Forming large titanium parts

Abstract

Apparatus for superplastically forming large parts from titanium comprising: a furnace (10) having an interior, the inside surface of the furnace (10) being contoured and finished so as to form a mold for the part to be superplastically formed; means (70) for heating the interior of the furnace (10); and a supply (60) of an inert gas. The surface of the mold is adapted to receive a substantially unformed titanium blank (80). The heating means (70) is adapted to heat the titanium blank (80) to the required temperature for superplastic forming. The supply (60) of the inert gas is operable to exert a pressure onto the surface of the titanium blank (80) furthermost from the surface of the mold such that the inert gas causes the titanium blank (80) to deform and take up the shape of the mold, thereby forming the required part. The heating means (70) includes one or more electrical induction coils positioned in the furnace (10) so as to be on the side of the titanium blank (80) furthermost from the mold when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mold, such that the or each induction coil induces a current in the titanium blank (80) which is heated thereby.

Description

[0001] This invention relates to the forming of large parts from titanium. More particularly, it relates to the forming of such parts with external dimensions of the order of meters.

[0002] Superplastic forming of titanium involves heating titanium sheet to a specified temperature (usually in the region of 950 degrees centigrade) and blow forming the heated titanium by using an inert gas, such as argon. This process is usually performed in either an industrial press or a furnace. However, the finite dimensions of these two items impose restrictions on the size of the parts that can be superplastically formed within them. Consequently, parts too large to be accommodated in existing presses or furnaces have to be fabricated from two or more smaller sections that have been superplastically formed individually. This increases both the time and cost associated with the forming of large parts from titanium. Furthermore, it may lead to dimensional inaccuracies in the finished part owing to its fabrication from a number of smaller constituent sections: unacceptably large dimensional variations resulting from the aggregate of the normal individual dimensional variations.

[0003] U.S. Pat. No. 5,209,093 discloses apparatus for superplastically forming large cylindrical structures, comprising:

[0004] (i) a generally cylindrical ceramic die having a radially inwardly facing surface defining the contours of a structure to be formed from a cylindrically rolled metal sheet positioned radially inwardly therefrom;

[0005] (ii) radiant heating means positioned radially inwardly of the rolled metal sheet for heating a medial portion thereof to a predetermined temperature at which it achieves a superplastic condition; and

[0006] (iii) means for introducing a pressurised gas to force the medial portion of the rolled metal sheet radially outwardly against the ceramic die when the medial portion of the rolled metal sheet has been heated to a superplastic condition.

[0007] The ceramic die may be formed of segments that mate along radial planes, for example 120 degree segments. This allows the die to be disassembled to enable removal of titanium structures formed therein which cannot be removed in any other way.

[0008] Radiant heating means tend to heat indiscriminately any object that lies in the path of radiation emitted from the heating means. Although this will result in the desired heating of the rolled metal sheet if the sheet is in the path of the emitted radiation, it may also result in objects being unnecessarily heated, such as the die or end portions of the apparatus. As a consequence, a greater amount of energy will be needed to operate the radiant heating means than would be the case if only the rolled metal sheet were heated. A corollary of this is that, for any given operating power of the heating means, the rolled metal sheet will take longer to reach the required temperature under the action of radiant heating means than would be the case if only the rolled metal sheet were heated.

[0009] It is an object of this invention to address these disadvantages.

[0010] According to a first aspect of this invention there is provided a method of superplastically forming large parts from titanium, comprising the steps of:

[0011] a) constructing a furnace, the inside surface of the furnace being contoured and finished so as to form a mould for the part to be superplastically formed;

[0012] b) introducing a substantially unformed titanium blank into the furnace;

[0013] c) heating the titanium blank to the required temperature for superplastic forming;

[0014] d) applying inert gas to the surface of the titanium blank furthermost from the surface of the mould, wherein the gas pressure is such that it causes the titanium blank to deform and take up the shape of the mould, thereby forming the required part, characterised in that the heating is by one or more electrical induction coils positioned on the side of the titanium blank furthermost from the mould such that the or each induction coil induces a current in the titanium blank which is heated thereby.

[0015] The substantially unformed blank may be substantially cylindrical and the or each induction coil may be positioned inside said substantially cylindrical blank.

[0016] The or each induction coil may be orientated such that its axis is substantially parallel to the axis of said substantially cylindrical blank.

[0017] There may be a plurality of such induction coils, the induction coils being distributed so that collectively they define an annulus that is substantially coaxial with the substantially cylindrical blank.

