Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PROCESSING BETA TITANIUM ALLOYS
INVENTOR
Brian J. Marquardt
FIELD OF INVENTION
The invention relates to a method for processing titanium alloys and,
more particularly, beta titanium alloys. The method of the present invention
includes cold working a beta titanium alloy and subsequently direct aging the
alloy
For less than 4 hours.
DESCRIPTION
The unique properties of titanium alloys allow their use in
applications requiring high corrosion resistance, high strength, and low
material
weight. For cost reasons, applications requiring corrosion resistance often
utilize
low-strength unalloyed titanium mill products. Unalloyed titanium may be
fabricated into equipment used in, for example, chemical processing,
desalination,
and power generation. In contrast, high performance applications often utilize
high-strength titanium alloys in a very selective manner depending on several
design factors including weight, strength, ductility, and reliability
requirements. To
meet the requirements of their specialized uses, alloys intended for high
performance applications normally are more stringently processed, with
resulting
additional cost, than titanium for corrosion service. Nevertheless, the
combination
of high strength and stifFness, favorable toughness, low density, and good
corrosion resistance inherent in various titanium alloys useful in low to
moderate
temperature applications allows substantial weight savings in aerospace
structures and other high-performance applications. Such weight savings often
justify the increased costs associated with processing the titanium alloys.
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A titanium alloy may be classified as one of several metallurgical
types, for example, alpha, near-alpha, alpha-beta, or beta. Beta titanium
alloys
are particularly useful in aerospace structures. Hot worked beta titanium
alloys
may be cold worked to final or near final form. The cold working process
imparts
high strength levels and/or a favorable ductility/strength relationship in the
alloys.
Certain "Aerospace Material Specifications", AMS 4957A and AMS 4958A, define
recommended processing conditions for the beta titanium alloy Ti-3AI-8V-6Cr-
4Zr-
4Mo (referred to herein as Ti-38-644 alloy) to produce round bar or wire
primarily
for use as aerospace coil springs. Typically, aerospace spring applications
require high tensile strength, low density and corrosion resistance. Ti-38-644
alloy includes, by weight, 3.0 to 4.0% aluminum, 7.5 to 8.5% vanadium, 5.5 to
6.5% chromium, 3.5 to 4.5% molybdenum, 3.5 to 4.5% zirconium, maximum
0.14% oxygen, maximum 0.05% carbon, maximum 0.03% nitrogen, and
remainder titanium. AMS 4957B requires certain additional restrictions on
alloy
composition, including maximum 0.30% iron, maximum 0.10% palladium,
maximum 300 ppm hydrogen, maximum 50 ppm yttrium, and maximum total
residual elements 0.40%. According to the AMS specifications, the alloy is
aged
by heating to a temperature within the range of 850°F to 1050°F
(454°C to 566°C)
and held at the selected temperature ~10°F (6°C) for six to
twenty hours. The
required minimum tensile properties, determined according to ASTM E8 or ASTM
EBM, as applicable, depend on the nominal diameter of the round bar or wire
final
product, but in no case are to be less than minimum tensile strength of 180
ksi,
minimum elongation of 8%, and minimum reduction of area ("RA") of 20%.
Whether a titanium alloy is of the alpha, near-alpha, alpha beta, or
beta metallurgical type is influenced by the chemical composition of the
alloy, the
applied heat treatment, and other factors. The metallurgical type designations
refer to the predominant crystalline phase present in the microstructure of
the
alloy at room temperature. Titanium metal has a close packed hexagonal crystal
structure ("hcp"), referred to as "alpha", at room temperature. This structure
may
be transformed to a body-centered cubic ("bcc") crystal structure ("beta") at
elevated temperatures. The temperature at which this transformation occurs is
referred to as the "beta transus temperature". The beta transus temperature
for a
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commercially pure titanium alloy is approximately 1625°F
(885°C). Certain
alloying elements added to pure titanium promote the formation of one or the
other of the alpha and beta crystal structures. Elements that favor the alpha
structure are referred to as "alpha stabilizers", and elements that favor the
beta
structure are referred to as "beta stabilizers". Aluminum, for example, is an
alpha
stabilizer and, therefore, adding aluminum to a titanium alloy increases the
beta
transus temperature. Chromium, iron, molybdenum, and vanadium are beta
stabilizers, and their addition lowers the beta transus temperature,
stabilizing the
beta structure at lower temperatures. The relative amounts of alpha and beta
stabilizers in an alloy and the heat treatment applied to the alloy determine
whether the microstructure of the alloy is predominantly single alpha phase,
single
beta phase, or a mixture of alpha and beta phases over a particular
temperature
range.
The properties of a titanium alloy are related to its microstructure.
Two-phase alpha-beta alloys generally exhibit tensile strengths greater than
single-phase alpha alloys or single-phase beta alloys. Also, alpha-beta alloys
can
be further strengthened by heat treatment because the microstructure may be
manipulated by controlling heating, quenching, and aging cycles.
