Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/12—Alloys based on aluminium with copper as the next major constituent
  • C22C21/16—Alloys based on aluminium with copper as the next major constituent with magnesium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/12—Alloys based on aluminium with copper as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/057—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with copper as the next major constituent

Definitions

• the present invention generally relates to aluminum alloy products and, more particularly, to such products, their methods of manufacture and use, particularly in the aerospace industry.
• AlLi Aluminum-lithium alloys
• AlLi alloys are very interesting in this respect, since lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each weight percent of lithium added.
• AlLi alloys are not yet used extensively in the aerospace industry because of the defects of the alloys developed until today, such as, for example, inadequate thermal stability, strong anisotropy and inadequate toughness.
• the history of the development of AlLi alloys is described, for example, in the "Aluminum-lithium alloys" chapter of "Aluminum and Aluminum Alloys” (ASM Specialty Handbook, 1994).
• the first aluminum-lithium alloys (Al-Zn-Cu-Li) were introduced in Germany in the 1920s, and were followed by the introduction of AA2020 alloy (Al-Cu-Li-Mn-Cd) at the end of the 1950s, and by the introduction of alloy 1420 (Al-Mg-Li) in the Soviet Union, in the mid-1960s.
• the only industrial applications of the AA2020 alloy were the horizontal wings and stabilizers of the Vigilante RA5C aircraft.
• the conventional composition of AA2020 alloy was (in percent by weight): Cu: 4.5, Li: 1.2, Mn: 0.5, Cd: 0.2. Among the reasons related to the limited applications of this alloy, it can be emphasized its low toughness.
• zirconium is used instead of manganese as a control agent for the granular structure.
• Blackenship declares: "Zirconium is the element of choice for the control of the granular structure in Al-Li-X alloys".
• Pickens' List of Refining Additives actually mixes elements used for foundry grain refining (such as TiB2) and elements used to control grain structure during operations transformation, such as zirconium.
• foundry grain refining such as TiB2
• zirconium conventional grain refiners such as Cr, Mn, Ti, B, Hf, V, TiB 2 and mixtures thereof can be used ", it is clear from the development history of AlLi alloys that a bias related to the use of any element other than Zr for the control of the granular structure exists for the skilled person.
• the Zr is used. It is likewise found in an alloy developed more recently (AA2050, see also WO2004 / 106570), the use of zirconium for grain refining, the addition of manganese to improve the toughness.
• AA2297 alloy which contains lithium, copper and manganese, optionally magnesium but no silver for which zirconium is also used for refining grain.
• No. 5,234,662 discloses a preferred composition of 1.6% by weight of Li, 3% by weight of Cu, 0.3% by weight of Mn and 0.12% by weight of Zr.
• Alloys AA2050 and AA2297 were mainly available for thick plates, thicker than 0.5 inches (12.7 mm).
• Another range of AlLi alloys, containing Zn has been described for example in US Patent No. 4,961,792 and US Patent No. 5,066,342, and developed in the early 1990s.
• the metallurgy of these alloys can not be compared with the metallurgy of the "Weldalite®" alloys, since the incorporation of a significant amount of zinc, and in particular the combination of zinc and magnesium, completely modifies the properties of the alloy, for example in terms of mechanical strength and corrosion resistance.
• the alloys In order to use AlLi alloys for fuselage applications, the alloys must offer the same performance, or even better performance, in terms of mechanical strength, damage tolerance, as the alloys not containing Li currently used. In particular, resistance to crack propagation is an important issue in these applications, and this explains why alloys known for their high tolerance to damage, such as AA2524 and AA2056, are traditionally used. Other desirable properties include weldability and corrosion resistance. Due to the growing trend of reducing costly mechanical fastening operations in the aerospace industry, weldable alloys such as AA6013, AA6056 or AA6156 are being introduced for fuselage panels. High corrosion resistance is also desirable in order to replace plated products with cheaper bare products.
• the yield strength anisotropy which, in turn, determines the anisotropy of the other mechanical properties, has been mentioned above.
• the low yield strength at intermediate test directions such as for example at 45 ° to the rolling direction, is the most obvious manifestation of anisotropy.
• the R curve test is a widely recognized means for characterizing the toughness properties.
• the curve R represents the evolution of the critical effective stress intensity factor for crack propagation as a function of the effective crack extension, under increasing monotonic stress. It allows the determination of the critical load for unstable failure for any configuration relevant to cracked aircraft structures.
