Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR IMPROVING MECHANICAL PROPERTIES OF A BETA
PROCESSED TITANIUM ALLOY ARTICLE
TECHNICAL FIELD
Embodiments described herein generally relate to methods for improving
mechanical
properties of a beta processed titanium alloy article. More particularly,
embodiments
herein generally describe methods for improving the ductility of a beta
processed Ti-
6246 article.
BACKGROUND OF THE INVENTION
Turbine engine designers are continuously looking for new materials with
improved
properties for reducing engine weight and obtaining higher engine operating
temperatures. In order to function in this environment, materials used must
have
sufficient creep strength to survive and perform properly, and must also
maintain
sufficient ductility, toughness, and strength at both room and elevated
temperatures to
be producible and fracture resistant in service. Titanium alloys (Ti alloys)
possess a
promising combination of low-temperature mechanical properties, and high
intermediate temperature strength and creep resistance. For these reasons, Ti
alloys
have the potential to replace nickel-based superalloys, which are currently
used to
make numerous turbine engine components.
Beta processed alpha-beta titanium alloys are one type of titanium alloy that
can be
used to manufacture components suitable for use in gas turbine engines. Alpha-
beta
titanium alloys are alloys having more titanium than any other element, and
which
form predominantly two phases upon heat treatment, an alpha phase and a beta
phase.
In alpha-beta (a-(3) titanium alloys, the alpha (a) phase is a hexagonal close
packed
(HCP) phase that is thermodynamically stable at lower temperatures, and the
beta ((3)
phase is a body centered cubic (BCC) phase that is thermodynamically stable at
temperatures above the "beta transus," which is a temperature that is
particular to the
alloy. Below the beta transus, a mixture of alpha and beta phases is
thermodynamically stable.
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In general, these alloys have excellent mechanical properties relative to
their weight,
at both room temperature and moderate elevated temperatures as high as about
1200 F (about 649 C). Such alloys can be used to make parts such as fan and
compressor disks, blisks, blades, vanes, shafts, and engine mounts, for
example.
Processing alpha-beta titanium alloys above the beta transus temperature can
result in
a greater than approximately 50 F (about 28 C) increase in creep strength when
compared to the same material processed below the beta transus temperature.
However, when alpha-beta titanium alloys are beta processed to increase high
temperature capability, the material can suffer from low ductility, on average
less than
about 3% for example, when tested at angles of from about 35 to about 55
degrees
from the grain flow. Minimum ductility is frequently encountered when the test
axis
angle approaches 45 degrees from the grain flow. See Krull, T. et al.,
Mechanical
Properties of (.3-processed Ti 6246; Ti-2003 Science and Technology;
Proceedings of
the 10th World Conference on Titanium, Hamburg, Germany; 13-18 July 2003.pp.
1871-1878.
Therefore, there remains a need for methods for making beta processed alpha-
beta
titanium alloys having increased temperature capability and acceptable
strength, while
maintaining adequate ductility.
BRIEF DESCRIPTION OF THE INVENTION
Embodiments herein generally relate to methods for improving mechanical
properties
of beta processed, alpha-beta titanium alloy articles comprising forging the
alloy
article above the beta transus to produce a post final forged article,
subjecting the post
final forged article to a post-forged cooling process to produce a post-forged
cooled
article, solution heat treating the post-forged cooled article to a
temperature below the
beta transus to produce a solution heat treated article, subjecting the
solution heat-
treated article to a controlled post-solution cooling process to produce a
post-solution
cooled article, and alpha phase precipitation treating the post-solution
cooled article to
obtain a final article having an average elongation value of at least about
3%.
Embodiments herein also generally relate to methods for improving mechanical
properties of a Ti-6Al-2Sn-4Zr-6Mo alloy article having a beta transus of
about
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1735 F (about 946 C) comprising forging the alloy article above the beta
transus to
produce a post final forged article, subjecting the post final forged article
to a post-
forged cooling process to produce a post-forged cooled article, solution heat
treating
the post-forged cooled article to a temperature below the beta transus to
produce a
solution heat treated article, subjecting the solution heat-treated article to
a controlled
post-solution cooling process to produce a post-solution cooled article, and
alpha
phase precipitation treating the post-solution cooled article to obtain a
final article
having an average elongation value of from about 5% to about 5.8%.
