U.S. patent application number 13/905986 was filed with the patent office on 2013-12-05 for aluminum alloy combining high strength, elongation and extrudability.
The applicant listed for this patent is Rio Tinto Alcan International Limited. Invention is credited to Raynald Guay, Alexandre Maltais, Nick C. Parson.
Application Number | 20130319585 13/905986 |
Document ID | / |
Family ID | 49668804 |
Filed Date | 2013-12-05 |
United States Patent
Application |
20130319585 |
Kind Code |
A1 |
Parson; Nick C. ; et
al. |
December 5, 2013 |
Aluminum Alloy Combining High Strength, Elongation and
Extrudability
Abstract
An aluminum alloy includes, in weight percent, 0.70-0.85 Si,
0.14-0.25 Fe, 0.25-0.35 Cu, 0.05 max Mn, 0.75-0.90 Mg, 0.12-0.18
Cr, 0.05 max Zn, and 0.04 max Ti, the balance being aluminum and
unavoidable impurities. The alloy may be suitable for extruding,
and may be formed into an extruded alloy product.
Inventors: |
Parson; Nick C.; (Ontario,
CA) ; Guay; Raynald; (Quebec, CA) ; Maltais;
Alexandre; (Chicoutimi, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rio Tinto Alcan International Limited |
Montreal |
|
CA |
|
|
Family ID: |
49668804 |
Appl. No.: |
13/905986 |
Filed: |
May 30, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61653531 |
May 31, 2012 |
|
|
|
Current U.S.
Class: |
148/690 ;
148/439; 148/689; 420/532; 420/534; 420/535 |
Current CPC
Class: |
C22C 21/02 20130101;
C22C 21/08 20130101; C22F 1/05 20130101; C22C 21/04 20130101 |
Class at
Publication: |
148/690 ;
148/689; 420/532; 420/535; 420/534; 148/439 |
International
Class: |
C22F 1/05 20060101
C22F001/05; C22C 21/02 20060101 C22C021/02; C22C 21/04 20060101
C22C021/04; C22C 21/08 20060101 C22C021/08 |
Claims
1. An aluminum alloy comprising, in weight percent, 0.70-0.85 Si,
0.14-0.25 Fe, 0.25-0.35 Cu, 0.05 max Mn, 0.75-0.90 Mg, 0.12-0.18
Cr, 0.05 max Zn, and 0.04 max Ti, the balance being aluminum and
unavoidable impurities.
2. The alloy of claim 1, wherein the unavoidable impurities may
each be present at a maximum weight percent of 0.05, and the
maximum total weight percent of the unavoidable impurities is
0.15.
3. The alloy of claim 1, wherein the Mn content is 0.03 max weight
percent.
4. The alloy of claim 1, wherein the alloy is extruded, and wherein
less than about 20% of a cross section of the extruded alloy has
undergone recrystallization over at least a portion of a length of
the extruded alloy.
5. The alloy of claim 4, wherein less than about 10% of the cross
section has undergone recrystallization over the at least a portion
of the length of the extruded alloy.
6. The alloy of claim 1, wherein the alloy is extruded, and wherein
less than about 20% of a cross section of the extruded alloy has
undergone recrystallization over an entire length of the extruded
alloy.
7. The alloy of claim 6, wherein less than about 10% of the cross
section has undergone recrystallization over the entire length of
the extruded alloy.
8. The alloy of claim 1, wherein the alloy has a tensile yield
strength of at least about 310 MPa.
9. The alloy of claim 1, wherein the alloy has a tensile elongation
of at least about 12%.
10. The alloy of claim 1, wherein the alloy has a fine Cr
dispersoid distribution.
11. The alloy of claim 1, wherein the alloy is extruded, wherein
the extruded alloy has a substantially non-recrystallized
microstructure, and wherein the alloy has a tensile yield strength
of at least about 310 MPa and a tensile elongation of at least
about 12%.
12. An extruded aluminum alloy product formed of an aluminum alloy
comprising, in weight percent, 0.70-0.85 Si, 0.14-0.25 Fe,
0.25-0.35 Cu, 0.05 max Mn, 0.75-0.90 Mg, 0.12-0.18 Cr, 0.05 max Zn,
and 0.04 max Ti, the balance being aluminum and unavoidable
impurities, wherein the unavoidable impurities may each be present
at a maximum weight percent of 0.05, and the maximum total weight
percent of the unavoidable impurities is 0.15, wherein the extruded
aluminum alloy product is homogenized prior to extrusion, and
wherein the extruded aluminum alloy product has a substantially
non-recrystallized microstructure, and wherein the extruded
aluminum alloy product has a tensile yield strength of at least
about 310 MPa and a tensile elongation of at least about 12%.
