Classifications

• • 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
  • C22F1/047—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 magnesium as the next major constituent
• • 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
• • 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
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent

Definitions

• Lithium magnesium aluminum alloy with improved toughness Lithium magnesium aluminum alloy with improved toughness
• the invention relates to aluminum-magnesium-lithium alloy products, more particularly, such products, their manufacturing and use processes, intended in particular for aeronautical and aerospace construction.
• Aluminum alloy rolled products are developed to produce high strength parts for the aerospace industry and the aerospace industry in particular.
• Aluminum alloys containing lithium 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.
• their performance compared with the other properties of use must reach that of the alloys commonly used, in particular in terms of a compromise between the static mechanical strength properties (yield strength in tension and in compression, breaking strength) and the properties of damage tolerance (toughness, fatigue crack propagation resistance), these properties being in general antinomic.
• These alloys must also have sufficient corrosion resistance, be able to be shaped according to the usual methods and have low residual stresses so that they can be machined integrally.
• GB Patent 1,172,736 teaches an alloy containing 4 to 7% by weight Mg, 1.5 - 2.6% Li, 0.2 - 1% Mn and / or 0.05 - 0.3% Zr, remaining aluminum useful for uses requiring high mechanical strength, good corrosion resistance, low density and high modulus of elasticity.
• No. 5,431,876 teaches a ternary alloy group of lithium aluminum and magnesium or copper, including at least one additive such as zirconium, chromium and / or manganese.
• US Pat. No. 6,551,424 discloses a process for the manufacture of aluminum-magnesium-lithium alloy products of composition (in% by weight) Mg: 3.0 - 6.0, Li: 0.4 - 3.0, Zn up to 2, 0, Mn up to 1.0, Ag up to 0.5, Fe up to 0.3, Si up to 0.3, Cu up to 0.3, 0.02 - 0.5 from a member selected from the group consisting of , Hf, Ti, V, Nd, Zr, Cr, Y, Be, including cold rolling in the lengthwise and in the widthwise directions.
• a member selected from the group consisting of , Hf, Ti, V, Nd, Zr, Cr, Y, Be including cold rolling in the lengthwise and in the widthwise directions.
• 6,461,566 discloses an alloy of composition (in% by weight) Li: 1.5 - 1.9, Mg: 4.1 - 6.0, Zn 0.1 - 1.5, Zr 0.05 - 0.3, Mn 0.01 - 0.8 H, 0.9 10 "5 - 4.5 10 " 5 and at least one element selected from the group Be 0.001 - 0.2, Y 0.001 - 0.5 and Se 0, 01 - 0.3.
• RU 2171308 discloses an alloy comprising (in% by weight) Li: 1.5 - 3.0, Mg: 4.5 - 7.0, Fe 0.01 - 0.15, Na: 0.001 - 0.0015 , H, 1.7 0 "5 - 4.5 10 " 5 and at least one member selected from the group Zr 0.05-0.15, Be 0.005-0.1, and Se 0.05-0.4 and minus one element selected from the group Mn 0.005-0.3, Cr 0.005-0.2, and Ti 0.005-0.2, remains aluminum.
• the patent RU2163938 describes an alloy containing (in% by weight) Mg: 2.0 - 5.8, Li: 1.3-2.3, Cu: 0.01-0.3, Mn: 0.03-0. , 5, Be: 0.0001 - 0.3, at least one of Zr and Se: 0.02 - 0.25 and at least one of Ca and Ba: 0.002 - 0.1, remain aluminum.
• the patent application DE 1 558 491 describes in particular an alloy containing (in% by weight) Mg: 4-7, Li: 1.5-2.6, Mn: 0.2-1.0, Zr 0.05 - 0.3 and / or Ti 0.05-0.15 or Cr 0.05-0.3. These alloys have not solved some problems and in particular their performance in terms of damage tolerance has not allowed their significant use in commercial aviation. It should also be noted that the manufacture of wrought products from these alloys has remained difficult and that the scrap rate is too high.
• a first subject of the invention is a wrought product made of aluminum alloy of composition, in% by weight,
• Another subject of the invention is a method of manufacturing a wrought product according to the invention comprising successively
• the temperature at a temperature below 150 ° C.
