EP0282421B1 - Aluminium alloy product containing lithium resistant to corrosion under tension and process for production - Google Patents

Description

The present invention relates to an Al alloy product containing lithium with high specific mechanical resistance and high damage tolerance, particularly resistant to corrosion under tension in the treated (quenched-tempered) state, in particular in the recrystallized state, and a process for obtaining such a product.

Obtaining alloys with high resistance to corrosion under stress is an essential objective for metallurgical semi-finished products intended for use in aeronautics or space.

Aluminum-lithium alloys which also exhibit excellent mechanical strength, toughness, ductility or fatigue properties (see Ph. MEYER, B. DUBOST - Al.Li Alloys III - Proceedings of the Third International Conference Sponsored by the Institute of Metals. Oxford July 8-11, 1985 - Baker Gregson Harris Peel London- 1986) are likely to exhibit corrosion resistance under insufficient stress, even in the rolling plane of thin sheets, when they are recrystallized.

This insufficiency is likely to limit their use; for example, the mere installation of rivets with strong interference can lead, as in the case of conventional alloys sensitive to corrosion under tension (CST) (see Kaneko Siemenz - Corrosion Thresholds for interference fit fasteners and cold worked holes - Stress Corrosion New Approaches ASTM - STP 610, 1976, pp. 252-266), to cracks due to corrosion under stresses, induced by residual riveting stresses.

The products according to the invention have a particular microstructure comprising, either in addition to the solid solution, numerous and fairly coarse precipitates of intermetallic phases rich in elements Al, Cu, Li, Mg and possibly Zn, or a solid solution obtained by dissolving at low temperature.

The corresponding process according to claims 1 to 3 of the invention essentially consists in dissolving the alloy under consideration at low temperature, generally incomplete. other parameters of the production range, in particular of income, being unchanged, compared to usual practice.

The invention applies to all aluminum-based alloys containing lithium, produced by molding, rapid solidification, ingot metallurgy or other production technique.
It applies in particular to alloys based on Al, the main elements of which are as follows (by weight%):
Li: 1.0 to 4.2%
Cu: 0 to 5.5%
Mg: 0 to 7.0%
Zn: 0 to 15.0%
with the following minor elements:
Zr: 0 to 0.2
Mn: 0 to 1
Cr: 0 to 0.3
Nb: 0 to 0.2
Ni: 0 to 0.5
Fe: 0 to 0.5
If: 0 to 0.5
Other items: <0.05 each
Rest Al.

It should preferably have:% Zn / 30 +% Mg / 18 +% Li / 4.2 +% Cu / 7 <1.

The products according to the invention preferably contain (by weight%) from 1.7 to 2.5 Li - 0.8 to 3% Mg - 1.0 to 3.5% Cu - up to 2% Zn, the rest consisting of Al, secondary elements such as Zr (0 to 0.20%), Mn, Cr, Ti and impurities whose total amount is less than or equal to 1% and are treated specifically. They have a microstructure which is particularly resistant to stress corrosion and which, in addition to the solid solution, contains numerous and fairly coarse precipitates of intermetallic phases rich in elements Al, Cu, Li, Mg and if necessary Zn if the contents of these elements d addition obey the following inequality determined after experimental study in metallography: $$\text{A> O where A =\% Cu +\% Li +\%}\frac{\text{<figure-callout id="2" label="Mg" filenames="imgf0002.png,imgf0003.png" state="{{state}}">Mg</figure-callout>}}{\text{2}}\text{+\%}\frac{\text{Zn}}{\text{3}}\text{- 2.7 - 3340 exp (}\frac{\text{-5960}}{\text{273 + T}}\text{)}$$

In this formula% Cu,% Li,% Mg,% Zn are the weight contents and T the effective solution temperature is T _MS , expressed in ° C. In this case these phases are of type R - Al₅ Cu (Li, Mg) and of type T₂ -Al₆ Cu (Li, Mg) in alloys 8090 and 2091 according to the designation of the Aluminum Association.

The chemical composition by weight of the 2091 alloy is as follows:
If ≦ 0.20%; Fe ≦ 0.30%; Li: 1.7-2.3%; Cu: 1.8-2.5%;
Mn ≦ 0.10; Mg: 1.1 - 1.9%; Cr ≦ 0.10%; Zn ≦ 0.25%; Zr: 0.04 -0.16%
Ti ≦ 0.10%; others: each ≦ 0.05%, total ≦ 0.15%, Al: remainder.

The metallographic and structural characteristics of these phases and their characteristic reticular distances in X-ray diffraction are similar to those given by the article by HK HARDY and JM SILCOK in the Al-Li-Cu system free of magnesium (Journal of the Institute of Metals, 1955-56, Vol 84, p. 423-425).

The volume fraction of these particles increases with the overall content of Li, Cu, Mg and Zn and is higher the lower the solution temperature, according to the invention. By metallographic and structural analysis, the applicant noted that the volume fraction of the particles, in% is fv = k · A if A > 0 with 2.0 ≦ k ≦ 4.0. This volume fraction must generally be greater than 0.6% and preferably between 1 and 4%, especially in alloy 2091. Below 0.6% the resistance to corrosion under stress may be insufficient on recrystallized products. ; above 4%, the mechanical characteristics of resistance and ductility become too weak.

• . un palier apparent ou pseudo-palier s'étendant entre la température de mise en solution réellement effectuée sur l'alliage et la température de fusion commençante de l'alliage.

The largest dimension of the largest particles exceeds 5 µm and preferably 10 µm.
This structure can be checked by differential thermal analysis or differential enthalpy analysis (DSC: Differential Scanning Calorimetry), the trace (thermogram) then having the following characteristics in the field of temperatures of solution and of melting beginning during a sample temperature rise programmed at a speed of 1 to 20 ° C / minute:

• . an apparent plateau or pseudo-plateau extending between the dissolution temperature actually carried out on the alloy and the starting melting temperature of the alloy.

This pseudo-level for which the thermogram obtained evolves substantially like the baseline of the differential enthalpy analysis device (determined with 2 identical inert samples or without sample no reference), the longer the lower the solution temperature. In addition, it appeared during tests that the temperature at the start of this plateau coincides in practice with the solution temperature according to the invention or annealing, if the alloy is not dissolved, this in the case where the Differential Enthalpy Analysis is performed after these thermal operations. Tempering does not significantly change the thermogram in this high temperature range. This method allows you to find with certainty the solution solution temperature, even annealing, practiced. It thus gives, on a product treated in the final state (dissolved, possibly soaked and hardened), the physical signature of the treatment according to the invention.

• . un picendothermique de fusion commençante de phase AlCuLiMg (R ou T₂ dans le domaine de composition préférentielle) dans la matrice Al vers 532 à 550°C (selon la composition de l'alliage) d'autant plus important en surface de pic (c'est-à-dire en chaleur, absorbée pour la fusion) que la fraction volumique de phase hors solution T₂ ou R est importante.

La surface de ce pic est donc, de ce fait, d'autant plus grande que la température de mise en solution selon l'invention, préalable à l'analyse thermique est faible et est inférieure à la température de mise en solution habituellement pratiquée sur l'alliage. Un alliage exempt de phases hors solution T₂ ou R, c'est-à-dire un alliage de composition telle que A < O ayant subi au préalable une mise en solution complète des particules grossières de phases T₂ à R à haute température selon la procédure normalement connue de l'homme de l'art ne présente pas de tel pic vers 532-550°C.This pseudo level follows a large picendothermal representing the re-solution of the small precipitates of the equilibrium phase formed during the rise in temperature of the sample in the area preceding that of the dissolving temperatures practiced on the alloy.

• . a picendothermic of beginning AlCuLiMg phase fusion (R or T₂ in the preferential composition range) in the Al matrix around 532 at 550 ° C. (depending on the composition of the alloy) all the more important at the peak surface (c ' that is to say in heat, absorbed for the fusion) that the volume fraction of phase out of solution T₂ or R is important.

The surface of this peak is therefore, as a result, the greater the lower the dissolution temperature according to the invention, prior to the thermal analysis, and is lower than the dissolution temperature usually practiced on the alloy. An alloy free of phases out of solution T₂ or R, that is to say an alloy of composition such as A <O having previously undergone complete dissolution of the coarse particles of phases T₂ at R at high temperature according to the procedure normally known to those skilled in the art does not exhibit such a peak at around 532-550 ° C.

