Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
METHODS FOR PROCESSING TITANIUM ALLOYS
INVENTORS
David J. Bryan
John V. Mantione
Jean-Philippe Thomas
STATEMENT REGARDING FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
[0001] This invention was made with United States government support under
N1ST Contract Number 70NANB7H7038, awarded by the National Institute of
Standards
and Technology (NISI), United States Department of Commerce. The United States
government may have certain rights in the invention.
BACKGROUND OF THE TECHNOLOGY
FIELD OF THE TECHNOLOGY
[0002] The present disclosure relates to methods for processing titanium
alloys.
DESCRIPTION OF THE BACKGROUND OF THE TECHNOLOGY
[0003] Methods for producing titanium and titanium alloys having coarse grain
(CG), fine grain (FG), very fine grain (VFG), or ultrafine grain (UFG)
microstructure
involve the use of multiple reheats and forging steps. Forging steps may
include one or
more upset forging steps in addition to draw forging on an open die press.
[0004] As used herein, when referring to the microstructure of titanium
alloys:
the term "coarse grain" refers to alpha grain sizes of 400 pm down to greater
than about
14 pm; the term "fine grain" refers to alpha grain sizes in the range of 14 pm
down to
greater than 10 pm; the term "very fine grain" refers to alpha grain sizes of
10 pm down
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to greater than 4.0 pm; and the term "ultrafine grain" refers to alpha grain
sizes of 4.0
pm or less.
[0005] Known commercial methods of forging titanium and titanium alloys to
produce coarse grain or fine grain microstructures employ strain rates of 0.03
s-1 to
0.10 s-1 using multiple reheats and forging steps.
[0006] Known methods intended for the manufacture of fine grain, very fine
grain, or ultrafine grain microstructures apply a multi-axis forging (MAF)
process at an
ultra-slow strain rate of 0.001 s-1 or slower (see, for example, G.
Salishchev, et. al.,
Materials Science Forum, Vol. 584-586, pp. 783-788 (2008)). The generic MAF
process
is described in, for example, C. Desrayaud, et. at, Journal of Materials
Processing
Technology, 172, pp. 152-156 (2006).
[0007] The key to grain refinement in the ultra-slow strain rate MAF process
is
the ability to continually operate in a regime of dynamic recrystallization
that is a result
of the ultra-slow strain rates used, i.e., 0.001 s-1 or slower. During dynamic
recrystallization, grains simultaneously nucleate, grow, and accumulate
dislocations.
The generation of dislocations within the newly nucleated grains continually
reduces the
driving force for grain growth, and grain nucleation is energetically
favorable. The ultra-
slow strain rate MAF process uses dynamic recrystallization to continually
recrystallize
grains during the forging process.
[0008] Relatively uniform cubes of ultrafine grain Ti-6-4 alloy (UNS R56400)
can be produced using the ultra-slow strain rate MAF process, but the
cumulative time
taken to perform the MAF steps can be excessive in a commercial setting. In
addition,
conventional large scale, commercially available open die press forging
equipment may
not have the capability to achieve the ultra-slow strain rates required in
such
.. embodiments and, therefore, custom forging equipment may be required for
carrying
out production-scale ultra-slow strain rate MAF.
[0009] Accordingly, it would be advantageous to develop a process for
producing titanium alloys having coarse, fine, very fine, or ultrafine grain
microstructure
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that does not require multiple reheats, accommodates higher strain rates,
reduces the
time necessary for processing, and/or eliminates the need for custom forging
equipment.
SUMMARY
[0010] According to a non-limiting aspect of the present disclosure, a method
of refining the grain size of a workpiece comprising a titanium alloy
comprises beta
annealing the workpiece. After beta annealing, the workpiece is cooled to a
temperature below the beta transus temperature of the titanium alloy. The
workpiece is
then multi-axis forged. Multi-axis forging comprises: press forging the
workpiece at a
workpiece forging temperature in a workpiece forging temperature range in the
direction
of a first orthogonal axis of the workpiece with a strain rate sufficient to
adiabatically
heat an internal region of the workpiece; press forging the workpiece at a
workpiece
forging temperature in the workpiece forging temperature range in the
direction of a
second orthogonal axis of the workpiece with a strain rate that is sufficient
to
adiabatically heat the internal region of the workpiece; and press forging the
workpiece
at a workpiece forging temperature in the workpiece forging temperature range
in the
direction of a third orthogonal axis of the workpiece with a strain rate that
is sufficient to
adiabatically heat the internal region of the workpiece. Optionally,
intermediate to
successive press forging steps, the adiabatically heated internal region of
the workpiece
is allowed to cool to a temperature at or near the workpiece forging
temperature in the
workpiece forging temperature range, and an outer surface region of the
workpiece is
heated to a temperature at or near the workpiece forging temperature in the
workpiece
forging temperature range. At least one of the press forging steps is repeated
until a
total strain of at least 1.0 is achieved in at least a region of the
workpiece. In another
non-limiting embodiment, at least one of the press forging steps is repeated
until a total
strain of at least 1.0 up to less than 3.5 is achieved in at least a region of
the workpiece.
In a non-limiting embodiment, a strain rate used during press forging is in
the range of
0.2 s-1 to 0.8 s-1.
[0011] According to another non-limiting aspect of the present disclosure, a
non-limiting embodiment of a method of refining the grain size of a workpiece
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comprising a titanium alloy includes beta annealing the workpiece. After beta
annealing, the workpiece is cooled to a temperature below the beta transus
temperature
of the titanium alloy. The workpiece is then multi-axis forged using a
sequence
comprising the following forging steps.
[0012] The workpiece is press forged at a workpiece forging temperature in a
workpiece forging temperature range in the direction of a first orthogonal A-
axis of the
workpiece to a major reduction spacer height with a strain rate that is
sufficient to
adiabatically heat an internal region of the workpiece. As used herein, a
major
reduction spacer height is a distance equivalent to the final forged dimension
desired for
each orthogonal axis of the workpiece.
[0013] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of a second
orthogonal B-axis
of the workpiece in a first blocking reduction to a first blocking reduction
spacer height.
The first blocking reduction is applied to bring the workpiece back to
substantially the
pre-forging shape of the workpiece. While the strain rate of the first
blocking reduction
may be sufficient to adiabatically heat an internal region of the workpiece,
in a non-
limiting embodiment, adiabatic heating during the first blocking reduction may
not occur
because the total strain incurred in the first blocking reduction may not be
sufficient to
significantly adiabatically heat the workpiece. The first blocking reduction
spacer height
is larger than the major reduction spacer height.
[0014] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of a third orthogonal
C-axis of
the workpiece in a second blocking reduction to a second blocking reduction
spacer
height. The second blocking reduction is applied to bring the workpiece back
to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
blocking reduction may be sufficient to adiabatically heat an internal region
of the
workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
reduction may not occur because the total strain incurred in the second
blocking
reduction may not be sufficient to significantly adiabatically heat the
workpiece. The
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second blocking reduction spacer height is greater than the major reduction
spacer
height.
[0015] The workpiece is press forged at a workpiece forging temperature in the
workpiece forging temperature range in the direction of the second orthogonal
B-axis of
the workpiece to the major reduction spacer height with a strain rate that is
sufficient to
adiabatically heat an internal region of the workpiece.
[0016] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of the third
orthogonal C-axis
of the workpiece in a first blocking reduction to the first blocking reduction
spacer height.
The first blocking reduction is applied to bring the workpiece back to
substantially the
pre-forging shape of the workpiece. While the strain rate of the first
blocking reduction
may be sufficient to adiabatically heat an internal region of the workpiece,
in a non-
limiting embodiment, adiabatic heating during the first blocking reduction may
not occur
because the total strain incurred in the first blocking reduction may not be
sufficient to
significantly adiabatically heat the workpiece. The first blocking reduction
spacer height
is larger than the major reduction spacer height.
[0017] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of the first
orthogonal A-axis of
the workpiece in a second blocking reduction to the second blocking reduction
spacer
height. The second blocking reduction is applied to bring the workpiece back
to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
blocking reduction may be sufficient to adiabatically heat an internal region
of the
workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
reduction may not occur because the total strain incurred in the second
blocking
reduction may not be sufficient to significantly adiabatically heat the
workpiece. The
second blocking reduction spacer height is larger than the major reduction
spacer
height.
[0018] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of the third
orthogonal C-axis
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of the workpiece in a major reduction to the major reduction spacer height
with a strain
rate that is sufficient to adiabatically heat an internal region of the
workpiece.
[0019] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of the first
orthogonal A-axis of
the workpiece in a first blocking reduction to the first blocking reduction
spacer height.
The first blocking reduction is applied to bring the workpiece back to
substantially the
pre-forging shape of the workpiece. While the strain rate of the first
blocking reduction
may be sufficient to adiabatically heat an internal region of the workpiece,
in a non-
limiting embodiment, adiabatic heating during the first blocking reduction may
not occur
because the total strain incurred in the first blocking reduction may not be
sufficient to
significantly adiabatically heat the workpiece. The first blocking reduction
spacer height
is larger than the major reduction spacer height.
[0020] The workpiece is press forged at the workpiece forging temperature in
the workpiece forging temperature range in the direction of the second
orthogonal B-
axis of the workpiece in a second blocking reduction to the second blocking
reduction
spacer height. The second blocking reduction is applied to bring the workpiece
back to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
blocking reduction may be sufficient to adiabatically heat an internal region
of the
workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
reduction may not occur because the total strain incurred in the second
blocking
reduction may not be sufficient to significantly adiabatically heat the
workpiece. The
second blocking reduction spacer height is larger than the major reduction
spacer
height.
[0021] Optionally, intermediate successive press forging steps of the
foregoing
method embodiment, the adiabatically heated internal region of the workpiece
is
allowed to cool to about the workpiece forging temperature in the workpiece
forging
temperature range, and the outer surface region of the workpiece is heated to
about the
workpiece forging temperature in the workpiece forging temperature range. At
least one
of the foregoing press forging steps of the method embodiment is repeated
until a total
strain of at least 1.0 is achieved in at least a region of the workpiece. In a
non-limiting
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embodiment of the method, at least one of the press forging steps is repeated
until a
total strain of at least 1.0 and up to less than 3.5 is achieved in at least a
region of the
workpiece. In a non-limiting embodiment, a strain rate used during press
forging is in
the range of 0.2 s-1 to 0.8 s-1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The features and advantages of apparatus and methods described
herein may be better understood by reference to the accompanying drawings in
which:
[0023] FIG. 1 is graph plotting a calculated prediction of the volume fraction
of
equilibrium alpha phase present in Ti-6-4, Ti-6-2-4-6, and Ti-6-2-4-2 alloys
as a function
of temperature;
[0024] FIG. 2 is a flow chart listing steps of a non-limiting embodiment of a
method for processing titanium alloys according to the present disclosure;
[0025] FIG. 3 is a schematic representation of aspects of a non-limiting
embodiment of a high strain rate multi-axis forging method using thermal
management
for processing titanium alloys for the refinement of grain sizes, wherein
FIGS. 2(a), 2(c),
and 2(e) represent non-limiting press forging steps, and FIGS 2(b), 2(d), and
2(f)
represent optional non-limiting cooling and heating steps according to non-
limiting
aspects of the present disclosure;
[0026] FIG. 4 is a schematic representation of aspects of a prior art slow
strain
rate multi-axis forging technique known to be used to refine grain size of
small scale
samples;
[0027] FIG. 5 is a flow chart listing steps of a non-limiting embodiment of a
method for processing titanium alloys according to the present disclosure
including
major orthogonal reductions to the final desired dimension of the workpiece
and first
and second blocking reductions;
[0028] FIG. 6 is a temperature-time thermomechanical process chart for a non-
limiting embodiment of a high strain rate multi-axis forging method according
to the
present disclosure;
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[0029] FIG. 7 is a temperature-time thermomechanical process chart for a non-
limiting embodiment of a multi-temperature high strain rate multi-axis forging
method
according to the present disclosure;
[0030] FIG. 8 is a temperature-time thermomechanical process chart for a non-
limiting embodiment of a through beta transus high strain rate multi-axis
forging method
according the present disclosure;
[0031] FIG. 9 is a schematic representation of aspects of a non-limiting
embodiment of a multiple upset and draw method for grain size refinement
according to
the present disclosure;
[0032] FIG. 10 is a flow chart listing steps of a non-limiting embodiment of a
method for multiple upset and draw processing titanium alloys to refine grain
size
according to the present disclosure;
[0033] FIG. 11(a) is a micrograph of the microstructure of a commercially
forged and processed Ti-6-2-4-2 alloy;
[0034] FIG. 11(b) is a micrograph of the microstructure of a 11-6-2-4-2 alloy
processed by the thermally managed high strain MAF embodiment described in
Example 1 of the present disclosure;
[0035] FIG. 12(a) is a micrograph that depicts the microstructure of a
commercially forged and processed Ti-6-2-4-6 alloy;
[0036] FIG. 12(b) is a micrograph of the microstructure of a Ti-6-2-4-6 alloy
processed by the thermally managed high strain MAF embodiment described in
Example 2 of the present disclosure;
[0037] FIG. 13 is a micrograph of the microstructure of a Ti-6-2-4-6 alloy
processed by the thermally managed high strain MAF embodiment described in
Example 3 of the present disclosure;
[0038] FIG. 14 is a micrograph of the microstructure of a Ti-6-2-4-2 alloy
processed by the thermally managed high strain MAF embodiment described in
Example 4 of the present disclosure, which applies equal strain on each axis;
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[0039] FIG. 15 is a micrograph of the microstructure of a 11-6-2-4-2 alloy
processed by the thermally managed high strain MAF embodiment, described in
Example 5 of the present disclosure, wherein blocking reductions are used to
minimize ,
bulging of the workpiece that occurs after each major reduction;
[0040] FIG. 16(a) is a micrograph of the microstructure of the center region
of a
Ti-6-2-4-2 alloy processed by the thermally managed high strain MAF embodiment
utilizing through beta transus MAF that is described in Example 6 of the
present
disclosure; and
[0041] FIG. 16(b) is a micrograph of the microstructure of the surface region
of
a Ti-6-2-4-2 alloy processed by the thermally managed high strain MAF
embodiment
utilizing through beta transus MAF that is described in Example 6 of the
present
disclosure.
[0042] The reader will appreciate the foregoing details, as well as others,
upon
considering the following detailed description of certain non-limiting
embodiments
according to the present disclosure.
DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS
[0043] In the present description of non-limiting embodiments, other than in
the
operating examples or where otherwise indicated, all numbers expressing
quantities or
characteristics are to be understood as being modified in all instances by the
term
"about". Accordingly, unless indicated to the contrary, any numerical
parameters set
forth in the following description are approximations that may vary depending
on the
desired properties one seeks to obtain by way of the methods according to the
present
disclosure. 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.
[0044] Also, any numerical range recited herein is intended to include all sub-
ranges subsumed therein. For example, a range of "1 to 10" is intended to
include all
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sub-ranges between (and including) the recited minimum value of 1 and the
recited
maximum value of 10, that is, having a minimum value equal to or greater than
1 and a
maximum value of equal to or less than 10. Any maximum numerical limitation
recited
herein is intended to include all lower numerical limitations subsumed therein
and any
minimum numerical limitation recited herein is intended to include all higher
numerical
limitations subsumed therein. Accordingly, Applicants reserve the right to
amend the
present disclosure, including the claims, to expressly recite any sub-range
subsumed
within the ranges expressly recited herein.
[0045] The grammatical articles "one", "a", "an", and "the", as used herein,
are
intended to include "at least one" or "one or more", unless otherwise
indicated. Thus, the
articles are used herein to refer to one or more than one (i.e., to at least
one) of the
grammatical objects of the article. By way of example, "a component" means one
or
more components, and thus, possibly, more than one component is contemplated
and may
be employed or used in an implementation of the described embodiments.
