Connection process

Abstract

Diffusion joining process in which, during the heating of a first joining object (W1) and a second joining object (W2) made of metal and in contact with each other, pressure is applied to the first joining object (W1) and the second joining object (W2) and atoms from the first joining object (W1) diffuse into the second joining object (W2), while atoms from the second joining object (W2) diffuse into the first joining object (W1), thereby joining the two joining objects (W1, W2) together, wherein the process comprises the following steps: Inserting the first connecting object (W1) and the second connecting object (W2) into a connecting container (12) and evacuating the connecting container (12) to a pressure on the order of 10 -1 Pa, Supplying nitrogen gas into the evacuated connecting vessel (12), whereby the pressure in the connecting vessel (12) is increased to 3 to 10 5 Pa is controlled, and Heating the first connecting object (W1) and the second connecting object (W2) inside the connecting container (12), in which the nitrogen gas is at a pressure of 3 to 10 5 Pa exhibits, whereby pressure is applied to the first connecting object (W1) and the second connecting object (W2), wherein the first connecting object (W1) consists of a steel material, a nickel alloy or a copper alloy, and wherein the second connecting object (W2) consists of a steel material, a nickel alloy or a copper alloy.

Description

Background of the invention

[0001] The present invention relates to a bonding method by which objects to be joined are joined by applying negative pressure and heating a plurality of bonding objects that are in contact with each other.

[0002] Diffusion bonding is a known method for joining such objects. In this process, the objects in contact are heated by applying electrical energy, high-frequency heating, or similar methods, causing atoms to distribute within the contact area. As a result, the two objects are bonded together. When electrical energy is applied, all types of metallic materials, including steel, nickel alloys, and copper alloys, are suitable as objects to be joined (conductive bodies).

[0003] In the event that diffusion bonding is carried out with the aforementioned metallic components, an oxide layer (oxide film) forms on the surface of the components if there is an excess of oxygen in the bonding environment. The temperature of this layer increases with the application of current. In this case, the diffusion of atoms is hindered by the oxide layer, raising concerns that the bond strength (bonding force) of the bonded area cannot be guaranteed.

[0004] To address this concern, the electrical current supply and the creation of a vacuum in such metallic bonding materials are generally carried out in a bonding vessel under a strong vacuum, or in a bonding vessel containing an inert gas atmosphere. In the case where a strong vacuum is created in the bonding vessel, a rotary pump and a diffusion pump are combined to evacuate the vessel. This reduces the pressure in the bonding vessel to approximately 10 -3 Pa is set. If it is necessary to lower the pressure further, a turbomolecular pump can be used in addition to the two pump types mentioned above.

[0005] In cases where an inert gas atmosphere is provided in the connecting vessel, argon gas, helium gas, or similar gases were primarily used as the inert gas. Depending on the circumstances, however, nitrogen was also used (see, for example, Japanese Patent Publication JP 2006-315040 A).

[0006] If a diffusion pump or a turbomolecular pump is used to generate a strong vacuum in the connecting vessel, the investment in equipment increases significantly due to the high cost of such pumps. If an inert gas atmosphere is used, it is not easy to completely remove the original atmosphere (oxygen) from the connecting vessel. Therefore, the formation of an oxide layer cannot be entirely avoided. Since the cost of argon, helium, or similar gases is comparatively high, the operating costs for carrying out the diffusion connection also increase.

[0007] Recently, attention has focused on nitrogen-doped stainless steel, which contains a specified amount of nitrogen to improve corrosion resistance. However, when this material is fusion-welded, it has been observed that the nitrogen content in the joint area decreases. This naturally leads to concerns that corrosion resistance may also decrease.

[0008] DE 44 30 779 C2 describes a method for producing a diffusion bond at low pressure. In one embodiment, the diffusion bonding process is carried out in a protective or reducing atmosphere, wherein the workpieces and a plate made of a superplastic alloy are enclosed in a chamber made of heat-resistant steel, which is filled with protective or reducing gas via a gas cylinder. Subsequently, the plate and the workpieces are heated, and pressure is applied perpendicular to the surface of the plate. Pressure and temperature are maintained until a diffusion bond between the plate and both workpieces is achieved.

