U.S. patent application number 12/413934 was filed with the patent office on 2009-07-30 for aluminum alloy extruded product exhibiting excellent impact cracking resistance and impact absorber.
This patent application is currently assigned to ASIN KEIKINZOKU CO., LTD.. Invention is credited to Shinji MAKINO.
Application Number | 20090191090 12/413934 |
Document ID | / |
Family ID | 40428779 |
Filed Date | 2009-07-30 |
United States Patent
Application |
20090191090 |
Kind Code |
A1 |
MAKINO; Shinji |
July 30, 2009 |
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.
Inventors: |
MAKINO; Shinji; (Imizu,
JP) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Assignee: |
ASIN KEIKINZOKU CO., LTD.
Imizu-shi
JP
|
Family ID: |
40428779 |
Appl. No.: |
12/413934 |
Filed: |
March 30, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2008/065483 |
Aug 29, 2008 |
|
|
|
12413934 |
|
|
|
|
Current U.S.
Class: |
420/532 ;
148/417 |
Current CPC
Class: |
C22C 21/10 20130101;
C22F 1/053 20130101 |
Class at
Publication: |
420/532 ;
148/417 |
International
Class: |
C22C 21/10 20060101
C22C021/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2007 |
JP |
2007-232102 |
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.
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.
* * * * *