U.S. patent application number 15/433381 was filed with the patent office on 2017-08-31 for forged aluminum alloy having excellent strength and ductility and method for producing the same.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Masayuki HORI, Katsushi MATSUMOTO, Manabu NAKAI, Hisao SHISHIDO.
Application Number | 20170247782 15/433381 |
Document ID | / |
Family ID | 59679363 |
Filed Date | 2017-08-31 |
United States Patent
Application |
20170247782 |
Kind Code |
A1 |
MATSUMOTO; Katsushi ; et
al. |
August 31, 2017 |
FORGED ALUMINUM ALLOY HAVING EXCELLENT STRENGTH AND DUCTILITY AND
METHOD FOR PRODUCING THE SAME
Abstract
Provided is a hot-forged 6xxx-series aluminum alloy having
excellent corrosion resistance and still having both high strength
and good ductility. A forged 6xxx-series aluminum alloy having a
specific chemical composition after solution treatment is further
subjected to warm working to introduce dislocations into the forged
aluminum alloy microstructure. This allows the forged aluminum
alloy after artificial aging to have a microstructure which has a
high dislocation density, includes a large proportion of small
angle grain boundaries, and has a high average number density of
precipitates. Thus, the resulting forged aluminum alloy has a 0.2%
yield strength of 400 MPa or more and an elongation of 10% or more
and combines properties necessary for suspension parts.
Inventors: |
MATSUMOTO; Katsushi;
(Kobe-shi, JP) ; SHISHIDO; Hisao; (Moka-shi,
JP) ; NAKAI; Manabu; (Inabe-shi, JP) ; HORI;
Masayuki; (Inabe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
59679363 |
Appl. No.: |
15/433381 |
Filed: |
February 15, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/08 20130101;
C22F 1/047 20130101; C22F 1/043 20130101; B21J 5/02 20130101; C22C
21/02 20130101; B22D 7/005 20130101 |
International
Class: |
C22F 1/047 20060101
C22F001/047; B21J 5/02 20060101 B21J005/02; C22F 1/043 20060101
C22F001/043; B22D 7/00 20060101 B22D007/00; C22C 21/08 20060101
C22C021/08; C22C 21/02 20060101 C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2016 |
JP |
2016-036603 |
Claims
1: A forged aluminum alloy having excellent strength and ductility,
the forged aluminum alloy comprising, in mass percent: Si in a
content of 0.7% to 1.5%; Mg in a content of 0.6% to 1.2%; Fe in a
content of 0.01% to 0.5%; at least one element selected from the
group consisting of: Mn in a content of 0.05% to 1.0%; Cr in a
content of 0.01% to 0.5%; and Zr in a content of 0.01% to 0.2%; and
Al and inevitable impurities, wherein: the forged aluminum alloy
having a microstructure in an observation plane at a center of a
thickness in a thickest portion of the forged aluminum alloy; the
microstructure has a dislocation density of from
1.0.times.10.sup.14 to 5.0.times.10.sup.16 per square meter on
average as measured by X-ray diffractometry; the microstructure
comprises small angle grain boundaries with a tilt angle of
2.degree. to 15.degree. in an average proportion of 50% or more as
measured by SEM-EBSD, the small angle grain boundaries being
present around grains having a misorientation of 2.degree. or more;
and the microstructure comprises precipitates in an average number
density of 5.0.times.10.sup.2 per cubic micrometer or more, the
precipitates being measurable by a transmission electron microscope
(TEM) at 300000-fold magnification.
2: The forged aluminum alloy according to claim 1, further
comprising, in mass percent, at least one element selected from the
group consisting of: Cu in a content of 0.05% to 1.0%; Ti in a
content of 0.01% to 0.1%; and Zn in a content of 0.005% to
0.25%.
3: The forged aluminum alloy according to claim 1, which has a
tensile strength of 420 MPa or more, a 0.2% yield strength of 400
MPa or more, and an elongation of 10% or more.
4: A method for producing a forged aluminum alloy having excellent
strength and ductility, the method comprising: preparing an
aluminum alloy ingot comprising, in mass percent: Si in a content
of 0.7% to 1.5%; Mg in a content of 0.6% to 1.2%; Fe in a content
of 0.01% to 0.5%; and at least one element selected from the group
consisting of: Mn in a content of 0.05% to 1.0%; Cr in a content of
0.01% to 0.5%; and Zr in a content of 0.01% to 0.2%, and Al and
inevitable impurities; subjecting the aluminum alloy ingot
sequentially to homogenization and hot-forging to give a forged
material; and subjecting the forged material sequentially to
solution treatment, quenching, warm working, and artificial aging
in the specified sequence, wherein: the forged material after
artificial aging has a microstructure in an observation plane at a
center of a thickness in a thickest portion of the forged material;
the microstructure has a dislocation density of from
1.0.times.10.sup.14 to 5.0.times.10.sup.16 per square meter on
average as measured by X-ray diffractometry; the microstructure
comprises small angle grain boundaries with a tilt angle of
2.degree. to 15.degree. in an average proportion of 50% or more as
measured by SEM-EBSD, the small angle grain boundaries being
present around grains having a misorientation of 20 or more; and
the microstructure comprises precipitates in an average number
density of 5.0.times.10.sup.2 per cubic micrometer or more, the
precipitates being measurable by a TEM at 300000-fold
magnification.
Description
FIELD OF INVENTION
[0001] The present invention relates to a forged aluminum alloy
having excellent strength and ductility, and a method for producing
the same. Hereinafter, "aluminum" is also simply referred to as
"AI".
[0002] As used herein, the term "forged material" refers to a
forged aluminum alloy produced (plastically worked) by hot
forging.
[0003] In the present the present invention, the term "forged
material" is used not as a term describing a production process of
a product, but as a term which is well-known to be generally used
as a technical term and/or a patent term for specifying the state
of the product.
[0004] Aluminum alloy materials, when having different plastic
working histories as in hot-forged materials, extruded materials
(extrusions), and rolled materials, quite differ from each other in
microstructures and properties, even when having identical alloy
chemical compositions. Thus, specifying or defining of the alloy
chemical compositions, microstructure, and properties of an
aluminum alloy material has no point unless the plastic working
history of the aluminum alloy material is specified.
[0005] Accordingly, the term "hot-forged material" or a synonym
thereof "forged material" is used in the appended claims and in the
following description so as to clearly distinguish a target
hot-forged aluminum alloy, to which the present invention is
applied, from other plastically worked materials and to clearly
specify the state of the substance (material).
BACKGROUND OF INVENTION
[0006] Reduction in body weight of and resulting improvements in
fuel efficiency of, automobiles and other transportation equipment
have been pursued so as to counter global environmental issues
caused typically by exhaust gases. To this end, 6xxx-series
(Al--Mg--Si) hot-forged aluminum alloys as prescribed in Aluminum
Association (AA) standards or Japanese Industrial Standards (JIS)
are used for structural components and structural parts of
automobiles and other transportation equipment and, in particular,
for automobile suspension parts such as upper arms and lower
arms.
[0007] Hot-forged 6xxx-series aluminum alloys, when used for these
structural components and structural parts, offer high strength and
high toughness and have relatively excellent corrosion resistance.
Hereinafter, such structural components and structural parts of
transportation equipment will be illustrated by taking automobile
suspension parts as an example.
[0008] For further weight reduction of automobiles, automobile
suspension parts require thinner thicknesses and still require
higher strength and higher toughness. The automobile suspension
parts also function as safety-related parts and require higher
corrosion resistance to intergranular corrosion (grain-boundary
corrosion) and to stress corrosion cracking so as to ensure
reliability as the safety-related parts. Accordingly, various
techniques have been developed to improve chemical compositions and
microstructures of material hot-forged 6xxx-series aluminum
alloys.
[0009] For example, Japanese Patent No. 5110938 proposes a
technique, in which a forged 6xxx-series aluminum alloy is
controlled to have a precipitate density of 1.5% or less in terms
of average area percentage, and grain-boundary precipitates are
controlled to be present at an average spacing between the
precipitates of 0.7 .mu.m or more, where the grain-boundary
precipitates are observed in a microstructure in a cross-sectional
region including a parting line formed upon forging.
