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
• • 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
  • 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-lithium alloys and, in particular, such products useful in the aeronautical industry.
• Al-Li alloys have long been recognized as an effective solution for reducing the weight of structural elements due to their low density.
• the different properties required for materials used in the aerospace industry such as high yield strength, high compressive strength, high damage tolerance and high corrosion resistance, have proven difficult. to obtain simultaneously.
• Al-Li alloys are particularly sensitive to crack bifurcation which is part of the damage tolerance problems limiting the use of Al-Li alloys (Hurtado, JA, de los Rios, ER, Morris, AJ, Crack deflection in Al-Li alloys for aircraft structures ", 18th Symposium of the International Committee on Aeronautical Fatigue, Melbourne, UNITED KINGDOM, 3-5 May 1995, pp. 107-136, 1995).
• 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 stress test. fatigue or tenacity.
• the crack bifurcation occurs at the microscopic ( ⁇ 100 ⁇ m), mesoscopic (100-1000 ⁇ m) or macroscopic scale (> 1 mm) scale, but it is considered harmful only if the direction of the crack remains stable after bifurcation (macroscopic scale). This phenomenon is of particular concern for fatigue tests in the LS direction for aluminum-lithium alloys.
• crack bifurcation is used here for the macroscopic crack bifurcation during fatigue or toughness testing in the LS direction, from the S direction to the L direction which occurs for rolled products whose thickness is from less than 30 mm.
• the crack bifurcation may occur in relation to the composition of the rolled product, its microstructure and the test conditions.
• Rolled products in AA7050 alloy can be considered as a product reference with a low tendency to crack bifurcation.
• a first object of the invention is a method of manufacturing a substantially non-recrystallized sheet having a thickness of at least 30 mm having a low propensity for crack bifurcation, the process comprising: a) casting a plate comprising 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, remains aluminum and unavoidable impurities, b) homogenizing said plate between 470 0 C and 510 0 C for a period of 2 to 30 hours, c) hot rolling said plate to obtain a sheet of at least 30 mm thick, with an exit temperature of at least 410 ° C., d) dissolving between 490 ° C.
• T in Kelvin
• T ref is a reference temperature set at 773 K
• e la quenched with cold water
• f the controlled traction of said sheet with a permanent deformation of 2 to 5%
• g the income of said sheet by heating between 130 0 C and 160 0 C for 5 to 60 hours.
• Another object of the invention is a substantially non-recrystallized sheet of thickness at least 30 mm, obtainable by the method according to the invention characterized in that it has a low propensity for crack bifurcation.
• Yet another object of the invention is a structural element obtained from a sheet according to the invention.
• Figure 1 Schematic representation of the location of the Sinclair sample.
• Figure 2 Sinclair sample geometry.
• Figure 3 Schematic representation of the mixed mode test conditions I and II used on the Sinclair sample.
• Figure 4 Schematic representation of the method of determining the deflection angle on a fractured Sinclair sample.
• Figure 5 Evolution of the deflection angle with the maximum equivalent stress intensity factor for two homogenization treatments applied to the same alloy and for a standard AA7050 alloy sheet.
• Figure 6 Geometry of the sample used for fatigue tests in the L-S direction.
• Figure 7 Photographs of samples after an L-S fatigue test.
• the mode I or mode by opening, is characterized in that one exerts a stress perpendicular to the faces of the crack.
• Mode II where plane bias mode, has a shear stress perpendicular to the crack front.
• the mode III or anti-plane biasing mode, is a mode in which the shear stress is parallel to the crack front.
• the Sinclair sample (6) is a SL sample and the initial crack corresponds to a 90 ° bifurcated crack in an LS sample. If the crack of the Sinclair sample is stable when subjected to a mixed mode I and II stress representative of the stress experienced by the bifurcated crack, then the bifurcated crack would have been stable and the sample has a high propensity to crack bifurcation.
• the geometry of the Sinclair sample is given in Figure 2.
• Six orifices (61) are used to attach the Sinclair sample to the test device. The sample is mechanically pre-milled, the length of the pre-crack is 7 mm.
• the Sinclair sample is subjected to mixed mode stress I and II in accordance with Figure 3.
• Two sample carriers (71) and (72) are used to subject the sample to mixed mode I and II stress.
• the samples are attached to the sample holders by the six orifices (61) to form an assembly that is stressed between the orifices (711) and (721).
