U.S. patent application number 12/044396 was filed with the patent office on 2008-08-28 for alluminum alloy pipe and aluminum alloy structural member for automobile using the same.
This patent application is currently assigned to Furukawa-Sky Aluminum Corp.. Invention is credited to Hiroshi Akiyama, Izuru Hori, Kazuhisa Kashiwazaki, Katsuhiro Shiotsuki, Ryo Shoji, Seizo Ueno, Toshiyasu Ukena.
Application Number | 20080202647 12/044396 |
Document ID | / |
Family ID | 37835480 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080202647 |
Kind Code |
A1 |
Kashiwazaki; Kazuhisa ; et
al. |
August 28, 2008 |
ALLUMINUM ALLOY PIPE AND ALUMINUM ALLOY STRUCTURAL MEMBER FOR
AUTOMOBILE USING THE SAME
Abstract
An Al--Mg-based aluminum alloy pipe for hot working, having an
alloy composition having from 2.5% by mass to 2.8% by mass of Mg,
0.25% by mass or less of Si, 0.35% by mass or less of Fe, and from
0.25% by mass to 0.35% by mass of Cr, with the balance being
inevitable impurities and Al, wherein an area ratio of cavities
after hot working is 2.3% or less; and an aluminum alloy structural
member for automobile using the same.
Inventors: |
Kashiwazaki; Kazuhisa;
(Tokyo, JP) ; Shoji; Ryo; (Tokyo, JP) ;
Ueno; Seizo; (Tokyo, JP) ; Akiyama; Hiroshi;
(Wako-shi, JP) ; Shiotsuki; Katsuhiro; (Wako-shi,
JP) ; Hori; Izuru; (Haga-gun, JP) ; Ukena;
Toshiyasu; (Tokyo, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
Furukawa-Sky Aluminum Corp.
Tokyo
JP
HONDA GIKEN KOGYO KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
37835480 |
Appl. No.: |
12/044396 |
Filed: |
March 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2005/017083 |
Sep 9, 2005 |
|
|
|
12044396 |
|
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Current U.S.
Class: |
148/552 |
Current CPC
Class: |
C22F 1/047 20130101;
C22C 21/06 20130101; B21C 37/065 20130101 |
Class at
Publication: |
148/552 |
International
Class: |
C22F 1/047 20060101
C22F001/047 |
Claims
1. An Al--Mg-based aluminum alloy pipe for hot working, having an
alloy composition comprising from 2.5% by mass to 2.8% by mass of
Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe, and
from 0.25% by mass to 0.35% by mass of Cr, with the balance being
inevitable impurities and Al, wherein an area ratio of cavities
after hot working is 2.3% or less.
2. The Al--Mg-based aluminum alloy pipe for hot working according
to claim 1, wherein the area ratio of cavities after hot working is
1.0% or less.
3. The Al--Mg-based aluminum alloy pipe for hot working according
to claim 1, wherein a minimum pipe wall thickness of the pipe after
pipe expanding by hot working is 83% or more of an average
thickness of the pipe wall thickness.
4. The Al--Mg-based aluminum alloy pipe for hot working according
to claim 2, wherein a minimum pipe wall thickness of the pipe after
pipe expanding by hot working is 90% or more of the average
thickness of the pipe wall thickness.
5. The Al--Mg-based aluminum alloy pipe for hot working according
to claim 3, wherein a crystal grain diameter after hot working of
the aluminum alloy pipe is 300 .mu.m or less.
6. The Al--Mg-based aluminum alloy pipe for hot working according
to claim 4, wherein a crystal grain diameter after hot working of
the aluminum alloy pipe is 300 .mu.m or less.
7. An Al--Mg-based aluminum alloy pipe obtained by hot working the
Al--Mg-based aluminum alloy pipe for hot working according to claim
1, which has a tensile strength from 175 to 235 MPa and a proof
stress from 70 to 95 MPa.
8. An automobile structural member made of an aluminum alloy,
obtained by hot working an Al--Mg-based aluminum alloy pipe, having
an alloy composition comprising from 2.5% by mass to 2.8% by mass
of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe,
and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 2.3% or less, a crystal grain
diameter after hot working of the aluminum alloy pipe is 300 .mu.m
or less, a minimum pipe wall thickness of the pipe after pipe
expanding by hot working is 83% or more of an average thickness of
the pipe wall thickness, and a tensile strength is from 175 to 235
MPa and a proof stress is from 70 to 95 MPa after hot working of
the aluminum alloy tube, respectively.
9. An automobile structural member made of an aluminum alloy,
obtained by hot working an Al--Mg-based aluminum alloy pipe, having
an alloy composition comprising from 2.5% by mass to 2.8% by mass
of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of Fe,
and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 1.0% or less, a crystal grain
diameter after hot working of the aluminum alloy pipe is 300 .mu.m
or less, a minimum pipe wall thickness of the pipe after pipe
expanding by hot working is 90% or more of an average thickness of
the pipe wall thickness, and a tensile strength is from 175 to 235
MP and a proof stress is from 70 to 95 MPa after hot working of the
aluminum alloy tube, respectively.
10. An aluminum alloy automobile structural member using the
Al--Mg-based aluminum alloy pipe for hot working according to claim
1 after hot working, wherein a tensile strength is from 175 to 235
MPa and a proof stress is from 70 to 95 MPa after the hot working,
respectively, and wherein fluctuations of the tensile strength and
proof stress are 10 MPa or less, respectively.
11. An aluminum alloy automobile structural member using the
Al--Mg-based aluminum alloy pipe for hot working according to claim
1 after extrusion followed by hot working, wherein a fatigue
strength upon 1.times.10.sup.7 times after hot working is 70 MPa or
more, and a fluctuation of the fatigue strength upon
1.times.10.sup.7 times after hot working is 20 MPa or less.
12. An aluminum alloy automobile structural member using the
Al--Mg-based aluminum alloy pipe for hot working according to claim
1 after hot working, wherein a tensile strength is from 175 to 235
MPa and a proof stress is from 70 to 95 MPa after the hot working,
respectively, fluctuations of the tensile strength and proof stress
are 10 MPa or less, respectively, a fatigue strength upon
1.times.10.sup.7 times after hot working is 70 MPa or more, and a
fluctuation of the fatigue strength upon 1.times.10.sup.7 times
after hot working is 20 MPa or less.
13. A structural member for motor bicycles or four-wheel
automobiles made of the aluminum alloy according to claim 10.
Description
TECHNICAL FIELD
[0001] The present invention relates to an aluminum alloy pipe and
an aluminum alloy structural member for automobile using the
same.
BACKGROUND ART
[0002] Automobile parts have been required to be light-weight in
recent years. For attaining the above, cast and die-cast articles
of an aluminum alloy have been used in place of parts manufactured
by welding and assembling a plurality of steel sheets or steel
pipes by pressing or bending. However, it is difficult to
manufacture thin articles of cast or die-cast aluminum alloys
having a relatively larger size, and the effect of making weight
lighter is not sufficient. Since the cast and die-cast articles
have low toughness as compared with draw materials such as extruded
materials or sheets, and the articles are not completely
appropriate for the parts required to have toughness.
[0003] On the other hand, as an example for using a drawn material,
forming a part having a complex shape by combining bending,
crushing and hydroforming (hydrostatic bulge forming) an aluminum
alloy pipe has been attempted. For example, methods for obtaining a
hollow aluminum member having a desired shape have been proposed by
combining bending and hydrostatic bulge forming (JP-A-6-226339
("JP-A" means unexamined published Japanese patent application))
and by combining crushing and hydrostatic bulge forming
(JP-A-11-104751). However, since the above-mentioned methods are
based on cold working, it was a problem that the material is
cracked when it is worked into a complex shape.
[0004] Thus, hot working has become to draw attention in recent
years. However, a convention aluminum alloy pipe has the following
problems that fatigue strength may be decreased due to coarsening
of crystal grains, fluctuations of tensile strength and fatigue
strength may be increased due to generation of cavities, and the
thickness of the pipe wall may be locally decreased. In particular,
cavities are conspicuously generated by working such as expanding
the aluminum alloy pipe by hot working at a temperature of
350.degree. C. or higher, and characteristics of the material is
deteriorated.
[0005] Therefore, it has been strongly desired to provide an
aluminum alloy pipe for hot working suitable for working into a
member having a specified shape, with maintaining a required
mechanical strength, such as the structural member for automobile.
Al--Mg-based alloys described in JIS 5052 and JIS 5154 are examples
of the conventional alloys being relatively excellent in strength
and workability. However, these conventional alloys are not
satisfactory for applying hot working, since they cause problems,
such as decrease of tensile strength and fatigue strength due to
coarsening of crystal grains and generation of cavities, as well as
reduction of local pipe wall thickness.
DISCLOSURE OF INVENTION
[0006] An object of the present invention is to provide an aluminum
alloy pipe favorable for manufacturing a member by hot working,
which is required to have a specified shape while a required
strength is maintained, such as an automobile structural member.
Another object of the present invention is to provide an automobile
structural member which has higher reliability and lower
fluctuations of strength and fatigue strength, using the aluminum
alloy pipe.
[0007] The inventors of the present invention have found, through
intensive studies on an aluminum alloy pipe for hot working, that,
when an aluminum alloy contains a predetermined amount of Mg, the
content of Cr is correlated with coarsening of crystal grains
occurring in the hot working process and the coarsening of the
crystal grains results in decrease of tensile strength and fatigue
strength. Further, the inventors have found that the content of Cr
and the amounts of Si and Fe as inevitable impurities are
correlated with the amount of cavities occurring in the hot working
process, and the cavities causes wider fluctuation of tensile
strength and wider fluctuation and the decrease of fatigue
strength, and further local decrease of the thickness of the
pipe.
[0008] In other words, the crystal grains are coarsened by hot
working when the content of Cr is too small. When the content of Cr
and the amounts of Si and Fe as inevitable impurities are too
large, on the other hand, the size and distribution density of
intermetallic compounds are so increased that the amount of
cavities generated by hot working are increased.
[0009] Thus, the inventors of the present invention found that the
crystal grains can be prevented from being coarsened by hot working
by limiting the amounts of Cr, Si and Fe in specific ranges in the
aluminum alloy pipe while the amount of cavities generated by hot
working can be reduced.
[0010] According to the present invention, there is provided the
following means:
[0011] (1) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg (magnesium), 0.25% by mass or less of Si (silicon),
0.35% by mass or less of Fe (iron), and from 0.25% by mass to 0.35%
by mass of Cr (chromium), with the balance being inevitable
impurities and Al (aluminum), wherein an area ratio of cavities
after hot working is 2.3% or less;
[0012] (2) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 1.0% or less;
[0013] (3) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 2.3% or less, and a minimum pipe wall
thickness of the pipe after pipe expanding by hot working is 83% or
more of an average thickness of the pipe wall thickness;
[0014] (4) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein the area ratio of
cavities after hot working is 1.0% or less, and a minimum pipe wall
thickness of the pipe after pipe expanding by hot working is 90% or
more of an average thickness of the pipe wall thickness;
[0015] (5) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe, and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 2.3% or less, a minimum pipe wall
thickness of the pipe after pipe expanding by hot working is 83% or
more of an average thickness of the pipe wall thickness, and a
crystal grain diameter after hot working of the aluminum alloy pipe
is 300 .mu.m or less;
[0016] (6) An Al--Mg-based aluminum alloy pipe for hot working,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 1.0% or less, a minimum thickness of
the pipe after pipe expanding by hot working is 90% or more of an
average thickness of the pipe wall thickness, and a crystal grain
diameter after hot working of the aluminum alloy pipe is 300 .mu.m
or less;
[0017] (7) An Al--Mg-based aluminum alloy pipe obtained by hot
working the Al--Mg-based aluminum alloy pipe for hot working
according to any one of (1) to (6), which has a tensile strength
from 175 to 235 MPa and a proof stress from 70 to 95 MPa;
[0018] (8) An automobile structural member made of an aluminum
alloy, obtained by hot working an Al--Mg-based aluminum alloy pipe,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 2.3% or less, a crystal grain
diameter after hot working of the aluminum alloy pipe is 300 .mu.m
or less, a minimum pipe wall thickness of the pipe after pipe
expanding by hot working is 83% or more of an average thickness of
the pipe wall thickness, and a tensile strength is from 175 to 235
MPa and a proof stress is from 70 to 95 MPa after hot working of
the aluminum alloy tube, respectively;
[0019] (9) An automobile structural member made of an aluminum
alloy, obtained by hot working an Al--Mg-based aluminum alloy pipe,
having an alloy composition comprising from 2.5% by mass to 2.8% by
mass of Mg, 0.25% by mass or less of Si, 0.35% by mass or less of
Fe and from 0.25% by mass to 0.35% by mass of Cr, with the balance
being inevitable impurities and Al, wherein an area ratio of
cavities after hot working is 1.0% or less, a crystal grain
diameter after hot working of the aluminum alloy pipe is 300 .mu.m
or less, a minimum pipe wall thickness of the pipe after pipe
expanding by hot working is 90% or more of an average thickness of
the pipe wall thickness, and a tensile strength is from 175 to 235
MPa and a proof stress is from 70 to 95 MPa after hot working of
the aluminum alloy tube, respectively;
[0020] (10) An aluminum alloy automobile structural member using
the Al--Mg-based aluminum alloy pipe for hot working according to
any one of (1) to (6) after hot working, wherein a tensile strength
is from 175 to 235 MPa and a proof stress is 70 to 95 MPa after the
hot working, respectively, and wherein fluctuations of the tensile
strength and proof stress are 10 MPa or less, respectively.
