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
  • C22F1/057—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
• • 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

Definitions

• This invention relates to a method of manufacturing highly worked pieces for mechanical engineering, and in particular aircraft construction, implementing type 2024 AlCuMg aluminium alloy sheets according to the Aluminum Association's registration.
• 2024 alloy is widely used for aircraft construction and its composition registered with the Aluminum Association is as follows (weight per cent): Si ⁇ 0.5 Fe ⁇ 0.5 Cu: 3.8-4.9 Mn: 0.3-0.9 Mg: 1.2-1.8 Zn ⁇ 0.25 Cr ⁇ 0.10 Ti ⁇ 0.15.
• Patent EP 0 473 122 describes a method of manufacturing alloy sheets composed of (weight per cent): Cu: 4-4.5 Mg: 1.2-1.5 Mn: 0.4-0.6 Fe ⁇ 0.12 Si ⁇ 0.05, including intermediate annealing at a temperature >488° C. It teaches that these sheets have improved toughness and resistance to crack propagation in comparison with conventional 2024.
• Patent application EP 0 731 185 describes sheets of modified 2024 alloy, subsequently registered with the Aluminum Association as 2024A, showing a reduced level of residual stress and improved toughness for thick sheets, and improved elongation for thin sheets.
• This application limits Mn content to 0.55% and Fe content to 0.25%, with the relation: 0 ⁇ Mn-2 Fe ⁇ 0.2 (Mn and Fe content being expressed in %).
• Patent application WO 96/29440 describes a method of manufacturing a product of type 2024 aluminium alloy, comprising hot rolling, annealing, cold rolling, solution treatment, quenching and minimum cold working, which may be stretching, planishing, or flattening, a process for improving forming ability.
• the application recommends a preferred alloy composition: Cu: 4.0-4.4, Mg: 1.25-1.5, Mn: 0.35-0.5, Si ⁇ 0.12, Fe ⁇ 0.08, Ti ⁇ 0.06.
• the intermediate annealing between hot rolling and cold rolling is described as favorable for mechanical strength and toughness.
• this additional and unusual process step has economic drawbacks.
• it does not solve the problem posed by the market, i.e. to supply sheets with characteristics such that the forming thereof be simplified.
• the sheets are in a temper characterized by good forming ability, but this temper is unstable (“W” temper), and forming must take place in as quenched condition, i.e. within a short time after quenching, roughly from about ten minutes up to a few hours.
• W temper unstable
• the sheet metal must be stored in a cold chamber at a sufficiently low temperature and for a sufficiently short duration in order to avoid natural aging.
• this solution heat treatment requires large furnaces, making the operation awkward, including with respect to the same operation performed on flat sheet metal.
• the possible need for a cold chamber increases the costs and drawbacks of the state of the art. For highly worked pieces, this operation may have to be repeated, if the material, in its present metallurgical temper, does not have sufficient forming ability allowing the desired shape to be obtained in a single operation.
• the only possible forming is roll forming.
• the roll formed sheet metal is then solution treated and quenched, and a second forming is carried out either in as quenched condition, or after storage in a cold chamber. Under all other circumstances, the sheet metal is directly solution treated and quenched before forming.
• the starting point is O temper sheet metal
• a first forming operation is carried out from this temper, and a second forming after solution treatment and quenching.
• This alternative is used when the target forming is too significant to be performed in a single operation from W temper, but may still be carried out in two passes starting from O temper.
• the sheet metal is admittedly less workable, but the O temper is easier to use than the W temper, which is unstable, and requires additional heat treatment.
• manufacturing sheet metal in O temper calls upon final annealing of the sheet metal as rolled, and therefore an additional manufacturing step, which is contrary to the objective of simplification this invention is aiming at.
• the object of the invention is therefore to simplify the method of manufacturing formed pieces, and in particular pieces highly worked in one or more processes, such as stretch forming, drawing, flow spinning, or bending, by associating an optimized chemical composition and specific manufacturing methods, allowing to avoid as much as possible solution treating formed sheet metal.
• the object of the invention is a method of manufacturing highly worked pieces of 2024 type AlCuMg alloy, comprising the following steps of:
• the alloy has a copper content between 3.9 and 4.3% (and even more preferably between 3.9 and 4.2%), a magnesium content between 1.2 and 1.4% (and even more preferably between 1.25 and 1.35%), a manganese content between 0.3 and 0.45%, an iron content of ⁇ 0.10%, a silicium content of ⁇ 0.10% (and preferably ⁇ 0.08%), a titanium, chromium and zirconium content of ⁇ 0.07% (preferably ⁇ 0.05%).