Many beta titanium alloys are alloyed with more than one beta
stabilizer. With sufficient quantities of beta stabilizer, and suitable
control over
heat treatment and cooling, beta phase may be retained at relatively low
temperatures, below the alloy's normal beta transus temperature. For example,
beta phase may be retained in a titanium alloy by rapid cooling from above and
through the beta transus temperatures, such as by quenching. However, the
titanium alloy must have sufficient quantities of beta stabilizers to prevent
the beta
phase from transforming to alpha phase by martensitic transformation. Titanium
alloys containing beta stabilizers in quantities sufficient to reduce the
alloy's
martensitic transformation temperature to below room temperature but not
sufficient to reduce the beta transus to below room temperature are known as
"metastable" beta titanium alloys. Metastable beta titanium alloys may
maintain at
least a portion of beta structure after heat treatment and cooling to room
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temperature. As used herein, references to a beta titanium alloy are to a
metastable beta titanium alloy as described above.
In addition, unless otherwise indicated, all numbers expressing
quantities of ingredients, time, temperatures, and so forth used in the
present
specification and claims are to be understood as being modified in all
instances by
the term "about." Accordingly, unless indicated to the contrary, the numerical
parameters set forth in the following specification and claims are
approximations
that may vary depending upon the desired properties sought to be obtained by
the
present invention. At the very least, and not as an attempt to limit the
application
of the doctrine of equivalents to the scope of the claims, each numerical
parameter should at least be construed in light of the number of reported
significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting
forth the broad scope of the invention are approximations, the numerical
values
set forth in the specific examples are reported as precisely as possible. Any
numerical value, however, may inherently contain certain errors necessarily
resulting from the standard deviation found in their respective testing
measurements.
An embodiment of the present invention comprises processing a
beta titanium alloy by a method including the steps of cold working the alloy
and
then direct aging the alloy for a total aging time of less than 4 hours. The
beta
titanium alloy may be, for example, Ti-38-644 alloy. The method may include
fabricating the alloy into an article of manufacture such as, for example, a
bar,
wire, a coil spring.
Another embodiment of the present invention is a method for
producing a spring or other article of manufacture from a beta titanium alloy.
The
beta titanium alloy may be, for example, the alloy comprising, by weight, 3.0%
to
4.0% aluminum, 7.5 to 8.5% vanadium, 5.5 to 6.5% chromium, 3.5 to 4.5%
molybdenum, 3.5 to 4.5% zirconium, and titanium. The alloy is hot worked, cold
worked to provide a 5% to 60% reduction, and direct aged for a total time of
less
than 4 hours. As used herein, cold working is defined as various working
processes performed at a temperature below an effective aging temperature of
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the alloy. Cold working of a titanium alloy, therefore, may be performed at
temperatures below the beta transus temperature of the alloy. Cold working
permanently deforms the work piece, which does not return to its original
shape
when the load causing the deformation is removed. The degree of cold working,
typically, is determined by the percentage reduction in cross-sectional area
of the
work piece. Thus, a 5°l° reduction achieved by cold working
refers to a reduction
of 5% in the cross-sectional area of the work piece upon cold working. Any
cold
working technique may be used in embodiments of the present invention. Useful
cold working techniques include, but are not limited to, compression
processes,
drawing, wire drawing, tube drawing, deep drawing, rolling, contour forming,
extruding, cold heading, swaging, coining, forging, tension processes, stretch
forming, and spinning.
Cold working may be used to improve the mechanical properties of
an alloy including hardness, yield strength, and tensile strength. Ductility,
however, may be reduced during cold working. Ductility is a measure of the
ability
of a material to deform plastically without fracture. Elongation or RA in
tensile
testing typically is used as a measure of ductility of material. The method of
the
present invention may be used to increase the strength of beta titanium alloys
while also maintaining good ductility and significantly increasing the aging
response of the alloy.
A beta titanium alloy was prepared and processed according to the
method of the present invention. Its properties were then compared with the
same alloy composition processed using a conventional method including cold
working and heat treating steps. This testing is described in greater detail
below:
A melt of a Ti 38-644 alloy was prepared and cast into an ingot. The
alloy had the average composition, in weight percentage, shown in Table 1. A
first ingot was hot rolled at a temperature not to exceed 1750°F,
annealed and air
cooled.
Ti AI V Cr Zr Mo O Fe C N
Bal. 3.42 7.84 5.95 3.98 4.15 0.08 0.13 0.01 0.006
Table 1: Composition of first Ingot
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A portion of the hot rolled, annealed, and air cooled ingot was
processed by the method of the present invention. Another portion of the hot
rolled, annealed, and air cooled ingot was processed in a conventional manner
for
comparison purposes. The portion processed in the conventional manner was hot
worked, then solution heat treated, and subsequently aged. The heat treatment
parameters were varied to assess the impact on mechanical properties. As is
known in the art, solution heat treating is a heat treatment step wherein an
alloy is
heated to a suitable temperature and held at the temperature for a time period
sufficient to cause one or more constituents of the alloy to enter solid
solution.