[0010] Japanese Patent No. 5723192 proposes another technique. This
technique relates to a forged 6xxx-series aluminum alloy formed by
subjecting an aluminum alloy extrusion to hot forging. The aluminum
alloy has an unrecrystallized region in the entire cross section
thereof. The unrecrystallized region includes small angle grain
boundaries with a tilt angle of from 2.degree. to less than
15.degree., and large angle grain boundaries with a tilt angle of
15.degree. or more. In the unrecrystallized region, areas
surrounded by boundaries with a tilt angle of 2.degree. or more
have an average grain size of 10 .mu.m or less. The
unrecrystallized region occupies 75% or more of the entire cross
section. The unrecrystallized structure region includes dispersed
particles having a maximum length of 10 nm to 800 nm in an average
number density of 10 per cubic micrometer. The unrecrystallized
structure region includes precipitates having a maximum length of
0.5 .mu.m or more in an average area percentage of 2.5% or
less.
[0011] In contrast, though not in the field of hot-forged aluminum
alloys, Japanese Unexamined Patent Application Publication (JP-A)
No. 2014-218685 and Japanese Patent No. 5082483 each disclose a
metallurgical technique so as to offer higher strength of aluminum
alloy materials. With this known technique, a 6xxx-series aluminum
alloy ingot is sequentially subjected to solution treatment
(solution heat treatment), repeatedly to warm forging at about
150.degree. C. to about 250.degree. C., and to artificial
aging.
SUMMARY OF INVENTION
[0012] However, even the improvements in chemical compositions and
microstructures of hot-forged 6xxx-series aluminum alloys, as
disclosed typically in Japanese Patent No. 5110938 and Japanese
Patent No. 5723192, are susceptible to improvement so as to give
forged aluminum alloys having strength and ductility both at
excellent levels, where the strength and ductility properties are
mutually contradictory and resist being compatible with each
other.
[0013] With the technique of subjecting a 6xxx-series aluminum
alloy ingot repeatedly to warm forging and then to artificial aging
so as to offer higher strength as disclosed typically in JP-A No.
2014-218685 and Japanese Patent No. 5082483, it has been considered
that the aluminum alloy ingot, when subjected to hot forging at a
high temperature of typically 500.degree. C. less effectively has
such higher strength. It is still unknown that this technique is
also effective for better mechanical properties of hot-forged
6xxx-series aluminum alloys.
[0014] The present invention has been made while focusing on these
circumstances and has an object to provide a forged 6xxx-series
aluminum alloy that has, as a precondition, excellent corrosion
resistance and still has strength and ductility both at excellent
levels (has both high strength and high elongation).
[0015] To achieve the object, the present invention provides a
forged aluminum alloy having excellent strength and ductility. The
forged aluminum alloy contains, in mass percent, Si in a content of
0.7% to 1.5%, Mg in a content of 0.6% to 1.2%, Fe in a content of
0.01% to 0.5%, and at least one element selected from the group
consisting of Mn in a content of 0.05% to 1.0%, Cr in a content of
0.01% to 0.5%, and Zr in a content of 0.01% to 0.2%, with the
remainder consisting of Al and inevitable impurities. The forged
aluminum alloy has a microstructure in an observation plane at a
center of the thickness in a thickest portion of the forged
aluminum alloy. The microstructure has a dislocation density of
from 1.0.times.10.sup.14 to 5.0.times.10.sup.16 per square meter on
average as measured by X-ray diffractometry. The microstructure
includes small angle grain boundaries with a tilt angle of
2.degree. to 15.degree. in an average proportion of 50% or more as
measured by SEM-EBSD analysis, where the small angle grain
boundaries are present around grains having a misorientation of
2.degree. or more. The microstructure includes precipitates
measurable with a transmission electron microscope (TEM) at
300000-fold magnification in an average number density of
5.0.times.10.sup.2 per cubic micrometer or more.
[0016] To achieve the object, the present invention also provides a
method for producing a forged aluminum alloy having excellent
strength and ductility. The method includes preparing an aluminum
alloy ingot. The aluminum alloy ingot contains, in mass percent, Si
in a content of 0.7% to 1.5%, Mg in a content of 0.6% to 1.2%, Fe
in a content of 0.01% to 0.5%, and at least one element selected
from the group consisting of Mn in a content of 0.05% to 1.0%, Cr
in a content of 0.01% to 0.5%, and Zr in a content of 0.01% to
0.2%, with the remainder consisting of Al and inevitable
impurities. The aluminum alloy ingot is subjected sequentially to
homogenization and hot forging to give a forged material. The
forged material is further subjected sequentially to solution
treatment, quenching, warm working, and artificial aging in the
specified sequence. The forged material (forged aluminum alloy)
after artificial aging has a microstructure in an observation plane
at a center of the thickness in a thickest portion of the forged
aluminum alloy. The microstructure has a dislocation density of
from 1.0.times.10.sup.14 to 5.0.times.10.sup.16 per square meter on
average as measured by X-ray diffractometry. The microstructure
includes small angle grain boundaries with a tilt angle of 20 to
15.degree. in an average proportion of 50% or more as measured by
SEM-EBSD analysis, where the small angle grain boundaries are
present around grains having a misorientation of 20 or more. The
microstructure includes precipitates measurable with a transmission
electron microscope (TEM) at 300000-fold magnification in an
average number density of 5.0.times.10.sup.2 per cubic micrometer
or more.
[0017] It has been found in the present invention that, when a
forged 6xxx-series aluminum alloy after solution treatment and
quenching is subjected to warm working to be imparted with working
strain, and is then subjected to artificial aging, the resulting
forged aluminum alloy has strength and ductility both at higher
levels (has both higher strength and better ductility) as compared
with regular equivalents to which working stain is not
imparted.
[0018] To offer or to ensure the advantageous effect, the present
invention specifies a microstructure in a central part of the
thickness in a thickest portion of the forged material after
artificial aging. Specifically, the present invention specifies the
average dislocation density, the average proportion of small angle
grain boundaries, and the average number density of precipitates in
the microstructure, as described above.
[0019] The present invention allows forged 6xxx-series aluminum
alloys to have strength and ductility both at higher levels, and
this enables further reduction in weight.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Some embodiments of the present invention will be
illustrated below.
[0021] Chemical Composition
[0022] Initially, the chemical composition of an aluminum alloy
constituting an ingot as a material for the forged material (forged
aluminum alloy) according to the present invention and for an
aluminum alloy ingot will be described below.
[0023] The chemical composition of the 6xxx-series
(Al--Mg--Si-series) aluminum alloy for use in the present invention
should be determined or specified so as to ensure higher strength
better ductility, and high corrosion resistance or durability to be
used typically as the forged suspension parts. Within the range of
6xxx-series aluminum alloy chemical compositions, the aluminum
alloy for use in the present invention contains, in a chemical
composition in mass percent Si in a content of 0.7% to 1.5%, Mg in
a content of 0.6% to 1.2%, Fe in a content of 0.01% to 0.5%, and at
least one element selected from the group consisting of Mn in a
content of 0.05% to 1.0%, Cr in a content of 0.01% to 0.5%, and Zr
in a content of 0.01% to 0.2%, with the remainder consisting of Al
and inevitable impurities.
[0024] For strength and other properties at higher levels, the
aluminum alloy may further contain, in mass percent, at least one
element selected from the group consisting of Cu in a content of
0.05% to 1.0%, Ti in a content of 0.01% to 0.1%, and Zn in a
content of 0.005% to 0.25%. All percentages in contents of
individual elements are by mass.
[0025] Next, critical significance and preferred ranges of contents
of the elements will be illustrated.
[0026] Si: 0.7% to 1.5%
[0027] Silicon (Si) precipitates, together with Mg, mainly as a
needle-like .beta.' phase in grains by artificial aging and is
necessary for imparting higher strength to automobile suspension
parts.
[0028] Si, if present in an excessively low content, may
precipitate in an excessively small amount upon artificial aging
and may fail to offer high strength.
[0029] In contrast, Si, if present in an excessively high content,
may cause coarse elementary Si particles to form and precipitate
upon casting and in the course of quenching after solution
treatment and may thereby cause the forged aluminum alloy to have
lower corrosion resistance and lower toughness. Such a large amount
of excessive Si may impede the forged aluminum alloy from having
high corrosion resistance, high toughness, and high fatigue
properties. In addition, this may also adversely affect hot
forgeability and workability to typically cause lower
elongation.
[0030] On the basis of these, the Si content is controlled in the
range of 0.7% to 1.5%.
[0031] Mg: 0.6% to 1.2%
[0032] Magnesium (Mg) also precipitates, together with Si, as a
needle-like .beta.' phase in grains by artificial aging (aging) and
is necessary for imparting higher strength and better ductility to
automobile suspension parts.
[0033] Mg, if present in an excessively low content, may
precipitate in an excessively small amount upon artificial aging
and may fail to offer high strength.