[0009] DE 29 40 959 C2 describes a method and a device for creating a shielding gas atmosphere in a welding booth for gas metal arc welding (GMAW). The welding booth is first placed under a relatively low vacuum in the range of 133 to 400 Pa before normal atmospheric pressure is restored in the welding booth by introducing the shielding gas. The shielding gas is not defined.

[0010] German patent DE 101 62 937 A1 discloses a method for joining dissimilar materials under a protective gas atmosphere. In metal inert gas (MIG) welding, an electric arc burns within a shielding gas mantle. Argon and / or helium are used as the inert and shielding gases. Additionally, an active gas is used, which advantageously influences the process. The active gas components of the shielding gas are one of the gases or a mixture of the gases O₂, CO₂, NO, N₂O, or N₂.

[0011] DE 601 10 312 T2 relates to an electrical connection method and a corresponding device for connecting several elements, wherein these are positioned so that their connection surfaces abut each other and a temporary connection is created by applying a pulsed current to the elements under pressure. In a subsequent step, the full connection strength is achieved by subjecting the temporarily connected elements to a heat treatment with mutual diffusion at a desired temperature and in an inert atmosphere.

[0012] Document DE 30 03 186 A1 discloses a method for joining components made of silicon composite materials, which are joined together by diffusion welding in a vacuum or under a protective gas atmosphere.

[0013] JP H08-257767 A discloses a diffusion bonding method in which the objects to be joined are placed in a chamber. The materials are clamped in such a way that a force is exerted on the bonding surface. A vacuum is created in the chamber, and the objects to be joined are heated and held at a predetermined temperature for a predetermined time to form the diffusion bond. After the predetermined time has elapsed, the chamber is cooled by the introduction of an inert gas.

[0014] Further diffusion bonding devices and methods are known from JP 2006 - 315 040 A, JP H01 - 169 111 A and JP S63 - 215 388 A. Summary of the invention

[0015] The object of the present invention is to propose a joining method which makes it easy to avoid the formation of an oxide layer on the objects to be joined.

[0016] The invention is intended in particular to propose a joining method with which the bond strength (bonding force) can also be ensured in the case that materials made of nitrogen-doped stainless steel are joined together.

[0017] Another object of the present invention is to provide a joining method that can be implemented at low cost.

[0018] These problems are essentially solved by the invention through the features of claim 1.

[0019] Advantageous embodiments of the invention are the subject of the dependent claims.

[0020] In the joining method according to the present invention, while a first joining object and a second joining object, which are in contact with each other, are heated, pressure is exerted on the first joining object and the second joining object, thereby joining the two joining objects together, wherein the joining method comprises the following steps: Arranging the first connecting object and the second connecting object in a connecting container and supplying nitrogen gas to the connecting container, whereby the pressure in the connecting container is increased to 3 to 10 5 Pa is being set up, and Heating the first connecting object and the second connecting object, whereby pressure is applied to the first connecting object and the second connecting object.

[0021] In cases where a nitrogen atmosphere is provided in the joining device within the pressure range described above, the partial pressure of oxygen is significantly reduced. This makes it easy to prevent the formation of an oxide layer on the surface of the objects being joined. Consequently, diffusion bonding can be promoted with high efficiency. As a result, the bond strength of the joint area is ensured.

[0022] Since there is no need to maintain a strong vacuum in the connecting vessel in this case, there is also no particular need to include an evacuation mechanism, such as an expensive diffusion pump or a turbomolecular pump, to generate a strong vacuum. This reduces equipment costs. Furthermore, the use of inexpensive nitrogen gas reduces the costs associated with diffusion bonding.

[0023] The objects to be joined are preferred if they do not tend to form a nitride layer when heated and are also capable of allowing nitrogen to enter a solid solution. Since neither the first nor the second object forms a nitride layer, diffusion between the objects is not restricted. Even if cavities exist between the first and second objects, and atmospheric nitrogen is present in these cavities, the nitrogen gas will form a solid solution on either the first or second object. Consequently, the cavities can simply be eliminated, and the bond strength in the joint area is increased.

