EP3818187B1 - Aluminum alloy and overaged aluminum alloy product made of such an alloy - Google Patents

Description

The invention relates to an aluminum alloy, in particular one from the 7000 group according to the classification of the Aluminum Association (AA). The invention also relates to an aged aluminum alloy product made from such an alloy.

For the aerospace industry, high-strength aluminum alloys are required primarily for the manufacture of load-bearing fuselage, wing and landing gear parts, which have a high level of strength under both static and dynamic loads. The required strength properties can be achieved by using alloys of the 7000 group according to the classification of aluminum alloys carried out by the Aluminum Association (AA).

Highly stressed parts in the aerospace industry are used, for example, from the alloys AA7075, AA7175, AA7475 and particularly preferably from the alloys AA7049 and AA7050 in the American region and from the alloys AA7010, AA7049A and AA7050A in the European region.

the end WO 02/052053 A1 a high-strength aluminum alloy of the aforementioned type with an increased zinc content compared to previous alloys of the same type, coupled with a reduced copper and magnesium content, is known. The copper and magnesium content in this known alloy together is less than 3.5% by weight. The copper content itself is given as 1.2-2.2% by weight, preferably 1.6-2.2% by weight. In addition to the elements zinc, magnesium and copper, this known alloy necessarily contains one or more elements from the group of zirconium, scandium and hafnium with maximum proportions of 0.4% by weight of zirconium, 0.4% by weight of scandium and 0, 3 wt% hafnium.

In EP 1 683 882 A1 discloses a deterrent-resistant aluminum alloy, from which highly stressed parts, for example for use in aerospace engineering and thus components with high static and dynamic strength properties with good fracture toughness and good stress-crack corrosion behavior, are made, these components also having a thickness of can be more than 200 mm. This known alloy consists of: 7 to 10.5% by weight Zn, 1.0 to 2.5% by weight Mg, 0.1 to 1.15% by weight Cu, 0.06 to 0.25 % By weight Zr, 0.02 to 0.15% by weight Ti as mandatory alloying elements, the sum of the alloying elements Zn + Mg + Cu being at least 9% by weight and with a remainder of Al and unavoidable impurities. In the manufacturing process described in this prior art, the semi-finished product made from this aluminum alloy is overaged in one or more stages in order to optimize the desired material properties. The fracture toughness determined on the semi-finished products made from this alloy in a neutral environment according to ASTM E399 is improved compared to the prior art.

The relevant properties include the fracture toughness and also the stress-crack corrosion resistance in an environment influenced by the environment (according to ASTM E1823: environment assisted cracking; EAC for short). For this purpose, stress-crack corrosion (SCC) is usually carried out in a salt water environment with the usual test set-up to determine the stress-crack corrosion resistance (SCC resistance). In the test setup, for example, a pre-notched specimen (e.g. ASTM G168-00) is subjected to a force that acts on the specimen in order, if the force is sufficient, to enlarge the notch or crack opening in such a way that crack formation occurs. As the crack length increases, the associated stress intensity factor (K-factor) decreases until the crack propagation finally comes to a standstill. A specimen is the more SRK-resistant, the less crack growth is observed or the higher the load necessary for crack propagation (in the form of the stress intensity factor K), i.e. the higher the stress intensity factor that a notched specimen can detect without crack propagation to be able to, can be exposed.

The SRK resistance of aluminum alloys can be very different for one and the same alloy depending on the environmental conditions in which the SRK test is carried out. The state of obsolescence of the semi-finished product or specimen also has an influence on the SRK resistance. In the case of an alloy according to AA7010, the SRK resistance increases significantly with increasing overaging of the specimen starting from the T6 state via the T76 state to the T74 state, in particular also in a salt water environment. Other 7xxx alloys also show basically the same behavior in the classic SRK test (ie in salt water). In the case of changed environmental conditions (eg high air humidity at increased temperature), it can be seen that, in particular, 7xxx alloys with a high zinc content generally tend to "environment assisted cracking" even when they are overaged (ie T7x). Due to hydrogen embrittlement, cracks propagate along the grain boundaries (see e.g. EASA Safety Information Bulletin No. 2018-04). For the AA7010 under such EAC ambient conditions, K _IEAC values between 6 and 7 MPa√m can be achieved _{in the T6 state; in the overaged state T74, the K IEAC} values increase to up to 25 MPa√m due to the obsolescence compared to the T6 state, however, significantly reduced strength. According to the above, the K-factor K _{IEAC is} a measure of the EAC resistance, since no crack propagation takes place _{for loads K I} <K _IEAC.