[0046] The present disclosure includes descriptions of various embodiments. It
is
to be understood that all embodiments described herein are exemplary,
illustrative, and
non-limiting. Thus, the invention is not limited by the description of the
various
exemplary, illustrative, and non-limiting embodiments. Rather, the invention
is defined
solely by the claims, which may be amended to recite any features expressly or
inherently
described in or otherwise expressly or inherently supported by the present
disclosure.
[0047] As such, and to the extent necessary, the disclosure as set forth
herein
supersedes any conflicting material.
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[0048] An aspect of the present disclosure is directed to non-limiting
embodiments of a multi-axis forging process for titanium alloys that includes
the
application of high strain rates during the forging steps to refine grain
size. These method
embodiments are generally referred to in the present disclosure as ''high
strain rate multi-
axis forging" or "high strain rate MAF". As used herein, the terms "reduction"
and "hit"
interchangeably refer to an individual press forging step, wherein a workpiece
is forged
between die surfaces. As used herein, the phrase "spacer height" refers to the
dimension
or thickness of a workpiece measured along one orthogonal axis after a
reduction along
that axis. For example, after a press forging reduction along a particular
axis to a spacer
height of 4.0 inches, the thickness of the press forged workpiece measured
along that axis
will be about 4.0 inches. The concept and use of spacer heights are well known
to those
having ordinary skill in the field of press forging and need not be further
discussed
herein.
[0049] It was previously determined that for alloys such as Ti-6AI-4V alloy
(ASTM Grade 5; UNS R56400), which also may be referred to as "Ti-6-4" alloy,
high
strain rate multi-axis forging, wherein the workpiece was forged at least to a
total strain
of 3.5, could be used to prepare ultrafine grain billets. This process is
disclosed in U.S.
Patent Application Serial No. 12/882,538, filed September 15, 2010, entitled
"Processing
Routes for Titanium and Titanium Alloys" ("the '538 Application"). Imparting
strain of at
least 3.5 may require significant processing time and complexity, which adds
cost and
increases the opportunity for unanticipated problems. The present disclosure
discloses a
high strain rate multi-axis forging process that can provide ultrafine grain
structures using
total strain in the range of from at least 1.0 up to less than 3.5.
[0050] Methods according to the present disclosure involve the application of
multi-axis forging and its derivatives, such as the multiple upset and draw
(MUD)
process
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disclosed in the '538 Application, to titanium alloys exhibiting slower
effective alpha
precipitation and growth kinetics than T1-6-4 alloy. In particular, Ti-6AI-2Sn-
4Zr-2Mo-
0.08Si alloy (UNS R54620), which also may be referred to as "Ti-6-2-4-2"
alloy, has slower
effective alpha kinetics than Ti-6-4 alloy as a result of additional grain
pinning elements
such as Si. Also, Ti-6A1-2Sn-4Zr-6Mo alloy (UNS R56260), which also may be
referred to
as "Ti-6-2-4-6" alloy, has slower effective alpha kinetics than 1-6-4 alloy as
a result of
increased beta stabilizing content. It is recognized that in terms of alloying
elements, the
growth and precipitation of the alpha phase is a function of the diffusion
rate of the alloying
element in the titanium-base alloy. Molybdenum is known to have one of the
slower
diffusion rates of all titanium alloying additions. In addition, beta
stabilizers, such as
molybdenum, lower the beta transus temperature (1-0) of the alloy, wherein the
lower To
results in general slower diffusion of atoms in the alloy at the processing
temperature for
the alloy. A result of the relatively slow effective alpha precipitation and
growth kinetics of
the Ti-6-2-4-2 and Ti-6-2-4-6 alloys is that the beta heat treatment that is
used prior to
MAF according to embodiments of the present disclosure produces a fine and
stable alpha
lath size when compared to the effect of such processing on Ti-6-4 alloy. In
addition, after
beta heat treating and cooling, the Ti-6-2-4-2 and Ti-6-2-4-6 alloys possess a
fine beta
grain structure that limits the kinetics of alpha grain growth.
[0051] The effective kinetics of alpha growth can be evaluated by identifying
the
slowest diffusing species at a temperature immediately below the beta transus.
This
approach has been theoretically outlined and experimentally verified in
literature (see
Semiatin et al., Metallurgical and Materials Transactions A: Physical
Metallurgy and
Materials Science 38 (4), 2007, pp. 910-921). In titanium and titanium alloys,
diffusivity
data for all of the potential alloying elements is not readily available;
however, literature
surveys such as that in Titanium (Second Edition, 2007), by Lutjering and
Williams,
generally agree to the following relative ranking for some common alloying
elements:
Dmo<DNb<DAI-Dv-Dsn-Dzr-DHf<Dcr--DNi-Ddr-Ddo-Dmn--DFe
[0052] Therefore, alloys such as Ti-6-2-4-6 alloy and Ti-6-2-4-2 alloy, which
contain molybdenum, show the desirable, slow alpha kinetics required to
achieve
ultrafine grain microstructures at comparatively lower strain than Ti-6-4
alloy where the
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kinetics are controlled by the diffusion of aluminum. Based on periodic table
group
relationships, one could also reasonably postulate that tantalum and tungsten
belong to
the group of slow diffusers.
[0053] In addition to the inclusion of slow diffusing elements to reduce the
effective kinetics of the alpha phase, reducing the beta transus temperature
in alloys
controlled by aluminum diffusion will have a similar effect. A beta transus
temperature
reduction of 100 C will reduce the diffusivity of aluminum in the beta phase
by
approximately an order of magnitude at the beta transus temperature. The alpha
kinetics in alloys such as ATI 425 alloy (Ti-4AI-2.5V; UNS 54250) and Ti-6-6-
2 alloy
(Ti-6AI-6V-2SN; UNS 56620) are likely controlled by aluminum diffusion;
however, the
lower beta transus temperatures of these alloys relative to Ti-6A1-4V alloy
also result in
the desirable, slower effective alpha kinetics. Ti-6A1-7Nb alloy (UNS R56700),
normally
a biomedical version of Ti-6AI-4V alloy, may also exhibit slower effective
alpha kinetics
because of the niobium content.
[0054] It was initially expected that alpha+beta alloys other than Ti-6-4
alloy could
be processed under conditions similar to those disclosed in the '538
Application at
temperatures that would result in similar volume fractions of the alpha phase.
For
example, according to predictions using PANDAT software, a commercially
available
computational tool available from Computherm, LLC, Madison, Wisconsin, USA, it
was
predicted that Ti-6-4 alloy at 1500 F (815.6 C) should have approximately the
same
volume fraction of the alpha phase as both Ti-6-2-4-2 alloy at 1600 F (871.1
C) and Ti-6-
2-4-6 alloy at 1200 F (648.9 C) See FIG. 1. However, both Ti-6-2-4-2 and Ti-6-
2-4-6
alloys cracked severely when processed in the manner in which Ti-6-4 alloy was
processed in the '538 Application using temperatures that it was predicted
would produce
a similar volume fraction of the alpha phase. Much higher temperatures,
resulting in lower
equilibrium volume fractions of alpha, and/or significantly reduced strain per
pass were
required to successfully process the Ti-6-2-4-2 and Ti-6-2-4-6 alloys.
[0055] Variations to the high strain rate MAF process, including alpha/beta
forging temperature(s), strain rate, strain per hit, hold time between hits,
number and
duration of reheats, and intermediate heat treatments can each affect the
resultant
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microstructure and the presence and extent of cracking. Lower total strains
were initially
attempted in order to inhibit cracking, without any expectation that ultrafine
grain structures
would result. However, when examined, the samples processed using lower total
strains
showed significant promise for producing ultrafine grain structures. This
result was entirely
unanticipated.
[0056] In certain non-limiting embodiments according to the present
disclosure, a
method for producing ultrafine grain sizes includes the following steps: 1)
selecting a
titanium alloy exhibiting effective alpha-phase growth kinetics slower than Ti-
6-4 alloy; 2)
beta annealing the titanium alloy to produce a fine, stable alpha lath size;
and 3) high
strain rate MAF (or a similar derivative process, such as the multiple upset
and draw
(MUD) process disclosed in the '538 Application) to a total strain of at least
1.0, or in
another embodiment, to a total strain of at least 1.0 up to less than 3.5. The
word "fine" for
describing the grain and lath sizes, as used herein, refers to the smallest
grain and lath
size that can be achieved, which in non-limiting embodiments is on the order
of 1 pm. The
word "stable" is used herein to mean that the multi-axis forging steps do not
significantly
coarsen the alpha grain size, and do not increase the alpha grain size by more
than about
100%.
[0057] The flow chart in FIG. 2 and the schematic representation in FIG. 3
illustrate aspects of a non-limiting embodiment according to the present
disclosure of a
method (16) of using a high strain rate multi-axis forging (MAF) to refine
grain size of
titanium alloys. Prior to multi-axis forging (26), a titanium alloy workpiece
24 is beta
annealed (18) and cooled (20). Air cooling is possible with smaller
workpieces, such as,
for example, 4 inch cubes; however, water or liquid cooling also can be used.
Faster
cooling rates result in finer lath and alpha grain sizes. Beta annealing (18)
comprises
.. heating the workpiece 24 above the beta transus temperature of the titanium
alloy of the
workpiece 24 and holding for a time sufficient to form all beta phase in the
workpiece 24. Beta annealing (18) is a process well-known to a person of
ordinary skill
and, therefore, is not described in detail herein. A non-limiting embodiment
of beta
annealing may include heating the workpiece 24 to a beta annealing temperature
that is
about 50 F (27.8 C) above the beta transus temperature of the titanium alloy
and
holding the workpiece 24 at the temperature for about 1 hour.
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[0058] After beta annealing (18), the workpiece 24 is cooled (20) to a
temperature below the beta transus temperature of the titanium alloy of the
workpiece 24. In a non-limiting embodiment of the present disclosure, the
workpiece is
cooled to ambient temperature. As used herein, "ambient temperature" refers to
the
.. temperature of the surroundings. For example, in a non-limiting commercial
production
scenario, "ambient temperature" refers to the temperature of the factory
surroundings.
In a non-limiting embodiment, cooling (20) can include quenching. Quenching
includes
immersing the workpiece 24 in water, oil, or another suitable liquid and is a
process
understood by a person skilled in the metallurgical arts. In other non-
limiting
embodiments, particularly for smaller sized workpieces, cooling (20) may
comprise air
cooling. Any method of cooling a titanium alloy workpiece 24 known to a person
skilled
in the art now or hereafter is within the scope of the present disclosure. In
addition, in a
certain non-limiting embodiments, cooling (20) comprises cooling directly to a
workpiece
forging temperature in the workpiece forging temperature range for subsequent
high
strain rate multi-axis forging.
[0059] After cooling (20) the workpiece, the workpiece is subjected to high
strain rate multi-axis forging (26). As is understood to those having ordinary
skill in the
art, multi-axis forging ("MAF"), which also may be referred to as "A-B-C"
forging, is a
form of severe plastic deformation. High strain rate multi-axis forging (26),
according to
a non-limiting embodiment of the present disclosure, includes heating (step 22
in FIG.
2) a workpiece 24 comprising a titanium alloy to a workpiece forging
temperature in a
workpiece forging temperature range that is within the alpha+beta phase field
of the
titanium alloy, followed by MAF (26) using a high strain rate. It is apparent
that in an
embodiment in which the cooling step (20) comprises cooling to a temperature
in the
workpiece forging temperature range, the heating step (22) is not necessary.
[0060] A high strain rate is used in the high strain rate MAF to adiabatically
heat an internal region of the workpiece. However, in non-limiting embodiments
according to the present disclosure, in at least the last cycle of A-B-C hits
of high strain
rate MAF in the cycle, the temperature of the internal region of the titanium
alloy
workpiece 24 should not exceed the beta transus temperature (Tp) of the
titanium alloy
workpiece. Therefore, in such non-limiting embodiments the workpiece forging
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temperature for at least the final cycle of A-B-C hits, or at least the last
hit of the cycle,
of high strain rate MAF should be chosen to ensure that during the high strain
rate MAF
the temperature of the internal region of the workpiece does not equal or
exceed the
beta transus temperature of the alloy. For example, in a non-limiting
embodiment
according to the present disclosure, the temperature of the internal region of
the
workpiece does not exceed 20 F (11.1 C) below the beta transus temperature of
the
alloy, i.e., To - 20 F (Tp -11.1 C), during at least the final high strain
rate cycle of A-B-C
hits in the MAF or during at least the last press forging hit when a total
strain of at least
1.0, or in a range of at least 1.0 up to less than 3.5, is achieved in at
least a region of
the workpiece.
[0061] In a non-limiting embodiment of high strain rate MAF according to the
present disclosure, a workpiece forging temperature comprises a temperature
within a
workpiece forging temperature range. In a non-limiting embodiment, the
workpiece
forging temperature range is 100 F (55.6 C) below the beta transus temperature
(To) of
the titanium alloy of the workpiece to 700 F (388.9 C) below the beta transus
temperature of the titanium alloy. In still another non-limiting embodiment,
the
workpiece forging temperature range is 300 F (166.7 C) below the beta transus
temperature of the titanium alloy to 625 F (347 C) below the beta transus
temperature
of the titanium alloy. In a non-limiting embodiment, the low end of a
workpiece forging
temperature range is a temperature in the alpha+beta phase field wherein
damage,
such as, for example, crack formation and gouging, does not occur to the
surface of the
workpiece during the forging hit.
[0062] In a non-limiting method embodiment shown in FIG. 2 applied to a Ti-6-
2-4-2 alloy, which has a beta transus temperature (To) of about 1820 F (996
C), the
workpiece forging temperature range may be from 1120 F (604.4 C) to 1720 F
(937.8 C), or in another embodiment may be from 1195 F (646.1 C) to 1520 F
(826.7 C). In a non-limiting method embodiment shown in FIG. 2 applied to a Ti-
6-2-4-6
alloy, which has a beta transus temperature (To) of about 1720 F (940 C), the
workpiece forging temperature range may be from 1020 F (548.9 C) to 1620 F
(882.2 C), or in another embodiment may be from 1095 F (590.6 C) to 1420 F
(771.1 C). In still another non-limiting embodiment, when applying the
embodiment
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shown in FIG. 2 to ATI 425 alloy (UNS R54250), which also may be referred to
as "Ti-
4A1-2.5V" alloy, and which has a beta transus temperature (TO of about 1780 F
(971.1 C), the workpiece forging temperature range may be from 1080 F (582.2
C) to
1680 F (915.6 C), or in another embodiment may be from 1155 F (623.9 C) to
1480 F
(804.4 C). In still another non-limiting embodiment, when applying the
embodiment of
the present disclosure of FIG. 2 to a Ti-6A1-6V-2Sn alloy (UNS 56620), which
also may
be referred to as "Ti-6-6-2" alloy, and which has a beta transus temperature
(1-0) of
about 1735 F (946.1 C), the workpiece forging temperature range may be from
1035 F
(527.2 C) to 1635 F (890.6 C), or in another embodiment may be from 1115 F
(601.7 C) to 1435 F (779.4 C). The present disclosure involves the application
of high
strain rate multi-axis forging and its derivatives, such as the MUD method
disclosed in the
'538 Application, to titanium alloys that posses slower effective alpha
precipitation and
growth kinetics than Ti-6-4 alloy.
[0063] Referring again to FIGS. 2 and 3, when the titanium alloy workpiece 24
is at the workpiece forging temperature, the workpiece 24 is subjected to high
strain rate
MAF (26). In a non-limiting embodiment according to the present disclosure,
MAF (26)
comprises press forging (step 28, shown in FIG. 3(a)) the workpiece 24 at the
workpiece forging temperature in the direction (A) of a first orthogonal axis
30 of the
workpiece using a strain rate that is sufficient to adiabatically heat the
workpiece, or at
least adiabatically heat an internal region of the workpiece, and plastically
deform the
workpiece 24.
