EP2703508B1 - Aluminium alloy resistant to intercrystalline corrosion - Google Patents

Description

The invention relates to an aluminum alloy, the use of an aluminum alloy strip or sheet, and a method for producing an aluminum alloy strip or sheet.

Aluminum magnesium (AlMg) alloys of type 5xxx are used in the form of sheets or plates or strips for the construction of welded or joined structures, in shipbuilding, automotive and aircraft construction. They are characterized by a particularly high strength, with increasing the magnesium content, the strength of the AlMg alloys increase. Typical examples of the 5xxx aluminum alloys are, for example, aluminum alloys of the type AA 5049, AA 5454 or AA 5918. The alloys are AlMg 2 Mn (5049) AlMg 3 Mn (5454) or AlMg 3.5 Mn (5918) aluminum alloys , The constant need for additional weight reduction requires aluminum alloys with higher strengths and thus with correspondingly higher magnesium (Mg-) held to provide the desired strengths. The problem with AlMgMn aluminum alloys with Mg contents of more than 2.4% by weight is that they are increasingly prone to intergranular corrosion when exposed to elevated temperatures for longer times. It has been found that in AlMgMn aluminum alloys containing more than 2.4% by weight of magnesium at temperatures of 70 to 200 ° C., β-Al ₅ Mg ₃ phases precipitate along the grain boundaries. If the Grain boundaries are consistently occupied with β-particles, in the presence of a corrosive medium, the dissolution of these β-phases can lead to a selective corrosion attack along the grain boundaries. As a result, aluminum alloys with increased Mg contents are either not usable in heat-stressed areas or have to have reduced Mg contents due to heat generation, thus minimizing the precipitation of β-Al ₅ Mg ₃ particles, and a continuous one Grain boundary occupancy with β-Al ₅ Mg ₃ particles fails. There are already proposed solutions to this problem in the prior art. For example, the German Offenlegungsschrift proposes DE 102 31 437 A1 to significantly reduce the susceptibility to intergranular corrosion by a specific aluminum alloy composition even after heat sensitization. It proposes the following aluminum alloy composition:

3.1% <Mg <4.5%,

0.4% <Mn <0.85%,

0.4% <Zn <0.8%,

0.06% <Cu <0.35%,

Cr <0.25%,

Fe <0.35%,

Si <0.2%,

Zr <0.25%,

Ti <0.3%,

Impurities ≤ 0.05% and in total max. 0.15%, balance Al.

The American Patent Publication US 2007/187009 A1 shows specific aluminum alloy compounds and corresponding mass loss measurements according to ASTM G67. For the measured samples C, D, E, F and J (after 100 hours at 100 ° C) a mass loss between 6-22 mg / cm ² is shown.

However, it has been found that the results with respect to the susceptibility to intergranular corrosion, which is measured and evaluated according to the standard ASTM G67, can be improved. In addition, the aluminum alloy allows a content of up to 0.25 wt .-% zirconium, which is considered in terms of recycling of the aluminum alloy as critical. From the international patent application WO 99/42627 In addition, a zirconium-containing aluminum alloy is known, which indeed achieved very good results in the ASTM G67 test, but is problematic because of the necessarily present zirconium content in use.

On this basis, the present invention has the object to provide an aluminum alloy is available, which has only a low tendency to intergranular corrosion, ie in the ASTM G67 test a mass loss value <15 mg / cm ² , at the same time high strength and good formability provides and Contains standard alloy components, so that the recycling of the aluminum alloy is simplified. In addition, a use of the aluminum alloy and a method for the production of products of the aluminum alloy to be proposed.

