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Title:
FORMING MULTI-WALLED COMPONENTS
Document Type and Number:
WIPO Patent Application WO/2008/142370
Kind Code:
A3
Abstract:
A method of forming a component having a plurality of walls comprises placing a blank (26) opposite a forming surface (14) of a die (10). The blank (26) comprises an outer wall (30) and an inner wall (28) in mutual contact between an inner side of the outer wall (30) and an outer side of the inner wall (28). Fluid pressure is applied to the inner side of the outer wall (30) to separate the outer wall (30) from the inner wall (28) and to form the outer wall (30) against the forming surface (14) while applying substantially equal fluid pressure to inner and outer sides of the inner wall (28), thus defining a gap between the inner and outer walls (28, 30). Fluid pressure is applied to the outer wall (30) through an aperture defined by an overlap portion (32) of the outer wall (30) extending outwardly beyond an edge of the inner wall (28) to an edge of the outer wall (30).

Inventors:
SLATER PAUL STUART (GB)
Application Number:
PCT/GB2008/001641
Publication Date:
January 15, 2009
Filing Date:
May 12, 2008
Export Citation:
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Assignee:
EXTEX LTD LIABILITY PARTNERSHI (GB)
SLATER PAUL STUART (GB)
International Classes:
B21C37/15; B21D26/051; F01N13/14
Foreign References:
US5836065A1998-11-17
JPH1052721A1998-02-24
DE4019899C11991-12-19
US5170557A1992-12-15
Attorney, Agent or Firm:
CUMMINGS, Sean, Patrick et al. (Fleet Place House2 Fleet Place, London EC4M 7ET, GB)
Download PDF:
Claims:

CLAIMS

1. A method of forming a component having a plurality of walls, the method comprising:

placing a blank opposite a forming surface of a die, which blank comprises an outer wall and an inner wall in mutual contact between an inner side of the outer wall and an outer side of the inner wall, wherein the forming surface allows space for deformation of the outer wall away from the inner wall; and

applying fluid pressure to the inner side of the outer wall to separate the outer wall from the inner wall and to form the outer wall against the forming surface while applying substantially equal fluid pressure to inner and outer sides of the inner wall, thus defining a gap between the inner and outer walls;

wherein fluid pressure is applied to the outer wall through an aperture defined in an overlap portion of the outer wall extending outwardly beyond an edge of the inner wall to an edge of the outer wall;

characterised in that an outward area of the overlap portion is restrained against deformation under said fluid pressure; and

an inward area of the overlap portion is permitted to deform under said fluid pressure to allow fluid pressure to be applied between the inner and outer walls.

2. The method of Claim 1 , wherein the overlap portion of the outer wall is opposed to a peripheral region of the forming surface, the peripheral region comprising:

an outward section fitting closely against the overlap portion to restrain local deformation of the opposed outward area of the overlap portion under said application of fluid pressure; and

an inward section spaced from the overlap portion to promote local deformation of the opposed inward area of the overlap portion under said application of fluid pressure.

3. The method of Claim 2, wherein the edges of the inner and outer walls lie opposed to the peripheral region.

4. The method of Claim 2 or Claim 3, wherein the inward section is wider than the outer section in the direction of separation of the outer wall from the inner wall.

5. The method of any of Claims 2 to 4, wherein the inward and outward sections of the peripheral region are separated by a step formation.

6. The method of Claim 5, wherein the step formation is a ramp.

7. The method of any of Claims 2 to 6, wherein at least one of the inward and outward sections of the peripheral region has a straight wall.

8. The method of any of Claims 2 to 7, wherein said edge of the outer wall is opposed to the outward section of the peripheral region, and the outward section of the peripheral region extends outwardly beyond said edge of the outer wall.

9. The method of any of Claims 2 to 8, wherein said edge of the inner wall is opposed to the inward section of the peripheral region, and the inward section of the peripheral region extends inwardly beyond said edge of the inner wall.

10. The method of Claim 9 wherein a marginal area of the inner wall opposed to the inward section of the peripheral region lies generally parallel to the opposed area of the inward section.

11. The method of any preceding claim, wherein fluid pressure is provided via a mandrel that seals with inward axial force against said edge of the outer wall.

12. The method of Claim 11 when appendant to any of Claims 8 to 11 , wherein the mandrel is a sliding fit with the outward section of the peripheral region that extends beyond said edge of the outer wall.

13. The method of Claim 11 or Claim 12, wherein the mandrel also applies lateral force to said edge of the outer wall, transverse to the direction of the inward axial force.

14. The method of Claim 13, wherein the mandrel has a distally-tapered edge co- operable with said edge of the outer wall.

15. The method of Claim 13 or Claim 14 when appendant to any of Claims 8 to 12, wherein the mandrel forces said edge of the outer wall against the outward section of the peripheral region.

16. The method of any of Claims 11 to 15, wherein the mandrel is driven to maintain inward axial force against said edge of the outer wall to feed the material of the outer wall inwardly as the outer wall is formed against the forming surface.

17. The method of any preceding claim, wherein the outer wall is separated from the inner wall by an inwardly-progressing peeling action starting from said edge of the inner wall and progressing to reduce the area of mutual contact between the inner and outer walls.

18. The method of Claim 17, wherein during the peeling action, fluid pressure is applied continuously to a receding interface between the inner and outer walls.

19. The method of any preceding claim, preceded by a preliminary forming operation that comprises:

placing the blank opposite a preliminary forming surface; and

applying fluid pressure to the inner side of the inner wall causing the inner wall to bear against the inner side of the outer wall, thereby deforming the inner and outer walls toward the preliminary forming surface until the outer wall conforms to the shape of the preliminary forming surface and the inner wall conforms to the shape of the outer wall.

20. The method of Claim 19, wherein during the preliminary forming operation, fluid pressure is provided via a mandrel that seals with inward axial force against said edge of the inner wall.

21. The method of Claim 20, wherein said mandrel is a sliding fit with the outer wall.

22. The method of Claim 20 or Claim 21 , wherein said mandrel also applies lateral force to said edge of the inner wall, transverse to the direction of the inward axial force.

23. The method of Claim 22, wherein said mandrel has a distally-tapered edge co- operable with said edge of the inner wall.

24. The method of Claim 22 or Claim 23, wherein said mandrel forces said edge of the inner wall against the inner side of the outer wall.

