EP4386097A1 - 7xxx wrought products with improved compromise of tensile and toughness properties and method for producing - Google Patents

7xxx wrought products with improved compromise of tensile and toughness properties and method for producing Download PDF

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Publication number
EP4386097A1
EP4386097A1 EP22213912.3A EP22213912A EP4386097A1 EP 4386097 A1 EP4386097 A1 EP 4386097A1 EP 22213912 A EP22213912 A EP 22213912A EP 4386097 A1 EP4386097 A1 EP 4386097A1
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EP
European Patent Office
Prior art keywords
product
mpa
hours
wrought
rolled
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EP22213912.3A
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German (de)
French (fr)
Inventor
Ricky WHELCHEL
Michael M NIEDZINSKI
Kenneth Paul Smith
Timothy Warner
Jean-Christophe Ehrstrom
Armelle Danielou
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Constellium Rolled Products Ravenswood LLC
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Constellium Rolled Products Ravenswood LLC
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Priority to PCT/EP2023/085062 priority Critical patent/WO2024126341A1/en
Publication of EP4386097A1 publication Critical patent/EP4386097A1/en
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    • 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
    • C22C21/18—Alloys based on aluminium with copper as the next major constituent with zinc
    • 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 present disclosure relates to a method for manufacturing 7xxx aluminum wrought product with improved compromise of tensile and toughness properties and excellent cryogenic properties and fatigue resistance in corrosive environment and more particularly to such manufacturing processes and rolled products and use, notably designed for aeronautical and aerospace engineering.
  • High strength 7xxx aluminum alloy products also designated as Al-Zn-Mg-Cu type alloy products, are extensively used in aerospace structure application, in which the material strength, fracture toughness, fatigue resistance, and corrosion resistance are required simultaneously. It is known that these various required properties cannot all be optimized at the same time and independently of each other during the fabrication of semi-finished products and structural elements for aeronautical construction.
  • chemical composition of the alloy or the parameters of product manufacturing processes are modified, several critical properties may tend to change in opposing directions. This is sometimes the case firstly for properties generally referred to as "static mechanical strength” (particularly the ultimate tensile stress UTS and the tensile yield stress TYS), and secondly for properties generally names as "damage tolerance" (particularly the toughness and the resistance to crack propagation).
  • Al-Zn-Mg-Cu alloys with high fracture toughness and high mechanical strength are described in the prior art.
  • US Patent 5,312,498 discloses a method of producing an aluminum-based alloy product having improved exfoliation resistance and fracture toughness which comprises providing an aluminum-based alloy composition consisting essentially of about 5.5-10.0% by weight of zinc, about 1.75-2.6% by weight of magnesium, about 1.8-2.75%) by weight of copper with the balance aluminum and other elements.
  • the aluminum-based alloy is worked, heat treated, quenched and aged to produce a product having improved corrosion resistance and mechanical properties.
  • US Patent 5,560,789 describes AA 7000 series alloys having high mechanical strength and a process for obtaining them.
  • the alloys contain, by weight, 7 to 13.5% Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others up to 0.05% each and 0.15% total, and remainder Al.
  • US Patent No 5,865,911 describes an aluminum alloy consisting essentially of (in weight %) about 5.9 to 6.7% zinc, 1.8 to 2.4% copper, 1.6 to 1.86% magnesium, 0.08 to 0.15% zirconium balance aluminum and incidental elements and impurities.
  • the '911 patent particularly mentions the compromise between static mechanical strength and toughness.
  • US Patent No 6,027,582 describes a rolled, forged or extruded Al-Zn-Mg-Cu aluminum base alloy products greater than 60 mm thick with a composition of (in weight %), Zn: 5.7-8.7, Mg: 1.7-2.5, Cu: 1.2-2.2, Fe: 0.07-0.14, Zr: 0.05-0.15 with Cu + Mg ⁇ 4.1 and Mg>Cu.
  • US Patent No 6,972,110 teaches an alloy, which contains preferably (in weight %) Zn: 7-9.5, Mg: 1.3-1.68 and Cu 1.3-1.9 and encourages keeping Mg +Cu ⁇ 3.5.
  • PCT Patent Application No WO2004090183 discloses an alloy comprising essentially (in weight percent): Zn: 6.0 - 9.5, Cu: 1.3 - 2.4, Mg: 1.5 - 2.6, Mn and Zr ⁇ 0.25 but preferably in a range between 0.05 and 0.15 for higher Zn contents, other elements each less than 0.05 and less than 0.25 in total, balance aluminum, wherein (in weight percent): 0.1 [Cu] + 1.3 ⁇ [Mg] ⁇ 0.2[Cu] + 2.15, preferably 0.2[Cu] + 1.3 ⁇ [Mg] ⁇ 0.1[Cu] + 2.15.
  • US 20050006010 discloses a method for producing a high strength AI-Zn- Cu-Mg alloy with an improved fatigue crack growth resistance and a high damage tolerance, comprising the steps of casting an ingot with the following composition (in weight percent) Zn 5.5-9.5, Cu 1.5-3.5, Mg 1.5-3.5, Mn ⁇ 0.25, Zr ⁇ 0.25, Cr ⁇ 0.10, Fe ⁇ 0.25, Si ⁇ 0.25, Ti ⁇ 0.10, Hf and/or V ⁇ 0.25, other elements each less than 0.05 and less than 0.15 in total, balance aluminum, homogenizing and/or pre-heating the ingot after casting, hot working the ingot and optionally cold working into a worked product of more than 50 mm thickness, solution heat treating, quenching the heat treated product, and artificially aging the worked and heat-treated product, wherein the aging step comprises a first heat treatment at a temperature in a range of 105 ° C. to 135 ° C. for more than 2 hours and less than 8 hours and a second heat treatment at a higher temperature than 135
  • EP 1544315 discloses a product, especially rolled, extruded or forged, made of an AlZnCuMg alloy with constituents having the following percentage weights: Zn 6.7 - 7.3; Cu 1.9 - 2.5; Mg 1.0 - 2.0; Zr 0.07 - 0.13; Fe less than 0.15; Si less than 0.15; other elements not more than 0.05 to at most 0.15 per cent in total; and aluminum the remainder, wherein Mg/Cu ⁇ 1.
  • the product is preferably treated by solution heat treatment, quenching, cold working and artificial aging.
  • US Patent No 8,277,580 teaches a rolled or forged Al-Zn-Cu-Mg aluminum-based alloy wrought product having a thickness from 2 to 10 inches.
  • the product has been treated by solution heat-treatment, quenching and aging, and the product comprises (in weight- %): Zn 6.2-7.2, Mg 1.5-2.4, Cu 1.7-2.1.
  • Fe 0-0.13, Si 0-0.10, Ti 0-0.06, Zr 0.06-0.13, Cr 0-0.04, Mn 0-0.04, impurities and other incidental elements ⁇ 0.05 each.
  • US Patent No 8,673,209 discloses aluminum alloy products about 4 inches thick or less that possesses the ability to achieve, when solution heat treated, quenched, and artificially aged, and in parts made from the products, an improved combination of strength, fracture toughness and corrosion resistance, the alloy consisting essentially of: about 6.8 to about 8.5 wt. % Zn, about 1.5 to about 2.00 wt. % Mg, about 1.75 to about 2.3 wt. % Cu; about 0.05 to about 0.3 wt. % Zr, less than about 0.1 wt. % Mn, less than about 0.05 wt. % Cr, the balance Al, incidental elements and impurities and a method for making same.
  • WO2019/007817 concerns an extruded, rolled and/or forged aluminum-based alloy product having a thickness of at least 25 mm comprising (in weight %): Zn 6.70 -7.40; Mg 1.50 -1.80; Cu 2.20 -2.60, with a Cu to Mg ratio of at least 1.30; Zr 0.04 -0.14; Mn 0 -0.5; Ti 0 -0.15; V 0 -0.15; Cr 0 -0.25; Fe 0 -0.15; Si 0 -0.15; impurities ⁇ 0.05 each and ⁇ 0.15 total.
  • the present invention relates to a method for manufacturing a high strength 7xxx aluminum wrought products by controlling the composition, in particular the quantity of Fe and Si and the fabrication parameters, in particular artificial aging conditions, to provide improved compromise between tensile yield strength and toughness with improved cryogenic fracture toughness, and fatigue resistance in corrosive environment.
  • the invention also concerns a rolled 7xxx product which can be obtained by the method of the invention.
  • cryogenic temperature is defined in accordance with the present invention to include temperatures significantly below room temperature and typically below - 100°C (173 K). Thus, the temperatures at which hydrogen (-253°C/20 K), oxygen (-183°C/90 K) and nitrogen (-196°C/77 K) become liquid at atmospheric pressure are included as cryogenic temperatures. For purposes of experimental evaluation, a temperature of -196°C/77K is considered as a cryogenic temperature.