According to a first teaching of the present invention, the above-described object for an aluminum alloy is achieved by comprising alloy components which have the following composition in% by weight:

2.91% ≤ Mg ≤ 4.5%,

0.5% ≤ Mn ≤ 0.8%,

0.05% ≤ Cu ≤ 0.30%,

0.05% ≤ Cr ≤ 0.30%,

0, 05% ≤ Zn ≤ 0.9%,

Fe ≤ 0.40%,

Si ≤ 0.25%,

Ti ≤ 0.20%,

Residual Al and impurities individually less than 0.05% in total max. 0.15%, and wherein for the alloy components Zn, Cr, Cu and Mn:

(2.3 *% Zn + 1.25 *% Cr + 0.65 *% Cu + 0.05 *% Mn) + 2.4 ≥% Mg.

"% Zn", "% Cr", "% Cu", "% Mn" and "% Mg" correspond to the contents of the alloy component in weight percent, respectively. The composition of the invention is based on the finding that the alloying components Zn, Cr, Cu and Mn at magnesium contents of at least 2.91 wt .-% suppresses the precipitation of β-Al ₅ Mg ₃ particles, by the presence of these alloying elements, the formation of τ phases supported. These τ-phases of the type AlCuMgZn suppress the β-phase formation to a considerable extent, so that even at higher Mg contents there is only a very slight tendency to form β-phase or β-Al ₅ Mg ₃ particles at the grain boundaries , In addition, in the presence of the alloying elements Cr and Mn, it is also possible to form ε-phases of the type AlCrMgMn, which likewise suppress the β-phase formation. As a result, the corresponding aluminum alloy is not so susceptible to intergranular corrosion. In addition, it has been found that the compensation efficiency of the individual alloy components

Zn, Cr, Cu and Mn are different. The alloying component zinc can for example serve to compensate for 2.3 times the amount of magnesium above 2.91% by weight of Mg, so that the resulting aluminum alloy shows only a very slight tendency to intergranular corrosion. The efficiency of suppressing the intercrystalline corrosion or the excretion of β-phases decreases with the alloying components chromium, copper and manganese. As a result, aluminum alloys can be provided in any case, which have relatively high magnesium contents and thus show higher strengths without these tend to intercrystalline corrosion after exposure to temperature. Higher strengths with comparable corrosion resistance is achieved at a Mg content of at least 3.0 wt .-%.

In order to be able to produce the aluminum alloy according to the invention economically and, moreover, to have no negative effects with regard to formability and no or only slight changes in the physical properties of the aluminum alloy, for example during casting and rolling, it is advantageous according to a first embodiment of the aluminum alloy according to the invention, the following applies to the alloy components Zn, Cr, Cu and Mn:

(2.3 *% Zn + 1.25 *% Cr + 0.65 *% Cu + 0.05 *% Mn) + 1.4 ≤% Mg.

Hereby, for an embodiment of the present invention, an upper limit of the addition of the alloy components Zn, Cr, Cu and Mn is given to one To achieve the most economical production of aluminum alloy. Additions beyond this upper limit show no additional positive effect on the resistance to intergranular corrosion. In addition, unwanted side effects due to high contents of the alloy components in this embodiment of the aluminum alloy according to the invention can be excluded.

Preferably, according to a further embodiment of the aluminum alloy according to the invention, the alloying component Cu has the following content in% by weight:

0.05% ≤ Cu ≤ 0.20%,

to make the aluminum alloy generally more corrosion resistant.

According to a next embodiment of the aluminum alloy according to the invention, the formability can be maximized by the alloying component Cr having the following content in% by weight:

0.05% ≤ Cr ≤ 0.20%.

According to a further embodiment of the aluminum alloy according to the invention, an aluminum alloy which is further optimized with respect to the addition of alloy components and which is resistant to intercrystalline corrosion is provided in that the alloy components Mg and Zn have the following contents in% by weight:

2.91% ≤ Mg ≤ 3.6%

0.05% ≤ Zn ≤ 0.75%.

The reduction of the upper limit of the magnesium content allows a further reduction of the maximum zinc concentration, so that a cost-optimized aluminum alloy with very high resistance to intergranular corrosion can be provided. The Mg content of this embodiment is preferably 3.0% by weight to 3.6% by weight, in particular 3.4% by weight to 3.6% by weight.

