Forming of gas injection/exhaustion channels in diffusion bonded materials

Abstract

 This invention relates to the forming of gas injection/exhaustion channels in sheets of material (5,6), having superplastic forming characteristics which are diffusion bonded in selected areas. A titanium or titanium alloy tube (41) is subsequently inserted in the channel which allows gas pressure to be applied thereby causing superplastic deformation of the materials, the channel being in communication with the areas between the sheets (5,6) which are not diffusion bonded together. The channel is formed by heating the appropriate region of the sheets (5,6) and driving a punch (8) into that region. The invention is particularly appropriate when the materials are of small gauge due to the gas flow limitations associated with small diameter conventional ceramic tubes.

Description

[0001] This invention relates to the forming of gas injection/exhaustion channels in diffusion bonded superplastically formable materials to enable a pipe or tube to be inserted between the materials. This allows gas pressure to be applied to cause the desired superplastic deformation of the materials. The invention is particularly applicable to thin sheets of material, ie those having a gauge of 1 mm or less.

[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, from multiple sheets of metal, of 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 manufacture structures of a complex nature it is often a requirement that the 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 is often used in these processes because in its received state it has the characteristics needed for superplastic forming, and further 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 and diffusion bonding has enabled our company to design and, using multiple sheets of metal, to manufacture components of complex structure that are essentially of one piece construction.
One known such method of manufacture is as follows. Two sheets of superplastically formable and diffusion bondable material, which will form the internal structure of the finished component, hereafter referred to as the core sheets, are selectively interlaid with stop-off material. Two further sheets of superplastically deformable and diffusion bondable material are positioned one each side of the core sheets in the form of a four sheet pack. These outer sheets will form the outer surface of the finished component, and are hereafter referred to as the skin sheets. Ceramic tubes are positioned between the sheets of the four sheet pack in rebates which are machined in the sheets to accommodate the tubes. The pack is then placed in a form tool in a platen press and heated to 930°C. An inert gas is injected into the space between each skin sheet and core sheet. The pressure exerted by this gas causes the skin sheets to bow outwards and conform to the shape of the cavity of the form tool while at the same time causing the core sheets to be diffusion bonded in areas where stop-off material is not applied, and forming a gas-tight seal with the sheets around the tubes. When these steps have been completed, a gas is injected into the spaces between the core sheets where they are not diffusion bonded. The pressure exerted by the gas causes the core sheets to be moved apart and form substantially rectangular cells which occupy the space between the skin sheets. These cells are formed by the continued application of pressure from the gas which causes parts of the surfaces of the core sheets to become parallel and adjacent to the skin sheets and to be diffusion bonded to them to form cell ceilings and floors while at the same time causing other parts of the surfaces of the core sheets which, due to forming, are now vertically adjacent to one another, to also be diffusion bonded to form cell walls.

[0006] Often there is a requirement for the sheets making up the component to be thin, for example gauges of 1 mm or less, in order to manufacture a component of low mass. However, the ceramic tubes exhibit gas flow limitations when they have a diameter small enough to be accommodated in the rebates for use in forming components of such gauges.

[0007] In attempting to overcome this problem we have tried a method in which a stainless steel member is positioned between two diffusion bondable and superplastically formable materials in a stopped-off area during diffusion bonding in a diffusion bonding tool. The member is then removed and a titanium tube inserted in the aperture which is welded gas tightedly to the surrounding sheets. The titanium tube provides a convenient means by which pressurising gas can be applied in between the materials thereby causing them to superplastically deform. The method is particularly appropriate to metals of small gauge. However, as diffusion bonding occurs between the two diffusion bondable and superplastically formable materials in the diffusion bonding tool, the materials form around the stainless steel member which results in the outer surfaces of the diffusion bonded materials (the pack) being uneven. This is due to the increased thickness of the pack in the region where the member is present. This unevenness of the outer surfaces of the pack makes it difficult to diffusion bond more than one pack in a bonding tool at a given time. Obviously the bonding of more than one pack at a given time is desirable because it is far more efficient in terms of time taken and energy used.

[0008] One object of the present invention is to provide an improved method and apparatus for enabling a gas to be injected between diffusion bonded sheets of a component thereby allowing them to be superplastically formed.

[0009] Another object of the present invention is to provide a method and apparatus for enabling gas injection which is particularly appropriate to materials of small gauge.

[0010] A further object of the present invention is to provide an improved method and apparatus for gas injection which is not incompatible with the diffusion bonding of more than one pack in a diffusion bonding tool at a given time.

[0011] According to one aspect of the present invention there is provided a device for forming an aperture in an edge region of two sheets of material which are diffusion bonded in selected areas, and at least one of which is superplastically formable, the device including heating element means for heating the edge region of said sheets, and punch means for being driven into the edge region of said sheets and forming said aperture therein such that, when said sheets are heated to an appropriate temperature and pressurised gas is applied to the aperture, the at least one of the two sheets undergoes superplastic deformation.

[0012] Preferably, said punch means is driven into said edge region hydraulically.

[0013] Advantageously, said punch means, said heating element means and said sheets are supported in a frame.

