EP1683882B2 - Aluminium alloy with low quench sensitivity and process for the manufacture of a semi-finished product of this alloy - Google Patents

Description

The invention relates to a quench-resistant aluminum alloy for the production of high-strength, low-intrinsic forgings and high-strength extruded and rolled products. Furthermore, the invention relates to a method for producing a semi-finished product from such an aluminum alloy.

For the aerospace industry, high-strength aluminum alloys are needed for the production of, in particular, load-bearing fuselage, wing and chassis parts, which have high strength both under static and dynamic load. The required strength properties can be achieved in the abovementioned semi-finished products by using alloys of the 7000 group (7xxx alloy) in accordance with the classification of aluminum alloys by the Aluminum Association (AA).

For example, die forgings for high-stress aerospace parts are made of the AA 7075, AA 7175, AA 7475 and more preferably AA 7049 and AA 7050 alloys in the Americas and the AA 7010, AA 7049A and AA 7050A alloys used in the European area.

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

In order that the semi-finished products produced from one of the abovementioned alloys, that is to say for example the forgings, the extruded profiles or the rolled plates, obtain the desired strength, the semi-finished products are subjected to a special heat treatment. This involves quenching of solution annealing temperature, usually associated with subsequent cold working at average thicknesses greater than 50 mm. Cold forming serves to reduce the stresses induced during quenching. The step of cold forming can be done by a cold heading or by stretching the semi-finished product typically by 1-3%. The semi-finished products produced should be as low in natural stress as possible in order to minimize undesirable distortion during further processing of the semi-finished products. In addition, the semi-finished products and, accordingly, also the prefabricated parts made therefrom should be low-stress, in order to give the designer the opportunity to utilize the entire material potential. For this reason, for example, for the alloys AA 7050 and AA 7010, the process steps to be used for producing parts for aerospace engineering and also the maximum thickness of the semi-finished products used for the production of the parts are standardized or prescribed. The maximum permissible thickness is 200 mm and assumes that after quenching the semi-finished product is necessarily subjected to a cold-forming step for the abovementioned reasons. In extruded and rolled products, cold forming is relatively easy to achieve due to the generally simple geometry of stretching in the longitudinal direction. In the case of geometrically complicated forgings, on the other hand, it is only possible to achieve a uniformly high degree of compression - if at all - with great effort. In the course of designing larger aircraft, larger and especially thicker forgings are needed.

US 4 629 517 A discloses an aluminum product having good strength properties and good corrosion resistance when in the T76 state. The aluminum product consists of an AA 7xxx alloy containing 0.08 wt% Si, 0.29 wt% Fe, 0.20 wt% Cu, 0.03 wt% Mn, 2.5 Wt% Mg, 0.03 wt% Cr, 7.1 wt% Zn, 0.12 wt% Zr and 0.06 wt% Ti. However, the method described in this document is only suitable for producing aluminum products having a thickness of a few millimeters. The alloy described in this document is not suitable for the production of semi-finished products with medium and larger thicknesses and the required strengths.

In " Aluminum and Aluminum Alloys "(1993) edited by JR Davies - ASM International Materials Park (pages 265-274 ) describes processes that can be used to produce aluminum semi-finished products with thicknesses of up to 114 mm. Among others, the already mentioned AA 7xxx alloys are used for this purpose. However, this document does not disclose with which alloy and by which method aluminum semi-finished products of greater thickness than those specified in the document can be produced.

It is therefore an object of the invention to have a largely quenching insensitive, high-strength aluminum alloy with the same or better strength properties as the alloys AA 7010 and AA 7050 available, which has low internal Abschreckeigenspannungen at high thicknesses after cold forming and also from the semi-finished with a medium thickness can be produced with high strength and fracture toughness without necessarily requiring a cold forming step to reduce residual stresses induced during quenching.

Furthermore, the invention has the object, a method for producing a semifinished product with to suggest the desired properties of this alloy.

The alloy-related object is achieved by a high-strength, quench-resistant aluminum alloy having the features of claim 1.

The method-related object is achieved by a method according to claim 9 or claim 12.

The terms used in the context of these embodiments with regard to the thickness are defined below: Medium-sized semi-finished products have a thickness of between 50 and 180 mm. Semi-finished products with a greater thickness have a coating thickness of> 180 mm.

