US20190040508A1

US20190040508A1 - Thick plates made of al-cu-li alloy with improved fatigue properties

Info

Publication number: US20190040508A1
Application number: US16/074,661
Authority: US
Inventors: Jean-Christophe Ehrstrom; Carla DA FONSECA BARBATTI
Original assignee: Constellium Issoire SAS
Current assignee: Constellium Issoire SAS
Priority date: 2016-02-03
Filing date: 2017-02-03
Publication date: 2019-02-07
Also published as: BR112018014770A2; CN108603253B; BR112018014770B1; US20240035138A1; FR3047253A1; CA3012956A1; WO2017134405A1; CN108603253A; FR3047253B1; EP3411508B1; EP3411508A1; CA3012956C

Abstract

The invention relates to a rolled product having a thickness of at least 50 mm made of aluminium alloy comprising, in % by weight, 2.2% to 3.9% of Cu, 0.7% to 1.8% of Li, 0.1% to 0.8% of Mg, 0.1% to 0.6% of Mn; 0.01% to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element, if it is chosen, being 0.2% to 0.8% for Zn and 0.1% to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element, if it is chosen, being 0.04% to 0.18% for Zr, 0.05% to 0.3% for Cr and for Sc, 0.05% to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si, the remainder being aluminium and inevitable impurities, having a content of less than 0.05% each and 0.15% in total; characterized in that its granular structure is predominantly recrystallised between ¼ and ½ thickness. The invention also relates to the process for manufacturing such a product. The products according to the invention are advantageously used in aircraft construction, in particular for the production of an aircraft wing spar or rib.

Description

SCOPE OF THE INVENTION

The present invention generally relates to thick plates of Al—Cu—Li alloy and in particular such products used in the aeronautics and aerospace industry.

STATE OF THE ART

Products, especially thick rolled products, typically at least 50 mm thick, made of aluminum alloy are developed for the production by cutting, surface milling or mass machining of high-strength parts intended in particular for the aeronautics industry, the aerospace industry or mechanical engineering.
Aluminum alloys containing lithium are of great interest in this respect because lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each percent of lithium weight added. For these alloys to be selected for aircraft, their performance as compared to the other usual properties must attain that of alloys in regular use, in particular in terms of the balance between static mechanical strength properties (yield strength and ultimate tensile strength) and damage tolerance properties (toughness, resistance to fatigue crack initiation and propagation), these properties being in general in opposition to each other. For thick products, these properties must in particular be obtained at quarter and at mid-thickness. These alloys must also have sufficient corrosion resistance, allowing them to be shaped using the usual processes and to have low residual stresses in order to be able to be machined integrally.
U.S. Pat. No. 5,032,359 describes a vast family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, makes it possible to increase the mechanical strength.
U.S. Pat. No. 7,229,509 describes an alloy including (% by weight): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain refining agents such as Cr, Ti, Hf, Sc, V, especially having a toughness K_1C(L)>37.4 MPa√m for a yield strength R_p0,2(L)>448.2 MPa (products with a thickness greater than 76.2 mm) and in particular a tenacity K_1C(L)>38.5 MPa√m for a yield strength R_p0,2(L)>489.5 MPa (products with a thickness of less than 76.2 mm).
AA2050 alloy comprises (as a percentage by weight): (3.2-3.9) Cu, (0.7-1.3) Li, (0.20-0.6) Mg, (0.20-0.7) Ag, 0.25 max. Zn, (0.20-0.50) Mn, (0.06-0.14) Zr and alloy AA2095 (3.7-4.3) Cu, (0.7-1.5) Li, (0.25-0.8) Mg, (0.25-0.6) Ag, 0.25 max. Zn, 0.25 max. Mn, (0.04-0.18) Zr. Products made of AA2050 alloy are known for their quality in terms of static mechanical strength and toughness, especially for thick rolled products and are selected in some aircraft.
For certain applications, it may be advantageous to further improve the properties of these products, in particular with regard to the propagation of fatigue cracks.
This is because for an airplane the interval between two operations to inspect the structure depends on the speed and the way the cracks propagate in the materials used for the structure and it is advantageous to use products for which cracks spread slowly and predictably. Improvement in the propagation properties of fatigue cracks therefore relates in particular to the speed of propagation and the direction of propagation.
Patent application WO2009103899 describes a substantially non-recrystallized rolled product comprising as a percentage by weight: 2.2 to 3.9% by weight of Cu, 0.7 to 2.1% by weight of Li; 0.2 to 0.8% by weight of Mg; 0.2 to 0.5% by weight of Mn; 0.04 to 0.18% by weight of Zr; less than 0.05% by weight of Zn and, optionally, 0.1 to 0.5% by weight of Ag, the remainder being aluminum and unavoidable impurities, having a low propensity for crack bifurcation during a fatigue test in the LS direction.
Crack bifurcation, crack deflection, crack rotation, or crack branching are terms used to express the propensity for the propagation of a crack to deviate from the expected plane of fracture perpendicular to the load applied during a fatigue or toughness test. Crack bifurcation occurs on a microscopic scale (<100 μm), a mesoscopic scale (100-1000 μm) or a macroscopic scale (>1 mm), but it is considered harmful only if the direction of the crack remains stable after bifurcation (macroscopic scale). The term crack bifurcation is used here for macroscopic crack bifurcation during fatigue or toughness testing in the direction L-S, from the direction S to the direction L which occurs for rolled products whose thickness is fat least 50 mm.
There is a need for a rolled product made of lithium aluminum alloy for aeronautics applications, particularly for integrally machined parts, having improved fatigue crack propagation properties and having a low propensity for crack bifurcation.

