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
  • C22C21/10—Alloys based on aluminium with zinc 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
  • C22F1/053—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 zinc as the next major constituent

Definitions

• the invention relates to rolled, spun or forged products in Al-Zn-Cu-Mg alloy treated by dissolving, quenching, cold working and tempering, and in particular structural elements elaborated from such products and intended for aircraft construction.
• U.S. Patent 5,865,911 (Aluminum Company of America) discloses an Al-Zn-Cu-Mg alloy of composition Zn 5.9 - 6.7, Mg 1.6 - 1.86, Cu 1.8 - 2.4, Zr 0.08 - 0.15 for the manufacture of structural elements for aircraft. These structural elements are optimized to show high mechanical strength, toughness and fatigue resistance.
• the patent application WO 02/052053 describes three alloys of Al-Zn-Cu-Mg type. of composition (a) Zn 7.3 + Cu 1.6; (b) Zn 6.7 + Cu 1.9; (c) Zn 7.4 + Cu 1.9; each of these three alloys also containing Mg 1.5 + Zr 0.11. She describes also suitable thermomechanical treatment processes for the manufacture of structural elements for aircraft.
• Also known alloy 7040 whose standardized chemical composition is: Zn 5.7 - 6.7 Mg 1.7 - 2.4 Cu 1,5 - 2,3 Zr 0.05 - 0.12 If ⁇ 0.10 Fe ⁇ 0.13 Ti ⁇ 0.06 Mn ⁇ 0.04 other elements ⁇ 0.05 each and ⁇ 0.15 in total.
• alloy 7475 whose standardized chemical composition is: Zn 5.2 - 6.2 Mg 1.9-2.6 Cu 1.2-1.9 Cr 0.18-0.25 If ⁇ 0.10 Fe ⁇ 0.12 Ti ⁇ 0.06 Mn ⁇ 0.06 other elements ⁇ 0.05 each and ⁇ 0.15 in total.
• alloys of the 2xxx series for example alloy 2324.
• Alloys conventionally used for fuselage structure elements belong to the 2xxx series, for example alloy 2024.
• the present invention aims to obtain aircraft structural elements, and fuselage elements, of Al-Zn-Cu-Mg alloy, having, for example, compared with the prior art, an improved mechanical strength, for a tolerance to comparable damage, and sufficient formability.
• the subject of the invention is a wrought product, in particular a laminated, spun or forged product, made of an alloy of composition (% by weight): Zn 6.7 - 7.3 Cu 1.9 - 2.5 Mg 1.0 - 2.0 Zr 0.07 - 0.13 Fe ⁇ 0.15 If ⁇ 0, 1 5 other elements ⁇ 0.05 each and ⁇ 0.15 in total, the rest Al in which Mg / Cu ⁇ 1, treated by dissolving, quenching, cold working, and tempering. Cold working can be obtained by controlled pulling and / or cold processing, for example rolling or drawing.
• the invention also relates to a structural element for construction aeronautical component, in particular an aircraft fuselage element, or an underside element aircraft wing, or an integral structure element for an aircraft, manufactured from a wrought product, and in particular from such a rolled product or spun.
• the static mechanical characteristics ie the ultimate tensile strength R m , the yield strength R p0,2 and the elongation at break A, are determined by a tensile test according to the standard EN 10002-1, the location and direction of specimen collection being defined in EN 485-1.
• the fatigue strength is determined by a test according to ASTM E 466, and the fatigue crack growth rate (so-called da / dn test) according to ASTM E 647.
• the curve R is determined according to ASTM standard E 561. From the curve R, the critical stress intensity factor K c is calculated, ie the intensity factor which causes the instability of the crack. .
• the stress intensity factor K CO is also calculated by assigning to the critical load the initial length of the crack at the beginning of the monotonic loading. These two values are calculated for a specimen of the desired shape. K app means the Kco corresponding to the test piece used for the R curve test.
• the size of the crack at the end of the fatigue pre-cracking step is W / 4 for the test pieces of the type M (T), and W / 2 for CT type specimens, where W is the specimen width as defined in ASTM E 561.
• spun product includes so-called “stretched” products, i.e. products that are developed by spinning followed by stretching.