The improved strength properties compared to alloy AA7010 EP 1 683 882 A1 known alloy (AA7037) surprisingly does not show the EAC resistance to be expected with increasing overaging, as can be observed with a test piece made of the alloy AA7010. _{Even in the overaged T7452 condition, an EAC resistance of only about K IEAC} = 6 to 7 MPa√m can be achieved with the alloy according to AA 7037 in a damp environment at elevated temperatures (50 ° C, 85% relative humidity).

Proceeding from this discussed prior art, the invention is therefore based on the object of proposing an aluminum alloy from which an aluminum alloy product with strength values comparable to those that an alloy product from the alloy AA7037 has, can be produced, which, however, has an improved EAC resistance under environmental influences that promote crack initiation and crack propagation.

• 0,04 - 0,1 Gew.-% Si,
• 0,8 - 1,8 Gew.-% Cu,
• 1,5 - 2,3 Gew.-% Mg,
• 0,15 - 0,6 Gew.-% Ag,
• 7,05 ― 9,2 Gew.-% Zn,
• 0,08 ― 0,14 Gew.-% Zr,
• 0,02 - 0,08 Gew.-% Ti
• max. 0,35 Gew.-% Mn,
• max. 0,1 Gew.-% Fe,
• max. 0,06 Gew.-% Cr,
• optional 0,0015 - 0,008 Gew.-% Be,
• Rest Aluminium nebst unvermeidbaren Verunreinigungen.

According to the invention, this object is achieved by an aluminum alloy with the following composition:

• 0.04-0.1 wt% Si,
• 0.8-1.8% by weight Cu,
• 1.5-2.3 wt% Mg,
• 0.15-0.6% by weight Ag,
• 7.05 - 9.2% by weight Zn,
• 0.08-0.14% by weight Zr,
• 0.02-0.08 wt% Ti
• max. 0.35% by weight Mn,
• max. 0.1% by weight Fe,
• max. 0.06 wt.% Cr,
• optionally 0.0015 - 0.008% by weight Be,
• The remainder of aluminum and unavoidable impurities.

In the case of the alloys described in the context of these explanations, unavoidable impurities can be present per element with a maximum of 0.05% by weight and a total of 0.15% by weight.

Surprisingly, it was found for semifinished products made from such an alloy that the EAC resistance, despite a relatively high Zn content, is significantly improved even under environmental influences that promote cracking corrosion compared to the values achievable with samples made from the AA7037 alloy. Nevertheless, the mechanical strength values are sufficiently high. The yield point R _p0.2 amounts to more than 440 MPa and can reach values of 460 MPa and more in a forged part with a thickness of 150 mm. The fracture toughness is more than 20 MPa√m and can reach values of 25 MPa√m and more.

The SRK resistance when an EAC test (ASTM E1823; ASTM G168) is carried out in an environment with a humidity of 85% and a temperature of 50 ° C surprisingly shows that with an applied load of K _I = 20 MPa√m with a test duration of 30 days, no crack propagation can be observed. Therefore, even under these environmental conditions, an alloy product made from the alloy according to the invention is overaged to the T7xxx state compared to that of previously known alloys, such as AA7037 or with regard to the AA7010 for parts with a greater thickness (thickness ≥ 100 mm, in particular also ≥ 150 mm) significantly improved. This alloy and the semi-finished products and products made from it prove to be particularly insensitive to quenching. This means that even as a result of a greater thickness (cross-sectional area) in the case of parts made from the alloy in the more central sections, due to their slower cooling, no, at least no noteworthy, losses in terms of their strength are to be accepted. The result is that these parts have high strengths even with large cross-sections. Especially in the case of high-strength aluminum alloy products, such as those used in the aerospace industry, the EAC resistance in such an environment (85% humidity at 50 ° C) is of particular interest. This result is surprising because the EAC resistance of an alloy product made from an AA7037 alloy in the same overaging state did not suggest this. Finally, for the alloy product of alloy AA7037 in the same overaging condition, only an EAC resistance of about 6 to 7 MPa√m was found with the same overaging.

So while alloy products made from the aluminum alloy AA7037 achieve stress intensity _factors K IEAC of around 6 to 7 MPa√m in the EAC tests, these values are significantly more than 20 MPa√m for aluminum alloy products made from the alloy according to the invention in the same state of overaging. The K IEAC values achieved in aluminum alloy products made from the alloy _{according to the invention are in relation to the fracture toughness K Ic} at room temperature at about 70% and more. In many cases, the K _IEAC value should even _{correspond to the K IC} value (and therefore cannot be determined experimentally for technical reasons), since crack propagation or propagation was not observed over the duration of the test (more than 30 days) could be. The special EAC resistance was not to be expected in view of the high Zn content. According to the prevailing teaching, higher Zn contents impair the EAC resistance.

An aluminum alloy product made from the aluminum alloy according to the invention is preferably overaged to the state T74, T7451, T7452 or T7454. In this state, the aluminum alloy product still has sufficient mechanical strength values and the desired SRK resistance both in the classic immersion test in salt water solution and in an environment that promotes EAC due to hydrogen, such as in an environment with 85% humidity and a temperature of 50 ° C an overaging that does not reach the state T74 or T74xx, higher mechanical strength values can be achieved, but then the SRK / EAC resistance is generally not achieved to the desired extent. On the other hand, aging beyond T74 / T74xx leads to a further decrease in the mechanical strength values with generally improved SRK / EAC properties.

According to one embodiment of this aluminum alloy, it contains 0.35 to 0.6% by weight Ag, in particular 0.40 to 0.50% by weight Ag. Interestingly, it has been shown that the above-described properties are established in an alloy with this Ag content, especially with regard to the EAC resistance. In this configuration of the alloy, the preferred Zn / Mg ratio is more than 3.4 up to and including 4.95. A Zn-Mg ratio between 3.5 and 4.25 is preferred. The preferred copper content of this alloy configuration is between 0.8 and 1.35% by weight Cu, in particular between 0.9 and 1.2% by weight Cu, combined with an Mn content between 0.18 and 0.3% by weight .-%, in particular 0.2 to 0.25% by weight and a Zn content between 7.1 to 8.9% by weight. In such an aluminum alloy, when the Cu content is greater than 1.35% by weight and in the range from more than 1.35 to 1.8% by weight, the alloy product has comparable alloy product properties when the Mn content is less than 0.1% by weight, in particular less than 0.05% by weight.

These special properties - high strength values and a special EAC resistance - have an alloy with a lower Ag content compared to the configuration described above, namely if this is less than 0.35% by weight Ag, but more than 0.15 % By weight. The Cu and Zn content corresponds to the alloy richer in Ag, the Zn / Mg ratio being between 3.9 and 4.3. The above description of these exemplary embodiments makes it clear that the desired effects extend over the entire range of the claimed alloy

The special properties of the alloy product made from this alloy can be seen in the very narrow spectrum of the elements involved in the alloy. Because only with this composition can the desired EAC resistance be set due to an aging of the alloy product made from the alloy in the state T74 / T74xx.

Be can optionally be involved in the alloy. The introduction of Be into the melt serves to reduce its susceptibility to oxidation. Be can be involved in the construction of the alloy between 0.0015 and 0.008, in particular in the range 0.0015 to 0.0035, for the purposes mentioned.