  • Room temperature is defined in accordance with its common usage and includes temperatures from about 20°C to about 25°C. For purposes of experimental evaluation, a temperature of 22°C is considered to be room temperature.
  • the present invention is directed to a process for the manufacture of a wrought 7xxx aluminum-based alloy product comprising the steps of:
  • the process is advantageously made with a bath of molten alloy comprising Si ⁇ 0.03 wt. % and Fe ⁇ 0.05 wt. %.
  • the sum Fe+Si content is ⁇ 0.07 wt. %, even more preferably ⁇ 0.06 wt. %.
  • the sum Fe+Si content is ⁇ 0.03 wt. %, even more preferably ⁇ 0.04 wt. %.
  • the present invention is directed to a rolled product with a thickness t in millimeter of at least 25 mm, preferentially from 25 mm to 200 mm, comprising (in weight-%)
  • the rolled product according to the invention displays a surprising small drop in both fracture toughness and elongation from room temperature down to liquid nitrogen temperature.
  • the rolled product according to the invention displays a toughness K1c(T-L) in MPa. ⁇ m, at cryogenic temperature, i.e at temperature below 100°C, typically at -196°C, measured according to ASTM standard E399 2020 which is reduced by less than 10 %, preferably by less than 8%, more preferably by less than 7 % in comparison with K1c(T-L) in MPa. ⁇ m, measured at room temperature according to ASTM standard E399 -2020.
  • the rolled product according to the invention displays a toughness K1c(T-L) in MPa. ⁇ m, at cryogenic temperature, i.e at temperature below 100°C, typically at - 196°C, measured according to ASTM standard E399 -2020 higher than -15 ⁇ t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than - 0.15*t+55 MPa, where t is the thickness of the rolled product in mm.
  • the rolled product comprises Si ⁇ 0.03 wt. % and Fe ⁇ 0.05 wt. %.
  • the rolled product has a thickness from 70 mm to 160 mm, preferably from 70 mm to 102 mm.
  • the rolled product comprises a sum of Fe+Si content which is ⁇ 0.07 wt. %, even more preferably ⁇ 0.06 wt. %.
  • the product according to the invention is used in structural member used at cryogenic temperature, in particular for cryogenic tanks.
  • Said stress relieved wrought product is then artificially aged with a total equivalent aging time t(eq) at 155°C from 24 hours to 45 hours, preferably from 24 hours to 34 hours, even more preferably from 26 hours to 30 hours
  • Aging treatment is advantageously carried out in two steps, with a first step at a temperature comprised from 110 to 130 °C for 3 to 20 hours, preferably for 3 to 10 hours and a second step at a temperature comprised from 140 to 170 °C for 6 to 90 hours and preferably between 150 and 165 °C for 9 to 50 hours.
  • the product of the invention with his specific composition in particular with a Fe and Si content ⁇ 0.08 wt. %, when combined with an appropriately designed artificial aging treatment, makes it possible to obtain a product with a better compromise between tensile yield strength and toughness, in particular a better compromise between tensile yield strength in the rolling direction (L direction) and toughness in L-T direction.
  • the present alloy according to the invention contains from 6.65 to 7.45 wt. % Zn.
  • a minimum value of Zn content of 6.65 wt. %, preferably 6.90 wt. %, more preferably 7.10 wt. % is needed to obtain sufficient strength; however, Zn should not exceed 7.45 wt. %, preferably 7.30 wt. %, more preferably 7.25 wt. % to obtain the sought balance of properties, in particular toughness and elongation.
  • the present alloy according to the invention contains from 1.35 to 1.75 wt. % Mg.
  • a minimum Mg content of 1.35 wt. % and preferably 1.40 wt. % or 1.50 wt. % or even 1.60 wt. % is needed to obtain sufficient strength.
  • the Mg content should not exceed 1.75 wt. % and preferably 1.70 wt. % to obtain the sought balance of properties in particular toughness and elongation.
  • the alloy of the present invention further contains from 0.04 to 0.14 wt.% Zr, which is typically used for grain size control.
  • the Zr content should preferably comprise at least about 0.07 wt. %, and preferentially about 0.09 wt.% in order to limit the recrystallization, but should advantageously remain below about 0.12 wt.% in order to reduce problems during casting.
  • Manganese up to 0.5 wt.% may be added but it is preferentially avoided and is generally kept below about 0.05 wt.%, preferentially below about 0.04 wt.%. and more preferentially below about 0.03 wt.%.
  • the present alloy may contain iron and silicon which affect fracture toughness properties. It was observed that iron and silicon content (e.g. Fe + Si content) must not exceed about 0.08 wt. %, preferably about 0.07 wt. %, more preferably 0.06 wt. % and even more preferably 0.05 wt. % to achieve a better compromise between tensile yield strength and toughness, small drop (less than 10%) in fracture toughness at cryogenic temperature, and very little increase in fatigue crack growth rate under high humidity conditions when compared to fatigue under ambient lab conditions. The inventors found that for the selected composition and Fe+ Si content below 0.08 wt.
  • iron and silicon content e.g. Fe + Si content
  • iron and silicon content is from 0.03% to 0.08 wt. %, preferably from 0.03% to 0.07 wt. %, more preferably from 0.03% to 0.06 wt. % and even more preferably from 0.03% to 0.05 wt. %.
  • the present alloy may contain an iron and silicon content (e.g. Fe + Si content) from 0.04% to 0.08 wt. %, preferably from 0.04% to 0.07 wt. %, more preferably from 0.04% to 0.06 wt. % and even more preferably 0.04% to 0.05 wt. %.
  • the present alloy may include up to 0.05 wt. % Si, preferentially up to 0.03 wt. %.
  • the present alloy may include up to 0.05 wt. % Fe, preferentially up to 0.03 wt. %.
  • the present alloy may include incidental impurities.
  • incidental impurities can include relatively small amounts, less than 0.05 wt. %, or less than 0.01 wt. % of other elements with a total of less than 0.15 wt% total of the total weight of the 7xxx aluminum alloy product. Incidental impurities can be present without departing from the scope of the invention.
  • the rolled product with a thickness of at least 25 mm, preferentially from 25 mm to 200 mm more preferably from 70 mm to 200 mm, or from 70 to 160 mm, even more preferably from 70 mm to 102 mm according to the invention has advantageously the following properties: A K 1C toughness in the L-T direction at room temperature measured according to ASTM E399-2020 of at least -0.25 *t +65 MPa.Vm, more preferably of at least -0.25 *t +68 MPaVm, and even more preferably of at least -0.25 ⁇ t +72 MPaVm, t being the thickness of the rolled product in mm.
  • the rolled product has advantageously a yield strength of at least 450 MPa in the rolling direction L.
  • a yield strength in the transverse direction LT of at least 410 MPa, even more preferably of at least 420 MPa.
  • the rolled product according to the present invention or the product obtained by the process according to the invention is advantageously used as or incorporated in structural members for the construction of aircraft or spacecraft.
  • the products according to the invention are used in wing ribs, spars and frames.
  • the rolled products according to the present invention are welded with other rolled products to form wing ribs, spars and frames.
  • the rolled product according to the invention the product obtained by the process according to the invention is advantageously used in structural member used at cryogenic temperature, typically cryogenic tank.
  • Tensile and K IC testing was performed both at room temperature (22°C) and at cryogenic temperature, precisely at liquid nitrogen temperature (-196°C). Tensile tests were performed according to ASTM B557. Samples were taken at mid thickness (t/2) and quarter thickness (t/4). Tensile tests were performed in the transverse direction (LT), short transverse direction (ST). [Table 3] Ref. Thickness Test Plane Direct.
  • Figure 1 shows the evolution of toughness K 1c (T-L) at cryogenic temperature versus the thickness of the plate. It can be observed that A1 and B1 specimen display a toughness K1c(T-L) in MPa. ⁇ m, at cryogenic temperature higher than -15*t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than - 0.15*t+55 MPa, where t is the thickness of the product in mm.
  • Alloy F composition is according to the invention but sample F1 is processed with a shorter artificial aging which does not permit to obtain the excellent compromise TYS (L) - K 1c (L-T).
  • Table 8 Prior art Reference Prior art Prior art Prior art Prior art Prior art Reference C1 D1 E1 F1 Alloy (Table 7) C D E F Homogenization 15h 479°C 16h 471C-482°C 479°C 479°C Hot rolling Entry hot Rolling temperature 420 °C - 440 °C 416 °C - 438 °C 400°C - 440°C
  • Figure 2 shows the compromise at Room temperature (RT) between tensile yield strength in the rolling direction (L) and toughness in L-T direction, measured respectively at quarter thickness and mid thickness for A1 and C1 plate.
  • RT Room temperature
  • L tensile yield strength in the rolling direction
  • L-T direction toughness in L-T direction
  • Figure 3 shows the compromise between tensile yield strength in the transverse direction (LT) and the toughness in T-L, measured at room temperature and at liquid nitrogen temperature of -196°C. It can be observed that A1 plates exhibits a lower drop in toughness than D1 plate. It is attributed to the chemical composition, in particular the lower Fe + Si content.