The alloy is then cooled rapidly so as to hold the one or more constituents in
solution. Solution heat treatment typically is performed on an alloy to
improve
ductility at a given strength.
Several variations of the conventional heat treatment process were
compared with the process of the present invention. Table 2 includes the
results
of room temperature tensile testing of the alloy of Table 1 processed by the
conventional heat treatment process under various conditions. All tensile
properties reported in Table 2 were determined in accordance with ASTM E 8.
Tensile testing was used to determine ultimate tensile strength ("UTS"), 0.2%
yield
strength, elongation, and RA of the test pieces. RA and elongation are
measures
of ductility of the test pieces. Elongation is the amount of extension of a
test piece
when stressed. In tensile testing, elongation is the increase in gage length,
measured after fracture of the specimen with the gage length, usually
expressed
as a percentage of the original gage length as marked on the test piece.
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Room Temperature Tensile Data As-Rolled and Heat Treated
Solution Cooling Aging CoolingUTS 0.2% Elong.RA Modulus
Heat After Temp. After (ksi)YS (%) (l)
Treatment Solution & Time Age (ksi)
Temp.
& Time at
temperature
as-rolled as-rolledas-rolledAs-rolled128 126 30 67 12.5
1400F / water None None 130 128 24 61 12.8
1 hr quench
1400F / water 900F air 148 138 17 47 13.8
1 hr quench / 8 cool
hr
1400F / air cool 900F air 174 160 17 37 14.1
20 min / 8 cool
hr
1400F / air cool 900F air 193* 180 5* 3* 14.9
20 min / 16 cool
hr
1400F / air cool 900F air 193* 179 5* 4* 14.8
20 min / 24 cool
hr
1400F / air cool 950F air 167 155 20 46 13.8
20 min / 8 cool
hr
1400F / air cool 950F air 184 170 18 43 14.9
20 min / 16 cool
hr
1400F / air cool 950F air 186 174 14 35 14.8
20 min / 24 cool
I hr
= rauea near puncn marK
Table 2: Properties of Conventionally Processed Ti-38-644 Alloy
The test pieces listed in Table 2 were hot rolled from 4 inch diameter
billet to 0.569 inch diameter bar and solution heat treated prior to aging.
The data
in Table 2 clearly shows that long aging times, more than 8 hours, are
required to
achieve high strength, greater than 180 ksi, in the alloy. For both of the
tested
solution heat treatment processes (1400°F (760°C) for one hour
and 1400°F
(760°C) for 20 minutes), the conventional process required more than 8
hours of
aging to achieve the minimum tensile strength for Ti-38-644 bar and,wire
specified
in AMS 4957A and AMS 4958B. AMS 4958A specifies that the beta titanium alloy
must receive no more than 5% cold work reduction after hot rolling and
solution
heat treating. AMS 4958A also requires that the alloy be subjected to aging
temperatures for at least 12 hours. Additionally, due to solution heat
treatment
and aging at elevated temperatures, an oxide layer may form on the alloy
surface.
AMS 4958A requires an acid pickling step to remove this layer.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
The aging time of an alloy may be measured and expressed by
different criteria. For example, the length of the aging process may be
measured
as the total time the alloy is exposed to the aging temperature in a furnace,
or as
the total time that the surface or an internal portion of the alloy is
maintained
within the aging temperature range. Unless otherwise noted, all aging times
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reported herein for embodiments of the present invention are the total time
that
the alloy is exposed to an environment at approximately the desired aging
temperature. Aging of the test piece samples listed in the examples was
performed in a laboratory oven. More efficient means for heating an alloy,
such
as, for example, a production convection oven, may allow faster heat transfer
to
the alloy and thereby reduce the minimum aging time necessary to impart
desired
properties to the alloy. The method of the present invention is not limited to
the
embodiments described herein, including the particular aging equipment used,
but
includes various other embodiments. Thus, the embodiments of the present
invention presented herein are only examples of the invention, and the scope
of
the invention is not so restricted.
An embodiment of the process of the present invention includes
direct aging a beta titanium alloy for less than 4 hours after a step of cold
working.
Prior to cold working, the beta titanium alloy may be hot worked. After hot
working, and prior to cold working, the alloy also may be annealed. A
preferred
annealing temperature for beta titanium alloys is 1425°F
(774°C). Strength and
ductility has been shown to be nearly identical for test pieces that have been
annealed and test pieces that have not been annealed prior to cold working and
aging by the process of the present invention.