[0024] A steel material is proposed as the preferred material. Steel materials such as carbon steel, alloy steel, stainless steel, or similar contain nitrogen because they are melted and refined in the surrounding environment. Furthermore, nitrogen-doped stainless steels, which are positively doped with nitrogen, also contain nitrogen and therefore do not form a nitride layer when heated. They are also capable of causing nitrogen to enter a solid solution. Specifically, in the case of such materials, the nitrogen entering a solid solution in the bonding elements eliminates voids. In particular, in the case of nitrogen-doped stainless steel, the nitrogen entering a solid solution prevents a decrease in the nitrogen content, thus ensuring corrosion resistance.

[0025] Other examples of materials that do not form a nitride layer when heated and are able to cause nitrogen to enter a solid solution include copper, nickel, and alloys of these elements.

[0026] The joining method can be carried out using a joining device in which pressure is applied to the first joining object and the second joining object during the heating of a first joining object and a second joining object that are in contact with each other, thereby joining the two joining objects together, wherein the device has the following elements: a heating mechanism for heating the first connecting object and the second connecting object, a connecting container for holding the first connecting object and the second connecting object, a nitrogen supply source for supplying nitrogen gas to the connecting container, a draining mechanism for draining the connecting container, and a pressure control mechanism for controlling the pressure of the nitrogen gas supplied from the nitrogen supply source and introduced into the connecting container, the pressure control mechanism adjusts the pressure of the nitrogen gas to 3 to 10 5 Pa adjusts.

[0027] This design allows for the simple and cost-effective avoidance of oxide and nitride layer formation on objects being joined. This promotes diffusion bonding and enables efficient, low-cost bonding.

[0028] For the drainage mechanism in the design described above, a relatively inexpensive rotary pump (rotary lobe pump) is preferably proposed. As a result, equipment investments can be reduced and kept inexpensive. Preferably, the rotary pump is used in conjunction with a mechanical booster pump (pressure booster pump).

[0029] In the design described above, the objects to be joined are heated by applying electrical energy to them. Alternatively, the objects to be joined can be heated by high-frequency induction heating.

[0030] Since, according to the present invention, diffusion bonding is carried out in a nitrogen atmosphere within a defined pressure range, the formation of an oxide or nitride layer on the surfaces of the objects to be joined can be avoided in the manner described above. This allows the diffusion bonding to be carried out efficiently and the bonding strength in the bonded area can be increased.

[0031] Since it is not necessary to create a strong vacuum in the connecting vessel, there is no particular need to install an expensive evacuation mechanism to provide a strong vacuum. Accordingly, equipment investments are reduced. Furthermore, the use of inexpensive nitrogen gas can lower the costs required for diffusion bonding.

[0032] Further developments, advantages, and possible applications of the present invention will also become apparent from the following description of exemplary embodiments and the drawings. All features described and / or illustrated, individually or in any combination, constitute the subject matter of the invention, irrespective of their compilation in the claims or their cross-references. Brief description of the drawings Fig. Figure 1 is a schematic view of the essential elements of a connecting device, Fig. Figure 2 is a schematic representation of the essential elements, showing a typical state in which the connecting device is in accordance with Fig. 1. Used to heat objects to be joined while the objects are pressed, Fig. Figure 3 is a table that shows the relationship between the type of atmospheric gas in the connecting device and its pressure, as well as the nitrogen and oxygen partial pressures. Fig. Figure 4 is a diagram illustrating the relationship between the connection temperature and the connection efficiency of the connected objects, and Fig. Figure 5 is a diagram showing the relationship between the tensile strength of the joined objects, which were joined at 1000°C, and the atmospheric gas. Description of preferred embodiments

[0033] An embodiment of the present invention is explained below with reference to the accompanying drawings and described in detail, with reference to the relationship between a joining method according to the present invention and a device for carrying out such a method.

[0034] Fig. Figure 1 is a schematic representation of essential elements of a typical connecting device 10. The connecting device 10 comprises a connecting container 12, which has an opening and closing door (not shown), and a first electrode 14 and a second electrode 16, which are housed in the connecting container 12. The first electrode 14 and the second electrode 16, together with a power source 18, which will be described later, form a heating source.

