Note: Descriptions are shown in the official language in which they were submitted.
~as~sss
~ACKGRDUND OF THE INVENTION
The present invention relates to a process for fabricating
metallic structures utilizing superplastic forming and forging. For
many years it has been known that certain metals, such as titanium and
many of its alloys, exhibit superplasticity. Superplasticity is the
capability of a material to develop unusually high tensile elongations
with reduced tendency towards necking. This capability is exhibited by
only a few metals and alloys and within limited temperature and strain
rate range. An example of the superplastic forming process is disclosed
in U. S. Patent No. 3,340,101, to Fields, Jr., et al.
However, superplastic forming by its very nature, (i.e. reduced
tendency toward necking) produces a constant overall deformation such
that the thickness of the final structure is substantially the same
throughout. Accordingly, superplastic forming is not used to fabricate
many variable thickness fittings and clips which typically are machined
from bar, plate, or forging stock at high cost and with attendant substantial
waste of material.
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SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to
efficiently fabricate complex variable thickness structures.
It is another object of the present invention to make
metallic structures in a single operation by a combination of super-
plastic foTming and forging.
It is still another object of the present invention to
fabricate deep drawn variable thickness parts.
Briefly, in accordance with the invention, there is
provided a method for making metallic structures which combines
superplastic forming and forging. A metal preform having superplastic
; characteristics and a shaping member which substantially defines the
inal configuration o the preform are provided. The preform is brought
to within a temperature range suitable for superplastic forming.
Pressure is applied to the preform to cause at least a portion thereof
to expand superplastically. At least a portion of the preform is
orged against the shaping member.
In a preferred embodiment, two shaping members are provided
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and the preform is superplastically expanded and deformed against at
least one of the shaping members and forged between the sh3ping members.
Cptimally, the temperature range suitable for superplastic forming of
the preform is also suitable for forging of the preform.
Other objects and advantages of the invention will become
apparent upon reading the following detailed description and upon
reference to the drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional diagr~matic illustration of a
first embodiment of the present invention illustrating the initial position
of the preform relative to the shaping members at A, an intermediate
position upon completion of superplastic forming at B, and the final
formed structure after forging at C;
Figure 2 is a cross-sectional diagramatic illustration of a
second embodiment of the present invention illustrating the initial
position of the preform relative to the shaping members at A, an inter-
mediate position upon completion of superplastic forming at B, and the
final formed structure after completion of forging at C;
Pigure 3 is a cross-sectional diagramatic illustration of a
third embodiment of the present invention illustrating the initial position
of the preform relative to the shaping members at A, intermediate positions
of the preform at B shown by the broken lines which illustrates a position
of the preform during superplastic forming and the solid lines which
illustrate the position of the preform after completion of superplastic
fo m ing, and the final formed structure after completion of forging at C;
Figure 4 is a cross-sectional diagramatic illustration of a
fourth embodiment of the present invention illustrating the initial
position of the preform relative to the shaping members at A, an inter-
mediate position of the preform after completion of forging at B, and
the final formed structure after completion of superplastic forming at C.
While the invention will be described in connection with the
preferred embodiments, it will be understood that it is not intended to
limit the invention to those embodiments. ~n the contrary, it is intended
to cover all alternatives, modifications, and equivalents that may be
included within the spirit and scope of the invention as defined by the
appended claims.
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DEIAILED DESCRIPTION OF IHE INVENTION
In order for superplastic forming to be successful, it is
necessary ~o use a material that is suitable. The extent to which any
material selected will exhibit superplastic properties is predictable
in general terms from a determination of its strain rate sensitivity
and a design determination of the permissible variation of wall thickness.
Strain ra~e sensitivity can be defined as m where
m = d ln a
d ln e
and a is stress in pounds per square inch and ~ is strain rate in reciprocal
minutes. Strain rate sensitivity may be determined by a simple and now well
recognized torsion test described in the article "Determination of Strain-
Hardening Characteristics by Torsion Testing," by D. S. Pields, Jr., and
W. A. Backofen, published in the proceedings of the ASTM, 1957, Volume 57,
pages 1259-1272. A strain rate sensitivity of about 0.5 or greater can be
expected to produce satisfactory results with the larger the value (to a
maximum of 1) the greater the superplastic properties. Maximum strain rate
sensitivity in metals is seen to occur, if at all, as metals are deformed
near the phase transformation temperature. Accordingly, the temperature
immediately below the phase transformation temperature can be expected to
produce the greatest strain rate sensitivity. For titanium and its alloys
the temperature range within which superplasticity can be observed is about
1450F to about 1850F depending upon the specific alloy used.