25. The method of any of Claims 20 to 24, wherein said mandrel is driven to maintain inward axial force against said edge of the inner wall to feed the material of the inner and outer walls inwardly as the inner and outer walls are deformed under said fluid pressure.

26. The method of any of Claims 19 to 25, wherein the preliminary forming operation is performed at a substantially higher fluid pressure than the subsequent forming operation that deforms the outer wall away from the inner wall, and wherein the fluid pressure used in the subsequent forming operation is insufficient to deform the combined thicknesses of the inner and outer walls.

27. A method of forming a component having more than one wall, comprising:

placing a blank comprising an outer wall and an inner wall opposite a first forming surface;

in a first forming operation to produce an intermediate blank, applying internal fluid pressure to the inner wall causing the inner wall to bear against the outer wall whereby the inner and outer walls are deformed toward the first forming surface until the outer wall conforms to the shape of the first forming surface and the inner wall conforms to the resulting inner shape of the outer wall;

placing the intermediate blank opposite a second forming surface allowing space for further deformation of the outer wall; and

in a second forming operation, applying internal fluid pressure to the outer wall to form the outer wall against the second forming surface while applying

substantially equal fluid pressure to both sides of the inner wall, thus defining a gap between the inner and outer walls;

wherein in the second forming operation, internal fluid pressure is applied to the outer wall through an aperture defined in an overlap portion of the outer wall extending beyond an inward margin of the inner wall to an outward margin of the outer wall;

characterised in that an outward part of the overlap portion is restrained against deformation under said fluid pressure; and

an inward part of the overlap portion is permitted to deform under said fluid pressure to allow fluid pressure to be applied between the inner and outer walls.

28. The method of any preceding claim, wherein the blank is tubular, with the inner wall being defined by an inner tube and the outer wall being defined by an outer tube around the inner tube.

29. The method of any preceding claim, further comprising severing the overlap portion of the outer wall level with, or inboard of, said edge of the inner wall.

30. The method of Claim 29 when appendant to any of Claims 2 to 28, wherein severing the overlap portion leaves behind a marginal area of the inner wall that was opposed to the inward section of the peripheral region.

31. The method of any preceding claim, wherein the aperture is defined by an axial gap bordered by said edge of the inner wall.

32. The method of any preceding claim, wherein the aperture communicates with a radial gap bordered by said edge of the inner wall.

33. A die for forming a component having a plurality of walls, the die comprising a forming surface for forming a blank by allowing space for deformation of an outer wall of the blank away from an inner wall of the blank, wherein a peripheral region of the forming surface comprises:

an outward section adapted to fit closely against the outer wall of the blank to restrain local deformation of the opposed area of the outer wall under fluid pressure; and

an inward section spaced from the outer wall to promote local deformation of the opposed area of the outer wall under said fluid pressure, the inward section communicating with a major portion of the forming surface that is spaced further from the outer wall of the blank and is disposed inwardly of the inward section.

34. The die of Claim 33, wherein the inward section is wider than the outer section in the direction of deformation of the outer wall away from the inner wall.

35. The die of Claim 33 or Claim 34, wherein the inward and outward sections of the peripheral region are separated by a step formation.

36. The die of Claim 35, wherein the step formation is a ramp.

37. The die of any of Claims 33 to 36, wherein at least one of the inward and outward sections of the peripheral region has a straight wall.

38. A forming apparatus for forming a component having a plurality of walls, the apparatus comprising the die of any of Claims 33 to 37 and further comprising at least one mandrel for sealing against a blank opposed to a forming surface of the die and applying fluid pressure to the blank.

39. A component having a plurality of walls and made in accordance with the method of any of Claims 1 to 32, with the die of any of Claims 33 to 37 or with the apparatus of Claim 38.

Description:

FORMING MULTI-WALLED COMPONENTS

This invention relates to components having more than one wall, especially double- walled components, and in particular to methods for their manufacture. The invention particularly relates to methods for forming such components by fluid pressure in a die, especially in a hydroforming process.

Although not limited to that application, the invention will be described herein in relation to a double-walled tubular component for the exhaust system of an internal combustion engine. However whilst the invention is particularly suitable for making tubular components like a pipe, the invention can be applied to non-tubular double- walled components like insulated panels too.

The invention will be described particularly in relation to an exhaust downpipe for a vehicle engine, to be positioned immediately downstream of an exhaust manifold and immediately upstream of a catalytic converter. It is also possible for a 'close-coupled' catalytic converter to be integrated with a downpipe immediately downstream of an exhaust manifold.

Double-walled exhaust system components are in widespread use. Their use has been driven by the need for thermal control, and particularly by the need for insulation defined within a gap between the walls. That gap is usually filled with air as the insulator but insulating filler materials can be used instead.

One purpose of insulation in an exhaust system can be to reduce the temperature of the outer wall to reduce heat transmission from the exhaust system. For example, insulating sections of an exhaust system can reduce the temperature of air within the engine compartment of a vehicle to ease cooling. Moreover, insulation can help to reduce the risk of burns caused by contact with an exposed exhaust component, particularly on a motorcycle. However where an insulated exhaust component is positioned upstream of a catalytic converter, the main purpose of insulation is to keep heat in the exhaust gas. This achieves fast 'light-off of the catalyst following engine start, when exhaust emissions are a particular problem, and helps to keep the catalyst at its working temperature thereafter.

For the purpose of fast light-off, it is beneficial for the inner wall in contact with the exhaust gas to have low thermal capacity by being as thin as possible, and for there

to be minimal heat conduction through the inner wall. If the temperature of the outer wall is comparatively low as a result, that is a beneficial side-effect of insulation. The outer wall can be as thick as may be necessary to lend mechanical support to the thin inner wall. However it is not essential that there is any difference in thickness between the inner and outer walls.

Hydroforming has been used for several years to make single-walled exhaust system components. The process involves expanding a tubular blank under internal fluid pressure within a die cavity until the wall of the component assumes the shape of an opposed forming surface of the die. Hydroforming is well-suited to the mass production of tubular components having intricate and complex shapes, with the advantage of making seamless near-finished components in a single operation.