The quenching-resistant alloy according to the invention can also be used to produce semi-finished products in a thickness of more than 200 mm, in particular 250 mm and more, with the desired high static and dynamic strength properties while at the same time having good fracture toughness and good stress-crack corrosion behavior. Only at these larger thicknesses a cold forming step is advantageously carried out to reduce the quench-induced residual stresses.

In addition, semi-finished products produced for average thicknesses of the alloy may be mildly mild after solution annealing. B. can be cooled in a glycol-water mixture without the very good material properties are significantly impaired after a subsequent thermal aging. For this reason, the step of cold forming is omitted for medium thicknesses, since the residual stresses induced by the mild cooling are not critically low. Therefore, it is possible with this alloy to produce semi-finished products in the middle thickness range in a simpler and more cost-effective manner, namely without an otherwise necessary cold forming step.

The above-described advantageous properties of the alloy can also be utilized to simplify the manufacturing process of a part, for the manufacture of which a semifinished product of greater initial thickness is required and which has an average thickness after being machined. Such for example forged semi-finished product of greater thickness is pre-machined after the step of hot forming. The preprocessing is designed so that the semifinished product to be quenched in the course of the heat treatment undergoes such a thickness reduction, which is anyway necessary for the production of the finished part, that the semi-finished product is subjected to a heat treatment with mild quenching (glycol / water quenching) without carrying out a cold forming step otherwise necessary for larger thicknesses. Mixture) can be subjected.

With the alloy according to the invention thus semi-finished products with a medium thickness can be gently quenched by glycol-water mixtures, while semifinished products of greater thickness such mild quenching due to the required Mindestabkühlgeschwindigkeit is no longer appropriate, so that they are quenched in water. As a result, these semi-finished products are then subjected to cold forming, such as upsetting or stretching by 1-5%.

The above-mentioned properties of the semifinished product produced from this alloy are unexpected since, contrary to the specifications resulting from the prior art, the copper content is significantly lower than was the case with previously known high-strength aluminum alloys. According to a preferred embodiment, the copper content is only 0.8-1.1 wt%. Thus, the copper content is only about 50% of the preferred copper content of the WO 02/052053 A1 known aluminum alloys. That nevertheless very high strength values are achieved, is surprising. These properties are believed to be due to the balanced composition of the alloy constituents, which includes the relatively high levels of zinc and the magnesium content adjusted to it. The balanced composition of the alloying elements permitted within narrow limits provides that the sum of the elements magnesium, copper and zinc is at least 9% by weight. In addition, the alloy has a zinc: magnesium ratio of between 4.4 and 5.3. It has been found that the desired strength properties can only be achieved if the elements magnesium, copper and zinc in the sum more than 9 wt .-% and the zinc: magnesium ratio. These characteristics of the alloy are a measure of how the products made with the alloy have the desired strength properties. This regulation also determines the hardenability of the semi-finished products produced with the alloy.

Particularly high static and dynamic strength properties and a special Abschreckunempflindlichkeit with high fracture toughness is obtained when the copper content is 0.8 to 1.1 wt .-% and the magnesium content 1.6 to 1.8 wt .-%. Thus, the copper content is well below the maximum solubility for copper in the presence of the aforementioned magnesium content. This has the consequence that the proportion of insoluble, copper-containing phases is very low, taking into account the übri alloying and accompanying elements. This immediately results in an improvement in dynamic properties and fracture toughness.

To further increase the strength of the alloy, silver addition may be advantageous. For economic reasons, the content will be limited to 0.2-0.7%, in particular 0.20-0.40% by weight.

The manganese content of the alloy was limited to a maximum of 0.5% by weight. Manganese precipitates in Al-Zn-Cu-Mg alloys in the homogenization of the continuous cast ingots in the form of finely divided manganese aluminides, which may also contain some of the iron present as an impurity in the alloy. These manganese aluminides are helpful in the recrystallization control of the microstructure during the heat treatment of the transformed semifinished product. Experience has shown that the hardenability of an Al-Zn-Cu-Mg alloy decreases with increasing manganese content. For this reason, the manganese content is limited.

