EP0717784B1 - Aluminium-silicon alloy sheet for mechanical, aircraft and space applications - Google Patents

Abstract

PCT No. PCT/FR95/00693 Sec. 371 Date Jan. 22, 1996 Sec. 102(e) Date Jan. 22, 1996 PCT Filed May 29, 1995 PCT Pub. No. WO95/34691 PCT Pub. Date Dec. 21, 1995The invention relates to an aluminum alloy sheet heat treated by natural aging, quenching and possibly tempering so as to obtain a yield strength greater than 320 MPa, for use in mechanical, naval, aircraft, or spacecraft construction, with a composition (by weight) of: Si: 6.5 to 11% Mg: 0.5 to 1.0% Cu: <0.8% Fe: <0.

Description

FIELD OF THE INVENTION

The invention relates to the field of alloy sheets medium and high strength aluminum used in the mechanical, aeronautical and space engineering and in armament.

PRIOR ART

For many years, high-strength aluminum alloys have been used in aeronautical and space construction, essentially Al-Cu alloys of the 2000 series (according to the designation of the Aluminum Association in the USA), for example alloys 2014 , 2019 and 2024, and Al-Zn-Mg and Al-Zn-Mg-Cu alloys of the 7000 series, for example alloys 7020 and 7075.
The choice of an alloy and a transformation range, in particular of heat treatment, results from an often delicate compromise between various properties of use such as static mechanical characteristics (resistance to rupture, elastic limit, modulus d (elasticity, elongation), resistance to fatigue, important for airplanes subjected to repeated cycles of takeoff-landing, toughness, that is to say resistance to the propagation of cracks, and corrosion under stress. In addition, the ability of the alloy to be cast, rolled and heat treated under good conditions, its density and possibly its weldability must be taken into account.
For more than thirty years, continuous progress has been made to improve the properties of the 2000 and 7000 alloys used in thin sheets for the fuselage of aircraft and in medium and thick sheets for the wings or cryogenic tanks of launchers and missiles, in the aim, in particular, to lighten the structures without compromising the other properties.
An important step in the reduction has been accomplished with the development of aluminum-lithium alloys. Thus, an alloy 8090 with 2.6% of lithium leads to a specific modulus (ratio of the modulus of elasticity to the density) higher by approximately 20% than that of 2024 and 24% than that of 7075. The alloys with higher copper content and lower lithium content, like 2095, have also been developed because of their good compromise between density, modulus of elasticity and solderability. In this case, the gain on the specific module is around 12% compared to 2219. However, these alloys are still little used, mainly because of their high manufacturing cost.

