Solid-state welding method and part formed therewith

Abstract

A method of joining a first metal component and a second metal component by solid-state welding, the method comprising the steps of: removing a surface oxide layer from each of the first and second components, thereby preparing respective surfaces to be joined; coating each of the surfaces to be joined with a metal layer; applying stress to press together the two coated surfaces to be joined, to form a joint; and heat treating the joint.

Description

[0001] SOLID-STATE WELDING METHOD AND PART FORMED THEREWITH

[0002] Field of the Invention

[0003] The present invention relates to a method of solid-state welding of metals. In particular, it relates to a method of solid-state welding which can be performed in open air, i.e. without a vacuum.

[0004] Background of the Invention

[0005] In many instances, metal components can only be joined by solid-state bonding techniques. This can be the case, for example, for high performance or high strength joints or complicated geometries of dissimilar metals with distinct melting points. Typical joining methods which may be used in such cases include diffusion welding / bonding, hot press welding and brazing. However, these methods typically require a vacuum environment in order to avoid undesirable oxidation. Reaching a suitable temperature in a vacuum can be a time-consuming and expensive process. The apparatus required to perform such bonding in a vacuum can also be expensive, especially if large industrial components are to be joined.

[0006] The presently-disclosed method of joining seeks to overcome such issues, by providing a solid-state joining method which can avoid the effects of oxidation without requiring a vacuum, whilst achieving welds of a similar quality to those obtained by diffusion welding in a vacuum.

[0007] Summary of the Invention

[0008] Aspects and embodiments of the present invention are set out in the appended claims.

[0009] According to a first aspect of the invention there is provided a method of joining a first metal component and a second metal component by solid-state welding, the method comprising the steps of: removing a surface oxide layer from each of the first and second components, thereby preparing respective surfaces to be joined; coating each of the surfaces to be joined with a metal layer; applying stress to press together the two coated surfaces to be joined, to form a joint; and heat treating the joint.

[0010] The first and second components may be formed of the same metal. Alternatively, the first and second components may be formed of dissimilar metals, having different melting points.

[0011] The method may be performed at atmospheric pressure.

[0012] Optionally the method may be performed in open air.

[0013] Optionally the method may be performed in an inert gas environment.

[0014] The step of removing a surface oxide layer may comprise mechanical polishing and / or electropolishing.

[0015] The coating step may comprise at least one of: electroplating; sputtering; spark coating; and spraying.

[0016] Optionally the metal layer may be formed of a softer material than the first and second components.

[0017] Optionally the metal layer may have a thickness of at least 20 microns.

[0018] Preferably the metal layer has a higher oxidation resistance than the first and second components.

[0019] Optionally the metal layer comprises at least one of: silver, nickel, iron, chromium, copper or titanium.

[0020] The step of applying stress may be performed at ambient temperature.

[0021] Alternatively, the step of applying stress may be performed at or below an elevated temperature of approximately one-third of the lowest melting point in Kelvin of the first and second components if they are made of dissimilar metals; or at or below an elevated temperature of approximately one-third of the melting point in Kelvin of the first and second components if they are made of the same metal.

[0022] Optionally the step of applying stress may be performed for a time period of the order of 60 s or less, before the step of heat treating the joint is initiated. However, longer periods of applying stress are also possible, before the step of heat treating the joint is initiated.

[0023] The step of applying stress may comprise applying uniaxial stress or multiaxial stress.

[0024] The step of heat treating the joint may comprise increasing the temperature of the joint.

[0025] Optionally the step of increasing the temperature of the joint may comprise increasing the temperature to an elevated temperature at or below approximately 80% of the lowest melting point in Kelvin of the first and second components if they are made of dissimilar metals; or to an elevated temperature at or below approximately 80% of the melting point in Kelvin of the first and second components if they are made of the same metal.

[0026] Optionally, while heat treating the joint, at least some of the applied stress may be maintained on the joint, at least to begin with.

[0027] Optionally the method may further comprise reducing the applied stress during or after increasing the temperature of the joint.

[0028] Optionally the applied stress may be reduced upon the joint reaching a predetermined temperature.

[0029] Optionally the applied stress may be reduced in a stepwise manner.

[0030] Optionally the applied stress may be reduced by means of a plurality of stepwise reductions while the temperature of the joint is progressively increased, each stepwise reduction in the applied stress being upon a corresponding temperature of the joint being reached.

[0031] Optionally the heat treating may be performed using induction heating or resistive heating.

[0032] Alternatively the heat treating may be performed in a furnace.