Hydroforming techniques have also been used in the manufacture of double-walled components. For example, a single-walled hydroformed component can be assembled with another single-walled component to make a double-walled component. However, the full benefit of hydroforming requires both walls to be assembled before forming in the die, preferably allowing both walls to be hydroformed together or successively. This is not straightforward: self-evidently, the walls are not the same size; they may also be quite different in shape. For example, the inner wall may be smoothly curved to promote gas flow within and the outer wall may be ribbed to impart rigidity to the unit. There is also the challenge of how to apply internal fluid pressure selectively to one wall rather than the other wall.

An early attempt to address these issues is US 5170557 to Benteler Industries, Inc. US 5170557 is also concerned with the problem of achieving fast catalyst light-off in an exhaust system. To solve this, US 5170557 proposes a double-walled elongate tubular blank in which an inner wall is defined by an inner tube nested within an outer tube that defines an outer wall. The inner tube, but not the outer tube, is perforated by a series of holes extending along its length. The inner and outer tubes are of substantially the same length and extend the full length of the blank; indeed, they define its length. Consequently, the ends of the inner and outer tubes are substantially aligned with each other.

The blank is placed in a die cavity having a forming surface that allows space for radially-outward deformation of the outer tube. Hydraulic fluid is supplied under pressure to the interior of the inner tube through a mandrel that is forced into sealing

engagement with nested ends of the inner and outer tubes. A peripheral region of the die closely surrounds and hence restrains the ends of the inner and outer tubes against radial expansion in order to maintain the seal with the mandrel. The opposed nested ends at the other end of the blank are sealed in similar fashion.

The holes in the inner tube wall expose the outer tube to internal fluid pressure while balancing pressure on the inner and outer surfaces of the inner tube. As a result, the outer tube lifts radially off the inner tube to be formed against the forming surface, while the inner tube 'floats' without deflection so that an annular gap is opened up between the inner and outer tubes. That gap defines an insulating jacket around the inner tube to retain the heat of exhaust gas that flows within the inner tube in use.

Benteler's proposal in US 5170557 is merely a partial solution that leaves some problems unsolved and introduces other problems of its own. For example:

• There is no provision for hydroforming the inner tube: hydroforming the inner tube is precluded by the holes spaced along its length that are necessary to allow the outer tube to be hydroformed. Hydroforming of the inner tube would only be possible before the holes are made, which presents challenges as to how to make holes in a workpiece that is then of potentially complex shape. It is also likely to be impossible to insert an already-hydroformed inner tube into an outer tube due to clashes of shape and size.

• Creating the holes in the inner tube involves additional manufacturing operations. For example, the holes can be drilled in the inner tube before the inner tube is inserted into the outer tube. Another approach is to drill the holes when the inner tube is in situ within the outer tube. Theoretically this can be done by drilling radially outwardly from the inside of the inner tube to penetrate the wall of the inner tube but not the wall of the outer tube. In practice, the lack of space within the inner tube necessitates drilling radially inwardly from outside through the combined thicknesses of the walls of the outer and inner tubes, and then plugging the holes in the outer tube with welds. This process has to be done with such accuracy that it is fraught with difficulty, and it adds yet more manufacturing operations. It also risks weakening the outer tube.

• The outer tube will tend to shorten so that its ends pull away from the sealing mandrels as material is drawn axially inward from the ends by the radial expansion of the central portion of the outer tube. This compromises sealing, hence reducing the maximum fluid pressure that can be applied to the blank. Reduced fluid pressure limits the wall thickness that can be formed and the degree of deformation and intricacy that can be achieved in the finished component.

• The end portions of the inner and outer tubes will tend to lock together under the applied internal fluid pressure. So, as the outer tube shortens, it will tend to shorten the inner tube too. This introduces a risk of unwanted deformation of the inner tube, particularly at any weak points such as the locations of the holes. The tubes will shorten in any event if inward axial thrust is exerted through the mandrels to maintain their seal against the tubes and to feed in material to assist radial expansion of the outer tube. That axially inward thrust increases the risk of deformation of the inner tube, such as buckling around the holes.

• Unless the holes through the wall of the inner tube are blocked in yet further operations such as welding or inserting an insert, those holes will remain in the finished component. The annular gap between the inner and outer tubes will therefore be in fluid communication with the interior of the inner tube in use. It is possible for hot exhaust gas to leak into the gap through the holes, reducing the effectiveness of insulation and requiring the ends of the gap to be sealed. It is also possible for the holes to interfere with gas flow within the inner tube, giving rise to a risk of increased turbulence, back-pressure, resonance and noise. Another problem is that some other applications of double-walled components, such as in a heat exchanger, may completely prohibit mixing of fluids between the inner tube and the annular gap. The method disclosed in US 5170557 is not capable of making components for such applications.

• The ends of the gap may easily be sealed because the end portions of the inner and outer tubes are held together to restrain them against radial expansion. However, this fails to take account of differential thermal

expansion between the hot inner tube and the relatively cool outer tube, giving rise to undesirable thermal stresses in the unit.

Benteler's later proposal in EP 0627272 (equivalents: US 5363544 and US 5475911 ) is a development of US 5170557 that allows hydroforming of inner and outer tubes in successive expansion stages, each performed in a respective die cavity.

In a first expansion stage in EP 0627272, fluid pressure is applied to the inner tube through a mandrel that has an annular radially-expandable o-ring seal capable of sealing, when expanded, against the inner surface of the inner tube. Holes through the wall of the inner tube are confined to positions near the ends of the blank so that in readiness for the first expansion stage, the seal of the mandrel can be advanced axially inwardly to an inboard position beyond the holes. In this way, internal fluid pressure can be applied only to the inner tube during the first expansion stage because the holes that communicate with the outer tube are then outboard of the seal.

In the first expansion stage of EP 0627272, the blank is placed in a first die cavity and the mandrel is advanced into the inner tube until the seal is beyond the holes. Advancing the mandrel to this extent also flares the ends of the inner and outer tubes by virtue of a distally-tapering frusto-conical surface of the mandrel, situated proximally with respect to the seal. The flared ends cooperate with locating surfaces on the die to locate the blank with respect to the die cavity during subsequent expansion.

The seal is then expanded so that internal fluid pressure can be applied through the mandrel to the inner tube, causing the inner tube to apply pressure in turn to the outer tube. Continued and increasing application of internal fluid pressure expands the inner and outer tubes toward the forming surface of the first die cavity. Expansion ceases when the outer tube conforms to the shape of that forming surface and the inner tube conforms to the shape of the outer tube. This forms an intermediate blank.