[0035] A nitrogen tank 22 (nitrogen supply source) for supplying nitrogen gas to the connecting container 12 via a gas supply line 20 is connected to the connecting container 12. Furthermore, a mass flow controller (hereinafter referred to as MFC) 24 for controlling the flow rate of the nitrogen gas and a pressure control valve 26 are provided in this order on the side of the nitrogen tank 22 in the gas supply line 20.

[0036] Furthermore, a first outlet line 28 and a second outlet line 29 are connected to the connecting vessel 12 to discharge gases from the connecting vessel 12. A rotary pump 30 is provided in the first outlet line 28, while a mechanical booster pump 31 is provided in the second outlet line 29. Accordingly, gases are removed from the connecting vessel 12 by the rotary pump 30 and the mechanical booster pump 31.

[0037] As is generally known, rotary pumps 30 are relatively inexpensive among the various pump types. However, the pressure cannot simply be reduced using such a rotary pump 30. In the present invention, when the pressure control valve 26 is closed and the connecting tank 12 is evacuated, the pressure is on the order of 10 -1 Pa.

[0038] A pressure sensor 32 is connected to the connecting tank 12 to measure the pressure in the connecting tank 12. The pressure sensor 32 and the pressure control valve 26 are electrically connected to a control circuit 38 via corresponding signal lines 34 and 36.

[0039] The first electrode 14 is fixed and positioned in the connecting container 12 on a base 40 so that it points upwards from it, as shown in Fig. Figure 1 shows the first electrode 14 being electrically connected to a positive terminal of the power source 18 via a conducting wire 42.

[0040] On the other hand, a second electrode 16, which has essentially the same structure as the first electrode 14, is arranged opposite the first electrode 14. The second electrode 14 is also attached to one end of a rod 46 of a hydraulic cylinder 44. Specifically, the second electrode 16 moves closer to and away from the first electrode 14 in accordance with the forward and backward movements of the rod 46.

[0041] The second electrode 16 is electrically connected to the negative terminal of the power source 18 via a conducting wire 48. Fig. Figure 1 represents a state in which an electrically open circuit is formed.

[0042] The connecting device 10 is essentially constructed as described above. Next, the operation and function of the device will be explained with reference to a connecting procedure carried out with it.

[0043] First, the door of the connecting container 12 is opened, and a first connecting object W1 and a second connecting object W2 are inserted through the open door into the connecting container 12 and positioned on the first electrode 14. Both the first connecting object W1 and the second connecting object W2 are made of nitrogen-doped stainless steel, and their surfaces are aligned.

[0044] Next, the door is closed and, after the connecting tank 12 has been hermetically sealed, the rotary pump 30 and the mechanical booster pump 31 are operated, thereby evacuating the connecting tank 12 through the first outlet line 28 and the second outlet line 29. Naturally, the pressure regulating valve 26 is closed at this time. As described above, the pressure in the connecting tank 12 is reduced and eventually reaches an order of magnitude of 10. -1 Pa.

[0045] Information about the pressure in the connecting vessel 12 is transmitted to the control circuit 38 via the pressure sensor 32 and the signal line 34. When the control circuit 38 determines that the pressure in the connecting vessel 12 is essentially constant, the pressure control valve 26 is opened by a command signal sent via the signal line 36. As a result, the nitrogen gas begins to flow from the nitrogen tank 22 through the gas supply line 20. The nitrogen gas is introduced into the connecting vessel 12 via the MFC 24 and the pressure control valve 26.

[0046] At this point, the control circuit 38 regulates the nitrogen supply rate so that the pressure in the connecting vessel 12 is maintained at a substantially constant, fixed pressure between 3 and 10. 5Pa is reached. Specifically, the control circuit 38 generates a command signal based on the information about the pressure in the connecting vessel 12, which is obtained via the pressure sensor 32 and the signal line 34, to control the opening degree of the pressure control valve 26 via the signal line 36. If the pressure in the connecting vessel 12 exceeds a defined upper limit, the supply rate of the nitrogen gas is reduced by decreasing the opening degree of the pressure control valve 26, while conversely, if the pressure in the connecting vessel is lower than a lower limit, the opening degree of the pressure control valve 26 is increased and the supply rate of the nitrogen gas is increased.

