WO2011012906A1

WO2011012906A1 - Heating apparatus and method

Info

Publication number: WO2011012906A1
Application number: PCT/GB2010/051261
Authority: WO
Inventors: Frank Richard Hall; Peter Julian Spence; Christopher A. Kenward
Original assignee: The University Of Wolverhampton
Priority date: 2009-07-30
Filing date: 2010-07-30
Publication date: 2011-02-03
Also published as: GB201203401D0; GB2485323B; GB2485323A8; GB2485323A

Abstract

A heating apparatus (100) arranged, in use, to accommodate an electrically conductive work-piece (300) comprising two or more parts (302,304) of which at least one part is a sheet of conductive material (302) of up to 3mm thick, and the apparatus comprising at least one coil (102) having a first face (106) arranged, in use, to be adjacent, the electrically conductive work-piece (300) of which the part adjacent the first face is a sheet (302) of conductive material of up to 3mm thickness, and a second face (108) on an opposite side of the coil (102) from the first face (106), and the apparatus (100) also comprising a flux- intensifier (110) which is provided adjacent the second face (108) and the radial outer face (112) and a temperature sensor (410) arranged to measure the temperature of the work- piece (300) wherein, in use, the coil (102) is arranged to have electric current passed therethrough and the coil (102) and flux-intensifier (110) are arranged to concentrate the resultant magnetic flux in a localised surface region of roughly between 0.1mm and 0mm diameter of the electrically conductive work-piece (300) in order to heat the two or more parts of the work-piece in the localised region to a substantially uniform temperature suitable for diffusion bonding across the localised region (307) of the two or more parts of the work-piece (300) in a time between roughly a few milliseconds and roughly 5 minutes and wherein the apparatus is arranged to control the temperature according to the temperature sensor (410)

Description

HEATING APPARATUS AND METHOD

Field of the invention

This invention relates and apparatus arranged to locally heat, perhaps rapidly, a work- piece and associated methods. In particular the invention relates to the heating of a conductive work-piece. In particular, but not exclusively, the invention relates to an apparatus and method arranged to rapidly perform diffusion bonding at one or more localised regions of the work-piece. In particular, but not exclusively, this invention relates to an apparatus arranged to rapidly join sheets of metal and/or metal alloys at one or more localised regions using diffusion bonding. The apparatus may also relate to a testing apparatus and method. Background of the invention

Certain applications, such as aero-space fabrication, heat exchangers, laminate die tooling and cellular material fabrication demand what may be termed high quality and high integrity bonding of parts that can be provided by processes such as diffusion bonding. However, prior art systems for providing such bonding are costly, usually requiring high pressure chambers and the like. Furthermore, the processes are slow, thereby increasing costs further, in view of the need to heat the pieces being bonded to a high temperature and pressure and maintaining this over what would generally be thought of as a long period of time in order to allow atoms in the material in the pieces being bonded to migrate across the interference boundary.

Summary of the invention

According to a first aspect of the invention there is provided a heating apparatus that can accommodate an electrically conductive work-piece comprising two or more parts of which at least one part being a sheet of conductive material, and comprising at least one coil having a first face arranged, in use, to be proximal to the electrically conductive work-piece of which the part proximal the first face being a sheet of conductive material, and a second face on an opposite side of the coil from the first face, and the apparatus also comprising a flux-intensifier which is provided adjacent the second face and the radial outer face wherein, in use, the coil is arranged to have electric current passed therethrough and the coil and flux-intensifier are arranged to concentrate the resultant magnetic flux in a localised region of a work-piece adjacent the first face of the coil in order to rapidly heat the localised region. Such an apparatus is advantageous as it can rapidly generate localised heating within a conducting work-piece situated adjacent the first face.

Such an apparatus is believed to be advantageous as the intensifier concentrates the magnetic flux in a localised region thereby heating a work-piece, in which current can be induced, in that region. It has been found the arrangement of the flux-intensifier and coil focuses magnetic flux generated by the coil to rapidly and locally heat a work-piece adjacent a first face of the coil in an area substantially below a central region of the coil.

The skilled person will appreciate that the depth of penetration of the magnetic flux into the work-piece, which may be characterised by the skin depth, determines the depth below the surface of the work-piece to which the localised region extends. The penetration depth increases as the frequency of the coil current decreases, but the resultant magnetic flux varies more slowly and induces a smaller voltage and therefore a smaller heating effect in the localised area of the work-piece. Larger coils may increase the heating effect for greater penetration depths but reduce the degree of localisation of the heating. The coil may be hollow and may be arranged to have a cooling fluid passed therethrough. Such an arrangement is convenient since it allows a higher current to be passed through the coil. The skilled person will appreciate that the flux-intensifier concentrates current flow as well as magnetic flux in regions of the coil on which it is not present. As such the heating in those regions of the coil may be mitigated by the use of a cooling fluid within the coil.