The method according to claim 1 of the invention consists of dissolution carried out in a range of temperatures T _MS lower than the usual dissolution temperature which the person skilled in the art considers to be the highest. possible to obtain the maximum mechanical resistance, due to the increased dissolution of the hardening elements.

T _MS must be less than T _M (in ° C) = 474 + 18.2% Li - 2% Cu (% Cu-1.7) +% Mg (-17.6 + 3.6% Li + 4.3% Cu) - 3% Zn
where% Li,% Cu,% Mg,% Zn are the% by weight of the alloying elements mentioned, but must remain greater than or equal to 460 ° C. and preferably to 480 ° C.

The dissolution time can be the same as that usually practiced at high temperature on aluminum-lithium alloys according to the prior art, generally from 10 min to 7 hours depending on the products (thin sheet to thick forged).

If the dissolution is carried out at too high a temperature, this results in a very significant loss of resistance to corrosion under stress; on the other hand, if it is carried out at too low a temperature, the mechanical resistance characteristics are insufficient.

Dissolution is followed by quenching carried out under the usual conditions.
The income treatment is not modified compared to the usual practices for aluminum alloys containing lithium.

The dissolution is preferably preceded during the manufacturing range of a possible hot keeping (with or without plastic deformation).
This hot keeping is preferably practiced in a temperature range between 490 and 250 ° C, more particularly between 450 ° C and 350 ° C, for a time between 1 h and 48 hours, preferably between 6 h and 24 hours.
However, the maximum temperature of this hot keeping must be less than or equal to that of the subsequent dissolution.

This keeping hot may possibly be multi-level, provided that the last level is carried out according to the invention.
It is preferably applied after the hot deformation phase for wrought alloys.
It can possibly be followed by a cold deformation.

If the alloy is cold deformed and if this deformation requires intermediate annealing, the last of them will be carried out under the conditions defined above.

The cooling rate after keeping hot must be greater than 10 ° C / hour and preferably greater than 25 ° C / h. This speed is the average speed between the temperature for keeping hot and 100 ° C., the cooling speed below 100 ° C. being not critical.

The cooling can be carried out in an oven, under a draft, in calm air, in water, or by any other technique allowing the desired cooling rates to be obtained.

If the hot keeping is carried out at too high a temperature, the resistance to corrosion under tension is greatly reduced. If the hot keeping is carried out at too low a temperature, this results in difficulties for the subsequent cold deformation or even a reduction in the resistance to corrosion under stress.

• . La figure 1 représente la micrographie optique dans le plan long-travers court d'un alliage traité hors l'invention.
• . La figure 2 représente la micrographie dans le plan long-travers court d'un alliage traité conformément à l'invention.
• . La figure 3 représente la micrographie dans le plan long-travers long d'un alliage traité conformément à l'invention.
• . La figure 4 représente divers thermogrammes d'un alliage 2091 mis en solution à diverses températures (exemple 2).
• . La figure 5 représente les courbes d'évolution de la vitesse de propagation (da/dn) d'une fissure de fatigue en traction ondulée : σ = 90 ± 40 MPa en fonction du Δ K dans les sens LT et TL, pour les alliages selon l'invention (cas A), hors l'invention (cas B), correspondant à l'exemple 3 et pour l'alliage de référence (2024).
• . La figure 6 représente une pièce matricée traitée selon l'invention et la position relative des éprouvettes de traction et de corrosion sous tension (exemple 5).
• . La figure 7 représente la structure de l'alliage traité selon l'invention correspondant à l'exemple 6.

The invention will be better understood with the aid of the following examples illustrated by FIGS. 1 to 7.

• . FIG. 1 represents the optical micrograph in the long-through-short plane of an alloy treated outside the invention.
• . FIG. 2 represents the micrograph in the long-through-short plane of an alloy treated in accordance with the invention.
• . FIG. 3 represents the micrograph in the long-through long plane of an alloy treated in accordance with the invention.
• . FIG. 4 represents various thermograms of an alloy 2091 dissolved in various temperatures (example 2).
• . Figure 5 represents the curves of evolution of the speed of propagation (da / dn) of a fatigue crack in wavy traction: σ = 90 ± 40 MPa according to the Δ K in the directions LT and TL, for the alloys according to the invention (case A), outside the invention (case B), corresponding to Example 3 and for the reference alloy (2024).
• . FIG. 6 represents a stamped part treated according to the invention and the relative position of the tensile and corrosion test specimens under tension (example 5).
• . FIG. 7 represents the structure of the alloy treated according to the invention corresponding to example 6.