[0034] In contrast, Mg, if present in an excessively high content,
may cause coarse Mg-containing compounds to be formed in grains and
at grain boundaries to lower corrosion resistance and toughness. In
addition, such excessive Mg may cause the forged aluminum alloy to
have excessively high strength (yield strength) at high
temperatures, and this may adversely affect hot forgeability and
workability.
[0035] On the basis of these, the Mg content is controlled in the
range of 0.6% to 1.2%.
[0036] Fe: 0.01% to 0.5%
[0037] Iron (Fe) combines with Si to form intermetallic compounds
as dispersed particles (dispersoids), and impedes grain boundary
migration ater recrystallization, thereby restrains
recrystallization and eliminates or minimizes coarsening of grains.
This advantageously contributes to refinement of grains.
[0038] In contrast, Fe, if present in an excessively high content,
tends to form coarse compounds in grains and at gram boundaries and
to cause the forged aluminum alloy to have lower corrosion
resistance and toughness. Since such intermetallic compounds formed
by Fe readily contain Si, the needle-like .beta.' phase, which is
formed by artificial aging and which requires Si, is decreased.
This tends to cause the forged aluminum alloy to have lower
strength.
[0039] On the basis of these, the Fe content is controlled in the
range of 0.01% to 0.5%.
[0040] At least one element selected from Mn in a content of 0.05%
to 1.0%, Cr in a content of 0.01% to 0.5%, and Zr in a content of
0.01% to 0.2%
[0041] As with Fe, manganese (Mn), chromium (Cr), and zirconium
(Zr) combine with Si to form intermetallic compounds as dispersed
particles (dispersoids), and impede grain boundary migration after
recrystallization, thereby restrain recrystallization and eliminate
or minimize coarsening of grains. This advantageously contributes
to refinement of grains.
[0042] In contrast, Mn, Cr, and Zr, if each present in an
excessively high content, tend to form coarse compounds in grains
and at gram boundaries and to cause the forged aluminum alloy to
have lower corrosion resistance and toughness. Since such
intermetallic compounds formed by these elements readily contain
Si, the needle-like .beta.' phase, which is formed by artificial
aging and which requires Si, is decreased. This tends to cause the
forged aluminum alloy to have lower strength.
[0043] On the basis of these, the content or contents of at least
one of these elements is controlled so that the Mn content falls in
the range of 0.05% to 1.0%, the Cr content falls in the range of
0.01% to 0.5%, and the Zr content falls in the range of 0.01% to
0.2%.
[0044] At least one element selected from Cu in a content of 0.05%
to 1.0%, Ti in a content of 0.01% to 0.1%, and Zn in a content of
0.005% to 0.25%
[0045] Copper (Cu), titanium (Ti), and zinc (Z) are equieffective
elements to allow the forged material to have strength and
toughness at higher levels. When these effects are expected, the
forged aluminum alloy may contain one or more of these elements
selectively.
[0046] Cu offers solid-solution strengthening, thereby contributes
to better strength and toughness of the forged material, and
effectively significantly promotes age hardening of the final
product upon aging. Cu, if present in an excessively low content,
may fail to offer these effects on strength improvements. In
contrast, Cu, if present in an excessively high content, may cause
the forged aluminum alloy microstructure to have significantly high
susceptibility (sensitivity) to stress corrosion cracking and to
intergranular corrosion and may thereby cause the forged aluminum
alloy to have lower corrosion resistance and durability. On the
basis of these, the content of Cu, when to be contained, may be
controlled in the range of 0.05% to 1.0%.
[0047] Zn precipitates and forms Zn--Mg precipitates finely in a
high density upon artificial aging and allows the forged aluminum
alloy to have better strength and toughness. In addition, solute Zn
lowers the potential in grains and causes corrosion not to initiate
from grain boundaries, but to be present as general corrosion. This
effectively results in reduction of intergranular corrosion and
stress corrosion cracking. However, Zn, if present in an
excessively high content, may cause the forged aluminum alloy to
have remarkably lower corrosion resistance. On the basis of these,
the content of Zn, when to be contained, may be controlled in the
range of 0.005% to 0.25%.
[0048] Ti effectively refines grains of the ingot, allows the
forged material microstructure to be fine grains, and allows the
forged aluminum alloy to have better strength and toughness. Ti, if
present in an excessively low content, may fail to offer these
effects. However, Ti, if present in an excessively high content,
may form coarse precipitates to lower the workability. On the basis
of these, the content of Ti, when to be contained, may be
controlled in the range of 0.01% to 0.1%.
[0049] It is accepted that the forged aluminum alloy contains other
impurity elements as the inevitable impurities constituting part of
the remainder of the alloy chemical composition, as long as the
impurity elements are in regular amounts according typically to the
upper limit specifications in JIS standards, within ranges not
adversely affecting properties of the forged aluminum alloy
according to the present invention. Such other impurities tend to
be contained typically from scrap as a raw material for
melting.
[0050] For example, impurity elements listed below may be contained
up to the after-mentioned contents. Hydrogen is readily
contaminated as an impurity and, particularly when the forged
material is worked at a low reduction ratio (working ratio),
bubbles derived from hydrogen do not undergo compression bonding in
working such as forging and cause blisters, which act as fracture
origins. This element thereby causes the forged aluminum alloy to
have significantly lower toughness and fatigue properties. In
particular, the influence of hydrogen is significant typically in
suspension parts prepared so as to have higher strength.
Accordingly, the hydrogen content is preferably minimized to 0.25
ml or less per 100 g of Al.
[0051] Scandium (Sc), vanadium (V), and hafnium (Hf) also tend to
be contaminated as impurities and adversely affect the properties
of suspension parts. To eliminate or minimize this, the total
content of these elements may be controlled to less than 0.3%.
[0052] Boron (B), if preset in a content greater than 500 ppm,
forms coarse precipitates and thereby lower the workability. On the
basis of this, the acceptable content of boron is 500 ppm or
less.
[0053] Microstructure
[0054] On the precondition that the forged aluminum alloy has the
above-mentioned alloy chemical composition, the present invention
specifies the microstructure of the forged material (forged
aluminum alloy) in an observation plane at the center of the
thickness (central part of the thickness) of a thickest portion.
The forged material is for use to constitute structural components
and structural parts of automobiles and other transportation
equipment, in particular for use typically in forged automobile
suspension parts. The specifying is performed so as to allow the
forged material to have strength and ductility both at higher
levels (to have both higher strength and better ductility).
[0055] Initially, the microstructure is specified so as to have a
dislocation density of 1.0.times.10.sup.14 to 5.0.times.10.sup.16
per square meter on average, as measured by X-ray
diffractometry.
[0056] The microstructure is also specified to include small angle
grain boundaries with a tilt angle of 2.degree. to 15.degree. in
average proportion of 50% or more, where the small angle grain
boundaries are present around grains having a misorientation of
2.degree. or more, as measured by SEM-EBSD analysis.
[0057] In addition, the microstructure is specified to include
precipitates in an average number density of 5.0.times.10.sup.2 per
cubic micrometer or more, where the precipitates are measurable
with a TEM at 300000-fold magnification.
[0058] Assume that a forged 6xxx-series aluminum alloy having the
alloy chemical composition after solution treatment and quenching
is subjected to warm working to be imparted with working strain,
and is then subjected to artificial aging. In this case, the
resulting forged material (forged aluminum alloy) has strength and
ductility both at higher levels, as compared with regular forged
materials to which no working strain is imparted.
[0059] This is probably because as follows. Heating before the warm
working allows uniform, fine .beta.' phases to precipitate in
grains of the forged material. Thereafter the warm working imparts
working strain to the forged material to thereby introduce and
enhance dislocations. The dislocations restrain .beta.' phases from
precipitating heterogeneously upon artificial aging and thereby
allows the forged aluminum alloy to have strength and ductility
both at higher levels.
[0060] It is also speculated as follows. The uniform, fine .beta.'
phases pin the dislocations introduced by the application of
working strain by the action of the subsequent warm working, where
the .beta.' phases have been precipitated in the grains by heating
before warm working. This restrains the recovery of dislocations
upon artificial aging, ensures work hardening in a sufficient
quantity, and contributes to better ductility.
[0061] To offer or ensure these advantageous effects, the present
invention specifies the average dislocation density, the average
proportion of small angle grain boundaries, and the average number
density of precipitates, as described above, on the microstructure
at the central part of the thickness in a thickest portion of the
forged material after artificial aging.
[0062] The conditions on the microstructure will be described
sequentially below.
