Aluminum Alloy Extruded Product Exhibiting Excellent Impact Cracking Resistance And Impact Absorber

Abstract

An aluminum alloy extruded product that exhibits excellent impact cracking resistance is formed of an aluminum alloy that includes 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of International Patent Application No. PCT/JP2008/065483, having an international filing date of Aug. 29, 2008, which designated the United States, the entirety of which is incorporated herein by reference. Japanese Patent Application No. 2007-232102 filed on Sep. 6, 2007 is also incorporated herein by reference in its entirety.

BACKGROUND

[0002] The present invention relates to an Al--Zn--Mg aluminum alloy extruded product. More particularly, the invention relates to an aluminum alloy extruded product that exhibits excellent toughness when subjected to impact (e.g., dynamic load or static load), and an impact absorber using the same.

[0003] The aluminum alloy extruded product according to the invention is suitably applied to vehicular parts and the like for which excellent impact energy absorption characteristics are desired.

[0004] In the field of vehicles, an improvement in collision safety and a reduction in vehicle weight have been strongly desired from the viewpoint of an improvement in occupant protection and a reduction in fuel consumption.

[0005] Collision safety may be improved by allowing a frame member or the like that forms a vehicle to be deformed and absorb impact energy when subjected to impact in an amount equal to or more than a given value while exhibiting frame rigidity during a minor collision.

[0006] The deformation pattern of the frame member or the like when absorbing the impact energy is classified as transverse collapse (crush) in which the frame member is deformed in the transverse direction or axial collapse (crush) in which the frame member is deformed in the longitudinal direction. In either case, a material that exhibits high toughness and rarely produces cracks during collapse is desired.

[0007] Japanese Patent No. 3772962 discloses an Al--Zn--Mg aluminum alloy extruded product that has a fiber structure and is provided with improved transverse collapse characteristics by means of overaging.

[0008] It is necessary for a vehicular member to contribute to a reduction in vehicle weight while exhibiting rigidity during a minor collision. However, the strength (particularly proof stress) of a material decreases due to overaging as compared with the maximum strength of the material.

[0009] According to Japanese Patent No. 3772962, the deformation form differs between the case where the aluminum alloy extruded product shrinks in the shape of bellows due to a compressive load applied in the extrusion axial direction and the case where the aluminum alloy extruded product is deformed due to a compressive load applied in the transverse direction. Japanese Patent No. 3772962 states that the aluminum alloy extruded product is provided with improved transverse collapse characteristics.

[0010] Japanese Patent No. 3772962 also states that it is necessary to add one or more of 0.2 to 0.7 mass % of Mn, 0.03 to 0.3 mass % of Cr, and 0.05 to 0.25 mass % of Zr so that the aluminum alloy extruded product has a fiber structure. However, Mn and Cr may increase the quench sensitivity of the aluminum alloy, and may form a crystallized product that decreases the toughness, strength, and extrudability of the aluminum alloy.

SUMMARY

[0011] According to one aspect of the invention, there is provided an aluminum alloy extruded product that exhibits excellent impact cracking resistance, the aluminum alloy extruded product being formed of an aluminum alloy that comprises 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

[0012] According to another aspect of the invention, there is provided an impact absorber obtained by welding an extruded product produced by extruding an aluminum alloy to another member, allowing the resulting product to stand at room temperature for one or more days, and subjecting the resulting product to artificial aging, the aluminum alloy comprising 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 shows the chemical compositions of aluminum alloys used for experimental evaluation.

[0014] FIG. 2 shows the evaluation results for aluminum alloy extruded products, wherein Alloys 1-1 and 1-2 only differ in T5 condition.

[0015] FIG. 3 shows the relationship between a proof stress and a Charpy impact value.

[0016] FIG. 4 shows the difference in Charpy impact value between alloys having a similar proof stress due to a T5 condition and a cooling rate.

[0017] FIG. 5 shows the relationship between a proof stress and an energy absorption amount.

[0018] FIG. 6 shows the relationship between the Zr content and a recrystallization thickness.

[0019] FIG. 7 shows an example of the cross section of an extruded product used for evaluation.