EXAMPLE 1

Two 1.6 mm thick sheets of the following composition (by weight%):
Li: 2.07 - Cu: 2.15 - Mg: 1.53 - Zr: 0.10 - Ti: 0.03
Fe: 0.04 - If 0.03 -
stay Al
were processed as follows:
annealing 1 h 450 ° C + 12 h 400 ° C followed by dissolving (according to the invention) or 530 ° C, quenched in cold water, drawn up by 2% and returned 12 h at 135 ° C.

The microstructures obtained are given in FIG. 1 with regard to the dissolution at 530 ° C. and in FIGS. 2 and 3 with regard to the dissolution at 500 ° C. The coarse particles, whose size is clearly greater than 5 µm, essentially consist of the R-Al₅Cu phase (Li, Mg) ₃ out of solution (point verified by quantitative analysis using a Castaing electron microprobe and by X-ray diffraction according to the Seeman method -Bohlin).
Their average surface fraction on polished sections (equal to the volume fraction in the bulk sample), measured by analysis of quantitative images on an IBAS KONTRON device, is 0.53% after dissolving at 530 ° C (k ≃ 2, 9) and 2.3% after dissolving at 500 ° C.
(k ≃ 2.7) with an accuracy of approximately + or - 10% on this average value.

EXAMPLE 2

• échantillons et référence (Aluminium raffiné) usinés sous forme de disques de diamètre 5 mm et d'épaisseur 1 mm
• balayage d'azote sec dans la cellule
• vitesse de montée en température de 5°C/min entre 400 et 590°C.

The same alloy as above (alloy 2091) was dissolved at various temperatures between 490 ° C and 535 ° C after annealing for 1 hour at 400 ° C and cold rolling, quenching with water and tempering for 12 hours at 135 ° C, before undergoing a differential thermal analysis on a DUPONT de NEMOURS DSC 910 device controlled by a DSC 990 programmer under the following conditions:

• samples and reference (Refined aluminum) machined in the form of discs with a diameter of 5 mm and a thickness of 1 mm
• dry nitrogen sweep through the cell
• temperature rise rate of 5 ° C / min between 400 and 590 ° C.

• La courbe (1) correspond à mise en solution à 490°C.
• La courbe (2) correspond à mise en solution à 510°C.
• La courbe (3) correspond à mise en solution à 520°C.
• La courbe (4) correspond à mise en solution à 530°C.

The thermograms obtained are shown in Figure 4.
On these thermograms the abscissa represents the temperature in ° C and the ordinate the power (in mW) released or absorbed respectively in the exothermic (+) or endothermic (-) direction. The baseline of the device (LB) is shown in broken lines.

• Curve (1) corresponds to dissolution at 490 ° C.
• Curve (2) corresponds to dissolution at 510 ° C.
• Curve (3) corresponds to dissolution at 520 ° C.
• Curve (4) corresponds to dissolution at 530 ° C.

On each thermogram we see that the temperature of the start of the detectable pseudo-level (I) - substantially straight part very slightly endothermic compared to the baseline of the device determined beforehand - corresponds, with the accuracy of the measurement and determining the phase transformation temperatures by intersection of the tangents to the thermogram, to the effective solution temperature according to the invention, and this better than 3 ° C.
We also note the narrow peak (II) of beginning fusion of the eutectic constituents, which begins around 535 ° C and ends just before the equilibrium fusion of the alloy (solidus). The latter is marked by a very deep and progressive picendotherm (III).
The starting melting peak (endothermic) appears, after thermal analysis, much deeper in the alloys treated according to the invention, than in the alloy treated at 530 ° C according to the conventional solution treatment.

The combination of this differential thermal analysis method and the metallographic analysis of Example 1 therefore make it possible to characterize in a reliable and new way the products produced according to the invention which is the subject of the main patent.

