The present invention relates to bonding of components by solid-state bonding, in which atoms from different components become interspersed across a bonding boundary by solid-state diffusion.

Traditional methods of forming solid-state bonds between plates include Hot Isostatic Pressing diffusion bonding (HIP-DB) and uniaxial pressure diffusion. In both processes, the whole volume of the plates needs to be brought up to the process temperature, and potentially the surrounding environment as well. This means that processing times are often long

(typically 10 ⁵ -10 ⁶ seconds), energy consumption high (driving up cost), and subsequent heat treatment procedures may be needed to correct for undesirable side-effects caused by the bonding (e.g. to reverse undesirable softening of metals). Where processing times are shortened, higher pressures may need to be applied across the bonding surfaces to

compensate, which can lead to undesirable distortion and corresponding inaccuracies in the geometry of the final product. High bonding pressures and distortion effects also limit the extent to which internal cavities may be present in the structure under pressure during bonding (e.g. they may limit the dimensions of internal cavities that are achievable). Finally, apparatus for implementing the above processes tends to be complex, expensive and/or inflexible (e.g. limited to relatively simple geometries).

It is an object of the invention to provide an alternative approach for solid-state bonding which at least partially addresses one or more of the above issues and/or other issues.

According to an aspect of the invention, there is provided a method of bonding, comprising: providing a first component having a first surface; providing a second

component having a second surface; pressing the first component against the second component such that the first surface is directly in contact with the second surface; and passing an electrical current through the first component and the second component to cause localised heating in an interfacial region, the interfacial region being between a bulk of the first component and a bulk of the second component, the localised heating being controlled to cause formation of a solid-state bond between the first component and the second component in the interfacial region, wherein an electrical property dependent on an electrical resistance of the interfacial region is monitored and used to control the localised heating.

Thus, a method is provided in which solid-state bonding is achieved by targeted and highly controlled heating of a localised interfacial region between the components to be bonded (the first component and the second component). This approach means that most of the first component, most of the second component, and the surrounding environment can all be kept at a temperature that is well below the temperature within the interfacial region during the bonding process. Processing times and/or energy consumption can thus be reduced significantly in comparison with alternative approaches that require more widespread heating (e.g. HIP-DB and uniaxial pressure diffusion). Processing times can be reduced to the order of 10 ² seconds for example. A further benefit of the reduced processing time and/or reduced spatial extent of the heating is that additional corrective heat treatments (e.g. to correct for softening) can be avoided or reduced. This is applicable to, for example, precipitation hardened alloys like CuCrZr, which soften when exposed to high processing temperatures for a long time.

In comparison with HIP-DB, the apparatus for implementing the above method can be less complex and costly because there is no need to encapsulate the components to be bonded in a specially made container. In the above method, intimate contact is only required between the first surface and the second surface (i.e. between the first component and the second component).

In comparison with alternative solid-state bonding methods that have low processing times but require high pressures, the pressures needed between the first and second components to implement the above method are considerably lower (e.g. 5 to 50MPa), thereby reducing volumetric distortions and improving accuracy of the final product. The lower pressures also simplifies bonding of complex geometries (including geometries with internal cavities). The method is not limited to forming joints between laps, butts or plates, which can be the case for alternative approaches using higher pressures.

Control of the localised heating based on the electrical property that is dependent on the resistance of the interfacial region allows a high degree of control over the formation of the solid-state bond, without requiring complex additional sensing apparatus. In some embodiments, the electrical property comprises a voltage required to pass the electrical current through the first component and the second component. Monitoring such a voltage can be implemented straightforwardly and efficiently. The monitored voltage provides rapid feedback about the state of the solid state bond due to the dependence of the voltage on the resistance in the interfacial region. In the case where a constant level of electrical current is maintained during the formation of the solid-state bond, it is possible to monitor the resistance of the interfacial region accurately simply by monitoring the voltage required to maintain the current. Alternatively or additionally, a mathematical function, such as a ratio, of the current and voltage may be used that is particularly sensitive to the quality of the bond as it is formed. The ratio of voltage to current, for example, provides a direct measure of the electrical resistance of the interfacial region. As the resistance is correlated with the quality of the solid-state bond, and will reduce as the solid-state bonding progresses (and the quality of the bond increases), the monitoring of the electrical property thus provides a sensitive measure of the extent to which the solid-state bond has been formed. The localised heating can thus be controlled so as to be responsive to the actual state of the solid-state bond, rather than relying on prior experience (e.g. to estimate a duration for the heating process that should normally be sufficient to reach a desired level of bonding), calibration experiments (e.g. running repeated trials at various bond temperatures until a satisfactory result is achieved), or investigative sensing after the bonding process has been completed to assess the quality of the bond. The approach of the present disclosure thus allows solid-state bonding to be implemented with high reliability while minimising a duration of the bonding process. Furthermore, the approach provides a measure of bond quality/completeness that can be material independent. The approach also provides a simple means to measure bond completeness/quality between individual plates in a stack comprising multiple plates.