  • compositions A, C, G, H, J, K, M were cast and transformed; respective details of composition and process are listed in Tables 14 and 15. All these cast were transformed into plates with thicknesses from 70 to 102 mm.
  • Alloys C, G, H correspond to prior art alloys and J, K and M are compositions according to the invention.
  • Alloy A is similar from example 1.
  • samples having a composition according to the invention and processed according to the invention are represented with a black diamond (A1-a, J1-A, K2-a).
  • Samples having a composition according to the invention but processed differently are represented by an empty diamond (M2-b).
  • a triangle symbol is used to represent compositions which differ from the invention by Fe+Si content above 0.08 wt. % (C1-a and C1-b).
  • a round circle symbol is used to represent compositions which differ from the invention by main alloying elements (G2-a, H2-a).
  • Triangle or round circle symbols are empty if the process differs from the invention, i.e. equivalent time at 155°C below 23 h or with full symbol if the process is according to the invention.

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Abstract

The present invention relates to wrought 7xxx aluminum alloy product comprising in weight % Zn 6.7 - 7.4, Mg 1.35 - 1.75, Cu 1.85 - 2.35, Zr 0.04 - 0.14, Mn 0 - 0.5, Ti 0 - 0.15, V 0 - 0.15, Cr 0 - 0.25, Fe ≤0.05, Si≤0.05 with a Fe+Si ≤ 0.08 % and artificially aged wherein the total equivalent aging time t(eq) at 155°C is comprised from 24 hours to 45 hours which permits to optimize the compromise between tensile yield strength and toughness with improved cryogenic fracture toughness and fatigue resistance in corrosive environment.

Description

    FIELD OF THE INVENTION
  • The present disclosure relates to a method for manufacturing 7xxx aluminum wrought product with improved compromise of tensile and toughness properties and excellent cryogenic properties and fatigue resistance in corrosive environment and more particularly to such manufacturing processes and rolled products and use, notably designed for aeronautical and aerospace engineering.
    • BACKGROUND
  • High strength 7xxx aluminum alloy products, also designated as Al-Zn-Mg-Cu type alloy products, are extensively used in aerospace structure application, in which the material strength, fracture toughness, fatigue resistance, and corrosion resistance are required simultaneously. It is known that these various required properties cannot all be optimized at the same time and independently of each other during the fabrication of semi-finished products and structural elements for aeronautical construction. When the chemical composition of the alloy or the parameters of product manufacturing processes are modified, several critical properties may tend to change in opposing directions. This is sometimes the case firstly for properties generally referred to as "static mechanical strength" (particularly the ultimate tensile stress UTS and the tensile yield stress TYS), and secondly for properties generally names as "damage tolerance" (particularly the toughness and the resistance to crack propagation). Moreover, some other properties such as resistance to fatigue, resistance to corrosion, formability and elongation at failure are related to these properties in a complex and often unpredictable manner. Therefore, optimization of all properties of a material for mechanical construction, for example in the aerospace sector, often requires a compromise between several key parameters.
  • Al-Zn-Mg-Cu alloys with high fracture toughness and high mechanical strength are described in the prior art.
  • US Patent 5,312,498 discloses a method of producing an aluminum-based alloy product having improved exfoliation resistance and fracture toughness which comprises providing an aluminum-based alloy composition consisting essentially of about 5.5-10.0% by weight of zinc, about 1.75-2.6% by weight of magnesium, about 1.8-2.75%) by weight of copper with the balance aluminum and other elements. The aluminum-based alloy is worked, heat treated, quenched and aged to produce a product having improved corrosion resistance and mechanical properties.
  • US Patent 5,560,789 describes AA 7000 series alloys having high mechanical strength and a process for obtaining them. The alloys contain, by weight, 7 to 13.5% Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others up to 0.05% each and 0.15% total, and remainder Al.
  • US Patent No 5,865,911 describes an aluminum alloy consisting essentially of (in weight %) about 5.9 to 6.7% zinc, 1.8 to 2.4% copper, 1.6 to 1.86% magnesium, 0.08 to 0.15% zirconium balance aluminum and incidental elements and impurities. The '911 patent particularly mentions the compromise between static mechanical strength and toughness.
  • US Patent No 6,027,582 describes a rolled, forged or extruded Al-Zn-Mg-Cu aluminum base alloy products greater than 60 mm thick with a composition of (in weight %), Zn: 5.7-8.7, Mg: 1.7-2.5, Cu: 1.2-2.2, Fe: 0.07-0.14, Zr: 0.05-0.15 with Cu + Mg < 4.1 and Mg>Cu.
  • US Patent No 6,972,110 teaches an alloy, which contains preferably (in weight %) Zn: 7-9.5, Mg: 1.3-1.68 and Cu 1.3-1.9 and encourages keeping Mg +Cu≤ 3.5.
  • PCT Patent Application No WO2004090183 discloses an alloy comprising essentially (in weight percent): Zn: 6.0 - 9.5, Cu: 1.3 - 2.4, Mg: 1.5 - 2.6, Mn and Zr < 0.25 but preferably in a range between 0.05 and 0.15 for higher Zn contents, other elements each less than 0.05 and less than 0.25 in total, balance aluminum, wherein (in weight percent): 0.1 [Cu] + 1.3 < [Mg] < 0.2[Cu] + 2.15, preferably 0.2[Cu] + 1.3 < [Mg] < 0.1[Cu] + 2.15.
  • US 20050006010 discloses a method for producing a high strength AI-Zn- Cu-Mg alloy with an improved fatigue crack growth resistance and a high damage tolerance, comprising the steps of casting an ingot with the following composition (in weight percent) Zn 5.5-9.5, Cu 1.5-3.5, Mg 1.5-3.5, Mn<0.25, Zr<0.25, Cr <0.10, Fe<0.25, Si<0.25, Ti<0.10, Hf and/or V<0.25, other elements each less than 0.05 and less than 0.15 in total, balance aluminum, homogenizing and/or pre-heating the ingot after casting, hot working the ingot and optionally cold working into a worked product of more than 50 mm thickness, solution heat treating, quenching the heat treated product, and artificially aging the worked and heat-treated product, wherein the aging step comprises a first heat treatment at a temperature in a range of 105 ° C. to 135 ° C. for more than 2 hours and less than 8 hours and a second heat treatment at a higher temperature than 135 ° C. but below 170 ° C. for more than 5 hours and less than 15 hours.
  • EP 1544315 discloses a product, especially rolled, extruded or forged, made of an AlZnCuMg alloy with constituents having the following percentage weights: Zn 6.7 - 7.3; Cu 1.9 - 2.5; Mg 1.0 - 2.0; Zr 0.07 - 0.13; Fe less than 0.15; Si less than 0.15; other elements not more than 0.05 to at most 0.15 per cent in total; and aluminum the remainder, wherein Mg/Cu<1. The product is preferably treated by solution heat treatment, quenching, cold working and artificial aging.
  • US Patent No 8,277,580 teaches a rolled or forged Al-Zn-Cu-Mg aluminum-based alloy wrought product having a thickness from 2 to 10 inches. The product has been treated by solution heat-treatment, quenching and aging, and the product comprises (in weight- %): Zn 6.2-7.2, Mg 1.5-2.4, Cu 1.7-2.1. Fe 0-0.13, Si 0-0.10, Ti 0-0.06, Zr 0.06-0.13, Cr 0-0.04, Mn 0-0.04, impurities and other incidental elements <=0.05 each.
  • US Patent No 8,673,209 discloses aluminum alloy products about 4 inches thick or less that possesses the ability to achieve, when solution heat treated, quenched, and artificially aged, and in parts made from the products, an improved combination of strength, fracture toughness and corrosion resistance, the alloy consisting essentially of: about 6.8 to about 8.5 wt. % Zn, about 1.5 to about 2.00 wt. % Mg, about 1.75 to about 2.3 wt. % Cu; about 0.05 to about 0.3 wt. % Zr, less than about 0.1 wt. % Mn, less than about 0.05 wt. % Cr, the balance Al, incidental elements and impurities and a method for making same.
  • WO2019/007817 concerns an extruded, rolled and/or forged aluminum-based alloy product having a thickness of at least 25 mm comprising (in weight %): Zn 6.70 -7.40; Mg 1.50 -1.80; Cu 2.20 -2.60, with a Cu to Mg ratio of at least 1.30; Zr 0.04 -0.14; Mn 0 -0.5; Ti 0 -0.15; V 0 -0.15; Cr 0 -0.25; Fe 0 -0.15; Si 0 -0.15; impurities ≤ 0.05 each and ≤ 0.15 total.
