METHOD FOR MANUFACTURING AN ALUMINUM-COPPER-LITHIUM ALLOY HAVING IMPROVED COMPRESSIVE STRENGTH AND IMPROVED TOUGHNESS

Abstract

The invention relates to a manufacturing method in which an alloy is prepared that comprises 3.5 to 4.7 wt % of Cu; 0.6 to 1.2 wt % of Li; 0.2 to 0.8 wt % of Mg; 0.1 to 0.2 wt % of Zr; 0.0 to 0.3 wt % of Ag; 0.0 to 0.8 wt % of Zn; 0.0 to 0.5 wt % of Mn; at most 0.20 wt % of Fe+Si; optionally an element selected from Cr, Sc, Hf and V, the amount of said element, if selected, being from 0.05 to 0.3 wt % for Cr and for Sc, 0.05 to 0.5 wt % for Hf and for V; the other elements being at most 0.05 wt % each and 0.15 wt % in total, a refiner is introduced, the alloy is cast in a crude form, homogenized, hot-worked, solution heat-treated, quenched, cold-worked, and tempered, in which the refiner contains particles of TiC and/or the cold working is between 8 and 16%. The products obtained by the method according to the invention have an advantageous compromise between mechanical strength and toughness.

Description

FIELD OF THE INVENTION

The invention relates to a method for manufacturing products made of aluminum-copper-lithium alloys, in particular, such products intended for aeronautical and aerospace construction.

PRIOR ART

Aluminum alloy products are developed to produce high strength parts intended in particular for the aircraft industry and the aerospace industry.

Aluminum alloys containing lithium are of great interest in this regard, as lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each weight percent lithium added. For these alloys to be selected in aircrafts, their performance in relation to other properties of use must reach that of commonly used alloys, in particular in terms of compromise between the properties of static mechanical strength (tensile and compressive yield strength, ultimate tensile strength) and damage tolerance properties (toughness, resistance to the fatigue crack propagation), these properties being generally mutually exclusive. For some parts such as the upper wing skin, the compressive yield strength as well as the toughness in plane stress are essential properties. These mechanical properties should moreover preferably be stable over time and have good thermal stability, that is to say not be significantly modified by aging at operating temperature.

These alloys must also have sufficient corrosion resistance, be able to be shaped according to the usual methods and have low residual stresses so that they can be fully machined.

Finally, they must be able to be obtained by robust manufacturing methods, in particular, the properties must be able to be obtained on industrial tools for which it is difficult to guarantee temperature homogeneity within a few degrees for large parts.

U.S. Pat. No. 5,032,359 describes a large family of aluminum-copper-lithium alloys wherein the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, allows to increase the mechanical strength.

U.S. Pat. No. 5,455,003 describes a method for manufacturing Al—Cu—Li alloys which have improved mechanical strength and improved toughness at cryogenic temperature, in particular thanks to suitable working hardening and ageing. This patent recommends in particular the composition, in percentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6 and Zn=0-0.75.

U.S. Pat. No. 7,438,772 describes alloys comprising, in weight percentage, Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourages the use of higher lithium content due to degradation of the compromise between toughness and mechanical strength.

U.S. Pat. No. 7,229,509 describes an alloy comprising (wt %): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain refiner agents such as Cr, Ti, Hf, Sc, V.

Patent application US 2009/142222 A1 describes alloys comprising (in wt %), 3.4 to 4.2% of Cu, 0.9 to 1.4% of Li, 0.3 to 0.7% of Ag, 0.1 to 0.6% of Mg, 0.2 to 0.8% of Zn, 0.1 to 0.6% of Mn and 0.01 to 0.6% of at least one element for controlling the granular structure. This application also describes a method for manufacturing extruded products.

Patent application WO2009/036953 relates to an aluminum alloy product for structural elements having a chemical composition comprising, by weight Cu from 3.4 to 5.0, Li from 0.9 to 1.7, Mg from 0.2 to 0.8, Ag from about 0.1 to 0.8, Mn from 0.1 to 0.9, Zn up to 1.5, and one or more elements selected from the group consisting of: (Zr about 0.05 to 0.3, Cr 0.05 to 0.3, Ti about 0.03 to 0.3, Sc about 0.05 to 0.4, Hf about 0.05 to 0.4), Fe<0.15, Si<0.5, normal and unavoidable impurities.

