Patent Application
20240035138
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.
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 K1C(L)>37.4 MPa√m for a yield strength Rp0,2(L)>448.2 MPa (products with a thickness greater than 76.2 mm) and in particular a tenacity K1C(L)>38.5 MPa√m for a yield strength Rp0,2(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.
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:
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.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
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 Rm, the conventional yield strength at 0.2% elongation (Rp0.2) 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 (K1C) 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, p156, 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,
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
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
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.
Five Al—Cu—Li alloy slabs referenced A, B, C, D and E were cast. Their composition is given in Table 1.
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 mm2 in the L-TC plane at various positions in the thickness. The results obtained are given in Table 2.
Samples were mechanically tested to determine their static mechanical properties and toughness. Ultimate tensile strength Rm, conventional 0.2% elongation yield strength Rp0.2 and elongation at rupture A are given in Table 3, and toughness K1C is given in Table 4.
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.
In addition, to examine the susceptibility to crack deflection, 6 L-S samples according to
The predominantly recrystallized product according to the invention has particularly advantageous fatigue crack propagation.
Three Al—Cu—Li alloy slabs referenced F, G and H were cast. Their composition is given in Table 6.
14 mm×50 mm×56 mm samples were machined at mid-width (L/2) and quarter-thickness (T/4) of the casting slabs.
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−1 respectively.
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 mm2 in the L-TC plane. The results obtained are given in Table 7.
Samples F and G are predominantly recrystallized.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1650850 | Feb 2016 | FR | national |
This application is a Continuation Application of U.S. application Ser. No. 16/074,661, filed 1 Aug. 2018, which is a National Stage entry of International Application No. PCT/FR2017/050255, filed 3 Feb. 2017, which claims priority to French Patent Application No. 16/50850, filed 3 Feb. 2016, the entire contents of each of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16074661 | Aug 2018 | US |
| Child | 18448998 | US |
