Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
:~OS~25~.~
BACKGROUND OF THE INVENTION
For many years it has been known that certain metals, such
as titanium and many of its alloys, exhibit superplasticity. Super-
plasticity is the capability of a material to develop unusually high
tensile elongations with reduced tendency toward necking. This cap-
ability is exhibited by only a few metals and alloys and within a
limited temperature and strain rate range. Titanium and titanium -
alloys have been observed to exhibit superplastic characteristics
equal to or greater than those of any other metals. With suitable
titanium alloys, overall increase in surface areas up to 300% are
possible.
The advantages of superplastic forming are numerous: very
complex shapes and deep drawn parts can be readily formed; low de-
formation stresses are required to form the metal at the superplastic
temperature range, thereby permitting forming of parts under low
pressures which minimize tool deformation and wear, allows the use
of inexpensive tooling materials, and eliminateQ 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 problem with compre~sion buckles or galling are encountered.
However, when superplastic forming of titanium and similar reactive
metals, it is necessary to heat and form in a controlled environment
to ensure cleanliness of the titanium which is particularly sensitive
to oxygen, nitrogen, and water vapor content in the air at elevated
temperatures. unless the titanium is protected, it becomes
embrittled and its integrity destroyed.
Diffusion bonding refers to the metallurgical joining of sur-
faces of similar or dissimilar metals by applying heat and pressure
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for a time duration so as to cause co-mingling of atoms at the joint
interface. Diffusion bonding is accomplished entirely in the solid-
state at or above one-half the base metal melting point (absolute).
Actual times, temperatures, and pressures will vary from metal to
metal. The joining surface must be brought within atomic distances
by application of pressure. Adequate pressure must also be provided
to cause some plastic flow to fill normal void areas. If pressures
are too low, small voids will remain at the joint interface and the
joint strength will be less than the maximum obtainable. The applica-
tion of pressure also breaks up the surface oxides and surfaceasperities so as to present clean surfaces for bonding. The elevated
temperatures used ~or diffusion bonding serve the functions of
accelerating diffusion of atoms at the joint interfaces and providing
a metal softening which aids in surface deformation so as to allow
more intimate contact for atom bonding and movement across the joint
interface. The elevated temperature and application of pressure also
resultg in diffusion of the surface contaminants into the base metal
during bonding which allows metal atom-to-atom bonding and thereby
strengthens the bond. Sufficient time must be allowed to ensure the
strengthening of the bond by diffusion of atoms across the joint
interface. A protective atmosphere for bonding i8 required when
titanium and other similar reactive metals are to be bonded.
The process of superplastic forming of metals and diffusion
bonding having individually been dis~losed in the prior art. U.S.
; patent No. 3,340,101 to Fields, Jr., et al discloses a method of
superplastic forming where a metal is conditioned to exhibit its
effective strain rate sensitivity and then formed by application of
vacuum solely or in combi~ation with a metal dye. Patents relating
to qolid-state or diffusion bonding include U.S. patent ~o. 3,145,466;
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~s~s~
3,180,02~; 3,044,160; 2,850,798; and 3,170,234. However, the prior
art does not disclose combining these two processes.
In my co-pending Canadian application No. 233,354 there is
disclosed a method for superplastic forming of metals with concurrent
diffusion bonding. The crux of that invention is the superplastic
forming of a metal blank against a shaping member and another metal
workpiece so that the m~stal blank is formed and diffusion bonded to
the other metal workpiece in one operation. Though a hollow metal
structure bonded at peripheral edges can be formed as disclosed in
10 that application, the process is not designed for the forming of
sandwich structures. The forming of ~andwich structures according
to the present invention requires separate superplastic forming and
joining stages and a technique for joining only ~elective areas of
the metal blanks employed.
The present invention obviates the prior art problems of
forming sandwich structures. A sandwich structure normally comprises
a core between face sheets. Previous to applicant's invention,
fabrication of sandwich structures typically took the approach of
first rolling metal foil or ribbon, forming and joining the foil
20 sheet into the desired cellular core, and then attaching the core to
face sheet~ by bracing or spot welding. Problems with the prior art
method include the cost of the core due to excess material usage and
the great difficulty, excess time consumption, and cost of fabrica-
tion of the sandwich shape. Additionally, a separate operation is
xequired to join a close out or attachment to the sandwich structure.
Fabrication of an unusual shape for the sandwich structure such as a
taper, is nearly impossible.