[0064] High strain rates and fast ram speeds are used to adiabatically heat
the
internal region of the workpiece in non-limiting embodiments of high strain
rate MAF
according to the present disclosure. In a non-limiting embodiment according to
the
present disclosure, the term "high strain rate" refers to a strain rate in the
range of about
0.2 s-1 to about 0.8 s-1. In another non-limiting embodiment according to the
present
disclosure, the term "high strain rate" refers to a strain rate in the range
of about 0.2 s-1
to about 0.4 s-1.
[0065] In a non-limiting embodiment according to the present disclosure using
a high strain rate as defined hereinabove, an internal region of the titanium
alloy
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workpiece may be adiabatically heated to about 200 F (111.1 C) above the
workpiece
forging temperature. In another non-limiting embodiment, during press forging
an
internal region is adiabatically heated to a temperature in the range of about
100 F
(55.6 C) to about 300 F (166.7 C) above the workpiece forging temperature. In
still
another non-limiting embodiment, during press forging an internal region is
adiabatically
heated to a temperature in the range of about 150 F (83.3 C) to about 250 F
(138.9 C)
above the workpiece forging temperature. As noted above, in non-limiting
embodiments, no portion of the workpiece should be heated above the beta
transus
temperature of the titanium alloy during the last cycle of high strain rate A-
B-C MAF hits,
or during the last hit on an orthogonal axis.
[0066] In a non-limiting embodiment, during press forging (28), the workpiece
24 is plastically deformed to a reduction in height or another dimension that
is in the
range of 20% to 50%, i.e., the dimension is reduced by a percentage within
that range.
In another non-limiting embodiment, during press forging (28), the workpiece
24 is
plastically deformed to a reduction in height or another dimension in the
range of 30% to
40%.
[0067] A known ultra-slow strain rate (0.001 s-1 or slower) multi-axis forging
process is depicted schematically in FIG. 4. Generally, an aspect of multi-
axis forging is
that after every three-stroke, (i.e., "three-hit") cycle by the forging
apparatus (which may
be, for example, an open die forge), the shape and size of the workpiece
approaches
that of the workpiece just prior to the first hit of that three-hit cycle. For
example, after a
5-inch sided cube-shaped workpiece is initially forged with a first "hit" in
the direction of
the "a" axis, rotated 90 and forged with a second hit in the direction of the
orthogonal
"b" axis, and then rotated 90 and forged with a third hit in the direction of
the orthogonal
"c" axis, the workpiece will resemble the starting cube and include
approximately 5-inch
sides. In other words, although the three-hit cycle has deformed the cube in
three steps
along the cube's three orthogonal axes, as a result of the repositioning of
the workpiece
between individual hits and selection of the reduction during each hit, the
overall result
of the three forging deformations is to return the cube to approximately its
original shape
and size.
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[0068] In another non-limiting embodiment according to the present disclosure,
a first press forging step (28), shown in FIG. 2(a), also referred to herein
as the "first
hit", may include press forging the workpiece on a top face down to a
predetermined
spacer height while the workpiece is at a temperature in the workpiece forging
temperature range. As used herein the term "spacer height" refers to the
dimension of
the workpiece on the completion of a particular press forging reduction. For
example,
for a spacer height of 5 inches, the workpiece is forged to a dimension of
about 5
inches. In a specific non-limiting embodiment of the method of the present
disclosure, a
spacer height is, for example, 5 inches. In another non-limiting embodiment, a
spacer
height is 3.25 inches. Other spacer heights, such as, for example, less than 5
inches,
about 4 inches, about 3 inches, greater than 5 inches, or 5 inches up to 30
inches are
within the scope of embodiments herein, but should not be considered as
limiting the
scope of the present disclosure. Spacer heights are only limited by the
capabilities of
the forge and optionally, as will be seen herein, the capabilities of the
thermal
management system according to non-limiting embodiments of the present
disclosure to
maintain the workpiece at the workpiece forging temperature. Spacer heights of
less
than 3 inches are also within the scope of embodiments disclosed herein, and
such
relatively small spacer heights are only limited by the desired
characteristics of a
finished product. The use of spacer heights of about 30 inches, for example,
in
methods according to the present disclosure allows for the production of
billet-sized
(e.g., 30-inch sided) cube-shaped titanium alloy forms having fine grain size,
very fine
grain size, or ultrafine grain size. Billet-sized cube-shaped forms of
conventional alloys
have been employed as workpieces that are forged into disk, ring, and case
parts for
aeronautical or land-based turbines, for example.
[0069] The predetermined spacer heights that should be employed in various
non-limiting embodiments of methods according to the present disclosure may be
determined by a person having ordinary skill in the art without undue
experimentation
on considering the present disclosure. Specific spacer heights may be
determined by a
person having ordinary skill without undue experimentation. Specific spacer
heights are
dependent upon a specific alloy's susceptibility to cracking during forging.
Alloys that
have a higher susceptibility to cracking will require larger spacer heights,
i.e., less
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deformation per hit to prevent cracking. The adiabatic heating limit must also
be
considered when choosing a spacer height because, at least in the last cycle
of hits, the
workpiece temperature should not surpass the Tp of the alloy. In addition, the
forging
press capability limit needs to be considered when selecting a spacer height.
For
example, during the pressing of a 4-inch sided cubic workpiece the cross-
sectional area
increases during the pressing step. As such, the total load that is required
to keep the
workpiece deforming at the required strain rate increases. The load cannot
increase
beyond the capabilities of the forging press. Also, the workpiece geometry
needs to be
considered when selecting spacer heights. Large deformations may result in
bulging of
the workpiece. Too great a reduction could result in a relative flattening of
the
workpiece, so that the next forging hit in the direction of a different
orthogonal axis could
result in bending of the workpiece.
[0070] In certain non-limiting embodiments, the spacer heights used for each
orthogonal axis hit are equivalent. In certain other non-limiting embodiments,
the
.. spacer heights used for each orthogonal axis hits are not equivalent. Non-
limiting
embodiments of high strain rate MAF using non-equivalent spacer heights for
each
orthogonal axis are presented below.
[0071] After press forging (28) the workpiece 24 in the direction of the first
orthogonal axis 30, i.e., in the A-direction shown in FIG. 2(a), a non-
limiting embodiment
of a method according to the present disclosure optionally further comprises a
step of
allowing (step 32) the temperature of the adiabatically heated internal region
(not
shown) of the workpiece to cool to a temperature at or near the workpiece
forging
temperature in the workpiece forging temperature range, which is shown in FIG.
3(b).
In various non-limiting embodiments, internal region cooling times, or
"waiting" times,
.. may range, for example, from 5 seconds to 120 seconds, from 10 seconds to
60
seconds, or from 5 seconds to 5 minutes. In various non-limiting embodiments
according to the present disclosure, an "adiabatically heated internal region"
of a
workpiece, as used herein, refers to a region extending outwardly from a
center of the
workpiece and having a volume of at least about 50%, or at least about 60%, or
at least
about 70%, or at least about 80% of the workpiece. It will be recognized by a
person
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skilled in the art that the time required to cool the internal region of a
workpiece to a
temperature at or near the workpiece forging temperature will depend on the
size,
shape, and composition of the workpiece 24, as well as on conditions of the
atmosphere
surrounding the workpiece 24.
[0072] During the internal region cooling period, an aspect of a thermal
management system 33 according to certain non-limiting embodiments disclosed
herein
optionally comprises heating (step 34) an outer surface region 36 of the
workpiece 24 to
a temperature at or near the workpiece forging temperature. In this manner,
the
temperature of the workpiece 24 is in a uniform or near uniform and
substantially
isothermal condition at or near the workpiece forging temperature prior to
each high
strain rate MAF hit. It is recognized that it is within the scope of the
present disclosure
to optionally heat (34) the outer surface region 36 of the workpiece 24 after
each A-axis
heat, after each B-axis hit, and/or after each C-axis hit. In non-limiting
embodiments,
the outer surface of the workpiece optionally is heated (34) after each cycle
of A-B-C
hits. In still other non-limiting embodiments, the outer surface region
optionally is be
heated after any hit or cycle of hits, as long as the overall temperature of
the workpiece
is maintained within the workpiece forging temperature range during the
forging
process. The times that a workpiece should be heated to maintain a temperature
of the
workpiece 24 in a uniform or near uniform and substantially isothermal
condition at or
near the workpiece forging temperature prior to each high strain rate MAF hit
may
depend on the size of the workpiece, and this may be determined by a person
having
ordinary skill without undue experimentation. In various non-limiting
embodiments
according to the present disclosure, an "outer surface region" of a workpiece,
as used
herein, refers to a region extending inwardly from an outer surface of the
workpiece and
having a volume of at least about 50%, or at least about 60%, or at least
about 70%, or
at least about 80% of the workpiece. It is recognized that at any time
intermediate
[0073] In non-limiting embodiments, heating (34) an outer surface region 36 of
the workpiece 24 may be accomplished using one or more surface heating
mechanisms
38 of the thermal management system 33. Examples of possible surface heating
mechanisms successive press forging steps, the entire workpiece may be placed
in a
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furnace or otherwise heated to a temperature with the workpiece forging
temperature
range.
[0074] In certain non-limiting embodiments, as an optional feature, between
each of the A, B, and C forging hits the thermal management system 33 is used
to heat
the outer surface region 36 of the workpiece, and the adiabatically heated
internal
region is allowed to cool for an internal region cooling time so as to return
the
temperature of the workpiece to a substantially uniform temperature at or near
the
selected workpiece forging temperature. In certain other non-limiting
embodiments
according to the present disclosure, as an optional feature, between each of
the A, B,
and C forging hits the thermal management system 33 is used to heat the outer
surface
region 36 of the workpiece, and the adiabatically heated internal region is
allowed to
cool for an internal region cooling time so that the temperature of the
workpiece returns
to a substantially uniform temperature within the workpiece forging
temperature range.
Non-limiting embodiments of a method according to the present disclosure
utilizing both
(1) a thermal management system 33 to heat the outer surface region of the
workpiece
to a temperature within the workpiece forging temperature range and (2) a
period during
which the adiabatically heated internal region cools to a temperature within
the
workpiece forging temperature range may be referred to herein as "thermally
managed,
high strain rate multi-axis forging".38 include, but are not limited to, flame
heaters
adapted for flame heating; induction heaters adapted for induction heating;
and radiant
heaters adapted for radiant heating of the outer surface of the workpiece 24.
Other
mechanisms and techniques for heating an outer surface region of the workpiece
will be
apparent to those having ordinary skill upon considering the present
disclosure, and
such mechanisms and techniques are within the scope of the present disclosure.
A
non-limiting embodiment of an outer surface region heating mechanism 38 may
comprise a box furnace (not shown). A box furnace may be configured with
various
heating mechanisms to heat the outer surface region of the workpiece using one
or
more of flame heating mechanisms, radiant heating mechanisms, induction
heating
mechanisms, and any other suitable heating mechanism known now or hereafter to
a
person having ordinary skill in the art.
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[0075] In another non-limiting embodiment, the temperature of the outer
surface region 36 of the workpiece 24 optionally is heated (34) and maintained
at or
near the workpiece forging temperature and within the workpiece forging
temperature
range using one or more die heaters 40 of a thermal management system 33. Die
heaters 40 may be used to maintain the dies 42 or the die press forging
surfaces 44 of
the dies at or near the workpiece forging temperature or at temperatures
within the
workpiece forging temperature range. In a non-limiting embodiment, the dies 42
of the
thermal management system are heated to a temperature within a range that
includes
the workpiece forging temperature down to 100 F (55.6 C) below the workpiece
forging
temperature. Die heaters 40 may heat the dies 42 or the die press forging
surface 44
by any suitable heating mechanism known now or hereafter by a person skilled
in the
art, including, but not limited to, flame heating mechanisms, radiant heating
mechanisms, conduction heating mechanisms, and/or induction heating
mechanisms.
In a non-limiting embodiment, a die heater 40 may be a component of a box
furnace
(not shown). While the thermal management system 33 is shown in place and
being
used during the cooling steps (32),(52),(60) of the multi-axis forging process
(26) shown
in FIGS. 2(b), (d), and (f), it will be recognized that the thermal management
system 33
may or may not be in place during the press forging steps (28),(46),(56)
depicted in
FIGS. 2(a), (c), and (e).
[0076] As shown in FIG. 3(c), an aspect of a non-limiting embodiment of a
multi-axis forging method (26) according to the present disclosure comprises
press
forging (step 46) the workpiece 24 at a workpiece forging temperature in the
workpiece
forging temperature range in the direction (B) of a second orthogonal axis 48
of the
workpiece 24 using a strain rate that is sufficient to adiabatically heat the
workpiece 24,
or at least an internal region of the workpiece 24, and plastically deform the
workpiece
24. In a non-limiting embodiment, during press forging (46), the workpiece 24
is
deformed to a plastic deformation of a 20% to 50% reduction in height or
another
dimension. In another non-limiting embodiment, during press forging (46), the
workpiece 24 is plastically deformed to a plastic deformation of a 30% to 40%
reduction
in height or another dimension. In a non-limiting embodiment, the workpiece 24
may be
press forged (46) in the direction of the second orthogonal axis 48 to the
same spacer
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height used in the first press forging step (28). In another non-limiting
embodiment, the
workpiece 24 may be press forged in the direction of the second orthogonal
axis 48 to a
different spacer height than is used in the first press forging step (28). In
another non-
limiting embodiment, the internal region (not shown) of the workpiece 24 is
adiabatically
heated during the press forging step (46) to the same temperature as in the
first press
forging step (28). In other non-limiting embodiments, the high strain rates
used for
press forging (46) are in the same strain rate ranges as disclosed for the
first press
forging step (28).
[0077] In a non-limiting embodiment, as shown in FIGS. 2(b) and (d), the
workpiece 24 may be rotated (50) between successive press forging steps (e.g.,
(28),(46),(56)) to present a different orthogonal axis to the forging
surfaces. This
rotation may be referred to as "A-B-C" rotation. It is understood that by
using different
forge configurations, it may be possible to rotate the ram on the forge
instead of rotating
the workpiece 24, or a forge may be equipped with multi-axis rams so that
rotation of
neither the workpiece nor the forge is required. Obviously, the important
aspect is the
relative change in position of the workpiece and the ram being used, and
rotating (50)
the workpiece 24 may be unnecessary or optional. In most current industrial
equipment
set-ups, however, rotating (50) the workpiece to a different orthogonal axis
in between
press forging steps will be required to complete the multi-axis forging
process (26).
[0078] In non-limiting embodiments in which A-B-C rotation (50) is required,
the
workpiece 24 may be rotated manually by a forge operator or by an automatic
rotation
system (not shown) to provide A-B-C rotation (50). An automatic A-B-C rotation
system
may include, but is not limited to including, free-swinging clamp-style
manipulator
tooling or the like to enable a non-limiting thermally managed high strain
rate multi-axis
forging embodiment disclosed herein.
[0079] After press forging (46) the workpiece 24 in the direction of the
second
orthogonal axis 48, i.e., in the B-direction, and as shown in FIG. 3(d),
process (20)
optionally further comprises allowing (step 52) an adiabatically heated
internal region
(not shown) of the workpiece to cool to a temperature at or near the workpiece
forging
temperature, which is shown in FIG. 3(d). In certain non-limiting embodiments,
internal
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region cooling times, or waiting times, may range, for example, from 5 seconds
to 120
seconds, or from 10 seconds to 60 seconds, or from 5 seconds up to 5 minutes.
It will
be recognized by an ordinarily skilled person that the minimum cooling times
are
dependent upon the size, shape, and composition of the workpiece 24, as well
as the
characteristics of the environment surrounding the workpiece.
