Patent Application
20060088726
1. Field of the Invention
The present invention relates to an aluminum alloy that has very high corrosion resistance and good deformation properties. Heat-exchanger components can be produced from an aluminum alloy of this type, especially heat-exchanger hollow sections or heat-exchanger tubes, which can be joined by brazing with collecting tubes and plates to form a heat exchanger.
2. Description of the Related Art
Aluminum heat exchangers have found broad application, e.g., in motor vehicle manufacture, due especially to their light weight. A heat exchanger of this type must have sufficient corrosion resistance and high heat-exchange efficiency for use in the automotive industry. Good deformation properties and sufficient strength of the aluminum material are also important for the manufacture of the heat exchanger.
In regard to corrosion resistance, motor vehicle manufacturers Previously required that heat exchangers withstand the so-called SWAAT test for at least 20 days. The SWAAT test is used to test corrosion resistance in an extreme atmosphere. In this test, the heat-exchanger components are exposed to an artificially produced seawater that has been adjusted to ca. pH 2.9 with acetic acid. If a heat exchanger, for example, one made of aluminum heat-exchanger components that have been joined by brazing, can endure this highly corrosive atmosphere for more than 20 days, it is assumed that it can withstand the demands made on it in the motor vehicle. Since failures of heat exchangers due to the appearance of various corrosion phenomena have nevertheless occurred in the past, automotive manufacturers have intensified the test conditions. A heat exchanger is now required to have a life of more than 40 days under SWAAT test conditions.
Good test results, i.e., a life of more than 20 days under SWAAT conditions, were obtained in the past by zinc coatings or chromating of the aluminum surface of the heat-exchanger components. However, due to restrictions imposed the Used Automobile Act, chromating is no longer used. Zinc coatings for heat-exchanger elements are widely used, since zinc coating can be advantageously integrated in the production process. For example, zinc is applied to an extruded section by arc spraying immediately after the section ha6 left the extruder and is still hot. Coating thicknesses of 10 g/m2 are customary. If larger amounts of zinc are applied, zinc oxide or zinc hydroxide can form during the brazing process at the brazing joint, which makes the heat-exchanger tube a sacrificial anode at the joint. This can result in the heat-exchanger tube losing its connection with the plate. Reduction of the amount of zinc does not lead to better corrosion resistance values, since the technical limit is reached at a coating thickness of about 8 g/m2. At lower amounts, uniform coating of the surface can no longer be guaranteed, which in turn leads. to worse corrosion results. This means that the more stringent SWAAT conditions cannot be satisfied solely by altering the zinc coating.
A great deal of effort has already been devoted to the production of heat-exchanger elements from corrosion-resistant aluminum alloys that do not need a zinc coating. A pure aluminum-based alloy that contains very small amounts of impurities exhibits very good corrosion resistance. However, an alloy of this type has very low strength values and is thus unsuitable for industrial applications. It is well known that strength can be increased by adding a wide variety of alloying elements to the aluminum. However, these have a variably strong effect on the corrosion-resistance properties to be achieved in the resulting alloy.
European Patent EP 996 754 B1describes an aluminum alloy that has greater corrosion resistance than the standardized alloy 3102, an AlMnO.4 alloy. This is attributed especially to such alloying elements as zirconium, chromium, and zinc, which the alloy preferably contains in concentrations of 0.1 to 0.18 wt.%. Furthermore, the concentrations of iron, silicon and manganese are kept low. European Patent EP 1 017 865 B1 discloses a comparable corrosion-resistant aluminum alloy in which the zirconium is replaced by titanium. However, these aluminum alloys are insufficiently strong for many applications due to their low concentrations of manganese.
Another European Patent, EP 866 746 B1, discloses an aluminum alloy for heat-exchanger tubes in which the iron and silicon concentrations as well as the zinc concentration are strictly controlled. The copper concentration is preferably 0.5 to 1 wt.%. The possible concentration of manganese is given as 0.7 to 1.5 wt.%. No information and especially no comparable SWAAT test results are provided about the life of heat-exchanger tubes of this type. High copper concentrations generally lead to copper precipitates, which can then act as corrosion cells.
The primary object of the present invention is to make available an aluminum alloy, especially for heat-exchanger components, which has a high level of corrosion resistance and at the same time exhibits improved deformation properties for extrusion.
In accordance with the invention, this object is achieved with an aluminum alloy produced from an aluminum material with at least 99.85 wt.% aluminum, i.e., with a maximum of 0.15 wt.% of unavoidable impurities, including:
by the addition of manganese, copper, and titanium to produce the following aluminum alloy composition:
An aluminum alloy with minimal concentrations of iron, silicon, zinc, chromium, and zirconium impurities is preferred. These impurities are already contained in the base alloy. To keep these impurities as low as possible, a pure aluminum material that contains at least 99.85 wt.% aluminum is used as the base material. This base material is produced from fresh metal and generally contains not more than 0.06 wt.% silicon and 0.06 wt.% iron. The desired alloying components manganese, copper, and titanium are added to a base alloy of this type during the casting process. In this regard, titanium serves as a grain refiner and is present in optimum concentrations of 0.003 to 0.01 wt.%.
Copper is added in amounts of 0.15 to 0.5 wt.%, preferably 0.2 to 0.4 wt.%, and especially 0.3 wt.%. The copper component in the aluminum alloy does not in itself improve the corrosion properties of the aluminum alloy. Only the form of corrosion that occurs is changed. If the alloy contains no copper or only very small concentrations of copper, the aluminum alloy shows a tendency towards pitting corrosion. The copper concentrations present in the alloys in accordance with the invention result, if any corrosion occurs, in galvanic corrosion. In contrast to pitting corrosion, which occurs in discrete points, galvanic corrosion is uniformly distributed over the entire surface and has a significantly smaller negative effect on the aluminum components. More than 0.5 wt.% copper should not be added to the alloy, since otherwise compressibility is adversely affected due to the appearance of AlCu phases.
Manganese is present in the aluminum alloy as a strengthening component and affects the.mechanical properties of the alloy. Very high manganese concentrations greater than 0.7 wt.%, as specified in the previously cited EP 866 746, lead to relatively large precipitations in the alloy, which can then act as corrosion cells. Moreover, aluminum alloys with high concentrations of manganese are relatively difficult to deform, i.e., an aluminum alloy of this type can be formed into sections only at low extrusion speeds, which results in high die wear, especially in the case of thin-walled heat-exchanger components. Studies have shown that manganese concentrations of 0.2 wt.% or more —up to less than 0.7 wt.% —result in an aluminum alloy that exhibits sufficient strength combined with good deformability.
A tested heat exchanger with multichamber hollow sections made from an aluminum alloy of the invention showed no corrosion phenomena at all after 40 days in a SWAAT test. This can be additionally attributed to the fact that the joining members of the multichamber hollow section in the aluminum heat exchanger, namely, the collecting tubes and plates, are made of an aluminum material that is less noble than the aluminum alloy of the invention. For example, especially good results were obtained when the aluminum material of the joining members is an aluminum alloy with a zinc content greater than 0.1 wt.%, and preferably 1-2 wt.%, and/or with a copper content of less than 0.15 wt.%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 049 748.6 | Oct 2004 | DE | national |
| 20 2004 015 805.1 | Oct 2004 | DE | national |