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~so~
SUMMARY OF THE I~VENTIO~
It is, therefore, an objec~ of the present invention to com-
bine the processe~ of metal joining and superplastic forming to form
metal sandwich structures.
It i9 another object of the present invention to form metal
sandwich structure~ in one operation, thereby significantly lowering
the cost, difficulty, and time involved.
It i~ yet another object of the pre~ent invention to form a
sandwich structure and join an attachment or close-out in the same
operation.
It is still another objsct of the present invention to heat,
~uperplastically form, and join the sandwich in the same apparatus,
thereby s2ving fabrication time and equipment costs.
Briefly, in accordance with the invention, there is provided
a method for making a metallic sandwich structure from a plurality
of metal blank workpieces initially positioned in a 4tacked position.
~he blanks are joined at selected area~. At least one of the blanks
i~ superplastically formed against a shaping member to form the
desired ~andwich structure. The core configuration is determined by
the location, size, and shape of the joined areas.
Other objectq and advantages of the invention will become
apparent upon reading the following detailed description -and upon
reference to the drawings.
B~I~F D~SCRIPTION OF THE DRAWINGS
Figure 1 i9 an exploded view of a three-piece metal sheet
a~sembly treated for 3elective diffusion bonding prior to insertion
in ~he forming apparatus:
Figure 2 is a cros~-sectional elevational view of a preferred
embodiment of a forming apparatus used for fabrication of metal
~ oso~s~
sandwich structures with the three piece metal sheet assembly of
Figure 1 inserted therein;
Figure 3 illustrates the fully expanded three-piece metal
sheet assembly within the forming apparatus of Figure 2 with broken
lines added to delineate the final position of the component metal
sheets of the expanded joined assembly:
Figure 4 is a detail view of an inflation tube connection to
the three-sheet assembly:
Figures 5 and 6 are cross-sectional elevational views of a
modified forming apparatus with a three-piece metal sheet assembly
inserted therein in its initial position in Figure 5 and in its final
expanded po~ition in Figure 6
Figures 7 and 8 are cross-sectional elevational views of a
modified forming apparatus with a two-piece metal sheet assembly
inserted therein in its initial position in Figure 7 and in its final
expanded position in Figure 8;
Figure 9 is a cross-sectional elevational view of a modified
forming apparatus illustrating a tapered three-piece metal sheet
assembly in final formed position with an attachment joined to the
sandwich structure at the broken lines;
Figure 10 i8 a perspective view with the ends cut off of the
formed sandwich structuxe of Figure 9:
Figure 11 is a fragmentary cro~s-sectional elevational view
of a modified forming apparatus illustrating the positioning of an
inflation t~be relative to a four-piece metal sheet assembly:
Fîgure 12 is a cross-sectional elevational view of a sandwich
structure formed from a four-piece metal sheet assembly: and
Figure 13 is a fragmentary perspective view of the sandwich
structure shown in Figure 12.
~o~o
While the invention will be described in connection with the
preferred procedures, it will be understood that it is not intended
to limit the invention to those procedures. On the contrary, it is
intended to cover all alternatives, modifications, and equivalents
that may be included within the ~pirit and scope of the invention as
defined by the appended claim~
D13TAILED Dl~SCRIPTION OF THE INV~TION
In order for superplastic forming to be successful, it is
necessary to u~e a material that is suitable. The extent to which
the material selected will exhibit superplastic properties is pre-
dictable in general terms from a determination of the strain rate
sensitivity and a design determination of the permissible variation
in wall thickness. Strain rate sensitivity can be defined as m where
m egual~ dl~ and ~ i~ stre~s in pounds per square inch and e is
strain rate in re~procal minutes. Strain rate sensitivity may be
determined by a ~imple and now well recognized torsion test described
in the article, "Determi.nation of Strain Hardening Characteristics
by Torsion Testing" by D.S. Fields, Jr., and W.A. Backofen, published
in the Proceedinga of the A.S.T.M., 1957, Volume 57, page~ 1259-1272.
A strain rate sensitivity of about 0.5 or greater can be expected to
produce satisfactory results. The larger the value (to a maximum of
one3 the greater the superplastic properties. Maximum ~train rate
~ensitivity in metals is seen to occur, if at all, as metals are de-
- formed near the phase transformation temperature. Accordingly, the
temperature immediately below the phase transformation temperature
can be expected to produce the greatest ~train rate sensitivi~y. For
titanium alloys, the temperature range in which superplasticity can
be observed is about 1450 F. to about 1850 F. depending on the
~pecific alloy u~ed.
