EP1231290A1 - Method for making a high strength, wrought AlZnMgCu alloy product - Google Patents

Abstract

Fabrication of a welded aluminum alloy product comprises: casting of a blank in alloy of a given composition; possibly the homogenization of the blank; hot transformation of the blank by rolling, drawing or forging; putting in solution and hardening of the product obtained; possibly the application of a controlled traction leading to a permanent elongation of 1-5 %; tempering of the product at a temperature and a duration such that the product attains the peak elasticity limit in compression. The composition of the Al-Zn-Mg-Cu alloy is (% by wt.): (a) Zn : 7.0 - 11.0; (b) Mg : 1.8 - 3.0; (c) Cu : 1.2 - 2.6; (d) one at least of the elements Mn (0.05 - 0.4), Cr (0.05 - 0.3), Zr (0.05 - 0.20), Hf (0.05 - 0.5), V (0.05 - 0.3), Ti (0.01 - 0.2) and Sc (0.05 - 0.3); (e) the rest being Al and inevitable impurities.

Description

Field of the invention

The invention relates to the manufacture of wrought products by rolling, spinning or forging of aluminum alloy of the AlZnMgCu type with high mechanical resistance, used in particular in aeronautical construction, in particular for the upper surfaces of airplane wings.

State of the art

Alloys of the Al-Zn-Mg-Cu type have been used for more than 50 years in the aeronautical construction, and in particular for the wing upper surfaces. So we used alloys 7075, 7178, 7050, 7150 and, more recently, alloys 7055 and 7449. These alloys were most often used in the T6 state, i.e. at an income corresponding to the maximum of the elastic limit in traction, or to an over-income state T76, T79 or T77 to obtain better corrosion resistance. As Illustrative of this state of the art, there may be mentioned EP 0020505 patents Boeing relating to alloy 7150, US patents 4,477,292, US 4,832,758, US 4,863,528 and US 5,108,520 from Alcoa on the T77 treatment, patent EP 0377779 Alcoa relating to a manufacturing process for alloy 7055, and the patent application EP 0670377 from the applicant describing a process for the manufacture of sheet metal alloy 7449.

The properties of alloy 7449 developed by the applicant for sheets for the upper surface of the sails are studied in the communication by T. Warner et al. "Aluminum alloy developments for affordable airframe structures", Conference on Synthesis, Processing and Modeling of Advanced Materials, ASM International, Paris, June 25-27, 1997, pp. 77-88. Figure 2 of the article, reproduced as figure 1 below, represents the typical properties of thick sheets between 15 and 40 mm in this alloy, namely the breaking strength and the elastic limit in direction L in tension, the elastic limit in compression in direction L and the voltage corrosion threshold (direction ST), in state T651 and in state T7x51 at improved corrosion resistance. This condition has been identified in publications by the same authors as T7951 (or T79511 for spun products), by example in the communication by F. Heymès et al. "New aluminum semi-products for airframe application ”, METEC Congress, Düsseldorf, June 1999, which takes the same figure. Figure 1 below shows that the elastic limit in compression in state T79 is less than in state T6. In this state T7951, the sheets in 7449, compared to 7150 in the T651 state usually used for extrados of commercial aircraft wings, a 10% gain in elasticity limit in compression, improved resistance to flaking and stress corrosion and fatigue, without reducing tolerance to damage.

In summary, the state of the art shows on the one hand that the mechanical resistance to compression is an essential property for the upper surfaces of fenders, and that, share the manufacturers of high resistance alloys offer for this application produced either in the T6 state, corresponding to the peak of elastic limit in tension, or an over-tempered T7 state with better corrosion resistance, but with a loss of mechanical strength.

Subject of the invention

The invention aims to further improve the mechanical strength in compression high resistance 7000 alloy products intended in particular for upper surfaces of aircraft wings without loss on other properties of use

• la coulée d'une ébauche en alliage de composition (% en poids) : Zn : 7,0 - 11,0 Mg : 1,8 - 3,0 Cu : 1,2 - 2,6 l'un au moins des éléments Mn (0,05 - 0,4), Cr (0,05 - 0,30), Zr (0,05 - 0,20), Hf (0,05 - 0,5), V (0,05 - 0,3), TI (0,01 - 0,2) et Sc (0,05 - 0,3), reste aluminium et impuretés inévitables,
• éventuellement l'homogénéisation de cette ébauche,
• la transformation à chaud de cette ébauche par laminage, filage ou forgeage,
• la mise en solution et la trempe du produit obtenu,
• éventuellement une traction contrôlée conduisant à un allongement permanent compris entre 1 et 5 %,
• le revenu du produit à une température et d'une durée telles que le produit atteigne le pic de limite d'élasticité sens L en compression.