EXAMPLE 3

A 2091 alloy with a composition by weight: 1.95% Li - 2.10% Cu - 1.5% Mg - 0.08% Zr - 0.04% Fe - 0.04% Si - aluminum residue is cast in trays 800 × 300 mm² section, homogenized 24 hours at 527 ° C, scalped, then hot rolled between 470 and 380 ° C up to 3.6 mm thick and wound in a coil. It is then kept hot according to the invention 1 h 450 ° C. followed by 12 hours at 400 ° C (with oven cooling between the two stages). Cooling after keeping hot is carried out at a speed in the region of 35 ° C / hour to a temperature of 100 ° C.
After keeping hot, the sheets are cold rolled to 1.6 mm.
A part of the thin sheets thus produced is then dissolved according to the invention (case A): 20 min at 500 ° C ± 2 ° C, quenched in cold water, depleted and pulled by 2%, finally returned 12 h at 135 ° C.
Another part of the sheets is dissolved outside the invention (case B) 20 min at 528 ° C ± 2 ° C and then undergoes the same completion as in case A described above. In this case of alloy: T _M = 505.5 ° C.
The structure of the alloy is recrystallized with fine and equiaxed grains (average size: 20 µm).

The properties obtained in the two cases in the Long (L), Travers-Long (TL) and 60 ° directions of the rolling direction (60 ° / L) are reported in Table I.
It will be noted that the treatment according to the invention provides a very strong improvement in the resistance to CST in the rolling plane while at the same time retaining good levels of mechanical properties.

The crack propagation results provided by FIG. 5 confirm the good level of the fatigue properties of the alloy treated according to the invention, which are superior to those of the reference alloy: 2024.

EXAMPLE 4

A 2091 alloy with a composition: 2.2% Li - 2.3% Cu - 1.6% Mg - Zr 0.10% - Fe 0.04% - If 0.03%, remains aluminum, is cast in ingot of section 100 × 300 mm², homogenized 24h at 527 ° C, scalped, hot rolled between 470 and 380 ° C up to 3.6 mm. Part of the sheets (marked C) is then kept hot according to the invention: 24 hours at 415 ° C., cooling by quenching with cold water.
D sheets: they are kept hot outside the invention: 24 h 415 ° C with cooling of 8 ° C / h between 415 and 100 ° C.

The two types of sheet are then cold rolled up to 1.6 mm. The sheets are dissolved in accordance with the invention 20 min at 510 ° C, quenched in cold water, de-plumped and pulled, then returned 12 h at 135 ° C.
In this case T _M = 511.6 ° C.

The stress corrosion and mechanical strength properties measured are given in Table II.

EXAMPLE 5

A 2091 alloy of composition (by weight) 2.0% Li - 1.8% Cu - 1.4% Mg - 0.12% Zr - 0.06% Fe - 0.04% Si is cast in Ø50 mm billets (induction heating; spinning at 430 ° C).
This bar is machined to lengths of 500 mm; these lengths were reheated and stamped in several passes between 490 and 400 ° C. Before the last stamping pass, the parts are kept hot according to the invention for 6 hours at 450 ° C. and deformed at this temperature. They then undergo cooling, the speed of which is greater than 100 ° C./h up to 100 ° C. according to the invention.
The parts are then dissolved at 503 ° C ± 2 ° C for 4 hours according to the invention, soaked in cold water and returned 24 hours to 190 ° C (in this case T _M = 506.3 ° C). These parts (see fig. 6) are characterized in tension and stress corrosion.
The tensile test pieces (sites A, B and C) are taken outside the intersections of the ribs. On the other hand, the corrosion corrosion test specimens cut across the ribs (site D).
The results are reported in Table III.

EXAMPLE 6

An alloy of composition (by weight): 2.5% Li - 1.2% Cu - 1.0% Mg 0.06% Zr - 1.5% Zn - 0.06% Fe - 0.04% Si is poured into a 300 × 100 mm² section plate, homogenized for 24 hours at 535 ° C (with rise in homogenization temperature at 25 ° C / h from 500 ° C). It is then scalped, reheated to 490 ° C, hot rolled between 480 and 300 ° C up to 3.6 mm. The raw hot rolling product thus obtained is then kept hot for 1 hour at 450 ° C., cooled by quenching in cold water and cold rolled from 3.6 to 1.2 mm.
The sheets thus obtained are dissolved in an oven in a salt bath for 20 min at 485 ° C., quenched in cold water, drawn by 1.5% and returned 12 h at 190 ° C. (in this case T _M = 512, 7 ° C).
The structure obtained is recrystallized (see fig. 7).
The properties obtained are given in Table IV.

