CA2049520A1 - Aluminum-lithium alloy having improved properties - Google Patents

Abstract

Abstract Aluminum alloys exibiting improved properties such as tensile strength, ductility and toughness comprise 1.0 - 3.0%
lithium, 1.0 - 3.5% copper, 0.1 - 3.0 % magnesium, 0.30 -0.90 % vanadium and/or 0.05 - 0.20 % beryllium, 0 - 0.30 %
boron, and the balance aluminum. The alloys may also contain up to 0.15 % zirconium. They are particularly useful for aerospace and like metal parts.

Description

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Aluminum-Lithium Alloy Having Improved Properties Background of the Invention The present invention relates to Al-Cu-Li-Mg based alloys having improved properties such as tensile strength, ductility and toughnessO

Aluminum-lithium alloys are important commercial products because they offer the promise of substantial weight savings, particularly in aerospace applications, by virtue of their reduced density and increased elastic modulus compared with conventional aluminum alloys. The increased strength of Al-Li alloys is attributed to precipitation of ~'(Al3Li); however, ~' precipitation also lowers ductility and toughness by strain localization and PFZ (precipitate free zone) formation. In recent years, research on Al-Li alloys has concentrated on improving ductility and toughness in two alloys systems; the quaternary Al-Li-Cu-Mg alloys (AA 8090 type) in which the precipitation of S'(A12CuMg) can occur, and also alloys of the AA 2090 type.

Recently, Al-Li alloys containing both Cu and Mg have been commercialized. These include AA (Aluminum Association) 8090, 2091 and 2090 series alloys. Alloy 8090, as disclosed in U.S. Patent No. 4,588,553 to Evans et: al., contains 1.0 to 1.5 % Cu, 2.0 to 2.8 % Li and 0.4 to 1.0 % Mg. This alloy was designed for good exfoliation corrosion resistance, good damage tolerance and good mechanical strength in aerospace applications. Alloy 2091, with 1.5 - 3.4 % Cu, 1.7 - 2.9 % Li and 102 - 2.7 % Mg, was designed as a high strength, high ductility alloy. IIowever, there is still a need for further improvements to the age hardening behaviour, microstructure and mechanical properties, in particular ductility and impact toughness of such alloys.

Several other patents relating to Al-Cu-Li-My alloys also exist. For instance U.S. Patent 2,915,390 describes a wrought aluminum base alloy containing copper, manganese, cadmium, lithium, magnesium and zinc. It is characterized by a high tensile and yield strength. It may have added thereto grain refining elements such as 0.002 to 0.05 ~ boron or 0.01 to 0.1 % vanadium.
U.S. Patent 4,661,172 relates to an aluminum alloy containing zirconium, lithium, magnesium and copper. It may also contain components such as Yanadium and beryllium.
~owever, these alloys must be produced using a rapid quench technique in order to obtain particles of intermetallics having the specified widths of less than 0.1 micron.
U.S. Patent 4,851,192 relates to an aluminum alloy with structures giving the increased electrical resistivity re~uired for good resistance weldability. It consists essentially of 1.0 5.0 % Li and one or a plurality of members selected from a group consisting of not more than 0.20 % Ti, 0.05 - 0.40 % Cr, 0.05 - 0.30 % Zr, 0.05 - 0.35 % V
and 0.05 - Q.30 % W. It may further include 0.05 - 5.0 % Cu 15 and/or 0.05 - 8.0 % Mg.
It is the object of the present invention to utilize additions of beryllium or vanadium, preferably together with boron, to improve the age hardening, microstructure, tensile properties and fracture behaviour of conventionally cast Al-Li 20 alloys of the 8090 and 2090 types.
Summary of the Invention According to the present invention, novel aluminum alloys are produced which comprise 1.0 - 3.0 % lithium, 1.0 - 3.5 %
copper, 1.0 - 3.0 % magnesium and an additive selected from 25 0.30 - 0.90 ~ vanadium and/or 0.05 - 0.20 ~ beryllium. Boron in an amount up ta 0.30 % may be added, particularly in combination with the vanadium. The balance of the alloy consists essentially of aluminum with the usual minor impurities~ All percentages are percentages by weight.
The aluminum base alloys in accordance with the present invention may be prepared by the addition of vanadium and/or beryllium and boron to alloys of the AA 8090, 2090, 2091 series of alloys.
According to a preferred embodiment the base allcy is 35 AA 8090, containing 2.0 - 2.8 % lithium, 1.0 - 1.5 % copper, 0.4 - 1.0 % magnesium and up to 0.15 % zirconium. When vanadium is used as the additive, it is preferably present in 2 ~ 2 ~