[0020] FIG. 8 schematically shows an axial compression evaluation method.

[0021] FIG. 9 shows a load curve with respect to an axial collapse stroke.

[0022] FIGS. 10A and 10B show photographs that indicate the presence or absence of cracks during axial collapse.

[0023] FIGS. 11A, 11B, and 11C show PFZ measurement examples.

[0024] FIGS. 12A and 12B show the shape of samples used for a welding test.

[0025] FIG. 13 shows the chemical compositions (mass %) of aluminum alloys used for a welding test.

[0026] FIG. 14 shows the hardness distribution when an extruded product was welded without cooling the extruded product and subjected to artificial aging immediately after welding.

[0027] FIG. 15 shows the hardness distribution when an extruded product was welded without cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0028] FIG. 16 shows the hardness distribution when an extruded product was welded without cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for seven days.

[0029] FIG. 17 shows the hardness distribution when an extruded product was welded while water-cooling the extruded product and subjected to artificial aging immediately after welding.

[0030] FIG. 18 shows the hardness distribution when an extruded product was welded while water-cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0031] FIG. 19 shows the hardness distribution when an extruded product was welded while water-cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for seven days.

[0032] FIG. 20 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded without cooling the extruded product, and subjected to artificial aging immediately after welding.

[0033] FIG. 21 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded without cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for one day.

[0034] FIG. 22 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded without cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0035] FIG. 23 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded without cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for seven days.

[0036] FIG. 24 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded while water-cooling the extruded product, and subjected to artificial aging immediately after welding.

[0037] FIG. 25 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded while water-cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for one day.

[0038] FIG. 26 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded while water-cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0039] FIG. 27 shows the hardness distribution when an extruded product was subjected to artificial aging after extrusion, welded while water-cooling the extruded product, and subjected to artificial aging after being allowed to stand at room temperature for seven days.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0040] An object of the invention is to provide an Al--Zn--Mg aluminum alloy extruded product that exhibits high strength, excellent toughness, and excellent extrusion productivity. The invention is particularly effective for improving axial collapse characteristics.

[0041] Various Al--Zn--Mg aluminum alloys have been studied as aluminum alloys that exhibit high strength.

[0042] These alloys generally have an Mg content of 0.8 to 1.6% and exhibit a relatively high proof stress, but exhibit poor toughness and extrudability.

[0043] The inventor of the invention conducted extensive studies in order to achieve high strength, excellent toughness, and high productivity while relatively reducing the Mg content to 0.50 to 0.75 mass % (hereinafter simply referred to as "%") to complete the invention.

[0044] According to one embodiment of the invention, there is provided an aluminum alloy extruded product that exhibits excellent impact cracking resistance, the aluminum alloy extruded product being formed of an aluminum alloy that comprises 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

[0045] Mg is the main component that improves the strength of the aluminum alloy. On the other hand, if the Mg content is too high, the aluminum alloy tends to produce cracks during collapse, and exhibit poor extrudability.

[0046] In the invention, the Mg content is set at 0.50% or more in order to maintain the proof stress of the aluminum alloy, and is set at 0.75% or less in order to prevent cracks and a decrease in extrudability.

[0047] Zn shows aging characteristics in the presence of Mg. The Zn content is set at 4.5% or more in order to ensure the proof stress.

[0048] If the Zn content is more than 6.5%, the corrosion resistance of the aluminum alloy may deteriorate.

[0049] Cu added in a small amount improves the general corrosion resistance of the aluminum alloy. Cu also reduces the potential difference between the inside and the outside of the crystal grains to improve the stress corrosion cracking resistance of the aluminum alloy.

[0050] Therefore, the Cu content is set at 0.10% or more.

[0051] If the Cu content is more than 0.20%, the corrosion resistance of the aluminum alloy may deteriorate due to a local potential.

[0052] When the Cu content is 0.10 to 0.20%, Cu improves the proof stress of the aluminum alloy.

[0053] Ti refines the crystal grains of the aluminum alloy ingot. The Ti content is preferably 0.005% or more. The crystal grain refinement effect of Ti is saturated when the Ti content is 0.05% or more.

