EP0208631A1 - Aluminium alloys with a high lithium and silicon content, and process for their manufacture - Google Patents

Abstract

The invention relates to alloys based on Al with high contents of Li and Si, containing (by weight%): from 3.6 to 8% Li from 5 to 14% Si from 0 to 1% of each of the following elements : Fe, Co, Ni, Cr, Mn, Zr, V, Ti, Nb, Mo, O ₂ , Sc and from 0 to 2% of Cu, Mg and / or Zn the total quantity of these optional secondary elements being less than 6% remains Al and Impurities (each ≤ 0.05%, total ≤ 0.15%. The contents of Li and Si are linked by the formula% Li: 0.4% Si + k with -1 ≤ k ≤ 5

The products are obtained by rapid solidification processes and contain from 15 to 60% by volume of T phase (Al, Si. Li), in particles of 0.01 to 10 µm.

The hot transformation temperature must remain below 400 ° C.

The products obtained have an average to high mechanical resistance and a very high specific modulus (E / d).

Description

The field of the invention relates to alloys based on Al on high Li and Si contents and having an average to high mechanical strength, a very low density and a high Young modulus; a process for obtaining it uses rapid solidification (atomization, hyper quenching on a metal substrate, etc.) densification and hot shaping.

• - la fragilité lors de la coulée semi-continue en lingots
• - la mauvaise aptitude à la mise en forme à chaud par suite d'une faible ductilité
• - la grande fragilité intergranulaire à l'état trempé et revenu due à la précipitation d'une fraction volumique très élevée (> 30%) de phase métastable δ'Al₃Li cohérente avec la matrice et très facilement cisaillable par les dislocations
• - la grande sensibilité à la corrosion spontanée à la température ambiante due à la présence de la phase d'équilibre δ Al Li aux joints de grains et dans la matrice.

According to the known prior art, it is known that alloys with a Li content greater than about 3% (by weight) have manufacturing difficulties due in particular to:

• - brittleness during semi-continuous ingot casting
• - poor aptitude for hot forming due to low ductility
• - the great intergranular brittleness in the quenched and tempered state due to the precipitation of a very high volume fraction (> 30%) of metastable phase δ'Al ₃ Li consistent with the matrix and very easily shearable by dislocations
• - the high sensitivity to spontaneous corrosion at room temperature due to the presence of the equilibrium phase δ Al Li at the grain boundaries and in the matrix.

To solve these problems, metallurgists have proposed additions of a few% of hardening elements such as Cu, Mg, Zn and other minor elements controlling the recrystallization or the grain size of the alloy, such as Mn, Cr, Ti, etc ...

These alloys also contain very low Fe and Si contents (less than 0.1% by weight).

However, such alloys, even if they practically reach the mechanical strength levels of conventional aeronautical alloys (2024, 2214.7075) have a decrease in density (d) and an increase in the elastic modulus (E) limited to about 12% each, ie an increase in the specific module (E / d) of less than 25%.

It has been shown that the joint addition of Li and Si in alloys of Al obtained by conventional solidification leads to a bad compromise mechanical strength-ductility-density (FWGAYLE, Aluminum Lithium alloys Proceeding of the ist International Al-Li Conference Ed. by TH SANDERS, Jr and EA STARKE, Jr The Metallurgical Society of AIME, 1981 p.119-139) .

The Applicant has found that it is possible to obtain sane specific modulus much greater than 25% on Al-Li-Si alloys containing large amounts of Si and Li, while retaining acceptable mechanical characteristics, resistance satisfactory spontaneous corrosion, and good formability.

This objective is achieved by the choice of a specific composition, the use of rapid solidification and techniques of powder metallurgy, and finally a shaping at controlled temperature.

la quantité totale de ces éléments secondaires optionnels étant inférieure à 5%, reste Al et impuretés (chacune ≤ 0,05%, total ≤ 0,15%.The alloys according to the invention contain (% by weight):

the total amount of these optional secondary elements being less than 5%, Al and impurities remain (each ≤ 0.05%, total ≤ 0.15%.

avec

et de préférence

The Li content is preferably linked to the Si content by the following formula:

with

and preferably

The Li content is preferably kept between 4 and 7%

The total content of secondary elements (other than Li and Si) is preferably kept below 2X.