The features and advantages of embodiments of the present
invention may be better understood by reference to the accompanying figures,
in
which:
Figure 1 is a graph depicting the effect of aging time on UTS, 0.2%
yield strength, elongation, and RA of a Ti-38-644 alloy subject to 13% or 15%
cold
work reduction and aged at 950°F (510°C);
Figure 2 is a graph depicting the effect of aging time and aging
temperature on UTS of a Ti-38-644 alloy subjected to 13% or 15% cold work
reduction and aged at 950°F (510°C), 1000°F
(538°C) and 1050°F (566°C); and
Figure 3 is a graph depicting the effect of aging time and aging
temperature on the RA of a Ti-38-644 alloy subjected to 13% or 15% cold work
reduction and aged at 950°F (510°C), 1000°F
(538°C), or 1050°F (566°C).
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Test pieces of the alloy of Table 1 were processed according to the
method of present invention. It will be understood that the method, of the
present
invention is applicable to other alloy compositions and is not limited to the
application of the method described herein. By employing the present
invention, a
~ relatively high strength beta titanium alloy may be produced in a relatively
short
time while maintaining ductility. Embodiments of the present invention are
listed
in Tables 3-9. In each case, the test pieces were direct aged for a total
aging time
of less than 4 hours after a cold work step. Direct aging an alloy includes
aging
the alloy after working without an intermediate heat treating step, such as
solution
heat treating. Direct aging does not preclude other processing steps from
being
performed after cold working the alloy and prior to aging the alloy. These
processes may be, for example, mechanical processes, such as shaving, or
chemical processes, such as, pickling. The tables list the process steps
employed
and the mechanical properties of the processed alloy samples derived from
tensile
testing at room temperature.
Tables 3-9 list embodiments of the method of the present invention
applied to the beta titanium alloy having the composition of Table 1. The
amount
of cold work may be to any degree and, preferably, in an embodiment of the
method of the present invention the beta-titanium alloy is cold worked from at
least a 5% reduction to a 60% reduction. Even more preferably, cold working
the
beta-titanium alloy comprises less than a 35% reduction. More preferably, an
embodiment of the method of the present invention includes cold working the
beta-titanium alloy to a reduction between 15% and 35%. With regard to Table
3,
the test pieces were hot rolled, cold drawn to provide 8% reduction and then
direct
aged at the temperatures and for the times shown in the tables. The test
pieces
described in Table 3 also were annealed and centerless ground prior to cold
drawing. The embodiments listed in Table 3 produced high strength (UTS greater
than 170 ksi) and maintained ductility (greater than 8% elongation and greater
than 20% RA) with less than four hours of direct aging. UTS values greater
than
180 ksi and as high as 199 ksi were realized in the listed embodiments. The
highest UTS values were realized at aging temperatures of 950°F
(510°C), at
which a UTS of 199 ksi was achieved at a total aging time of only 166 minutes.
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The highest ductility as measured by elongation and RA was realized at the
higher
aging temperature of 1050°F (566°C).
Aging Temp.Aging UTS UTS 0.2% 0.2% Elong.RA Modulus
(F) [C] Time (ksi)(MPa)YS YS (%) (%)
(minutes) (ksi) (MPa)
- 0 140.5969 132.5 913 19 61 12.0
950 [510] 166 199.01372 182.2 1256 14 41 14.4
950 [510] 170 197.51362 180.6 1245 14 35 13.4
1000 [538] 125 186.71287 168.7 1163 18 42 14.2
1000 [538] 200 186.01282 167.5 1155 18 41 14.9
1050 [565] 133 175.11207 156.9 1082 20 49 14.4
1050 [565] 182 172.81191 155.3 1071 21 52 14.5
I I I ~
Table 3: Tensile Testing Results for Embodiments of the Present Invention with
8% Cold Work Reduction
Table 4 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 13% reduction, and direct aged.
Additionally, the embodiments described in Table 4 were annealed and
centerless
ground after hot rolling and prior to cold drawing. The embodiments of the
method of the present invention in Table 4 displayed significantly increased
strength after only 20 minutes of total aging time. With further aging at
aging
temperatures of 950°F (510°C) and 1000°F (538°C)
the strength increased to a
value greater than required by AMS 4958A and 4957B specifications. The test
pieces aged at 1050°F (565°C), however, did not obtain strength
as high as the
test pieces aged at lower aging temperatures. The test pieces aged at
1050°F
(565°C) did maintain a greater degree of ductility as measured by
elongation and
RA.
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Aging Temp.Aging UTS UTS 0.2% 0.2% Elong.RA Modulus
(F) [C] Time (ksi) (Mpa) YS YS (%) (%)
(minutes) (ksi) (MPa)
as-drawn As-drawn145.3 1002 137.5 948 17 55 11.0
950 [510] 20 172.8 1191 163.1 1124 21 50 13.7.
