Voltage-reduced sonotrode, ultrasonic processing device with sonotrode and use of the sonotrode

Abstract

The present invention relates to a sonotrode for the ultrasonic processing of metals, which has a resonance frequency in the ultrasonic range, wherein the sonotrode has a welding face (5) which is provided for coming into contact with the metal to be processed, wherein, when the sonotrode oscillates with the resonance frequency and the welding face (5) is in contact with the metal to be processed, the welding face (5) heats up and a temperature gradient is formed between the welding face (5) and sections (6) of the sonotrode adjacent to the welding face, wherein as a consequence of the temperature gradient the welding face (5) is subjected to a voltage, characterised in that means for reducing the welding face voltage during processing are provided.

Description

[0001] Stress-reduced sonotrode, ultrasonic processing device with sonotrode and use of the sonotrode

[0002] The present invention relates to a sonotrode for the ultrasonic processing of metals, which has a resonant frequency in the ultrasonic range. The sonotrode has a welding surface designed to come into contact with the metal to be processed. For processing, the sonotrode is excited with an acoustic ultrasonic oscillation with a frequency close to the resonant frequency. A standing wave then forms in the sonotrode, which has at least two oscillation maxima and at least one oscillation node.

[0003] When ultrasonically processing metals, such as metal cables, very high energies must be applied to melt the metal elements to be joined. This is the case, for example, when welding metal cables. It is known to manufacture corresponding sonotrodes from hardenable steel and heat-treat them to achieve a hardness of up to 54 to 62 HRC.

[0004] Due to the very high energy required for welding, a sudden temperature rise occurs between the metallic elements being welded, as well as at the interface between the sonotrode and the metallic element. The weld surface can briefly rise to temperatures exceeding 300 °C.

[0005] 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 short service lives, which in turn increases processing costs. It has therefore already been proposed to equip the sonotrode as a reversible sonotrode with two welding surfaces, so that if one welding surface develops defects, the sonotrode can be rotated and the other welding surface used. This has led to considerable cost reductions. However, each welding surface has a reduced service life when processing metals compared to sonotrode service lives achieved when welding plastics. Furthermore, the defects resulting from the abrupt temperature increase can in some cases be so severe that they impair the vibration behavior, meaning that the entire sonotrode, i.e.The unused welding surface of the reversible sonotrode also becomes unusable. In the worst case, the material on the welding surface breaks during the first use of the welding surface, and the sonotrode becomes worn and can no longer be used.

[0006] Based on the described prior art, it is therefore an object of the present invention to provide a sonotrode which has a longer service life, in particular when welding metals.

[0007] According to the invention, this object is achieved by providing means for reducing the stresses on the welding surface during processing.

[0008] As expected, the welding surface heats up when the sonotrode oscillates at or near the resonant frequency and the welding surface is in contact with the metal being processed. A temperature gradient develops between the welding surface and the adjacent sections of the sonotrode. Temperature differences within the sonotrode material inevitably lead to thermal stresses within the material. Typically, compressive, tensile, and shear stresses arise. These initially lead to elastic deformations, which disappear when the sonotrode is not in use. However, if the deformation is too great, the material of the sonotrode can change and even break. While this risk is negligible when welding plastics, it can be significant when welding metals.

[0009] Extensive investigations have surprisingly shown that it is less the high temperature of the sonotrode during welding that is responsible for premature wear, but rather the extreme temperature gradient that develops between the welding surface and the sections of the sonotrode adjacent to the welding surface. Welding occurs in such a short time that only the welding surface and a directly adjacent thin material layer are heated to such high temperatures, while deeper material layers of the sonotrode cannot keep up with the extreme temperature increase in such a short time. Therefore, the means according to the invention for reducing the welding surface tension during welding increase the service life of sonotrodes intended for ultrasonic processing of metals.

[0010] In a preferred embodiment, when the sonotrode is excited at the resonant frequency, a standing wave will form along a sonotrode axis, with the welding surface not arranged perpendicular to the sonotrode axis. This welding surface arrangement has proven advantageous when machining metal elements. Ideally, the welding surface runs parallel or substantially parallel to the sonotrode axis.

[0011] When welding plastic films, it is often advantageous if the welding surface has some structure. For example, it is common practice to create narrow grooves in the welding surface, arranged very closely together. This concentrates the energy applied to the film on specific areas.