The reduced effect of manganese with regard to microstructural control is compensated by a zirconium additive. This is according to a preferred embodiment, 0.14 - 0.20 wt .-%. Zirconium also precipitates out of the microstructure during the homogenization of the continuous casting ingots in the form of zirconium aluminides. These aluminides are generally more finely dispersed than the manganese aluminides. Therefore, they are especially helpful in terms of recrystallization control. The zirconium aluminides formed are not coarsened by the intended heat treatment and are stable in the selected temperature ranges, unlike manganese aluminides. For this reason, zirconium is a necessary component of the alloy.

The titanium contained in the alloy is primarily used for grain refining during continuous casting. Preference is given to adding from 0.03 to 0.1% by weight of titanium, in particular from 0.03 to 0.06% by weight, of titanium.

Optionally, the alloy may contain 0.001-0.03 wt% boron. Furthermore, the alloy can max. 0.2% by weight of cerium and max. 0.30 wt .-% scandium.

The desired properties are achieved if the specified alloying constituents are used proportionally in the specified range. With an alloy in which one or more alloying constituents have a content which is outside the stated range, semi-finished products can no longer be produced with the required properties.

• Gießen von Barren aus der Legierung;
• Homogenisierung der gegossenen Barren bei einer Temperatur, die möglichst dicht unter 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; und
• Warmaushärten des abgeschreckten Halbzeuges einstufig oder mehrstufig, wobei Aufheizraten, Haltezeiten und Temperaturen auf die Optimierung der Eigenschaften eingestellt werden.

The semi-finished products are manufactured from this alloy with the following steps:

• Casting bars of the alloy;
• Homogenization of the cast ingots at a temperature which is as close as possible to the melting temperature of the alloy for a heating and holding time which is sufficient to achieve the most uniform and fine distribution of the alloying elements in the cast structure, preferably at 460-490 ° C .;
• Hot working of the homogenised billets by forging, extrusion and / or rolling in the temperature range of 350 - 440 ° C;
• Solution heat treatment of the hot-formed semifinished product at temperatures which are high enough to dissolve the alloying elements necessary for hardening evenly distributed in the structure, preferably at 465-500 ° C .;
• Quenching the solution-annealed semi-finished products in water at 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; and
• Heat curing the quenched semi-finished product in one or more stages, with heating rates, holding times and temperatures are set to optimize the properties.

Particularly preferred is a method in which the thermosetting of the quenched semifinished product takes place in two stages, wherein in the first stage, the semi-finished product is heated to a temperature of more than 100 ° C and held for more than eight hours at this temperature and in the second stage heated to more than 130 ° C and heated for more than five hours. These two steps can be carried out immediately after each other. Without prejudice to the desired properties of the semifinished product, the semifinished product treated with the first stage can also be cooled and the second stage of the thermosetting can be carried out at a later time.

For larger thicknesses, it may be necessary, in spite of the quenching resistance of the alloy, to subject the semi-finished product to reducing the residual stresses resulting from quenching to a cold working step after the quenching step. Appropriately, this is done by upsetting or stretching the semi-finished product by typically 1 - 5%.

EXAMPLES:

Two typical alloy compositions of the claimed aluminum alloy were used to make specimens to perform the necessary strength studies. The two alloys Z1, Z2 had the following composition: Si Fe Cu Mn mg Cr Zn Ti Zr Ti + Zr Alloy Z1 0.05 0.05 0.95 0.39 1.70 0,002 8.35 0,035 0.12 0,155 Alloy Z2 0.04 0.07 0.90 0,004 1.65 0.001 8.50 0,025 0.12 0.145

Alloys Z1, Z2 were cast on an industrial scale to 370 mm diameter continuous casting blocks. The continuous casting blocks were homogenized to compensate for the crystallization induced crystallization. The blocks were homogenized in two stages in a temperature range of 465 ° C - 485 ° C and cooled.

Example 1:

After dressing the cast skin of the blocks thus produced, the homogenized blocks were preheated to 370 ° C and remolded into open-die forgings 250 mm thick and 500 mm wide.

Subsequently, the forgings made of Alloy Z1 and Z2 were solution-annealed for at least 4 hours at 485 ° C, quenched in water at room temperature and then cured between 100 ° C and 160 ° C warm, wherein the hot curing has been carried out in two stages. In the first stage, the semi-finished product was heated to more than 100 ° C and held at this temperature for more than eight hours. The second stage carried out after the first stage was carried out at a temperature of more than 130 ° C for more than five hours.