[0054] Mn, Cr, and Zr (transition elements) are bonded to Al and form minute compounds to suppress recrystallization so that a fiber metal structure is obtained.

[0055] However, Mn and Cr affect the quench sensitivity of the aluminum alloy to a large extent during press quenching. Therefore, when it is desired to subject the aluminum alloy to press quenching by means of forced air cooling and achieve a stable and high strength by means of the subsequent artificial aging, it is preferable to reduce the Mn content and the Cr content as much as possible. Therefore, it is preferable that each of Mn content and the Cr content ideally is reduced to an unavoidable impurity level. Mn content is set to 0.05% or less, preferably 0.001 to 0.02%, more preferably 0.001 to 0.01%. Cr content is set to 0.03% or less, preferably 0.001 to 0.02%, more preferably 0.001 to 0.01%.

[0056] On the other hand, Zr affects the quench sensitivity of the aluminum alloy to only a small extent as compared with Mn and Cr. Therefore, the inventor conducted studies in order to obtain a fiber structure by adjusting only the Zr content. As a result, the inventor found that a fiber structure can be obtained by adjusting the Zr content to 0.17 to 0.23%, and preferably 0.19 to 0.23%.

[0057] In this case, the Mn content is preferably 0.05% or less, more preferably 0.03% or less, and ideally 0.01% or less.

[0058] The Cr content is preferably 0.03% or less, and more preferably 0.01% or less.

[0059] Fe and Si tend to be contained in aluminum metal as impurities. Since Fe and Si may form Al--Fe, Al--Fe--Si, Al--Mn--Fe, and Al--Cr--Fe crystallized products which may decrease the toughness of the aluminum alloy, it is preferable that the Fe content be 0.20% or less and the Si content be 0.10% or less.

[0060] When the components of the aluminum alloy are adjusted as described above, an aluminum alloy extruded product that exhibits excellent toughness is obtained by subjecting the extruded product to press quenching by means of forced air cooling immediately after extrusion. Specifically, the aluminum alloy extruded product is subjected to forced air cooling at a cooling rate of 30.degree. C./min or more immediately after extrusion until the material surface temperature is reduced to 100.degree. C. or less, and then subjected to artificial aging.

[0061] In this case, if the aluminum alloy extruded product is subjected to artificial aging so that the aluminum alloy extruded product has a proof stress approximately equal to a peak strength that is equal to or larger than a value 0.9 times the maximum strength of the aluminum alloy extruded product, the aluminum alloy extruded product exhibits excellent toughness even at the maximum strength. An aluminum alloy extruded product having a hollow cross section collapses in the shape of bellows during axial collapse. In this case, the aluminum alloy extruded product according to the invention shows high energy absorbing characteristics so that only a small number of cracks occur even in such an axial deformation pattern.

[0062] An improvement in axial collapse characteristics is also affected by a precipitate-free zone (PFZ). It is preferable that the PFZ be 100 nm or less.

[0063] In an aluminum alloy structure, a large number of precipitates and crystallized products are produced in the crystal grains. A precipitate-free zone (PFZ) where the number of precipitates and crystallized products is very small is present on each side of the crystal grain boundary.

[0064] The width of the PFZ (i.e. the width of the PFZ in the direction that crosses the crystal grain boundary) may be determined by photographing the aluminum alloy structure using an electron microscope, defining a PFZ using an imaginary line that is drawn corresponding to the boundary between an area in which the number of precipitates, crystallized products, and the like is very small and an area in which precipitates, crystallized products, and the like are observed, and measuring the width of the PFZ.

[0065] When the width of PFZ is 100 nm or less, and preferably 60 nm or less, cracks occur to only a small extent during axial collapse.

[0066] The collapse characteristics of the aluminum alloy can also be improved by dividing and refining coarse compounds that are crystallized when casting an extrusion billet.

[0067] A cast billet of a 7000 series Al--Zn--Mg alloy has been generally homogenized at 450 to 480.degree. C. The inventor found that coarse compounds crystallized during casting are not sufficiently divided and refined in such a temperature range.

