EP1683882A1 - Aluminium alloy with low quench sensitivity and process for the manufacture of a semi-finished product of this alloy - Google Patents

Abstract

An aluminum alloy for production of high-strength forged pieces that are low in inherent tension, and high-strength extruded and rolled products, comprises 7.0-10.5 wt.% zinc (Zn); 1-2.5 wt.% magnesium (Mg); 0.1-1.15 wt.% copper (Cu); 0.06-0.25 wt.% zirconium; 0.02-0.15 wt.% titanium; =0.5 wt.% manganese; =0.6 wt.% silver; =0.10 wt.% silicon; =0.10 wt.% iron; =0.04 wt.% chrome; hafnium, scandium, strontium and/or vanadium at =1 wt.%. An aluminum alloy for production of high-strength forged pieces that are low in inherent tension, and high-strength extruded and rolled products, comprises 7.0-10.5 wt.% zinc (Zn); 1-2.5 wt.% magnesium (Mg); 0.1-1.15 wt.% copper (Cu); 0.06-0.25 wt.% zirconium (Zr); 0.02-0.15 wt.% titanium (Ti); =0.5 wt.% manganese (Mn); =0.6 wt.% silver (Ag); =0.10 wt.% silicon (Si); =0.10 wt.% iron (Fe); =0.04 wt.% chrome; hafnium (Hf), scandium (Sc), strontium (Sr) and/or vanadium (V) with a summary content of =1 wt.%, and contaminants at proportions of =0.05 wt.% per element with a total contaminant proportion of =0.15 wt.%. A remaining amount by wt.% is aluminum (Al); and a sum of the alloy elements Zn, Mg and Cu is >=9 wt.%. An independent claim is also included for a method for the production of a high-strength semifinished product low in inherent tension, up to greater thickness values, comprising providing an aluminum alloy; hot forming homogenized bars via forging, extrusion and/or rolling, at 350-440[deg]C; solution heat treating the hot-formed semi-finished product at a temperature sufficiently high to bring the alloy elements necessary for hardening into solution uniformly distributed in the structure; quenching the solution heat treated semi-finished products in a quenching medium comprising water, in a water/glycol mixture, or in a salt mixture at 100-170[deg]C; cold forming the quenched semi-finished product to reduce a set of inherent tensions that occurred during quenching in the quenching medium; and artificial aging the quenched semi-finished product, in at least one stage, where a heating rate, holding time, and temperature is adjusted for optimization of the properties.

Description

The invention relates to a quench-insensitive 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.

WO 02/052053 A1 discloses a high-strength aluminum alloy of the aforementioned type with a zinc content, increased with respect to earlier 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.

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 inner Abschreckeigenspannungen at high thicknesses after cold forming and also from the semi-finished with a medium thickness with high strength and fracture toughness can be made without causing any reduction of quenching induced residual stresses mandatory cold forming step is needed.

Furthermore, the invention has for its object to propose a method for producing a semifinished product with 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 11 or claim 14.

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 production 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 required 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 aluminum alloys known from WO 02/052053 A1. 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. It has been shown that the desired strength properties are only achieved can be, if the elements magnesium, copper and zinc in the sum more than 9 wt .-% have. This feature of the alloy is 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 .-%. This corresponds to a zinc: magnesium ratio of 4.4 - 5.2. 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, even taking into account the other 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 formed semi-finished 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 in the Usually finely dispersed than the Manganaluminide. 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.

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
• Thermosetting the quenched semi-finished product in one or more stages, heating rates, hold times, and temperatures are set to optimize 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 prepare 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 0.005 0.005 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 two-stage in a temperature range homogenized from 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, 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:

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 for the two transverse directions at least 4% elongation at break (A ₅ ). 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 _IC values are given below:

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:

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. To subsequent curing, which was carried out in two stages according to the parameters mentioned above, the strength properties at room temperature were determined in the sample layers "long" (L), "long-transverse" (LT) and "short-transverse" (ST). The results are listed in the following table for alloy Z1:

The results indicate that the R _p02 and R _m values are lower for all three load directions and the lowest value for the short transverse direction (ST) was 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 _IC in the test specimens LT and TL was determined according to ASTM-E 399. The K _IC values are given in the following table:

Example 2:

In another series of tests, forgings with a thickness of 150 mm and a width of 500 mm were made of Alloy Z1 and, according to the example described above Solution annealing in water or in a water-glycol mixture with about 20% or about 40% quenched and warm-stored as described above. A forging was additionally cold-crushed after quenching in water. Tensile samples taken from "long" (L), "long-transverse" (LT) and "short-transverse" (ST) forgings were used to show the influence of different cooling media. The average strength properties of Alloy Z1 for a thickness of 150 mm for different cooling treatments are shown below:

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 _IC in the test specimens LT and TL was determined according to ASTM-E 399. The K _IC values are shown in the following table:

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:

The results indicate that the R _p02 and R _m values are highest in the L direction and lowest in the ST direction with values of 600 MPa and 609 MPa, respectively, 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. On compact tensile specimens (W = 50 mm) from the same open-die forgings, the fracture toughness K _IC in the test specimens LT and TL was determined according to ASTM-E 399. The average fracture mechanics properties of Alloy Z1 and Z2 for a thickness of 250 mm with water quenching are given in the following table:

FIG. 1 shows a diagram for illustrating the strength behavior of various AA 7xxx alloys as a function of the average cooling rate during quenching from the solution annealing temperature. 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 results as shown in FIG Form was not predictable. 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 (15)

Non-quenching aluminum alloy for the production of high-strength, low-intrinsic forgings and high-strength extruded and rolled products consisting of: 7.0 to 10.5% by weight of zinc, 1.0 to 2.5% by weight of magnesium, - 0.1 - 1.15 wt .-% copper - 0.06 - 0.25 wt .-% zirconium, 0.02-0.15% by weight of titanium, - Max. 0.5% by weight of manganese, - Max. 0.6% by weight of silver, - Max. 0.10% by weight of silicon - Max. 0.10% by weight of iron, - Max. 0.04 wt .-% chromium - and optionally one or more elements of the group hafnium, scandium, strontium and / or vanadium with a total content of max. 1.0% by weight - in addition to other impurities with proportions of max. 0.05 wt .-% per element and a total content of max. 0.15% by weight, - rest: aluminum, - Wherein the sum of the alloying elements zinc and magnesium and copper is at least 9 wt .-%. Aluminum alloy according to claim 1, characterized in that the zinc: magnesium ratio of the alloy is between 4.4 and 5.3. Aluminum alloy according to claim 2, characterized in that the alloy contains 1.6 to 1.8% by weight of magnesium and 0.8 to 1.1% by weight of copper. Aluminum alloy according to Claim 1 or 2, characterized in that the aluminum alloy contains 0.8-1.1% by weight of copper and 0.3-0.5% by weight of manganese. Aluminum alloy according to claim 1 or 2, characterized in that the aluminum alloy 0.8 to 1.1 wt .-% copper and max. Contains 0.03 wt .-% manganese. Aluminum alloy according to claim 1 or 2, characterized in that the aluminum alloy contains 0.2-0.3% by weight of copper and 0.25-0.40% by weight of silver. Aluminum alloy according to one of claims 1 to 6, characterized in that the aluminum alloy contains 0.10 - 0.15 wt .-% titanium. Aluminum alloy according to one of claims 1 to 6, characterized in that the aluminum alloy contains 0.001 to 0.03 wt .-% boron. Aluminum alloy according to one of claims 1 to 8, characterized in that the alloy is max. 0.30 wt.% Scandium and max. 0.2 wt .-% vanadium or hafnium or cerium. Aluminum alloy according to one of claims 1 to 9, characterized in that the iron and silicon content in each case max. 0.08 wt .-% is. A method for producing a high-strength, low-intrinsic semi-finished product up to larger thicknesses of an aluminum alloy according to one of claims 1 to 10, comprising the following steps: - Hot forming of the homogenized 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, in a water-glycol mixture or in a salt mixture with temperatures between 100 ° C and 170 ° C; - Cold forming the quenched semi-finished product for reduction the residual stresses created during quenching in the quench medium; and - Heat curing of the quenched semi-finished product in one or more stages, with heating rates, holding times and temperatures are set to optimize the required material properties. A method according to claim 11, characterized in that the step of cold forming by upsetting or stretching of the semi-finished product is performed. A method according to claim 11 or 12, characterized in that the cold forming rate is 1 - 5%. A method for producing a high-strength, low-energy, medium-thickness aluminum-alloy semi-finished product according to any one of claims 1 to 10, comprising the following steps: - Hot forming of the homogenized 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, in a water-glycol mixture or in a salt mixture with temperatures between 100 ° C and 170 ° C; and - Heat curing of the quenched semi-finished product in one or more stages, with heating rates, holding times and temperatures are set to optimize the required material properties. A method according to claim 14, characterized in that after the step of hot forming a semifinished product of greater thickness is present, which is machined before the subsequent heat treatment by way of pre-machining to reduce by the machining, the thickness of the semifinished product to the extent that this pre-processed semifinished product an average thickness and the subsequent heat treatment is carried out according to the requirements for semi-finished products of medium thickness.

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