• the inventive method allows for possibly using cladded plates, e.g. sheets coated with a cladding of an alloy having better corrosion resistance, as is the case usually for aircraft fuselage coating sheets.
• a first characteristic of the invention consists in using an alloy modified with respect to traditional 2024.
• the first modification consists in reducing the Si and Fe content to less than 0.25 and 0.20% respectively, and preferably to less than 0.10%.
• Mn content is also reduced to less than 0.5%, and preferably to less than 0.45%.
• Cu content is also slightly reduced and maintained at less than 4.5%, and preferably at less than 4.3%, or even 4.2%.
• Mg content is also slightly reduced, and maintained at less than 1.5%, preferably between 1.2 and 1.4%, or even between 1.25 and 1.35%.
• the alloy is cast into plates, which may be homogenized at a temperature between 460 and 510° C. (preferably between 470 and 500° C.) for 2 to 12 hrs (preferably 3 to 6 hrs). Plates may be scalped. Hot rolling is done at an input temperature between 430 and 470° C., and preferably between 440 and 460° C.
• the output temperature of the coils is preferably at a higher temperature than the usual temperature, >300° C., and preferably >310° C., especially in case part of the forming is done before solution treatment.
• the coils are coiled. At this stage, they are elongated by more than 13.5%, and often more than 15% in the L and TL directions. They may be cold rolled if the required thickness cannot be achieved by hot rolling. Next, the coils are cut into sheets.
• a first alternative of the invention consists in carrying out forming, through stretch forming, drawing, flow spinning, or bending, directly in this F temper without annealing or any other prior treatment.
• the partially shaped sheet is then solution treated at a temperature between 480 and 500° C. for a duration between 5 min and 1 hr, then quenched, generally with cold water.
• Forming takes place in two or more passes.
• the piece in as quenched condition (less than one hour) can immediately undergo another forming, or else it is transferred into a cold chamber at a temperature of less than 10° C., and preferably of less than 0° C., and formed after leaving the cold chamber.
• Sheets cladded on one or two sides can be used, as is the case most frequently for aircraft fuselage panels, cladded with an alloy of the 1000 series, e.g. the alloys 1050, 1100, 1200, 1135, 1145, 1170, 1175, 1180, 1185, 1188, 1199, 1230, 1235, 1250, 1285, 1350, or 1435.
• a second alternative consists in carrying out the forming on sheets having undergone solution treatment and quenching.
• Forming can be done in T3 or T4 temper (quenched and aged with or without subsequent strain hardening), or, for more deeply worked pieces, in W temper, i.e. less than one hour after quenching, or on a sheet stored in a cold chamber immediately after quenching.
• These sheets in T3 or T4 temper have a forming ability characterized by at least one of the following three properties:
• the LDH value is greater than 40 mm for a thickness of less than 4 mm, or greater than 74 mm for a thickness greater than 4 mm,
• Pieces made from sheets both in T3 or T4 temper and in W temper show only very little deterioration of damage tolerance after the last forming operation, if the amplitude thereof is less than 6%.
• R p0.2 yield strength at 0.2% permanent elongation (MPa);
• R m ultimate tensile strength (MPa)
• A elongation after failure (%), sometimes represented by the symbol “A%”;
• a g non proportional elongation under maximum load, also called distributed elongation (%).
• Distributed elongation is the difference of elongation between the beginning and the end of the plastic flow range, i.e. the permanent set range before contraction, of the strain curve.
• the LDH (limit dome height) parameter is widely used for evaluating the drawing ability of sheets with thickness from 0.5 to 2 mm. It is the subject of many publications, in particular:
• the LDH test is a drawing test wherein the blank is clamped peripherally by a retaining ring.
• the pressure of the blankholder providing this clamping is 240 MPa.
• This blank the size of which is 500 ⁇ 500 mm, is stressed by equiaxial bi-expansion. Lubrication between the punch and the sheet is provided by a plastic film and grease.
• the LDH value is the ultimate punch displacement, i.e. the drawing near depth. Three tests are averaged.
• Resilience R c is determined by a tensile bending test allowing to compare the resilience of various subtle differences (same sheet thickness) for a given strain.
• the predefined tensile stress is kept constant throughout bending, by means of the hydraulic servovalve regulation of the tension jack.
• the regulation loop incorporates the tensile stress by measuring via a piezoelectric transducer (Kistler washer). Tensile stress depends on the alloy and the thickness of the test piece.