The intermediate blank of EP 0627272 is then ready for a second expansion stage, which is similar to the expansion process of US 5170557. Thus, the intermediate blank is placed in a second die cavity whose forming surface allows space for radially-outward deformation of the outer tube. Again, the flared ends locate the intermediate blank with respect to the second die cavity during subsequent

expansion. Hydraulic fluid is supplied under pressure to the interior of the inner tube through a mandrel that is forced into sealing engagement with the flared end of the inner tube. The holes near the ends of the inner tube wall are now exposed to the fluid pressure within the inner tube; as in US 5170557, those holes expose the inner surface of the outer tube to the fluid pressure while balancing pressure on the inner and outer surfaces of the inner tube. As a result, the outer tube lifts radially off the inner tube to be formed against the forming surface of the second die cavity and an annular gap is opened up between the inner and outer tubes. Finally, the flared ends of the workpiece are cut off.

EP 0627272 suffers from most of the problems of US 5170557 and introduces other problems:

• Again, creating holes through the wall of the inner tube involves additional manufacturing operations and the holes remain in the finished component.

• As the end portions of the inner and outer tubes are pressed together, there is no provision for differential thermal expansion between the inner and outer tubes in use of the component.

• The flared ends of the inner and outer tubes lock to the die under pressure and so reduce the ability of the tube material to flow axially inwardly as their central portions expand radially. This limits the wall thickness that can be formed and the extent of deformation that is possible. Nevertheless, the tubes will still try to shorten as material is drawn axially inwardly from their ends by the radial expansion of their central portions, and under the inward axial thrust exerted through the mandrels to maintain the seal during the second expansion stage. Also, as any material of the outer tube flows axially inwardly in the second expansion stage, it will apply axial inward pressure to the inner tube too. These factors again risk unwanted deformation of the inner tube, such as buckling, particularly at any weak points such as the locations of the holes.

• The o-ring seals used in the first expansion stage are susceptible to wear and damage, for example from the edges of the holes past which the seals must move on each insertion of the mandrels. Also, radially-acting o-ring seals are

not well supported axially and cannot sustain very high pressures. This reduces the maximum fluid pressure that can be applied to the blank, hence limiting the wall thickness that can be formed and the degree of deformation and intricacy that can be achieved in the finished component.

• A considerable length may need to be cut off the ends of the workpiece after the second expansion stage to create a usable component. This is wasteful of material.

US 6519851 to Daimler-Benz is a recent example of the use of a double-walled tubular intermediate blank in which radial holes are disposed near an end of the inner tube. Again, in the second expansion stage, hydraulic fluid is supplied under pressure to the interior of the inner tube through a mandrel that is forced into sealing engagement with an end of the inner tube. Again, the radial holes expose the inner surface of the outer tube to the fluid pressure while balancing pressure on the inner and outer surfaces of the inner tube. The main difference over EP 0627272 is that the mandrel is forced inwardly during the second expansion stage to promote inward flow of material This axial inward pressure risks buckling or other unwanted deformation of the inner tube at weak points such as the radial holes. The effect is exacerbated because the inner and outer tubes are locked together at their ends, so that inward axial force is applied by the outer tube to the inner tube during the second expansion stage when the outer tube expands radially and shortens axially.

Finally, Benteler has proposed a solution in US 5836065 that does away with radial holes in the inner tube. In this proposal, the ends of the inner and outer tubes are first flared together on the outside of a first die cavity, in which initial simultaneous forming of both tubes takes place with internal fluid pressure being applied to the inner tube. The flared end of the outer tube overlaps the flared end of the inner tube and so extends radially outside and axially beyond the flared end of the inner tube. In the second expansion stage, the tubes are placed in a second die cavity and a mandrel seals within the overlapping flared end of the outer tube, outboard of the flared end of the inner tube. In doing so, the mandrel flares the outer tube a little further than the inner tube to create a gap between the flared ends of the inner and outer tubes. When hydraulic fluid is introduced under pressure through the mandrel, that fluid is forced through the gap and between the inner and outer tubes to expand the outer tube away from the inner tube.

Again, the flared ends of the tubes reduce the ability of the tube material to flow axially inwardly as the central portions of the tubes expand radially, hence limiting the wall thickness that can be formed and the extent of deformation that is possible. Moreover it is not clear how fluid will flow between the tubes in the die cavity from the gap between the flared ends of the tubes: the gap tapers to nothing where the tubes are pinched together at the inner margin of their flared ends, and internal fluid pressure acting on the inner tube will tend to force the tubes together at that point. It appears that fluid can only flow out of the gap between the flared ends of the tubes by deflecting the inner tube inwardly, in an uncontrolled manner, against the internal pressure that acts on the inner tube until the pressures equalise on the inner and outer surfaces of the inner tube.

It is against this background that the present invention has been devised.

Broadly, the invention contemplates a method of forming a component having a plurality of walls, the method comprising: placing a blank opposite a forming surface of a die, which blank comprises an outer wall and an inner wall in mutual contact between an inner side of the outer wall and an outer side of the inner wall, wherein the forming surface allows space for deformation of the outer wall away from the inner wall; and applying fluid pressure to the inner side of the outer wall to separate the outer wall from the inner wall and to form the outer wall against the forming surface while applying substantially equal fluid pressure to inner and outer sides of the inner wall, thus defining a gap between the inner and outer walls; wherein fluid pressure is applied to the outer wall through an aperture defined in an overlap portion of the outer wall extending outwardly beyond an edge of the inner wall to an edge of the outer wall; characterised in that an outward area of the overlap portion is restrained against deformation under said fluid pressure; and an inward area of the overlap portion is permitted to deform under said fluid pressure to allow fluid pressure to be applied between the inner and outer walls.

Preferably, the overlap portion of the outer wall is opposed to a peripheral region of the forming surface, the peripheral region comprising: an outward section fitting closely against the overlap portion to restrain local deformation of the opposed area of the outer wall under said application of fluid pressure; and an inward section spaced from the overlap portion to promote local deformation of the opposed area of the outer wall under said application of fluid pressure.