[0012] The recesses can also be designed as grooves, whereby the grooves are best arranged parallel to each other.

[0013] It has been shown that this surface structure is also advantageous for sonotrodes used for welding metals in order to optimally transfer the vibration amplitude of the sonotrode to the component.

[0014] However, these surface structures lead to an additional stress increase on the welding surface due to the previously mentioned temperature gradients.

[0015] To reduce the welding surface tension, several recesses of depth t and width a can be introduced, where a > 3 x t. The recesses are therefore advantageously very wide, at least 3 times as wide as they are deep. If the recesses are narrower, the greatest stresses occur at the base of the recesses, which can lead to defects. The recesses can be circular, for example. The width a then corresponds to the diameter of the circular shape.

[0016] Furthermore, it is advantageous if the grooves are curved in a cross-sectional view. It is best if the radius of curvature is r > 1.5 mm and even better if r > 3 mm. Furthermore, a preferred embodiment provides for the grooves to be arranged in a plane that runs perpendicular to the sonotrode axis, with the welding surface preferably being arranged parallel to the sonotrode axis.

[0017] In a further particularly preferred embodiment, the sonotrode comprises a sonotrode main body and a welding section having the welding surface, wherein a connecting web is provided which connects the sonotrode main body and the welding section to one another, wherein, as a means for reducing the welding surface tension, the connecting web has 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. By reducing the cross-sectional area of ​​the connecting web, the heat introduced during welding is dissipated to the sonotrode main body less quickly, so that the part of the connecting web facing the welding surface heats up more quickly, thereby reducing the temperature gradient in the region of the welding surface.

[0018] For example, the connecting web can have a first notch in a first side surface. By introducing the notch, the cross-sectional area of ​​the connecting web and thus the welding surface tension are reduced, as demonstrated by tests. It is particularly preferred if a second notch is provided in a second side surface of the connecting web. The two side surfaces of the connecting web can be arranged parallel to each other.

[0019] In a preferred embodiment, the width of the connecting web in the region of the at least one notch is at least 5%, preferably at least 10% and most preferably at least 15% smaller than the length of the welding surface.

[0020] Furthermore, the first and / or second notch can have a curved notch base in a sectional view perpendicular to the sonotrode axis.

[0021] It is advantageous if the first and / or second notch runs parallel to the sonotrode axis.

[0022] Furthermore, as a means of reducing the welding surface tension, at least one slot can be provided in the welding surface, which divides the welding surface into several welding surface segments, wherein the at least one slot preferably runs parallel to the sonotrode axis. By dividing the welding surface into individual welding surface segments, the maximum welding surface tension is significantly reduced. To ensure that the slot does not have a negative impact on the welding result, the slot width b should be < 0.3 mm and preferably < 0.15 mm. To ensure that the individual welding surface segments do not collide during processing due to the development of heat, the width b should preferably be > 0.025 mm and particularly preferably > 0.05 mm.

[0023] The slot width b is the width of the slot in the area of ​​the welding surface. In principle, the slot can taper or widen within the material.

[0024] Furthermore, it is advantageous if the slot has a depth t which is at least 10 times, preferably at least 20 times, greater than the width b.

[0025] Alternatively, the slot may also be formed with a depth t of at least 2 mm and preferably of at least 5 mm.

[0026] In a further preferred embodiment, the slot has a curved slot base, wherein the slot has a base section having the slot base, in which the slot has a width bc which is greater than the width b and preferably at least by a factor of 5 greater than the width b.

[0027] The present invention also relates to an ultrasonic processing device with a sonotrode according to the invention.

[0028] It is preferably provided that a converter, which converts an electrical alternating voltage into a mechanical oscillation, is coupled to the sonotrode, and a generator for generating the electrical alternating voltage with an amplitude A o is provided, wherein a control unit is provided as a means for reducing the welding surface tension, which before the electrical alternating voltage with the amplitude A o generates an alternating electrical voltage with the amplitude Av during a time interval t, where A v < A o . t is preferably more than 0.2 seconds and particularly preferably more than 0.4 seconds.