[0021] (11) An aluminum alloy automobile structural member using
the Al--Mg-based aluminum alloy pipe for hot working according to
any one of (1) to (6) after extrusion followed by hot working,
wherein a fatigue strength upon 1.times.10.sup.7 times after hot
working is 70 MPa or more, and a fluctuation of the fatigue
strength upon 1.times.10.sup.7 times after hot working is 20 MPa or
less;
[0022] (12) An aluminum alloy automobile structural member using
the Al--Mg-based aluminum alloy pipe for hot working according to
any one of (1) to (6) after hot working, wherein a tensile strength
is from 175 to 235 MPa and a proof stress is from 70 to 95 MPa
after the hot working, respectively, fluctuations of the tensile
strength and proof stress are 10 MPa or less, respectively, and a
fatigue strength upon 1.times.10.sup.7 times after hot working is
70 MPa or more, and a fluctuation of the fatigue strength upon
1.times.10.sup.7 times after hot working is 20 MPa or less; and
[0023] (13) A structural member for motor bicycles or four-wheel
automobiles, made of the aluminum alloy according to any one of
(10) to (12).
[0024] The Al--Mg-based aluminum alloy pipe of the present
invention can prevent coarsening of the crystal grains after hot
working with lower generations of cavities, while required strength
for the automobile structural member is maintained. According to
the aluminum alloy pipe, it is possible to provide an automobile
structural member having small fluctuation of properties while
required tensile strength, proof stress and fatigue strength are
maintained after hot working, and to improve reliability of the
automobile structural member.
[0025] Other and further features and advantages of the invention
will appear more fully from the following description,
appropriately referring to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1(a) is a front view schematically illustrating an
expansion die, and FIG. 1(b) is a cross section along the line A-A
in FIG. 1(a).
[0027] FIGS. 2(a), 2(b), 2(c) and 2(d) schematically illustrate an
example of the pipe expanding process.
[0028] FIG. 3(a) is a front view schematically illustrating a round
pipe (alloy pipe) obtained by expanding of the pipe, and FIG. 3(b)
is a cross section along the line B-B in FIG. 3(a).
[0029] FIG. 4 schematically illustrates sampling positions of the
round pipe shown in FIGS. 3(a) and 3(b).
[0030] FIG. 5 is a perspective view schematically illustrating the
positions for measuring the pipe wall thickness of the round pipe
shown in FIGS. 3(a) and 3(b).
[0031] FIG. 6(a) is a front view of the die for forming a
trapezoidal pipe of the alloy, and FIG. 6(b) is a cross section
along the line C-C in FIG. 6(a).
[0032] FIG. 7(a) schematically illustrates a trapezoidal pipe
formed by hot working by the process in FIGS. 6(a) and 6(b), and
FIG. 7(b) is a cross section along the line D-D in FIG. 7(a).
BEST MODE FOR CARRYING OUT THE INVENTION
[0033] The present invention will be described in detail below.
[0034] A specified amount of Mg is added to a material in order to
attain desired mechanical strength for the automobile structural
member. Cr is added for suppressing coarsening of crystal grains
from occurring during hot working. The amounts of Cr, Si and Fe are
defined for reducing the size and distribution density of
intermetallic compounds that serve as cause of cavities during hot
working.
[0035] Thus, it is possible to provide an extruded pipe of the
aluminum alloy suitable for manufacturing a member required to have
a complex shape, while a required strength of the material for the
automobile structural member is obtained. The composition of the
aluminum alloy used for the aluminum alloy pipe of the present
invention will be described in detail below.
[0036] While Mg improves the strength of the alloy by solid
solution strengthening, 2.5% by mass or more of Mg should be added
for ensuring strength necessary for the automobile structural
member. However, when the amount of addition of Mg exceeds 2.8% by
mass, hot deformation resistance increases to make working
difficult while stress-corrosion cracking is liable to occur.
Accordingly, the content of Mg is from 2.5% by mass to 2.8% by
mass.
[0037] Cr is an element that improves the strength of the base
alloy while crystal grains are suppressed from being coarsened by
hot working. While an amount of addition of 2.5% by mass or more of
Cr is necessary for suppressing crystal grains from being coarsened
by hot working. However, when Cr exceeding 0.35% by mass is added,
coarse intermetallic compounds of Al--Cr-based alloys are
crystallized and toughness and fatigue characteristics of the
material are largely deteriorated.
[0038] Si and Fe are impurity elements inevitably mingled from
starting materials such as ground metals and scraps of aluminum in
most cases, and form cause of cavities by hot working by forming
intermetallic compounds such as Al--Fe, Al--Fe--Si and Mg--Si-based
compounds. However, the size and distribution density of the
intermetallic compound are reduced to enable the cavities to be
suppressed from being generated by hot working, when the contents
of Si and Fe are suppressed to be 0.25% by mass or less and 0.35%
by mass or less, respectively.
[0039] In the present invention at least one of the elements
selected from Ti and B is preferably added in a minute amount to
the composition of the Al alloy.