[0047] For example, if the nitrogen atmosphere is maintained at a pressure of 3 Pa, the oxygen partial pressure in the connecting vessel 12 will be significantly lower compared to an air or argon atmosphere at the same pressure. Specifically, the oxygen partial pressure in this case is equivalent to the oxygen partial pressure in a case where the pressure is reduced to 3 × 10 -3 Pa is set when the atmosphere is evacuated. This also applies in the case where a nitrogen atmosphere is formed while the pressure in the connecting vessel is reduced from 12 to 10. 5 When Pa is set, the partial pressure of oxygen becomes significantly lower than in an air atmosphere or an argon gas atmosphere at the same pressure.

[0048] By using nitrogen gas as the atmospheric gas, the oxygen partial pressure in the connection vessel 12 can thus be achieved in the same order of magnitude as an oxygen partial pressure in a case where the interior of the connection vessel 12 is evacuated to a vacuum, even when the pressure of the connection environment is relatively high.

[0049] Next, as in Fig. As shown in Figure 2, the hydraulic cylinder 44 is operated and its rod 46 is moved downwards and forwards. As a result, the second electrode 16 presses the second connection object W2 against the first connection object W1, and an electrically closed circuit is formed.

[0050] In this state, a current I is applied from the power source 18. The current I is returned to the power source 18 via the first electrode 14, the first connection object W1, the second connection object W2, and the second electrode 16. Specifically, electrical energy is supplied to both the first connection object W1 and the second connection object W2, which is associated with a temperature increase of the first connection object W1 and the second connection object W2. Furthermore, atoms from the first connection object W1 diffuse into the second connection object W2, while atoms from the second connection object W2 diffuse into the first connection object W1.

[0051] As described above, in the present embodiment, electrical energy is supplied to the first joining object W1 and the second joining object W2 in a nitrogen environment controlled at a defined pressure. In other words, the current is supplied in a state where the partial pressure of oxygen is extremely low. Consequently, the formation of an oxide film on the surfaces of the first joining object W1 and the second joining object W2 can be prevented. For this reason, the diffusion of atoms is readily promoted, and as a result, a solid-state bond (diffusion bond) can be efficiently established. Therefore, the bond strength in the bonded area can be ensured.

[0052] Since nitrogen gas is present in the surroundings, even when nitrogen from the atomic diffusion zone is desorbed into the first and second bonded objects (together with nitrogen-doped stainless steel), the nitrogen from the surroundings of the diffusion zone is replenished. Because the nitrogen is forced to enter a solid solution within the diffusion zone, a decrease in the nitrogen content is prevented.

[0053] In the event that voids exist between the adjoining end surfaces, for example due to warping (curvature) or similar defects of the first joining object W1 or the second joining object W2, and if diffusion bonding is carried out in an argon or helium environment, the first joining object W1 and the second joining object W2 will be joined in a state in which argon or helium gas is trapped in these voids. Specifically, voids remain between the first joining object W1 and the second joining object W2, raising concerns that the bond strength may be reduced.

[0054] In contrast, in the present embodiment, diffusion bonding is carried out in a nitrogen environment. Since the first bonding object W1 and the second bonding object W2 are in a state in which nitrogen is caused to enter a solid solution, any nitrogen gas remaining in the cavities is consumed by becoming solidly dissolved in the first bonding object W1 and the second bonding object W2. For this reason, it becomes possible to eliminate the cavities between the first bonding object W1 and the second bonding object W2, thus increasing the bond strength between the two bonded objects W1 and W2.

[0055] Furthermore, the standard production energy required to form a nitride film on stainless steel with nitrogen is higher than the standard production energy required to form an oxide film. This means that the nitrogen gas in the environment is less likely to react with the first compound object W1 and the second compound object W2. Consequently, nitrogen gas tends to be inert with respect to the first compound object W1 and the second compound object W2. Specifically, the formation of a nitride film on the first compound object W1 and the second compound object W2 is prevented.