The flux-intensifier may be further provided adjacent to at least an annular outer region of the first face of the coil. Such an arrangement is advantageous because the magnetic flux may then be further concentrated in a localised region of the work-piece and so a higher temperature may be achieved rapidly. Such an arrangement is also advantageous because the same magnetic flux density in the localised region may be achieved at a lower coil current. Conveniently, the flux intensifier is arranged to heat a localised surface region between roughly 3mm and 10mm in diameter. Other embodiments may be arranged to heat a localised surface region between roughly 5mm and 7mm in diameter. Other embodiments may be arranged to heat a localised surface region of roughly 6mm. The skilled person will appreciate that values in between these ranges may also be suitable.

The arrangement of the flux intensifier will be such that the localised region is rapidly heated to a temperature suitable for the process being undertaken between roughly a few milliseconds and 5 minutes. In further embodiment, the arrangement of the flux intensifier is such that the localised region is rapidly heated to a high temperature suitable for the process being undertaken between roughly a second and 1 minute. In yet a further embodiments, the localised region is rapidly heated to a high temperature suitable for the process being undertaken in roughly 30 seconds. Thus, the skilled person will appreciate that the time periods discussed herein may be thought of as being rapidly heated and that times within these ranges are also possible.

In some embodiments, the flux intensifier may not extend for the entire radial outer face of the coil. For example, the flux intensifier may extend to cover roughly 50% of the radial outer face. In other embodiments, the flux intensifier may extend for roughly any of the following percentages of the radial outer face: 75%, 90%, 100%.

Moreover, in some the flux intensifier may not extend to cover the entire second face of the coil. For example, the flux intensifier may extend to cover roughly 50% of the second face of the coil. In other embodiments, the flux intensifier may cover roughly any of the following percentages of the second face: 75%, 90%, 100%.

The apparatus may also comprise an indenter arranged to apply localised pressure to a work-piece that is being heated. Such an indenter is particularly advantageous when the apparatus is being used to diffusion bond the work-piece since increased pressure within the work-piece may accelerate that rate at which bonding occurs.

In one embodiment, the indenter is arranged to pass through the coil such the coil substantially surrounds the indenter. Such an arrangement is convenient since localised pressure may be applied in a region at which the heating of the work-piece is occurring.

The indenter is advantageously fabricated from a non- electrically conducting material such that eddy current heating does not occur within the indenter due to proximity with the coil.

Conveniently, the indenter is fabricated from a ceramic material.

Conveniently, the indenter is arranged to apply pressures of between roughly 50OkPa and 3.5Mpa. Other embodiments may be arranged to apply pressures of between roughly IMPa and 3MPa. Other embodiments may be arranged to apply a pressure of roughly 2MPa. The skilled person will appreciate that values in between these ranges may also be suitable. In some embodiments, which may be in addition to the indenter or instead of the indenter, a pressure housing may be provided around the work-piece and arranged to be pressurized to a pressure above the atmosphere surrounding the work-piece. Such embodiments, are believed advantageous, particularly if the work-piece comprises sheets of material which are being diffusion bonded, since it will tend to force the sheets together thereby facilitating close contact between the sheets.

A work-piece may comprise sheets of electrically conductive material, and further the sheets of material may be made from different materials. The sheets may be arranged in a stack or layered. The depth of penetration of the magnetic flux into the work-piece may be sufficient such that the localised region extends through the sheet closest to the coil, but the skilled person will appreciate that the depth of penetration may further extend into one or more sheets adjacent to the first sheet in the stack.

Typically, the sheets are from roughly 0.1mm to roughly 5mm thick. However, other embodiments may be arranged to heat a sheet from roughly 0.2mm to roughly 3mm thick. Yet further embodiments may be arranged to heat a sheet of from roughly 0.5mm to roughly lmm thick. The skilled person will appreciate that other thicknesses within these ranges are also possible. In one embodiment the apparatus is arranged to heat a sheet of up to roughly 3mm.

The apparatus may also comprise a bladder in which a work-piece may be enclosed during heating thereof. Conveniently, the bladder is sealably closeable in a gas impermeable manner such that an environment is provided around the work-piece. Embodiments having such a bladder are believed to be advantageous not only through the provision of a suitable environment but also for the unconnected advantage of ease of handling of a heated work-piece. The bladder may be in addition to the pressure housing. As such, the bladder may be placed within the pressure housing. In other embodiments, the pressure housing may provide the bladder. Hereinafter, reference to bladder may be replaced mutatis mutandis with reference to pressure housing. The bladder may be fabricated from a silica material, or other such material that is capable of withstanding the temperature of the heated work-piece. In some embodiments, the bladder is provided from a woven material which may be particularly advantageous in embodiments which are not pressurized above the surrounding atmosphere.

The apparatus may be arranged to control the atmosphere within the bladder. The apparatus may be arranged to substantially or at least partially evacuate the bladder and/or fill the bladder with an environment comprising a gas, or a mixture of gases which may provide an inert atmosphere (such as argon); or may be one that is non- oxidizing and/or non-nitriding (such as Nitrogen and/or Hydrogen); or the like. Thus, the pressure within the bladder may be below the surrounding environment.