SUBJECT OF THE INVENTION

A first subject of the invention is a rolled product with a thickness of at least 50 mm made of aluminum alloy comprising as a percentage by weight 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% each and 0.15% in total; characterized in that its grain structure is predominantly recrystallized between ¼ and ½ thickness.
A second subject of the invention is a method of manufacturing a plate according to the invention, comprising:

- a) casting of a slab, made of aluminum alloy comprising, as a percentage by weight, 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% by weight each and 0.15% in total;
- b) homogenizing said slab at a temperature of at least 490° C.,
- c) hot rolling said plate to obtain a plate of at least 50 mm thick,
- d) solution heat treatment between 490° C. and 540° C.,
- e) quenching with cold water,
- f) controlled stretching of the said plate with a permanent set of 1 to 7%,
- g) artificial aging of said plate by heating between 130° C. and 160° C. for 5 to 60 hours, characterized in that the sum of the contents of the elements Zr, Cr, Sc, Hf, and V is less than 0.08% by weight and/or in that in step b) the homogenization temperature is at least 520° C. for a period of at least 20 hours and in step c) the hot rolling exit temperature is less than 390° C.

Yet another subject of the invention is the use of a plate according to the invention for producing an aircraft wing spar or an aircraft wing rib.

DESCRIPTION OF THE FIGURES

FIG. 1: Diagram of the CT test piece used for fatigue crack growth tests. The dimensions are given in mm.

FIG. 2. Fatigue crack growth rates obtained on CCT test pieces for reference plate E and plate C according to the invention.

FIG. 3a -Plate A, according to the invention, after fatigue test on the CT test piece for 6 test pieces.

FIG. 3b -Reference plate D, after fatigue test on CT test piece for 6 test pieces.

FIG. 4—Fatigue crack growth rates obtained with the CT test piece.