  • None of these documents disclose the benefit of combining an aluminum alloy comprising in weight % Zn Zn 6.65 - 7.45, Mg 1.35 - 1.75, Cu 1.85 - 2.35, Zr 0.04 - 0.14, Mn 0 - 0.5, Ti 0 - 0.15, V 0 - 0.15, Cr 0 - 0.25, Fe≤0.05, Si≤0.05% with a Fe+Si ≤ 0.08 % and artificially aged wherein the equivalent aging time t(eq) at 155°C is comprised from 24 hours to 45 hours which permits to optimize the compromise between tensile yield strength and toughness with improved cryogenic fracture toughness and fatigue resistance in corrosive environment.
    • SUMMARY OF THE INVENTION
  • The present invention relates to a method for manufacturing a high strength 7xxx aluminum wrought products by controlling the composition, in particular the quantity of Fe and Si and the fabrication parameters, in particular artificial aging conditions, to provide improved compromise between tensile yield strength and toughness with improved cryogenic fracture toughness, and fatigue resistance in corrosive environment. The invention also concerns a rolled 7xxx product which can be obtained by the method of the invention.
  • The term "cryogenic temperature" is defined in accordance with the present invention to include temperatures significantly below room temperature and typically below - 100°C (173 K). Thus, the temperatures at which hydrogen (-253°C/20 K), oxygen (-183°C/90 K) and nitrogen (-196°C/77 K) become liquid at atmospheric pressure are included as cryogenic temperatures. For purposes of experimental evaluation, a temperature of -196°C/77K is considered as a cryogenic temperature. Room temperature is defined in accordance with its common usage and includes temperatures from about 20°C to about 25°C. For purposes of experimental evaluation, a temperature of 22°C is considered to be room temperature.
  • The present invention is directed to a process for the manufacture of a wrought 7xxx aluminum-based alloy product comprising the steps of:
    1. a) preparing a bath of molten alloy metal comprising in weight-% (wt. %)
      • Zn 6.65 - 7.45
      • Mg 1.35 - 1.75
      • Cu 1.85 - 2.35
      • Zr 0.04 - 0.14
      • Mn 0-0.5
      • Ti 0 - 0.15, preferably from 0.02 to 0.06 wt. %,
      • V 0 -0.15
      • Cr 0 - 0.25
      • Fe ≤0.05
      • Si ≤0.05
      • Fe + Si ≤ 0.08,
      • Other impurities <0.05 each and <0.15 total, remainder aluminum.
    2. b) casting said molten alloy metal to obtain a slab or a billet;
    3. c) homogenizing said ingot or billet to obtain a homogenized ingot or billet;
    4. d) hot working said homogenized slab or billet to obtain a wrought product, such as an extruded, rolled and/or forged product, with a final thickness of at least 25 mm, preferably from 25 mm to 200 mm;
    5. e) solution heat treating and quenching said wrought product to obtain a quenched wrought product;
    6. f) stress relieving the quenched wrought product to obtain a stress relieved wrought product;
    7. g) artificial aging said stress relieved wrought product wherein the total equivalent aging time t(eq) at 155°C is comprised from 24 hours to 45 hours, preferably from 24 hours to 34 hours, even more preferably from 26 hours to 30 hours,
      the total equivalent time t(eq) at 155°C being defined by the formula: t eq = ∫ exp − 16000 / T dt exp − 16000 / T ref
      Figure imgb0001
    where T is the instantaneous temperature in Kelvin during aging and Tref is a reference temperature selected at 155 °C (428 K), t(eq) is expressed in hours.
  • The process is advantageously made with a bath of molten alloy comprising Si ≤0.03 wt. % and Fe ≤ 0.05 wt. %.
  • Preferably, the sum Fe+Si content is ≤ 0.07 wt. %, even more preferably ≤ 0.06 wt. %.
  • Preferably, the sum Fe+Si content is ≥ 0.03 wt. %, even more preferably ≥ 0.04 wt. %.
  • The present invention is directed to a rolled product with a thickness t in millimeter of at least 25 mm, preferentially from 25 mm to 200 mm, comprising (in weight-%)
    • Zn 6.65 - 7.45
    • Mg 1.35 -1.75
    • Cu 1.85-2.35
    • Zr 0.04 - 0.14
    • Mn 0-0.5
    • Ti 0 - 0.15
    • V 0 - 0.15
    • Cr 0 - 0.25
    • Fe ≤0.05
    • Si≤0.05
    • Fe + Si ≤ 0.08
    • Other impurities < 0.05 each and <0.15 total, remainder aluminum,
    • and wherein the toughness K1c (L-T) in MPa.√m, at room temperature, measured according to ASTM standard E399 (2020) is higher than -0.25*t +65 MPaVm , preferably higher than - 0.25*t+68 MPaVm , even more preferably -0.25 *t +72 MPaVm.
  • In a preferred embodiment, the rolled product according to the invention displays a surprising small drop in both fracture toughness and elongation from room temperature down to liquid nitrogen temperature. The rolled product according to the invention displays a toughness K1c(T-L) in MPa.√m, at cryogenic temperature, i.e at temperature below 100°C, typically at -196°C, measured according to ASTM standard E399 2020 which is reduced by less than 10 %, preferably by less than 8%, more preferably by less than 7 % in comparison with K1c(T-L) in MPa.√m, measured at room temperature according to ASTM standard E399 -2020.
  • Preferably, the rolled product according to the invention displays a toughness K1c(T-L) in MPa.√m, at cryogenic temperature, i.e at temperature below 100°C, typically at - 196°C, measured according to ASTM standard E399 -2020 higher than -15∗t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than - 0.15*t+55 MPa, where t is the thickness of the rolled product in mm.
  • Preferably, the rolled product comprises Si ≤0.03 wt. % and Fe ≤ 0.05 wt. %.
  • In a preferred embodiment, the rolled product has a thickness from 70 mm to 160 mm, preferably from 70 mm to 102 mm.
  • Preferably, the rolled product comprises a sum of Fe+Si content which is ≤ 0.07 wt. %, even more preferably ≤ 0.06 wt. %.
  • Preferably, the rolled product comprises a sum of Fe+Si content which is ≥ 0.03 wt. %, even more preferably ≥ 0.04 wt. %.
  • Preferably, the rolled product comprises a sum of Fe+Si content which is from 0.03 wt. % to 0.08 wt. %, preferably from 0.03 wt. % to 0.07 wt. %, even more preferably from .03 wt. % to 0.06 wt. %.
  • Preferably, the rolled product comprises a Zn content from 6.90 to 7.30 wt. %, more preferably from 7.10 to 7.25 wt.%.
  • Preferably, the rolled product comprises a Ti content which is <_ 0.06%, preferably from 0.02 to 0.06 wt. %, even more preferably from 0.03 to 0.05 wt. %.
  • The rolled product according to the present invention or the product obtained by the process according to the invention is advantageously used as or incorporated in structural members for the construction of aircraft or spacecraft.
  • In an advantageous embodiment, the product according to the invention is used in structural member used at cryogenic temperature, in particular for cryogenic tanks.
    • BRIEF DESCRIPTION OF THE DRAWINGS
    • [Fig. 1] Figure 1 shows the evolution of toughness K1c (T-L) at cryogenic temperature versus the thickness of the plate.
    • [Fig. 2] Figure 2 shows the compromise at Room temperature (RT) between tensile yield strength in the rolling direction (L) and the toughness in L-T, measured at mid thickness.
    • [Fig. 3] Figure 3 shows the evolution of the compromise TYS in the rolling direction and toughness K1c (T-L) at room temperature and cryogenic temperature.
    • [Fig.4] Figure 4 shows the evolution of toughness K1c (L-T) at Room temperature versus the sum Fe +Si for different compositions.
    • [Fig. 5] Figure 5 shows the evolution of toughness K1c (L-T) at Room temperature versus the sum Fe +Si for different artificial aging conditions
    • [Fig. 6] Figure 6 shows the evolution of toughness K1c (L-T) versus the thickness of the plate.
    • [Fig 7] Figure 7 shows the fatigue crack growth rate (L-T) for a reference product in normal ambient air humidity condition and humid air condition.
    • [Fig 8] Figure 8 shows the fatigue crack growth rate (L-T) for a product according to the invention in normal ambient air humidity condition and humid air condition.
    DETAILED DESCRIPTION
  • Unless otherwise indicated, all the indications relating to the chemical composition of the alloys are expressed as a mass percentage by weight based on the total weight of the alloy. Alloy designation is in accordance with the regulations of The Aluminium Association, known to those skilled in the art.
  • The definitions of tempers are laid down in NF EN 515 (2017). Unless mentioned otherwise, static mechanical characteristics, i.e., the ultimate tensile strength UTS, the tensile yield stress TYS and the elongation at fracture E, are determined by a tensile test according to standard ASTM B557, the location at which the pieces are taken and their direction being defined in ASTM B557. Unless otherwise specified, the definitions of standard EN 12258 apply. The fracture toughness K1C is determined according to ASTM standard E399-2020. Materials are tested at t/2 up to 102 mm (4 inch) thickness, and t/4 for thicknesses greater than 102 mm (4 inch). For S-L, samples are all tested at t/2. Unless otherwise specified, C(T) specimen have a width W=101.6 mm (4 inch) and B = 50.8 mm (2 inch).
