[0001] This invention relates to an improved method of forming components from materials
having superplastic characteristics, and in particular to the use of additional sheets
of material to improve the strength characteristics of formed components.
[0002] Metals having superplastic characteristics, such as titanium and many of its alloys,
have a composition and microstructure such that, when heated to within an appropriate
range of temperature and when deformed within an appropriate range of strain rate,
they exhibit the flow characteristics of a viscous fluid. The condition in which these
characteristics are attained is known as superplasticity, and, in this condition,
the metals may be deformed so that they undergo elongations of several hundred percent
without fracture or significant necking. This is due to the fine, uniform grain structures
of superplastically formable metals which, when in the condition of superplasticity,
allow grain boundary sliding by diffusion mechanisms so that the individual metal
crystals slide relative to one another.
[0003] Diffusion bonding is often combined with superplastic forming to enable the manufacture
of multi-sheet components of complex structure. The diffusion bonding process concerns
the metallurgical joining of surfaces by applying heat and pressure which results
in the co-mingling of atoms at the joint interface, the interface as a result becoming
metallurgically undetectable. In order to achieve structures of a complex nature it
is often a requirement that the multi-sheet metals are not bonded at all their contacting
areas, and therefore bond inhibitors (commonly known as stop-off or stopping-off materials)
are applied to selected areas by, for example, a silk screen printing process.
[0004] Titanium, in sheet form, has in its received state the characteristics needed for
superplastic forming, and because it will absorb its own oxide layer at high temperature
in an inert atmosphere to provide an oxide-free surface, it is also particularly amenable
to diffusion bonding under pressure. The optimum temperature for this self-cleaning
is 930°C which is also the optimum superplastic forming temperature. Thus, superplastic
forming and diffusion bonding of titanium components can be carried out at the same
time.
[0005] The ability to combine superplastic forming (SPF) and diffusion bonding (DB) has
enabled our company to design multi-sheet components of complex structure that are
essentially of one-piece construction. Such structures have a potential application
in aircraft manufacture, for example the manufacture of wing leading and trailing
edge control surfaces and canards which must have smooth, aerodynamic skin surfaces
and be strong and light in weight.
[0006] Many aircraft components require structures of variable gauge. Attachment points
on wing sections, for example, often require locally thickened regions, for example.
[0007] Further, if large recesses or protrusion are required in a surface to be made by
superplastic forming, the extreme, localised elongations required can result in the
weakening (or necking) of the surface in these areas. Such surfaces are sometimes
used in aerodynamic or hydrodynamic applications (see our European Patent Application
No. 0 267 023, for example). The problem of weakening of the surface can be reduced
by providing locally thickened areas of the sheet from which the surface is formed.
[0008] Previously such thickening or strengthening has been achieved by using uniformly
thickened sheets in the multi-sheet SPF/DB process. However, it is then necessary
to add an additional, onerous step to the production process of chemical-milling in
order to reduce the thickness of the sheets in areas where strengthening is not required.
This chemical milling step is time consuming, wasteful of material and produces hazardous
waste products.
[0009] According to the present invention there is provided a method of manufacturing a
component from a main sheet of superplastically formable material and at least one
additional sheet of superplastically formable material having a face of smaller area
than a face of said main sheet, the method including the steps of overlaying said
face of the at least one additional sheet on said face of the main sheet prior to
superplastically forming the component, and superplastically forming the component
such that after forming said face of the at least one additional sheets is substantially
in contact with said face of said main sheet.
[0010] Optionally said component is manufactured from a plurality of main sheets over at
least one of which at least one additional sheet of material is laid; selected areas
of said main and additional sheets are pretreated with a bond inhibitor or to which
a stop-off material has been applied;
[0011] and said main and additional sheets are laid over one another and diffusion bonded
as required and superplastically formed in a mould to form a component with a cellular
internal structure and having a thickened region where said additional sheet is present.
[0012] Optionally said at least one additional sheet of material is positioned on said main
sheet in a region which when the main and additional sheets are positioned on a form
tool will coincide with a region on the form tool which defines a recess or protrusion,
the main and additional sheets being formed into said component by placing them in
the form tool and applying heat and pressure.
[0013] Conveniently said face of the at least one additional sheet is spot welded to said
face of the main sheet prior to superplastic forming.
[0014] Alternatively said face of the at least one additional sheet is line bonded to said
face of the main sheet prior to superplastic forming.
[0015] Optionally said face of the at least one additional sheet is diffusion bonded to
said face of the main sheet prior to superplastic forming.
[0016] Hereinafter the term "doubler" is used to refer to additional sheet(s) of material.
[0017] For a better understanding of the invention, embodiments of it will now be described
by way of example only, and with particular reference to the accompanying drawings
in which:-
Figs. 1A, 1B, 1C and 1D show the formation of a component by a conventional SFP/DB
process,
Figs. 2A, 2B, and 2C illustrate the application of a core-sheet reinforcing doubler,
Fig. 3 illustrates the application of a skin-sheet reinforcing doubler,
Fig. 4 shows an aircraft wing with a leading edge configuration which is manufacutred
in accordance with the invention,
Fig. 5 shows a main sheet onto which additional sheets (doublers) have been placed,
Fig. 6 shows a form tool for forming the wing leading edge of Fig. 4, and
Fig. 7 shows sheets positioned for forming in the form tool of Fig. 6.
