Metal treatment – Compositions – Heat treating
Patent
1985-06-24
1988-07-19
Dean, R.
Metal treatment
Compositions
Heat treating
148159, 148417, C22F 104
Patent
active
047582863
DESCRIPTION:
BRIEF SUMMARY
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 bet
REFERENCES:
patent: 4526630 (1985-07-01), Field
Alloys Index, vol. 9, American Society for Metals and Metal Society, pp. E-21, E-22, 1982.
NASA Contractor Report 3578, P. P. Pizzo, 1982.
Dubost Bruno
Meyer Philippe
Cegedur Societe de Transformation de l'Aluminium Pechiney
Dean R.
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