• Yet another object of the invention is the use of a product of the invention for producing aircraft structural elements.
• Figure 1 Curve R in the L-T direction (CCT760 specimen).
• FIG. 1 Curve R in the T-L direction (specimen CCT760).
• alloys are in accordance with the regulations of The Aluminum Association, known to those 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 given in the European standard EN 515.
• the static mechanical characteristics in tension in other words the tensile strength R m , the conventional yield stress at 0.2% elongation R p0.2 , and the elongation at break A% are determined by a tensile test according to standard NF EN ISO 6892-1, the sampling and the direction of the test being defined by the EN 485-1 standard.
• the critical stress intensity factor Kc in others the intensity factor which makes the crack unstable, is calculated from the curve R.
• the stress intensity factor Kco 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 Kco factor corresponding to the specimen that was used to perform the curve test R.
• Kc e ff represents the factor Kc corresponding to the specimen that was used to perform the R curve test.
• e ff ( ma x) represents the crack extension of the last valid point of the curve R.
• the length of the curve R - namely the maximum crack extension of the curve - is a parameter that is in itself important, in particular for fuselage design.
• EN 12258 Unless otherwise specified, the definitions of EN 12258 apply.
• a "structural element” or “structural element” of a mechanical construction is called 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 realized.
• 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 the upper or lower wing skin, the stringers or stiffeners, the ribs and spars) and the stabilizer composed in particular of horizontal and vertical stabilizers (horizontal or vertical). stabilizers), as well as floor beams, seat tracks and doors.
• a selected class of aluminum alloys which contain specific and critical amounts of magnesium, lithium, zirconium, titanium, iron and silicon makes it possible to produce wrought products having an improved property compromise. in particular between mechanical strength and damage tolerance, while having a good corrosion performance.
• the magnesium content of the products according to the invention is between 4.0 and 5.0% by weight. In an advantageous embodiment of the invention, the magnesium content is at least 4.3% by weight or preferably 4.4% by weight. A maximum content of 4.7% by weight or preferably 4.6% by weight of magnesium is preferred.
• the lithium content of the products according to the invention is between 1.0 and 1.6% by weight.
• the present inventors have found that a limited lithium content, in the presence of certain addition elements, makes it possible to very significantly improve the fracture toughness and the speed of propagation of fatigue cracks, which largely compensates for the slight increase in density and the decrease in static mechanical properties.
• the maximum lithium content is 1.5% by weight and preferably 1.45% by weight or preferably 1.4% by weight.
• a minimum lithium content of 1.1% by weight and preferably 1.2% by weight is advantageous, in particular to improve the resistance to intergranular corrosion.
• the zirconium content of the products according to the invention is between 0.05 and 0.15% by weight and the titanium content is between 0.01 and 0.15% by weight.
• the presence of these elements associated with the transformation conditions used advantageously makes it possible to maintain a granular structure substantially not recrystallized.
• the present inventors have found that it is not necessary to add scandium in these alloys to obtain the desired substantially non-recrystallized granular structure and that the addition of scandium could even prove to be harmful by making the alloy particularly fragile and difficult to cold roll up to thicknesses less than 3 mm.
• the scandium content is therefore less than 0.01% by weight.
• the titanium content is between 0.01 and 0.05% by weight.
• the alloy contains at least one of Mn and Cr with a content, in% by weight Mn: 0.05 - 0.5 or 0 , 05 - 0.3 and Cr: 0.05 - 0.3, an element not selected from Mn and Cr having a content of less than 0.05% by weight.
• the improvement of the hot ductility facilitates hot deformation, which makes it possible to reduce the scrap rate during the processing.
• the alloy contains at least one of Ag and Cu with, if selected, in% by weight Cu: 0.05 - 0.3 and Ag: 0, 05 - 0.3, an element not selected from Ag and Cu having a content of less than 0.05% by weight.
• the Ag content and / or the Cu content are less than 0.05% by weight.
• the wrought products according to the invention contain a small amount of iron and silicon, the content of these elements being between 0.02 and 0.2% by weight.
