 and Cu 1.3-1.9, and encourages to keep Mg + Cu < 3.5. The' 110 patent discloses the use of a three-step aging treatment to improve stress corrosion cracking resistance. Three-step aging times are long and difficult to master, and it is desirable to obtain high corrosion resistance without the necessity of such heat treatment.

PCT patent application WO2004090183 discloses an alloy consisting essentially of (in weight%): zn: 6.0-9.5, Cu: 1.3-2.4, Mg: 1.5-2.6, Mn and Zr <0.25, but preferably in the range 0.05 to 0.15 for higher Zn contents, less than 0.05 each and less than 0.25 in total of other elements, balance aluminium, wherein (in wt%): 0.1[ Cu ] +1.3< [ Mg ] <0.2[ Cu ] +2.15, preferably 0.2[ Cu ] +1.3< [ Mg ] <0.l [ Cu ] + 2.15.

U.S. patent application No 2005/006010, a method of producing a high strength Al-Zn-Cu-Mg alloy with improved fatigue crack propagation resistance and high damage tolerance, comprising the steps of: casting an ingot having the following composition (in weight%): zn 5.5-9.5, Cu 1.5-3.5, Mg 1.5-3.5, Mn <0.25, Zr <0.25, Cr <0.10, Fe <0.25, Si <0.25, Ti <0.10, Hf and/or V <0.25, other elements each less than 0.05 and less than 0.15 in total, balance aluminium, homogenising and/or preheating the ingot after casting, hot rolling the ingot and optionally cold rolling to a worked product having a thickness of more than 50mm, solution heat treating, quenching the heat treated product, and artificially ageing the worked and heat treated product, wherein the ageing step comprises a first heat treatment at a temperature in the range 105 ℃ to 135 ℃ for more than 2 hours and less than 8 hours and a second heat treatment at a temperature above 135 ℃ but below 170 ℃ for more than 5 hours and less than 15 hours. Also, such three-step aging times are long and difficult to master.

EP patent 1544315 discloses a product made of an AlZnCuMg alloy, in particular rolled, extruded or forged, having the following composition in weight percent: zn

6.7-7.3; 1.9-2.5 of copper; 1.0-2.0 of magnesium; 0.07-0.13 of zirconium; fe is less than 0.15; si is less than 0.15; the total amount of other elements is not more than 0.05% and at most 0.15%; the balance being aluminum. The product is preferably subjected to solution heat treatment, quenching, cold rolling and artificial aging treatment.

U.S. patent No 8,277,580 teaches a rolled or forged Al-Zn-Cu-Mg aluminum based alloy forged product having a thickness of 2 to 10 inches. The product is subjected to solution heat treatment, quenching and aging treatment, and comprises (by weight percent): zn 6.2-7.2, Mg 1.5-2.4, Cu 1.7-2.1, Fe 0-0.13, Si 0-0.10, Ti 0-0.06, Zr 0.06-0.13, Cr 0-0.04, Mn 0-0.04, impurities and other incidental elements each < 0.05.

U.S. patent No 8,673,209 discloses aluminum alloy products having a thickness of about 4 inches or less and methods of making the same; the product is capable of achieving a combination of strength, fracture toughness and corrosion resistance when solution heat treated, quenched and artificially aged, as well as in parts made from the product, the alloy consisting essentially of: about 6.8 to about 8.5 wt.% Zn, about 1.5 to about 2.00 wt.% Mg, about 1.75 to about 2.3 wt.% copper; about 0.05 to about 0.3 wt.% Zr, less than about 0.1 wt.% Mn, less than about 0.05 wt.% Cr, and the balance Al, incidental elements, and impurity composition.

None of the documents describing high strength 7xxx alloy products describes an alloy product having no tendency to crack deviation and having a low fatigue crack propagation rate, together with high strength, high toughness properties and high corrosion resistance.

The problem addressed by the present invention is to obtain a thick rolled product of the 7XXX alloy series having an improved fatigue crack propagation rate without increasing the crack departure tendency, while maintaining a good balance between mechanical strength, fracture toughness, corrosion resistance, quench sensitivity, fatigue resistance and residual stress levels. By thick rolled product is meant a product having a thickness of at least 80mm or even at least 100 mm.