Alternative approaches based exclusively on temperature control may be restricted by the lower spatial resolution of the temperature measurement.

In an embodiment, a duration of the heating is controlled based on the monitored electrical property, which may include stopping the bonding process when the monitored electrical property moves past a predetermined threshold value. The bonding process may thus be stopped at precisely the point where the solid-state bond has reached a desired level of quality and does not need to be continued further to a predetermined minimum time based on prior trials. Cost and throughput can thus be improved, as well as minimizing side effects of the bonding process, such as undesirable softening in the interfacial region or nearby.

In another embodiment, the controlling of the localised heating comprises controlling a level of the electrical current in response to a determination of how the electrical property is changing as a function of time. This approach can be used to detect whether the solid-state bonding process is progressing abnormally, for example too slowly or too quickly, allowing corrective action to be taken in good time (e.g. by increasing or decreasing the electrical current being driven through the interfacial region). This approach can also be used to detect when a bonding process is nearing completion by sensing when structural properties in the interfacial region are no longer changing significantly with time. In an embodiment, the controlling of the localised heating comprises reducing or stopping the electrical current when it is detected that a rate of change of the electrical property is below a threshold value. Methods of the present disclosure are particularly beneficial in the context of manufacturing devices in which a laminate structure has a plurality of laminated components defining one or more internal cavities (e.g. flow channels for a coolant in a heat sink or heat exchanger). The presence of such cavities makes the structure more vulnerable to damage by distortion during pressing of the first component against the second component. The lower pressures and high level of control provided by methods of the present disclosure reduce or remove the risk of such damage and/or allow a wider variety of internal cavities (e.g. larger or more complex internal cavities) to be formed with low or no risk of damage or distortion to surrounding elements. The region of the bond is defined by the geometry of the interfacial region and where the current is made to flow.

According to an aspect of the invention, there is provided an apparatus for bonding, comprising: a first electrode; a second electrode; a clamping arrangement configured to clamp a first component and a second component between the first electrode and the second electrode, the first component having a first surface and the second component having a second surface and the clamping being such that the first component is pressed against the second component such that the first surface is directly in contact with the second surface; and a heating unit configured to pass an electrical current through the first component and the second component to cause localised heating in an interfacial region between a bulk of the first component and a bulk of the second component, the heating unit being configured to control the localised heating to cause formation of a solid-state bond between the first component and the second component in the interfacial region, wherein the heating unit is configured to monitor an electrical property dependent on an electrical resistance of the interfacial region and use the monitored electrical property to control the localised heating.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic side sectional view of respective portions of first and second components to be bonded; a magnified view of a selected region is shown within the lower broken line ellipse;

Figure 2 depicts the arrangement of Figure 1 with respective first and second surfaces of the first and second components in direct contact and solid-state bond formation in progress;

Figure 3 shows the arrangement of Figure 2 after formation of the solid-state bond; Figure 4 depicts an apparatus for performing a method of bonding; a magnified view of a region containing first and second components is shown within the rightmost broken line ellipse;

Figure 5 is a graph depicting variation with time of monitored characteristics during an example implementation of a method of forming a solid-state bond;

Figure 6 depicts a standardized geometry for measuring shear strength of a bond;

Figure 7 is a graph showing the results of testing tensile stress and tensile strain for the bonds formed using nine different combinations of parameters;

Figure 8 is a graph depicting the results of lap shear strength testing for two different combinations of bonded first and second components, including between similar materials and between dissimilar materials;

Figure 9 is a graph showing the variation of applied voltage and current as a function of time for bonding applied to four different sized interfacial regions;

Figure 10 is a graph depicting stable power requirements as a function of bond area; Figure 11 is a schematic perspective view of a stack of plates for testing the effect of internal cavities on the method;

Figure 12 is an image showing the result of microstructural examination revealing continuous material across a bonded interfacial region underneath an internal cavity in a geometry of the type depicted in Figure 11;

Figures 13 and 14 depict images of individual layers of a laminate structure prior to bonding to form the laminate structure;

Figure 15 depicts a laminate structure formed by bonding layers of the type depicted in Figures 13 and 14.