OBJECT OF THE INVENTION

The Applicant, continuing its research into alloys to lighten the structures of aircraft, has noticed that another category of alloys usually used in molded form, Al-Si alloys of the 4000 series, not only makes it possible to improve significantly, between 3 and 10%, the specific modulus compared to alloys 2000 and 7000, but also had a bundle of properties in terms of toughness, resistance to fatigue and corrosion under stress meeting the severe requirements of aeronautical construction, without causing difficult problems in casting, rolling and heat treatment. In addition, these alloys have a much better weldability than most of the 2000 and 7000, and at least equivalent to the alloys of these series specially dedicated to welding, such as alloys 2219 and 7020. Finally, they have a much better temperature resistance than that of most alloys 2000 and 7000, and at least equivalent to that of alloys in these series specially studied for their temperature resistance, such as alloys 2019 and 2618.
Al-Si alloys are widely used for the production of molded parts. However, in this form, they have far lower mechanical strength, fatigue and toughness properties than the wrought and transformed alloys 2000 and 7000 used in structural parts. In rare cases, they can be used in laminated form, in particular for covering plated sheets intended for the manufacture of brazed heat exchangers. Alloys 4343, 4104, 4045 and 4047 are thus used, for example, the properties sought in this case being essentially a low melting temperature and good wettability.
Al-Si alloys can also be spun in the form of bars or profiles which, due to their good resistance to wear and temperature, are used in mechanical parts such as connecting rods, brake master cylinders, drive shafts , bearings and various components of motors and compressors. One of the alloys used for this purpose is 4032.
French patent FR 2291284 describes the manufacture of AlSi alloy sheets containing 4 to 15% Si by continuous casting between two cooled cylinders. This casting mode is intended to increase the elongation at break, and therefore the formability. They are not high-strength sheets usable in structural applications, since the sheets are simply annealed and the elastic limits exemplified do not exceed not 220 MPa.
But no one until now has had the idea of developing, thanks to a judicious choice of composition and an appropriate range of heat treatment, sheets of Al-Si alloys with high mechanical resistance usable for structural applications , in particular in mechanical, naval or aeronautical construction, by mechanical or welded assemblies.
The subject of the invention is therefore sheets heat treated by dissolving, quenching and optionally tempering so as to obtain an elastic limit R _0.2 greater than 320 MPa, intended for mechanical, naval, aeronautical or space construction made of alloy of following composition (by weight): Yes 6.5 to 11% Mg 0.5 to 1.0% Cu <0.8% Fe <0.3% Mn <0.5% and / or Cr <0.5% Sr 0.008 to 0.025% Ti <0.02% the total of the other elements being less than 0.2%, the rest being aluminum.
The silicon content is preferably between 6.5 and 8%, corresponding to that of the AS7G alloy.
Another object of the invention is the use of especially medium or thick sheets of this alloy for the undersides of aircraft wings, of particularly thin sheets for the coating of aircraft fuselages, of sheets for the manufacture of tanks cryogenic rockets, floors and skips of industrial vehicles and hulls or supersructures of boats.

Another object of the invention is the method of manufacturing sheets according to claim 9.

DESCRIPTION OF THE INVENTION

The sheets according to the invention have silicon contents generally corresponding to the fields of alloys AS7G and AS9G according to French standard NF A 57-702 or the designations A 357 and A 359 of the Aluminum Association.
Magnesium should not exceed 1% to avoid the formation of the insoluble intermetallic compound Mg ₂ Si. Copper must be limited to 0.8% to avoid the formation of insoluble phases Mg ₂ Si and Q (AlMgSiCu). This content also makes it possible to limit the sensitivity to intercrystalline corrosion.
Iron is also limited to 0.3%, and preferably 0.08%, as it is in 7000 alloys for heavy plate, when good toughness and / or good elongation. The presence of titanium is linked to the refining of the titanium plates, identical to that used for current medium and high strength alloys.

As is usually the case with quality molding alloys, it is necessary to modify the alloy to avoid the formation of primary silicon and to obtain a finely dispersed fibered eutectic structure. For this operation, strontium is preferable to sodium which could cause hot fragility during processing.
The sheets according to the invention can be obtained by vertical casting of plates, hot rolling up to 6 mm, optionally cold rolling in the case of thin sheets, dissolving between 545 and 555 ° C, quenching with cold water, maturation at room temperature and / or tempering between 6 and 24 hours at a temperature between 150 and 195 ° C.
Hot rolling can be preceded by homogenization between 530 and 550 ° C for a duration of less than 20 h, short enough to avoid a globalization of the fibrous eutectic and a marked coalescence of the manganese and / or chromium, when the alloy contains it. In the absence of homogenization, a very fine, non-globular eutectic microstructure is obtained in the final state, which has a favorable effect on the toughness.
One can thus obtain in the state T6 an elastic limit higher than 320 and even 340 MPa, an elongation higher than 6% in the direction TL and 9% in the direction L, and a tenacity, measured by the critical factor of intensity of constraints K1c, greater than 20 MPaVm.
Under these conditions, the alloy is weldable by conventional TIG or MIG processes, continuous or pulsed, depending on whether it is a thin or thick sheet, and its density is always lower than that of traditional 2000 and 7000 alloys. as well as Al-Li alloys with a lithium content of less than 1%