[0080] During the optional internal region cooling period, an optional aspect
of
a thermal management system 33 according to certain non-limiting embodiments
disclosed herein comprises heating (step 54) an outer surface region 36 of the
workpiece 24 to a temperature in the workpiece forging temperature range at or
near
the workpiece forging temperature. In this manner, the temperature of the
workpiece 24
is maintained in a uniform or near uniform and substantially isothermal
condition at or
near the workpiece forging temperature prior to each high strain rate MAF hit.
In non-
limiting embodiments, when using the thermal management system 33 to heat the
outer
surface region 36, together with allowing the adiabatically heated internal
region to cool
for a specified internal region cooling time, the temperature of the workpiece
returns to a
substantially uniform temperature at or near the workpiece forging temperature
between
each A-B-C forging hit. In another non-limiting embodiment according to the
present
disclosure, when using the thermal management system 33 to heat the outer
surface
region 36, together with allowing the adiabatically heated internal region to
cool for a
specified internal region cooling time, the temperature of the workpiece
returns to a
substantially uniform temperature within the workpiece forging temperature
range prior
to each high strain rate MAF hit.
[0081] In a non-limiting embodiment, heating (54) an outer surface region 36
of
the workpiece 24 may be accomplished using one or more outer surface heating
mechanisms 38 of the thermal management system 33. Examples of possible
heating
mechanisms 38 may include, but are not limited to, flame heaters adapted for
flame
heating; induction heaters adapted for induction heating; and/or radiant
heaters adapted
for radiant heating of the workpiece 24. A non-limiting embodiment of a
surface heating
mechanism 38 may comprise a box furnace (not shown). Other mechanisms and
techniques for heating an outer surface of the workpiece will be apparent to
those
having ordinary skill upon considering the present disclosure, and such
mechanisms
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and techniques are within the scope of the present disclosure. A box furnace
may be
configured with various heating mechanisms to heat the outer surface of the
workpiece,
and such heating mechanisms may comprise one or more of flame heating
mechanisms, radiant heating mechanisms, induction heating mechanisms, and/or
any
other heating mechanism known now or hereafter to a person having ordinary
skill in
the art.
[0082] In another non-limiting embodiment, the temperature of the outer
surface region 36 of the workpiece 24 may be heated (54) and maintained at or
near the
workpiece forging temperature and within the workpiece forging temperature
range
using one or more die heaters 40 of a thermal management system 33. Die
heaters 40
may be used to maintain the dies 42 or the die press forging surfaces 44 of
the dies at
or near the workpiece forging temperature or at temperatures within the
workpiece
forging temperature range. Die heaters 40 may heat the dies 42 or the die
press forging
surfaces 44 by any suitable heating mechanism known now or hereafter by a
person
skilled in the art, including, but not limited to, flame heating mechanisms,
radiant heating
mechanisms, conduction heating mechanisms, and/or induction heating
mechanisms.
In a non-limiting embodiment, a die heater 40 may be a component of a box
furnace
(not shown). While the thermal management system 33 is shown in place and
being
used during the equilibration and cooling steps (32),(52),(60) of the multi-
axis forging
process (26) shown in FIGS, 2(b), (d), and (f), it is recognized that the
thermal
management system 33 may or may not be in place during the press forging steps
(28),(46),(56) depicted in FIGS. 2(a), (c), and (e) .
[0083] As shown in FIG. 3(e), an aspect of an embodiment of multi-axis forging
(26) according to the present disclosure comprises press forging (step 56) the
workpiece 24 at a workpiece forging temperature in the workpiece forging
temperature
range in the direction (C) of a third orthogonal axis 58 of the workpiece 24
using a ram
speed and strain rate that are sufficient to adiabatically heat the workpiece
24, or at
least adiabatically heat an internal region of the workpiece, and plastically
deform the
workpiece 24. In a non-limiting embodiment, the workpiece 24 is deformed
during press
forging (56) to a plastic deformation of a 20% to 50% reduction in height or
another
dimension. In another non-limiting embodiment, during press forging (56) the
workpiece
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is plastically deformed to a plastic deformation of a 30% to 40% reduction in
height or
another dimension. In a non-limiting embodiment, the workpiece 24 may be press
forged (56) in the direction of the third orthogonal axis 58 to the same
spacer height
used in the first press forging step (28) and/or the second forging step (46).
In another
non-limiting embodiment, the workpiece 24 may be press forged in the direction
of the
third orthogonal axis 58 to a different spacer height than used in the first
press forging
step (28). In another non-limiting embodiment according to the disclosure, the
internal
region (not shown) of the workpiece 24 is adiabatically heated during the
press forging
step (56) to the same temperature as in the first press forging step (28). In
other non-
limiting embodiments, the high strain rates used for press forging (56) are in
the same
strain rate ranges as disclosed for the first press forging step (28).
[0084] In a non-limiting embodiment, as shown by arrow 50 in FIGS. 3(b), 3(d),
and 3(e) the workpiece 24 may be rotated (50) to a different orthogonal axis
between
successive press forging steps (e.g., 46,56). As discussed above, this
rotation may be
referred to as A-B-C rotation. It is understood that by using different forge
configurations, it may be possible to rotate the ram on the forge instead of
rotating the
workpiece 24, or a forge may be equipped with multi-axis rams so that rotation
of
neither the workpiece nor the forge is required. Therefore, rotating 50 the
workpiece 24
may be unnecessary or an optional step. In most current industrial set-ups,
however,
rotating 50 the workpiece to a different orthogonal axis between press forging
steps will
be required to complete the multi-axis forging process (26).
[0085] After press forging 56 the workpiece 24 in the direction of the third
orthogonal axis 58, i.e., in the C-direction, and as shown in FIG. 3(e),
process 20
optionally further comprises allowing (step 60) an adiabatically heated
internal region
(not shown) of the workpiece to cool to a temperature at or near the workpiece
forging
temperature, which is indicated in FIG. 3(f). Internal region cooling times
may range, for
example, from 5 seconds to 120 seconds, from 10 seconds to 60 seconds, or from
5
seconds up to 5 minutes, and it is recognized by a person skilled in the art
that the
cooling times are dependent upon the size, shape, and composition of the
workpiece
24, as well as on the characteristics of the environment surrounding the
workpiece.
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[0086] During the optional cooling period, an optional aspect of a thermal
management system 33 according to non-limiting embodiments disclosed herein
comprises heating (step 62) an outer surface region 36 of the workpiece 24 to
a
temperature at or near the workpiece forging temperature. In this manner, the
temperature of the workpiece 24 is maintained in a uniform or near uniform and
substantially isothermal condition at or near the workpiece forging
temperature prior to
each high strain rate MAF hit. In non-limiting embodiments, by using the
thermal
management system 33 to heat the outer surface region 36, together with
allowing the
adiabatically heated internal region to cool for a specified internal region
cooling time,
the temperature of the workpiece returns to a substantially uniform
temperature at or
near the workpiece forging temperature between each A-B-C forging hit. In
another
non-limiting embodiment according to the present disclosure, by using the
thermal
management system 33 to heat the outer surface region 36, together with
allowing the
adiabatically heated internal region to cool for a specified internal region
cooling time,
the temperature of the workpiece returns to a substantially isothermal
condition within
the workpiece forging temperature range between successive A-B-C forging hits.
[0087] In a non-limiting embodiment, heating (62) an outer surface region 36
of
the workpiece 24 may be accomplished using one or more outer surface heating
mechanisms 38 of the thermal management system 33. Examples of possible
heating
mechanisms 38 may include, but are not limited to, flame heaters for flame
heating;
induction heaters for induction heating; and/or radiant heaters for radiant
heating of the
workpiece 24. Other mechanisms and techniques for heating an outer surface of
the
workpiece will be apparent to those having ordinary skill upon considering the
present
disclosure, and such mechanisms and techniques are within the scope of the
present
disclosure. A non-limiting embodiment of a surface heating mechanism 38 may
comprise a box furnace (not shown). A box furnace may be configured with
various
heating mechanisms to heat the outer surface of the workpiece using one or
more of
flame heating mechanisms, radiant heating mechanisms, induction heating
mechanisms, and/or any other suitable heating mechanism known now or hereafter
to a
person having ordinary skill in the art.
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[0088] In another non-limiting embodiment, the temperature of the outer
surface region 36 of the workpiece 24 may be heated (62) and maintained at or
near the
workpiece forging temperature and within the workpiece forging temperature
range
using one or more die heaters 40 of a thermal management system 33. Die
heaters 40
may be used to maintain the dies 42 or the die press forging surfaces 44 of
the dies at
or near the workpiece forging temperature or at temperatures within the
temperature
forging range. In a non-limiting embodiment, the dies 42 of the thermal
management
system are heated to a temperature within a range that includes the workpiece
forging
temperature to 100 F (55.6 C) below the workpiece forging temperature. Die
heaters
.. 40 may heat the dies 42 or the die press forging surface 44 by any suitable
heating
mechanism known now or hereafter by a person skilled in the art, including,
but not
limited to, flame heating mechanisms, radiant heating mechanisms, conduction
heating
mechanisms, and/or induction heating mechanisms. In a non-limiting embodiment,
a
die heater 40 may be a component of a box furnace (not shown). While the
thermal
management system 33 is shown in place and being used during the equilibration
steps
(32),(52),(60) of the multi-axis forging process show in FIGS. 2(b), (d), and
(f), it will be
recognized that the thermal management system 33 may or may not be in place
during
the press forging steps 28,46,56 depicted in FIGS. 2(a), (c), and (e).
[0089] An aspect of the present disclosure includes a non-limiting embodiment
wherein one or more of the press forging steps along the three orthogonal axes
of a
workpiece are repeated until a total strain of at least 1.0 is achieved in the
workpiece.
The total strain is the total true strain. The phrase "true strain" is also
known to a
person skilled in the art as "logarithmic strain" or "effective strain".
Referring to FIG. 2,
this is exemplified by step (g), i.e., repeating (step 64) one or more of
press forging
steps (28),(46),(56) until a total strain of at least 1.0, or in the range of
at least 1.0 up to
less than 3.5 is achieved in the workpiece. It is further recognized that
after the desired
strain is achieved in any of the press forging steps (28) or (46) or (56) and
further press
forging is unnecessary, and the optional equilibration steps (i.e., allowing
the internal
region of the workpiece to cool to a temperature at or near the workpiece
forging
temperature (32) or (52) or (60) and heating the outer surface of the
workpiece (34) or
(54) or (62) to a temperature at or near the workpiece forging temperature)
are not
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needed, the workpiece can simply be cooled to ambient temperature, in a non-
limiting
embodiment, by quenching in a liquid, or in another non-limiting embodiment,
by air
cooling or any faster rate of cooling.
[0090] It will be understood that in a non-limiting embodiment, the total
strain is
the total strain in the entire workpiece after multi-axis forging, as
disclosed herein. In
non-limiting embodiments according to the present disclosure, the total strain
may
comprise equal strains on each orthogonal axis, or the total strain may
comprise
different strains on one or more orthogonal axes.
[0091] According to a non-limiting embodiment, after beta annealing, a
workpiece may be multi-axis forged at two different temperatures in the alpha-
beta
phase field. For example, referring to FIG. 3, repeating step (64) of FIG. 2
may include
repeating one of more of steps (a)-(optional b), (c)-(optional d), and (e)-
(optional f) at a
first temperature in the alpha-beta phase field until a certain strain is
achieved, and then
repeating one or more of steps (a)-(optional b), (c)-(optional d), and (e)-
(optional f) at a
second temperature in the alpha-beta phase field until after a final press
forging step
(a), (b), or (c) (i.e., (28),(46), (56)) a total strain of at least 1.0, or in
the range of at least
1.0 up to less than 3.5, is achieved in the workpiece. In a non-limiting
embodiment, the
second temperature in the alpha-beta phase field is lower than first
temperature in the
alpha-beta phase field. It is recognized that conducting the method so as to
repeat one
or more of steps (a)-(optional b), (c)-(optional d), and (e)-(optional f) at
more than two
MAF press forging temperatures is within the scope of the present disclosure
as long as
the temperatures are within the forging temperature range. It is also
recognized that, in
a non-limiting embodiment, the second temperature in the alpha-beta phase
field is
higher than the first temperature in the alpha-beta phase field.
[0092] In another non-limiting embodiment according to the present disclosure,
different reductions are used for the A-axis hit, B-axis hit, and C-axis hit
to provide
equalized strain in all directions. Applying high strain rate MAF to introduce
equalized
strain in all directions results in less cracking of, and a more equiaxed
alpha grain
structure for, the workpiece. For example, non-equalized strain may be
introduced into
a cubic workpiece by starting with a 4-inch cube that is high strain rate
forged on the A-
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axis to a height of 3.0 inches. This reduction on the A-axis causes the
workpiece to
swell along the B-axis and the C-axis. If a second reduction in the B-axis
direction
reduces the B-axis dimension to 3.0 inches, more strain is introduced in the
workpiece
on the B-axis than on the A-axis. Likewise, a subsequent hit in the C-axis
direction to
reduce the C-axis dimension to 3.0 inches would introduce more strain into the
workpiece on the C-axis than on the A-axis or B-axis. As another example, to
introduce
equalized strain in all orthogonal directions, a 4-inch cubic workpiece is
forged ("hit") on
the A-axis to a height of 3.0 inches, rotated 90 degrees and hit on the B-axis
to a height
of 3.5 inches, and then rotated 90 degrees and hit on the C-axis to a height
of 4.0
inches. This latter sequence will result in a cube having approximately 4 inch
sides and
including equalized strain in each orthogonal direction of the cube. A general
equation
for calculating reduction on each orthogonal axis of a cubic workpiece during
high strain
rate MAF is provided in Equation 1.
Equation 1: strain = -In(spacer height/starting height)
A general equation for calculating the total strain is provided by Equation 2:
Equation 2: total strain = Id -In (spacer height/starting height)
Different reductions can be performed by using spacers in the forging
apparatus that
provide different spacer heights, or by any alternate manner known to a person
having
ordinary skill in the art.
[0093] In a non-limiting embodiment according to the present disclosure,
referring now to FIG. 5, and considering FIG. 3, a process (70) for the
production of
ultra-fine grain titanium alloy includes: beta annealing (71) a titanium alloy
workpiece;
cooling (72) the beta annealed workpiece 24 to a temperature below the beta
transus
temperature of the titanium alloy of the workpiece; heating (73) the workpiece
24 to a
workpiece forging temperature within a workpiece forging temperature range
that is
within an alpha+beta phase field of the titanium alloy of the workpiece; and
high strain
rate MAF (74) the workpiece, wherein high strain rate MAF (74) includes press
forging
reductions to the orthogonal axes of the workpiece to different spacer
heights. In a non-
limiting embodiment of multi-axis forging (74) according to the present
disclosure, the
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workpiece 24 is press forged (75) on the first orthogonal axis (A-axis) to a
major
reduction spacer height. The phrase "press forged ... to major reduction
spacer height",
as used herein, refers to press forging the workpiece along an orthogonal axis
to the
desired final dimension of the workpiece along the specific orthogonal axis.
Therefore,
the term "major reduction spacer height" is defined as the spacer height used
to attain
the final dimension of the workpiece along each orthogonal axis. All press
forging steps
to major reduction spacer heights should occur using a strain rate sufficient
to
adiabatically heat an internal region of the workpiece.
[0094] After press forging (75) the workpiece 24 in the direction of the first
orthogonal A-axis to a major reduction spacer height as shown in FIG. 3(a),
the process
(70) optionally further comprises allowing (step 76, indicated in FIG. 3(b))
an
adiabatically heated internal region (not shown) of the workpiece to cool to a
temperature at or near the workpiece forging temperature. Internal region
cooling times
may range, for example, from 5 seconds to 120 seconds, from 10 seconds to 60
seconds, or from 5 seconds up to 5 minutes, and a person having ordinary skill
will
recognize that required cooling times will be dependent upon the size, shape,
and
composition of the workpiece, as well as the characteristics of the
environment
surrounding the workpiece.