• the values of the stress intensity factor and the crack extension are actual values as defined in ASTM E561.
• a first subject of the invention is an aluminum-based alloy comprising 2.1 to 2.8% by weight of Cu, 1.1 to 1.7% by weight of Li, 0.1 to 0.8% by weight of Ag, 0.2 to 0.6% by weight of Mg, 0.2 to 0.6% by weight of Mn, an amount of Fe and Si of less than or equal to 0.1% by weight each, and unavoidable impurities at a content of less than or equal to 0.05% by weight each and 0.15% by weight in total, the alloy being substantially free of zirconium, which means that the zirconium content is less than 0, 04% by weight.
• Another object of the invention is a method of manufacturing an aluminum alloy sheet having high strength and toughness, wherein:
• said plate is hot-rolled and optionally cold-rolled into a sheet, with an initial rolling temperature of 450 to 490 ° C., (d) said sheet is dissolved at 480 to 520 ° C. for 15 minutes at 4 ° C. hours
• Yet another object of the invention is a rolled, spun or forged product comprising an alloy according to
• Yet another object of the invention is a structural element intended for aeronautical construction comprising a product according to the invention. Description of figures
• Figures 1 to 5 relate to certain aspects of the invention described herein. These are illustrative and are in no way limiting.
• Figure 3 Evolution of the cracking velocity in the T-L direction when the amplitude of the stress intensity factor varies.
• Figure 4 Evolution of the cracking velocity in the LT direction when the amplitude of the stress intensity factor varies.
• Figure 5 Relative evolution of R p o, 2 as a function of the orientation with respect to the rolling direction.
• the static mechanical characteristics in other words the ultimate ultimate strength R m , the tensile yield strength RpO, 2 and the elongation at break A, are determined by a tensile test according to the EN 10002-1 standard, the location at which the parts are taken and their meaning defined by EN 485-1.
• the cracking rate (using the da / dN - ⁇ K test) is determined according to ASTM E 647.
• a curve giving the effective stress intensity factor as a function of the effective crack extension, known as the R curve. is determined according to ASTM E 561.
• the critical stress intensity factor K c in other words the intensity factor which makes the crack unstable, is calculated from the curve R.
• the factor of stress intensity K C o is also calculated by assigning the initial crack length at the beginning of the monotonic load, to the critical load. These two values are calculated for a specimen of the required form.
• K app represents the K C o factor corresponding to the specimen that was used to perform the R curve test.
• K eff represents the K c factor corresponding to the specimen that was used to perform the R curve test.
• ⁇ a ⁇ ff (max) represents the crack extension of the last point of the curve R, valid according to the ASTM E561 standard. The last point is obtained either at the time of the sudden rupture of the test piece, or possibly at the moment when the stress on the uncracked ligament exceeds on average the limit of elasticity of the material.
• the crack size at the end of the pre-fatigue cracking stage is W / 3 for M (T) type specimens, where W is the width of the specimen as defined in ASTM E561.
• the width of the specimen used in a toughness test may have a _
• a "structural element” or “structural element” of a mechanical construction is called a mechanical part, the failure of which is likely to endanger the safety of the said construction, its users, its users or others.
• these structural elements include the elements that make up the fuselage (such as fuselage skin (fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frames), wings (such as wing skin), stiffeners (stiffeners), ribs (ribs) and spars) and empennage including horizontal stabilizers and vertical stabilizers (horizontal or vertical stabilizers), as well as floor beams, seat tracks and doors "sheet” means a rolled product not exceeding 12.7 mm or 0.5 inches thick.
• the aluminum-copper-lithium-silver-magnesium-manganese alloy according to one embodiment of the invention advantageously has the following composition:
• Table 1 Composition ranges of alloys according to
• the invention (% by weight, the remainder being Al)
• the alloy being substantially free of zirconium.
• substantially free of zirconium it should be understood that the zirconium content should be less than 0.04% by weight, preferably less than 0.03% by weight, and even more preferably less than 0.01% by weight. % in weight.
• the inventors have discovered that the low zirconium content makes it possible to improve the toughness of Al-Cu-Li-Ag-Mg-Mn alloys; in particular, the length of the curve R is significantly increased.
• the use of manganese in place of zirconium to control the granular structure has several additional advantages such as obtaining a recrystallized structure and isotropic properties for a thickness of between 0.8 and 12.7 mm, or between 1/32 and 1/2 inch.
• Iron and silicon generally affect toughness properties.