However, the properties of the products obtained are only satisfactory if the alloys sc..t produced by rapid solidification at cooling rates from the liquid state greater than 1000 ° C / sec. by any known means (solidification on a wheel, atomization, etc.) This operation preferably takes place under an inert atmosphere, for example argon or helium. The alloys thus obtained are then consolidated by the known techniques of powder metallurgy, for example according to the range: possible grinding, cold compaction, degassing under possible vacuum, hot compression and drawing by spinning, forging, stamping or any other technique. , with a wrought rate (initial cross-section / final cross-section) generally greater than 8.

However, during these various hot forming operations, the product temperature must remain below 400 ° C, and preferably 350 ° C, to obtain acceptable mechanical characteristics. The products are generally used, as indicated, in the raw state of hot transformation, or after a slight additional deformation at a lower temperature, which makes it possible to improve both the flatness, the straightness or the dimensional tolerances and the mechanical resistance characteristics.

In the state of use, the products thus obtained have a large volume fraction, between 15 and 60%, preferably between 20 and 50%, of particles essentially consisting of a phase of cubic structure, with a parameter close to 0 , 59 at 0.60 nm, identified as phase T -Al ₂ Li ₃ Si ₂ or Al Li Si, according to the authors.

This phase, distributed homogeneously, has a size of between 0.01 and 10 μm, more generally between 0.01 and 5 μm: it is believed that this phase contributes to cold hardening and to the average temperatures of the alloy, its fine and homogeneous precipitation being accentuated by an income between room temperature and 350 ° C, preferably between 150 and 250 ° C. The microstructure may optionally include a very fine globular precipitation of phase δ '(Al ₃ Li) whose diameter is less than 50 nm and also a slight precipitation of free Si or of phase 6 Al Li.

The amount of phase présente 'present is less than 10% (by volume). Finally, the products thus obtained are characterized by an extremely fine grain whose size is less than 20 μm, and generally less than 10 μm.

If k is less than the lower limit, we cause the appearance of Si particles, to the detriment of the T phase, which decreases the mechanical characteristics and specific elastic properties.

If k exceeds the upper limit, the precipitation of the phase 6 _A l Li which is spontaneously corrodible and also of the phase 6 'Al ₃ Li embrittling is favored.

The Applicant has on the other hand, found that, for an equal composition, the hardness of the products is higher the smaller the size of the phase T particles (Al, Li, Si); in particular the very rapid solidification of the thin ribbons (20 to 30 μm thick) on a metallic substrate ("melt spinning") leads, on the substrate side, to T-phase particle sizes of 0.01 to 0.5 μm.

The microhardness is then greater than about 40% than that obtained on the outer face of the thicker ribbons or on powders obtained by atomization, for which the size of phase T particles is of the order of 0.5 to 5 μm.

The invention will be better understood using the following examples:

Example 1

Alloys, the composition of which is given in Table 1, were obtained in the form of powder, by centrifugal spraying with helium, the latter being sieved to 200 μm maximum.

These powders were made from cast ingots, made with a pure base having an Fe content <0.05%.

• mise en conteneur en Al-Mg Ø 42 x 100 mm
• dégazage 24h sous 1 à 10-¹ p_a
• préchauffage 1h 20 à 250°C
• filage direct à 250°C en barres cylindriques 0 9 mm (rapport de filage λ = 22)

The following range has been applied:

• containerized in Al-Mg Ø 42 x 100 mm
• 24h degassing under 1 to 10- ¹ p _a
• preheating 1h 20 to 250 ° C
• direct spinning at 250 ° C in cylindrical bars 0 9 mm (spinning ratio λ = 22)

The outlet temperature being approximately 330 ° C.

