Patent Application
20170326677
The invention relates to a method for resistance welding a sandwich metal sheet to at least one further metallic component, wherein the sandwich metal sheet has two metallic cover layers and one thermoplastic layer arranged between the metallic cover layers, in which that region of the sandwich metal sheet which is to be welded is heated in such a way that the thermoplastic layer softens and is displaced from the welding region by compressing the cover layers, and the cover layers with the further component are welded to one another by means of a flow of electrical current in a first circuit via a first welding electrode arranged on the side of the sandwich metal sheet and a second welding electrode arranged on the side of the further metallic component. Furthermore, the invention relates to a device for resistance welding a sandwich metal sheet which has a thermoplastic layer arranged between metallic cover layers, having at least one further metallic component, having a first welding electrode which can be arranged on the side of the sandwich metal sheet and having a second welding electrode which can be arranged on the side of the further metallic component, having means for making available a first circuit, which means conduct the welding current at least via the first and second welding electrodes, and having means for displacing the plastic layer of the sandwich metal sheet from that region of the sandwich metal sheet which is to be welded.
The increasing demand for lightweight designs in the field of motor vehicles is placing increasing emphasis on the use of sandwich metal sheets which have a thermoplastic layer between two thin metallic cover layers, in order to further increase the potentials for saving weight in motor vehicle design by using sandwich metal sheets. Sandwich metal sheets can make available various properties which are frequently mutually exclusive and which provide new potentials for saving weight. For example, owing to the plastic layer, sandwich metal sheets have a significantly lower weight than solid metal sheets and at the same time make available high rigidity values. Furthermore, the sandwich metal sheets are sound-damping and provide a high level of rigidity. A disadvantage with sandwich metal sheets is, however, that they have an electrically insulating plastic layer which causes problems during fusion welding processes with regard to the formation of a satisfactory welded connection. Owing to the lack of suitability of the sandwich metal sheets for welding, for example for resistance welding or resistance spot welding to other metallic components, sandwich metal sheets are therefore frequently bonded or joined to one another mechanically.
German laid-open patent application DE 10 2011 109 708 A1 discloses a method for joining a sandwich metal sheet to a further metallic component, in which method the intermediate layer is fused on in the connecting region and displaced from the connecting region, with the result that subsequently a welded connection can be generated by establishing an electrical contact between the component and the cover layers of the sandwich metal sheet. It is proposed to heat the joint regions by means of electrodes or pressing elements whose temperature can be controlled. The welding electrodes or pressing elements are for this purpose provided, for example, with heating elements. The design of the welding electrodes is therefore relatively complicated. Furthermore, the speed of heating of the thermoplastic layer can be increased even further, with the result that shorter cycle times can be achieved.
Furthermore, U.S. Pat. No. 4,650,951 discloses a method for resistance welding two composite steel sheets, which method uses two welding electrodes which are heated and thereby heat and displace the plastic layer located between the cover layers, before the actual welding starts.
Furthermore, German patent application DE 10 2013 108 563, which was not published before the priority date of the present document, discloses a method in which two circuits are used to connect a sandwich metal sheet to a further metallic component by resistance welding. However, in this method a current bridge which is connected to the further metallic component is necessary to manufacture the second circuit. In addition, it has become apparent that the service life of the welding electrodes is in need of improvement.
Taking this as the basis, the object of the present invention is to make available a method for resistance welding of sandwich metal sheets and a corresponding device with which short cycle times, compact design and a process-reliable welded connection can be achieved.
The specified object is achieved according to a first teaching of the present invention with a method of the generic type by virtue of the fact that that region of the sandwich metal sheet which is to be welded is heated by a flow of current in a second circuit which comprises the first welding electrode and an electrical conductor arranged between the first welding electrode and the sandwich metal sheet.
It has become apparent that when a second circuit which comprises the first welding electrode and an electrical conductor arranged between the first welding electrode and the sandwich metal sheet is used, the welding region with a compact design can easily be heated. This is because, for example by virtue of a high electrical resistance, the electrical conductor can carry out the heating of the welding region without, for example, additional current bridges being necessary at the components to be welded. The heat is generated here by means of the contact resistance between the first welding electrode and the electrical conductor arranged between the first welding electrode and the sandwich metal sheet and/or by means of the electrical resistance of the electrical conductor arranged between the first welding electrode and the sandwich metal sheet itself. In this context, by providing the second circuit it is possible to achieve short cycle times, with the result that the method can be carried out economically. In addition, the wear and tear of the first welding electrode is reduced and its service life can be increased by means of the conductor arranged between the first welding electrode and the sandwich metal sheet. Finally, the electrical conductor can remain between the first welding electrode and the sandwich metal sheet even during the welding process itself and can influence said welding process in a positive way.
