Patent Application
20250137102
The invention relates to thin sheets of aluminum-copper-lithium alloy, more particularly such products and the methods for manufacturing and using same, intended in particular for aeronautical and aerospace construction for fuselage sheet applications.
Continuous research efforts are being made in order to develop materials that can simultaneously reduce the weight and increase the efficacy of high-performance aircraft structures. Aluminum-lithium (AlLi) alloys are highly advantageous in this regard, since lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each weight percentage of lithium added.
EP 1 891 247 discloses a low-density aluminum-based alloy useful in an aircraft structure for fuselage sheet applications having high mechanical strength, high toughness and high corrosion resistance, containing, as % by weight, 2.7 to 3.4 Cu, 0.8 to 1.4 Li, 0.1 to 0.8 Ag, 0.2 to 0.6 Mg and an element such as Zr, Mn, Cr, Sc, Hf, Ti or a combination of these, the quantity of which, as % by weight, is 0.05 to 0.13 for Zr, 0.05to 0.8 for Mn, 0.05 to 0.3 for Cr and Sc, 0.05 to 0.5 for Hf and 0.05 to 0.15 for Ti. The quantity of Cu and of Li is determined in accordance with the formula Cu (% by weight) +5/3 Li (% by weight)<5.
The patent U.S. Pat. No. 5,455,003 describes a method for manufacturing Al—Cu—Li alloys that have improved mechanical strength and toughness at cryogenic temperature, in particular by means of suitable working and aging. This patent recommends in particular the composition, as percentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6 and Zn=0-0.75. U.S. Pat. No. 5,455,003 encourages developing non-recrystallized products for obtaining the expected properties under cryogenic conditions and using the addition of Zr and Ti.
Fuselage sheets may be stressed in several directions and isotropic thin sheets having high properties and balanced in mechanical strength in the directions L and TL and in toughness for the directions L-T and T-L are highly sought after. In addition, it has been found that thin sheets obtained with certain alloys having high properties at certain thicknesses, for example 4 mm, can, in some cases, have less high or anisotropic properties at another thickness, for example 2.5 mm. It is often not advantageous industrially to use different alloys for different thicknesses and an alloy making it possible to achieve high isotropic properties whatever the thickness would be particularly advantageous.
The patent EP 1 966 402 describes an alloy comprising 2.1 to 2.8% by weight Cu, 1.1 to 1.7% by weight Li, 0.1 to 0.8% by weight Ag, 0.2 to 0.6% by weight Mg, 0.2 to 0.6% by weight Mn, a quantity of Fe and Si of less than or equal to 0.1% by weight each, and unavoidable impurities in a proportion of less than or equal to 0.05% by weight each and 0.15% by weight in total, the alloy being substantially free from zirconium, particularly adapted for obtaining recrystallized thin sheets.
WO2016/051099 describes a sheet with a thickness of 0.5 to 9 mm with an essentially recrystallized annular structure made from aluminum-based alloy comprising 2.8 to 3.2% by weight Cu, 0.5 to 0.8% by weight Li, 0.1 to 0.3% by weight Ag, 0.2 to 0.7% by weight Mg, 0.2 to 0.6% by weight Mn, 0.01 to 0.15% by weight Ti, a quantity of Zn of less than 0.2% by weight, a quantity of Fe and of Si of less than or equal to 0.1% by weight each, and unavoidable impurities in a proportion of less than or equal to 0.05% by weight each and 0.15% by weight in total, said sheet being obtained by a method comprising casting, homogenization, hot rolling and optionally cold rolling, solution heat treatment, quenching and aging.
There is a need for thin sheets having high yield strength (to withstand buckling) as well as high plane strain toughness, in particular having a high apparent breaking stress intensity factor (Kapp) and high Δaeff_max values; Δaeff_max represents the crack extension of the last point of the curve R, valid in accordance with ASTM E561-20.