• the amount of iron should be limited to 0.1% by weight (preferably 0.05% by weight) and the amount of silicon should be limited to 0.1% by weight (preferably 0.05% by weight).
• the unavoidable impurities should be limited to 0.05% by weight each and 0.15% by weight in total. If the alloy has no other additive element, the rest is made of aluminum.
• the inventors have discovered that if the copper content is greater than 2.8% or even 2.6% or even 2.5% by weight, the toughness properties may in some cases fall rapidly, whereas, if the copper is less than 2.1% or even 2.2% or even 2.3% by weight, the mechanical strength is too low.
• lithium content With regard to the lithium content, a higher lithium content to 1.7% or even 1.6% or even 1.5% by 'weight leads to thermal stability problems. A lithium content of less than 1.1% or even 1.2% or even 1.3% by weight results in inadequate mechanical strength and lower density gain. It has been discovered by the inventors that, if the silver content is less than 0.1% or even 0.2% by weight, the mechanical strength obtained does not satisfy the desired properties. The silver content, however, must be kept below 0.8% or even 0.6% or even 0.4% by weight because a high amount of silver increases the density of the alloy and also its cost. .
• the alloy according to the invention can be used to manufacture extruded, forged or rolled products.
• the alloy according to the invention is used to manufacture sheets.
• the products according to the invention have a very high tenacity.
• Zr is a peritectigue element that is generally enriched at the grain center and depleted at the grain boundaries, while Mn, which is a eutectic element with a partition coefficient close to one, is distributed much more evenly. .
• the different behavior of Zr and Mn during solidification could be related to the different effect observed in terms of toughness.
• the recrystallization rate of the products according to the invention is greater than 80%.
• the inventors have discovered that the homogenization temperature should preferably be between 480 and 520 ° C. for 5 to 60 hours, and even more preferably between 490 and 510 ° C. for 8 to 20 hours. In the course of the invention, the inventors have observed that homogenization temperatures above 520 ° C tended to reduce the toughness performance in some cases. The inventors believe that there is a relationship between the technical effect of the homogenization conditions and the behavior during the solidification described above.
• the initial hot rolling temperature is preferably 450 to 490 ° C.
• the hot rolling is preferably carried out to obtain a thickness of between about 4 and 12.7 mm.
• a cold rolling step may optionally be added, if necessary.
• the sheet obtained has a thickness of between 0.8 and 12.7 mm, and the invention is more advantageous for sheets of 1.6 to 9 mm thick, and even more advantageous. for sheets 2 to 7 mm thick.
• the product according to the invention is then dissolved, preferably by heat treatment between 480 and 52O 0 C for 15 min to 4 h, and then quenched with water at room temperature.
• the product then undergoes a controlled pull of 1 to 5% and preferably 2 to 4%. If the traction is greater than 5%, the mechanical properties may not be sufficiently improved and one may encounter industrial difficulties such as a high implementation, which would increase the cost of the product.
• An income is produced at a temperature between 140 and 170 ° C. for 5 to 80 hours and, more preferably, between 140 and 155 ° C. for 20 to 80 hours. The lowest solution temperatures in this range generally favor high toughness.
• the revenue step is divided into two steps: a pre-revenue step prior to a welding operation, and a final heat treatment of an element. welded structural.
• the characteristics of the sheets obtained with the present invention comprise at least one of the following characteristics:
• the tensile yield strength R p o, 2 in the direction L is preferably at least 390 MPa or even 400 MPa
• the breaking strength R m in the direction L is preferably at least 410 MPa or even 420 MPa
• the tensile yield strength R p o, 2 at 45 ° with respect to the direction of rolling is at least equal to the tensile yield strength R p o, 2 in the direction LT.
• the difference between the tensile yield strength R p o, 2 at 45 ° with respect to the direction of rolling and the tensile yield strength R p o, 2 in the direction TL defined by (Rp 0 , 2 (TL) - R p0, 2 (45 °)) / R p0, 2 (TL) is between + 5% and -5% and preferably between + 3% and -3%.
• K app in the direction TL is preferably at least 100 MPaVm and preferably at least 120 MPaVm;
• - K ef f in the direction TL is at least 120 MPaVm and preferably at least
• the crack extension of the last valid point of the curve R in the direction TL is preferably at least 60 mm and preferably at least 80 mm;
• the recrystallization rate of the sheets according to the invention is greater than 80%.