The electrical conductor arranged between the first welding electrode and the sandwich metal sheet can have, for example, a specific resistance which is higher than that of the welding electrodes, of the cover layers of the sandwich metal sheet and/or of the further metallic component.
According to a first refinement of the method according to the invention, a conductor ribbon is used as an electrical conductor arranged between the first welding electrode and the sandwich metal sheet. A conductor ribbon or contact-forming ribbon can, for example, be easily guided around the first welding electrode or encompass it so that the contact region of the welding electrode which would normally make contact with the sandwich metal sheet to be welded is covered by the conductor ribbon.
According to a further refinement of the method according to the invention, the conductor ribbon is arranged with a ribbon-guiding system between the first welding electrode and the sandwich metal sheet. By means of a ribbon-guiding system, the conductor ribbon can be provided in a particularly compact and reliable fashion between the first welding electrode and the sandwich metal sheet. In addition, the ribbon-guiding system can be embodied in such a way that the conductor ribbon can be moved with respect to the first welding electrode for the purpose of renewal. As a result, different locations of the conductor ribbon are positioned between the first welding electrode and the sandwich metal sheet, so that a change in the properties of the conductor ribbon in the region between the first welding electrode and the sandwich metal sheet can be counteracted. If the welding electrodes are made available, for example, by welding tongs, the ribbon-guiding system can be integrated, for example, into the welding tongs.
According to a further refinement of the method according to the invention, the electrical conductor which is arranged between the first welding electrode and the sandwich metal sheet makes contact directly with the first welding electrode and/or the sandwich metal sheet. The resistance in the first and second circuits and the heating can therefore be controlled well and a process-reliable method can be made available.
According to a further refinement of the method according to the invention, the flow of current for welding in the first circuit passes via the first welding electrode, the electrical conductor arranged between the first welding electrode and the sandwich metal sheet, the sandwich metal sheet, the further metallic component and the second welding electrode which is in contact with the further metallic component. The electrical conductor arranged between the first welding electrode and the sandwich metal sheet therefore does not need to be removed again during the welding. Instead, the electrical conductor can remain between the first welding electrode and the sandwich metal sheet. As a result, on the one hand, a simple method with particularly short cycle times can be made available. In addition, wear and tear on the first welding electrode is reduced by it being covered with the electrical conductor, with the result that long service lives can be achieved. Finally, the electrical conductor arranged between the first welding electrode and the sandwich metal sheet can be selected in such a way that it can also influence the welding process in a positive way, for example the formation of the welding nugget can be optimized.
According to a further refinement of the method according to the invention, the flow of current for heating in the second circuit passes via the first welding electrode, the electrical conductor arranged between the first welding electrode and the sandwich metal sheet, and a shunt conductor. The second circuit therefore does not comprise, in particular, the second welding electrode, the sandwich metal sheet and/or the further metallic component. For example, the flow of current for carrying out heating in the second circuit is provided with respect to the first welding electrode in the same way as the flow of current for welding in the first circuit is provided to the first welding electrode. For example, the electrical conductor arranged between the first welding electrode and the sandwich metal sheet is connected directly to the shunt conductor, for example a shunt current ribbon.
According to a further refinement of the method according to the invention, the first circuit is at least temporarily disconnected while the region of the sandwich metal sheet which is to be welded is heated by the flow of current in the second circuit. The first circuit is preferably permanently disconnected while that region of the sandwich metal sheet which is to be welded is heated by the flow of current in the second circuit. It is therefore possible to prevent undesired secondary currents. For example, it is possible to prevent the flow of current for carrying out heating in the second circuit from flowing via the first circuit, that is to say, in particular, via the second welding electrode. This is advantageous, for example, if multipoint welding is carried out.