[0033] By way of practical examples, the first component may be made of steel (e.g. gear steel), preferably heat-treated steel (e.g. heat-treated gear steel), and the step of heat treating the joint may comprise increasing the temperature to no higher than 200 degrees Celsius, preferably no higher than 150 degrees Celsius. Optionally the second component may be made of aluminium, and the metal layer may be formed of silver.

[0034] The first and second components may form a bimetallic gear when joined together - for example with the first component being an outer part of the gear and the second component being an inner part of the gear.

[0035] In other examples the first component may be made of steel and the second component may be made of tungsten.

[0036] According to a second aspect of the invention there is provided a part formed by the method of the first aspect, wherein the part comprises first and second components joined by the method. For example, the part may be a bimetallic gear.

[0037] The invention extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings.

[0038] Brief Description of the Figures

[0039] Embodiments of the invention will now be described by way of example only with reference to the attached figures in which:

[0040] Figure 1 is a flow diagram providing an overview of the presently-disclosed method;

[0041] Figure 2 shows an example apparatus for mechanical removal of an oxide layer;

[0042] Figure 3 shows an example apparatus for electrolytic removal of an oxide layer;

[0043] Figure 4 shows an example apparatus for electroplating components;

[0044] Figure 5 shows an example micrograph of a component comprising bulk metal and a deposited metal layer;

[0045] Figure 6 shows an example apparatus for performing uniaxial press welding;

[0046] Figure 7 shows an example apparatus for performing multiaxial press welding;

[0047] Figure 8 shows an example plot of temperature and stress applied to components during joining and strengthening of a joint over time;

[0048] Figure 9a shows an example plot of applied stress against temperature;

[0049] Figure 9b shows an example plot of applied temperature over time;

[0050] Figure 10 shows an optical image, SEM image, and atomic percentage plots of a steel-steel welded part; Figure 11a shows a steel-steel welded part after tensile testing;

[0051] Figure 11b shows the stress-strain plot of the tensile testing of the sample of Figure 11a;

[0052] Figure 12a shows an optical image of a welded steel-tungsten part;

[0053] Figure 12b shows an SEM image of the part of Figure 12a;

[0054] Figure 12c shows a micrograph of the part of Figure 12a;

[0055] Figure 12d shows a welded steel-tungsten part after tensile testing; and

[0056] Figure 13 illustrates an exemplary method of joining components to form a bimetallic gear.

[0057] In the figures, like elements are indicated by like reference numerals throughout.

[0058] Detailed Description

[0059] The present embodiments represent the best ways known to the Applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.

[0060] Method overview

[0061] Figure 1 is a flow diagram providing an overview of the main steps of the presently- disclosed solid-state welding method 10, by means of which a first metal component and a second metal component are joined together with a near oxide-free strong joint. The first and second components may be made of the same metal or dissimilar metals. In forming the joint, a bonding surface (also known as a faying surface) of the first component is joined to a bonding surface of the second component. The first step 12 of the method is removal of any surface oxide from the bonding surface of the first component and from the bonding surface of the second component, thereby preparing respective oxide-free surfaces to be joined. This is followed by a step 14 of coating each of these oxide-free surfaces with a thin protective layer of an appropriate metal, thereby achieving oxide-free preservation of the surfaces to be joined. There then follows a step 16 of pressing together the coated surfaces to be joined (i.e. applying stress to press the respective protective layers together), thereby initiating the welding process, sealing the joint and preventing oxidation of the protective layers. Then, in a further step 18, the joint is strengthened by heat treating it (typically by increasing the temperature of the joint), and optionally changing (typically reducing) the applied stress during the heat treatment. Each of these stages will be described in detail below.

[0062] Advantageously, the present method may be performed under atmospheric pressure, in open air, without any significant loss of joint strength compared to diffusion welding in a vacuum. Accordingly, to implement the present method a sufficiently large non-vacuum furnace or induction heating unit and a hydraulic press can be used, which are commonly available for hot forging metallic components and are significantly less expensive than obtaining and / or using a vacuum furnace. Alternatively, the present method may be performed in an inert gas environment, for example.

[0063] Surface oxide removal from the surfaces to be joined

[0064] The first step 12 of removing any surface oxide from the bonding surface of each component to be joined can be carried out, for example, by mechanical polishing and / or electropolishing. Figure 2 illustrates an example of mechanical polishing (or ‘grinding’) of a component 102 using a grinding wheel 106. By way of example a P4000 grit paper may be used. The surface of the component 102 which is to be joined to another component is pushed against the rotating grinding wheel 106 to grind away the surface until the surface oxide layer has been removed.