Embodiments of the invention may provide the indenter in addition to reducing the inside of the bladder to below the pressure of the surrounding environment.

The skilled person will appreciate that in embodiments in which the pressure within the bladder is less than that outside the bladder a positive pressure will be exerted on a work-piece provided within the bladder. Such a pressure may be in addition or instead of pressure applied by an indenter and may be advantageous in embodiments which are diffusion bonding parts within the work-piece. The bladder may be flexible and/or arranged to maintain at least a partial vacuum in order to maximise the positive pressure that may be exerted on a work-piece provided within the bladder. Further the bladder may comprise an insulating layer, which may be an inside layer. Such an arrangement may be advantageous to prevent or reduce heat transfer from the work-piece to the bladder. For example the bladder may comprise an outside PTFE (polytetrafluoroethylene) layer and an inside insulation layer. Such an arrangement may be advantageous in providing a low cost and flexible bladder which may be used to provide pressure to the work-piece particularly for diffusion bonding.

In some embodiments, the inside of the bladder is filled with an environment comprising a gas, or a mixture of gases, that provide a non-reactive atmosphere (which may or may not be an inert atmosphere). For example, the atmosphere may be one that is non- oxidizing and/or non-nitriding. In a further example, a vacuum may be provided. In some embodiments a gas may be introduced in conjunction with the bladder being evacuated and such an embodiment may be a way of providing the desired environment within the bladder. The gas introduced into the bladder may be any of those discussed above.

A temperature sensor may be provided and arranged to take the temperature of the work-piece during heating thereof. The temperature sensor may be a thermocouple. Alternatively, the temperature sensor may be a non-contact sensor, such as for example, an IR (Infra-red) optical sensor.

Such a temperature sensor is advantageous as it helps to ensure that the temperature in the work-piece can be accurately controlled. The skilled person will appreciate that it is desirable to increase the temperature in a work-piece to increase thermal energy but that exceeding phase transition temperatures may result in detrimental changes within the material.

The apparatus may comprise a controller arranged to control the current applied to the coil. The temperature sensor may be arranged to provide a temperature input signal to the controller. In some embodiments, the controller may be arranged to operate a feed- back loop in which the current applied to the coil is controlled according to the temperature input signal.

The controller may be arranged to raise the temperature of a work-piece to a predetermined temperature. Subsequently, the controller may be arranged to maintain the work-piece at, or at least around, the predetermined temperature for a predetermined time.

The flux-intensifier may be fabricated from a Soft Magnetic Composite (SMC) material which is also sometimes known as a 3D composite in which particles of magnetiseable material are held within an insulating matrix. In one particular embodiment, the flux- intensifier is fabricated from the material Ferrotron™. Such an arrangement is advantageous as it can help to reduce, perhaps significantly, the generation of eddy currents within the flux-intensifier. Ferrotron™ will be widely recognized by the skilled person but is available from companies such as Inductoheat (Tewkesbury) Ltd of Unit 23 Cotteswold Dairy Industrial Estate. Northway Lane. Tewkesbury. Glos GL20 8JE (WWW.inducto-heat.co.uk/fluxtrol.htm). The coil is conveniently fabricated from copper. More specifically the coil may be made from OFC (Oxygen Free Copper) and/or OFHC (Oxygen Free High-Thermal Conductivity Copper).

The coil may be arranged such that the coolant fluid is water and more specifically, the coolant fluid is de-mineralised water. It is advantageous if the cooling fluid has a high thermal conductivity but a low viscosity.

The apparatus may be arranged to move the work-piece relative to the coil (or visa versa) in order to heat the work-piece in further areas. In such an arrangement, the work- piece may be stepped through a plurality of positions and paused at each position. The controller may be arranged to pause the relative work-piece coil position in order to heat the work-piece for a predetermined time, until a predetermined action occurs (such as the formation of a diffusion bond), or the like. The apparatus may be arranged such that movement is along a predetermined path.

The coil may be arranged to have a voltage of substantially in the range 1 OO volts to 400 volts applied across it. Some embodiments may be arranged to have a voltage of substantially in the range of 200 volts to 300 volts applied across the coil. Some embodiments may be arranged to have a voltage of roughly 250volts applied across the coil. Voltages in between these ranges may also be used.

The coil may be arranged to have a current of substantially in the range of 50 amps to 150 amps passed through it. Some embodiments may be arranged to have a current of substantially in the range of 75amps to 125amps passed through the coil. Some embodiments of the invention may be arranged to have a current of roughly lOOamps passed through the coil. Currents in between these ranges may also be used. The coil may be arranged to be driven by an AC voltage having a frequency of substantially in the range of 15kHz to 45kHz. Some embodiments may be arranged to drive the coil with an AC voltage having a frequency of substantially in the range of 23kHz to 37kHz. Some embodiments of the invention may be arranged to drive the coil with an AC voltage having a frequency of roughly 3OkHz. Frequencies in between these ranges may also be used.