Figs. 1A to 1D illustrate a known 4-sheet SPF/DB process for making a component having
a cellular internal structure. 4 titanium alloy sheets, 1,2,3, and 4, are laid one
on top of the other but the sheets 2 and 3 are separated by a pattern of stop-off
material leaving a grid of untreated areas where line bonds 5 in the final structure
are required. (See Fig. 1A). The two outer sheets 1 and 4 will after formation become
the skin of the component and are hereinafter referred to as skin sheets, whereas
the two inner sheets 2 and 3 will become the core or support walls of the component
and are hereinafter referred to as core sheets.
[0018] Fig. 1B shows a cross-section through the assembly of skin and core sheets transverse
to the line bonds 5. The core sheets 2 and 3 of this 4-sheet "pack" are pressed together
at 930°C so that diffusion bonding between the core sheets 2 and 3 takes place at
the line bonds but is inhibited elsewhere wherever the stop-off material is laid.
[0019] Fig. 1C shows how the bonded pack is clamped between the two halves of a nickel chromium
steel mould 6, heated to 930°C and an inert gas introduced under pressure via suitable
gas pipe connections to regions between the 4-sheets of the pack. As the inert gas
is introduced initially to the regions between the skin and core sheets, and subsequently
between the core sheets, the sheets deform superplastically and bow out towards the
inner mould surfaces, as shown by the dotted lines. The core sheets 2 and 3 do not
separate at the diffusion bonded line bonds 5. Eventually the structure superplastically
deforms to correspond to the inner shape of the mould but, because of the line bonding,
vertical walls of titanium alloy are formed at regular intervals throughout the structure
as shown in Fig. 1D.
[0020] As stated previously, although this conventional technique is satisfactory for many
applications, sometimes locally thickened regions are required. For example, for high
strength cellular structures it may be desired to have thickened vertical or horizontal
walls. This can be achieved by the method described below.
[0021] In Fig. 2a is shown a plan view of a core sheet 7 of superplastically formable and
diffusion bondable material having an inner area X bounded by a border 8 which is
to be superplastically deformed and an outer area Y bounded by a border 9 which is
to cooperate with the clamping surfaces of a forming tool, such as that shown at 6
in Fig. 1C, and via which inflating gas will be introduced to the inner surface 8
by means of breakthrough areas (not shown). A bond line 10 defines an area of the
area X which during subsequent diffusion bonding bonds to an adjacent identical core
sheet. A stop-off material, or bond inhibitor, is applied by any known method, for
example silk screen printing, to all areas of the inner area X except the bond line
and a rectangular area Z bounded by a border 11 (shown dotted) in the centre of the
area X.
[0022] The area Z is that part of the finished cellular structure where thickening is required.
A rectangular doubler 12 of identical superplastically deformable diffusion bondable
material to that of the core sheet 7 and having chamfered edges and dimensions identical
to the area Z is laid over the area Z. The bond line 10 is continued over the surface
of the doubler 12 which is otherwise treated with stop-off.
[0023] The sheet 7 and its overlaid doubler will be assembled into a bond pack in the manner
shown in Fig. 1B. The sheet and doubler will be positioned as one of the core sheets,
say 2 in Fig. 1B. An identical combination of core sheet and doubler will form the
other core sheet, in this case 3 in Fig. 1B, and so that the two doublers will be
in contact. Prior to the skin sheets 1 and 4 being overlaid onto the core sheets 2
and 3, the core sheets 2 and 3 are subjected to diffusion bonding pressure at 930
degrees C. The doublers diffusion bond to their respective core sheets 7 in the regions
Z. Each pair of core sheets and doublers also diffusion bond to each other along the
bond line 10. The pack is then assembled as shown in Figure 1B.
[0024] After assembly the bond pack is placed in a steel mould tool, for example 6 in Fig.
1C, being clamped by the mould faces at the area Y. The temperature of the tool is
raised to 930 degrees C in a suitable furnace and Argon gas is initially introduced
to the regions between the skin and core sheets 1, 2 and 3, 4 to blow them under uniform
pressure to the designed shape, for example as shown in Fig. 1D. When the skin sheets
1 and 4 are formed, the regions between the core sheets 2 and 3 are similarly blown
under pressure, and form the vertical walls of the component. In addition to shaping,
the application of pressure may also result in the diffusion bonding of surfaces as
they come into contact with one another. In this case, because of the existence of
the doublers, the cross-section of the formed support wall will be thicker than the
formed skin in those regions. Note that after the SPF operation the plan view of the
doubler, originally area Z, will be reduced to an area Z¹ bounder by a border 11¹.