950 [510] 166 203.5 1403 187.1 1290 14 32 15.0
950 [510] 170 202.9 1399 185.8 1281 15 36 15.1
1000 [538] 20 168.7 1163 156.8 1081 24 51 14.4
1000 [538] 125 189.9 1309 172.1 1186 18 44 14.7
1000 [538] 200 189.8 1308 173.3 1195 16 41 15.0
1050 [565] 20 164.4 1133 151.3 1043 26 51 14.4
1050 [565] 133 178.7 1232 161.7 1115 20 47 14.4
1050 [565] 182 176.6 1217 159.3 1098 20 52 ~ 14.0
Table 4: Tensile Testing Results for Embodiments of the Present Invention with
13% Cold Work Reduction
Table 5 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 13% reduction, and direct aged, in a
fashion
similar to the embodiments shown in Table 4. However, the test pieces listed
in
Table 5 were not annealed and centerless ground prior to cold drawing.
Nevertheless, the embodiments of the invention listed in Table 5 produced test
pieces that exhibited high strength and ductility. The embodiments of Table 5
produced very high strength (UTS above 190 ksi) in the beta titanium alloy
when
aged for as short as 69 to 72 minutes. The results show that the annealing
step
may be excluded in embodiments of the present invention without significant
affect on mechanical properties when the invention is applied to the beta
titanium
alloy of Table 1.
Aging Temp.Aging UTS UTS 0.2% 0.2% Elong.RA Modul
Time YS YS us
(F) [C] (ksi) (Mpa) (%) (%)
(minutes) (ksi) (MPa)
as-drawn As-drawn 147.2 1015141.0 972 18 67 12.4
950 [510] 69 199.3 1374181.0 1248 18 37 14.6
950 [510] 94 199.7 1377181.7 1253 17 42 15.1
1000 [538]72 194.7 1342176.8 1219 20 43 14.5
1000 [538]89 190.2 1311173.3 1195 20 37 14.6
1000 [538]125 190.8 1315172.8 1191 16 45 14.5
1000 [538]200 191.8 1322173.8 1198 16 46 15.1
1050 [565]81 179.0 1234162.2 1118 24 57 15.0
1050 [565]88 178.9 1233161.6 1114 24 57 14.6
Table 5: Tensile Testing Results for Embodiments of the Nresent Invention witn
13% Cold Work Reduction and Without Anneal
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Table 6 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 15% reduction, and direct aged.
Additionally, the test pieces of Table 6 were not annealed and centerless
ground
prior to cold drawing. Certain embodiments of the present invention in Table 6
included aging times less than 60 minutes. The embodiments including cold
working to 15% reduction showed higher strengths than the embodiments
including cold working to only 8% reduction, without a corresponding loss of
ductility. The embodiments cold worked to 15% reduction achieved UTS greater
than 190 ksi after aging for only 45 minutes of total aging time at
900°F (482°C)
and 950°F (510°C), and achieved UTS greater than 200 ksi after
aging for only 60
minutes of total aging time at the same temperatures.
Aging Aging UTS 0.2% Elong. RA
Temp Time (ksi) YS (%) (%)
(F) [C] (minutes) (ksi)
- 0 148.4 146.3 19.3 65.9
900 [482]45 192.5 177.2 15.8 45.2
900 [482]60 206.1 190.4 11.4 40.6
900 [482]60 200.5 189.3 13.4 40.0
900 [482]120 212.2 192.9 16.3 35.7
~
950 [510]30 179.4 164.5 17.2 50.5
950 [510]45 190.3 172.2 16.9 45.7
950 [510]60 195.2 174.8 15.8 40.6
950 [510]60 197.5 186.4 13.7 37.8
950 [510]60 195.2 183.5 13.5 37.6
950 [510]156 207.6 187.0 14.8 37.0
1000 45 187.8 167.7 18.2 45.6
[538]
1000 60 188.4 175.8 15.8 44.0
[538]
1000 60 188.3 175.7 16.8 ~ 44.6
[538]
Table 6: Tensile Testing Results for Embodiments of the Present Invention with
15% Cold Work Reduction
Table 7 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 19%, and direct aged. Additionally, the
embodiments described in Table 7 were annealed and centerless ground prior to
cold drawing.
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Aging Aging UTS UTS 0.2% 0.2% Elong.RA Modulus
Temp. Time (ksi) (MPa) YS YS (%) (%)
(F) [C] (minutes) (ksi)(MPa)
as-drawn 0 153.3 1057 141.0972 13 57 13.3
950 [510]166 210.2 1449 193.31333 12 27 14.2
950 [510]170 209.4 1444 191.61321 14 31 14.7
1000 [538]72 191.7 1322 173.81198 22 47 15.4
1000 [538]89 196.9 1357 179.31236 19 32 15.3
1000 [538]125 196.5 1355 179.11235 14 33 14.1
1000 [538]200 196.0 1351 178.61231 15 40 14.4
1050 [565]81 183.8 1267 166.61149 22 54 14.1
1050 [565]88 186.3 1284 169.01165 23 52 15.1
1050 [565]133 183.1 1262 165.41140 20 54 13.6
1050 [565]182 181.7 1253 164.51134 20 50 15.1
I
I axle ~: I ensue I estlng Kesuits for tmbodlments of the Present Invention
with
19% Cold Work Reduction
Table 8 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 20% reduction, and direct aged.
