Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Patent
1997-04-04
1999-01-12
Phipps, Margery
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
148689, 148690, 148439, 148700, 148701, C22F 104
Patent
active
058581340
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns high strength AlSiMgCu aluminium alloy products designated by the 6000 series of the international nomenclature of the "United States Aluminum Association", for structural applications, in particular in the aeronautical industry.
DESCRIPTION OF RELATED ART
Some alloys in the 6000 series have superior properties which render them suitable for the most demanding structural applications.
Thus United States patent U.S. Pat. No. 4,082,578 from ALCOA describes two families of alloys, subsequently registered with the Aluminum Association and designated 6009 and 6010, the first with superior formability and the second with superior mechanical strength. These alloys have good dent resistance, stress corrosion resistance and exfoliation resistance, as well being well suited to resistance spot welding. They are thus particularly suitable for automobile construction (bodywork and bumpers).
These alloys have the following composition (by weight):
In some cases, in the T6 temper (in the Aluminum Association designation), an ultimate tensile strength R.sub.m of 400 MPa and a yield strength of 370 MPa at 0.2%, R.sub.0.2, can be exceeded.
U.S. Pat. No. 4,614,552 from ALCAN concerns aluminium alloy sheets, also for automobile bodywork, with the following composition:
This alloy was subsequently registered under designation AA 6111. In common with alloys 6009 and 6010 above, it does not have good resistance to intercrystalline corrosion in the T6 temper.
U.S. Pat. No. 4,589,932 from ALCOA proposes an alloy for automobile, rail, naval or aeronautical construction which was subsequently registered under designation AA 6013, with the following composition:
The alloy is solution heat treated at 549.degree. C. to 582.degree. C., this temperature being close to the solidus temperature.
The sheets obtained compare vary favourably as regards yield strength and toughness, with coated alloy 2024 which is currently used for aircraft fuselages. Further, the manufacturing costs are lower.
However, some studies published in the scientific press have shown that this alloy has a high sensitivity to intercrystalline corrosion in the T6 temper (see T. D. Burleigh, "Microscopic Investigation of the Intergranular Corrosion of 6013-T6", in ICAA3, Trondheim 1992, p 435).
Our European patent EP-A-0 173 632 concerns extruded or forged products of an alloy with composition:
That alloy, subsequently registered under designation AA 6056, has very good mechanical properties for both strength and ductility:
Our studies have shown that this alloy is also sensitive to intercrystalline corrosion in the T6 temper, with analogous results to those of 6013 (see M. Reboul et al., "Stress Corrosion Cracking of High Strength Al Alloys), in ICAA3, Trondheim 1992, p 455).
SUMMARY OF THE INVENTION
It has been noticed that the use of a particular region within the composition range of 6000 alloys containing Si, Mg and Cu, combined with a particular intercrystalline corrosion desensitising treatment, can produce both mechanical properties equivalent to those of alloy 2024 in the T3 temper and a considerably improved resistance to intercrystalline corrosion in the non coated temper, meaning that alloys of this type treated in this fashion are particularly suitable for the production of aircraft fuselages and, more generally, to high strength structural applications.
The invention thus provides a process for the production of wrought products of high strength AlSiMgCu aluminium alloy with good intercrystalline corrosion resistance, comprising the following steps: Mg/Si<1 470.degree. C. to 570.degree. C.; C. to 570.degree. C.; 150.degree. C. to 250.degree. C., preferably in the range 165.degree. C. to 220.degree. C., for a period which is in the range 30 h to 300 h, preferably in the range 70 h to 120 h, measured as an in equivalent period at 175.degree. C.
Preferably, annealing comprises a further temperature plateau at a higher temperature which is in the range 185.degree. C. to 250.degree. C., the equivalent peri
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Bechet Denis
Warner Timothy
Pechiney Rhenalu
Phipps Margery
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