[0068] In order to sufficiently divide and refine coarse compounds, the homogenization temperature of the billet is preferably set at 480 to 550.degree. C., and more preferably 520 to 550.degree. C.

[0069] If the homogenization temperature is less than 480.degree. C., coarse compounds may not be sufficiently divided and refined. If the aluminum alloy is held at a temperature of more than 550.degree. C. for a given period of time, local melting may occur.

[0070] When the extruded product using the aluminum alloy according to the invention is welded after extrusion, the effect of heat during welding can be suppressed by allowing the extruded product to stand at room temperature for one or more days after welding, and subjecting the resulting product to artificial aging.

[0071] According to another embodiment of the invention, there is provided an impact absorber obtained by welding an extruded product produced by extruding an aluminum alloy to another member, allowing the resulting product to stand at room temperature for one or more days, and subjecting the resulting product to artificial aging, the aluminum alloy comprising 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

[0072] According to the invention, an extruded product that rarely produces cracks during collapse can be obtained by maintaining a high proof stress by optimizing the Mg content and the Zn content, reducing the Mn content and the Cr content (ideally to an unavoidable impurity level), and setting the Zn content at 0.17 to 0.23%. As mentioned above, Mn content is set to 0.05% or less, preferably 0.001 to 0.02%, more preferably 0.001 to 0.01%. Cr content is set to 0.03% or less, preferably 0.001 to 0.02%, more preferably 0.001 to 0.01%.

[0073] In particular, the invention is effective for axial collapse. Therefore, the extruded product according to the invention may be applied to an impact-absorbing crush member provided in front of a side member of a vehicle.

[0074] The aluminum alloy extruded product according to the invention is further described below based on comparison with comparative examples.

[0075] A columnar billet (diameter: 204 mm) in which the amounts of chemical components were adjusted as shown in FIG. 1 (table) was cast, and homogenized at 520.degree. C. for 12 hours.

[0076] Note that the amounts of chemical components shown in FIG. 1 indicate analytical values (mass %).

[0077] An extruded product having a triple hollow cross section shown in FIG. 7 was extruded using the resulting billet.

[0078] The extruded product shown in FIG. 7 is an example of a crush member provided in front of a side member of a vehicle. In FIG. 7, the dimension a is 150 mm, the dimension b is 80 mm, and the thickness t is 2 mm.

[0079] FIG. 2 (table) shows an extruded product cooling rate by means of forced air cooling immediately after extrusion, an artificial aging (T5) condition, and extruded product evaluation results.

[0080] Examples 1 to 6 correspond to the extruded products according to the invention.

[0081] In Example 1, the PFZ was compared under two aging conditions in order to check the effect of artificial aging on toughness. The components of the alloy of Comparative Example 1 were the same as those of Example 1. In Comparative Example 1, the cooling rate during press quenching after extrusion was reduced to 15.degree. C./min (i.e., 30.degree. C./min or less) to check the effect of the quenching rate on the PFZ and toughness.

[0082] In FIG. 2, the term "extruded product cooling rate" refers to a cooling rate when cooling a product extruded from an extrusion press end by means of forced air cooling (i.e., press quenching).

[0083] The term "T5 condition" refers to an artificial aging condition after press quenching. A low temperature-long time condition (90.degree. C..times.4 hours+145.degree. C..times.9 hours) and a high temperature-short time condition (100.degree. C..times.4 hours+160.degree. C..times.4 hours) were compared and evaluated.

[0084] The low temperature-long time artificial aging condition refers to two-stage aging that includes aging at 85 to 95.degree. C. for 4 to 8 hours and aging at 130 to 150.degree. C. for 6 to 15 hours. The high temperature-short time artificial aging condition refers to two-stage aging that includes aging at 95 to 105.degree. C. for 3 to 6 hours and aging at 150 to 170.degree. C. for 3 to 7 hours.

[0085] The mechanical characteristics were evaluated in accordance with JIS Z 2241. The term "proof stress" refers to a 0.2% proof stress.

[0086] The toughness of the extruded product was evaluated in accordance with JIS B 7722 using a Charpy impact tester (manufactured by JT Tohsi Inc.).