In a further refinement, the aluminum alloy according to the invention can be further optimized with respect to its strength by virtue of the content of the alloying component Mg being at least 3.6% by weight and not more than 4.5% by weight. The increased magnesium contents cause a significant increase in the strength of the aluminum alloy with good formability. Due to the specific composition of the aluminum alloy according to the invention, this aluminum alloy shows only small mass losses <15 mg / cm ² despite the high Mg contents and is therefore free from intercrystalline corrosion according to ASTM G67. Preferably, the Mg content is limited to a maximum of 4.0 wt .-% in order to improve the corrosion behavior.

As already stated above, the aluminum alloys according to the invention are characterized in that, in addition to very good strength and formability, they also have a very good resistance to intergranular corrosion. In this respect, the above-indicated task according to Another teaching of the invention by the use of an aluminum alloy strip or sheet of a aluminum alloy according to the invention for the production of suspension and structural components in vehicle, aircraft or shipbuilding solved.

Chassis and structural components of vehicles, automobiles or aircraft are frequently exposed to heat sources, for example the exhaust gases of the internal combustion engine or other heat sources, so that aluminum alloys, which tend to undergo intercrystalline corrosion after a heat treatment, can not normally be used here. However, the use of an aluminum alloy strip or sheet according to the invention for the production of chassis and structural components also allows the use of higher-strength aluminum magnesium alloys with magnesium contents of at least 2.91 wt .-% in these applications due to the very good resistance to intergranular corrosion. The higher strength aluminum bands or sheets allow the reduction of wall thickness due to the increased strength. In this respect, they contribute to the further weight reduction of vehicles, ships or even aircraft.

Preferably, an aluminum alloy strip or sheet is used consisting of the aluminum alloy according to the invention for producing a chassis and structural part, which is arranged in the region of the engine, the exhaust system or other heat sources of a motor vehicle. A typical example of this is a spring or wishbone of a motor vehicle. Areas of this component, especially if they are located close to the engine, are permanently exposed to increased heat input. Especially in the automotive industry, but also in the construction of trains, airplanes and ships open up by the use of bands and sheets of erfindungemäßen aluminum alloy new applications, which are characterized by an increased heat input.

Particularly advantageous is the use of an aluminum alloy strip or sheet consisting of the aluminum alloy according to the invention, when the chassis or structural components have at least one weld. Welds are generally areas in which heat has entered the metal. This heat input can lead to intercrystalline corrosion, if the aluminum alloy tends to. In the case of the aluminum alloys according to the invention, by contrast, the β-phase precipitate responsible for the intercrystalline corrosion is largely suppressed, so that the component can be readily welded and yet it does not tend to intergranular corrosion.

Finally, the use of an aluminum alloy strip or sheet of the aluminum alloy of the invention is particularly advantageous when the wall thickness of the aluminum alloy strip or sheet is 0.5 mm to 8 mm, optionally 1.5 to 5 mm. These wall thicknesses are very well suited to provide the necessary strength for a chassis or structural part.

• Gießen eines Walzbarrens
• Homogenisieren des Walzbarrens bei 500 bis 550 °C für mindestens 2 Stunden,
• Warmwalzen des Walzbarrens zu einem Warmband bei Warmwalztemperaturen von 280 °C bis 500 °C,
• Kaltwalzen des Warmbandes mit oder ohne Zwischenglühung auf Enddicke und
• Weichglühen des Kaltbandes bei 300 bis 400 °C in einem Batchofen.

According to another teaching of the present invention, an economical manufacturing method for an aluminum alloy strip or sheet is now given, which consists of the aluminum alloy according to the invention. This procedure comprises the following steps:

• Pouring a rolled bar
• Homogenizing the rolling ingot at 500 to 550 ° C for at least 2 hours,
• Hot rolling the billet to a hot strip at hot rolling temperatures of 280 ° C to 500 ° C,
• Cold rolling of the hot strip with or without intermediate annealing to final thickness and
• Soft annealing of the cold strip at 300 to 400 ° C in a batch oven.

