Stress reducing sonotrode, ultrasonic machining device with sonotrode, and use of sonotrode

Abstract

The present invention relates to a sonotrode for ultrasonic machining of metals, having a resonant frequency in the ultrasonic range, the sonotrode having a welding surface (5) intended to come into contact with the metal to be machined, characterized in that when the sonotrode vibrates at its resonant frequency and the welding surface (5) comes into contact with the metal to be machined, the welding surface (5) heats up and a temperature gradient is formed between the welding surface (5) and a section (6) of the sonotrode adjacent to the welding surface, as a result of which the welding surface (5) develops stresses, and means are provided for reducing the stresses in the welding surface during machining.

Description

[Technical Field]

[0001] The present invention relates to a sonotrode for ultrasonic machining of metals, the sonotrode having a resonant frequency in the ultrasonic range, the sonotrode having a welding surface intended to come into contact with the metal to be machined. For machining, the sonotrode is excited with acoustic ultrasonic vibrations at a frequency close to the resonant frequency. A standing wave is then formed in the sonotrode, the standing wave having at least two vibration maxima and at least one vibration node. [Background technology]

[0002] During ultrasonic machining of metals, such as metal cables, very high energies must be provided to melt the metal elements to be joined, as is the case when welding metal cables. It is known to manufacture the corresponding sonotrodes from hardenable steel and heat treat them to achieve hardnesses of up to 54-62 HRC.

[0003] The very high energy required for welding causes a rapid temperature rise between the metal elements to be welded and also at the interface between one sonotrode and the other. The welding surfaces can briefly reach temperatures above 300°C.

[0004] When welding metals, defects in the sonotrode often occur after a relatively short time and are essentially limited to the welding surface. This results in a shortened service life and increased processing costs. Therefore, it has been proposed to equip the sonotrode with two welding surfaces as a reversible sonotrode, so that if a defect occurs in one welding surface, the sonotrode can be rotated and the other welding surface can be used. This significantly reduces costs. However, the service life of each welding surface is reduced when welding metals compared to the service life of sonotrodes when welding plastics. Furthermore, defects caused by a sudden increase in temperature can sometimes be severe, affecting vibration behavior and rendering the entire sonotrode unusable, including the unused welding surface of a reversible sonotrode. In the worst case scenario, the material of the welding surface breaks off when the welding surface is first used, causing the sonotrode to wear out and become unusable. Summary of the Invention [Problem to be solved by the invention]

[0005] Based on this prior art, the object of the present invention is to provide a sonotrode with a longer service life, especially when welding metals. [Means for solving the problem]

[0006] According to the present invention, this objective is achieved by providing a means for reducing stress on the weld surface during processing.

[0007] As expected, when the sonotrode vibrates at or near its resonant frequency and the welding surface comes into contact with the workpiece metal, the welding surface heats up. A temperature gradient forms between the welding surface and the section of the sonotrode adjacent to it. The temperature difference within the sonotrode material inevitably leads to thermal stresses within the material. As a rule, compressive, tensile, and shear stresses are generated. These initially result in elastic deformations that disappear again when the sonotrode is not in use. However, if the deformations are too great, the sonotrode material may change or even break. While this risk is negligible when welding plastics, it can be significant when welding metals.

[0008] Extensive investigations have surprisingly shown that the cause of premature wear is not the high temperature of the sonotrode during welding, but rather the extreme temperature gradient that develops between the weld surface and the section of the sonotrode adjacent to the weld surface. The welding process is carried out in such a way that only the thin layer of material directly adjacent to the weld surface rises to a high temperature in a short period of time, while the deeper material layers of the sonotrode cannot keep up with the extreme temperature rise in such a short period of time.

[0009] The measures according to the invention for reducing stresses on the weld surface during welding therefore increase the lifespan of sonotrodes intended for ultrasonic machining of metals.