[0065] As an alternative or subsequent technique to mechanical polishing, Figure 3 illustrates an apparatus 110 which can be used for electropolishing, to remove the surface oxide layer from a component 102. The apparatus 110 is an electrolysis apparatus, comprising a bath 112 holding an electrolyte solution 114, which is typically formed of an acidic solution. The component 102 is connected to a power source 118 to form an anode. The power source 118 is further connected to a cathode 116, which is typically formed of stainless steel. Electrical current is conducted from the component 102, acting as an anode, through the electrolyte solution 114, to the cathode 116. This current causes metal oxide ions on the surface of the component to be removed. This process is performed until the metal oxide layer has been removed from the component 116. This method offers the advantage of a leaving a smooth upper surface.

[0066] Using either or both of the above techniques, or another suitable technique, the oxide layer is removed from the bonding surface of the first component 102 to be joined, and similarly from the bonding surface of the second component which is to be joined to the bonding surface of the first component 102. It will be appreciated that if one of the components is formed of an inert metal such as gold, silver or platinum, then it will already be oxide free and so will not require this oxide removal step to be performed. Metal layer deposition on the surfaces to be joined

[0067] Once the surface oxide layer has been removed from each component, in the second step 14 the clean oxide-free surface of each of the first and second components is preserved. This is achieved by coating each of the surfaces to be joined with a thin metal layer. The deposition of the metal layer can be performed by electroplating, sputtering, spark coating or spraying. In the presently-preferred embodiment the metal layer is deposited by electroplating, as this is a relatively quick and easy method resulting in the deposition of a smooth and even layer.

[0068] Figure 4 shows a schematic diagram of an apparatus 210 which can be used to electroplate the surface of a component 102. This is a similar arrangement to that shown in Figure 3 for electropolishing, comprising a bath 212 holding a volume of electrolyte 214, but in this case the component 102 is connected to the power source 218 such that it forms a cathode. When current is passed through the metallic anode 216, metal ions in the electrolyte solution are deposited on the component 102 (forming the cathode). This process is typically performed until a metal layer of approximately 40 pm has been deposited. As a minimum, at least 20 pm to 30 pm of metal layer is deposited; at least this thickness is required to prevent oxidation of the surface of the component 102 and to allow joining of the deposited metal layer to a counterpart metal layer deposited on the second component. A thicker layer can be deposited, for example up to around 1 mm or 2 mm, but it is not necessary to deposit such a thick layer to achieve the required effect, and doing so may be time-consuming and increase costs. Figure 5 shows an example micrograph of a section of the component 102 comprising underlying base metal 1022 and the deposited metal layer 1024 on the surface to be joined.

[0069] The metal layer 1024 is an appropriate metal chosen for having a low reactive tendency with oxygen at room temperature (in other words, it has good oxidation resistance at room temperature; that is to say it has a high oxidation energy) and is metallurgically compatible with the base metal 1022 of the substrate (the component 102). Examples of metals with good oxidation resistance at room temperature include silver and nickel. Nickel is particularly well-suited to deposition by electroplating. Other metals such as chromium, copper or titanium can also be used. Even metals such as iron, which are not typically considered to have high oxidation resistance, can be used, especially if the next step of joining the two components is performed sufficiently quickly after the deposition of the metal layers. The metal layer 1024 is also preferably softer than the base metal 1022, so that (as will be discussed below) any plastic deformation under stress, in the joining stage, will preferentially occur in the metal layer 1024 rather than in the base metal 1022.

[0070] Joining together the coated surfaces

[0071] The next stage 16 is the pressing together of the coated surfaces to be joined, thereby initiating the welding process, sealing the joint by local plastic deformation, and preventing oxidation of the protective layers. Such welding may be referred to herein as “press welding”, and examples are shown in Figures 6 and 7.

[0072] The first exemplary arrangement 120 for performing such welding is a conventional hydraulic press, as illustrated in Figure 6. The method comprises placing the first component 102 and second component 104 to be joined between a first pressure plate 122a and a second pressure plate 122b, where the pressure plates 122a, 122b are configured to exert stress in opposing directions, i.e. in a compressive manner, on the components 102, 104, thereby applying a uniaxial load. (The plates 122a, 122b may alternatively be configured as a first pressure plate 122a and a fixed stage 122b, or vice versa, which exert the same effective compressive stress on the components to be joined.) As illustrated, the first component 102 comprises base metal 1022 and the deposited metal layer 1024, and the second component 104 comprises base metal 1042 and the metal layer 1044. The components 102, 104 are arranged such that the metal layers 1024, 1044 (i.e. the surfaces to be joined) face and abut one another. The uniaxial stress is applied perpendicular to the metal layers 1024, 1044 (i.e. the surfaces to be joined), so that they are pushed together. The metal layers 1024, 1044 join to become an ‘interlayer’ 1028.