[0063] Dislocation Density
[0064] The present invention specifies and controls the dislocation
density in an observation plane at the center of the thickness in a
thickest portion of the forged material to be in the range of
1.0.times.10.sup.14 to 5.0.times.10.sup.16 per square meter on
average, as measured by X-ray diffractometry. This control is
performed for higher strength and better ductility of the forged
material, in combination with other microstructure controls such as
controls on the average proportion of small angle grain boundaries,
among grain boundaries, and the average number density of
precipitates.
[0065] According to the present invention, the forged material
after solution treatment and quenching is subjected to warm working
to impart working strain (distortion) to the forged material to
thereby introduce dislocations again to the forged material. This
controls the forged material to have a dislocation density within
the specified range. The configuration thus restrains heterogeneous
deformation up to a high strain region or up to rupture, where the
deformation is caused by the application of external force upon use
typically as an automobile suspension part, and allows the forged
aluminum alloy to develop excellent work hardening properties
(lower yield ratio and higher elongation). This allows the forged
aluminum alloy to have high strength in terms of 0.2% yield
strength of 400 MPa or more and good ductility in terms of an
elongation of 10% or more. This specification (condition) works in
combination with other microstructure conditions or controls, such
as conditions on the average proportion of small angle grain
boundaries in grain boundaries and the average number density of
precipitates.
[0066] The forged aluminum alloy, if having an excessively low
dislocation density less than 1.0.times.10.sup.14 per square meter,
may have inferior work hardening properties as equivalent to
conventional forged materials to which no strain is imparted by the
warm working. This may cause early rupture in the high strain
region upon the application of external force, when the forged
aluminum alloy is used typically as an automobile suspension
part.
[0067] In contrast, the forged aluminum alloy, if having an
excessively high dislocation density greater than
5.0.times.10.sup.16 per square meter, may include smaller amounts
of dislocations introduced and accumulated in a high strain region
upon the application of external force, when the forged aluminum
alloy is used typically as an automobile suspension part. This may
also cause early rupture in the high strain region.
[0068] Dislocation Density Measurement Method
[0069] Although measurement of dislocation density typically with a
transmission electron microscope is widely employed, the present
invention employs X-ray diffractometry to measure the dislocation
density more simply and more reproducibly. Of dislocations, there
are "forest" dislocations which are regions (cell walls and shear
zones) where linear or streaky dislocations are densely present.
The transmission electron microscopic analysis hardly distinguishes
such forest dislocations from each other, and this can cause
measurement errors in determination of the dislocation density
.rho.. In contrast, X-ray diffractometry advantageously less causes
errors even in analysis of such forest dislocations, because the
dislocation density .rho. is calculated from half peak widths of
diffraction peaks from individual planes in a crystallographic
texture, as described below.
[0070] When plastic deformation is applied by forging and the warm
working to introduce dislocations into a forged material, the
resulting forged material has a structure in which lattice
distortions are formed as centering around the dislocations.
Depending on the arrangement of the dislocations, small angle grain
boundaries, cell structures or other structures develop. When such
dislocations and domain structures with them are grasped on the
basis of X-ray diffraction patterns, distinctive expanses and
shapes according to diffraction indices appear in diffraction
peaks. The shape (line profile) of such diffraction peaks are
analyzed via line profile analysis to determine the dislocation
density.
[0071] Specifically, the analysis may be performed in the following
manner. Initially, the forged material after subjected to solution
treatment and quenching is subjected to warm working to be imparted
with strain, and is then subjected to artificial aging. From the
forged material after artificial aging in a longitudinal section at
any position of the thickest portion, three measurement samples
(test specimens) including a central part of the thickness are
sampled. The samples are, in a word, sliced in parallel with the
forged material surface and polished so that the center of the
thickness is exposed as an observation plane.
[0072] Namely, the term "central part of the thickness" refers to a
plane parallel to a forged material surface at the center of the
thickness (corresponding to the center of the sheet or plate
thickness in a sheet or plate) of the forged material, in a plan
view, where the plane extends approximately in parallel with the
forged material surface (for example, a horizontal surface) at the
center of the thickness.
[0073] Of each test specimen, the microstructure of the surface
(the plane at the center of the thickness position) is analyzed by
X-ray diffractometry, on the basis of which half peak widths of
diffraction peaks respectively from (111), (200), (220), (311),
(400), (331), (420), and (422) planes (planes of orientations) are
determined, where these orientations of planes are principal
orientations in the crystallographic texture of the surface
portion. The half peak width of the diffraction peak from each
plane increases with an increasing dislocation density .rho.. Of
each test specimen, the rolling surface to be measured by X-ray
diffractometry may be as-sampled, or may have undergone cleaning
without etching.
[0074] Next, a lattice distortion (crystal distortion) .epsilon. is
determined from the half peak width of the diffraction peak from
each plane by Williamson-Hall analysis, on the basis of which the
dislocation density .rho. can be calculated according to the
following expression. The dislocation densities .rho. are
determined on the three samples sampled from the central parts of
the thickness and are averaged to give an average of dislocation
density .rho.:
.rho.=16.1.epsilon..sup.2/b.sup.2
where .rho. represents the dislocation density; .epsilon.
represents the lattice distortion; and b represents the magnitude
of the Burgers vector. The magnitude of the Burgers vector herein
is defined to be 28635.times.10.sup.-10 m.
[0075] The Williamson-Hall analysis is a known line profile
analysis technique, which is widely used for determining
dislocation densities and grain sizes from the relationship between
half peak widths and diffraction angles in two or more
diffractions. In addition, a series of processes for determining
dislocation density by X-ray diffractometry is also publicly known.
In the present invention, the dislocation density is referred to as
"dislocation density measured by X-ray diffractometry", where
"X-ray diffractometry" herein is employed as a generic name of the
series of processes for determining dislocation density by X-ray
diffractometry.
[0076] Average Proportion of Small Angle Grain Boundaries
[0077] To offer higher strength and better ductility of the forged
material, the present invention specifies the average proportion of
small angle grain boundaries with a tilt angle of 2.degree. to
15.degree. to be 50% or more, where the small angle grain
boundaries are present around grains having a misorientation of
2.degree. or more, in an observation plane at the center of the
thickness (central part of the thickness) in a thickest portion of
the forged material, as measured by electron back-scatter
diffraction analysis with canning electron microscopy (SEM-EBSD).
The present invention employs this configuration (control) in
combination with other microstructure controls such as controls on
the average dislocation density and the average number density of
precipitates.
[0078] The control of the proportion of small angle grain
boundaries to be high as in the specified range allows the
microstructure to uniformly deform without local concentration or
focusing of strain when external force is applied upon use
typically as an automobile suspension part. This eliminates or
minimizes local rupture and allows the forged material to have high
strength in terms of 0.2% yield strength of 400 MPa or more and
good ductility in terms of elongation of 10% or more, as employed
in combination of other microstructure controls (conditions) such
as controls on the average dislocation density and the average
number density of precipitates.
[0079] In contrast, the forged material, if having an average
proportion of small angle grain boundaries less than 50%, does not
undergo the mechanism to achieve the high strength and the high
elongation, but has a lower elongation, as with conventional forged
materials.
[0080] As used herein, the term "small angle grain boundary" refers
to a grain boundary between grains with a small difference (tilt
angle) in crystal orientations of 2.degree. to 15.degree., out of
crystal orientations measured by the after-mentioned SEM-EBSD
analysis.
[0081] In contrast, the term "large angle grain boundary" refers to
a grain boundary between grains having a difference (tilt angle) in
the crystal orientations of from greater than 15.degree. to
90.degree..
[0082] In the present invention, the proportion of small angle
grain boundaries with a tilt angle of 2.degree. to 15.degree.,
which is the average proportion of the small angle grain
boundaries, refers to and is defined as the proportion of the total
length of the measured small angle grain boundaries (total length
of grain boundaries between all measured small angle grains) to the
total length of grain boundaries between measured grains with a
difference in crystal orientation of 2.degree. to 90.degree. (total
length of grain boundaries of all measured grains). Namely, the
proportion (%) of the small angle grain boundaries with a tilt
angle of 2.degree. to 15.degree. to be specified can be calculated
according to the expression: [(Total length of grain boundaries
with a tilt angle of 2.degree. to 15.degree.)/Total length of grain
boundaries with a tilt angle of 2.degree. to
90.degree.)].times.100. The average of the calculated values is
controlled to be 50% or more. The upper limit of the proportion of
small angle grain boundaries with a tilt angle of 2.degree. to
15.degree. is about 90%, in consideration of limitations in
production or hot forging.