[0010] In a preferred embodiment, when the sonotrode is excited at a resonant frequency, a standing wave is formed along the sonotrode axis, and the welding surface is not arranged perpendicular to the sonotrode axis, which has proven to be advantageous when machining metal components. It is best if the welding surface runs parallel or essentially parallel to the sonotrode axis.

[0011] When welding plastic films, it is often advantageous if the welding surface has a specific structure. For example, it is known to introduce into the welding surface several narrow grooves arranged very close to each other, which concentrates the energy introduced into the film in a specific area.

[0012] Some recesses may also be configured as grooves, whereby the grooves are preferably arranged parallel to one another.

[0013] This surface structure has also been shown to be advantageous for metal welding sonotrodes in order to optimally transfer the vibration amplitude of the sonotrode to the part.

[0014] However, these surface structures introduce additional stress buildup at the weld surface due to the aforementioned temperature gradients.

[0015] To reduce the stresses on the weld surface, several recesses can be introduced, for example, with a width a and a depth t, where a>3×t. These recesses are therefore advantageously very wide, with a width at least three times the depth. If the recesses are relatively narrow, the greatest stresses will occur at the bottom of the recess, which may lead to defects. The recesses can be, for example, circular. The width a then corresponds to the diameter of the circle.

[0016] Furthermore, it is advantageous if the groove is curved in cross section, with a radius of curvature r>1.5 mm, and especially best if r>3 mm.

[0017] Furthermore, in a preferred embodiment, the grooves are each arranged in a plane extending perpendicular to the sonotrode axis, and the welding surface is preferably arranged parallel to the sonotrode axis.

[0018] In a further particularly preferred embodiment, the sonotrode comprises a sonotrode body and a welding section with the welding surface, and is provided with a connecting web interconnecting the sonotrode body and the welding section, the connecting web having a cross-sectional area parallel to the welding surface that is at least 5%, preferably at least 10%, and most preferably at least 15% smaller than the welding surface, as a means for reducing stresses in the welding surface. By reducing the cross-sectional area of ​​the connecting web, less heat introduced during welding is dissipated into the sonotrode body, resulting in a faster heating of the part of the connecting web facing the welding surface and therefore a reduced temperature gradient in the area of ​​the welding surface.

[0019] For example, the connecting web can have a first notch on its first side. Tests have shown that introducing the notch reduces the cross-sectional area of ​​the connecting web and, therefore, the stress on the welding surface. It is particularly preferred if a second notch is provided on the second side of the connecting web. The two sides of the connecting web can be arranged parallel to each other.

[0020] In a preferred embodiment, the width of the connecting web in the region of the at least one cutout is at least 5%, preferably at least 10%, most preferably at least 15% smaller than the length of the welding surface.

[0021] Furthermore, the first cutout and / or the second cutout can have a curved cutout bottom in a cross section perpendicular to the sonotrode axis.

[0022] It is advantageous if the first cutout and / or the second cutout extend parallel to the sonotrode axis.

[0023] Additionally, as a means for reducing stress in the welding surface, at least one slot can be provided in the welding surface, dividing the welding surface into several welding surface segments, with at least one slot preferably extending parallel to the sonotrode axis. By dividing the welding surface into individual welding surface segments, maximum welding surface stress is significantly reduced. To prevent the slot from adversely affecting the welding results, the slot width b should be less than 0.3 mm, preferably less than 0.15 mm. To prevent the individual welding surface segments from colliding during processing due to heat generation, the width b should preferably be greater than 0.025 mm, and particularly preferably greater than 0.05 mm.

[0024] The slot width b is understood to be the width of the slot in the area of ​​the weld surface. In principle, the slot can be narrow or wide in the material.

[0025] Furthermore, it is advantageous if the slot has a depth t that is at least 10 times, preferably at least 20 times, greater than the width b.

[0026] Alternatively, the slots may also be formed with a depth t of at least 2 mm, preferably at least 5 mm.