Disclosure of Invention

It is an object of the present invention to provide an Al-Zn-Cu-Mg alloy with a specific composition range and preparation method enabling improved fatigue crack propagation rates for thick rolled products without increasing the tendency for crack deviation.

It is another object of the present invention to provide a method of producing a wrought aluminium product, which method enables an improved compromise improved fatigue crack propagation rate without increasing the tendency for crack deviation.

To achieve these and other objects, the invention relates to a rolled product having a thickness of at least 80mm, comprising (in weight%):

Zn 6.85–7.25，

Mg 1.55–1.95，

Cu 1.90–2.30，

Zr 0.04–0.10，

Ti 0–0.15，

Fe 0–0.15，

Si 0–0.15，

each of other elements is less than or equal to 0.05, the total amount is less than or equal to 0.15, and the balance is Al,

wherein more than 75% of the grains in the middle of the thickness are recrystallized or 30 to 75% of the grains in the middle of the thickness are recrystallized and the aspect ratio of the non-recrystallized grains in the L/ST cross section is less than 3.

To achieve these and other objects, the present invention relates to a method for preparing a rolled aluminium-based alloy product, comprising the steps of:

a) casting ingot, said ingot comprising (in wt%)

Zn 6.85–7.25，

Mg 1.55–1.95，

Cu 1.90–2.30，

Zr 0.04–0.10，

Ti 0–0.15，

Fe 0–0.15，

Si 0–0.15，

Each of the other elements is less than or equal to 0.05, the total amount is less than or equal to 0.15, and the balance is Al;

b) homogenizing the ingot;

c) hot rolling the homogenized ingot into a rolled product having a final thickness of at least 80 mm;

d) solution heat treating and quenching the product;

e) stress relieving the solution heat treated and quenched product;

f) artificially aging the stress-relieved product;

wherein the hot rolling start temperature is controlled to obtain more than 75% recrystallized grains at the middle of the thickness or 30 to 75% recrystallized grains at the middle of the thickness after step f and the aspect ratio of non-recrystallized grains in L/ST cross section is less than 3.

Drawings

FIG. 1 shows C (T) specimens used for the fatigue crack growth rate test. The conical range of the origin at ± 20 ° at the intersection of the line through the center of the hole and the axis of symmetry of the specimen for the deviation of the crack from the norm is plotted with a thick line.

FIG. 2a is a schematic representation of a C (T) specimen prior to fatigue testing and used for crack deviation criteria. Fig. 2b shows a crack specimen without crack deviation tendency: the cracks remain within the cone. Fig. 2c shows a specimen with a tendency to crack deviation.

FIG. 3 is a sample of alloy A after the fatigue crack propagation rate test.

FIG. 4 is a sample of alloy B after the fatigue crack propagation rate test.

FIG. 5 is a sample of alloy C after the fatigue crack growth rate test.

Detailed Description

Unless otherwise indicated, all statements relating to the chemical composition of the alloy are expressed as mass percentages by weight, based on the total weight of the alloy. In the expression Cu/Mg, Cu denotes the Cu content in weight%, and Mg denotes the Mg content in weight%. The alloy designations conform to the aluminum Association (aluminum Association) specifications known to those skilled in the art. The definition of tempering is specified in EN 515 (1993).

Unless otherwise stated, the static mechanical properties, i.e. ultimate tensile strength UTS, tensile yield stress TYS and elongation at break E, are determined by tensile testing according to standard NF EN ISO 6892-1(2016), the sampling position and its orientation being defined in standard EN 485 (2016).

The definition of the standard EN 12258 applies unless otherwise stated.

Symbol is used to denote "multiply".

Fracture toughness K_1CDetermined according to ASTM standard E399 (2012).

Unless otherwise stated, EAC under high stress and humid environmental conditions was tested at constant strain, using a load of about 80% of ST direction TYS, at the middle of the thickness on a tensile sample, at 85% relative humidity and a temperature of 70 ℃, as described in standard ASTM G47. The minimum life without failure after environmental cracking (EAC) corresponds to the minimum number of days to failure for 3 specimens per plate.