• the shaping of the products according to the invention can advantageously be carried out by stretch-forming, deep-drawing, pressing, spinning, profile rolling or folding, these techniques being known to those skilled in the art.
• all known and possible riveting, bonding and welding techniques suitable for aluminum alloys can be used, if desired.
• the products according to the invention can be attached to stiffeners or frames, for example by riveting, gluing or welding.
• the inventors have discovered that if welding is chosen, it may be preferable to use low temperature welding techniques, which help to ensure that the thermally affected area is as limited as possible. In this respect, laser welding and friction stir welding often give particularly satisfactory results.
• the products according to the invention before or after shaping, can advantageously be subjected to an income to confer improved static mechanical properties. This income can also advantageously be conducted on an assembled structural element if desired.
• the products according to the invention are preferably used for the manufacture structural elements for aeronautical construction.
• a structural element, formed of a sheet according to the invention and stiffeners or frames, these stiffeners or frames being preferably made of extruded profiles, can be used in particular for the manufacture of aeronautical fuselage as well as any other use where the present properties could be advantageous.
• a fuselage panel is produced comprising at least one sheet according to the invention.
• the inventors have discovered that the products of the invention offer a particularly favorable compromise between the static mechanical properties, the high tenacity and the density.
• high tensile and high yield strength sheets generally have low toughness.
• the properties of high tenacity, and in particular the very long curve R favor the industrial application to aircraft fuselage parts.
• the density of the products is less than about 2.69 g / cm 3 and preferably less than about 2.66 g / cm 3 .
• the products of the invention generally do not induce any particular problem during subsequent surface treatment operations conventionally used in aeronautical construction, in particular for mechanical or chemical polishing, or treatments intended to improve the adhesion of the polymer coatings. Resistance to intergranular corrosion of the products' of the invention is generally high; for example, only pits are generally detected when the metal is subjected to a corrosion test.
• a sheet of the invention may be used without being plated on either side with an aluminum alloy lightly loaded with alloying elements.
• Example A is a reference AA2098 alloy.
• the chemical compositions of the various alloys tested are given in Table 2.
• the granular structure of the samples was characterized from microscopic observation of cross sections after anodic oxidation, under polarized light, or after chromatic attack.
• a recrystallization rate was determined. The recrystallization rate is defined as the surface fraction occupied by recrystallized grains. For samples B, C and D the recrystallization rate was 100%. For samples A # 1 and A # 2, the recrystallization rate was less than 20%.
• the samples were mechanically tested to determine their static mechanical properties as well as their resistance to crack propagation. Tensile yield strength, ultimate strength and elongation at break are given in Table 5. Table 5: Mechanical Properties of Samples
• the static mechanical properties of the samples according to the invention are comparable to those of a conventional 2XXX damage tolerant alloy, and are lower than those of high strength alloys such as 7475 or 2098 (A).
• the resistance of the comparative alloy B is lower than that of the alloy according to the invention (C), which may be related to the absence of silver in the comparative alloy B.
• the inventors consider that that the lower copper content and zirconium content of the sample according to the invention explain the lower strength compared to alloy 2098 (sample A).
• the anisotropy of the sample C according to the invention is very weak as illustrated in FIG. 5 which represents the normalized evolution of the elastic limit as a function of the orientation.
• the tensile strength limit at 45 ° is slightly greater than the tensile yield strength in the TL direction, the difference between these values, defined by (R p0 , 2 (TL) - R p0 , 2 ( 45 °)) / Rp 0 , 2 (TL) is thus -0.3%. In comparison, this difference is 13.2% for the reference sample A # 2 (AA2098).
• the sample C according to the invention has high toughness properties.
• FIGS. 1 and 2 The curves R of the samples A # 1, B and C are given in FIGS. 1 and 2, for the directions T-L and LT, respectively.
• FIG. 1 clearly shows that the crack extension of the last valid point of the curve R ( ⁇ a e ff (ma ⁇ )) is much greater for the sample C of the invention than for the reference sample A # 1 .
• This parameter is at least as critical as K app values because, as explained in the description of the prior art, the length of the curve R is an important parameter for the design of the fuselage.
• Figure 2 shows the same trend, but the difference is smaller because the LT direction inherently gives better results. Table 6 summarizes the results of the toughness tests.
• Figures 3 and 4 show the evolution of the cracking rate in the T-L and L-T orientation, respectively, as the magnitude of the stress intensity factor varies.
• the cracking rate of the sample C is of the same order of magnitude as in that typically obtained for alloys AA6156 and AA2056.

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