EP0787217B1 - METHOD FOR MAKING AlSiMgCu ALLOY PRODUCTS HAVING ENHANCED INTERCRYSTALLINE CORROSION RESISTANCE - Google Patents

Description

Technical area

The invention relates to the field of alloy products high-strength aluminum AlSiMgCu, belonging to the 6000 series according to the international aluminum nomenclature Association in the United States, and intended for applications structural, in particular in aeronautical construction.

State of the art

Among the 6000 series alloys, some have high characteristics which make them suitable for the most demanding structural applications.

Thus, US patent 4082578 of ALCOA describes two families of alloys, subsequently registered with the Aluminum Association under the numbers 6009 and 6010, the first favoring formability and the second mechanical strength. These alloys have good resistance to indentation, stress corrosion and exfoliating corrosion, as well as good spot welding ability, which makes them particularly suitable for automobile construction (bodywork and bumpers).
These alloys have the following composition (by weight): Yes 0.4 - 1.2% Mg 0.4 - 1.1% Cu 0.1 - 0.6% Mn 0.2 - 0.8% Fe 0.05 - 0.35% In certain cases, one can exceed in state T6 (according to the designation of the Aluminum Association) 400 MPa for the breaking strength R _m and 370 MPa for the elastic limit at 0.2% R _0.2 .

ALCAN patent US 4,614,552 covers aluminum alloy sheets, also intended for automobile bodywork, of composition: Yes 0.60 - 1.0% Mg 0.62 - 0.82% Cu 0.65 - 0.79% Mn 0.10 - 0.50% Fe <0.40% Ti <0.10% other <0.05% each and <0.15% in total. This alloy was subsequently registered under the designation AA 6111. Like the alloys 6009 and 6010 mentioned above, it does not exhibit good resistance to intercrystalline corrosion in the T6 state.

The US patent 4589932 of ALCOA proposes for the automobile, railway, naval or aeronautical construction, an alloy, subsequently registered under the designation AA 6013, of composition: Yes 0.4 - 1.2% and preferably 0.6 - 1% Mg 0.5 - 1.3% " 0.8 - 1.2% Cu 0.6 - 1.1% Mn 0.1 - 1% " 0.2 - 0.8% Fe <0.5% Cr <0.10% Ti <0.10% Zn around 0.25% The alloy undergoes dissolution between 549 and 582 ° C, this temperature being close to the temperature of the solidus.
The sheets obtained compare very favorably, in terms of elastic limit and toughness, to the plated alloy 2024 commonly used for the fuselage of aircraft, and, moreover, the manufacturing cost is lower.
However, a certain number of studies published in the scientific press show a high sensitivity to intercrystalline corrosion of this alloy in the T6 state (cf. TD BURLEIGH "Microscopic investigation of the intergranular corrosion of 6013-T6" in ICAA3 Trondheim 1992 , p. 435).

Patent EP 173632 of the applicant relates to extruded or stamped alloy products of composition: Yes 0.9 - 1.3% and preferably 1 - 1.15% Mg 0.7 - 1.1% " 0.8 - 1% Cu 0.3 - 1.1% " 0.8 - 1% Mn 0.5 - 0.7% Zr 0.07 - 0.2% " 0.08 - 0.12% Fe <0.30% Zn <0.7% " 0.3 - 0.6% having an essentially non-recrystallized structure. This alloy, subsequently registered under the designation AA 6056, has very high mechanical characteristics, both in strength and in ductility:
R _m > 420 MPa R _0.2 > 380 MPa A> 10%
The applicant's studies show that this alloy is also sensitive to intercrystalline corrosion in the T6 state, with results similar to those of 6013 (cf. M. REBOUL et al. "Stress Corrosion cracking of high strength Al alloys" in ICAA3 Trondheim 1992, p. 455).

Subject of the invention

The Applicant has noticed that the use of a particular domain within the composition domain 6000 alloys loaded with Si, Mg and Cu, associated with a special treatment for corrosion desensitization intercrystalline, allowed to obtain both mechanical properties equivalent to those of the alloy 2024 in T3 state and corrosion resistance significantly improved intercrystalline in the unplated state, which makes alloys of this type thus treated particularly suitable for making fuselages aircraft and, more generally, for structural applications   high resistance.