• the present inventors believe that the presence of these elements can contribute, by forming intermetallic phases and / or by contributing to the formation of dispersoids especially in the presence of manganese, to improve the properties of damage tolerance by avoiding the localization of the deformation.
• the Fe content and / or the Si content are in% by weight Fe: 0.04-0.15; If: 0.04-0.15
• the Fe content and / or the Si content is less than 0.15% by weight and preferably less than 0.1% by weight. weight.
• the Zn content is at most 0.5% by weight. In an advantageous embodiment of the invention, the Zn content is less than 0.2% by weight and preferably less than 0.05% by weight.
• the deliberate addition of Zn is typically not desirable because this element can contribute to degrade the hot ductility while not providing any advantage for the resistance to intergranular corrosion. In addition the addition of Zn contributes to increase the density of the alloy which is most often not desirable.
• the other elements have a content of less than 0.05% by weight, each.
• the products according to the invention have a maximum content of 5 ppm of Be and preferably 2 ppm of Be and / or a maximum content of 10 ppm of Na and / or a maximum content of 20 ppm of It.
• the wrought products according to the invention are preferably spun products such as profiles, rolled products such as sheets or thick plates and / or forged products.
• the process for manufacturing the products according to the invention comprises the successive steps of producing a bath of liquid metal so as to obtain an aluminum alloy of composition according to the invention, casting said alloy in raw form, optionally homogenization of the product thus cast, hot deformation and optionally cold, the dissolution of the product thus deformed, and quenching, optionally the cold deformation of the product so dissolved and quenched and the tempering at a temperature below 150 ° C.
• a bath of liquid metal is produced so as to obtain an aluminum alloy of composition according to the invention.
• the liquid metal bath is then cast in a raw form, typically a rolling plate, a spinning billet or a forging blank.
• the raw form is then optionally homogenized so as to reach a temperature of between 450 ° C. and 550 ° C. and preferably between 480 ° C. and 520 ° C. for a period of between 5 and 60 hours.
• the homogenization treatment can be carried out in one or more stages.
• the hot deformation is carried out directly after a simple reheating without performing homogenization.
• the hot deformation typically by spinning, rolling and / or forging, is preferably carried out with an inlet temperature above 400 ° C and advantageously above 430 ° C or even 450 ° C.
• the present inventors have found that even in carrying out these intermediate heat treatments, it was not possible for them to cold-roll industrial sheets of reference alloys to a thickness of 2 mm, whereas this step proved achievable with alloy sheets according to the invention.
• the sheets according to the invention have a preferred thickness of at least 0.5 mm and preferably at least 0.8 mm or 1 mm.
• the product is dissolved and quenched.
• the dissolution is carried out, according to the composition of the product, at a temperature between 370 and 500 ° C. Quenching is carried out with water and / or air. It is advantageous to perform quenching in the air because the intergranular corrosion properties are improved.
• the product thus dissolved and quenched can optionally be further deformed cold.
• Planing or straightening steps are typically performed at this stage, but it is also possible to carry out further deformation so as to further improve the mechanical properties.
• the metallurgical state obtained for the rolled products is advantageously a T6 or T6X or T8 or T8X state and for the advantageously spun products a T5 or T5X state in the case of quenching on a press or a T6 or T6X or T8 or T8X state.
• the product finally undergoes an income at a temperature below 150 ° C.
• the income is carried out in three stages, a first stage at a temperature of between 70 and 100.degree. C., a second stage at a temperature of between 100 and 140.degree. ° C and a third bearing at a temperature between 90 to 110 ° C, the duration of these bearings being typically 5 to 50 hours.
• substantially non-recrystallized granular structure means a non-recrystallized granular structure content at mid-thickness greater than 70% and preferably greater than 85%.
• the rolled products according to the invention have particularly advantageous characteristics.
• the rolled products preferably have a thickness of between 0.5 mm and 15 mm, but products with a thickness greater than 15 mm, up to 50 mm or even 100 mm or more may have advantageous properties.
• the laminates obtained by the process according to the invention have, for a thickness of between 0.5 and 15 mm, at least one property of static mechanical resistance among the properties (i) to (iii) and at least one property at mid-thickness. of damage tolerance among properties (iv) to (vi)
• the rolled products according to the invention exhibit an improvement in the isotropy of the mechanical properties, in particular the toughness.