The bars obtained were air-cooled, and characterized by density measurement, Young's modulus, tensile tests (long sense) and micrographic examinations.

Table I brings together the targeted chemical compositions determined by atomic absorption and the results obtained (average of 5 tests). The oxygen content is around 0.5%.

The phase T present was coarse (average size 2 μm, maximum size 5 μm), but homogeneously dispersed except for a few large particles of phase T (100 to 200 μm) whose presence explains the low elongations observed (primers premature rupture). Despite this, we note the interesting level of mechanical characteristics obtained in particular on the Al-6Li-10Si alloy and the significant plastic difference, as well as the significant variations in density and Young's modulus.

• - l'absence quasi-complète de phase δ' Al₃Li et phase à Al Li
• - une taille de grains de l'alliage de 2 à 5 µm.

Micrographic examinations in the raw spinning state reveal:

• - the almost complete absence of phase δ 'Al ₃ Li and phase to Al Li
• - a grain size of the alloy of 2 to 5 μm.

Example 2

Alloys Al, Li, Si including the compositions given in Example 1, were cast in strips of 10 mm x 40 μm approximately, of cross section, on a copper wheel 0 480 mm rotating at 1000 rpm, since 730 to 830 ° C they were characterized by Vickers microhardness under 10 g micrographic examination by optical microscopy, electron and X-ray diffraction in the raw casting state, and after heat treatment of tempering from 1 to 10 h between 200 and 350 ° C, to assess the hot stability and the structural evolution.

The compositions and the results are given in Table II.

The entire ribbon of composition A and the wheel side of the ribbons B, C, D over 20 to 30 μm, exhibited a fine T-phase structure (size <0.4 μm) in the raw casting state, and even after income.

The external part of the ribbons B, C, D and the entire thickness of the ribbons (E, F) had a coarse structure of the order of 1 µm on average (maximum size of 4 µm) in the raw state of casting and after income.

The volume fraction of the precipitates, evaluated by quantitative analysis of images, did not vary significantly during the incomes.

It is noted that the hardness increases with the contents of Li and Si, and the volume fraction of phase T, at least as long as it remains in the form of fine particles.

The fine structures (wheel side) give the alloys according to the invention a very high level of hardness after tempering at 200 ° C, and this remains high even after tempering at 350 ° C, unlike the alloys outside the invention.

Example 3:

Part of the ingots used for the preparation of the powders of Example 1 and which had been cast in cylindrical shells of dimensions Ø 55mm x 175mm with a slow cooling rate (5 ° C / sec. Approximately) typical of conventional casting , were peeled to Ø 48mm then reheated to 400 ° C for 1 hour, then spun to 400 ° C in 0 9mm cylindrical bars and air-cooled.

The mechanical tensile characteristics, measured in the long direction on 3 test pieces per alloy are given in Table III. It has been found that these products have a crippling brittleness which exhibits premature rupture during loading, and practically zero ductility.

The microstructure of these products presents in particular very coarse particles of phase T (Al, Li, Si) of very heterogeneous sizes, rather coarse, of several µm to several hundreds of µm, and clearly higher than 10 µm on average, associated with a small amount of phaseδ Al Li.

This example shows the need to use a method involving rapid solidification for the alloys according to the invention.