[0018] According to another aspect of this invention there is provided apparatus for superplastically forming large parts from titanium comprising: a furnace having an interior, the inside surface of the furnace being contoured and finished so as to form a mould for the part to be superplastically formed; means for heating the interior of the furnace; and a supply of an inert gas, wherein the surface of the mould is adapted to receive a substantially unformed titanium blank, the heating means is adapted to heat the titanium blank to the required temperature for superplastic forming when that titanium blank is positioned in the furnace for superplastic forming in the mould, and the supply of the inert gas is operable to exert a pressure onto the surface of the titanium blank furthermost from the surface of the mould when that titanium blank is positioned in the furnace for superplastic forming in the mould such that the inert gas causes the titanium blank to deform and take up the shape of the mould, thereby forming the required part, characterised in that the heating means include one or more electrical induction coils positioned in the furnace so as to be on the side of the titanium blank furthermost from the mould when that titanium blank is positioned in the furnace for superplastic forming in the mould, such that the or each induction coil induces a current in the titanium blank which is heated thereby.

[0019] The or each induction coil may extend the full height of the surface of the mould.

[0020] The substantially unformed blank may be substantially cylindrical and the or each induction coil may be positioned inside said substantially cylindrical blank when that titanium blank is positioned in the furnace for superplastic forming in the mould.

[0021] The or each induction coil may be orientated such that its axis is substantially parallel to the axis of said substantially cylindrical blank when that titanium blank is positioned in the furnace for superplastic forming in the mould.

[0022] There may be a plurality of such induction coils, the induction coils being distributed so that collectively they define an annulus that is substantially coaxial with the substantially cylindrical blank when that titanium blank is positioned in the furnace for superplastic forming in the mould.

[0023] The contoured inside of the furnace surface may be formed of a ceramic material. The contoured inside of the furnace surface may be formed of a metallic or nonmetallic material that may be magnetiseable or non-magnetiseable.

[0024] The furnace may be constructed of a plurality of furnace walls or furnace wall sections.

[0025] The inert gas may be argon.

[0026] To assist the deformation of the titanium blank under the action of the inert gas, a vacuum may be applied to the side of the blank that is adjacent the mould.

[0027] The titanium blank may be partially formed, before it is introduced into the furnace.

[0028] Specific embodiments of the present invention are now described by way of example and with reference to the accompanying drawings, of which:

[0029] FIG. 1 is a transverse cross-section of a furnace for forming a large diameter titanium tube by superplastic forming;

[0030] FIG. 2 is an enlarged view of the detail within the circle labelled A of FIG. 1;

[0031] FIG. 3 is a plan view of the apparatus of FIG. 1;

[0032] FIG. 4 is a plan view of another furnace, wall sections of the furnace being in mutual abutment; and

[0033] FIG. 5 is another plan view of the other furnace, the wall sections being spaced apart.

[0034] FIGS. 1 and 3 show a cylindrical furnace 10 which is constructed predominantly from three furnace wall sections 30, each moulded from the same ceramic material. Collectively, the three sections 30 form the curved surfaces of a cylinder whose longitudinal axis lies vertically.

[0035] The inside surface of the cylinder formed by the three ceramic wall sections 30 is shaped and finished so as to form a mould suitable for forming titanium.

[0036] The furnace 10 rests on a base plate 40, which in turn is situated on legs 50. The centre of the base plate 40 includes a lower argon gas inlet aperture 60 which is connected to a controllable supply of argon gas.

[0037] FIG. 1 also shows a heating assembly 70. This assembly consists of a circular top plate 71 with a central top aperture 72 and several induction coils. The induction coils are attached to the underside of the top plate 71 and are orientated so as to lie vertically and are circumferentially spaced from one another so as to form a circular array that is coaxial with the circular top plate 71. The induction coils themselves are not shown, but each of them has a support member 73, of which two are shown in FIG. 1. An annular bottom plate 74 is attached to the lower end of the support members 73 and is also coaxial with the top plate 71.

[0038] The heating assembly 70 may be inserted into and withdrawn from the cylindrical furnace 10. FIG. 1 shows the heating assembly 70 inserted into the furnace 10. In this position, the top plate 71 of the heating assembly 70 rests on and is coaxial with the cylinder formed by the three ceramic wall sections 30. The induction coils extend the full height of the inside surface of the sections 30.

[0039] The method of operation is described now with reference to FIG. 1. A substantially unformed titanium blank 80 (in the form of a cylinder fabricated from a length of titanium sheet) is placed around the annulus defined by the induction coils. The positioning of the blank 80 relative to the heating assembly 70 is such that the top edge of the blank 80 contacts an O-ring seal situated in the top plate 71 of the heating assembly 70 and the bottom edge of the blank 80 contacts an O-ring seal situated in the bottom plate 74 of the heating assembly 70. FIG. 2 shows the O-ring seal 90 included in the top plate 71. This provides airtight contact between this plate 71 and the blank 80. A similar arrangement is included in the bottom plate 74. Although FIG. 2 shows the top edge of the blank 80 in contact with the top plate 71, it is envisaged that, when the blank is at ambient temperature, a gap should exist between the top edge of the blank 80 and the top plate 71 to allow for thermal expansion of the blank 80.