the range of 0.40 - 0.80 % and it i5 preferably used together with 0.10 - 0.30 % boron.
The alloys of the present invention may be processed in accordance with conventional practices and techniques for Al-Li alloys.
The mechanical properties of alloys, in particular aluminum alloys, can generally be improved by the refinement of both the grain structure, and the size and distribution of the precipitate particles of intermetalIic phases. When alloying elements are added to refine the grain structure of castings, usually in concentrations of 0.10 to 0.30 %, the additives are referred to as grain refiners or hardeners. The effect of these additives is developed during the liquid-solid transformation or solidification of the alloy.
In age-hardening processes, the hardening or strengthening of the alloy results from the precipitation of intermetallic precipitates in the solid state, such as the ~-phase (Al2Cu) during the age~hardening of commercial A1-Cu alloys. The refinement of the precipitate is not a grain refining process, but occurs within existing grains, and generally strengthens the alloy undergoing the aging treatment.
The refinement of the precipitate is a nucleation rate-dependent phenomenon, and the refinement increases with increasing nucleation rate.
The app;icant has developed a thermodynamic-kinetic theory for the nucleation process in alloys and this is known as the "nucleation entropy theory" (see Bibliograph on page 10). According to this theory, the nucleation entropy increases with the number of additional elements incorporated in the precipitate, and the increase in nucleation rate (and thus precipitate refinement) increases exponentially with the nucleation entropy. It has been shown that the addition of 0.2 % beryllium can significantly improve the age-hardening response of a binary Al - 3% Cu alloy by refining the e-phase~
which has been shown to incorporate beryllium to form (Al,Be)2Cu.

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The major strengthening phase in Al-Li alloys is the ~'(Al3Li) phase. However, this phase is readily sheared by dislocations and its deformation mode results in strain localization that is responsible for the low ductility of the Al-Li alloys. The addition of sufficient amounts of copper and magnesium to Al-Li alloys results in the formation of the S' phase (A12CuMg), which homogenizes the slip and strain distribution throughout the alloy by providing a dispersion of particlesO The nucleation of the S' phase is sluggish, and is generally accelerated by a prior-aging deformation stage (stretching operation). The deformation introduces defects (dislocations) into the lattice, which promotes the heterogeneous nucleation of S' particles. However, the prior-aging deformation procedure is not always feasible, and severely restricts the commercial potential for the alloy's products.
In the present invention, it has been found that the elements beryllium and vanadium are able to increase the nucleation rate of the precipitates, both S' phase and ~' phase. The beryllium enhances the refinement of ~' and S' precipitates, thereby improving the age-hardening of the alloys. Since vanadium is a transition element with variable valency, it is believed that its atoms have a greater probability of adjusting their bond types to be incorporated into the structures of various compounds, such as ~' and S', to increase the nucleation entropy and nucleation rate. The variability of vanadium in bonding modes is evident in the different Al-V intermetallics and structures that can form, g Al1lV, Al45V7, Al23V4, Al3V, Al8V5, etc. It is believed that the vanadium is incorporated into the ~' phase to form Al3(Li,V), which increases the nucleation rate for refinement.
The addition of at least 0.60 % vanadium was found to improve the age-hardening and mechanical properties of the 8090 alloy signi~icantly more than the 0.30 % vanadium addition. This is consistent with the nucleation entropy model, since a higher vanadium content in the Al-V alloy system is required to nucleate the Al3V phase instead of th~

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Al23V4 and Al45V7 phases. The Al3V phase has a higher nucleation entropy than the latter two, and better refines the structure of the alloy. The above A1 V compounds ultimately transform by a series of peritectic reactions to the Al11V phase.
Since the beneficial effects of the Be and V additions to the gO90 alloy is associated with the refinement of the ~' and S' phase precipitates, other Al-Li-Cu-Mg base alloys precipitating ~' and S' phase can also be improved by appropriate amounts of beryllium and vanadium.
Brief Description of the Drawinqs The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings in which:
Fig. 1 shows a series of age hardening curves for different alloys;
Fig. 2 shows age hardening curves for additional alloys;
Fig. 3a is an optical micrograph of an 8090 alloy aged for 30 minutes at 385C;
Fig. 3b is an optical micrograph of an 8090 alloy containing 0.60 ~ vanadium and aged for 30 minutes at 385C;
Fig. 3c is an optical micrograph of an 8090 alloy containing 0.15 % beryllium and aged for 30 minutes at 385C;
Fig. 4a is a SEM fractograph of an optimally age-hardened 25 8090 alloy;
Fig. 4b is a SEM fractograph of an optimally age-hardened 8090 alloy containing 0.60 % vanadium;
Fig. 5a is an optical micrograph of an as-cast 8090 -0.60 % vanadium alloy without boron, and Fig. 5b is an optical micrograph of an as-cast 8090 -0.60 % vanadium alloy with 0.10 % boron.
Example 1 An as-cast Al-Li-Cu-Mg-Zr alloy in the form of an 8090 base alloy was obtained from Alcan International in Kingston.
Al-5% vanadium and Al-5.23% beryllium master alloys were used in the preparation of the base alloys containing 0.30 % V, 0.60 % V, 0.90 % V, 0.15 % Be, and 0.30 % V ~ 0.10 % Be ë~ 2 ~