[0083] Measurement of Average Proportion of Small Angle Grain
Boundaries by SEM-EBSD Analysis
[0084] The average proportion of small angle grain boundaries with
a tilt angle of 2.degree. to 15.degree. around grains with a
misorientation of 2.degree. or more in the forged material
microstructure is measured by analyzing a microstructure of the
forged material after artificial aging by SEM-EBSD analysis, where
the microstructure is in an observation plane of the center of the
thickness (central part of the thickness) in a thickest
portion.
[0085] Specifically, the measurement may be performed in the
following manner. Three measurement samples including a central
part of the thickness are sampled from a longitudinal section at
any position of the thickest portion of the forged material after
artificial aging, in the same manner as with the sampling of
samples for dislocation density measurement. The samples are
polished so that an observation plane at the center of the
thickness is exposed.
[0086] Electron beams are applied at a pitch of 1.0 .mu.m to a
measurement region in the observation plane of the forged material
using a SEM-EBSD system. The measurement region is a rectangular
region having a long side length of 1000 .mu.m and a cross side
length of 320 .mu.m.
[0087] The average proportion of small angle grain boundaries per
each sample is measured in the above manner, and the resulting
three measurements of the three samples are averaged (divided by
3).
[0088] The SEM-EBSD (EBSP) analysis is a versatile crystal
orientation analysis technique using a scanning electron microscope
(SEM) including an electron back scattering (scattered) diffraction
pattern (EBSD) analysis system.
[0089] More specifically, the samples to be observed by SEM-EBSD
analysis may be prepared in the following manner. The observation
samples (cross-sectional microstructures) are further mechanically
polished and then electrically etched to have a mirror surface.
Each of the resulting samples is set in a lens barrel of the SEM,
and electron beams are applied to the mirror surface of the sample
to project an EBSD (EBSP) on the screen. An image of this is taken
by a highly sensitive camera and captured as an image into a
computer. In the computer, the image is analyzed and compared with
patterns obtained by simulation on known crystal systems, and on
the basis of the comparison, crystal orientations are determined.
The determined crystal orientations are recorded as
three-dimensional Eulerian angles typically with position
coordinates (x, y, z). This process is automatically performed on
all measurement points, and gives crystal orientation data at
several tens of thousands to several hundreds of thousands of
points upon the completion of measurement. On the basis of the
crystal orientation data, grains are distinguished, and the
misorientations of grain boundaries are analyzed.
[0090] Precipitates
[0091] For higher strength and better ductility of the forged
material, the present invention specifies the average number
density of precipitates to be 5.0.times.10.sup.2 per cubic
micrometer or more, where the precipitates are measurable with a
TEM at 300000-fold magnification in an observation plane at the
center of the thickness (central part of the thickness) in a
thickest portion of the forged material. This control is employed
in combination with the controls on the average dislocation density
and the average proportion of small angle grain boundaries.
[0092] This allows the forged material to have high strength in
terms of 0.2% yield strength of 400 MPa or more and good ductility
in terms of elongation of 10% or more, as employed in combination
with other microstructure controls (conditions) such as conditions
on the average dislocation density and the average proportion of
small angle grain boundaries in grain boundaries.
[0093] As used herein, the term "precipitates measurable with a TEM
at 300000-fold magnification" refers to all precipitates that can
be measured identified) with a TEM at 300000-fold magnification,
regardless of chemical compositions.
[0094] Specifically, the term refers to all precipitates which have
granular or massive, rod-like, needle-like, or any other isolated
indefinite (complicated) shapes, can be distinguished (identified)
typically from the matrix, grain boundaries, and dislocations, and
can be observed (determined) by analysis of TEM images.
[0095] The minimum size of such precipitates measurable with a TEM
at 300000-fold magnification is 5 nm or more in terms of average
equivalent circle diameter. Precipitates having a size smaller than
this are not measurable and are out of the measurement range.
[0096] In this connection, the substantial upper limit in size of
the precipitates measurable with a TEM is 1000 nm. This is because
automobile suspension part forged materials produced by a common
procedure include (are controlled to include) approximately no
coarse precipitates having an average equivalent circle diameter
greater than 1000 nm, where the coarse precipitates will cause
fracture.
[0097] As used herein, the "equivalent circle diameter" refers to a
circle equivalent diameter obtained by processing images of the
identified precipitates, calculating the area of each individual
precipitate in the TEM view field, and determining the diameter of
a circle having an area equivalent to the area of the precipitate
(diameter of the equivalent circle).
[0098] The "precipitates" in the present invention are
intermetallic compounds having chemical compositions mainly
including Mg--Si or Al--Mg--Si--Cu, Al--Mn, Al--Cr, A--Zr, or
chemical compositions corresponding to them, except for further
including Fe. These intermetallic compounds are derived from the
alloy chemical composition and formed upon artificial aging.
[0099] The forged material has significantly higher strength (bake
hardenability (BH)) when the fine precipitates measurable with the
TEM is present (is controlled to be present) in a high average
number density of 5.0.times.10.sup.2 per cubic micrometer or
more.
[0100] While its mechanism is still unknown, these precipitates
enhance or improve the bake hardenability probably for the
following reason. Such transition element-containing dispersed
particles having the size and being present in the number density
contribute particularly to better work hardening properties upon
the application of prestrain, and to restrainment of recovery of
dislocations upon artificial aging, where the dislocations are
introduced by the application of the prestrain.
[0101] In addition, the fine precipitates as above also have an
excellent effect of not causing deterioration in elongation of the
forged material.
[0102] Assume that the forged material includes precipitates
measurable with a TEM at 300000-fold magnification at the central
part of the thickness of the thickest portion in a low average
number density of less than 5.0.times.10.sup.2 per cubic
micrometer. In this case, the forged material does not undergo the
mechanism of bake hardening to achieve the high strength, but has a
lower elongation, as with conventional forged materials.
[0103] The upper limit of the average number density of
precipitates may be about 1.0.times.10.sup.5 per cubic micrometer
in consideration of limitations in production or hot forging.
[0104] Measurement of Average Number Density of Precipitates
[0105] The average number density of precipitates as specified in
the present invention is measured by measuring or analyzing the
microstructure of the forged material after artificial aging in an
observation plane at the center of the thickness of the thickest
portion with a transmission electron microscope (TEM; such as
field-emission transmission electron microscope (FE-TEM)) at
300000-fold magnification.
[0106] Specifically, the measurement may be performed in the
following manner. Three measurement samples including a central
part of the thickness are sampled from the forged material after
artificial aging in a longitudinal section at any position in a
thickest portion, and thin-film samples for TEM observation are
prepared from the measurement samples so that an observation plane
at the center of the thickness is exposed.
[0107] The TEM thin-film samples are prepared by mechanically
polishing the measurement samples so as to have a dimension of 0.05
mm in both thickness directions from the center of the thickness
(to have a thickness of 0.1 mm), and thinning the same by twin-et
electropolishing into thin films each having a thickness
(dimension) of 100 nm from the center of the thickness.
[0108] In addition, photos of microstructures of the thin films
(samples) are taken using a TEM at 300000-fold magnification, and
images thereof are processed, on the basis of which the number of
all precipitates which can be identified (distinguished) in the
measurement view field is counted, where the total area of the
observation view fields is 0.5 .mu.m.sup.2 or more.
[0109] The average number density (number per cubic micrometer) of
precipitates in the measurement view field is determined. The
average number density measurement is performed on the three
samples sampled from the central part of the thickness, the
measured three average number densities are averaged, and the
average is defined as the average number density (number per cubic
micrometer) of precipitates.
[0110] As described above, the microstructure and properties of the
forged material as specified in the present invention are the
microstructure and properties of a forged material obtained by
subjecting a forged material after solution treatment and quenching
to warm working to impart strain to the forged material, and then
subjecting the resulting forged material to artificial aging.
[0111] Forged Material Measurement Portion
[0112] The measurements of the microstructure and properties are
performed at a portion corresponding to the central part of the
thickness in a thickest portion of the forged material after
artificial aging. When the forged material has a simple shape,
so-called "type I", such as rod-like, plate-like, circular, or
cylindrical shape, the central part of the thickness of the forged
material to be measured can be specified relative to the center of
the forged material.
[0113] However, the automobile suspension parts representatively
have a complicated shape as follows. This shape is an approximately
triangular shape as a whole in a plan view. Ball joints at the
three apices of the triangle are coupled to each other though arms.