Suitably, the edges of the inner and outer walls lie opposed to the peripheral region, whose inward section may be wider than the outer section. For example, the inward and outward sections of the peripheral region may be separated by a step formation such as a ramp. At least one of the inward and outward sections of the peripheral region preferably has a straight wall, and the area of the inner wall opposed to the inward section of the peripheral region preferably lies generally parallel to the opposed area of the inward section.

Said edge of the outer wall is preferably opposed to the outward section of the peripheral region, and the outward section of the peripheral region extends outwardly beyond said edge of the outer wall. Similarly, said edge of the inner wall is preferably opposed to the inward section of the peripheral region, and the inward section of the peripheral region extends inwardly beyond said edge of the inner wall.

Advantageously, fluid pressure is provided via a mandrel that seals with inward axial force against said edge of the outer wall. For optimum location, the mandrel is preferably a sliding fit with an outward section of the peripheral region extending beyond said edge of the outer wall. It is also preferred that the mandrel applies lateral force to said edge of the outer wall, transverse to the direction of the inward axial force. To this end, the mandrel preferably has a distally-tapered edge co-operable with said edge of the outer wall, and is arranged to force said edge of the outer wall against the outward section of the peripheral region.

To drive flow of material during forming, the mandrel is preferably driven to maintain inward axial force against said edge of the outer wall to feed the material of the outer wall inwardly as the outer wall is formed against the forming surface.

In the method of the invention, the outer wall is advantageously separated from the inner wall by an inwardly-progressing peeling action starting from said edge of the inner wall and progressing across the area of mutual contact between the inner and outer walls. During that peeling action, fluid pressure is preferably applied continuously to a receding interface between the inner and outer walls.

The method of the invention may involve a preliminary forming operation that comprises: placing the blank opposite a preliminary forming surface; and applying fluid pressure to the inner side of the inner wall causing the inner wall to bear against the inner side of the outer wall, thereby deforming the inner and outer walls toward

the preliminary forming surface until the outer wall conforms to the shape of the preliminary forming surface and the inner wall conforms to the shape of the outer wall.

During the preliminary forming operation, fluid pressure is preferably provided via a mandrel that seals with inward axial force against said edge of the inner wall. Advantageously for good location, that mandrel may be a sliding fit with the outer wall. Again, that mandrel may apply lateral force to said edge of the inner wall, transverse to the direction of the inward axial force. To that end, the mandrel preferably has a distally-tapered edge co-operable with said edge of the inner wall to, for example, forces the edge of the inner wall against the inner side of the outer wall. If the mandrel is driven to maintain inward axial force against said edge of the inner wall, it helps to feed the material of the inner and outer walls inwardly as the inner and outer walls are deformed under said fluid pressure.

Where performed, the preliminary forming operation is preferably performed at a substantially higher fluid pressure than the subsequent forming operation that deforms the outer wall away from the inner wall. Moreover it is preferred that the fluid pressure used in the subsequent forming operation is insufficient to deform the combined thicknesses of the inner and outer walls.

Within the inventive concept, the invention can also be expressed as a two-stage method of forming a component having more than one wall, comprising: placing a blank comprising an outer wall and an inner wall opposite a first forming surface; in a first forming operation to produce an intermediate blank, applying internal fluid pressure to the inner wall causing the inner wall to bear against the outer wall whereby the inner and outer walls are deformed toward the first forming surface until the outer wall conforms to the shape of the first forming surface and the inner wall conforms to the resulting inner shape of the outer wall; placing the intermediate blank opposite a second forming surface allowing space for further deformation of the outer wall; and in a second forming operation, applying internal fluid pressure to the outer wall to form the outer wall against the second forming surface while applying substantially equal fluid pressure to both sides of the inner wall, thus defining a gap between the inner and outer walls; wherein in the second forming operation, internal fluid pressure is applied to the outer wall through an aperture defined in an overlap portion of the outer wall extending beyond an inward margin of the inner wall to an outward margin of the outer wall; characterised in that an outward part of the overlap

portion is restrained against deformation under said fluid pressure; and an inward part of the overlap portion is permitted to deform under said fluid pressure to allow fluid pressure to be applied between the inner and outer walls.

In the preferred embodiment of the invention illustrated herein, the blank is tubular, with the inner wall being defined by an inner tube and the outer wall being defined by an outer tube around the inner tube.

The method of the invention preferably further comprises severing the overlap portion of the outer wall level with, or inboard of, the edge of the inner wall. Severing the overlap portion suitably leaves behind a marginal area of the inner wall that was opposed to the inward section of the peripheral region of the forming surface. This creates a joint that can accommodate differential thermal expansion between the inner and outer walls in use of the component.

Preferably, the aperture is defined by an axial gap bordered by said edge of the inner wall. The aperture may also, or alternatively, be defined by a radial gap bordered by said edge of the inner wall.

The invention extends to a blank or a die adapted for use in the methods of the invention and to a forming apparatus adapted to operate in accordance with the methods of the invention.

For example, a die in accordance with the invention suitably comprises a forming surface for forming a blank by allowing space for deformation of an outer wall of the blank away from an inner wall of the blank, wherein a peripheral region of the forming surface comprises: an outward section adapted to fit closely against the outer wall of the blank to restrain local deformation of the opposed area of the outer wall under fluid pressure; and an inward section spaced from the outer wall to promote local deformation of the opposed area of the outer wall under said fluid pressure, the inward section communicating with a major portion of the forming surface that is spaced further from the outer wall of the blank and is disposed inwardly of the inward section.

A forming apparatus of the invention combines this die with at least one mandrel for sealing co-operation with a blank opposed to a forming surface of the die and for applying fluid pressure to the blank.

The invention also encompasses a component having a plurality of walls and made in accordance with the methods of the invention or with the blank or the die or the forming apparatus of the invention.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a sectional view of a tubular double-walled blank in a first die ready for first-stage expansion of the inner and outer tubes;

Figure 2 is a sectional view of the blank and first die of Figure 1 after first- stage expansion of the inner and outer tubes to form an intermediate blank;

Figure 3 is a sectional view of the intermediate blank of Figure 2 in a second die ready for second-stage expansion of the outer tube;

Figure 4 is an enlarged detail view of an end portion of the intermediate blank in the second die of Figure 3, before second-stage expansion;

Figure 5 is a sectional view of the intermediate blank and second die of Figure 3 after second-stage expansion of the outer tube; and

Figure 6 is an enlarged detail view corresponding to Figure 4 but after second-stage expansion of the outer tube in accordance with Figure 5.