[0029] Although more energy is required due to the time interval preceding the actual welding process, during which the sealing surface and thus also the areas adjacent to the sealing surface are already heated, the very abrupt temperature gradient in the sealing surface area can be mitigated because the areas adjacent to the welding surface have already been heated before the actual welding process begins. The amplitude Av can be increased during the time interval t from a minimum value Amin, which can be zero but is preferably greater than 0, to Av. Tests have shown that a linear increase in the amplitude is particularly advantageous.

[0030] During the welding process, a significant amount of heat energy is transferred to the sonotrode. Since the sonotrode is usually connected to a converter, either directly or via an amplitude transformer, and this converter contains temperature-sensitive piezo elements, there are known designs in which the welding surface is cooled, for example, between individual welding steps. However, this has been shown to be detrimental to the service life of the sonotrode, as the temperature gradient at the welding surface is then extremely high during the next welding step.

[0031] In an alternative embodiment of the sonotrode according to the invention, a cooling device is provided as a means for reducing the welding surface tension. This cooling device cools the sonotrode in the region of the vibration node, whereby the cooling device ideally does not cool the welding surface. The aim is therefore not to cool the welding surface and the sections of the sonotrode immediately adjacent to the welding surface, but to keep them at an elevated temperature. However, it must be ensured that a converter arranged on the end of the sonotrode facing away from the welding surface is not excessively heated. As a rule, it must be ensured that the converter is not heated above 50 °C. Cooling devices are known to those skilled in the art. For example, fans directed towards the region of the vibration node, compressed air, or a water-cooled holder acting in the vibration node are suitable here.It is important that the sonotrode is not cooled at the welding surface, since it is precisely there that a temperature increase is desired in order to reduce the temperature gradient.

[0032] In a preferred embodiment, at least one fan is provided, which directs an air flow onto the sonotrode, not onto the welding surface, but onto a section spaced apart from the welding surface. This section can be positioned, for example, at the oscillation node of the standing wave that forms when the sonotrode is excited at the appropriate ultrasonic frequency. Furthermore, a barrier can be provided, which is arranged between the welding surface and this section in such a way that the barrier largely prevents the air flow caused by the fan from reaching the welding surface. The barrier preferably does not come into contact with the sonotrode. The barrier can be designed, for example, as a partition wall or a partition curtain.

[0033] In an alternative embodiment, a heating device is used as a means for reducing the welding surface tension, which heating device is arranged and designed such that the welding surface can be heated. Heating devices are known to those skilled in the art. For example, the heating device can comprise a heat 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 can be provided that is directed onto the welding surface or the area immediately adjacent to the welding surface. Induction heating or the use of infrared radiation is also possible.

[0034] In a preferred embodiment, an infrared radiator is provided which is either directed towards the welding surface or is directed towards a mirror which is arranged such that infrared radiation emanating from the infrared radiator is reflected onto the welding surface.

[0035] Heating by induction has proven to be very advantageous, as it allows very high temperatures to be achieved in the immediate vicinity of the welding surface in very short cycle times. For this purpose, an induction coil is positioned so that its windings encircle parts of the sonotrode. For example, the induction coil can encircle the welding section. The induction coil can have multiple windings. The windings can be spiral and arranged in a single plane. Alternatively, the windings can also be arranged in a helix.

[0036] The cooling device and / or the heating device can 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. Alternatively, the cooling device and / or the heating device can also be movable relative to the sonotrode in order to be positioned relative to the sonotrode during phases in which the sonotrode is not in contact with the metal to be machined in such a way that they can efficiently perform the cooling and / or heating function.

[0037] For example, the heating device could be a heating plate, which is brought into contact with the welding surface whenever the welding surface is not in contact with the metal to be processed, in order to preheat the welding surface—as well as the areas immediately adjacent to the welding surface. If the welding surface has a structure, such as the grooved structure according to the invention, the heating plate should have a corresponding structure.

[0038] Further advantages, features, and possible applications will become clear from the following description of preferred embodiments and the accompanying figures. They show:

[0039] Figure 1 is a side view of a first embodiment of the sonotrode according to the invention,

[0040] Figure 1 a is a detailed enlargement of area A of Figure 1 ,

[0041] Figure 2 is a perspective view of a second embodiment of a sonotrode according to the invention,

[0042] Figure 2a is a front view of the sonotrode of Figure 2,

[0043] Figure 2b is a detailed enlargement of area X of Figure 2a,

[0044] Figure 3 is a perspective view of a third embodiment of a sonotrode according to the invention,

[0045] Figure 3a is a front view of the sonotrode of Figure 3,

[0046] Figure 3b is a detailed enlargement of area X of Figure 3a and

[0047] Figure 4 is a schematic representation of the separation of cooling device and heating device.