[0073] Figure 7 illustrates a cross-section of an alternative exemplary arrangement 220 for applying a multiaxial load. This arrangement comprises an upper die 222a and a lower die 222b. The arrangement further comprises a mandrel 224 within an expanding cylindrical die 226. The mandrel 224 has a tapered configuration such that when the upper die 222a exerts a stress downwards, this moves the mandrel 224 downwards, having the effect of exerting a radially outwards stress on the expanding die 226. In the illustrated arrangement 220, the expanding cylindrical die 226 is positioned within a cylindrical pipe 200, to be joined to a series of blocks 202, 204, 206, 208. The radial stress exerted by the expanding cylindrical die 226 is used to join the blocks 202, 204, 206, 208 to the pipe 200 via metallic interlayers 2124, 2168 (comprising deposited metal layers on the surfaces to the joined - namely the surfaces of the blocks 202, 204, 206, 208 and the outer surface of the pipe 200). Additionally, the blocks 202 and 204 are joined to one another via the deposited interlayer 2024 (comprising metal layers deposited on the surfaces of the blocks 202, 204 to be joined), and the blocks 206 and 208 are joined to one another via the deposited interlayer 2068 (comprising metal layers deposited on the surfaces of the blocks 206, 208 to be joined). These joints are joined due to the downward stress exerted directly on the blocks 202, 204, 206, 208 by the upper die 222a and lower die 222b. This is an example of a configuration by which multiaxial loading can be used to join components having more complicated shapes and / or arrangements.

[0074] In any configuration used, the applied compressive stress is chosen to be sufficiently high to seal the joint, to prevent oxygen from entering the joint, but not so high as to cause severe distortion of the components. The applied stress is chosen in dependence on the material properties and arrangement of the components to be joined. For example, a lower stress is used for high-precision joints. Typical stresses applied may be in the range of 50 MPa to 700 MPa.

[0075] Referring further to the example of uniaxial pressing of Figure 6 for the sake of simplicity (but understating that the present principles also apply to the multiaxial pressing of Figure 7, or to other pressing configurations), the initial joining of the abutting coated surfaces by press welding is typically instantaneous (e.g. occurring in a matter of seconds). Furthermore, under the applied stress, the metal layers 1024, 1044 (forming interlayer 1028) typically undergo plastic deformation, further strengthening the joint. The metal layers 1024, 1044 are typically softer than the bulk metals 1022, 1042 of the components 102, 104 being joined. This may be a consequence of the deposition method of the metal layers 1024, 1044 resulting in a soft layer, while the bulk metal 1022, 1042 may have undergone hardening treatments. In some cases, the metal chosen to be deposited as a metal layer 1024, 1044 may simply be a softer metal. In either case, plastic deformation preferentially occurs in the metal layers 1024, 1044 (i.e. in the interlayer 1028) rather than in the bulk metals 1022, 1042 of the components 102, 104.

[0076] Importantly, the present press welding can be performed under ambient conditions. This removes the need for a vacuum furnace, which would typically have a high outlay cost as well as being expensive and time-consuming to run. As the surfaces of the base metals 1022, 1042 of each component 102, 104, are protected from oxidation by the interlayer 1028 (formed of the deposited metal layers 1024, 1044 pressed together), the press welding can be performed without vacuum conditions, for example in open air, under atmospheric pressure. This means that the present method can be operated in a much more cost- effective manner, and more quickly. Furthermore, it is more easily scalable. Alternatively, the present method may be performed in an inert gas environment, which is also a cost- effective alternative to vacuum conditions.

[0077] Additionally, by virtue of the metal layers 1024, 1044 being relatively soft compared to the base metals 1022, 1042, the metal layers 1024, 1044 and interlayer 1028 preferentially undergo plastic deformation. This means a relatively low stress is required for pressing the components together. This means that the method can be used for fine assembly processes without deforming the components to be joined.

[0078] Moreover, the pressing step can be performed at room temperature, or at a relatively low temperature - for example, no greater than one third of the lowest melting point in Kelvin (K) of the bulk metals 1022, 1042 if they are dissimilar metals; or no greater than one third of the melting point in Kelvin (K) of the bulk metals 1022, 1042 if they are the same metal. This further reduces the likelihood that oxidation will occur, or at least limits the extent to which oxidation will occur.

[0079] Strengthening of the joint by heat treating

[0080] The next stage 18 of the process is strengthening of the joint by heat treating it. This typically comprises increasing the temperature of the joint (including the interlayer 1028) to a higher temperature than that of the initial press welding of step 16, whilst maintaining (at least to begin with) at least some of the applied compressive stress on the joint. As discussed below, the applied stress may be reduced during the heat treatment of the joint.

