Note: Descriptions are shown in the official language in which they were submitted.
lOS713Z :.
BACKGROUND OF THE-I~VEN~IO~
The forming of titanium alloys into cclplex configurations by
present day processes, for forming parts requiring large tensile elon-
gations, is extremely difficult and in some cases cannot be achievéd.
Limited tensile e ongations, high yield, and moderate modulus of elas-
ticity impose practical limits for ambient temperature forming, and ex-
cessive spring-back frequently requires elevated temperature creep
sizing. In some parts, forming is done in a 1200 - 1400F temperature
range to increase the allowable defo~mation and to mim mize spring-back
and sizing problems. However, even with the use of these moderately
high temperatures, an extremely expensive integrally heated double
action fonning tool is required. Even with these àdvanced techniques,
forming of titanium alloys is still severly limited and c~npromises in
the design of structural hardware are often necessary with attendant
:15 decrease in efficiency and increase in weight.
For several years it has been known that titani~n and many of
its alloys exhibit superplasticity. Superpla~cticity is the capability,
of a material to develop unusually high tensile elongations with reduced
tendency toward necking, a capability exhibited by only a few metals and
~0 alloys and within a limited temperature and strain rate range. Titanium
and titanium alloys have been observed to exhibit superplastic charac-
teristics equal to or greater than those of any other metals. With suit-
able titanium alloys, overail increase in surface area of up to 300 per-
cent are possible.
The advantages of superplastic foTming are numerous: Very
canplex shapes and deep drawn parts can be readily fonned; low defonnation
stresses are required to form the metal at the superplastic temperature
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range, thereby permitting forming of parts under low pressures
(as 15 psi) which minimize tool deformation and wear, allows the
use of inexpensive tooling materials, and eliminates creep in the
tool; single male or female tools can be used; no spring-back
occurs; no Bauschinger effect develops; multiple parts of different
geometry can be made during a single operation; very small radii
can be formed; and no problems with compression buckles or galling
are encountered. However, prior to applicants' invention super-
plastic forming of titanium and similar reactive metals was
impractical because of the high forming temperatures required and
the relatively long time in forming. Titanium at the superplastic
forming temperature has a strong affinity for most elements. The
heating and forming atmosphere is critical to titanium cleanliness
which is particularly sensitive to oxygen, nitrogen, and water
vapor content. Unless the titanium is protected, it becomes
embrittled and its integrity destroyed. Coating materials cannot
be used for protection at the superplastic forming temperatures
as the coating and associated binders react with and contaminate
the titanium alloy in any type of environment.
The present invention relates generally to a method and
apparatus for superplastic forming of metals in a controlled
environment. More specifically, the present invention relates to
superplastic forming of metal blank into a desired shape by heating
the metal blank in a controlled environment and applying a fluid
pressure loading to the metal blank causing it to form against a
shaping member.
A method for superplastic forming of metals has been disclosed
in U.S. patent No.3,340,101 to Fields, Jr. et al. This patent dis-
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closes heating or otherwise conditioning a metal to exhibit its effec-
tive strain rate sensitivity and then placing the metal in an apparatus
for forming. Forming is usually acca~plished by a vacuum exerting ten-
sile stress on the metal. ~Yever, a male die member can be utilized
S to initially defo~m the metal befoTe application of the vacuum, or the
male die member can be used in combinatio~l with the application of pos-
itive pressure. However, this method would not be suitable for super-
plastic forming of titanium because of the contamination that would
result to the surface integrity of the metal due to the heating and
forming without a controlled environment. In fact, the patent does not
list titanium as one of the metals having superplastic properties and
discusses fo~ming temperatures in the rangé of 600 F. as opposed to
the approximately 1450 ~ - 1850 F. required by titanium and its al-
loys. No mention is made in the patent as to protection from contam-
ination. Additionally, fo~ming time, especially with thicker metal
sheet is quite lengthy as the amount of differential pressure is lim-
ited.
U. S. patent No. 3,605,477 to Carlson discloses apparatus for
hot forming titanium alloy blanks where the blanks are coated with a
high temperature lubricant, preheated to a forming temperature of about
1,000 to 1,500 F., removed and placed in forming equipment in contact
only with mated heated fo~ming tools. The forming equipment is main-
tained at the foTmong t~mperature during forming. It is disclosed to
use an argon atmosphere in the heater when preheating the titanium
bIanks to prevent contamination. However the blank is removed from the
heater to separate forming equipment wheTe it is formed into the desired
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shape ~ithout the benefit of the controiled environment. For protec-
tic~, a high te~perature lubricant is formed on the titanium sheet.
This method while suitable for hot forming of titanium, would be im-
practical and unsuccessful for superplastic forming. In the super-
~lastic foxming temperature range of approximately 1450 F to 1850 F,
the high temperature foTming lubricant itself contaminates the titanium
~heet regardless of the environment. In any case, the heater is sepa-
rate from the forming apparatus and once the titanium sheet is removed
fram the heater it would be contaminated.
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SUM~RY OF THE INVENTION
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It is, therefore, an o~ject of the present invention to suc-
~essfully deform metal blank against and into inti~ate contact with a die
~ving a surface area extraordinarily greater than the original surface
S area of the sheet without contaminating the meta] surfaces.
It is another object of the present invention to heat and form
the metal in the same apparatus.
It is yet another object of the present invention to Teduce
the forming time in superplastic forming.
It is still another object of the present invention to effi-
ciently seal the forming apparatus.
Briefly, in accordance with the invention, there is provided
foIming apparatus where a sheet metal diaphram is formed about a shaping
D~mber. The diaphram is located in an enclosure and is formed under
1~ tensile stress by a fluid pressure loadIng. Heating means such as press
heating platens are provided to heat the metal diaphram to a suita~le
~orming temperature. Means is provided to contTol the fluid pressure in
~he enclosure. Heating s d forming of the diaphram takes place in a
controlled environment of inert gas or inert gas on one side of the dia-
~hram and vacuum on the other.
cn~er objects s d adv stages of the invention will become ap-
- y~Tent upon reading the follo~ng detailed description and upon refer-
ence to the drawings.
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BRIEF DESCRIPTION OF THE D ~ YINGS
Figure 1 is a perspective view of the basic forming apparatus
employed in superplastic forming of metals in a controlled environment
with portions broken away to show internal details;
Figure 2 is a cross-sectional elevational view of the appa-
ratus shown in Figure 1 mounted between heating platens of a press;
Figure 3 is a cross-sectional elevational view of a portîon
of the forming apparatus below the metal diaphram of a modified appa-
; ratus illustrating the original position of the metal to be formed, an
inteTmediate position, and the final position of the metal as formed;
Figure 4 is a cross-sectional elevational view of a modified
apparatus for forming the diaphram into a complex shape;
Figure 5 is a detail view of a sealing method for the form-
ing apparatus;
Figure 6 is a detail view of an alterna~0 seal arrangement
for the forming apparatus.
While the invention will be described in connection with the
preferred embodiment~ it will be understood that it is not intended to
limit the invention to those embodiments. On the contrary, it is in-
~0 tended to cover all alternatives, modifications, and equi~alents that- may be included within the spirit and scope of the invention as defined
by the appended claims.
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DETAILED DESCRIPTION OF THE INVENTION
:
In order for superplastic forming to be successful, it is necessary
to use a material that is suitable. The extent to which any material selectPd
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 in wall thickness. Strain rate sensitivity can be
defined as m where
d ln 6-
m d ln ~
and ~ is stress in pounds per square inches 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 "Detenmination
of Strain -- Hardening Characteristics by Torsion Testing," by D. S. Fields,
Jr., and W. A. Backofen, published in the proceedings of the AS~M, i957,
Yol. 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
rste sensitivity in metals is seen to occur, if at all, as metals are deformed
near ~he phase transformation temperature. Accordingly, the temperature
i ediately belbw the phase transformation temperature can be expected to
produce the greatest strain rate sensitivity. For titanium and its alloys
the temperature range which superplasticity can be observed is about 1450F.
to about 1850F. depending on the specific alloy used.