[0048] Figures 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 come into contact with a converter, via which an ultrasonic vibration can be coupled into the sonotrode, so that it forms a standing wave, with a vibration maximum being formed in the rear end face 2 and the front end face 3, while a vibration node is formed in the middle in the horizontal direction of Figure 1. The sonotrode 1 has a circumferential bead 4 in the region of the vibration node, on which the sonotrode can be held, e.g. supported, without the vibration behavior of the sonotrode being excessively influenced by the holder.

[0049] The sonotrode is essentially cylindrical, except for the section located at the front end face 3. It has a nearly cylindrical sonotrode main body 8. The section of the sonotrode main body 8 facing the front end face 3 has two diametrically opposed connecting webs 6, which connect the sonotrode main body to two welding sections, each comprising a welding surface 5.

[0050] For welding metals, a welding surface 5 is pressed against the metal elements to be welded, while the sonotrode 1 is set into ultrasonic vibration.

[0051] If one welding surface 5 is worn, the sonotrode can be rotated by 180° about its sonotrode axis s connecting the rear end face 2 and the front end face 3, so that the second welding surface 5 can then be used.

[0052] During welding, the welding surface 5 is heated to very high temperatures in a short time, which leads to a very large temperature gradient between the welding surface 5 and the section of the connecting web 6 facing the welding surface.

[0053] This enormous temperature gradient leads to stresses (also referred to as mechanical stresses) within the material. In detail, the stress state at any specific point within the sonotrode material can be described by a stress tensor. This stress tensor usually contains at least six different stress values. To characterize the stress state more easily, it is common to calculate a scalar equivalent stress. This can be done, for example, using the Mises equivalent stress, named after Richard von Mises. The present invention is also based on this Mises equivalent stress. The aim of the invention is to reduce the maximum Mises equivalent stress that occurs at any point on the welding surface 5.

[0054] In the embodiment shown in Figures 1 and 1a, grooves 7 have been introduced into the welding surface 5. The individual grooves have a depth t and a width a. The width a is significantly larger, namely at least three times as large as the depth t. The grooves 7 are arranged parallel to one another. Each groove runs skew relative to the sonotrode axis S, with each groove 7 lying in a plane perpendicular to the sonotrode axis S.

[0055] 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, between which a standing wave forms along the sonotrode axis s during operation. In the region where the vibration node is located, a bead 104 is provided, on which the sonotrode 101 can be held.

[0056] Here, too, the sonotrode 101 is essentially cylindrical in shape, with two diametrically opposed connecting webs 106 extending from the sonotrode axis radially outward over the outer surface of the cylindrical section of the sonotrode 101 at the front end, where the front end face 103 is arranged, reaching as far as the two welding sections, each of which has a welding surface 105. In the embodiment shown, the welding surface 105 is constructed in the same way as the welding surface 5 of the embodiment shown in Figures 1 and 1a. It has a length 1s (shown in Figure 2a) and a width bs (shown in Figure 1). In addition, the connecting webs 106 have two opposing notches 109. Calculations and experiments have shown that these notches significantly reduce the material stresses in the area of ​​the welding surface 105.The notches 109 are preferably aligned parallel to the sonotrode axis s. In the embodiment shown, 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 resonant frequency of approximately 20 kHz. If a sonotrode is manufactured with a higher resonant frequency, the same effect can be achieved with a smaller notch depth. The notch depth can also be selected to be larger. The notches significantly reduce the cross-sectional area of ​​the connecting web, thereby significantly reducing the heat flow from the welding surface 105 to the sonotrode main body, which reduces the temperature gradient, especially in the immediate vicinity of the welding surface 105. Furthermore, the mechanical stress near the welding surface is reduced overall because the material in the region of the notch provides a lower counterforce.

[0057] Figure 2b shows a view perpendicular to the sonotrode axis, from which it can be seen that the notch 109 has a curved notch base.

[0058] 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, between which the sonotrode axis s extends. During operation, a standing, longitudinal wave is formed, at the vibration node of which the bead 204 is arranged, on which the sonotrode can be held. Here, too, no connecting web 206 is provided to connect the welding section 210, which has the welding surface 205, to the sonotrode main body 208. As in the embodiment shown in Figures 2, 2a, and 2b, a notch 209 is provided here. In contrast to the previous embodiments, 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 so as not to adversely affect the welding result produced by the contact of the welding surface 205 with the material to be welded. On the one hand, the slot width b should be as small as possible, but on the other hand, it must ensure that the individual welding surface segments do not collide with one another during the expected heating of the welding surface.

[0059] The slot has a slot depth ST. Furthermore, the slot widens toward the slot base and has a maximum width St>.

[0060] Figure 4 shows a schematic representation of an embodiment of a sonotrode 11 according to the invention. The sonotrode 11 consists of 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 which engages at a vibration node of the sonotrode. The entire sonotrode 11 can be moved up or down in the direction of the arrow in order to increase or decrease the distance between the welding surface 18 and the counter tool 12. The material to be processed, i.e. in this case two metal parts which are to be joined together by ultrasonic welding with the help of the sonotrode 11, is inserted between the welding surface 18 and the counter tool 12.

[0061] In the position shown in Figure 4, the welding surface 18 can be heated to preheat the entire welding section 15 so that during welding the temperature gradient in the immediate vicinity of the welding surface 18 does not become too large.

[0062] Heating can be achieved, for example, using hot air or an infrared radiator. However, care must be taken to ensure that only the welding surface 18 and, if applicable, also the welding section 15 are heated. The main section 13, which is generally connected to a converter (not shown) with temperature-sensitive piezo elements at its end facing away from the welding section 18, should not be heated under any circumstances. While the highest possible temperature is therefore desired in the area of ​​the welding section 15 even before the actual welding process, the temperature of the main section 13 should remain as low as possible in order not to damage the piezo elements of the converter. In a preferred embodiment, it may therefore be necessary to cool the main section 13. For example, the holder 16 can be water-cooled or cooling air streams can be supplied to this area.In order to keep the main section 13 relatively cool on the one hand, and simultaneously heat the welding surface 18 as much as possible, especially when working with hot air or cooling air, it is necessary to prevent this air from reaching the wrong section. Therefore, a separating element 17 is provided, which, without touching the connecting section 14, is arranged essentially surrounding it and largely prevents air flow from the welding section 15 to the main section 13 and in the reverse direction.

[0063] The voltage reduction according to the invention significantly extends the service life of the sonotrode. It is advantageous to implement as many of the described measures as possible in combination, even if each measure alone already improves the sonotrode's service life.

[0064] List of reference symbols

[0065] 1 , 11 , 101 , 201 Sonotrode

[0066] 2, 102, 202 Rear frontal surface

[0067] 3, 103, 203 Frontal surface

[0068] 4, 104, 204 vibration nodes

[0069] 5, 18 105, 205 welding surface

[0070] 6, 14, 106, 206 connecting bridge

[0071] 7 groove

[0072] 8, 108, 208 Sonotrode main body

[0073] 12 Counter tool

[0074] 13 Main section

[0075] 15 welding section

[0076] 16 Bracket

[0077] 17 Separator

[0078] 109, 209 notch

[0079] 110, 210 welding section

[0080] 211 slot

[0081] 212 Welding surface segment

[0082] 213 Welding surface segment a Width of the grooves b Slot width

[0083] B v Width of connecting bar in the area of ​​the notch bs Width of the sealing surface

[0084] Is length of the sealing surface k tNotch depth kh Notch height s Sonotrode axis

[0085] Sb widened slot width s t Slot depth t Depth of grooves

Claims

Patent claims 1 . Sonotrode for the ultrasonic processing of metals, which has a resonance frequency in the ultrasonic range, wherein the sonotrode has a welding surface (5) which is intended to come into contact with the metal to be processed, wherein, when the sonotrode oscillates at the resonance frequency and the welding surface (5) is in contact with the metal to be processed, the welding surface (5) heats up and a temperature gradient forms between the welding surface (5) and sections (6) of the sonotrode adjacent to the welding surface, wherein as a result of the temperature gradient the welding surface (5) is subjected to a stress, characterized in that means for reducing the welding surface tension during processing are provided.