[0040] Ti is an element usually added for industrial manufacture of
billets by casting, since it has various advantages such as an
effect for fining the cast structure, an effect for preventing
cracks from being generated in the ingot, an effect for improving
workability of hot working, and an effect for homogenizing
mechanical properties of the product. The fining effect becomes
insufficient when the amount of addition of Ti is too small, while
toughness and fatigue characteristics are largely deteriorated due
to crystallization of coarse intermetallic compounds when the
amount of addition is too large. Accordingly, the amount of
addition of Ti is preferably suppressed in the range from 0.001% by
mass to 0.2% by mass. While B may be added alone, it is preferable
to add B and Ti together since the effect for fining the cast
structure is more enhanced. The content of B is preferably 0.02% by
mass or less.
[0041] As inevitable impurities mingled from the aluminum ingot and
scraps other than Si and Fe, the contents of Mn, Cu and Zn are
0.10% by mass or less, respectively, and permisible contents of the
other inevitable impurities are 0.05% by mass or less.
[0042] The ingot of the aluminum alloy having the above-mentioned
composition is extruded to a predetermined size after a
homogenization, and is molded into an extruded pipe. The extruded
pipe is directly used, or is subjected to annealing, if necessary.
In the present invention, a drawn pipe after cold working is also
used for the aluminum alloy pipe to be subjected to hot working.
The drawn pipe manufactured by cold working is directly used, or is
subjected to annealing, if necessary.
[0043] Since the crystal grains are coarsened in hot working
thereafter when the working rate by cold working is small, a
working rate by cold working of at least 20% is necessary.
[0044] Hot working of the aluminum alloy pipe of the present
invention may be applied by a conventional pipe expanding method
using the die heated at a temperature from 380 to 550.degree. C.,
preferably from 420 to 530.degree. C. The methods described in
following examples are used for defining the characteristics of the
tube after hot working.
[0045] The pipe expanding method is able to form the aluminum alloy
pipe into round pipes, rectangular pipes such as square or
trapezoidal pipes and into complex shapes by partially combining
these shapes by introducing air under pressure, and thus alloy
pipes having various three-dimensional shapes are obtained.
Accordingly, the aluminum alloy tube of the present invention is
not restricted to apply for structural members such as the
automobile structural member, but is applicable to members for
motor bicycles, four-wheel automobiles and the like that require
such working method.
[0046] A fluctuation of the pipe wall thickness caused during pipe
expanding of the aluminum alloy pipe by hot working is related to
the abundance ratio of cavities, and the pipe wall thickness is
reduced at the parts containing many cavities. When the area ratio
of the cavity is large, the strength of the portion containing many
cavities is locally reduced. As a result, stress is concentrated at
the portion having a high cavity area ratio, and a pipe wall
thickness of the portion rapidly decreases to consequently increase
a fluctuation of the pipe wall thickness. The portion where the
thickness is reduced may serve as cause of fatigue breaking that
finally causes fatigue breaking. Furthermore, localization of the
cavity itself may be a cause of fluctuations of the strength and
fatigue strength of the material.
[0047] Accordingly, it is preferable to suppress the amount of the
cavity to be low. In the aluminum alloy pipe of the present
invention, the area ratio of the cavity is suppressed to be 2.3% or
less (preferably 1.0% or less) by defining the contents of Cr, Fe
and Si as described above. Consequently, a fluctuation of the pipe
wall thickness is reduced to permit the minimum pipe wall thickness
to be 83% or more (preferably 90% or more) of the average pipe wall
thickness. It is also possible to suppress fluctuations of the
strength of the material and fatigue strength, and to provide
Al--Mg-based alloy pipes preferable for hot working, automobile
structural member made of the aluminum alloy, and the like.
[0048] When the crystal grain diameter of the aluminum alloy pipe
after hot working (In the present invention, the grain diameter
refers to an average value obtained by a line intersection method
for measuring the diameter in two directions in the direction of
the pipe wall thickness and in the direction of the circumference,
unless otherwise stated) is too large, the fatigue strength is so
extremely reduced that the alloy is not suitable for using as the
automobile structural member. The fatigue strength required for the
automobile structural member is satisfied by suppressing the
crystal grain diameter to be 300 .mu.m or less.
[0049] While the strength of the aluminum alloy pipe after hot
working is mainly determined by the amount of Mg, the tensile
stress is determined in the range from 175 to 235 MPa (preferably
from 185 to 225 MPa), and the proof stress is determined in the
range from 70 to 95 MPa (preferably from 75 to 90 MPa) considering
the balance between the strength and workability of hot working.
The strength is insufficient for use as the automobile structural
member when the tensile strength is 175 MPa or less or the proof
stress is 70 MPa or less, while workability of hot working becomes
poor when the tensile strength exceeds 235 MPa or the proof stress
exceeds 95 MPa.
[0050] Fluctuations of the tensile strength and proof stress after
hot working are correlated to the abundance ratio of the cavity,
and a smaller amount of the cavity reduces the fluctuation (in the
present invention, fluctuation refers to the difference of the
minimum value and maximum value of the measured values at least at
four points, unless otherwise stated). Accordingly, in the material
within the scope of the present invention, the fluctuation of the
tensile strength can be suppressed to be 10 MPa or less, the
fluctuation of the proof stress can be suppressed to be 10 MPa or
less, and the fluctuation of the fatigue strength can be suppressed
to be 20 MPa or less by suppressing the amount of the cavity.
[0051] The present invention will be described in more detail based
on examples given below, but the invention is not meant to be
limited by these.
EXAMPLES
Manufacture of Aluminum Extruded Pipe and Test
(Examples of Manufacture)
[0052] The alloy having the composition shown in Table 1 was melted
and cast into a billet with a diameter of 260 mm, which was
homogenized at 530.degree. C. for 4 hours. The billet was heated at
480.degree. C., extruded at an extrusion rate of 5 m/minute, and
formed into a round pipe with an outer diameter of 95 mm and a pipe
wall thickness of 3.5 mm. This round pipe (outer diameter 95 mm,
pipe wall thickness 3.5 mm) was cut into a length of 300 mm, and
the piece of the round pipe was heated at 500.degree. C. and was
inserted into a die heated at 500.degree. C. as shown in FIGS. 1(a)
and 1(b). FIG. 1(a) is a front view of the die 1. Die 1 has a pipe
insertion part 2. A division part is shown by the reference numeral
1a in FIG. 1(a). FIG. 1(b) shows a cross section of the die.
[0053] Then, the alloy pipe (round pipe) was inserted into the die
shown in FIGS. 1(a) and 1(b), and was expanded by the steps
described in FIGS. 2(a), 2(b), 2(c) and 2(d). After inserting a
pipe 3 made of an alloy A to D, K or L, respectively, as shown in
FIG. 2(a), the pipe was held between the dies 1 as shown in FIG.