Tensile samples were taken from the thermoset open-die forgings to determine the room temperature strength properties in the "long" (L), "long-transverse" (LT) and "short-transverse" (ST) sample plies. The average strength properties of alloy Z1 and Z2 for a thickness of 250 mm with water quenching are given in the following table: alloy load direction _Rp02 (MPa) R _m (MPa) A ₅ (%) Z 1 L 504 523 11.2 LT 502 533 5.2 ST 498 522 8.0 Z 2 L 520 528 8.6 LT 508 530 4.0 ST 511 525 5.1

The results indicate that the R _p02 and R _m values are nearly identical for all three load _directions and are above 490 MPa and tensile strength above 520 MPa for the yield strength (R _p02 ). The A ₅ values are highest for the L direction and reach at least 4% elongation at break (A ₅ ) for the two transverse directions. On compact tensile specimens (W = 50 mm) from the same _{open-die forgings} , the fracture toughness K _IC in the LT and TL test specimens was determined according to ASTM-E 399. The K _lC values are given below: alloy test direction location K _IC (MPa -√m) R _p0.2 (MPa) Z 1 LT edge 30.5 529 LT core 32.9 504 TL edge 23.1 516 TL core 20.4 502 Z 2 LT edge 30.3 514 LT core 35.9 520 TL edge 23.6 514 TL core 21.8 508

On round specimens, the stress cracking corrosion resistance was determined for the LT and ST layers in accordance with ASTM G47 (alternating dip test). The results are given below for the alloy Z1: load direction Voltage (MPa) Duration (days) Electric conductivity (% IACS) LT 320 > 30 34.7 ST 320 > 30

For both test directions, life of more than 30 days results at a stress of 320 MPa. Typical specifications for high strength Al alloys, such as AA 7050, require these lives at minimum stresses of 240 MPa. This means that the new alloy, despite significantly higher strength compared to the alloy AA 7050 at the same time has a stress corrosion cracking resistance, which is well above the minimum value for AA 7050.

Similarly, forgings were made with the same parameters from alloy Z1. In addition, after solution annealing and quenching in the short transverse direction (ST), the forgings were cold-rolled to reduce the detonation ring tensions. After the subsequent curing, which was carried out in two stages according to the aforementioned parameters, the strength properties were determined at room temperature in the sample layers "long" (L), "long-transverse" (LT) and "short-transverse" (ST). The results are listed in the following table for alloy Z1: alloy load direction _Rp02 (MPa) R _m (MPa) A ₅ (%) Z 1 L 504 523 11.2 LT 502 533 5.2 ST 498 522 8.0 Z 1 + cold diving L 448 501 11.1 LT 468 516 6.7 ST 417 498 10.8

The results indicate that the _p02 R - and R are _m values for all three directions of load low and the lowest value for the short transverse (ST) has been found. The A ₅ values are highest for the L direction and reach at least 6% elongation at break (A ₅ ) for the two transverse directions. By shortening the second curing step, the strength drop can be reduced. On compact tensile specimens (W = 50 mm) from the same open-die forgings, the fracture toughness K _lc in the test specimens LT and TL was determined according to ASTM-E 399. The K _lC values are given in the following table: alloy test direction location K _lC (MPa √m) R _p0.2 ( _Mpa ) Z 1 LT edge 30.5 529 LT core 32.9 504 TL edge 23.1 516 TL core 20.4 502 Z 1 + cold diving LT edge 38.9 485 LT core 42.2 448 TL edge 23.9 474 TL core 21.9 468

Example 2:

In a further series of tests, open-die forgings with a thickness of 150 mm and a width of 500 mm were produced from Alloy Z1 and, according to the example described above, after solution heat treatment in water or in a water-glycol mixture with about 20% or approx. 40% quenched and warmed up as previously described. A forging was additionally cold-crushed after quenching in water. Tensile specimens taken from the forgings in the "long" (L), "long-transverse" (LT) and "short-transverse" (ST) directions, the influence of different cooling media was shown. The average strength properties of Alloy Z1 for a thickness of 150 mm for different cooling treatments are shown below: quenching load direction _Rp02 (MPa) R _m (MPa) A ₅ (%) Water (RT) L 551 573 10, 3 LT 515 544 7.5 ST 505 549 8.0 Water (RT) + cold diving L 491 537 12.8 LT 465 520 8.7 ST 430 513 8.5 Water / glycol (16-20%) L 545 566 12.5 LT 520 547 7.2 ST 512 548 8.3 Water / glycol (38-40%) L 503 529 12.2 LT 493 525 5.0 ST 487 526 5.6

The results show that a reduction of the cooling rate by glycol additives has hardly any influence on the strength values of the alloy. The ductility decreases only minimally with decreasing cooling rate or increasing glycol content.