[0056] After a specified time has elapsed, the supply of current I from the power source 18 is interrupted, and the rod 46 of the hydraulic cylinder 44 is retracted upwards so that it comes to the position indicated in Fig. The state shown in Figure 1 returns. As a consequence, the electrical energy of the first connecting object W1 and the second connecting object W2 is stopped, and simultaneously, the diffusion of atoms ceases. The solid-state connection (diffusion connection) between the first object W1 and the second object W2 is complete. Specifically, the first connecting object W1 and the second connecting object W2 are connected to each other across their entire surfaces.

[0057] As described above, the present invention avoids a reduction in the nitrogen content in the diffusion area (joint area) and allows sufficient diffusion of atoms. Consequently, the corrosion resistance and joint strength in the joint area can be increased.

[0058] As explained above, the present embodiment does not require the use of expensive gases such as argon, helium, or the like. Furthermore, since there is no need to reduce the pressure in the connecting vessel, it is sufficient to attach the rotary pump 30 and the mechanical booster pump 31 to the connecting vessel 12. In other words, there is no need to provide an expensive diffusion pump or a turbomolecular pump. Consequently, the equipment investment required to form the connecting device 10 can be reduced, and diffusion connecting can be implemented at low cost.

[0059] In the embodiment described above, nitrogen-doped stainless steel was used for the first connection object W1 and the second connection object W2. However, it is also possible to use various other types of steel materials, including stainless steel, nitrogen-doped stainless steel, nickel alloys, copper alloys, and the like. In this case as well, the formation of an oxide or nitride film is avoided. Since such materials allow nitrogen to enter a solid solution, residual voids in the connection area can be avoided, thus ensuring the connection strength. Naturally, the quality of the connection area is also excellent. Furthermore, it is possible to manufacture the first connection object W1 and the second connection object W2 from different metals.In other words, the present invention is also applicable when heterogeneous metals are to be joined together.

[0060] In the present embodiment, the pressure in the connecting vessel 12 is controlled so that it remains essentially constant. However, the control can also be implemented such that the pressure repeatedly fluctuates between 3 and 10. 5 Pa varies, that is, from a lower limit of 3 Pa to an upper limit of 10 5 Pa.

[0061] Furthermore, in this embodiment, both connecting objects W1 and W2 are heated by the electrical current supplied to the first connecting object W1 and the second connecting object W2. However, it is understood that other heating methods, such as high-frequency induction heating or the like, can also be used.

[0062] Instead of the rotary pump 30, a diffusion pump or a turbomolecular pump or the like can also be used. Example 1:

[0063] The in the Fig. 1 and Fig. The connection device shown in Figure 2 was connected to a gas analysis system. Specifically, an outlet line was connected to the connection vessel 12, and a gas analysis device and a pressure measuring device, along with a computer, were connected to the outlet line.

[0064] In this state, the atmospheric gas and the pressure in the connecting vessel 12 were modified in various ways, and the partial pressures of nitrogen and oxygen were determined. The results are presented in Fig. 3 summarized. From Fig. 3. It follows that the oxygen partial pressure in a case where the pressure in the nitrogen atmosphere was set to 3 Pa is equivalent to the oxygen partial pressure in a case where the pressure was set to 3 × 10 -3 Pa is set.

[0065] Out of Fig. 3. It also follows that the oxygen partial pressure in a case where the pressure in the nitrogen atmosphere is 10 5 The partial pressure of oxygen (Pa) is significantly lower than that of oxygen in an argon atmosphere at the same pressure. This means that nitrogen gas can produce a lower partial pressure of oxygen than argon gas, thus preventing oxidation of the connected objects. Example 2:

[0066] The base surfaces of cylindrical, column-shaped connecting objects made of SUS304 (stainless steel), with a length of 30 mm and a base diameter of 12 mm, were brought into contact with each other. While they were pressed in this state at 5.2 MPa through the first electrode 14 and the second electrode 16 of the joining device 10, an electric current was applied at connection temperatures of 700°C, 800°C, 900°C, 1000°C, and 1100°C. This was measured using an R-type thermocouple, which was attached by spot welding near the joining surface. This joined the two connecting objects together. The connecting objects were heated to the connection temperature at a rate of approximately 100°C / minute and held at the connection temperature for 20 minutes. The connecting container 12 was filled with a nitrogen atmosphere at pressures of 5 Pa and 10 2 Pa and 10 5 Pa filled.