13. The extruded aluminum alloy product of claim 12, wherein less
than about 20% of a cross section of the extruded aluminum alloy
product has undergone recrystallization over at least a portion of
a length of the extruded aluminum alloy product.
14. The extruded aluminum alloy product of claim 13, wherein less
than about 10% of the cross section has undergone recrystallization
over the at least a portion of the length of the extruded aluminum
alloy product.
15. The extruded aluminum alloy product of claim 12, wherein less
than about 20% of a cross section of the extruded aluminum alloy
product has undergone recrystallization over the entire length of
the extruded aluminum alloy product.
16. The extruded aluminum alloy product of claim 15, wherein less
than about 10% of the cross section has undergone recrystallization
over the entire length of the extruded aluminum alloy product.
17. The extruded aluminum alloy product of claim 12, wherein the
extruded aluminum alloy product has a minimum cross-sectional
thickness greater than 6.30 mm.
18. A method of forming an extruded product comprising: extruding
an aluminum alloy having a composition, in weight percent,
0.70-0.85 Si, 0.14-0.25 Fe, 0.25-0.35 Cu, 0.05 max Mn, 0.75-0.90
Mg, 0.12-0.18 Cr, 0.05 max Zn, and 0.04 max Ti, the balance being
aluminum and unavoidable impurities; and quenching the alloy after
extruding at a rate of at least 10.degree. C./sec.
19. The method of claim 18, wherein the extrusion is performed at
an extrusion ratio of less than about 40/1, and the extruded
product has a minimum cross-sectional thickness of at least 6.30
mm, and wherein less than about 20% of a cross section of the
extruded product has undergone recrystallization over at least a
portion of a length of the extruded product, and wherein the
extruded product has a tensile yield strength of at least about 310
MPa and a tensile elongation of at least about 12%.
20. The method of claim 18, further comprising homogenizing the
alloy prior to extruding.
21. The method of claim 18, wherein the quenching comprises press
quenching performed by using water mist, spray or quench bath.
22. The method of claim 18, further comprising artificially aging
the alloy after quenching, wherein the artificial aging is carried
out for 2-24 hours at an aging temperature of 160-200.degree.
C.
23. The method of claim 18, wherein less than about 20% of a cross
section of the extruded product has undergone recrystallization
over at least a portion of a length of the extruded product.
24. The method of claim 23, wherein less than about 10% of the
cross section has undergone recrystallization over the at least a
portion of the length of the extruded product.
25. The method of claim 18, wherein the extruded product has a
tensile yield strength of at least about 310 MPa and a tensile
elongation of at least about 12%.
26. The method of claim 18, wherein the alloy is extruded to a
minimum thickness of at least 6.30 mm.
27. The method of claim 18, wherein the extrusion is performed at
an extrusion ratio of less than about 40/1.
28. The method of claim 18, wherein the extrusion is performed with
an extrusion strain of less than about 3.7.
29. The method of claim 18, wherein the quenching is at a rate of
at least 50.degree. C./sec.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority to and is a
non-provisional filing of U.S. Provisional Application No.
61/653,531, filed May 31, 2012, which application is incorporated
by reference herein and made part hereof.
FIELD OF THE INVENTION
[0002] The present invention relates generally to an aluminum alloy
having high strength, elongation and extrudability, and in some
specific aspects, to an aluminum alloy for use in extrusion and
other applications, as well as methods for processing such
alloys.
BACKGROUND
[0003] AA6061 is a widely accepted alloy for structural extrusions.
There is extensive literature on AA6061 aluminum alloys, including
U.S. Pat. Nos. 6,364,969 and 6,565,679. It is typically supplied to
meet minimum properties associated with the AA6061 T6 temper:
[0004] 240 MPa YS-260 MPa UTS and 8% elongation for section
thickness <=6.30 mm [0005] 240 MPa YS-260 MPa UTS-10% elongation
for section thickness >6.30 mm
[0006] The alloy composition can be improved using relatively low
levels of Mg and Si in order to optimise extrusion speed while
still meeting these mechanical property targets. An example of this
is U.S. Pat. No. 6,565,679. For thick section applications (i.e.