On compact tensile specimens (W = 50 mm) from the same open-die forgings, the fracture toughness K _lc in the test specimens LT and TL was determined according to ASTM-E 399. The K _lC values are shown in the following table: quenching test direction K _lC (MPa √m) _Rp02 (MPa) Water (RT) LT 36.8 551 TL 23.8 515 Water (RT) + cold diving LT 39.1 491 TL 24.1 465 Water / glycol (16-20%) LT 28.2 545 TL 20.7 520 Water glycol (38-40%) LT 35.4 503 TL 18.5 493

There is no clear dependency on the cooling rate for the L-T position, whereas for the T-L layer there is a trend towards slightly lower values with decreasing cooling rate.

Example 3:

To determine the strength properties, the alloy Z1 was cast analogously to the first example and produced blocks for extrusion in yet another example.

After dressing the cast skin, the homogenized blocks were preheated to over 370 ° C and extruded into extruded profiles having a rectangular cross section of 40 mm thickness and a width of 100 mm.

The profiles were then solution-annealed for at least 4 hours at 485 ° C., quenched in water at room temperature and then heated between 100 ° C. and 160 ° C. in two stages (first stage:> 100 ° C.,> 8 h, second stage:> 130 ° C,> 5h) cured.

Tear samples were taken from the thermoset extruded sections to determine the room temperature strength properties in the "long" (L), "long-transverse" (LT), and "short-transverse" (ST) sample plies. The average strength properties of alloy Z1 for an extruded rectangular profile (40 x 100 mm) with water cooling followed by stretching are shown in the following table: load direction _Rp02 (MPa) R _m (MPa) A ₅ (%) L 600 609 9.3 LT 554 567 7.1 ST 505 561 7.5

The results indicate that the _p02 R - R and _m are values with values of 600 MPa and 609 MPa in the L direction and lowest in the ST-direction lowest, with values of 505 MPa and 561 MPa. The A ₅ values are highest for the L direction and reach at least 7% elongation at break (A ₅ ) for the two transverse directions. Compact fracture specimens (w = 50 mm) from the same extruded section were used to determine the fracture toughness K _lc in the LT and TL test specimen according to ASTM-E 399. The average fracture mechanics properties of Alloy Z1 extruded rectangular profile (40 x 100 mm) with water quenching are shown in the following table: test direction K _lC (MPa √m) _Rp02 (MPa) LT 50.9 600 TL 30.7 554

FIG. 1 FIG. 12 is a graph showing the strength behavior of various AA 7xxx alloys as a function of the average cooling rate during quenching from the solution annealing temperature. FIG. It is clearly recognizable in this presentation that the loss of strength when using the claimed aluminum alloy is considerably lower even at low cooling rates than in the case of the comparative alloys AA 7075, AA 7010 and AA 7050.

The strength values of the products / semi-finished products produced with the claimed alloy as determined in the context of the description of the invention are considerably improved, in particular with regard to the stress-crack corrosion resistance compared to products of prior art alloys, which is a result which could not be foreseen in the form shown. The results presented are also interesting in that the strength values described can be represented in particular in the case of hot curing carried out only in two stages.

Claims (13)

1. Aluminium alloy that is not sensitive to quenching and is used for producing high-strength forged pieces that are low in internal stress, and for high-strength extruded and rolled products, the said alloy being composed of:

   - 7.0 - 10.5 percent by weight zinc,

   - 1.0 - 2.5 percent by weight magnesium,

   - 0.1 -1.15 percent by weight copper,

   - 0.06 - 0.25 percent by weight zirconium,

   - 0.02 - 0.15 percent by weight titanium,

   - Maximum of 0.5 percent by weight manganese,

   - Maximum of 0.6 percent by weight silver,

   - Maximum of 0.10 percent by weight silicon,

   - Maximum of 0.10 percent by weight iron,

   - Maximum of 0.04 percent by weight chrome

   - and, as an option, one or more elements form the group hafnium, scandium, strontium and/or vanadium with a summary content of a maximum of 1.0 percent by weight

