Patent Application
20240287665
The invention relates in general to worked products made from aluminum-copper-lithium alloys, and more particularly such products in the form of profiles intended to make stiffeners in aeronautical construction.
A continuous research effort is carried out in order to develop materials that can simultaneously reduce the weight and increase the effectiveness of the structures of high-performance airplanes. The aluminum alloys containing lithium are of great interest in this regard, since lithium can reduce the density of the aluminum by 3% and increase the modulus of elasticity by 6% for each percent by weight of lithium added. In order for these alloys to be selected in airplanes, their performance must reach that of the alloys routinely used, in particular in terms of compromise between the properties of static mechanical strength (yield point, ultimate tensile strength) and the properties of damage tolerance (toughness, resistance to the propagation of fatigue cracks), these properties being in general antinomic. These alloys must moreover have sufficient resistance to corrosion, be able to be formed according to the usual methods and have low residual stresses in such a way as to be able to be machined integrally.
A plurality of Al—Cu—Li alloys for which an addition of silver is carried out are known.
The U.S. Pat. No. 5,032,359 describes a vast family of aluminum-copper-lithium alloys in which the addition of magnesium and of silver, in particular between 0.3 and 0.5 percent by weight, allows to increase the mechanical strength. These alloys are often known under the trade name Weldalite™.
The U.S. Pat. No. 5,198,045 describes a family of Weldalite™ alloys comprising (in % by weight) (2.4-3.5)Cu, (1.35-1.8)Li, (0.25-0.65)Mg, (0.25-0.65)Ag, (0.08-0.25) Zr. The worked products manufactured with these alloys combine a density of less than 2.64 g/cm3 and an attractive compromise between the mechanical strength and the toughness.
The U.S. Pat. No. 7,229,509 describes a family of Weldalite™ alloys comprising (in % 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, (up to 0.4) Zr or other elements such as Cr, Ti, Hf, Sc and V. The examples presented have an improved compromise between the mechanical strength and the toughness but their density is greater than 2.7 g/cm3.
The patent application WO2007/080267 describes a Weldalite™ alloy not containing zirconium intended for fuselage sheets comprising (in % by weight) (2.1-2.8)Cu, (1.1-1.7) Li, (0.2-0.6) Mg, (0.1-0.8) Ag, (0.2-0.6) Mn.
Moreover, the alloy AA2196 is known, comprising (in % by weight) (2.5-3.3)Cu, (1.4-2.1) Li, (0.25-0.8) Mg, (0.25-0.6) Ag, (0.04-0.18) Zr and at most 0.35 Mn.
The limitation of the quantity of silver is economically very favorable. However, it is noted that the products according to the prior art made of alloy substantially not containing any silver do not allow to obtain properties as advantageous as those of the products made with alloys containing silver such as the alloy AA2196.
There is a need for products made of aluminum-copper-lithium alloy, namely extruded profiles, having a reduced density and properties substantially equivalent to those of known products containing silver, in particular in terms of compromise between the properties of static mechanical strength and the properties of damage tolerance. The thermal stability, the resistance to corrosion, the machinability and the density of these products namely must also be satisfactory with respect to those of known products containing silver while having a low density.
A first object of the invention is a product made of alloy containing aluminum comprising, in % by weight,
A second object of the invention is a method for manufacturing an extruded, rolled and/or forged product containing aluminum alloy comprising the steps of:
Yet another object of the invention is a structural element incorporating at least one product according to the invention.
Unless otherwise mentioned, all the indications relating to the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The designation of the alloys is done in accordance with the regulations of The Aluminum Association, known to a person skilled in the art. The density depends on the composition and is determined by calculation rather than by a method of measuring weight. The values are calculated in accordance with the procedure of The Aluminum Association, which is described on pages 2-12 and 2.13 of “Aluminum Standards and Data”. The definitions of the metallurgical states are indicated in the European standard EN 515 (2009).