• a displacement transducer connected to the acquisition computer, enables continuous test parameter control and calculates the test piece's bending angle.
• a forming punch integral with the upper frame of the tension machine, is used as a support for the test piece.
• Each folded sample is checked after disassembly using a sensor contour follower. This measuring apparatus allows to evaluate the final angle as well as the radius of curvature obtained.
• the stretching applied to the test piece, corresponding to the desired plastic flow, is determined using the rational tension curve by graphically noting the stress equivalent to the strain rate aimed at.
• the initial rate of strain, defining the bending stress, was kept constant during the test at 0.2%.
• R e ⁇ f - ⁇ 0 180 - ⁇ 0 ⁇ ⁇
• ⁇ f angle ⁇ ⁇ measured ⁇ ⁇ by ⁇ ⁇ the ⁇ ⁇ contour ⁇ ⁇ follower ⁇ ⁇ ( ° )
• ⁇ 0 angle ⁇ ⁇ measured ⁇ ⁇ during ⁇ ⁇ bending ⁇ ⁇ by ⁇ ⁇ the ⁇ ⁇ PC ⁇ ⁇ ( ° )
• R e ⁇ springback ⁇ ⁇ ( 0 ⁇ ⁇ for ⁇ ⁇ no ⁇ ⁇ springback ⁇ ⁇ and ⁇ ⁇ 1 ⁇ ⁇ for ⁇ ⁇ full ⁇ ⁇ springback ) .
• R e 1 1 - R 0 R f ⁇ ⁇
• R 0 punch ⁇ ⁇ radius
• R f radius ⁇ ⁇ measured ⁇ ⁇ by ⁇ ⁇ the ⁇ ⁇ contour ⁇ ⁇ follower
• R e springback ⁇ ⁇ ( 0 ⁇ ⁇ for ⁇ ⁇ no ⁇ ⁇ and ⁇ ⁇ 1 ⁇ ⁇ for ⁇ ⁇ full ⁇ ⁇ springback ) .
• the formats are analyzed using the automatic CamSys system near the cracking area.
• the Asame-CamSys software makes it possible to create a mapping of the strains of the areas measured as described by J. H. Vogel and D. Lee “The automated measurement of strains from three dimensional deformed surfaces”, J. O. M., vol. 42, 1990, pp. 8-13. Limit strains before local contraction are thus estimated and transferred onto a forming diagram with the coordinates ⁇ 1 and ⁇ 2 .
• Examples 1a, 1b, 1k, 1L, 1m, 1n, 1p, and 1q correspond to this invention.
• Examples 1c, 1d, 1e, 1f, 1g, 1h, 1i, and 1j correspond to prior art.
• the inventive method provides a better ability to forming in F temper, expressed as A% of LDH or forming limit diagram, than the prior art method. More particularly, a cold rolled coil according to the invention has an LDH value greater than 42 mm and preferably greater than 44 mm, whereas a hot rolled coil has an LDH value greater than 73 and preferably greater than 75 mm. It also appears that for a given thickness, the preferred composition yields better forming ability than the traditional composition.
• the LDH value and the forming limit diagram level are lower for a strain hardened sheet than for a sheet that has only undergone hot rolling; this effect is well known.
• the applicant was surprised to notice that for a given process (hot rolling or hot rolling with subsequent cold rolling) and at comparable thickness, the LDH value, which is one of the parameters relevant for measuring forming ability, increases significantly when the chemical composition is within a preferred range: Cu 3.9-4.3 and preferably 3.9-4.2, Mg 1.2-1.4 and preferably 1.25-1.35, Mn 0.30-0.45, Si ⁇ 0.10 and preferably ⁇ 0.08, Fe ⁇ 0.10.
• forming ability is further improved when certain alloying and impurity elements are strictly controlled, as follows: Zn ⁇ 0.20%, Cr ⁇ 0.07% and preferably ⁇ 0.05%, Zr ⁇ 0.07% and preferably ⁇ 0.05%, Ti 0.07% and preferably ⁇ 0.05%.
• strain started exactly 30 minutes after the end of quenching.
• Examples 2a, 2b, 2e, 2j, 2k, 2n correspond to this invention.
• Examples 2h, 2L, 2m, 2p correspond to prior art.
• Examples 3s, 3t, 3u, 3v, 3w, 3x correspond to this invention.
• Examples 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3L, 3m, 3n, 3p, 3q, 3r correspond to prior art.
• Examples 3a, 3b, 3c, 3d correspond to examples 2h, 2f, 2L, and 2m of example 2; they appear here by way of comparison in order to represent a prior art W temper 2024.
• the method results in an improvement of forming ability as characterized by the parameters that have just been listed. It is possible to carry out forming much stricter than in prior art T3 temper, or even to eliminate solution treatment because the inventive method results in a T3 temper product with forming ability properties at least as good as that of the prior art method W temper product.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Metal Rolling (AREA)

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Cited By (18)

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• 1999
  • 1999-04-12 FR FR9904685A patent/FR2792001B1/fr not_active Expired - Lifetime
• 2000
  • 2000-04-06 GB GB0008506A patent/GB2352453A/en not_active Withdrawn
  • 2000-04-07 BR BR0001563-6A patent/BR0001563A/pt not_active IP Right Cessation
  • 2000-04-10 DE DE0001045043T patent/DE00420071T1/de active Pending
  • 2000-04-10 DE DE60020188T patent/DE60020188T2/de not_active Expired - Lifetime
  • 2000-04-10 EP EP00420071A patent/EP1045043B1/de not_active Expired - Lifetime
  • 2000-04-12 JP JP2000110617A patent/JP2000328211A/ja active Pending
• 2003
  • 2003-03-07 US US10/382,519 patent/US20030140990A1/en not_active Abandoned

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