FIG. 5: Different modes of crack growth on the CT test piece according to FIG. 1, having a rear face (1), a lower face (22) and an upper face (21). Directions S and L are indicated. FIG. 5a : low propensity for crack bifurcation and fracture by the rear face (1), 5 b: high propensity for crack bifurcation and fracture by the lower face (22), 5 c: low propensity for crack bifurcation, fracture by the upper face (21) but distance d on which the crack is neither in the initial direction S nor in the direction L of at least 5 mm.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise stated, all the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed as a percentage by weight is multiplied by 1.4. Alloy designation is made in accordance with the regulations of The Aluminium Association, known to specialists in the field. Definitions of the tempers are given in European standard EN 515.
Unless otherwise specified, the static mechanical properties, in other words the ultimate tensile strength R_m, the conventional yield strength at 0.2% elongation (R_p0.2) and elongation at rupture A %, are determined by a tensile test according to standard EN ISO 10002-1, with sampling and test direction being defined by standard EN 485-1. Unless otherwise specified, the definitions of standard EN 12258-1 apply.
The fatigue crack growth rate (da/dN) is determined according to standard ASTM E 647. The stress intensity factor (K_1C) is determined according to standard ASTM E 399.
For thick aluminum alloy products, those skilled in the art look for a nonrecrystallized grain structure because it is known in particular to be favorable to toughness and resistance to fatigue crack growth (see, for example, the reference article “Application of Modern Aluminum Alloys to Aircraft”, Prog. Aerospace Sci. Vol 32 pp 131-172, 1996, E. A Starke and J. T Staley, p 156, and R. J. H. Wanhill and G. H. Bray, “Fatigue Crack Growth Behavior of Aluminum-Lithium Alloys”, in Aluminium-Lithium alloys Processing, Properties and Applications, Chapter 12 pages 381-413, Elsevier 2014, p 386).
The present inventors found, surprisingly, that rolled products having a thickness of at least 50 mm made of aluminum-copper-lithium-magnesium-manganese alloy have advantageous properties when the grain structure is predominantly recrystallized between ¼ and ½ thickness. In this way, surprisingly, for the thick products according to the invention, fatigue crack growth resistance is improved while the compromise between mechanical strength and toughness is not significantly downgraded. By grain structure predominantly recrystallized between ¼ and ½ thickness, is meant a grain structure whose recrystallization rate is at least 50% between ¼ and ½ thickness, i.e. at least 50% of grains between ¼ and ½ thickness are recrystallized. Preferably the recrystallization rate between ¼ and ½ thickness is at least 55%. Advantageously, the thickness of the products according to the invention ranges between 80 and 130 mm. The products according to the invention have a copper content of between 2.2 and 3.9% by weight. Preferably, the copper content is at least 2.8% by weight and preferentially at least 3.2% by weight. Advantageously, the maximum copper content is 3.8% by weight.
The products according to the invention have a lithium content of between 0.7 and 1.8% by weight. Preferably, the lithium content is at least 0.8% by weight and preferentially at least 0.9% by weight. Advantageously, the maximum lithium content is 1.5% by weight, preferentially 1.1% and preferably 0.95% by weight.
The products according to the invention have a magnesium content of between 0.1 and 0.8% by weight. Preferably, the magnesium content is at least 0.2% by weight and preferentially at least 0.3% by weight. Advantageously, the maximum magnesium content is 0.7% by weight and preferably 0.6% by weight.
The products according to the invention have a manganese content of between 0.1 and 0.6% by weight. Preferably, the manganese content is at least 0.2% by weight and preferentially at least 0.3% by weight. Advantageously, the maximum manganese content is 0.5% by weight and preferably 0.4% by weight.
The products according to the invention contain at least one element chosen from Zn and Ag, the amount of said element, if it is chosen, being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, these elements being particularly useful for hardening the alloy. Preferably, only one of these elements is added, the second being maintained at a content of less than 0.05% by weight.
Optionally, the products according to the invention contain at least one element chosen from Zr, Cr, Sc, Hf, and V, the quantity of said element, if it is chosen, being 0.04 to 0.18% and preferably 0.04 to 0.15% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V. These elements contribute to control of the grain structure.
There are mainly two embodiments of the invention.
In a first embodiment of the invention, the predominantly recrystallized grain structure according to the invention is obtained by a selection of the conversion parameters, in particular homogenization and hot rolling conditions. In this first embodiment, the sum of the content of the elements Zr, Cr, Sc, Hf, and V is preferably at least 0.08% by weight. Preferably, the Zr content in this first embodiment is from 0.08 to 0.10% by weight.
In a second embodiment, the predominantly recrystallized grain structure according to the invention is obtained by limiting the content of elements acting on control of the grain structure. In this second embodiment, the sum of the content of elements Zr, Cr, Sc, Hf, and V is less than 0.08% by weight. In a particular realization of this second embodiment, the Zr content is 0.04 to 0.07% by weight, and preferably 0.05 to 0.07% by weight. In another realization of this second embodiment, no Zr is added, the Zr content is less than 0.05% by weight, preferentially less than 0.04% by weight and more preferentially still less than 0.02% by weight. In some cases, these two embodiments can also be combined.
The products according to the invention contain 0.01 to 0.15% by weight of titanium, this element being especially useful for controlling the grain structure during casting. Preferably, the titanium content ranges between 0.01 and 0.05% by weight. The content of iron and silicon impurities must be limited to avoid downgrading the fatigue and toughness properties. According to the invention, the products according to the invention contain less than 0.1% Fe and less than 0.1% Si. Preferentially, the iron content is less than 0.08% by weight and preferably less than 0.06% by weight. Preferentially, the silicon content is less than 0.07% by weight and preferably less than 0.05% by weight. The other elements present are unavoidable impurities whose content is each less than 0.05% by weight and 0.15% by weight in total. An element not chosen from Cr, Sc, Hf, V, Ag and Zn therefore has a content of less than 0.05% by weight and preferably less than 0.03% by weight. If Zr is not chosen, its content is less than 0.04% by weight and preferably less than 0.02% by weight.
The products according to the invention have satisfactory properties in terms of compromise between mechanical strength and toughness and very advantageous properties in terms of fatigue crack growth rate and in terms of sensitivity to crack deflection.
Advantageously then, the products according to the invention have,