2(b). After sealing both ends of the die with a seal mold 4, the
pipe 3 was expanded by hot working as shown in FIG. 2(c) by
applying a pneumatic pressure of 1.5 MPa through an air induction
inlet 5 to mold into an alloy pipe (round pipe) 6 (example). Each
alloy pipes made of an alloy E to J, M or N, respectively, was also
molded for comparative (comparative example).
[0054] While pieces for various test were sampled from the
periphery of the expanded tube, the degree of strain of each test
piece at the sampling position was about 27%. Many cavities are
generated by hot expansion of the pipe as compared with uniaxial
tension processing under high temperature. As a result, the
influence of the cavity becomes more clear. Each of outer diameters
of the alloy pipe after hot working was as shown in FIGS. 3(a) and
3(b) (FIG. 3(a) denote a front view and FIG. 3(b) denotes a cross
section), and the time required for working was about 5
seconds.
(Measurement of Cavity Area Ratio)
[0055] A test piece for observing the micro-texture (20 mm.times.20
mm) was cut from a surface perpendicular to the extrusion or draw
direction of the extruded pipe or drawn pipe at the position 6a of
the alloy pipe 6 obtained by hot working as shown in FIG. 4. The
surface of the test piece was polished, and five fields of view
were photographed with a magnification of 100 with an optical
microscope. The photographic image was analyzed to measure the
cavity area ratios, and an average of the measured values is shown
in Table 2 as the cavity area ratio (%).
[0056] Local reduction of the pipe wall thickness occurs when the
cavity area ratio exceeds 2.3%, and a fluctuation of the pipe wall
thickness increases.
(Measurement of Crystal Grain Diameter)
[0057] A test piece for observing the micro-texture (20 mm.times.20
mm) was cut from the position 6a of the material after hot working
as shown in FIG. 4, and the crystal grain diameter was measured
from five fields of vision of the photograph taken with a
magnification of 100 with an optical microscope. The crystal grain
diameter was measured in two directions of the direction of
thickness and the circumference direction using an intersection
method, and an average of the values was calculated. The average
value of the five fields of vision is shown in Table 2.
[0058] The fatigue strength decreases when the crystal grain
diameter exceeds 300 .mu.m. In addition, when the crystal grain
diameter exceeds 300 .mu.m, the surface of the aluminum alloy pipe
after hot working is roughened and appearance of the product is
impaired while the fatigue strength is decreased and secondary
workability of the product is deteriorated.
(Tensile Test)
[0059] A JIS No. 12 test piece was cut from the position 6b of the
pipe after hot working in a longitudinal direction as shown in FIG.
4, and the test piece was subjected to a tensile test according to
JIS Z2241. The results are shown in Table 2.
[0060] Workability during hot working fluctuates when the tensile
strength is less than 175 MPa or the proof stress is less than 70
MPa, while reliability decreases when the alloy is formed into an
aluminum alloy pipe for hot working.
(Stress-Corrosion Cracking)
[0061] A test piece was cut from the position 6b of the pipe after
hot working as shown in FIG. 4, and was subjected to a
stress-corrosion cracking test according to JIS H8711. Generation
of cracks was observed by alternate immersion for 30 days.
[0062] The sample in which cracks are generated within 30 days in
the alternate immersion test is likely to generate stress-corrosion
cracks when used. The sample showing no generation of cracks is
denoted by ".smallcircle.", while the sample showing the generation
of cracks is denoted by "x" in Table 2.
(Measurement of Pipe Wall Thickness)
[0063] The pipe wall thickness of each of three test pieces was
measured at each 8 points with a uniform distance of 450 by taking
the minimum thickness portion (the position 6c for measuring the
pipe wall thickness) as a reference position along the
circumference in the perspective view (FIG. 5) of the alloy pipe 6
after hot working, and the results of measurement are shown in
Table 3. The minimum value and average value of the pipe wall
thickness were calculated, and the results are shown in Table
2.
[0064] The ratio (%) of the average pipe wall thickness to the
minimum pipe wall thickness is defined as a ratio of a pipe wall
thickness. Fluctuations of the tensile strength and fatigue
strength increase when the ratio of the pipe wall thickness is 83%
or less.
<Manufacture of Automobile Structural Member and Test>
(Example of Manufacture)
[0065] The alloy having the composition shown in Table 1 was melted
and cast into a billet with a diameter of 260 mm, and was
homogenized at 530.degree. C. for 4 hours. The billet was heated at
480.degree. C. and, after extruding into an extruded pipe having a
predetermined size at an extrusion rate of 5 m/minutes, the pipe
was drawn at a cold working ratio of 35% to manufacture a round
pipe with an outer diameter of 95 mm and a pipe wall thickness of
3.5 mm.
[0066] The drawn round pipe (outer diameter 95 mm, pipe wall
thickness 3.5 mm) manufactured as described above was cut into a
piece with a length of 300 mm, and the piece of the round pipe was
heated at 500.degree. C. The piece was inserted into an insertion
part 11 of the die 10 heated at 500.degree. C. as shown in FIGS.
6(a) and 6(b), and both ends of the die were sealed by the same
procedure as shown in FIGS. 2(a) to 2(d). The reference numeral 10a
in FIG. 6(a) shows the dividing position of the die. The pipe was
subjected to hot working for forming a trapezoidal pipe (a pipe
worked into a trapezoid shape) 12 by applying a pneumatic pressure
of 1.5 MPa in the pipe by the same procedure shown in FIGS. 2(a) to
2(d). The time required for working was about 5 seconds. Each of
automobile structural members made of the alloy A to D, K or L,
respectively, was molded (examples). Each of members made of the
alloy E to J, M or N, respectively, was also molded for comparison
(comparative examples).
[0067] The front view (i.e. view from the surface P) of the
trapezoidal pipe and cross section thereof are as shown in FIGS.
7(a) and 7(b). While the cross section of the automobile structural
member is not specifically restricted to the trapezoid shape and
may be various shapes. In this example, a die for working the
article into the trapezoid shape was used as a representative
example. The cavity area ratio was measured for all the faces of P,
Q, R and S surfaces according to the method described below, the
crystal grain diameter was observed only on P face where the
crystal grain is liable to be coarse, and the tensile
characteristics and fatigue characteristics were measured only on P
face where the stress is most likely concentrated by forming into
automobile parts.