2. Sonotrode according to claim 1, characterized in that when the sonotrode is excited with the resonance frequency, a standing wave is formed along a sonotrode axis (s), wherein the welding surface (5) is not arranged perpendicular to the sonotrode axis, wherein preferably the welding surface (5) runs parallel or substantially parallel to the sonotrode axis (s).

3. Sonotrode according to one of the preceding claims, characterized in that as a means for reducing the welding surface tension, a plurality of depressions of depth t and width a relative to one another are introduced into the welding surface, where a > 3 xt.

4. Sonotrode according to claim 3, characterized in that the depressions are designed as grooves (7), wherein the grooves (7) are preferably arranged parallel to one another.

5. Sonotrode according to claim 4, characterized in that the grooves (7) are curved in a cross-sectional view, wherein preferably the radius of curvature is r > 1.5 mm and particularly preferably r > 3 mm.

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

7. Sonotrode according to one of the preceding claims, characterized in that the sonotrode has a sonotrode main body (8) and a welding section having the welding surface (5), wherein a connecting web (6) is provided which connects the sonotrode main body (8) and the welding section to one another, wherein as a means for reducing the welding surface tension, the connecting web (6) has a cross-sectional area parallel to the welding surface (5) which is at least 5%, preferably at least 10% and most preferably at least 15% smaller than the welding surface (5).

8. Sonotrode according to claim 7, characterized in that a first notch (109) is arranged in a first side surface of the connecting section, wherein preferably the connecting web (6) has a second notch (109) in a second side surface.

9. Sonotrode according to claim 7, characterized in that the first and / or second notch (109) has a curved notch base in a sectional view perpendicular to the sonotrode axis (s).

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

11. Sonotrode according to one of the preceding claims, characterized in that as a means for reducing the welding surface tension at least one slot (211) is provided in the welding surface (5), which separates the welding surface (5) into several welding surface segments (212, 213), wherein the at least one slot (211) preferably runs parallel to the sonotrode axis (s).

12. Sonotrode according to claim 10, characterized in that the at least one slot (21 1 ) has a width b which is less than 0.3 mm and preferably less than 0.15 mm, wherein the width b is greater than 0.025 mm and preferably greater than 0.05 mm.

13. Sonotrode according to claim 10 or 11, characterized in that the slot (21 1 ) has a depth s t which is at least 10 times larger, preferably at least 20 times larger, than the width b.

14. Sonotrode according to claim 10, 11 or 12, characterized in that the slot has a curved slot base, wherein the slot has a base section having the slot base, in which the slot has a width bc which is greater than the width b and preferably at least by a factor of 5 greater than the width b.

15. Sonotrode according to one of the preceding claims, characterized in that when the sonotrode is excited with the resonance frequency, a standing wave with at least one oscillation node is formed along a sonotrode axis and a cooling device is provided as a means for reducing the welding surface tension, which cooling device cools the sonotrode in the region of the oscillation node, wherein preferably the cooling device does not cool the welding surface.

16. Sonotrode according to one of the preceding claims, characterized in that a heating device is provided as a means for reducing the welding surface tension, which heating device is arranged and designed such that the welding surface can be heated.

17. Ultrasonic processing device with a sonotrode according to one of the preceding claims, wherein a converter which converts an electrical alternating voltage into a mechanical oscillation is coupled to the sonotrode, and a generator for generating the electrical alternating voltage with an amplitude A o is provided, wherein a control unit is provided as a means for reducing the welding surface tension, which before the electrical alternating voltage with the amplitude A o An alternating electrical voltage with amplitude A is generated v generated during a time interval t, where A v < A o where t is preferably greater than 0.2 s and more preferably greater than 0.4 s.

18. Ultrasonic processing device according to claim 14, characterized in that the control unit is designed such that the amplitude A v during the time interval t from a minimum value A min, which is preferably 0, on A o is increased, whereby the amplitude is preferably increased linearly.

19. Use of a sonotrode according to one of the preceding claims for joining two metallic materials by means of ultrasound, wherein preferably the both metallic materials are arranged between the sonotrode and a counter tool and, to connect the two metallic materials, the counter tool and the sonotrode are moved relative to each other, whereby the sonotrode is not rotated about the sonotrode axis.

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