• structural element or “structural element” of a mechanical construction a mechanical part whose failure is likely to endanger the security 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 fuselage stringers, watertight bulkheads (bulkheads), circumferential frames, wings (such as wing skin, stiffeners, ribs (ribs) and spars) and the empennage including stabilizers horizontal and vertical (horizontal or vertical stabilizers), as well as floor beams, seat rails and doors.
• integral structure the structure of a part of an airplane that has been designed to ensure as much as possible the continuity of the material on a dimension as large as possible to reduce the number of points mechanical assembly.
• An integral structure may be manufactured either by machining in the mass, either by using shaped parts obtained for example by spinning, forging or molding, or by welding of structural elements made of weldable alloys. We thus obtain elements of size structure larger and in one piece, without mechanical assembly or with a fewer mechanical assembly points compared to an assembled structure in which sheets, thin or strong depending on the destination of the structural element (for example: fuselage element or wing element), are fixed, the most often by riveting, on stiffeners and / or frames (which can be made by machining from spun or rolled products).
• the present invention can be applied to an aluminum alloy containing between 6.7% and 7.3% zinc.
• the zinc content must be high enough to ensure good mechanical properties, but if it is too high, the sensitivity of the quenching alloy increases, which risks, particularly for thick products, degrade the compromise of the properties concerned.
• the product is a sheet of thickness less than 20 mm. In an other advantageous embodiment of the invention, the product is a thick plate of thickness greater than about 20 mm.
• the chemical composition of the Al-Zn-Cu-Mg alloy has been chosen so as to the Mg / Cu ratio of the alloy which is the subject of the invention is less than 1. preferred, this ratio is maintained at a value below 0.9. A lower value at 0.85 or even about 0.8 is preferred.
• the Applicant has found that a zirconium content of between 0.07 and 0.13% gives access for this composition to Al-Zn-Cu-Mg major elements, to a better compromise between R p0.2 , toughness ( at ambient or cold temperature) and fatigue resistance (in particular speed of propagation of fatigue cracks). Above 0.12%, there is a significant risk of forming Al 3 Zr type primary phases, unless the cooling is sufficiently rapid; in the case of semi-continuous casting, such a sufficient speed can be achieved, especially when casting billets.
• the Zr content must be less than 0.12%, and the best results were obtained with a content between 0.07 and 0.09%.
• a zirconium content up to 0.13% may be suitable for billets.
• the silicon and iron contents must be maintained each below 0.15%, and preferably below 0.10%, to have good tenacity.
• the content of iron does not exceed 0.07%, and the silicon content does not exceed 0.06%.
• the alloy according to the invention can be cast according to one of the known techniques of those skilled in the art to obtain a raw form, such as a spinning billet, or a rolling plate. This raw form, possibly after scalping, is then homogenized, typically for a period of 15 to 16 hours at a temperature between 470 and 485 ° C.
• the raw form is then hot processed to form spun products (including bars, tubes or profiles), hot-rolled sheet or parts forged.
• the hot rolling of thick products according to the invention could occur at a temperature of about 350 ° C, much lower than traditionally practiced for this type of product (which is about 415 to 440 ° C), without affecting the compromise of properties required for thick products used in aircraft structures.
• the hot transformation can possibly be followed by a transformation Cold.
• a transformation Cold For example, spun and drawn tubes can be made.
• spun and drawn tubes can be made.
• This solution can be in any suitable oven, such as air oven (horizontal or vertical), or oven bath of salts.
• this dissolution in solution is carried out at a temperature between 470 and 480 ° C, and preferably between 475 and 480 ° C for a period duration of at least 4 hours.
• the dissolution temperature is between 470 ° C and 475 ° C, and the dissolution time, the optimum value of which depends on the thickness of the product is typically at least one hour.
• the products are quenched, preferably in a liquid medium such as water, said liquid preferably having a temperature not exceeding 40 ° C.
• the products are then generally subjected to controlled traction with a permanent elongation of the order of 1 to 5%, and preferably 1.5 to 3%.
• the products are subject to a treatment of income, which influences in a important on the final properties of the product.
• income which influences in a important on the final properties of the product.
• the product according to the invention leads to new products which have particularly interesting characteristics for aeronautical construction.