[0040] The heating assembly 70 and the blank 80 are then inserted into the centre of the furnace 10. To facilitate this insertion, two of the ceramic wall sections 30 are pivotably mounted, thereby serving as doors to the furnace 10. This arrangement is shown in FIG. 3, the pivotably mounted sections being labelled 30a, 30b.

[0041] Closing the two doors 30a, 30b causes the top and bottom edges of the inside surface of all three ceramic sections 30 to abut the top and bottom edges respectively of the titanium blank 80. The inclusion of seals 90 in the top and bottom edge of the inside surfaces of the sections 30 provides for an airtight contact against the blank 80. This completes the arrangement shown in FIG. 1. The titanium blank 80 is firmly held in position by the airtight abutment of the three circular sections 30 on its outside surface and the airtight abutment of the top plate 71 and bottom plate 74 on its inside surface.

[0042] Argon gas is introduced into the furnace 10 through the lower aperture 60 in the base plate 40. The argon gas replaces air that was previously inside the furnace by forcing that air out of the top aperture 72. The top aperture 72 is then closed by any known means, such as a bung or a cut-off valve, and the introduction of argon gas is ceased.

[0043] The electrical induction coils are then operated. A current flows in each coil and this results in a respective associated magnetic field being set up around the coil. The current in each coil is in the same direction, thus causing each respective field to be orientated in the same direction. As a result, a substantially toroidal magnetic field is set up around the annular arrangement of the coils. Magnetic flux of this field passes, in an axial direction, through the titanium blank 80 that is adjacent and surrounds the annular arrangement of the coils, thereby causing a current to be induced in the titanium blank 80. This induced current results in the titanium blank 80 being heated. Positioning the annular arrangement of coils inside the titanium blank 80 does not optimise the induction heating effect of the coils as far as heating the titanium blank 80 is concerned. This is because the flux density outside the annular arrangement of coils is less than the flux density inside the annular arrangement of coils. Positioning the annular arrangement of coils inside the titanium blank 80 therefore puts the titanium blank 80 in a weaker part of the field. However, by positioning the annular arrangement of coils inside the titanium blank 80, it is possible to more accurately predict how the titanium blank will be heated, as each of the coils is at a known and easily verifiable distance from the surface of the blank 80. Furthermore, the coils may be more easily replaced in the event of failure, or altered in order to achieve different heating characteristics. One such alteration may be to move some of the turns of one or more coils apart and others of the turns of the or each coil together, in order to achieve a different heating profile of the titanium blank 80.

[0044] Using induction is advantageous as compared with using radiant heating means. Induction coils may be used to avoid heating parts of the apparatus that need not be heated, for example the circular top plate 71 or the base plate 40, if such parts are fabricated from non-magnetiseable material. The use of induction coils therefore results in improved efficiency and a shorter heating time for any given operating power of the induction coils. A shorter heating time is advantageous in reducing the thermal stress to which components of the apparatus are subjected. This may result in prolonging the useful life of the components, or in the use of cheaper components. For example conventional O-ring seals may be used to provide a gas-tight seal whilst permitting movement of the blank due to thermal expansion. It will be appreciated that high temperature, mechanical-type seals may hinder such expansion and increase the likelihood of the blank 80 buckling.

[0045] Once the titanium blank 80 has been heated to the required temperature for superplastic forming, more argon gas is introduced into the furnace via the aperture 60. This is continued such that the pressure of the argon on the inside surface of the titanium blank 80 is greater than the pressure against the outside surface of the blank 80, the two spaces being sealed from one another in an airtight fashion as previously described. This pressure difference causes the titanium blank 80 to deform outwards and take up the shape of the mould comprised of the inside surface of the three ceramic sections 30. To further increase the pressure difference across the two surfaces of the titanium blank 80, it is envisaged that a vacuum may be applied to the outside surface of the titanium blank 80. Techniques for achieving this are known to the skilled addressee.

[0046] The heating is then stopped, allowing the superplastically formed part to cool prior to removal from the furnace 10 and the heating apparatus 70.

[0047] In an alternative embodiment, shown in FIG. 4, the furnace wall sections 30 are not pivotably mounted. Instead, the furnace wall sections 30 are surrounded by a cylindrical outer wall 100. The cylindrical outer wall 100 is coaxial with the wall sections 30 and has an internal diameter that is greater than the external diameter of the wall sections 30. Thus, when the wall sections 30 are in mutual abutment, there is an annular space 110 between the wall sections 30 and the cylindrical outer wall 100. Six actuators 120, only three of which are shown, are mounted on the outer surface of the outer cylindrical wall 100. A pair of actuators 120 are provided for each wall section 30: an upper actuator 120 and a lower actuator 120.