>6.30 mm or 0.25 in.) such as anti-lock brake actuator units or
heavily machined engineering parts, a higher yield strength is
beneficial to improve machinability and also to allow some weight
reduction. Uniformity of grain structure is also important to
provide uniform machinability, and also because such parts are
often anodized, and a mixed recrystallized and non-recrystallized
or "fibrous" grain structure can lead to an undesirable visual
appearance. For this reason, a predominantly fibrous grain
structure with a thin surface recrystallized layer is preferred for
such applications. Often the approach to increasing strength in
6XXX alloys is to increase additions of both magnesium and silicon
to achieve the required strength levels, but this can be
detrimental due to the increased flow stress and reduced melting
point of the alloy.
[0007] The present invention is provided to address at least some
of these problems and other problems, and to provide advantages and
aspects not provided by prior alloys, processing methods, and
articles. A full discussion of the features and advantages of the
present invention is deferred to the following detailed
description.
SUMMARY OF THE INVENTION
[0008] The following presents a general summary of aspects of the
invention in order to provide a basic understanding of the
invention. This summary is not an extensive overview of the
invention. It is not intended to identify key or critical elements
of the invention or to delineate the scope of the invention. The
following summary merely presents some concepts of the invention in
a general form as a prelude to the more detailed description
provided below.
[0009] Aspects of the invention relate to an extrudable aluminum
alloy composition comprising, in weight percent:
TABLE-US-00001 Si 0.70-0.85; Fe 0.14-0.25; Cu 0.25-0.35; Mn 0.05
max; Mg 0.75-0.90; Cr 0.12-0.18; Zn 0.05 max; and Ti 0.04 max;
the balance being aluminum and unavoidable impurities.
[0010] According to one aspect, the unavoidable impurities may each
be present at a maximum weight percent of 0.05, and the maximum
total weight percent of the unavoidable impurities is 0.15.
According to another aspect, the Mn content may be 0.03 max weight
percent.
[0011] According to a further aspect, the composition may be
provided in the form of a billet, ingot, or similar article.
[0012] According to yet another aspect, the alloy may be extruded,
and the extruded alloy is processed so as to give a substantially
non recrystallized structure containing deformed grains from the
original billet. In one embodiment, less than about 20% of the
cross section of the extruded alloy has undergone
recrystallization. In one embodiment, less than about 10% of the
cross section has undergone recrystallization. Such
recrystallization percentages may be over at least a portion of the
length of the extruded alloy, over a majority of the length of the
extruded alloy, or over the entire length of the extruded alloy
product.
[0013] According to a still further aspect, the alloy has a tensile
yield strength of at least about 310 MPa and/or a tensile
elongation of at least about 12%.
[0014] Additional aspects of the invention relate to a method for
processing an alloy as described above. Such processing includes
extruding the composition, press quenching and artificially aging
the alloy. The term "press quenching" refers to quenching
immediately after the metal exits the extrusion die. Prior to
extruding, the alloy may also be homogenized. The extruded alloy is
then quenched at a rate >10.degree. C./sec, such as by using
water mist, spray or quench bath. The quenching may be performed at
a rate >50.degree. C./sec in another embodiment. The alloy may
be processed to achieve artificial aging, which may be carried out
for about 2-24 hours at an aging temperature of, for example,
160-200.degree. C. The method according to such aspects may produce
an extruded aluminum alloy that may have properties as described
above.
[0015] According to one aspect, the extrusion may be performed at
an extrusion ratio of less than about 40/1 and/or with an extrusion
strain of less than about 3.7. According to another aspect, the
extruded product may have a minimum thickness of at least 6.30 mm
or 0.25 in.
[0016] Further aspects of the invention relate to an aluminum
extrusion or extruded aluminum alloy product formed of an alloy as
described above. The extrusion may also be processed as in the
method as described above and may have properties as described
above.
[0017] According to one aspect, the extruded products may have a
substantially non-recrystallized microstructure. For example, in
one embodiment, less than about 20% of the extrusion cross section
has undergone recrystallization. In another embodiment, less than
about 10% of the extrusion cross section has undergone
recrystallization. According to a further aspect, the extrusion may
have a tensile yield strength of at least about 310 MPa in
combination with a tensile elongation of at least about 12%
[0018] The alloy may be used in a wide range of extruded
applications and other product forms such as sheet plate or
forgings.