[0027] In a further preferred embodiment, the slot has a curved slot bottom, the slot has a base with a slot bottom, and the slot has a width b that is greater than width b, preferably at least 5 times greater than width b. G It has.

[0028] The invention also relates to an ultrasonic machining device comprising a sonotrode according to the invention.

[0029] In this case, preferably, a transducer that converts an electrical alternating voltage into mechanical vibrations is connected to the sonotrode, a generator for generating an electrical alternating voltage having an amplitude Ao is provided, and a control unit is provided as means for reducing the stress on the welding surface. This control unit generates an electrical alternating voltage having an amplitude Av during a time interval t until an electrical alternating voltage having an amplitude Ao is generated, where Av < Ao. In this case, t is preferably longer than 0.2 seconds, particularly preferably longer than 0.4 seconds.

[0030] For the time interval before the actual welding process, the welding surface, and thus the area adjacent to the welding surface, is already heated, but more energy is required. Also, since the area adjacent to the welding surface is currently heated before the actual welding process starts, a very steep temperature gradient in the area of the welding surface can be alleviated. The amplitude Av can be increased from a minimum value Amin, which can be zero but is preferably greater than 0, to Av during the time interval t. As experiments have shown, a linear increase in the amplitude is particularly advantageous.

[0031] During welding, a significant amount of thermal energy is transmitted into the sonotrode. The sonotrode is usually connected directly or via an amplitude transformer to a transducer, and since this transducer has a temperature-sensitive piezo element, there are known embodiments where the welding surface is cooled between individual welding steps, for example. However, it has been shown that this is harmful to the life of the sonotrode because the temperature gradient on the welding surface is extremely high during the subsequent welding step.

[0032] In an alternative embodiment of the sonotrode according to the present invention, a cooling device is provided as a means for reducing stresses at the welding surface, cooling the sonotrode in the region of the vibration nodes, preferably without cooling the welding surface. The objective is therefore to keep the welding surface and the section of the sonotrode immediately adjacent to the welding surface hot, without cooling them, while still ensuring that the transducer located at the end of the sonotrode facing away from the welding surface is not overheated. In principle, it is necessary to ensure that the transducer does not heat up to more than 50°C. Cooling devices are known to those skilled in the art. For example, a fan, compressed air, or a water-cooled holder acting on the vibration nodes are possible. It is essential that the sonotrode is not cooled at the welding surface, precisely because a temperature increase is desired to reduce the temperature gradient.

[0033] In a preferred embodiment, at least one fan is provided to direct an air flow over the sonotrode, but not over the welding surface, and over a section away from the welding surface. This section can be located, for example, within the oscillation node of a standing wave formed when the sonotrode is excited at a suitable ultrasonic frequency. Furthermore, a barrier can be provided between the welding surface and this section, so that the barrier largely prevents the air flow caused by the fan from reaching the welding surface. The barrier preferably does not contact the sonotrode. The barrier can be implemented, for example, as a bulkhead or bulkhead curtain.

[0034] In an alternative embodiment, a heating device is used as a means for reducing stress in the welding surface, and the heating device is arranged and configured so that the welding surface can be heated. Heating devices are known to those skilled in the art. For example, the heating device may comprise a heating stamp that is brought into contact with the welding surface or the connecting web before or during processing in order to heat the welding surface or the area immediately adjacent to the welding surface. Alternatively, a hot air stream may be provided that is directed onto the welding surface or the area immediately adjacent to the welding surface. Induction heating or infrared radiation may also be used.

[0035] In a preferred embodiment, an infrared emitter is provided, which is positioned so that it is directed towards the welding surface or towards a mirror so that infrared light emitted from the infrared emitter is reflected onto the welding surface.

[0036] Heating by induction has proven to be very advantageous, as very high temperatures can be reached in the immediate vicinity of the weld surface in very short cycle times. For this purpose, an induction coil is positioned so that its windings surround a portion of the sonotrode. For example, the induction coil can circulate around the weld section. The induction coil can have multiple windings. The windings can be formed as a spiral and arranged in one plane. Alternatively, the windings can be arranged as a helix.