WO2016/051099 discloses data for curve R in
Testing curve R is a widely recognized means for characterizing the toughness properties. Curve R represents the change in the effective stress intensity factor critical for crack propagation as a function of the effective crack extension, under increasing monotonic stress. It makes it possible to determine the critical load for unstable rupture for any configuration relevant to cracked aircraft structures. The stress intensity factor and crack extension values are effective values as defined in ASTM E561-20. Conventional analysis, generally used, of the tests performed on panels with a central crack gives an apparent breaking stress intensity factor (Kapp). This value does not necessarily vary significantly as a function of the length of the curve R. However, the length of the curve R—namely the maximum crack extension of the curve Δaeff_max—is an important parameter in itself for fuselage design, in particular for panels including fixed stiffeners.
A first object of the invention is a thin sheet with a thickness of less than 12.7 mm, with an essentially recrystallized granular structure, made from aluminum-based alloy comprising, as % by weight, 2.5 to 3.5% Cu, 0.7 to 0.9% Li, 0.3 to 0.5% Mg, 0.2 to 0.5% Mn, 0.25 to 0.65% Zn, 0.01 to 0.15% Ti, 0 to 0.07% Ag, a quantity of Fe and of Si of less than or equal to 0.1% each, and unavoidable impurities in a proportion of less than or equal to 0.05% each and 0.15% by weight in total, the remainder aluminum.
A second object of the invention is a method for manufacturing a thin sheet with a thickness of less than 12.7 mm, from aluminum alloy, wherein, successively
Yet another object of the invention is the use of a thin sheet according to the invention or obtained according to the method of the invention for fuselage structure elements.
Unless specified otherwise, all of 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 alloys are designated in accordance with the Aluminum Association rules, known to a person skilled in the art. The definitions of the metallurgical tempers are indicated in European standard EN 515-2017.
In the context of the present invention, essentially non-recrystallized granular structure means a granular structure such that the degree of recrystallization at half thickness is less than 30% and preferably less than 10%, and essentially recrystallized granular structure means a granular structure such that the degree of recrystallization at half thickness is greater than 70% and preferably greater than 90%. The degree of recrystallization is defined as the fraction of surface area over a metallographic section occupied by recrystallized grains. The grain sizes are measured in accordance with ASTM E112-2013.
Unless mentioned to the contrary, the static mechanical characteristics, in other words the ultimate breaking strength Rm, the tensile elastic limit Rp0.2 and the elongation at break A, are determined by a tensile test in accordance with EN 6892-1-2019, the location at which the parts are taken and their direction been defined by EN 485-1-2016.
A curve giving the effective stress intensity factor as a function of the effective crack extension, known as the R curve, is determined in accordance with ASTM E 561-20. The critical stress intensity factor KC, in other words the intensity factor that makes the crack unstable, is calculated from the R curve. The stress intensity factor KC0 is also calculated by attributing the initial crack length at the commencement of the monotonic load to the critical load. The two values are calculated for a test piece with the required shape. Kapp represents the factor KC0 corresponding to the test piece that was used to carry out the R-curve test. Keff represents the factor KC corresponding to the test piece that was used to carry out the R-curve test. Δaeff_max represents the crack extension of the last valid point on the curve R, valid in accordance with ASTM E561-20. The last valid point is obtained either at the moment of abrupt rupture of the test piece, or optionally at the moment when the stress on the non-cracked ligament exceeds on average the elastic limit of the material. Unless mentioned to the contrary, the crack size at the end of the pre-fatigue-cracking stage is W/3 for test pieces of type M(T), wherein W is the width of the test piece as defined in ASTM E561-20.
It should be noted that the width of the test piece used in a toughness test may have a substantial influence on the R curve measured in the test. Since fuselage sheets are large panels, only the toughness results obtained on sufficiently large samples, such as samples having a width greater than the than or equal to 400 mm, are judged significant for evaluating toughness. For this reason, only CCT760 test samples, which have a width of 760 mm, were used for evaluating toughness. The initial crack length is 2ao=253 mm.