[0087] The energy absorption amount and cracks of the extruded product were evaluated by applying an axial compression load in the extrusion direction (see FIG. 8) so that the extruded product (L=170 mm) was compressed by 120 mm, and calculating the area from a graph of a displacement stroke and a load KN (see FIG. 9) (the graph shown in FIG. 9 corresponds to Example 1-1).

[0088] FIGS. 10A and 10B show the appearance of the extruded products thus compressed.

[0089] Cracks did not occur in the extruded products of the examples (see FIG. 10B). On the other hand, cracks occurred in the extruded products indicated by "Bad" in FIG. 2 (see FIG. 10A).

[0090] FIGS. 11A to 11C show PFZ evaluation examples. FIG. 11A shows the extruded product of Example 1-1, FIG. 11B shows the extruded product of Example 1-2, and FIG. 11C shows the extruded product of Comparative Example 1.

[0091] The criteria ("Good" and "Bad") shown in the table are described below.

[0092] The mechanical characteristics were evaluated as "Good" when the proof stress was 200 MPa or more (generally required in this field), and evaluated as "Bad" when the proof stress was less than 200 MPa.

[0093] A case where the Charpy impact value was 37 J/cm.sup.2 or more was evaluated as "Good", and a case where the Charpy impact value was less than 37 J/cm.sup.2 was evaluated as "Bad" for the same reason as that for the mechanical characteristics.

[0094] The recrystallization thickness from the product surface was evaluated as follows. When recrystallization has occurred, cracks tend to spread in the recrystallized layer. Even if significant cracks do not occur, microcracks tend to occur in the recrystallized layer. Therefore, a case where the recrystallization thickness was 100 .mu.m or less was evaluated as "Good", and a case where the recrystallization thickness was more than 100 .mu.m was evaluated as "Bad".

[0095] An energy absorption amount of about 11,000 J is generally required for a crush member of a normal vehicle. Therefore, a case where the energy absorption amount was 11,000 J or more was evaluated as "Good", and a case where the energy absorption amount was less than 11,000 J was evaluated as "Bad".

[0096] The extruded products of the examples and the extruded products of the comparative examples are compared below. In Comparative Example A, since the Zr content was 0.13% (i.e., <0.17%), the surface recrystallization thickness of the extruded product was larger than those of the examples. Moreover, since the Si content was 0.15% (i.e., >0.10%) and the Fe content was 0.28% (i.e., >0.20%), the extruded product of Comparative Example A exhibited poor toughness.

[0097] In Comparative Example B, since the Mg content was 0.45% (i.e., <0.50%) and the Zn content was 4.42% (i.e., <4.5%), the extruded product had a proof stress as low as 188 MPa.

[0098] In Comparative Example B, since the Zr content was 0.14% (i.e., <0.17%), the extruded product had a large surface recrystallization thickness.

[0099] In Comparative Example C, since the Mg content was 0.83% (i.e., >0.75%) and the Zn content was 6.61% (i.e., >6.5%), the extruded product exhibited poor toughness although the proof stress was relatively high (339 MPa).

[0100] Note that the extruded products of the examples had a proof stress in the range from 200 to 310 MPa.

[0101] In Comparative Example D, since the Mn content was 0.41% (i.e., >0.05%) and the Cr content was 0.25% (i.e., >0.05%), cracks occurred in the surface recrystallized area in the axial collapse test although the Zr content was 0.18% (indicated by "Fair" in FIG. 2).

[0102] Comparative Example E shows a typical composition example of a JIS 7003 alloy. The alloy of Comparative Example E exhibited poor toughness and produced cracks in the axial collapse test.

[0103] In FIG. 3, the mechanical characteristics (proof stress) and the toughness (Charpy impact value) are plotted based on the results shown in FIG. 2. It was confirmed that the proof stress and the toughness have a negative correlation when the proof stress is in the range from 180 to 350 MPa.