The dimensions shown in the drawings are merely for the purpose of illustration and do not limit the scope of the invention. Also, the drawings can be regarded as sectional views through a die and its die cavity or as a plan view of a die half that co- operates with a matching die half (not shown) to define a complete die cavity.

Referring firstly to Figure 1 , a first die 10 contains a die cavity 12 that defines a first forming surface 14. The die cavity 12 includes an elongate enlarged central region 16 that is slightly curved along its length to produce a component with similar general curvature. The die cavity 12 also includes peripheral regions defined by cylindrical channels 18 that lead from the opposed ends of the enlarged central region 16 to openings 20 in opposed external faces 22, 24 of the die 10. In the example shown,

the channels 18 are of circular cross-section but they need not be of that cross- sectional shape.

A double-walled elongate tubular blank 26 comprises an inner tube 28 defining an inner wall nested within and in close contact with an outer tube 30 defining an outer wall. The tubes 28, 30 may be of stainless steel or of any other suitable material. They are of circular cross-section in this example: other cross-sections are possible. The blank 26 is slightly curved to fit within the correspondingly-curved enlarged central region 16 of the die cavity 12, whereupon end portions of the blank 26 are received snugly within the channels 18 to locate the blank 26 in the die 10. The enlarged central region 16 of the die cavity 12 allows space for radially-outward deformation of the opposed central portion of the inner and outer tubes 28, 30 under internal fluid pressure. Engagement of the end portions of the blank 26 within the channels 18 locates the blank 26 in the die 10 during that radial expansion.

The outer tube 30 of the blank 26 extends the full length of the blank 26 and when placed in the die 10, terminates in the channels 18 slightly inboard of the opposed external faces 22, 24 of the die 10. The inner tube 28 is notably shorter than the outer tube 30 and is disposed substantially centrally with respect to the length of the outer tube 30. Consequently, at each end of the blank 26, an overlap portion 32 of the outer tube 30 extends beyond an axially-inward edge 34 defined by an end of the inner tube 28 to an axially outward edge 36 defined by the corresponding end of the outer tube 30. The inner tube 28 nevertheless extends beyond the enlarged central region 16 of the die cavity 12 to terminate in the channels 18 inboard of the ends of the outer tube 30.

In a first forming operation, the blank 26 is placed in the die cavity 12 as described and the die 10 is closed, for example by clamping together mirror-image halves of the die 10 that co-operate to define the die cavity 12. Mandrels 38, 40 are then inserted into the openings 20 in the opposed external faces 22, 24 of the die 10, and from there into the open ends of the outer tube 30 that are supported within the channels 18. The mandrels 38, 40 are of circular cross-section and are a close sliding fit within the ends of the outer tube 30. Each mandrel 38, 40 has a flat distal face 42 encircled by a chamfered frusto-conical sealing edge 44 that tapers distally.

Hydraulic actuators (not shown) force the mandrels 38, 40 into sealing engagement with the opposed ends of the inner tube 28. The mandrels 38, 40 stabilise the wall of

the inner tube 28, which is restrained against expansion by the surrounding channels 18 of the die cavity 12 acting via the outer tube 30. The frusto-conical sealing edges 44 of the mandrels 38, 40 flare the ends of the inner tube 28 slightly, engaging those ends with the inner surface of the outer tube 30 and forcing both tube walls against the inner surface of the surrounding channel 18.

An excellent, stable seal results, enabling the application of very high fluid pressure to the interior of the inner tube 28. The use of such high pressure allows forming of thick-walled components if required and, of course, eases forming of components having two walls or, even, more than two walls. It also enables substantial deformation of the blank 26, suiting the production of intricate components.

One of the mandrels 38, shown to the left in Figure 1 , has a central longitudinal duct 46 for injecting a forming liquid such as water into the interior of the inner tube 28. The other mandrel 40 merely applies opposing thrust and seals the other end of the inner tube 28. However that mandrel 40 has a central longitudinal duct 48 to evacuate air through a valve (not shown) as the inner tube 28 fills with forming liquid injected by the other mandrel 38. It is of course possible for both mandrels 38, 40 to provide for liquid injection and air exhaust if required.

As the mandrel 38 injects forming liquid to apply internal fluid pressure to the inner tube 28, the surrounding peripheral regions of the die cavity 12 defined by the associated channel 18 restrains the ends of the inner and outer tubes 28, 30 against radial expansion in order to maintain the seal of the inner tube 28 with the mandrel 38. The other end of the inner tube 28 is kept sealed to the associated mandrel 40 in similar fashion.

Internal fluid pressure applied through the mandrel 38 to the inner tube 28 causes the inner tube 28 to apply pressure in turn to the inner surface of the outer tube 30. Continued and increasing application of internal fluid pressure expands the inner and outer tubes 28, 30 together toward the forming surface 14 of the die cavity 12. Expansion ceases when the outer tube 30 conforms to the shape of the forming surface 14 and the inner tube 28 conforms to the resulting internal shape of the outer tube 30. This forms an intermediate blank 50 as shown in Figure 2.

As the central portion of the blank 26 undergoes expansion during the first forming operation, the mandrels 38, 40 continue to apply inward axial pressure to the ends of

the inner tube 28. This axial thrust exerted by the mandrels 38, 40 forces the ends of the inner tube 28 toward the enlarged central region 16 of the die cavity 12. Where they are locked to the outer tube 30 by the aforesaid engagement, the ends of the inner tube 28 carry the end portions of the outer tube 30 with them.

The channels 18 are therefore axial infeed zones in which the thrust exerted by the mandrels 38, 40 promotes axially inward flow of material from the end portions of the blank 26 toward the enlarged central region 16 of the die cavity 12 as the central portion of the blank 26 expands within the die cavity 12. This supports the flow of material from the end portions of the blank 26 that is drawn axially inwardly by expansion of the central portion of the blank 26 under internal fluid pressure. This flow of material is important to resist excessive local thinning of the blank 26 that may otherwise lead to failure of the component when forming or in use, particularly if the component is intricate in shape.