The apparatus may be arranged to perform diffusion bonding within the work-piece. According to a second aspect of the invention there is provided a diffusion bonding apparatus arranged to cause a diffusion bond in a work-piece comprising two plates of material, wherein the apparatus comprises at least one coil having a first face arranged, in use, to be proximal a work-piece and a second face on an opposite side of the coil from the first face, and the apparatus also comprising a flux-intensifier which is provided adjacent the second face and the radial outer face wherein, in use, the coil is arranged to have electric current passed therethrough and the coil and flux-intensifier are arranged to concentrate the resultant magnetic flux in a localised region of a work-piece adjacent the first face of the coil in order to heat the localised region. According to a third aspect of the invention there is provided a method of heating a work-piece comprising the following steps:

1. positioning the coil of an apparatus according to the first aspect of the invention adjacent a work-piece;

2. applying an AC voltage across the coil to induce a current within the work- piece thereby heating the work-piece.

The method may be used to manufacture cellular materials from a work-piece comprising a plurality of electrically conductive layers by selectively bonding those layers. Subsequent to bonding the structure may be inflated such that the resultant material has a cellular structure. The method may be used to bond two more work pieces together. For example, the method may be used to create a diffusion bond or assist a diffusion bond between the work pieces or to otherwise adhere the work pieces. The method may be arranged to hold the work-piece at around a phase transition temperature of the material from which the work-piece is fabricated.

According to a fourth aspect of the invention there is provided a method of creating a diffusion bond in a work-piece comprising two sheets of material, the method comprising the following steps:

1. positioning the coil of the apparatus of the second aspect of the invention adjacent the work-piece; 2. applying an AC voltage across the coil in order to induce a current in the work-piece thereby heating the work-piece;

3. holding the work-piece at a predetermined temperature for a predetermined length of time.

Such a method may create a localised bond in an area substantially underneath a central region of the coil. A skilled person will appreciate that such a method may be readily extended to create a diffusion bond in a multilayer work-piece or a work-piece comprising more than two sheets or layers of material by repeating the steps of the method. A third and any further sheets or layers may be present at the start of the method and/or may be added between repeats of the steps of the method.

In one embodiment of the method the penetration depth of the magnetic flux, and consequently the greatest depth at which a localised bond may form, may be typically 3mm. A skilled person will appreciate that extending the method to create a diffusion bond in a multilayer work-piece may be advantageous in allowing a work-piece of final thickness greater than such a penetration depth to be prepared by repeating steps of the method as layers are added to the work-piece. In other embodiments, the method may be arranged such the penetration depth is roughly any of the following depths: lmm, 2mm, 4mm, 5mm, 10mm (or any depth in between these).

The method may comprise holding the work-piece in a bladder. The bladder may contain an inert atmosphere.

The method may also comprise applying localised pressure to the work-piece by using an indenter to press the work-piece.

The method may further comprise moving the relative position of the coil and the work- piece in order to create further bonds. In one embodiment it is the work-piece that is moved relative to the coil. However, in other embodiments, it may be the coil that is moved relative to the work-piece.

The method may be arranged to bond Titanium sheets. However, the method may also be applicable to other materials such as stainless steel, chrome moly steel, aluminium, or any other suitable metal or metal alloy. The method may also be applicable to materials such as polymers, glasses or ceramics that are or have been made conductive, for example by doping. In particular, but not exclusively, the method may be applicable to doped ceramics, such as antimony-doped tin oxide.

According to a fifth aspect of the invention there is provided a method of diffusion bonding a work-piece comprising two sheets of material, the method comprising inductively heating a localised region of the work-piece and mechanically applying localised pressure to that region using a member.

The member may be termed an indenter.

According to a sixth aspect of the invention there is provided a kit comprising an apparatus according to the first or second aspects of the invention together with a bladder arranged to house a work-piece. The bladder may be as described in relation to earlier aspects of the invention.

According to a seventh aspect of the invention there is provided a heating apparatus comprising at least one coil having a first face arranged, in use, to be proximal to a work-piece and a second face on an opposite side of the coil from the first face, and the apparatus also comprising a flux-intensifier which is provided adjacent the second face and the radial outer face wherein, in use, the coil is arranged to have electric current passed therethrough and the coil and flux-intensifier are arranged to concentrate the resultant magnetic flux in a localised region of a work-piece adjacent the first face of the coil in order to heat the localised region.

The skilled person will appreciate that a feature discussed in relation to any one of the above aspects of the invention may be applied mutatis mutandis to any of the other aspects of the invention.