The tendency of crack deflection was observed using the L-S Compact tensile (Compact Tension) C (T) fatigue specimens defined in ASTM E647. The term "deviation" is not meant herein to mean the meaning as set forth in ASTM E647-15 (which definition focuses on the accuracy of measurement of fatigue crack propagation rate), but rather means that the crack remains within a cone range of + -20 deg., preferably + -15 deg., having its origin at the intersection of a line through the hole center and the symmetry axis of the specimen, as shown by line A-A in FIG. 1. The width W of the sample (c (t)) was 40mm and the thickness B was 10 mm. The samples used are depicted in FIG. 1, which also shows the cone range of. + -. 20 ℃ in bold lines. For the samples used, L-48 mm, W-40 mm, Z-50 mm, C-22 mm, B-10 mm. The method of evaluating crack deviation is shown in fig. 2. Fig. 2a schematically shows a CT specimen before a fatigue test. FIG. 2b shows a crack specimen without crack deviation tendency: the cracks remain within the cone indicated by the bold line. Fig. 2c shows a specimen with a tendency to crack deviation.

The term "structural element" is a term well known in the art and refers to a component for mechanical construction whose static and/or dynamic mechanical properties are particularly important to the structural performance and are usually specified or structurally calculated for it. These components are typically such components: its rupture may seriously compromise the safety of the mechanical structure, its user or a third party. In the case of an aircraft, the structural element comprises the following elements: fuselage (e.g., fuselage skin), stringers, bulkheads, circumferential bulkheads (circular lattice frames), wing components (e.g., wing skin, stringers or stiffeners, ribs, spars), empennage (e.g., horizontal and vertical stabilizers), floor beams, seat tracks, and doors.

The alloy of the invention has a specific composition and microstructure, which makes it possible to obtain products having a very low fatigue crack propagation rate and no tendency for cracks to deviate.

A minimum Zn content of 6.85 and preferably 6.90 or even 6.90 is required to obtain sufficient strength. However, the Zn content should not exceed 7.25 and preferably 7.20 or even 7.15 to obtain the sought balance of properties, in particular toughness and elongation.

A minimum Mg content of 1.55 and preferably 1.60 or even 1.65 is required to obtain sufficient strength. However, the Mg content should not exceed 1.95, preferably 1.90 or even 1.85, in order to obtain the sought balance of properties, in particular toughness and elongation, and to avoid quench sensitivity.

A minimum Cu content of 1.90 and preferably 1.95 or 2.00, or even 2.05 is required to obtain sufficient strength and also to obtain sufficient EAC performance. However, the Cu content should not exceed 2.30 and particularly preferably 2.25 to avoid quench sensitivity. In one embodiment, the Cu maximum content is 2.20.

In order to obtain a product with low susceptibility to EAC under high stress and humid environmental conditions and avoid quench susceptibility, the sum of Cu + Mg is preferably controlled to be 3.8 to 4.2.

The alloys of the present invention also contain 0.04 to 0.10 weight percent zirconium, which is commonly used for grain size control. Control of the zirconium content in combination with hot rolling conditions is important to obtain the desired microstructural properties of the invention, i.e. more than 75% recrystallized grains in the middle of the thickness or 30 to 75% recrystallized grains in the middle of the thickness and non-recrystallized grains with an aspect ratio in L/ST cross-section of less than 3.

The Zr content should preferably comprise at least about 0.05 wt.%, but should advantageously be kept below about 0.08 or even 0.07 wt.%.

Titanium (along with incidental elements such as boron or carbon) can typically be added during casting to limit the as-cast grain size, if desired. The present invention can typically accommodate up to about 0.15 wt.%, preferably up to about 0.06 wt.% Ti. In a preferred embodiment of the invention, the Ti content is from about 0.02 to about 0.06 wt% and preferably from about 0.03 to about 0.05 wt%.

The alloys of the present invention may further comprise other elements to a lesser extent and in some embodiments, on a less preferred basis. Iron and silicon generally affect fracture toughness properties. The iron and silicon content should generally be kept low, with an iron content of at most 0.15% by weight, preferably not more than about 0.13% by weight or preferably not more than about 0.10% by weight, and a silicon content of preferably not more than about 0.10% by weight or preferably about 0.08% by weight. In one embodiment of the invention, the iron and silicon content is 0.07 wt.% or less.

The other elements are impurities or incidental elements, each in a maximum amount of 0.05 wt.% and a total amount of 0.15 wt.% or less, preferably each in a maximum amount of 0.03 wt.% and a total amount of 0.10 wt.% or less.