[0104] The proof stress in the range from 230 to 280 MPa is plotted in a graph shown in FIG. 4. In Example 1-2 in which high-temperature aging was performed, the Charpy value decreased to 39.1 J/cm.sup.2 from 40.1 J/cm.sup.2 of Example 1-1 (low-temperature aging and high-speed quenching) (.tangle-solidup.2.5%).

[0105] In Comparative Example 1 (low-temperature aging and low-speed quenching), the Charpy value decreased to 36.9 J/cm.sup.2 from 40.1 J/cm.sup.2 (.tangle-solidup.8.0%).

[0106] The PFZ increased from 51 nm (Example 1-1) to 88 nm (Example 1-2) and 109 nm (Comparative Example 1) (i.e., toughness was affected) along with a decrease in the Charpy value, as shown in FIG. 1A (Example 1-1), FIG. 11B (Example 1-2), and FIG. 11C (Comparative Example 1).

[0107] Specifically, it was found that low temperature-long time aging and high-speed quenching at a cooling rate of 30.degree. C./min or more are desired in order to improve the toughness when the proof stress is in the range from 230 to 280 MPa.

[0108] The relationship between the proof stress and the energy absorption amount is plotted in FIG. 5. An approximately positive correlation is observed in FIG. 5. Specifically, the energy absorption amount increases as the proof stress increases.

[0109] However, when cracks have occurred in the product (in particular, when a biased load is applied to the product (e.g., a load is applied diagonally), the product tends to buckles from the cracks even if the energy absorption amount is sufficient.

[0110] Therefore, good results are not necessarily obtained even if the energy absorption amount is large. Specifically, the presence or absence of cracks is an important evaluation item. As shown in FIG. 2, cracks occurred in most of the comparative examples.

[0111] The components used in Comparative Example D were almost the same as those of Example 1 excluding Mn and Cr. However, since Mn and Cr were added in large amounts, Al--Mn(Cr)--Fe crystallized products were produced so that microcracks occurred.

[0112] Therefore, although the metal structure is refined by adding Mn and Cr, it is desirable not to add Mn and Cr since unnecessary crystallized products are produced.

[0113] FIG. 6 shows the relationship between the Zr content and the surface recrystallization thickness of the product. In FIG. 6, the recrystallization thickness indicates an average value.

[0114] As shown in FIG. 6, recrystallization is suppressed by adding Zr.

[0115] As shown in FIG. 6, the recrystallization thickness was 100 .mu.m or less when the Zr content was 0.17% or more (the remaining portion was a fiber structure). Since the fiber structure has a small grain size, cracks are prevented when an impact propagates.

[0116] Moreover, since the recrystallized layer has a small thickness, stress-corrosion cracking occurs to only a small extent.

[0117] When applying the aluminum alloy extruded product according to the invention to an impact absorbing member of a vehicle, the extruded product may be welded to another member of the vehicle.

[0118] The effect of heat during welding on the aluminum alloy extruded product according to the invention was investigated.

[0119] Test samples shown in FIGS. 12A and 12B were produced using an extruded product having a double hollow cross section shown in FIG. 7.

[0120] Aluminum plates 2 were T1G-welded to the upper and lower open ends of an extruded product 1 having a length L.sub.0 of 200 mm.

[0121] In FIG. 12A, a welded area is indicated by reference numeral 3, and a rib la of the hollow portion of the extruded product is indicated by a dotted line.

[0122] In FIG. 12A, the aluminum plates 2 were T1G-welded without cooling the extruded product. In FIG. 12B, the aluminum plates 2 were T1G-welded while water-cooling the periphery of the extruded product using a cooler 4 disposed at an interval L.sub.1 of about 25 mm from the upper and lower aluminum plates 2.

[0123] FIG. 13 (table) shows the components of aluminum alloys used to investigate the effect of welding.

[0124] The aluminum alloy of Example 7 corresponds to the aluminum alloy according to the invention, the aluminum alloy of Comparative Example F corresponds to a JIS 7003 alloy, and the aluminum alloy of Comparative Example G corresponds to a JIS 7N01 alloy.

[0125] In FIG. 13, the Zn/Mg ratio indicates the mass ratio of Zn to Mg.