The arms each include ribs and a web, where the ribs are in the
periphery and have a narrow width and a large thickness, and the
web is in the central portion and has a wide width and a small
thickness. The arms each have an approximately H-shaped or
approximately U-shaped cross section.
[0114] Accordingly, the "central part of the thickness" of such an
automobile suspension part is defined herein as the center of the
thickness at any position of the thick ribs, where the thick ribs
are taken as the thickest portion of the forged material.
Production Method
[0115] Next, a method according to the present invention for
producing a forged aluminum alloy will be illustrated. The
production process itself for the forged aluminum alloy according
to the present invention can be performed by a common procedure, in
which an aluminum alloy ingot having the chemical composition is
subjected sequentially to homogenization and hot forging to give a
forged material, and the forged material is subjected sequentially
to solution treatment, quenching and artificial aging. Namely, the
forged aluminum alloy can be produced without hot extrusion of the
ingot.
[0116] However, the production method (production process) employs
preferred production conditions as described below, such as warm
working to be performed after solution treatment and quenching and
before artificial aging. Such production conditions are preferred
so as to allow the forged aluminum alloy to have the microstructure
and to have strength and ductility both at higher levels (higher
strength and better ductility) on the precondition that the forged
aluminum alloy has high corrosion resistance. The resulting forged
aluminum alloy is suitable for use typically as or in an automobile
suspension part.
[0117] Casting
[0118] Casting of a molten aluminum alloy, which is melted and
adjusted to have an aluminum alloy chemical composition within the
specific range, may be performed by a common melt casting
technique. The melt casting technique may be selected as
appropriate typically from continuous casting-directed rolling,
semicontinuous casting (direct chill (DC) casting), and hot top
casting.
[0119] However, the casting of the molten aluminum alloy having an
aluminum alloy chemical composition within the specific range is
preferably performed at an average cooling rate of 100.degree. C./s
or more, for refinement of precipitates and decrease of secondary
dendrite arm spacing (SDAS).
[0120] Homogenization
[0121] The homogenization (soaking) of the ingot after casting may
be performed by holding the ingot in a temperature range of
450.degree. C. to 580.degree. C. for 2 hours or longer. The
homogenization, if performed at a temperature lower than
450.degree. C., may fail to homogenize the ingot due to such
excessively low temperature. In contrast, the homogenization, if
performed at a temperature higher than 580.degree. C., may cause
burning of the ingot surface. Extrusion after homogenization and
before hot forging is not necessary, but may be performed when
desired.
[0122] Hot Forging
[0123] After reheating the ingot after homogenization, the hot
forging is preferably performed under conditions at a material
temperature of from 430.degree. C. to 550.degree. C., a forming die
temperature of from 100.degree. C. to 250.degree. C., a minimum
reduction of wall thickness of 25% or more, and a maximum reduction
of wall thickness of 90% or less.
[0124] The hot forging may be performed using a mechanical press or
using an oil hydraulic press so as to forge the ingot to a final
product shape (or a near net shape) of an automobile suspension
part. The hot forging may be performed multiple times as including
upset, rough forging, and finish forging, without reheating, or
with reheating as needed, during forging.
[0125] The hot forging, if performed at a minimum reduction of wall
thickness less than 1%, may fail to give the automobile suspension
part having the above-mentioned complicated shape with good shape
precision by forging, where the minimum reduction of wall thickness
is considered as a hot forging reduction ratio. In contrast, the
hot forging, if performed at a maximum reduction of wall thickness
greater than 90%, may hardly restrain recrystallization and may
highly possibly cause coarse recrystallized grains to be
formed.
[0126] The hot forging, if performed at a forging end temperature
after final forging of lower than 300.degree. C., may impede
restrainment of recrystallization during forging and solution
treatment processes, and this may cause a deformed microstructure
to be recrystallized to form coarse grains. These coarse grains, if
formed, may impede the forged material from having higher strength
and better ductility and may cause the forged material to have
lower corrosion resistance, even when the forged material is
controlled to have the above-mentioned microstructure. In addition,
hot forging, if performed at such a low temperature, may impede
refinement of grains in the entire region in a cross section of the
forged material. In contrast, the hot forging, if performed at a
material temperature higher than 550.degree. C., may highly
possibly cause burning of the forged material surface and cause
coarse recrystallized grains to be formed.
[0127] Solution Treatment and Quenching Treatment
[0128] The forged material after the hot forging is subjected to
solution treatment and quenching treatment. In the solution
treatment, the forged material is preferably held in a temperature
range of 530.degree. C. to 570.degree. C. for a time of 1 hour to 8
hours. The solution treatment, if performed at an excessively low
temperature and/or for an excessively short time, may become
insufficient and may cause insufficient solid-solution of Mg--Si
compounds. This may cause the compounds to precipitate in an
excessively small amount in the subsequent artificial aging and
cause the forged material to have lower strength. The solution
treatment may be performed for a long holding time, but may offer
saturated effects when performed for a time longer than 8
hours.
[0129] After the solution treatment, the forged material may be
subjected to quenching preferably at an average cooling rate of
25.degree. C./s or more in a temperature range of from 500.degree.
C. down to 100.degree. C. The cooling in the quenching treatment is
preferably performed by water cooling, and particularly performed
by water cooling (water tank immersion) in which cooling water is
circulated with bubbling. This is preferred for ensuring the
above-mentioned average cooling rate and for performing homogeneous
cooling in which strain on the forged material is eliminated or
minimized. The quenching treatment, if performed at an excessively
low cooling rate, may cause precipitation typically of Mg--Si
compounds and Si at grain boundaries and may thereby cause the
product after artificial aging to be susceptible to grain boundary
fracture and to have toughness and fatigue properties at lower
levels. In addition, such quenching treatment at an excessively low
cooling rate may cause Mg--Si compounds and Si, which are stable
phases, to be formed also in grains in the course of cooling. This
may cause a .beta. phase and a .beta.' phase to precipitate in
smaller amounts upon artificial aging and may cause the forged
material to have lower strength.
[0130] In contrast, the quenching, if performed at an excessively
high cooling rate (if the forged material is cooled excessively
rapidly), may cause hardening strain during quenching to be formed
in a large amount, and this may disadvantageously require an extra
straightening process after quenching, or may cause the
straightening process to include steps in a larger number. In
addition, such quenching performed at an excessively high cooling
rate gives higher (greater) residual stress and may cause the
product to have dimensional precision and shape precision at lower
levels. In consideration of these, the quenching is preferably
performed as hot-water quenching at 30.degree. C. to 85.degree. C.
at which temperature quenching strain is relaxed. This is preferred
for shortening the product production process and lowering the
cost. The hot-water quenching, if performed at a temperature lower
than 30.degree. C., may cause greater quenching strain. The
hot-water quenching, if performed at a temperature higher than
85.degree. C., may cause the forged material to have toughness,
fatigue properties, and strength at lower levels, due to an
excessively low cooling rate.
[0131] Warm Working
[0132] In the present invention, the hot-forged material thus
obtained after solution treatment and quenching treatment is
subjected to warm working prior to artificial aging so as to be
imparted with strain and to be allowed to have the specified
microstructure and to have higher strength and better
ductility.
[0133] The warm working may be performed within 48 hours after
solution treatment and quenching treatment. The heating before warm
working is preferably performed in a furnace in a temperature range
of 140.degree. C. to 220.degree. C. for a placing time (holding
time) in the furnace of 20 minutes to 120 minutes. Preferably
immediately after the heating, the forged material is subjected to
warm working without delay. The holding in the furnace herein is
performed by raising the work in temperature for 19 minute to 60
minutes and holding the work at the attained temperature for 1
minute to 60 minutes.
[0134] During the heating/holding under the conditions, a uniform
fine .beta.' phase precipitates previously in grains. This heat
treatment performed before warm working restrains heterogeneous
precipitation of the .beta.' phase due to dislocations introduced
by the subsequent warm working. The already-precipitated .beta.'
phase pins the dislocations, thereby restrains recovery of
dislocations upon artificial aging and ensures work hardening in an
sufficient magnitude.
[0135] In contrast, the heat treatment if performed by placing the
work in the furnace for an excessively short time, may cause the
.beta.' phase to little precipitate during temperature rise and
heating/holding in the heat treatment before warm working, but may
allow dislocations introduced by the subsequent warm working to act
as precipitation sites upon artificial aging and to heterogeneously
precipitate. In addition, the dislocations may promote diffusion of
elements to cause the precipitates to coarsen and to disperse
sparsely, and this may possibly cause the forged material to have
lower strength.