   - and, as an option, 0.001 - 0.03 percent by weight boron,

   - and, as an option, a maximum of 0.2 percent by weight cerium and a maximum of 0.30 percent by weight scandium,

   - and, as an option, a maximum of 0.2 percent by weight cerium

   - together with other contaminants with proportions of a maximum of 0.05 percent by weight per element and an overall proportion of a maximum of 0.15 percent by weight,

   - remainder: aluminium,

   - wherein the sum of the alloy elements zinc and magnesium and copper is at least 9 percent by weight and

   - wherein the ratio of zinc to magnesium in the alloy is between 4.4 and 5.3.
2. Aluminium alloy according to claim 1, characterised in that the alloy contains 1.6 - 1.8 percent by weight magnesium and 0.8 - 1.1 percent by weight copper.
3. Aluminium alloy according to claim 1, characterised in that the aluminium alloy contains 0.8 - 1.1 percent by weight copper and 0.3 - 0.5 percent by weight manganese.
4. Aluminium alloy according to claim 1, characterised in that the aluminium alloy contains 0.8 - 1.1 percent by weight copper and a maximum of 0.03 percent by weight manganese.
5. Aluminium alloy according to claim 1, characterised in that the aluminium alloy contains 0.2 - 0.3 percent by weight copper and 0.25 - 0.40 percent by weight silver.
6. Aluminium alloy according to one of claims 1 to 5, characterised in that the aluminium alloy contains 0.10 - 0.15 percent by weight titanium.
7. Aluminium alloy according to one of claims 1 to 6, characterised in that the alloy contains a maximum of 0.30 percent by weight scandium and a maximum of 0.2 percent by weight vanadium or hafnium or cerium.
8. Aluminium alloy according to one of claims 1 to 7, characterised in that the iron and silicon content is a maximum of 0.08 percent by weight in each case.
9. Method for producing a high-strength, semi-finished product that is low in internal stress by using an aluminium alloy in accordance with one of claims 1 to 8, the said method including the following steps:

   - hot forming the homogenised bars by means of forging, extruding and/or rolling within the temperature range of 350 - 440° C;

   - solution treating the hot-formed semi-finished product at temperatures that are high enough to bring the alloy elements necessary for hardening into solution evenly distributed in the structure, preferably at 465 - 500° C;

   - quenching the solution heat treated semi-finished product in water, in a water glycol mixture or in a saline mixture at temperatures between 100° C and 170° C;

   - cold forming the quenched semi-finished product to reduce the internal stress generated during the quenching process in the quenching medium; and

   - artificially ageing the quenched semi-finished product in one step or in multiple steps, wherein heating rates, holding times and temperatures are set to optimise the required material characteristics.
10. Method according to claim 9, characterised in that the cold forming step is carried out by means of upsetting or drawing out the semi-finished product.
11. Method according to claim 9 or 10, characterised in that the cold forming rate is 1 - 5 %.
12. Method for producing a high-strength, semi-finished product with a heat treatable thickness of 50 - 180 mm from an aluminium alloy in accordance with one of claims 1 to 8, the said method including the following steps:

    - hot forming of the homogenised bars by means of forging, extruding and/or rolling within the temperature range of 350 - 440° C;

    - solution treating the hot-formed semi-finished product at temperatures that are high enough to bring the alloy elements necessary for hardening into solution evenly distributed in the structure, preferably at 465 - 500° C;

    - quenching of the solution heat treated semi-finished product in water, in a water glycol mixture or in a saline mixture at temperatures between 100° C and 170° C; and

    - artificially ageing the quenched semi-finished product in one step or in multiple steps, wherein heating rates, holding times and temperatures are set to optimise the required material characteristics.
13. Method according to claim 12, characterised in that after the heat forming step, there is produced a semi-finished product, which has a heat treatable thickness of more than 180 mm and which, before the subsequent heat treatment, is machined by way of a preliminary machining process in order to reduce the thickness of the semi-finished product by means of the machining process, such that the said preliminarily machined semi-finished product has a heat treatable thickness of between 50 and 180 mm and the subsequent heat treatment is carried out corresponding to the requirements for semi-finished products with a heat treatable thickness of between 50 and 180 mm.

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