[0037] The cooling and / or heating device may be coupled to the sonotrode in such a way that the cooling and / or heating device moves together with the sonotrode relative to the counter-tool, or alternatively, the cooling and / or heating device may also be movable relative to the sonotrode so that it can be positioned relative to the sonotrode during stages when it is not in contact with the workpiece metal in such a way that the sonotrode can efficiently perform its cooling and / or heating functions.

[0038] For example, the heating device may be a hot plate that is brought into contact with the welding surface whenever the welding surface is not in contact with the workpiece metal to preheat the welding surface, not just the area immediately adjacent to the welding surface. If the welding surface has a structure, such as a groove structure according to the present invention, the hot plate should have a corresponding structure.

[0039] Further advantages, features and possible applications will become apparent from the following description of the preferred embodiments and the associated drawings. [Brief explanation of the drawings]

[0040] [Figure 1] FIG. 1 is a side view of a first embodiment of a sonotrode according to the present invention. [Figure 1a] FIG. 1a is a detailed enlarged view of area A of FIG. [Figure 2] FIG. 2 is a perspective view of a second embodiment of a sonotrode according to the invention. [Figure 2a] FIG. 2a is a front view of the sonotrode of FIG. [Figure 2b] FIG. 2b is a detailed enlarged view of area X of FIG. 2a. [Figure 3] FIG. 3 is a perspective view of a third embodiment of a sonotrode according to the invention. [Figure 3a] FIG. 3a is a front view of the sonotrode of FIG. [Figure 3b] FIG. 3b is a detailed enlarged view of area X of FIG. 3a. [Figure 4] FIG. 4 is a schematic diagram of the separation of the cooling and heating devices. DETAILED DESCRIPTION OF THE INVENTION

[0041] 1 and 1a show a first embodiment of a sonotrode 1 according to the invention. The sonotrode 1 has a rear end face 2 and a front end face 3. The rear end face 2 is intended to be in contact with a transducer, via which ultrasonic vibrations can be coupled into the sonotrode, which forms a standing wave, whereby vibration maxima are formed at the rear end face 2 and the front end face 3, while vibration nodes are formed in the middle in the horizontal direction in FIG. 1. The sonotrode 1 has a circumferential expansion 4 in the region of the vibration nodes, whereby the sonotrode can be held, e.g., supported, without its vibration behavior being too strongly influenced by the holder.

[0042] Apart from the section arranged on the front end face 3, the sonotrode is essentially cylindrical. The sonotrode has a substantially cylindrical sonotrode body 8. The section of the sonotrode body 8 facing the front end face 3 has two diametrically opposed connecting webs 6 that connect the sonotrode body to two welding sections, each with a welding surface 5.

[0043] In metal welding, the sonotrode 1 is set to ultrasonic vibration while the welding surface 5 is pressed against the metal elements to be welded. If one welding surface 5 is worn, the sonotrode can be rotated 180° about the sonotrode axis s connecting the rear end face 2 and the front end face 3, so that the second welding surface 5 can be used.

[0044] During welding, the welding surface 5 is heated to a very high temperature for a short time, which results in a very large temperature gradient between the welding surface 5 and the section of the connecting web 6 facing the welding surface.

[0045] This large temperature gradient induces stresses (also called mechanical stresses) within the material. Specifically, the stress state at each specific point within the sonotrode material can be described by a stress tensor. This stress tensor typically contains at least six different stress values. To more easily characterize the stress state, it is common to calculate the scalar stress. This can be done, for example, using the von Mises equivalent stress, named after Richard von Mises. The present invention is also based on this von Mises equivalent stress. The objective of the present invention is to reduce the maximum von Mises equivalent stress occurring at any point on the weld surface 5.