- (i) for a thickness of between 50 and 75 mm, at quarter thickness, a yield strength R_p0.2(LT)≥435 MPa and preferably R_p0.2(LT)≥455 MPa and a toughness K1C (T−L)≥28 MPa√m and advantageously such that K_1C(T−L)≥30 MPa√m,
- (ii) for a thickness of between 76 and 102 mm, at quarter thickness, a yield strength R_p0.2(LT)≥435 MPa and preferably R_p0.2(LT)≥455 MPa and a toughness K1C (T−L)≥25 MPa√m and advantageously such that K_1C(T−L)≥27 MPa√m,
- (iii) for a thickness of between 103 and 130 mm, at quarter thickness, a yield strength R_p0.2(LT)≥428 MPa and preferably R_p0.2(LT)≥448 MPa and a toughness K_1C(T−L)≥23 MPa√m and advantageously such that K_1C(T−L)≥25 MPa√m,
- (iv) for a thickness greater than 130 mm, at quarter thickness, a yield strength R_p0.2(LT)≥428 MPa and preferably R_p0.2(LT)≥448 MPa and a toughness K_1C(T−L)≥21 MPa√m and advantageously such that K_1C(T−L)≥23 MPa√m, and they have a fatigue crack growth rate measured according to standard ASTM E647 on CCT test pieces, with a central crack, of width 100 mm and thickness 6.35 mm taken at mid-thickness in the direction L-S of less than 10⁻⁴mm/cycle for a 4K=20 MPa√m and preferentially less than 9.10⁻⁵mm/cycle for aΔK=20 MPa√m.