(Measurement of Cavity Area Ratio)
[0068] Test pieces (20 mm.times.20 mm) for observing the
micro-texture were cut from a surface perpendicular to the
extrusion or draw direction of the pipe at the position 12a shown
in FIG. 7(a), on each surface of P, Q, R and S surfaces of the
hot-worked material for the automobile parts as shown in FIG. 7(b).
After polishing the surface of the test piece, five fields of
vision of each surface were photographed with an optical
microscope. The cavity area ratios were measured with respect to
the five fields of vision on each observation surfaces P, Q, R and
S using an image analyzer. The average cavity area ratios are shown
in Table 4.
[0069] A fluctuation of the pipe wall thickness increases when the
area ratio of generation of the cavity exceeds 2.3% (even at one
position on any one of the observation faces P, Q, R and S) to
cause local reduction of the pipe wall thickness and decrease of
the tensile strength and fatigue strength.
(Measurement of Crystal Grain Diameter)
[0070] A test piece (20 mm.times.20 mm) for observing the
micro-texture was cut from the position 12a of P-surface of a
hot-worked material for automobile parts as shown in FIGS. 7(a) and
7(b) in two directions of the direction of the pipe wall thickness
and direction along the circumference. The test piece was
photographed with a magnification of 100 with an optical microscope
to determine the crystal grain diameter. The results of observation
on five fields of vision are shown in Table 5 as the results of
measurement of the average grain diameter.
[0071] The fatigue strength decreases when the crystal grain
diameter exceeds 300 .mu.m. The sample with a crystal grain
diameter of 300 .mu.m or less is shown by ".smallcircle." and the
sample with the crystal grain diameter exceeding 300 .mu.m is shown
by "x" in Table 4.
(Tensile Strength Test)
[0072] JIS No. 5 sample pieces were cut from the position 12b on
P-surface of the hot-worked material for the automobile part in the
longitudinal directions as shown in FIGS. 7(a) and 7(b), and each
sample was subjected to the tensile strength test according to JIS
Z2241. The results are shown in Table 5.
[0073] Workability by hot working fluctuates when the tensile
strength, proof stress and fluctuations thereof are out of the
ranges from 175 to 235 MPa, from 70 to 95 MPa and 10 MPa or less,
respectively, while reliability of the material decreases when it
is used for the automobile member. The sample with a tensile
strength in the range from 175 to 235 MPa, proof stress in the
range from 70 to 95 MPa and fluctuations thereof in the range of 10
MPa or less is shown by ".smallcircle.", and the sample out of the
above-mentioned ranges is shown by "x" in Table 4.
(Fatigue Strength)
[0074] JIS No. 1 sample pieces were cut from the position 12b on
P-surface of the hot-worked material for automobile parts in the
longitudinal directions as shown in FIGS. 7(a) and 7(b), and each
sample was subjected to the plane bend fatigue test according to
JIS Z2275 to determine the fatigue strength upon 1.times.10.sup.7
times of bending. The results are shown in Table 6.
[0075] The sample with fatigue strength of less than 70 MPa or
fluctuation thereof of exceeding 20 MPa is of problem with respect
to the service life and safety of parts, and reliability for use as
the automobile structural member or automobile parts decreases. The
sample with the fatigue strength of 70 MPa or more and fluctuation
thereof of 20 MPa or less is shown by ".smallcircle.", while the
sample out of the above-mentioned ranges is shown by "x" in Table
4.
(Stress-Corrosion Cracking Test)
[0076] Sample pieces were cut from the position 12b on P-surface of
the hot worked material for automobile parts as shown in FIGS. 7(a)
and 7(b), and the sample was subjected to the stress-corrosion
cracking test according to JIS H8711. Generation of cracking was
observed from the alternate immersion test for 30 days.
[0077] The sample that generates cracking within 30 days in the
alternate immersion test is liable to generate stress-corrosion
cracking in practical uses. The sample with no generation of
cracking is shown by ".smallcircle.", while the sample that
generates cracking is shown by "x" in Table 4.
TABLE-US-00001 TABLE 1 Alloy composition (% by mass) Alloy No. Mg
Cr Si Fe Mn Cu Ti Al A 2.8 0.28 0.23 0.33 0.01 0.01 0.01 Balance B
2.6 0.28 0.23 0.20 0.01 0.01 0.01 Balance C 2.5 0.28 0.14 0.34 0.01
0.01 0.01 Balance D 2.6 0.29 0.14 0.19 0.01 0.01 0.01 Balance E 3.2
0.3 0.2 0.25 0.02 0.01 0.02 Balance F 2.8 0.45 0.2 0.25 0.02 0.01
0.02 Balance G 2.6 0.28 0.35 0.2 0.02 0.01 0.02 Balance H 2.6 0.28
0.2 0.45 0.02 0.01 0.02 Balance I 2.3 0.27 0.15 0.2 0.01 0.01 0.01
Balance J 2.7 0.21 0.21 0.26 0.01 0.01 0.01 Balance K 2.7 0.3 0.24
0.33 0.08 0.08 0.02 Balance L 2.8 0.35 0.25 0.35 0.1 0.09 0.03
Balance M 2.8 0.34 0.25 0.49 0.09 0.08 0.03 Balance N 3.2 0.35 0.39
0.35 0.1 0.09 0.03 Balance
TABLE-US-00002 TABLE 2 Evaluation results of aluminum extruded
pipes Crystal Ratio of pipe wall thickness (%) (the results Cavity
grain Tensile Proof Stress- of measurements are shown in table 3)
Alloy area ratio diameter strength stress corrosion First Second
Third Total No. (%) (.mu.m) (MPa) (MPa) cracking sample sample
sample evaluation A 0.9 80 220 85 .smallcircle. 92.2 91.7 92.2
.smallcircle..smallcircle. B 0.8 73 198 80 .smallcircle. 93.7 94.1
93.8 .smallcircle..smallcircle. C 0.7 76 183 72 .smallcircle. 96.5
96.1 96.8 .smallcircle..smallcircle. D 0.5 75 196 79 .smallcircle.