Unless otherwise mentioned, the static mechanical characteristics, in other words the ultimate tensile strength Rm, the conventional elastic limit at 0.2% of elongation Rp0.2 (“elastic limit”) and the elongation at rupture A, are determined by a tensile test according to the standard EN 10002-1 (2001), the sampling and the direction of the test being defined by the standard EN 485-1 (2016).
The stress intensity factor (KQ) is determined according to the standard ASTM E 399 (2012). Thus, the proportion of the test pieces defined in paragraph 7.2.1 of this standard is always verified just like the overall procedure defined in paragraph 8. The standard ASTM E 399 (2012) gives in paragraphs 9.1.3 and 9.1.4 criteria that allow to determine whether KQ is a valid value of K1C. Thus, a value K1C is always a value KQ the converse not being true. In the context of the invention, the criteria of paragraphs 9.1.3 and 9.1.4 of the standard ASTM E399 (2012) are not always met, however for a given test-piece geometry, the values of KQ presented are always comparable to each other, the geometry of the test piece allowing to obtain a valid value of K1C not always being accessible given the constraints related to the dimensions of the sheets or profiles.
Unless otherwise mentioned, the definitions of the standard EN 12258 (2012) apply. The thickness of the profiles is defined according to the standard EN 2066:2001: the transverse cross-section is divided into elementary rectangles having dimensions A and B; A always being the greatest dimension of the elementary rectangle and B being able to be considered as the thickness of the elementary rectangle.
Here, “structural element” or “structural element” of a mechanical construction designates a mechanical part for which the static and/or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural calculation is usually prescribed or carried out. These are typically elements, the failure of which may put in danger the safety of said construction, of its users, of its customers or of others. For an airplane, these structural elements comprise in particular the elements that make up the fuselage (such as the fuselage skin), the fuselage stiffeners or stringers, the bulkheads, the fuselage frames (circumferential frames), the wings (such as the wing skin), the stiffeners (stringers or stiffeners), the ribs and spars and the empennage composed namely of horizontal and vertical stabilizers, as well as the floor profiles (floor beams), the seat rails (seat tracks) and the doors.
According to the invention, a selected class of aluminum alloys containing specific and critical concentrations of copper, lithium, magnesium, zinc, manganese and zirconium but substantially not containing any silver allow to prepare worked products having namely an improved compromise between toughness and mechanical strength with respect to that of products substantially not containing silver.
The present inventors have noted that surprisingly, it is possible for products to obtain a compromise between the properties of static mechanical strength and the properties of damage tolerance at least equivalent to that obtained with an aluminum-copper-lithium alloy containing silver, such as namely the alloy AA2196, by carrying out a narrow selection of the quantities of lithium, copper, magnesium, manganese, zinc and zirconium.
The concentration of copper in the products according to the invention is between 2.5 and 3.4% by weight. In an advantageous embodiment of the invention, the concentration of copper is at least 2.8 or preferably at least 2.9% by weight and/or at most 3.2 and preferably at most 3.1% by weight.
The concentration of lithium in the products according to the invention is between 1.6 and 2.2% by weight. Advantageously, the concentration of lithium is between 1.65% and 1.8% by weight. Preferably, the concentration of lithium is at most 1.75% by weight.
The concentration of magnesium in the products according to the invention is between 0.4 and 0.9% by weight and preferably it is at least 0.5% by weight and, even more preferably greater than 0.6% by weight. Advantageously, the concentration of magnesium is at most 0.8% by weight. The present inventors have noted that when the concentration of magnesium is less than 0.30% by weight the advantageous compromise between the mechanical strength and the damage tolerance is not obtained.
The concentration of manganese in the products according to the invention is between 0.2 and 0.6% by weight and, preferably, it is at least 0.3% by weight and, even more preferably at least 0.33% by weight and more preferably at least 0.4% by weight. In another embodiment, the concentration of manganese is between 0.2 and 0.4% by weight, preferably between 0.25 and 0.35% by weight. The present inventors have noted that when the concentration of manganese is less than 0.2% by weight, the advantageous toughness KQ (L−T), in the direction L−T, according to the invention is not obtained.