Fig. 1:

Ein Diagramm zum Darstellen der EAC-Beständigkeit in Form der Plateau-Rissgeschwindigkeiten sowie der K_IEAC-Beständigkeit einer herkömmlichen Legierung AA7010 bei unterschiedlichen Alterungs- bzw. Überalterungszuständen,

Fig. 2:

ein Diagramm zum Darstellen der Versuchsergebnisse eines EAC-Tests unter Umgebungseinfluss (50°C / 85% relativer Luftfeuchte) zweier Vergleichsproben aus einer Legierung AA7037 und

Fig. 3 - 6:

Diagramme entsprechend demjenigen der Figur 2, darstellend die Versuchsergebnisse von jeweils zwei bis vier Probenkörpern aus den erfindungsgemäßen Legierungen.

The invention is described below on the basis of exemplary embodiments. Reference is made to the accompanying figures, which show the following results for tests carried out with test specimens according to ASTM G168 at ambient conditions of 50 ° C and 85% relative humidity:

Fig. 1:

A diagram to show the EAC resistance in the form of the plateau crack speeds and the K _IEAC resistance of a conventional AA7010 alloy with different aging or overaging conditions,

Fig. 2:

a diagram to show the test results of an EAC test under the influence of the environment (50 ° C / 85% relative Humidity) of two comparison samples made of an alloy AA7037 and

Fig. 3 - 6:

Diagrams corresponding to that of the Figure 2 , representing the test results of two to four test specimens from the alloys according to the invention.

• Gießen von Barren aus der Legierung;
• Homogenisierung der gegossenen Barren bei einer Temperatur, die möglichst dicht unter der Anschmelztemperatur der Legierung liegt für eine Aufheiz- und Haltezeit, die ausreichend ist, eine möglichst gleichmäßige und feine Verteilung der Legierungselemente im Gussgefüge zu erreichen, bevorzugt bei 460 - 490 °C;
• Warmumformen der homogenisierten Barren durch Schmieden, Strangpressen und/oder Walzen im Temperaturbereich von 350 - 440 °C;
• Lösungsglühen des warmumgeformten Halbzeuges bei Temperaturen, die hoch genug sind, um die für die Aushärtung notwendigen Legierungselemente gleichmäßig im Gefüge verteilt in Lösung zu bringen, bevorzugt bei 465 - 500 °C;
• Abschrecken der lösungsgeglühten Halbzeuge in Wasser mit einer Temperatur zwischen Raumtemperatur und 100 °C oder in einem Wasser-Glykol-Gemisch oder in einem Salzgemisch mit Temperaturen zwischen 100 °C und 170 °C;
• ggf. Kaltstauchen (d.h. Endzustand T7x52 alt. T7x54) oder Recken (d.h. Endzustand T7x51) des Produkts mit Stauch-/Reckgraden vorzugsweise im Bereich 1 bis 5 %; und
• mehrstufiges Warmauslagern des abgeschreckten Halbzeuges zum Überaltern des Halbzeuges zum Zustand T74 bzw. T7452/T7454/T7451.

Test specimens were produced from the comparison alloys and the test alloy as follows:

• Casting ingots from the alloy;
• Homogenization of the cast ingots at a temperature that is as close as possible to the melting temperature of the alloy for a heating and holding time that is sufficient to achieve the most even and fine distribution of the alloying elements in the cast structure, preferably at 460 - 490 ° C;
• Hot forming of the homogenized bars by forging, extrusion and / or rolling in the temperature range of 350 - 440 ° C;
• Solution annealing of the hot-formed semi-finished product at temperatures high enough to bring the alloying elements necessary for hardening into solution evenly distributed in the structure, preferably at 465-500 ° C;
• Quenching the solution-annealed semi-finished products in water with a temperature between room temperature and 100 ° C or in a water-glycol mixture or in a salt mixture with temperatures between 100 ° C and 170 ° C;
• possibly cold upsetting (ie final state T7x52 alt. T7x54) or stretching (ie final state T7x51) of the product with degrees of compression / stretching, preferably in the range 1 to 5%; and
• Multi-stage artificial aging of the quenched semi-finished product to age the semi-finished product to the condition T74 or T7452 / T7454 / T7451.