Brief description of the drawings

There now follows, by way of example only, a detailed description of embodiments of the invention of which:

Figure 1 shows, schematically, a cross section through a first embodiment of the invention (along line AA of Figure 2); Figure 2 shows, schematically, a plan view of the embodiment of Figure 1 having a flux intensifier removed for reasons of clarity;

Figures 3a and 3b schematically show an embodiment adjacent a work-piece including how lines of flux emanate from a coil thereof;

Figures 3c and 3d schematically show an embodiment adjacent a work-piece with alternative coil and flux intensifier arrangements; Figures 4a and 4b show, schematically, a further embodiment of the invention adjacent a work-piece with alternative bladder arrangements;

Figure 5 shows a flow chart of how the apparatus of Figure 4 may be used;

Figure 6 shows, schematically, a further embodiment of the invention adjacent a work-piece;

Figure 7a shows a cross-section through a good bond generated by an apparatus according to an embodiment of the invention; and

Figure 7b shows a failed bond caused due to excessive heat within the materials. Detailed description of the drawings

A heating apparatus 100 is shown in Figure 1, in cross section, and in plan view in Figure 2. The heating apparatus 100 comprises a coil 102 which is hollow. The coil and the internal void 104 are, in this embodiment, each square in cross section. However, and as shown by example in Figures 3c and 3d, other embodiments of the invention may provide other cross sectional shapes and indeed the coil and void need not have the same cross sectional shape.

Looking at Figure 1, the coil 102 has a first face 106 which is the lower-most face as viewed in the Figure. The coil 102 also has a second face 108 which is the upper-most face of the coil as viewed in the Figure and as such it will be seen that the second face 108 is on an opposite side of the coil from the first face 106.

Again looking at Figure 1, a flux-intensifier 110 is provided adjacent to the second face 18 and radial outer face 112 of the coil 102. Thus, it will be seen that the inner radial face 114 and the first face 106 do not have, or at least do not substantially have a portion of the flux-intensifier 110 adjacent. For reasons of clarity, the flux- intensifier 110 has been omitted from Figure 2. The void 104 within the coil 102 is arranged to have cooling fluid passed therethrough and circulated in order to remove heat when the apparatus 100 is in use. As such inlet 116 and outlet 118 pipes are connected to the coil 102 in order that the fluid can be passed around the void 104.

In some embodiments, the fluid used to cool the coil 102 is de-mineralised water. However, other fluids may be suitable and may for example be oils (such as mineral oil or silicone oil) or any other fluid having a suitable thermal capacity and suitably low viscosity.

It should also be noted that in the embodiment that is being described, the coil does not extend for 360° and there is a gap 120 in the coil 102 in a region in the vicinity of the inlet 116 and outlet 118 pipes. The gap 120 allows an electric circuit to be formed by the inlet pipe 116, the coil 102 and the outlet pipe 118. The skilled person will appreciate that, with the coil arrangement shown, if there were no gap then electric current would not flow around the coil 102. In other embodiments, it may be possible to provide other arrangements of coil such as in a helical arrangement, etc. in which case the coil may be provided around a full 360° of rotation.

In some embodiments, the coil 102 is fabricated from OFHC Copper (Oxygen Free high Conductivity Copper). This material will be familiar to the skilled person as providing Copper of a particularly high conductivity and chemical purity and which typically have an Oxygen content of less than roughly 0.001% Oxygen. However, in other embodiments, the coils may be provided from other materials.

In some embodiments, the flux-intensifier 110 is fabricated from what may be termed a 3D composite material, or a SMC (Soft Magnetic Composite) material as exemplified by the material Ferrotron™. Such materials may be summarised as comprising a matrix of insulation coated magnetic material. However, in other embodiments, the flux- intensifier may be constructed from laminations. The skilled person will appreciate that the use of an SMC or laminations is arranged to help prevent the occurrence of eddy currents in the flux-intensifier which would otherwise be caused by current flowing through the coil 102.

Figures 3 a and 3b show an example of how the apparatus described in relation to Figures 1 and 2 may be used to heat a work-piece 300. For reasons of clarity, only the coil 102 and flux-intensifier 110 are shown in the Figures, but the skilled person will appreciate that the remainder of the apparatus described in relation to Figures 1 and 2 will also be provided. Further, it should be noted that the work-piece 300 comprises two conducting sheets 302, 304 adjacent one another.

In use, an alternating current is passed around the coil 102 which causes a magnetic flux to be generated from the coil. Proximity of the coil 102 and the work-piece 300 causes the generated flux to pass through the work-piece 300. The coil 102 and flux-intensifier 110 are arranged so that the magnetic flux generated by the current is concentrated in a localised region 307 of the work piece 300. Since the sheets 302, 304 constituting the work-piece 300 conduct, the magnetic flux causes an electric current to flow in the sheets at the localised region thereby generating heat therein.

The flux-intensifier 110 prevents magnetic flux from emanating from the coil 102 in the areas that it covers the coil 102. As such, more flux is concentrated underneath the coil 102 from the first face 106. This concentration causes localised heating in the work- piece 300 in a region 307 substantially underneath a centre region 306 of the coil 102.