• These products may be in the form of sheets, including fuselage sheets, thick sheets for intrados or for integral structures, or in the form of profiles, or in the form of forged parts.
• the rolled products according to the invention whether they are thick or thin, have a yield strength R p0.2 (L) of at least 460 MPa, preferably at least 480 MPa, and even more preferentially of at least 500 MPa.
• sheets having a thickness greater than about 20 mm, having a yield strength R p0.2 (L) of at least 520 MPa, a K app are obtained.
• K app LT
• the value of K app (LT) is stable, or even higher, cold compared to its temperature value. room. Specifically, this value is slightly increased from room temperature when measured at -54 ° C. This is particularly interesting since -54 ° C is about the typical temperature reached by the structural elements during the flight of a civilian jet aircraft.
• the products according to the invention advantageously replace the alloy structural elements known alloys 2x24, for example alloy 2024 or 2324.
• rolled products according to the invention may have a thickness of less than 10 mm and thus be used as a fuselage liner. They can also have a thickness greater than about 10 mm and thus be used as intrados. of the rolled products with a thickness greater than about 40 mm may be used for manufacture of structural elements by integral machining, as described below. Rolled products with a thickness greater than about 60 mm can be used for the manufacture of stiffeners or frames, especially for large aircraft capacity.
• the products according to the invention can be plated on at least one face according to methods and with the alloys usually used to plate the products in Al-Zn-Cu-Mg type alloys. This is particularly interesting for sheet metal used for the manufacture of aircraft fuselage elements, which must withstand the corrosion.
• a useable plating alloy is 7072.
• a particularly advantageous use of the products according to the invention is related to the concept of the integral structure in aeronautical construction.
• a big part of the aircraft structures are dimensioned according to a compromise between damage tolerance and resistance to static loads.
• the requirements of tolerance for damage are specified for example in the article "Damage Tolerance Certification of Commercial Aircraft by T. Swift, ASM Handbook vol. 19 (1996), pp 566 - 576.
• Sizing under static loadings is explained by example in the book “Airframe Stress Analysis and Sizing” by Mr. Niu, Hong Kong Conmilit Press Ltd, 1999, in particular pages 607 to 654. From the point of view of the material, it is known that, generally, the damage tolerance of the alloys of the series 7xxx, and in particular their toughness, decreases when their yield strength increases.
• a toughness gain of x% to constant yield strength as with the alloy according to the invention can be translated weight gain of the same level, or even higher if allowing a load stronger on the part considered also allows the lightening of other components.
• a yield strength gain of x%, with a constant damage tolerance can result in a weight gain of the order of x / 3% to x%.
• x is typically between 15 and 30%.
• the continuity between the stiffeners and the skin means that the damage tolerance becomes more critical than in a component assembled by riveting. Indeed, given stress, the stress intensity factor increases sharply when the stiffener is crossed by a crack, since it must be admitted that this stiffener will necessarily be cracked.
• the present inventors have found that products with high tenacity, for a given elastic limit, are particularly well suited to the manufacture of integral structures.
• fuselage barge panels or wing covers are manufactured by integral machining of products according to one of the preceding embodiments, given that said products, and in particular the plates for machining advantageously have a thickness of at least 40 mm; this value also depends on the type of aircraft, and especially its size.
• the product according to the invention with a yield strength R p0.2 (L) at half thickness of at least 540 MPa and a K app toughness (LT) measured on a specimen of type M (T).
• the Applicant has found that refining the grain at a reduced level compared to the usual practice during casting provides a compromise of properties, including a level of tenacity, particularly interesting.
• a TiC refining agent for example adding A13% Ti0.15% C thread
• the solidification germ obtained with this approach having a different germination-growth compromise germs obtained by refining for example with A15% Ti1% B (ie a seed type TiB 2 ).
• the level of this refining can be quantified by the amount of C added, since it corresponds indirectly to the amount of added solidification nuclei, as well as by the amount of free Ti (not combined with C) in the alloy.
• the stoichiometry of the seed is not definitively quantified, it can be considered that the seed is composed of TiC, each C atom combining with a Ti atom to form said seeds.
• affinants Al - x% Ti - y% C there are different types of affinants Al - x% Ti - y% C, in general the Ti being added in excess of C.