The method suitable for preparing the rolled product of the invention comprises: (a) casting an ingot made of the alloy of the present invention, (b) homogenizing the ingot at a temperature of about 460 to about 510 ℃, or preferably about 470 to about 500 ℃, preferably in at least one step, for 5 to 30 hours, (c) hot rolling the homogenized ingot in one or more stages by rolling, preferably at an inlet temperature of about 280 to about 420 ℃, until a final rolled product having a thickness of at least 80mm, (d) solution heat treating, typically 1 to 10 hours, at a temperature of 460 to about 510 ℃, or preferably from about 470 to about 500 ℃, depending on the thickness, and quenching, preferably water quenching at room temperature, (e) stress relieving by controlled stretching or compression at a permanent deformation rate of preferably less than 5% and preferably 1 to 4%, and (f) artificial aging.

The hot rolling inlet temperature is controlled to achieve the desired microstructural properties of the present invention, which refer to more than 75% recrystallized grains in the middle of the thickness or 30 to 75% recrystallized grains in the middle of the thickness with non-recrystallized grains having an aspect ratio of less than 3 in the L/ST cross-section. Advantageously, the hot rolling start temperature is at least 145 Zr^-0.313-20, preferably at least 145 Zr^-0.313-10. Preferably, the hot rolling start temperature is at most 145 × Zr^-0.313+20, preferably at least 145 × Zr^-0.313+10. Zr is the weight percent concentration of zirconium in the alloy.

The invention is particularly applicable to thick gauges greater than about 80 mm. In a preferred embodiment, the rolled product of the invention is a plate comprising the alloy of the invention and having a thickness of 80 to 200mm, or advantageously 100 to 180 mm. An "overaged" temper ("type T7") is advantageously used to improve corrosion behavior in the present invention. Tempers which may be suitable for the products of the invention include, for example, T6, T651, T73, T74, T76, T77, T7351, T7451, T7452, T7651, T7652 or T7751, preferably tempers T7351, T7451 and T7651. The ageing treatment is advantageously carried out in two steps, the first of which is carried out at a temperature of 110-130 ℃ for 3 to 20 hours, preferably 4 or 5 to 12 hours, and the second of which is carried out at a temperature of 140-170 ℃, preferably 150-165 ℃ for 5 to 30 hours.

In an advantageous embodiment, the equivalent ageing time t (eq) at 155 ℃ is from 8 to 35 or 30 hours, preferably from 12 to 25 hours.

The equivalent time at 155 ℃ t (eq) is defined by the following formula:

wherein T is the instantaneous temperature during annealing in ° K, T_refA reference temperature at 155 ℃ (428 ° K) was selected. t (eq) expressed in hours.

With the narrow composition range of the present invention, products with low tendency to crack deflection and very low fatigue crack propagation rate can be obtained. Thus, for the product of the invention, during the fatigue crack propagation rate test according to the standard ASTM E647, the crack remains within a cone range of ± 20 °, as shown in fig. 2B, preferably within a cone range of ± 15 °, the origin of which is at the intersection of a line through the centre of the specimen hole and the axis of symmetry of the specimen, and the da/dN at Δ K15 MPa m is less than 2.010 at the middle of the thickness on the L-S C (T) fatigue specimen (W40 mm, B10 mm)^-4mm/cycle, preferably less than 1.010^-4mm/cycle, and more preferably less than 0.910^-5mm/cycle.

The narrow compositional range of the alloys of the present invention, selected primarily for the tradeoff between strength and toughness, provides rolled products with unexpectedly high EAC performance under high stress and humid environmental conditions.

The product of the invention also preferably has one, more preferably two, most preferably three of the following properties:

a) after environmental induced cracking (EAC), a minimum life without failure of at least 20 days, preferably at least 30 days, under conditions of high stress, Short Transverse (ST) stress level of 80% of the tensile yield strength of the product in the ST direction, and in a humid environment at a temperature of 70 ℃ and a relative humidity of 85%,

b) a conventional tensile yield strength measured in the direction of L at a quarter of the thickness of at least 515-0.279 t MPa, preferably 525-0.279 t MPa, even more preferably 535-0.279 t MPa (t being the thickness of the product in mm),

c) k in the L-T direction measured at one quarter of the thickness_1CThe toughness is at least 32-0.1 x t MPa m, preferably 34-0.1 x t MPa m, even more preferably 36-0.1 x t MPa m (t being the product thickness in mm).

