WO2018122261A1

WO2018122261A1 - Method and device for fastening at least two workpieces

Info

Publication number: WO2018122261A1
Application number: PCT/EP2017/084641
Authority: WO
Inventors: Georg KIROV; Alois Birgmann
Original assignee: Lkr Leichtmetallkompetenzzentrum Ranshofen Gmbh
Priority date: 2016-12-27
Filing date: 2017-12-27
Publication date: 2018-07-05
Also published as: EP3562606B1; EP3342503A1; EP3562606A1

Abstract

The invention concerns a method for fastening at least two workpieces (1,2) together by a punch rivet (3) wherein at least one workpiece (1) is a metallic workpiece (1) and wherein the at least one metallic workpiece (1) is heated to an elevated temperature for setting the punch rivet (3). Further, the invention concerns a device (10) for fastening at least two workpieces (1,2) together by a punch rivet (3)wherein at least one workpiece (1) is a metallic workpiece (1),comprising a die (5), a riveting punch (6), the riveting punch (6) being moveable toward the die (5), a blank holder (7) for holding the workpieces (1,2) during movement of the riveting punch (6) toward the die (5) for setting the punch rivet (3) to fasten the workpieces (1,2), wherein a heating element (8) is provided for heating at least one of the workpieces (1,2).

Description

Method and device for fastening at least two workpieces

The invention concerns a method for fastening at least two workpieces together by a punch rivet wherein at least one workpiece is a metallic workpiece.

Furthermore, the invention concerns a device for fastening at least two workpieces together by a punch rivet wherein at least one workpiece is a metallic workpiece.

Punch riveting is a joining method which gains increasing importance, in particular in automotive industry. Usually, so called C-rivets are used, for example punch rivets according to EP 0 720 695.

In automotive industry, punch riveting is in particular used to fasten two or more sheets of metallic workpieces together. Thereby, quality parameters are of course very important. In order to be applicable on the large scale in automotive industry, no significant mechanical and/or aesthetic defects are allowable.

Traditionally, many parts of automobiles have been made from steel. Today, many efforts are undertaken in order to replace parts of steel by light weight parts made from aluminum or its alloys as well as magnesium and its alloys. However, sheets of heat treated aluminium alloys can show severe cracks after joining by punch riveting. Such cracks are not admissible for quality reasons.

An object of the invention is to provide a method for fastening at least two metallic workpieces together by a punch rivet which allows fastening workpieces from sheets of heat treated aluminium alloys and other alloys which are difficult to fasten by punch riveting without significant appearance of cracks in the fastening region.

A further object of the invention is to provide a device for a corresponding method.

According to the invention, a method for fastening at least two workpieces together by a punch rivet is provided wherein at least one workpiece is a metallic workpiece and wherein the at least one metallic workpiece is heated to an elevated temperature for setting the punch rivet. According to the invention, at least one of the workpieces to be fastened, in particular the at least one metallic workpiece is heated to an elevated temperature for setting the punch rivet. Thereby, the at least one workpiece can be heated only before setting the punch rivet so that the punch rivet is set at a still elevated temperature during the setting of the punch rivet. However, it is also possible to begin the heating before setting the punch rivet and to continue the heating during setting of the punch rivet. In any case, heating of the at least one workpiece leads, in comparison with room temperature, to a higher ductility of the at least one workpiece which is made of a metal or an alloy. This, in turn, allows providing connected workpieces which are free from cracks in the fastening region. This is one of the quality criterions for parts used in automotive industry.

If metallic workpieces are to be fastened, the workpieces will be in conductive contact at least during the riveting process. Therefore, it is sufficient to heat at least only one workpiece to an elevated temperature for setting the punch rivet. For example, a lower workpiece can be heated to a desired temperature for setting the punch rivet whereas an upper workpiece receives heating indirectly by contact with the lower workpiece. It is also possible to combine at least one metallic workpiece with one or more non-conductive workpieces. For example, one or more metallic workpieces can be combined with workpieces made of polymers or polymers comprising reinforcing particles.

Heating only one workpiece of the workpieces to be connected is, hence, principally sufficient. However, if short cycle times are desired, it is advantageously to heat all workpieces to be connected simultaneously. This allows faster reaching of temperature conditions within the workpieces for a setting of a punch rivet.