Unless specified otherwise, the definitions of EN 12258 apply. “Structure element” or “structural element” of a mechanical construction means here a mechanical part the failure of which is liable to put in danger the safety of said construction, of its utilizers, of its users or others. For an aircraft, these structure elements comprise the fuselage structure elements. A fuselage structure element comprises the elements that make up the fuselage such as the fuselage skin, the fuselage stiffeners or stringers, the airtight partitions (bulkheads), and the fuselage frames (the circumferential frames).
“Sheet” or “thin sheet” used indifferently in this description means a laminated product not exceeding 12.7 mm or 0.5 inches in thickness. The sheets according to the invention have a thickness of between 0.5 and 12.7 mm, preferentially between 0.5 mm and 9 mm, more preferentially between 1.5 mm and 6 mm.
According to the present invention, a selected class of aluminum alloys containing specific and critical quantities of copper, lithium, magnesium, zinc, and manganese but essentially not containing silver makes it possible to prepare thin sheets having an improved compromise between toughness and mechanical strength, and an improved value of Δaeff_max. corresponding to the crack extension of the last valid point of the R curve.
Preferably, the thin sheet with a thickness of less than 12.7 mm, with an essentially recrystallized granular structure, made from aluminum-based alloy comprising, as % by weight, 2.5 to 3.5% Cu, 0.7 to 0.9% Li, 0.3 to 0.5% Mg, 0.2 to 0.5% Mn, 0.25 to 0.65% Zn, 0.01 to 0.15% Ti, 0 to 0.07% Ag, a quantity of Fe and of Si of less than or equal to 0.1% each, and unavoidable impurities in a proportion of less than or equal to 0.05% each and 0.15% by weight in total, the remainder aluminum, is in the T8 metallurgical temper. This means that the thin sheet has undergone solution heat treatment, cold deformation and aging.
The copper content is from 2.5 to 3.5% by weight. If the copper content is greater than 3.5% by weight, it is not possible to obtain sufficient toughness. Preferentially, the copper content is no more than 3.4%, 3.3%, 3.2% or 3.1%. If the copper content is below 2.5% it is too low. Preferably the copper content is at least 2.7% or even at least 2.8% by weight in order to obtain sufficient mechanical strength. In an advantageous embodiment of the invention, the copper content is 2.8 to 3.1% by weight.
The lithium content is from 0.7% to 0.9% by weight. In an advantageous embodiment of the invention, the lithium content is from 0.7% to 0.8%, preferentially from 0.70 to 0.80%. Adding lithium can help to increase the mechanical strength and toughness, an excessively high or excessively low content does not make it possible to obtain a high toughness value and/or a sufficient elastic limit.
The magnesium content is from 0.3% to 0.5% by weight. In an advantageous embodiment of the invention, the magnesium content is from 0.30% to 0.45% by weight, preferentially from 0.35% to 0.45% by weight.
The manganese content is from 0.2 to 0.5% by weight and preferably from 0.20% to 0.45% by weight, and even more preferably from 0.25% to 0.45% by weight. In one embodiment of the invention, the manganese content is no more than 0.45% by weight. Adding manganese to the claimed quantity make it possible to control the essentially recrystallized granular structure at half thickness while maintaining a homogeneous structure through the thickness.
The silver content is less than or equal to 0.07% by weight, preferentially less than or equal to 0.05% by weight, even more preferentially less than or equal to 0.04% by weight, or even 0.03% by weight.
The zinc content is from 0.25 to 0.65% by weight, preferably from 0.45% to 0.65% by weight. According to the present invention, it was observed that the presence of zinc in proportions between 0.25% and 0.65% combined with a silver content of less than or equal to 0.07% by weight, preferably less than or equal to 0.04% or even 0.03%, makes it possible to obtain a higher tensile elastic limit close to the aging peak.