[0136] The warm working is preferably performed at a reduction
ratio (working ratio) of 5% to 30%. The warm working, if performed
at a reduction ratio less than 5%, may impart a smaller quantity of
strain to the forged material, may thereby introduce a smaller
amount of dislocations into the forged material, and may fail to
offer effective dislocation hardening. In contrast, the warm
working, if performed at a reduction ratio greater than 300%, may
cause an increased magnitude of accumulated strain to cause the
driving force in recovery upon artificial aging to increase. Thus,
the warm working less effectively offers strength improvement,
because the hardening by dislocation hardening is saturated in
amount (magnitude).
[0137] In addition, the warm working, if performed to a large
magnitude, may increase misorientations of grain boundaries formed
by recovery after warm working and/or during artificial aging. This
may reduce the proportion of small angle grain boundaries, increase
the amount of preferential precipitation at grain boundaries, and
cause the forged material to have lower strength contrarily.
[0138] The warm working may be performed by a procedure according
to the shape of the forged material. When the forged material has a
simple shape such as a rod-like, plate-like, circular, or
cylindrical shape, the warm working procedure can be selected
typically from rolling with rolls and press forming. When the
forged material has a complicate shape such as a shape of the
automobile suspension part, the warm working procedure can be
selected typically from warm closed die forging and warm open die
forging.
[0139] When a high reduction ratio within the range is desired, the
warm working is preferably performed so that the forged material is
allowed to have a final product shape by the warm working, namely,
the hot forging is performed so that the forged material is allowed
to have a near net shape, although this depends on the reduction
ratio and working procedure in the warm working.
[0140] Artificial Aging
[0141] The forged material after the warm working is subjected to
artificial aging. To eliminate or minimize the progress of natural
aging at room temperature, the artificial aging is preferably
performed immediately after the warm working typically within one
hour as a rough reference. The artificial aging conditions are
preferably selected within a temperature range of 100.degree. C. to
250.degree. C. and within a holding time range of 20 minutes to 8
hours.
[0142] However, even within the condition ranges, optimum
conditions should be selected corresponding to the chemical
composition of the forged material and the conditions of previous
processes such as hot forging, solution treatment, quenching
treatment, and cold or warm working. Assume that the artificial
aging is performed under conditions not corresponding to the
chemical composition and the previous process conditions at an
excessively low or an excessively high temperature, or for an
excessively short holding time. In this case, the artificial aging
may impede the forged material from having the desired, specified
microstructure and from having high tensile strength, high yield
strength, and high elongation.
[0143] The homogenization and solution treatment as mentioned above
may be performed using an apparatus selected as appropriate
typically from air furnaces, induction heating furnaces (induction
heaters), and salt-bath furnaces. The artificial aging may be
performed using an apparatus selected as appropriate typically from
air furnaces, induction heating furnaces, and oil baths.
[0144] The forged material according to the present invention, when
for use in or as automobile suspension parts, may be subjected to
processing such as machining and surface treatment as appropriate
before and/or after the artificial aging.
[0145] The present invention will be illustrated in further detail
with reference to several examples (working examples) below. It
should be noted, however, that the examples are by no means
intended to limit the scope of the present invention; that various
changes and modifications can naturally be made therein without
deviating from the spirit and scope of the present invention as
described herein; and all such changes and modifications should be
considered to be within the scope of the present invention.
EXAMPLES
[0146] Next, the present invention will be illustrated with
reference to several examples (working examples). Forged materials
as materials for automobile suspension parts were produced in the
following manner. Initially, hot-forged materials having aluminum
alloy chemical compositions given in Table 1 were prepared and
subjected to solution treatment and quenching under identical
conditions. The hot-forged materials were then subjected
sequentially to warm working and artificial aging under individual
different conditions given in Table 2 to give the forged materials.
The resulting forged materials were subjected to measurements and
evaluations on microstructure, mechanical properties, and corrosion
resistance as indicated in Table 2.
[0147] Specifically, ingots having chemical compositions
corresponding to the forged aluminum alloy chemical compositions
given in Table 1 were prepared by casting via semicontinuous
casting at an average cooling rate of 100.degree. C./s or more, in
common in each sample. All the aluminum alloy samples given in
Table 1 had a hydrogen content of 0.10 to 0.15 ml per 100 g of Al
in common. The symbol "-" in element contents in Table 1 indicates
that the content of an element in question is below the detection
limit.
[0148] In common in each sample, the outer surface of each of the
aluminum alloy ingots was faced by a thickness of 3 mm and cut into
a round rod-like billet having a length of 120 mm and a diameter of
75 mm. The billet was homogenized at 520.degree. C. for 5 hours and
thereafter cooled via forced wind cooling at a cooling rate of
10.degree. C./hr or more using a fan.
[0149] The ingot after homogenization was subjected to hot forging,
in which forging was performed three times down to a final wall
thickness via mechanical press forming using upper and lower dies
in common in each sample. The forging was performed under common
conditions at a forging start temperature in the range of
500.degree. C. to 520.degree. C., a forming die temperature in the
range of 170.degree. C. to 200.degree. C., and a wall thickness
change of 75% (greater than 25%) in the central part of the forged
material.
[0150] In these hot forging operations, each sample was formed into
a hot-forged material having a near net shape corresponding to a
reduction ratio in after-mentioned warm forging, so as to have a
final forged material shape in common in each sample.
[0151] These forged materials were, in common in each sample,
subjected to solution treatment at 550.degree. C. for 5 hours using
an air furnace and then subjected to the water cooling (water tank
immersion) at an average cooling rate of 25.degree. C./s or more in
the temperature range of from 500.degree. C. down to 100.degree.
C.
[0152] The hot-forged materials (after solution treatment and
quenching treatment) obtained in the above manner were subjected
sequentially to warm working and artificial aging under the
conditions given in Table 2 to form the individual
microstructures.
[0153] The warm forging was performed in the following manner.
Initially, the forged materials were heated under the heating
conditions before warm working given in Table 2 and subjected to
warm working at the heating temperature before warm working and the
reduction ratio each given in Table 2, via mechanical press forming
using upper and lower dies.
[0154] The produced forged materials had a suspension part shape in
common in each sample. The suspension part shape is an
approximately triangular shape as a whole in a plan view. Ball
joints at the three apices of the triangle are coupled to each
other through arms. The arms each include ribs and a web, where the
ribs are in the periphery and have a narrow width and a large
thickness (height) of 60 mm, and the web is in the central portion
and has a wide width and a small thickness (height) of 31 mm. The
arms each have an approximately H-shaped cross section.
[0155] The forged materials prepared so as to have the different
microstructures as above were subjected to measurements and
evaluations on microstructure, mechanical properties, and
resistance to intergranular stress corrosion cracking by the
following methods. The results of these are presented in Table
2.
[0156] Microstructure
[0157] The microstructures specified in the present invention were
individually measured by the measurement method. Specifically,
samples were sampled from a longitudinal section of any central
part of the thickness of the rib, which is the thickest portion, of
any of the approximately H-shaped arms of the forged material. Of
the samples, the average dislocation density (per square meter),
the average proportion (%) of small angle grain boundaries with a
tilt angle of 2.degree. to 15.degree. around grains having a
misorientation of 2.degree. or more, and the average number density
of precipitates (number per cubic micrometer) were measured
according to the above-mentioned procedures.
[0158] Mechanical Properties
[0159] A sample was sampled from a central part of the thickness in
any portion of the rib, which is the thickest portion, of the
forged material. From the sample, three tensile test specimens (L
direction) having an outer diameter of 5 mm and a gauge length of
25 mm were prepared so as to include the center of the thickness at
a central position of the thickness direction and to have its L
direction (longitudinal direction) extending along the longitudinal
direction of the forged material. The mechanical properties, such
as 0.2% yield strength (MPa) and elongation (%), of the test
specimens were measured at room temperature, and the measured
values at the three points (three test specimens) were averaged.
The tensile speed was set to 5 mm/min at a stress up to the 0.2%
yield strength, and to 20 mm/mm at a stress equal to or higher than
the 0.2% yield strength.
[0160] Acceptance criteria for such forged materials for automobile
suspension parts were a 0.2% yield strength of 400 MPa or more and
an elongation of 10% or more.
[0161] Corrosion Resistance
[0162] As the corrosion resistance, resistance to intergranular
corrosion (grain boundary corrosion) was evaluated in conformity
with the alternate immersion test prescribed in JIS H 8711.