• the angle ⁇ of application of the load between a plane perpendicular to the initial direction of crack and the direction of the stress is 75 °. It may be noted that the angle ⁇ is the angle complementary to the angle of inclination of the crack with respect to the axis of stress.
• the stress intensity factors K 1 and Kn are obtained according to
• ⁇ is the angle between a plane perpendicular to the initial crack direction and the direction of the stress.
• K eff V ((1 - V ⁇ ) K, 1 + (1 - v *) K n 2 + (1 + V) K1)
• K eff max is the maximum stress intensity factor during a fatigue cycle, it corresponds to the maximum load P max .
• the deflection angle ⁇ between the initial crack direction and the direction of the deflected crack allows a quantitative evaluation of the propensity for crack bifurcation. It is measured as described in Figure 4.
• Figure 4 is a representation of a broken Sinclair sample (61). The profile (65) of the broken sample is measured using a profilometer with steps of 0.5 mm. The data obtained is smoothed by a sliding average over three points. The deflection angle is measured for each set of three points. The maximum deflection angle between the end of the mechanical crack (69) and a distance of 32 mm from the edge of the sample is the value of ⁇ .
• 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 are typically elements whose failure is likely to endanger the safety of 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 the floor beams, the seat tracks and the doors
• a laminate product essentially unreinforced at least 30 mm thick according to the invention has a low propensity for crack bifurcation through the combination of a carefully selected composition and specific steps of the manufacturing process
• the aluminum-lithium alloy laminate according to the invention 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.
• 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 remains aluminum and unavoidable impurities.
• the content of iron and silicon is at most 0.15% by weight each or preferably 0.10% by weight and the content of the other unavoidable impurities is at most 0.05% by weight each and 0 15% by weight in total.
• a refining agent containing titanium is added during casting.
• the titanium content is preferably between 0.01 and 0.15% by weight and preferably between 0.01 and 0.04% by weight.
• the copper content is preferably at least 2.7% by weight or even at least 3.2% by weight so as to achieve sufficient strength.
• the lithium content is preferably at least 0.8% by weight and even more preferably at least 0.9% by weight, so as to obtain a low density.
• the maximum lithium content is limited to 1.8 wt% or even 1.4 wt% and more preferably 1.25 wt%.
• the invention is particularly advantageous for alloys which simultaneously contain a high lithium content and a high copper content, because these alloys have a very favorable compromise of mechanical properties but are particularly sensitive to the bifurcation of cracks.
• the content of Li and Cu, expressed in% by weight are in accordance with Li + Cu> 4 and preferably Li + Cu> 4.3. However, if the alloy simultaneously contains a very high content of Li and Cu, burning phenomena can occur during homogenization.
• the contents of Li and Cu, expressed in% by weight are in accordance with Li + 0.7 Cu ⁇ 4.3 and preferably Li + 0.5 Cu ⁇ 3.3.
• Manganese is an essential component of the laminated product according to the invention and its content is carefully selected, preferably between 0.3 and 0.5% by weight. Carefully controlled distribution of manganese dispersoids obtained through the combination of selected content and thermo-mechanical processing conditions helps to avoid stress localization and grain boundary constraints. Although not related to any specific theory, the inventors believe that the distribution of the manganese-containing dispersoids obtained according to the invention contributes to the low propensity for crack bifurcation. The performance in terms of strength and toughness observed by the inventors are generally difficult to achieve for alloys containing no silver, especially when the permanent deformation after controlled pulling is less than 3%.
• the present inventors believe that silver plays a role during the formation of the copper-containing hardening phases formed during natural or artificial aging, and, in particular, allows the formation of finer phases and also allows a more homogeneous distribution of these phases. .
• the advantageous effect of silver is observed when the silver content is at least 0.1% by weight and preferably at least 0.2% by weight. An excessive addition of silver would probably be prohibitively expensive in many cases because of the high price of silver, and it is advantageous not to exceed a content of 0.5% by weight and preferably 0.3% by weight. in weight.
• the addition of magnesium improves the mechanical strength and decreases the density. Too high an addition of Mg can, however, be detrimental to toughness.
• the Mg content is at most 0.4% by weight. The present inventors believe that the addition of Mg may also play a role in the formation of copper-containing phases.
• An alloy containing controlled amounts of alloying elements is cast as a plate.
• the plate is homogenized at a temperature between 470 0 C and 510 0 C for 2 to
• the present inventors have found that a homogenization temperature greater than about 510 ° C. causes a higher propensity for crack bifurcation.
