EP0162096B1 - Aluminium alloys containing lithium, magnesium and copper - Google Patents

Abstract

Aluminium alloys containing essentially additions of Li, Mg and Cu, and optionally minor additions of Cr, Zr, Ti, Mn which have high specific mechanical characteristics, a low density and a good corrosion resistance. The alloys according to the invention contain in percentage by weight: 1.8 to 3.5 of Li; 1.4 to 6.0 Mg; 0.2 to 1.6 of Cu with Mg-Cu >= 1.5; Cr up to 0.3; Mn up to 1; Zr up to 0.2; Ti up to 0.1 and/or Be up to 0.02, Fe up to 0.20; Si up to 0.12; Zn up to 0.35. The homogenizing and soluting treatments must be sufficiently carried out to dissolve the quaternary intermetal phases (Al, Li, Mg, Cu) of a grain size larger than 5 mum. These alloys present a mechanical characteristics/density compromise higher than that of known Al Cu Mg alloys and those containing Li.

Description

The present invention relates to alloys based on AI, containing Li, Mg and Cu and having mechanical characteristics equivalent to those of aluminum alloys with conventional structural hardening at medium strength with a density reduced by at least minus 9% compared to these conventional alloys.

It is known to metallurgists that the addition of lithium decreases the density and increases the modulus of elasticity and the mechanical resistance of aluminum alloys. This explains the designers' interest in these alloys for applications in the aeronautical industry, and more particularly, for lithium aluminum alloys containing other addition elements such as magnesium or copper. However, such lithium alloys must imperatively have ductility and toughness at least equivalent, with mechanical strength equal to that of conventional aeronautical alloys such as alloys 2024-T4 or T351, 2214-T6 (51), 7175-T73 (51) or T7652 and 7150-T651 (according to the nomenclature of the Aluminum Association), which is not the case for known lithium alloys.

In the Aluminum-Lithium-Magnesium system, the only known industrial alloy is the Soviet alloy 01420, of nominal composition (by weight%): Li = 2.0 to 2.2; Mg = 5.0 to 5.4; Mn = 0 to 0.6; Zr = 0 to 0.15. This alloy gives thin sheets and spun products treated in the T6 state (16 h at 170 ° C) moderately high mechanical tensile properties (FRIDLYANDER et al. Met. Science and Heat Treatment n ° 3-4, April 1968, page 212 - Metalov Translator I. Term Obrab Metallov No. 3, page 5052, March 1968) and lower than those of conventional aeronautical alloys. Furthermore, the study of the statistical laws of modification of characteristics of alloys of the AI-Li-Mg-Zr system as a function of their Li and Mg contents (IN FRIDLYANDER et al. “Zavod. Lab. •, July 1974, T7 , page 847) shows that it is not possible to increase the compromise between mechanical strength and elongation of this alloy up to the level of conventional aeronautical alloys, by reducing the contents of Lithium and Magnesium. These trends are confirmed by the results of SANDES (NADC Contract final report n ° N 622 69-74-C-0438, June 1976) showing that the compromise between elastic limit and toughness of products spun with AI-Li-Mg alloys is d as much higher than the lithium content and, to a lesser extent, the magnesium content are low. In particular, the authors show that alloys with high overall lithium-magnesium contents have, in the quenched-tempered state, a compromise between mechanical strength, ductility and toughness much lower than that of conventional alloys of the 2000 and 7000 series.

More recently, metallurgists have proposed new compositions of aluminum-lithium alloys with copper (Cu = 1.5 to 3%) and magnesium (Mg = 0.5 to 1.4%) with low density and high mechanical resistance . It is, in particular, the experimental alloy F92 (British specification DXXXA) of nominal composition (by weight%): Li = 2.5; Cu = 1.2; Mg = 0.7; Zr = 0.12, including the compromises of typical mechanical characteristics announced in 1983 by British ALCAN on thin sheets in the T8 state (Rm = 500 MPa; Rp 0.2 = 420 MPa; A = 6%) and on thick sheets in the T651 state (Rm = 520 MPa; Rp 0.2 = 460 MPa; A = 7%) show that this alloy has a compromise between mechanical strength and ductility even lower than that of aeronautical alloys of the 2000 and 7000 series, as all the other alloys of the AILiCu and AILiCuMg systems with a lithium content greater than 2% known to date.

During metallurgical tests, we found and tested new compositions of industrial alloys of the AI-Li-Mg-Cu system (-Cr, Mn, Zr, Ti) more efficient than the alloys of the AICuMg systems (2024). AILiCu and AILiMg, and that the known alloys of the AILiCuMg system, from the point of view of the compromise between mechanical strength, density and resistance to intergranular or laminating corrosion.

These new alloys according to the invention have the following weight compositions:

The content of main elements is preferably held individually or in combination between 2.3 to 3.3% for Li, 1.4 and 2.4% for Mg and 0.25 and 1.2% for Cu. The Zr content is preferably between 0.08 and 0.18%.

avec 8,5 ≤ K ≤ 11,5 et de préférence 9 ≤ K ≤ 11.To obtain a better compromise, mechanical strength-density, we must also observe the following relationship:

with 8.5 ≤ K ≤ 11.5 and preferably 9 ≤ K ≤ 11.

The alloys according to the invention have their optimal level of resistance and ductility after homogenization treatments of the cast products and dissolving of the transformed products comprising at least one level at a temperature θ (in ° C) of the order of 0 = 535-5 (% Mg) for a sufficient time so that after quenching, the intermetallic compounds of the quaternary phases (AILiCuMg) detectable during micrographic examination or by electronic or ionic microanalysis (SIMS) have a size less than 5 µm. Homogenization can be done in a temperature range between 8 + 10 (° C) and 0 ―20 (° C); the dissolution is preferably carried out between 0 + - 10 ° C.