Embodiments herein also generally relate to methods for improving mechanical
properties of a Ti-6Al-2Sn-4Zr-6Mo alloy article having a beta transus of
about
1735 F (about 946 C) comprising forging the alloy article to a temperature of
from
about 1745 F to about 1825 F (about 952 C to about 996 C) to produce a post
final
forged article, subjecting the post final forged article to a post-forged
cooling process
comprising cooling the post-forged article at a post-forged cooling rate of
from about
150 F/minute to about 400 F/minute (about 83 C/minute to about 222 C/minute)
while the temperature of the post final forged article is between about the
beta transus
and about 700 F (about 371 C) to produce a post-forged cooled article,
solution heat
treating the post-forged cooled article to a temperature below the beta
transus of from
about 165 F to about 225 F (about 92 C to about 125 C) for about 4 hours to
produce
a solution heat treated article, subjecting the solution heat-treated article
to a
controlled post-solution cooling process comprising cooling the solution heat-
treated
article at a controlled post-solution cooling rate of from about 50 F/minute
to about
200 F/minute (about 28 C/minute to about 111 C/minute) to produce a post-
solution
cooled article, and alpha phase precipitation treating the post-solution
cooled article at
a temperature of from about 1100 F to about 1350 F (593 C to about 732 C) for
about 8 hours to obtain a final article having an average elongation value of
from
about 5% to about 5.8%.
These and other features, aspects and advantages will become evident to those
skilled
in the art from the following disclosure.
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BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing out and
distinctly
claiming the invention, it is believed that the embodiments set forth herein
will be
better understood from the following description in conjunction with the
accompanying figures, in which like reference numerals identify like elements.
FIG. 1 is a ductility plot reflecting the results described in the EXAMPLE.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments described herein generally relate to methods for improving
mechanical
properties of a beta processed titanium alloy article. In particular,
embodiment
described herein generally relate to methods for improving the ductility of Ti-
6246
(Ti-6A1-2Sn-4Zr-6Mo) articles. While the description herein focuses on Ti-
6246,
those skilled in the art will understand that the methods herein should not be
limited to
such, and may be equally applicable to any alpha-beta titanium alloy such as,
but not
limited to, Ti-6242 (Ti-6A1-2Sn-4Zr-2Mo), Ti-6-22-22S (Ti-6A1-2Sn-2Zr-2Mo-2Cr-
0.25Si), and Ti-17 (Ti-5Al-4Mo-4Cr-2Sn-2Zr).
As set forth herein below, Ti-6246 (referred to henceforth as the "alloy") can
be
processed to improve mechanical properties, and in particular, ductility,
without
negatively impacting other properties below acceptable limits. Initially, the
alloy can
be forged in the beta phase field to produce a post final forged article. Ti-
6246 has a
beta transus temperature of about 1735 F (about 946 C) and forging can be
carried
out from about 10 F to about 90 F (about 6 C to about 50 C) above the beta
transus
temperature, or from about 1745 F to about 1825 F (about 952 C to about 996
C).
This forging temperature can help assure that the alloy is substantially all
in the beta
phase while minimizing excessive beta grain growth. Because it is desirable
that the
finished machined article within the forging envelope remains above the beta
transus
temperature substantially throughout the forging process, heated dies can be
used.
Those skilled in the art of forging will understand that a variety of specific
die
temperatures and strain rates can be employed to achieve this effect.
Independent of
the particular forging operation performed, the embodiments described herein
below
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can be applied post-forging. In general, however, beta forging of alpha-beta
titanium
alloys can employ a height reduction in the post final forged article of at
least about
30%.
Upon completion of the final forging step, the post final forged article can
be
subjected to a post-forged cooling process using a variety of cooling
techniques
known to those skilled in the art, such as, but not limited to, fan air, oil,
gas, and water
quenching, to produce a post-forged cooled article. The cooling rate of this
post-
forged cooling process can be controlled to maintain a balance between
strength and
ductility in the final article. In one embodiment, from the beta transus to
about 700 F
(about 371 C), the post-forged cooling rate may generally be from about
150 F/minute to about 400 F/minute (about 83 C/minute to about 222 C/minute).
This post-forged cooling rate can be maintained until the article reaches a
temperature
of about 700 F (about 371 C). Below approximately 700 F (about 371 C), the
cooling rate is less significant and the article can be cooled at any rate.
It should be noted that the cooling rate used for the post-forged cooling
process can be
dependent on several factors. In some instances, the post-forged cooling rate
can be
outside of the previously described range of from about 150 F/minute to about
400 F/minute (about 83 C/minute to about 222 C/minute) due to such factors as
forging process, article section size, and post-forged cooling configurations,
for
example. For the purposes of the embodiments herein, the cooling rate of the
post-
forged cooling process has a secondary effect on ductility, with the primary
effect
being attributable to the subsequent heat treatment described below.