[0067] For comparison purposes, the conditions described above were replicated except for the fact that the pressures were reduced to 10 -2 Pa, 5 Pa, 10 2 Pa and 10 5 The Pa settings were adjusted, the device was manufactured, and the interior was supplied with ambient air. The connection of the components described above was then carried out.

[0068] The relationship between the joining efficiency, obtained by dividing the joining strength of each of the joining objects by the strength of SUS304, and the joining temperature is shown in the Fig. The tensile strength of the bonded objects, joined together at 1000°C, is shown in diagram 4. Fig. 5 shown. Fig. As a result of 4, the bonding efficiency is increased in conjunction with an increase in the bonding temperature and also with a decrease in atmospheric gas pressure. At 1000°C and 1100°C, a strength equivalent to that of SUS304 was obtained. As can be seen from Fig. 5, and assuming that the pressure in the connecting container 12 was the same, a higher bond strength was obtained in the case of a nitrogen environment than in an air atmosphere.

[0069] In a comparison of the oxidized state of the joined objects after the joining process, it was also found that when the joining was carried out in a 10 5 When the Pa atmosphere was used, a thick, black-colored oxidation film formed. On the other hand, at 10 2 At 5 Pa, a dark green colored oxidation film was observed, while at 5 Pa no oxidation film was present and a metallic shine was observed.

[0070] In contrast, even when the reaction was carried out in a nitrogen atmosphere, the oxidation state was significantly lower at the same pressure than when the reaction was carried out in an air atmosphere, although a similar trend was observed. This can be attributed to the fact that the partial pressure of oxygen was reduced by the nitrogen environment.

[0071] Using a scanning electron microscope (SEM), the tensile fracture surface of each of the joined objects, which were heated to 1000°C under a 10 5 Pa and 5 Pa nitrogen atmosphere or under a 10 5 Pa and 10 -2The bonding of objects in an air atmosphere was investigated. It was found that, in both air and nitrogen atmospheres, the depressions became larger as the ambient gas pressure decreased. Furthermore, a reduction in the number of inclusions within the depressions was observed. At the same pressure, compared to objects bonded in air, the depressions in nitrogen atmospheres were larger, and the number of inclusions was lower. These observations indicate that a better bond is obtained at lower ambient gas pressures, while at the same pressure, a better bond is achieved when nitrogen gas is used instead of air.

Claims

[1] Diffusion joining process in which, during the heating of a first joining object (W1) and a second joining object (W2) which are made of metal and are in contact with each other, pressure is applied to the first joining object (W1) and the second joining object (W2) and atoms from the first joining object (W1) diffuse into the second joining object (W2), while atoms from the second joining object (W2) diffuse into the first joining object (W1), thereby joining the two joining objects (W1, W2) together, the process comprising the following steps: Inserting the first connecting object (W1) and the second connecting object (W2) into a connecting container (12) and evacuating the connecting container (12) to a pressure on the order of 10 -1 Pa, Supplying nitrogen gas into the evacuated connecting vessel (12), whereby the pressure in the connecting vessel (12) is increased to 3 to 10 5 Pa is controlled, and Heating the first connecting object (W1) and the second connecting object (W2) inside the connecting container (12), in which the nitrogen gas is at a pressure of 3 to 10 5 Pa exhibits, whereby pressure is applied to the first connecting object (W1) and the second connecting object (W2), wherein the first connecting object (W1) consists of a steel material, a nickel alloy or a copper alloy, and wherein the second connecting object (W2) consists of a steel material, a nickel alloy or a copper alloy. [2] Method according to claim 1, characterized by , that the heating of the first connecting object (W1) and the second connecting object (W2) is carried out by supplying electric current. [3] Method according to claim 1, characterized by , that the heating of the first connecting object (W1) and the second connecting object (W2) is carried out by high-frequency heating. [4] Method according to any one of the preceding claims, characterized by , that for the first connecting object (W1) and / or the second connecting object (W2) materials are used which are able to cause nitrogen to enter a solid solution. [5] Method according to claim 4, characterized by , that nitrogen-doped stainless steel is used for the first connecting object (W1) and / or the second connecting object (W2).

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