The products according to the invention also have advantageous properties in terms of their propensity for crack bifurcation. Macroscopic crack bifurcation during fatigue tests in the direction L-S from the direction S to the direction L was assessed in two ways.
In the first method, at least 6 L-S CT test pieces, 10 mm thick and 50 mm total width (40 mm between the axis of the holes and the rear face of the test piece) made according to FIG. 1 are fatigue tested. at a maximum load of at least 3000 N, and a load ratio of R=0.1, to cover in the course of the test the field of 4K ranging from 10 to 40 MPa√m, where 4K is the variation in the stress intensity factor in a load cycle. On the test pieces, the face in which the fracture takes place is observed. This is illustrated in FIG. 5. When the fracture takes place via the rear face (1), as in FIG. 5A, crack bifurcation was low. When the fracture takes place via the upper (21) or lower (22) face, as in FIG. 5B or 5C, crack bifurcation was more significant. For the products according to the invention, the propensity for crack bifurcation is low and the fracture during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a batch of at least 6 CT test pieces of thickness 10 mm and total width 50 mm, is mainly via the rear face.
In a second method, the propensity for crack bifurcation is assessed by measuring the distance d over which the crack is neither in the initial direction S nor in the direction L, for an L-S CT test piece, of thickness 10 mm and 50 mm total width made according to FIG. 1 and are fatigue tested at the maximum load of at least 3000 N, and a load ratio of R=0.1. FIG. 5c shows an example of the evaluation of this distance: when the crack deviates, it does not immediately take direction L and it is therefore possible to measure distance d. It is considered that the crack is in the direction S or in the direction L when it does not deviate from this direction by more than 10°. For the products according to the invention, the propensity for crack bifurcation is low and during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a CT test piece of thickness 10 mm and total width 50 mm the distance d over which the crack is neither in the initial direction S nor in the direction L is at least 5 mm and preferably at least 10 mm.
The method for manufacturing a plate of predominantly recrystallized grain structure with a thickness of at least 50 mm according to the invention comprises the steps of casting, homogenization, hot rolling, solution heat treatment, quenching, controlled stretching and aging. An alloy containing controlled quantities according to the invention of alloy elements is cast in the form of a slab.
The slab is homogenized at a temperature of at least 490° C. Preferably, homogenization time is at least 12 hours. The homogenization can be carried out in one or more stages. According to the first embodiment of the invention, homogenization comprises at least one step at a temperature of at least 520° C. and preferably at least 530° C., the time during which the temperature is greater than 520° C. being at least 20 hours and preferably at least 30 hours. A hot rolling step is performed after reheating if necessary to obtain plates having a thickness of at least 50 mm According to the first embodiment of the invention, the hot rolling exit temperature is less than 390° C., and preferentially less than 380° C. The combination, in particular, of the conditions of the homogenization step and the hot rolling step of the first embodiment makes it possible to obtain a final structure after aging which is predominantly recrystallized, especially for products for which the sum of the content of the elements Zr Cr, Sc, Hf, and V is at least 0.08% by weight. Surprisingly, the inventors found that the conditions according to this first embodiment make it possible to reduce the propensity for crack bifurcation.
According to the second embodiment, the sum of the content of elements Zr, Cr, Sc, Hf, and V is less than 0.08% by weight and the hot rolling exit temperature is preferably at least 400° C. and preferably at least 420° C.
The plates undergo solution heat treatment by heating between 490 and 540° C., preferably for 15 minutes to 4 hours, the solution heat treatment parameters depending on the thickness of the product.
Quenching with cold water is carried out after the solution heat treatment.
The product then undergoes controlled stretching with a permanent set of between 1% and 7% and preferably between 2% and 6%. Aging is carried out at a temperature of between 130° C. and 170° C. and preferably at a temperature of between 140° C. and 160° C. for a period of 5 to 60 hours, resulting in a T8 state. In some cases, and particularly for some preferred compositions, aging is preferably between 140 and 160° C. for 12 to 50 hours.
The products according to the invention are advantageously used in aircraft construction. The use of the products according to the invention for producing an aircraft wing spar or an aircraft wing rib is particularly advantageous. The use of the products according to the invention for producing an aircraft wing spar is preferred, advantageously for the lower part, i.e. that in connection with the lower surface of the wing, of a welded spar.