The concentration of zirconium in the products according to the invention is between 0.08 and 0.18% by weight and, preferably, it is from 0.12 to 0.16% by weight and, even more preferably, from 0.14 to 0.15% by weight. In another embodiment, the concentration of zirconium is advantageously between 0.09 and 0.12% by weight, preferably between 0.09 and 0.11% by weight, or even between 0.09 and 0.10% by weight.
The concentration of zinc is less than 0.4% by weight, preferably it is from 0.05 to 0.35% by weight. Advantageously, the concentration of zinc is 0.2 to 0.3% by weight which can contribute to achieving the desired compromise between the toughness and the mechanical strength.
The concentration of silver is less than 0.15% by weight, preferably less than 0.10% by weight and, even more preferably less than 0.05% by weight. The present inventors have noted that the advantageous compromise between the mechanical strength and the damage tolerance known for alloys typically containing 0.2 to 0.4% silver by weight can be obtained for alloys substantially not containing any silver with the composition selection carried out.
The sum of the concentration of iron and of the concentration of silicon is at most 0.20% by weight. Preferably, the concentrations of iron and of silicon are each at most 0.08% by weight. In an advantageous realization of the invention, the concentrations of iron and of silicon are at most 0.06% and 0.04% by weight, respectively.
The alloy also contains at least one element that can contribute to controlling the grain size chosen from Ti, Cr, Sc, Hf and V, the quantity of the element, if it is chosen, being from 0.01 to 0.15% by weight, preferably 0.01 to 0.05% by weight for Ti; from 0.01 to 0.15% by weight, preferably 0.02 to 0.1% by weight for Sc; 0.01 to 0.5% by weight, preferably 0.02 to 0.1% by weight for Hf and from 0.01 to 0.3% by weight, preferably from 0.02 to 0.1% by weight for Cr and from 0.01 to 0.3% by weight, preferably from 0.01 to 0.05% by weight for V. Preferably, between 0.02 and 0.04% titanium by weight is chosen.
The alloy according to the invention is particularly intended for manufacturing rolled, extruded and/or forged products and, even more particularly, extruded products. The products according to the invention have a particularly advantageous compromise between mechanical strength and toughness.
The products according to the invention, have in an extruded, solution heat treated, quenched, stretched and aged state, in particular for thicknesses of up to 50 mm or between 8 and 50 mm, or even between 15 and 35 mm, a conventional elastic limit measured at 0.2% of elongation in the direction L, Rp0.2 (L), of at least 510 MPa and a toughness KQ (L−T), in the direction L−T, of at least 21 MPa√m and such that KQ (L−T)>−0.2667*Rp0.2 (L)+169. According to a particularly advantageous embodiment, they have, in the conditions described above, a conventional elastic limit measured at 0.2% of elongation in the direction L, Rp0.2 (L), of at least 525 MPa and a toughness KQ (L−T), in the direction L−T, of at least 23 MPa√m and such that KQ (L−T)>−0.2667*Rp0.2 (L)+171. In the present invention, the test pieces used for the measurements of KQ are of the CT type having a thickness of 20 mm and a width of 50 mm.
The method for manufacturing the products according to the invention comprises steps of production, casting, rolling, extrusion and/or forging, solution heat treatment, quenching, stress relief and aging.
In a first step, a bath of liquid metal is produced in such a way as to obtain an aluminum alloy having a composition according to the invention.
The bath of liquid metal is then cast as an unwrought product typically a rolling ingot, an extrusion billet or a forging blank.
The unwrought product is then homogenized at a temperature between 450° C. and 550° and preferably between 520° C. and 530° C. for a time between 6 and 15 hours.
After homogenizing, the unwrought product is optionally cooled to ambient temperature before being preheated in preparation for being hot worked. The hot working is carried out by rolling, extrusion and/or forging in such a way as to obtain a rolled, extruded and/or forged product, preferably an extruded product.