The intermediate blank 50 is then ready for a second forming operation shown in Figures 3 to 6. Here, the intermediate blank 50 is placed in a second die 52 whose die cavity 54 defines a second forming surface 56. Again, the cavity 54 of the second die 52 includes an elongate enlarged central region 58 that is slightly curved along its length, matching the general curvature of the die cavity 12 of the first die 10. The die cavity 54 also includes peripheral regions defined by cylindrical channels 60, 62 that lead from the opposed ends of the enlarged central region 58 to openings in the opposed external faces 64, 66 of the die. The channels 60, 62 again receive the end portions of the intermediate blank 50 in a snug fit that locates the intermediate blank 50 with respect to the die cavity 54.

The most obvious difference between the second die cavity 54 shown in Figures 3 to 6 and the first die cavity 12 shown in Figures 1 and 2 is that the enlarged central region 58 of the second die cavity 54 is wider than the corresponding central region 16 of the first die cavity 12. The increased width of the second die cavity 54 allows space for radially-outward deformation of the outer tube 30 of the intermediate blank 50 in the second forming step as will be described. Otherwise, the first and second die cavities 12, 54 are of similar shape. However this is a simple example for the purpose of illustration: in practice, the shape of the second forming surface 56 could be considerably different in detail to that of the first forming surface 14. For instance, the second forming surface could be shaped to impart relief dents or flats to give

clearance for fastenings or other structures in use, or to create platforms for supporting the component in use.

A less obvious but much more important difference between the second die cavity 54 and the first die cavity 12 lies in the inner surface shape of the cylindrical channel 60.

As best shown in the enlarged detail view of Figure 4, the channel 60 leading axially inwardly from the external face 64 of the second die 52 has a stepped longitudinal section. The other channel 62 at the other end of the die 52 can be similarly shaped but this description will now concentrate upon channel 60 shown to the left in Figures 3 to 6.

Specifically, a step is defined by a shallowly-curved frusto-conical ramp formation 68 in the inner surface of the channel 60. The ramp formation 68 flares out between a relatively narrow outer section 70 of the channel 60 and a relatively wide inner section 72 of the channel 60. Consequently, the ramp formation 68 is disposed between the external face 64 of the second die 52 and the enlarged central region 58 of its die cavity 54. Moreover, the ramp formation 68 lies between the end 34 of the inner tube 28 and the end 36 of the outer tube 30 when the intermediate blank 50 is positioned in the die cavity 54 ready for the second forming operation.

The outer section 70 of the channel 60 extends axially outwardly from the ramp formation 68 to the external face 64 of the second die 52 and has parallel sides. This outer section 70 of the channel 60 receives an end portion of the intermediate blank 50 - specifically an outer end portion of the overlap portion 32 - in a snug fit that helps to locate the intermediate blank 50 with respect to the die cavity 54 in the second forming operation. The outer tube 30 of the intermediate blank 50 again terminates inboard of the external face 64 so that the end 36 of the outer tube 30 lies supported within the narrow outer section 70 of the channel 60.

The inner section 72 of the channel 60 extends axially inwardly from the ramp formation 68 to the enlarged central region 58 of the die cavity 54 and also has parallel sides in this embodiment, although that feature is not critical. The inner tube 28 terminates inboard of the ramp formation 68 so that the end 34 of the inner tube 28 lies within the wider inner section 72 of the channel. As a result, an outer part 74 of the inner section 72 of the channel 60 aligns with an inner part 76 of the overlap portion 32 of the outer tube 30, at which the intermediate blank 50 is single-walled.

Importantly, the relatively wide inner section 72 of the channel 60 defines a radial gap 78 around the single-walled inner part 76 of the overlap portion 32. That gap 78 leaves the inner part 76 of the overlap portion 32 unsupported at the start of the second forming operation, and hence susceptible to radially outward deformation under internal fluid pressure.

The difference in width between the inner and outer sections 72, 70 of the channel 60 may be very small, in the order of tens of microns. So, the step defined by the ramp formation 68 need not be high.

In the second forming operation, the intermediate blank 50 is placed in the cavity of the second die 52 as described and the die 52 is closed, for example by clamping together mirror-image halves of the die 52 that co-operate to define the second die cavity 54. The relatively narrow outer section 70 of the channel 60 locates the intermediate blank 50 with respect to the second die cavity 54. Similar location is effected by the other channel 62 of the die 52. Mandrels 80, 82 are then inserted into the openings in the opposed external faces 64, 66 of the die 52. The mandrels 80, 82 are again of circular cross-section and each mandrel 80, 82 has a flat distal face 84 encircled by a chamfered frusto-conical sealing edge 86 that tapers distally. However in this instance, the mandrels 80, 82 are a close sliding fit within the channels 60, 62 rather than within the ends of the outer tube 30.

Hydraulic actuators (not shown) force the mandrels 80, 82 into sealing engagement with the opposed ends of the outer tube 30. As before, the mandrels 80, 82 continuously apply inward axial thrust to maintain a seal and to promote axially inward flow of the material of the intermediate blank 50 during expansion - but in this case the outer tube 30 only. The mandrels 80, 82 stabilise the wall of the outer tube 30, which is restrained against expansion by the surrounding channels 60, 62 of the die cavity 54. The frusto-conical sealing edges 86 of the mandrels 80, 82 flare the ends of the outer tube 30 slightly, forcing the outer tube wall against the inner surface of the surrounding channel 60, 62.

Again, an excellent, stable seal results, enabling the application of very high fluid pressure to the interior of the outer tube 30. However, the second forming operation will generally require substantially lower internal fluid pressure than the first forming operation, typically about half the pressure of the first forming operation. This is

simply because the second forming operation aims to form only one wall thickness rather than a double wall thickness.

Again, one of the mandrels 80, shown to the left in Figures 3 to 6, has a central longitudinal duct 88 for injecting a forming liquid such as water into the interior of the intermediate blank 50. The other mandrel 82 applies opposing thrust, seals the other end of the intermediate blank 50 and has a central longitudinal duct 90 to evacuate air through a valve (not shown) as the intermediate blank 50 fills with forming liquid injected by the other mandrel 80. Of course, the mandrel 82 could have means for liquid injection if desired.

As the mandrel 80 injects forming liquid to apply internal fluid pressure to the intermediate blank 50, the relatively narrow outer section 70 of the channel 60 restrains the opposed outer part of the overlap portion 32 of the outer tube 30 against radial expansion in order to maintain the seal of the outer tube 30 with the mandrel 80. The other end of the outer tube 30 is kept sealed to the mandrel 82 in similar fashion.