  • For extrusion, the thickness of the extruded products is defined according to standard NF EN 2066 (2002). The cross section is divided into elementary rectangles of dimensions A and B; A always being the largest dimension of the elementary rectangle and B being regarded as the thickness of the elementary rectangle.
  • The term "structural member" is a term well known in the art and refers to a component used in mechanical construction for which the static and/or dynamic mechanical characteristics are of particular importance with respect to structure performance, and for which a structure calculation is usually prescribed or undertaken. These are typically components the rupture of which may seriously endanger the safety of the mechanical construction, its users or third parties. In the case of an aircraft, structural members comprise members of the fuselage (such as fuselage skin), stringers, bulkheads, circumferential frames, wing components (such as wing skin, stringers or stiffeners, ribs, spars), empennage (such as horizontal and vertical stabilizers), floor beams, seat tracks, and doors.
  • When used in this description the symbol * means multiply by.
  • When used in this description, the term "about" means a value with a tolerance of +/-5% of the displayed value.
  • The process of the invention comprises different steps.
  • A first step aims at preparing a bath of molten alloy metal comprising, or advantageously consisting essentially of in weight % (wt. %) Zn 6.65 - 7.45, Mg 1.35 -1.75, Cu 1.85 - 2.35, Zr 0.04 - 0.14, Mn 0 - 0.5, Ti 0 - 0.15, V 0 - 0.15, Cr 0 - 0.25, Fe≤0.05, Si≤0.05% with a Fe+Si ≤ 0.08 % and other impurities < 0.05 each and < 0.15 total, remainder aluminum. Said molten alloy metal is then cast to obtain a slab or a billet. A slab according to the invention is parallelepipedal; it can be also called "ingot".
  • Said slab or billet is then homogenized. In a preferred embodiment, the homogenization is preferably performed with at least one step at a temperature from about 450°C to about 510 °C typically for 5 to 30 hours or preferentially from about 470°C to about 500 °C for 8 to 20 hours or even more preferably between 470°C to 490°C for 8 to 20 hours.
  • Said homogenized slab or billet is hot worked to obtain a wrought product with a final thickness of at least 25 mm, preferably from about 25 mm to about 200 mm, more preferably from about 70 mm to about 160 mm, even more preferably from about 70 mm to about 102 mm. Said wrought product is a rolled product or an extruded product or a forged product. The forged product can be obtained directly by forging the billet or ingot or from the rolled product (e.g. first rolled and then forged), or from the extruded product (e.g. first extruded then forged). In one embodiment, the homogenized slab is hot rolled to obtain a rolled product with a final thickness of at least 25 mm, preferably from about 25 mm to about 200 mm, more preferably from about 70 mm to about 160 mm, even more preferably from about 75 mm to about 102 mm or between about 70 mm to about 80 mm. Hot rolling is preferably done in one or multiple stages with an entry temperature preferably comprised from about 380°C to about 460 °C and preferentially from about 400°C to about 450 °C.
  • Said wrought product (e.g. rolled product or an extruded product and/or a forged product) is solution-heat treated to obtain a solution heat treated wrought product, preferably at a temperature from 460°C to about 510 °C or preferentially from about 470°C to about 500 °C or even more preferably between 470°C to 490°C typically for 1 to 10 hours depending on thickness.
  • Said solution heat treated wrought product is then quenched to obtain a quenched wrought product, preferably in water at room temperature.
  • Said quenched wrought product is then stress relieved to obtain a stress relieved wrought product. Stress relief is obtained by controlled stretching or compression with a permanent plastic deformation of preferably less than 5% and preferentially from 1 to 4%, more preferentially between 2 to 3 %.
  • Said stress relieved wrought product is then artificially aged with a total equivalent aging time t(eq) at 155°C from 24 hours to 45 hours, preferably from 24 hours to 34 hours, even more preferably from 26 hours to 30 hours
  • The total equivalent time t(eq) at 155°C being defined by the formula: t eq = ∫ exp − 16000 / T dt exp − 16000 / T ref
    Figure imgb0002
     where T is the instantaneous temperature in °K during aging and Tref is a reference temperature selected at 155 °C (428 °K), t(eq) is expressed in hours.
  • The total equivalent aging time t(eq) at 155°C is at least 24 hours, preferably at least 25 hours, or 26 hours or even more preferably at least 27 hours. The total equivalent aging time t(eq) at 155°C is less than 45 hours, preferably less than 40 hours, 39 hours, 38 hours, 37 hours, 36 hours, 35 hours, 34 hours, 33 hours, 32 hours, 31 hours, or even more preferably less than 30 hours. Any combination of minimum and maximum disclosed total equivalent aging time t(eq) at 155°C above listed can permit to obtain the best compromise of toughness and yield strength.
  • Aging treatment is advantageously carried out in two steps, with a first step at a temperature comprised from 110 to 130 °C for 3 to 20 hours, preferably for 3 to 10 hours and a second step at a temperature comprised from 140 to 170 °C for 6 to 90 hours and preferably between 150 and 165 °C for 9 to 50 hours.
  • The product of the invention with his specific composition, in particular with a Fe and Si content ≤ 0.08 wt. %, when combined with an appropriately designed artificial aging treatment, makes it possible to obtain a product with a better compromise between tensile yield strength and toughness, in particular a better compromise between tensile yield strength in the rolling direction (L direction) and toughness in L-T direction.
  • The present alloy according to the invention contains from 6.65 to 7.45 wt. % Zn. A minimum value of Zn content of 6.65 wt. %, preferably 6.90 wt. %, more preferably 7.10 wt. % is needed to obtain sufficient strength; however, Zn should not exceed 7.45 wt. %, preferably 7.30 wt. %, more preferably 7.25 wt. % to obtain the sought balance of properties, in particular toughness and elongation.
  • The present alloy according to the invention contains from 1.35 to 1.75 wt. % Mg. A minimum Mg content of 1.35 wt. % and preferably 1.40 wt. % or 1.50 wt. % or even 1.60 wt. % is needed to obtain sufficient strength. However, the Mg content should not exceed 1.75 wt. % and preferably 1.70 wt. % to obtain the sought balance of properties in particular toughness and elongation.
  • The present alloy according to the invention contains from 1.85 to 2.35 wt. % Cu. A minimum Cu content of 1.85 wt. %, preferably of 2.0 wt. % or even 2.1 wt. % is needed to obtain sufficient strength. However, the Cu should not exceed 2.35 wt. % and preferably 2.30 or even more preferably 2.25 wt. % to avoid quench sensitivity.
  • The alloy of the present invention further contains from 0.04 to 0.14 wt.% Zr, which is typically used for grain size control. The Zr content should preferably comprise at least about 0.07 wt. %, and preferentially about 0.09 wt.% in order to limit the recrystallization, but should advantageously remain below about 0.12 wt.% in order to reduce problems during casting.
  • Titanium, associated with either boron or carbon can usually be added up to 0.15 wt.% if desired during casting in order to limit the as-cast grain size. The present invention may typically accommodate up to about 0.06 wt. % or about 0.05 wt.% Ti. In a preferred embodiment of the invention, the Ti content is from about 0.02 wt.% to about 0.06 wt.% and preferentially from about 0.03 wt.% to about 0.05 wt.%.
  • Manganese up to 0.5 wt.% may be added but it is preferentially avoided and is generally kept below about 0.05 wt.%, preferentially below about 0.04 wt.%. and more preferentially below about 0.03 wt.%.
  • Vanadium up to 0.15 wt.% may be added but it is preferentially avoided and is generally kept below about 0.05 wt.%, preferentially below about 0.04 wt.%. and more preferentially below about 0.03 wt.%.
  • Chromium up to 0.25 wt.% may be added but it is preferentially avoided and is generally kept below about 0.05 wt.%, preferentially below about 0.04 wt.%. and more preferentially below about 0.03 wt.%.
  • The present alloy may contain iron and silicon which affect fracture toughness properties. It was observed that iron and silicon content (e.g. Fe + Si content) must not exceed about 0.08 wt. %, preferably about 0.07 wt. %, more preferably 0.06 wt. % and even more preferably 0.05 wt. % to achieve a better compromise between tensile yield strength and toughness, small drop (less than 10%) in fracture toughness at cryogenic temperature, and very little increase in fatigue crack growth rate under high humidity conditions when compared to fatigue under ambient lab conditions. The inventors found that for the selected composition and Fe+ Si content below 0.08 wt. % in combination with an appropriately designed aging treatment, with a total equivalent aging time t(eq) at 155°C from 24 hours to 45 hours, it is possible to achieve a significant improvement in the compromise tensile yield strength - toughness, toughness and elongation at cryogenic temperature and fatigue crack rate under high humidity conditions.