[0014] Conveniently, an inert gas is applied to said edge region during said heating.

[0015] According to a second aspect of the invention there is provided a me!hod of forming a gas injection/exhaustion channel to inject gas into or exhaust gas from an area between two sheets of material which are diffusion bonded together in selected regions, said selected regions excluding said area, and at least one of said two sheets being superplastically formable, the method including the steps of heating a region of said sheets where said channel is required; driving a punch into said region and thereby forming a channel therein; and positioning a tube to communicate with said channel such that, when said sheets are heated to an appropriate temperature and pressurised gas is applied to the tube, the at least one of the two sheets undergoes superplastic deformation.

[0016] Preferably said tube comprises either titanium or titanium alloy.

[0017] Advantageously said tube is gas tightedly welded around its circumference to at least one of the two sheets.

[0018] Optionally, diffusion bonding of said selected areas defines after said superplastic deformation a series of cells, and preferably two further sheets are positioned one either side of said two sheets, and the cells are diffusion bonded to said further sheets.

[0019] Preferably an inert gas is applied to said region during the heating thereof.

[0020] For a better understanding of the invention, an embodiment of it will now be described by way of example only and with reference to the accompanying drawings, in which:-

Figure 1 shows a side elevation of an aperture making device in which two diffusion bonded sheets of titanium alloy are positioned;

Figure 2 shows a perspective view of the device of Figure 1;

Figure 3 shows a cross-sectional view along the line A-A in Figure 1;

Figure 4 shows a side elevation of two sheets of titanium alloy being diffusion bonded in a bonding tool;

Figure 5 shows a cross-sectional view of the two diffusion bonded titanium alloy sheets into which a titanium tube has been inserted at one end.

Figure 6 shows the superplastic formation of a component manufactured from four sheets of titanium alloy; and

Figure 7 shows the hot isostatic pressing of the four sheets.

[0021] To improve understanding of the drawings, like elements which appear in more than one figure are designated by the same reference number.

[0022] The aperture making device is indicated generally at 1 in Figures 1 -3. The device 1 includes a frame 2 in which a heating element 3 is housed. A recess 4 runs along the length of the front face of the frame 2 and the heating element 3 in which edge portions of two diffusion bonded titanium alloy sheets 5 and 6 are accommodated. A hydraulic cylinder 7 is attached to the rear face of the frame 2. The cylinder 7 controls the longitudinal movement of a punch 8 contained in the bore 9 thereof. The punch 8 and the frame 2 are made from a material which has good hot hardness and high strength characteristics at high temperature, for example P1050 or P1051 manufactured by Cars of Sheffield, UK. The frame 2 accommodates the cylinder 7 in such a way that the bore 9, and hence the punch 8, has a passage to the edge portions of the titanium alloy sheets 5 and 6 contained in the recess 4.

[0023] The operation and some additional features of the device 1 will now be described in relation to a diffusion bonding and superplastic forming process.

[0024] Two sheets of diffusion bondable and superplastically formable titanium alloy 5 and 6 are selectively interlaid with a stop-off material 10 which prevents diffusion bonding in the areas where it is applied. Two further pairs of sheets 11 and 13 are stopped off in a similar manner. The three pairs of sheets 5 and 6, 11, and 13 are then stacked one on top of the other, each pair being separated from the other by respective layers of stop off material 15 to prevent the separate pairs of sheets bonding together. The thus formed stack is then placed in the cavity 17 which is defined by the top tool 19 and the bottom tool 21 of a diffusion bonding tool shown generally at 23 in Figure 4. The cavity 17 is pressurised by an inert gas (shown by vertical arrows 25) from a pipe 27 connected to a pressure pump (not shown). The gas enters space 29 and exerts pressure on a diaphragm 31 located in the top tool 19 and made of, for example, titanium or stainless steel (which is superplastically formable) which in turn presses on the stack. A pipe 33 is connected to a vacuum pump (not shown) to evacuate the part of the cavity 17 below the diaphragm 31 containing the pack. Heaters (not shown) are positioned in the walls of top tool 19 so that the stack can be sufficiently heated to enable diffusion bonding to occur in each of the pairs of sheets 5 and 6, 11, and 13 in areas where stop-off material has not been applied when pressure is exerted by the gas from the pump via the pipe 27.

[0025] The stack is then removed from the diffusion bonding tool 23 and each of the pairs of sheets 5 and 6, 11, and 13 (which will form the core sheets of respective finished components) are subsequently processed in the same manner. For the sake of brevity only the processing of the pair of sheets 5 and 6 will be described hereinafter.