EXAMPLES Example 1: homogenized sheet

Plates with a cross section of 380 x 120 mm of alloy of the following composition (by weight) were produced by vertical casting: Yes 6.77% Mg 0.59% Cu 0.24% Fe 0.06% Mn 0.31% Sr 0.016% Ti 0.01% the total of the other elements being less than 0.2% and the rest being aluminum.
The alloy was homogenized at 550 ° C for 8 hours, after a temperature rise of 4 hours, reheated for 2 hours at 500 ° C, then hot rolled up to 20 mm thick on a reversible rolling mill. Cut sheets were placed in solution for 2 hours at 550 ° C, soaked in water and subjected to an income of 8 hours at 175 ° C, ie a state T651 according to the designations of the Aluminum Association.
The alloy has a density of 2,678 and a modulus of elasticity E of 74,100 MPa, or a specific module of 27,670 MPa, was measured on the sheet by the hysteresis loop method in tension. 2.770, 72,500 MPa and 26,175 MPa respectively for a sheet of the same thickness in 2024 alloy in the T351 state, an increase of 5.7% in the specific module. This increase is more than 9% higher compared to alloy 2219 for welded construction.
The mechanical characteristics, compared to those of a sheet in 2024 T351, are as follows: alloy meaning R _0.2 MPa R _m MPa AT % meaning K _1c MPavm invention L 358 386 9.4 LT 20 " TL 350 386 6.6 TL 19 2024 L 350 485 18.0 LT 35 " TL 345 489 17.1 TL 32

Example 2: non-homogenized sheet

With the same alloy as in Example 1, the same operations are carried out, except that the plate does not undergo homogenization before the reheating preceding the hot rolling. A modulus of elasticity of 74,170 MPa is measured on the 20 mm thick sheet, an increase of 5.7% in the specific module compared to the 2024 T351.
The mechanical characteristics measured on the 20 mm sheet are as follows: meaning R _0.2 MPa R _m MPa AT % meaning K _1c MPavm L 359 384 10.0 LT 22.1 TL 346 383 6.9 TL 19.1

We find that the absence of homogenization has an effect favorable on elongation and tenacity. An exam compared micrographic shows that the average size of silicon particles, which was around 7 microns for the homogenized sheet becomes less than 4 microns for the non-homogenized sheet.

Claims (11)

1. High strength aluminum alloy sheet metal heat treated by solution heat treating, quenching and possibly ageing to obtain a yield strength R_0.2 greater than 320 MPa, for use in mechanical construction, shipbuilding, and the aeronautic and space industries, with composition (by weight) Si 6.5 to 11% Mg 0.5 to 1.0% Cu < 0.8% Fe < 0.3% Mn < 0.5% and/or Cr: 0.5% Sr 0.008 to 0.025% Ti < 0.02% total other elements 0.2% the remainder aluminum.
2. Sheet metal according to claim 1, characterized in that the Si content is between 6.5 and 8%.
3. Sheet metal according to one of claims 1 and 2, characterized in that the iron content is less than 0.08%.
4. Use of sheet metal according to one of claims 1 to 3 for the manufacture of aircraft lower wing skins.
5. Use of sheet metal according to claims 1 to 3 for the skin of aircraft fuselages.
6. Use of sheet metal according to one of claims 1 to 3 for the manufacture of rocket cryogenic tanks.
7. Use of sheet metal according to one of claims 1 to 3, for the manufacture of industrial vehicle floors or bins.
8. Use of sheet metal according to one of claims 1 to 3 for the manufacture of ship hulls and superstructures.
9. Process for manufacturing sheet metal according to one of claims 1 to 3, comprising the following steps:

   cast a slab

   reheat to between 480 and 520°C

   hot and possibly cold rolling

   solution heat treat between 545 and 555°C

   quench in cold water

   and natural and/or artificial ageing
10. Process according to claim 9, characterized in that it includes homogenization between 530 and 550°C for not more than 20 h, before reheating.
11. Process according to one of claims 9 and 10, characterized in that it comprises 6h to 24h of ageing at between 150 and 195°C.