The method according to the invention can be applied to parts of any shape. Preferably, the parts to be connected are sheets, for example sheets with a thickness between 0.1 mm and 5.0 mm. The at least one workpiece can be heated with any conventional heating means, for example an oven or a gas burner. However, in order to exactly control a temperature for setting the punch rivet, the at least one workpiece is preferably heated electrically. An electrical heating generally allows a very exact temperature control within the workpieces to be fastened. Thereby, it is sufficient to heat only a lower metallic workpiece. For electrical heating, the at least one workpiece is contacted by electrodes. The electrodes can be made from any material. Preferably, the electrodes are copper electrodes. By contacting the at least one workpiece with electrodes and by applying a voltage, the at least one workpiece is heated as a resistance heater.

In a preferred embodiment, the at least one metallic workpiece is brought into contact with at least one second workpiece on a top surface and the at least one metallic workpiece is contacted on an opposite lower surface by copper electrodes in order to heat the at least one metallic workpiece. In this embodiment, only the lower surface of the at least one metallic workpiece is contacted by copper electrodes for electrically heating this workpiece. In this case, the electrodes, preferably copper electrodes, are completely located below the at least one metallic workpiece. This configuration can be easily adapted with conventional punch rivet setting devices. For example, the copper electrodes can be arranged to surround a die a wherein the copper electrodes are optionally adjustable to project up to 0.2 mm over the die.

In a further embodiment of the invention, the temperature of the least one workpiece is set to a temperature of 100°C to 250°C, in particular 120°C to 180°C, preferably 130°C to 160°C. In particular for aluminium alloys a temperature of at least 100°C is necessary in order to achieve a good ductility for setting a punch rivet. Temperatures higher than 200°C are not useful as the ductility would be too large. Preferred temperature regions are within 120°C to 180°C, in particular 130°C to 160°C.

A method according to the invention can be applied to all metallic materials regardless whether steel, aluminium and its alloys or other light weight metals and its alloys like magnesium and its alloys. Preferable, the method is applied to sheets made from heat treated aluminium alloys which are otherwise difficult to fasten with punch rivets without cracks developing. As outlined, however, it is also possible to combine one or more metallic workpieces with non-conductive workpieces.

In a further aspect, the invention provides a device for fastening at least two workpieces together by a punch rivet wherein at least one workpiece is a metallic workpiece, comprising a die, a riveting punch, the riveting punch being moveable toward the die, a blank holder for holding the workpieces during movement of the riveting punch toward the die for setting the punch rivet to fasten the workpieces, wherein a heating element is provided for heating at least one of the workpieces.

A device according to the invention allows to heat metallic workpieces to be fastened by a punch rivet before and, if applicable, during setting of the rivet. Due to the heating of at least one workpiece, it is possible to produce in particular connected sheets of heat treated aluminium alloys which are at least almost free of cracks in the fastening region. The device of the invention is in particular suited for realizing a method according to the invention.

Preferably, the heating element is an electrical heating element. The heating element can comprise copper electrodes for contacting at least one workpiece. The copper electrodes are then part of an electrical circuit so that the workpiece to be heated can be supplied with a voltage. This leads to a heating of the at least one workpiece.

If copper electrodes are provided, preferably also copper blocks are provided which are in contact with the copper electrodes. Copper is a relatively good electrical conductor.

However, the current intensity and current flow needed to heat at least one metallic workpiece made from aluminium alloy would lead to an overheating of the copper electrodes. Therefore, copper blocks are provided which reduce local heating of the electrodes. In this respect, the device can also comprise a cooling circuit wherein the copper blocks are coolable by the cooling circuit. In this variant, very high current densities and current flows are possible which leads to high heating rates for the at least one workpiece to be heated. On the other hand, the electrodes withstand very high current intensities during heating of the at least one workpiece.

Despite the copper electrodes being in contact with coolable copper blocks, the electrodes may wear in the course of time. Therefore, the copper electrodes are exchangeable. In one embodiment of the device according to the invention, the heating element comprises copper electrodes isolated against a direct contact with each other wherein the copper electrodes surround the die with a predetermined protrusion of top surfaces of the electrodes so that only the at least one metallic workpiece can be contacted at a lower surface of the at least one metallic workpiece. This embodiment can applied easily to conventional devices of the prior art. The copper electrodes can be arranged completely below the first workpiece of the workpieces to be connected. In particular, the copper electrodes can be arranged around the die so that the copper electrodes slightly protrude the die in order to contact a workpiece being positioned next to the die. An electrical connection between the electrodes is established by the at least one metallic workpiece during the rivet setting process.