[0095] During the optional internal region cooling time period, an aspect of a
thermal management system 33 according to non-limiting embodiments disclosed
herein may comprise heating (step 77) an outer surface region 36 of the
workplace 24
to a temperature at or near the workpiece forging temperature. In this manner,
the
temperature of the workpiece 24 is maintained in a uniform or near uniform and
substantially isothermal condition at or near the workpiece forging
temperature prior to
each high strain rate MAF hit. In certain non-limiting embodiments using the
thermal
management system 33 to heat the outer surface region 36, together with
allowing the
adiabatically heated internal region to cool for a specified internal region
cooling time,
the temperature of the workpiece returns to a substantially uniform
temperature at or
near the workpiece forging temperature intermediate each of the A, B, and C
forging
hits. In other non-limiting embodiments according to the present disclosure
using the
thermal management system 33 to heat the outer surface region 36, together
with
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allowing the adiabatically heated internal region to cool for a specified
internal region
cooling time, the temperature of the workpiece returns to a substantially
uniform
temperature within the workpiece forging temperature range intermediate each
of the A,
B, and C forging hits.
[0096] In a non-limiting embodiment, heating (77) an outer surface region 36
of
the workpiece 24 may be accomplished using one or more outer surface heating
mechanisms 38 of the thermal management system 33. Examples of possible outer
surface heating mechanisms 38 include, but are not limited to, flame heaters
adapted
for flame heating; induction heaters adapted for induction heating; and
radiant heaters
adapted for radiant heating of the workpiece 24. Other mechanisms and
techniques for
heating an outer surface region of the workpiece will be apparent to those
having
ordinary skill upon considering the present disclosure, and such mechanisms
and
techniques are within the scope of the present disclosure. A non-limiting
embodiment of
an outer surface region heating mechanism 38 may comprise a box furnace (not
shown). A box furnace may be configured with various heating mechanisms to
heat the
outer surface region of the workpiece using, for example, one or more of flame
heating
mechanisms, radiant heating mechanisms, induction heating mechanisms, and/or
any
other suitable heating mechanism known now or hereafter to a person having
ordinary
skill in the art.
[0097] In another non-limiting embodiment, the temperature of the outer
surface region 36 of the workpiece 24 may be heated (34) and maintained at or
near the
workpiece forging temperature and within the workpiece forging temperature
range
using one or more die heaters 40 of a thermal management system 33. Die
heaters 40
may be used to maintain the dies 42 or the die press forging surfaces 44 of
the dies at
or near the workpiece forging temperature or at temperatures within the
workpiece
forging temperature range. In a non-limiting embodiment, the dies 42 of the
thermal
management system are heated to a temperature within a range that includes the
workpiece forging temperature down to 100 F (55.6 C) below the workpiece
forging
temperature. Die heaters 40 may heat the dies 42 or the die press forging
surface 44
by any suitable heating mechanism known now or hereafter by a person skilled
in the
art, including, but not limited to, flame heating mechanisms, radiant heating
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mechanisms, conduction heating mechanisms, and/or induction heating
mechanisms.
In a non-limiting embodiment, a die heater 40 may be a component of a box
furnace
(not shown). While the thermal management system 33 is shown in place and
being
used during the cooling steps of the multi-axis forging process, it is
recognized that the
.. thermal management system 33 may or may not be in place during the press
forging
steps.
[0098] In a non-limiting embodiment, after the press forging to a major
reduction spacer height (75) on the A-axis (see FIG.3), which is also referred
to herein
as reduction "A", and after the optional allowing (76) and heating (77) steps,
if applied,
subsequent press forgings to blocking reduction spacer heights, which may
include
optional heating and cooling steps, are applied on the B and C axes to "square-
up" the
workpiece. The phrase "press forging to a ... blocking reduction spacer
height",
otherwise referred to herein as press forging to a first blocking reduction
spacer height
((78),(87),(96)) and press forging to a second blocking reduction spacer
((81),(90),(99)),
is defined as a press forging step that is used to reduce or "square-up" the
bulging that
occurs near the center of any face after press forging to major reduction
spacer height.
Bulging at or near the center of any face results in a triaxial stress state
being
introduced into the faces, which could result in cracking of the workpiece.
The steps of
press forging to a first reduction spacer height and press forging to a second
blocking
reduction spacer height, also referred to herein a first blocking reduction,
second
blocking reduction, or simply blocking reductions are employed to deform the
bulged
faces, so that the faces of the workpiece are flat or substantially flat
before the next
press forging to a major reduction spacer height along an orthogonal axis. The
blocking
reductions involve press forging to a spacer height that is greater than the
spacer height
used in each step of press forging to a major reduction spacer height. While
the strain
rate of all of the first and second blocking reductions disclosed herein may
be sufficient
to adiabatically heat an internal region of the workpiece, in a non-limiting
embodiment,
adiabatic heating during the first blocking and second blocking reductions may
not occur
because the total strain incurred in the first and second blocking reductions
may not be
sufficient to significantly adiabatically heat the workpiece. Because the
blocking
reductions are performed to spacer heights that are greater than those used in
press
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forging to a major reduction spacer height, the strain added to the workpiece
in a
blocking reduction may not be enough to adiabatically heat an internal region
of the
workpiece. As will be seen, incorporation of the first and second blocking
reductions in
a high strain rate MAF process, in a non-limiting embodiment results in a
forging
sequence of at least one cycle consisting of: A-B-C-B-C-A-C, wherein A, B, and
C
cornprise press forging to the major reduction spacer height, and wherein B,
C, C, and
A comprise press forging to first or second blocking reduction spacer heights;
or in
another non-limiting embodiment at least one cycle consisting of: A-B-C-B-C-A-
C-A-B,
wherein A, B, and C comprise press forging to the major reduction spacer
height, and
wherein B, C, C, A, A, and B comprise press forging to first or second
blocking
reduction spacer heights.
[0099] Referring again to FIGS. 3 and 5, in a non-limiting embodiment, after
the step of press forging to a major reduction spacer height (75) on the first
orthogonal
axis (an A reduction), and, if applied, after the optional allowing (76) and
heating (77)
steps, as described above, the workpiece is press forged (78) on the B-axis to
a first
blocking reduction spacer height. While the strain rate of the first blocking
reduction
may be sufficient to adiabatically heat an internal region of the workpiece,
in a non-
limiting embodiment, adiabatic heating during the first blocking reduction may
not occur
because the strain incurred in the first blocking reduction may not be
sufficient to
significantly adiabatically heat the workpiece. Optionally, the adiabatically
heated
internal region of the workpiece is allowed (79) to cool to a temperature at
or near the
workpiece forging temperature, while the outer surface region of the workpiece
is
heated (80) to a temperature at or near the workpiece forging temperature. All
cooling
times and heating methods for the A reduction (75) disclosed hereinabove and
in other
embodiments of the present disclosure are applicable for steps (79) and (80)
and to all
optional subsequent steps of allowing the internal region of the workpiece to
cool and
heating the outer surface region of the workpiece.
[0100] The workpiece is next press forged (81) on the C-axis to a second
blocking reduction spacer height that is greater than the major reduction
spacer height.
The first and second blocking reductions are applied to bring the workpiece
back to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
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blocking reduction may be sufficient to adiabatically heat an internal region
of the
workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
reduction may not occur because the strain incurred in the second blocking
reduction
may not be sufficient to significantly adiabatically heat the workpiece.
Optionally, the
adiabatically heated internal region of the workpiece is allowed (82) to cool
to a
temperature at or near the workpiece forging temperature, while the outer
surface
region of the workpiece is heated (83) to a temperature at or near the
workpiece forging
temperature.
[0101] The workpiece is next pressed forged to a major reduction spacer
height (84) in the direction of the second orthogonal axis, or B-axis. Press
forging to a
major reduction spacer height on the B-axis (84) is referred to herein as a B
reduction.
After the B reduction (84), optionally, the adiabatically heated internal
region of the
workpiece is allowed (85) to cool to a temperature at or near the workpiece
forging
temperature, while the outer surface region of the workpiece is heated (86) to
a
temperature at or near the workpiece forging temperature.
[0102] The workpiece is next press forged (87) on the C-axis to a first
blocking
reduction spacer height that is greater than the major reduction spacer
height. While
the strain rate of the first blocking reduction may be sufficient to
adiabatically heat an
internal region of the workpiece, in a non-limiting embodiment, adiabatic
heating during
the first blocking reduction may not occur because the strain incurred in the
first
blocking reduction may not be sufficient to significantly adiabatically heat
the workpiece.
Optionally, the adiabatically heated internal region of the workpiece is
allowed (88) to
cool to a temperature at or near the workpiece forging temperature, while the
outer
surface region of the workpiece is heated (89) to a temperature at or near the
workpiece
forging temperature.
[0103] The workpiece is next press forged (90) on the A-axis to a second
blocking reduction spacer height that is greater than the major reduction
spacer height.
The first and second blocking reductions are applied to bring the workpiece
back to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
blocking reduction may be sufficient to adiabatically heat an internal region
of the
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workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
reduction may not occur because the strain incurred in the second blocking
reduction
may not be sufficient to significantly adiabatically heat the workpiece.
Optionally, the
adiabatically heated internal region of the workpiece is allowed (91) to cool
to a
temperature at or near the workpiece forging temperature, while the outer
surface
region of the workpiece is heated (92) to a temperature at or near the
workpiece forging
temperature.
[0104] The workpiece is next press forged to a major reduction spacer height
(93) in the direction of the third orthogonal axis, or C-axis. Press forging
to the major
reduction spacer height on the C-axis (93) is referred to herein as a C
reduction. After
the C reduction (93), optionally, the adiabatically heated internal region of
the workpiece
is allowed (94) to cool to a temperature at or near the workpiece forging
temperature,
while the outer surface region of the workpiece is heated (95) to a
temperature at or
near the workpiece forging temperature.
[0105] The workpiece is next press forged (96) on the A-axis to a first
blocking
reduction spacer height that is greater than the major reduction spacer
height. While
the strain rate of the first blocking reduction may be sufficient to
adiabatically heat an
internal region of the workpiece, in a non-limiting embodiment, adiabatic
heating during
the first blocking reduction may not occur because the strain incurred in the
first
blocking reduction may not be sufficient to significantly adiabatically heat
the workpiece.
Optionally, the adiabatically heated internal region of the workpiece is
allowed (97) to
cool to a temperature at or near the workpiece forging temperature, while the
outer
surface region of the workpiece is heated (98) to a temperature at or near the
workpiece
forging temperature.
[0106] The workpiece is next press forged (99) on the B-axis to a second
blocking reduction spacer height that is greater than the major reduction
spacer height.
The first and second blocking reductions are applied to bring the workpiece
back to
substantially the pre-forging shape of the workpiece. While the strain rate of
the second
blocking reduction may be sufficient to adiabatically heat an internal region
of the
workpiece, in a non-limiting embodiment, adiabatic heating during the second
blocking
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reduction may not occur because the strain incurred in the second blocking
reduction
may not be sufficient to significantly adiabatically heat the workpiece.
Optionally, the
adiabatically heated internal region of the workpiece is allowed (100) to cool
to a
temperature at or near the workpiece forging temperature, while the outer
surface
region of the workpiece is heated (101) to a temperature at or near the
workpiece
forging temperature.
[0107] Referring to FIG. 5, in non-limiting embodiments, one or more of press
forging steps (75), (78), (81), (84), (87), (90), (93), (96), and (99) are
repeated (102)
until a total strain of at least 1.0 is achieved the titanium alloy workpiece.
In another
non-limiting embodiment, one or more of press forging steps (75), (78), (81),
(84), (87),
(90), (93), (96), and (99) are repeated (102) until a total strain in a range
of at least 1.0
up to less than 3.5 is achieved in the titanium alloy workpiece. It will be
recognized that
after achieving the desired strain of at least 1.0, or alternatively the
desired strain in a
range of at least 1.0 up to less than 3.5, in any of the press forging steps
(75), (78),
(81), (84), (87), (90), (93), (96), and (99), the optional intermediate
equilibration steps
(i.e., allowing the internal region of the workpiece to cool (76), (79), (82),
(85), (88), (91),
(94), (97), or (100), and heating the outer surface of the workpiece (77),
(80), (83), (86),
(89), (92), (95), (98), or (101)) are not needed, and the workpiece can be
cooled to
ambient temperature. In a non-limiting embodiment, cooling comprise liquid
quenching,
such as, for example, water quenching. In another non-limiting embodiment,
cooling
comprises cooling with a cooling rate of air cooling or faster.
[0108] The process described above includes a repeated sequence of press
forging to a major reduction spacer height followed by press forging to first
and second
blocking reduction spacer heights. A forging sequence that represents one
total MAF
cycle as disclosed in the above-described non-limiting embodiment may be
represented
as A-B-C-B-C-A-C-A-B, wherein the reductions (hits) that are in bold and
underlined are
press forgings to a major reduction spacer height, and the reductions that are
not in
bold or underlined are first or second blocking reductions. It will be
understood that all
press forging reductions, including press forging to major reduction spacer
heights and
the first and second blocking reductions, of the MAF process according to the
present
disclosure are conducted with a high strain rate that is sufficient to
adiabatically heat the
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internal region of the workpiece, e.g., and without limitation, a strain rate
in the range of
0.2 s-1 to 0.8 s-1, or in the range of 0.2 s-1 to 0.4 s-1. It will also be
understood that
adiabatic heating may not substantially occur during the first and second
blocking
reductions due to the lower degree of deformation in these reductions, as
compared to
the major reductions. It also will be understood that, as optional steps,
intermediate
successive press forging reductions the adiabatically heated internal region
of the
workpiece is allowed to cool to a temperature at or near the workpiece forging
temperature, and the outer surface of the workpiece is heated to a temperature
at or
near the workpiece forging temperature utilizing the thermal management system
disclosed herein. It is believed that these optional steps may be more
beneficial when
the method is used to process larger sized workpieces. It is further
understood that the
A-B-C-B-C-A-C-A-B forging sequence embodiment described herein may be repeated
in whole or in part until a total strain of at least 1.0, or in the range of
at least 1.0 up to
less than 3.5, is achieved in the workpiece.
[0109] Bulging in the workpiece results from a combination of surface die lock
and the presence of hotter material near the center of the workpiece. As
bulging
increases, each face center is subjected to increasingly triaxial loads that
can initiate
cracking. In the A-B-C-B-C-A-C-A-B_sequence, the use of blocking reductions
intermediate each press forging to a major reduction spacer height reduces the
tendency for crack formation in the workpiece. In a non-limiting embodiment,
when the
workpiece is in the shape of a cube, the first blocking reduction spacer
height for a first
blocking reduction may be to a spacer height that is 40-60% larger than the
major
reduction spacer height. In a non-limiting embodiment, when the workpiece is
in the
shape of a cube, the second blocking reduction spacer height for the second
blocking
reduction may be to a spacer height that is 15-30% larger than the major
reduction
spacer height. In another non-limiting embodiment, the first blocking
reduction spacer
height may be substantially equivalent to the second blocking reduction spacer
height.
[0110] In non-limiting embodiments of thermally managed, high strain rate
multi-axis forging according to the present disclosure, after a total strain
of at least 1.0,
or in the range of at least 1.0 up to less than 3.5, the workpiece comprises
an average
alpha particle grain size of 4 pm or less, which is considered to be an ultra-
fine grain
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(UFG) size. In a non-limiting embodiment according to the present disclosure,
applying
a total strain of at least 1.0, or in the range of at least 1.0 up to less
than 3.5, produces
grains that are equiaxed.
[0111] In a non-limiting embodiment of a process according to the present
disclosure comprising multi-axis forging and use of the optional thermal
management
system, the workpiece-press die interface is lubricated with lubricants known
to those of
ordinary skill, such as, but not limited to, graphite, glasses, and/or other
known solid
lubricants.
