Heat treated and aged Al-base alloys containing lithium, magnesium and copper and process

The present invention relates to Al-base alloys essentially containing additions of Li, Mg and Cu and possibly minor additions of Cr, Zr, Ti and Mn, which have high specific mechanical characteristics, a low density and good resistance to corrosion. The alloys according to the invention contain (in % by weight): Li 1.8 to 3.5; Mg 1.4 to 6.0; Cu 0.2 to 1.6 with Mg/Cu.gtoreq.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 homogenization and solution treatments must be taken to a sufficiently advanced stage to dissolve the quaternary intermetallic phases (Al, Li, Mg, Cu) which are larger than 5 .mu.m in size. Such alloys present a compromise between their mechanical characteristics and their density, which is higher than that of the knwn Al Cu Mg alloys, and alloys containing Li.

The present invention relates to Al-base alloys containing Li, Mg and Cu 
and having mechanical characteristics equivalent to those of conventional 
aluminium alloys with precipitation hardening and of average strength, 
with a density which is reduced by at least 9% with respect to such 
conventional alloys. 
Metallurgists are aware that the addition of lithium reduces density and 
increases the modulus of elasticity and the mechanical strength of 
aluminium alloys. That explains the attraction to designers of such alloys 
for uses in the aeronautical industry and more particularly 
lithium-bearing aluminium alloys containing other additive elements such 
as magnesium or copper. However, it is absolutely essential that such 
lithium-containing alloys enjoy ductility and tenacity that are at least 
equivalent, with the same mechanical strength, to that of conventional 
aeronautical alloys such as alloys 2024-T4 or T351, 2214-T6(51), 
7175-T73(51) or T7652 and 7150-T651 (using the Aluminium Association 
nomenclature), which is not the case with the known lithium-containing 
alloys. 
In the aluminium-lithium-magnesium system, the only known industrial alloy 
is the Soviet alloy 01420, of the following nominal composition (in % by 
weight): Li=2.0 to 2.2; Mg=5.0 to 5.4; Mn=0 to 0.6; Zr=0 to 0.15. That 
alloy imparts mediumly elevated tensile mechanical properties to thin 
plates and extruded products which have been subjected to treatment, in 
state T6 (16 hours at 170.degree. C.) (FRIDLYANDER et al. Met. Science and 
Heat Treatment No 3-4, April 1968, page 212, translation of Metalov. i. 
Term. Obrab. Metallov No 3, page 5052, March 1968), with such 
characteristics being worse than those of the conventional aeronautical 
alloys. Moreover, study in regard to the statistical laws in respect of 
modification of the characteristics of alloys of the Al-Li-Mg-Zr system, 
in dependence on their contents of Li and Mg (I. N. FRIDLYANDER et al. 
"Zavod. Lab.", July 1974, T7, page 847) shows that it is not possible to 
enhance the compromise between mechanical strength and elongation of that 
alloy, to the level of the conventional aeronautical alloys, by reducing 
the amounts of lithium and magnesium. Those trends are confirmed by the 
results obtained by SANDES (final report NADC Contract No N 622 
69-74-C-0438, June 1976), showing that the compromise between elastic 
limit and tenacity of the extruded products of Al-Li-Mg alloys becomes 
higher in proportion to a reducing amount of lithium and, to a lesser 
degree, a reducing amount of magnesium. In particular, the authors show 
that alloys with high overall proportions of lithium+magnesium, in the 
quenched and tempered state, have a compromise as between mechanical 
strength, ductility and tenacity, which is much lower than that of the 
conventional alloys of series 2000 and 7000. 
More recently, metallurgists have proposed novel compositions of 
aluminium-lithium alloys containing copper (Cu=1.5 to 3%) and magnesium 
(Mg=0.5 to 1.4%), of low density and high mechanical strength. That is in 
particular experimental alloy F92 (British Specification DXXXA), of the 
following nominal composition (in % by weight): Li=2.5; Cu=1.2; Mg=0.7; 
Zr=0.12, wherein the compromises in respect of type mechanical 
characteristics as announced in 1983 by British ALCAN, on thin sheets in 
the state T8 (Rm=500 MPa; Rp 0.2=420 MPa; A=6%) and on thick sheets in the 
state T651 (Rm=520 MPa; Rp 0.2=460 MPa; A=7%) show that that alloy has a 
compromise as between mechanical strength and ductility, which is even 
lower than that of the aeronautical alloys of series 2000 and 7000, like 
all the other alloys of AlLiCu and AlLiCuMg systems with a lithium content 
of more than 2%, which are known to date. 