The magnetic flux lines are schematically shown in Figure 3b and it will be seen that there is a concentration of flux within the region 307 within the work-piece 300. In the localised region 307, the distribution of magnetic flux may be particularly concentrated, and therefore the heating effect strongest, within a substantially annular or toroidal zone rather than at a centre region of the localised region. However in thermally and electrically conductive work-pieces the stronger heating effect within the substantially annular or toroidal zone may be rapidly conducted into the centre of the localised region because the volume at the centre of the substantially annular or toroidal zone is small. Typically, the coil is on the order of lmm to 2mm away from the surface of the work- piece 300. The skilled person will appreciate that larger distances (such as roughly 3mm, 4mm, 5mm or the like) may be used but that the proximity of the coil to the work- piece determines the coupling and therefore the heating that the work-piece undergoes. As such the apparatus may be arranged to control the distance between the first face of the coil and the work-piece.

Figure 3c shows an alternative arrangement of the coil and flux-intensifier wherein the flux-intensifier 111 may be further provided adjacent to at least an annular outer region of the first face of the coil 102 wherein the radial distance from a central region of the coil is in the region of the radius of the coil. This arrangement may further increase the concentration of the magnetic flux in a localised region of the work-piece. For example, the flux intensifier may be provided at radial distances of greater than 50% of the radius. In other embodiments, the flux intensifier may be provided at greater than 75% of the radius, or greater than 90% of the radius.

Figure 3d shows an alternative arrangement of the coil and flux-intensifier wherein the coil 103 and void 105 have a rectangular cross section so as to reduce the diameter of the coil while maintaining the coil bore diameter, and the flux-intensifier 113 having a complementary shape. This arrangement may further increase the concentration of the magnetic flux in a localised region of the work-piece.

Other embodiments may have other cross sections of coil. For example, circular cross sections may also be used.

Figure 3 is used to show the principles of how the coil 102 may be used to heat a work- piece 300. Figures 4 and 5 are used to explain details of how the coil 102 may be used to diffusion bond two sheets of electrically conductive material, such as metallic material, such as titanium.

Figure 4a shows a work-piece 400 also composed of two sheets 402, 404. The work- piece 400 is maintained within a bladder 406 of flexible, gas impermeable, material. The bladder 406 is arranged to withstand the temperatures to which the work-piece 400 is heated and an example of a suitable material is a woven silica based fabric. Alternative, or additional, embodiments may employ a PTFE (polytetrafluoroethylene) sheet or other suitable material. Seals 408 are provided (shown schematically in the Figure) in order to seal the bladder 406 from the surrounding environment. In some embodiments, the inside of the bladder 406 is filled with an inert atmosphere, such as argon.

The bladder 406 of Figure 4a may provide a pressure housing in which the internal environment is above the pressure of the surrounding atmosphere in order to assist in pushing the two sheets 402, 404; the two sheets having had the air removed from between them before being placed within the bladder.

Figure 4b shows another embodiment of the bladder in which it has been formed as an envelope 450 and as such, the seals 408 may be replaced by a bonding, or other joining, of the material of the bladder 406. Other parts of Figure 4b are referenced with similar reference numerals. In this embodiment, the bladder 450 and the apparatus may be placed within a pressure chamber 454 which is able to exert a pressure greater than that inside the bladder 450 onto the bladder. Again, this can assist the close coupling of the two sheets 402, 404.

In other embodiments in which the bladder 406, 450 is flexible, the internal environment is below the pressure of the surrounding atmosphere. In such an embodiment, the surrounding atmosphere may then compress the work-piece.

Further, Figure 4a shows that the bladder has a substantial thickness such that the bladder itself has insulating properties to insulate the heated work-piece. In other embodiments (such as exemplified by Figure 4b), the bladder may be fabricated from a thinner material. In such embodiments, the bladder is likely to be lined with an insulating material placed between the work-piece and the bladder to thermally insulate the bladder from the work-piece; it will be appreciated that the work-piece undergoes significant heating during operation of the coil 102. Such insulating material is shown at 452 in Figure 4b. For reasons of clarity the bladder 450 is shown significantly away from the work- piece 400 and insulating material 452 in Figure 4b. This is not likely to be the case especially in embodiments in which the inside of the bladder 450 is at a lower pressure than the surrounding atmosphere.

In further embodiments, the inside of the bladder is evacuated. In some embodiments, the gap between the two plates 402, 404 is evacuated whilst the bladder is itself filled with an inert atmosphere. Evacuating between the plates can help increase the contact pressure between the plates 402, 404 and thus increase the rate of diffusion bonding.

Also shown in the Figure is a thermocouple 410 which is used to measure the temperature of the work-piece 400. In other embodiments a temperature probe other than a thermo-couple may be used. For example an optical IR (infra-red) probe may be used to remotely sense the temperature of the work-piece. Indeed, in some embodiments, no temperature sensor may be used. The skilled person will appreciate that the temperature sensor 410 shown in this Figure may be applied mutatis mutandis to any of the other embodiments. The thermocouple 410 is also connected to a controller 412 which is arranged to control the current flowing through the coils 102 and as such can provide a feedback loop which is used to control heating of the work-piece 400.