• coulée d'une plaque ou billette de composition (en poids): Si 0,7 - 1,3% Mg 0,6 - 1,1% Cu 0,5 - 1,1% Mn 0,3 - 0,8% Zr < 0,20% Fe < 0,30% Zn < 1% Cr < 0,25% Ag < 1% autres éléments < 0,05% chacun et < 0,15% au total reste aluminium.
  avec:   Mg/Si < 1
• homogénéisation de cette plaque ou billette à une température comprise entre 470 et 570°C
• corroyage à chaud et éventuellement à froid
• mise en solution à une température comprise entre 540 et 570°C
• trempe
• revenu comportant au moins un palier à une température comprise entre 150 et 250°C, et de préférence entre 165 et 220°C, et d'une durée comprise entre 30h et 300h, de préférence entre 70 et 120h en durée équivalente à 175°C.
  Le revenu comporte, de préférence, un autre palier à température plus élevée comprise entre 185 et 250°C, la durée équivalente à 175°C étant toujours, pour l'ensemble des 2 paliers, comprise entre 30 et 300h.

The subject of the invention is therefore a method of manufacturing wrought products of AlSiMgCu aluminum alloy with high mechanical strength and having good resistance to intercrystalline corrosion, comprising the following steps:

• pouring a composition plate or billet (by weight): Yes 0.7 - 1.3% Mg 0.6 - 1.1% Cu 0.5 - 1.1% Mn 0.3 - 0.8% Zr <0.20% Fe <0.30% Zn <1% Cr <0.25% Ag <1% other elements <0.05% each and <0.15% in total remains aluminum.
  with: Mg / Si <1
• homogenization of this plate or billet at a temperature between 470 and 570 ° C
• hot and possibly cold working
• dissolved at a temperature between 540 and 570 ° C
• quenching
• tempering comprising at least one level at a temperature between 150 and 250 ° C, and preferably between 165 and 220 ° C, and with a duration between 30h and 300h, preferably between 70 and 120h in duration equivalent to 175 ° vs.
  The tempering preferably comprises another level at a higher temperature between 185 and 250 ° C, the duration equivalent to 175 ° C being always, for all 2 levels, between 30 and 300 hours.

The subject of the invention is also a laminated product or aluminum alloy spun of the above-mentioned composition,   desensitized to intercrystalline corrosion (in the sense of the US Department of Defense standard MIL-H-6088) and having, in this desensitized state, a conductivity at least 0.5 MS / m higher than that measured at state T6.

It also relates to a fuselage element airplane or structural element of a road vehicle or rail made from products according to the invention or of products produced according to the process of the invention.

Description of the invention

The alloys according to the invention having an Mg / Si ratio <1 have a rather higher silicon content, since the Mg composition ranges are typical of alloys 6000 series. Surprisingly better resistance to intercrystalline corrosion by increasing the Si content, while this is deemed to act in the sense opposite. Thus, Kemal NISANCIOGLU in the SINTEF Report A 820/3 du 23/8/1982 "Intercrystalline, stress and exfoliation corrosion of AlMgSi alloys. A litterature survey. "ISBN n ° 82-0595-2860-6, p.7, mentions that "the susceptibility to intercrystalline corrosion (in state T6) increases with the Si content, especially for alloys where Si is in excess over stoichiometric content ".

It can be seen that with alloys located in the same composition ranges, but with an Mg / Si ratio> 1, the particular income does not provide a satisfactory desensitization to corrosion intercrystalline. There are indeed traces locally of intercrystalline attack. Desensitization could without doubt be obtained, but at the cost of degradation unacceptable mechanical characteristics.

We also observe, for the alloys according to the invention having a Mg / Si ratio <1 and desensitized to intercrystalline corrosion, numerous intergranular precipitates in the form of planchettes, whereas these are rather in the form of needles with state T6. At least some of these board-shaped precipitates contain AlMgSiCu quaternary compounds.
Furthermore, the desensitized alloys according to the invention have a higher electrical conductivity of at least 0.5 MS / m compared to the electrical conductivity in the T6 state when the income applied is of the two-bearing type and of 1 MS / m in the case of single income.

The Cu content must be> 0.5% to have both sufficient mechanical characteristics and good stability thermal of the alloy. Beyond 1.1%, we risk seeing problems of stress corrosion and exfoliating corrosion, as well as a decrease in the tenacity to because of primary copper particles.

An addition of Zn at a content between 0.15 and 1% has, for an identical composition and income, an influence positive on the resistance to intercrystalline corrosion. In addition, an addition of around 0.5% Ag allows improve mechanical characteristics.