[0112] In certain non-limiting embodiments of methods according to the
present disclosure, the workpiece comprises a titanium alloy selected from
alpha+beta
titanium alloys and metastable beta titanium alloys. In another non-limiting
embodiment, the workpiece comprises an alpha+beta titanium alloy. In still
another
non-limiting embodiment, the workpiece comprises a metastable beta titanium
alloy. In
a non-limiting embodiment, a titanium alloy processed by the method according
to the
present disclosure comprises effective alpha phase precipitation and growth
kinetics
that are slower than those of Ti-6-4 alloy (UNS R56400), and such kinetics may
be
referred to herein as "slower alpha kinetics". In a non-limiting embodiment,
slower
alpha kinetics is achieved when the diffusivity of the slowest diffusing
alloying species in
the titanium alloy is slower than the diffusivity of aluminum in Ti-6-4 alloy
at the beta
transus temperature (To). For example, Ti-6-2-4-2 alloy exhibits slower alpha
kinetics
than 11-6-4 alloy as a result of the presence of additional grain pinning
elements, such
as silicon, in the Ti-6-2-4-2 alloy. Also, Ti-6-2-4-6 alloy has slower alpha
kinetics than
Ti-6-4 alloy as a result of the presence of additional beta stabilizing alloy
additions, such
as higher molybdenum content than 1-6-4 alloy. The result of slower alpha
kinetics in
these alloys is that beta annealing the Ti-6-2-4-6 and 11-6-2-4-2 alloys prior
to high
strain rate MAF produces a relatively fine and stable alpha lath size and a
fine beta-
phase structure as compared with Ti-6-4 alloy and certain other titanium
alloys
exhibiting faster alpha phase precipitation and growth kinetics than Ti-6-2-4-
6 and Ti-6-
2-4-2 alloys. The phrase "slower alpha kinetics" is discussed in further
detail earlier in
the present disclosure. Exemplary titanium alloys that may be processed using
embodiments of methods according to the present disclosure include, but are
not limited
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to, Ti-6-2-4-2 alloy, Ti-6-2-4-6 alloy, All 425 alloy (Ti-4A1-2.5V alloy), Ti-
6-6-2 alloy,
and Ti-6A1-7Nb alloy.
[0113] In a non-limiting embodiment of the method according to the present
disclosure, beta annealing comprises: heating the workpiece to a beta
annealing
temperature; holding the workpiece at the beta annealing temperature for an
annealing
time sufficient to form a 100% titanium beta phase microstructure in the
workpiece; and
cooling the workpiece directly to a temperature at or near the workpiece
forging
temperature. In certain non-limiting embodiments, the beta annealing
temperature is in
a temperature range of the beta transus temperature of the titanium alloy up
to 300 F
(111 C) above the beta transus temperature of the titanium alloy. Non-limiting
embodiments include a beta annealing time from 5 minutes to 24 hours. A person
skilled in the art, upon reading the present description, will understand that
other beta
annealing temperatures and beta annealing times are within the scope of
embodiments
of the present disclosure and that, for example, relatively large workpieces
may require
relatively higher beta annealing temperatures and/or longer beta annealing
times to
form a 100% beta phase titanium microstructure.
[0114] In certain non-limiting embodiments in which the workpiece is held at a
beta annealing temperature to form a 100% beta phase microstructure, the
workpiece
may also be plastically deformed at a plastic deformation temperature in the
beta phase
field of the titanium alloy prior to cooling the workpiece to a temperature at
or near the
workpiece forging temperature or to ambient temperature. Plastic deformation
of the
workpiece may comprise at least one of drawing, upset forging, and high strain
rate
multi-axis forging the workpiece. In a non-limiting embodiment, plastic
deformation in
the beta phase region comprises upset forging the workpiece to a beta-upset
strain in
the range of 0.1 to 0.5. In certain non-limiting embodiments, the plastic
deformation
temperature is in a temperature range including the beta transus temperature
of the
titanium alloy up to 300 F (111 C) above the beta transus temperature of the
titanium
alloy.
[0115] FIG. 6 is a temperature-time thermomechanical process chart for a non-
limiting method of plastically deforming the workpiece above the beta transus
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temperature and directly cooling to the workpiece forging temperature. In FIG.
6, a non-
limiting method 200 comprises heating 202 a workpiece comprising a titanium
alloy
having alpha precipitation and growth kinetics that are slower than those of
11-6-4 alloy,
for example, to a beta annealing temperature 204 above the beta transus
temperature
206 of the titanium alloy, and holding or "soaking" 208 the workpiece at the
beta
annealing temperature 204 to form an all beta titanium phase microstructure in
the
workpiece. In a non-limiting embodiment according to the present disclosure,
after
soaking 208, the workpiece may be plastically deformed 210. In a non-limiting
embodiment, plastic deformation 210 comprises upset forging. In a non-limiting
embodiment, plastic deformation 210 comprises upset forging to a true strain
of 0.3. In
a non-limiting embodiment, plastically deforming 210 comprises thermally
managed
high strain rate multi-axis forging (not shown in FIG. 6) at a beta annealing
temperature.
[0116] Still referring to FIG. 6, after plastic deformation 210 in the beta
phase
field, in a non-limiting embodiment the workpiece is cooled 212 to a workpiece
forging
temperature 214 in the alpha+beta phase field of the titanium alloy. In a non-
limiting
embodiment, cooling 212 comprises air cooling or cooling at a rate faster than
achieved
through air cooling. In another non-limiting embodiment, cooling comprises
liquid
quenching, such as, but not limited to, water quenching. After cooling 212,
the
workpiece is high strain rate multi-axis forged 214 according to certain non-
limiting
embodiments of the present disclosure. In the non-limiting embodiment of FIG.
6, the
workpiece is hit or press forged 12 times, i.e., the three orthogonal axes of
the
workpiece are non-sequentially press forged a total of 4 times each. In other
words,
referring to FIGS. 2 and 6, the cycle including steps (a)-(optional b), (c)-
(optional d), and
(e)-(optional f) is performed 4 times. In the non-limiting embodiment of FIG.
6, after a
multi-axis forging sequence involving 12 hits, the total strain may be equal
to, for
example, at least 1.0, or may be in the range of at least 1.0 up to less than
3.5. After
multi-axis forging 214, the workpiece is cooled 216 to ambient temperature. In
a non-
limiting embodiment, cooling 216 comprises air cooling or cooling at a rate
faster than
achieved through air cooling, but other forms of cooling, such as, but not
limited to, fluid
or liquid quenching are within the scope of embodiments disclosed herein.
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[0117] A non-limiting aspect of the present disclosure includes high strain
rate
multi-axis forging at two temperatures in the alpha+beta phase field. FIG. 7
is a
temperature-time thermomechanical process chart for a non-limiting method
according
to the present disclosure that comprises multi-axis forging the titanium alloy
workpiece
at a first workpiece forging temperature; optionally utilizing a non-limiting
embodiment of
the thermal management feature disclosed hereinabove; cooling to a second
workpiece
forging temperature in the alpha+beta phase; multi-axis forging the titanium
alloy
workpiece at the second workpiece forging temperature; and optionally
utilizing a non-
limiting embodiment of the thermal management feature disclosed herein.
[0118] In FIG. 7, a non-limiting method 230 according to the present
disclosure
comprises heating 232 the workpiece to a beta annealing temperature 234 above
the
beta transus temperature 236 of the alloy and holding or soaking 238 the
workpiece at
the beta annealing temperature 234 to form an all beta phase microstructure in
the
titanium alloy workpiece. After soaking 238, the workpiece may be plastically
deformed
240. In a non-limiting embodiment, plastic deformation 240 comprises upset
forging. In
another non-limiting embodiment, plastic deformation 240 comprises upset
forging to a
strain of 0.3. In yet another non-limiting embodiment, plastically deforming
240 the
workpiece comprises high strain multi-axis forging (not shown in FIG. 7) at a
beta
annealing temperature.
[0119] Still referring to FIG. 7, after plastic deformation 240 in the beta
phase
field, the workpiece is cooled 242 to a first workpiece forging temperature
244 in the
alpha+beta phase field of the titanium alloy. In non-limiting embodiments,
cooling 242
comprises one of air cooling and liquid quenching. After cooling 242, the
workpiece is
high strain rate multi-axis forged 246 at the first workpiece forging
temperature, and
optionally a thermal management system according to non-limiting embodiments
disclosed herein is employed. In the non-limiting embodiment of FIG. 7, the
workpiece
is hit or press forged at the first workpiece forging temperature 12 times
with 90 rotation
between each hit, i.e., the three orthogonal axes of the workpiece are press
forged 4
times each. In other words, referring to FIG. 2, the cycle including steps (a)-
(optional b),
(c)-(optional d), and (e)-(optional f) is performed 4 times. In the non-
limiting
embodiment of FIG. 7, after high strain rate multi-axis forging 246 the
workpiece at the
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first workpiece forging temperature, the titanium alloy workpiece is cooled
248 to a
second workpiece forging temperature 250 in the alpha+beta phase field. After
cooling
248, the workpiece is high strain rate multi-axis forged 250 at the second
workpiece
forging temperature, and optionally a thermal management system according to
non-
limiting embodiments disclosed herein is employed. In the non-limiting
embodiment of
FIG. 7, the workpiece is hit or press forged at the second workpiece forging
temperature
a total of 12 times. It is recognized that the number of hits applied to the
titanium alloy
workpiece at the first and second workpiece forging temperatures can vary
depending
upon the desired true strain and desired final grain size, and that the number
of hits that
is appropriate can be determined without undue experimentation upon
considering the
present disclosure. After multi-axis forging 250 at the second workpiece
forging
temperature, the workpiece is cooled 252 to ambient temperature. In non-
limiting
embodiments, cooling 252 comprises one of air cooling and liquid quenching to
ambient
temperature.
[0120] In a non-limiting embodiment, the first workpiece forging temperature
is
in a first workpiece forging temperature range of more than 100 F (55.6 C)
below the
beta transus temperature of the titanium alloy to 500 F (277.8 C) below the
beta transus
temperature of the titanium alloy, i.e., the first workpiece forging
temperature T1 is in the
range of To- 100 F >11 To - 500 F. In a non-limiting embodiment, the second
workpiece forging temperature is in a second workpiece forging temperature
range of
more than 200 F (277.8 C) below the beta transus temperature of the titanium
alloy to
700 F (388.9 C) below the beta transus temperature, i.e., the second workpiece
forging
temperature T2 is in the range of To - 200 F> Tp - 700 F. In a non-limiting
embodiment, the titanium alloy workpiece comprises Ti-6-2-4-2 alloy; the first
workpiece
temperature is 1650 F (898.9 C); and the second workpiece forging temperature
is
1500 F (815.6 C).
[0121] FIG. 8 is a temperature-time thermomechanical process chart of a non-
limiting method embodiment according to the present disclosure for plastically
deforming a workpiece comprising a titanium alloy above the beta transus
temperature
and cooling the workpiece to the workpiece forging temperature, while
simultaneously
employing thermally managed high strain rate multi-axis forging on the
workpiece
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according to non-limiting embodiments herein. In FIG. 8, a non-limiting method
260 of
using thermally managed high strain rate multi-axis forging for grain refining
of a
titanium alloy comprises heating 262 the workpiece to a beta annealing
temperature
264 above the beta transus temperature 266 of the titanium alloy and holding
or soaking
268 the workpiece at the beta annealing temperature 264 to form an all beta
phase
microstructure in the workpiece. After soaking 268 the workpiece at the beta
annealing
temperature, the workpiece is plastically deformed 270. In a non-limiting
embodiment,
plastic deformation 270 may comprise thermally managed high strain rate multi-
axis
forging. In a non-limiting embodiment, the workpiece is repetitively high
strain rate
multi-axis forged 272 using the optional thermal management system as
disclosed
herein as the workpiece cools through the beta transus temperature. FIG. 8
shows
three intermediate high strain rate multi-axis forging 272 steps, but it will
be understood
that there can be more or fewer intermediate high strain rate multi-axis
forging 272
steps, as desired. The intermediate high strain rate multi-axis forging 272
steps are
intermediate to the initial high strain rate multi-axis forging step 270 at
the soaking
temperature and the final high strain rate multi-axis forging step in the
alpha+beta phase
field 274 of the titanium alloy. While FIG. 8 shows one final high strain rate
multi-axis
forging step wherein the temperature of the workpiece remains entirely in the
alpha+beta phase field, it will be understood on reading the present
description that
more than one multi-axis forging step could be performed in the alpha+beta
phase field
for further grain refinement. According to non-limiting embodiments of the
present
disclosure, at least one final high strain rate multi-axis forging step takes
place entirely
at temperatures in the alpha+beta phase field of the titanium alloy workpiece.
[0122] Because the multi-axis forging steps 270,272,274 take place as the
temperature of the workpiece cools through the beta transus temperature of the
titanium
alloy, a method embodiment such as is shown in FIG. 8 is referred to herein as
"through
beta transus high strain rate multi-axis forging". In a non-limiting
embodiment, the
thermal management system (33 of FIG. 3) is used in through beta transus multi-
axis
forging to maintain the temperature of the workpiece at a uniform or
substantially
uniform temperature prior to each hit at each through beta transus forging
temperature
and, optionally, to slow the cooling rate. After final multi-axis forging 274
the workpiece
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forging temperature in the alpha+beta phase field, the workpiece is cooled 276
to
ambient temperature. In a non-limiting embodiment, cooling 276 comprises air
cooling.
[0123] Non-limiting embodiments of multi-axis forging using a thermal
management system, as disclosed hereinabove, can be used to process titanium
alloy
workpieces having cross sections greater than 4 square inches using
conventional
forging press equipment, and the size of cube-shaped workpieces can be scaled
to
match the capabilities of an individual press. It has been determined that
alpha
lamellae or laths from the 13-annealed structure break down easily to fine
uniform alpha
grains at workpiece forging temperatures disclosed in non-limiting embodiments
herein.
It has also been determined that decreasing the workpiece forging temperature
decreases the alpha particle size (grain size).
[0124] While not wanting to be held to any particular theory, it is believed
that
grain refinement that occurs in non-limiting embodiments of thermally managed,
high
strain rate multi-axis forging according to the present disclosure occurs via
meta-
dynamic recrystallization. In the prior art slow strain rate multi-axis
forging process,
dynamic recrystallization occurs instantaneously during the application of
strain to the
material. It is believed that in high strain rate multi-axis forging according
to the present
disclosure, meta-dynamic recrystallization occurs at the end of each
deformation or
forging hit, while at least the internal region of the workpiece is hot from
adiabatic
heating. Residual adiabatic heat, internal region cooling times, and external
surface
region heating influence the extent of grain refinement in non-limiting
methods of
thermally managed, high strain rate multi-axis forging according to the
present
disclosure.
[0125] The present inventors have further developed alternate methods
according to the present disclosure providing certain advantages relative to a
process
as described above including multi-axis forging and using a thermal management
system and a cube-shaped workpiece comprising a titanium alloy. It is believed
that
one or more of (1) the cubical workpiece geometry used in certain embodiments
of
thermally managed multi-axis forging disclosed herein, (2) die chill (i.e.,
allowing the
temperature of the dies to dip significantly below the workpiece forging
temperature),
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and (3) use of high strain rates may disadvantageously concentrate strain
within a core
region of the workpiece.
[0126] The alternate methods according to the present disclosure can achieve
generally uniform fine grain, very fine grain, or ultrafine grain size
throughout a billet
size titanium alloy workpiece. In other words, a workpiece processed by such
alternate
methods may include the desired grain size, such as an ultrafine grain
microstructure,
throughout the workpiece, and not only in a central region of the workpiece.
Non-
limiting embodiments of such alternate methods comprise "multiple upset and
draw"
steps performed on billets having cross-sections greater than 4 square inches.