The method of how the apparatus in Figure 4 would be used is now described with the aid of the flow chart of Figure 5.

In step 500 the two sheets 402, 404 are placed adjacent one another and placed within the bladder 40. Air is then removed from the bladder and from between the sheets 402, 404 to form the work-piece 400. The bladder 40 which is then filled with an inert atmosphere and sealed in step 502. Next 504, the bladder is placed underneath the coil arrangement. The cooling fluid is turned on in step 506 and circulated through the inlet and outlet pipes and through the coil 102. Subsequent to the circulation of the cooling fluid, an AC current is passed 508 around the coil and is fed to the coil along the inlet and outlet pipes. In the embodiment being described, a voltage of roughly 250volts is applied across the coil 102 and a current of roughly lOOamps at a frequency of roughly 30kHz passes around the coil. The arrangement of the coil and flux-intensifier concentrates the magnetic flux generated by the current at a localised region of the work piece adjacent a centre region of the coil 306. The magnetic flux causes an electric current to flow in the sheets at the localised region thereby generating heat at the localised region.

The temperature of the work-piece is increased until it reaches a predetermined temperature 508 as measured by the temperature probe 410. Once at the predetermined temperature a closed-loop control circuit maintains 510 the work-piece 400 at roughly that temperature by intermittently turning current on and off in the coil 102. The temperature is maintained for a predetermined time 512 after which the work-piece is allowed to cool 514. In some embodiments, when the temperature falls below a predetermined temperature then the work-piece may be re-heated by passing current through the coil 102. In other embodiments, current may simply be passed after a further time has elapsed. The further time may or may not be the same as the predetermined time. In other embodiments, the current within the coil 102 may be varied in order to control the temperature within the work-piece; such an arrangement may be termed direct-current control.

In some embodiments, the predetermined and/ or the further time is roughly 6 seconds. However, in other embodiments, the predetermined and/or the further time may be roughly 10 seconds, 20 seconds, 30 seconds, 60 seconds or any time between. In yet further embodiments, the predetermined time may be of up to 5 minutes. When the sheets 402, 404 are titanium the predetermined temperature is likely to be on the order of roughly 95O⁰C. However, the skilled person will appreciate that other temperatures are possible, particularly if the material of the work-piece is changed. In the predetermined time a diffusion bond (which may be a high integrity diffusion bond) is created in the region of the work-piece 400 adjacent a centre region 306 of the coil 102. If further diffusion bonds are desired 516 between the sheets 402, 404 then the work-piece 400 and the bladder 406 are moved 518 relative to the coil 102 and the heating process repeated. Such relative movement of the work-piece 4— and the coil may be under the control of a CNC (Computer Numerically Controlled) machine.

If no further bonds are needed then the work-piece 400 is removed 520 from the bladder 406. Figure 6 shows a further embodiment of how the apparatus of Figures 1 and 2 may be used to create a diffusion bond in a work-piece 400 comprising two sheets 402, 404. Parts similar to those shown in Figure 4 are denoted with the same reference numerals.

In this embodiment, an anvil 600 is provided below the bladder 406 and arranged to provide a surface against which pressure may be applied.

An indenter 602 comprising a rod is used to apply pressure to the work-piece 400. The coil 102 and flux-intensifier 110 are arranged such that the indenter 602 can pass through a central region of the coil 102 and thereby contact a work-piece adjacent a first face of the coil 102 underneath the central region 306 of the coil.

The indenter is arranged to apply pressure to the work-piece which should increase the rate of diffusion bonding thereby reducing the time predetermined time at which the work-piece is maintained at a predetermined temperature in step 512 of the method. In some embodiments, the indenter 602 is arranged to apply roughly 2MPa to the work- piece 400. Pressure generated by the indenter may be provided in addition to, or instead of, pressure applied by the surrounding atmosphere and/or pressure chamber 454; ie embodiments that use an indenter may not need to rely on pressure differential to apply pressure to the work-piece 300.

In order not to under-go significant heating from the magnetic flux produced by the coil 102 the indenter 602 is fabricated from a non-conducting material, such as a ceramic. Suitable materials include aluminate, alumina and zirconia. The indenter 602 is likely to be exposed to elevated temperatures via conduction due to contact with the work-piece and/or bladder. Figure 7a shows a Scanning Electron Microscope (SEM) cross section through a successful diffusion bond created between the two parts 302, 304 of the work-piece 300. It will be appreciated that because a successful bond has been created the grain structure across the bond as if the resultant bond were a single piece of metal and the bond is consequently not visible. The material shown in Figure 7 is α-phase Titanium alloy.