Embodiments of the present disclosure implement solid-state bonding, which is described with reference to Figures 1-3 below.

Figure 1 depicts a portion of a first component 11 and a portion of a second component 12 suitable for solid-state bonding using methods of the present disclosure. The first component 11 is facing the second component 12 and slightly spaced apart therefrom ready for bonding. The first component 11 has a first surface 21. The second component 12 has a second surface 22. The first surface 21 faces the second surface 22. The magnified portion in Figure 1 schematically depicts how the first surface 21 and second surface 22 are rough at small length scales.

Figure 2 depicts the arrangement of Figure 1 after the first component 11 has been moved into contact with the second component 12 and is being pressed against the second component 12. No filler material or other material has been provided between the first component 11 and the second component 12. The first surface 21 is in direct (i.e. intimate) contact with the second surface 22. The pressing of first component 11 against the second component 12 applies pressure to material in an interfacial region 23. As will be described below with reference to Figure 4, the interfacial region 23 is furthermore heated. The combination of high pressure and high temperature causes solid-state bonding between the two components. The solid-state bonding arises by movement of atoms to form a continuous material in the interfacial region. This may occur by movement of vacancies 14, as depicted schematically in Figure 2 or by kinetic input to allow surface plasticisation and direct mixing. Figure 3 depicts the arrangement of Figure 2 after the solid-state bond formation has been completed.

Solid-state bonds are desirable because they provide strong bonding (with the bonded interfacial region often being stronger than portions of the material not directly involved with formation of the bond), large surfaces can be bonded in one go, no intermediate materials with different properties are required (allowing higher levels of continuity in physical properties across the interfacial region), and geometries can be maintained more easily.

Figure 4 depicts an apparatus 100 for performing methods of bonding according to embodiments of the present disclosure. The apparatus 100 comprises a first electrode 31 and a second electrode 32. A clamping arrangement 33 is provided to clamp a first component 11 and a second component 12 between the first electrode 31 and the second electrode 32. The first and second components 11,12 may be configured as described above, including respective first and second surfaces 21,22. The clamping is such that the first component 11 is pressed against the second component 12 and the first surface 21 is directly in contact with the second surface 22. Various techniques may be used to implement the clamping arrangement. Pressures to be applied between the first and second surfaces 21,22 will depend on the nature of the materials to be bonded, as well as on dwell time and heating

temperatures, but will typically be less than lOOMPa, optionally between 2MPa and lOOMPa, optionally between 5MPa and 70MPa, optionally between 20MPa and 60MPa.

A heating unit 34 is provided for passing an electrical current through the first component 11 and the second component 12. An electrical circuit is thus formed which includes a path from the first electrode 31 through the first component 11 and the second component 12 to the second electrode 32. The first and second electrodes 31, 32 may be directly in contact respectively with the first and second components 11, 12 or one or more intermediate (electrically conductive) layers may be provided. The electrical current is relatively large, e.g. higher than lkA/cm ² and is applied via a relative low potential difference (applied voltage) across the stack of components (e.g. less than 4V). The current meets resistance at the interfacial region 23 to be bonded, resulting in localised Joule heating in this region.

The electrical current causes localised heating in the interfacial region 23. The interfacial region 23 is located between a bulk of the first component 11 and a bulk of the second component 12, encompassing both the first surface 21 and the second surface 22. Where the first and second components 11, 12 are planar, the interfacial region will consist of a thin planar slab in the region of the interface between the first and second components 11 , 12 (extending slightly to either side of the interface so at to encompass the region that is subjected to the localised heating to a significant extent). The localised heating occurs because a resistivity in the interfacial region 23 (which before the solid-state bond may comprise gaps or other imperfections) is higher than in the bulk of the first and second components 11,12. For a given electrical current density, the amount of Joule heating per unit volume will thus be higher in the interfacial region 23 than elsewhere - the heating is thus localised in the sense that the heating is higher in the interfacial region 23 than elsewhere.

In the example of Figure 4 a laminated structure comprises three components (in the form of planar plates) is processed (compressed and heated to form bonding). The upper two components correspond to the first and second components 11 and 12 for the purposes of the present discussion, but the method may cause bonding not only between the first and second components 11 and 12 but also between the second component 12 and a further component 13 (or indeed between any number of further components). In the case where further components (such as component 13) are provided, each interface between different components will correspond to the“interfacial region” where a solid-state bond may be formed.

