Aluminum alloy and method of manufacturing extrusion using same

Abstract

A high-strength aluminum alloy exhibiting excellent stress corrosion cracking resistance and excellent extrudability, and a method for producing an extruded shape using the same are disclosed. The aluminum alloy includes 1.6 to 2.6 mass % of Mg, 6.0 to 7.0 mass % of Zn, 0.5 mass % or less of Cu, and 0.01 to 0.05 mass % of Ti, with the balance being Al and unavoidable impurities.

Claims

1. A method for producing an aluminum alloy extruded shape comprising: casting a billet using an aluminum alloy consisting of 1.6 to 2.6 mass % of Mg, 6.0 to 7.0 mass % of Zn, 0.15 to 0.4 mass % of Cu, 0.01 to 0.05 mass % of Ti, 0.10 to 0.30 mass % of Zr and 0.10 to 0.3 mass % of Mn, the total of Zr and Mn being 0.34 to 0.6 mass %, with the balance being Al and unavoidable impurities; homogenizing the billet at 500 to 560 C.; preheating the billet to 400 C. or more; extruding the homogenized billet to obtain an extruded shape having a temperature between 500 and 585 C., the extruded shape being cooled at a cooling rate of 50 to 500 C./min immediately after the extrusion so as to obtain the extruded shape having a recrystallization ratio of 7% or less and a 0.2% proof stress of 470 MPa or more; and performing one cycle of an accelerated test that includes immersing a sample to which a stress equal to 80% of the proof stress is applied in a 3.5% NaCl aqueous solution at 25 C. for 10 minutes, and allowing the sample to air-dry at a room temperature of 25 C. and a humidity of 40% for 50 minutes, wherein the aluminum alloy extruded shape causes no stress corrosion cracking after 720 cycles.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the chemical components of the aluminum alloys of the examples.

(2) FIG. 2 shows the chemical components of the aluminum alloys of the comparative examples.

(3) FIG. 3 shows the extrusion conditions and the evaluation results when using the aluminum alloys of the examples.

(4) FIG. 4 shows the extrusion conditions and the evaluation results when using the aluminum alloys of the comparative examples.

(5) FIG. 5 illustrates an example of the cross-sectional shape of an extruded shape.

(6) FIG. 6 illustrates an example of the cross-sectional shape of an extruded shape.

(7) FIG. 7 illustrates an example of the cross-sectional shape of an extruded shape.

DESCRIPTION OF EMBODIMENTS

(8) A molten metal of each aluminum alloy shown in FIG. 1 (Examples 1 to 13) and a molten metal of each aluminum alloy shown in FIG. 2 (Comparative Examples 1 to 28) were prepared, and cast into a billet.

(9) Note that the content (mass %) of each component shown in FIGS. 1 and 2 indicates the analytical value after casting.

(10) Each cast billet (diameter: 8 inches) was extruded.

(11) FIGS. 3 and 4 show the billet homogenization temperature (HOMO temperature), the extrusion conditions, and the evaluation results.

(12) FIGS. 3 and 4 show the optimum ranges of the HOMO temperature and the extrusion conditions, and each data indicates the measured value.

(13) In FIGS. 3 and 4, the billet temperature refers to the billet preheating temperature before extrusion, and the temperature of the extruded shape refers to the surface temperature of the extruded shape measured immediately after extrusion.

(14) The cooling rate after extrusion refers to the cooling rate until the temperature of the extruded shape reached 200 C. or less when air was blown against the extruded shape using a fan immediately after extrusion.

(15) SCC refers to the stress corrosion cracking resistance. The stress corrosion cracking resistance was evaluated as described below.

(16) Specifically, an accelerated test was performed by immersing a sample (to which a stress equal to 80% of the proof stress was applied) in a 3.5% NaCl aqueous solution at 25 C. for 10 minutes, and allowing the sample to dry at a temperature of 25 C. (room temperature) and a humidity of 40% for 50 minutes (=1 cycle). When stress corrosion cracking was not observed after 720 cycles, it was determined that the sample had stress corrosion cracking resistance sufficient for a structural material (e.g., bumper reinforcement).

(17) In FIGS. 3 and 4, .sub.B refers to tensile strength, .sub.0.2 refers to proof stress, and refers to elongation. These mechanical properties were measured using a JIS Z 2241 No. 5 specimen that was cut from the extruded shape.