The products according to the invention can be sheets rolled or extruded sections. The alloy is cast in plates (for sheets) or in billets (for profiles) and its transformation range is relatively classic up to final income. Homogenization takes place between 480 and 570 ° C for a period between 5 and 50h. We then proceed to working by hot rolling or spinning, then, in the case sheets, cold rolling to a thickness included between 0.5 and 15 mm. We then perform a solution thrust at a temperature close to solidus, between 540 and 575 ° C, then water quenching with a speed of cooling dependent on the thickness of the product.  

Income is a special heat treatment which allows both to obtain the mechanical characteristics required while desensitizing the alloy to corrosion intercrystalline. This treatment can either be a treatment single-bearing at a temperature between 150 and 250 ° C, and preferably between 165 and 220 ° C, or a two-bearing treatment, one of the bearings being at a temperature between 150 and 250 ° C (preferably 165 and 220 ° C) and the other at a higher temperature, between 170 and 270 ° C.

The processing time depends on the temperature. We can reduce this duration to a time equivalent to 175 ° C t _eq , linked to the temperature T of the plateau in ° K and to the duration t of treatment at this temperature (the duration of temperature rise being taken into account in the calculation equivalent time) by the relation: (t eq / 448) exp (-Q / 448R) = t / T exp (-Q / RT) in which Q = 145000 J / mol and R is the constant of ideal gases.
For the two-bearing treatments, it is found that partial desensitization to intercrystalline corrosion is obtained for t _eq > 30 h and a total desensitization for t _eq > 70 h. By partial desensitization is meant the absence of intercrystalline ramifications of length greater than 20 microns, on a polished section produced following the test according to the American military standard MIL-H-6088. Desensitization is considered complete in the absence of ramifications larger than 5 microns.
It is not recommended to exceed an equivalent time of 120 h, because there is then a too significant degradation of the elastic limit which falls clearly below 300 MPa. The optimum desensitization range is between 70 and 120 h for two-bearing treatments and between 150 and 250 h for single-bearing treatments. Following this income, it can be seen that the difference in conductivity with state T6 is always greater than 0.5 MS / m.
One can also practice a single-bearing treatment, but, to be effective, it must have an equivalent duration greater than that of a two-bearing treatment, which generally leads to lower mechanical characteristics. This equivalent duration is preferably between 150 and 250 h. In this case, the deviation in conductivity from the T6 state is at least 1 MS / m.
The products produced according to the invention have a good elastic modulus and an excellent specific modulus (quotient of the modulus by density) taking into account their lower density than that of alloys 2000 for example. Thus, for sheets of 1.6 mm thickness, a module of 71 GPa was measured, barely less than the module of sheets of the same thickness in bare 2024 alloy, and clearly greater than that of the plated 2024 usually used for fuselage commercial aircraft.
These products also exhibit, thanks to the high temperature tempering, good thermal stability which makes them suitable, for example, for use in the fuselage of supersonic aircraft.

Examples Example 1

limite élastique R_0,2 = 375 MPa

résistance à la rupture R_m = 417 MPa

allongement A = 14%

Sa conductivité électrique est de 24,0 MS/m.
Différents traitements thermiques ont été effectués sur ces tôles pour essayer de les désensibiliser à la corrosion intercristalline. On a utilisé, pour qualifier cette sensibilité, soit un test nommé "Interneutre", correspondant à la norme militaire américaine MIL-H-6088, soit un test interne nommé "Interano", consistant en une attaque anodique de l'échantillon, pendant 6h, en milieu chlorures - perchlorates et sous une densité de courant de 1 mA/cm², suivie d'un examen en coupe micrographique.
Les températures équivalentes de revenu ainsi que les résultats en matière de caractéristiques mécaniques dans le sens travers et corrosion intercristalline sont rassemblés dans le tableau 1. An alloy of composition has been developed in the form of a plate: Yes 0.79% Mg 0.94% Cu 1.0% Mn 0.58% Fe 0.22% Zn 0.15% therefore with an Mg / Si ratio = 1.2.
The plate was homogenized 21 h at 530 ° C, peeled, then hot and cold rolled to a thickness of 1.6 mm. The solution was carried out at 550 ° C for 1 hour.
The standard income for such an alloy, leading to the T6 state, would be 8 hours at 175 ° C. and the transverse mechanical characteristics obtained in this case are:

elastic limit R _0.2 = 375 MPa

breaking strength R _m = 417 MPa

elongation A = 14%

Its electrical conductivity is 24.0 MS / m.
Various heat treatments have been carried out on these sheets to try to desensitize them to intercrystalline corrosion. To qualify this sensitivity, we used either a test called "Interneutre", corresponding to the US military standard MIL-H-6088, or an internal test called "Interano", consisting of an anodic attack on the sample, for 6h , in chlorides - perchlorates medium and at a current density of 1 mA / cm ² , followed by an examination in micrographic section.
The equivalent tempering temperatures as well as the results in terms of mechanical characteristics in the cross direction and intercrystalline corrosion are collated in Table 1.

Example 2

pour A    R_0,2 = 350 MPa R_m = 380 MPa A = 13%

pour B   R_0,2 = 363 MPa R_m = 400 MPa A = 14%

Les conductivités à l'état T6 des alliages A et B sont respectivement de 24,3 et 24,7 MS/m.
Différents traitements thermiques de revenu ont été effectués sur ces tôles pour essayer de les désensibiliser à la corrosion intercristalline, qui a été qualifiée par des tests accélérés "Interneutre" et "Interano".
Les temps équivalents à 175°C, les caractéristiques mécaniques dans le sens travers, la conductivité électrique et la sensibilité à la corrosion intercristalline ont été rassemblés dans les tableaux 2 (pour l'alliage A) et 3 (pour l'alliage B). Two alloys A and B of the following composition were produced in the form of a plate: AT B Yes 0.95 0.82 Mg 0.87 0.80 Cu 0.80 1.0 Mn 0.63 0.58 Fe 0.20 0.21 Mg / Si 0.91 0.98 The plates were homogenized 21 h at 530 ° C, peeled, then hot and cold rolled to a thickness of 1.6 mm. The dissolution was carried out at 550 ° C for 1 hour for alloy A and at 570 ° C for 1 hour for alloy B. The standard tempering to lead to state T6 is 8 hours at 175 ° C and the mechanical characteristics in the cross direction are then:

for A R _0.2 = 350 MPa R _m = 380 MPa A = 13%

for B R _0.2 = 363 MPa R _m = 400 MPa A = 14%

The conductivities in the T6 state of the alloys A and B are respectively 24.3 and 24.7 MS / m.
Various tempering heat treatments have been carried out on these sheets in an attempt to desensitize them to intercrystalline corrosion, which has been qualified by accelerated "Interneutre" and "Interano" tests.
The times equivalent to 175 ° C., the mechanical characteristics in the transverse direction, the electrical conductivity and the sensitivity to intercrystalline corrosion have been collated in Tables 2 (for alloy A) and 3 (for alloy B).

Example 3

An alloy of composition has been developed in the form of a plate: Yes 0.924 Mg 0.860 Cu 0.869 Mn 0.550 Fe 0.192 Zn 0.152 Zr 0.103 Or 0.017 Ti 0.020 Cr 0.004 therefore with a Mg / Si ratio = 0.93
The plate was homogenized at 530 ° C, peeled, hot rolled to a thickness of 35 mm, dissolved in 550 ° C and quenched. We compared samples having undergone a conventional tempering corresponding to a T6 state with samples having undergone a desensitization treatment to intercrystalline corrosion according to the invention, with a double-stage tempering of 6 h at 175 ° C. + 2 h at 220 ° C.
The mechanical characteristics measured in the long and cross-long direction are as follows: sense L sense TL R _0.2 R _m AT R _0.2 R _m AT MPa MPa % MPa MPa % state T6 368 380 13.0 356 394 9.6 according to invention 315 344 11.5 316 349 9.0 The samples treated according to the invention show in the "Interano" and "Interneutre" tests an absence of sensitivity to intercrystalline corrosion, unlike the T6 samples.