The die and the riveting punch are preferably positioned on two carriers wherein the carriers are electrically isolated against each other and wherein the carriers are connected in order to support the die and the riveting punch. These carriers can be made of steel or another metal or alloy. In particular, the carriers can be connected to a power supply so that a high current can be provided at the copper electrodes despite a low volume of the same. Preferably, the copper electrodes are formed as blocks surrounding the die around a circumference of the die wherein the blocks surround the die essentially completely. In this way, a homogeneous heating of the at least one metallic workpiece can be provided. The blocks can comprise one or more cavities so that the blocks are coolable, for example by water.

The device can comprise a temperature sensor. The temperature sensor can be located in a copper block or in the die. A temperature sensor allows for an exact temperature control, in particular as regards heating of the copper blocks on the one hand and cooling of the copper blocks on the other hand. In this connection, the temperature of the copper blocks can be exactly controlled which allows cycle times for setting a punch rivet of below three seconds.

In the following, the invention is further illustrated in an example. Thereby, reference is made to the enclosed drawings. In the drawings show:

Fig. 1 a device according to the invention;

Fig. 2a and 2b dies;

Fig. 3a and 3b sheets of aluminium alloys fastened at room temperature;

Fig. 4a to 4d sheets of aluminium alloy fastened at 150°C; Fig. 5a to 5d sheets of aluminium alloy fastened at 150°C;

Fig. 6 a further device according to the invention;

Fig. 7 a front view of the device according to Fig. 6;

Fig. 8 a part of the device according to Fig. 6.

In Fig. 1 , a device 10 for punch riveting is depicted. The device 10 comprises a die 5 and a riveting punch 6. The riveting punch 6 is movable up and down as indicated by the arrow in Fig. 1 . The device 10 further comprises two or more electrodes 4 for which are made from copper or another high conducting material. Copper, however, is the preferred choice for the electrodes 4. The electrodes 4 are connected to one or more copper blocks 9. The copper blocks 9, in turn, are connected to a cooling circuit (not depicted). In particular, the copper blocks 9 are structured so that water can pass through internal channels of the copper blocks 9. However, it is also possible that the copper blocks 9 are cooled by simply winding some loops of a flexible tube around the copper blocks 9 so that cooling is achieved in this way. Moreover, the copper blocks 9 may alternatively also be connected indirectly or directly to a cooling circuit. In any case, the copper blocks 9 can be cooled by a further element of the device 10.

For fastening two or more workpieces 1 , 2 like sheets of an aluminium alloy, the same are positioned between the die 5 and the riveting punch 6. The riveting punch 6 carries a punch rivet 3. In order to hold the workpieces 1 , 2 in position during the fastening process, the workpieces 1 , 2 are hold by a blank holder 7. Before the riveting punch 6 is lowered in order to place the punch rivet 3 within the workpieces 1 , 2, a lower workpiece 1 is heated by a heating element 8. The heating element 8 comprises the copper electrodes 4. As shown in Fig. 1 , the lower workpiece 1 is placed on the die so that simultaneously the electrodes 4 are conducted. However, the electrodes could also be flexible so that the electrodes 4 are positioned on a lateral side of the lower workpiece 1 . It is also possible that the electrodes 4 are clamping element which clamp both workpieces 1 , 2 so that both workpieces 1 , 2 are electrically conducted.

For setting the punch rivet 3, first at least the lower workpiece 1 is heated by the heating element 8 with the electrodes 4. The lower workpiece 1 is heated by resistance heating. The heating is continued at least until the riveting punch 6 is lowered. Although not necessary, the heating can also be continued during the fastening process. However, usually that is not necessary as the riveting process is much faster than a temperature loss of the workpieces 1 , 2. As can be seen in Fig. 1 , the workpieces 1 , 2 to be fastened are in close contact with each other before the riveting process. As both workpieces 1 , 2 are made of a metallic material, i.e., a metal or alloys, the upper workpiece 2 will also be heated even when only the lower workpiece 1 is contacted with the electrodes 4. In this manner, the workpieces 1 , 2 are heated to suited temperatures. Suited temperatures for later setting of the punch rivet 3. For aluminium alloys which are heat treated, suitable temperatures are in the temperature region 100°C to 200°C. In particular suited temperatures lie between 130°C and 180°C.

