High strength aluminum alloy extruded material with excellent corrosion resistance and favorable quenching properties and manufacturing method therefor

Abstract

An aluminum alloy extruded material that exhibits high strength by air cooling immediately after extrusion processing and excellent stress corrosion cracking resistance, and a method for manufacturing the same are disclosed. The material includes, by mass: 6.0 to 8.0% of Zn, 1.50 to 2.70% of Mg, 0.20 to 1.50% of Cu, 0.005 to 0.05% of Ti, 0.10 to 0.25% of Zr, 0.3% or less of Mn, 0.05% or less of Cr, 0.25% or less of Sr, and 0.10 to 0.50% in total among Zr, Mn, Cr and Sr, with the balance being Al and unavoidable impurities.

Claims

1. An aluminum alloy extruded material, comprising by mass: 6.0 to 6.81% of Zn, 2.40 to 2.70% of Mg, 0.20 to 1.50% of Cu, 0.005 to 0.05% of Ti, and 0.10 to 0.25% of Zr, 0.3% or less of Mn, 0.05% or less of Cr, 0.03 to 0.25% of Sr, and 0.22 to 0.50% in total among Zr, Mn, Cr and Sr, with the balance being Al and unavoidable impurities, wherein the aluminum alloy extruded material exhibits a stress corrosion cracking resistance in which, under a stress of 80% of the proof stress, no crack occurs in test pieces which are subjected to 720 cycles, in each cycle of which the test pieces are immersed in a water solution of 3.5% NaCl at 25° C. for 10 minutes, then left at 25° C. and a humidity of 40% for 50 minutes, and then let dry naturally.

2. The aluminum alloy extruded material as defined in claim 1, wherein Cr not being included therein and a total amount of Zr, Mn, and Sr is within a range of 0.22 to 0.50%.

3. The aluminum alloy extruded material as defined in claim 1, wherein Cr and Mn are not included and a total amount of Zr and Sr is within a range of 0.22 to 0.50%.

4. The aluminum alloy extruded material as defined in claim 1, wherein the amount of Cu is within a range of more than 0.4% to less than 0.8%.

5. The aluminum alloy extruded material as defined in claim 1, wherein a recrystallization depth on a surface of the extruded material is 150 μm or less.

6. The aluminum alloy extruded material as defined in claim 1, wherein the extruded material has a tensile strength of 480 MPa or more and a 0.2% proof stress of 450 MPa or more.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates the composition of each aluminum alloy used for evaluation.

(2) FIG. 2 illustrates manufacturing conditions of billets and extruded materials.

(3) FIG. 3 illustrates the evaluation results for each extruded material.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(4) A molten aluminum alloy with alloy components listed in the table of FIG. 1 was prepared to be cast into a columnar billet at a casting rate listed in the table of FIG. 2.

(5) In the table of FIG. 2, the homo temperature indicates homogenizing conditions of the billet. Samples were cut from the surface of the billets. The surfaces of the samples were mirror-polished and then etched by Keller's reagent (0.5% HF). Average crystalized grain diameters of the cast billets were observed by an optical microscope.

(6) The average crystalized grain diameters each was measured by subjecting a 100× microscope image to image-processing.

(7) The billet was pre-heated at a BLT temperature shown in the table of FIG. 2 and extruded into an extruded material having a U-shaped or channel cross section and 3 to 4 mm in thickness.

(8) Immediately after extrusion, the extruded material was air cooled (fan air cooled) at the cooling rate shown in the table of FIG. 2, and then was subjected to two-stage artificial aging treatment under the heat treatment conditions shown in the table of FIG. 2.

(9) The evaluation results are shown in the table of FIG. 3.

(10) Each item was evaluated as described below.

(11) No. 5 tension test pieces were prepared from the extruded material in accordance with Japanese Industrial Standard JIS-Z2241, and T5 tensile strength (MPa), T5 proof stress (0.2%, MPa), and T5 extension (%) were measured using a tension tester that conforms to the JIS standard.

(12) Under a stress of 80% relative to the proof stress, the test pieces were subjected to 720 cycles of a process described later to examine SCC resistance (stress corrosion cracking resistance). The test pieces without cracks were regarded to attain the target. For the test pieces cracked in a smaller number than 720 cycles, the number of cycles in which crack occurred were counted.

(13) <Test Conditions For One Cycle>

(14) The test pieces were immersed with a water solution of 3.5% NaCl at 25° C. for 10 minutes, then left at 25° C. and a humidity of 40% for 50 minutes, and then let dry naturally.

(15) The surface of the extruded material was mirror-polished and etched in a water solution of 3% NaOH. Then, the average thickness of the recrystallized layer on the surface of the extruded material was measured as a recrystallization depth with a 100× optical microscope image.

(16) The evaluation results of FIG. 3 show that the extruded materials of aluminum alloys in examples 1 to 8 attained all the targets of tensile strength of 480 MPa or more, a 0.2% proof stress of 450 MPa or more, extension of 10% or more, and SCC resistance of 720 cycles or more.

(17) The proof stress is preferably 460 MPa or more.

(18) The examples 1 to 8 were free of Cr. Further, the examples 1, 2, and 7 were free of Mn.

(19) The example 8 was free of Sr.

(20) The examples 3, 4, 5 and 7, which contained Cu of more than 0.4%, exhibited relatively high values in tensile and proof strengths.

(21) Comparative examples 9 to 12, 14, and 15 did not reach the target of SCC resistance.

(22) This may be because the amount of Cu exceeds 1.50%.

(23) For the comparative example 13, cooling rate after extrusion processing was low and the strength was insufficient.

(24) The comparative example 14 contained 0.26% Cr.

(25) The aluminum alloy extruded material according to the invention exhibits high strength and excellent corrosion resistance, and thus may be used as structural members for vehicles and industrial machines.

(26) Although only some embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within scope of this invention.