Patent application WO 2012/085359 A2 relates to a method for manufacturing rolled products made of an aluminum-based alloy comprising 4.2 to 4.6 wt % of Cu, 0.8 to 1.30 wt % of Li, 0.3 to 0.8 wt % of Mg, 0.05 to 0.18 wt % of Zr, 0.05 to 0.4 wt % of Ag, 0.0 to 0.5 wt % of Mn, at most 0.20 wt % of Fe+Si, less than 0.20 wt % of Zn, at least one element selected from Cr, Se, Hf and Ti, the amount of said element, if selected, being 0.05 to 0.3 wt % for Cr and for Se, 0.05 to 0.5 wt % for Hf and from 0.01 to 0.15 wt % for Ti, the other elements at most 0.05 wt % each and 0.15 wt % in total, the remainder being aluminum, comprising the steps of preparation, casting, homogenization, rolling with a temperature greater than 400° C., solution heat-treating, quenching, tensioning between 2 and 3.5% and ageing.

Patent application US2012/0225271 A1 relates to wrought products with a thickness of at least 12.7 mm containing from 3.00 to 3.80 wt % of Cu, from 0.05 to 0.35 wt % of Mg, from 0.975 to 1.385 wt % of Li, wherein −0.3 Mg−0.15Cu+1.65≤Li≤−0.3 Mg−0.15Cu+1.85, from 0.05 to 0.50 wt % of at least one grain structure control element, wherein the grain structure control element is selected from the group consisting of Zr, Sc, Cr, V, Hf, other rare earth elements, and combinations thereof, up to 1.0 wt % of Zn, up to 1.0 wt % of Mn, up to 0.12 wt % of Si, up to 0.15 wt % of Fe, up to 0.15 wt % of Ti, up to 0.10 wt % of other elements with a total not exceeding 0.35 wt %.

Application WO 2013/169901 describes alloys comprising, in percentage by weight, 3.5 to 4.4% of Cu, 0.65 to 1.15% of Li, 0.1 to 1.0% of Ag, 0.45 to 0.75% of Mg, 0.45 to 0.75% of Zn and 0.05 to 0.50% of at least one element for the control of granular structure. The alloys advantageously have a Zn to Mg ratio comprised between 0.60 and 1.67.

There is a need for aluminum-copper-lithium alloy products having ever more improved properties compared to those of known products, in particular in terms of compromise between the properties of static mechanical strength, in particular the tensile and compressive yield strength and the properties of damage tolerance, in particular toughness, thermal stability, corrosion resistance and machinability, while having a low density.

In addition, there is a need for a method for manufacturing these products that is robust, reliable and economical.

Object of the Invention

A first object of the invention is a method for manufacturing a product based on an aluminum alloy wherein, successively,

• • a) a liquid metal bath based on aluminum is prepared comprising 3.5 to 4.7 wt % of Cu; 0.6 to 1.2 wt % of Li; 0.2 to 0.8 wt % of Mg; 0.1 to 0.2 wt % of Zr; 0.0 to 0.3 wt % of Ag; 0.0 to 0.8 wt % of Zn; 0.0 to 0.5 wt % of Mn; at most 0.20 wt % of Fe+Si; optionally an element selected from Cr, Sc, Hf and V, the amount of said element, if selected, being from 0.05 to 0.3 wt % for Cr and for Sc, 0.05 to 0.5 wt % for Hf and for V; other elements at most 0.05 wt % each and 0.15 wt % in total and the remainder being aluminum;
  • b) a refiner is introduced into said bath so that the Ti content is comprised between 0.01 to 0.15 wt %;
  • c) a crude form is cast from said liquid metal bath;
  • d) said crude form is homogenized at a temperature comprised between 450° C. and 550° C. and preferably between 480° C. and 530° C. for a period comprised between 5 and 60 hours;
  • e) said homogenized crude form is hot-worked, preferably by rolling;
  • f) the hot-worked product is solution heat-treated between 490° C. and 530° C. for 15 min to 8 hours and said solution heat-treated product is quenched;
  • g) said product is cold-worked with a cold working of 2 to 16%;
  • h) a ageing is carried out wherein said product thus cold-worked reaches a temperature comprised between 130° C. and 170° C. and preferably between 140° C. and 160° C. for 5 to 100 hours and preferably 10 to 70 hours;
    wherein said refiner contains particles of the TiC type and/or said cold working is of 8 to 16%.