Additionally, the test pieces of Table 8 were not annealed and centerless
ground
prior to cold drawing. The embodiments of the present invention in Table 8
produced an increase in UTS of approximately 5% and an increase in 0.2% yield
strength of 6% over the embodiments employing a cold work of 15% reduction.
Cold working to 20% reduction reduced ductility by either 5% (as measured by
elongation) or 9% (as measured by RA).
Aging Aging UTS 0.2% Elong. RA
Temp Time (ksi) YS (%) (%)
(F) [C] (minutes) (ksi)
- 0 155.2 152.0 16.4 63.5
900 [482]45 201.1 185.9 15.3 40.6
900 [482]120 216.0 199.4 9.3 36.4
950 [510]30 188.1 173.9 17.3 50.3
950 [510]45 200.8 184.0 17.4 43.8
950 [510]60 205.0 187.2 13.2 36.9
950 [510]156 214.8 196.3 14.5 32.5
1000 [538]45 194.2 174.7 17.2 40.4
1000 [538]60 196.5 176.9 18.0 40.0
Table 8: Tensile Testing Results for Embodiments of the Present Invention with
20% Cold Work Reduction
Table 9 lists embodiments of the present invention wherein test
pieces were hot rolled, cold drawn to 25% reduction, and direct aged.
Additionally, the embodiments described in Table 9 were not annealed and
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centerless ground prior to cold drawing. The embodiments of the present
invention listed in Table 9 on average show an increase in UTS of
approximately
7% and an increase in 0.2% yield strength of 9% over the embodiments which
utilized cold working to 15% reduction. Cold working to 25% reduction reduced
ductility by either 11 % (as measured by elongation) or 2% (measured by RA) as
compared to embodiments utilizing cold working to 15% reduction.
Aging Aging UTS 0.2% Elong. RA
Temp Time (ksi) YS (%) (%)
(F) [C] (minutes) (ksi)
- 0 162.5 159.4 16.9 64.0
900 [482]45 207.2 193.0 13.7 43.8
900 [482]120 220.9 204.6 15.2 34.9
950 [510]30 194.2 180.8 16.9 48.7
950 [510]45 205.1 189.9 15.4 43.2
950 [510]60 207.6 189.3 14.0 39.4
950 [510]156 212.7 193.7 16.4 33.8
1000 [538]45 I ~ 99.3 L 181.7-- 16.0~ 46.5
I -~
Table 9: Tensile Testing Results for Embodiments of the Present Invention with
25% Cold Work Reduction
The tensile properties of embodiments of the present invention
including the step of cold working to 13% or 15% reduction are shown
graphically
in Figures 1 to 3. Figure 1 graphically depicts the effect of aging time on
samples
of Ti-38-644 beta titanium alloy having the composition shown in Table 1 and
wherein the method included a step of cold working to a 13 or 15% reduction.
The UTS and 0.2% yield strength increase rapidly for at least the first 60
minutes
of total aging time. For these embodiments, UTS of the test pieces reached 180
ksi in approximately 30 minutes of total aging time. These test pieces were
aged
in a conventional laboratory testing oven. Production aging furnaces would
likely
more efficiently heat articles and, therefore, in a production furnace it is
expected
that total aging times in the method of the present invention necessary to
reach
high strength (180 ksi, for example) may be reduced, possibly by two thirds or
more in some cases.
The aging of the beta titanium alloy may be conducted at an
temperature below the beta transus. Preferably, the aging of the beta titanium
alloy occurs at a temperature between 800°F (427°C) and
1100°F (538°C). For
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some applications, the aging of the beta titanium alloy may occur between
800°F
(427°C) and 1000°F (538°C), and more preferably between
900°F (482°C) and
1000°F (538°C).
It can been seen in Figure 1 that ductility of the test pieces, as
measured by elongation or RA, decreased with total aging time. However,
ductility decreases slowly with total aging time, and UTS of over 200 ksi was
achieved while maintaining relatively good ductility. For certain uses, such
as in
the production of suspension springs for automobile, snow mobile, motorcycle
and
other recreational vehicles and valve springs for piston engines, short aging
times
are preferred. Automobile production lines may include installations for
winding
and aging springs as required for production. The springs may be, for example,
wound and then aged on a conveyer belt as the belt passes through an aging
furnace. Preferably in these and other applications, aging of the beta-
titanium
alloy will be for a period of less than 3 hours. More preferably the aging of
the
beta-titanium alloy will be for a period of less than 2 hours, and even more
preferably for some time sensitive applications the aging will be for a period
of
less than 1 hour or more preferably for less than 45 minutes. Alloys produced
by
the present invention may also be useful in other.applications than springs,
such
as, for example, in the biomedical industry for surgical instruments or
implants.