  • The inventors found that it is possible to obtain a satisfying compromise even if Fe+Si content is above 0.03 wt. %, or above 0.04 wt. % or above 0.05 wt. %.
  • In one embodiment of the present invention, iron and silicon content (e.g. Fe + Si content) is from 0.03% to 0.08 wt. %, preferably from 0.03% to 0.07 wt. %, more preferably from 0.03% to 0.06 wt. % and even more preferably from 0.03% to 0.05 wt. %. In another embodiment, the present alloy may contain an iron and silicon content (e.g. Fe + Si content) from 0.04% to 0.08 wt. %, preferably from 0.04% to 0.07 wt. %, more preferably from 0.04% to 0.06 wt. % and even more preferably 0.04% to 0.05 wt. %. In another embodiment, the present alloy may contain an iron and silicon content (e.g. Fe + Si content) from 0.05% to 0.08 wt. %, preferably from 0.05% to 0.07 wt. %, more preferably from 0.05% to 0.06 wt. %.
  • The present alloy may include up to 0.05 wt. % Si, preferentially up to 0.03 wt. %. The present alloy may include up to 0.05 wt. % Fe, preferentially up to 0.03 wt. %.
  • The present alloy may include incidental impurities. The term "incidental impurities" can include relatively small amounts, less than 0.05 wt. %, or less than 0.01 wt. % of other elements with a total of less than 0.15 wt% total of the total weight of the 7xxx aluminum alloy product. Incidental impurities can be present without departing from the scope of the invention.
  • The term "comprising" shall be construed to mean that no intentional additional of other elements are added beyond those recited in order to provide the novel and basic features of the present invention. It is understood that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of such elements may, nevertheless, find their way into the final alloy product. It is to be understood, however, that the scope of this invention should not/cannot be avoided through the mere addition of any such element(s) in quantities that would not otherwise impact on the combination of properties desired and attained herein.
  • The present invention finds particular interest for rolled product, in particular with a thickness of at least 25 mm, preferentially from 25 mm to 200 mm, more preferably from 70 mm to 200 mm, or from 70 mm to 160 mm, even more preferably from 70 mm to 102 mm.
  • The rolled product with a thickness of at least 25 mm, preferentially from 25 mm to 200 mm more preferably from 70 mm to 200 mm, or from 70 to 160 mm, even more preferably from 70 mm to 102 mm according to the invention has advantageously the following properties:
    A K1C toughness in the L-T direction at room temperature measured according to ASTM E399-2020 of at least -0.25 *t +65 MPa.Vm, more preferably of at least -0.25 *t +68 MPaVm, and even more preferably of at least -0.25 ∗t +72 MPaVm, t being the thickness of the rolled product in mm.
  • Preferably, the rolled product has advantageously a yield strength of at least 450 MPa in the rolling direction L. Preferably a yield strength in the transverse direction LT of at least 410 MPa, even more preferably of at least 420 MPa.
  • In a preferred embodiment, the rolled product according to the invention displays a surprising small drop in both fracture toughness and elongation from room temperature down to liquid nitrogen temperature. The rolled product according to the invention displays a toughness K1c(T-L) in MPa.√m, at cryogenic temperature, measured according to ASTM standard E399 2020 which is reduced by less than 10 %, preferably by less than 8%, more preferably by less than 7 % in comparison with K1c(T-L) in MPa.√m, measured at room temperature according to ASTM standard E399 -2020. Preferably, the rolled product according to the invention displays a toughness K1c(T-L) in MPa.√m, at cryogenic temperature, measured according to ASTM standard E399 - 2020 higher than -15*t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than -0.15*t+55 MPa, where t is the thickness of the rolled product in mm.
  • The rolled product according to the present invention or the product obtained by the process according to the invention is advantageously used as or incorporated in structural members for the construction of aircraft or spacecraft.
  • In an advantageous embodiment, the products according to the invention are used in wing ribs, spars and frames. In embodiments of the invention, the rolled products according to the present invention are welded with other rolled products to form wing ribs, spars and frames. In another embodiment, the rolled product according to the invention the product obtained by the process according to the invention is advantageously used in structural member used at cryogenic temperature, typically cryogenic tank.
    • EXAMPLES Example 1
  • Two ingots were cast with two compositions A and B according to the invention. The ingots were homogenized and hot rolled to 76.2 mm (A1) and 152.4 mm (B3) in thickness, followed by a solution heat treatment, stretching and final aging according to the process of the invention. The chemistry and process information for these two lots are shown respectively in Table 1 and Table 2. [Table 1]
    (% wt) Si Fe Cu Mn Mg Zn Zr Ti Si+Fe
    A 0.02 0.03 2.15 - 1.65 7.21 0.10 0.03 0.05
    B 0.02 0.04 2.10 - 1.64 7.05 0.10 0.03 0.06
    [Table 2]
    Invention Invention
    Reference A1 B3
    Alloy A B
    Homogenization 16h - 471°C- 482°C
    Hot rolling : Entry hot Rolling temperature 406°C 409°C
    Final thickness (mm) 76.2 152.4
    Solution heat Treatment 4.5h at 479°C 7h 479°C
    Stretch 2.25%
    Aging First step aging 4h 121°C
    Ramp between first and second step 6°C/h
    Second step Aging 12h 163°C
    Total Equivalent Time at 155C 27.7 h
  • Tensile and KIC testing was performed both at room temperature (22°C) and at cryogenic temperature, precisely at liquid nitrogen temperature (-196°C). Tensile tests were performed according to ASTM B557. Samples were taken at mid thickness (t/2) and quarter thickness (t/4). Tensile tests were performed in the transverse direction (LT), short transverse direction (ST). [Table 3]
    Ref. Thickness Test Plane Direct. Temperature UTS TYS E%
    mm °C MPa MPa %
    A1 76,2 t/2 LT 22 502 447 14,8
    -196 623 537 13
    ST 22 507 434 8,5
    -196 632 514 8
    B3 152,4 t/2 LT 22 497 434 11
    -196 621 524 8
    ST 22 483 412 6,8
    -196 581 466 4
    [Table 4]
    Ref. Thickness mm Test Plane Direct. Temperature °C UTS TYS E%
    MPa MPa %
    A1 76,2 t/4 LT 22 521 463 14
    -196 637 549 11
    B3 152,4 22 504 439 12
    -196 612 524 8
    [Table 5]
    Ref. Thick. mm Test Plane T. °C TYS (LT) TYS (ST)
    MPa % by varying temperature from RT to -196°C MPa % by varying temperature from RT to -196°C
    A1 76.2 t/2 22 447 434
    -196 537 20.1 % 514 18.4%
    t/4 22 463
    -196 549 18.9%
    B3 152,4 t/2 22 434 412
    -196 524 20.7% 466 13.1%
    t/4 22 439
    -196 524 19.4%
  • It is observed an increase in yield strength between the cryogenic temperature and ambient temperature. The increase is more than 12 % in the ST direction and more than 18% in the LT direction.
  • K1C tests were performed in the T-L or S-L directions according to ASTM E399 using a C(T) specimen. [Table 6]
    Ref. Thick mm Temp. °C K1c T-L K1c S-L
    Test Plane MPa.√m % by varying Temp. from RT to -196°C Test Plane MPa.√m % by varying Temp. from RT to -196°C
    A1 76.2 22 t/2 48.7 t/2 52.4 *
    -196 t/2 47.0 -3.4 % t/2 47.1 -10.0 %
    B3 152,4 22 t/4 40,8 t/2 39,7
    -196 t/4 38,1 -6.5 % t/2 34,0 -14.4 %
    * Kq
  • It is observed a slight drop in toughness between the cryogenic temperature and ambient temperature. The drop increases with plate gauge and remains less than 7% for K1c (T-L) and even less than 4% at 76.2mm in this orientation and less than 15% for K1c (S-L) and even less than 10.5% at 76.2mm in this orientation.
  • Figure 1 shows the evolution of toughness K1c (T-L) at cryogenic temperature versus the thickness of the plate. It can be observed that A1 and B1 specimen display a toughness K1c(T-L) in MPa.√m, at cryogenic temperature higher than -15*t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than - 0.15*t+55 MPa, where t is the thickness of the product in mm.