Another object of the invention is a product that can be obtained by the method according to the invention and such that it is a rolled product with a thickness comprised between 8 and 50 mm and having, at mid-thickness,

K_app(L−T)≥−0.5Rc_p0.2(L)+375,

preferably K_app(L−T)≥−0.5Rc_p0.2(L)+386

• • with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and
  • Rc_p0.2(L), expressed in MPa, the compressive yield strength measured at 0.2% compression according to standard ASTM E9 (2018).

Yet another object is an aircraft structure member, preferably an aircraft upper wing skin element, comprising a product according to the invention.

DESCRIPTION OF THE FIGURES

FIG. 1: Compromise between the toughness K_app L−T and the compressive yield strength Rc_p0.2 L of the alloys of Example 1.

FIG. 2: Graph showing the difference between the value of K_app (L−T) measured according to the alloys of example 1 and the value calculated according to the formula −0.5 R_cp0.2(L)+386 as a function of the conventional yield strength R_p0.2 measured in the longitudinal direction of the product.

FIG. 3: Compromise between the toughness Kapp L−T and the compressive yield strength Rc_p0.2 L of the alloys of Example 2.

FIG. 4: Graph showing the difference between the value of K_app (LT) measured according to the alloys of example 2 and the value calculated according to the formula −0.5 R_cp0.2 (L)+375 as a function of the conventional yield strength R_p0.2 measured in the longitudinal direction of the product.

DESCRIPTION OF THE INVENTION

Unless otherwise indicated, all indications relating to the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed in wt % is multiplied by 1.4. The designation of the alloys is made in accordance with the regulations of The Aluminum Association, known to the person skilled in the art. When the concentration is expressed in ppm (parts per million), this indication also refers to a mass concentration.

Unless otherwise indicated, the definitions of metallurgical states given in European standard EN 515 (1993) apply.

The tensile static mechanical features, in other words the ultimate tensile strength R_m, the conventional yield strength at 0.2% elongation R_p0.2, and the elongation at rupture A %, are determined by a tensile test according to standard NF EN ISO 6892-1 (2016), the sampling and direction of the test being defined by standard EN 485 (2016). R_p0.2 (L) means R_p0.2 measured in the longitudinal direction.

The compressive yield strength Rc_p0.2 was measured at 0.2% compression according to standard ASTM E9-09 (2018). Rc_p0.2 (L) means Rc_p0.2 measured in the longitudinal direction. The stress intensity factor (K_1C) is determined according to standard ASTM E 399 (2012). The standard ASTM E 399 (2012) gives the criteria that allow determining whether K_Q is a valid value of K_1C. For a given test specimen geometry, the values of K_Q obtained for different materials are comparable with each other provided that the yield strengths of the materials are of the same order of magnitude.

Unless otherwise indicated, the definitions of standard EN 12258 (2012) apply.

The values of the apparent stress intensity factor at rupture (K_app) and the stress intensity factor at rupture (K_c) are as defined in standard ASTM E561.

A curve giving the effective stress intensity factor as a function of the effective crack extension, known as the curve R, is determined according to standard ASTM E 561 (ASTM E 561-10-2).

The critical stress intensity factor K_C, in other words the intensity factor which makes the crack unstable, is calculated from the curve R. The stress intensity factor K_CO is also calculated by assigning the length of the initial crack at the beginning of the monotonic load, to the critical load. These two values are calculated for a test specimen of the required shape. K_app represents the factor K_CO corresponding to the test specimen which was used to perform the test of curve R. K_eff represents the factor K_C corresponding to the test specimen which was used to perform the test of curve R.

A mechanical part for which the static and/or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural calculation is usually required or performed is here called “structure element” or “structural element” of a mechanical construction. These are typically elements the failure of which is likely to endanger the safety of said construction, its users, its customers or others. For an airplane, these structure elements comprise in particular the elements that compose the fuselage (such as the fuselage skin), the stiffeners or stringers of the fuselage, the watertight bulkheads, the circumferential frames of the fuselage, the wings (such as the upper or lower wing skin), the stiffeners (or stringers), the ribs and spars and the empennage in particular composed of horizontal and vertical stabilizers, as well as floor beams, seat tracks and doors.