[0019] Other features and advantages of the invention will be
apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] To allow for a more full understanding of the present
invention, it will now be described by way of example, with
reference to the accompanying drawings in which:
[0021] FIGS. 1a and 1b are micrographs illustrating the grain
structure of one embodiment of an extruded alloy according to
aspects described herein; and
[0022] FIGS. 2a and 2b are micrographs illustrating the grain
structure of one embodiment of an extruded alloy according to
aspects described herein.
DETAILED DESCRIPTION
[0023] In general, the alloy composition of the present invention
uses a combination of a low magnesium content and a high silicon
content, whereas the conventional approach to increasing strength
in AA6061 is to increase both Mg and Si. The resultant alloy may
have a solution temperature lower than the high Mg-high Si alloys
typically used for similar applications, allowing for more
efficient use of the alloy additions. The resultant alloy may also
have high mechanical strength and improved extrudability over
alternate compositions capable of similar strength levels. The
alloy also utilizes Cr addition, and the high silicon content and
low homogenisation temperature combine to promote a fine Cr
dispersoid distribution in the ingot, which increases Zener pinning
and suppresses recrystallization and promotes a recovered fibrous
grain structure. The latter may, in turn, provide superior
ductility for an equivalent yield strength. Additionally, the alloy
may achieve these strength and ductility increases with excellent
efficiency of utilisation of the alloy additions for strengthening
and little, if any, detriment to extrudability.
[0024] The alloy may include silicon in an amount of 0.70-0.85 wt.
% or about 0.70-0.85 wt. % in one embodiment. As stated above, this
level of silicon is increased with respect to the silicon levels
typically used in commercial AA6061 alloys. Additionally, this
silicon content may assist in increasing strength, lowering
solution temperature, and promoting a fine Cr dispersoid
distribution in the ingot.
[0025] The alloy may include iron in an amount of 0.14-0.25 wt. %
or about 0.14-0.25 wt. % in one embodiment.
[0026] The alloy may include copper in an amount of 0.25-0.35 wt. %
or about 0.25-0.35 wt. % in one embodiment.
[0027] The alloy may include manganese in an amount of 0.05 max wt.
% Mn or about 0.05 max wt. % Mn in one embodiment. In another
embodiment, the alloy may include manganese in an amount of 0.03
max wt. % or about 0.03 max wt. %.
[0028] The alloy may include magnesium in an amount of 0.75-0.90
wt. % or about 0.75-0.90 wt. % in one embodiment. As stated above,
this amount of magnesium is similar to the amount of magnesium in
AA6061.
[0029] The alloy may include chromium in an amount of 0.12-0.18 wt.
% or about 0.12-0.18 wt. % in one embodiment. As stated above, this
level of chromium is increased with respect to the chromium levels
in AA6061. A fine Cr dispersoid distribution in the alloy can
increase Zener pinning and suppress recrystallization, as well as
promote a recovered fibrous grain structure.
[0030] The alloy may include zinc in an amount of 0.05 max wt. % or
about 0.05 max wt. % in one embodiment.
[0031] The alloy may include titanium in an amount of 0.04 max wt.
% or about 0.04 max wt. % in one embodiment.
[0032] The balance of the alloy includes aluminum and unavoidable
impurities. The unavoidable impurities may each be present at a
maximum weight percent of 0.05 or about 0.05, and the maximum total
weight percent of the unavoidable impurities may be 0.15 or about
0.15, in one embodiment. Additionally, the alloy may include
further alloying additions in another embodiment.
[0033] The alloy may be used in forming a variety of different
articles, and may be initially produced as a billet. The term
"billet" as used herein may refer to traditional billets, as well
as ingots and other intermediate products that may be produced via
a variety of techniques, including casting techniques such as
continuous or semi-continuous casting and others. Further
processing may be used to produce articles of manufacture using the
alloy, such as extruded articles, which may be produced by
extruding the billet to form the extruded article. It is understood
that an extruded article may have a constant cross section in one
embodiment, and may be further processed to change the shape or
form of the article, such as by cutting, machining, connecting
other components, or other techniques.