The rolled or extruded products and desensitized to intercrystalline corrosion according to the invention are particularly well suited to the production of structural parts for aeronautics, in particular fuselages, and for road and rail vehicles. HEAT TREATMENT t _eq (h) R0.2 (MPa) RM (MPa) AT (%) CI SENSITIVITY 6h 175 ° C + 30 min 200 ° C 9.7 367 396 12.7 Yes 6h 175 ° C + 2h 200 ° C 20.8 363 386 11.9 Yes 6h 175 ° C + 8h 200 ° C 65.2 330 371 11.5 Yes 6h 175 ° C + 30 min 220 ° C 21.8 326 379 11.8 Yes 6 h 175 ° C + 2 h 220 ° C 69.3 314 363 11.8 Yes 6 h 175 ° C + 30 min 250 ° C 119.4 304 348 11.3 partial 6 h 175 ° C + 2 h 250 ° C 459.5 277 328 10.7 partial 100 h at 175 ° C 100 351 380 13 Yes 8h at 185 ° C 18.3 360 398 6.7 Yes 8h at 220 ° C 253.3 290 343 6 Yes HEAT TREATMENT t _eq (h) R0.2 (MPa) RM (MPa) AT (%) CI SENSITIVITY J MS / m 6h 175 ° C + 4h 200 ° C 35.6 322 370 11.4 Yes 24.6 6h 175 ° C + 8h 200 ° C 65.2 319 361 10 partial 24.7 6h 175 ° C + 30 min 220 ° C 21.8 338 376 11.4 Yes 24.5 6h 175 ° C + 2h 220 ° C 69.3 310 349 10.1 no 25.1 6h 175 ° C + 30 min 250 ° C 119.4 288 331 10.1 no 25.8 6h 175 ° C + 2h 250 ° C 459.5 241 300 10.2 no 26.7 8h at 185 ° C 18.3 349 388 11.1 Yes 24.3 8h at 200 ° C 59.2 322 353 10.3 partial 24.7 8h at 220 ° C 253.3 272 323 9.5 no 25.8 HEAT TREATMENT t _eq (h) R0.2 (MPa) RM (MPa) AT (%) SENSITIVITY CI J MS / m 6h 175 ° C + 2h 220 ° C 69.3 313 374 11 partial 25.1 6h 175 ° C + 30 min at 250 ° C 119.4 282 345 11 no 25.4

Claims (11)

1. A process for the production of high strength AlSiMgCu aluminium alloy products with good intergranular corrosion resistance, comprising the following steps:

   casting a plate or billet with the following composition (by weight): Si 0.7 - 1.3% Mg 0.6 - 1.1% Cu 0.5 - 1.1% Mn 0.3 - 0.8% Zr < 0.20% Fe < 0.30% Zn < 1% Ag < 1% Cr < 0.25% other elements < 0.05% each and < 0.15% in total remainder: aluminium;
   with: Mg/Si < 1

   homogenising in the range 470°C to 570°C;

   hot working, and optionally cold working;

   solution heat treating in the range 540°C to 570°C;

   quenching;

   ageing, comprising at least one temperature plateau in the range 150°C to 250°C preferably in the range 165°C to 220°C, the total period, measured as the equivalent period at 175°C, being in the range 30 h to 300 h.
2. A process according to claim 1, characterized in that the Zn content is in the range 0.15% to 1%.
3. A process according to claim 1 or claim 2, characterized in that the ageing comprises a plateau at a temperature which is in the range 150°C to 250°C, preferably in the range 165°C to 220°C, and a further plateau at a higher temperature which is in the range 170°C to 270°C.
4. A process according to claim 3, characterized in that the equivalent period at 175°C for ageing is in the range 30 h to 120 h.
5. A process according to claim 4, characterized in that the equivalent period at 175°C for ageing is in the range 70 h to 120 h.
6. A process according to claim 1 or claim 2, characterized in that the ageing comprises a single plateau and its equivalent period at 175°C is in the range 150 h to 250 h.
7. A rolled or extruded product of high strength AlSiMgCu aluminium with the following composition (by weight): Si 0.7 - 1.3% Mg 0.6 - 1.1% Cu 0.5 - 1.1% Mn 0.3 - 0.8% Zr < 0.20% Fe < 0.30% Zn < 1% Ag < 1% Cr < 0.25% other elements < 0.05% each and < 0.15% in total with: Mg/Si < 1, which has been desensitised to intercrystalline corrosion within the meaning of standard MIL-H-6088, and in the desensitised temper has an electrical conductivity which is at least 0.5 MS/m higher than that measured in the T6 temper.
8. An aircraft fuselage element formed from rolled or extruded products produced by a process according to any one of claims 1 to 6.
9. An aircraft fuselage element formed from rolled or extruded products according to claim 7.
10. A structural element for a rail or road vehicle produced from rolled or extruded products produced by the process of any one of claims 1 to 6.
11. A structural element for a rail or road vehicle formed from products according to claim 7.