According to the present invention, a selected class of aluminum alloys containing in particular specific and critical amounts of lithium, copper, magnesium, and zirconium allows to prepare, under certain processing conditions, products, in particular rolled products, having an improved compromise between toughness, tensile and compressive yield strength. The present inventors have observed that, surprisingly, it is possible to improve, for the products produced from these alloys, the properties of use, in particular those making the products suitable for the production of structure elements in the aeronautical and aerospace fields. In particular, the products according to the invention are particularly well adapted to the production of aircraft upper wing skin elements since they have a particularly improved compromise compressive yield strength Rc_p0.2 (L)−toughness Kapp (L−T).

The invention relates in particular to a method wherein an alloy is prepared comprising 3.5 to 4.7 wt % of Cu; 0.6 to 1.2 wt % of Li; 0.2 to 0.8 wt % of Mg; 0.1 to 0.2 wt % of Zr; 0.0 to 0.3 wt % of Ag; 0.0 to 0.8 wt % of Zn; 0.0 to 0.5 wt % of Mn; at most 0.20 wt % of Fe+Si; optionally an element selected from Cr, Sc, Hf and V; the amount of said element, if selected, being 0.05 to 0.3 wt % for Cr and for Sc, 0.05 to 0.5 wt % for Hf and for V; other elements at most 0.05 wt % each and 0.15 wt % in total, a refiner is introduced, the alloy is cast in a crude form, homogenized, hot-worked, solution heat-treated, quenched, cold-worked and tempered, wherein the refiner contains particles of TiC and/or the cold working is comprised between 8 to 16%.

The copper content of the products according to the invention is comprised between 3.5 and 4.7 wt %, preferably between 4.0 and 4.6 wt %. In a particularly advantageous embodiment, the copper content is comprised between 4.1 and 4.5 wt %, preferably between 4.2 and 4.4 wt %. The increase in the copper content contributes to an improvement in the tensile and compressive yield strength. However, copper, in an excessively high quantity, induces a decrease in the toughness in plane stress Kapp.

The lithium content of the products according to the invention is comprised between 0.7 to 1.2 wt %. Advantageously, the lithium content is comprised between 0.8 and 1.0 wt %; preferably between 0.85 and 0.95 wt %. The increase in the lithium content has a favorable effect on the density, however the present inventors have observed that for the alloys according to the invention, the selected lithium content allows an improvement in the compromise between mechanical strength, in particular the tensile and compressive yield strength, and toughness. A very high lithium content can lead to a degradation of the toughness.

The magnesium content of the products according to the invention is comprised between 0.2% and 0.8 wt %. Preferably, the magnesium content is at least 0.3% or even 0.4% or 0.5 wt %, which simultaneously improves static mechanical strength and toughness. Preferably, the magnesium content is less than 0.7 wt % or even 0.65 wt %. Indeed, a high magnesium content can induce a degradation of the toughness.

The alloy may contain zinc up to 0.8 wt %. In an advantageous embodiment, the Zn content is comprised between 0.05 and 0.6 wt %, preferably 0.2 and 0.5 wt % and, more preferably still, between 0.30 and 0.40 wt %. In another embodiment, the alloy contains less than 0.05 wt % of Zn, preferably less than 0.02 wt %.

The alloy may also contain up to 0.3 wt % of silver. In one embodiment, the alloy comprises more than 0.05 wt %, preferably more than 0.1% and more preferably still from 0.2 to 0.3 wt % of Ag. In one embodiment the maximum Ag content is 0.27 wt %.

The presence of zinc and/or silver allows to obtain a compressive yield strength having a value close to that of the tensile yield strength. In one embodiment, the Ag content is of 0.1 to 0.27 wt % and/or the Zn content is of 0.2 to 0.40 wt %. The alloy may also contain up to 0.5 wt % of manganese. Advantageously, the manganese content is comprised between 0.05 and 0.4 wt %. In one embodiment the manganese content is comprised between 0.2 and 0.37 wt % and preferably between 0.25 and 0.35 wt %. In another embodiment the manganese content is comprised between 0.1 and 0.2 wt % and preferably between 0.10 and 0.20 wt %. In particular, the addition of Mn allows to obtain high toughness. However, if the Mn content is too high, the fatigue life can be significantly reduced.

The Zr content of the alloy is comprised between 0.1 and 0.2 wt %. In an advantageous embodiment, the Zr content is comprised between 0.10 and 0.15 wt %, preferably between 0.11 and 0.14 wt %.