    • Example 2
  • Four ingots of aluminum alloys C, D, E, F were cast, homogenized and hot rolled to respectively 75 mm (C1), 69.9 mm (D1), 76 mm (E1 and F1) in thickness, followed by a solution heat treatment, stretching and a final aging. The chemistry and process information for these four lots are shown respectively in Table 8 and Table 9. Alloy C corresponds to alloy P of example 3 cited in EP 1 544 315 , alloy D is representative of AA7050, alloys E and F correspond respectively to alloy E and C of example 1 of WO2019/007817 . The chemistry and process information for these reference samples are listed in the tables below. Alloy F composition is according to the invention but sample F1 is processed with a shorter artificial aging which does not permit to obtain the excellent compromise TYS (L) - K1c (L-T). [Table 7]
    (% wt) Si Fe Cu Mn Mg Zn Zr Ti Si+Fe
    C 0.05 0.06 2.05 - 1.64 7.08 0.08 - 0,11
    D 0.05 0.07 2.19 0.01 2.22 6.20 0.09 0.03 0,12
    E 0.027 0.046 2.49 - 1.66 7.09 0.09 0.030 0,07
    F 0.029 0.03 2.11 1.69 7.24 0.10 0.041 0,06
    [Table 8]
    Prior art Reference Prior art Prior art
    Reference C1 D1 E1 F1
    Alloy (Table 7) C D E F
    Homogenization 15h 479°C 16h 471C-482°C 479°C 479°C
    Hot rolling Entry hot Rolling temperature 420 °C - 440 °C 416 °C - 438 °C 400°C - 440°C 400°C - 440°C
    Final Thickness (mm) 75 70 76 76
    Solution heat Treatment 4h 479 °C 4h 479 °C 479°C 479°C
    Stretch 2% 2.1% 2.0-2.5% 2.0-2.5%
    Aging - First step aging 6h 120 °C 4h 121 °C 4h 120°C 6h 120°C
    Ramp between first and second step 15°C/h 8°C/h
    Second step Aging 5h, 10h, 15h at 160 °C 4.7 h at 174°C 15, 20, 24, 32h at 155°C 15h at 155°C
    Total Equivalent Time at 155°C 9.0h, 16.7h, 24.4h 30.3 h 17, 22, 27, 35h 17h
  • Tensile strength and toughness of C1, E1 and F1 are taken from the prior art. For purpose of comparison, sample A1 was tested similarly at t/2 for comparison. [Table 9]
    Ref. Thick. Temp. Aging Equiv. time at 155C UTS (L) TYS (L) E% (L) K1C L-T
    mm °C h t/4 t/4 t/4 t/2
    MPa MPa % (MPa.Vm)
    A1 Inv 76 RT 27.7 507 467 17.5 64.0
    C1 Ref 75 9.0 561 542 9.7 30.1
    16.7 549 525 10.3 32.8
    24.4 537 507 11.3 34.6
    E1 Ref 76 17 558 537 12.9 38.2
    22 545 515 13.6 40.2
    27 524 479 13.9 45.1
    35 516 473 13.6 51.1
    F1 Ref 76 17 547 519 14.0 43.2
  • Yield strength of Plate D1 was tested in LT direction and K1C test was performed according to ASTM E399 using a C(T) specimen, taken at t/2. Obtained properties have been compared to inventive A1 plate according to example 1. [Table 10]
    Ref. Thick. Test Plane Temp. UTS (LT) TYS (LT) E% (LT) TYS Difference by changing from RT to -196°C
    mm °C MPa MPa % %
    D1 70 t/4 22 536 476 13.5
    -196 657 560 7.8 17.6%
    [Table 11]
    Ref. Thickness Test Plane Temperature K1c T-L K1c T-L Difference by changing from RT to -196°C
    mm °C MPa.√m %
    D1 70 t/2 22 35.1
    -196 29.7 -15.7%
  • Figure 2 shows the compromise at Room temperature (RT) between tensile yield strength in the rolling direction (L) and toughness in L-T direction, measured respectively at quarter thickness and mid thickness for A1 and C1 plate. The significantly better compromise between toughness and yield strength of A1 is attributed to the Fe + Si low content and aging optimization.
  • Figure 3 shows the compromise between tensile yield strength in the transverse direction (LT) and the toughness in T-L, measured at room temperature and at liquid nitrogen temperature of -196°C. It can be observed that A1 plates exhibits a lower drop in toughness than D1 plate. It is attributed to the chemical composition, in particular the lower Fe + Si content.
    • Example 3
  • Different ingots, compositions A, C, G, H, J, K, M were cast and transformed; respective details of composition and process are listed in Tables 14 and 15. All these cast were transformed into plates with thicknesses from 70 to 102 mm.
  • Alloys C, G, H correspond to prior art alloys and J, K and M are compositions according to the invention. Alloy A is similar from example 1.
  • Two types of aging conditions were evaluated corresponding respectively to a total equivalent time at 155°C higher than 24h ("a") according to the invention and below 23h ("b") for comparison. Tensile tests and toughness measurements were performed at room temperature. Results are illustrated in Table 16. Properties of alloy A presented in example 2 are also mentioned in Table 14. [Table 12]
    Alloy Si Fe Cu Mn Mg Zn Ti Zr Si+Fe
    C Ref 0.05 0.06 2.05 1.64 7.08 0.08 0,11
    G Ref 0,03 0,05 2,08 0,01 2,19 6,1 0,02 0,10 0,09
    H Ref 0,02 0,03 2,3 2,17 6,3 0,04 0,10 0,05
    J Inv 0,02 0,04 2,15 1,67 7,2 0,03 0,10 0,06
    K Inv 0,03 0,05 2,14 1,65 7,1 0,03 0,10 0,07
    M Inv 0,03 0,04 2,16 1,65 7,1 0,04 0,10 0,07
    [Table 13]
    Reference A1, J1, C1 K2, G2, H2, M2
    Homogenization 15h-16h 471°C- 482°C
    Hot rolling Entry temperature 416 °C - 440 °C
    Thickness 70-80 mm (See Table 14) 95-102 mm (See Table 14)
    Solution heat Treatment 4h - 4.5 h 479 °C 6h - 479 °C
    Stretch 2-2.25%
    Aging See Table 14 for equivalent time at 155°C
    [Table 14]
    Ref. Thick. Fe+Si Aging total Equiv. Time at 155C UTS (L) TYS (L) E% (L) K1C (L-T)
    (mm) (wt.%) (h) (MPa) (MPa) (%) (MPaVm)
    (t/4) (t/4) (t/4) (t/2)
    A1-a inv 76.2 0.05 27.7 507 467 17.5 64.0
    J1-a 76.2 0.06 27.7 513 466 15 59.6
    K2-a 101.6 0.07 27.7 506 467 17.5 63.9
    G2-a Ref 100 0.09 42.2 514 466 11.08 35.2
    H2-a 100 0.05 42.2 518 469 14.5 39.0
    C1- a 75 0.11 24.4 537 507 11.3 34.6
    C1-b 75 0.11 16.7 549 525 10.3 32.8
    M2-b 101.6 0.07 18.3 538 515 13 36.1
  • Figure 4 to Figure 6 illustrate the data of Table 14.
  • On figures 4 to 6, samples having a composition according to the invention and processed according to the invention are represented with a black diamond (A1-a, J1-A, K2-a). Samples having a composition according to the invention but processed differently are represented by an empty diamond (M2-b). A triangle symbol is used to represent compositions which differ from the invention by Fe+Si content above 0.08 wt. % (C1-a and C1-b). A round circle symbol is used to represent compositions which differ from the invention by main alloying elements (G2-a, H2-a). Triangle or round circle symbols are empty if the process differs from the invention, i.e. equivalent time at 155°C below 23 h or with full symbol if the process is according to the invention.
  • Figure 4 represents the effect of Fe+Si content on K1c (L-T) for products artificially aged with a total equivalent time at 155°C higher than 24h. For thickness range between about 70 mm to about 102 mm, it is observed that for all cases, decreasing the amount of Fe+Si permits to improve the toughness K1c (L-T). However, the trend is significantly more marked on samples A1-a, J1-a, K2-a processed according to the invention, artificially aged with a total equivalent time at 155°C from 24h to 45 hours and with a composition, in particular a Fe+Si content less or equal to 0.08 wt.%.
  • For samples having a composition according to the invention and those having major elements expect Fe, Si and/or (Fe+Si) content according to the invention, and artificially aged according to the invention (samples A1-a, J1-A, K2-A and C1-a), slope Δ1 is representative of the effect of (Fe+Si) on K1c L-T. Sample C1-a, aged with an equivalent time of 24.4h at 155°C exhibits a lower toughness K1c (L-T). The inventors attribute this behavior to its Fe+Si content.
  • For samples artificially aged according to the invention whose composition is outside the invention but for which Fe, Si and/or (Fe+Si) content are according to the invention (H2-a and G2-a) slope Δ2 is representative of the effect of major alloying elements on K1c L-T. The slope Δ2 is lower than slope Δ1. The inventors attribute this behavior to the synergetic effect of composition selection, in particular Zn and Mg content, and Fe+Si content.
  • Figure 5 represents the effect of Fe+Si content on K1c (L-T). Arrows (ΔT1) from M2-b to K2-a) and (ΔT2) from C1-b to C1-a represent respectively the effect of total equivalent time at 155°C at iso Fe+Si content for a composition according to the invention or the prior art. It is well observed that increasing total equivalent time at 155°C of aging permits to increase the toughness. However, this trend is more pronounced for the composition with a Fe+Si content below 0.08 wt. % according to the invention (arrow ΔT1).