[0034] The alloy may have a substantially non-recrystallized
structure containing deformed grains from the original billet. As
described above, the formation of fine Cr dispersoids can assist in
achieving this microstructure by suppressing recrystallization of
the grain structure during the extrusion (or other hot
deformation). In one embodiment, less than about 20% of the cross
section of the entire extrusion has undergone recrystallization. In
another embodiment, less than about 10% of the cross section of the
entire extrusion has undergone recrystallization. It is understood
that the "entire" extrusion or the "entire length" of the
extrusion, as used herein, refers to the entire salable length of
the extrusion. In a further embodiment, the above amounts of
recrystallization may occur over a majority (>50%) of the
length, or over at least a portion of the length of the extrusion.
In yet another embodiment, the above amounts of recrystallization
may occur as an average across the entire salable length of the
extrusion.
[0035] In one embodiment, the alloy or an article produced from the
alloy, has a tensile yield strength of at least about 310 MPa and a
tensile elongation of at least about 12%.
[0036] The alloy may be processed using one or more of a variety of
techniques, such as to form an article and/or achieve desired
properties. As described above, such processing may include
extruding the alloy or forming the alloy into an article using a
different technique. The alloy may be used for thick gauge
extrusions in one embodiment, which have minimum thicknesses
greater than 6.30 mm or 0.25 in., although the alloy may be used in
other applications as well. Additionally, an extrusion ratio of
about 40/1 or less and/or an extrusion strain of less than about
3.7 may be used in one embodiment. In one embodiment, the alloy
processing may include press quenching and/or artificial aging
techniques. The term "press quenching" refers to quenching
immediately after the metal exits the extrusion die. Prior to
extruding, the alloy may also be homogenized in one embodiment, for
example, by heating to about 550-575.degree. C. for about 2-8 hours
or another effective homogenization cycle. In one embodiment, the
extruded alloy may be quenched (e.g., by press quenching) after
extrusion, such as by using water mist, spray, and/or quench bath.
The cooling rate achieved by such quenching may be at least
10.degree. C./sec in one embodiment, or may be at least 50.degree.
C./sec on another embodiment. It is noted that the quench rates
reported herein were measured for cooling between 510.degree. C.
(i.e., close to the typical exit temperature) and 200.degree. C. An
in situ solution treatment may also be accomplished in connection
with the quenching. Additionally, in one embodiment, the alloy may
be processed to achieve artificial aging, such as by heating for
2-24 hours at an aging temperature of, for example, 160-200
.degree. C. Other processing techniques may be used in further
embodiments.
EXAMPLE 1
[0037] The following example illustrates beneficial properties that
can be obtained with embodiments of the invention. Four alloy
compositions, control (standard high speed AA6061) and alloys A, B,
and C were DC cast as 101 mm diameter billets, homogenised and
cooled at 350.degree. C./h. A series of three extrusion tests were
conducted using a 780-tonne extrusion press. In each case, the
extrusion was water quenched and aged for 8 h/170.degree. C.
Tensile properties were measured on each extrusion and grain
structures were assessed metallographically for the % of the cross
section that was recrystallized. The alloy compositions and test
results are summarised in Table 1.
[0038] The control alloy is typical of a dilute AA6061 alloy used
for general applications, with a magnesium content close to the
AA6061 specification minimum and silicon content close to the
balanced level associated with Mg.sub.2Si. The Cr content is
<0.10 wt %, which is intended to give adequate toughness for
structural applications without compromising quench sensitivity and
extrudability. The experimental alloys A, B, and C all had
increased Cr additions relative to AA6061, which, as described
above, can help to promote a non-recrystallized grain structure.
Alloy A has the Cr level is raised from 0.08 to 0.15 wt % relative
to the base alloy AA6061. Alloy B is a typical AA6061 composition
used commercially in order to try and achieve higher mechanical
properties and has increased Mg and Si levels for this purpose.
Alloy C has similar Mg content as the control alloy AA6061 but the
silicon content is significantly higher and the Cr content is
higher as well.
TABLE-US-00002 TABLE 1 Extrusion Test Results R = 70/1, TB 480 C. R
= 70/1, TB 520 C. R = 22/1, TB 500 C. Alloy Mg Si Cu Mn Fe Cr
.DELTA.P % YS % El % RX .DELTA.V % YS % El % RX .DELTA.P % YS % El
% RX Control 0.80 0.56 0.20 0.01 0.17 0.08 . . . 264 11.4 100 100
274 15.4 100 . . . 255 19.8 80 A 0.81 0.55 0.29 0.02 0.17 0.15 8.8
268 10.4 90 80 289 12.1 95 8.2 284 18.5 27 B 0.98 0.69 0.29 <.01
0.17 0.15 9.6 276 10.3 90 50 308 11.3 95 7.5 303 16.5 22 C 0.82
0.78 0.30 0.01 0.17 0.15 5.2 302 10.9 80 60 339 10 80 4.4 327 16.2
20
[0039] Three trials were conducted, using different processing
parameters. A summary of the individual trial conditions
follows:
[0040] Billet temperature 480.degree. C., ram speed 5-10 mm/s,
extrusion ratio 70/1, profile 3.times.42 mm. The cooling rate
during quenching is estimated at 300.degree. C./sec between
510.degree. C. and 200.degree. C. Breakthrough pressure and tensile
properties were measured. The breakthrough pressure values at 8
mm/s ram speed were compared, and the % increase in breakthrough
pressure compared to the control alloy is presented in column
.DELTA.P% in Table 1.
[0041] Billet Temperature 520.degree. C., ram speed 5-9 mm/s,
extrusion ratio 70/1, profile 3.times.42 mm. The cooling rate
during quenching is estimated at 300.degree. C./sec between
510.degree. C. and 200.degree. C. The maximum ram speed attainable
for each alloy without encountering hot tearing was assessed and
the relative extrusion speed vs. the control is expressed as a
percentage in column .DELTA.V%.
[0042] Billet temperature 500.degree. C., ram speed 8 mm/s,
extrusion ratio 22/1, profile 50.times.8 mm. The cooling rate
during quenching is estimated at 158.degree. C./sec between
510.degree. C. and 200.degree. C. The breakthrough pressure was
recorded and the % increase in breakthrough pressure vs. the
control alloy is expressed as .DELTA.P% in Table 1.
[0043] [41] The yield strength (YS), elongation (% El) and amount
of recrystallization (% RX) were measured for all alloys tested in
all three trials. These results are also reported in Table 1.
[0044] In test 1, alloy C was the closest of the four alloys to
meeting the property targets of 310 MPa YS and 12% elongation but
did not quite meet these targets, although the property levels
achieved were superior to the standard AA6061 control and alloys A
and B. Surprisingly, the pressure increase for alloy C compared to
the control alloy was lower than alloys A and B.
[0045] In test 2, all four alloys exhibited a strength increase
caused at least partially by the increased solutionizing effect due
to the higher preheat temperature. Alloy B was close to the
property targets but alloy C gave the highest yield strength, well
in excess of 310 MPa, and gave a higher tearing speed than alloy
B.
[0046] In test 3, alloy B was again close to the property targets,
and alloy C again had the highest yield strength and exceeded the
target strength and elongation.
[0047] In both trials 1 and 2, the extrusions were predominantly
recrystallized. In trial 3, the lower extrusion ratio produced a
substantially non-recrystallized or fibrous grain structure with a
shallow recrystallized layer at the surface (expressed as % RX in
Table 1--e.g., 100% indicates the full cross section was
recrystallized, 20% indicates 20% of the cross section was
recrystallized and 80% was non recrystallized. This resulted in a
significant improvement in elongation for all four alloys and all
four met the 12% elongation target. At the same time, the billet
temperature was intermediate between tests 1 and 2, which in turn
gave intermediate solutionizing and yield strength values. Under
these conditions, alloy C was the only composition to meet the
yield strength and elongation targets. Again, the increase in
extrusion breakthrough pressure for alloy C was lower than for
alloys A and B, which was unexpected.
[0048] Overall, alloy C gave the best combination of yield strength
and ductility in all conditions and met the target property values
of 310 MPa YS-12% El when the extrusion conditions were controlled
to give a substantially fibrous grain structure. At the same time,
surprisingly, alloy C required lower breakthrough pressure than
alloys A and B, which can permit the alloy to be extruded faster at
lower cost. These benefits were obtained with Alloy C for both
thick gauge (more than 6.30 mm or 0.25 in. minimum thickness) and
thin gauge (6.30 mm or 0.25 in. or less minimum thickness) alloys.
Alloy C also exhibited superior hot tearing speed to alloy B, which
represents a typical high strength AA6061 used in North America
today.
EXAMPLE 2
[0049] Alloy composition D (0.84 wt. % Mg, 0.77 wt. % Si, 0.29 wt.
% Cu, 0.18 wt. % Fe, 0.14 wt. % Cr) was DC cast and homogenized as
described above with respect to Example 1. The billets were
extruded into a 3.times.42 mm profile at a billet temperature of
500.degree. C. using a ram speed of 5 mm/s. The quench rate at the
press exit was varied on successive billets by applying a slow air
quench, a fast air quench, and a standing wave water quench to give
quench rates of 2.degree. C./sec, 8.degree. C./sec and 300.degree.
C./sec. The material was aged for 8 hrs/170.degree. C. Table 2
shows tensile properties and % recrystallization values of these
samples.
TABLE-US-00003 TABLE 2 Quench Test Results (YS and UTS in MPa)
Quench Quench Rate .degree. C./sec YS UTS % El % RX slow air 2 252
300 10 30 fast air 8 287 327 12 35 water 300 306 337 13 34
[0050] As seen in Table 2, the cross section was at least 30%
recrystallized in all samples due to the narrow section thickness,
and the 310 MPa target yield strength was not achieved. However, it
is clear from the data in Table 2 that fast quenching as achieved
by water quenching gives superior strength and ductility compared
to air quenching. Thus, a minimum quench rate of at least
10.degree. C./sec is desirable. While this test was conducted on a
thin gauge alloy, the result would apply to thick gauge alloys
(>6.30 mm) as well.
EXAMPLE 3
[0051] Alloy composition D (0.84 wt. % Mg, 0.77 wt. % Si, 0.29 wt.
% Cu, 0.18 wt. % Fe, 0.14 wt. % Cr) was cast and homogenized as
described in Example 2 and extruded into a 50.times.8 mm profile
(extrusion ratio of 22/1) using billet temperatures ranging from
475-520.degree. C. and ram speeds from 4-10 mm/sec in order to
assess the effect of process conditions on mechanical properties.
The extrusion was water quenched at the press and subsequently aged
for 8 hrs at 170.degree. C. The cooling rate during quenching is
estimated at 158.degree. C./sec between 510.degree. C. and
200.degree. C. Tensile testing was conducted using the full section
thickness of 8 mm and the grain structure was assessed at front and
back positions along the extruded length. The results of this
testing are summarized in Table 3 below.
TABLE-US-00004 TABLE 3 Extrusion Test Results ram Billet speed exit
MPa % RX % RX Temp .degree. C. mm/s temp .degree. C. YS UTS % El
front back 520 4 515 345.7 372.6 15.3 9.6 9.6 520 6 516 345.3 375.4
14.3 7.7 13.4 500 6 515 344.7 371 15.1 7.7 14.4 500 8 528 346.5
375.5 16 7.7 13.4 475 8 516 340.6 369 15.8 9.6 13.4 475 10 519
342.2 373.6 15.5 9.6 15.3
[0052] All the ram speed/billet temperature combinations resulted
in an exit temperature >510.degree. C. which is normally
considered the target for medium strength 6XXX alloys. Typical
longitudinal grain structures exhibited by the tested alloy are
shown in FIGS. 1a and 1b, which illustrate the microstructure of a
sample extruded at 520.degree. C. with a ram speed of 6 mm/sec at
the front (FIG. 1a) and back (FIG. 1b) of the extruded sample. As
seen in FIGS. 1a and 1b, the section core was observed to be
fibrous (non-recrystallized), and there was a thin surface
recrystallized layer. The depth of this layer is expressed as a %
of the section thickness in Table 3 (%RX). The yield strength and
elongation values achieved over a wide range of press conditions
were well in excess of the 310 MPa and 12% targets. The depth of
recrystallization increased from front to back of the extruded
length, which is normal for direct extrusion. The maximum
recrystallization recorded was 15.3% at the back of the extrusion
produced at the highest ram speed.
EXAMPLE 4
[0053] Alloy D (0.84 wt. % Mg, 0.77 wt. % Si, 0.29 wt. % Cu, 0.18
wt. % Fe, 0.14 wt. % Cr) was cast and homogenized as described in
Example 3 and then extruded into a 66.times.18 mm profile with an
extrusion ratio of 7/1. Billet temperatures ranged from 505 to
523.degree. C. and the ram speed was varied from 10-30 mm/s which
resulted in exit temperatures in excess of 510.degree. C. The
extrusion was water quenched at the press and subsequently aged for
8 hrs at 170.degree. C. The cooling rate during quenching is
estimated at 128.degree. C./sec between 510.degree. C. and
200.degree. C. The test results are summarized in Table 4.
TABLE-US-00005 TABLE 4 Extrusion Test Results ram Billet speed exit
% RX % RX Temp .degree. C. mm/s temp .degree. C. YS UTS % El front
back 521 10 -- 369.3 401 16.9 0.9 2.1 523 20 537 366.2 396.5 13.4
2.6 3.4 505 30 535 368.9 394.8 15.7 2.1 4.3 521 25 542 368.5 396.8
12.5 1.7 4.3
[0054] The section was machined to 12 mm thickness around the
centerline for tensile testing. On this profile, a yield strength
in excess of 360 MPa was achieved with elongation values >12%.
Typical longitudinal grain structures exhibited by the tested alloy
are shown in FIGS. 2a and 2b, which illustrate the microstructure
of a sample extruded at 521.degree. C. with a ram speed of 10
mm/sec at the front (FIG. 2a) and back (FIG. 2b) of the extruded
sample. Again the structure was predominantly fibrous with only a
thin recrystallized surface layer.
[0055] The results from Examples 2-4 indicate that with a press
water quench combined with thick section extrusions, e.g., 8-18 mm,
Alloy D can achieve an excellent combination of strength and
ductility. The water quench prevents waste of the Mg, Si and Cu
added to the alloy by inhibiting precipitation of coarse
non-hardening solute phases during quenching. Compared to the
thinner 3 mm profile, the lower strain during extrusion associated
with the 8 mm and 18 mm profiles maintained the % recrystallization
<20% and allowed a good yield strength and ductility balance to
be achieved. Accordingly, the various embodiments of the alloy
described above can produce excellent yield strength and ductility
balance when used for thick gauge extrusions, such as having an
extrusion thickness of 6.30 mm or 0.25 in.
[0056] Further, as described above, the lower strain during
extrusion associated with the thicker gauge profiles assisted in
maintaining the recrystallization below 20%. The strain in
extrusion is proportional to log.sub.e (extrusion ratio) where the
extrusion ratio is the cross sectional area of the press container
/cross section of the profile. The extrusion ratios and
corresponding strain values for the three profiles tested in
Examples 1-4 were as follows:
TABLE-US-00006 Billet Size Extrusion Ratio Extrusion Strain 42
.times. 3 mm 70/1 4.2 50 .times. 8 mm 22/1 3.1 66 .times. 18 mm 7/1
1.9
Thus, the various embodiments of the alloy described above can
produce excellent yield strength and ductility balance when
extruded using an extrusion ratio of less than about 40/1 and/or an
average extrusion strain of less than about 3.7. It is understood
that while the extrusion ratio of less than about 40/1 and the
average extrusion strain of less than about 3.7 are shown in the
above example for producing thick gauge extrusions, this same
extrusion rate and extrusion strain may be used by those skilled in
the art in producing smaller gauge extrusions, and similar benefits
may be expected.
[0057] The embodiments described herein can provide advantages over
existing alloys, extrusions, and processes, including advantages
over typical AA6061 alloys. For example, alloys described herein
may have a solution temperature lower than the high Mg-high Si
alloys typically used for similar applications, allowing for more
efficient use of the alloy additions. Alloys described herein may
also have high mechanical strength and improved extrudability over
alternate compositions capable of similar strength levels. Further,
alloys described herein utilize Cr additions, and the high silicon
content and low homogenisation temperature combine to promote a
fine Cr dispersoid distribution in the ingot, which increases Zener
pinning and suppresses recrystallization and promotes a recovered
fibrous grain structure. This may, in turn, provide superior
ductility for an equivalent yield strength. Still further benefits
and advantages are recognizable to those skilled in the art.
[0058] While the invention has been described with respect to
specific examples including presently preferred modes of carrying
out the invention, those skilled in the art will appreciate that
there are numerous variations and permutations of the above
described systems and methods. Thus, the spirit and scope of the
invention should be construed broadly as set forth in the appended
claims. All compositions herein are expressed in weight percent,
unless otherwise noted. It is understood that compositions and
other numerical values modified by the term "about" herein may
include variations beyond the exact numerical values listed.
* * * * *