Contrary to previous experience, the aluminum alloy according to the invention does not require any specific heat treatment step, for example a solution annealing step at the end of the production process, but the aluminum alloy can be produced highly economically with conventional equipment, for example batch ovens. It is also conceivable to provide a direct casting of the strip in place of the casting of a rolling bar, which is then subsequently hot and / or cold rolled.

The invention will now be explained in more detail with reference to embodiments. <b> Table 1 </ b> alloy ST5049 ST5454 ST5918 V1 V2 V3 V4 conv. conv. conv. erf. erf. erf. erf. alloy components mg 2.05 2.90 3.45 2.91 3.42 3.75 3.77 Mn 0.95 0.80 0.55 0.56 0, 6 0, 66 0.66 Si 0.15 0.15 0.15 0.13 0.12 0.12 0.12 Fe 0.4 0.30 0.30 0.24 0.24 0.24 0.25 Cu 0.06 0.03 0.02 0.15 0.2 0.25 0.13 Cr 0.01 0.07 0.16 0,065 0.11 0.16 0.16 Ti 0.01 0.01 0.01 0,013 0,014 0,014 0.016 Zn 0 0.00 0.00 0.4 0.5 0, 6 0.61 minimal compensation 2.9 3.45 2.91 3.42 3.75 3.77 Mg compensation 2,547 2.6405 3.5155 3.8475 4.1755 4.1205

First, Table 1 shows the chemical analyzes of the standard alloys ST 5049, ST 5454 and ST 5918 and the aluminum alloys V1, V2, V3 and V4 according to the invention. In addition, in Table 1, the value of the amount of magnesium compensated by the alloy components, which is referred to as "Mg Compensation" and calculated by the following formula, is given.

(2.3 *% Zn + 1.25 *% Cr + 0.65 *% Cu + 0.05 *% Mn) + 2.4.

The minimum compensation is the value of the "compensated" Mg content, which is compensated at least by the alloy components Zn, Cr, Cu and Mn got to. The value given in Table 1 therefore corresponds to the Mg content of the respective aluminum alloys.

Since the Mg compensation value is only relevant for aluminum alloys with magnesium contents of at least 2.91% by weight, this value is not recorded for the standard alloy ST 5049. The other standard alloys ST 5454 and ST 5918 have a Mg compensation value which is below the magnesium content of the alloy. As is known, these alloys tend to intergranular corrosion under certain conditions. The reason is considered that the Mg content of these aluminum alloys is not sufficiently compensated. The situation is different for the aluminum alloys V1, V2, V3 and V4 according to the invention, whose Mg compensation value is significantly higher than the Mg content of the respective aluminum alloy in% by weight. <b> Table 2 </ b> measurand R _p0,2 R _m A _g A _50mm alloy MPa MPa % % ST5049 conv 99 215 16.4 21.9 ST5454 conv 118 246 17.4 21.8 ST5918 conv 129 264 18.1 19.8 V1 erf 115 246 16.2 20.7 V2 erf 125 271 18.5 21.3 V3 erf 132 288 15.8 20.6 V4 erf 133 289 18.7 22.0

Rolling ingots were cast from all seven aluminum alloys and the ingots were cast at temperatures of 500 to Homogenized 550 ° C for at least two hours. The billets thus produced were hot rolled to hot strip at hot rolling temperatures of 280 ° C to 500 ° C and then cold rolled to final thickness, with intermediate annealing and final annealing of the cold strip at temperatures between 300 and 400 ° C in a batch oven. The strip thickness was 1.5 mm.

Sheets were removed from the strips produced and their mechanical properties were determined in a tensile test according to DIN EN 10002-1 perpendicular to the rolling direction. The measured values are shown in Table 2. They show that the exemplary embodiment V1 according to the invention has, for example, a significantly higher tensile strength and yield strength than the standard alloy ST 5049. The elongation values Ag for the uniform elongation and A _{50 mm of} the alloy strips according to the invention and the standard alloys do not differ significantly, so that it can be assumed that the aluminum alloys according to the invention have an identical formability as the standard alloys.