The
multiple upset and draw steps are intended to impart uniform fine grain, very
fine grain,
or ultrafine grain microstructure throughout the workpiece, while preserving
substantially
the original dimensions of the workpiece. Because these alternate methods
include
Multiple Upset and Draw steps, they are referred to herein as embodiments of
the
"MUD" method. The MUD method includes severe plastic deformation and can
produce
uniform ultrafine grains in billet-size (e.g., 30 inch (76.2 cm) in length)
titanium alloy
workpieces. In non-limiting embodiments of the MUD method according to the
present
disclosure, strain rates used for the upset forging and draw forging steps are
in the
range of 0.001 s-1 to 0.02 s-1. In contrast, strain rates typically used for
conventional
open die upset and draw forging are in the range of 0.03 s-1 to 0.1 s-1. The
strain rate
for MUD is slow enough to prevent adiabatic heating in the workpiece in order
to keep
the forging temperature in control, yet the strain rate is acceptable for
commercial
practices.
[0127] A schematic representation of non-limiting embodiments of the MUD
method is provided in FIG. 9, and a flow chart of certain embodiments of the
MUD
method is provided in FIG. 10. Referring to FIGS. 9 and 10, a non-limiting
method 300
for refining grains in a workpiece comprising a titanium alloy using multiple
upset and
draw forging steps comprises heating an elongate titanium alloy workpiece 302
to a
workpiece forging temperature in the alpha+beta phase field of the titanium
alloy. In a
non-limiting embodiment, the shape of the elongate workpiece is a cylinder or
a
cylinder-like shape. In another non-limiting embodiment, the shape of the
workpiece is
an octagonal cylinder or a right octagon.
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[0128] The elongate workpiece has a starting cross-sectional dimension. For
example, in a non-limiting embodiment of the MUD method according to the
present
disclosure in which the starting workpiece is a cylinder, the starting cross-
sectional
dimension is the diameter of the cylinder. In a non-limiting embodiment of the
MUD
method according to the present disclosure in which the starting workpiece is
an
octagonal cylinder, the starting cross-sectional dimension is the diameter of
the
circumscribed circle of the octagonal cross-section, i.e., the diameter of the
circle that
passes through all the vertices of the octagonal cross-section.
[0129] When the elongate workpiece is at the workpiece forging temperature,
the workpiece is upset forged 304. After upset forging 304, in a non-limiting
embodiment, the workpiece is rotated 90 degrees to the orientation 306 and
then is
subjected to multiple pass draw forging 312. Actual rotation of the workpiece
is
optional, and the objective of the step is to dispose the workpiece into the
correct
orientation (refer to FIG. 9) relative to a forging device for subsequent
multiple pass
draw forging 312 steps.
[0130] Multiple pass draw forging comprises incrementally rotating (depicted
by
arrow 310) the workpiece in a rotational direction (indicated by the direction
of arrow
310), followed by draw forging 312 the workpiece after each increment of
rotation. In
non-limiting embodiments, incrementally rotating 310 and draw forging 312 is
repeated
until the workpiece comprises the starting cross-sectional dimension. In a non-
limiting
embodiment, the upset forging and multiple pass draw forging steps are
repeated until a
total strain of at least 1.0 is achieved in the workpiece. Another non-
limiting
embodiment comprises repeating the heating, upset forging, and multiple pass
draw
forging steps until a total strain in the range of at least 1.0 up to less
than 3.5 is
achieved in the workpiece. In still another non-limiting embodiment, the
heating, upset
forging, and multiple pass draw forging steps are repeated until a total
strain of at least
10 is achieved in the workpiece. It is anticipated that when a total strain of
10 is
imparted to the MUD forging, an ultrafine grain alpha microstructure is
produced, and
that increasing the total strain imparted to the workpiece results in smaller
average grain
sizes.
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[0131] An aspect of the present disclosure is to employ a strain rate during
the
upset and multiple pass drawing steps that is sufficient to result in severe
plastic
deformation of the titanium alloy workpiece, which, in non-limiting
embodiments, further
results in ultrafine grain size. In a non-limiting embodiment, a strain rate
used in upset
forging is in the range of 0.001 s".1 to 0.003 s-1. In another non-limiting
embodiment, a
strain rate used in the multiple pass draw forging steps is the range of 0.01
s-1 to 0.02
s'. It was disclosed in the '538 Application that strain rates in these ranges
do not
result in adiabatic heating of the workpiece, which enables workpiece
temperature
control, and were found sufficient for an economically acceptable commercial
practice.
[0132] In a non-limiting embodiment, after completion of the MUD method, the
workpiece has substantially the original dimensions of the starting elongate
article, such
as, for example, cylinder 314 or octagonal cylinder 316. In another non-
limiting
embodiment, after completion of the MUD method, the workpiece has
substantially the
same cross-section as the starting workpiece. In a non-limiting embodiment, a
single
upset requires numerous draw hits and intermediate rotations to return the
workpiece to
a shape including the starting cross-section of the workpiece.
[0133] In a non-limiting embodiment of the MUD method wherein the
workpiece is in the shape of a cylinder, for example, incrementally rotating
and draw
forging further comprises multiple steps of rotating the cylindrical workpiece
in 150
increments and subsequently draw forging, until the cylindrical workpiece is
rotated
through 360 and is draw forged at each increment. In a non-limiting
embodiment of the
MUD method wherein the workpiece is in the shape of a cylinder, after each
upset
forge, twenty-four draw forging steps with intermediate incremental rotation
between
successive draw forging steps are employed to bring the workpiece to
substantially its
starting cross-sectional dimension. In another non-limiting embodiment,
wherein the
workpiece is in the shape of an octagonal cylinder, incrementally rotating and
draw
forging further comprises multiple steps of rotating the cylindrical workpiece
in 45
increments and subsequently draw forging, until the cylindrical workpiece is
rotated
through 360 and is draw forged at each increment. In a non-limiting
embodiment of the
MUD method wherein the workpiece is in the shape of an octagonal cylinder,
after each
upset forge, eight forging steps separated by incremental rotation of the
workpiece are
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employed to bring the workpiece substantially to its starting cross-sectional
dimension.
It was observed in non-limiting embodiments of the MUD method that
manipulation of
an octagonal cylinder by handling equipment was more precise than manipulation
of a
cylinder by handling equipment. It also was observed that manipulation of an
octagonal
cylinder by handling equipment in a non-limiting embodiment of a MUD method
was
more precise than manipulation of a cube-shaped workpiece using hand tongs in
non-
limiting embodiments of the thermally managed high strain rate MAF process
disclosed
herein. It will be recognized on considering the present description that
other draw
forging sequences, each including a number of draw forging steps and
intermediate
incremental rotations of a particular number of degrees, may be used for other
cross-
sectional billet shapes so that the final shape of the workpiece after draw
forging is
substantially the same as the starting shape of the workpiece prior to upset
forging.
Such other possible sequences may be determined by a person skilled in the art
without
undue experimentation and are included within the scope of the present
disclosure.
[0134] In a non-limiting embodiment of the MUD method according to the
present disclosure, a workpiece forging temperature comprises a temperature
within a
workpiece forging temperature range. In a non-limiting embodiment, the
workpiece
forging temperature is in a workpiece forging temperature range of 100 F (55.6
C)
below the beta transus temperature (To) of the titanium alloy to 700 F (388.9
C) below
the beta transus temperature of the titanium alloy. In still another non-
limiting
embodiment, the workpiece forging temperature is in a temperature range of 300
F
(166.7 C) below the beta transus temperature of the titanium alloy to 625 F
(347 C)
below the beta transus temperature of the titanium alloy. In a non-limiting
embodiment,
the low end of a workpiece forging temperature range is a temperature in the
alpha+beta phase field at which substantial damage does not occur to the
surface of the
workpiece during the forging hit, as may be determined without undue
experimentation
by a person having ordinary skill in the art.
[0135] In a non-limiting embodiment of the MUD method according to the
present disclosure, the workpiece forging temperature range for a Ti-6-2-4-2
alloy,
which has a beta transus temperature (T) of about 1820 F (993.3 C), may be,
for
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example, from 1120 F (604.4C) to 1720 F (937.8 C), or in another embodiment
may be
from 1195 F (646.1 C) to 1520 F (826.7 C).
[0136] Non-limiting embodiments of the MUD method comprise multiple
reheating steps. In a non-limiting embodiment, the titanium alloy workpiece is
heated to
the workpiece forging temperature after upset forging the titanium alloy
workpiece. In
another non-limiting embodiment, the titanium alloy workpiece is heated to the
workpiece forging temperature prior to a draw forging step of the multiple
pass draw
forging. In another non-limiting embodiment, the workpiece is heated as needed
to
bring the actual workpiece temperature back to or near the workpiece forging
temperature after an upset or draw forging step.
[0137] It was determined that embodiments of the MUD method impart
redundant work or extreme deformation, also referred to as severe plastic
deformation,
which is aimed at creating ultrafine grains in a workpiece comprising a
titanium alloy.
Without intending to be bound to any particular theory of operation, it is
believed that the
round or octagonal cross sectional shape of cylindrical and octagonal
cylindrical
workpieces, respectively, distribute strain more evenly than workpieces of
square or
rectangular cross sectional shape across the cross-sectional area of the
workpiece
during a MUD method. The deleterious effect of friction between the workpiece
and the
forging die is also reduced by reducing the area of the workpiece in contact
with the die.
[0138] In addition, it was also determined that decreasing the temperature
during the MUD method reduces the final grain size to a size that is
characteristic of the
specific temperature being used. Referring to FIG. 10, in a non-limiting
embodiment of
a method 400 for refining the grain size of a workpiece, after processing the
workpiece
by the MUD method at the workpiece forging temperature, the temperature of the
workpiece may be cooled 416 to a second workpiece forging temperature. In a
non-
limiting embodiment, after cooling the workpiece to the second workpiece
forging
temperature, the workpiece is upset forged at the second workpiece forging
temperature 418. The workpiece is rotated 420 or otherwise oriented relative
to the
forging press for subsequent draw forging steps. The workpiece is multiple-
step draw
forged at the second workpiece forging temperature 422. Multiple-step draw
forging at
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the second workpiece forging temperature 422 comprises incrementally rotating
424 the
workpiece in a rotational direction (refer to FIG. 9) and draw forging at the
second
workpiece forging temperature 426 after each increment of rotation. In a non-
limiting
embodiment, the steps of upset, incrementally rotating 424, and draw forging
are
repeated 426 until the workpiece comprises the starting cross-sectional
dimension. In
another non-limiting embodiment, the steps of upset forging at the second
workpiece
temperature 418, rotating 420, and multiple step draw forging 422 are repeated
until a
total strain of at least 1.0, or in the range of 1.0 up to less than 3.5, or
up to 10 or
greater is achieved in the workpiece. It is recognized that the MUD method can
be
continued until any desired total strain is imparted to the titanium alloy
workpiece.
[0139] In a non-limiting embodiment comprising a multi-temperature MUD
method embodiment, the workpiece forging temperature, or a first workpiece
forging
temperature, is about 1600 F (871.1 C), and the second workpiece forging
temperature
is about 1500 F (815.6 C). Subsequent workpiece forging temperatures that are
lower
than the first and second workpiece forging temperatures, such as a third
workpiece
forging temperature, a fourth workpiece forging temperature, and so forth, are
within the
scope of non-limiting embodiments of the present disclosure.
[0140] As forging proceeds, grain refinement results in decreasing flow stress
at a fixed temperature. It was determined that decreasing the forging
temperature for
sequential upset and draw steps keeps the flow stress constant and increases
the rate
of microstructural refinement. It is anticipated that in non-limiting
embodiments of MUD
according to the present disclosure, a total strain of at least 1.0, in a
range of at least
1.0 up to less than 3.5, or up to 10 results in a uniform equiaxed alpha
ultrafine grain
microstructure in titanium alloy workpieces, and that the lower temperature of
a two-
temperature (or multi-temperature) MUD method can be determinative of the
final grain
size after a total strain of up to 10 is imparted to the MUD forging.
[0141] An aspect of the present disclosure includes the possibility that after
processing a workpiece by the MUD method, subsequent deformation steps are
performed without coarsening the refined grain size, as long as the
temperature of the
workpiece is not subsequently heated above the beta transus temperature of the
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titanium alloy. For example, in a non-limiting embodiment, a subsequent
deformation
practice after the MUD method may include draw forging, multiple draw forging,
upset
forging, or any combination of two or more of these forging techniques at
temperatures
in the alpha+beta phase field of the titanium alloy. In a non-limiting
embodiment,
subsequent deformation or forging steps include a combination of multiple pass
draw
forging, upset forging, and draw forging to reduce the starting cross-
sectional dimension
of the cylinder-like or other elongate workpiece to a fraction of the cross-
sectional
dimension, such as, for example, but not limited to, one-half of the cross-
sectional
dimension, one-quarter of the cross-sectional dimension, and so forth, while
still
maintaining a uniform fine grain, very fine grain, or ultrafine grain
structure in the
titanium alloy workpiece.
[0142] In a non-limiting embodiment of a MUD method, the workpiece
comprises a titanium alloy selected from the group consisting of an alpha+beta
titanium
alloy and a metastable beta titanium alloy. In another non-limiting embodiment
of a
.. MUD method, the workpiece comprises an alpha+beta titanium alloy. In still
another
non-limiting embodiment of the multiple upset and draw process disclosed
herein, the
workpiece comprises a metastable beta titanium alloy. In a non-limiting
embodiment of
a MUD method, the workpiece is a titanium alloy selected from a Ti-6-2-4-2
alloy, a Ti-6-
2-4-6 alloy, ATI 425 titanium alloy (Ti-4A1-2.5V), and a Ti-6-6-2 alloy.
[0143] Prior to heating the workpiece to the workpiece forging temperature in
the alpha+beta phase field according to MUD embodiments of the present
disclosure, in
a non-limiting embodiment the workpiece may be heated to a beta annealing
temperature, held at the beta annealing temperature for a beta annealing time
sufficient
to form a 100% beta phase titanium microstructure in the workpiece, and cooled
to
ambient temperature. In a non-limiting embodiment, the beta annealing
temperature is
in a beta annealing temperature range that includes the beta transus
temperature of the
titanium alloy up to 300 F (111 C) above the beta transus temperature of the
titanium
alloy. In a non-limiting embodiment, the beta annealing time is from 5 minutes
to 24
hours.
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[0144] In a non-limiting embodiment, the workpiece is a billet that is coated
on
all or certain surfaces with a lubricating coating that reduces friction
between the
workpiece and the forging dies. In a non-limiting embodiment, the lubricating
coating is
a solid lubricant such as, but not limited to, one of graphite and a glass
lubricant. Other
lubricating coatings known now or hereafter to a person having ordinary skill
in the art
are within the scope of the present disclosure. In addition, in a non-limiting
embodiment
of the MUD method using cylinder-like or other elongate-shaped workpieces, the
contact area between the workpiece and the forging dies is small relative to
the contact
area in multi-axis forging of a cube-shaped workpiece. For example, with a 4
inch cube,
two of the entire 4 inch by 4 inch faces of the cube is in contact with the
die. With a 5
foot long billet, the billet length is larger than a typical 14 inch long die,
and the reduced
contact area results in reduced die friction and a more uniform titanium alloy
workpiece
microstructure and macrostructure.
[0145] Prior to heating the workpiece comprising a titanium alloy to the
workpiece forging temperature in the alpha+beta phase field according to MUD
embodiments of the present disclosure, in a non-limiting embodiment the
workpiece is
plastically deformed at a plastic deformation temperature in the beta phase
field of the
titanium alloy after being held at a beta annealing time sufficient to form
100% beta
phase in the titanium alloy and prior to cooling the alloy to ambient
temperature. In a
non-limiting embodiment, the plastic deformation temperature is equivalent to
the beta
annealing temperature. In another non-limiting embodiment, the plastic
deformation
temperature is in a plastic deformation temperature range that includes the
beta transus
temperature of the titanium alloy up to 300 F (111 C) above the beta transus
temperature of the titanium alloy.