The optimal durations of heat treatment for homogenization at temperature 0 are 0.5 to 8 hours for alloys produced by rapid solidification (atomization - splat cooling - or any other means) and 12 to 72 hours for molded products or produced in semi-continuous casting.

These alloys have their optimal mechanical properties after tempering of durations of 8 to 48 hours at temperatures between 170 and 220 ° C and it is preferable to subject the products of adequate shape (sheets, bars, widgets) to work hardening giving rise to plastic deformation of 1 to 5% (preferably 2 to 4%) between quenching and tempering, which further improves the mechanical strength of the products.

Under these conditions, the alloys according to the invention have a higher mechanical strength than that of the alloy AILiMgMn 01420, which did not make it possible to predict the results of the studies available on the system. We have found that the alloys according to the invention have a compromise between mechanical characteristics and density greater than that of the known AILiCuMg alloys (with low magnesium contents). They also have a satisfactory resistance to intergranular or laminating corrosion much higher than that of the known alloys AICuMg, AILiCu, and AILiCuMg.

These alloys are therefore particularly advantageous for the manufacture of molded or wrought semi-finished products (produced by semi-continuous casting, rapid atomization or solidification, etc.) whether they are, for example, spun, rolled, forged or matrixes used in particular in the aeronautical or space industries.

In particular, it was surprisingly found that the alloys according to the invention, very loaded with Li and Mg, were pourable without major difficulty in semi-continuous casting in the form of billets or trays of industrial format (absence of cracks and porosities).

The invention will be better understood and illustrated with the aid of the following examples:

Examples

We have produced by semi-continuous casting 200 mm diameter billets made of aeronautical aluminum alloys of known compositions and of various lithium alloys according to the invention. These billets have undergone long-term homogenization at a temperature sufficient to dissolve almost all of the eutectic phases and transformed, after peeling, into strips of width 100 mm and thickness 13 mm.

The largets were dissolved in the conditions deemed optimal from the point of view of the dissolution of the phases rich in main addition elements (Li, Cu, Mg, Zn), then soaked in cold water (20 ° C ), before undergoing controlled traction at 2% residual deformation and different tempering temperatures in a ventilated oven for a period of 24 hours. Certain spun strips have not been pulled between quenching and tempering, so as to highlight the influence of the work hardening between quenching and tempering on the mechanical properties.

All the widgets thus produced were characterized by tensile tests and density measurement. Sensitivity tests for intergranular corrosion according to AIR 9048 standard (6 hour continuous immersion in NaCI-H ₂ O _{2 solution} ) and to flaky corrosion according to EXCO test (96 hour continuous immersion according to ASTM G 34-79 standard) were also carried out.

Table 1 gives the chemical compositions of the alloys measured by atomic absorption and spark emission spectrometry, and their characteristics (coefficient K) with respect to the domains according to the invention.

Table II gives the mechanical traction characteristics and the density as a function of the chemical composition of the widgets, for the various heat treatments carried out and the rate of work hardening between quenching and tempering. The elastic limit (Rp 0.2), the breaking load (Rm) and the elongation at break (A%) are given.

Table III gives the results obtained during the corrosion tests.

The results of the sensitivity tests to intergranular corrosion and to laminating corrosion carried out, for certain states of tempering in the T651 state, show that the alloys according to the invention have improved corrosion resistance compared to conventional 2000 series alloys and known lithium alloys, which are less loaded with Mg.

All the results obtained therefore show that the alloys according to the invention have mechanical strengths of levels comparable to those of the alloys of the 2000 series without lithium currently used in aeronautics, and greater than those of known AI-Li-Mg alloys. (eg alloy 01420) with the advantage of a significantly lower density than that of conventional alloys and lower than that of the known lithium alloys of the AI-Li-Cu, AI-Li-Cu-Mg systems. They also show the advantage of hardening between quenching and tempering on the mechanical properties.

Claims (12)

1. AI-base alloy having high strength and high ductility characterized in that it contains (in % by weight) :

2. Alloy according to claim 1 characterised in that it contains from 2.3 to 3.3 % Li.

3. Alloy according to claim 1 characterised in that it contains from 0.25 to 1.2% Cu.

4. Alloy according to one of claims 1 to 3 characterised in that it contains from 2.3 to 3.3 % Li, 0.25 to 1.2 % Cu and 1.4 to 2.4 % Mg.

5. Alloy according to one of claims 1 to 4 characterised in that :

with 8.5 ≤ K ≤ 11.5.

6. Alloy according to claim 5 characterised in that 9 ≤ K ≤ 11.

7. A process for the heat treatment of the alloys according to one of claims 1 to 6 comprising a homogenization operation, a solution treatment, a quenching operation and a tempering operation, characterised in that the alloy is subjected to homogenization and solution treatment at a temperature (in °C) of the order of 0 = 535 - 5 (% Mg).

8. A process according to claim 7 characterised in that the duration of the homogenization operation and the solution treatment is sufficiently long that, after the quenching operation, the residual quaternary intermetallic phases (AI, Li, Mg, Cu) are less than 5 µm in size.

9. A process according to claim 7 or claim 8 characterised in that the homogenization operation is carried out in the temperature range which is defined by θ + 10 (°C) and 8―20 (°C).

10. A process according to claim 7 or claim 8 characterised in that the solution treatment is carried out in the temperature range defined by 8 + 10 (°C) and 0 - 10 (°C).

11. A process according to one of claims 7 to 10 characterised in that the tempering operation is effected at from 170 to 220 °C for a period ranging from 8 to 48 hours.

12. A process according to one of claims 7 to 11 characterised in that plastic deformation of from 1 to 5 % and preferably from 2 to 4 % is applied to the treated product between the quenching and tempering operations.