Specifically, stress of 300 MPa was applied to a test specimen for
stress corrosion cracking resistance evaluation (C-ring test
specimen for stress corrosion cracking (SCC) testing), and a time
(in day) until intergranular corrosion cracking occurred was
measured, regardless of the size of cracking. A sample undergoing
intergranular corrosion cracking within a time period shorter than
30 days was evaluated as having poor corrosion resistance (Poor),
and a sample undergoing intergranular corrosion cracking within a
time period from 30 days to shorter than 60 days was evaluated as
having good corrosion resistance (Good).
[0163] As clearly demonstrated by Tables 1 and 2, the examples
according to the present invention (Examples) had chemical
compositions within the ranges specified in the present invention
and underwent warm working and artificial aging under conditions
within the preferred ranges. As presented in Table 2, the examples
have microstructures as specified in the present invention and each
have a dislocation density in the range of 1.0.times.10.sup.14 to
5.0.times.10.sup.16 per square meter on average as measured by
X-ray diffractometry. The examples also have an average proportion
of small angle grain boundaries with a tilt angle of 2.degree. to
15.degree. of 50% or more, as measured by SEM-EBSD analysis, where
the small angle grain boundaries are present around grains having a
misorientation of 2.degree. or more. In addition, the examples have
an average number density of precipitates measurable with a TEM at
300000-fold magnification of 5.0.times.10.sup.2 per cubic
micrometer or more.
[0164] As a result, the examples have, as a precondition, excellent
corrosion resistance, still have high strength in terms of 02%
yield strength of 400 MPa or more and good ductility in terms of
elongation of 10% or more, and can combine properties necessary as
suspension parts.
[0165] In contrast, as in Comparative Examples 13 to 19 in Table 2,
samples having alloy chemical compositions within the range
corresponding to the alloy number 1 in Table 1, but being produced
through warm working under conditions out of the preferred ranges
have microstructures at the central part of the thickness, which do
not meet conditions specified in the present invention. As a
result, these comparative examples have, in common, a 0.2% yield
strength and an elongation at significantly lower levels as
compared with the examples.
[0166] Comparative Example 13 did not undergo warm working before
artificial aging.
[0167] Comparative Example 14 underwent warm working at an
excessively low heating temperature.
[0168] Comparative Example 15 underwent warm working at an
excessively high heating temperature.
[0169] Comparative Example 16 underwent warm working for an
excessively short heating-holding time.
[0170] Comparative Example 17 underwent warm working for an
excessively long heating-holding time.
[0171] Comparative Example 18 underwent warm working at an
excessively low reduction ratio.
[0172] Comparative Example 19 underwent warm working at an
excessively high reduction ratio.
[0173] Comparative Examples 20 to 23 in Table 2 underwent warm
working under conditions within the preferred ranges, but have
alloy chemical compositions out of the specified ranges, and have
such microstructures at the central part of the thickness as not to
meet the conditions on microstructures as specified in the present
invention. As a result, these comparative examples have, in common,
a 0.2% yield strength and an elongation at significantly lower
levels, as compared with the examples.
[0174] Comparative Example 20 has a chemical composition
corresponding to the alloy number 11 in Table 1 and has a Mg
content lower than the lower limit.
[0175] Comparative Example 21 has a chemical composition
corresponding to the alloy number 12 in Table 1 and has a Si
content lower than the lower limit.
[0176] Comparative Example 22 has a chemical composition
corresponding to the alloy number 13 in Table 1 and does not
contain Fe.
[0177] Comparative Example 23 has a chemical composition
corresponding to the alloy number 14 in Table 1 and contains none
of Mn, Cr, and Zr.
[0178] These results demonstrate critical significance of the
conditions as specified in the present invention on chemical
compositions and microstructures to give forged 6xxx-series
aluminum alloys having excellent corrosion resistance and still
having both high strength and good ductility.
TABLE-US-00001 TABLE 1 Chemical composition of forged 6xxx series
aluminum alloy Alloy (in mass percent, the remainder including Al)
number Mg Si Fe Mn Cr Zr Cu Ti Zn 1 0.8 1.0 0.25 -- 0.08 -- -- 0.02
-- 2 0.8 1.0 0.14 0.20 -- 0.03 -- -- -- 3 0.7 1.1 0.14 0.25 0.03
0.02 0.4 0.02 -- 4 1.2 1.1 0.14 0.08 0.15 -- 0.3 0.02 0.05 5 1.1
0.8 0.14 0.46 -- -- -- 0.02 -- 6 0.7 1.3 0.02 0.25 0.02 -- 0.3 0.04
-- 7 0.6 1.2 0.14 0.06 0.40 -- -- 0.02 -- 8 0.7 1.1 0.14 0.15 0.03
0.18 0.4 0.01 -- 9 0.7 1.1 0.14 0.25 0.03 -- 0.7 0.02 -- 10 0.7 1.1
0.14 0.25 0.03 -- 0.4 0.02 0.20 11 0.4 1.0 0.18 0.08 -- -- 0.1 0.02
-- 12 1.0 0.5 0.20 0.15 0.03 0.02 0.1 0.015 0.1 13 0.6 0.7 -- 0.08
-- -- 0.1 0.02 -- 14 0.6 0.9 0.25 -- -- -- 0.1 0.025 --
TABLE-US-00002 TABLE 2 Production method of forged 6xxx series
aluminum alloy Warm working conditions after solution treatment
Heating conditions Artificial aging conditions Alloy before warm
working after warm working number in Temperature Placing time Warm
working ratio Temperature (.degree. C.) Category No. Table 1
(.degree. C.) (min) (%) for time (hr) Examples 1 1 160 100 30
170.degree. C. for 8 hr 2 2 180 60 20 190.degree. C. for 1 hr 3 3
140 110 15 180.degree. C. for 3 hr 4 4 220 40 20 230.degree. C. for
0.4 hr 5 5 190 20 25 200.degree. C. for 1 hr 6 6 170 120 10
180.degree. C. for 2 hr 7 7 180 90 5 190.degree. C. for 2 hr 8 8
170 80 30 200.degree. C. for 0.5 hr 9 9 200 30 15 220.degree. C.
for 0.5 hr 10 10 150 120 10 180.degree. C. for 5 hr Comparative 13
1 -- -- -- 190.degree. C. for 4 hr Examples 14 1 120 30 5
190.degree. C. for 4 hr 15 1 250 90 10 190.degree. C. for 4 hr 16 1
140 5 5 160.degree. C. for 4 hr 17 1 220 180 10 190.degree. C. for
4 hr 18 1 150 20 2 190.degree. C. for 4 hr 19 1 140 30 50
190.degree. C. for 4 hr 20 11 160 60 10 190.degree. C. for 4 hr 21
12 160 60 10 190.degree. C. for 4 hr 22 13 160 60 10 190.degree. C.
for 4 hr 23 14 160 60 10 190.degree. C. for 4 hr Properties of
forged 6xxx series aluminum alloy after artificial aging Average
proportion Average Resistance Dislocation of small number 0.2% to
Alloy density angle grain density of Yield Elonga- intergranular
number in (average) boundaries precipitates stength tion corrosion
Category No. Table 1 .times.10.sup.15/m.sup.2 %
.times.10.sup.2/.mu.m.sup.3 MPa % cracking Examples 1 1 4.71 78
35.1 429 16 Good 2 2 2.45 81 41.3 414 17 Good 3 3 2.13 72 29.6 411
17 Good 4 4 1.32 69 18.1 403 16 Good 5 5 1.18 64 12.8 401 17 Good 6
6 3.42 75 33.4 425 16 Good 7 7 1.39 58 26.3 408 17 Good 8 8 1.85 74
24.9 410 17 Good 9 9 1.13 63 46.6 416 16 Good 10 10 2.98 71 31.9
421 16 Good Comparative 13 1 0.03 47 4.6 364 18 Good Examples 14 1
0.42 63 3.6 383 18 Good 15 1 0.53 54 2.9 378 17 Good 16 1 0.07 68
1.6 366 9 Good 17 1 0.65 61 3.7 372 17 Good 18 1 0.05 49 4.6 364 18
Good 19 1 1.42 64 4.7 391 17 Good 20 11 0.31 60 3.7 377 16 Good 21
12 0.18 67 2.5 352 17 Good 22 13 0.09 52 5.4 386 16 Good 23 14 0.06
53 5.2 390 16 Good
[0179] The present invention can provide forged 6xxx-series
aluminum alloys having excellent corrosion resistance and still
having both high strength and good ductility. The present invention
therefore enlarges applications of hot-forged 6xxx-series aluminum
alloys to transportation equipment such as automobile suspension
parts and has significant industrial value.
* * * * *