[0046] Thus, in the embodiment shown in Figures 1 and 1a, grooves 7 are introduced into the welding surface 5. Each groove has a depth t and a width a. The width a is significantly greater, i.e., at least three times the depth t. The grooves 7 are arranged parallel to one another. Each groove 7 extends obliquely to the sonotrode axis s, and each groove 7 lies in a plane perpendicular to the sonotrode axis s.

[0047] Figures 2, 2a and 2b show a second embodiment of a sonotrode 101. Figure 2 shows a perspective view. Here too, the sonotrode has a rear end face 102 and a front end face 103, and during operation a standing wave is formed along the sonotrode axis s between the rear end face 102 and the front end face 103. In the region where the vibration nodes are located, an expansion part 104 is provided in which the sonotrode 101 can be held.

[0048] Here too, the sonotrode 101 is essentially cylindrical, with two diametrically opposite connecting webs 106 extending radially outward from the sonotrode axis at the front end face 103, which extend over the sides of the cylindrical section of the sonotrode 101 and terminate in two welding sections, each having a welding surface 105. In the illustrated embodiment, the welding surfaces 105 are configured exactly like the welding surfaces 5 of the embodiment shown in Figures 1 and 1a. The welding surfaces 105 have a length ls (shown in Figure 2a) and a width b s (shown in FIG. 1 ). In addition, the connecting web 106 has two opposing notches 109. Calculations and experiments have shown that these notches significantly reduce the stress in the material in the region of the welding surface 105. These notches 109 are preferably aligned parallel to the sonotrode axis s. In the illustrated embodiment, the notches have a notch depth of approximately 2.5 mm and are spaced from the welding surface 105 by a notch height of 2.4 mm. The sonotrode shown has a resonance frequency of approximately 20 kHz. If the sonotrode is manufactured with a higher resonance frequency, a smaller notch depth can be used to achieve the same effect. A larger notch depth can also be selected. The notches significantly reduce the cross-sectional area of ​​the connecting web, thereby significantly reducing the heat dissipation from the welding surface 105 to the sonotrode body, thereby reducing the temperature gradient, especially in the immediate vicinity of the welding surface 105. Additionally, the overall mechanical stress near the weld surface is reduced because the material in the area of ​​the notch provides a lower reaction force.

[0049] FIG. 2b shows a view perpendicular to the sonotrode axis, from which it can be seen that the notch 109 has a curved notch bottom.

[0050] Figures 3, 3a, and 3b show a third embodiment of the invention. The sonotrode 201 has a rear end face 202 and a front end face 203, with a sonotrode axis s extending between the rear end face 202 and the front end face 203. During operation, a longitudinal standing wave is formed, with a bulge 204 located at a vibration node of the standing wave, on which the sonotrode can be held. Again, no connecting web 206 is provided, connecting the welded section 210 with the welded surface 205 to the sonotrode body 208. As in the embodiment shown in Figures 2, 2a, and 2b, a cutout 209 is provided here.

[0051] In contrast to the previous embodiment, the welding surface 205 has a slot 211, which divides the welding surface 205 into welding surface segments 212 and 213. The slot has a very small width b in order not to adversely affect the welding result caused by contact between the material to be welded and the welding surface 205. On the one hand, the slot width b should be as small as possible, but on the other hand, it must be ensured that the individual welding surface segments do not collide with each other when the welding surface is heated as expected. The slot has a slot depth ST Furthermore, the slot widens towards the bottom of the slot to a maximum width S b It has.

[0052] Figure 4 shows a schematic diagram of a sonotrode 11 according to an embodiment of the invention. The sonotrode 11 comprises a welding section 15 with a welding surface 18, a main section 13, and a connecting section 14 connecting the main section 13 and the welding section 15. The sonotrode is held by a holder 16 that engages with the vibration nodes of the sonotrode. The entire sonotrode 11 can be moved up and down in the direction of the arrow to increase or decrease the distance between the welding surface 18 and the counter tool 12. The material to be machined, i.e., two metal parts in this case, is inserted between the welding surface 18 and the counter tool 12 to be joined by ultrasonic welding using the sonotrode 11.