The subject of the invention is a method of manufacturing a wrought product of aluminum alloy Al-Zn-Mg-Cu with high mechanical strength, comprising

• casting an alloy alloy blank (% by weight): Zn: 7.0 - 11.0 Mg: 1.8 - 3.0 Cu: 1.2 - 2.6 at least one of the elements Mn (0.05 - 0.4), Cr (0.05 - 0.30), Zr (0.05 - 0.20), Hf (0.05 - 0.5), V (0.05 - 0.3), TI (0.01 - 0.2) and Sc (0.05 - 0.3), aluminum residue and unavoidable impurities,
• possibly the homogenization of this draft,
• the hot transformation of this blank by rolling, spinning or forging,
• dissolving and quenching the product obtained,
• possibly controlled traction leading to permanent elongation of between 1 and 5%,
• the income of the product at a temperature and a duration such that the product reaches the peak of elastic limit direction L in compression.

Another subject of the invention is the rolled, spun or forged product obtained by said process.

Yet another object of the invention is the structural element for construction mechanical, and in particular aeronautical, made from at least one product laminated, spun or forged obtained by said process.

Description of the figures

Figure 1 represents the typical properties, namely the elastic limit in tension (direction L), the tensile strength (direction L), the elastic limit in compression (direction L) and the corrosion threshold under tension (direction ST), of sheets thickness between 15 and 40 mm in alloys 7150-T651, 7449-T651 and 7449-T7951 according to the prior art.

FIG. 2 represents the time-temperature domain of the income of the process according to the invention.

FIG. 3 represents the breaking strength and the elastic limit in tension of 38 mm thick sheets of alloy 7449 of Example 1 as a function of time equivalent to 120 ° C of tempering for different tempering temperatures.

Figure 4 shows the tensile strength (direction L) and the limit tensile and compressive strength (L direction) of 38 mm 7449 alloy sheets of example 2 as a function of the time equivalent to 120 ° C. of the income.

FIG. 5 represents the compressive elastic limit of the sheets of alloys A and

B of Example 3 as a function of the time equivalent to 120 ° C. of the income.

Description of the invention

The invention is based on highlighting a difference between the resistance peak mechanical tensile obtained income, which corresponds to what is called usually the state T6, and the peak of mechanical resistance in compression. Well that it has been known for a very long time that the extrados of wings work especially in compression, and that therefore the elastic limit in compression is a property dimensioning for the structural elements of this wing part, the metallurgists have always used tensile strength to define the T6 state of maximum resistance to income.

The inventors have found that between states T6 and T79 of the prior art, there exists a metallurgical state for which the elastic limit in compression passes through a peak lying between 20 to 25 MPa above the compressive elasticity limits of these two states.

The process according to the invention applies to alloys of the Al-Zn-Mg-Cu type with a content high in zinc, between 7 and 11%, with a magnesium content included between 1.8 and 3%, and preferably between 1.8 and 2.4%, and a copper content between 1.2 and 2.6% and preferably between 1.6 and 2.2%. Below a zinc content of 7%, the invention seems to be of little interest because it is no longer used in construction aeronautics of such alloys for the manufacture of structural elements used in compression. Above a zinc content of 11%, we encounter difficulties during the industrial casting of sufficiently large plates or billets for the production of sheets, profiles or forgings suitable for the manufacture of said structural elements.

The process according to the invention applies in particular to the alloys used for the manufacture of aircraft wing upper surface elements, for example alloys 7055, 7349 and 7449, in the form of wrought products, that is to say rolled, extruded or forged. This method comprises, in known manner, the manufacture of a blank, namely a plate for rolled products, a billet for extruded products or a block of forge for forged products. This blank is preferably homogenized to a temperature close to the starting melting temperature of the alloy, such as indicated in patent application EP 0670377. It is then transformed by hot rolling, spinning or forging, to the desired size. The product obtained is dissolved, also at a temperature fairly close to the temperature of starting melting of the alloy, this temperature being controlled by analysis differential enthalpy. Dissolution is followed by quenching, generally with cold water. The quenched product is preferably subjected to controlled traction leading to a permanent elongation of between 1 and 5%.