The alloy also contains titanium, the Ti content is comprised between 0.01 and 0.15 wt %, preferably between 0.02 and 0.08 wt %. In one embodiment, the refiner introduced into the aluminum alloy bath contains particles of the TiC type. Advantageously the refiner has the formula AlTi_xC_y which is also written ATxCy where x and y are the contents of Ti and C in wt % for 1 wt % of Al, and x/y>4. Against all expectations, the present inventors have observed that, in the particular case of the present alloy, the presence in the refiner and therefore in the alloy of particles of TiC at the origin of a particular refining of the alloy during casting (AlTiC refining), allows to obtain a product having an optimized compromise of properties. In particular, the presence of particles of TiC in the grain refining rod and in the alloy of an embodiment of the method of the present invention allows an improvement in the compromise between the toughness K_app L−T and the compressive yield strength R_cp0.2 L.

The sum of the iron content and the silicon content is at most 0.20 wt %. Preferably, the iron and silicon contents are each at most 0.08 wt %. In an advantageous embodiment of the invention the iron and silicon contents are at most 0.06% and 0.04 wt %, respectively. A controlled and limited iron and silicon content helps improve the compromise between mechanical strength and damage tolerance.

The alloy may also contain at least one element which can contribute to the control of the grain size selected from Cr, Sc, Hf and V, the amount of said element, if selected, being from 0.05 to 0.3 wt % for Cr and for Sc and 0.05 to 0.5 wt % for Hf and for V.

The content of the alloy elements can be selected to minimize the density. Preferably, the additive elements contributing to increase the density such as Cu, Zn, Mn and Ag are minimized and the elements contributing to decrease the density such as Li and Mg are maximized so as to achieve a density less than or equal to 2.73 g/cm³ and preferably less than or equal to 2.72 g/cm³.

The content of the other elements is at most 0.05 wt % each and 0.15 wt % in total. The other elements are typically unavoidable impurities.

The method for manufacturing products according to the invention comprises the steps of preparation, casting, introducing a refiner, homogenization, hot working, solution heat-treating and quenching, cold working and ageing.

In a first step, a liquid metal bath is prepared so as to obtain an aluminum alloy of a composition according to the invention. A refiner is then introduced into said bath so that the Ti content is comprised between 0.01% to 0.15 wt %, optionally the refiner contains particles of the TiC type. Advantageously, the Ti content is comprised between 0.02 and 0.08 wt %, preferably between 0.03 and 0.06 wt %. In one embodiment the refiner contains particles of the TiC type. Advantageously, the refiner containing particles of the TiC type is introduced in a form and an amount such that an amount of TiC identical to that added with a refiner AT3C0.15 at a rate of 2 to 5 kg/t of aluminum alloy is added. Preferably, the refiner containing particles of the TiC type is introduced in the form of AT3C0.15 at a rate of 2 to 5 kg/t of aluminum alloy.

The liquid metal bath is then cast in the form of crude form, preferably in the shape of an ingot for rolling.

The crude form is then homogenized so as to reach a temperature comprised between 450° C. and 550° and preferably between 480° C. and 530° C. for a period comprised between 5 and 60 hours. The homogenization treatment can be carried out in one or more stages.

After homogenization, the crude form is generally cooled to room temperature before being preheated in order to be hot-worked. The hot working can in particular be an extrusion or a hot rolling. Preferably, this is a hot rolling step. The hot rolling is carried out to a thickness preferably comprised between 8 and 50 mm and in a preferred manner between 15 and 40 mm.

The product thus obtained is then solution heat-treated to reach a temperature comprised between 490 and 530° C. for 15 min to 8 h, then quenched typically with water at room temperature.

The product then undergoes cold working with a cold working of 2 to 16%. In one embodiment the cold working is a controlled tensioning with a permanent set of 2 to 6%, preferably from 2.0% to 4.0%. In one embodiment said product is cold-worked with a cold working rate comprised between 8 to 16%. In one embodiment, the cold working is carried out in two steps: the product is first of all cold rolled with a thickness reduction rate comprised between 8 and 12%, preferably 9 and 11%, then subsequently tensioned in a controlled manner with a permanent set comprised between 0.5 and 4%, preferably between 0.5 and 2%.

The product is then subjected to an ageing step carried out by heating at a temperature comprised between 130 and 170° C. and preferably between 140 and 160° C. for 5 to 100 hours and preferably 10 to 70 hours. In a particularly advantageous embodiment, the ageing is carried out at a temperature comprised between 140 and 155° C., preferably between 145 and 150° C., preferably for 18 to 22 hours.