[0126] Test samples were produced using an extruded product (without artificial aging) that was extruded under the same conditions as in Example 1-1 without cooling the extruded product during welding or while cooling the extruded product during welding. The test samples were subjected to artificial aging within 2 to 3 hours after welding, or subjected to artificial aging after being allowed to stand at room temperature for three or seven days after welding. FIGS. 14 to 19 show the hardness (Hv) distributions of the test samples.

[0127] FIG. 14 shows the hardness distributions of the test samples that were welded without cooling the extruded product and then subjected to artificial aging under the same conditions as in Example 1-1. The horizontal axis indicates the measurement position starting at about 5 mm from the upper end of the upper plate (=0 mm). The hardness of the test sample was measured at intervals of 10 mm toward the lower plate.

[0128] The above hardness distribution measuring method and artificial aging conditions were also applied to other test samples. FIG. 15 shows the hardness distributions of the test samples that were welded without cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0129] FIG. 16 shows the hardness distributions of the test samples that were welded without cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for seven days.

[0130] FIG. 17 shows the hardness distributions of the test samples that were welded while cooling the extruded product and subjected to artificial aging immediately after welding.

[0131] FIG. 18 shows the hardness distributions of the test samples that were welded while cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for three days.

[0132] FIG. 19 shows the hardness distributions of the test samples that were welded while cooling the extruded product and subjected to artificial aging after being allowed to stand at room temperature for seven days.

[0133] FIGS. 14 to 16 that indicate the hardness distributions when the extruded product was not cooled during welding were compared with FIGS. 17 to 19 that indicate the hardness distributions when the extruded product was cooled during welding. The aluminum alloy of Comparative Example F (corresponding to JIS 7003 alloy) was affected to a relatively small extent by heat during welding when subjected to artificial aging after welding. On the other hand, the aluminum alloy of Example 7 according to the invention was affected to a large extent by heat during welding (see FIG. 14). The hardness of the aluminum alloy of Example 7 was locally low even after the aluminum alloy was subjected to artificial aging.

[0134] When comparing FIG. 17 with FIG. 14, the cooled area was less affected by welding. However, the area other than the cooled area had low hardness.

[0135] However, when the aluminum alloy according to the invention was subjected to artificial aging after being allowed to stand at room temperature for a given period time after welding, the aluminum alloy recovered hardness so that the effect of heat during welding decreased (see FIGS. 15 to 16, and 18 to 19).

[0136] When comparing FIG. 14 with FIG. 15 and FIG. 17 with FIG. 18, the hardness was recovered to a large extent when the aluminum alloy was allowed to stand at room temperature for three days. Therefore, it is desirable to allow the aluminum alloy to stand for one day or more.

[0137] The JIS 7003 alloy (Comparative Example E) tended to produce cracks during axial collapse. Therefore, the JIS 7003 alloy cannot be applied to an impact absorbing member of a vehicle. On the other hand, when the aluminum alloy according to the invention was welded after extrusion and subjected to artificial aging after being allowed to stand at room temperature for a given period time, the effect of heat during welding was suppressed, and stable axial collapse characteristics were achieved.

[0138] In order to check the characteristics of the aluminum alloy according to the invention, the extruded product was subjected to artificial aging, welded, and then subjected to artificial aging. The presence or absence of the effect of heat during welding was evaluated.

[0139] FIGS. 20 to 23 show the hardness distributions when the extruded product subjected to artificial aging was welded without cooling the extruded product, and then subjected to artificial aging. The artificial aging conditions were the same as in Example 1-1.

[0140] FIG. 20 shows the hardness distribution when the extruded product was subjected to artificial aging immediately after welding.

[0141] FIG. 21 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for one day after welding.

[0142] FIG. 22 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for three days after welding.

[0143] FIG. 23 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for seven days after welding.

[0144] FIGS. 24 to 27 show the hardness distributions when the extruded product subjected to artificial aging was welded while cooling the extruded product, and then subjected to artificial aging.

[0145] FIG. 24 shows the hardness distribution when the extruded product was subjected to artificial aging immediately after welding.

[0146] FIG. 25 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for one day after welding.