Cther variables have been found to affect strain rate sensitivity
and therefore should be considered in selecting a suitable metal material.
Decreasing grain size results in correspondingly higher values for strain
rate sensitivity. It has been found that the m-value reaches a peak at an
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intermediate value of strain rate ~approximately 10-4 in./in./sec.). For
maximum stable deformation, superplastic forming should be done at this
strain rate. Too great a variance from the optimum strain rate may result
in a loss of superplastic properties.
Turning first to Figure 1, there is sh~wn a first embodiment of
the present invention. Preform 10 is preferably a metal blank in the form
of a sheet having upper and lower opposed principal surfaces 12 and 14.
Any metal that exhibits suitable superplastic properties can be used, but
the present invention is particularly concerned with titanium or an alloy
thereof, such as Ti-6Al-4V. Additionally, it is preferable that the metal
used for preform 10 be capable of plastic deformation under compressive
pressure at obtainable economical temperatures (titanium d the afore-
mentioned alloy meet this qualification). The initial thickness of
preform 10 is determined by the dimensions of the part to be formed.
; 15 Preform 10 is supported on shaping member 20. Shaping member 20
defines a chamber 22 d female die surface 24. Die surface 24 has a
projecting portion 25 thereon. A hold-dcwn ring 30 acts as a clamping
means for the preform 10. A single continuous edge of preform 10 is
effectively constrained between hold-down ring 30 and shaping member 20.
A punch or shaping member 40 has a male die surface 42 which preferably
is in mating relationship with die surface 24.
The dimensions of shaping members 20 d 40 are such that they
are complementary to the shape desired to be formed, i.e., the unconstrained
; portion of preform 10 would conform to die surface 24 on surface 14 d
` 25 to die surface 42 of punch 40 on surface 12. A primary consideration in
selection of a suitable shaping member alloy is reactivity with the metal
to be formed at forming temperatures. When the metal to be formed is
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titanium or an alloy thereof, iron base alloys with low nickel
content and modest ~arb~n con~ent~-~(as 0.2 - 0.5% carbon) have
been successful. Since forming loads are relatively low, creep
strength and mechanical properties are fairly unimportant.
Figure lB illustrates the superplastic forming of
preform 10. While in this embodiment superplastic forming occurs
before forging, the sequencing i5 not critical. Either operation
could be conducted initially followed by the other, or in some
cases both operations could be conducted concurrently. When the
steps of superplastic forming and forging are conducted con-
currently, it is to different portions of the preform.
For superplastic forming, preform 10 must be brought to
within a temperature range at which it exhibits superplastic
characteristics, if it is not already in that range. Various
heating methods can be used for heating preform 10 to the desired
temperature range twhere the metal would be in a plastic state
having a suitable strain rate sensitivity). Thus, the forming
apparatus can be placed between heating platens (not shown) such
as disclosed in U.S. Patent No. 3,934,441 to Hamilton, et al.
This method is advantageous as it also heats shaping members 20
and 40 so that the areas of preform 10 contacted by shaping
members 20 and 40 during forming (and forging) do not have their
temperatures substantially affected.
Forming of preform 10 into the basic configuration can
be accomplished by pressure from punch 40 or by a pressure
differential around preform 10. Such a pressure differential
method is disclosed in U.S. Patent No. 3,934,441 to Hamilton, et
al. It has been found that differential pressures that can be
used for superplastic forming normally vary from 15 psi to 300 psi.
When a differential pressure is used, the preform acts as a
diaphragm. As shown in Figure lB, this embodiment uses male die
member 40 which is forced against preform 10 at a rate such as
to cause superplastic forming. This rate should be such that the
superplastic ..........................