The rolled product of the invention is advantageously used as a structural element for the manufacture of aircraft, or incorporated therein.

In an advantageous embodiment, the product of the invention is used for wing ribs, spars and bulkheads. In an embodiment of the invention, the rolled product of the invention is welded with other rolled products to form ribs, spars and bulkheads.

These and other aspects of the invention are explained in more detail by the following illustrative and non-limiting examples.

Examples

Example 1

Two ingots were cast, one of the product (a) of the invention and one of the comparative example (B), having the following composition (table 1):

table 1: composition (wt.%) of inventive and control castings.

Alloy (I)	Si	Fe	Cu	Mg	Zn	Ti	Zr
								A	0.03	0.04	2.13	1.75	7.05	0.04	0.06
B	0.05	0.09	1.64	2.25	6.10	0.02	0.11

The pastilles were then peeled and homogenized at about 475 ℃. The ingot was hot rolled into a plate with a thickness of 102mm (alloy A) or 110mm (alloy B). The hot rolling inlet temperature of alloy A was 350 ℃ and the hot rolling inlet temperature of alloy B was 440 ℃. The plates were solution heat treated at a cooking temperature (soak temperature) of about 475 ℃. The plate is quenched and stretched at a permanent elongation of 2.0 to 2.5%.

The control panels were subjected to two-step aging: 4 hours at 120 ℃ and then 15 hours at 155 ℃ for a total equivalent time of 155 ℃ for 17 hours to obtain a T7651 temper.

The plate made of alloy a has more than 75% recrystallized grains in the middle of the thickness, while the plate of alloy B is substantially non-recrystallized, with a volume fraction of recrystallized grains below 35% in the middle of the thickness.

The samples were mechanically tested in the L and LT directions at quarter thickness and in the ST direction at the middle of the thickness to determine their static mechanical properties and their fracture toughness. Table 2 provides the tensile yield strength, ultimate strength and elongation at break.

Table 2: static mechanical Properties of the samples

Table 3 provides the fracture toughness test results.

Table 3: fracture toughness Properties of the samples

The EAC under high stress and humid environmental conditions was measured with ST direction tensile test specimens, which are described in ASTM G47. The test stress and environment are different from ASTM G47 and a t/2 load of about 80% of ST direction TYS at 85% relative humidity and 70 ℃ temperature is used. The three samples for each panel are given the number of days to failure,

the results are given in table 4.

TABLE 4 EAC results under high stress and humid ambient conditions

For the SCC test under ASTM G47, the plate made from alloy a withstood an average of 33 days at a stress of 350 MPa.

The L-S fatigue crack propagation rate was measured according to standard ASTM E647 at a maximum load of 4KN and R ═ 0.1, at the middle of the thickness and at one quarter of the thickness in the L-S direction of a CT specimen (CT10W40, thickness 10mm, width 40 mm). The results are given in table 5.

TABLE 5L-S fatigue crack growth Rate test results (da/dN at 15MPa v m)

The L-S fatigue crack growth rate on the CT specimen was reduced by up to at least 3 times for alloy A of the present invention as compared to alloy B.

The image of the alloy a crack specimen is shown in fig. 3. None of the crack specimens exhibited a tendency to crack deflection and the cracks were maintained within a cone of ± 15 °. The crack specimens of alloy B are shown in fig. 4, and the cracks remain within the cone range of ± 20 °, but not within the cone range of ± 15 °.

Example 2

Casting another ingot having a composition according to invention (C), (table 6):

table 6: composition of casting C (% by weight).

Alloy (I)	Si	Fe	Cu	Mg	Zn	Ti	Zr
								C	0.03	0.04	2.15	1.65	7.11	0.03	0.10

The pastilles were then peeled and homogenized at 475 ℃. The ingot was hot rolled into a plate with a thickness of 152 mm. The hot rolling inlet temperature was 420 ℃. The plates were solution heat treated at a warm-up temperature of about 475 ℃. The plate is quenched and stretched at a permanent elongation of 2.0 to 2.5%.

Due to the high hot rolling temperature, the microstructure of the plate differs from the present invention, the resulting plate having less than 20% recrystallized grains in the middle of the thickness.