98.9 99.2 98.9 .smallcircle..smallcircle. E 0.8 75 245 98 x 94.0
93.8 94.2 x F 1.7 75 231 91 .smallcircle. 87.3 87.0 87.3
.smallcircle. G 1.4 73 202 82 .smallcircle. 88.1 88.2 88.1
.smallcircle. H 1.5 74 199 81 .smallcircle. 88.1 87.9 88.2
.smallcircle. I 0.5 85 171 68 .smallcircle. 99.2 99.3 99.0 x J 0.7
320 203 83 .smallcircle. 96.6 96.4 96.3 x K 2.1 70 226 91
.smallcircle. 84 84.3 84.4 .smallcircle. L 2.3 68 229 95
.smallcircle. 83.4 83.2 83.0 .smallcircle. M 2.7 78 231 94
.smallcircle. 81.8 82.2 81.5 x N 3.0 72 247 99 x 80.2 80.9 81.3
x
TABLE-US-00003 TABLE 3 Evaluation results of aluminum extruded
pipes (data of pipe wall thickness) Thickness for every measuring
Minimum Average Ratio of Alloy No. of positions (mm) thickness
thickness thickness No. measurement 1 2 3 4 5 6 7 8 (mm) (mm) (%) A
1 2.39 2.46 2.59 2.70 2.77 2.73 2.60 2.50 2.39 2.59 92.2 2 2.38
2.48 2.63 2.68 2.76 2.75 2.59 2.49 2.38 2.60 91.7 3 2.39 2.49 2.59
2.68 2.77 2.75 2.60 2.47 2.39 2.59 92.2 B 1 2.45 2.53 2.57 2.74
2.75 2.70 2.61 2.56 2.45 2.61 93.7 2 2.46 2.54 2.58 2.73 2.74 2.70
2.61 2.55 2.46 2.61 94.1 3 2.46 2.51 2.57 2.76 2.76 2.71 2.64 2.56
2.46 2.62 93.8 C 1 2.52 2.58 2.63 2.67 2.70 2.63 2.59 2.57 2.52
2.61 96.5 2 2.51 2.59 2.62 2.67 2.71 2.61 2.60 2.58 2.51 2.61 96.1
3 2.53 2.60 2.62 2.65 2.71 2.62 2.60 2.58 2.53 2.61 96.8 D 1 2.58
2.59 2.61 2.63 2.63 2.62 2.61 2.59 2.58 2.61 98.9 2 2.59 2.60 2.62
2.62 2.64 2.62 2.60 2.60 2.59 2.61 99.2 3 2.58 2.60 2.62 2.62 2.63
2.63 2.60 2.60 2.58 2.61 98.9 E 1 2.43 2.50 2.59 2.71 2.76 2.63
2.58 2.49 2.43 2.59 94.0 2 2.43 2.52 2.58 2.72 2.76 2.64 2.57 2.50
2.43 2.59 93.8 3 2.44 2.50 2.59 2.72 2.75 2.65 2.58 2.50 2.44 2.59
94.2 F 1 2.26 2.43 2.62 2.73 2.79 2.76 2.69 2.42 2.26 2.59 87.3 2
2.25 2.42 2.64 2.75 2.77 2.75 2.68 2.43 2.25 2.59 87.0 3 2.26 2.42
2.64 2.74 2.78 2.75 2.69 2.42 2.26 2.59 87.3 G 1 2.29 2.44 2.68
2.75 2.77 2.75 2.67 2.44 2.29 2.60 88.1 2 2.29 2.44 2.69 2.74 2.76
2.73 2.66 2.45 2.29 2.60 88.2 3 2.28 2.43 2.67 2.73 2.76 2.73 2.66
2.45 2.28 2.59 88.1 H 1 2.28 2.42 2.64 2.75 2.77 2.74 2.67 2.44
2.28 2.59 88.1 2 2.27 2.41 2.65 2.74 2.76 2.73 2.66 2.45 2.27 2.58
87.9 3 2.28 2.41 2.64 2.73 2.76 2.74 2.66 2.45 2.28 2.58 88.2 I 1
2.59 2.60 2.61 2.62 2.64 2.62 2.60 2.60 2.59 2.61 99.2 2 2.58 2.60
2.62 2.62 2.63 2.63 2.60 2.59 2.59 2.61 99.3 3 2.58 2.59 2.61 2.62
2.63 2.62 2.60 2.59 2.58 2.61 99.0 J 1 2.52 2.57 2.62 2.66 2.69
2.64 2.60 2.58 2.52 2.61 96.6 2 2.51 2.58 2.61 2.65 2.70 2.62 2.59
2.57 2.51 2.60 96.4 3 2.51 2.57 2.61 2.66 2.71 2.62 2.60 2.57 2.51
2.61 96.3 K 1 2.17 2.47 2.65 2.71 2.78 2.75 2.62 2.52 2.17 2.58
84.0 2 2.18 2.47 2.65 2.73 2.78 2.73 2.64 2.50 2.18 2.59 84.3 3
2.18 2.48 2.68 2.73 2.76 2.73 2.62 2.51 2.18 2.58 84.4 L 1 2.16
2.35 2.62 2.78 2.86 2.88 2.72 2.34 2.16 2.59 83.4 2 2.15 2.36 2.60
2.76 2.89 2.85 2.70 2.37 2.15 2.59 83.2 3 2.15 2.34 2.63 2.80 2.87
2.88 2.71 2.35 2.15 2.59 83.0 M 1 2.12 2.41 2.69 2.86 2.88 2.79
2.64 2.34 2.12 2.59 81.8 2 2.13 2.40 2.66 2.87 2.90 2.80 2.62 2.35
2.13 2.59 82.2 3 2.11 2.39 2.67 2.86 2.91 2.82 2.64 2.32 2.11 2.59
81.5 N 1 2.08 2.32 2.68 2.88 2.95 2.85 2.67 2.32 2.08 2.59 80.2 2
2.09 2.31 2.67 2.85 2.92 2.84 2.70 2.30 2.09 2.59 80.9 3 2.11 2.33
2.68 2.85 2.90 2.86 2.70 2.33 2.11 2.60 81.3
TABLE-US-00004 TABLE 4 Evaluation results of automobile structural
members Crystal grain Tensile Proof Fatigue Stress- Alloy Cavity
area ratio (%) diameter strength stress strength corrosion Total
No. P Q R S (.mu.m) (MPa) (MPa) (MPa) cracking evaluation A 0.9 0.4
0.7 0.2 .smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. B 0.8 0.4 0.6 0.2 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. C 0.8 0.3 0.4 0.2 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. D 0.7 0.3
0.4 0.1 .smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. E 0.8 0.4 0.5 0.3 .smallcircle. x x
.smallcircle. x x F 1.8 1.0 1.3 0.4 .smallcircle. x x x
.smallcircle. x G 1.5 0.8 1.0 0.4 .smallcircle. x x x .smallcircle.