(designated as 30V, 60V, 90V, 15Be, and 30VlOBe respectively).
The alloys were prepared in graphite crucibles by induction melting under argon to prevent the possible loss of lithium.
The melts were heated to well above the liquidus temperature (-800C), maintained for about 10 min. to ensure complete homogenization and then poured into graphite molds 25 mm (dia.) x 70 mm (length) at room temperature. The nominal compositions of the alloys in this study are given in Table 1.
Table 1 - Nominal Compositions of Alloys _ .
Alloy Composition (wt%) I _ Li Cu Mg Zr V BeA1 _ 80902.47 1.24 0.77 0.10 Bal.
I _ ~
30V2.33 1.17 0.73 0.09 0.30 _Bal.
_ 60V2.21 1.11 0.69 0.09 0.60 _Bal.
90V2.11 1.05 0.65 0.08 0.90 _Bal.
30VlOBe 2.29 1.15 0.71 0.09 0.30 0.10 Bal.
_ 15Be 2.401.20 0.75 0.10 _ 0.15 Bal.
. _ The as-cast ingots were given solution heat treatments for 4 hours at 590C, followed by a quench in iced brine and aging at a temperature o 190C for 22.5 hours. This was established to be the optimum temperature and time for the aging process.
Figure 1 shows that the 0.15 % beryllium and 0.60 %
vanadium additions increased the peak hardness level of the 8090 alloy by approximately 30 and 20 Vickers hardness points respectively. The combined addition of 0.30 % v~nadium and 0.10 % beryllium did not significantly change the peak hardness level resulting from the 0.30 % vanadium addition alone, which is about 10 hardness points above the 8090 alloy.
Figure 2 shows that the 0.15 % beryllium and the 0.60 %
vanadium additions each result in higher peak hardness levels ~fl~

than for the 8090 alloy deformed (by rolling) by 4%. This shows that the pre-aging deformation stage ~stretching~ is not required to develop the high mechanical properties for 8090 alloy.
The tensile properties for several of the above obtained age-hardened alloys are shown in Table 2 below.
Table 2 . _ _ _ _ _ Alloy 0.2% YS (MPa) UTS (MPa)E (%) 8090 372 4~8 2O5 ~ _ _ . . _ *8090 + 4% def. 429 501 2.0 8090 + 0.15% Be 480 528 2.5 _ .
8090 + 0.60 % V 459 512 4.6 8090 + 0.90 % V 366 450 2.0 * Aged 10 hr.
The deformed 8090 alloy was aged for only 10 hours to avoid over-aging, since the deformation greatly accelerates the age-hardening process. The 0.2 % yield strength for the 8090 alloy was increased by the 0.15 % beryllium and the 0.60 ~ vanadium additions by approximat:ely 29 % and 23 %
respectively. Both the above beryllium and vanadium additions resulted in yield strengths higher than that obtained for the 8090 base alloy by the prior aging deformation treatment. The o.90 % vanadium addition fell slightly below that of the 8090 base alloy, and it is believed that this may be attributed to the increase in number and size of the aluminum-vanadium intermetallic (Al11~) particles, identified by energy dispersive analysis (EDS), that form in the alloy.
Figures 3a, b and c are optical micrographs, magnification 750X, of the alloys, over-aged by aging at 385C
for 30 minutes. The over-aging treatment was required for optical resolution of the precipitate particles. It is evident that the 0.15 ~ beryllium and 0.60 % vanadium additions to the 8090 alloy significantly refined the precipitate particles, identified by energy dispersive h i, ~ 2 ~

