GB2261183A - Diffusion bonding superplastic materials - Google Patents

Abstract

This invention relates to an improved method of providing apertures through which pressurising gas for superplastically forming a component may be provided. A stainless steel member 5 is positioned between two 1, 7 diffusion bondable and superplastically formable materials in a stopped-off area during diffusion bonding. 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 when the materials are of small gauge due to the gas flow limitations associated with small diameter conventional ceramic tubes. <IMAGE>

Description

DIFFUSION BONDING OF SUPERPLASTICALLY FORMABLE MATERIALS This invention relates to a method of diffusion bonding superplastically formable materials such that subsequently a pipe or tube may be inserted between the materials which allows gas pressure to be applied which causes superplastic deformation of the materials. The invention is particularly applicable to the diffusion bonding of thin sheets of material, ie those having a gauge of 1 mm or less.

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.

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.

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 9300C 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.

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; these 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 tube. The pack is then placed in a form tool in a heated platen press that is 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 tube. 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.

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.

An object of the present invention is to provide an improved means for enabling a gas to be injected between diffusion bonded sheets of a component thereby allowing them to be superplastically formed.

According to the present invention there is provided a method of diffusion bonding two materials, at least one of which is superplastically formable, the method including the steps of positioning a member between said two materials such that the member occupies a region which extends to a periphery of at least one of the two materials; diffusion bonding the materials in selected areas, the said selected areas excluding said region; removing said member; and positioning a tube to communicate with said region such that, when said materials are heated to an appropriate temperature and pressurised gas is applied to the tube, the at least one of the materials undergoes superplastic deformation.

Preferably said tube comprises either titanium or titanium alloy.

Advantageously, said tube is gas tightedly welded around its circumference to at least one of the two materials.

Conveniently, said member comprises stainless steel.

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

Preferably said member has a higher coefficient of linear expansion than said tube.

For a better understanding of the invention, an embodiment of it will now be described by way of example only and with particular reference to the accompanying drawings, in which: Figure 1 shows a plan view of part of a sheet of titanium alloy to which a stop-off material has been applied and on which a stainless steel pin has been laid; Figure 2 shows in side elevation two sheets of titanium alloy being diffusion bonded in a bonding tool; Figure 3 shows a partial view of the edge of the diffusion bonded sheets viewed along the arrow in Figure 1; Figure 4 shows a cross-sectional partial view taken along the line A-A of Figure 1; Figure 5 shows a cross-sectional whole view taken along the line A-A of Figure 1 of the two 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.

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

Figure 1 shows part of a sheet 1 of titanium alloy which has been coated with stop-off material in selected areas 3 by a known silk screen printing process. A cylindrical pin 5 of stainless steel, such as EN58B, or material having similar properties is placed on the sheet 1 in a position where a gas injection point is required for subsequent superplastic forming of the sheet 1. One beneficial property for the pin 5 is that it should have a high scaling temperature, i.e. it resists oxidisation at high temperatures, and another is that it should have a higher coefficient of linear expansion than the titanium sheets. A second sheet of titanium alloy 7 is placed on top of the stopped-off sheet 1, and the thus formed pack placed on a packer 8 in a tool shown generally at 9 in Figure 2.

The pin 5 is tack welded at point 11 to the packer 8. The top tool 13 of the diffusion bonding tool 9 together with bottom tool 15 defines a cavity 17 in which the sheets 1 and 7 are positioned. The cavity 17 is purged by an inert gas from a pipe 21 connected to a pressure pump (not shown). An inert gas also enters space 23 (defined by seals 24) from a pipe 25 and exerts pressure (shown by vertical arrows 26) on a diaphragm 27 located in the top tool 13 and made of, for example, titanium or stainless steel (which is superplastically formable) which in turn presses on the sheets 1 and 7. Heaters (not shown) are positioned in the walls of top tool 13 so that the sheets 1 and 7 can be sufficiently heated to enable diffusion bonding to occur in areas where stop-off material has not been applied when pressure is exerted by the gas from the pump via the pipe 21.

Because the pin 5 has a higher coefficient of linear expansion than the titanium sheets 1 and 7, as the pin 5 is heated it expands at a greater rate than the sheets 1 and 7, and therefore forces them apart.

The pack is then removed from the tool 9. when the pack has cooled the pin 5 may be easily withdrawn from the pack due to the pin's higher coefficient of linear expansion, thereby leaving an aperture 29 through which gas can be injected into the stopped-off (and therefore non-diffusion bonded) areas 3.

Figures 3 and 4 show the aperture 29.

Figure 5 shows a titanium tube 31 which has been inserted in the aperture 29. As an alternative to insertion, the tube 31 may instead be positioned to abut the aperture 29. The tube 31 is secured in any suitable air tight manner, such as by welding at points 32, to the edges of sheets 1 and 7.

Two further sheets 33 and 35 of titanium alloy are now positioned one on each side of the core sheets 1 and 7 respectively. The assembly of the four sheets 1, 7, 33 and 35 and the tube 31 is then positioned between the two form tools 37 and 39 of a heated platen press shown generally at 41 in Figure 6. The outer sheets 33 and 35 may have already been formed to conform to the inner shape of the form tools 37 and 39, or they may be superplastically formed at this stage by methods well known in the art. Pressurised gas is applied via the tube 31 which feeds the areas which have been stopped-off between the inner or core sheets 1 and 7. The exertion of gas pressure in these areas causes the sheets 1 and 7 to be inflated so that they bow outward and form rectangular cells 43.The pairs of opposing walls of the cells 43 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 43. This may be done in the heated platen press 41, or by removing the assembly from the press 41 and subjecting it to hot isostatic pressing (a technique well known in the field of powder metallurgy).

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.

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 (10)

1. A method of diffusion bonding two materials, at least one of which Is superplastically formable, the method including the steps of positioning a member between said two materials such that the member occupies a region which extends to a periphery of at least one of the two materials; diffusion bonding the materials in selected areas, the said selected areas excluding said region; removing said member; and positioning a tube to communicate with said region such that, when said materials are heated to an appropriate temperature and pressurised gas is applied to the tube, the at least one of the materials undergoes superplastic deformation.

2. A method according to claim 1, wherein said tube comprises titanium.

3. A method according to claim 1, wherein said tube comprises titanium alloy.

4. A method according to claim 1, 2 or 3, wherein said tube is gas tightedly welded around its circumference to at least one of the two materials.

5. A method according to any preceding claim, wherein said member comprises stainless steel.

6. A method according to any preceding claim, wherein the diffusion bonding of said selected areas defines after said superplastic deformation a series of cells.

7. A method according to claim 6, wherein two sheets are positioned one either side of said two materials, and the cells are diffusion bonded to said sheets.

8. A method according to any preceding claim, wherein said member has a higher coefficient of linear expansion than said tube.

9. A method of diffusion bonding two materials substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

10. A component manufactured by a method as claimed in any one of the preceding claims.