The L-S fatigue crack growth rate was measured according to standard ASTM E647 on CT specimens (CT10W40, thickness 10mm, width 40mm) at the middle of the thickness and one quarter of the thickness in the L-S direction at a maximum load of 4KN and R ═ 0.1. The results are given in table 7.

Table 7 shows the results of the L-S fatigue crack growth rate test (da/dN at 15MPa v m)

The image of the panel C crack sample is shown in fig. 4. The cracked specimens showed a tendency for crack deviation and the cracks were not maintained within a cone range of ± 20 °.

All documents mentioned herein are specifically incorporated by reference herein in their entirety.

As used herein and in the claims that follow, articles such as "the", "a", and "an" may mean singular or plural.

In this specification and the claims that follow, to the extent a range of numerical values is recited, such values are intended to be inclusive of the precise value and values approximating the value, with insubstantial variations from the recited value.

Claims

1. A rolled aluminium-based alloy product having a thickness of at least 80mm, comprising (in wt%):

Zn 6.85–7.25，

Mg 1.55–1.95，

Cu 1.90–2.30，

Zr 0.04–0.10，

Ti 0–0.15，

Fe 0–0.15，

Si 0–0.15，

2. The product of claim 1, wherein Cu 1.95-2.25, and preferably Cu: 2.00-2.20.

3. product according to claim 1 or 2, wherein the crack remains within a conical range of ± 20 ° and preferably ± 15 ° at the middle of the thickness on an L-S C (T) fatigue specimen with W-40 mm and B-10 mm, the origin of said conical range being at the intersection of a line through the centre of the specimen hole and the axis of symmetry of the specimen, and da/dN at Δ K-15 MPa vm being less than 2.010 ° during the fatigue crack propagation speed test according to standard ASTM E647^-4mm/cycle, preferably less than 1.010^-4mm/cycle and more preferably less than 0.910^-5mm/cycle.

4. The product according to any one of claims 1 to 3, wherein the product has at least one of the following properties:

a) after environmental induced cracking (EAC), at high stress, Short Transverse (ST) stress levels of 80% of the tensile yield strength of the product in the ST direction, and at a temperature of 70 ℃

The minimum life without failure under humid conditions with a relative humidity of 85% is at least 30 days, preferably at least 40 days,

b) a conventional tensile yield strength measured in the direction of L at a quarter of the thickness is at least 515-0.279 t MPa, preferably 525-0.279 t MPa, even more preferably 535-0.279 t MPa, t being the thickness of the product in mm,

c) k in the L-T direction measured at one quarter of the thickness_1CThe tenacity is at least 32-0.1 × t MPa √ m, preferably 34-0.1 × t MPa √ m, even more preferably 36-0.1 × t MPa √ m, t being the product thickness in mm.

5. The product according to any of claims 1 to 4, wherein its thickness is 80 to 200mm, or advantageously 100 to 180 mm.

6. A structural component suitable for use in aircraft construction, wherein the structural component is used in a rib, spar and bulkhead, the structural component comprising a product according to any of claims 1 to 5.

7. A method of preparing a rolled aluminium-based alloy product comprising the steps of:

a) casting ingot, said ingot comprising (in wt%)

Zn 6.85–7.25，

Mg 1.55–1.95，

Cu 1.90–2.30，

Zr 0.04–0.10，

Ti 0–0.15，

Fe 0–0.15，

Si 0–0.15，

b) homogenizing the ingot;

d) solution heat treating and quenching the product;

e) stress relieving the solution heat treated and quenched product;

f) artificially aging the stress-relieved product;

8. The method of claim 7, wherein the hot rolling start temperature is at least 145 x Zr^-0.313-20 and preferably at least 145 Zr^-0.313-10。

9. The method of claim 7 or 8, wherein the hot rolling start temperature is at most 145 x Zr^-0.313+20 and preferably at least 145 × Zr^-0.313+10。

10. The method according to any one of claims 7 to 9, wherein the equivalent ageing time t (eq) is between 8 and 30 hours and preferably between 12 and 25 hours, the equivalent time t (eq) at 155 ℃ being defined by the formula:

wherein T is the instantaneous temperature during annealing in ° K, and T_refA reference temperature at 155 ℃ (428 ° K) was chosen, and t (eq) expressed in hours.

11. The method of any one of claims 7 to 10, wherein the solution heat treatment temperature is 460 to about 510 ℃, or preferably about 470 to about 500 ℃.