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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Various Al—Zn—Mg aluminum alloys have been studied as aluminum alloys that exhibit high strength.
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.
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.
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.
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.
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.
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.
If the Zn content is more than 6.5%, the corrosion resistance of the aluminum alloy may deteriorate.
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.
Therefore, the Cu content is set at 0.10% or more.
If the Cu content is more than 0.20%, the corrosion resistance of the aluminum alloy may deteriorate due to a local potential.
When the Cu content is 0.10 to 0.20%, Cu improves the proof stress of the aluminum alloy.
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.
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.
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%.
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%.
In this case, the Mn content is preferably 0.05% or less, more preferably 0.03% or less, and ideally 0.01% or less.
The Cr content is preferably 0.03% or less, and more preferably 0.01% or less.
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.
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° C./min or more immediately after extrusion until the material surface temperature is reduced to 100° C. or less, and then subjected to artificial aging.
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.
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.
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.
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.
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.
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.
A cast billet of a 7000 series Al—Zn—Mg alloy has been generally homogenized at 450 to 480° C. The inventor found that coarse compounds crystallized during casting are not sufficiently divided and refined in such a temperature range.
In order to sufficiently divide and refine coarse compounds, the homogenization temperature of the billet is preferably set at 480 to 550° C., and more preferably 520 to 550° C.
If the homogenization temperature is less than 480° C., coarse compounds may not be sufficiently divided and refined. If the aluminum alloy is held at a temperature of more than 550° C. for a given period of time, local melting may occur.
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.
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.
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%.
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.
The aluminum alloy extruded product according to the invention is further described below based on comparison with comparative examples.
A columnar billet (diameter: 204 mm) in which the amounts of chemical components were adjusted as shown in
Note that the amounts of chemical components shown in
An extruded product having a triple hollow cross section shown in
The extruded product shown in
Examples 1 to 6 correspond to the extruded products according to the invention.
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° C./min (i.e., 30° C./min or less) to check the effect of the quenching rate on the PFZ and toughness.
In
The term “T5 condition” refers to an artificial aging condition after press quenching. A low temperature-long time condition (90° C.×4 hours+145° C.×9 hours) and a high temperature-short time condition (100° C.×4 hours+160° C.×4 hours) were compared and evaluated.
The low temperature-long time artificial aging condition refers to two-stage aging that includes aging at 85 to 95° C. for 4 to 8 hours and aging at 130 to 150° 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° C. for 3 to 6 hours and aging at 150 to 170° C. for 3 to 7 hours.
The mechanical characteristics were evaluated in accordance with JIS Z 2241. The term “proof stress” refers to a 0.2% proof stress.
The toughness of the extruded product was evaluated in accordance with JIS B 7722 using a Charpy impact tester (manufactured by JT Tohsi Inc.).
The energy absorption amount and cracks of the extruded product were evaluated by applying an axial compression load in the extrusion direction (see
Cracks did not occur in the extruded products of the examples (see
The criteria (“Good” and “Bad”) shown in the table are described below.
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.
A case where the Charpy impact value was 37 J/cm2 or more was evaluated as “Good”, and a case where the Charpy impact value was less than 37 J/cm2 was evaluated as “Bad” for the same reason as that for the mechanical characteristics.
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 μm or less was evaluated as “Good”, and a case where the recrystallization thickness was more than 100 μm was evaluated as “Bad”.
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”.
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.
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.
In Comparative Example B, since the Zr content was 0.14% (i.e., <0.17%), the extruded product had a large surface recrystallization thickness.
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).
Note that the extruded products of the examples had a proof stress in the range from 200 to 310 MPa.
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
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.
In
The proof stress in the range from 230 to 280 MPa is plotted in a graph shown in
In Comparative Example 1 (low-temperature aging and low-speed quenching), the Charpy value decreased to 36.9 J/cm2 from 40.1 J/cm2 (▴8.0%).
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
Specifically, it was found that low temperature-long time aging and high-speed quenching at a cooling rate of 30° 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.
The relationship between the proof stress and the energy absorption amount is plotted in
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.
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
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.
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.
As shown in
As shown in
Moreover, since the recrystallized layer has a small thickness, stress-corrosion cracking occurs to only a small extent.
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.
The effect of heat during welding on the aluminum alloy extruded product according to the invention was investigated.
Test samples shown in
Aluminum plates 2 were T1G-welded to the upper and lower open ends of an extruded product 1 having a length L0 of 200 mm.
In
In
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.
In
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.
The above hardness distribution measuring method and artificial aging conditions were also applied to other test samples.
When comparing
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
When comparing
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.
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.
As shown in
The effects of the alloy components are considered below based on
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.
Therefore, the aluminum alloy is affected by heat during welding when the Zn/Mg ratio is larger than or smaller than a given range.
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.
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.
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.
Number | Date | Country | Kind |
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2007-232102 | Sep 2007 | JP | national |
Number | Date | Country | |
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Parent | PCT/JP2008/065483 | Aug 2008 | US |
Child | 12413934 | US |