The present inventors have observed that, surprisingly, the method according to the invention allows to obtain an advantageous product. Thus the specific and critical contents of the alloy of the present invention associated with a particular manufacturing method allow to achieve excellent properties. In particular, the product according to the invention is advantageously a rolled product with a thickness comprised between 8 and 50 mm and having, at mid-thickness,

K_app(L−T)≥−0.5Rc_p0.2(L)+375,

preferably K_app(L−T)≥−0.5Rc_p0.2(L)+386

• • even more preferably K_app(L−T)≥−0.5 Rc_p0.2(L)+391,
  • with K_app (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and
  • Rc_p0.2(L) expressed in MPa, the compressive yield strength measured at 0.2% compression according to standard ASTM E9 (2018).

Advantageously, the product according to the invention is a rolled product with a thickness comprised between 8 and 50 mm and having, at mid-thickness,

K_app(L−T)≥−0.5R_cp0.2(L)+375

and a yield strength value R_p0.2(L) of at least 580 MPa, preferably 600 MPa, even more preferably 615 MPa,

• • with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and
  • Rc_p0.2(L) expressed in MPa, the compressive yield strength measured at 0.2% compression according to standard ASTM E9 (2018),
  • and R_p0.2(L) the conventional yield strength at 0.2% elongation measured in the longitudinal direction of the product, determined by a tensile test according to standard NF EN ISO 6892-1 (2016).

The inventors have in particular observed that, surprisingly, the combination of the introduction into the liquid metal bath of a refiner containing particles of the TiC type so that the Ti content is comprised between 0.01 to 0.15 wt % and a cold working after solution heat-treating with a cold working rate comprised between 8 to 16% is advantageous. In particular, this combination allows to obtain a rolled product with a thickness comprised between 8 and 50 mm, at mid-thickness

K_app(L−T)≥−0.5Rc_p0.2(L)+386,

preferably K_app(L−T)≥−0.5Rc_p0.2(L)+391

• • with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and
  • Rc_p0.2(L) expressed in MPa, the compressive yield strength measured at 0.2% compression according to ASTM E9 (2018).

Advantageously, the combination comprising the introduction into the liquid metal bath of a refiner containing particles of TiC type so that the Ti content is comprised between 0.01 to 0.15 wt % and a cold working after solution heat-treating with a cold working rate comprised between 8 to 16% allows to obtain, for a rolled product with a thickness comprised between 8 and 50 mm, at mid-thickness,

K_app(L−T)≥−0.5Rc_p0.2(L)+386,

preferably K_app(L−T)≥−0.5Rc_p0.2(L)+391

and a yield strength value R_p0.2(L) of at least 600 MPa, even more preferably of at least 615 MPa,

• • with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and
  • Rc_p0.2 (L) expressed in MPa, the compressive yield strength measured at 0.2% compression according to ASTM E9 (2018), and
  • R_p0.2 (L) the conventional yield strength at 0.2% elongation measured in the longitudinal direction of the product, determined by a tensile test according to standard NF EN ISO 6892-1 (2016).

The alloy products according to the invention allow in particular the manufacture of structure elements, in particular aircraft structure elements. In an advantageous embodiment, the preferred aircraft structure element is an aircraft upper wing skin element.

These and other aspects of the invention are explained in more detail using the following illustrative and non-limiting examples.

EXAMPLES

Example 1

In this example, two plates with a thickness of 406 mm for each of the alloys, the composition of which is given in Table 1, were cast. Alloy 1 was refined using 2.7 kg/t AT3B. Alloy 2 was refined using 4 kg/t AT3C0.15.

TABLE 1
Composition in wt % of alloys 1 and 2
Alloy	Si	Fe	Cu	Mn	Mg	Zn	Ti	Zr	Li	Ag
1	0.02	0.03	4.3	0.31	0.60	0.35	0.03	0.12	0.91	0.24
2	0.02	0.04	4.3	0.14	0.61	0.36	0.05	0.13	0.88	0.25

The plates were homogenized at about 510° C. The homogenized plates were hot rolled at an input temperature of about 450° C. and an output temperature of about 390° C. to obtain for each alloy sheets of thickness 28 mm The sheets were solution heat-treated at about 510° C. for 3 h, quenched with water at 20° C. One sheet of each alloy 1 and 2 was then cold rolled with a thickness reduction rate of 10% (“LAF 10%” condition) followed by tensioning with a permanent elongation of about 1%. For each alloy another sheet was also tensioned with a permanent set of 3% without prior cold rolling.