analysis (EDS) as S' phase (Al2CuMg,lath-like) and the ~' phase (Al3Li,spheroidal). The higher peak hardness levels and higher strengths resulting from the beryllium and vanadium additions to 8090 can be attributed to the refinement of the precipitate particles.
Figures 4a and 4b are scanning electron microscope (SEM) micrographs (fractographs), showing the fracture faces o~ the tensile test specimens of the above optimally age-hardened alloys. The 8090 alloy shows a much coarser structure and most grains exhibit cleavage-like features (Fig. 4a), characteristic of brittle ~racturesO The fractograph of the 8090 alloy containing 0.60 % vanadium shows a much finer structure, the individual grains exhibiting dimple-like features (Fig. 4b), characteristic of ductile fractures. The higher ductility obtained with the 8090 alloy containing the vanadium addition is consistent with the above fracture modes exhibited in the fractographs, indicating the vanadium addition has refined the precipitates and correspondingly improved the ductility of the alloy.
The optimally age hardened alloys were tested for impact toughness with energy values measured by the Charpy V-notch method. The energy values ar~ given in Joules, and the results are shown in Table 3 below.
Table 3 . _ _ __ --_ _. _ 25 Alloy Mold, diameter Cooling Rate CVN
(C/sec) (J) _ I
8090 graphite, 25 mm 128 4.5 _ 8090 + 0.15% Be graphite, 25 mm 128 4.5 _ __ _ 8090 + 0.60% V graphite, 25 mm 128 2.3 _ _ _ _ .
8090 + 0060% V steel, 15 mm 340 4.5 _ . .

30 0.10~ B graphite, 25 mm 128 5.0 2 ~

The cooling rates shown are calculated rates, based on a heat balance calculation, and were obtained by using molds of different diameters and materials. The 0.60 % vanadium addition decreased the toughness of the 8090 base alloy by about 50 %, from 4.5 to 2.3 Joules. The toughness for the V-containing alloy was restored by the faster solidification rate ~340C/sec.) obtained by using a steel mold of smaller diameter. The addition of 0.10 % boron to the 8090 + 0.60 %
vanadium alloy increased the toughness value above that for 10 the 8090 base alloy to 5.0 Joules. The increased cooling rate has the effect of significantly decreasing the size of the A111V particles that form in the alloys.
Figures 5a and 5b are optical micrographs showing the microstructures of the as-cast alloys. Both 8090 + 0.60 %
15 vanadium and the 8090 + 0.60 % vanadium + 0.10 % boron alloys were solidified at the slowest solidification rate of 128C/sec. The boron addition modified the morphology of the intermetallic A111V particles, making the particles slightly smaller, but less faceted and angular, and fewer plate-like particles are evident. The presence of fewer plate-like particles and the less faceted, more rounded shapes, diminishes the ability of the intermetallic particles to act as stress risers, which results in the improved toughness values.

Claims (7)

1. An aluminum-lithium alloy having improved age-hardening and mechanical properties, consisting essentially of 1.0 - 3.0 % lithium, 1.0 - 3.5 % copper, 0.1 - 3.0 %
magnesium, 0.30 - 0.90 % vanadium and/or 0.05 - 0.20 %
beryllium, 0 - 0.30 % boron, and the balance aluminum.

2. An alloy as claimed in claim 1 consisting essentially of 2.0 - 2.8 % lithium, 1.0 - 1.5 % copper, 0.40 - 1.0 %
magnesium, 0.30 - 0.90 % vanadium and/or 0.10 - 0.20 %
beryllium, 0 - 0.30 % boron, and the balance aluminum.

3. An alloy as claimed in claim 2 consisting essentially of 2.0 - 2.8 % lithium, 1.0 - 1.5 % copper, 0.40 - 1.0 %
magnesium, 0.30 - 0.90 % vanadium, 0.10 - 0.30 % boron, and the balance aluminum.

4. An alloy as claimed in claim 1, 2 or 3 wherein the vanadium is present in an amount of 0.4 - 0.8 %.

5. An alloy as claimed in claim 2 consisting essentially of 2.0 - 2.8 % lithium, 1.0 - 1.5 % copper, 0.40 - 1.0 %
magnesium, 0.10 - 0.20 % beryllium and the balance aluminum.

6. An alloy as claimed in claim 2 consisting essentially of 2.0 - 2.8 % lithium, 1.0 - 1.5 % copper, 0.4 - 1.0 %
magnesium, 0.30 - 0.90 % vanadium, 0.1 - 0.2 % beryllium, 0 -0.30 % boron, and the balance aluminum.

7. An alloy as claimed in claim 1, 2, 3, or 5 wherein up to 0.15 % zirconium is present.