EXAMPLES

Example 1

Five Al—Cu—Li alloy slabs referenced A, B, C, D and E were cast. Their composition is given in Table 1.

TABLE 1

Composition (% by weight) of the different slabs.

	Si	Fe	Cu	Mn	Mg	Ti	Zr	Li	Ag	Zn

A	0.03	0.04	3.57	0.38	0.33	0.03	0.08	0.87	0.35	0.05
B	0.03	0.04	3.59	0.38	0.33	0.03	0.08	0.92	0.36	0.03
C	0.03	0.04	3.68	0.38	0.34	0.03	0.08	0.92	0.38	0.04
D	0.02	0.01	3.50	0.55	0.33	0.03	0.08	0.88	0.36	0.04
E	0.03	0.05	3.53	0.38	0.40	0.03	0.09	0.89	0.38	<0.05

Slab A was homogenized in two stages of 36 hours at 504° C. and 48 hours at 530° C. Slabs B and C were homogenized in two stages of 8 hours at 496° C. and 34 hours at 530° C. Slab D was homogenized for 12 hours at 505° C. Slab E was homogenized in two stages of 8 hours at 500° C. and 36 hours at 527° C.
Slab A was hot rolled to a 100 mm thick plate; the hot roll input temperature was 410° C., and the hot roll exit temperature was 361° C.
Slab B was hot rolled to a 102 mm thick plate; the hot roll input temperature was 406° C., and the hot roll exit temperature was 350° C.
Slab C was hot rolled to a 102 mm thick plate; the hot roll input temperature was 410° C., and the hot roll exit temperature was 360° C.
Slab D was hot rolled to a 100 mm thick plate; the hot roll input temperature was 505° C., and the hot roll exit temperature was 520° C.
Slab E was hot rolled to a 100 mm thick plate; the hot roll input temperature was 481° C., and the hot roll exit temperature was 460° C.
The plates thus obtained underwent solution hardening for 2 hours at 525° C. and were then quenched with cold water.
The plates that had undergone solution hardening and quenching underwent controlled stretching, with a permanent elongation of 4% and were aged for 18 hours at 155° C. (A, B, C and E) or 24 hours at 155° C. (D).
The recrystallization rate of the plates thus obtained was measured on metallographic sections of area 0.5×1 mm²in the L-TC plane at various positions in the thickness. The results obtained are given in Table 2.

TABLE 2

Recrystallization rate measurements (%)

	A	B	C	D	E

¼ Thickness	67	75	72	13	<15
½ Thickness	60	58	60	5	<15

Samples were mechanically tested to determine their static mechanical properties and toughness. Ultimate tensile strength R_m, conventional 0.2% elongation yield strength R_p0.2and elongation at rupture A are given in Table 3, and toughness K_1Cis given in Table 4.

TABLE 3

Static mechanical properties measured at ½ thickness (T/4) and at mid-thickness (T/2).

T/4

T/2

L

LT

L

LT

R_m

R_p0,2

R_m

R_p0,2

R_m

R_p0,2

R_m

R_p0,2

Sample

MPa

A (%)

MPa

A (%)

MPa

A (%)

MPa

A (%)

A	514	480	6.2	519	465	7.2	511	477	7.5	505	453	8.8
B
C	516	483	9.2	516	458	6.3	523	492	8.6	500	449	6.1
D	509	478	11.3	517	460	9.3
E	527	492	9.6	527	459	7.0	530	492	9.5	505	444	6.9

TABLE 4

Stress intensity factor (K_1C) measured at ¼
thickness (T/4) and at half-thickness (T/2) determined
according to standard ASTM E 399.