The alloy compositions of the comparative alloys and the test alloys in% by weight are as follows: Si Cu Mg Ag Zn Zr Mn Fe Cr Ti Al AA 7010 0.1 1.65 2.3 - 6.3 0.11 0.02 0.08 0.02 0.03 rest AA 7037 0.04 0.9 1.65 - 8.5 0.12 0.29 0.07 0.01 0.05 rest E1 0.07 1.1 1.9 0.21 7.5 0.11 0.22 0.05 - 0.03 rest E2 0.07 1.05 1.9 0.45 7.5 0.11 0.22 0.06 - 0.03 rest E3 0.07 1.1 2.2 0.45 7.5 0.11 0.22 0.06 - 0.03 rest E4 0.07 1.55 1.75 0.45 7.5 0.11 - 0.08 0.01 0.03 rest E5 0.07 1.1 1.7 0.45 8.3 0.11 0.22 0.08 - 0.03 rest E6 0.07 1.55 1.75 0.2 7.5 0.12 - 0.05 - 0.03 rest

The samples in condition T7452 were subjected to an EAC test in accordance with ASTM E1681 using DCB samples in accordance with ASTM G168 in the present case at a relative humidity of 85% and a temperature of 50 ° C. At the start of the test, the stresses on the samples with cracks were between 20 and 30 MPa√m, depending on the fracture toughness determined. The investigations regarding the EAC behavior on the DCB samples were carried out in an SL orientation. The K _IEAC values therefore relate to this orientation. The SL orientation is the direction in which a sample is most susceptible to failure due to EAC. The specimen is loaded in the ST direction of the forging (in the direction of least expansion). Thus, crack formation in the L-direction (direction of greatest expansion) is to be expected. Therefore, the EAC tests were carried out on SL-oriented samples.

Figure 1 Using the sample made from the alloy AA7010, shows the influence of overaging on the increase in the K _IEAC values and the simultaneous decrease in the initial crack propagation rate. While the K _IEAC values in condition T6 are low and do not meet the requirements (K _IEAC of 5 MPa√m), the EAC resistance improves with increasing age. In state T7452 the K _IEAC value is 24 MPa√m. The mechanical strength values of this alloy are only up to that Condition T76 acceptable and have a fracture toughness K _IC of about 21 MPa√m and a yield point R _p0.2 of 470 MPa. In state T7452, the K _IEAC value of 24 MPa√m is relatively high, as is the K _IC value of around 32 MPa√m, but the yield strength R _{p0.2 of} 420 MPa is not sufficient.

Alloy AA7037, which has already been improved in terms of strength compared to alloy AA7010, shows _{adequate mechanical strength values even in condition T7452 with a yield point R p0.2} of 450 MPa and more and a fracture toughness K _IC of around 30 MPa√m, but no EAC that meets the requirements -Resistance, see Figure 2 . The K _IEAC value is around 6 MPa√m.

In contrast to this, as can be seen from the diagram of the Figure 3 recognizable, IEAC values of more than 20 MPa√m achieved with the sample E1 from the alloy K _{according to} the invention, it being noted for this sample that crack propagation could not be observed within the test duration of 30 days in the mentioned EAC environment. The failure of crack propagation or crack propagation in the EAC-conducive environment (85% humidity, 50 ° C) becomes clear from the point clustering of the different samples, which are merely the result of scatter in the crack length measurements. A typical stress-crack resistance behavior that leads to crack propagation and breakage is shown in the diagram of Figure 2 recognizable from the sample made from the alloy AA7037. The yield point R _p0.2 for E1 is around 480 MPa. The fracture toughness K _IC is around 26 MPa√m (SL sample position).

Figure 4 FIG. 13 shows a diagram corresponding to that of FIG Figure 3 with the results of a sample of alloy E2. In this sample, too, no crack propagation could be observed within the test duration of 30 days. The EAC resistance is reflected in the achieved K _IEAC values of more than 35 MPa√m.

Figure 5 shows a further diagram of the type mentioned above with the achieved K _IEAC values of about 20 MPa√m with four samples from the Alloy E4 have been achieved. In this sample, too, no crack growth could be determined within the test duration of 30 days.

The K _IEAC values from the diagram in FIG Figure 6 refer to. These are between approximately 22 to 26 MPa√m. The accumulation of points in this diagram also makes it clear that crack growth could not be observed within the duration of the test.