The post-forged cooled article can then be solution heat treated to a
temperature of
from about 165 F to about 225 F (about 92 C to about 125 C) below the beta
transus,
and held for about 4 hours to produce a solution heat-treated article. This
solution
heat-treated article can then be subjected to a controlled post-solution
cooling process
to produce a post-solution cooled article. Methods suitable for use in the
solution
heating process will be known to those skilled in the art. Examples of
solution heat-
treating methods can include heat-treating in air, vacuum, or inert (i.e.
argon)
atmospheres. The controlled post-solution cooling process can have the most
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significant impact on achieving the desired ductility and may again involve a
variety
of cooling techniques known to those skilled in the art, such as fan air, oil,
gas, and
water quenching. The controlled post solution-cooling rate may be from about
50 F/minute to about 200 F/minute (about 28 C/minute to about 111 C/minute).
The configuration of the post forged cooled article, which may involve rough
machining after the final forge operation, and the specific cooling method,
may be
selected to achieve the desired controlled post-solution cooling rate range.
In
portions of the article where ductility is of less concern, controlled post-
solution
cooling rates above the desired range are acceptable. Similarly, controlled
post-
solution cooling rates that fall below the desired range are acceptable in
portions of
the article where lower strength is allowable.
After the controlled post-solution cooling, the post-solution cooled article
may be
subjected to an alpha phase precipitation treatment at a temperature of from
about
1100 F (about 593 C) to about 1350 F (about 732 C) for a period of about 8
hours,
followed by uncontrolled cooling to about room temperature, to produce a final
article. This precipitation treatment can provide the final article with its
desired
strength and can be carried out in multiple steps to facilitate manufacturing.
For
example, the alpha phase precipitation may be split into two or more exposures
to
relieve residual stresses generated during machining or joining operations.
In final articles resulting from the previously described process, strength is
reduced,
creep is maintained and 45 degree angle ductility is improved to at least
about 3%
when compared with conventionally beta processed and heat treated alpha beta
titanium alloys. As used herein throughout, ductility elongation values are
measured
using a room temperature tensile test elongation taken at 45 degrees to the
grain flow.
More specifically, the resulting final article can have an average elongation
value of at
least about 5%, and in one embodiment from about 5% to about 5.8%.
EXAMPLE
A systematic study of variable thermal response relationships was performed on
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Ti-6Al-2Sn-4Zr-6Mo (nominal) alloy, which has a beta transus (0t) temperature
of
about 1745 F (952 C). The study was carried out as follows:
A forging billet was cut to a diameter of about 8 inches (about 20.3 cm) and a
length
of about 10.5 inches (about 26.7 cm) and heated to a nominal temperature of
beta
transus ((3t) plus 50 F (28 C).
The heated billet was then beta forged into a pancake shape having a diameter
of
about 13 inches (about 33.0 cm) and a thickness of about 4 inches (10.2 cm),
which
equated to a forging height reduction of about 2.5:1, or about 60%.
The final forged article was then subjected to a post-forged cooling process.
Air
cooling was used to cool the article at a rate of about 40 F (about 22 C) per
minute
until the article reached room temperature.
The post-forged cooled article was then cut into four pieces, each piece
comprising
about 1/8, or less, of the article. The four pieces were then solution heat
treated for
approximately 4 hours with each piece being treated at a different temperature
as
outlined below:
Piece 1: 1695 F (924 C), or (3t - 50 F (28 C)
Piece 2: 1645 F (896 C), or (3t - 100 F (56 C)
Piece 3: 1595 F (868 C), or (3t - 150 F (83 C)
Piece4: 1545 F (841 C), or (3t - 200 F (111 C)
The solution heat treated pieces were then subjected to a controlled post
solution
cooling process in which the solution heat treated pieces were cooled at one
of the
following nominal rates:
about 40 F/min (about 22 C/min)
about 100 F/min (about 56 C/min)
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about 170 F/min (about 94 C/min), or
about 600 F/min (about 333 C/min)
Once cooled to about room temperature, each piece was subjected to an alpha
phase
precipitation process for about 8 hours, wherein the pieces were heat treated
at one of
the following temperatures followed by air cooling to about room temperature:
1100 F(593 C), 1150 F(621 C), or 1200 F(649 C)
The resulting final articles were then sectioned to produce tensile test
specimens at
nominally 45 degrees to the grain flow.
A room temperature tensile test was carried out using ASTM E8. The resulting
elongation data was found to be highly dependent on cooling rate from the
solution
temperature as shown in Figure 1. For each cooling rate, the data shown in
Figure 1
represents an average of between 6 and 9 data points gathered during repeated
test
carried out as previously described.
This written description uses examples to disclose the invention, including
the best
mode, and also to enable any person skilled in the art to make and use the
invention.
The patentable scope of the invention is defined by the claims, and may
include other
examples that occur to those skilled in the art. Such other examples are
intended to be
within the scope of the claims if they have structural elements that do not
differ from
the literal language of the claims, or if they include equivalent structural
elements
with insubstantial differences from the literal language of the claims.
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