The alloy variant V2 also provides a higher tensile strength and a higher yield strength compared to the standard alloy ST 5454. For the uniform elongation Ag and the elongation A _50mm also arise for the inventive variant V2 almost identical values for standard alloy ST 5454. The same applies to the variants V3 and V4, which, when compared to conventional aluminum alloy variant ST 5918 improved tensile and yield strengths. As a result, the aluminum alloys according to the invention have very good mechanical characteristics and can be processed identically to the comparable standard alloys.

The embodiments according to the invention and the conventional embodiments have now been subjected to a corrosion test according to ASTM G67, with which the susceptibility of an aluminum alloy to intergranular corrosion can be measured by measuring the mass loss. In this test, test strips 50 mm long and 60 mm wide are cut from the sheet or strip and stored with or without thermal pretreatment in concentrated nitric acid at 30 ° C for 24 hours. Nitric acid preferentially dissolves β-phases from the grain boundaries and thus causes a significant mass loss during the subsequent weight measurement, provided that β-phases precipitated in the sample along the grain boundaries are present.

In order to determine the susceptibility to intergranular corrosion also in heat-stressed applications, the samples were subjected to a pre-treatment in the form of storage at elevated temperatures prior to a mass loss measurement according to ASTM G67. For this purpose, the samples were stored for 17, 100 and 500 hours at 130 ° C and then subjected to the mass loss test. In addition, however, a storage was carried out for 100 hours at 100 ° C in order to achieve the comparability of the aluminum alloys according to the invention with those known from the prior art aluminum alloys. <b> Table 3 </ b> alloy outsourcing ST5049 ST5454 ST5918 V1 V2 V3 V4 Without 1.1 1.1 1.3 1.3 1.6 2.0 1.8 17h 130 ° C 1.0 1.4 2.3 1.4 1.8 2.4 1.9 100h 130 ° C 1.0 5.6 11.3 1.5 2.4 3.5 2.9 500h 130 ° C 1.1 16.2 30.9 1.9 6.7 8.3 8.9 100h 100 ° C 1.0 2.1 5.2 1.4 2.1 2.6 2.1

Table 3 shows the respective experimental conditions of aging and the measured mass loss according to a test according to ASTM G67 in mg / cm ² . According to ASTM G67, intergranular corrosion-resistant aluminum alloys achieve 1 to 15 mg / cm ² mass loss, whereas unstable 25 to 75 mg / cm ² mass loss.

It can be clearly seen that the standard alloy ST 5049, which has a relatively low magnesium content of 2.05% by weight, has the highest resistance to intergranular corrosion. Even with storage times of 500 hours at 130 ° C, this aluminum alloy does not change its corrosion behavior in the test. On the other hand, it also has the lowest mechanical strength values.

Unlike the other hand, the standard alloy ST 5454 and ST 5918. standard alloy behave The ST 5454 was rated at 500 hours presensitization at 130 ° C a mass loss of 16.2 mg / cm ^2. The mass loss of the ST 5918 also shows a very marked increase in the storage of the samples for 100 hours or for 500 hours at 130 ° C Mass loss after storage in concentrated nitric acid to a maximum of 30.9 mg / cm ² . If one compares the aluminum alloys according to the invention during storage for 500 hours at 130.degree. C., then these are significantly more stable to intercrystalline corrosion in spite of similarly high magnesium contents.

The maximum mass loss of the aluminum alloy V4 according to the invention was 8.9 mg / cm ² at 500 hours at 130 ° C. and thus lower by more than a factor of three than the standard alloy ST 5918. According to ASTM G67, it is considered stable to intercrystalline corrosion because of its Mass loss is less than 15 mg / cm ² . Despite higher magnesium contents and higher strength values compared to the respective standard alloys ST5454 or ST5918, the aluminum alloy according to the invention is distinguished by excellent resistance to intergranular corrosion.