1050~5~
Other variables have been found to effect strain rate sen-
sitivity and therefore should be considered in selecting a suitable
metal material. Decreasing grain size results in correspondingly
higher values for strain rate sensitivity. Additionally strain rate
and material texture effect the strain rate ~ensitivity. It has been
found that for titanium the m value reaches a peak at an intermediate
value of strain rate (approximately 10 in./in./sec.). For maximum
~table deformation, superplastic forming should be done at or near
this ~train rate. Too great a variance from the optimum strain rate
may re~ult in loss of superpla-~tic properties.
Diffu~ion bonding, where separate elements form a single uni-
; tary ma~s therefrom, may be practiced in a wide variety of metals and
alloy~. However, the quality of the bond and the parameter~ employed
will neces~arily vaxy for each particular choice of workpiece material~
Among the metals or alloys which may be joined by solid state dif- ~`
fus~on bonding are aluminium, stainles~ steel, titanium, nickel,
` tantalum, molybdenum, zirconium, columbium, and beryllium.
The present invention i~ particularly directed to the reactive
metals whose surfaces would be contaminated at the elevated tempera-
tures required for superplastic forming and diffu~ion bonding.
Titanium and its alloys are examples of ~uch metals which have also
been found to be particularly well suited for a proces~ of the present
invention in that the~alloys exhibit very high superplastic pro-
perties in a temperature range suitable for diffuqion bonding, i.e.
1450F. to about 1850P. depending on the ~pecific alloy used.
Turning first to Figure 1, there is shown an exploded view
of a three-piece metal sheet assembly to be formed into a ~andwich
structure according to the present invention. ~he assembly is made
up of metal blanks 10, 12 and 14, all preferably in the form of sheets
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having upper and lower opposed principal surfaces 15 and 16, 17 and
18, and 19 and 20, respectively (see also Figure 4). The number of
sheets used will vary depending on load condi~ions and design require-
ments However, a minimum of two sheets must be utilized and normally
no moxe than four sheets would be used. The metal sheets must have
the ability to be joined by such methods as bracing, welding, or
diffusion bonding. Depending on the number of sheets to be expanded,
at least one of the sheets muqt exhibit superplastic properties. Any
metal tha~ exhibits suitable superplastic properties within a work-
10 able temperature range can be used for such sheet, but the present --
invention is particularly concerned with metals that exhibit super-
plastic properties ~ithin the temperature range required for diffusion
bonding and that are subject to contamination at forming temperatures,
as titanium or an alloy thereo such as Ti-6Al-4V. When using Ti-
6Al-4V, the forming temperature is preferably approximately 1700F.
Initial thickness of metal blankq 10, 12 and 14 are determined by
the dimenqions of the part to be formed.
In order to join only selected areas of the metal qheets, a
- preferred step is to apply a suitable stop-off material to thoqe
areas within the stack where no attachment or joining between the
sheets is desired. Thus, areas 30, 32, and 34 are covered with a
stopooff material to pre~ent bonding in those areas. Other areas on
surfaces 15, 18 and 20 could also be so covered for prevention of
joining. Alternatively, the metal sheet structure could be spot
welded or brazed at those areas where joining i8 desired. Addition-
ally, as hereinafter explained, the metal sheet structure or stack
40 could be diffusion bonded at selected areas by selective applica-
tion of pressureO
Figure 2 illustrates a preferred forming apparatus generally
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indicated at 42 for carrying out the invention. Upper tooling frame
44 has preferably integral side walls 45 in the form of a ring which
can be of any desired shape. Lower tooling frame 46 which prefer-
ably has the same outer area dimensions as upper frame 42 can be flat
and act as a base as illustrated for supporting the stack of metal
blanks 40. For purposes of the claims, both upper tooling frame 44
and lower tooling frame 46 are to be considered shaping members as
both combine to form the desired shaped structure. The inner surface
of upper tooling frame 44 defines an inner chamber 48 and a female die
surface. One or more male die members (not shown) can be provided in
chamber 48 to vary the shape of the part to be formed. The stack of
metal blanks 40, which is supported on lower tooling frame 46, covers
chamber 48. The metal blanks of t~e stack must all be of a material
suitable ~or joining such aq by welding, brazing, or diffusion
bonding. At least one of the outer metal blanks, and most likely
the inner blanks, must have an effective strain rate sensitivity for
exhibiting superplastic properties at a desired forming temperature
and preferably within a temperature range required for diffusion
bonding of the stack. This is shown in Figure 3 by the expanded
stack 40 where both sheet~ 12 and 14 have been superplastically
expanded, while sheet 10 of the formed stack has been sub~tantially
unchanged. Initial thicknesses of the sheets of stack 40 are deter-
mined by the dimension~ of the parts to be formed. The joining
method to be used, namely welding, brazing, or diffusion bonding,
depend~ on the material selected for the metal blanks, the temperature
required for superplastic forming, and the desired strength. However,
especially for titanium, diffu~ion bonding is preferred as this
results in the strongest joining and the bonding temperature is
generally suitable for superplastic forming.