• the present inventors believe that high homogenization temperatures affect the size and distribution of manganese-containing dispersoids.
• a hot rolling step is performed after reheating if necessary to obtain sheets having a thickness of at least 30 mm.
• a hot rolling exit temperature of at least 410 ° C., preferably of at least 430 ° C., and preferably of at least 450 ° C., is necessary to obtain a substantially non-recrystallized product after solution dissolution.
• product essentially not recrystallized a product whose recrystallization rate is less than 10% to quarter and half thickness (T / 4 and T / 2).
• the sheets are dissolved by heating between 490 and 540 0 C for 15 minutes to 4 hours and quenched with cold water. The dissolution parameters depend on the thickness of the product.
• the total equivalent time for homogenization and dissolution t (eq) does not exceed 30h and preferably 2Oh.
• the equivalent time t (eq) at 500 ° C. is defined by the formula: where T is the instantaneous temperature expressed in Kelvin which changes with time t (in hours) and T ref is a reference temperature of 500 ° C. (773 K). t (eq) is expressed in hours.
• the formula giving t (eq) takes into account the heating and cooling phases.
• Quenching with cold water is carried out after dissolution.
• rapid quenching is performed.
• Fast quenching means that the cooling rate is as high as possible given the thickness of the sheet.
• vertical immersion quenching is preferably carried out by horizontal spraying quenching. The present inventors have observed that fast quenched products have a lower propensity to crack bifurcation. The present inventors believe that this effect could be related to a lower precipitation at the grain boundaries.
• the product then undergoes a controlled pull with a permanent deformation of between 2% and 5% and preferably between 3% and 4%.
• the income is made at a temperature between 130 0 C and 160 0 C for a period of 5 to 60 hours, resulting in a T8 state.
• the income is preferably carried out between 140 and 160 ° C. for 12 to 50 hours. Lower tempering temperatures generally favor higher toughness.
• the propensity for crack bifurcation is also observed for fatigue tests in the LS direction.
• a low propensity for crack bifurcation also means that for the products according to the invention, a crack bifurcation is observed on less than 20% and preferably less than 10% of the samples of a batch of at least 4 LS samples.
• a2 the breaking strength R m at T / 4 and T / 2 is at least 490 MPa, preferably at least 495 MPa or even at least 500 MPa in the direction L.
• bl the tenacity KlC: in the direction LT at T / 4 and T / 2 is at least 31 MPaVm, preferably at least 32 MPaVm or even at least 33 MPaVm.
• Toughness KlC in the TL direction at T / 4 and T / 2 is at least 28 MPaVm and preferably at least 29 MPaVm or even at least 30 MPaVm.
• b3 the tenacity KlC: in the direction SL at T / 4 and T / 2 is at least 25 MPaVm and preferably at least 26 MPaVm or even at least 27 MPaVm.
• Other advantageous properties of the products according to the invention whose thickness is greater than 100 mm include at least one of characteristics a4 and a5 and at least one of characteristics b4, b5 and b6 in the T8 state, where the characteristics a4, a5, b4, b5 and b6 are defined by: a4: the elastic limit Rpo, 2 to T / 4 and T / 2 is at least 440 MPa, preferably at least 445 MPa or even at least 450 MPa in the L direction.
• a5 the breaking strength R m at 174 and T / 2 is at least 475 MPa, preferably at least 480 MPa or even at least 485 MPa in the direction L.
• b4 the tenacity KlC: in the direction LT to T / 4 and T / 2 is at least 26 MPaVm, preferably at least 27 MPaVm or even at least 28 MPaVm.
• b5 the tenacity KlC: in the direction TL at T / 4 and T / 2 is at least 25 MPaVm and preferably at least 26 MPaVm or even 27 MPaVm.
• the tenacity KlC: in the direction SL to T / 4 and T / 2 is at least 24 MPaVm and preferably at least 25 MPaVm or even at least 26 MPaVm.
• the products according to the invention have a high resistance to corrosion.
• the products according to the invention tested under the conditions MASTMAAS IS (Modified ASTM Acetic Acid Sait Intermittent Spray) according to the ASTM G85 standard reach the EA level and preferably the P level (pitting alone).
• the stress corrosion resistance according to the ASTM G47 standard of the products according to the invention reaches a 30-day hold for ST samples subjected to a stress of 300 MPa and preferably at a stress of 350 MPa.
• a structural member made of a laminate according to the present invention may typically include a spar, rib or frame for aircraft construction in a preferred manner.