The product then undergoes an income to obtain the elastic limit peak in compression direction L. The income can be mono-level, that is to say comprising a temperature rise ramp, linear or not as a function of time, a constant temperature within the temperature tolerance limit of the oven used, and a cooling to room temperature. In the case of a single-tier income, the bearing is at a temperature between 120 and 150 ° C, and of a duration included in the AEFG parallelogram in FIG. 2, and preferably between 120 and 145 ° C of a duration included in the parallelogram ABCD of figure 2. The income can also be bi-level, with a first level at a temperature between 80 and 120 ° C, and a second level at a higher temperature, between 120 and 160 ° C. It can also be tri-level, with a first and a second level within the same limits as two-level income, and a third level at a temperature lower than the second, between 100 and 140 ° C. Considering of the time required, in industrial ovens, for temperature rises, it it is hardly possible to have stages of less than 2 hours duration, and preferably at 5 a.m.

In all cases, we can reduce the two parameters temperature and duration to a single parameter, the time equivalent to 120 ° C, defined by the formula: t (eq) = ∫exp (-16000 / T) dt exp (-16000 / T ref ) in which T is the temperature of the tempering stage in ° K, t the treatment time in hours and T _ref the reference temperature taken here at 120 ° C, or 393 ° K. The duration of the tempering is between 100 and 250 h of time equivalent to 120 ° C., and 50 to 200 h longer than the equivalent time corresponding to the tempering T651. The duration of tempering necessary to reach the peak in compression depends on the composition of the alloy, and in particular on the Cu / Mg ratio, the necessary duration increasing with this ratio.

The wrought product, and in particular the rolled, extruded or forged product obtained by the process according to the invention can be advantageously used for manufacturing structural elements, particularly in aircraft construction. Thanks to the increase in the elastic limit in compression which results from the process according to the invention, a structural element made from at least one spun, laminated product or forged according to the invention, shows with respect to a structural element of the same dimension and made from wrought, extruded or forged products according to the prior art better resistance during compressive stress. In an achievement preferred of the invention, the structural element is an aircraft wing upper surface.

Examples Example 1

7449 alloy sheets 38 mm thick were produced. The composition of the alloy was as follows (% by weight): Zn = 8.11 Mg = 2.19 Cu = 1.94 Si = 0.04 Fe = 0.07 Zr = 0.09 Cr = 0.005 Ti = 0.025 aluminum residue and impurities (<0.05 each).

The sheets have undergone a pre-enlargement to pass from a plate width of 1100 mm to 2500 mm, hot rolling up to 38 mm with exit temperature at 378 ° C, dissolution at 475 ° C, quenching in cold water, and traction controlled at 2.8% permanent elongation after waiting 1 hour after quenching.

Samples taken at mid-thickness of the sheets were subjected to 11 different incomes, of the mono- or bi-bearing type, mentioned in Table 1. The ramps of ascent and descent between bearings were respectively 16 ° C / h and 65 ° C / h, corresponding to observable speeds on industrial heat treatment furnaces. For each income, the time equivalent to 120 ° C t _eq was calculated according to the formula: t (eq) = ∫exp (-16000 / T) dt exp (-16000 / T ref ) in which T is the temperature of the tempering stage in ° K, t the treatment time in hours and T _ref the reference temperature taken here at 120 ° C, or 393 ° K.

The 11 revenues tested are between T651 income and T7951 art income previous, and their parameters, as well as the corresponding equivalent times, are shown in Table 1.