K_1cMPa√m)

T/4

T/2

Sample	L-T	T-L	S-L	L-S

A	32.3	35.1
B
C	36.0	29.1	27.7	48.9*
D	40.0*	32.9	31.2
E	37.7	29.8	31.0

*Crack deflection at 90°

Fatigue crack growth tests on L-S test pieces were performed on samples from plates C and E. The tests were performed according to standard ASTM E647. These tests are carried out on CCT test pieces, with central crack, width 100 mm and thickness 6.35 mm.
FIG. 2 shows the fatigue crack growth rate results for the samples tested with the CCT test piece. The results are summed up in table 5 below.

TABLE 5

L-S test piece fatigue crack growth rate test
results according to standard ASTM E647.

	C	E

da/dn for ΔK = 10 MPa√m	6.5 10⁻⁵	1.2 10⁻⁴
[mm/cycle]
da/dn for ΔK = 20 MPa√m	8.0 10⁻⁵	1.4 10⁻⁴
[mm/cycle]
da/dn for ΔK = 30 MPa√m	1.5 10⁻⁴	2.2 10⁻⁴
[mm/cycle]

In addition, to examine the susceptibility to crack deflection, 6 L-S samples according to FIG. 1 were taken from plates A (samples A1, A2, B1, B2, C1, C2) and D (samples 84A1, 84A2, 84B1, 84B2, 84C1, 84C2) and subjected to a fatigue growth test at a maximum load of 4000 N, or 3000 N when specified, and a load ratio of R=0.1. Markings 84A2 and A2, B2 and C2 were tested at 3000 N maximum force rather than 4000 N. The conditions make it possible, during the test, to cover the field of ΔK ranging from 10 to 40 MPa√m, where ΔK is the variation in the stress intensity factor in a load cycle. On this other geometry, the difference in fatigue crack growth rate between the recrystallized alloy and the non-recrystallized alloy is illustrated in FIG. 4.
FIGS. 3a and 3b show the samples from plates A and D respectively after the fatigue test. The samples from plate A according to the invention have a progressive crack bifurcation with, in 4 cases out of 6 (C1, C2, B1, A2), a fracture by the rear face of the test piece. Distance d over which the crack is neither in the initial direction S nor in the direction L is at least 15 mm for all the samples from plate A, because in no case does the crack join direction L. All samples from plate D have a high propensity for crack bifurcation with a fracture always on the upper or lower face of the test piece and a distance d over which the crack is neither in the initial direction S or in the direction L less than 3 mm: for all samples the crack moves directly from the initial direction S to the perpendicular direction L.
FIG. 4 shows the fatigue crack growth rate results measured by the crack opening method, when tested on CT test pieces. These tests also show that the fatigue crack growth rate is significantly slower, in the direction L-S, for plate A according to the invention.
The predominantly recrystallized product according to the invention has particularly advantageous fatigue crack propagation.

Example 2

Three Al—Cu—Li alloy slabs referenced F, G and H were cast. Their composition is given in Table 6.

TABLE 6

Composition (% by weight) of the different slabs.

	Si	Fe	Cu	Mn	Mg	Ti	Zr	Li	Ag

F	0.03	0.04	3.04	0.28	0.44	0.03	—	0.71	0.22
G	0.03	0.04	3.61	0.37	0.35	0.03	0.06	0.88	0.36
H	0.03	0.05	3.55	0.38	0.32	0.03	0.08	0.87	0.36

14 mm×50 mm×56 mm samples were machined at mid-width (L/2) and quarter-thickness (T/4) of the casting slabs. FIG. 6 shows such samples having a thickness C 14 mm and width B 50 mm Samples were homogenized in two 5 stages increments at 505° C. and 12 hours at 525° C.
The samples were hot worked by twin punching using a machine of the “Servotest”® type. Temperature and rate of deformation were 400° C. and 1 s⁻¹respectively. FIG. 6 illustrates such deformation by twin punching. The final thickness of the deformed portion of width W (W=15 mm) was 3.6 mm, which represents a total reduction of about 74%. Such deformation is representative of industrial deformation by hot rolling of a rolling ingot of about 400 mm to a final thickness of about 100 mm.
The samples thus obtained underwent solution hardening for 2 hours at 525° C. and were then quenched with cold water and aged.
The recrystallization rate at mid-thickness of the samples so obtained was measured on metallographic sections of area 0.5×1 mm²in the L-TC plane. The results obtained are given in Table 7.