Other variables have been found to affect strain rate sensitivity
and therefore should be considered in selecting a suitable metal material.
Decreasing grain siæe results in correspondingly higher values for strain
rate sensitivity. Additionally, strain rate and material texture affect
the strain rate sensitivity. It has been found that the m-value reaches a
peak at an intermediate value of strain rate (approximately 10 in./in.Isec.).
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For maximum stable deformation superplastic forming should be done at this
ætrain rate. Too great a variance from the optimum strain rate may result
in a loss of superplastic properties. The present invention is directed to
metals whose surfaces would be contaminated at the elevated temperatures
required for superplastic forming. Titanium and its alloys are examples
of such metals.
Turning first to Figures l and 2, there is shown an example of
the forming apparatus generally indicated at 10 for carrying out t'ne invention.On a base plate 12 is suitably mounted, as by welding, support tooling
frame 14. Tooling frame 14 could also be integral with base plate 12. Tooling
frame 14 is in the form of a ring which can be any desired shape, and with
base plate 12 defines an inner chamber 18 and a female die surface or shap-
ing member 20. The dimensions of tooling frame 14 and base plate 12 are
such that the shaping member 20 is complementary to the shape desired to be
formed. One or more male die members 22 can be provided in chamber 18 to
vary the shape of the part to be formed. A primary consideration in selection
of a suitable shaping member alloy is reactivity with the metal to be formed
at forming temperature. When the metal to be formed i9 titBnium or an alloy
thereof, iron base alloys with low nickel content and modest carbon content
(as 0.2-0.5% carbon) have been successful. Since forming loads are very low,
creep strength and mechanical propertles are relatively unimportant.
Metal blank 24, preferably in the form of a sheet having upper and
lower opposed surfaces 26 and 28 respectively, is supported on tooling frame 14
and covers chamber 18. Any metal blank that exhibits suitable superplastic
properties can be used, but the present invention is particularly concerned
with such metals that are subject to contamination at forming temperature,
such as titanium or an alloy thereof as Ti-6Al-4V. The initial thickness of
diaphram 24 is determined by the dimensions of the part to be formed.
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Upper support tooling frame 30 is mounted over the metal blank 24. Prefer-
ably upper framR 30 is dimensionally the same as the lower frame 14 and is
mounted in alignment therewith. Tooling frame 30 and diaphram 24 define
a chamber 32. Chamber 32 is covered by upper plate 34 which is mounted
on upper support tooling frame 30.
The weight of upper plate 34 and support tooling 30 acts as a
clamping means for the metal diaphram 24. A single continuous edge of the
diaphram 24 is effectively constrained between upper support tooling frame
30 and lower support tooling frame 14. This insures that the final part
will be stretched rather than drawn. Should it be desired, additional
tightening means such as bolts (not shown) can be employed to more effect-
ively constrain the di~phram 24. As shown in Figure 2, an additional
tightening means employed is a hydraulic press (not shown) having platens 40.
The superplastic forming apparatus lO is placed between platens 40 and
compressed thereby assuring that the diaphram 24 is effectively constrained
and the chambers sealed from the air. This arrangement is particularly
sdvantageous as the platens can be made of ceramic material and resistance
heated wires 42 can be provided in the platens 40 for heating the metal
blank.24 to the forming temperature. Heat from the resistance wires 42 is
transmitted through plates 12 and 34 to the metal diaphram 24. Other heating
methods c~uld be used with the forming apparatus 10 ordinarily ~urrounded by
a heating means if the heating platens are not used.
For contamination prevention of the metal diaphram 24 while heating
and forming, an environmental control system is provided. The purpose of the
system is to expose the metal diaphram 24 only to inert gas or a vacuum while
heating and forming. The metal diaphram 24 will not react with the inert
gas due to the nature of the inert gas, even at elevated forming temperatures.
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In a high vacuum, there are substantially no elements for the
diaphram 24 to react with. Thus, in this environment, contamin-
ation of the metal diaphram 24 will be prevented. Line 50 is
connected to a source of pressurized inert gas at one end (not
shown) and into a T-junction member 51 at the other end. The
inert gas used is preferably argon in liquid form. Member 51
is connected to two parallel lines 52 and 54 by elbow joints 53
and 55. Line 52 is connected through an orifice 60 in upper
tooling frame 30 to chamber 32. For governing the flow of
inert gas through line 52 into chamber 32 a valve 56 is mounted
in line 52. A pressure gage 62 is also provided in line 52 to
indicate up-stream pressure. Line 54 is connected to chamber 18
through an orifice 64 in lower support tooling frame 14. A
valve 58 is connected in the line 54 for regulating flow of
inert gas into chamber 18. Line 70, which is connected to the
opposite side of upper tooling frame 30, through orifice 72
functions as an outlet for inert gas from chamber 32. A valve
74 is provided in the line 70 to govern flow of inert gas through
the outlet. A pressure gauge 76 is also connected in line 70
to provide an indication of pressure downstream. Line 80
functions as either an inert gas vent or a vacuum inlet. Line
80 is shown mounted to lower tooling frame 14 through orifice 82.
However, it could just as easily be mounted to base plate 12.
If line 80 functions as a vacuum inlet, a suction pump (not
shown) would be employed in line 80 for creating the vacuum
in chamber 18.
Forming of the diaphram 24 is produced by the pressure
differential between chambers 18 and 32. This pressure loading
can be accomplished in a variety of ways. For example, a
constant positive pressure can be maintained in chamber 32
while a vacuum is applied to chamber 18, or positive pressure
in chamber 32 can be increased to greater than the positive
pressure in chamber 18, or positive pressure in chamber 32
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could be increased at the same time a vacuum is applied to
chamber 18. By using the metal blank 24 as a diaphram which
divides two pressure chambers, forming time can be reduced
because a vacuum can be applied to one chamber and positive
pressure to the other. This allows increase of the pressure
differential which increases the strain rate. This is very
significant with a thick diaphram. However, the usable strain
rate should not be exceeded. Differential pressures used
normally vary from 15 psi to 150 psi. Forming times, depending
on diaphram thickness and differential pressure, may vary from
10 minutes to 16 hours.
Figure 3 illustrates the forming of the metal diaphram 24.
The original position of the diaphram is shown at a, intermediate
positions at b and c, and the final position of the metal
diaphram as formed at d. During forming, the pressure above the
diaphram 24 in chamber 32 is greater than that below the
diaphram 24 in chamber 18. Inert gas in chamber 18 is forced
out through vents 90 as the metal diaphram 24 deforms due to the
pressure differential.
Figure 4 illustrates a modification of the present invention.
The forming apparatus here employed is used to form a beaded or
ridged shaped of form from the diaphram. The base plate tool
100 is a preferably unitary structure that replaces the base
plate 12 and lower support tooling frame 14 of the Figure 1
embodiment. Base plate tool 100 has a plurality of cavities
102 equal to the number of ridges desired to be formed in
diaphram 24. Cavities 102 replace the chamber 18 of the
embodiment in Figure 1. Inert gas is transmitted from line
54 to cavities 102 by a conduit 104 formed in base plate tool
30 100. Conduit 104 has individual openings 106 for each cavity
102. Conduit 110 is a vacuum inlet to the cavities 106 and is
connected to a suction pump (not shown). Separate channels
112 are provided in conduit 110 for drawing out the inert gas
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in cavities 106 and for application of vacuum to cavities 106.
The diameters of openings 106 and channels 112 are less than
the thickness of the diaphram 24 as formed to minimize material
flow therein.
Referring now to Figures 1, 5, and 6, there are shown
three sealing methods for sealing chambers 18 and 32. These
seal methods are optional in that the forming apparatus 10 is
sealed by compression from the press platens 40. However,
especially when a vacuum is used, it is desirable to have very
effective sealing to prevent entrance into chambers 18 and 32
of any contaminating air. Such contamination, if minimal,
results in extra labor in cleaning the surface of the formed
part, and if more than minimal, may result in the formed part
being unsatisfactory for use. The technique illustrated in
Figure 1 uses a pure titanium O-ring 120 which can be combined
with an elevated temperature glass base type sealant 122, both
of which overlie the periphery of the upper side of the lower
tooling support frame 14. The elevated temperature sealant can
also be placed on the bottom and top sides around the periphery
20 of the upper tooling frame 30 as shown at 124 and 125 respectively
in contact with the diaphram 24. The titanium O-ring is
extremely soft at the forming temperatures and therefore effects
a very good seal. Another technique shown in Figure 5 uses
sharp hard projections 130 that run continuously around the
perimeter of the tooling 14 and 30 that penetrate into the
softer diaphram 24 at the elevated forming temperatures thereby
effecting a seal. In E`igure 6 is shown another method where
only the lower tooling 14 is provided with an additional sealing
feature. Tooling 14 is provided at its upper side with two
30 sharp projections 140 and 142 which run continuously around
the perimeter of the tooling 14 and which penetrate into the
metal diaphram 24 at the forming temperature. These pro-
jections 140 and 142 contain inert gas in a cavity, 144. Inert
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gas is transmitted to cavity 144 via internal conduit 146
which is connected to line 148 leading to a source of pressurized
inert gas. Various combinations of the illustrated sealing
techniques are also within the scope of this invention.
OPERATION
Referring to Figures 1 and 2, base plate 12 and lower
tooling 14 and associated gas lines 52 and 54 are assembled.
Sealing means such as sealer 122 and O-ring 120 are applied to
lower frame 14 if desired. Shaping member 22 is positioned
inside frame 14 and on base plate 12. A suitable metal blank
24 is placed over the frame 14 enclosing chamber 18. Optionally,
sealant can be placed on the lower and upper sides of upper
frame 30. Upper frame 30 with the connected gas lines 70 and
80 is placed over the metal blank 24. Upper plate 34 is placed
over upper frame 30 enclosing chamber 32. The entire forming
apparatus 10 is placed inside a press with heated top and bottom
ceramic platens 40. Pressure is applied by the press on the
forming apparatus 10 for an effective seal. Inert gas is
applied to both upper and lower chambers, 32 and 18 respectively,
to protect the metal blank 24 from contamination during heating
and forming. The temperature of the metal blank 2~ is raised
by the heating apparatus 42 in platens 40 to a suitable forming
temperature. A pressu~e differential across the principal
surfaces of the metal blank 24 causes the metal blank 24 to form
against the shaping member 22, and the uncovered portions of
lower frame 14 and base plate 12 which may also be shaping
members. The pressure differential can be generated by a
vacuum in lower chamber 18, increased inert gas pressure in
upper chamber 32, or both. The temperature in heating platens
40 is reduced and the metal blank 24 is cooled with the inert
gas atmosphere (or vacuum) retained though reduced. The press
is raised, forming apparatus 10 disassembled, and the part
removed and trimmed to size.
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Thus, it is apparent that there has been provided, in
accordance with the invention, a method and apparatus for
controlled environment superplastic forming of metals that fully
satisfies the objectives, aims, and advantages set forth above.
While the invention has been described in con~unction
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 as fall within the
spirit and scope of the appended claims.
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