In each case, the static characteristics in tension in the L direction were measured (tensile strength R _m , elastic limit R _0.2 and elongation A) on TOR 6 test pieces taken from the core of the sheets. The results are the average of at least two measurements; they are shown in Table 1, as well as in Figure 3. Returned
^1st level Returned
2 ^nd level Teq at 120 ° C
(H) R _{0.2 (L)}
(Mpa) R _{m (L)}
(MPa) AT
(%) 24h-120 ° C 24 617 661 12 48h-120 ° C 48 623 661 12 96h-120 ° C 96 624 655 12 6h-135 ° C 29 616 655 12 12h-135 ° C 55 619 651 11 24h-135 ° C 108 619 651 11 48h-135 ° C 215 611 642 11 24h-120 ° C 5h-150 ° C 125 620 649 11 24h-120 ° C 9h-150 ° C 196 613 642 11 24h-120 ° C 13h-150 ° C 265 607 636 10 24h-120 ° C 17h-150 ° C 336 595 627 10

It can be seen that in the vicinity of the peak, it is the incomes at lower temperature, that is to say at 120 ° C., which lead to the highest values of R _0.2 and R _m . With regard to two-tier income, it is the temperature of the second tier which is decisive for this effect. Furthermore, the peaks for R _0.2 and R _m are neighboring, but not exactly in the same place. We can define the treatment T651 at the peak as the treatment allowing to approach the maximum potential value of R _0.2 and

R _m to less than 5 MPa, while remaining industrially acceptable. In this case, it is a 48 h treatment at 120 ° C.

Example 2

In the same way as in Example 1, samples were taken in 38 mm thick sheets of alloy 7449 of composition: Zn = 8.38 Mg = 2.15 Cu = 1.96 Si = 0.04 Fe = 0.06 Zr = 0.11 remains aluminum and impurities (<0.5% each).

These samples are subjected to 8 different incomes, between the T651 income defined in example 1 and the T7951 income. The temperatures and durations of these 8 returns, as well as the corresponding times at 120 ° C., are shown in Table 2. Returned Settings Equivalent time A (T651) 48h-120 ° C 48 B 12h-135 ° C 48 VS 18h-135 ° C 78 D 24h-135 ° C 102 E 30h-135 ° C 130 F 24h-120 ° C + 5.5h-150 ° C 130 G 24h-120 ° C + 11h-150 ° C 222 H (T7951) 24h-120 ° C + 17h-150 ° C 321

In addition to the mechanical tensile characteristics, the compressive elastic limit in the L direction was measured on test specimens with a diameter of 13 mm and a length of 25 mm, as well as the electrical conductivity on samples taken from the surface. The results, averages of two measurements, are shown in Table 3, and in Figure 4 for R _m and R _0.2 in tension, and R _0.2 in compression. Returned R _m jitters.
(MPa) R _0.2 trac.
(MPa) AT
(%) R _0.2
comp (MPa) Conduct.
(MS / m) AT 638 676 12.4 596 18.4 B 639 673 11.8 599 18.7 VS 637 668 12.0 611 19.1 D 634 663 11.0 614 19.7 E 633 663 10.5 615 20.0 F 635 662 11.2 613 20.1 G 619 648 10.5 608 21.2 H 597 621 10.7 590 21.9

It is noted that the income leading to the peak of elastic limit in compression (sense L) is located at an equivalent time of the order of 150 h, that is to say at a time intermediate equivalent between a T651 income and a T7951 income. The beach interesting is between 100 and 250 h of time equivalent to 120 ° C, or 50 to 200 h more than that of T651 income. This income at peak compression leads to a gain 19 MPa compared to T651 income and 25 MPa compared to T7951 income.

Example 3

In the same way as in the previous examples, the sheets were made from two alloys 7449, the thicknesses and compositions of which are given in Table 4. Alloy e (mm) Yes Fe Cu mg Zn Zr Ti AT 30 0.049 0,075 1.87 2.35 8.38 0.11 0.03 B 23 0,045 0.068 1.95 2.27 8.31 0.10