According to a further refinement of the method according to the invention, the disconnection of the first circuit takes place by spacing apart the second welding electrode from the further metallic component or a separate switch. For example, a spacer element can be provided which permits the spacing apart. The spacer element can make contact here with the further component with an electrically insulated contact region. If a switch is provided, it can be embodied, for example, mechanically or electrically. For example, the switch can comprise a thyristor. The disconnection of the first circuit can thus be implemented in a particularly simple and process-reliable fashion. The disconnection can be controlled, for example, by means of a weld controller, for example as a function of a threshold value, for instance of a resistance threshold value. For example, the disconnection can be canceled if the electrical resistance between the first and the second welding electrodes drops.
According to a further refinement of the method according to the invention, the cover layers of the sandwich metal sheet are compressed by the first welding electrode and a spacer element arranged on the side of the further metallic component. For example, the spacer element simultaneously permits the second welding electrode to be spaced apart from the further metallic component for the disconnection of the first circuit. For example, the spacer element is spring-mounted. For example, the spacer element is part of welding tongs. In this context, the spring stiffness is, for example, sufficiently large to permit the cover layers to be compressed with the first welding electrode without contact between the second welding electrode and the further metallic component. For example, the spacer element enables contact between the second welding electrode and the further metallic component when a sufficiently large force is applied, for example also the welding force applied to the welding electrodes.
Alternatively, it is also possible for the cover layers of the sandwich metal sheet to be compressed by the first welding electrode and the second welding electrode.
According to a further refinement of the method according to the invention, the electrical conductor which is arranged between the first welding electrode and the sandwich metal sheet is adapted to the resistance welding to be carried out. For example, the material, the quality of the material, the thickness and/or the resistance of the electrical conductor can be adapted. Therefore, a suitable electrical conductor can be selected depending on the specific welding process. For example, the electrical conductor is selected in such a way that when welding is carried out the two metallic cover layers of the sandwich metal sheet are connected to the further metallic component. As a result of the fact that the first welding electrode makes contact with the sandwich metal sheet via the electrical conductor, but the second welding electrode does not, an asymmetrical embodiment of welding electrodes is effectively produced. For example, the electrical conductor which is arranged between the first welding electrode and the sandwich metal sheet can be selected in such a way that the embodiment of the welding nugget is optimized, for example is shifted in the direction of the sandwich metal sheet, with the result that a process-reliable connection of the metallic cover sheets of the sandwich metal sheet can take place.
According to a further refinement of the method according to the invention, electrical properties of the sandwich metal sheet are measured at least temporarily. It is therefore possible, for example, for a contact of the metallic cover layers of the sandwich metal sheet to be defined in the region which is to be welded. For example, the measurement is carried out during the heating of the welding region and the compression of the cover layers of the sandwich metal sheet. For example, a resistance-measuring process is carried out which includes the electrical resistance between the cover layers of the sandwich metal sheet. For example, the electrical resistance between the first and second welding electrodes is measured. If the cover layers have metallic contact, the resistance drops abruptly, with the result that metallic contact can be inferred.
According to a second teaching of the present invention, the indicated object is achieved by means of a device of the generic type by virtue of the fact that a second circuit is provided, wherein the second circuit comprises the first welding electrode and an electrical conductor which can be arranged between the first welding electrode and the sandwich metal sheet, with the result that that region of the sandwich metal sheet which is to be welded can be heated by a flow of current in the second circuit.
As already stated above, it is therefore easily possible to achieve heating of the welding region with a compact design. In this context, short cycle times can be achieved by means of the second circuit. In addition, if the electrical conductor is arranged between the first welding electrode and the sandwich metal sheet, this reduces the wear and tear on the first welding electrode and can increase the service life.
With respect to further advantageous refinements of the device, reference is made to the description of the embodiments of the method and the advantages thereof.
In this context, the description of method steps according to preferred embodiments of the method according to the invention is also intended to disclose corresponding means for carrying out the method steps by means of preferred embodiments of the device according to the invention. Likewise, the disclosure of means for carrying out a method step is intended to disclose the corresponding method step.
For example, in one refinement of the device according to the invention a ribbon-guiding system is provided with which the conductor ribbon can be arranged between the first welding electrode and the sandwich metal sheet.
In one refinement of the device according to the invention the electrical conductor which can be arranged between the first welding electrode and the sandwich metal sheet can preferably be arranged in such a way that the electrical conductor can make contact directly with the first welding electrode and/or the sandwich metal sheet.