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strain rate is not exceeded. Forming times depend upon diaphragm thickness,
material superplastic properties, and the pressure ~or rate of die 40
movement) used and may vary from 10 minutes to 16 hours. As can be seen
in Figure lB, the unconstrained portion of preform 10 is superplastically
formed against die surface 42 and preferably in sufficient amount to also
deform against die surface 24. The superplastically formed preform 10
has a uniform thickness. However, a part of preform 10 does not contact
the lower essentially recessed portion 27 of die surface 24 due to the
uniform deformation of superplastic forming, i.e. the remaining portion
of die surface 24 is in contact with preform 10 so that punch 40 cannot
be moved further downward without a substantial increase of pressure
which would exceed the strain rate necessary for superplastic forming.
The completion of the process is shown in Figure lC. The pressure
applied by punch 40 (a differential pressure could also be used to forge
preform 10 but would not be a desired approach because of the extremely
large gas pressures required with consequent sealing problems and the
fact that gas pressure would be uniform over the surface of preform 10)
is increased and sustained allowing creep to occur as in conventional "hot
die" or isothermal forging such that preform 10 is forged between shaping
members 20 and 40 from the configuration of Pigure IB to that of Figure lC
(this forces flow of preform 10 against recessed portion 27). This forging
is similar to that disclosed in U.S. Patent No. 3,519,523 to Moore, et al
where the preform is in a condition of low strength and high ductility
when forged. The forging is in hot dies at a forging tem~erature within
about 350F of but not exceeding on a sustained basis the normal recrystalliza-
tion temperature of the alloy, while inhibiting substantial grain growth.
Optimally, the temperature range used for superplastic forming of preforn
10 would also be suitable for forging of the preform 10. Typically, with
Ti-6Al-4V, a temperature of about 1700F can be used for both the forging
and superplastic forming steps. The forging pressure that can be used can
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vaTy and it depends upon many parameters such as the paTticular metal or
alloy used for preform 10, how formable it is at the forming temperature,
thickness of preforn 10, ~mount of defoTmation required for prefoTm 10,
and desired time of processing, etc. Applicants have found that for
titanium and its alloys, and particularly the Ti-6Al-4V alloy, the range
of pressure than can be used is 1500-10,000 psi, with the prefeTTed range
being about 2000-6000 psi, with the lower end of the preferred range
producing better results. Depending upon the configuratian, this pressure
is noTmally applied for 4-5 hours, but could be as low as one-half hour
when simple shapes are to be fabricated.
While the part to be foTmed as shown in Figure lC could not be
accomplishèd by orging alone due to the large stretching required (see
Figure lB), a high degree of forging is possible. This is due to the
typically low flow stresses of a superplastic material. Thus the forging
loads can be sustained for a prolonged time period to capitalize on the
available low flow stresses of the superplastic preform. The heated dies
prevent undesirable cooling of the part to be forged. It should be noted
that the flow stresses are lower at lower strain rates. This permits
reduced pressures to cause the forging (albeit at lower strain rates) and
the forging of relatively thin members.
As can be seen in Figure lC, the part formed has a variable
thickness, having its greatest thickness along the continuous edge
constrained between ring 30 and shaping member 20 (where such portion
is not to be trimmed frcm the completed part), its thinnest section where
it overlles protruding portion 25 of die surface 24, and a portion having
an inte~mediate thickness which overlies the remaining portion of die
surface 24 of shaping mcnber 20.
When the preform 10 is a reactive metal such as titanium and its
alloys, whose surface would be contaminated at the elevated temperatures
required for superplastic forming, the present method would be
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accomplished in an inert atmosphere. A contamination prevention system
which could be used to provide such an inert atmosphere is disclosed in
U. S. Patent No. 3,934,441 to Hamilton, et al.
After the forming operation, the part 10 is remo~ed, trimmed,
cleaned, and further processed as required for its intended application.
Tooling can be heated and cooled for each part produced or it can be
maintained at the processing temperature range and each part produced,
ejected,and removed and a subsequent sheet inserted and formed immediately
thereafter.
Additional embodiments of the present invention are illustrated
;~ in Figures 2, 3, and 4. The previous discussion of the requirements for
superplastic forming and forging such as elevated temperatures, suitable
preform material, and necessary pressure are also as should be understood
applicable to these embo~iments.