Fig. 2B is a view then on section A-A in Fig. 2A and shows the formed thickened support
wall 13. In contrast, the thickness of the skin at 14 is thin and corresponds approximately
to the thickness of the skins 1 and 4 in Fig. 1B before DB/SPF. Regions 15, where
the support wall joins the outer skin 14 are also thickened due to the doubler and
the dimensions of this region 15 are clearly determined by the width of the doubler
in relation to the height of the SPF mould tool.
[0025] Fig. 2C is a section on B-B in Fig. 2A after SPF/DB and shows the formed structure
in the region of a non-thickened support wall 16.
[0026] Where it is specifically desired to increase the thickness of the skin sheets 1 or
4 in Fig. 1B, suitable shaped doublers are located on those sheets prior to core sheet
bonding. Fig. 3 shows a cross-section through a formed cellular structure in which
doublers had been applied to both upper and lower skins (1 and 4 in Fig. 1B) in the
region of a support wall formed by the core sheets (2 and 3 in Fig. 1B). The skin
reinforcing doublers have resulted in thickened areas 17 which due to the constraints
of the mould tool have resulted in uniformly flat outer skins but humped inner surfaces
in the region of the support walls 18.
[0027] It will be appreciated that many modifications, variations and improvements to the
method according to this embodiment may now be devised. For example, skin doublers
may now be employed in structures made from any plurality of SPF sheets not just 4-sheet
cellular structures. When used to reinforce outer skins in 4-sheet structures, the
doublers need not reinforce regions adjacent inner support walls formed from core
sheets, but may thicken any area of the outer skin. Many other variations will now
suggest themselves to those skilled in the art.
[0028] Doublers may also be employed in the manufacture of surfaces which require large
recesses or protrusions to avoid localised weakening in these areas due to the excessive
localised elongation. Such a surface is shown in Figure 4 at 50 and forms part of
the leading edge of a wing 52.
[0029] Figure 5 shows the main sheet 54 from which the surface is to be formed, and onto
which three additional sheets (doublers) 56, 58 and 60 have been laid. The additional
sheets 56, 58 and 60 are positioned in the region where the recess is required in
the finished component. The sheets 56, 58 and 60 are attached to the main sheet and
to one another by spot welding, line bonding or diffusion bonding. The methods of
spot welding and line bonding, for example at opposing sides of the mound formed by
the additional sheets, allow all the sheets to move relative to one another where
they are not bonded during subsequent superplastic forming. If the sheets are diffusion
bonded in their contacting areas then this relative movement will not be possible.
[0030] The lower tool of a form tool for the manufacture of the surface 50 is shown at 62
in Figure 6. The region 64 will define the recess in the wing leading edge.
[0031] Figure 7 shows the assembly of Figure 5, positioned between the two members of a
form tool 66. The assembly is heated to a temperature at which the sheets of material
become superplastic (930°C for example). Gas pressure is applied to the sheets via
a gas feed line (not shown). This forces the sheets into the cavity of the lower tool
62, and hence forms the leading edge for the wing 52 - as illustrated in Figure 4.
[0032] The doublers are positioned so that they coincide with the region 64 of the lower
tool 62 (which forms the recess in the wing leading edge). It will be apparent that
where the sheets 54 - 60 are forced against region 64, they undergo a greater degree
of elongation than in other areas (which can result in localised thinning). The presence
of the doublers compensates for this and result in the finished component having an
approximately uniform thickness.
1. A method of manufacturing a component from a main sheet of superplastically formable
material (7,54) and at least one additional sheet of superplastically formable material
(12,56,58,60) having a face of smaller area than a face of said main sheet, the method
including the steps of overlaying said face of the at least one additional sheet on
said face of the main sheet prior to superplastically forming the component, and superplastically
forming the component such that after forming said face of the at least one additional
sheets is substantially in contact with said face of said main sheet.
2. A method according to claim 1 wherein said component is manufactured from a plurality
of main sheets (1,2,3,4,) over at least one of which at least one additional sheet
of material (12) is laid; selected areas of said main and additional sheets are pretreated
with a bond inhibitor or to which a stop-off material has been applied; and said main
and additional sheets are laid over one another and diffusion bonded as required and
superplastically formed in a mould (6) to form a component with a cellular internal
structure and having a thickened region (15,17) where said additional sheet is present.
3. A method according to claim 1, wherein said at least one additional sheet of material
is positioned on said main sheet in a region which when the main and additional sheets
are positioned on a form tool will coincide with a region on the form tool (62) which
defines a recess or protrusion (64), the main and additional sheets being formed into
said component by placing them in the form tool and applying heat and pressure.
4. A method according to claim 1 or claim 3, wherein said face of the at least one additional
sheet is spot welded to said face of the main sheet prior to superplastic forming.
5. A method according to claim 1 or claim 3, wherein said face of the at least one additional
sheet is line bonded to said face of the main sheet prior to superplastic forming.
6. A method according to claim 1 or claim 3, wherein said face of the at least one additional
sheet is diffusion bonded to said face of the main sheet prior to superplastic forming.