The titanium content is from 0.01% to 0.15% by weight. Advantageously, the titanium content is at least 0.02% by weight and preferably at least 0.03% by weight. In an advantageous embodiment of the invention, the titanium content is no more than 0.1% by weight and preferably no more than 0.05% by weight. Adding titanium helps to control the granular structure, in particular during casting.
The proportions of iron and silica are each no more than 0.1% by weight. In an advantageous embodiment of the invention, the proportions of iron and silicon are no more than 0.08% and preferably no more than 0.04% by weight. A controlled and limited iron and silicon content helps to improve the compromise between mechanical strength and tolerance to damage.
Unavoidable impurities on maintained at a proportion of less than or equal to 0.05% by weight each and 0.15% by weight in total. The remainder is aluminum.
The composition of the alloy is substantially free from zirconium. “Substantially free from zirconium” must be understood to mean that zirconium is not an addition element intentionally added. It may however be present by way of impurities in a proportion of zirconium of less than or equal to 0.05% by weight, preferably less than or equal to 0.04% by weight, even more preferably less than or equal to 0.03% by weight, or even less than or equal to 0.01% by weight.
The thin sheets according to the invention are in the T8 temper:
The method for manufacturing the sheets according to the invention comprises steps of melting, casting, rolling, solution heat treatment, quenching, controlled traction and aging.
In a first step, a liquid metal bath is prepared so as to obtain an aluminum alloy with the composition according to the invention. The liquid metal bath is next cast in the form of a rolling plate.
The rolling plate is next homogenized at a temperature of between 480° C. and 535° C. and preferably between 490° C. and 530° C. and preferably between 500° C. and 520° C. The duration of homogenization is preferably between 5 and 60 hours. In the context of the invention, an excessively low homogenization temperature or the absence of homogenization does not make it possible to achieve improved isotropic properties compared with those of the known products, in particular in terms of mechanical strength in the directions L and TL and of toughness for the directions L-T and T-L, and this over the whole of this thickness range.
After homogenization, the rolling plate is in general cooled to ambient temperature before being preheated with a view to being deformed hot. The objective of preheating is to reach a temperature preferably between 400 and 500° C. allowing deformation by hot rolling.
Hot and optionally cold rolling is implemented so as to obtain a sheet with a thickness of less than 12.7 mm, preferably between 0.5 and 9 mm.
Advantageously, during hot rolling, a temperature above 400° C., preferably above 450° C. is maintained until the thickness of 20 mm. Intermediate heat treatments during rolling and/or after rolling can be implemented in some cases. However, preferably, the method does not comprise any intermediate heat treatment during rolling and/or after rolling.
The sheet thus obtained is next solution heat treated by heat treatment between 450 and 535° C., preferably between 490° C. and 530° C., and preferably between 500° C. and 520° C., preferably for 5 minutes to 2 hours, and then quenched. Advantageously, the duration of solution heat treatment is no more than 1 hour so as to minimize surface oxidation. It is known to persons skilled in the art that the precise solution heat treatment conditions must be selected according to the thickness and composition so as to put the hardening elements in solid solution.
The sheet next undergoes cold deformation by controlled traction with a permanent deformation of 0.5 to 5% and preferentially from 1 to 3%. Known steps such as rolling, flattening, smoothing, straightening and shaping can optionally be implemented after solution heat treatment and quenching and before or after controlled traction; however, total cold deformation after solution heat treatment and quenching must remain less than 15% and preferably less than 10%.
Aging is implemented, comprising heating at a temperature of between 130 and 170° C. and preferably between 150 and 160° C., for 5 to 100 hours and preferably from 10 to 40 hours.
Advantageously, the aging treatment is implemented so as to obtain an equivalent duration t1eq150° at 150° C. of between 10 hours and 80 hours, preferentially from 40 hours to 75 hours, even more preferentially from 55 hours to 75 hours.
