U.S. patent application number 15/824149 was filed with the patent office on 2018-06-07 for ecae materials for high strength aluminum alloys.
The applicant listed for this patent is Honeywell International Inc.. Invention is credited to Frank C. Alford, Lucia M. Feng, Stephane Ferrasse, Wayne D. Meyer, Marc D. Ruggiero, Susan D. Strothers, Patrick K. Underwood.
Application Number | 20180155811 15/824149 |
Document ID | / |
Family ID | 62240347 |
Filed Date | 2018-06-07 |
United States Patent
Application |
20180155811 |
Kind Code |
A1 |
Ferrasse; Stephane ; et
al. |
June 7, 2018 |
ECAE MATERIALS FOR HIGH STRENGTH ALUMINUM ALLOYS
Abstract
A method of forming a high strength aluminum alloy. The method
comprises subjecting an aluminum material containing at least one
of magnesium, manganese, silicon, copper, and zinc at a
concentration of at least 0.1% by weight to an equal channel
angular extrusion (ECAE) process. The method produces a high
strength aluminum alloy having an average grain size from about 0.2
.mu.m to about 0.8 .mu.m and a yield strength from about 300 MPa to
about 650 MPa.
Inventors: |
Ferrasse; Stephane;
(Spokane, WA) ; Strothers; Susan D.; (Mead,
WA) ; Underwood; Patrick K.; (Spokane Valley, WA)
; Ruggiero; Marc D.; (Liberty Lake, WA) ; Meyer;
Wayne D.; (Cheney, WA) ; Feng; Lucia M.;
(Fremont, CA) ; Alford; Frank C.; (Spokane Valley,
WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Honeywell International Inc. |
Morris Plains |
NJ |
US |
|
|
Family ID: |
62240347 |
Appl. No.: |
15/824149 |
Filed: |
November 28, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62429201 |
Dec 2, 2016 |
|
|
|
62503111 |
May 8, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22F 1/053 20130101;
B21C 23/002 20130101; C22C 21/06 20130101; C22F 1/043 20130101;
C22C 21/10 20130101; B21C 23/001 20130101; C22F 1/002 20130101;
C22F 1/047 20130101; C22C 21/02 20130101 |
International
Class: |
C22F 1/053 20060101
C22F001/053; C22F 1/047 20060101 C22F001/047; C22F 1/043 20060101
C22F001/043; C22C 21/10 20060101 C22C021/10; C22C 21/06 20060101
C22C021/06; C22C 21/02 20060101 C22C021/02 |
Claims
1. A method of forming a high strength aluminum alloy, the method
comprising: heating an aluminum material containing aluminum as a
primary component and at least one of magnesium, manganese,
silicon, copper, and zinc as a secondary component at a
concentration of at least 0.1% by weight to a temperature from
about 400.degree. C. to about 550.degree. C. to form a heated
aluminum material; quenching the heated aluminum material to room
temperature to form a cooled aluminum material; subjecting the
cooled aluminum material to an equal channel angular extrusion
(ECAE) process while maintaining the cooled aluminum material at a
temperature from about 20.degree. C. to about 200.degree. C. to
form a high strength aluminum alloy, wherein the high strength
aluminum alloy has an average grain size from about 0.2 .mu.m to
about 0.8 .mu.m in diameter and a yield strength from about 300 MPa
to about 650 MPa.
2. The high strength aluminum alloy of claim 1, wherein the
aluminum material contains from about 2.0 wt. % to about 7.5 wt. %
zinc and from about 0.5 wt. % to about 4.0 wt. % magnesium.
3. The high strength aluminum alloy of claim 1, wherein the
aluminum material contains from about 0.3 wt. % to about 3.0 wt. %
magnesium and from about 0.2 wt. % to about 2.0 wt. % silicon.
4. The high strength aluminum alloy of claim 1, wherein the
aluminum material contains from about 0.5 wt. % to about 7.0 wt. %
copper.
5. The high strength aluminum alloy of claim 1, wherein the
aluminum material contains from about 0.5 wt. % to about 7.0 wt. %
magnesium and from about 0.1 wt. % to about 2.0 wt. %
manganese.
6. The method of claim 1, wherein the ECAE process is completed
within 24 hours of the quenching step.
7. The method of claim 1, wherein the ECAE process includes at
least two ECAE passes.
8. The method of claim 1, further comprising subjecting the cooled
aluminum material to an aging step before the ECAE process.
9. The method of claim 8, wherein the aging step includes heating
the cooled aluminum material to a temperature from about 80.degree.
C. to about 200.degree. C. for from about 15 minutes to about 40
hours.
10. The method of claim 1, wherein the aluminum material containing
aluminum as a primary component and zinc and magnesium as secondary
components has a yield strength from about 400 MPa to about 650
MPa.
11. The method of claim 1, wherein the aluminum material containing
aluminum as a primary component and magnesium and silicon as
secondary components has a yield strength from about 300 MPa to
about 600 MPa.
12. The method of claim 1, wherein the aluminum material containing
aluminum as a primary component and copper as a secondary component
has a yield strength from about 300 MPa to about 600 MPa.
13. The method claim 1, wherein the high strength aluminum alloy
containing aluminum a primary component and magnesium and manganese
as secondary components has a yield strength from about 300 MPa to
about 500 MPa.
14. A high strength aluminum alloy material comprising: an aluminum
material containing at least one of magnesium, manganese, silicon,
copper, and zinc at a concentration of at least 0.1% by weight,
wherein the aluminum material has an average grain size from about
0.2 .mu.m to about 0.8 .mu.m in diameter; and an average yield
strength from about 300 MPa to about 650 MPa.
15. The high strength aluminum alloy of claim 14, wherein the
aluminum material contains aluminum as a primary component and at
least one of magnesium, manganese, silicon, copper, and zinc as a
secondary component.
16. The high strength aluminum alloy of claim 14, wherein the
aluminum material containing aluminum as a primary component and
zinc and magnesium as secondary components has a yield strength
from about 400 MPa to about 650 MPa.
17. The high strength aluminum alloy of claim 14, wherein the
aluminum material containing aluminum as a primary component and
magnesium and silicon as secondary components has a yield strength
from about 300 MPa to about 600 MPa.
18. The high strength aluminum alloy of claim 14, wherein the
aluminum material containing aluminum as a primary component and
copper as a secondary component has a yield strength from about 300
MPa to about 650 MPa.
19. The high strength aluminum alloy of claim 14, wherein the
aluminum material containing aluminum a primary component and
magnesium and manganese as secondary components has a yield
strength from about 300 MPa to about 550 MPa.
20. A device case formed of the high strength aluminum alloy of
claim 14.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Provisional Application
No. 62/429,201, filed Dec. 2, 2016 and also claims priority to
Provisional Application No. 62/503,111, filed May 8, 2017, both of
which are herein incorporated by reference in their entireties.
TECHNICAL FIELD
[0002] The present disclosure relates to high-strength aluminum
alloys which may be used, for example, in devices requiring high
yield strength. More particularly, the present disclosure relates
to high-strength aluminum alloys that have high yield strength and
which may be used to form cases or enclosures for electronic
devices. Methods of forming high-strength aluminum alloys and
high-strength aluminum cases or enclosures for portable electronic
devices are also described.
BACKGROUND
[0003] There is a general trend toward decreasing the size of
certain portable electronic devices, such as laptop computers,
cellular phones, and portable music devices. There is a
corresponding desire to decrease the size of the outer case or
enclosure that holds the device. As an example, certain cellular
phone manufacturers have decreased the thickness of their phone
cases, for example, from about 8 mm to about 6 mm. Decreasing the
size, such as the thickness, of the device case may expose the
device to an increased risk of structural damage, both during
normal use and during storage between uses, specifically due to
device case deflection. Users handle portable electronic devices in
ways that put mechanical stresses on the device during normal use
and during storage between uses. For example, a user putting a
cellular phone in a back pocket of his pants and sitting down puts
mechanical stress on the phone which may cause the device to crack
or bend. There is thus a need to increase the strength of the
materials used to form device cases in order to minimize elastic or
plastic deflection, dents, and any other types of damage.
SUMMARY
[0004] Disclosed herein is a method of forming a high strength
aluminum alloy. The method comprises subjecting an aluminum
material containing at least one of magnesium, manganese, silicon,
copper, and zinc at a concentration of at least 0.1% by weight to a
temperature from about 400.degree. C. to about 550.degree. C. to
form a heated aluminum material. The method further includes
quenching the solutionized aluminum material to below about room
temperature to form a cooled aluminum material. The method also
includes subjecting the aluminum alloy to an equal channel angular
extrusion (ECAE) process while maintaining the cooled aluminum
material at a temperature between about 20.degree. C. and
200.degree. C. to form a high strength aluminum alloy. The high
strength aluminum alloy has an average grain size from about 0.2
.mu.m to about 0.8 .mu.m in diameter and a yield strength greater
than about 300 MPa.
[0005] Also disclosed herein is a high strength aluminum alloy
material comprising an aluminum material containing at least one of
magnesium, manganese, silicon, copper, and zinc at a concentration
of at least 0.1% by weight. The high strength aluminum alloy
material has an average grain size from about 0.2 .mu.m to about
0.8 .mu.m in diameter and a yield strength greater than about 300
MPa.
[0006] While multiple embodiments are disclosed, still other
embodiments of the present invention will become apparent to those
skilled in the art from the following detailed description, which
shows and describes illustrative embodiments of the invention.
Accordingly, the drawings and detailed description are to be
regarded as illustrative in nature and not restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a flow chart showing an embodiment of a method of
forming a high-strength aluminum alloy.
[0008] FIG. 2 is a flow chart showing an alternative embodiment of
a method of forming a high-strength aluminum alloy.
[0009] FIG. 3 is a flow chart showing an alternative embodiment of
a method of forming a high-strength aluminum alloy.
[0010] FIG. 4 is a flow chart showing an alternative embodiment of
a method of forming a high-strength metal alloy.
[0011] FIG. 5 is a schematic view of a sample equal channel angular
extrusion device.
[0012] FIG. 6 is a schematic of a flow path of an example material
change in an aluminum alloy undergoing heat treatment.
[0013] FIG. 7 is a graph comparing Brinell hardness to yield
strength in an aluminum alloy.
[0014] FIG. 8 is a graph comparing natural aging time to Brinell
hardness in an aluminum alloy.
[0015] FIG. 9 is a schematic illustrated various orientations of a
sample material prepared for thermomechanical processing.
[0016] FIGS. 10A to 10C are optical microscopy images of an
aluminum alloy that has been processed using exemplary methods
disclosed herein.
[0017] FIG. 11 is an image of an aluminum alloy that has been
processed using exemplary methods disclosed herein.
[0018] FIGS. 12A and 12B are optical microscopy images of an
aluminum alloy that has been processed using exemplary methods
disclosed herein.
[0019] FIGS. 13A and 13B are optical microscopy images of an
aluminum alloy that has been processed using exemplary methods
disclosed herein.
[0020] FIG. 14 is a graph comparing material temperature to Brinell
hardness in an aluminum alloy processed using exemplary methods
disclosed herein.
[0021] FIG. 15 is a graph comparing processing temperature to
tensile strength in an aluminum alloy processed using exemplary
methods disclosed herein.
[0022] FIG. 16 is a graph comparing the number of extrusion passes
to the resulting Brinell hardness of an aluminum alloy processed
using exemplary methods disclosed herein.
[0023] FIG. 17 is a graph comparing the number of extrusion passes
to the resulting tensile strength of an aluminum alloy processed
using exemplary methods disclosed herein.
[0024] FIG. 18 is a graph comparing various processing routes to
the resulting tensile strength of an aluminum alloy processed using
exemplary methods disclosed herein.
[0025] FIG. 19 is a photograph of an aluminum alloy that has been
processed using exemplary methods disclosed herein.
[0026] FIGS. 20A and 20B are photographs of an aluminum alloy that
has been processed using exemplary methods disclosed herein.
[0027] FIG. 21 is a graph comparing annealing temperature to
Brinell hardness in an aluminum alloy processed using exemplary
methods disclosed herein.
DETAILED DESCRIPTION
[0028] Disclosed herein is a method of forming an aluminum (Al)
alloy that has high yield strength. More particularly, described
herein is a method of forming an aluminum alloy that has a yield
strength from about 300 MPa to about 650 MPa. In some embodiments,
the aluminum alloy contains aluminum as a primary component and at
least one secondary component. For example, the aluminum alloy may
contain magnesium (Mg), manganese (Mn), silicon (Si), copper (Cu),
and/or zinc (Zn) as a secondary component at a concentration of at
least 0.1 wt. % with a balance of aluminum. In some examples, the
aluminum may be present at a weight percentage greater than about
70 wt. %, greater than about 80 wt. %, or greater than bout 90 wt.
%. Methods of forming a high strength aluminum alloy including by
equal channel angular extrusion (ECAE) are also disclosed. Methods
of forming a high strength aluminum alloy having a yield strength
from about 300 MPa to about 650 MPa, including by equal channel
angular extrusion (ECAE) in combination with certain heat treatment
processes, are also disclosed. In some embodiments, the aluminum
alloy may be cosmetically appealing. For example, the aluminum
alloy may be free of a yellow or yellowish color.
[0029] In some embodiments, the methods disclosed herein may be
carried out on an aluminum alloy having a composition containing
aluminum as a primary component and zinc and magnesium as secondary
components. For example, the aluminum alloy may contain zinc in the
range from 2.0 wt. % to 7.5 wt. %, from about 3.0 wt. % to about
6.0 wt. %, or from about 4.0 wt. % to about 5.0 wt. %; and
magnesium in the range from 0.5 wt. % to about 4.0 wt. %, from
about 1.0 wt. % to 3.0 wt %, from about 1.3 wt. % to about 2.0 wt.
%. For example, the aluminum alloy may be one of an Al 7xxx series
of alloys. In some embodiments, the methods disclosed herein may be
carried out with an aluminum alloy having a Zinc-to-magnesium
weight ratio from about 3:1 to about 7:1, from about 4:1 to about
6:1, or about 5:1. In some embodiments, the methods disclosed
herein may be carried out on an aluminum alloy having magnesium and
zinc and having copper (Cu) in limited concentrations. For example,
copper may be present at a concentration of less than about 1.0 wt.
%, less than 0.5 wt. %, less than 0.2 wt. %, less than 0.1 wt. %,
or less than 0.05 wt. %.
[0030] In some embodiments, the aluminum alloy may have a yield
strength from about 400 MPa to about 650 MPa, from about 420 MPa to
about 600 MPa, or from about 440 MPa to about 580 MPa. In some
embodiments, the methods disclosed herein may be carried out with
an aluminum alloy in the Al 7xxx series and form an aluminum alloy
having a submicron grain size less than about 1 .mu.m in diameter.
For example, the grain size may be from about 0.2 .mu.m to about
0.8 .mu.m.
[0031] In some embodiments, the methods disclosed herein may be
carried out on an aluminum alloy having a composition containing
aluminum as a primary component and magnesium and silicon as
secondary components. For example, the aluminum alloy may have a
concentration of magnesium of at least 1.0 wt. %. For example, the
aluminum alloy may have a concentration of magnesium in the range
from about 0.3 wt. % to about 3.0 wt. %, 0.5 wt. % to about 2.0 wt.
%, or 0.5 wt. % to about 1.5 wt. % and a concentration of silicon
in the range from about 0.2 wt. % to about 2.0 wt. % or 0.4 wt. %
to about 1.5 wt. %. For example, the aluminum alloy may be one of
an Al 6xxx series alloy. In some embodiments, the aluminum alloy
may have a yield strength from about 300 MPa to about 600 MPa, from
about 350 MPa to about 600 MPa, or from about 400 MPa to about 550
MPa.
[0032] In some embodiments, the methods disclosed herein may be
carried out on an aluminum alloy having aluminum as a primary
component and copper as a secondary component. For example, the
aluminum alloy may have a composition containing a concentration of
copper in the range from about 0.5 wt. % to about 7.0 wt. % or from
about 2.0 wt. % to about 6.5 wt. %. For example, the aluminum alloy
may be one of an Al 2xxx series alloy. In some embodiments, the
aluminum alloy may have a yield strength from about 300 MPa to
about 650 MPa, from about 350 MPa to about 600 MPa, or from about
350 MPa to about 550 MPa.
[0033] In other embodiments, the methods disclosed herein may be
carried out on an aluminum alloy having aluminum as a primary
component and magnesium and manganese as secondary components. For
example, the aluminum alloy may have a composition containing a
concentration of magnesium in the range from about 0.5 wt. % to
about 7.0 wt. %, from about 1.0 wt. % to about 5.5 wt. %, or from
about 4.0 wt. % to about 5.5 wt. % and manganese in the range from
about 0.1 wt. % to about 2.0 wt. % or from about 0.25 wt. % to
about 1.5 wt. %. For example, the aluminum alloy may be one of an
Al 3xxx series or Al 5xxx series alloy. In some embodiments, the
aluminum alloy may have a yield strength from about 300 MPa to
about 550 MPa, from about 350 MPa to about 500 MPa, or from about
400 MPa to about 500 MPa.
[0034] A method 100 of forming a high strength aluminum alloy
having magnesium and zinc is shown in FIG. 1. The method 100
includes forming a starting material in step 110. For example, an
aluminum material may be cast into a billet form. The aluminum
material may include additives, such as other elements, which will
alloy with aluminum during method 100 to form an aluminum alloy. In
some embodiments, the aluminum material billet may be formed using
standard casting practices for an aluminum alloy having magnesium
and zinc, such as an aluminum-zinc alloy. However, in other
embodiments, the aluminum material billet may be formed using
standard casting practices for an aluminum alloy having magnesium,
manganese, silicon, copper, and/or zinc.
[0035] After formation, the aluminum material billet may optionally
be subjected to a homogenizing heat treatment in step 112. The
homogenizing heat treatment may be applied by holding the aluminum
material billet at a suitable temperature above room temperature
for a suitable time to improve the aluminum's hot workability in
following steps. The temperature and time of the homogenizing heat
treatment may be specifically tailored to a particular alloy. The
temperature and time may be sufficient such that the secondary
components are dispersed throughout the aluminum material to form a
solutionized aluminum material. For example, the secondary
components may be dispersed throughout the aluminum material such
that the solutionized aluminum material is substantially
homogenous. In some embodiments, a suitable temperature for the
homogenizing heat treatment may be from about 300.degree. C. to
about 500.degree. C. The homogenizing heat treatment may improve
the size and homogeneity of the as-cast microstructure that is
usually dendritic with micro and macro segregations. Certain
homogenizing heat treatments may be performed to improve structural
uniformity and subsequent workability of billets. In some
embodiments, a homogenizing heat treatment may lead to the
precipitation occurring homogenously, which may contribute to a
higher attainable strength and better stability of precipitates
during subsequent processing.
[0036] In some embodiments, after the homogenizing heat treatment,
the aluminum material billet may be subjected to solutionizing in
step 114. The goal of solutionizing is to dissolve the additive
elements, such as magnesium, manganese, silicon, copper, and/or
zinc into the aluminum material to form an aluminum alloy. A
suitable solutionizing temperature may be from about 400.degree. C.
to about 550.degree. C., from about 420.degree. C. to about
500.degree. C., or from about 450.degree. C. to about 480.degree.
C. Solutionizing may be carried out for a suitable duration based
on the size, such as the cross sectional area, of the billet. For
example, the solutionizing may be carried out for from about 30
minutes to about 8 hours, from 1 hour to about 6 hours, or from
about 2 hours to about 4 hours, depending on the cross section of
the billet. As an example, the solutionizing may be carried out at
450.degree. C. to about 480.degree. C. for up to 8 hours.
[0037] The solutionizing may be followed by quenching, as shown in
step 116. For standard metal casting, heat treatment of a cast
piece is often carried out near the solidus temperature (i.e.
solutionizing) of the cast piece, followed by rapidly cooling the
cast piece by quenching the cast piece to about room temperature or
lower. This rapid cooling retains any elements dissolved into the
cast piece at a higher concentration than the equilibrium
concentration of that element in the aluminum alloy at room
temperature.
[0038] In some embodiments, aging may be optionally carried out
after the aluminum alloy billet is quenched and before the ECAE
process, as shown in step 118. In one example, aging may be carried
out using a one-step heat treatment. In some embodiments, the
one-step heat treatment may be carried out at temperatures from
about 80.degree. C. to about 200.degree. C. for a duration of 0.25
hours to about 40 hours. In other examples, aging may be carried
out using a two-step heat treatment. For example, a first heat
treatment step may be carried out at temperatures from about
80.degree. C. to about 100.degree. C., from about 85.degree. C. to
about 95.degree. C., or from about 88.degree. C. to about
92.degree. C., for a duration of from 1 hour to about 50 hours,
from about 8 hours to about 40 hours, or from about 10 hours to
about 20 hours. In some embodiments, a second heat treatment step
may be carried out at temperatures from about 100.degree. C. to
about 170.degree. C., from about 100.degree. C. to about
160.degree. C., or from about 110.degree. C. to about 160.degree.
C. for a duration of from 20 hours to about 100 hours, from about
35 hours to about 60 hours, or from about 40 hours to about 45
hours. For example, the first step may be carried out at about
90.degree. C. for about 8 hours and the second step may be carried
out at about 115.degree. C. for about 40 hours or less. Generally,
a first aging heat treatment step may be carried out at a lower
temperature and for less time than the temperature and duration
that the second artificial aging heat treatment step is carried out
at. In some embodiments, the second aging heat treatment step may
include temperatures and time that are less than or equal to
conditions suitable for aging an aluminum alloy to peak hardness
(i.e., peak aging).
[0039] In some embodiments, the aluminum alloy billet may be
subjected to severe plastic deformation such as equal channel
angular extrusion (ECAE), as shown in step 120. For example, the
aluminum alloy billet may be passed through an ECAE device to
extrude the aluminum alloy as a billet having a square or circular
cross section. The ECAE process may be carried out at relatively
low temperatures compared to the solutionizing temperature of the
particular aluminum alloy being extruded. For example, ECAE of an
aluminum alloy having magnesium and zinc may be carried out at a
temperature of from about 0.degree. C. to about 200.degree. C.,
from about 20.degree. C. to about 150.degree. C., or from about
20.degree. C. to about 125.degree. C., or about room temperature,
for example, from about 20.degree. C. to about 35.degree. C. In
some embodiments, during the extrusion, the aluminum alloy material
being extruded and the extrusion die may be maintained at the
temperature that the extrusion process is being carried out at to
ensure a consistent temperature throughout the aluminum alloy
material. That is, the extrusion die may be heated to prevent the
aluminum alloy material from cooling during the extrusion process.
In some embodiments, the ECAE process may include one pass, two or
more passes, or four or more extrusion passes through the ECAE
device.
[0040] Following severe plastic deformation by ECAE, the aluminum
alloy may optionally undergo further plastic deformation, such as
rolling in step 122, to further tailor the aluminum alloy
properties and/or change the shape or size of the aluminum alloy.
Cold working (such as stretching) may be used to provide a specific
shape or to stress relief or straighten the aluminum alloy billet.
For plate applications where the aluminum alloy is to be a plate,
rolling may be used to shape the aluminum alloy.
[0041] FIG. 2 is a flow chart of a method 200 of forming a high
strength aluminum alloy. The method 200 includes forming a starting
material in step 210. Step 210 may be the same as or similar to
step 110 described herein with respect to FIG. 1. In some
embodiments, the starting material may be an aluminum material
billet formed using standard casting practices for an aluminum
material having magnesium and zinc, such as aluminum-zinc alloys.
However, in other embodiments, the aluminum material billet may be
formed using standard casting practices for an aluminum alloy
having magnesium, manganese, silicon, copper, and/or zinc.
[0042] The starting material may be optionally subjected to a
homogenizing heat treatment in step 212. This homogenizing heat
treatment may be applied by holding the aluminum material billet at
a suitable temperature above room temperature to improve the
aluminum's hot workability. Homogenizing heat treatment
temperatures may be in the range of 300.degree. C. to about
500.degree. C. and may be specifically tailored to particular
aluminum alloys.
[0043] After the homogenzing heat treatment, the aluminum material
billet may be optionally subjected to a first solutionizing in step
214. The goal of solutionizing is to dissolve the additive
elements, such as magnesium, manganese, silicon, copper, and/or
zinc, zincmagnesium to form an aluminum alloy. A suitable first
solutionizing temperature may be from about 400.degree. C. to about
550.degree. C., from about 420.degree. C. to about 500.degree. C.,
or from about 450.degree. C. to about 480.degree. C. Solutionizing
may be carried out for a suitable duration based on the size, such
as the cross sectional area, of the billet. For example, the first
solutionizing may be carried out for from about 30 minutes to about
8 hours, from 1 hour to about 6 hours, or from about 2 hours to
about 4 hours, depending on the cross section of the billet. As an
example, the first solutionizing may be carried out at 450.degree.
C. to about 480.degree. C. for up to 8 hours.
[0044] The first solutionizing may be followed by quenching, as
shown in step 216. This rapid cooling retains any elements
dissolved into the cast piece at a higher concentration than the
equilibrium concentration of that element in the aluminum alloy at
room temperature.
[0045] In some embodiments, after the aluminum alloy billet is
quenched, aging may optionally be carried out in step 218. In one
example, aging may be carried out using a one-step heat treatment.
In some embodiments, the one-step heat treatment may be carried out
at temperatures from about 80.degree. C. to about 200.degree. C.
for a duration of 0.25 hours to about 40 hours. Aging may be
carried out using a two-step heat treatment. In some embodiments, a
first heat treatment step may be carried out at temperatures from
about 80.degree. C. to about 100.degree. C., from about 85.degree.
C. to about 95.degree. C., or from about 88.degree. C. to about
92.degree. C., for a duration of from 1 hour to about 50 hours,
from about 8 hours to about 40 hours, or from about 8 hours to
about 20 hours. In some embodiments, a second heat treatment step
may be carried out at temperatures from about 100.degree. C. to
about 170.degree. C., from about 100.degree. C. to about
160.degree. C., or from about 110.degree. C. to about 160.degree.
C. for a duration of from 20 hours to about 100 hours, from about
35 hours to about 60 hours, or from about 40 hours to about 45
hours. For example, the first step may be carried out at about
90.degree. C. for about 8 hours and the second step may be carried
out at about 115.degree. C. for about 40 hours or less. Generally,
a first aging heat treatment step may be carried out at a lower
temperature and for less time than the temperature and duration
that the second artificial aging heat treatment step is carried out
at. In some embodiments, the second aging heat treatment step may
include temperatures and time that are less than or equal to
conditions suitable for artificially aging an aluminum alloy.
[0046] As shown in FIG. 2, after quenching in step 216, or after
the optional aging in step 218, the aluminum alloy may be subjected
to a first severe plastic deformation process, such as an ECAE
process, in step 220. ECAE may include passing the aluminum alloy
billet through an ECAE device in a particular shape, such as a
billet having a square or circular cross section. In some
embodiments, this first ECAE process may be carried out at
temperatures below the homogenizing heat treatment but above the
artificial aging temperature of the aluminum alloy. In some
embodiments, this first ECAE process may be carried out at
temperatures of from about 100.degree. C. to about 400.degree. C.,
or from about 150.degree. C. to about 300.degree. C., or from about
200.degree. C. to about 250.degree. C. In some embodiments, the
first ECAE process may refine and homogenize the microstructure of
the alloy and may provide a better, more uniform, distribution of
solutes and microsegregations. In some embodiments, this first ECAE
process may be performed on an aluminum alloy at temperatures
higher than 300.degree. C. Processing aluminum alloys at
temperatures higher than about 300.degree. C. may provide
advantages for healing of cast defects and redistribution of
precipitates, but may also lead to coarser grain sizes and may be
more difficult to implement in processing conditions. In some
embodiments, during the extrusion process, the aluminum alloy
material being extruded and the extrusion die may be maintained at
the temperature that the extrusion process is being performed at to
ensure a consistent temperature throughout the aluminum alloy
material. That is, the extrusion die may be heated to prevent the
aluminum alloy material from cooling during the extrusion process.
In some embodiments, the first ECAE process may include one, two or
more, or four or more extrusion passes.
[0047] In some embodiments, after a first severe plastic
deformation, the aluminum alloy may be optionally subjected to a
second solutionizing in step 222. The second solutionizing may be
carried out on the aluminum alloy at similar temperature and time
conditions as the first solutionizing. In some embodiments, the
second solutionizing may be carried out at a temperature and/or
duration that are different than the first solutionizing. In some
embodiments, a suitable second solutionizing temperature may be
from about 400.degree. C. to about 550.degree. C., from about
420.degree. C. to about 500.degree. C., or from about 450.degree.
C. to about 480.degree. C. A second solutionizing may be carried
out for a suitable duration based on the size, such as the cross
sectional area, of the billet. For example, the second
solutionizing may be carried out for from about 30 minutes to about
8 hours, from 1 hour to about 6 hours, or from about 2 hours to
about 4 hours, depending on the cross section of the billet. In
some embodiments, the second solutionizing may be from about
450.degree. C. to about 480.degree. C. for up to 8 hours. In
various embodiments, the second solutionizing may be followed by
quenching.
[0048] In some embodiments, after the second solutionizing and/or
the quenching, the aluminum alloy may be optionally subjected to a
second severe plastic deformation step, such as an ECAE process, in
step 226. In some embodiments, the second ECAE process may be
carried out at lower temperatures than that used in the first ECAE
process of step 220. For example, the second ECAE process may be
carried out at temperatures greater than 0.degree. C. and less than
200.degree. C., or from about 20.degree. C. to about 125.degree.
C., or from about 20.degree. C. to about 100.degree. C., or about
room temperature, for example from about 20.degree. C. to about
35.degree. C. In some embodiments, during the extrusion, the
aluminum alloy material being extruded and the extrusion die may be
maintained at the temperature that the extrusion process is being
carried out at to ensure a consistent temperature throughout the
aluminum alloy material. That is, the extrusion die may be heated
to prevent the aluminum alloy material from cooling during the
extrusion process. In some embodiments, the second ECAE process may
include one pass, two or more passes, or four or more extrusion
passes through the ECAE device.
[0049] In some embodiments, after the aluminum alloy is submitted
to a second severe plastic deformation step such as ECAE, a second
aging process may be optionally carried out in step 228. In one
example, aging may be carried out using a one-step heat treatment.
In some embodiments, the one-step heat treatment may be carried out
at temperatures from about 80.degree. C. to about 200.degree. C.
for a duration of 0.25 hours to about 40 hours. In some
embodiments, aging may be carried out using a two-step heat
treatment. In some embodiments, a first heat treatment step may be
carried out at temperatures from about 80.degree. C. to about
100.degree. C., from about 85.degree. C. to about 95.degree. C., or
from about 88.degree. C. to about 92.degree. C., for a duration of
from 1 hour to about 50 hours, from about 8 hours to about 40
hours, or from about 8 hours to about 20 hours. In some
embodiments, a second heat treatment step may be carried out at
temperatures from about 100.degree. C. to about 170.degree. C.,
from about 100.degree. C. to about 160.degree. C., or from about
110.degree. C. to about 160.degree. C. for a duration of from 20
hours to about 100 hours, from about 35 hours to about 60 hours, or
from about 40 hours to about 45 hours. For example, the first aging
step may be carried out at about 90.degree. C. for about 8 hours
and the second aging may be carried out at about 115.degree. C. for
about 40 hours or less. In some embodiments, the second step may
include temperatures and time that are less than or equal to
conditions suitable for artificially aging an aluminum alloy to
peak hardness (i.e., peak hardness).
[0050] Following method 200, the aluminum alloy may optionally
undergo further plastic deformation, such as rolling to change the
shape or size of the aluminum alloy.
[0051] A method 300 of forming a high strength aluminum alloy is
shown in FIG. 3. The method 300 may include casting a starting
material in step 310. For example, an aluminum material may be cast
into a billet form. The aluminum material may include additives,
such as other elements, which will alloy with the aluminum during
method 310 to form an aluminum alloy. In some embodiments, the
aluminum material billet may be formed using standard casting
practices for an aluminum alloy having magnesium and zinc, such as
aluminum-zinc alloys, for example Al 7xxx series aluminum alloys.
However, in other embodiments, the aluminum material billet may be
formed using standard casting practices for an aluminum alloy
having at least one of magnesium, manganese, copper, and/or zinc
such as, for example, Al 2xxx, Al 3xxx, Al 5xxx, or Al 6xxx series
alloys.
[0052] After formation, the aluminum material billet may be
subjected to a homogenizing heat treatment in step 312. The
homogenizing heat treatment may be applied by holding the aluminum
material billet at a suitable temperature above room temperature to
improve the aluminum's hot workability in following steps. The
homogenizing heat treatment may be specifically tailored to a
specific aluminum alloy. For example, the temperature may vary
depending on the composition of the aluminum alloy or which series
of alloy is used. In some embodiments, a suitable temperature for
the homogenizing heat treatment may be from about 300.degree. C. to
about 500.degree. C.
[0053] After the homogenizing heat treatment, the aluminum material
billet may be subjected to a first solutionizing in step 314 to
form an aluminum alloy. The first solutionizing may be similar to
that described herein with respect to steps 114 and 214. A suitable
first solutionizing temperature may be from about 400.degree. C. to
about 550.degree. C., from about 420.degree. C. to about
500.degree. C., or from about 450.degree. C. to about 480.degree.
C. A first solutionizing may be carried out for a suitable duration
based on the size, such as the cross sectional area, of the billet.
For example, the first solutionizing may be carried out for from
about 30 minutes to about 8 hours, from 1 hour to about 6 hours, or
from about 2 hours to about 4 hours, depending on the cross section
of the billet. As an example, the solutionizing may be carried out
at 450.degree. C. to about 480.degree. C. for up to 8 hours. The
solutionizing may be followed by quenching. During quenching, the
aluminum alloy billet is rapidly cooled by quenching the aluminum
alloy billet is cooled to about room temperature or lower. This
rapid cooling retains any elements dissolved into the aluminum
alloy at a higher concentration than the equilibrium concentration
of that element in the aluminum alloy at room temperature. In some
embodiments, the quenching may occur within 24 hours of the first
solutionizing.
[0054] In some embodiments, after the aluminum alloy is quenched,
aging may optionally be carried out in step 316. In one example,
aging may be carried out using a one-step heat treatment. In some
embodiments, the one-step heat treatment may be carried out at
temperatures from about 80.degree. C. to about 200.degree. C. for a
duration of 0.25 hours to about 40 hours. In some embodiments,
aging may be carried out with two heat treatment steps that form
the artificial aging step. In some embodiments, a first heat
treatment step may be carried out at temperatures from about
80.degree. C. to about 100.degree. C., from about 85.degree. C. to
about 95.degree. C., or from about 88.degree. C. to about
92.degree. C., for a duration of from 1 hour to about 50 hours,
from about 8 hours to about 40 hours, or from about 8 hours to
about 20 hours. In some embodiments, a second heat treatment step
may be carried out at temperatures from about 100.degree. C. to
about 170.degree. C., from about 100.degree. C. to about
160.degree. C., or from about 110.degree. C. to about 160.degree.
C. for a duration of from 20 hours to about 100 hours, from about
35 hours to about 60 hours, or from about 40 hours to about 45
hours. For example, the first step may be carried out at about
90.degree. C. for about 8 hours and the second step may be carried
out at about 115.degree. C. for about 40 hours or less. Generally,
a first aging heat treatment step may be carried out at a lower
temperature and for less time than the temperature and duration
that the second aging heat treatment step is carried out at. In
some embodiments, the second aging heat treatment step may include
temperatures and time that are less than or equal to conditions
suitable for aging an aluminum alloy to peak hardness (i.e., peak
aging).
[0055] After aging, the aluminum alloy billet may be subjected to
severe plastic deformation, such as a first ECAE process, in step
318. For example, the aluminum alloy billet may be passed through
an ECAE device to extrude the aluminum alloy as a billet having a
square or circular cross section. In some embodiments, a first ECAE
process may be carried out at elevated temperatures, for example,
temperatures below the homogenizing heat treatment but above the
aging temperature of a particular aluminum-zinc alloy. In some
embodiments, the first ECAE process may be carried out with the
aluminum alloy maintained at temperatures from about 100.degree. C.
to about 400.degree. C., or from about 200.degree. C. to about
300.degree. C. In some embodiments, the first ECAE process may be
carried out with the aluminum alloy maintained at temperatures
higher than 300.degree. C. Temperatures at this level may provide
certain advantages, such as healing of cast defects and
redistribution of precipitates, but may also lead to coarser grain
sizes and may be more difficult to implement in processing
conditions. In some embodiments, during the extrusion, the aluminum
alloy material being extruded and the extrusion die may be
maintained at the temperature that the extrusion process is being
carried out at to ensure a consistent temperature throughout the
aluminum alloy material. That is, the extrusion die may be heated
to prevent the aluminum alloy material from cooling during the
extrusion process. In some embodiments, the first ECAE process may
include one pass, two or more passes, or four or more extrusion
passes through the ECAE device.
[0056] In some embodiments, after severe plastic deformation, the
aluminum alloy may be subjected to a second solutionizing in step
320. A suitable second solutionizing temperature may be from about
400.degree. C. to about 550.degree. C., from about 420.degree. C.
to about 500.degree. C., or from about 450.degree. C. to about
480.degree. C. A second solutionizing may be carried out for a
suitable duration based on the size, such as the cross sectional
area, of the billet. For example, the second solutionizing may be
carried out for from about 30 minutes to about 8 hours, from 1 hour
to about 6 hours, or from about 2 hours to about 4 hours, depending
on the cross section of the billet. In some embodiments, the second
solutionizing may be from about 450.degree. C. to about 480.degree.
C. for up to 8 hours. The second solutionizing may be followed by
quenching.
[0057] In some embodiments, after the second solutionizing and/or
the quenching, a second aging heat treatment step may be carried
out in step 322. In one example, aging may be carried out using a
one-step heat treatment. In some embodiments, the one-step heat
treatment may be carried out at temperatures from about 80.degree.
C. to about 200.degree. C. for a duration of 0.25 hours to about 40
hours. In some embodiments, the second aging may be carried out
using a two-step heat treatment. In some embodiments, a first heat
treatment step may be carried out at temperatures from about
80.degree. C. to about 100.degree. C., from about 85.degree. C. to
about 95.degree. C., or from about 88.degree. C. to about
92.degree. C., for a duration of from 1 hour to about 50 hours,
from about 8 hours to about 40 hours, or from about 8 hours to
about 20 hours. In some embodiments, a second heat treatment step
may be carried out at temperatures from about 100.degree. C. to
about 170.degree. C., from about 100.degree. C. to about
160.degree. C., or from about 110.degree. C. to about 160.degree.
C. for a duration of from 20 hours to about 100 hours, from about
35 hours to about 60 hours, or from about 40 hours to about 45
hours. For example, the first aging step may be carried out at
about 90.degree. C. for about 8 hours and the second aging may be
carried out at about 115.degree. C. for about 40 hours or less. In
some embodiments, the second step may include temperatures and time
that are less than or equal to conditions suitable for aging an
aluminum alloy to peak hardness (i.e., peak hardness).
[0058] In some embodiments, after the second aging process, the
aluminum alloy may be subjected to a second severe plastic
deformation process, such as a second ECAE process, in step 324. In
some embodiments, the second ECAE process may be carried out at
lower temperatures than that used in the first ECAE process. For
example, the second ECAE process may be carried out at temperatures
greater than 0.degree. C. and less than 200.degree. C., or from
about 20.degree. C. to about 125.degree. C., or about room
temperature, for example from about 20.degree. C. to about
35.degree. C. In some embodiments, during the extrusion, the
aluminum alloy material being extruded and the extrusion die may be
maintained at the temperature that the extrusion process is being
carried out at to ensure a consistent temperature throughout the
aluminum alloy material. That is, the extrusion die may be heated
to prevent the aluminum alloy material from cooling during the
extrusion process. In some embodiments, the second ECAE process may
include one pass, two or more passes, or four or more extrusion
passes through the ECAE device.
[0059] Following the second severe plastic deformation, the
aluminum alloy may optionally undergo further plastic deformation
in step 326, such as rolling, to change the shape or size of the
aluminum alloy.
[0060] A method of forming a high strength aluminum alloy is shown
in FIG. 4. The method 400 includes forming a starting material in
step 410. Step 410 may be the same or similar to steps 110 or 210
described herein with respect to FIGS. 1 and 2. In some
embodiments, the starting material may be an aluminum material
billet formed using standard casting practices for an aluminum
material having magnesium, manganese, copper, and/or zinc. After
the starting material is cast, a homogenizing heat treatment may
optionally be employed in step 412. Step 412 may be the same or
similar to steps 112 or 212 described herein with respect to FIGS.
1 and 2.
[0061] After the homogenizing heat treatment, the aluminum material
may be subjected to a first solutionizing in step 414, to form an
aluminum alloy. A suitable first solutionizing temperature may be
from about 400.degree. C. to about 550.degree. C., from about
420.degree. C. to about 500.degree. C., or from about 450.degree.
C. to about 480.degree. C. A first solutionizing may be carried out
for a suitable duration based on the size, such as the cross
sectional area, of the billet. For example, the first solutionizing
may be carried out for from about 30 minutes to about 8 hours, from
1 hour to about 6 hours, or from about 2 hours to about 4 hours,
depending on the cross section of the billet. As an example, the
solutionizing may be carried out at 450.degree. C. to about
480.degree. C. for up to 8 hours. The solutionizing may be followed
by quenching, as shown in step 416.
[0062] In some embodiments, after the solutionizing and quenching,
the aluminum alloy billet may be subjected to a severe plastic
deformation process in step 418. In some embodiments, the severe
plastic deformation process may be ECAE. For example, the aluminum
alloy billet may be passed through an ECAE device having a square
or circular cross section. For example, an ECAE process may include
one or more ECAE passes. In some embodiments, the ECAE process may
be carried out with the aluminum alloy billet at temperatures
greater than 0.degree. C. and less than 160.degree. C., or from
about 20.degree. C. to about 125.degree. C., or about room
temperature, for example from about 20.degree. C. to about
35.degree. C. In some embodiments, during the ECAE, the aluminum
alloy billet being extruded and the extrusion die may be maintained
at the temperature that the extrusion process is being carried out
at to ensure a consistent temperature throughout the aluminum alloy
billet. That is, the extrusion die may be heated to prevent the
aluminum alloy from cooling during the extrusion process. In some
embodiments, the ECAE process may include one pass, two or more
passes, or four or more extrusion passes through the ECAE
device.
[0063] In some embodiments, after the aluminum alloy is subjected
to severe plastic deformation in step 418, aging may be carried out
in step 420. In one example, aging may be carried out using a
one-step heat treatment. In some embodiments, the one-step heat
treatment may be carried out at temperatures from about 80.degree.
C. to about 200.degree. C. for a duration of 0.25 hours to about 40
hours. In some embodiments, aging may be carried out using a
two-step heat treatment. In some embodiments, a first heat
treatment step may be carried out at temperatures from about
80.degree. C. to about 100.degree. C., from about 85.degree. C. to
about 95.degree. C., or from about 88.degree. C. to about
92.degree. C., for a duration of from 1 hour to about 50 hours,
from about 8 hours to about 40 hours, or from about 8 hours to
about 20 hours. In some embodiments, a second heat treatment step
may be carried out at temperatures from about 100.degree. C. to
about 170.degree. C., from about 100.degree. C. to about
160.degree. C., or from about 110.degree. C. to about 160.degree.
C. for a duration of from 20 hours to about 100 hours, from about
35 hours to about 60 hours, or from about 40 hours to about 45
hours. For example, the first aging step may be carried out at
about 90.degree. C. for about 8 hours and the second aging may be
carried out at about 115.degree. C. for about 40 hours or less. In
some embodiments, the second step may include temperatures and time
that are less than or equal to conditions suitable for aging an
aluminum alloy to peak hardness (i.e., peak hardness).
[0064] Following aging, the aluminum alloy may optionally undergo
further plastic deformation in step 422, such as rolling, to change
the shape or size of the aluminum alloy billet.
[0065] The methods shown in FIGS. 1 to 4 may be applied to aluminum
alloys having one or more additional components. For example, the
aluminum alloys may contain at least one of magnesium, manganese,
silicon, copper, and zinc. In some embodiments, the methods of
FIGS. 1 to 4 may be applied to aluminum alloys that are suitable
for use in portable electronic device cases due to high yield
strength (i.e., a yield strength from 300 MPa to 650 MPa), a low
weight density (i.e., about 2.8 g/cm.sup.3), and relative ease of
manufacturing to complex shapes.
[0066] In addition to the mechanical strength requirements, there
may also be a desire for the aluminum alloy to meet particular
cosmetic appearance requirements, such as a desired color or shade.
For example, in the portable electronics area, there may be a
desire for an outer alloy case to have a specific color or shade
without the use of paint or other coatings.
[0067] Therefore, the specific alloy used in various applications
may depend on the characteristics desired. For example, it has been
found that copper-containing aluminum alloys often display a
yellowish color after being anodized. In other examples where a
yellowish color is not desired, aluminum-zinc alloys may be used
due to a lower concentration of copper. To facilitate the desired
coloring characteristics in aluminum-zinc alloys, the concentration
of copper must be kept relatively low. For example, in some
embodiments, the concentration of copper may be less than about 0.5
wt. %. The weight percentages and weight ratio of zinc and
magnesium in the aluminum alloy may also be carefully controlled.
For example, zinc and magnesium may cause an increase in strength
by forming zinc-magnesium precipitates such as MgZn.sub.2 that
increase the strength of the aluminum alloy by precipitation
hardening. However, too high of a concentration of zinc and
magnesium may, in some embodiments, decrease the resistance of the
alloy to stress corrosion during specific manufacturing steps, such
as anodizing.
[0068] As-cast yield strengths for aluminum alloys containing and
Magnesium have been found to be between about 50 MPa and 450 MPa.
As-cast yield strengths for aluminum alloys containing copper have
been found to be between about 50 MPa and 400 MPa. As-cast yield
strengths for aluminum alloys containing magnesium and manganese
have been found to be between about 50 MPa and 350 Mpa. Using the
methods disclosed herein, it has been found possible to further
increase the strength of aluminum alloys, thus the resulting alloy
may be attractive for use in electronic device cases. For example,
using the methods described with reference to FIGS. 1 to 4, yield
strengths of 300 MPa to 650 MPa, 300 MPa to 500 MPa, 350 MPa to 600
MPa, and 420 MPa to 500 MPa have been achieved with aluminum alloys
containing at least one of magnesium, manganese, silicon, copper,
and zinc.
[0069] As described herein the mechanical properties of these
aluminum alloys can be improved by subjecting the alloy to severe
plastic deformation (SPD). As used herein, severe plastic
deformation includes extreme deformation of bulk pieces of
material. In some embodiments, ECAE provides suitable levels of
desired mechanical properties when applied to the materials
described herein.
[0070] ECAE is an extrusion technique which consists of two
channels of roughly equal cross-sections meeting at a certain angle
comprised practically between 90.degree. and 140.degree.. An
example ECAE schematic of an ECAE device 500 is shown in FIG. 5. As
shown in FIG. 5, an exemplary ECAE device 500 includes a mold
assembly 502 that defines a pair of intersecting channels 504 and
506. The intersecting channels 504 and 506 are identical or at
least substantially identical in cross-section, with the term
"substantially identical" indicating the channels are identical
within acceptable size tolerances of an ECAE apparatus. In
operation, a material 508 is extruded through channels 504 and 506.
Such extrusion results in plastic deformation of the material 508
by simple shear, layer after layer, in a thin zone located at the
crossing plane of the channels. In some embodiments, then channels
504 and 506 intersect at an angle of about 90.degree. to produce a
sufficient deformation (i.e., true shear strain). For example, a
tool angle of 90.degree. may result in true strain that is about
1.17 per each ECAE pass. However, it is to be understood that an
alternative tool angle, for example an angle greater than
90.degree., can be used (not shown).
[0071] ECAE provides high deformation per pass, and multiple passes
of ECAE can be used in combination to reach extreme levels of
deformation without changing the shape and volume of the billet
after each pass. Rotating or flipping the billet between passes
allows various strain paths to be achieved. This allows control
over the formation of the crystallographic texture of the alloy
grains and the shape of various structural features such as grains,
particles, phases, cast defects or precipitates. Grain refinement
is enabled with ECAE by controlling three main factors: (i) simple
shear, (ii) intense deformation and (iii) taking advantage of the
various strain paths that are possible using multiple passes of
ECAE. ECAE provides a scalable method, a uniform final product, and
the ability to form a monolithic piece of material as a final
product.
[0072] Because ECAE is a scalable process, large billet sections
and sizes can be processed via ECAE. ECAE also provides uniform
deformation throughout the entire billet cross-section because the
cross-section of the billet can be controlled during processing to
prevent changes in the shape or size of the cross-section. Also,
simple shear is active at the intersecting plane between the two
channels.
[0073] ECAE involves no intermediate bonding or cutting of the
material being deformed. Therefore, the billet does not have a
bonded interface within the body of the material. That is, the
produced material is a monolithic piece of material with no bonding
lines or interfaces where two or more pieces of previously separate
material have been joined together. Interfaces can be detrimental
because they are a preferred location for oxidation, which is often
detrimental. For example, bonding lines can be a source for
cracking or delamination. Furthermore, bonding lines or interfaces
are responsible for non-homogeneous grain size and precipitation
and result in anisotropy of properties.
[0074] In some instances, the aluminum alloy billet may crack
during ECAE. In certain aluminum alloys, a high diffusion rate of
constituents in the aluminum alloy may affect processing results.
In some embodiments, carrying out ECAE at increased temperatures
may avoid cracking of the aluminum alloy billet during ECAE. For
example, increasing the temperature that the aluminum alloy billet
is held at during extrusion may improve the workability of the
aluminum alloy and make the aluminum alloy billet easier to
extrude. However, increasing the temperature of the aluminum alloy
generally leads to undesirable grain growth, and in heat treatable
aluminum alloys, higher temperatures may affect the size and
distribution of precipitates. The altered precipitate size and
distribution may have a deleterious effect on the strength of the
aluminum alloy after processing. This may be the result when the
temperature and time used during ECAE are above the temperature and
time that correspond to peak hardness for the aluminum alloy being
processed, i.e. above the temperature and time conditions that
correspond to peak aging. Carrying out ECAE on an aluminum alloy
with the alloy at a temperature too close to the peak aging
temperature of the aluminum alloy may thus not be a suitable
technique for increasing the final strength of certain aluminum
alloys even though it may improve the billet surface conditions
(i.e. reduce the number of defects produced).
[0075] Processing an aluminum alloy via ECAE with the aluminum
alloy held at about room temperature after an initial solutionizing
and quenching may provide a suitable process for increasing the
strength of the aluminum alloy. This technique may be fairly
successful when a single ECAE pass is conducted almost immediately
(i.e, within one hour) after the initial solutionizing and
quenching treatments. However, this technique is not generally
successful for certain alloy compositions or when multiple passes
of ECAE are used. For example, for aluminum alloys having zinc and
magnesium in weight concentrations close to the upper level for the
Al 7xxx series (i.e., zinc and magnesium values of about 6.0 wt. %
and 4.0 wt. %, respectively), it has been found that a single pass
ECAE may not adequately increase the alloy strength or provide a
sufficiently fine submicron structure.
[0076] In some embodiments, it may be beneficial to perform aging
on an aluminum alloy before cold-working the alloy and if the alloy
has been subjected to an initial solutionizing and quenching. One
example of such an alloy is an aluminum alloy having magnesium and
zinc and a low concentration of Cu. Aging may be beneficial in
certain embodiments because the effects of cold working certain
aluminum alloys, such as, for example, those in the Al 7xxx series,
after solutionizing are the opposite of some other heat treatable
aluminum alloys, such as Al 2xxx series alloys. For example, cold
work may reduce the maximum attainable strength and toughness in
overaged tempers of aluminum alloys. The negative effect of cold
work before aging certain aluminum alloys is attributed to the
nucleation of coarse precipitates on dislocations. The approach of
using ECAE directly after solutionizing and quenching and before
aging may therefore require particular parameters. This effect is
shown further in the examples below.
[0077] Keeping the above considerations in mind, it has been found
that particular processing parameters may improve the outcome of
ECAE processes for aluminum alloys having magnesium, manganese,
silicon, copper, and/or zinc. These parameters are outlined further
in the examples below.
Process Parameters for ECAE
[0078] Pre-ECAE Heat Treatment
[0079] It has been discovered that producing stable Guinier Preston
(GP) zones and establishing thermally stable precipitates in an
aluminum alloy before performing ECAE may improve workability
which, for example, may lead to reduced billet cracking during
ECAE. In some embodiments, this is accomplished by performing heat
treatment such as artificial aging before carrying out ECAE. In
some embodiments, artificial aging incorporates a two-step heat
treatment which limits the effects of unstable precipitation at
room temperature (also referred to as natural aging). Controlling
precipitation is important for ECAE processing of aluminum alloys
having magnesium and zinc alloys because these alloys have a fairly
unstable sequence of precipitation, and high deformation during
ECAE makes the alloy even more unstable unless the processing
conditions and order of heat treatment are carefully
controlled.
[0080] The effects of heat and time on precipitation in an aluminum
alloy having magnesium and zinc have been evaluated. The sequence
of precipitation in an aluminum alloy having magnesium and zinc is
complex and dependent on temperature and time. First, using high
temperature heat treatment such as solutionizing, solutes such as
magnesium and/or zinc are put in solution by distributing
throughout the aluminum alloy. The high temperature heat treatment
is often followed by rapid cooling in water or oil, also known as
quenching, to hold the solutes in solution. At relatively low
temperatures for long time periods and during initial periods of
artificial aging at moderately elevated temperatures, the principal
change is a redistribution of solute atoms within the solid
solution lattice to form clusters termed Guinier Preston (GP) zones
that are considerably enriched in solute. This local segregation of
solute atoms produces a distortion of the alloy lattice. The
strengthening effect of the zones is a result of the additional
interference with the motion of dislocations when they cut the GP
zones. The progressive strength increase with aging time at room
temperature (defined as natural aging) has been attributed to an
increase in the size of the GP zones.
[0081] In most systems as aging time or temperature are increased,
the GP zones are either converted into or replaced by particles
having a crystal structure distinct from that of the solid solution
and also different from the structure of the equilibrium phase.
Those are referred as "transition" precipitates. In many alloys,
these precipitates have a specific crystallographic orientation
relationship with the solid solution, such that the two phases
remain coherent on certain planes by adaptation of the matrix
through local elastic strain. Strength continues to increase as the
size and number of these "transition" precipitates increase, as
long as the dislocations continue to cut the precipitates. Further
progress of the precipitation reaction produces growth of
"transition" phase particles, with an accompanying increase in
coherency strains until the strength of interfacial bond is
exceeded and coherency disappears. This usually coincides with the
change in the structure of the precipitate from "transition" to
"equilibrium" form and corresponds to peak aging, which is the
optimum condition to obtain maximum strength. With loss of
coherency, strengthening effects are caused by the stress required
to cause dislocations to loop around rather than to cut
precipitates. Strength progressively decreases with growth of
equilibrium phase particles and an increase in inter-particle
spacing. This last phase corresponds to overaging and in some
embodiments is not suitable when the main goal is to achieve
maximum strength.
[0082] In an aluminum alloy having magnesium and zinc, the GP zones
are very small in size (i.e. less than 10 nm) and quite unstable at
room temperature. As shown in the examples provided herein, a high
level of hardening occurs after the alloy has been held at room
temperature for a few hours after quenching, a phenomenon called
natural aging. One reason for this hardening in an aluminum alloy
having magnesium and zinc is the fast diffusion rate of zinc, which
is the element with the highest diffusion rate in aluminum. Another
factor is the presence of magnesium which strongly influences the
retention of a high concentration of non-equilibrium vacancies
after quenching. magnesium has a large atomic diameter that makes
the formation of magnesium-vacancy complexes and their retention
during quenching easier. These vacancies are available for zinc to
diffuse into and form GP zones around the magnesium atoms. Extended
aging time and temperatures above room temperature (i.e. artificial
aging) transform the GP zones into the transition precipitate
called .eta.' or M', the precursor of the equilibrium MgZn.sub.2
phases termed .eta. or M. For aluminum alloys having a higher
magnesium content (e.g. greater than 2.0 wt. %), the precipitation
sequence includes the GP zone transforming into a transition
precipitate called T' that becomes the equilibrium
Mg.sub.3Zn.sub.3Al.sub.2 precipitate called T at extended aging
time and temperature. The precipitation sequence in Al 7xxx can be
summarized in the flow schematic shown in FIG. 6.
[0083] As shown in the flow schematic in FIG. 6, the GP zone
nucleates homogeneously within the lattice and the various
precipitates develop sequentially. However, the presence of grain
boundaries, subgrain boundaries, dislocations and lattice
distortions alters the free energy of zone and precipitate
formation and significant heterogeneous nucleation may occur. This
has two consequences in an aluminum alloy having magnesium and
zinc. First, there is the potential for creating a non-homogeneous
distribution of GP zones and precipitates, either of which may
become a source for defects during cold or hot working. Second,
heterogeneously nucleated precipitates at boundaries or
dislocations are usually larger and do not contribute as much to
the overall strength and therefore potentially decrease the maximum
attainable strength. These effects may be enhanced when extreme
levels of plastic deformation are introduced, for example during
ECAE, directly after the solutionizing and quenching steps for at
least the following reasons.
[0084] First, ECAE introduces a high level of subgrain, grain
boundaries and dislocations that may enhance heterogeneous
nucleation and precipitation and therefore lead to a non-homogenous
distribution of precipitates. Second, GP zones or precipitates may
decorate dislocations and inhibit their movement which leads to a
reduction in local ductility. Third, even at room temperature
processing, there is some level of adiabatic heating occurring
during ECAE that provides energy for faster nucleation and
precipitation. These interactions may happen dynamically during
each ECAE pass. This leads to potentially detrimental consequences
for the processing of a solutionized and quenched aluminum alloy
having magnesium and zinc during ECAE.
[0085] Some of the potentially detrimental consequences are as
follows. A propensity for surface cracking of the billet due to a
loss in local ductility and heterogeneous precipitate distribution.
This effect is most severe at the top billet surface. Limitation of
the number of ECAE passes that can be used. As the number of passes
increases the effects become more severe and cracking becomes more
likely. A decrease in the maximum achievable strength during ECAE,
partly due to heterogeneous nucleation effects and partly due to
limitation of the number of ECAE passes, which affects the ultimate
level of grain size refinement. An additional complication arises
with the processing of solutionized and quenched aluminum-zinc
alloys, such as Al 7xxx series alloys, due to the fast kinetics of
precipitation even at room temperature (i.e. during natural aging).
It has been found that the time between the solutionizing and
quenching steps and ECAE may be important to control. In some
embodiments, ECAE may be conducted relatively soon after the
quenching step, for example, within one hour.
[0086] Stable precipitates may be defined as precipitates that are
thermally stable in an aluminum alloy even when the aluminum alloy
is at a temperature and time that is substantially close to
artificial peak aging for its given composition. In particular,
stable precipitates are precipitates that will not change during
natural aging at room temperature. Note that these precipitates are
not GP zones but instead include transition and/or equilibrium
precipitates (e.g. .eta.' or M' or T' for aluminum-zinc alloys).
The goal of heating (i.e. artificial aging) is to eliminate most of
the unstable GP zones, which may lead to billet cracking during
ECAE, and replace these with stable precipitates, which may be
stable transition and equilibrium precipitates. It may also be
suitable to avoid heating the aluminum alloy to conditions that are
above peak aging (i.e. overaging conditions), which may produce
mostly equilibrium precipitates that have grown and become too
large, which may decrease the aluminum alloy final strength.
[0087] These limitations may be avoided by transforming most of the
unstable GP zones into stable transition and/or equilibrium
precipitates before performing the first ECAE pass. This may be
accomplished, for example, by conducting a low temperature heat
treatment (artificial aging) after or immediately after the
solutionizing and quenching step, but before the ECAE process. In
some embodiments, this may lead to most of the precipitation
sequence occurring homogenously, contributing to a higher
attainable strength and better stability of precipitates for ECAE
processing. Furthermore, the heat treatment may consist of a
two-step procedure that includes a first step that includes holding
the material at a low temperature of 80.degree. C. to 100.degree.
C. for less than or about 40 hours, and a second step that includes
holding the material at a temperature and time that are less or
equal than the peak aging conditions for the given an aluminum
alloy having magnesium and zinc, for example holding the material
between 100.degree. C. and 150.degree. C. for about 80 hours or
less. The first low temperature heat treatment step provides a
distribution of GP zones that is stable when the temperature is
raised during the second heat treatment step. The second heat
treatment step achieved the desired final distribution of stable
transition and equilibrium precipitates.
[0088] In some embodiments, it may be advantageous to increase the
uniformity and achieve a predetermined grain size of the alloy
microstructure before conducting the final ECAE process at low
temperature. In some embodiments, this may improve the mechanical
properties and workability of the alloy material during ECAE as
demonstrated by a reduced amount of cracking.
[0089] Aluminum alloys having magnesium and zinc are characterized
by heterogeneous microstructures with large grain sizes and a large
amount of macro and micro segregations. For example, the initial
cast microstructure may have a dendritic structure with solute
content increasing progressively from center to edge with an
interdendritic distribution of second phase particles or eutectic
phases. Certain homogenizing heat treatments may be performed
before the solutionizing and quenching steps in order to improve
structural uniformity and subsequent workability of billets. Cold
working (such as stretching) or hot working is also often used to
provide a specific billet shape or to stress relief or straighten
the product. For plate applications such as forming a phone case,
rolling may be used and may lead to anisotropy of the
microstructure and properties in the final product even after heat
treatments such as solutionizing, quenching and peak aging.
Typically, grains are elongated along the rolling direction but are
flattened along the thickness as well as the direction transverse
to the rolling direction. This anisotropy is also reflected in the
precipitate distribution, particularly along the grain
boundaries.
[0090] In some embodiments, the microstructure of an aluminum alloy
having magnesium and zinc with any temper, such as for example T651
may be broken down, refined, and made more uniform by applying a
processing sequence that includes at least a single ECAE pass at
elevated temperatures, such as below 450.degree. C. This step is
may be followed by solutionizing and quenching. In another
embodiment, a billet made of the aluminum alloy having magnesium
and zinc may be subjected to a first solutionizing and quenching
step, followed by a single pass or multi-pass ECAE at moderately
elevated temperatures between 150.degree. C. and 250.degree. C.,
followed by a second solutionizing and quenching step. After either
of the above mentioned thermo-mechanical routes, the aluminum alloy
can be further subjected to ECAE at a low temperature, either
before or after artificial aging. In particular, it has been
discovered that the initial ECAE process at elevated temperatures
helps reduce cracking during a subsequent ECAE process at low
temperatures of a solutionized and quenched aluminum alloy having
magnesium and zinc. This result is described further in the
examples below.
[0091] In some embodiments, ECAE may be used to impart severe
plastic deformation and increase the strength of aluminum-zinc
alloys. In some embodiments, ECAE may be performed after
solutionizing, quenching and artificial aging is carried out. As
described above, an initial ECAE process carried out while the
material is at an elevated temperature may create a finer, more
uniform and more isotropic initial microstructure before the second
or final ECAE process at low temperature.
[0092] There are two main mechanisms for strengthening with ECAE.
The first is refinement of structural units, such as the material
cells, sub-grains and grains at the submicron or nanograined
levels. This is also referred as grain size or Hall Petch
strengthening and can be quantified using Equation 1.
.sigma. y = .sigma. 0 + k y d Equation 1 ##EQU00001##
Where .sigma..sub.y is the yield stress, .sigma..sub.0 is a
material constant for the starting stress or dislocation movement
(or the resistance of the lattice to dislocation motion), k.sub.y
is the strengthening coefficient (a constant that is specific to
each material), and d is the average grain diameter. Based on this
equation, strengthening becomes particularly effective when d is
less than 1 micron. The second mechanism for strengthening with
ECAE is dislocation hardening, which is the multiplication of
dislocations within the cells, subgrains, or grains of the material
due to high straining during the ECAE process. These two
strengthening mechanisms are activated by ECAE and it has been
discovered that certain ECAE parameters can be controlled to
produce particular final strengths in the aluminum alloy,
particularly when extruding aluminum-zinc alloys that have
previously been subjected to solutionizing and quenching.
[0093] First, the temperatures and time used for ECAE may be less
than those corresponding to the conditions of peak aging for the
given aluminum alloy having magnesium and zinc. This involves
controlling both the die temperature during ECAE and potentially
employing an intermediate heat treatment in between each ECAE pass,
when an ECAE process including multiple passes is performed, to
maintain the material being extruded at a desired temperature. For
example, the material being extruded may be kept maintained at a
temperature of about 200.degree. C. for about 2 hours between each
extrusion pass. In some embodiments, the material being extruded
may be kept maintained at temperature of about 120.degree. C. for
about 2 hours in between each extrusion pass.
[0094] Second, in some embodiments, it may be advantageous to
maintain the temperature of the material being extruded at as low a
temperature as possible during ECAE to get the highest strength.
For example, the material being extruded may be maintained at about
room temperature. This may result in an increased number of
dislocations formed and produce a more efficient grain
refinement.
[0095] Third, it may be advantageous to perform multiple ECAE
passes. For example, in some embodiments, two or more passes may be
used during an ECAE process. In some embodiments, three or more, or
four or more passes may be used. In some embodiments, a high number
of ECAE passes provides a more uniform and refined microstructure
with more equiaxed high angle boundaries and dislocations that
result in superior strength and ductility of the extruded
material.
[0096] In some embodiments, ECAE affects the grain refinement and
precipitation in at least the following ways. In some embodiments,
ECAE has been found to produce faster precipitation during
extrusion, due to the increased volume of grain boundaries and
higher mechanical energy stored in sub-micron ECAE processed
materials. Additionally, diffusion processes associated with
precipitate nucleation and growth are enhanced. This means that
some of the remaining GP zones or transition precipitates can be
transformed dynamically into equilibrium precipitates during ECAE.
In some embodiments, ECAE has been found to produce more uniform
and finer precipitates. For example, a more uniform distribution of
very fine precipitates can be achieved in ECAE submicron structures
because of the high angle boundaries. Precipitates can contribute
to the final strength of the aluminum alloy by decorating and
pinning dislocations and grain boundaries. Finer and more uniform
precipitates may lead to an overall increase in the extruded
aluminum alloy final strength.
[0097] There are additional parameters of the ECAE process that may
be controlled to further increase success. For example, the
extrusion speed may be controlled to avoid forming cracks in the
material being extruded. Second, suitable die designs and billet
shapes can also assist in reducing crack formation in the
material.
[0098] In some embodiments, additional rolling and/or forging may
be used after the aluminum alloy has undergone ECAE to get the
aluminum alloy closer to the final billet shape before machining
the aluminum alloy into its final production shape. In some
embodiments, the additional rolling or forging steps can add
further strength by introducing more dislocations in the
micro-structure of the alloy material.
[0099] In the examples described below, Brinell hardness was used
as an initial test to evaluate the mechanical properties of
aluminum alloys. For the examples included below, a Brinell
hardness tester (available from Instron.RTM., located in Norwood,
Mass.) was used. The tester applies a predetermined load (500 kgf)
to a carbide ball of fixed diameter (10 mm), which is held for a
predetermined period of time (10-15 seconds) per procedure, as
described in ASTM standard. Measuring Brinell hardness is a
relatively straightforward testing method and is faster than
tensile testing. It can be used to form an initial evaluation for
identifying suitable materials that can then be separated for
further testing. The hardness of a material is its resistance to
surface indentation under standard test conditions. It is a measure
of the material's resistance to localized plastic deformation.
Pressing a hardness indentor into the material involves plastic
deformation (movement) of the material at the location where the
indentor is impressed. The plastic deformation of the material is a
result of the amount of force applied to the indentor exceeding the
strength of the material being tested. Therefore, the less the
material is plastically deformed under the hardness test indentor,
the higher the strength of the material. At the same time, less
plastic deformation results in a shallower hardness impression; so
the resultant hardness number is higher. This provides an overall
relationship, where the higher a material's hardness, the higher
the expected strength. That is, both hardness and yield strength
are indicators of a metal's resistance to plastic deformation.
Consequently, they are roughly proportional.
[0100] Tensile strength is usually characterized by two parameters:
yield strength (YS) and ultimate tensile strength (UTS). Ultimate
tensile strength is the maximum measured strength during a tensile
test and it occurs at a well-defined point. Yield strength is the
amount of stress at which plastic deformation becomes noticeable
and significant under tensile testing. Because there is usually no
definite point on an engineering stress-strain curve where elastic
strain ends and plastic strain begins, the yield strength is chosen
to be that strength where a definite amount of plastic strain has
occurred. For general engineering structural design, the yield
strength is chosen when 0.2% plastic strain has taken place. The
0.2% yield strength or the 0.2% offset yield strength is calculated
at 0.2% offset from the original cross-sectional area of the
sample. The equation that may be used is s=P/A, where s is the
yield stress or yield strength, P is the load and A is the area
over which the load is applied.
[0101] Note that yield strength is more sensitive than ultimate
tensile strength due to other microstructural factors such as grain
and phase size and distribution. However, it is possible to measure
and empirically chart the relationship between yield strength and
Brinell hardness for specific materials, and then use the resulting
chart to provide an initial evaluation of the results of a method.
Such a relationship was evaluated for the materials and examples
below. The data was graphed and the results are shown in FIG. 7. As
shown in FIG. 7, it was determined that for the materials
evaluated, a Brinell hardness above about 111 HB corresponds to YS
above 350 MPa and a Brinell hardness above about 122 HB corresponds
to YS above 400 MPa.
EXAMPLES
[0102] The following non-limiting examples illustrate various
features and characteristics of the present invention, which is not
to be construed as limited thereto.
Example 1: Natural Aging in an Aluminum Alloy Having Magnesium and
Zinc
[0103] The effect of natural aging was evaluated in an aluminum
alloy having aluminum as a primary component and magnesium and zinc
as secondary components. For this initial assay, Al 7020 was chosen
because of its low Cu weight percentage and the zinc to magnesium
ratio from about 3:1 to 4:1. As discussed above, these factors
affect the cosmetic appearance for applications such as device
casings. The composition of the sample alloy is displayed in Table
1 with a balance of aluminum. It should be noted that zinc (at 4.8
wt. %) and magnesium (at 1.3 wt. %) are the two alloying elements
present in the highest concentrations and the Cu content is low (at
0.13 wt. %).
TABLE-US-00001 TABLE 1 Composition of Al 7020 Starting Material
(Weight Percentage) Si Fe Cu Mn Mg Cr Zn Zr Ti + Zr Ag 0.1 0.28
0.13 0.25 1.3 0.12 4.8 0.13 0.16 0
[0104] The as-received Al 7020 material was subjected to a
solutionizing heat treatment by holding the material at 450.degree.
C. for two hours and then was quenched in cold water. The sample
material was then kept at room temperature (25.degree. C.) for
several days. The Brinell hardness was used to evaluate the
stability of the mechanical properties of the sample material after
being stored at room temperature for a number of days (so called
natural aging). The hardness data is presented in FIG. 8. As shown
in FIG. 8, after only one day at room temperature there was already
a substantial increase in hardness from 60.5 HB to about 76.8 HB;
about a 30% increase. After about 5 days at room temperature, the
hardness reached 96.3 HB and remained fairly stable, showing
minimal changes when measured over 20 days. The rate of increase in
hardness indicates an unstable supersaturated solution and
precipitation sequence for Al 7020. This unstable supersaturated
solution and precipitation sequence is characteristic of many Al
7xxx series alloys.
Example 2: Example of Anisotropy of Microstructure in the Initial
Alloy Material
[0105] The aluminum alloy formed in Example 1 was subjected to hot
rolling to form the alloy material into a billet followed by
thermo-mechanical processing to the T651 temper that includes
solutionizing, quenching, stress relief by stretching to an
increase of 2.2% greater than the starting length and artificial
peak aging. The measured mechanical properties of the resulting
material are listed in Table 2. The yield strength, ultimate
tensile strength and Brinell hardness of the Al 7020 material are
347.8 MPa, 396.5 MPa and 108 HB respectively. The tensile testing
was conducted with the example material at room temperature using
round tension bars with threaded ends. The diameter of the tension
bars were 0.250 inch and the gage was length 1.000 inch. The
geometry of round tension test specimens is described in ASTM
Standard E8.
TABLE-US-00002 TABLE 2 Mechanical Properties of Al 7020 Material in
Example 2 Percent Temper YS (MPa) UTS (MPa) Elongation (%) Hardness
(HB) T651 347.8 396.5 14.4 108
[0106] FIG. 9 illustrates the planes of an example billet 602 to
show the orientation of a top face 604 of the billet 602. The arrow
606 shows the direction of rolling and stretching. The first side
face 608 is in the plane parallel to the rolling direction and
perpendicular to the top face 604. The second side face 610 is in
the plane perpendicular to the rolling direction of arrow 606 and
the top face 604. Arrow 612 shows the direction normal to the plane
of the first side face, and arrow 614 shows the direction normal to
the plane of the second side face 610. An optical microscopy image
of the grain structure of the Al 7020 material from Example 2 is
shown in FIGS. 10A to 10C. FIGS. 10A to 10C show the microstructure
of Al 7020 with a T651 temper across the three planes shown in FIG.
9. Optical microscopy was used for grain size analysis. FIG. 10A is
an optical microscopy image of the top face 604 shown in FIG. 9 at
.times.100 magnification. FIG. 10B is an optical microscopy image
of the first side face 608 shown in FIG. 9 at .times.100
magnification. FIG. 10C is an optical microscopy image of the
second side face 610 shown in FIG. 9 at .times.100
magnification.
[0107] As shown in FIGS. 10A to 10C, an anisotropic fibrous
microstructure consisting of elongated grains is detected. The
original grains are compressed through the billet thickness, which
is the direction normal to the rolling direction, and elongated
along the rolling direction during thermo-mechanical processing.
The grain sizes as measured across the top face are large and
non-uniform around 400 to 600 .mu.m in diameter with a large aspect
ratio of average grain length to thickness ranging between 7:1 to
10:1. The grain boundaries are difficult to resolve along the two
other faces shown in FIGS. 10B and 10C, but clearly demonstrate
heavy elongation and compression as exemplified by thin parallel
bands. This type of large and non-uniform microstructure is
characteristic in aluminum alloys having magnesium and zinc and
having a standard temper such as T651.
Example 3: ECAE of as Solutionized and Quenched Al 7020
Material
[0108] A billet of Al 7020 material with the same composition and
T651 temper as in Example 2 was subjected to solutionizing at a
temperature of 450.degree. C. for 2 hours and immediately quenched
in cold water. This process was carried out to retain the maximum
number of elements added as solutes, such as zinc and magnesium, in
solid solution in the aluminum material matrix. It is believed that
this step also dissolved the (ZnMg) precipitates present in the
aluminum material back into the solid solution. The resulting
microstructure of the Al 7020 material was very similar to the one
described in Example 2 for aluminum material that had the temper
T651, and consisted of large elongated grains parallel to the
initial rolling direction. The only difference is the absence of
fine soluble precipitates. The soluble precipitates are not visible
by optical microscopy because they are below the resolution limit
of 1 micron; only the large (i.e. greater than 1 micron in
diameter) non soluble precipitates are visible. Thus, the results
of Example 3 illustrate that the after solutionizing and quenching
steps the grain size and anisotropy of the initial T651
microstructure remained unchanged.
[0109] The Al 7020 material was then shaped into three billets,
i.e. bars, with a square cross-section and a length that is greater
than the cross-section, and ECAE was then performed on the billets.
The first pass was performed within 30 minutes after the
solutionizing and quenching to minimize the effect of natural
aging. Furthermore, ECAE was conducted at room temperature to limit
the temperature effects on precipitation. FIG. 11 shows a
photograph of a first billet 620 of Al 7020 after having undergone
one pass, a second billet 622 having undergone two passes, and a
third billet 624 having undergone three passes. The ECAE process
was successful for the first billet 620 after one pass. That is, as
shown in FIG. 11, the billet did not crack after one ECAE pass.
However, heavy localized cracking at the top face of the billet
occurred in the second billet 622 that was subjected to two passes.
FIG. 11 shows the cracks 628 in the second billet 622 that
developed after two passes. As also shown in FIG. 11, the third
billet 624, which was subjected to three passes, also exhibited
cracks 628. As shown in FIG. 11, the cracks intensified to such an
extent that one macro-crack 630 ran through the entire thickness of
the third billet 624 and split the billet into two pieces.
[0110] The three sample billets were further submitted to a
two-step peak aging treatment consisting of a first heat treatment
step with the samples held at 90.degree. C. for 8 hours followed by
a second heat treatment step with the samples held at 115.degree.
C. for 40 hours. Table 3 displays Brinell hardness data as well as
tensile data for the first billet 620. The second billet 622 and
the third billet 624 had too deep of cracking and the machine
tensile test could not be conducted for these samples. All
measurements were conducted with the sample material at room
temperature.
TABLE-US-00003 TABLE 3 Test Results After Various Numbers of ECAE
Passes and aging treatment Number Brinell of ECAE Hardness YS UTS
Surface Sample passes (HB) (MPa) (MPa) condition Billet 620 1 127
382 404 good Billet 622 2 132 n/a n/a crack at top Billet 624 3 138
n/a n/a crack through sample
[0111] As shown in Table 3, a steady increase in hardness from
about 127 to 138 was recorded with an increasing number of ECAE
passes. The material hardness after each pass was higher than the
hardness value for material having only the T651 temper condition,
as shown in Example 2. Yield strength data for the first sample
after one pass also showed an increase in yield strength when
compared to material having only the T651 temper. For example, the
yield strength increased from 347.8 MPa to 382 MPa.
[0112] This example demonstrated the ability of ECAE to improve
strength in aluminum-zinc alloys as well as certain limitations due
to billet cracking during ECAE processing. The next examples
illustrate techniques that can be used to improve the overall
processing when applying ECAE to Al alloys at low temperatures and,
can increase the Al alloy material strength without cracking the
material.
Example 4: Multi-Step ECAE of as-Solutionized and Quenched
Samples--Effect of Initial Grain Size and Anisotropy
[0113] To evaluate the potential effect of the initial
microstructure on the processing results, Al7020 material with the
T651 temper of Examples 1 and 2 was submitted to a more complex
thermo-mechanical processing route than in Example 3. In this
Example, ECAE was performed in two steps, one before and one after
a solutionizing and quenching step with each step including an ECAE
cycle having multiple passes. The first ECAE cycle was aimed at
refining and homogenizing the microstructure before and after the
solutionizing and quenching step, whereas the second ECAE cycle was
conducted at a low temperature to improve the final strength as in
Example 3.
[0114] The following process parameters were used for the first
ECAE cycle. Four ECAE passes were used, with a 90 degree rotation
of the billet between each pass to improve the uniformity of
deformation and as a result the uniformity of microstructure. This
is accomplished by activating simple shear along a three
dimensional network of active shear planes during multi-pass ECAE.
The Al 7020 material that formed the billet was maintained at a
processing temperature of 175.degree. C. throughout the ECAE. This
temperature was chosen because it is low enough to give submicron
grains after ECAE, but is above the peak aging temperature and
therefore provides an overall lower strength and higher ductility,
which is favorable for the ECAE process. The Al 7020 material
billets did not suffer any cracking during this first ECAE
cycle.
[0115] After the first ECAE process, solutionizing and quenching
was carried out using the same conditions as described in Example 3
(i.e. the billet was held at 450.degree. C. for 2 hours followed by
immediate quenching in cold water). The microstructure of the
resulting Al 7020 material was analyzed by optical microscopy and
is shown in FIGS. 12A and 12B. FIG. 12A is the resulting material
at .times.100 magnification and FIG. 12B is the same material at
.times.400 magnification. As shown in FIGS. 12A and 12B, the
resulting material consists of fine isotropic grain sizes of 10-15
.mu.m throughout the material in all directions. This
microstructure was formed during the high temperature solution heat
treatment by recrystallization and growth of the submicron grains
that were initially formed by the ECAE. As shown in FIGS. 12A and
12B, the resulting material contains grains that are much finer and
the material possesses a better isotropy in all directions than the
solutionized and quenched initial microstructure of Example 3.
[0116] After the solutionizing and quenching, the samples were
again deformed via another process of ECAE, this time at a lower
temperature than used in the first ECAE process. For comparison,
the same process parameters used in Example 3 were used in this
second ECAE process. The second ECAE process was performed at room
temperature with two passes as soon as possible after the quench
step (i.e. within 30 minutes of quenching). The overall ECAE
processing was discovered to have improved results using the second
ECAE process as the lower temperature ECAE process. In particular,
unlike in Example 3, the billet in Example 4 did not crack after
two ECAE passes conducted with the billet material at lower
temperature. Table 4 shows tensile data collected after the sample
material had been subjected to two ECAE passes.
TABLE-US-00004 TABLE 4 Results of Al 7020 Material After Two ECAE
Cycles, With Second ECAE Cycle Having Two Passes Number of Brinell
YS UTS Surface ECAE passes Hardness (HB) (MPa) (MPa) condition 2
133 416 440 good
[0117] As shown in Table 4, the resulting material also had a
substantial improvement over material that has only had a T651
temper condition. That is, the Al 7020 material that underwent the
two step ECAE process had a yield strength of 416 MPa and an
ultimate tensile strength of 440 MPa.
[0118] Example 4 demonstrates that the grain size and isotropy of
the material before ECAE can affect the processing results and
ultimate attainable strength. ECAE at relatively moderate
temperatures (around 175.degree. C.) may be an effective method to
break, refine and uniformize the structure of Al 7xxx alloy
material and make the material better for further processing. Other
important factors for processing Al 7xxx with ECAE are the
stabilization of GP zone and precipitates prior to ECAE processing.
This is described further in the following examples.
Example 5: ECAE of Artificially Aged Al 7020 Samples Having Only
T651 Temper
[0119] In this Example, the Al 7020 alloy material of Example 1 was
submitted to an initial processing that included solutionizing,
quenching, stress relief by stretching to 2.2% greater than the
starting length, and artificial peak aging. Artificial peak aging
of this Al 7020 material consisted of a two-step procedure that
included a first heat treatment at 90.degree. C. for 8 hours
followed by a second heat treatment at 115.degree. C. for 40 hours,
which is similar to a T651 temper for this material. Peak aging was
started within a few hours after the quenching step. The Brinell
hardness of the resulting material was measured at 108 HB and the
yield strength was 347 MPa (i.e. similar to the material in Example
2). The first heat treatment step is used to stabilize the
distribution of GP zones before the second heat treatment and to
inhibit the influence of natural aging. This procedure was found to
encourage homogeneous precipitation and optimize strengthening from
precipitation.
[0120] Low temperature ECAE was then conducted after the artificial
peak aging. Two ECAE process parameters were evaluated. First, the
number of ECAE passes was varied. One, two, three, and four passes
were tested. For all ECAE cycles, the material billets were rotated
by 90 degrees between each pass. Second, the effect of material
temperature during ECAE was varied. The ECAE die and billet
temperatures evaluated were 25.degree. C., 110.degree. C.,
130.degree. C., 150.degree. C., 175.degree. C., 200.degree. C., and
250.degree. C. Both Brinell hardness and tensile data were taken
with the sample material at room temperature after certain
processing conditions in order to evaluate the effects on
strengthening. Optical microscopy was used to create images of
samples of the resulting material and is shown in FIGS. 13A and
13B.
[0121] As an initial observation, no cracking was observed in the
material of any of the sample billets, even for billets that
underwent ECAE processing at room temperature. This example
contrasts with Example 3, where ECAE was conducted right after the
unstable solutionized and quenched state and cracking occurred in
the second and third samples. This result shows the effect of the
stabilization of GP zones and precipitates on the processing of Al
7xxx series alloy material. This phenomenon is specific to Al 7xxx
alloys due to the nature and fast diffusion of the two main
constitutive elements, zinc and magnesium.
[0122] FIGS. 13A and 13B show typical microstructures of the Al
7020 alloy material after undergoing ECAE as analyzed by optical
microscopy. FIG. 13A shows the material at room temperature after
being subjected to four ECAE passes at room temperature and after
being held at about 250.degree. C. for one hour. FIG. 13B shows the
material at room temperature after being subjected to four ECAE
passes at room temperature and after being held at 325.degree. C.
for one hour. From these images, it was discovered that the
submicron grain size was stable up to about 250.degree. C. After
being held at about 250.degree. C. for one hour, the average
measured grain size was submicron (less than 1 .mu.m in diameter).
The measured average grain size was from about 0.1 .mu.m to about
0.8 .mu.m in diameter. After being held at about 300.degree. C. to
about 325.degree. C. for the same amount of time, full
recrystallization occurred and the submicron grain size grew into a
uniform and fine recrystallized microstructure with grain sizes of
about 5-10 .mu.m. The grain size increased slightly, up to about
10-15 .mu.m, after heat treatment at temperatures as of about
450.degree. C., which is in the typical temperature range for
solutionizing (see Example 4). This structural study shows that
hardening due to grain size refinement by ECAE can be most
effective when ECAE is performed at temperatures below about
250.degree. C. to 275.degree. C., i.e. when the grain size is
submicron.
[0123] Table 5 contains the measured results of Brinnell hardness
and tensile strength as a result of varying the temperature of the
Al 7020 alloy material during ECAE.
TABLE-US-00005 TABLE 5 Effect of Al 7020 Material Temperature
During ECAE on Final Yield Strength UTS YS % UTS % Process YS (MPa)
(MPa) increase increase T651 temper 347.8 396.5 4 ECAE pass at
125.degree. C. 417 474 19.9 19.5 4 ECAE pass at 100.degree. C. 447
483 28.5 21.8 4 ECAE pass at 25.degree. C. 488 493 40.3 24.3
[0124] FIGS. 14 and 15 show the measured results of the material
formed in Example 5 as graphs showing the effect of ECAE
temperature on the final Brinell hardness and tensile strength. All
samples shown in FIGS. 14 and 15 were subjected to a total of 4
ECAE passes with intermediate annealing at a given temperature for
short periods lasting between 30 minutes and one hour. As shown in
FIG. 14, hardness was greater than material having only the T651
temper when the material underwent ECAE while the material
temperature during extrusion was less or equal to about 150.degree.
C. Furthermore, strength and hardness was higher as the billet
material processing temperature was reduced, with the greatest
increase shown from 150.degree. C. to about 110.degree. C. The
sample that had the greatest final strength was the sample that
underwent ECAE with the billet material at room temperature. As
shown in FIG. 15 and Table 5, this sample had a resulting Brinell
hardness around 140 HB and YS and UTS equal to 488 MPa and 493 MPa
respectively. This shows a nearly 40% increase in yield strength
above material having only a standard T651 temper. Even at
110.degree. C., which is near the peak aging temperature for this
material, YS and UTS are respectively 447 MPa and 483 MPa. Some of
these results can be explained as follows.
[0125] Holding the Al 7020 alloy material at temperatures from
about 115.degree. C. to 150.degree. C. for a few hours corresponds
to an overaging treatment in Al 7xxx alloys when precipitates have
grown larger than during conditions of peak aging, which gives peak
strength. At temperatures of about 115.degree. C. to about
150.degree. C., the ECAE extruded material is still stronger than
material having only undergone the T651 temper because the strength
loss due to overaging is compensated by grain size hardening due to
ECAE. The strength loss due to overaging is rapid, which explains
the lowered final strength when the material is held at
temperatures increasing from 110.degree. C. to about 150.degree.
C., as shown in FIG. 14. Above about 200.degree. C. to about
225.degree. C., strength loss is not only caused by overaging but
also by the growth of the submicron grain size. The effect is also
observed at temperatures above 250.degree. C. where
recrystallization starts to occur.
[0126] Temperatures around 110.degree. C. to about 115.degree. C.
are near the conditions for peak aging of Al 7xxx (i.e. the T651
temper) and the increased strength above the strength of material
having only a T651 temper is due mainly to grain size and
dislocation hardening by ECAE. When the Al 7020 alloy material is
at temperatures below about 110.degree. C. to about 115.degree. C.,
precipitates are stable and in the peak aged condition. As the
material is lowered to temperatures near room temperature, ECAE
hardening becomes more effective because more dislocations and
finer submicron grain sizes are created. The rate of strength
increase when the material is processed around room temperature is
more gradual compared to temperatures between about 110.degree. C.
and 150.degree. C.
[0127] FIGS. 16 and 17 and Table 6 show the effect of the number of
ECAE passes on the attainable strength of the Al 7020 alloy.
TABLE-US-00006 TABLE 6 Effect of Number of ECAE Passes on Al 7020
Material Final Yield Strength UTS % Process YS (MPa) UTS (MPa) YS %
increase increase T651 Temper 347.8 396.5 1 ECAE pass 408 415 17.3%
4.7% 2 ECAE passes 469 474 34.8% 19.5% 3 ECAE passes 475 483 36.6%
21.8% 4 ECAE passes 488 493 40.3% 24.3%
[0128] The samples used to create the data in the graphs of FIGS.
16 and 17 were extruded with the sample material at room
temperature and the billet was rotated by 90 degrees between each
pass. A gradual increase in strength and hardness was observed with
an increasing number of ECAE passes. The largest increase in
strength and hardness occurred after the material had undergone
between one and two passes. In all cases, the final yield strength
was over 400 MPa, specifically 408 MPa, 469 MPa, 475 MPa and 488
MPa after one, two, three and four passes respectively. This
example shows that the mechanisms of refinement into submicron
grain size that include dislocation generation and interaction and
creation of new grain boundaries become more effective with
increasing levels of deformation by simple shear during ECAE. A
lower billet material temperature during ECAE can also lead to
increased strengths as described earlier.
[0129] As shown in Example 5, improvements in strength were
achieved without cracking the material by performing ECAE after
artificial aging that used a two-step aging procedure to stabilize
GP zones and precipitates. Avoiding cracking of the billet enables
a lower ECAE processing temperature and allows for a higher number
of ECAE passes to be used. As a consequence, higher strengths can
be formed in the Al 7020 alloy material.
Example 6: Comparison of Various Processing Routes
[0130] Table 7 and FIG. 18 display strength data comparing the
various processing routes described in Examples 3, 4 and 5. Only
the samples that were subjected to ECAE at room temperature are
compared, showing one and two passes.
TABLE-US-00007 TABLE 7 Comparison of Final Strength of Al 7020
Material After Various Processing Routes YS UTS (MPa) (MPa) Example
3 1 ECAE pass after solutionizing 382 404 and quenching Example 5 1
ECAE pass after aging 408 415 Example 4 2 ECAE passes after initial
ECAE and 416 440 solutionizing and quenching Example 5 2 ECAE
passes after aging 469 474
[0131] As shown in FIG. 18 and Table 7, applying ECAE to Al 7020
alloy material samples that have both been solutionized and aged
(i.e. Examples 3 and 4) does not result in as high a final strength
when compared to applying ECAE to artificially aged samples (i.e.
Example 5) for the same given number of passes. Namely, compare 382
MPa (Example 3) to 408 MPa (Example 5) for one ECAE pass and 416
MPa (Example 4) to 469 MPa (Example 5) for two passes. This
comparison shows that standard cold working of solutionized and
quenched Al 7xxx is generally not as effective as, for example, for
Al 2xxx series alloys. This is generally attributed to a coarser
precipitation on dislocation. This trend appears to apply also to
extreme plastic deformation for Al 7xxx series alloys at least for
the first two passes. This comparison indicates that a processing
route that involves stabilization of precipitation by artificial
aging before applying ECAE has more advantages than a route using
ECAE directly after the solutionizing and quenching steps. The
advantages have been shown to lead to better surface conditions,
such as less cracking, for the material being extruded and allow
the material to reach a higher strength for a given deformation
level.
Example 7: Result of Conducting ECAE on Al 7020 Plates
[0132] The procedure described in Example 5 was applied to material
formed into plates rather than bars, as shown in FIG. 10. FIG. 19
shows an example plate 650 having a length 652, a width 654, and a
thickness less than either the length 652 or width 654. In some
embodiments, the length 652 and width 654 may be substantially the
same such that the plate is a square in the plane parallel to the
length 652 and the width 654. Often the length 652 and width 654
are substantially larger than the thickness, for example, by a
factor of three. This shape may be more advantageous for
applications such as portable electronic device casings as it is a
near net shape. ECAE was conducted after the same initial
thermomechanical property treatment used in Example 5:
solutionizing, quenching, stress relief by stretching to 2.2% and a
two-step peak aging comprising a first heat treatment at 90.degree.
C. for 8 hours followed by a second heat treatment at 115.degree.
C. for 40 hours. The plate 650 in FIG. 19 is a plate of Al 7020
alloy shown after the material was subjected to ECAE.
[0133] Workability of the plate 650 was good with no severe
cracking at all temperatures, including at room temperature. The
results of hardness and strength testing of the plate 650 are
contained in Table 8. As shown in Table 8, hardness and strength
tests were taken after applying one, two, and four ECAE passes and
tensile data after two and four ECAE passes. Table 8 shows that the
results of applying ECAE to plates were similar to those for ECAE
bars. In particular, yield strength (YS) in the material that was
extruded as a plate was well above 400 MPa
TABLE-US-00008 TABLE 8 Measured Values for Plates After ECAE was
Applied Brinell Hardness (HB) YS (MPa) UTS (MPa) 1 ECAE pass 130
n/a n/a 2 ECAE pass 133.5 452 456 4 ECAE pass 140.6 490 502
Example 8: Effect of Rolling after ECAE
[0134] FIGS. 20A and 20B show Al 7020 alloy material that has
undergone ECAE with the material formed as a plate 660. After ECAE,
the plate 660 was rolled. Rolling reduced the thickness of the
plate up to 50%. When multiple rolling passes are used to gradually
reduce the thickness to a final thickness, the mechanical
properties are often slightly better during the final rolling step
as compared to the initial rolling pass after the plate 660 has
undergone ECAE, as long as rolling is conducted at relatively low
temperatures close to room temperature. This example demonstrates
that an aluminum alloy having magnesium and zinc that has undergone
ECAE has the potential to undergo further processing by
conventional thermomechanical processing to form a final desirable
near net shape if needed. Some example thermomechanical processing
steps may encompass rolling, forging, stamping or standard
extrusion, for example, as well as standard machining, finishing
and cleaning steps.
Example 9: Effect of ECAE on Al 6xxx Series Alloy Material
[0135] ECAE processing was tested on other types of heat treatable
alloys. An example of ECAE processing on Al 6061, a heat treatable
Al 6xxx series alloy, is described first. The starting material was
an as-received Al 6061 billet, in an as-cast and homogenized
condition. The composition of the Al 6061 starting material
containing aluminum as a primary component and magnesium and
silicon as secondary components is included in Table 9.
TABLE-US-00009 TABLE 9 Composition of Al 6061 Starting Material
(Weight Percentage) Si Fe Cu Mn Mg Cr Other Al 0.62 <0.05
<0.05 0.28 1.01 0.21 <0.05 Balance
[0136] Initial heat treatments were performed to evaluate the
effect of temperature and time on hardness, precipitation and
microstructure of the Al 6061 starting material.
[0137] Heat treatment 1 (HT 1) comprised solutionizing the starting
material at 530.degree. C. for 3 hours, immediately followed by
water quenching. This treatment helped dissolve the precipitates
into solution. The measured hardness after HT1 was 60.5 HB.
[0138] Heat treatment 2 (HT 2) comprised solutionizing the starting
material at 530.degree. C. for 3 hours, immediately followed by
water quenching and then peak aging at 175.degree. C. for 8 hours
in air. This process produced an equilibrium solid solution matrix
containing many small and uniformly spaced precipitate particles of
about 0.05-0.1 .mu.m in diameter. This range of processing
temperature and time is comparable to a heat treatment for
producing a T6 temper in an Al 6061 alloy. The measured hardness
after HT 2 was 92.6 HB. This hardness value is comparable to the
ASTM standard value of 95 HB for a T6 temper. The final measured
strength was a UTS of 310 MPa and a YS of 275 MPa, which are
comparable to a standard Al 6061 having the T6 temper condition.
These values are included in Table 10 below.
[0139] Heat treatment 3 (HT 3) comprised solutionizing the starting
material at 530.degree. C. for 3 hours, immediately followed by
water quenching and then artificial over-aging at 400.degree. C.
for 8 hours in air. This process caused small soluble precipitates
to grow and coalesce into large precipitates having a diameter of
about 1-5 .mu.m on average. In general, large precipitates provide
minimal strengthening effects. The measured hardness of the
material after HT 3 was low, around 30 HB. The heat treatment
process used and the resulting hardness value is similar to
material that has undergone an O temper. The final measured
strength was also comparable to a standard Al 6061 alloy having an
O temper. The UTS was 125 MPa and the YS was 55 MPa. These values
are included in Table 10 below.
[0140] Heat treatment 4 (HT 4) comprised solutionizing the starting
material at 530.degree. C. for 3 hours, immediately followed by
water quenching and natural aging at room temperature. This
produced very fine precipitate particles from the supersaturated
solid solution. After one month, the hardness of this material
increased slowly from 60.5 to 71.5 HB and leveled off at this
hardness value. After the initial one month, a duration of several
days passed before an additional change in hardness was
observed.
[0141] The measured results of the Al 6061 material that underwent
HT 4 show that compared to Al 7020, precipitation proceeds at a
slower rate in Al 6061 compared to Al 7020. As a result, during
ECAE processing, the Al 6061 alloy was less sensitive to cracking,
in particular after a solutionizing and quenching step. From these
measurements it was shown that it is possible to perform
multiple-pass ECAE on an Al 6061 alloy that has undergone one of at
least two initial conditions: either directly after solutionizing
and quenching or after a process that includes solutionizing,
quenching and aging.
Effect of ECAE Processing on Al 6061 Alloy Material
[0142] Two examples of ECAE in combination with heat treatment were
studied. In ECAE process A, which included solutionizing,
quenching, peak aging and ECAE, a billet of Al 6061 material was
subjected to HT 2 described above, followed by 4 ECAE passes with
the die at temperature of less than 175.degree. C. An increase in
strength of the Al 6061 alloy material was attained. The final UTS
of the material was 430.25 MPa and the YS was 403.3 MPa. The
results are contained in Table 10.
[0143] In ECAE process B, solutionizing, quenching and ECAE was
used. In this example, a billet of Al 6061 material was first
subjected to HT 1 as described above. Two ECAE processes with 4 and
6 passes, respectively, were then conducted with the die maintained
at a temperature below 175.degree. C. The die and billet of Al 6061
material were heated during the ECAE process to a temperature
between about 100.degree. C. and about 140.degree. C. That is, the
die was heated during the ECAE process, and the billet of Al 6061
alloy material was heated to a temperature close to the temperature
of the die (within 50.degree. C. of the temperature of the die) for
between about 5 minutes and one hour between each pass. Heating the
die and billet between each ECAE pass maintained the billet at a
more uniform temperature throughout the extrusion process. This
intermediate heating step between each pass can also provide some
annealing of the Al 6061 material in between each pass. A hardness
of 133 HB was measured after the Al 6061 material underwent ECAE.
This represented an increase in hardness by a factor 1.25-1.4 and
4-4.3 compared to the T6 and O tempers respectively. The hardness
increase is believed to be due to the combined effect of the ECAE
and dynamic precipitation caused during deformation and
intermediate annealing applied between each ECAE pass. Measurements
of the final material strength and hardness are contained in Table
10. The final UTS of 456.5 MPa and YS of 443 MPa of the Al 6061
material after undergoing ECAE process B represents an increase in
UTS of 46% and a YS of 60% above that of standard Al 6061 having a
T6 temper, and an increase in the UTS of 262% UTS and YS of 700%
higher than that of standard Al 6061 having an O temper. Although
the strength of the Al 6061 material increased, the percent
elongation (around 13%) was comparable to that of a standard Al
6061 T6 (12%).
TABLE-US-00010 TABLE 10 Effect of ECAE on Hardness and Tensile
Strength Compared to Standard Al 6061 YS UTS Elongation Hardness
(ksi) UTS (ksi) YS (MPa) (MPa) (%) (HB) Al 6061 After ECAE 58.5
62.4 403.3 430.25 13 127 process A Al 6061 After ECAE 64.26 66.21
443.07 456.52 13 133 process B Standard A16061-O (HT 3) 8 18.1 55
125 25 30 Standard A16061-T6 (HT 2) 39.9 45 275 310 12 95
[0144] It was also found that including an annealing treatment with
the Al 6061 alloy material held at a low temperature after ECAE can
further augment the increase in strength of ECAE on an Al 6061
alloy material. FIG. 21 is a graph showing the effect of annealing
temperatures between 100.degree. C. and 400.degree. C. for a total
heat treatment time of one hour on the final Brinell hardness
measured in samples that had first undergone ECAE process B
described above. For heat treatment carried out at temperatures
between 100.degree. C. and 175.degree. C. for one hour, the Brinell
hardness increased to a value of about 143 HB, compared to an
initial value of 133 HB measured immediately after the material
underwent ECAE process B.
Example 10: Effect of ECAE on Al 2xxx Series Alloy Material
[0145] The effect of ECAE on another heat treatable Al alloy was
tested. In this example, an Al 2xxx series alloy, Al 2618, was
used. The composition of the Al 2618 starting material containing
aluminum as a primary component and copper as a secondary component
is included in Table 11. The Al 2618 starting material was shaped
as a billet was in the as-cast and homogenized condition.
TABLE-US-00011 TABLE 11 Composition of Al 2618 Starting Material
(Weight Percentage) Si Fe Cu Mn Mg Cr Ni Zn Ti V B Ga Pb Li Zr Al
0.22 1.1 2.4 0 1.45 0 1.06 0.01 0.07 0 0.006 0.02 0 0 0 Balance
[0146] Initial heat treatment tests were performed to evaluate the
effect of temperature and time on the precipitation dynamics of an
Al 2618 alloy. The Al 2618 alloy contains various types of
precipitates including CuMgAl.sub.2, FeNiAl.sub.9 and (Cu,
Fe)Al.sub.6. The main soluble second phase, which is affected by
solutionizing and aging treatment, is CuMgAl.sub.2.
[0147] Heat treatment A (HT A) comprised solutionizing at
530.degree. C. for 24 hours, immediately followed by water
quenching. The heat treatment dissolved the soluble precipitates
back into solution. The measured hardness after HT A was 72.6-76
HB
[0148] Heat treatment B (HT B) comprised solutionizing at
530.degree. C. for 24 hours, immediately followed by water
quenching in boiling water and artificial peak aging at 200.degree.
C. for 20 hours in air. This produced an equilibrium solid solution
matrix containing many small and uniformly spaced precipitate
particles principally CuMgAl.sub.2 having a diameter of about
0.05-0.1 .mu.m. This range of temperature and time is used in Al
2618 to get the standard T6 temper. The measured hardness of the
material after HT B was 114-119 HB, which was close to the ASTM
standard value of 115 HB for the standard T61 temper.
[0149] Heat treatment C (HT C) comprised solutionizing at
530.degree. C. for 24 hours, immediately followed by water
quenching and annealing at 385.degree. C. for 4 hours in air. This
heat treatment allowed precipitates to grow and coalesce into large
sizes. In this example, most soluble precipitates such as
CuMgAl.sub.2 were over one micron in diameter and had lost most of
their strengthening ability. The measured hardness of the final
material was around 47.5 HB. The heat treatment process used here
and the resulting hardness value were similar to the standard O
temper.
[0150] Heat treatment D (HT D) comprised solutionizing at
530.degree. C. for 24 hours, immediately followed by water
quenching then natural aging at room temperature. This heat
treatment was used to gauge how fast precipitation from solid
solution occurs. After 2 weeks, the hardness increased from 72.6 HB
to 82 HB and, after 3-4 weeks, the hardness further increased to
100 HB. Comparing these results to Examples 1 and 9 above, for Al
2618, precipitation happens faster than in Al6061 but slower than
in Al 7020.
Effect of ECAE Processing on Al 2618 Alloy Material
[0151] The effect of ECAE on the Al 2618 alloy material after heat
treatment was investigated. For this test, Al 2618 material that
had undergone heat treatment A was used. The temperature of the Al
2618 material during ECAE, the number of ECAE passes used, and the
time and temperature of post ECAE annealing were varied to evaluate
the effect of each parameter on the final strength of the Al 2618
material.
[0152] Carrying out ECAE while maintaining the Al 2618 material at
a temperature above 150.degree. C. and less than 230.degree. C.
provided a balance between material strength and good billet
surface conditions. The higher processing temperature used for ECAE
on the Al 2618 material was used due to the better thermal
stability and a higher range of temperature and time needed for
precipitation to occur, which is the result of a higher amount of
Ni and Fe present in the Al 2618 alloy than in many other
alloys.
[0153] The best strength results were attained when using 1 or 2
passes, compared to a greater number of passes, such as 4 passes.
The measured results are included in Table 12 below. ECAE
influences not only grain refinement but also the extent and
dynamics of precipitation. Precipitation happens dynamically during
ECAE and precipitates interact with the newly created dislocations
and finer grain sizes. As shown by the measured results, this
effect is the strongest when only a few passes are used, such as 1
or 2 passes. When additional passes are used, the additional passes
can increase the rate of dissolution and size of precipitates
thereby reducing their contribution to overall strength in the Al
2618 alloy.
[0154] The effect of post-ECAE annealing for a total duration of
one hour was also measured in relation to a change in temperature
between 100.degree. C. and 400.degree. C. For temperatures less
than about 200.degree. C., annealing further increases the strength
of solutionized alloy material that undergoes ECAE for any number
of passes. The measured results are shown in Table 12. The effect
of pose-ECAE annealing is most pronounced for annealing
temperatures between 100.degree. C. and 150.degree. C.
[0155] As shown in the measured values in Table 12, the most stable
and highest hardness was obtained using 1 and 2 passes of ECAE.
Using 2 passes, a final hardness as high as 158-160 HB can be
attained even after annealing at 200.degree. C. for one hour.
Overall, the increase in hardness from an Al 2618 material having a
standard T6 temper is 32.7% after 1 pass, 42.8% after 2 passes, and
23.5% after 4 passes. The increase in YS from the T6 temper was 37%
for 1 pass, 53% for 2 passes, and 10% for 4 passes. One reason for
this additional increase is believed to be further precipitation,
distribution and growth of second phase material that remained in
solid solution after ECAE, and the interaction of these
particulates with the dislocation lines and newly grain boundaries
produced by mechanical deformation.
TABLE-US-00012 TABLE 12 Hardness and Strength Comparison of
ECAE-Processed Al 2618 to Standard Al 2618 YS UTS YS UTS Elongation
Hardness (ksi) (ksi) (MPa) (MPa) (%) (HB) HT A, ECAE (1 pass) n/a
n/a n/a n/a n/a 130 HT A, ECAE (1 pass), and 73.54 80.06 507 552 14
158 anneal at 150.degree. C. for 1 hour HT A, ECAE (2 passes) 81.3
84.85 560 585 10 160 HT A, ECAE (2 passes) and 82.1 86.18 566 594
11 170 anneal at 150.degree. C. for 1 hour HT A, ECAE (4 passes)
56.73 62.66 391 432 14 134 HT A, ECAE (4 passes) and 59.1 69.2
407.5 477 14 147 anneal at 150.degree. C. for 1 hour Standard Al
2618-O (at 25.degree. C.) 11 25 76 172 18 47.5 Standard Al 2618-T6
(at 25.degree. C.) 53.7 63.1 370 435 10 119 (n/a: not measured)
Example 11: ECAE of Al 2xxx Series Alloy Material
[0156] Another heat treatable Al alloy from the Al 2xxx series was
tested after ECAE; in this case Al 2219. The composition of the
starting material containing aluminum as a primary component and
copper as a secondary component is given in Table 13. The Al 2219
alloy starting material was in the as-cast and homogenized
condition prior to any heat treatment. Initial heat treatment tests
were performed to evaluate the effect on precipitation of soluble
phases within Al 2219.
[0157] Heat treatment AA (HT AA) comprised solutionizing at
537.degree. C. for 24 hours immediately followed by water
quenching. This heat treatment dissolved all soluble precipitates
back into solution. The measured hardness after HT AA was 74.1
HB.
[0158] Heat treatment BB (HT BB) comprised solutionizing at
537.degree. C. for 24 hours immediately followed by water quenching
and artificial peak aging at 190.degree. C. for 29 hours in air.
This produced an equilibrium solid solution matrix containing many
small and uniformly spaced Al--Cu--Fe--Mn precipitates. The
measured hardness of the material after HT BB was 115 HB, which was
close to the ASTM standard value of 115 HB for this material having
the T6 temper.
[0159] Heat treatment CC (HT CC) comprised solutionizing at
537.degree. C. for 24 hours immediately followed by water quenching
and annealing at 400.degree. C. for 2 hours in air. This heat
treatment allowed precipitates to grow and coalesce to large sizes
of several microns and thereby, the benefits from precipitation
strengthening were low. The measured hardness of the material after
HT CC was around 45 HB. This heat treatment corresponds to that
used in the low strength O temper for Al 2219.
[0160] Heat treatment D (HT D) comprised solutionizing at
537.degree. C. for 24 hours immediately followed by water quenching
and natural aging at room temperature. This process was used to
evaluate the dynamics of precipitation from solid solution at room
temperature. After 3 weeks, the hardness of the material remained
stable at 74.1 HB. This indicated that Al 2219 has a slow
precipitation rate, when compared to Al alloys in the Al 7xxx
series.
Effect of ECAE Processing on Al 2219 Alloy Material
[0161] ECAE was conducted on Al 2219 alloy material that had
undergone the HT AA heat treatment. The billet of Al 2219 material
and die were heat treated prior to and in between ECAE passes to
temperatures between 150.degree. C. and 275.degree. C., more
specifically between 175.degree. C. and 250.degree. C. The highest
strength levels in the ECAE conditions were found after 1 and 2
ECAE passes for this type of heat treatment sequence. The final
results for tensile strength and Brinell hardness after 1 and 2
ECAE passes are included in Table 20. For comparison, data for the
strength and hardness of an Al 2219 material with the O temper and
T6 temper that has undergone standard thermomechanical processing
(TMP) are also shown.
[0162] The hardness increased to 130 and 139 HB after 1 and 2 ECAE
passes respectively. This is an increase by a factor of
.times.1.13-1.21 and .times.2.9-3.1 compared to the standard T6 and
O temper conditions respectively. Tensile testing confirmed the
increase in strength as well. The largest increase was seen in the
yield strength of 415 MPa for 1 pass and 365 MPa for 2 passes,
which is about 26% (2 passes) to 43% (1 pass) higher than the T6
temper and 420% (2 passes) to 490% (1 pass) higher than the O
temper. The ductility level of the material remained good
throughout the processing steps and was similar to the T6
condition.
[0163] A low temperature heat treatment (annealing) was tested
after the ECAE in order to test the effect on the final strength.
The optimal temperature and time range of post-ECAE annealing was
between 100.degree. C. and 200.degree. C. and 0.5 hours and up to
50 hours respectively. Data for the heat treatment conducted at
150.degree. C. for 6 hours are displayed in table 20 for 1 and 2
passes. The largest strength improvement of about 8-9% in YS and
UTS was observed after 2 ECAE passes. The additional strength
increase resulted from the precipitation of additional second
phases remaining in solid solution after ECAE.
TABLE-US-00013 TABLE 13 Composition of Al 2219 Starting Material
Others Si Fe Cu Mn Mg V Ti + Zr Total Al 0.07 0.17 6.23 0.31 0.011
0.11 0.21 <0.15 Remainder
TABLE-US-00014 TABLE 14 Tensile Strength Data for Al 2219 after
ECAE vs Standard Al 2219 YS UTS YS UTS Elongation Brinell (ksi)
(ksi) (MPa) (MPa) (%) Hardness HT AA and ECAE Al 2219 60.2 68 415.1
469 18 139 (1 pass) HT AA and ECAE Al 2219 62 71 427.5 490 17 145
(1 pass) and annealing at 150.degree. C. for 6 hours HTAA and ECAE
Al 2219 53 59 365.4 407 19 130 (2 passes) HTAA and ECAE Al 2219 58
64 399.9 441 15 133 (2 passes) and annealing at 150.degree. C. for
6 hours Standard Al 2219-O 10.2 24.7 70.3 170 18 45 Standard Al
2219-T62 42.1 60.2 290.3 415 10 115
Example 12: Effect of ECAE on Non Heat-Treatable Alloys (Al 5xxx
Series Alloys)
[0164] The effect of ECAE on Al 5083, an Al alloy in the Al 5xxx
series, was measured. Table 15 displays the composition of the Al
5083 alloy material containing aluminum as a primary component and
magnesium and manganese as secondary components used in this
example. Like most wrought Al alloys in the 5xxx series, Al 5083 is
mostly based on the Al--Mg binary system and does not show
appreciable precipitation hardening characteristics, which is
expected for Al alloys having magnesium at concentrations below 7
wt. %. For this reason, Al 5083 is referred to as a
non-heat-treatable Al alloy, in which heat treatments such as
solutionizing, quenching and age hardening generally do not create
fine soluble precipitates. Common second phases in Al 5083 are, for
example, Mg.sub.2Al or MnAl.sub.6. These second phases are
non-soluble and are created during the initial casting and cooling
steps, and stay mostly stable in size and number during subsequent
heat treatments.
[0165] In non-heat-treatable Al alloys, because precipitation
hardening is generally not very effective, one way to increase
strength is by dislocation hardening. In dislocation hardening, a
high amount of dislocations are introduced into the material grains
during hot or cold working using TMP techniques such as rolling,
forging or drawing. These TMP techniques introduce strain into the
processed material, for example, by reducing the thickness of a
sample while other dimensions increase. The amount and density of
dislocations in the resulting material is directly related to the
amount of strain introduced into the material and therefore also
related to the amount of mechanical deformation of the material. In
practice, often the achievable mechanical deformation of the
material may be limited, such as for fairly thick plates, for
example greater than 0.5-1 inch thick. In such an example, the
final strength of the material depends on how fine the initial
grain size is in the material before applying TMP techniques, which
is often set by the casting process.
[0166] ECAE as described above offers two strengthening mechanisms:
grain size (Hall Petch) hardening and dislocation hardening. This
means that ECAE offers an additional strengthening mechanism over
standard TMP methods. That is, ECAE provides a strengthening
mechanism in addition to Hall Petch hardening. ECAE also does not
change the billet thickness or shape dimensions, so large billets
can be strengthened throughout the thickness of the billet while
also introducing a very high level of strain.
[0167] In this example, an as-cast and homogenized Al 5083 material
having the composition listed in Table 15 with aluminum present as
a primary component was processed. In order to limit surface
cracking of the Al 5083 billet during ECAE, the ECAE die and the
billet of Al 5083 material that was being extruded were heated
during the extrusion. A suitable temperature range for maintaining
the Al 5083 material at during ECAE was found to be between
150.degree. C. and 275.degree. C., from about 175.degree. C. to
about 250.degree. C. Multiple passes of ECAE were tested, and the
Al 5083 material measured after the total number of passes was
between 4 and 6. Table 14 shows the resulting tensile strength data
for Al 5083 having undergone 4 passes versus a standard Al 5083
with either the O temper (fully annealed) or the H116 temper (cold
rolled). Increases in strength and hardness were measured after the
material underwent ECAE, with a sharp increase in both yield
strength (399 MPa which was a 77% increase over the H116 temper)
and ultimate tensile strength (421 MPa which was a 37.8% increase
over the H116 temper).
[0168] It was further shown that additional strengthening could be
introduced by using TMP techniques such as rolling or forging of
the Al 5083 material after the ECAE process. Table 14 shows an
example that included additional cold rolling of the Al 5083
material to a 35% height reduction that was performed after ECAE.
The final YS and UTS were 418 MPa and 441 MPa respectively. In this
example, the microstructure of the Al 5083 alloy after ECAE but
before cold rolling had a relatively fine submicron grain size and
additional dislocations were imparted during the rolling step to
further contribute to the final strength. Factors that can be
controlled to reduce forming defects in the material during cold
rolling include percent height reduction of the material per pass,
the diameter of the roller used, trimming of sharp edges and
corners, and the roller temperature.
TABLE-US-00015 TABLE 15 Composition of Al 5083 Starting Material
(Weight Percent) Others Si Fe Cu Mn Mg Cr Zn Ti Total Al 0.28 0.224
0.066 0.639 4.551 0.105 0.104 0.0234 0.0621 Balance
TABLE-US-00016 TABLE 16 Tensile Strength Data for Al 5083 After
ECAE vs Standard Al 5083 UTS YS (ksi) UTS (ksi) YS (MPa) (MPa)
Standard Al 5083-O 21 42.1 145 290 Standard Al 5083-H116 32.66
44.24 225 305 ECAE 57.84 60.99 399 421 ECAE and 35% Height 60.58 64
418 441 Reduction with Cold Rolling
Example 13: Effect of ECAE on Non Heat-Treatable Alloys (Al 5xxx
and Al 3xxx Series Alloys)
[0169] In this example, 2 more non heat treatable Al alloys, namely
Al 5456 from the Al 5xxx series and Al 3004 from the Al 3xxx
series, were processed using ECAE according to a similar process
used in Example 12 above, with some alterations. The composition of
the starting Al alloys containing aluminum as a primary component
and magnesium and manganese as secondary components used in this
example is given in Tables 17 and 18. In table 17, "Others Each" is
the maximum weight percentage of any single element other than
those listed, and "Others Total" is the maximum combined weight
percentage of all elements other than those listed.
[0170] The total number of ECAE passes used was between 4 and 6
passes. A suitable process temperature was found to be between
100.degree. C. and 275.degree. C., from about 150.degree. C. to
about 225.degree. C., which provided good surface conditions for
the billet.
[0171] The final measured tensile properties are given in Tables 19
and 20. Measurements for Al 3004 and Al 5456 having commercial
tempers are also given for comparison, either for the fully
annealed condition (O temper) or for various degrees of strain
working, for example, the H116 temper for Al 5456 and H38 temper
for Al 3004. As shown in the measured values contained in Tables 19
and 20, ECAE improved the YS and UTS values, about 1.5-8 times for
YS and about 1.3-1.4 times for UTS, above the standard strain
worked tempers H116 or H38. The strength increases were greater
when compared to the O temper.
[0172] As described in Example 12, it was shown to be advantageous
to subject the material to cold rolling after conducting ECAE, in
order to increase the final strength of the Al alloy further. Cold
rolling with a 40% reduction in billet height was used. The
resulting mechanical properties are shown at the bottom row of
Table 19.
[0173] It should be noted that a YS above 350 MPa is relatively
high for Al alloys from the Al 3xxx and Al 5xxx series which are
typically weaker than those from the Al 2xxx and Al 7xxx series.
The resulting strength increase in the Al 3xxx and Al 5xxx series
alloys that is imparted by the process in this example means that a
user can choose from a wider range of alloys when deciding on an Al
alloy having a strength above a particular value. In other words, a
wider range of Al alloys having a desired strength can be formed
from alloys in a series other than just the Al 2xxx and Al 7xxx
series. An alloy that may be more suitable because of a particular
feature, such as its cosmetic appeal, but that previously was not
suitable because of for example a lower strength, may be processed
using the techniques described above, resulting in a material that
has more of the desired properties than before.
TABLE-US-00017 TABLE 17 Composition of Starting Material of Al 5456
(Weight Percent) Others Others Si Fe Cu Mn Mg Cr Zn Ti Each Total
Al 0.117 0.158 0.009 0.663 5.24 0.084 0.013 0.021 0.015 <0.045
Balance
TABLE-US-00018 TABLE 18 Composition of Starting Material of Al 3004
(Weight Percent) Others Si Fe Cu Mn Mg Cr Zn Total Al 0.19 0.41
0.16 1.31 1.06 0.27 0.12 <0.15 Balance
TABLE-US-00019 TABLE 19 Tensile Strength Data for Al 5456 After
ECAE vs Standard Al 5456 YS (ksi) UTS (ksi) YS (MPa) UTS (MPa)
Standard Al 5456-O 23.2 45 160 310 Standard Al 5456-H116 37.1 50.8
255 350 Al 5456 After ECAE 62.9 65.8 434 454 Al 5456 After ECAE and
65.4 67.6 451 466 cold rolling
TABLE-US-00020 TABLE 20 Tensile Strength Data for Al 3004 After
ECAE vs Standard Al 3004 YS (ksi) UTS (ksi) YS (MPa) UTS (MPa)
Standard Al 3004-O 10.2 26.1 70 180 Standard Al 3004-H38 36.3 41.3
250 285 Al 3004 After ECAE 55.1 57.2 380 394
[0174] Various modifications and additions can be made to the
exemplary embodiments discussed without departing from the scope of
the present invention. For example, while the embodiments described
above refer to particular features, the scope of this invention
also includes embodiments having different combinations of features
and embodiments that do not include all of the above described
features.
* * * * *