• the rolled products according to the invention which have been air quenched have a weight loss of less than 20 mg / cm 2 and preferably less than 15 mg / cm 2 after the intergranular corrosion test NAMLT ("Nitric Acid Mass”). Loss Test "ASTM-G67).
• the wrought products according to the invention are advantageously used to produce aircraft structural elements, in particular aircraft.
• Preferred aircraft structural elements are in particular a fuselage skin advantageously obtained with sheets having a thickness of 0.5 to 12 mm according to the invention, a fuselage frame, a stiffener or a fuselage rail advantageously obtained with profiles according to the invention or a rib.
• alloys A to C are reference alloys.
• the plates were heated and hot rolled to a thickness of about 4 mm. Cold rolling tests up to 2 mm thickness were carried out after a heat treatment consisting of two successive one-hour steps at 340 ° C. followed by 1 hour at 400 ° C. Only the alloy sheets according to the invention could be successfully cold-rolled to the final thickness, the reference alloy sheets being broken to a thickness of 2.6 mm. After hot rolling and possibly cold rolling, the sheets were dissolved at 480 ° C. for 20 minutes, this treatment being preceded by a heat treatment consisting of two successive steps of one hour at 340 ° C. followed by 1 hour at 400 ° C. After dissolution, the sheets were air-soaked and glued. The yield was made during 10 h at 85 ° C. followed by 16 h at 120 ° C. followed by 100 h at 100 ° C.
• the granular structure of all the samples was substantially non-recrystallized, the recrystallization rate at mid-thickness being less than 10%.
• Figure 3 shows the improvement of the compromise between yield strength and toughness.
• the improvement of K app (LT) is greater than 25% whereas the reduction in elastic limit is less than 15% relative to the alloy sheet C.
• the length of the curve R is also significantly improved thus Aa eff ( m ax) (TL) is improved by more than 30%.
• the crack propagation rate was determined according to E647 standard on 160 mm wide CCT test pieces.
• the alloy sheets according to the invention quenched in air have a low sensitivity to intergranular corrosion for a thickness of 4 mm and are not sensitive to intergranular corrosion for a thickness of 2 mm.
• Example 2 In this example, ingots were cast to evaluate the hot ductility and the intergranular corrosion properties of different alloys. The size of the ingots after scalping was in mm of 255 x 180 x 28.
• the hot ductility was evaluated on test pieces machined in the ingots after a homogenization of 12 h at 505 ° C.
• the hot ductility test was carried out using a servo hydraulic machine supplied by Servotest Testing Systems Ltd on specific specimens with a thickness of 20 mm at a deformation rate of 1 s- 1 . in compression a sample containing two holes Due to compression, the material between the holes expands at a controlled rate of deformation Test conditions are described in the article by A. Deschamps et al. in the journal Materials Science and Engineering A319-321 (2001) 583-586. The standard measurement of surface area reduction ( ⁇ / ⁇ ) by image analysis makes it possible to evaluate the ductility at the considered temperature. The results obtained at 450 ° C. and 475 ° C. are shown in Table 8. Table 8 - Hot Ductility (AA / Ap) (%)
• the alloys E and F which contain Mn and Cr have advantageous heat ductility while the hot ductility of the reference alloy I containing 0.6% by weight of Zn is the weakest of the tested alloys.
• the ingotins were hot-rolled to a thickness of 4 mm.
• the sheets thus obtained were dissolved at 480 ° C., this treatment being preceded by a heat treatment consisting of two successive steps of one hour at 345 ° C. followed by 1 hour at 400 ° C. After dissolution, the sheets were air quenched and glided by controlled traction with a permanent elongation of 2%.
• the yield was made during 10 h at 85 ° C. followed by 16 h at 120 ° C. followed by 100 h at 100 ° C.
• Alloy G which differs from alloy D in particular by a lower copper content, has a particularly low weight loss.
• the alloy I which contains Zn is not distinguishable from the G alloy in terms of resistance to intergranular corrosion.
• Alloy H which has a lower lithium content than the other alloys tested, has a higher weight loss.

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• 2011
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