[0081] The temperature of the joint may be raised by induction heating or resistive heating, for example. Alternatively, the joined components 102, 104 may be placed in a furnace to raise their temperature via furnace heating. In some implementations - for example for components that are too large to fit inside a furnace in an economically-viable or procedurally-viable manner - a mobile induction heating source may be moved along the joint to apply localised heat treatment instead.

[0082] The rate of increase in temperature, to a holding temperature at which the joint is then held, is typically as fast as possible to minimize energy requirements and to preserve the microstructure. For example the heating rate may be 5°C / s. The temperature of the joint is typically tracked, for example by placing a thermocouple at the location of the joint. The temperature is raised to a value no greater than 80% of the lowest melting temperature in Kelvin (K) of the bulk metals 1022, 1042 of the components 102, 104. That is to say, in the case of the components 102, 104 being made of dissimilar metals, the temperature is raised to a value no greater than 80% of the melting temperature in Kelvin of whichever of the bulk metals 1022, 1042 of the components 102, 104 has the lowest melting temperature. For example, the temperature may be raised to a value in Kelvin of around two thirds of the lowest melting temperature. If the components 102, 104 are made of the same metal, the temperature is raised to a value no greater than 80% of the melting point in Kelvin (K) of the bulk metals 1022, 1042. For example, the temperature may be raised to a value in Kelvin of around two thirds of the melting point.

[0083] Increasing the temperature has the effect of increasing diffusion across the joint, leading to hot diffusion welding. In particular, diffusion occurs from the first metal layer 1024 to the second metal layer 1044, and vice versa. This also acts to close any interfacial micro-voids and dissolve any interfacial oxide which may have formed at room temperature, thereby facilitating the production of a strong and oxide-free joint. The joining of the surfaces preferably takes place soon after the deposition of the metal layers 1022, 1042, so that any oxide layer which may form will only be very thin. Importantly, diffusion typically further takes place between the bulk metals 1022, 1042 and the metal interlayer 1028 to cause alloying / interdiffusion. This allows the disappearance of the joining line as grains migrate across the welding line. This creates a seamless, homogeneous, indistinguishable grain structure across the welding interface and the matrix of the bulk metal.

[0084] Typically, the pressing stage 16 can be of the order of a minute (60 s) or less before the strengthening stage 18 is commenced. The heating of the strengthening stage 18 and the pressing stage 16 are typically overlapping processes rather than consecutive discrete stages. The joining of the two metal layers 1024, 1044 is expected to occur instantaneously, so the heating can commence almost immediately after the pressing stage 16.

[0085] In order to prevent plastic deformation but promote diffusion, as the temperature is increased, the applied stress may be decreased. For example, Figure 8 illustrates a relatively simple example of this, showing a plot of temperature and applied stress over time. The solid line of Figure 7 shows the change in temperature over time, showing a progressive increase from the initial press welding temperature (e.g. room temperature, which may be considered to be, for example, 20°C) up to an elevated temperature. Merely by way of example, the elevated temperature may be 1000°C. The components are held at this elevated temperature for a duration of time, in order to promote strengthening of the bond by promoting diffusion.

[0086] The dashed line in Figure 8 shows the stress applied to the components over time. This starts at a higher value (the stress under which the press welding of step 16 is carried out) when the temperature is at the lower value (for example, room temperature). Merely by way of example, this initial stress may be 90 MPa. Once the temperature has reached a predetermined value, in this case the elevated holding temperature, the stress is reduced to a lower stress (e.g. in a stepwise manner as illustrated). By way of example, this lower stress may be 20 MPa. The components are held at this lower stress while being held at the elevated temperature. This is to avoid distortion or deformation of the components. The temperature of the components may, for example, be raised to the elevated temperature at a rate of approximately 5°C / s (thus it may only take a matter of minutes to raise the temperature). The components may typically be heated for a period of time of the order of a few hours; for example, 2 or 3 hours. In some cases, the heating may only be for less than an hour, for example 20, 30 or 40 minutes. The heating time is preferably minimised to the shortest time period over which effective joint strengthening can occur.

[0087] After the heating, the components are returned to room temperature. This may occur simply due to air cooling, for example; or alternatively an active cooling mechanism may be utilized.

[0088] The temperature and timings of the heating stage will depend on the properties of the components to be joined. For example, a lower temperature and / or time may be used when a component has been heat treated or strengthened (for example, dispersion- strengthened), to prevent these properties being adversely affected. Additionally, the heating and stress profile may be determined so as to preserve the grain structure of the bulk metals.