The sheets underwent a single-stage ageing as indicated in Table 2. Samples were taken at mid-thickness to measure the static mechanical features in tension and in compression as well as the toughness K_Q. The test specimens used for the toughness measurement had a width W=40 mm and a thickness B=20 mm. The measurements taken were valid according to the ASTM E399 standard. The toughness in plane stress was also measured at mid-thickness during tests of curve R with CCT test specimens 406 mm wide and 6.35 mm thick. The results are shown in Table 2 and FIG. 1.

FIG. 2 shows the difference between the measured value of Kapp (L−T) and the value calculated according to the formula “−0.5 R_cp0.2(L)+386” as a function of the conventional yield strength R_p0.2(L) measured in the longitudinal direction L of the product.

TABLE 2
Ageing conditions and mechanical properties obtained for the different sheets.
								Kq
								L-T,
					Rp0.2	Rcp0.2	Kapp	mean
			Cold		(L)	(L)	L-T	value,
Alloy	Refiner	Ref	working	Ageing	(MPa)	(MPa)	(MPa✓m)	(MPa✓m)
1	AT3B	1-A	Tension 3	15 h 155° C.	598	593	76	30.63
				20 h 155° C.	599	599	69	29.93
				25 h 155° C.	604	613	63	27.61
1	AT3B	1-B	LAF 10% +	15 h 147° C.	610	602	78
			Tension 1%	20 h 147° C.	629	620	75	25.85
				25 h 147° C.	632	622	69	25.96
2	AT3C0.15	2-C	Tension 3	20 h 155° C.	598	599	84	28.71
				25 h 155° C.	603	603	80	26.81
				30 h 155° C.	613	606	78	26.87
2	AT3C0.15	2-D	LAF 10 +	16.5 h 147° C.	620	605	90
			Tension 1%	18 h 147° C.		611	88	29.79
				20 h 147° C.	628	615	93.5	26.99
				22 h 147° C.		619	86	26.25

Example 2

Plates with a section of 406×1520 mm, the composition of which is given in Table 3, were cast. The refiner used was AT3B.

TABLE 3
Composition in wt % of alloys 3, 4 and 5
Alloy	Si	Fe	Cu	Mn	Mg	Zn	Ti	Zr	Li	Ag
3	0.05	0.05	4.5	0.37	0.35	0.02	0.03	0.11	1.02	0.21
4	0.03	0.05	4.5	0.34	0.71	0.04	0.04	0.11	1.02	0.21
5	0.03	0.04	4.6	0.35	0.23	0.04	0.02	0.14	1.05	0.22

The plates were homogenized at about 510° C. After homogenization, the plates were hot rolled to obtain sheets having a thickness of 25 mm. The sheets were solution heat-treated for 5 hours at about 510° C., quenched in cold water. One plate of each alloy was cold rolled with a thickness reduction rate of 10% (“LAF 10%” condition), followed by tensioning with a permanent elongation of about 1.2%. Another plate of each alloy was tensioned with a permanent elongation without prior cold rolling. The values of the permanent elongations are shown in Table 4.

The sheets then underwent an ageing comprised between 10 hours and 25 hours at 155° C. as indicated in Table 2. Samples were taken at mid-thickness to measure the tensile, compressive static mechanical features as well as the toughness in plane stress K_app (L−T). The test specimens used for the toughness measurement are CCTs with a width W=406 mm and a thickness B=6.35 mm. The results obtained are presented in Table 4 and FIG. 3. FIG. 4 shows the difference between the measured value of Kapp (L−T) and the value calculated according to the formula −0.5 Rcp0.2(L)+375 as a function of the conventional yield strength Rp0.2 measured in the longitudinal direction L of the product.

TABLE 4
Ageing conditions and mechanical properties
obtained for sheets made of alloy 3, 4 and 5.
			Final			Rp0.2	Rcp0.2	Kapp
			thickness	Cold		(L)	(L)	L-T
Alloy	Refiner	Ref.	(mm)	working	Ageing	(MPa)	(MPa)	(MPa✓m)
3	AT3B	3-A	25	4% tension	22 h	631	630	56
					155° C.
		3-B	22.5	10% LAF +	10 h	617	633	61
				1.2% tension	155° C.
4	AT3B	4-A	25	3.1% tension	25 h	637	643	48
					155° C.
		4-B	22.5	10% LAF +	10 h	668	642	57
				1.2% tension	155° C.
5	AT3B	5-A	25	3.1% tension	25 h	638	651	41
					155° C.
		5-B	22.5	10% LAF +	10 h	657	648	52
				1.2% tension	155° C.