In particular, the comparisons with the prior art results for high magnesium aluminum alloys show that in the selected aluminum alloy range a significant increase in the resistance of the aluminum alloys to intergranular corrosion can be achieved without having to sacrifice recycling or high manufacturing costs.

Finally, it was also shown that the use of highly economical batch ovens can be used to carry out soft annealing to high magnesium and intercrystalline corrosion to provide stable aluminum alloys and alloy products. Until now, it had been assumed that solution annealing in a continuous process line was necessary to achieve resistance to intergranular corrosion.

Claims (11)

1. Aluminium alloy comprising alloy components, which have the following composition in % by weight:
   
           2.91 % ≤ Mg ≤ 4.5 %,
   
           0.5% ≤ Mn ≤ 0.8 %,
   
           0.05 ≤ Cu ≤ 0.30 %,
   
           0.05 % ≤ Cr ≤ 0.30 %,
   
           0.05 % ≤ Zn ≤ 0.9 %,
   
           Fe ≤ 0.40 %,
   
           Si ≤ 0.25 %,
   
           Ti ≤ 0.20 %,
   
   the balance Al and impurities individually less than 0.05 % and in total a maximum of 0.15 % and wherein the following applies to the alloy components Zn, Cr, Cu and Mn:
   
           (2.3* %Zn + 1.25* %Cr + 0.65* %Cu + 0.05* %Mn) + 2.4 ≥ %Mg.
   
   
2. Aluminium alloy according to Claim 1, characterised in that the following further applies to the alloy components Zn, Cr, Cu and Mn:
   
           (2.3* %Zn + 1.25* %Cr + 0.65* %Cu + 0.05* %Mn) + 1.4 ≤ %Mg.
   
   
3. Aluminium alloy according to Claim 1 or 2, characterised in that the alloy component Cu has the following content in % by weight:
   
           0.05 % ≤ Cu ≤ 0.20 %.
   
   
4. Aluminium alloy according to any one of Claims 1 to 3, characterised in that the alloy component Cr has the following content in % by weight:
   
           0.05 % ≤ Cr ≤ 0.20 %.
   
   
5. Aluminium alloy according to any one of Claims 1 to 4, characterised in that the alloy components Mg and Zn have the following contents in % by weight:
   
           2.91 % ≤ Mg ≤ 3.6 %,
   
           0.05 % ≤ Zn ≤ 0.75 %.
   
   
6. Aluminium alloy according to any one of Claims 1 to 5, characterised in that the content of the alloy component Mg is at least 3.6 % by weight and a maximum of 4.5 % by weight.
7. Use of an aluminium alloy strip or sheet of an aluminium alloy according to any one of Claims 1 to 6 for producing chassis and structural components in vehicle, aircraft or ship construction.
8. Use according to Claim 7, characterised in that the aluminium alloy strip or sheet is used for producing a chassis or structural component which is arranged in the region of the engine, the exhaust gas system or other heat sources of a motor vehicle.
9. Use according to Claim 7 or 8, characterised in that the chassis or structural components have at least one weld seam.
10. Use according to any one of Claims 7 to 9, characterised in that the wall thickness of the aluminium alloy strip or sheet is from 0.5 mm to 8 mm, optionally from 1.5 to 5 mm.
11. Method for producing an aluminium alloy strip or sheet from an aluminium alloy according to any one of Claims 1 to 6 having the following steps:

    - casting a rolling ingot,

    - homogenising the rolling ingot at from 500 to 550 °C for at least 2 hours,

    - hot-rolling the rolling ingot to form a thermal strip at hot rolling temperatures of from 280 °C to 500 °C,

    - cold-rolling the hot strip with or without intermediate annealing to a final thickness, and

    - soft-annealing the cold strip at from 300 °C to 400 °C in a batch furnace.