The heating unit 34 controls the localised heating to cause formation of a solid-state bond between the first component 11 and the second component 12 in the interfacial region 23. The localised heating may be controlled to ensure that no melting occurs in the interfacial region 23 during formation of the solid-state bond (thereby improving the quality of the solid-state bond and reducing or avoiding any geometric distortions in the region of the bond). The control may be implemented using a data processing unit within the heating unit 34 and/or by dedicated digital or analogue circuitry within the heating unit 34. In

embodiments, the control comprises control of the electrical current to provide an optimal level of heating during the solid-state bond formation. In some embodiments, the heating unit 34 controls the localised heating by monitoring as a function of time an electrical property dependent on a resistance of the interfacial region 23. As described in the introductory part of the description, this approach makes it possible to control the localised heating to be responsive to the actual state of the solid-state bond. In an embodiment, the electrical property comprises either of, or a mathematical function of both of, the electrical current and a voltage required to maintain the electrical current. In one embodiment, the electrical current is maintained at a constant level during at least a portion of the localised heating and the monitoring of the electrical property comprises monitoring a voltage required to maintain the electrical current at the constant level. In an embodiment, the monitored electrical property comprises a ratio of the electrical current (whether kept constant or not) to the required voltage or a ratio of the required voltage to the electrical current. The ratio of the applied voltage to current is proportional to the resistance in the electrical circuit, which is typically dominated by the resistance through the interfacial region, at least during early stages of the bond formation).

In some embodiments, including the example of Figure 4, a pressurization control unit 35 is provided to provide an additional level of control. The heating unit 34 and the pressurization control unit 35 may thus cooperate with each other to control the temperature and pressure conditions of the solid-state bond forming process to achieve optimal results.

In the example of Figure 4, the solid-state bond is formed within an environment control unit 36. The environment control unit 36 allows the solid-state bond to be formed in a controlled gaseous environment that is different in composition or pressure to atmospheric conditions. For example, the controlled gaseous environment may comprise a vacuum environment in which pressure is maintained below atmospheric pressure.

In some embodiments, including the example of Figure 4, a temperature sensor 37 is provided to monitor a temperature of one or more of the first component 11 , the second component 12, and the interfacial region 23. The temperature sensor 37 may use a contact method (e.g. using a thermocouple) or a non-contact method (e.g. a pyrometer). The monitored temperature may be input to the heating unit 34 and used to control the localised heating to further improve the efficiency with which the solid-state bond is formed (e.g. by ensuring that the measured temperature follows a desired trajectory as a function of time and/or does not exceed predefined upper and/or lower limits). The temperature measurement may allow the heating unit 34 to control the current using a feedback system, such as a proportional-integral-derivative (PID) controller. The current may thus be controlled precisely to give high accuracy in the temperature maintained in the interfacial region 23.

Figure 5 depicts variation with time of monitored characteristics during an example implementation of a method of forming a solid-state bond. The application of heating and pressure allows rapid diffusion of atoms in the interfacial region 23 without melting of the material. This allows a bond to be formed in the solid-state. The time required for this bond to be formed is of the order of 10 ² -10 ³ seconds in total, and the electrical resistance of the interfacial region 23 is found to decrease as the bond is formed, which is characterised by a drop in the voltage required to maintain a current through the interfacial region 23. Four vertical axes A-D are shown. Curve A represents variation of a temperature of the interfacial region 23 measured by the temperature sensor 37, as indicated by corresponding axis A.

Curve B represents variation of pressure between the first and second components 11, 12 being bonded, as measured by the pressurization control unit 35 and indicated by

corresponding axis B. Curve C represents voltage applied to the circuit to drive the electrical current through the first and second components 11, 12, as measured by the heating unit 34 and indicated by corresponding axis C. Curve D represents the amplitude of the electrical current (in this case DC), as measured by the heating unit 34 and indicated by corresponding axis D. A trajectory of a set temperature is indicated by broken line curve E, which underlies the curve of measured temperature versus time for most of the period in which the solid-state bond is being formed (due to control by the PID).