Claims

1. A method for manufacturing a product based on an aluminum alloy wherein, successively,

a) a liquid metal bath based on aluminum is prepared comprising 3.5 to 4.7 wt % of Cu; 0.6 to 1.2 wt % of Li; 0.2 to 0.8 wt % of Mg; 0.1 to 0.2 wt % of Zr; 0.0 to 0.3 wt % of Ag; 0.0 to 0.8 wt % of Zn; 0.0 to 0.5 wt % of Mn; at most 0.20 wt % of Fe+Si; optionally an element selected from Cr, Sc, Hf and V, the amount of said element, if selected, being from 0.05 to 0.3 wt % for Cr and for Sc, 0.05 to 0.5 wt % for Hf and for V; other elements at most 0.05 wt % each and 0.15 wt % in total and the remainder being aluminum;

b) a refiner is introduced into said bath so that the Ti content is comprised between 0.01 to 0.15 wt %;

c) a crude form is cast from said liquid metal bath;

d) said crude form is homogenized at a temperature comprised between 450° C. and 550° C. and optionally between 480° C. and 530° C. for a period comprised between 5 and 60 hours;

e) said homogenized crude form is hot-worked, optionally by rolling;

f) the hot-worked product is solution heat-treated between 490° C. and 530° C. for 15 min to 8 hours and said solution heat-treated product is quenched;

g) said product is cold-worked with a cold working of 2 to 16%;

h) aging is carried out wherein said product thus cold-worked reaches a temperature comprised between 130° C. and 170° C. and optionally between 140° C. and 160° C. for 5 to 100 hours and optionally 10 to 70 hours;

wherein said refiner contains particles of the TiC type and/or said cold working is of 8 to 16%.

2. The method according to claim 1, wherein the refiner containing particles of the TiC type is introduced in a form and an amount such that an amount of TiC identical to that added with a refiner AT3C0.15 at a rate of 2 to 5 kg/t of aluminum alloy is added.

3. The method according to claim 1 wherein the cold working of g comprises:

g1) said product is cold rolled with a thickness reduction rate comprised between 8 to 12%;

g2) said product is tensioned in a controlled manner with a permanent set comprised between 0.5 and 4%.

4. The method according to claim 1, wherein the aging is carried out at a temperature comprised between 140 and 155° C., optionally between 145 and 150° C., optionally for 18 to 22 hours.

5. The method according to claim 1, wherein the copper content is comprised between 4.0 and 4.6 wt % and optionally between 4.1 and 4.5 wt %.

6. The method according to claim 1, wherein the manganese content is comprised between 0.05 and 0.4 wt %.

7. The method according to claim 1 wherein the Ag content is of 0.1 to 0.27 wt % and/or the Zn content is of 0.2 to 0.40 wt %.

8. A product based on an aluminum alloy that can be obtained by the method according to claim 1.

9. The product according to claim 8 comprising a rolled product with a thickness comprised between 8 and 50 mm and having, at mid-thickness,

Kapp(L−T)≥−0.5Rcp0.2(L)+375,

optionally Kapp(L−T)≥−0.5Rcp0.2(L)+386

with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and

Rcp0.2(L), expressed in MPa, the compressive yield strength measured at 0.2% compression according to standard ASTM E9 (2018).

10. The product according to claim 8 comprising a rolled product with a thickness comprised between 8 and 50 mm and having, at mid-thickness,

Kapp(L−T)≥−0.5Rcp0.2(L)+386,

optionally Kapp(L−T)≥−0.5Rcp0.2(L)+391, and

Rp0.2(L)>600 MPa, optionally 615 MPa,

with Kapp (L−T) expressed in MPa√m, the value of the apparent stress intensity factor at rupture defined according to standard ASTM E561 (2015) measured on CCT test specimens of width W=406 mm and thickness B=6.35 mm, and

Rcp0.2(L), expressed in MPa, the compressive yield strength measured at 0.2% compression according to standard ASTM E9 (2018), and Rp0.2 (L) the conventional yield strength at 0.2% elongation measured in the longitudinal direction of the product, determined by a tensile test according to standard NF EN ISO 6892-1 (2016).

11. An aircraft structure element, optionally an aircraft upper wing skin element, comprising a product according to claim 9.