• the invention is particularly advantageous for parts of complex shape obtained by integral machining, used in particular for the manufacture of aircraft wings and for any other use for which the properties of the products according to the invention are advantageous. .
• Plate A was homogenized according to the invention for 12 hours at 500 ° C. (rise rate: 15 ° C./h, time equivalent to 500 ° C.:16.7h).
• Plate B (reference) was homogenized for 8 hours at 500 0 C and then for 36 hours at 530 ° (rise rate: 15 ° C / h, time equivalent to 500 0 C: 140h).
• Plate A was hot rolled to a 60 mm thick sheet and the hot rolling exit temperature was 466 ° C. The sheet thus obtained was dissolved for 2 hours at 50 ° C. rise: 50 ° C / h, time equivalent to 500 ° C: 2.9h) and quenched with cold water.
• Plate B was hot rolled to a 65 mm thick sheet and the hot rolling exit temperature was 494 ° C.
• the sheet thus obtained was dissolved for 2 hours at 526 ° C. rise: 50 ° C / h, time equivalent to 500 0 C: 6h) and quenched with cold water. Both sheets were controlled in a controlled manner, with a permanent elongation of 3.5% and were 18 hours at 155 ° C.
• the sheets from plates A and B are referenced sheet A-60 and sheet B-60, respectively.
• the total equivalent time at 773 K for homogenization and dissolution t (eq) was therefore 19.6 h and 146 h for sheets A-60 and B-60, respectively.
• the samples were mechanically tested to determine their static mechanical properties and toughness.
• the tensile strength R m , the conventional yield stress at 0.2% elongation R p o, 2 and the elongation at break A are given in Table 2 and the toughness Kic is given in the table. 3.
• Sheet A-60 has a deflection angle ⁇ greater than 20 ° for a value 10 MPa Vm, which demonstrates a low propensity for crack bifurcation. This result was confirmed by fatigue tests on LS specimens.
• FIGS. 7b show, respectively, the four samples from sheets A-60 and B-60 after the fatigue test. The results are consistent with those obtained in tests on SL samples under mixed mode I and II stress: all samples from sheet B-60 show a severe crack bifurcation while the samples from sheet A-60 show only mode I crack propagation.
• Plate A ' was homogenized according to the invention for 12 hours at 500 ° C. (rise speed: 15 ° C./h, time equivalent to 500 ° C.:16.7h). Plate C (reference) was homogenized for 8 hours at 500 ° C and then for 36 hours at 530 ° (rise rate: 15 ° C / h, time equivalent to 500 ° C: 140h). The plate A 'was hot rolled to a sheet thickness of 30 mm and the hot rolling output temperature was 466 ° C. The sheet thus obtained was dissolved for 2 hours at 505 ° C. (speed rise: 50 ° C / h, time equivalent to 500 0 C: 3.0h) and quenched with cold water.
• Plate C was hot rolled to a sheet thickness of 30 mm and the hot rolling output temperature was 474 ° C.
• the sheet thus obtained was dissolved for 5 hours at 525 ° C. rise: 50 ° C / h, time equivalent to 500 0 C: 15.7h) and quenched with cold water.
• the two sheets were controlled in a controlled manner, with a permanent elongation of 3.5% and were tempered for 18 hours at 155 ° C.
• the sheets from the plates A 'and C are referenced sheet A'-30 and sheet metal. C-30, respectively.
• the plates D and E were homogenized for 15 hours at 492 ° C. (rise rate: 15 ° C./hr, time equivalent to 500 ° C.:11.5h).
• Plate D was hot rolled to a 25 mm thick sheet and the hot rolling output temperature was 430 ° C.
• the sheet thus obtained was dissolved for 5 hours at 510 ° C. rise: 50 ° C / h, time equivalent to 500 0 C: 8.4h) and quenched with cold water.
• Plate E was hot-rolled to a sheet thickness of 30 mm and the hot rolling exit temperature was 411 ° C.
• the sheet thus obtained was dissolved for 4.5 h at 510 ° C. ( rise rate: 50 ° C / h, time equivalent to 500 ° C: 7.6h) and quenched with cold water.
• the two sheets were fractionated in a controlled manner, with a permanent elongation of 4.3% and were tempered for 24 hours at 150 ° C.
• FIGS. 8a, 8b, 8c and 8d show, respectively, the four samples from plates A '-30, C-30, D-25 and E-30 after the fatigue test.

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