(18) The recrystallization ratio in the microstructure was determined by observing the cross section orthogonal to the extrusion direction using a microscope, and calculating the area ratio of the recrystallization area.

(19) The extrudability was determined to be normal when a surface defect having a depth of 0.5 mm or more was not observed when an extruded shape having the cross-sectional shape illustrated in FIGS. 5 to 7 was extruded, and the presence or absence of a surface defect (tearing or pickup) having a depth of 0.5 mm or more was evaluated.

(20) Discussion

(21) In Examples 1 to 13, a proof stress of 470 MPa or more that is necessary for achieving a reduction in weight of a bumper reinforcement and the like was obtained, and excellent stress corrosion cracking resistance was achieved.

(22) In Comparative Examples 1 and 2 in which the HOMO temperature was more than 560 C., the billet was locally melted to only a small extent, and could be extruded. However, since the amount of precipitates produced during homogenization was small, and recrystallization of the extruded shape could not be suppressed, the stress corrosion cracking resistance deteriorated.

(23) In Comparative Examples 3, 5, 8, 10 to 15, and 20, since the Mg content was less than 1.6%, a proof stress of 470 MPa or more could not be obtained.

(24) In Comparative Examples 16 and 25, since the Zn content was more than 7.0%, the stress corrosion cracking resistance deteriorated.

(25) In Comparative Examples 4 and 6 in which the Mg content was less than 1.6%, and the Zn content was more than 7.0%, the desired strength was obtained, but the stress corrosion cracking resistance deteriorated.

(26) In Comparative Example 7, since the Zn content was less than 6.0%, the desired strength could not be obtained.

(27) In Comparative Example 9, since the cooling rate after extrusion was less than 50 C./min, the desired strength could not be obtained.

(28) In Comparative Examples 17 and 18 in which the HOMO temperature did not fall within the optimum range, the desired strength could not be obtained when the HOMO temperature was lower than the optimum range, and the billet was locally melted, and could not be extruded when the HOMO temperature was higher than the optimum range.

(29) In Comparative Example 19, since the Mg content was less than 1.6%, and the cooling rate after extrusion was less than 50 C./min, the desired strength could not be obtained.

(30) In Comparative Example 21, since the Mg content was less than 1.6%, the desired strength could not be obtained. Moreover, since the total content of Mn, Cr, and Zr was less than 0.15%, recrystallization of the extruded shape could not be suppressed, and the stress corrosion cracking resistance deteriorated.

(31) In Comparative Example 22, since the Mg content was less than 1.6%, the desired strength could not be obtained. Moreover, since water cooling was performed after extrusion, the cooling rate exceeded 500 C./min, and the material became brittle due to incorporation of hydrogen. As a result, the stress corrosion cracking resistance deteriorated.

(32) In Comparative Examples 23 and 24, since the temperature of the extruded shape was higher than 585 C., pickup or tear occurred on the surface of the extruded profile.

(33) In Comparative Example 26, since the Mg content was less than 1.6%, the desired strength could not be obtained. Moreover, since the Zn content was more than 7.0%, sufficient stress corrosion cracking resistance could not be obtained.

(34) In Comparative Examples 27 and 28, since the Mg content was less than 1.6%, the desired strength could not be obtained. Moreover, since the total content of Mn, Cr, and Zn was less than 0.15%, recrystallization of the extruded shape could not be suppressed, and the stress corrosion cracking resistance deteriorated.

(35) The aluminum alloy according to the embodiments of the invention exhibits excellent extrudability, and makes it possible to produce an extruded shape having the cross-sectional shape illustrated in FIGS. 5 to 7.

(36) FIG. 5 illustrates an example of an extruded shape having a triple-hollow cross-sectional shape that is used for a bumper reinforcement and the like. When the dimension a is more than 40 mm and 75 mm or less, and the dimension b is 120 mm or less, it is possible to produce an extruded shape wherein 3t.sub.18, 1t.sub.26, 1t.sub.3(t.sub.31, t.sub.32, . . . )6.

(37) The extrudable dimensional range is similarly shown in FIGS. 6 and 7.

INDUSTRIAL APPLICABILITY

(38) A high-strength extruded shape exhibiting excellent stress corrosion cracking resistance can be obtained using the aluminum alloy according to the embodiments of the invention. The extruded shape may be applied to a vehicular structural member and the like.