[0089] In some implementations, the applied stress may be adjusted in a stepwise manner. This is illustrated in Figures 9a and 9b. Figure 9a shows a plot of applied stress against temperature, and Figure 9b shows a plot of temperature over time. As shown in Figure 9a, for each temperature applied to the components to be joined, there is defined a corresponding stress to be applied to the components; the higher the temperature, the lower the stress (and vice versa). For example, at room temperature, a stress of 200 MPa can be applied; when the temperature is raised to 600°C, the stress is lowered to 150 MPa; when the temperature is raised to 800°C, the stress is lowered to 70 MPa; and when the temperature is raised to 1000°C, the stress is lowered to 20 MPa. As such, the temperature is gradually raised, while the stress is reduced in a stepwise manner. In other words, the applied stress is reduced by means of a plurality of stepwise reductions while the temperature of the joint is progressively increased, each stepwise reduction in the applied stress being upon a corresponding temperature of the joint being reached.

[0090] Figure 9b shows how the temperature profile varies over time. In particular, the heating is a gradual increase over 2 hours from room temperature (approximately 20°C) to 1000°C, at which temperature the system is held for approximately 20 minutes. The temperature of the system is then allowed to cool back down to room temperature.

[0091] It should be understood that these particular values of stress, temperature and time are exemplary only, and can be adjusted for particular components and arrangements. The next section provides some particular examples.

[0092] Examples and experimental results

[0093] The following section provides examples of the use of the present method in joining components made of the same metal (e.g. both made of steel), and in joining dissimilar metals (e.g. steel to tungsten, and steel to aluminium). An example of the use of the present method in forming a bimetallic gear is also described.

[0094] Steel to steel

[0095] A first example relates to the joining of two steel bars, in particular two cylindrical bars of 316L stainless steel. These were cleaned and then submerged into an electroplating chemical solution, in which a thin (~40 pm) nickel layer was electroplated onto the end surfaces to be joined. Joining of the electroplated surfaces was then performed by applying (in substantially the manner of Figure 6) a uniaxial compressive stress of 90 MPa at 20°C under atmospheric pressure in a conventional hydraulic press within a hot air furnace. The temperature of the steel bars was raised to 1000°C at a heating rate of 5°C / s by the furnace (thus taking 196 s), and once a temperature of 1000°C was reached, the applied uniaxial stress was reduced to 20 MPa (in substantially the manner illustrated in Figure 8). The temperature of 1000°C and the stress of 20 MPa were maintained for 900 s (15 minutes), and then the stress was removed and the joined components were allowed to air cool naturally to room temperature.

[0096] Figure 10 shows an optical image of the bonded bars. The bonding interface is indicated, and a scanning electron microscope (SEM) image of the grain structure across the interlayer zone is further shown in Figure 10. This shows a seamless, homogeneous, indistinguishable grain structure across the welding interface and the matrix at the end of the open-air welding process. Additionally, no oxide particles or micro-voids could be discerned along the joint interfaces. Figure 10 also shows a map of atomic percentages of Ni and Fe, and a plot of the atomic percentages of Ni, Cr and Fe. These show continuous uniform interdiffusion between the electroplated nickel layers (forming the nickel interlayer) and the steel matrix.

[0097] Figure 11a shows an image of a welded sample after tensile testing, and Figure 11b shows a stress-strain plot of the tensile test, including micrographs of the Ni-rich interlayer. Firstly, it should be noted that the break point was not located at the bonding line, which means that the bonding line does not form a weak point of the joint, consistent with the homogeneous, indistinguishable grain structure across the welding interface and the metal matrix. This nearly perfect joint microstructure offers high joint strength, -97% of that of the base material. Additionally, the overall plastic deformation across the welded sample is less than 1%. Accordingly, this open-air welding achieved an oxide-free homogeneous microstructure, high joint strength (close to or exceeding the base material strength) with small joint distortion. This means that large safety-critical components can be welded with high precision using only a conventional hydraulic press with a large hot air furnace. Put another way, the present method enables metals to be welded to a similar level of quality as diffusion welding in a vacuum chamber, but potentially at only about 10% of the associated cost, and requiring only about 10% of the time.

[0098] By way of example, the method may be used for welding stainless steel components for heat exchangers, which require high joint performance to prevent corrosion cracking at the joints, or for making other large industrial parts. The present method offers a cheaper and quicker method of bonding such components, while maintaining high joint performance. Steel to tungsten

[0099] In a further example, steel and tungsten can be joined by the present method, for example to produce large industrial parts. Conventional fusion welding is not appropriate for steel- tungsten joints due to the very different melting temperatures of the dissimilar metals (tungsten has a melting temperature of ~3,400°C and steel has a melting temperature of ~1 ,600°C). Other solid state welding techniques include friction-welding and diffusion welding; however, friction-based welding is limited to simple thin sheet or tube geometries while diffusion welding is generally limited to small sizes and is difficult (and expensive) to scale up, particularly because of the need for a vacuum chamber of suitable size.