[0026] One edge of the pair of sheets 5 and 6 is then positioned and clamped in the recess 4 of aperture making device 1, the central axis of the punch 8 being aligned with a position on the pair of sheets 5 and 6 where a gas injection point is required for subsequent superplastic forming (this position, obviously, will be such that the injection point communicates with an area that has been stopped off and therefore not diffusion bonded). The edge of the pair of sheets 5 and 6 are locally heated by the heating element 3 to the required temperature (for example 800°C to 850°C) at which titanium alloy becomes sufficiently soft for the punch 8 to easily penetrate it. When the required temperature is reached, the hydraulic cylinder 7 drives the punch 8 in the direction of arrow 35 in Figure 3 towards and into the edge of the pairs of sheets 5 and 6 in order to form an aperture therein through which a gas can be injected. During this process argon gas, or any other suitable inert gas, is applied through a purging channel 37 which runs through the frame 2 and the heating element 3 to a region 39 of the recess 4 in which the penetration takes place. The argon gas prevents the pair of sheets 5 and 6, the punch 8 and the bore 9 from oxidising during the heating. As can be seen in Figures 1-3, the recess 4 has greater height at region 39 in order to allow for the circulation of argon gas, and to accommodate any local increase in volume of the pair of sheets 5 and 6 due to the punch 8 being driven into them.

[0027] Figure 5 shows a titanium or titanium alloy tube 41 which has been inserted in the aperture formed in the pair of sheets 5 and 6. As an alternative to insertion, the tube 41 may instead be positioned to abut the aperture 29. The tube 41 is secured in any suitable air tight manner, such as by welding at points 42, to the edges of sheets 5 and 6.

[0028] Two further sheets 43 and 45 of titanium alloy are now positioned one on each side of the core sheets 5 and 6 respectively. The assembly of the four sheets 5, 6, 43 and 45 and the tube 41 is then positioned between the two form tools 47 and 49 of a heated platen press shown generally at 51 in Figure 6. The outer sheets 43 and 45 may have already been formed to conform to the inner shape of the form tools 47 and 49, or they may be superplastically formed at this stage by methods well known in the art. Pressurised gas is applied via the tube 41 which feeds the areas which have been stopped-off between the inner or core sheets 5 and 6. The exertion of gas pressure in these areas causes the sheets 5 and 6 to be inflated so that they bow outward and form rectangular cells 53. The pairs of opposing walls of the cells 53 form the support walls and the interior surfaces (or ceilings and floors) of the finished component respectively. Diffusion bonding then occurs between the exterior and interior surfaces of the component, and between the adjacent walls of cells 53. This may be done in the heated platen press 51, or by removing the assembly from the press 51 and subjecting it to hot isostatic pressing (a technique well known in the field of powder metallurgy).

[0029] Hot isostatic pressing involves the evacuation of the area between the exterior and interior surfaces of the component and the application of an isostatic pressure while maintaining the component at a required constant temperature. The arrows in Figure 7 show the force being exerted by the pressurising gas on the exterior and interior of the component in a hot isostatic press. An advantage of using a hot isostatic press for diffusion bonding is that it obviates the need for using the highly stressed form tools. The bonding pressures act isostatically, and therefore do not require mechanical reaction.

[0030] When the diffusion bonding is completed by either of the above methods, the atoms of the exterior and interior surfaces of the component are interdiffused thus forming a metallurgically bonded layer.

Claims

1. A device for forming an aperture in an edge region of two sheets of material which are diffusion bonded in selected areas, and at least one of which is superplastically formable, characterised in that the device includes heating element means (3) for heating the edge region of said sheets (5,6), and punch means (8) for being driven into the edge region of said sheets and forming said aperture therein such that, when said sheets (5,6) are heated to an appropriate temperature and pressurised gas is applied to the aperture, the at least one of the two sheets undergoes superplastic deformation.

2. A device according to claim 1, characterised in that said punch means (8) comprises hydraulic punch means (7,8,9).

3. A device according to claim 1 or 2, characterised in that said punch means (8), said heating element means (3) and said sheets (5,6) are supported in a frame (12).

4. A device according to claim 1, 2 or 3, characterised in that the device further includes means (37) for applying inert gas to said edge region during said heating.

5. A method of forming a gas injection/exhaustion channel to inject gas into or exhaust gas from an area between two sheets of material which are diffusion bonded together in selected regions, said selected regions excluding said area, and at least one of said two sheets being superplastically formable characterised in that, the method includes the steps of heating a region of said sheets (5,6) where said channel is required; driving a punch (8) into said region and thereby forming a channel therein; and positioning a tube (41) to communicate with said channel such that, when said sheets are heated to an appropriate temperature and pressurised gas is applied to the tube (41), the at least one of the two sheets (5,6) undergoes superplastic deformation.

6. A method according to claim 5 characterised in that said tube (41) comprises titanium.

7. A method according to claim 5 characterised in that said tube (41) comprises titanium alloy.

8. A method according to claim 5, 6 or 7 characterised in that said tube is gas (41) tightedly welded around its circumference to at least one of the two sheets (5,6).

9. A method according to claims 5, 6, 7 or 8, characterised in that the diffusion bonding of said selected regions defines after said superplastic deformation a series of cells (53).

10. A method according to claim 9, characterised in that two further sheets (43,45) are positioned one either side of said two sheets (5,6) and the cells (53) are diffusion bonded to said further sheets (43,45).

11. A method according to any one of claims 5 to 10, characterised in that an inert gas is applied to said region during the heating thereof.

12. A method according to any one of claims 5 to 11, characterised in that the sheets (5,6) are heated to substantially between 800°C and 850°C.

13. A component manufactured by a method as claimed in any one of claims 5 to 12.

Drawing