[0053] In the position shown in FIG. 4, the welding surface 18 can be heated to preheat the entire welding section 15 to prevent excessive temperature gradients in the immediate vicinity of the welding surface 18 during welding.

[0054] Heating can be carried out, for example, using hot air or infrared heaters. However, care must be taken to ensure that only the welding surface 18, and possibly also the welding section 15, is heated. The main section 13, which is usually connected at its end facing away from the welding section 18 to a transducer (not shown) with a temperature-sensitive piezo element, should not be heated under any circumstances. Therefore, even before the actual welding process, a high temperature as high as possible is desired in the area of ​​the welding section 15, but the temperature of the main section 13 should remain as low as possible so as not to damage the piezo element of the transducer.

[0055] Therefore, in a preferred embodiment, it may be necessary to cool the main section 13. For example, the holder 16 may be water-cooled, or a cooling air flow may be provided in this area. In order to keep the main section 13 relatively cool on the one hand and heat the welding surface 18 as much as possible on the other hand, it is necessary to prevent the hot or cold air from reaching the wrong section, especially when working with hot or cold air. Therefore, a separating element 17 is provided, which is arranged so as to essentially surround the connecting section 14 without touching it, and which largely prevents air from flowing in the opposite direction from the welding section 15 to the main section 13.

[0056] The stress reduction according to the invention significantly increases the service life of the sonotrode. Although each measure alone already leads to an improvement in the sonotrode service life, it is advantageous to implement as many of the measures described as possible in combination. [Explanation of symbols]

[0057] 1 Sonotrode 2 Rear end surface 3 Front end surface 4 vibrational nodes 5 Welding Mask 6 Connect Web 7 grooves 8 Sonotrode body 11 Sonotrode 12 Counter Tools 13 Main Section 14 Connecting Web 15 Welding Section 16 Holder 17 Separation elements 18 Welding Mask 101 Sonotrode 102 Rear end face 103 Front end face 104 vibrational nodes 105 Welding Mask 106 Connected Web 108 Sonotrode body 109 Notch 110 Welding Section 201 Sonotrode 202 Rear end surface 203 Front end face 204 Vibration Nodes 205 Welding Mask 206 Connected Web 208 Sonotrode body 209 Notch 210 Welding Section 211 Slots 212 Welding Surface Segment 213 Welding Surface Segment A Groove width B Slot width B v Width of the connecting web in the area of ​​the cutout b s Welding surface width l s Welding surface length k t Notch depth k h Notch height s Sonotrode axis s b Widened slot width s t Slot Depth t Groove depth

Claims

1. A sonotrode for ultrasonic machining of metals, having a resonant frequency in the ultrasonic range, with a welding surface (5) intended to contact the metal to be machined, 1. A sonotrode comprising: a sonotrode vibrating at a resonant frequency, wherein when the sonotrode vibrates and the welding surface (5) contacts the metal to be worked, the welding surface (5) heats up, a temperature gradient is formed between the welding surface (5) and a section (6) of the sonotrode adjacent to the welding surface (5), the welding surface (5) develops stress as a result of the temperature gradient, and means are provided for reducing the stress in the welding surface (5) during working.

2. 2. A sonotrode according to claim 1, characterized in that, when the sonotrode is excited at the resonant frequency, a standing wave is formed along the sonotrode axis (s), and the welding surface (5) is not arranged perpendicular to the sonotrode axis (s), and the welding surface (5) preferably extends parallel or essentially parallel to the sonotrode axis (s).

3. 3. A sonotrode according to claim 1 or 2, characterized in that, as a means for reducing stresses in the weld surface, several recesses of depth t and width a are introduced into the weld surface, where a>3*t.

4. 4. A sonotrode according to claim 3, characterized in that the several recesses are configured as grooves (7), the grooves (7) being preferably arranged parallel to one another.