Claims (15)

1. Process for the production of aluminium alloys containing 1 to 4.2% (by weight) of Li, up to 5.5% Cu, up to 7% Mg, up to 15% Zn, up to 0.2% Zr, up to 1% Mn, up to 0.3% Cr, up to 0.2% Nb, up to 0.5% Ni, up to 0.5% Fe, up to 0.5% Si and other elements up to 0.05% each, the remainder being Al, making it possible to desensitize them to corrosion under tension, comprising at least the hot shaping of a moulded or hot worked product, an optional cold hammering, a dissolving, a hardening, an optional controlled cold hammering and a tempering, characterized in that dissolving takes place in a temperature range between 460°C and T_M(°C) = 474 + 18.2 (%Li) - 2(%Cu)(% Cu-1.7) + (%Mg)(-17.6+3.6(%Li) + 4.3 (%Cu)) -3(%Zn).

2. Process according to claim 1, characterized in that dissolving is preceded, in a prior production stage, by maintaining hot at between 250 and 490°C (with or without simultaneous plastic deformation) at an average cooling rate, after maintaining hot and up to 100°C, exceeding 10°C/h and preferably 25°C/h.

3. Process according to claim 2, characterized in that maintaining hot takes place at between 450 and 350°C.

4. Process according to either of the claims 2 or 3, characterized in that maintaining hot lasts between 1 and 48 hours and preferably between 6 and 24 hours.

5. Process according to any one of the claims 2 to 4, characterized in that the maintaining hot temperature is equal to or below that of the dissolving temperature.

6. Product obtained according to any one of the claims 1 to 5, characterized in that the thermograms obtained by differential enthalpy analysis have a pseudo-plateau starting the effective dissolving of the product and which is equal to or below:
T_M(°C) = 474 + 18.2%Li - 2%Cu(%Cu-1.7)+%Mg(3.6%Li+4.3%Cu-17.6)-3%Zn and finishes at the initial melting temperature of the alloy.

7. Product according to claim 6, characterized in that the pseudo-plateau visible on the thermograms is followed by an initial narrow melting peak between 532 and 550°C.

8. Product according to either of the claims 6 or 7, characterized in that the composition complies with the inequation $$\frac{\text{\%Zn}}{\text{30}}\text{+}\frac{\text{\%Mg}}{\text{18}}\text{+}\frac{\text{\%Li}}{\text{4.2}}\text{+}\frac{\text{\%Cu}}{\text{7}}\text{< 1}$$

9. Product according to any one of the claims 1 to 5 containing (in % by weight) 1.7 to 2.5% Li, 0.8 to 3.0% Mg, 1.0 to 3.5% Cu, up to 2% Zn, 0 to 0.2% Zr and in total 1% of other elements, the remainder being Al and in that it contains undissolved intermetallic phases rich in the elements Al, Li, Cu, and Mg and, if appropriate, Zn in the form of coarse particles, whose volume fraction fv (in %) is substantially equal to:

and 2.0 ≦ K ≦ 4.0

10. Product according to claim 9, characterized in that the size of the coarsest particles exceeds 5µm.

11. Product according to either of the claims 9 or 10, characterized in that the size of the coarsest particles exceeds 10µm.

12. Product according to any one of the claims 9 to 11, characterized in that the undissolved particles are constituted by the R or T2 phase rich in the elements Al, Cu, Li and Mg and their volume fraction exceeds 0.6%.

13. Product according to any one of the claims 9 to 12, characterized in that the volume fraction of the undissolved phases is between 1 and 4%.

14. Product according to any one of the claims 9 to 13, characterized in that its structure is recrystallized.

15. Product according to any one of the claims 9 to 14, characterized in that its composition is that of alloy 2091, as defined by the Aluminum Association.

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