As noted above, the relatively wide inner section 72 of the channel 60 enables the opposed unsupported single-wall inner part 76 of the overlap portion 32 of the outer tube 30 to expand radially under internal fluid pressure. The extent of radial expansion of the outer tube 30 at that location is determined by the height of the step defined by the ramp formation 68 of the channel 60.

It should be noted that the outer tube 30 deforms preferentially here at the single-wall inner part 76 of the overlap portion 32 in relation to the inboard region of the intermediate blank 50 where the wall thicknesses of the inner and outer tubes 28, 30 are superimposed. This is because the same internal fluid pressure is applied to the single wall of the outer tube 30 at the inner part 76 of the overlap portion 32 as to the superimposed double walls of the inner and outer tubes 28, 30 inboard of the inner part 76 of the overlap portion 32. If that pressure is selected as being sufficient to deform a single wall but insufficient to deform a double wall, there is no possibility that the double-wall region will deform before the single-wall region of the intermediate blank 50.

So, when forming liquid is supplied under sufficient pressure to the interior of the intermediate blank 50, the unsupported inner part 76 of the overlap portion 32

exposed to the fluid pressure deforms radially outwardly. Initially this deformation occurs between the ramp formation 68 of the channel and the end 34 of the inner tube 28, until the outer tube 30 conforms to the local shape of the channel 60. As best shown in Figure 6, an annular axial aperture or gap 92 thereby opens between the end 34 of the inner tube 28 and the part of the outer tube 30 that lies against the ramp formation 68. Moreover, this deformation lifts the outer tube 30 off the end 34 of the inner tube 28, defining a radial gap 94 between the inner and outer tubes 28, 30 that unlocks the engagement between the tubes 28, 30 that was effected by the mandrels 38, 40 before the first forming operation.

As the outer tube 30 continues to deform between the start point shown in Figures 3 and 4 and the end point shown in Figures 5 and 6, the outer tube 30 lifts off the inner tube 28 with an efficient and effective 'peeling 1 action caused by direct application of fluid pressure to the interface between the tubes 28, 30. In that progressive action, the area of contact at the interface between the inner and outer tubes 28, 30 recedes axially inwardly along the channel 60. Quickly, a narrow annular conduit 96 is formed between the tubes 28, 30 that extends axially inwardly from the radial gap 94 at the end of the inner tube 28 to the enlarged central portion 58 of the die cavity 54. The peeling action then continues across the enlarged central portion 58 to free the central portion of the outer tube 30 from the central portion of the inner tube 28 for expansion against the second forming surface 56 defined by the cavity 54 of the second die 52.

Once the annular conduit 96 is fully formed between the inner and outer tubes 28, 30 in the channel 60, forming liquid flowing from the conduit 88 of the mandrel 80 can pass through the axial gap 92, the radial gap 94 and the conduit 96. This liquid then feeds the continued expansion of the outer tube 30 until the second forming operation is complete. Thus, the outer tube 30 lifts radially off the inner tube 28 to be formed against the second forming surface 56 of the die cavity 54 and an annular void 98 is opened up between the inner and outer tubes 28, 30. That void 98 can extend the full length of the intermediate blank 50 apart from its outward extremities where the intermediate blank 50 is supported by the channels 60, 62 of the second die 52.

As soon as any part of the inner tube 28 is no longer supported as the outer tube 30 peels away and expands during the second forming operation, pressure is balanced on the inner and outer surfaces of the inner tube 28. This ensures that the inner tube

28 will not deform during the second forming operation, allowing dimensional certainty. It must also be noted that the axial gap 92, the radial gap 94, the annular conduit 96 and the void 98 de-couple the inner and outer tubes. In this way, the material of the outer tube 30 can flow inwardly under internal expansion and the inward thrust of the mandrels 80, 82 without deforming, or being restricted by, the inner tube 28.

When removed from the second die at the end of the second forming operation, the ends of the workpiece can be cut off with minimal waste. If cut along the line 100 of Figure 6, at or just inboard of the end 34 of the inner tube 28, a telescopic sliding joint is created between the concentric ends of the inner and outer tubes 28, 30. This joint elegantly accommodates differential thermal expansion between the inner tube 28, which will be very hot in use, and the outer tube 30 which will be substantially cooler in view of the insulating air within the void 98 that retains the heat of exhaust gas flowing within the inner tube 28 in use. If it is preferred to seal the void, however, it is a simple matter to insert a sealant into the radial gap 94 between the concentric ends of the inner and outer tubes 28, 30.

The skilled reader will appreciate that the invention very elegantly solves all of the problems of the prior art discussed above. There is no need to create holes in the inner tube, hence avoiding unnecessary manufacturing operations and weakening of the blank or the finished component. The absence of holes in the inner tube also avoid disruption to gas flow in exhaust systems and widens the potential applications of the component. Excellent seal performance allows high internal fluid pressure to be applied. The inner and outer walls of the component are free to move relative to each other during the second forming operation and, if required, in use. In particular, inward axial flow of material during radial expansion is eased and that flow is more easily promoted by the thrust of the mandrels.

The embodiment described above employs an annular axial aperture or gap between the end of the inner tube and the part of the outer tube that lies against the ramp formation. In a broad sense, the invention does not require the axial aperture or gap to extend the full width or circumference of the blank. For example, a longitudinal flute would be enough to admit forming liquid to feed expansion of the outer tube throughout the second forming operation. However it may be advantageous if the axial aperture or gap extends the full width or circumference of the blank because this decouples the inner and outer walls at that location.

Whilst described herein in relation to a double-walled component made in two forming operations, it would be possible to apply the principles of the invention to the manufacture of a component having three or more walls. For example, the walls of a triple-walled component could be expanded in three successive forming operations with three dies and three sets of successively wider mandrels.

It is also possible to use the same die for the successive forming operations, adapting the die cavity with inserts or providing movable die walls to define the first, second or subsequent forming surfaces used in those forming operations.

Components having two or more walls may be used in other applications. For example, a double-walled pipe could be used as a heat exchanger in aircraft engines where hot lubricating oil circulating in a jacket between inner and outer tubes transfers heat to kerosene fuel flowing within the inner tube. Double-walled pipes can also be used to give redundancy or additional protection, or in any application where multiple fluid flows are necessary but space is at a premium.