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~OS025i0
As previously mentioned, stop-off can be used at areas 30,
32 and 34 to prevent bonding at those areas. The particular stop- -
off selected must prevent bonding and be compatible with the métal
or metals of the stack (nonreactive with the stack metals and minimal
diffusion into the stack metals). For titanium metal blanks, suitable
stop-off materials are graphite, boron nitride, and yttria. Typically,
when using yttria stop-off, the stop-off pattern on the blanks is
sprayed with a solution of yttria and a binder therefor. The binder
holds the yttria in position during bonding and eventually vaporizes
10 below forming temperature. ~ -
Joining the stack 40 at selected locations, when by brazing
or spot welding i8 normally accomplished outside of the forming
apparatus 42 before the stack is placed therein. When diffusion
bonding, the unjoined stack 40 i~ preferably bonded after placement
in the forming apparatus 42, thereby saving fabrication time and
equipment costs (although the ~tack could be diffusion bonded as by
press bonding or roll bonding before placing it in the forming
apparatus 42). The weight of upper tooling frame 44 acts as a clam-
ping means for the stack 40. A single continuous edge of the stack
40 i~ effectively constrained between the upper tooling frame 44 and
the lower tooling frame 46. This insures that those portions of the
blanks of the stack to be formed, will be stretched rather than
; drawn. Should it be desired, additional tightening means such as
bolts (not shownl can be employed to more effectively constrain the
~tack 40. Another additional tightening means that could be employed
is a preqs (not shown), preferably hydraulic, having platens 50.
Forming apparatu~ 42 i~ positioned between platens 50 and compressed
thereby as~uring that stack 40 is effectively constrained and cham-
ber 48 qealed fr~m the air. This arrangement is particularly advan-
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tageous as the platens 50 can be made of ceramic material and
resistance heated wires 52 can be provided therein for heating stack
40 to the forming temperature. Other heating methods could be used
with lthe forming apparatus 42 ordinarily surrounded by a heating
means if the heating platens are not used.
For contamination prevention and diffusion bonding of the
~tack 40 when unjoined within the forming apparatus 42, an environ-
metal control ~ystem is provided. The purpose of the system is to
expoae the ~tack 40 only to inert gas or vacuum while heating, forming
and bonding, and optionally, to diffusion bond the stack by fluid
pressure. The metal blanks of stack 40 will not react with inert
gas due to the nature of inert gas, even at the elevated forming and
bonding temperatures. In a high vacuum, there are substantially no
elements for the stack 40 to react with. Thus, in this environment,
contamination of the stack 40 will be prevented.
Line 52 i8 connected to a source of pressurized inert gas at
one end (not shown) and to chamber 48 through orifice 54 in upper
tooling frame 44. A valve 56 for governing the flow of inert gas
through line 52 into chamber 48 and a pre~qure gauge 58 to indicate
pressure are provided. The inert gas used is preferably argon in
liquia form. Line 52 also functions as an outlet for inert gas in
chamber 48 and could also be connected to a source of vacuum, such
as a suction pump (not shown) for creating vacuum in chamber 48.
When line 52 ia uqed as an outlet, valve 56 governs flow of inert
gas from chamber 48. An additional line 60 is optimally provided on
; the opposite ~ide of tooling frame 44 and functions a~ an outlet for
inert gas in chamber 48. Line 60 is connected to chamber 48 through
orifice 62 in upper tooling frame 44. Line 60 has a valve 64 pro-
vided therein for regulation of inert gaq flow from chamber 48.
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Line 60 can simply function as a vent or be connected to a source of
vacuum, such as a suction pump (not shown).
AS previously stated, the contamination prevention system
can also function as a means for gas pressure diffusion bonding
stack 40. Thus, when stack 40 is placed in forming apparatus 42 as
illustrated in Figure 2, the stack can be heated in an inert gas
atmosphere to a suitable diffusion bonding temperature (approximately
1700F. when the metal blank3 of stack 40 are of Ti-6Al-4V) by heat
generated from heating platens 50, and then applying pressure to
stack 40 by increasing the pregsure in chamber 48 by adding additional
pressurized inert gas through line 52 while maintaining line 60 closed
by valve 64. In this manner, the untreated areas of stack 40 will
; be diffusion bonded by the application of such pres~ure, which i8
preferably approximately 500 p3i for Ti-6Al-4V, for a ~uitable forming
t~me, which depends on the thickness of stack 40 and may vary from
30 minutes to 12 hours. The edges of the blanks of stack 40 may also
be diffusion bonded if desired by virtue of sealing pressure thereon
! in the form of the weight of upper tooling 44 and optionally pressure
from a press and/or a clamping means. After diffusion bonding stack
. 20 40, excess inert gas would be removed from chamber 48 through lines
52 and 60 to allow for inflation of stack 40.
For expansion of stack 40 to the configuration shown in
Figure 3 expan~ion tubes 72 and 74 are provided, the details of which
are best illustrated in Figures 1 and 4. Expansion tube 72 (and
likewise expansion tube 74 located on the opposite side of stack 40)
is positioned between metal blanks 10 and 14 and protrudes into a
channel 75 defined by recesses 76 and 78 and that portion of ~urface
20 of metal blank 14 which overlies recess 78. Recesses 77 and 79
are provid0d on the opposite sides of blanks 10 and 12 respectively
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~ s~so
to provide a channel for inflation tube 74. The positioning of
expansion tube 72 in such a channel 75 prevents compression of tube
72 by the tooling frames 44 and 46. Also, by locating tube 72 so
that it protrude~ only partially into channel 75, inert gas will flow
fairly evenly between the metal blanks of stack 40, in this case on
either side of blank 12 as shown by arrows 80 and 82. As shown in :-
Figure 2 expansion tubes 72 and 74 have valves 84 and 86 therein
respectively for governing flow of inert gas therethrough, and pres-
sure gauges 88 and 90 for indicating pressure.
Expansion tubes 72 and 74 can also serve to draw off vaporized
binder. To this end, tube 72 could act as an inlet and tube 72 an
outlet with inert ga~ flow being transmitted through stack 40 prior
; to expansion of ~tack 40 to draw off the vaporized binder.
A pair of lateral grooves 71 (not shown) and 73 are preferably
provided on opposite sides of lower tooling frame 46 with groove 71
in alignment with recess 77 and groove 73 in alignment with recess
76. The grooves 71 and 73 are provided to insure that passage of
inert gas from inflation tubes 72 and 74 betwaen the metal blanks of
stack 40 is not prevented from reaching treated areas 30 and 34 by
pinching of the ~tack due to the pre~sure exerted by upper and lower
tooling frames 44 and 46. The width of grooves 71 and 73 are prefer-
ably the same as recesses 76 and 77, but grooves 71 and 73 terminate
further inward in stack 40 so that pinching does not occur before
the inert gas flow reaches treated areas 30 and 34.
As Rhown in Figure 1 when treating as by stop-off selected
areas of stack 40 to prevent diffusion bonding thereon, additional
areas such aR that shown at 92 and 94 ~hould likewise be treated to
prevent bonding thereon so that the gas from inflation tubes 72 and
74 will reach the selected treated areas for expansion of stack 40.
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Apertures 96 are provided in treated areas 300 32 and 34 for further
transmitting the inert gas from inflation tubes 72 and 74 inside
stack 40 to the other treated areas and to insure equal pressure
inside stack 40 (in this case on either side of metal blank 12).
If pressure is unequal, the resulting core of the sandwich, metal
blank 12 in the configuration illustrated in Figure 3, would be dis-
torted with a consequent effect on load carrying ability on the final
sandwich structure.
In order to make the expanded metallic sandwich structure as
shown in Figure 3 according to applicants' inventive method, metal
blank workpieces 10, 12 and 14 are provided. Both sheets 12 and 14
must be of a material having an effective strain rate sensitivity
for superplastic forming. Optimally one or more of the blanks are
treated at specified locations such as 30, 32 and 34 so that when the
metal blanks are arranged in a stack 40 and diffusion bonded, only
; selected areas of the stack will be joined thereby. Alternately,
the stack 40 could be spot welded or brazed for selective joining.
When joining by diffu~ion bonding, the stack 40 would optimally be
placed in the forming apparatus 42 before forming. The pressure in
chamber 48 would be increased by flow of pressurized inert gas through
line 52 into chamber 48. Once ~hamber 48 had an inert gas atmosphere,
the stack 40 would ~e heated by resi~tance wires 52 in heating platens
50 to a temperature which optimally woula be suitable for both di-
ffu~ion bonding and superplastic forming, although the temperature
~o~ld later be raised or lowered if a different temperature is re-
quired for superplastic forming. The pressure in chamber 48 would
be increased b~ addgtional pressurized inert gas through line 52 to
a pressure suitable for diffusion bonding of stack 40. That pressure
would be maintained for a time duration sufficient for diffusion
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bonding. When the metal blanks of stack 40 are of Ti-4V-6Al alloy,
the temperature used would be approximately 1700F. and the pressure
about 500 psi. These values can of course be varied during forming
and bonding as long as they are maintained within a suitable range.
The time duration will vary depending upon the alloys used, tempera- ;
ture, pressure, and thickness of stack 40. Duration may vary from
30 minutes to 15 hours, but three hours is a fairly repre~entative
expectation. As previously stated, bonding temperature may vary
from 1450F. to about 1850F. Bonding pressure may vary from about
100 psi to 2000 psi or more with the preferred range being 150 psi
to 600 p3i.
Before expanding stack 40, the pressure in chamber 48 is
reduced thxough lines 52 and 60. When stack 40 is joined prior to
insertion in forming apparatus 42, the preceding di~fusion bonding
step in forming apparatus 42 would be omitted. At superplastic
forming temperature, which is approximately 1700F for Ti-6Al-4v
alloy (generally 1650-1750F.), stack 40 i~ expanded by flowing pres-
surized inert gas through lines 72 and 74 while optimally a vacuum
is applied to chamber 48 through lines 52 and 60. The pressurized
inert gas, which protects the interior of stack 40 from contamination
at the elevated forming temperatures, flows from tubes 72 and 74 into
channel~ 75, preferably on opposite sides of stack 40, whereupon the
.,
iner~ gas flows within stack 40~ Such pressurized inert gas within
stack 40 forces the expansion of stack 40 due to the pre~sure dif-
ferential between the interior of stack 40 and chamber 48. The pres-
~ure differential normally used for superplastic forming of Ti-6Al-4V
i3 normally in a range of from 25 to 250 psi. Metal blank 14 is
initially lifted by the pressure differential and pulls with it at
the ~elected joined area~ metal blank 12. Such expansion allows the
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pressurized inert gas to flow through apertures 96 to provide an
equal pressure within stack 40 so that the core (workpiece 12) is
formed uniformly. The equal pressure also retains metal blank 10
of stack 40 in its initial position, it being forced against the
base or lower tooling frame 46.
Figures 5 and 6 illustrate the use of a differently con-
figured lower tooling frame 100 having preferably integral sidewalls
102,104 which define a chamber 106. Tube lines 108 and 110 are pro-
vided in lower tooling frame 100 for creating an inert gas environ-
ment in chamber 106 and to act as vents or vacuum lines when super-
pla~tically expanding stack 40. If stack 40 is diffusion bonded in
forming apparatus 120, the pressure in both chambers 48 and 106 would
have to be increased, preferably equally, so that a suitable pressure
is applied to stack 40 for diffusion bonding. For superplastically
expanding stack 40, the pressure within stack 40 would be increased
by allowing flow of inert pressurized gas into c2tack 40 through lines
72 and 74 so that the pressure within stack 40 is greater than that
in chambers 48 and 106. Additionally, the precsure in chambers 48
and 106 ~ould have been reduced and optimally exposed to vacuum
through lines 108, 110, 52 and 60. As shown in Figure 6, since all
three of the-metal blanks of stack 40 would be superplastically
expanded, each of the blanks must be of a material with an effective
ætrain rate sensitivity for superplastic forming. As illustrated,
metal blank 14 is forced upwards into chamber 48, metal blank 10 is
forced d~wnward into chamber 106, and metal bLank 12, by virtue of
being selectively joined at specified locations to both metal blank
10 and metal blank 14 i deformed in both directions and forms the
core of the sandwich structure as shown.
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Figures 7 and 8 illustrate a modified forming apparatus 130
and the use of a different technique for selective joining. The use
of a two sheet stack 132 with metal blanks 134 and 136 is also
illustrated. Stack 132 could be ~oined as by diffusion bonding,
brazing, or spot welding prior to insertion in forming apparatus 130.
If diffusion bonded, the metal blanks 134 and 136 would first be
~electively treated with a suitable stop-off so that only certain
predetermined areas of the stack would be joined. Optimally, how-
ever, the stack 132 would be unjoined prior to insertion in the
forming apparatus 130. When such is the ca~e, stop-off need not be
applied.
Forming apparatus 130 utilizes an upper tooling frame 140
having a lower arcuate surface defined by a plurality of protuberances
142 spaced from each other by intermediate recesses or chambers 144.
Lower tooling frame 150 has a complimentary upper arcuate surface
152 *o that deined by protuberances 142. Inflation tube~ 160 and
162 are positioned between the two metal blanks 134 and 136.
Similar to recesses 76 and 77 in metal blank 10 (Figure 1) blanks
134 and 136 are provided with aligned recesQes (not shown) which
define a cylindrical chamber (not shown) in which tubes 160 and 162
are located~ Lines 164 and 166, like lines 52 and 60 of Figure 2,
provide an inert gas environment in chambers 144 and act as vents
or connection to ~ources of vacuum for drawing out the inert gas from
chamber~ 144 for superplastically expanding metal blank 134 within
those chambers. ~ach of tube~ 164 and 166 would be provided with a
valve (not shown) and pressure gauge (not ~hown) to control addition
and removal of inert gas in chambers 144~ Tubes 164 and 166 are
connected to bores 170 and 172 respectively which provide access to
chambers 144.
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~(~50ZS0
Depending on the thickness and desired curvature of stack
132, l:he stack can be preformed to this shape prior to insertion in
forming apparatus 130 by conventional sheet metal forming, as roll
forming or superplastic forming, or subsequent to insertion in
forming apparatus 130 by pressure applied to stack 132 by the pro-
tuberances 142 o~ upper tooling frame 140 and surface 152 of lower
tooling frame 150. Preforming in the forming apparatus 130 is pre-
ferable as the preforming and selective diffusion bonding are
accomplished simultaneously when the stack is also unjoined prior
to insertion in forming apparatus 130, thereby saving fabrication
time and equipment costs.
Using this forming apparatus 130, the unjoined stack can be
diffusion bonded by application of pressurs from upper tooling frame
140 and lower tooling frame 15~ at required temperature for a suit-
able time duration. By virtue of protuberances 142, the pressure is
only applied to selected areas of stack 132 so that only those areas
are diffusion bonded, thereby allowing for expansion in the unbonded
areas.
After di4fusion bonding (or insertion into forming apparatus
130 if the stack 132 has already been joined) and preforming, stack
132 is inflated by flow of inert gas through lines 160 and 162 so
that the unjoined areas of metal blank 134 are expanded into chambers
144. Optimally a vacuum would be applied to chambers 144 thrDugh
lines 164 and 166. As the only space provided for expansion is
chambers 144, only metal blank 134 is expanded and consequently only
that blan~ must be of a material suitable for superplastic forming.
It is seen from the above that diffusion bonding, pre-forming, and
superplastic expansion can all be done in the same apparatus in one
operation
1 ~ _
50~
It should be noted that the superplastic expansion could be
done prior to diffusion bonding of stack 132 by sealing the sur-
rounding area around stack 132 without applying pressure to stack 1327
Thus, upper tooling frame 140 could be applied to stack 132 con-
tacting at its protuberances 142 without sufficient pressure for
diffucion bonding. With this procedure, stack 132 would be expanded
into chambers 144 by superplastic inflation with protuberances 142
preventing expansion at other areas without applicatuon of pressure
by virtue of the position of tooling frame 140. After ~uch expansion,
sufficient pressure would be applied by tooling frames 140 and 150
at areas corresponding to the lower surface of protuberances 142 to
. diffusion bond those corresponding areas of stack 132.