A second embodiment of the present invention is shown in
Pigure 2. In this embodiment, the workpiece 10 is not clamped at its
periphery such as by hold-down ring 30 in Figure 1, but allowed to pull
i into the die cavity during the forming operation. Figure 2A illustrates
the initial position of preform 10 relative to shaping members 20 and 50.
Figure lB illustrates the preform 10 after its superplastic forming is
completed by male die member 50. The completely formed part 10 is shcwn
in Figure 2C where the forging has been accomplished by increased pressure
applied by shaping member 50 for the necessary time duration.
Pigure 3 illustrates another embodiment of the present invention.
In this embodiment, the initial position of preform 10 is shcwn in
Figure 3A. Preforn 10 has a single continuous edge thereof constrained
between shaping members 60 and a ring-like hold-down m~mber 62. Gas
lines 64 and 66 are provided in member 60. These can form part of the
contamination prevention system as previously discussed. A piston-like
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punch 70 is provided above preform 10 in the annular area defined by the
ring-like member 62. A cavity 72 is defined by shaping member 60. Punch 70
has a groove 74 on its contact surface 76.
Figure 3B illustrates the superplastic forming of preform 10
from its initial position to an intermediate position shown by the broken
lines of Figure 3B and to the final position shown by the unbroken lines.
Such superplastic forming is accomplished by gas pressure through lines 64
and 66 which are connected to a source (not shown) of inert gas. Such gas
pressure would also preferably be in the range of about 15-3Q0 psi. As
preform 10 deforms, inert gas is ve~ted from chamber 72 through vent
lines 76 and 78 in shaping member 60.
The preform 10 is formed to its final shape by a forging step
illustrated in Figure 3C. As shown, punch 70 moves downw~rd and applies
a forging pressure along its contact surface 7~ to preform 10. Such
forging pressure acts to compress the contacted portions of preform 10
forcing material flow up into groove 74. The portion of preform 10
which flows into groove 74 is shaped to co~form to groove 74 by virtue
of the plastic state of preform 10 due to the elevated temperature. As
the remaining portion of preform 10 which contacts surface 76 and does
not now into groove 74 is compressed, its thickness is less than the
portion of preform 10 which contacts the side walls 73 of chamber 72.
The portion o preform 10 which protrudes into groove 74 is of an
increased thickness which can vary depending upon the shape of groove 74.
Figure 4 illustrates another embodiment of the present invention.
As shown in Figure 4A, preform 10 in its initial position is constrained
between a lower shaping member 80 and an upper ring-like retaining member
82. Gas lines 84 and 86 are provided in retaining member 82 to provide
an inert atmosphere over preform 10. Shaping member 80 has a cavity 90
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defined therein. Cavity 90 has an upper tapered portion 92 and a lower
portion 94 of uniform width. Fluid lines 96 and 98 are provided at the
bottom of portion 94 of cavity 90. These lines are connected to a source
of vacuum (not shown). A piston-like punch or shaping member 100 having
s a contact surface made up of a tapered portion 102 which mates with
tapered portion 92 of cavity 90 and a level portion 104 is located in
the annular area 106 defined by retaining member 82.
As shown in Figure 4B, punch 100 is moved downward and applies
a compressive forging pressure to preform 10 where it contacts the walls
of portion 92 of cavity 90. The remaining portion 110 of prefoTm 10
extends into portion 94 of cavity 90.
Portion 110 of preform 10 is then superplastically formed as shown in
Figure 4C by application of vacuum ~positive pressure could also be applied above
portion 110 by application of gas through lines, not shown, which would
run through punch 100) through lines 96 and 98 and deforms to conform
to portion 94 of cavity 90. The portion of preform 10 which is contacted
by the tapered sides 102 of punch 100 is retained by pressure fram punch
100 and consequently does not have its thickness varied by superplastic
forming, i.e. portion 110 has its thickness reduced by its expansion to
conform to portion 94 of cavity 90.
Thus, it is apparent that there has been provided in accordance
with the invention, a method of making metallic structures which combines
superplastic forming and forging that fully satisfies the objectives,
aims, and advantages set forth above. While the invention has been
described in conjunc~cion with specific embodiments thereof, it is
evident that many alternatives, modifications, and variations will be
apparent to those skilled in the art in light of the foregoing description.
Accordingly, it is intended to embrace all such alternatives, modifications,
and variations that fall within the spirit and scope of the appended claims.
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