High-strength aluminum alloy extruded material with excellent corrosion resistance and method of producing the same

Abstract

The present invention provides a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance and secondary workability and suitably used as a structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts, and a method of manufacturing the same. The aluminum alloy extruded product has a composition containing 0.6 to 1.2% of Si, 0.8 to 1.3% of Mg, and 1.3 to 2.1% of Cu while satisfying the following conditional expressions (1), (2), (3), and (4), 3%≦Si %+Mg %+Cu %≦4%   (1) Mg %≦1.7×Si %   (2) Mg %+Si %≦2.7%   (3) Cu % / 2≦Mg %≦(Cu % / 2)+0.6%   (4) and further containing 0.04 to 0.35% of Cr, and 0.05% or less of Mn as an impurity, with the balance being aluminum and unavoidable impurities. The cross section of the extruded product has a recrystallized structure with an average grain size of 500 μm or less. The manufacturing method includes, when extruding the aluminum alloy into a solid product by using a solid die, extruding the aluminum alloy by using a solid die in which a bearing length (L) is 0.5 mm or more and the bearing length (L) and the thickness (T) of the solid product have a relationship expressed as “L≦5 T”, and, when extruding the aluminum alloy into a hollow product by using a porthole die or a bridge die, extruding the aluminum alloy while setting the ratio of the flow speed of the aluminum alloy in a non-joining section to the flow speed of the aluminum alloy in a joining section in a chamber, where the billet reunites after entering a port section of the die in divided flows and subsequently encircling a mandrel, at 1.5 or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance. More particularly, the present invention relates to a method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance and suitably used as a structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts. BACKGROUND ART [0002] A structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts is required to have performance such as (1) strength, (2) corrosion resistance, and (3) fracture mechanics properties (such as fatigue crack propagation resistance and fracture toughness). A recent material development trend involves overall evaluation including not only strength, but also production, assembly, and operation of the material. [0003] As high-strength aluminum alloys, an Al—Cu—Mg aluminum alloy (2000 series) an...

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Benefits of technology

[0009] The inventors of the present invention conducted experiments and examinations in order to solve the above-described problems and to obtain a corrosion-resistant, high-strength aluminum alloy extruded product exhibiting further stable extrudability based on the proposed aluminum alloy composition and extrusion conditions. As a result, the inventors found that an aluminum alloy extruded product exhibiting excellent corrosion resistance and high strength, showing improved extrudability, and having a fine recrystallization texture over the entire cross section of the extruded product can be obtained by extruding an aluminum alloy containing specific amounts of Si, Mg, Cu, and Cr, in which the content of Mn as an impurity is limited, under the proposed extrusion conditions.

[0010] The present invention has been achieved based on this finding. An object of the present invention is to provide an aluminum alloy extruded product which satisfies the strength and corrosion resistance required for a structural material for transportation equipment such as automobiles, railroad vehicles, and aircrafts without reducing the productivity during extrusion and ensures excellent quality in secondary working such as bending or machining, and a method of manufacturing the same.

[0011] In order to achieve the above object, a first aspect of the present invention provides a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, comprising an aluminum alloy which comprise, in mass %, 0.6 to 1.2% of Si, 0.8 to 1.3% of Mg, and 1.3 to 2.1% of Cu while satisfying the following conditional expressions (1), (2), (3), and (4), 3%≦Si %+Mg %+Cu %≦4%   (1) Mg %≦1.7×Si %   (2) Mg %+Si %≦2.7%   (3) Cu % / 2≦Mg %≦(Cu % / 2)+0.6%   (4) and further comprises 0.04 to 0.35% of Cr, and 0.05% or less of Mn as an impurity, with the balance being aluminum and unavoidable impurities, the aluminum alloy extruded product having a recrystallization texture with a grain size of 500 μm or less.

[0012] A second aspect of the present invention provides the high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance according to claim 1, wherein the aluminum alloy further comprises at least one of 0.03 to 0.2% of Zr, 0.03 to 0.2% of V, and 0.03 to 2.0% of Zn.

[0013] A third aspect of the present invention provides a method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, the method comprising: extruding a billet of the aluminum alloy according to claim 1 or 2 into a solid product by using a solid die, in which a bearing length (L) is 0.5 mm or more and the bearing length (L) and a thickness (T) of the solid product to be extruded have a relationship expressed as “L≦5 T”, to obtain a extruded solid product of which a cross-sectional structure has a recrystallized structure with a grain size of 500 μm or less.

[0015] A fifth aspect of the present invention provides a method of manufacturing a high-strength aluminum alloy extruded product exhibiting excellent corrosion resistance, the method comprising: extruding a billet of the aluminum alloy according to claim 1 or 2 into a hollow product by using a porthole die or a bridge die while setting a ratio of a flow speed of the aluminum alloy in a joining section to a flow speed of the aluminum alloy in a non-joining section in a weld chamber, where the billet reunites after entering a port section of the die in divided flows and subsequently encircling a mandrel, at 1.5 or less, to obtain a hollow extruded product of which a cross-sectional structure has a recrystallized structure with a grain size of 500 μm or less.

Problems solved by technology

However, these aluminum alloys do not necessarily exhibit sufficient corrosion resistance, and tend to produce cracks due to inferior extrudability.

Therefore, since these aluminum alloys must be extruded at a low extrusion rate, manufacturing cost is increased.

Moreover, it is difficult to extrude these aluminum alloys into a hollow product by using a porthole die or a spider die.

Therefore, since it is necessary to form a desired structure by combining solid profiles, the application range of these aluminum alloys is limited.

However, the 6000 series alloy exhibits insufficient strength in comparison with the 7000 series (Al—Zn—Mg) or 2000 series (Al—Cu) high-strength aluminum alloy.

However, these alloys do not necessarily exhibit strength and corrosion resistance sufficient to meet a demand for a reduction in the material thickness along with a reduction in the weight of vehicles.

Although the strength of this aluminum alloy extruded product is improved by this measure, a problem relating to extrudability, such as extrusion cracks, occurs depending on the conditions.

However, since the above aluminum alloy extruded product containing Mn has a recrystallized structure in the surface layer and a fibrous structure inside the product, the surface properties and the dimensional accuracy after secondary working are decreased if the recrystallization texture becomes coarse.

As a result, a severe dimensional tolerance may not be maintained or machinability may be decreased.

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