In a further refinement of the device according to the invention, a disconnection mechanism is preferably provided for the first circuit. For example, the disconnection mechanism of the first circuit comprises a spacer element, which can be arranged on the side of the further metallic component, for spacing apart the second welding electrode from the further metallic component or a separate switch.
In a further refinement of the device according to the invention, the means for displacing the plastic layer of the sandwich metal sheet from that region of the sandwich metal sheet which is to be welded advantageously comprise the first welding electrode and a spacer element, with the result that the cover layers of the sandwich metal sheet are compressed by the first welding electrode and the spacer element.
In a further refinement of the device according to the invention, a measuring device for measuring electrical properties of the sandwich metal sheet is advantageously provided, for example for measuring the electrical resistance.
In the text which follows, the invention will be explained in more detail on the basis of exemplary embodiments in conjunction with the drawing, in which:
In the device 1a, a second circuit is also provided which comprises the first welding electrode 4 and an electrical conductor 8 which can be arranged and is arranged between the first welding electrode 4 and the sandwich metal sheet 2. The electrical conductor 8 is embodied here as a conductor ribbon 8 which is arranged via a ribbon-guiding system 9 between the first welding electrode 4 and the sandwich metal sheet 2. In this context, the conductor ribbon 8 is in direct contact with the first welding electrode 4, on the one hand, and with the metallic cover layer 2a, on the other. The conductor ribbon 8 is additionally connected via a shunt conductor 10 to the power source 6. In this exemplary embodiment, the electrical wires 7 are insulated with respect to the conductor ribbon 8. A flow of current in the second circuit therefore passes via the electrical wires 7, the first welding electrode 4, the conductor ribbon 8 and the shunt conductor 10, with the result that the pre-heating current IV flows in the second circuit. In particular, current can pass to the conductor ribbon 8 only via the first welding electrode 4. Heat for softening the plastic layer 2c is generated by means of the contact resistance between the first welding electrode 4 and the conductor ribbon 8 and the material resistance of the conductor ribbon 8.
At the same time, the cover layers 2a, 2b of the sandwich metal sheet 2 are compressed by the first and the second welding electrodes 4, 5, with the result that the first welding electrode 4 displaces the plastic layer 2c in the welding region. The result of the displacement of the plastic layer 2c is illustrated here in
As a further difference, the second welding electrode is connected to a spacer element 14 via the elements 14c. The spacer element 14 has an insulating region 14a with which the spacer element makes contact with the component 3. The second welding electrode 5 is arranged spaced apart from the further component 3. The first circuit is disconnected by the insulating region 14a and the spacing apart, with the result that during the heating with the pre-heating current IV no undesired current flows via the welded connection 13 which has already been formed and via the spacer element 14 itself, such that a further separate switch 12 does not have to be provided.
Furthermore, the spacer element 14 also comprises spring elements 14b. The spring stiffness of the spring elements 14b of the spacer element 14 is configured here in such a way that during the heating with the pre-heating current IV the second welding electrode 5 is spaced apart the further metallic component 3 from the component 3 despite the application of a force in the direction of the component 3. The application of force compresses the cover layers 2a, 2b of the sandwich metal sheet, as illustrated in
The conductor ribbon 8 divides the current IS into the current IS1 through the first welding electrode and IS2 through the conductor ribbon 8. The current through the shunt conductor 10 can be ignored here. Therefore, both IS1 and IS2 contribute to the welding process. However, a partial current IS3 flows through the welded connection 13 which has already been formed and which therefore does not contribute to the welding process. IS3 is, however, very much smaller than IS1+IS2, and negligible. In addition, further secondary currents can be diverted through the sandwich metal sheet 2 or the component 3, which secondary currents can, however, also be ignored. The conductor ribbon can advantageously preferably be composed of three sections (not illustrated here) with different materials, which conductor ribbon is, for example, in the external sections/end sections composed of a material which is a good electrical conductor, for example a copper material, and the central region which is arranged, in particular, in the contact-forming region of the welding electrode 4 and the cover layer 2a, from another material, in particular with a high electrical resistance, for example from a steel material or from a tungsten material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 117 923.4 | Dec 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/075211 | 10/30/2015 | WO | 00 |