[0146] In a non-limiting embodiment of the MUD method, plastically deforming
the workpiece in the beta phase field of the titanium alloy comprises at least
one of
drawing, upset forging, and high strain rate multi-axis forging the titanium
alloy
workpiece. In another non-limiting embodiment, plastically deforming the
workpiece in
the beta phase field of the titanium alloy comprises multiple upset and draw
forging
according to non-limiting embodiments of the present disclosure, and wherein
cooling
the workpiece to a temperature at or near the workpiece forging temperature
comprises
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air cooling. In still another non-limiting embodiment, plastically deforming
the workpiece
in the beta phase field of the titanium alloy comprises upset forging the
workpiece to a
30-35% reduction in height or another dimension, such as length.
[0147] Another aspect of the MUD method of the present disclosure may
include heating the forging dies during forging. A non-limiting embodiment
comprises
heating dies of a forge used to forge the workpiece to temperature in a
temperature
range bounded by the workpiece forging temperature down to 100 F (55.6 C)
below the
workpiece forging temperature.
[0148] In non-limiting embodiments of the MUD method according to the
present disclosure, a method for production of ultra-fine grained titanium
alloys includes:
choosing a titanium alloy having slower alpha precipitation and growth
kinetics than
Ti-6-4 alloy; beta annealing the alloy to provide a fine and stable alpha lath
structure;
and high strain rate multi-axis forging the alloy, according to the present
disclosure, to a
total strain of at least 1.0, or in a range of at least 1.0 up to less than
3.5. The titanium
alloy may be chosen from alpha+beta titanium alloys and metastable beta
titanium
alloys that provide a fine and stable alpha lath structure after beta
annealing.
[0149] It is believed that the certain methods disclosed herein also may be
applied to metals and metal alloys other than titanium alloys in order to
reduce the grain
size of workpieces of those alloys. Another aspect of this disclosure includes
non-
limiting embodiments of a method for high strain rate multi-step forging of
metals and
metal alloys. A non-limiting embodiment of the method comprises heating a
workpiece
comprising a metal or a metal alloy to a workpiece forging temperature. After
heating,
the workpiece is forged at the workpiece forging temperature at a strain rate
sufficient to
adiabatically heat an internal region of the workpiece. After forging, a
waiting period is
employed before the next forging step. During the waiting period, the
temperature of
the adiabatically heated internal region of the metal alloy workpiece is
allowed to cool to
the workpiece forging temperature, while at least a one surface region of the
workpiece
is heated to the workpiece forging temperature. The steps of forging the
workpiece and
then allowing the adiabatically heated internal region of the workpiece to
equilibrate to
the workpiece forging temperature while heating at least one surface region of
the metal
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alloy workpiece to the workpiece forging temperature are repeated until a
desired
characteristic is obtained. In a non-limiting embodiment, forging comprises
one or more
of press forging, upset forging, draw forging, and roll forging. In another
non-limiting
embodiment, the metal alloy is selected from the group consisting of titanium
alloys,
zirconium and zirconium alloys, aluminum alloys, ferrous alloys, and
superalloys. In still
another non-limiting embodiment, the desired characteristic is one or more of
an
imparted strain, an average grain size, a shape, and a mechanical property.
Mechanical properties include, but are not limited to, strength, ductility,
fracture
toughness, and hardness,
[0150] The examples that follow are intended to further describe certain non-
limiting embodiments, without restricting the scope of the present invention.
Persons
having ordinary skill in the art will appreciate that variations of the
following examples
are possible within the scope of the invention, which is defined solely by the
claims.
EXAMPLE 1
[0151] A bar of Ti-6-2-4-2 alloy was processed according to a commercial
forging
process, identified in the industry by specification number AMS 4976, which is
typically
used to process Ti-6-2-4-2 alloy. By reference to the AMS 4976 specification,
those
having ordinary skill understand the specifics of the process to achieve the
mechanical
properties and microstructure set out in that the specification. After
processing, the alloy
was metallographically prepared and the microstructure was evaluated
microscopically.
As shown in the micrograph of the prepared alloy included as FIG. 11(a), the
microstructure includes alpha grains (the lighter colored regions in the
image) that are on
the order of 20 pm or larger.
[0152] According to a non-limiting embodiment within the present disclosure, a
4.0 inch cube-shaped workpiece of Ti-6-2-4-2 alloy was beta annealed at 1950 F
(1066 C)
for 1 hour and then air cooled to ambient temperature. After cooling, the beta
annealed
cube-shaped workpiece was heated to a workpiece forging temperature of 1600 F
(871.1 C) and forged using four hits of high strain rate MAF. The hits were to
the following
orthogonal axes, in the following sequence: A-B-C-A. The hits were to a spacer
height of
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3.25 inches, and the ram speed was 1 inch per second. There was no strain rate
control
on the press, but for the 4.0 inch cubes, this ram speed results in a minimum
strain rate
during pressing of 0.25 s-1. The time between successive orthogonal hits was
about 15
seconds. The total strain applied to the workpiece was 1.37. The
microstructure of the Ti-
6-2-4-2 alloy processed in this manner is depicted in the micrograph of FIG.
11(b). The
majority of alpha particles (lighter colored areas) are on the order of 4 pm
or less, which is
substantially finer than the alpha grains produced by the commercial forging
process
discussed above and represented by the micrograph of FIG. 11(a).
EXAMPLE 2
[0153] A bar of Ti-6-2-4-6 alloy was processed according to a commercial
forging
process typically used for T-6-2-4-6 alloy, i.e., according to specification
AMS 4981. By
reference to the AMS 4981 specification, those having ordinary skill
understand the
specifics of the process to achieve the mechanical properties and
microstructure set out in
that the specification. After processing, the alloy was metallographically
prepared and the
microstructure was evaluated microscopically. As shown in the micrograph of
the
prepared alloy shown in FIG. 12(a), the microstructure exhibits alpha grains
(the lighter
colored regions) that are on the order of 10 pm or larger.
[0154] In a non-limiting embodiment according to the present disclosure, a 4.0
.. inch cube-shaped workpiece of Ti-6-2-4-6 alloy was beta annealed at 1870 F
(1066 C) for
1 hour and then air cooled. After cooling, the beta annealed cube-shaped
workpiece was
heated to a workpiece forging temperature of 1500 F (815.6 C) and forged using
four hits
of high strain rate MAF. The hits were to the following orthogonal axes and
followed the
following sequence: A-B-C-A. The hits were to a spacer height of 3.25 inches,
and the
ram speed was 1 inch per second. There was no strain rate control on the
press, but for
the 4.0 inch cubes, this ram speed results in a minimum strain rate during
pressing of
0.25 s-1. The time between successive orthogonal hits was about 15 seconds.
The total
strain applied to the workpiece was 1.37. The microstructure of the alloy
processed in this
manner is depicted in the micrograph of FIG. 12(b). It is seen that the
majority of alpha
particles (lighter colored areas) are on the order of 4 pm or less, and in any
case are much
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finer than the alpha grains produced by the commercial forging process
discussed above
and represented by the micrograph of FIG. 12(a).
EXAMPLE 3
[0155] In a non-limiting embodiment according to the present disclosure, a 4.0
inch cube-shaped workpiece of Ti-6-2-4-6 alloy was beta annealed at 1870 F
(1066 C) for
1 hour and then air cooled. After cooling, the beta annealed cube-shaped
workpiece was
heated to a workpiece forging temperature of 1500 F (815.6 C) and forged using
three hits
of high strain rate MAF, one each on the A, the B, and the C axes (i.e., the
hits were to the
.. following orthogonal axes and in the following sequence: A-B-C). The hits
were to a
spacer height of 3.25 inches, and the ram speed was 1 inch per second. There
was no
strain rate control on the press, but for the 4.0 inch cubes, this ram speed
results in a
minimum strain rate during pressing of 0.25 s-1. The time between successive
hits was
about 15 seconds. After the A-B-C cycle of hits, the workpiece was reheated to
1500 F
(815.6 C) for 30 minutes. The cube was then high strain rate MAF with one hit
each on
the A, the B, and the C axes, i.e., the hits were to the following orthogonal
axes and in the
following sequence: A-B-C. The hits were to the same spacer height and used
the same
ram speed and time in between hits as used in the first A-B-C sequence of
hits. After the
second sequence of A-B-C hits, the workpiece was reheated to 1500 F (815.6 C)
for 30
minutes. The cube was then high strain rate MAF with one hit at each of the A,
the B, and
the C axes, i.e., an A-B-C sequence. The hits were to the same spacer heights
and used
the same ram speed and time in between hits as in the first sequence of A-B-C
hits. This
embodiment of a high strain rate multi-axis forging process imparted a strain
of 3.46. The
microstructure of the alloy processed in this manner is depicted in the
micrograph of
FIG.13. Ills seen that the majority of alpha particles (lighter colored areas)
are on the
order of 4 pm or less. It is believed likely that the alpha particles are
comprised of
individual alpha grains and that each of the alpha grains has a grain size of
4 pm or less
and is equiaxed in shape.
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EXAMPLE 4
[0156] In a non-limiting embodiment according to the present disclosure, a 4.0
inch cube-shaped workpiece of Ti-6-2-4-2 alloy was beta annealed at 1950 F
(1066 C)
for 1 hour and then air cooled. After cooling, the beta annealed cube-shaped
workpiece
was heated to a workpiece forging temperature of 1700 F (926.7 C) and held for
1 hour,
Two high strain rate MAF cycles (2 sequences of three A-B-C hits, for a total
of 6 hits)
were employed at 1700 F (926.7 C). The time between successive hits was about
15
seconds. The forging sequence was: an A hit to a 3 inch stop; a B hit to a 3.5
inch
stop; and a C hit to a 4.0 inch stop. This forging sequence provides an equal
strain to
all three orthogonal axes every three-hit MAF sequence. The ram speed was 1
inch per
second. There was no strain rate control on the press, but for the 4.0 inch
cubes, this ram
speed results in a minimum strain rate during pressing of 0.25 s-1. The total
strain per
cycle is less than forging to a 3.25 inch reduction in each direction, as in
previous
examples.
[0157] The workpiece was heated to 1650 F (898.9 C) and subjected to high
strength MAF for three additional hits (i.e., one additional A-B-C high strain
rate MAF
cycle). The forging sequence was: an A hit to a 3 inch stop; a B hit to a 3.5
inch stop;
and a C hit to a 4.0 inch stop. After forging, the total strain imparted to
the workpiece was
2.59.
[0158] The microstructure of the forged workpiece of Example 4 is depicted in
the
micrograph of FIG. 14. It is seen that the majority of alpha particles
(lighter colored
regions) are in a networked structure. It is believed likely that the alpha
particles are
comprised of individual alpha grains and that each of the alpha grains has a
grain size of
4 pm or less and is equiaxed in shape.
EXAMPLE 5
[0159] In a non-limiting embodiment according to the present disclosure, a 4.0
inch cube-shaped workpiece of Ti-6-2-4-2 alloy was beta annealed at 1950 F
(1066 C)
for 1 hour and then air cooled. After cooling, the beta annealed, cube-shaped
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workpiece was heated to a workpiece forging temperature of 1700 F (926.7 C)
and held
for 1 hour. MAF according to the present disclosure was employed to apply 6
press
forgings to a major reduction spacer height (A, B, C, A, B, C) to the cube-
shaped
workpiece. In addition, between each press forging to a 3.25 inch major
reduction spacer
height, first and second blocking reductions were conducted on the other axes
to "square
up" the workpiece. The overall forging sequence used is as follows, wherein
the bold and
underlined hits are press forgings to the major reduction spacer height: A-B-C-
B-C-A-C-
A-B-A-B-C-B-C-A-C.
[0160] The forging sequence, including major, first blocking, and second
blocking
spacer heights (in inches) that were utilized are outlined in the table below.
The ram
speed was 1 inch per second. There was no strain rate control on the press,
but for the
4.0 inch cubes, this ram speed results in a minimum strain rate during
pressing of 0.25 s-1.
The time elapsed between hits was about 15 seconds. The total strain after
thermally
managed MAF according to this non-limiting embodiment was 2.37.
Axes and Spacer Heights (Inches)
HIT A
1 3.25
2 4.25 ------------
3111 4 --------------- 3.25
5 4,75
4
7 3.25
---------------- 8 4.75
9 4
10 3.25
11 4,75
12 4
13 3.25
14 4.75
________________ 15 4 __
16 ------------------------------------- 3.25
Total
Strain 2.37
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[0161] The microstructure of the workpiece forged by the process described in
this Example 5 is depicted in the micrograph of FIG. 15. It is seen that the
majority of
alpha particles (lighter colored regions) are elongated. It is believed likely
that the alpha
particles are comprised of individual alpha grains and that each of the alpha
grains has a
grain size of 4 pm or less and is equiaxed in shape.
EXAMPLE 6
[0162] In a non-limiting embodiment according to the present disclosure, a 4.0
inch cube-shaped workpiece of Ti-6-2-4-2 alloy was beta annealed at 1950 F
(1066 C)
for 1 hour and then air cooled. Thermally managed high strain rate MAF,
according to
embodiments of the present disclosure, was performed on the workpiece,
including 6
hits (2 A-B-C MAF cycles) at 1900 C, with 30 second holds between each hit.
The ram
speed was 1 inch per second. There was no strain rate control on the press,
but for the
4.0 inch cubes, this ram speed results in a minimum strain rate during
pressing of 0.25 s-1.
The sequence of 6 hits with intermediate holds was designed to heat the
surface of the
piece through the beta transus temperature during MAF, and this may therefore
be
referred to as a through transus high strain rate MAF. The process results in
refining
the surface structures and minimizing cracking during subsequent forging. The
workpiece was then heated at 1650 F (898.9 C), i.e., below the beta transus
temperature for 1 hour. MAF according to embodiments of the present disclosure
was
applied to the workpiece, including 6 hits (two A-B-C MAF cycles) with about
15
seconds between hits. The first three hits (the hits in the first A-B-C MAF
cycle) were
performed with a 3.5 inch spacer height, and the second 3 hits (the hits in
the second A-
B-C MAF cycle) were performed with a 3.25 inch spacer height. The workpiece
was
heated to 1650 F and held for 30 minutes between the hits with the 3.5 inch
spacer and
the hits with the 3.25 inch spacer. The smaller reduction (i.e., larger spacer
height)
used for the first 3 hits was designed to inhibit cracking as the smaller
reduction breaks
up boundary structures that may lead to cracking. The workpiece was reheated
to
1500 F (815.6 C) for 1 hour. MAF according to embodiments of the present
disclosure
was then applied using 3 A-B-C hits (one MAF cycle) to 3.25 inch reductions
with 15
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seconds in between each hit. This sequence of heavier reductions is designed
to put
additional work into the non-boundary structures. The ram speed for all hits
described
in Example 6 was 1 inch per second.
[0163] A total strain of 3.01 was imparted to the workpiece of Example 6. A
representative micrograph from the center of the thermally managed MAF
workpiece of
Example 6 is shown in FIG. 16(a). A representative micrograph of the surface
of the
thermally managed MAF workpiece of Example 6 is presented in FIG.16(b). The
surface microstructure (FIG. 16(b)) is substantially refined and the majority
of the
particles and/or grains have a size of about 4 pm or less, which is an
ultrafine grain
microstructure. The center microstructure shown in FIG. 16(a) shows highly
refined
grains, and it is believed likely that the alpha particles are comprised of
individual alpha
grains and each of the alpha grains has a grain size of 4 pm or less and is
equiaxed in
shape.
[0164] It will be understood that the present description illustrates those
aspects of the invention relevant to a clear understanding of the invention.
Certain
aspects 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 only a limited number of
embodiments of the present invention are necessarily described herein, one 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.
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