The skilled person will appreciate that in order to increase the rate at which a diffusion bond occurs it is advantageous to increase the temperature at which the material is held to provide greater thermal energy to atoms. Further, the skilled person will also appreciate that materials often undergo phase changes at known temperatures which if exceeded change the material properties of the material. For example, in the case of Titanium a phase transition between the α phase and the β phase occurs at temperatures of somewhere between 95O⁰C and 1050⁰C depending upon the alloy composition. As such, it is advantageous to heat Titanium alloy close to 95O⁰C in order to expedite bonding without exceeding the phase transition temperature. The selection of 95O⁰C may be viewed as a conservative temperature selection since some alloys may allow further heating before passing the transition temperature.

The skilled person will appreciate that it is difficult to determine the transition temperature for some, if not all, alloys of Titanium. As such, it may be preferred to maintain a conservative temperature to which to heat the work-piece 300. Further, the skilled person will appreciate that for other metals or metal alloys that the transition temperature (should they exhibit one) is likely to be different to that of Titanium alloy. Figures 7a and 7b show Ti6A14V Titanium Alloy which is believed to have a transition temperature of between 982⁰C and 1010⁰C.

Figure 7b shows an SEM through a two parts 302, 304 of a work-piece which has been heated in excess of the β-phase transition and it can be seen that the grain structure of the material has changed and that a bond has not formed.

Moreover, it is believed that the apparatus causes uniform heating across the localised region. That is the temperature across the localised region is relatively constant. If there were variations in the temperature across the localised region it is likely that the SEM of Figure 7a would show regions which had not bonded (through being too cold) or which exhibited a β phase transition and had the grain structure of Figure 7b. However, as can be seen from Figure 7a the grain structure across the bond is relatively constant. For ease the distance of 1 Oμm has been highlighted on Figure 7a as L and on Figure 7b as X, noting that the horizontal scale is slightly different between the two Figures.

Claims

1. A heating apparatus arranged, in use, to accommodate an electrically conductive work-piece comprising two or more parts of which at least one part is a sheet of conductive material, and the apparatus comprising at least one coil having a first face arranged, in use, to be adjacent, the electrically conductive work-piece of which the part adjacent the first face is a sheet of conductive material, and a second face on an opposite side of the coil from the first face, and the apparatus also comprising a flux-intensifier which is provided adjacent the second face and the radial outer face and a temperature sensor arranged to measure the temperature of the work-piece wherein, in use, the coil is arranged to have electric current passed therethrough and the coil and flux-intensifier are arranged to concentrate the resultant magnetic flux in a localised surface region of roughly between 0.1mm and 10mm diameter of the electrically conductive work-piece in order to heat the two or more parts of the work-piece in the localised region to a substantially uniform temperature suitable for diffusion bonding across the localised region of the two or more parts of the work-piece in a time between roughly a few milliseconds and roughly 5minutes and wherein the apparatus is arranged to control the temperature according to the temperature sensor.

2. An apparatus according to claim 1 in which the coil is hollow and which may be further arranged to have a cooling fluid passed therethrough.

3. An apparatus according to claim 1 or 2 which comprises an indenter arranged to apply localised pressure to a work-piece that is being heated and in which the indenter may be arranged to pass through the coil such the coil substantially surrounds the indenter.

4. An apparatus according to any preceding claim which comprises a pressure housing arranged to surround the work-piece and arranged to be pressurized to above atmospheric pressure and/or depressurized to below atmospheric pressure.

5. An apparatus according to any preceding claim in which the apparatus comprises a bladder in which a work-piece may be enclosed during heating thereof.

6. An apparatus according to any preceding claim which comprises a controller arranged to control the current applied to the coil which may be arranged to operate a feed-back loop in which the temperature sensor provides a temperature input signal thereto whereby the controller is arranged to control the current applied to the coil according to the temperature input signal.

7. An apparatus according to any preceding claim in which the flux-intensifier is fabricated from a Soft Magnetic Composite (SMC) material.

8. The apparatus according to any preceding claim in which the work-piece is arranged to move relative to the coil (or visa versa) in order to heat the work-piece in further localised areas to a temperature suitable for diffusion bonding of each localised region of the parts of the work-piece.

9. A method of heating an electrically conductive work-piece comprising the following steps:

1. positioning the coil of an apparatus according any of claims 1 to 8 adjacent a work-piece;

10. A method according to claim 9 in which the work-piece is held at a predetermined temperature for a predetermined length of time.

11. A method according to claim 9 or 10 which creates a diffusion bond within the work-piece of a localised region of the parts of the work-piece in a time between a few milliseconds and 5minutes.

12. A method according to any of claims 9 to 11 further comprising holding the work-piece in a bladder, which may contain a non-oxidising and/or non-nitriding atmosphere.

13. A method according to any of claims 9 to 12 which comprises applying localised pressure to the work-piece by using an indenter to press the work-piece.

14. A method according to any of claims 9 to 13 which comprises moving the work- piece relative to the coil (or visa versa), with the work-piece being stepped through a plurality of positions and paused at each position in order to heat the work-piece in further areas,

15. A kit comprising an apparatus according to claim 1 together with a bladder arranged to house a work-piece.