[0048] Each pair of actuators 120 are positioned so that their lines of action pass radially through a respective one of the wall sections. Each actuator includes an actuator rod 125. The actuator rods 125 of each pair of actuators 120 pass through the cylindrical outer wall 100 and mechanically engage the respective wall section 30. Operation of the actuators 120 causes the wall sections 30 to be moved radially between a position in which they are in mutual abutment and a position in which they are spaced apart. FIG. 4 shows the wall sections 30 in mutual abutment. It will be appreciated that the heating and forming operations would be performed in this position. FIG. 5 shows the wall sections 30 spaced apart. It will be appreciated that it is in this position that the titanium blank would be inserted into the furnace 10, the heating assembly 70 (not shown) would be inserted into and withdrawn from the furnace 10, and the formed titanium part (not shown) would be withdrawn from the furnace 10.

Claims

1. A method of superplastically forming large parts from titanium, comprising the steps of:

a) constructing a furnace (10) with an inside surface which is contoured and finished so as to form a mould for the part to be superplastically formed;

b) introducing a substantially unformed titanium blank (80) into the furnace (10);

c) heating the titanium blank (80) to the required temperature for superplastic forming;

d) applying inert gas to the surface of the titanium blank (80) furthermost from the mould, wherein the gas pressure is such that it causes the titanium blank (80) to deform and take up the shape of the mould, thereby forming the required part, characterised in that the heating is by one or more electrical induction coils positioned on the side of the titanium blank (80) furthermost from the mould such that the or each induction coil induces a current in the titanium blank (80) which is heated thereby.

2. A method according to claim 1, wherein the or each induction coil extends the full height of the mould.

3. A method according to claim 1 or claim 2, wherein the substantially unformed blank (80) is substantially cylindrical and the or each induction coil is positioned inside said substantially cylindrical blank (80).

4. A method according to claim 3, wherein the or each induction coil is orientated such that its axis is substantially parallel to the axis of said substantially cylindrical blank (80).

5. A method according to claim 3 or claim 4, wherein there is a plurality of such induction coils, the induction coils being distributed so that collectively they define an annulus that is substantially coaxial with the substantially cylindrical blank (80).

6. A method according to any one of the preceding claims, wherein the furnace (10) is constructed from a plurality of furnace walls or furnace wall sections (30) which collectively form the inside surface of the furnace (10).

7. A method according to claim 6, wherein the or each furnace wall or furnace wall section (30) is formed of a ceramic material.

8. A method according to any one of the preceding claims, wherein the inert gas is argon.

9. Apparatus for superplastically forming large parts from titanium comprising: a furnace (10) having an interior, the inside surface of the furnace (10) being contoured and finished so as to form a mould for the part to be superplastically formed; means (70) for heating the interior of the furnace (10); and a supply (60) of an inert gas, wherein the surface of the mould is adapted to receive a substantially unformed titanium blank (80), the heating means (70) is adapted to heat the titanium blank (80) to the required temperature for superplastic forming when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould, and the supply (60) of the inert gas is operable to exert a pressure onto the surface of the titanium blank (80) furthermost from the surface of the mould when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould such that the inert gas causes the titanium blank (80) to deform and take up the shape of the mould, thereby forming the required part, characterised in that the heating means (70) include one or more electrical induction coils positioned in the furnace (10) so as to be on the side of the titanium blank (80) furthermost from the mould when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould, such that the or each induction coil induces a current in the titanium blank (80) which is heated thereby.

10. Apparatus according to claim 9, wherein the or each induction coil extends the full height of the mould.

11. Apparatus according to claim 9 or claim 10, wherein the substantially unformed blank (80) is substantially cylindrical and the or each induction coil is positioned in the furnace (10) so as to be inside said substantially cylindrical blank (80) when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould.

12. Apparatus according to claim 11 wherein the or each induction coil is orientated such that its axis is substantially parallel to the axis of said substantially cylindrical blank (80) when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould.

13. Apparatus according to claim 11 or claim 12, wherein there is a plurality of such induction coils, the induction coils being distributed so that collectively they define an annulus that is substantially coaxial with the substantially cylindrical blank (80) when that titanium blank (80) is positioned in the furnace (10) for superplastic forming in the mould.

14. Apparatus according to any one of claims 9 to 13, wherein the furnace (10) is constructed of a plurality of furnace walls or furnace wall sections (30), which collectively form the inside surface of the furnace (10).

15. Apparatus according to claim 14, wherein the or each furnace wall or furnace wall section (30) is formed of a ceramic material.

16. Apparatus according to any one of claims 9 to 15, wherein the inert gas is argon.