The above-discussed strength values of the test specimens from the comparison alloys and those of the alloys E1-E6 according to the invention are summarized in the table below: alloy EAC properties in T7452 Strength properties in T7452 K _IEAC / K _IC K _IEAC [MPa√m] K _{IC, SL} [MPa√m] R _{p0.2, L} [MPa] AA7010 24 32 420 ≈ 75% AA7037 6th 22nd 450 ≈ 27% E1 > 20 26th 482 ≥ 80% E2 No crack growth> 20 23 466 ≥ 90% E3 No crack growth> 20 20th 470 ≈ 100% E4 No crack growth> 20 21 487 ≥ 90% E5 No crack growth> 20 26th 467 ≥ 90% E6 No crack growth> 20 22nd 470 ≥ 90%

The description of the alloys according to the invention and of the outdated alloy products produced therefrom makes it clear that the EAC resistance of these alloy products is unexpectedly good.

Claims (10)

1. Aluminium alloy with

   0.04 - 0.1 % by weight Si,

   0.8 - 1.8 % by weight Cu,

   1.5 - 2.3 % by weight Mg,

   0.15 - 0.6 % by weight Ag,

   7.05 - 9.2 % by weight Zn,

   0.08 - 0.14 % by weight Zr,

   0.02 - 0.08 % by weight Ti,

   max. 0.35 % by weight Mn,

   max. 0.1 % by weight Fe

   max. 0.06 % by weight Cr,

   optional 0.0015 - 0.008 % by weight Be,

   remainder aluminium together with unavoidable impurities.
2. Aluminium alloy according to claim 1, characterised in that the alloy contains less than 0.35 % by weight Ag, in particular more than 0.15 - 0.26 % by weight Ag, in particular 0.2 - 0.23 % by weight Ag, 0.9 - 1.6 % by weight Cu, 7.15 - 8.3 % by weight Zn, in particular 7.3 - 7.8 % by weight, and a Zn/Mg proportion in the range from 3.6 up to and including 4.4, in particular in the range from 3.9 to 4.3.
3. Aluminium alloy according to claim 1, characterised in that it contains 0.35 - 0.55 % by weight Ag, in particular 0.40 - 0.50 % by weight Ag.
4. Aluminium alloy according to claim 3, characterised in that, in respect of the elements Zn and Mg, the aluminium alloy comprises a Zn/Mg proportion in the range of more than 3.4 up to and including 4.9.
5. Aluminium alloy according to claim 3, characterised in that it comprises 0.8 - 1.35 % by weight Cu, in particular 0.9 - 1.2 % by weight, Cu, 0.18 - 0.3 % by weight Mn, in particular 0.2 - 0.25 % by weight Mn, and 7.1 - 8.9 % by weight Zn.
6. Aluminium alloy according to claim 3, characterised in that it comprises more than 1.35 to max. 1.8 % by weight Cu, and less than 0.1 % by weight Mn, in particular less than 0.05 % by weight Mn.
7. Aluminium alloy according to any one of claims 1 to 6, characterised in that it contains 0.0015 - 0.0035 % by weight Be.
8. Aluminium alloy product, manufactured from an alloy according to any one of claims 1 to 7, characterised in that the aluminium alloy product is aged in accordance with T74xx.
9. Aluminium alloy product according to claim 8, characterised in that the aluminium alloy product undergoes plastic deformation after solution annealing and before the ageing, and is therefore aged in accordance with T7451 and/or T7452 respectively, or T7454.
10. Aged aluminium alloy product according to any one of claims 7 to 9, characterised in that the tensile yield strength R_p0.2 is at least 440 MPa, the fracture toughness (K_IC) is at least 20 MPa√m, and that no fissure propagation occurs after the performance of an EAC test in accordance with ASTM E1681 with the use of DCB samples in accordance with ASTM G168 under the following conditions:

    - in air at 50 °C

    - in an air humidity of 85%

    - under a stress of up to 20 MPa√m, and

    - a test duration of 30 days has been undertaken.

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