  • Samples A1-a, J1-a and K2-a whose thicknesses are between 75 mm and 102 mm present a toughness K1c (L-T) higher than -0.25t+65 MPa.√m, preferably higher than - 0.25t +68 MPa.√m, and even more preferably higher than -0.25t+72 MPa.√m, where t is the thickness plate in mm (figure 6).
  • Samples M2-b, K2-a, H2-a exhibit a lower toughness K1c (L-T) than samples according to the invention despite a Fe+Si content lower than 0.08%. The inventors attribute this behavior to the composition and/or the artificial aging conditions which do not fulfill a total equivalent time at 155°C between 24h and 45h.
    • Example 4
  • Fatigue crack growth rate of B3 plate mentioned in example 1 was evaluated and compared with a reference product O3 of similar thickness 152.4 mm (6 inch). Alloys O and B are according to the invention (Table 15) with a Fe+Si content below 0.08 wt. %. O3 plate is processed similarly to B3 plate, except the artificially aging conditions which gives an equivalent aging time at 155°C of 19.9h (Table 16). [Table 15]
    (% wt) Si Fe Cu Mn Mg Zn Zr Ti Si+Fe
    O Inv 0.03 0.05 2.2 0.0 1.7 7.1 0.10 0.03 0.07
    B Inv 0.02 0.04 2.1 0.0 1.6 7.0 0.10 0.03 0.06
    [Table 16]
    Reference Invention
    O3 B3
    Alloy O Alloy B
    Homogenization 16h - 471°C- 482°C
    Hot rolling: Entry hot Rolling temperature 405-415°C 409°C
    Final thickness (mm) 152.4 152.4
    Solution heat Treatment 7h at 479°C 7h 479°C
    Stretch
    2,25%
    Aging First step aging 4h 121°C
    Ramp 5°C/h 6°C/h
    Second step Aging 18.7 h 154°C 12h 163 °C
    Total Equivalent Time at 155C 19.9 h 27.7 h
  • Fatigue crack growth rate was evaluated based on ASTM E647. The coupons orientation is L-T. The standard Compact Tension, i.e. C(T), coupon dimension was used for test. The dimension B is 7.6 mm (0.3 inch) and W is 50.8 mm (2 inch) for all test coupons. The FCGR testing procedure was according to ASTM E647 in general with the following specific requirements: (1) stress ratio 0.1 and f=10 Hz; (2) Pre-cracking is conducted under constant load amplitude, and the starting ΔK reaches the ΔKi=10 MPa∗√m values at the end of pre-cracking. After pre-cracking, the testing is conducted under constant load amplitude at the same load as pre-cracking. The test was conducted at room temperature.
  • Two relative humidity (RH) conditions were evaluated for both plates: one under normal ambient air humidity condition (27-32% humidity, designated "standard air") and the other under Humid air conditions (92-93% humidity, designated "humid air").
  • Figure 7 and Figure 8 disclose respectively the fatigue crack growth rate evolution of plate O3 and B3 versus environment conditions.
  • It is observed that a product with a composition chosen according to the invention aged with an equivalent time at 155°C higher than 24h (B3, Figure 8) exhibits a similar fatigue crack growth rate whatever relative humidity considered. The product aged for an equivalent time at 155°C below 23h (O3, Figure 7) shows a significant dependence on humidity.

Claims (15)

  1. - A process for the manufacture of a wrought 7xxx aluminum-based alloy product comprising the steps of:
    a) preparing a bath of molten alloy metal comprising in weight-% (wt. %)
    Zn 6.65 - 7.45
    Mg 1.35 - 1.75
    Cu 1.85 - 2.35
    Zr 0.04 - 0.14
    Mn 0-0.5
    Ti 0 - 0.15, preferably from 0.02 to 0.06 wt. %,
    V 0 -0.15
    Cr 0 - 0.25
    Fe ≤0.05
    Si ≤0.05
    Fe + Si ≤ 0.08,
    Other impurities <0.05 each and <0.15 total, remainder aluminum.
    b) casting said molten alloy metal to obtain a slab or a billet
    c) homogenizing said ingot or billet to obtain a homogenized ingot or billet,
    d) hot working said homogenized slab or billet to obtain a wrought product, such as an extruded, rolled and/or forged product, with a final thickness of at least 25 mm;
    e) solution heat treating and quenching said wrought product to obtain a quenched wrought product;
    f) stress relieving the quenched wrought product to obtain a stress relieved wrought product;
    g) artificial aging said stress relieved wrought product wherein the total equivalent aging time t(eq) at 155°C is comprised from 24 hours to 45 hours,
    the total equivalent time t(eq) at 155°C being defined by the formula: t eq = ∫ exp − 16000 / T dt exp − 16000 / T ref
    Figure imgb0003
    where T is the instantaneous temperature in Kelvin during aging and Tref is a reference temperature selected at 155 °C (428 K), t(eq) is expressed in hours.
  2. - A process for the manufacture of a wrought 7xxx aluminum-based alloy product according to claim 1 wherein Si ≤0.03 wt. % and Fe ≤ 0.05 wt. %.
  3. - A process for the manufacture of a wrought 7xxx aluminum-based alloy product according to any of claim 1 to 2 wherein Fe+Si content is ≤ 0.07 wt. %, preferably ≤ 0.06 wt. %.
  4. - A process for the manufacture of a wrought 7xxx aluminum-based alloy product according to any of claims 1 to 3 wherein Fe+Si content is ≥ 0.03 wt. %, preferably ≥ 0.04 wt. %.
  5. - A process for the manufacture of a wrought 7xxx aluminum-based alloy product according to any of claims 1 to 4 wherein the total equivalent aging time t(eq) at 155°C is comprised from 24 hours to 34 hours, preferably from 26 hours to 30 hours.
  6. - A rolled product with a thickness t in millimeter of at least 25 mm, preferentially from 25 mm to 200 mm, comprising (in weight-%)
    Zn 6.65 - 7.45
    Mg 1.35 -1.75
    Cu 1.85-2.35
    Zr 0.04 - 0.14
    Mn 0-0.5
    Ti 0 - 0.15
    V 0 - 0.15
    Cr 0 - 0.25
    Fe ≤0.05
    Si≤0.05
    Fe + Si ≤ 0.08
    Other impurities < 0.05 each and <0.15 total, remainder aluminum,
    and wherein the toughness K1c(L-T) in MPa.Vm, at room temperature according to ASTM standard E399-2020 is higher than -0.25*t +65 MPa.Vm , preferably higher than - 0.25∗t+68 MPa.Vm , even more preferably -0.25 *t +72 MPa.Vm, where t is the thickness of the rolled product in mm.
  7. - A rolled product according to claim 6 wherein the toughness K1c(T-L) in MPa.√m, at cryogenic temperature, measured according to ASTM standard E399 2020), is reduced by less than 10 %, preferably less than 8%, more preferably less than 7 % in comparison with K1c(T-L) in MPa.√m, measured at room temperature according to ASTM standard E399 -2020.
  8. - A rolled product according to claims 6 or 7 wherein the toughness K1c(T-L) in MPa.√m, at cryogenic temperature, measured according to ASTM standard E399 -2020 is higher than -15*t +45 MPa.Vm , preferably higher than - 0.15*t+49 MPa.Vm, even more preferably higher than -0.15*t+55 MPa, where t is the thickness of the rolled product in mm.
  9. - A rolled product according to any of claims 6 to 8 with a thickness from 70 mm to 160 mm, preferably from 70 mm to 102 mm.
  10. - A rolled product according to any of claims 6 to 9 wherein Si ≤0.03 wt. % and Fe ≤ 0.05 wt. %.
  11. - A rolled product according to any of claims 6 to 10 wherein Fe+Si content is from 0.03 wt. % to 0.07 wt. %, preferably from 0.03 wt. % to 0.06 wt. %.
  12. - A rolled product according to any of claims 6 to 11 wherein Zn content is from 6.90 wt. % to 7.30 wt. %, more preferably from 7.10 to 7.25 wt.%.
  13. - A rolled product according to any of claims 6 to 12 wherein Ti content is ≤ 0.06%, preferably from 0.02 to 0.06 wt. %, even more preferably from 0.03 to 0.05 wt. %.
  14. - A structural member suitable for the construction of aircraft comprising a product according to any of claims 6 to 13 or obtainable by the process according to any of claims 1 to 5.
  15. - A structural member according to claim 14 wherein said structural member is used for components subjected to cryogenic temperatures, in particular cryogenic tank.
EP22213912.3A 2022-12-12 2022-12-15 7xxx wrought products with improved compromise of tensile and toughness properties and method for producing Pending EP4386097A1 (en)

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