5. 5. The sonotrode according to claim 4, wherein the grooves (7) are curved in cross section, the radius of curvature of which is preferably r>1.5 mm, particularly preferably r>3 mm.

6. 6. A sonotrode according to claim 4 or 5, characterized in that each groove (7) is arranged in a plane extending perpendicular to the sonotrode axis (s), the welding surface preferably being arranged parallel to the sonotrode axis (s).

7. 7. The sonotrode according to claim 1, characterized in that the sonotrode comprises a sonotrode body (8) and a welded section having the welding surface (5), and is provided with a connecting web (6) connecting the sonotrode body (8) and the welded section, the connecting web (6) having a cross-sectional area parallel to the welding surface (5) that is at least 5%, preferably at least 10%, most preferably at least 15% smaller than the welding surface (5) as a means for reducing stresses in the welding surface.

8. 8. The sonotrode according to claim 7, characterized in that a first notch (109) is arranged on a first side of the connection section and the connecting web (6) has a second notch (109) on a second side.

9. 8. Sonotrode according to claim 7, characterized in that the first notch and / or the second notch (109) have a curved notch bottom in a cross section perpendicular to the sonotrode axis (s).

10. 9. Sonotrode according to claim 7 or 8, characterized in that the first notch and / or the second notch (109) extend parallel to the sonotrode axis (s).

11. 11. A sonotrode according to any one of claims 1 to 10, characterized in that the welding surface (5) is provided with at least one slot (211) as a means for reducing stresses in the welding surface, said slot dividing the welding surface (5) into several welding surface segments (212, 213), said at least one slot (211) preferably extending parallel to the sonotrode axis (s).

12. 12. Sonotrode according to claim 11, characterized in that the at least one slot (21) has a width b less than 0.3 mm, preferably less than 0.15 mm, and said width b being greater than 0.025 mm, preferably greater than 0.05 mm.

13. The at least one slot (21) has a depth b that is at least 10 times, preferably at least 20 times, greater than its width b. ｓｔ 13. The sonotrode according to claim 11 or 12, characterized in that it comprises:

14. The slot has a curved slot bottom, the slot has a base with the slot bottom, and the slot has a width b that is greater than width b, preferably at least five times greater than width b. Ｇ 14. The sonotrode according to claim 11, 12 or 13, characterized in that it comprises:

15. 15. A sonotrode according to any one of claims 1 to 14, characterized in that when the sonotrode is excited at a resonant frequency, a standing wave having at least one vibration node is formed along the sonotrode axis, and a cooling device is provided as a means for reducing stresses on the welding surface, the cooling device cooling the sonotrode in the region of the at least one vibration node, and the cooling device preferably not cooling the welding surface.

16. 16. The sonotrode according to claim 1, wherein a heating device is provided as a means for reducing stress on the welding surface, the heating device being arranged and configured to heat the welding surface.

17. In an ultrasonic processing device, The ultrasonic processing device includes: A sonotrode according to any one of claims 1 to 16, a converter coupled to the sonotrode for converting an electrical alternating voltage into mechanical vibrations; a generator arranged to generate an electrical alternating voltage having an amplitude Ao; and a control unit provided as a means for reducing stress on the welding surface, the control unit generating an electrical AC voltage having the amplitude Av during a time interval t until the electrical AC voltage having the amplitude Ao is generated, where Av<Ao, and t is longer than 0.2 seconds, particularly preferably longer than 0.4 seconds.

18. 18. The ultrasonic machining apparatus according to claim 17, wherein the control unit is configured to increase the amplitude Av from a minimum value Amin, preferably 0, to Ao during the time interval t, the amplitude preferably increasing linearly.

19. 17. Use of a sonotrode according to any one of claims 1 to 16 for ultrasonically joining two metallic materials, the two metallic materials being preferably arranged between a sonotrode and a counter tool, and in order to join the two metallic materials, the counter tool and the sonotrode are moved relative to each other, the sonotrode not being rotated about the sonotrode axis.

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