For achieving the necessary temperature increase before and, if applicable, during the rivet setting process, a power source is provided for the electrodes 4. The power source can operate with up do 14V direct current and a current of to 10 kA. Due to the high current intensities, the copper blocks 9 are water cooled so that the device 10 operates despite very high current densities.

In Fig. 2a and 2b typical dies 5 are shown which were used in order for a punch rivet fastening of workpieces 1 , 2 without and with heating. The dies 5 are commercially available from Bollhoff GmbH. Fig. 2a depicts a so called PZ geometry; Fig. 2b depicts a so called FM geometry.

All punch riveting experiments were performed by using sheets of an aluminium alloy. The aluminium alloy was of the grade EN AW 7075 T6. This alloy was chosen because this alloy is setting the highest demands for mechanical joining. In Fig. 3a and 3b photographs of a top view of two workpieces 1 , 2 of an aluminium alloy EN AW 7076 T6 are shown. The workpieces 1 , 2 where fastened by a C-rivet at room temperature. As clearly visible, independent of the die 5, in both cases cracks are observed. This is the case for the FM geometry (Fig. 3a) as well as PZ geometry (Fig. 3b). In Fig. 4 to 4d corresponding photographs are shown for workpieces 1 , 2 of the same material but fastened at the temperature at 140°C (fig. 4a, 4b) and 150°C (Fig. 4c, 4d) with the FM geometry. In Fig. 5a to 5d analogous results are shown for the PZ geometry also at 140°C (Fig. 5a, 5b) and 150°C (Fig. 5c, 5d). Independent of the geometry, in both cases no cracks are observed. This clearly indicated that fastening of the workpieces 1 , 2 with a so called C-rivet at the temperature of about 140°C to 160°C leads to a crack free connection of the workpieces 1 , 2.

According to the invention, the temperature for the setting process can be adapted for the workpieces 1 , 2 to be fastened in order to achieve an optimum ductility during the setting process. Therefore, the number of possible combinations of rivets and workpieces 1 , 2 to be fastened is significantly increased.

In Fig. 6 a further device 10 according to the invention is shown. The device 10 comprises two or more carriers 1 1 . The carriers 1 1 are open formed at a front side. Generally, the carriers 1 1 are c-shaped although any other shape with an open front side is suited. In the device of Fig. 6, two carriers 1 1 are provided. Those two carriers 1 1 support on a lower side the die 5 and on an upper side the riveting punch unit. The two carriers 1 1 are usually made of a material having sufficient strength to provide a support during a riveting operation and, therefore, are usually made of steel. In this case, the two carriers are electrically isolated against each other. The carriers 1 1 are detachably connected to each other, for example by screws or any other suited connecting means.

The carriers 1 1 are connected to a main plate13. Beside the device 10 comprises at least a power source 14 comprising a control unit and a hydraulic unit 15 for moving the riveting punch 6.

At the lower front end of the carriers 1 1 two blocks 12 are positioned. The blocks 12 are usually made of a metal, in particular copper or an alloy of copper. The blocks 12 are arranged circumferentially around the die 5 and are electrically isolated against each other. Also, the die 5 is isolated against the blocks 12. In Fig. 6 two blocks 12 are shown although the device 10 can comprise more than two blocks 12. Each of the blocks 12 is connected to the power source 14 comprising a control unit so that the blocks 12 can be electrically heated by resistance heating. Moreover, the blocks 12 can comprise one or more cavities, for example a channel for a cooling liquid, so that the blocks 12 can be cooled. In addition, the control unit of the power source 14 is connected to the hydraulic unit 15 for regulating the same as shown in Fig. 7. Although not shown in Fig. 6, the device 10 can comprise on or more temperature sensors. The at least one temperature sensor is positioned in or near the heating zone, i.e., in one of the blocks 12 or in or nearby the die 5. The temperature sensor is in connection with the control unit of the power source 14 which also regulates cooling of the blocks 12 so that an exact temperature control is possible. The power unit 14 is connected with both of the carriers 1 1 electrically. When at least one metallic workpiece 1 like a sheet of a metal or an alloy is positioned on the blocks 12 and the die 5, the sheet gets into contact with the blocks 12 at least when the riveting punch 6 presses against the sheet. Then, an electrical contact is given and the sheet is heated electrically. In this respect, the carriers 12 are provided with a large cross section so that a high current can flow through the block 12 made of copper and a fast heating can be provided.