The equivalent duration teq150° is calculated at the temperature of 150° C. in accordance with the formula
where T°C.(t) (in ° C.) is the instantaneous temperature of the sheet that changes with time (in seconds). The calculation is made over the time interval (in seconds) corresponding to the duration of aging treatment.
The metallurgical temper of the thin sheet at the end of aging is preferentially a T8 temper.
In another embodiment, it is possible, after the cold deformation step by controlled traction, to implement a short treatment in accordance with the disclosure of EP2766503 in order to improve suitability of the product for shaping. After deformation, the sheet undergoes a final T8 aging.
The use of thin sheets according to the invention or obtained according to the method of the invention is advantageous in fuselage structure elements or in aerospace applications such as rocket manufacture.
An alloy D, the composition of which as % by weight is given in Table 1, was cast in the form of plates. The composition D corresponds to a composition according to the invention.
Two plates were homogenized at 508° C. for 12 hours, and then heated before hot rolling at 505° C. for approximately 28 hours in order to obtain two thin sheets with respective thicknesses of 4.2 mm and 2.5 mm. The sheets were next subjected to solution heat treatment at 505°, detensioning by traction of 2% and aging of 40 hours at 155° C. (time equivalent to 63 hours at 150° C.).
The transformation parameters are indicated in Table 2 below:
The granular structure of the samples at half thickness was characterized from the microscopic observation of metallographic sections in the L-TC direction after anodic oxidation under polarized light. The granular structure of the sheets is essentially recrystallized at half thickness.
The samples were tested mechanically in order to determine their static mechanical properties (Table 3) and their toughness (Tables 4 and 5). The toughness characteristics were measured at full thickness and after surfacing so as to have test pieces of 1.2 mm, the surfacing having been implemented on a single face; the test pieces have a width of 760 mm. Table 5 discloses Δaeff_max for each of the cases. Δaeff_max represents the crack extension of the last point on the R curve, valid according to ASTM E561-20. The last point is obtained either at the moment of the abrupt rupture of the test piece, or optionally at the moment when the strain on the non-cracked ligament exceeds the elastic limit of the material. The value of the tensile elastic limit of the material Rp0.2 is indicated in Table 4; it is measured after the R curve test, on a non-deformed zone of the test piece with CT760 toughness located above the initial crack.
The alloys A, B, C, D were cast in the form of plates. Their composition as % by weight as indicated in Table 6. The alloys A, B, C have compositions outside the invention. They represent the disclosure of WO2016/051099.
Ti aimed at 0.03% weight and Si, Fe aimed at ≤0.05% weight.
These plates were homogenized, hot rolled, solution heat treated, and then tractioned to obtain thin sheets A #1, B #1, C #1, D #1 of thickness 4.2 mm. The transformation conditions are indicated in Table 7. All the sheets have an essentially recrystallized structure at half thickness.
An aging kinetics was implemented at 155° C. on each of the sheets (Table 8).
It is noted that the sheet D #1 reaches a higher elastic limit Rp0.2 than the other sheets A #1, B #1, C #1. Its hardening kinetics is faster at the start of the aging kinetics.
The alloys A, E, F were cast in the form of plates. Their composition as % by weight is indicated in Table 9. The alloys A, E, F have compositions outside the invention. The alloy A represents the disclosure of WO2016/051099.
Ti aimed at 0.03% weight and Si, Fe aimed at ≤0.05% weight.
These plates were homogenized, hot rolled, solution heat treated, and then tractioned to obtain thin sheets of thickness 4 mm, 4.2 mm, 2 mm or 2.5 mm according to the case. The transformation conditions are indicated in Table 10. The sheets were subjected to aging making it possible to have a temper close to the aging peak.
All the sheets have an essentially recrystallized structure at half thickness. As with example 1, the sheets were characterized so as to measure the toughness (Table 11) and the values of Δaeff_max valid on CCT760 test pieces.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2200763 | Jan 2022 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2023/050105 | 1/26/2023 | WO |