• - une densité diminuée de 15 à 20% et un module d'Young augmenté de 15 à 35% par rapport à celle (ou celui) des alliages d'Al conventionnels élaborés par coulée classique en lingots tels que les 2024, le 6061, le 7075 selon les désignations de l'Aluminium Association. Le module spécifique se trouve augmenté de 30 à 60% environ ;
• - une résistance mécanique à froid comparable à celle des alliages d'Al corroyés de moyenne résistance, tels que le 2024-T4, 6061-T6, 7020-76, pas exemple pour les produits contenant des particules de phase T grossière(0,5 à 10 µm),et équivalente à celles des alliages à haute résistance (7075-T6, 2214-T6, 7010-T736 et 7150-T736 ou _T6) pour les produits contenant une phase T fine (0,01 à 0,5 µm);
• - une résistance mécanique à tiède ou à chaud supérieure à celle de tous les alliages d'Al connus élaborés par coulée semi-continue (par ex. les alliages 2214 ou 2219, selon la nomenclature de l'Aluminium Association), en particulier dans le domaine comprit entre 100 et 350°C;
• - une bonne résistance à la corrosion intergranulaire ou localisée malgré les teneurs élevées en Li, en l'absence de phase δ Al Li;
• - une ductilité à chaud ou à froid suffisante permettant leur mise en forme ou leur utilisation comme pièces mécaniques ou éléments de structure ; .
• - des propriétés mécaniques intéressantes obtenues même en l'absence de revenu.
  
  
  

The products obtained according to the invention have the following advantages:

• - a density reduced by 15 to 20% and a Young's modulus increased by 15 to 35% compared to that (or that) of conventional Al alloys produced by conventional casting in ingots such as 2024, 6061, 7075 according to the designations of the Aluminum Association. The specific module is increased by approximately 30 to 60%;
• - a mechanical resistance to cold comparable to that of wrought Al alloys of medium resistance, such as 2024-T4, 6061-T6, 7020-76, not example for products containing particles of coarse T phase (0.5 to 10 µm), and equivalent to those of high strength alloys (7075-T6, 2214-T6, 7010-T736 and 7150-T736 or _T 6) for products containing a fine T phase ( 0.01 to 0.5 µm);
• - a mechanical resistance to lukewarm or hot higher than that of all known Al alloys produced by semi-continuous casting (eg alloys 2214 or 2219, according to the nomenclature of the Aluminum Association), in particular in the range comprised between 100 and 350 ° C;
• - good resistance to intergranular or localized corrosion despite the high Li contents, in the absence of δ Al Li phase;
• - sufficient hot or cold ductility allowing their shaping or their use as mechanical parts or structural elements; .
• - interesting mechanical properties obtained even in the absence of income.
  
  
  

Claims (13)

1. Al-based alloy essentially comprising Li and Si, characterized in that it contains (by weight X):

the total quantity of these secondary optional elements being less than 5%, the remainder being constituted by Al and impurities (each ≤ 0.05 X, total ≤ 0.15%). 2. Alloy according to claim 1, characterized in that the contents of Li and Si are linked by the relation:% Li = 0.4% Si + k with

3. Alloy according to claim 2, characterized in that:

4. Alloy according to one of claims 1 to 3, characterized in that it contains from 4 to 7% of Li. 5. Alloy according to one of claims 1 to 4, characterized in that the total amount of optional secondary elements does not exceed 2 X. 6. Wrought product of composition according to one of claims 1 to 5, characterized in that it contains from 15 to 60X by volume of phase T. 7. Product according to claim 6, characterized in that it contains between 20 and 50X by volume of phase T. 8. Product according to one of claims 6 or 7, characterized in that the size of the particles T is between 0.01 to 10 µm (and preferably 0.01 µm to 5 µm). 9. Product according to one of claims 6 to 8, characterized in that the grain size of the alloy is less than 20 µm (and preferably 10 µm). 10. Method for obtaining an alloy or product according to one of claims 1 to 9, comprising melting, solidification, optional grinding, cold or hot compression, degassing under vacuum, compression and hot working, characterized in that the rate of solidification from the liquid state is greater than 1000 ° C / sec. 11. Method according to claim 10, characterized in that all the hot operations after solidification take place at a temperature below 400 ° C, and preferably 350 ° C. 12. Method according to claim 10, characterized in that the hot working is completed by a slight plastic deformation at a lower temperature. 13. Method according to one of claims 10 or 11, characterized in that one applies an income between 20 and 350 ° C and preferably 150 and 250 ° C, after hot or cold deformation.