In the course of metallurgical tests, we have found and experimented with 
novel compositions of industrial alloys of the system Al-Li-Mg-Cu (+Cr, 
Mn, Zr, Ti) with higher levels of performance than alloys of the systems 
AlCuMg (2024), AlLiCu and AlLiMg and than the known alloys of the system 
AlLiCuMg, from the point of view of the compromise between mechanical 
strength, density and resistance to inter-granular or flaking corrosion. 
The new alloys according to the invention are of the following compositions 
by weight: 
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Li 1.8 to 3.5% 
Mg 1.4 to 6.0% 
with Mg/Cu .gtoreq. 1.5 
Cu 0.2 to 1.6% 
Fe .ltoreq. 0.20% 
Si .ltoreq. 0.12% 
Cr 0 to 0.3% 
Mn 0 to 1.0% 
Zr 0 to 0.2% 
Zn 0 to 0.35% 
Ti 0 to 0.1% 
Be 0 to 0.02% 
other elements (impurities) 
each &lt; 0.05% 
total &lt; 0.15% 
balance: aluminium. 
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The amount of principal elements is preferably maintained individually or 
in combination between 2.3 and 3.3 for Li, 1.4 and 5% for Mg and 0.25 and 
1.2% for Cu. The amount of Zr is preferably between 0.08 and 0.18%. 
In order to achieve a better compromise in regard to mechanical strength 
and density, the following relationship must also be observed: 
EQU %Li(%Cu+2)+%Mg=K 
with 8.5.ltoreq.K.ltoreq.11.5 and preferably 9.ltoreq.K.ltoreq.11. 
The alloys according to the invention have their optimum strength and 
ductility after treatment for homogenization of the cast products and 
solution treatment of the transformed products comprising at least one 
stage at a temperature .theta. (in .degree.C.) of the order of 
.theta.=535-5 (% Mg) for a sufficient period of time that, after 
quenching, the intermetallic compounds of the quaternary phases (AlLiCuMg) 
which can be detected upon micrographic examination or by electronic or 
ionic microanalysis (SIMS) are of a size of less than 5 .mu.m. The 
homogenization treatment may be carried out in a temperature range of from 
.theta.+10 (.degree.C.) to .theta.-20 (.degree.C.), and the solution 
treatment is preferably carried out at between .theta..+-.10.degree. C. 
The optimum periods for thermal homogenization treatment at the temperature 
.theta. are from 0.5 to 8 hours for the alloys which are produced by rapid 
solidification (atomization-splat cooling--or any other means) and from 12 
to 72 hours for cast products or products which are produced by a 
semi-continuous casting operation. 
Those alloys have their optimum mechanical properties after tempering 
operations for periods of from 8 to 48 hours at temperatures of from 
170.degree. to 220.degree. C. and it is preferable for the products of 
suitable shape (sheets, bars, billets) to be subjected to a cold hardening 
operation which gives rise to a degree of plastic deformation of from 1 to 
5% (preferably from 2 to 4%) between quenching and tempering, which 
permits the mechanical strength of the products to be further improved. 
Under those conditions, the alloys according to the invention have a 
mechanical strength which is higher than that of the alloy AlLiMgMn 01420, 
which did not permit the results of the studies available on the system to 
be foreseen. We have found that the alloys according to the invention have 
a compromise as between mechanical characteristics and density, which is 
higher than that of the known AlLiCuMg alloys (with small proportions of 
magnesium). They also have a satisfactory degree of resistance to 
intergranular or flaking corrosion which is much higher than that of the 
known AlCuMg, AlLiCu and AlLiCuMg alloys. 
Those alloys are therefore a particularly attractive proposition for the 
production of cast or rolled semi-manufactured products (produced by 
semi-continuous casting, atomization or rapid solidification, etc.), 
whether such products are for example extruded, rolled, forged, stamped or 
drop forged products which are used in particular in the aeronautical and 
space industries. 
In particular, it was surprisingly found that the alloys according to the 
invention, with high contents of Li and Mg, could be cast without major 
difficulty in a semi-continuous casting process in the form of billets or 
plates of industrial format (no cracks and porosity). 
The invention will be better appreciated and illustrated by means of the 
following examples: