U.S. patent application number 10/634099 was filed with the patent office on 2004-02-12 for method for producing lightweight alloy stock for impact extrusion.
Invention is credited to Kramer, Lawrence Stevenson, Tack, William Troy.
Application Number | 20040025981 10/634099 |
Document ID | / |
Family ID | 46299710 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040025981 |
Kind Code |
A1 |
Tack, William Troy ; et
al. |
February 12, 2004 |
Method for producing lightweight alloy stock for impact
extrusion
Abstract
The present invention relates to a method for fabricating
lightweight alloy feedstock for impact extrusion or impact forging.
Specifically, the method for producing the working stock proposed
in the present invention enables the production of impact-extruded
components utilizing a highly alloyed starting stock. The high
strength properties attained in the final impact extrusion allow
part designers to utilize the economical process of impact
extrusion and provide high strength impact extruded components.
Inventors: |
Tack, William Troy;
(Glenelg, MD) ; Kramer, Lawrence Stevenson;
(Dayton, MD) |
Correspondence
Address: |
Wm. Troy Tack
3060 Route 97
Glenwood
MD
21738
US
|
Family ID: |
46299710 |
Appl. No.: |
10/634099 |
Filed: |
August 4, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10634099 |
Aug 4, 2003 |
|
|
|
09681076 |
Dec 22, 2000 |
|
|
|
6627012 |
|
|
|
|
Current U.S.
Class: |
148/550 |
Current CPC
Class: |
C22F 1/053 20130101;
C22F 1/04 20130101; C22C 21/10 20130101 |
Class at
Publication: |
148/550 |
International
Class: |
C22F 001/04 |
Claims
1. A method for producing a lightweight starting stock for impact
extrusion or impact forging comprising the following sequence: a)
mixing alloying elements into aluminum with the alloy composition
containing 5.0 to 12.0 wt % Zn, 1.0 to 3.5 wt % Mg, 0 to 2.8 wt %
Cu and 0.02 to 1.0 wt % of at least one grain refining element
selected from the group consisting of Zr, Sc, Zr, Mn, Ti, Hf and
other dispersoid forming elements and casting said alloying
elements to produce a billet, b) extruding said billet to provide
and extruded starting stock for impact extrusion, c) annealing said
starting stock to provide annealed starting stock for impact
extrusion d) impact extruding said starting stock into an impact
extruded component, e) solution heat treating and quenching said
impact extruded component and f) aging said impact extruded
component wherein said impact extruded component has a yield
strength of at least 85 ksi.
2. The method of claim 1 wherein said impact extruded component has
a yield strength of >90 ksi.
3. The method of claim 1 wherein the action of impact extruding is
conducted in multiple steps with intermediate annealing provided
after each impact extrusion to soften the alloy for the subsequent
impact extrusion.
4. The method of claim 1 wherein the billet is homogenized prior to
extrusion.
5. The method of claim 1 wherein machining operations are
introduced to advantageously shape the impact extrusion starting
stock for initial impact extrusions or to shape the impact
extrusion stock during the repeated impact extrusion steps.
6. A method for producing a lightweight starting stock for impact
extrusion or impact forging comprising the following sequence: a)
mixing alloying elements into aluminum with the alloy composition
containing 5.0 to 9.0 wt % Zn, 1.0 to 3.5 wt % Mg, 0 to 1.9 wt % Cu
and 0.02 to 1.0 wt % of at least one grain refining element
selected from the group consisting of Zr, Sc, Zr, Mn, Ti, Hf and
other dispersoid forming elements and casting said alloying
elements to produce a billet, b) extruding said billet to provide
and extruded starting stock for impact extrusion, c) annealing said
starting stock to provide annealed starting stock for impact
extrusion d) impact extruding said starting stock into an impact
extruded component, e) solution heat treating and quenching said
impact extruded component and f) aging said impact extruded
component wherein said impact extruded component has a yield
strength of at least 85 ksi.
7. The method of claim 6 wherein said impact extruded component has
a yield strength of >90 ksi.
8. The method of claim 6 wherein the action of impact extruding is
conducted in multiple steps with intermediate annealing provided
after each impact extrusion to soften the alloy for the subsequent
impact extrusion.
9. The method of claim 6 wherein the billet is homogenized prior to
extrusion.
10. The method of claim 6 wherein machining operations are
introduced to advantageously shape the impact extrusion starting
stock for initial impact extrusions or to shape the impact
extrusion stock during the repeated impact extrusion steps.
11. A method for producing a lightweight starting stock for impact
extrusion or impact forging comprising the following sequence: a)
mixing alloying elements into aluminum with the alloy composition
containing 8.4 to 12.0 wt % Zn, 1.0 to 3.5 wt % Mg, 0 to 2.8 wt %
Cu and 0.02 to 1.0 wt % of at least one grain refining element
selected from the group consisting of Zr, Sc, Zr, Mn, Ti, Hf and
other dispersoid forming elements and casting said alloying
elements to produce a billet, b) extruding said billet to provide
and extruded starting stock for impact extrusion, c) annealing said
starting stock to provide annealed starting stock for impact
extrusion d) impact extruding said starting stock into an impact
extruded component, e) solution heat treating and quenching said
impact extruded component and f) aging said impact extruded
component wherein said impact extruded component has a yield
strength of at least 85 ksi.
12. The method of claim 11 wherein said impact extruded component
has a yield strength of >90 ksi.
13. The method of claim 11 wherein the action of impact extruding
is conducted in multiple steps with intermediate annealing provided
after each impact extrusion to soften the alloy for the subsequent
impact extrusion.
14. The method of claim 11 wherein the billet is homogenized prior
to extrusion.
15. The method of claim 11 wherein machining operations are
introduced to advantageously shape the impact extrusion starting
stock for initial impact extrusions or to shape the impact
extrusion stock during the repeated impact extrusion steps.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Ser. No.
09/681,076, filed Dec. 22, 2000.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF INVENTION
[0003] The primary objective of this invention is to provide a
method for producing a suitable lightweight starting stock that can
be used for impact extrusion or impact forging. Another objective
is to provide a method for producing a lightweight aluminum
starting stock that provides high strength once the starting stock
is impact extruded. It is yet another objective to provide a method
for producing a lightweight aluminum alloy starting stock that can
be heat treated to provide high strength impact extrusions in a
number of unique sizes and shapes. A final objective is to provide
a method for producing a lightweight aluminum alloy starting stock
that, once fabricated into an impact extruded shape, provides
enhanced performance in the final product by virtue of higher
strength properties.
BRIEF SUMMARY OF THE INVENTION
[0004] This invention provides a method to produce an aluminum
alloy starting stock alloy that can readily be manufactured into
impact extrusions (also known as impact forgings) wherein said
impact extrusions are readily fabricated into cost effective
components that possess high strength. Another objective of this
invention is to attain strength levels heretofore unachievable in
aluminum alloy impact extrusions to enable designers to reduce the
cross section size of impact extruded components to reduce weight.
Another objective of this invention is to provide an aluminum alloy
starting stock that can be readily impact extruded into a number of
shapes that are capable of attaining strength properties greater
than or equal to that achieved in various steel alloys. In this
instance, the impact extrusion of this invention can be produced to
a similar size and shape as the steel impact extrusion. The high
strength properties would ensure similar or enhanced performance of
said impact extrusion while the inherently low density of the
aluminum alloy relative to steel would enable a weight reduction of
at least 60%.
[0005] While the scope of this invention is oriented toward the
development of starting stock that is suitable for the process of
impact extrusion, it should be noted that the strength levels
attained and the accompanying performance attributes can enable the
use of this invention for other downstream cold forming processes,
for example stamping, impact forging, rolling, swaging, explosive
forming and drawing. In these processes, the unique combination of
cold formability and ultra-high strength are anticipated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Not Applicable
DESCRIPTION OF PRIOR ART
[0007] Impact extrusion is accomplished by utilizing a punch that
is rapidly forced through a metal slug starting stock. The metal
slug can be constrained such that metal flows into a shape mandated
by the shape of the penetrating die, thereby causing metal to flow
upward around the punch. Alternatively, the metal slug can be
forced through an orifice of a die so that the slug takes the shape
of the die. A combination impact utilizes a penetrating punch in
combination with a die to accomplish both forward and reverse metal
flow. The process of impact extrusion is especially amenable to
producing cylindrical hollow shapes with a closed end. Applications
include automotive parts such as airbag canisters and shock
absorbers, components in household appliances and military
applications such as missile casings.
[0008] When utilizing aluminum alloy starting stock for the impact
extrusion process, soft alloys such as commercially pure aluminum
are used in most applications. Commercially pure aluminum, by
virtue of the low alloying content, has a combination of low
strength and high elongation. This highly ductile material is
amenable to large impact extrusion reductions that are advantageous
for a number of final part geometries. Because it is desirable to
blend the high rate production capabilities of impact extrusion
with the use of higher strength alloys, several endeavors have been
undertaken to develop improved alloys amenable to impact
extrusion.
[0009] U.S. Pat. No. 4,243,438 to Yanagida et. al. describes and
method of producing an impact extrusion using an aluminum alloy
slug comprised of <3.0% of at least one element selected from
cobalt and nickel. The primary focus of this invention is an
increase of ductility of the new alloy after annealing to affect an
improvement in fabrication characteristics. The highest strength
level reported in this invention is 20.7 kg/mm.sup.2 or 28.5
ksi.
[0010] U.S. Pat. No. 5,961,752 to Bergsma describes a high strength
Mg--Si type aluminum alloy, methods of casting and thermo
mechanical processing sequences. In Example 8, an aluminum alloy
with a composition of 0.91 Si, 0.17 Fe, 0.78 Cu 1.41 Mg, 0.22 0.10
V, 0.0006 Be, balance aluminum, was cast into an ingot, homogenized
and cold impact extruded into a hollow shape. Subsequent heat
treatment resulted in a yield strength value of 59 ksi, tensile
strength of 64 ksi and elongation of 18%.
[0011] U.S. Pat. No. 5,221,377 to Hunt et. al. provides an alloy
product consisting essentially of 7.6 to 8.4% Zn, 1.8 to 2.2% Mg,
2.0 to 2.6% Cu and at least one element selected from Zr, V, and Hf
in a total amount not exceeding 0.5%. High yield strength
properties were achieved in both plate (85-86 ksi) and extrusion
(89-90 ksi) products. Forged products are anticipated by Hunt in
claims 125 to 129, and anticipated yield strength values are
presented as a percentage increase over the properties of alloy
7050 forgings shown in Table 5 of the patent application. When
calculating the percent increase in yield strength, the highest
value in the table, 63 ksi, is multiplied by the highest percent
increase anticipated by Hunt (15%). Accordingly, the highest
strength anticipated is 63 ksi.times.1.15 or 72.5 ksi. It should be
noted that the production of extrusion, plate and forgings are
comprised of warm and or hot working operations. The use of this
alloy for an impact extrusion, a process performed at ambient
temperature, was neither taught nor anticipated by Hunt.
[0012] U.S. Ser. No. 09/681,076 to Tack et. al. provides a method
for producing a suitable lightweight starting stock that can be
used by gun manufacturers for gun frames and components. A
combination of alloying elements is blended with various processing
sequences to produce a high strength gun frame or gun component.
The primary emphasis is the use of conventional forging practices
that are conducted at an elevated temperature for the specific
purpose of producing gun frames and components.
[0013] Brief Description of Sequences
[0014] An objective of this invention is to provide an aluminum
alloy starting stock for impact extrusion that meets the following
criteria:
[0015] An optimum combination of alloying elements for good
strength and elongation,
[0016] A starting stock with a combination of a favorable
microstructure, a minimum amount of defects and a favorable heat
treatment practice to enable impact extrusion fabrication into
complex shapes without premature failure,
[0017] A suitable heat treatment sequence to impart high strength
on the impact extruded part.
[0018] Suggested processing steps and the accompanying purpose are
as follows:
1 Processing Step Purpose 1) Casting of Billet Provide mixture of
alloying elements 2) Homogenize Billet Refine as-cast structure 3)
Extrude into bar Breaks down the as-cast microstructure and orients
the grains parallel to the direction of subsequent impact
extrusion, heals any internal casting defects to provide a sound
starting stock for impact extrusion 4) Anneal extruded bar Softens
the extruded bar to provide a low yield strength and increased
elongation to enable impact extrusion fabrication into complex
geometries 5) Impact extrusion Provide final shape or intermediate
shape for the desired part geometry 6) Solution Heat Treat Place
alloying elements into solid solution 7) Quench Achieve metastable
solid solution 8) Artificial Age Promote precipitation
strengthening
[0019] While the aforementioned processing steps are the core
principle of this invention, it should be noted that the injection
of ancillary processing steps into the sequence are anticipated. As
one example, a machining operation can be used after extrusion or
annealing (steps 3 and 4) to further modify the starting stock in
preparation for impact extrusion. The impact extrusion step itself
can be conducted in one step or multiple steps. For multiple impact
extrusion steps, the initial starting stock can be annealed and
subjected to impact extrusion to attain and intermediate shape.
This intermediate shape can then be annealed and subjected to a
further reduction and shape change by utilizing another set of
dies. Once a multi-step impact extrusion is completed, the final
sequences of solution heat treatment, quenching and aging are
applied to achieve high strength in the final part. It is
acknowledged that machining operations can be applied throughout
the multi-step impact extrusion sequence, prior to the final heat
treatment or following the heat treatment sequence. Finally, other
forming processes such as drawing or ironing can be accomplished on
intermediate or final impact extrusion forms to achieve a final
shape.
DETAILED DESCRIPTION OF THE INVENTION
[0020] According to the present invention, a method for producing
lightweight starting stock for impact extrusion is provided. In a
preferred embodiment of the invention, an alloy is selected that is
comprised of primary elements Zn, Mg and Cu combined with grain
refining elements Zr, Cr and Sc, with the balance consisting of
aluminum. The elements are blended together in the appropriate
ratios and direct chill cast into billets. After the billet is
homogenized, it is heated to an elevated temperature and extruded.
The extrusion is then annealed to soften the starting stock for
subsequent impact extrusion. If multiple impact extrusion steps are
necessary, it is advantageous to anneal the intermediate forms
prior to each impact extrusion step to affect larger impact
extrusion reductions. Once the final shape is attained, the alloy
is solution heat treated, quenched and subjected to artificial
aging. Machining operations can be introduced at any point in the
manufacturing sequence to improve the impact extrusion stock
surface to enhance the initial fit-up into the next impact
extrusion die.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] In accordance with this invention, the impact extrusion
starting stock is an aluminum alloy consists essentially of about
5.0 to 11.0% Zn, 1.0 to 3.5% Mg, 0 to 2.8% Cu and lesser amounts of
grain and structure refining elements including Zr, Ti, Cr, Mn and
Sc.
EXAMPLE 1
[0022] Mechanical Properties of Starting Stock, Alloy 1: The alloy
formulation shown in Table 1 was direct chill cast into billets.
The alloy formulation was selected to provide a high level of
primary alloying additions Zn, Mg and Cu along with
dispersoid-forming elements Zr, Sc and Cr. The billets were then
homogenized, pre-heated to 650.degree. F. and extruded into round
bars with a 1.5-inch diameter. The 1.5-inch diameter bar was then
subjected to the following heat treatment:
[0023] Solution heat treatment at 875.degree. F. for 1 hour,
[0024] Water Quench,
[0025] Hold at Ambient Temperature for 72 Hours,
[0026] Age at 250.degree. F. for 8 hours followed by 320.degree. F.
for 8 hours (treatment A), or
[0027] Age at 250.degree. F. for 24 hours (treatment B).
2TABLE 1 Composition of Alloy 1 Starting Stock (weight %) Zn Mg Cu
Zr Ti Cr Sc Al 7.75 1.83 1.75 0.114 0.015 0.029 0.07 balance
[0028] As shown in Table 2, the mechanical properties of this alloy
formulation indicate that ultra-high strength is attainable. The
alloy also displays a good elongation considering the high strength
levels attained.
3TABLE 2 Mechanical Properties of Alloy 1 Starting Stock Ultimate
Tensile Heat Treatment Yield Strength (ksi) Strength (ksi)
Elongation (%) A 92.3 94.5 13.8 B 99.5 105.0 13.0
EXAMPLE 2
[0029] Mechanical Properties of Starting Stock, Alloy 2: The alloy
formulation shown in Table 3 was direct chill cast into billets.
The alloy formulation was selected to provide a high level of
primary alloying additions Zn, Mg and Cu and dispersoid-forming
element Zr. The billets were then homogenized, pre-heated to
700.degree. F. and extruded into round bars with a 2.25-inch
diameter. The 2.25-inch diameter bar was then subjected to the
following heat treatment:
[0030] Solution heat treatment at 875.degree. F. for 1 hour,
[0031] Water Quench,
[0032] Hold at Ambient Temperature for 48 Hours,
[0033] Age at 250.degree. F. for 24 hours.
4TABLE 3 Composition of Alloy 2 Starting Stock (weight %) Zn Mg Cu
Zr Ti Cr Al 8.17 2.22 1.65 0.099 0.039 0 balance
[0034] As shown in Table 4, the mechanical properties of this alloy
formulation indicate that ultra-high strength is attainable. The
alloy also displays a good elongation considering the high strength
levels attained.
5TABLE 4 Mechanical Properties of Alloy 2 Starting Stock Ultimate
Tensile Heat Treatment Yield Strength (ksi) Strength (ksi)
Elongation (%) B 100 105 7.0
EXAMPLE 3
[0035] Mechanical Properties of Starting Stock, Alloy 3: The alloy
formulation shown in Table 5 was direct chill cast into billets.
The alloy formulation was selected to provide a high level of
primary alloying additions Zn, Mg and Cu and dispersoid-forming
elements, Zr, Sc and Cr. For this alloy variant, an unusually high
alloying amount of Zn was provided for extra solid solution and
precipitation strengthening. The billets were then homogenized,
pre-heated to 700.degree. F. and extruded into round bars with a
2.25-inch diameter. The 2.25-inch diameter bar was then subjected
to the following heat treatment:
[0036] Solution heat treatment at 875.degree. F. for 1 hour,
[0037] Water Quench,
[0038] Hold at Ambient Temperature for 48 Hours,
[0039] Age at 250.degree. F. for 24 hours.
6TABLE 5 Composition of Alloy 3 Starting Stock (weight %) Zn Mg Cu
Zr Ti Sc Cr Al 9.1 2.4 1.66 0.096 0.04 0.09 0.04 balance
[0040] As shown in Table 6, the mechanical properties of this alloy
are remarkably high for an aluminum alloy as indicated by the yield
strength of 105 ksi and tensile strength of 111 ksi. The alloy also
displays a good elongation of 8.0%, a substantial accomplishment in
view of the fact that such high strength levels are attained.
7TABLE 6 Mechanical Properties of Alloy 3 Starting Stock Ultimate
Tensile Heat Treatment Yield Strength (ksi) Strength (ksi)
Elongation (%) B 105 111 8.0
EXAMPLE 4
[0041] Mechanical Properties of Starting Stock, Alloy 4: The alloy
formulation shown in Table 7 was direct chill cast into billets.
The alloy formulation was selected to provide an intermediate level
of primary alloying additions Zn and Mg; Cu was removed to create
an alloy formulation with good weldability. Dispersoid-forming
elements, Zr, Sc and Cr were added to provide grain refinement. The
billets were then homogenized, pre-heated to 750.degree. F. and
extruded into round tube with a 2.5-inch diameter, and drawn to a
1.375-inch diameter with a wall thickness of 0.060-inch. The
purpose of producing this shape is to progress into the practice of
multiple drawing and annealing steps to access the potential of
this alloy for impact forging applications. After the final drawing
pass, the following heat treatment practice was applied:
[0042] Solution heat treatment at 875.degree. F. for 1 hour,
[0043] Water Quench,
[0044] Hold at Ambient Temperature for 48 Hours,
[0045] Age at 250.degree. F. for 24 hours.
8TABLE 7 Composition of Alloy 4 Starting Stock (weight %) Zn Mg Cu
Zr Ti Cr Sc Al 5.09 1.83 0.316 0.115 0.023 0.055 0.071 balance
[0046] As shown in Table 8, the mechanical properties of this
weldable alloy variant are approximately double that of mainstay
weldable aluminum alloys such as 6061 and 7005. The alloy also
displays a good elongation considering the high strength levels
attained.
9TABLE 8 Mechanical Properties of Alloy 4 Starting Stock Ultimate
Tensile Heat Treatment Yield Strength (ksi) Strength (ksi)
Elongation (%) B 82.0 90.5 14.0
EXAMPLE 5
[0047] Impact Extrusion Trials of Alloy 1: Alloy 1 was provided to
an impact extrusion vendor in two different conditions:
[0048] Condition 1:
[0049] Cast into 7.5-inch diameter billet,
[0050] Extrude at 700.degree. F. into 2.25-inch diameter rod,
[0051] Anneal for 3 hours at 775.degree. F., cool at 50.degree. F.
per hour to 450.degree. F. and hold for 6 hours, cool to ambient
temperature,
[0052] Machine into appropriate starting stock size.
[0053] Condition 2:
[0054] Cast into 2.5-inch diameter billet,
[0055] Homogenize at 800.degree. F. for 24 hours,
[0056] Machine into appropriate starting stock size
[0057] Despite the fact that Alloy 1 contains a high alloying
content and can attain ultra-high strength levels, the impact
extrusion vendor was able to utilize the starting stock provided in
Condition 1 and conduct a multi-step impact extrude/anneal cycle to
achieve a hollow tube with a 2.25-inch diameter and 0.073" wall
thickness. The alloy provided in Condition 2 cracked at the onset
of the impact forging trial. This contrast in performance between
the two conditions highlights the importance of the extrusion and
annealing step for enhancing the impact extrusion capability of the
starting stock. It is believed that the extrusion step serves to
orient the grains along the axis of the extrusion and heals
internal defects that may cause premature cracking in subsequent
impact extrusion fabrication. In any event, the fact that a highly
alloyed variant can be successfully impact extruded is a remarkable
achievement.
[0058] Subsequent to the successful impact extrusion fabrication
trial with Alloy 1-Condition 1, the 2.25-inch diameter, 0.073-inch
impact extrusion was solution heat treated at 800.degree. F. for
1.0 hour, water quenched, and aged at 250.degree. F. for 24 hours.
Despite the potential for recrystallization and an accompanying
strength reduction, high strength properties were attained as shown
in Table 9.
10TABLE 9 Mechanical Properties of Alloy 1 Impact Extrusion
Ultimate Tensile Heat Treatment Yield Strength (ksi) Strength (ksi)
Elongation (%) B 91.3 101.4 13.5
[0059] Four different alloy variants within the stated range of
this invention were derived, cast, extruded into round bar,
heat-treated and tested for mechanical properties. An initial
assessment of the tensile properties indicated that ultra-high
yield strength is attainable in the extruded product form for each
of the alloy variants. While the tensile properties in the extruded
product form are encouraging, the challenge to develop a starting
stock for impact extrusion is to achieve two goals: 1) produce a
starting stock that can be readily impact extruded into challenging
geometries and 2) upon completion of impact extrusion, utilize heat
treatment practices to achieve high strength properties. To design
an alloy that is highly ductile and amenable to impact extrusion,
the approach is to minimize the amount of alloying additions that
increase strength and decrease ductility. In contrast, a part
designer that is seeking a high-strength, impact-extruded component
would attempt to utilize an alloy that has high alloying content.
The combination of an alloy that is readily impact extruded and
capable of reaching high strength levels has heretofore not been
achievable.
[0060] The difficulty of impact extruding a high strength alloy
variant was observed when the Alloy 1 variant was cast,
homogenized, machined and impact extruded. Impact extrusion of
Alloy 1 could not be accomplished as cracking occurred as the
impact loading was applied. Although Alloy 1 was readily extruded
at an elevated temperature into round bar and heat treated to
achieve high strength, the same alloy could not be impact extruded
at ambient temperature. This trial underlines the difficulty of
deriving an alloy that can be impact extruded and exhibit high
strength. Moreover, the impact extrusion trial highlights the
traditional approach for most impact-extruded parts: the impact
extruder selects low-strength alloy variants that are sufficiently
ductile for conversion into complex parts via impact extrusion.
[0061] It is well established that aluminum alloys are much
stronger in a wrought product form compared to the as-cast
condition. For example, aluminum alloys such as 6061 or 7075 are
not useful in the billet or ingot product form for direct use as a
final part; desirable mechanical properties only achieved by
various combinations of warm working and cold working to produce
final product forms such as plate and extrusions. Accordingly, when
alloy 1 was cast into a billet, homogenized, extruded into bar and
annealed, the additional warm working applied to the billet
provided starting stock for impact extrusion that was expected to
prematurely fail during impact extrusion by virtue of its higher
strength Surprisingly, only the lower-strength, as-cast and
homogenized starting stock failed. The extruded and annealed
starting stock, despite its higher strength afforded by warm
working, was successfully impact extruded into tube at ambient
temperature. This unexpected result of producing a highly alloyed
starting stock and providing secondary warm working and annealing
was surprising as the impact extrusion was successfully
accomplished. In particular, a multi-step impact extrusion sequence
resulted in a very thin wall tube without cracking or premature
failure.
[0062] Once the unexpected accomplishment of producing a complex,
thin-walled tube was performed, the next assessment was to
determine whether high strength was attainable in the final part.
Expectations of potential yield strength properties were set by a
review of prior art related to impact extrusion. U.S. Pat. No.
4,243,438 to Yanagida et. al. attained yield strength values as
high as 28.5 ksi. Because the present invention contains a higher
alloying content compared to that of Yanagida's alloy, a yield
strength value greater than 28.5 ksi was predicted. U.S. Pat. No.
5,961,752, to Bergsma, reported a yield strength value of 59 ksi
for an impact extrusion utilizing a 6XXX alloy.
[0063] U.S. Pat. No. 5,221,377 to Hunt et. al. has an alloy range
that overlaps with the present invention. Warm working processes
such as rolling and extrusion followed by heat treatment produced
yield strength values of approximately 90 ksi. Another warm working
process, forging, was anticipated by Hunt to provide a yield
strength value as high as 72.5 ksi. Hunt did not anticipate the
process of impact extrusion process, nor was a suitable process for
producing a suitable starting stock taught. Interestingly, the
expectations of final properties for the Hunt alloy subjected to
impact extrusion would be somewhat lower that anticipated for
forgings or less than 72.5 ksi. Because impact extrusion is
typically performed at ambient temperature, the stored energy from
the impact extrusion is much greater than that generated by a warm
working operation such as forging. This increased stored energy is
the driving force for a higher amount recrystallization during
subsequent solution heat treatment. Accordingly, the impact
extrusion yield strength of Hunt's alloy formulation would be
expected to be somewhat less than 72.5 ksi.
[0064] As shown in Table 9, a heat treatment for the Alloy 1 impact
extrusion resulted in a remarkable yield strength level of 91.3
ksi. This strength level far exceeds the 28.5 ksi yield strength
attained by Yanagida and the 59 ksi yield strength level attained
by Bergsma. Hunt did not anticipate a method to successfully
produce impact extrusion starting stock nor a suitable means to
achieve a high strength impact extruded product; a forging was
expected to achieve a yield strength of 72.5 ksi, thus a
cold-working operations such as impact extrusion would be expected
to provide a yield strength less than 72.5 ksi. In contrast, this
invention uncovered a method to successfully produce an impact
extrusion and achieve a yield strength value of 91.3 ksi in the
final part. This dual accomplishment of good impact extrusion
capability and properties that far exceed those reported or even
anticipated will result in numerous applications for high strength
impact extruded components.
* * * * *