[0100] Figure 12a shows a bar comprising a P91 steel component joined to a tungsten (W) component via a nickel interlayer. A nickel layer was deposited onto the ends of each of the components, and then they were pressed together and the joint strengthened by applying the stress and temperature profile of Figures 9a and 9b (as described above). In particular, a stress of 200 MPa was applied initially at room temperature, and then the stress was decreased as the temperature was increased. The joint was held at 1000°C for approximately 20 minutes before being cooled to room temperature. Figure 12b shows an SEM image of the grain structure across the diffusion layer and bulk metal and Figure 12c shows a micrograph of the grain structure across the diffusion layer and bulk metal. These show interdiffusion and grains which pass across the boundary, with no voids or oxide inclusions. Figure 12d further shows a sample which has undergone micro tensile testing, during which fracture occurred in a region other than the welding line. Additionally, a nominal ultimate tensile strength (UTS) of nearly 800 MPa was recorded for the sample welded in open air by the present method. By contrast, the previous best UTS (obtained from micro tensile testing) for a steel-tungsten join (welded in a vacuum) was -450 MPa.

[0101] Furthermore, the present method can be used to join dispersion strengthened steel, such as oxide dispersion strengthened steel (ODS). ODS steel contains nanosized oxide particles to strengthen the materials, and the strengthening can be deteriorated by conventional fusion-based welding techniques, many of which do not provide sufficiently strong joints in any case. The present method provides an effective and relatively low-cost and time-efficient method of welding such steels.

[0102] Steel to aluminium

[0103] A further example of metals which can be welded by the open-air welding technique as presently disclosed is aluminium and steel. By way of example, the aluminium component and steel component can both be mechanically polished, to remove a surface oxide layer from the surfaces to be joined. A silver layer is then deposited on each of said surfaces by electroplating (for example). The welding can be performed at a relatively low temperature, for example below 200°C, and in some instances below 150°C. A stress of up to 170MPa can be applied. A particular example of use of a steel-aluminium joint is the manufacture of lightweight bimetallic gears.

[0104] Bimetallic gears

[0105] Bimetallic gears consist of conventional gear steel in an outer ring (bearing the gear’s teeth) to provide the strength and wear resistance for torque transfer, while the inner core part of the gear is replaced with light alloy, e.g. Al, Mg or Ti, to reduce the overall weight and inertia of the gear. This arrangement can be over 50% lighter, and can reduce the inertia by more than 40%, compared to fully steel gears. Reducing the inertia of a gear enables it to accelerate or decelerate more quickly. Bimetallic gears can therefore provide lightweight transmission systems in vehicles with reduced weight, improved energy efficiency and reduced inertia; or can allow more gears to be included in a transmission system for the same overall weight as when fully steel gears are used. They can also lead to a reduction in noise / vi bration.

[0106] A current technical challenge for manufacturing bimetallic gears is achieving sufficiently good joint strength. Importantly, it is also challenging to perform a heat treatment strengthening process on steel to improve its wear-resistance in a bimetallic system, in which the two metals have distinct melting temperatures: the heat treatment temperature for steel is say 1000°C but aluminium melts at 640°C. As such, the heat treatment, typically a carbon-based hardening treatment, is typically carried out first, before joining. However, joining by hot forge welding requires a temperature above 400°C, which can be detrimental to (if not destroy) the effect of the heat treatment hardening of the steel. The present method offers a low-temperature technique (at less than 200°C, and preferably less than 150°C) for joining aluminium to heat-treated steel, which maintains the heat treatment hardening effects.

[0107] Figure 13 illustrates an exemplary method of joining a steel gear ring 302, which forms an outer part of a bimetallic gear, and an inner core part 304 (for example, formed of aluminium). The steel ring 302 has undergone machining and heat treatment strengthening (for example carburization), and the aluminium core 304 has undergone machining and heat treatment. The parts 302, 304 are plated with layers of silver in an electroplating arrangement akin to that described in relation to Figure 4. Typically a layer of silver of approximately 40 pm is applied to each surface to be joined. Press welding is then performed in a forging machine or hydraulic press 320, comprising a pressure plate 322 for applying stress to the components 302, 304. A compressive stress of up to 170 MPa is applied, at a temperature up to 200°C, for example up to 150°C. The compressive stress can be provided by the forging machine / hydraulic press or by thermal expansion, if the aluminium core part 304 is held at a lower temperature than the steel gear 302.