Figure 9 illustrates the forming of a variably shaped
: structure, here shown as a tapered sandwich ~tructure with an attach-
ment joined thereto. Applicant accompliahes tapering of the sand-
wich structure by suitably aesigning the corresponding shaping ~ur-
face of the upper tooling frame. In Figure 9, upper tooling frame
170 has an upper shaping surface 172 which angles downwardly from
one ~ide to the other (shown ai left to right). Thus, when the upper
~ 20 metal blank 174 of stack 176 is ~uperplastically expanded again~t
.~ surface 172, it is shaped into the taper of such surface. The core
formed by blank member 178 of stack 176 is also tapered by virtue
of its dependence upon the movement of upper blank 174.
~ttachment 180, shown in an arbitrary design, is joined to
the stack 176 on the upper member 174 along bond line 182 during the
uperplastic expansion of stack 176 in the same forming apparatus,
~ : thereby lowering fabrication time, apparatus cost, and forming dif-
~ ficulty. A3 set out in our prior co-pending application, Serial No.
511,900, attachment 180 could be positioned within a suitably shaped
- 20 -
10502S~
groove, here shown at 182, where the attachment may or may not
protrude from such groove, or be located in the forming chamber,
without the use of a groove so that it forms either a male or
female shaping surface itself. For purposes of the claims, when
a groove is used, it is considered part of the forming chamber,
so that the attachment is positioned within the chamber when in
the groove. In the embodiment illustrated, when stack 176 is
superplastically expanded i.t contacts attachment 180 along bond
line 182. By virtue of the superplastic forming temperature,
which attachment 180 is also heated to, and the pressure expand-
ing stack 176 which eventually forces upper member 174 to bear
against surface 172 and attachment 182, attachment 182 is diffu-
sion bonded to stack 176 along bond line 182 when the temperature
and pressure are maintained at a required diffusion bonding level -
for a suitable time duration. Optimally, the temperature and
pressure for superplastic forming of members 174 and 178 would
also be suitable for diffusion bonding so that the pressure or
temperature would not have to be increased or decreased after
forming in order to diffusion bond. The material selected for
attachment 182 should be one that is suitable for diffusion
bonding to the material of member 174, preferably a like mate-
rial. As the superplastic forming gas pressure typically used
j is 150 psi, a pressure considerably less than the 2000 psi used
,~ normally in diffusion bonding, the bond which results may not
develop full parent metal strength, but would likely be analo-
gous to a high quality braze joint. However, once the sandwich
~; structure is fully expanded, as shown in Figure 9, the pressure
can be increased within the expanded sandwich through inflation
tubes 190 and 192 to a level more suitable for complete diffusion
, 30 bonding.
A close-out such as shown at 194 can also be formed to
the..............................................................
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' ~:
,. - . . . . . . .
~ 05(~2~ 2 ~ -
expanded sandwich structure in the same operation by diffusion
bonding the close out 194, which comprises an unexpanded end of
stack 176, by heating stack 176 to the diffusion bonding temperature
with application of pressure by upper and lower tooling frames 170 --
and 171 respectively. The expanded sandwich structure 200 of Figure
9 removed from the forming apparatus is shown at Figure 10 with the
side ends cut off.
Figure 11 illustrate~ the positioning of an inflation tube ~
10 relative to a four-piece metal sheet assembly or stack 212 in a --
10forming apparatus having upper and lower chambers 214 and 216 -
respectively. Inflation tube 210 is positioned bet~een the upper
metal blank 217 and lower metal blank 218 with the lower metal blank
218 having a reces~ 220 at its periphery such as rece~s 76 shown in
Figure 1. Inner metal blanks 221 and 222 of stack 212 have a recess
such as that shown at 78 in Figure 1 which allows for positioning
of inflation tube 210 between members 217 and 218. Apertures 230,
for transmitting inflating gas, and stop-off 232 at areas for which
no bonding i8 desired, are also illustrated. Illu~trative views of
the final foxmed sandwich structure 240 of Figure 11 are shown at
Figure-Q 12 and 13.
Thus it is apparent that there ha~ been provided, in
accordance with the invention, a method for making metallic sandwich
structure~ from a plurality of workpieces that fully ~atisfies the
objective~, aim~, and ad~antages set forth above. While the inven-
tion has been de~cribed in conjunction with specific embodiments
thereof, it i~ evident that many alternatives, modifications, and
variations, will be apparent to those skilled in the art in light
of the foregoing de~cription. Accordingly, it is intended to embrace
all such alternatives, modifications, and variations which fall
within the spirit and scope of the appended claims.