In Fig. 8 a part of the device 10 according to Fig. 6 is shown, namely the blocks 12 or copper electrodes 4, respectively. As depicted, the blocks 12 surround the die 5 essentially completely. The blocks 12 extend slightly over an upper surface of the die 5, usually up to 1.0 mm. The blocks 12 can be height adjustable so that a distance between a metallic workpiece 1 and the die 5 can be adapted individually to a punch riveting process.

A setup according to Fig. 6 to 8 allows for fast riveting cycles with frequencies lower than three seconds as required by automotive industry. This is due to a rapid temperature adjustment provided by the combination of the heating of the blocks 12 and the cooling regulated by the control unit.

Claims

1 . Method for fastening at least two workpieces (1 , 2) together by a punch rivet (3) wherein at least one workpiece (1 ) is a metallic workpiece (1 ) and wherein the at least one metallic workpiece (1 ) is heated to an elevated temperature for setting the punch rivet (3).

2. Method according to claim 1 wherein the at least two workpieces (1 , 2) are heated to an elevated temperature.

3. Method according claim 1 or 2 wherein the at least two workpieces (1 , 2) are sheets.

4. Method according to one of claims 1 to 3 wherein the at least one workpiece (1 ) is heated electrically.

5. Method according to claim 4 wherein the at least one workpiece (1 ) is contacted by electrodes (4).

6. Method according to claim 5 wherein the electrodes are copper electrodes (4).

7. Method according to one of claims 1 to 6 wherein the temperature of at least one workpiece (1 ) is set to a temperature of 100°C to 250°C, in particular 120°C to 180°C, preferably 130°C to 160°C.

8. Method according to one of claims 1 to 7 wherein the at least two workpieces (1 , 2) are each made of an aluminum alloy.

9. Method according to one of claims 1 to 8 wherein the at least one metallic workpiece (1 ) is brought into contact with at least one second workpiece (2) on a top surface and wherein the at least one metallic workpiece (1 ) is contacted on an opposite lower surface by copper electrodes (4) in order to heat the at least one metallic workpiece (1 )-

10. Method according to claim 9 wherein the copper electrodes (4) are arranged to surround a die (5) wherein the copper electrodes (4) are optionally adjustable to project up to 0.2 mm over the die (5).

1 1 . Device (10) for fastening at least two workpieces (1 , 2) together by a punch rivet (3) wherein at least one workpiece (1 ) is a metallic workpiece (1 ), comprising a die (5), a riveting punch (6), the riveting punch (6) being moveable toward the die (5), a blank holder (7) for holding the workpieces (1 , 2) during movement of the riveting punch (6) toward the die (5) for setting the punch rivet (3) to fasten the workpieces (1 , 2), wherein a heating element (8) is provided for heating at least one of the workpieces (1 , 2).

12. Device (10) according to claim 1 1 wherein the heating element (8) is an electrical heating element.

13. Device (10) according to claim 1 1 or 12 wherein the heating element (8) comprises copper electrodes (4) for contacting at least one workpiece (1 ).

14. Device (10) according to claim 13 wherein copper blocks (9) are provided, the copper blocks (9) being in contact with the copper electrodes (4).

15. Device (10) according to claim 14 further comprising a cooling circuit wherein the copper blocks (9) are coolable by a cooling circuit.

16. Device (10) according to one of claims 1 1 to 15 wherein the heating element (8) comprises copper electrodes (4) isolated against a direct contact with each other wherein the copper electrodes (4) surround the die (5) with a predetermined protrusion of top surfaces of the copper electrodes (4) so that only the at least one metallic workpiece (1 ) can be contacted at a lower surface of the at least one metallic workpiece (1 ).

17. Device (10) according to one of claims 1 1 to 16 wherein the die (5) and the riveting punch (6) are positioned on two carriers (1 1 ) wherein the carriers (1 1 ) are electrically isolated against each other and wherein the carriers (1 1 ) are connected in order to support the die (5) and the riveting punch (6).

18. Device (10) according to one of claims 1 1 to 17 wherein the copper electrodes (4) are formed as blocks (12) surrounding the die (5) around a circumference of the die (5) wherein the blocks (12) surround the die (5) essentially completely.

19. Device (10) according to claim 18 wherein the blocks (12) comprise one or more cavities so that the blocks (12) are coolable.

20. Device (10) according to one of claims 1 1 to 19 comprising a temperature sensor.