[0108] This method has been shown to produce a weld without any interfacial voids or oxide inclusions. This has further been shown to achieve an ultimate tensile strength (UTS) of the joint of -170 MPa. This is only slightly lower than the UTS of pure aluminium, thereby demonstrating excellent properties of the joint.

[0109] Alternatives and modifications

[0110] It should be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

[0111] Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

[0112] Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

CLAIMS1. A method of joining a first metal component and a second metal component by solid- state welding, the method comprising the steps of: removing a surface oxide layer from each of the first and second components, thereby preparing respective surfaces to be joined; coating each of the surfaces to be joined with a metal layer; applying stress to press together the two coated surfaces to be joined, to form a joint; and heat treating the joint.

2. The method of claim 1 , wherein the first and second components are formed of the same metal.

3. The method of claim 1, wherein the first and second components are formed of dissimilar metals having different melting points.

4. The method of any preceding claim, wherein the method is performed at atmospheric pressure.

5. The method of claim 4, wherein the method is performed in open air.

6. The method of any of claims 1 to 4, wherein the method is performed in an inert gas environment.

7. The method of any preceding claim, wherein the step of removing a surface oxide layer comprises mechanical polishing and / or electropolishing.

8. The method of any preceding claim, wherein the coating step comprises at least one of: electroplating; sputtering; spark coating; and spraying.

9. The method of any preceding claim, wherein the metal layer is formed of a softer material than the first and second components.

10. The method of any preceding claim, wherein the metal layer has a thickness of atleast 20 microns.11 . The method of any preceding claim, wherein the metal layer has a higher oxidation resistance than the first and second components.12 The method of any preceding claim, wherein the metal layer comprises at least one of: silver, nickel, iron, chromium, copper or titanium.

13. The method of any preceding claim, wherein the step of applying stress is performed at ambient temperature.

14. The method of any of claims 1 to 12, wherein the step of applying stress is performed at or below an elevated temperature of approximately one-third of the lowest melting point in Kelvin of the first and second components if they are made of dissimilar metals; or wherein the step of applying stress is performed at or below an elevated temperature of approximately one-third of the melting point in Kelvin of the first and second components if they are made of the same metal.

15. The method of any preceding claim, wherein the step of applying stress is performed for a time period of the order of 60 s or less, before the step of heat treating the joint is initiated.

16. The method of any preceding claim, wherein the step of applying stress comprises applying uniaxial stress.

17. The method of any of claims 1 to 15, wherein the step of applying stress comprises applying multiaxial stress.

18. The method of any preceding claim, wherein the step of heat treating the joint comprises increasing the temperature of the joint.

19. The method of claim 18, wherein the step of increasing the temperature of the joint comprises increasing the temperature to an elevated temperature at or below approximately 80% of the lowest melting point in Kelvin of the first and second componentsif they are made of dissimilar metals; or wherein the step of increasing the temperature of the joint comprises increasing the temperature to an elevated temperature at or below approximately 80% of the melting point in Kelvin of the first and second components if they are made of the same metal.

20. The method of claim 18 or claim 19 wherein, while heat treating the joint, at least some of the applied stress is maintained on the joint, at least to begin with.

21. The method of claim 20, further comprising reducing the applied stress during or after increasing the temperature of the joint.

22. The method of claim 21 , wherein the applied stress is reduced upon the joint reaching a predetermined temperature.

23. The method of claim 21 or claim 22, wherein the applied stress is reduced in a stepwise manner.

24. The method of claim 23, wherein the applied stress is reduced by means of a plurality of stepwise reductions while the temperature of the joint is progressively increased, each stepwise reduction in the applied stress being upon a corresponding temperature of the joint being reached.

25. The method of any preceding claim, wherein the heat treating is performed using induction heating or resistive heating.

26. The method of any of claims 1 to 24, wherein the heat treating is performed in a furnace.

27. The method of any preceding claim, wherein the first component is made of steel, preferably heat-treated steel, and the step of heat treating the joint comprises increasing the temperature to no higher than 200 degrees Celsius, preferably no higher than 150 degrees Celsius.

28. The method of claim 27, wherein the second component is made of aluminium, and preferably wherein the metal layer is formed of silver.

29. The method of any preceding claim, wherein the first and second components form a bimetallic gear when joined together, preferably wherein the first component is an outer part of the gear and the second component in an inner part of the gear.

30. The method of any of claims 1 to 27, wherein the first component is made of steel and the second component is made of tungsten.

31. A part formed by the method of any preceding claim, wherein the part comprises first and second components joined by the method, optionally wherein the part is a bimetallic gear.

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