In the example of Figure 5, the temperature is increased quickly by localised heating using the electrical current and then maintained at a constant level before being reduced to an ambient level again. The solid-state bond is formed during time period X. The voltage (curve C) required to maintain the electrical current through the interfacial region 23 starts off relatively high and gradually decreases as the solid-state bond is formed before beginning to plateau in region Y. This behaviour is caused by the relatively high resistance of the interfacial region 23 at the start of the solid-state bond forming process and the much smaller resistance achieved once the solid-state bond is formed. The plateauing indicates that the solid-state bonding process is no longer making significant changes to the structure in the interfacial region 23, thus indicating that bond is well formed and the material properties have become continuous across the interfacial region 23.

In one class of embodiments, the controlling of the localised heating comprises controlling a duration of the heating based on the monitored electrical property. For example, the duration of the heating may be controlled by stopping the heating when the monitored electrical property moves past a predetermined threshold value (from below to above or from above to below). For example, in the example of Figure 5, the applied voltage may be the electrical property that is monitored and level Z2 may be the predetermined threshold value. During formation of the solid stage bond, the applied voltage falls from Zl towards Z2 and the heating is stopped when the voltage reaches Z2.

In an embodiment, the threshold level is determined based on a starting level (e.g. Zl) of the monitored electrical property. The threshold level may, for example, be a

predetermined proportion of the starting level of the monitored electrical property (e.g. Z2 = predetermined proportion multiplied by Zl). Alternatively or additionally, the controlling of the localised heating comprises controlling a level of the electrical current in response to a determination of how the electrical property is changing as a function of time. For example, the control may be implemented based on detecting when the electrical properties of the interfacial region 23 stop changing significantly (i.e. plateau). For example, the controlling of the localised heating may comprise controlling a level of the electrical current (e.g. to reduce or stop the heating) in response to a determination that the electrical property is changing relatively slowly as a function of time (e.g. that the rate of change of the electrical property as a function of time is below a threshold value).

In this example, the first component 11 and the second component 12 are planar and the first surface 21 and the second surface 22 are planar. Methods of the present disclosure may be applied to first and second components 11,12 having other geometries, for example non-planar first and/or second surfaces, such as curved first and/or second surfaces.

Additionally, a shape of the interfacial region (i.e. the region which is under pressure) when viewed along the line of application of the force pressing the first and second components 11,12 together may take various forms and aspect ratios. Typically, the aspect ratio (i.e. the ratio of a maximum width to a maximum length will not be so high that the interfacial region becomes too long and thin, such as to resemble a line for example. For example, in typical embodiments an aspect ratio of the interfacial region viewed along a line of application of a force acting between the first component 11 and the second component 12 during the pressing of the first component 11 against the second component 12 is no greater than 10:1 (e.g. not 11:1 or more), optionally no greater than 5:1.

Methods of the disclosure may process first and second components 11,12 having various different forms and compositions. It can be used to bond similar, as well as dissimilar, materials. The materials do not necessarily have to be electrically conducting at room temperature. In some embodiments, each of either or both of the first component 11 and the second component 12 consists of a continuously integral body, optionally a continuously integral metal body. The use of metal means the resistivity will be very low within the bulk of the metal body, which allows heating to be localised efficiently in the region to be heated (the interfacial region 23).

In some embodiments, each of either or both of the first component 11 and the second component 12 consists of a combination of an insulating material and a conductive material, for example an insulating material (e.g. plastic) matrix comprising dispersed conductive particles (e.g. metal particles), e.g. a doped plastic.

Further Examples and Validation

The shear strength of bonds formed using methods of the present disclosure were tested using the standardised geometry shown in Figure 6 (following standard ASTM D1002) with each of the first component 11 and the second component 12 being formed from CuCrZr and with a heat treatment being performed post bonding. The results are shown in the table below (with * indicated that shearing did not occur at the bond itself) .

In experiments 5, 6 and 7, the bond itself did not shear at all, failure instead occurring at a location other than the bond. This indicated that effective solid-state bonding had occurred for these experiments. Experiment 6 shows a particularly desirable combination of properties, with high shear strength being achieved with a very low dwell time (only 60s) and a tolerable reduction in thickness. Combining the results from all of the experiments suggest that the following combination of conditions would be particularly desirable: temperature = 600 degrees C, pressure = 30 MPa, dwell time = 60 or 300s, and surface roughness = 0.03 Ra.

Figure 7 shows the results of testing tensile stress and tensile strain for the nine experiments, indicating that the characteristics of Experiment 6 are particularly desirable, with high tensile strength being achieved with tolerance to significant tensile strain before failure.