[0147] FIG. 26 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for three days after welding.

[0148] FIG. 27 shows the hardness distribution when the extruded product was subjected to artificial aging after being allowed to stand at room temperature for seven days after welding.

[0149] As shown in FIGS. 20 to 27, the aluminum alloy of Example 7 was affected by heat during welding when the extruded product was subjected to artificial aging immediately after welding (see FIGS. 20 and 24). On the other hand, the effect of heat during welding can be suppressed by allowing the extruded product to stand at room temperature for one or more days after welding (see FIGS. 21 to 23, and 25 to 27). It was found that it is preferable to allow the extruded product to stand at room temperature for three or more days (see FIGS. 22 to 23, and 26 to 27).

[0150] The effects of the alloy components are considered below based on FIG. 13. The Zn/Mg ratio of Example 7 was 8.10, and the Zn/Mg ratio of Comparative Example F (JIS 7003 alloy) was 6.81. Cracks tend to occur during axial collapse when the Zn/Mg ratio is 6.81. On the other hand, the effect of heat during welding is small.

[0151] However, the aluminum alloy was affected by heat during welding in Comparative Example G (JIS 7N01) in which the Zn/Mg ratio was 3.71.

[0152] Therefore, the aluminum alloy is affected by heat during welding when the Zn/Mg ratio is larger than or smaller than a given range.

[0153] Unlike Comparative Examples F and G, Zn/Mg ratio is more than or equal to 8.0. in the all Examples 1 to 7. Since the evaluation of cracks during axial collapse respect to Example 7 is superior to that of Comparative Example F and G, Zn/Mg ratio is set at 8.0 or more in the embodiments of the present invention in order to reduce the cracks during axial collapse.

[0154] Since the extruded product using the aluminum alloy according to the invention rarely produces cracks during axial collapse, exhibits excellent impact absorbing characteristics, and can suppress the effect of heat during welding by allowing the extruded product to stand at room temperature for a given period of time after welding, the extruded product can be widely applied to an impact absorbing member of a vehicle and the like.

[0155] Although only some embodiments of the invention have been described in detail above, those skilled in the art would readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the invention. Accordingly, such modifications are intended to be included within the scope of the invention.

Claims

1. An aluminum alloy extruded product that exhibits excellent impact cracking resistance, the aluminum alloy extruded product being formed of an aluminum alloy that comprises 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.

2. The aluminum alloy extruded product as defined in claim 1, the aluminum alloy having an Mn content of 0.01 mass % or less and a Cr content of 0.01 mass % or less.

3. The aluminum alloy extruded product as defined in claim 1, the aluminum alloy extruded product having been subjected to forced air cooling at a cooling rate of 30.degree. C./min or more immediately after extrusion until a material surface temperature is reduced to 100.degree. C. or less, and then subjected to artificial aging.

4. The aluminum alloy extruded product as defined in claim 3, the aluminum alloy extruded product having been subjected to artificial aging so that the aluminum alloy extruded product has a proof stress approximately equal to a peak strength that is equal to or larger than a value 0.9 times a maximum strength of the aluminum alloy extruded product.

5. The aluminum alloy extruded product as defined in claim 4, the aluminum alloy extruded product having a precipitate-free zone (PFZ) of 100 nm or less.

6. The aluminum alloy extruded product as defined in claim 1, the aluminum alloy extruded product having a hollow cross section and exhibiting excellent impact cracking resistance during axial collapse along an extrusion direction.

7. An impact absorber obtained by welding an extruded product produced by extruding an aluminum alloy to another member, allowing the resulting product to stand at room temperature for one or more days, and subjecting the resulting product to artificial aging, the aluminum alloy comprising 0.50 to 0.75 mass % of Mg, 4.5 to 6.5 mass % of Zn, 0.10 to 0.20 mass % of Cu, 0.17 to 0.23 mass % of Zr, 0.005 to 0.05 mass % of Ti, 0.05 mass % or less of Mn, 0.03 mass % or less of Cr, 0.20 mass % or less of Fe, and 0.10 mass % or less of Si, with the balance being Al and unavoidable impurities.