These sheets were subjected to different incomes indicated in Table 5, the first 11 corresponding to alloy A and the last 7 to alloy B. We measured on test pieces with a diameter of 13 mm and a length of 25 mm, taken at core of the sheets, the elastic limit R _0.2 in compression direction L, as well as the modulus of elasticity in compression, also direction L. The results are shown in Table 5 and reported, for the elastic limit, on Figure 5 as a function of time equivalent to 120 ° C of the income. Returned
^1st level Returned
2 ^nd level Returned
^3rd tier R0.2 compr.
(MPa) Module
(MPa) 24h-80 ° C 24h-135 ° C 605 70281 24h-100 ° C 24h-135 ° C 602 71200 24h-120 ° C 24h-135 ° C 607 72335 24h-100 ° C 18h-140 ° C 603 70598 24h-100 ° C 7h-150 ° C 601 70618 24h-100 ° C 2,5h-160 ° C 607 72302 24h-100 ° C 30h-140 ° C 600 72806 24h-100 ° C 18h-140 ° C 24h-120 ° C 616 71621 24h-100 ° C 7h-150 ° C 24h-120 ° C 615 70862 24h-100 ° C 2,5h-160 ° C 24h-120 ° C 622 72569 T7951 587 24h-80 ° C 24h-135 ° C 635 72910 24h-120 ° C 24h-135 ° C 611 72222 24h-100 ° C 18h-140 ° C 614 73244 24h-100 ° C 7h-150 ° C 610 72349 24h-100 ° C 30h-140 ° C 596 70181 24h-100 ° C 7h-150 ° C 24h-120 ° C 621 71303 T7951 598

It is noted that the peak of elastic limit in compression is located for a time equivalent to 120 ° C between 100 and 200 h, and that tri-level revenues lead to higher values. In addition, we note in relation to income T7951 an increase in the elastic limit in compression of the order of 15 MPa for two-tier revenues, and 25 MPa for tri-tier revenues.

Claims (13)

Method for manufacturing a wrought product of aluminum alloy Al-Zn-Mg-Cu with high mechanical strength, comprising casting an alloy alloy blank (% by weight): Zn: 7.0 - 11.0 Mg: 1.8 - 3.0 Cu: 1.2 - 2.6 at least one of the elements Mn (0.05 - 0.4), Cr (0.05 - 0.3), Zr (0.05 - 0.20), Hf (0.05 - 0.5), V (0.05 - 0.3), Ti (0.01 - 0.2) and Sc (0.05 - 0.3), aluminum residue and unavoidable impurities, possibly the homogenization of this draft, the hot transformation of this blank by rolling, spinning or forging, dissolving and quenching the product obtained, possibly controlled traction leading to permanent elongation of between 1 and 5%, the income of the product at a temperature and a duration such that the product reaches the peak of elastic limit direction L in compression. Process according to claim 1, characterized in that the magnesium content of the alloy is between 1.8 and 2.4%. Method according to one of claims 1 or 2, characterized in that the copper content of the alloy is between 1.6 and 2.2%. Method according to one of claims 1 to 3, characterized in that the alloy is 7349 or 7449. Method according to one of claims 1 to 3, characterized in that the alloy is 7055. Method according to one of claims 1 to 5, characterized in that the tempering is a single-stage tempering at a temperature and a duration included in the AEFG parallelogram whose vertices have, in a temperature-duration diagram, the following coordinates: A: 120 ° C - 100 h E: 150 ° C - 5 h F: 150 ° C - 40 h G: 120 ° C -700 h Method according to claims 6, characterized in that the income is a single-stage income at a temperature and a duration included in the parallelogram ABCD whose vertices have, in a temperature-duration diagram, the following coordinates: A: 120 ° C - 100 h B: 145 ° C - 9 h C: 145 ° C - 22 h D: 120 ° C - 230 h Method according to one of claims 1 to 5, characterized in that the tempering has a duration in time equivalent to 120 ° C of between 100 and 250 h. Method according to one of claims 1 to 5, characterized in that the tempering has a duration in time equivalent to 120 ° C greater by 50 to 200 h than that corresponding to the state T651. Method according to one of claims 8 or 9, characterized in that the tempering is a two-stage tempering, comprising a first stage at a temperature between 80 ° C and 120 ° C, and a second stage at a temperature between 120 ° C and 160 ° C. Method according to one of claims 8 or 9, characterized in that the tempering is a tri-level tempering, comprising a first stage at a temperature between 80 ° C and 120 ° C, a second stage at a temperature between 120 ° C and 160 ° C, and a third level at a lower temperature than the second and between 100 ° C and 140 ° C. Rolled, extruded or forged product obtained by the process according to any one of claims 1 to 11. Structural element for mechanical construction and in particular aeronautics, made from at least one rolled, spun or forged product obtained by the method according to one of claims 1 to 11.

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