Figure 8 depicts the results of lap shear strength testing for two different combinations of bonded first and second components 11,12: the solid curve shows the result for first and second components 11, 12 have the same composition (both CuCrZr); and the broken line curve shows the result for first and second components 11, 12 have different compositions, CuCrZr and SS316 (316L austenitic stainless steel). The shear strength properties are very similar and show that the method achieves high quality bonding both where the same material is present either side of the bond and where different materials are present either side of the bond. The method was also shown to be effect for other combinations of materials, including bonding of CuCrZr to a Cu coated W plate (with the localised heating being controlled to prevent melting of Cu).

Figure 9 is a graph showing the variation of applied voltage and current as a function of time for four different sized interfacial regions (with different bond areas viewed along the direction of pressurization), as indicated in the inset. The solid line curves depict applied voltage and the broken line curves depict the resulting current. As discussed above with reference to Figure 5, the applied voltage decreases in each case as a function of time, due to the progressive reduction of resistance of the interfacial region. Each of the voltage curves plateaus from about 200s onwards, indicating that the solid-state bond is well established by this point. Figure 10 depicts the corresponding stable power required for each of the four examples shown in Figure 9. As can be seen, the required power rises quickly as a function of increasing bond area. Constraints in availability of time and/or power will thus set a limit on the maximum area that can be bonded effectively using these techniques.

The method of bonding advantageously achieves a high quality bond with minimal deformation of the structures being bonded (due to the combination of low applied pressure, no melting, short dwell times, and localised heating). This makes the approach particularly suitable for manufacturing complex structures, such as laminate structures having internal cavities such as internal channels. In alternative manufacturing methods, the internal cavity would tend to cause deformation or reduce the quality of bonding by compromising structural support of adjacent layers of the laminate during pressurization. The low levels of deformation achieved by methods of the present disclosure reduce such problems, allowing laminate structures with internal cavities to be produced more easily and/or with larger or more complex internal cavities.

These effects were confirmed by testing on stacks bonded with up to four plates using the optimised bonding conditions developed, with an internal plate 44 containing a cavity 40 to simulate internal cavities such as heat sink fluid flow features. The testing geometry for an example laminate structure 46 is depicted in Figure 11. The purpose of these tests was to determine whether machining fluid-flow features into one of the plates (plate 44) would have a detrimental impact on the bond strength of the subsequent plates (e.g. first and second components 11 and 12) in the stack due to maldistribution of the pressure/current field. This type of behaviour has been observed in uniaxial pressure diffusion bonding where the size of the cavity is limited to the thickness of the subsequent layer in the stack. For the new technique it was found that the cavity size/shape had no impact on the shear strength of the bond between the subsequent plates in the stack. Microstructural examination revealed a continuous material across the interfacial region 23, as shown in Figure 12.

Thus, embodiments may be provided in which a laminate structure 46 having a plurality of laminated components (e.g. plates) is manufactured. The laminate structure 46 comprises one or more internal cavities 40. The manufacturing process includes bonding a first component 11 of the laminated components to a second component 12 of the laminated components using any of the methods of the present disclosure. The pressing of the first component 11 against the second component 12 is such as to exert pressure on a region of the laminated structure 46 that encompasses at least a portion of one or more of the internal cavities (e.g. as shown in Figure 11, where the material surrounding the cavity 40 is subjected to pressure when the first and second components 11,12 are pressed together).

Figures 13 and 14 depict individual layers (examples of first and second components 11,12) of a laminate structure 46 having internal cavities, configured to function as a heat sink. The internal cavities function as internal channels to allow flow of a coolant fluid through the heat sink. Figure 15 depicts a bonded stack formed by using the solid-state bonding method of the present disclosure applied to multiple layers of the type depicted in Figures 13 and 14, thereby forming a laminate structure 46.

The technique is not necessarily limited to flat plates. By appropriate shaping of the electrodes, other geometries could be bonded using the same method. For example, a composite pipe made up of a number of concentric laminate pipes with flow features machined in them could be formed.

In other embodiments, the method is applied to local repair of failed regions in a bond formed at an earlier time. Failed regions will manifest as regions of higher resistivity and so will attract higher levels of heating than areas where the bond is still in good condition.

Applying the method to such a bond would cause heating to occur predominantly only where the bond needs repairing, thus reducing power and time requirements and allowing repairs to be made efficiently and/or at low cost, with minimal negative effects to regions of the structure that do not need repair.

The program leading to this application has received funding from the Euratom research and training programme 2014 - 2018 under grant agreement n° 633053.