Cold rolling for aluminum-lithium alloys

Details Cold rolling for aluminum-lithium alloys Cold rolling for aluminum-lithium alloys

1993-12-20

1994-12-20

Dean, Richard O.

Metal treatment

Process of modifying or maintaining internal physical...

With casting or solidifying from melt

148692, 148693, 148696, 148418, 148439, C22F 104

Patent

active

053743210

DESCRIPTION:

BRIEF SUMMARY
This invention relates to aluminium alloys containing lithium which are particularly suitable for aerospace construction and have been found to have improved cold rolling characteristics.
Such alloys are attractive in providing significant weight reduction, for example of up to 20%, over other aluminium alloys, and it is known that they can present high strength and stiffness and have good corrosion resistance properties. However, they have, in the past, in comparison with other aircraft alloys been liable to suffer from a reduction in fracture toughness and can be difficult to cold roll.
Working with small additions of magnesium, copper and zirconium, one successful aluminium-lithium alloy which has been developed commercially is that designated "8090" and described and claimed in EP-B-0088511. This known alloy has the following composition in weight percent:


______________________________________ lithium 2.0 to 2.8 magnesium 0.4 to 1.0 copper 1.0 to 1.5 zirconium up to 0.2 manganese 0 to 0.5 nickel 0 to 0.5 chromium 0 to 0.5 aluminium balance (except for incidental impurities) ______________________________________
This known alloy when measured against previous Al-Li alloys, such as X2020, demonstrates improved fracture toughness whilst not losing other desirable features such as adequate strength.
In EP-B-0088511, the importance of zirconium in controlling grain size and grain growth on recrystallisation is recognised, and the processing of an alloy ingot through the stages of homogenisation, hot working, cold rolling with inter-stage annealing, solution treatment, water quench and stretching is described.
EP-B-0124286 is concerned with an alloy closely similar to the 8090 alloy, except that the copper content thereof has been increased above that described in EP-B-0088511 to at least 1.6% by weight. This alloy is now recognised commercially as "8091". In this patent, the thermal history of the ingot is recognised as having an important bearing upon the isotropy of the final cold rolled sheet or strip, and also upon the ease with which subsequent cold rolling can be performed. Specifically, it is taught in that patent that the cast alloy should be homogenised, hot rolled, cold rolled, solution treated, cold water quenched, and then cold worked, e.g. by stretching.
It has now been found that by using appropriate processing and heat treatment conditions it is possible to produce a sheet or strip material from Al-Li alloys having improved cold rolling characteristics optionally with improved damage tolerance coupled with adequate strength for aerospace construction.
Since "damage tolerance" does not have a precise definition, a set of typical values for the aluminium alloys of the present invention are:


______________________________________ Tensile properties: 0.2% Proof strength >290 MPa Tensile strength >400 MPa Elongation to fracture >10% ______________________________________ Fracture toughness (Kc) measured according to ASTM 561: For a 1.6 mm thick sheet: Panel Width 760 mm 500 mm 400 mm ______________________________________ L-T orientation >105 MPa.sqroot.m >90 Mpa.sqroot.m >85 MPa.sqroot.m T-L orientation >95 MPa.sqroot.m >80 Mpa.sqroot.m >75 MPa.sqroot.m Fatigue crack growth: For a 1.6 mm thick sheet da/dn <0.7 .times. 10.sup.-4 mm/cycle ______________________________________ (Stress intensity factor range = 10 MPa.sqroot.m: stress ratio = 0.1)
With regard to fatigue crack growth, in a test of damage tolerance which is applicable to pressurised fuselage structures, a sample of sheet is subjected to a cyclic tensile stress to cause a fatigue crack to grow. The fatigue crack propagates approximately perpendicular to the axis of the tensile load and continues to grow in this direction until failure occurs. In a sheet of an Al-Li alloy the fatigue crack tends to deviate from the perpendicular direction to grow in a direction closer to parallel to the tensile axis, unless the alloy's composition an

REFERENCES:
patent: 4647318 (1987-03-01), Foreman
patent: 4652314 (1987-03-01), Meyer
patent: 4894096 (1990-01-01), Meyer
K. M. Gatenby; (A Thesis); The Development of Microstructure, Texture and Mechanical Properties During The Production Of Aluminum-Lithium Alloys; Sep. 1988.
Journal De Physique, Colloque C3, supplement au No. 9, Tome 48, Sep. 1987, Proceedings of the Fourth International Aluminium-Lithium Conference, Paris, 10-12 Jun. 1987, editions de Physique, (Paris, FR), M. Goncalves et al.: "Static recrystallization after hot working of Al-Li alloys". See pp. C3-171 to C3-177.

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