Figure 2 depicts the effect of aging time and temperature on UTS of
test pieces of the beta titanium alloy of Table 1 processed by embodiments of
the
present invention including cold working to 13% or 15% reduction. The
embodiments of the present invention employing aging at lower temperatures
achieved higher UTS. This may be expected due to crystalline growth at higher
temperatures and the lower volume of alpha phase present in the alloy as a
result
of the processing conditions, which both may adversely effect the strength of
a
beta titanium alloy.
Figure 3 depicts the effect of aging time and temperature on ductility
of test pieces of the beta titanium alloy of Table 1, as measured by reduction
in
area, using embodiments of the present invention including cold working to 13%
or 15% reduction. The embodiments of the present invention utilizing aging at
higher temperatures produced higher ductility in the test pieces over time.
This
CA 02468263 2004-05-25
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may be expected due to crystalline growth at higher temperatures which,
although
adversely effecting strength, enhances ductility of the beta titanium alloy.
A second titanium ingot was produced and processed according to
method of the present invention. The composition of the second ingot at three
locations is shown in Table 10. The composition of the alloy was tested in
three
locations to verify the composition and ensure a fairly consistent composition
throughout the ingot.
Source Ti AI V Cr Zr Mo O Fe C N
Top of Second Bal.3.65 7.956.16 4.06 4.08 0.1 0.05 0.010.01
Ingot
Middle of SecondBal.3.45 7.96.29 4.12 4.04 0.1 0.06 0.020.01
Ingot
Bottom of SecondBal.~ 7.856.43 4.14 3.98 0.1 0.06 0.010.01
Ingot ~ 3.34
I
Table 10: Comaosition
of Second Inaot
The second ingot was processed according to the method of the
present invention. The second ingot was hot rolled at a temperature not to
exceed 1825°F (996°C), annealed and air cooled. With regard to
Table 11, test
pieces produced from the second ingot were hot rolled, cold drawn to provide
16.5% reduction and then direct aged at the temperatures and for the times
shown in the table. The test pieces described in Table 11 also were annealed
at a
temperature not to exceed 1450°F (774°C) and air cooled prior to
cold drawing.
The embodiments listed in Table 11 produced higher strength (UTS greater than
190 ksi) and maintained ductility (greater than 8% elongation and greater than
20% RA) with less than 30 minutes of direct aging. UTS values greater than 200
ksi and as high as 220 ksi were realized in the listed embodiments. Again, the
highest UTS values were realized at the lower aging temperatures, 900°F
(482°C), at which a UTS of 220 ksi was achieved at a total aging time
of only 60
minutes. The highest ductility as measured by elongation and RA was realized
at
the higher aging temperature of 1050°F (566°C).
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Aging Aging UTS UTS 0.2% 0.2% Elong.RA
Temp Time (ksi) (MPa) YS YS (%) (%)
(F) [C] (minutes) (ksi) (MPa)
NA 0 164.2 1132 150.8 1040 16.1 52.5
NA 0 154.6 1066 149.2 1029 17.9 52.9
900 [482]30 205.5 1417 191.0 1317 11.5 33.3
900 [482]45 207.6 1431 191.7 1322 11.5 31.4
900 [482]45 216.0 1489 197.7 1363 10.9 29.1
900 [482]60 220.4 1519 202.7 1397 11.0 30.4
900 [482]60 216.2 1490 201.1 1386 10.5 28.0
NA 0 164.2 1132 150.8 1040 16.1 52.5
NA 0 154.6 1066 149.2 1029 17.9 52.9
950 [510]30 198.7 1370 182.7 1260 13.7 35.9
950 [510]30 198.7 1370 181.3 1250 14.3 35.0
950 [510]45 207.0 1427 191.7 1322 13.7 32.0
950 (510]45 205.1 1414 190.5 1313 12.6 30.8
950 [510]60 210.5 1451 192.6 1328 13.8 24.7
.
950 [510]60 209.3 1443 193.5 1334 13.1 29.8
NA 0 164.2 1132 150.8 1040 16.1 52.5
NA 0 154.6 1066 149.2 1029 17.9 52.9
1000 30 190.9 1316 175.2 1208 17.6 37.0
[538]
1000 45 197.8 1364 182.4 1257 14.0 36.8
[538]
1000 45 199.9 1378 182.9 1261 20.4 35.1
[538]
1000 60 201.5 1389 185.1 1276 - 34.5
[538]
1000 60 204.7 1411 189.5 1306 16.0 39.5
[538]
Table
11:
Tensile
Testing
Results
for
Embodiments
of
the
Present
Invention
produced
from
the
Second
Ingot
with
16.5%
Cold
Work
Reduction
Generally, the test pieces produced by an embodiment of the
process of the present invention as described in Table 11 achieved higher
tensile
strengths with shorter aging times than the test pieces produced by the
embodiments of the process of the present invention as described in Tables 3
to
9. However, generally, the ductility of the test pieces described in Table 11
were
lower. It is believed that the higher hot rolling temperature experienced by
the
second ingot produced the lower ductility since the higher processing
temperatures favored a larger prior beta grain size. The higher strength is
thought
to be associated with slower cooling after the anneal which allowed for some
aging prior to cold working.