TABLE 7

Recrystallization rate measurement (%)

	F	G	H

	½ Thickness	100	70	48

Samples F and G are predominantly recrystallized.

Claims

1. Rolled product with a thickness of at least 50 mm made of aluminum alloy comprising as a percentage by weight 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% each and 0.15% in total; comprising a grain structure predominantly recrystallized between ¼ and ½ thickness.

2. Rolled product according to claim 1, comprising a thickness between 80 and 130 mm.

3. Rolled product according to claim 1, wherein the maximum Li content is 1.5% by weight.

4. Rolled product according to claim 1, wherein the sum of the content of elements Zr, Cr, Sc, Hf and V is less than 0.08% by weight.

5. Rolled product according to claim 1 having

(i) for a thickness of between 50 and 75 mm, at quarter thickness, a yield strength R_p0.2(LT)≥435 MPa and optionally R_p0.2(LT)≥455 MPa and a toughness K_1C(T−L)≥28 MPa√m and advantageously such that K_1C(T−L)≥30 MPa√m,

(ii) for a thickness of between 76 and 102 mm, at quarter thickness, a yield strength R_p0.2(LT)≥435 MPa and optionally R_p0.2(LT)≥455 MPa and a toughness K_1C(T−L)≥25 MPa√m and advantageously such that K_1C(T−L)≥27 MPa√m,

(iii) for a thickness of between 103 and 130 mm, at quarter thickness, a yield strength R_p0.2(LT)≥428 MPa and optionally R_p0.2(LT)≥448 MPa and a toughness K_1C(T−L)≥23 MPa√m and advantageously such that K_1C(T−L)≥25 MPa√m,

(iv) for a thickness greater than 130 mm, at quarter thickness, a yield strength Rp0.2(LT)≥428 MPa and optionally Rp0.2(LT)≥448 MPa and a toughness K_1C(T−L)≥21 MPa√m and advantageously such that K_1C(T−L)≥23 MPa√m,

and having a fatigue crack growth rate measured according to standard ASTM E647 on CCT test pieces, with a central crack, of width 100 mm and thickness 6.35 mm taken at mid-thickness in the direction L-S of less than 10-4 mm/cycle for a ΔK=20 MPa√m.

6. Rolled product according to claim 1 having a low propensity for crack bifurcation wherein the fracture during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a batch of at least 6 CT test pieces of thickness 10 mm and total width 50 mm, is mainly via the rear face.

7. Rolled product according to claim 1 having a low propensity for crack bifurcation wherein during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a CT test piece of thickness 10 mm and total width 50 mm the distance d over which the crack is neither in the initial direction S nor in the direction L is at least 5 mm and optionally at least 10 mm.

8. Manufacturing process for a plate according to claim 1 comprising:

a) the casting of a slab, made of aluminum alloy comprising, as a percentage by weight, 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% by weight each and 0.15% in total;

b) homogenizing said slab at a temperature of at least 490° C.,

c) hot rolling said plate to obtain a plate of at least 50 mm thick,

d) solution heat treatment between 490° C. and 540° C.,

e) quenching with cold water,

f) controlled stretching of the said plate with a permanent set of 1 to 7%,

g) artificial aging of said plate by heating between 130° C. and 170° C. for 5 to 60 hours,

wherein the sum of the content of elements Zr, Cr, Sc, Hf, and V is less than 0.08% by weight and/or in that in b) the homogenization comprises at least one step for which the temperature is at least 520° C., the time during which the temperature is greater than 520° C. being at least 20 hours and in c) the hot rolling exit temperature is less than 390° C.

9. A plate according to claim 1 for producing an aircraft wing spar or an airplane wing rib.

10. A plate according to claim 9 for the lower part of a welded spar.