• the amount of added seed is proportional to the amount of refining (in kilograms) added per tonne of molten metal multiplied by y%, that is to say proportional to A (number of kilograms of refining added per tonne of metal) x y%.
• the addition of seeds can be quantified by specifying 3 g / t of added C (2 x 0.0015 kg / t).
• an affine containing titanium and carbon so that the amount of carbon added is between 0.4 and 3 g / t of carbon, preferably between 0.6 and 2 g / t, and so that the total Ti content in the final product is between 50 and 500 ppm (by weight), preferably between 150 and 300 ppm.
• N alloy was developed whose chemical composition was in accordance with the invention.
• the liquid metal was first treated in a holding furnace by injection of gas using an Irma ® type rotor, and then into a pocket type Alpur ®, both brands owned by the plaintiff.
• the refining was done in line, that is to say in the channel between the holding oven and the Alpur ® pocket, 1.1 kg / ton of Al-3% Ti-0.15% C wire (9.5 mm diameter). We sank a industrial size plate. It was relaxed for 10 hours at 350 ° C.
• the product thus cast was homogenized after scalping for 15 hours at a temperature between 471 ° C and 482 ° C (880 ° F to 900 ° F), then rolled to hot to a thickness of 5 mm (0.2 inches).
• the start temperature of rolling was 450 ° C (840 ° F), and the end-of-rolling temperature was 349 ° C (660 ° F).
• Sheets of width 178 mm (7 inches) and length 508 mm (20 inches) were taken. These coupons were dissolved in a salt bath oven for 1 hour at 472 ° C, then quenched with water, and triturated until a permanent deformation of 2%.
• the coupons thus obtained subsequently suffered a two-stage artificial aging treatment, the first step being 6 hours at 105 ° C, the second stage being 18 hours at 155 ° C, in order to reach the peak of mechanical properties.
• the alloy was cast by treating the liquid metal first in a holding furnace by injection of gas using an Irma ® type rotor, and then into a pocket type Alpur ®.
• the refining was done online, that is to say in the channel between the oven of maintenance and the Alpur ® bag, at a rate of 0.7 kg / tonne of AT5B wire (diameter 9.5 mm).
• the plates were relaxed for 10h at 350 ° C. These plates have undergone a homogenization for 12 hours at 500 ° C., followed by hot rolling (temperature end of rolling between 230 and 255 ° C) up to a thickness of 6 mm.
• 7xxx alloy plates according to the prior art have also been cast (reference G), in the same foundry device as the 2xxx alloy sheets described previously.
• a plate was obtained which was then homogenized for 24 hours. hours at 470 ° C then 24 hours at 495 ° C, then hot rolled (end temperature of rolling between 230 and 255 ° C) up to a thickness of 6 mm.
• a solution treatment of 1 hour at 450 ° C in a salt bath oven was then made on a coupon of 600 mm by 200 mm. This operation was followed quenching with water and pulling to obtain a permanent deformation of 2%.
• the coupon then underwent an artificial aging treatment of 5 hours at 100 ° C then 6 hours at 155 ° C, in order to reach the peak of mechanical properties (state T6).
• An AA7475 alloy plate was also cast (reference H) following conventional methods of the prior art.
• the plate thus obtained was homogenized for 9 hours at 480 ° C, then co-hot rolled at a temperature of about 270 ° C, with a sheet 7072, to obtain a plated sheet of thickness 4.5 mm.
• the 7072 plating was about 2% of the final thickness.
• the product thus obtained was dissolved in a salt bath oven for 45 minutes at 478 ° C, then quenched with water at a temperature of about 20 ° C, and then underwent a traction to obtain a permanent deformation of 2%. He then suffered a four-hour income operation at 120 ° C, then 24 hours at 162 ° C (state T76).
• the tensile strength R m (in MPa), the conventional yield stress at 0.2% elongation R p0.2 (in MPa) and the elongation at break A (in%) were measured by a tensile test according to EN 10002-1.
• the sheet according to the invention has in both directions measured tensile strength and yield strength much higher than those 2xxx alloy sheets.
• the elongation of the sheet according to the invention is lower that of the sheet E, but sufficient compared to the intended applications.
• the alloy according to the invention is present in both directions measured a breaking strength and a yield strength significantly improved, for comparable lengthening.