The product thus obtained is then solution heat treated by heat treatment between 490 and 550° C. for 15 min to 8 h, then quenched typically with water at ambient temperature.
The product then undergoes controlled stress relief, preferably by stretching, with a permanent set of 1 to 15% and preferably of 2 to 4%.
Advantageously, the extruded product has after the method steps described in detail above a thickness of up to 50 mm or between 8 and 50 mm, or even between 15 and 35 mm.
Aging is carried out comprising heating to a temperature between 140 and 170° C. for 5 to 70 hours in such a way that said product has a conventional elastic limit measured at 0.2% of elongation in the direction L, Rp0.2 (L), of at least 510 MPa and a toughness KQ (L−T), in the direction L−T, of at least 21 MPa √{square root over (m)} and such that KQ (L−T)>−0.2667*Rp0.2 (L)+169. The present inventors have noted that the compromise between mechanical strength and toughness can be improved by carrying out the aging at a temperature between 150 and 165° C. for a time with an equivalent ti at 160° C. between 15 and 28 h, preferably between 20 and 27 h, ti being defined by the formula:
where T (in Kelvin) is the instantaneous temperature of treatment of the metal, which changes over the time t (in hours), and Tref is a reference temperature set to 433K. According to a particularly advantageous embodiment, the extruded product with a conventional elastic limit measured at 0.2% of elongation in the direction L, Rp0.2 (L), of at least 525 MPa and a toughness KQ (L−T), in the direction L−T, of at least 23 MPa√m and such that KQ (L−T)>−0.2667*Rp0.2 (L)+171. In said embodiment, the extruded product advantageously has a thickness, up to 50 mm or between 8 and 50 mm, or even between 15 and 35 mm.
The products according to the invention can advantageously be used in structural elements, in particular of an airplane. Thus, an object of the invention is a structural element incorporating at least one product according to the invention or a product manufactured using a method according to the invention.
The use, of a structural element incorporating at least one product according to the invention or manufactured from such a product is advantageous, in particular for aeronautical construction. The products according to the invention are particularly advantageous for the creation of structural elements such as the stiffeners or the frames or for the manufacturing of airplane-wing lower-surface or upper-surface elements, preferably stiffeners, spars and ribs, or also the floor beams and the seat rails.
These aspects, as well as others of the invention are explained in more detail using the following illustrative and non-limiting examples.
In this example, a plurality of billets (384 mm in diameter) made from an Al—Cu—Li alloy, the composition of which is given in table 1, were cast (alloys 67, 74 a and b, 66, 68 and 69). The composition of two alloys of the prior art AA2196 have also been given in table 1 below.
The billets made of alloy 67, 74 a and b, 66, 68 and 69 were then homogenized from 8 to 10 h at 524° C. The billet made from alloy 2 was homogenized 8 h at 500° C. then 24 h at 527° C. while that made of alloy 5 was homogenized 8 h at 520° C.
After homogenizing, the billets were heated to 450° C.+/−40° C. and then hot extruded in order to obtain profiles W according to
The profiles were subjected to aging as indicated in table 2: 30 h at 152° C., 48 h at 152° C., 30 h at 160° C. For the alloys 2 and 5, the aging was carried out for 48 h at 152° C. The equivalent times ti at 160° C. were calculated while taking into account the time of rising to the aging plateau and while considering a rising speed of 20° C./h.
Samples taken at profile end were tested in order to determine their static mechanical properties as well as their toughness (Kq). The location of the samples is indicated by dotted lines in
The results obtained are given in table 2 below and illustrated by
| Number | Date | Country | Kind |
|---|---|---|---|
| 17/53133 | Apr 2017 | FR | national |
This application is a divisional of U.S. application Ser. No. 16/603,604, filed Oct. 8, 2019, which is a National Stage entry of International Application No. PCT/FR2018/050886, filed Apr. 9, 2018, which claims priority to France No. 17/53133, filed Apr. 10, 2017, the contents of which are incorporated herein by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16603604 | Oct 2019 | US |
| Child | 18649164 | US |