Table 12 shows the results of Rotating Beam Fatigue Testing on
articles prepared by the method of the present invention wherein the articles
were
hot rolled, cold drawn to 15% reduction, and direct aged at 950°F
(510°C) for one
hour. The Rotating Beam Fatigue Testing was conducted to determine the
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bending fatigue according to international testing standard ISO 1143 at a
frequency of 50 Hz, R= -1 and using a smooth bar. The results indicate the
number of cycles experienced for each specimen prior to failure or the total
number of cycles performed on the specimen if no failure occurred.
Max StressMax Stress
ksi Mpa Cycles Comment
73 500 13401000Run out, no failure
83 575 10017100Run out, no failure
87 600 10804700Run out, no failure
87 600 151900 Failure
91 625 620800 Grip Fail
94 650 525100 Failure
98 675 79300 Failure
102 700 395200 Failure
Table 12: Rotating Beam Fatigue Testing Results for Embodiments of the Present
Invention comprising 15% Cold Work Reduction and direct aging at 950°F
(510°C) for
one hour.
Table 13 shows the results of Load Controlled Axial Fatigue Testing
on articles prepared by the method of the present invention wherein the
articles
were hot rolled, cold drawn to 15% reduction, and direct aged at 950°F
(510°C)
for one hour. The Load Controlled Axial Fatigue Testing was conducted to
determine the fatigue of the articles according to ASTM E-466-96 with a
frequency
of 29 Hz at R= 0.1. The results indicate the number of cycles experienced for
each specimen prior to failure. Specimen prepared using different conditions
in
the method of the present invention, such as, a longer aging time, different
aging
temperature or difFerent degree of cold working, for example, may result in an
increase in the number of cycles prior to failure in the fatigue tests.
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Max StressMax Stress
ksi MPa Cycles Comment
142 979 2313507Failure
145 1000 286613 Failure
150 1034 170773 Failure
160 1103 22532
Failure
Table
13:
Load
Controlled
Axial
Fatigue
Testing
Results
for
Embodiments
of
the
Present
Invention 15% Cold
comprising Work Reduction
and direct
aging
at 950F
(510C)
for
one
hour.
Though the method of the present invention is described above with
respect to beta titanium alloys of certain compositions, it is believed that
the
method of the present invention has wider application, to the processing of
other
beta titanium alloys. For example, without limiting the method of the present
invention, some additional commercially available beta titanium alloys that
may
benefit from the present invention are titanium alloys having the following
nominal
compositions, in weight percentages. Ti-12Mo-6Zr-2Fe (an alloy comprising 12%
molybdenum, 6% zirconium, 2% iron and titanium, and which is available
commercially in at least one form as ALLVAC TMZF alloy); Ti-4.5Fe-6.8Mo-1.SAI
(an alloy comprising 4.5% iron, 6.8% molybdenum, 1.5% aluminum and titanium,
and which is available commercially in at least one form as TIMETAL LCB
alloy);
Ti-15Mo-2.6Nb-3AI-0.2Si (an alloy comprising 15% molybdenum, 2.6% niobium,
3% aluminum, 0.2% silicon and titanium, and which is available commercially in
at
least one form as TIMETAL 21 S alloy); Ti-15V-3Cr-3Sn-3AI (an alloy comprising
15% vanadium, 3% chromium, 3% tin, 3% aluminum and titanium, and which is
available commercially in at least one form as ALLVAC 15-3 alloy), Ti-11.SMo-
6Zr-4.5Sn (an alloy comprising 11.5% molybdenum, 6% zirconium, 4.5 tin and
titanium, and which is available commercially in at least one form as UNITEK
Beta
III alloy); and Ti-6V-6Mo-5.7Fe-2.7AI (an alloy comprising 6% vanadium, 6%
molybdenum, 5.7% iron, 2.7 aluminum and titanium, and which is available
commercially in at least one form as TIMETAL 125 alloy). The compositions of
the alloys presented above are nominal compositions, and the content of each
element may vary by at least as much as 2% or more and the alloys may also
include additional components.
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It is to be understood that the present description illustrates those
aspects of the invention relevant to a clear understanding of the invention.
Certain aspects of the invention that would be apparent to those of ordinary
skill in
the art and that, therefore, would not facilitate a better understanding of
the
invention have not been presented in order to simplify the present
description.
Although the present invention has been described in connection with certain
embodiments, those of ordinary skill in the art will, upon considering the
foregoing
description, recognize that many modifications and variations of the invention
may
be employed. All such variations and modifications of the invention are
intended
to be covered by the foregoing description and the following claims.