• the sheet according to the invention has a Kapp much higher than the alloy sheets 7xxx of the prior art, and of the same order of magnitude as 2xxx alloy sheets.
• Fatigue behavior was also tested according to ASTM E 647, by measuring the crack propagation velocity in sheet N compared with E, F and G.
• the specimens used were of type C (T), with W of 76.2 mm (3 inches).
• the comparative results are shown in Table 4.
• the sheet Y had a thickness of 6 mm.
• Fatigue results sheet metal da / dN (10) TL (10 -4 mm / cycles) da / dN (30) TL (10 -4 mm / cycles) ⁇ K at 100 ⁇ inch / cycle [MPa ⁇ m] N (invention) 1.4 29 27.5 Y (invention) 1.3 33 27 E (2024A) 1.4 30 27 BOY WUT 1.1 38 25.9
• the sheet according to the invention behaves at least as well in fatigue as the sheets according to the prior art.
• a 3.2 mm thick Y sheet had da / dN (10) TL of 1.7 10 -4 mm / cycles, da / dN (30) TL of 10 -4 mm / cycles, and ⁇ K at 100 ⁇ inch. / cycle of 28.3 MPa ⁇ m.
• An alloy M was developed whose chemical composition was in accordance with the invention.
• an alloy plate 2324 according to the prior art (reference I) was developed according to a conventional casting process.
• the chemical compositions of alloys M and I measured on a spectrometric counter taken from the casting channel, are collated in the table: Chemical composition Alloy Yes Fe Cu mn mg Zn Zr M 0.05 0.06 2.05 - 1.64 7.08 0.08 I (AA2324) ⁇ 0.10 ⁇ 0.12 3.8-4.4 0.3-0.9 1.2-1.8 ⁇ 0.20 ⁇ 0.05
• the alloy plates M were homogenized for 15 hours at 479 ° C, then slowly cooled to 420 - 440 ° C and rolled to a thickness of 25.4 mm.
• the output temperature of the hot rolling mill was 354 ° C, which is significantly lower than that which is usually practiced for this type of product.
• the sheets thus obtained were then subjected to solution treatment. at 479 ° C for 4 hours (total time about 1/3 of which was spent climbing temperature), then quenched, and tractionned so that the permanent deformation resulting from it being 2%.
• the sheets then underwent an artificial income treatment for 8 hours at 160 ° C.
• the alloy 1 (AA2324) was subjected to a conventional range until an alloy sheet AA 2324, thickness 25.4 mm in the T39 state, that is to say a homogenization step, was obtained. hot rolling, followed by dissolution and quenching, followed by cold working by about 9%, and controlled pulling to obtain a permanent deformation of between 1.5 and 3 %.
• breaking strength and yield strength of the sheet according to the invention are substantially higher than those of the sheet I used generally for these applications, and this for quite elongations comparable.
• the alloy according to the invention has under all conditions a toughness better than the conventional alloy 1.
• the alloy according to the invention has a K app (LT) at -54 ° C which is of the same order as at room temperature.
• Fatigue behavior was also tested according to ASTM E 647, by measuring the speed of propagation of cracks in sheet metal M by compared to sheet I.
• the specimens used were of type C (T), with B equal to 0.375 inches, and W equal to 101.6 mm (4 inches).
• exfoliating corrosion behavior of the plates of this test was evaluated according to ASTM G34; this test was done on the surface, and at mid-thickness, in the conditions adapted to the alloys 7xxx for the sheet metal M according to the invention, and under conditions suitable for 2xxx alloys for sheet I.
• the sample M according to the invention has been classified EA, both at the surface and at mid-thickness, while the sample I according to the prior art has been classified EA surface, and EB mid-thickness.
• the sheet according to the invention is therefore at least as powerful, if not more performing, in resistance to exfoliating corrosion, than the sheet according to the prior art.
• the sheet M is better for each of the following physical parameters: static mechanical characteristics, K app , fatigue resistance, crack propagation speed.
• Example 2 A P alloy similar to the M alloy of Example 2 was developed. from this alloy, following a manufacturing range similar to that of Example 2, fully hot rolled plates (inlet temperature: 420 - 440 ° C) with a thickness of 75 mm.

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