x H 1.5 0.7 1.1 0.4 .smallcircle. x x x .smallcircle. x I 0.6 0.3
0.3 0.1 .smallcircle. x x .smallcircle. .smallcircle. x J 0.8 0.4
0.4 0.2 x .smallcircle. .smallcircle. x .smallcircle. x K 2.2 1.1
1.5 0.5 .smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. L 2.3 1.2 1.6 0.6 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. M 2.8 1.4 2.0 0.7 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. x N 3.1 1.6 2.2 0.8
.smallcircle. .smallcircle. .smallcircle. .smallcircle. x x
TABLE-US-00005 TABLE 5 Evaluation results of automobile structural
members (data of crystal grain diameter, tensile strength and proof
stress) Crystal grain Tensile strength (MPa) Proof stress (MPa)
Alloy diameter Average Average No. (.mu.m) First Second Third
Fourth value Fluctuation First Second Third Fourth value
Fluctuation A 83 218 220 218 222 220 4 83 86 84 87 85 3 B 70 197
200 199 202 200 5 78 83 82 84 82 4 C 73 180 183 183 185 183 5 71 74
73 76 74 4 D 74 194 197 196 200 197 6 77 80 79 82 80 4 E 78 245 247
247 249 247 4 96 99 98 100 98 4 F 72 218 230 226 233 227 15 81 87
83 93 86 12 G 73 193 203 200 206 201 13 72 78 75 83 77 11 H 78 191
201 197 205 199 14 71 78 74 82 76 11 I 88 170 172 172 173 172 3 67
68 68 69 68 2 J 330 200 202 202 203 202 3 79 80 80 81 80 3 K 72 227
225 221 226 225 6 94 93 90 92 92 4 L 70 231 230 228 225 229 6 96 95
95 92 95 4 M 82 223 225 231 236 229 13 87 89 93 98 92 11 N 76 240
249 234 244 242 15 95 100 89 97 95 11
TABLE-US-00006 TABLE 6 Evaluation results of automobile structural
members (data of fatigue strength) Fatigue strength (MPa) Average
Alloy No. First Second Third Fourth value Fluctuation A 87 96 91 99
93 12 B 83 93 88 96 90 13 C 75 85 79 88 82 13 D 80 90 85 94 87 14 E
99 110 106 115 108 16 F 84 97 89 107 94 23 G 75 91 84 98 87 23 H 74
89 83 96 86 22 I 67 74 72 78 73 11 J 58 67 63 73 65 15 K 104 100 97
90 98 14.0 L 94 99 102 107 101 13.0 M 98 77 82 102 89 25.0 N 101
117 91 108 104 26.0
(Total Evaluation--Extruded Pipe)
[0078] The tests of the aluminum extruded pipes and the results
shown in Tables 1 to 3 are summarized below.
[0079] Each of the pipes of alloy A to D, K and L has the cavity
area ratio of 2.3% or less and the ratio of the pipe wall thickness
83% or more, without coarsening of the crystal grain. Accordingly,
the pipes maintain the required tensile strength as the aluminum
alloy for the automobile structural member, and no stress-corrosion
cracking occurs (evaluated as ".smallcircle." in the total
evaluation in Table 2). Among these pipes, those of alloy A to D
have the cavity area ratio of 1.0 or less and the thickness ratio
of 90% or more (evaluated as ".smallcircle..smallcircle." in total
evaluation in Table 2).
[0080] While all of the cavity area ratio, crystal grain diameter,
tensile strength, proof stress and local reduction of the pipe wall
thickness are satisfied in the pipe of alloy E, the
stress-corrosion cracking occurs due to high content of Mg. The
pipe of alloy I does not satisfy the required tensile strength as
an aluminum alloy pipe for the automobile structural member since
the content of Mg is small. The crystal grain is coarsened in the
pipe of alloy J due to a small content of Cr. Generation of the
cavity is high in the pipes of alloy M and N due to high contents
of Fe and Si, respectively, and the pipe wall thickness are locally
reduced, respectively (reduction of the ratio of pipe wall
thickness) (evaluated as "x" in the total evaluation in Table
2).
(Total Evaluation--Automobile Structural Member)
[0081] The test results of the structural members for automobile
shown in Tables 4 to 6 are summarized below.
[0082] Each of the structural members of alloy A to D, K and L has
the cavity area ratio of 2.3% or less and the ratio of the pipe
wall thickness of 83% or more. The crystal grain is not coarsened,
the member has the required tensile strength for the automobile
structural member with small fluctuation of the tensile strength,
and the required fatigue strength is ensured (evaluated as
".smallcircle." in the total evaluation in Table 4).
[0083] The member of alloy E satisfies all of the cavity area
ratio, crystal grain diameter, average of the tensile strength and
fluctuation thereof, average of the proof stress and the
fluctuation thereof, and average of the fatigue strength and the
fluctuation thereof. However, the stress-corrosion cracking occurs
due to high content of Mg. The member of alloy I does not satisfy
the required tensile strength for the automobile structural member
due to small content of Mg. The crystal grain is coarsened in the
member of alloy J due to small content of Cr. Generation of the
cavity is high in the members of alloys M and N due to high
contents of Fe and Si, respectively, and fluctuations of the
tensile strength, proof stress and fatigue strength are large. The
stress-corrosion cracking occurs in the member of alloy N due to a
larger content of Mg (evaluated as "x" in the total evaluation in
Table 4).
INDUSTRIAL APPLICABILITY
[0084] The aluminum alloy pipe of the present invention is suitable
for working into members required to have relatively complex shapes
while required strength is maintained such as structural member for
automobile.
[0085] Hot working of the aluminum alloy pipe permits highly
reliable members having complex shapes that are impossible to form
by cold or warm working and having a small fluctuation of
mechanical characteristics to be manufactured. Examples of such
member include structural members for automobiles and structural
members for motor-bicycles and four-wheel automobiles.
[0086] Having described our invention as related to the present
embodiments, it is our intention that the invention not be limited
by any of the details of the description, unless otherwise
specified, but rather be construed broadly within its spirit and
scope as set out in the accompanying claims.
* * * * *