CA2257251A1

CA2257251A1 - Diffusion welding process and device for joining sheet metal parts

Info

Publication number: CA2257251A1
Application number: CA002257251A
Authority: CA
Inventors: Hanns-Peter Liebig; Jan Bober; Jens Jacobsen; Timm Kuhne
Original assignee: Individual
Current assignee: Eckold GmbH and Co KG
Priority date: 1997-04-05
Filing date: 1998-04-03
Publication date: 1998-10-15
Also published as: EP0934132B1; ES2179494T3; CN1222872A; RU99100054A; ATE219708T1; DE59804575D1; PT934132E; JP2000511470A; DK0934132T3; EP0934132A1; WO1998045065A1; DE19714129A1

Abstract

In a diffusion welding process for joining superimposed sheet metal parts by welding, part of the material of the metal sheets is locally upset by means of a stamp and a die perpendicularly to the plane of the metal sheets, causing the material of the sheet metal parts to yield and diffusion welding the sheet metal parts at a weld. In order to achieve a sufficiently solid weld even when the parts to be joined are made of materials with a particular tendency to hardening at low temperatures or pre-hardened materials, the parts of the metal sheet surrounding the weld are pressed onto the die during welding with a force which is sufficient to prevent the material from yielding against the active direction of the stamp.

Description

CA 022~72~1 1998-12-04 Press-joining method and device for connecting sheet metal parts The invention relates to a press-jolning method S according to the precharacterizing clause of Patent Claim 1 and to a device for implementing the said method in accordance with the precharacterizing clause of Patent Claim 8.
EP 0 330 061 Bl discloses a method and a device for press-joining two or more even sheet metal parts lying one upon another, in which a tool set consisting of punch and die is used, whose die has a central anvil and die mould pieces which project vertically with respect to an anvil working face, which can preferably move out sideways during upsetting and delimit a mould cavity into which the join is made.
In the case of known press-joining, the sheet metal parts to be connected are simultaneously displaced at an optional joint location and then upset without heating whilst applying such pressures that positive and force-fitting seating of the sheet metal parts in one another in the form of a cup-like joint element is produced as a result of lateral material flow. The volume of material that is located at the joint location under a die active face is in this case initially only displaced. To this end, the sheet metal material is displaced out of the plane of the sheet metal parts, the base thickness of the joint produced still corresponding approximately to the overall initial sheet metal thickness. During further press-joining, the base of the cup of the joint element is clamped in between the punch active face and the anvil and, by means of an increasingly rising punch force, an upsetting operation with radial material flow in the bottom region is effected, the so-called spreading. The thickness of the bottom of the metal sheets is reduced by upsetting with respect to the initial sheet metal thickness, which is necessary in order to produce the necessary interlock by means of spreading.

CA 022~72~1 1998-12-04 In order to make this upsetting operation easier, the sheet metal parts to be joined can be cut into partially in the direction of displacement at the joint location, since then, during the upsetting operation, the base of the cup of the joint element remains largely free of the bond to the remaining material, or this bond is at least reduced. However, it is disadvantageous that a cut portion at the joining operation reduces the strength of the press-joint, and the joint is not gastight, so that joint elements which are produced without local cutting are preferred.
Until now, it has not been economically possible to use this press-joining of sheet metal parts in the case of sheet metal parts made of stainless steel with a predominantly austenitic basic structure.
The same is true in the case of other metals with a particularly pronounced tendency to work-hardening during a progressive deformation process, for example highly alloyed stainless steels, titanium, or of metals that are already pre-hardened, for example higher-strength steels. Here, the material-induced processing forces for joining by metal forming lie above the load limit of the joining tool or lead to the latter failing as early as after a few joining operations.
However, if too low processing forces are used, the load capacity of the joints is inadequate.
Furthermore, it has been established that, even given the greatest possible cold upsetting, only an inadequate interlock in the joint element is achieved.
Press-joining such sheet metal materials is therefore only possible at all when accompanied by cutting of the sheet metal parts, and at the expense of the disadvantages associated with this, the reduction of the processing forces which is achieved by the clltting not leading to any noticeable increase in the tool service life either.
A first object of the invention is therefore to provide a press-joining method of the generic type mentioned at the beginning with which an adequately CA 022~72~1 1998-12-04 strong joint can be produced even in the case of parts to be joined which consist of materials with a particular tendency to work-hardening, or pre-hardened materials.
A second object is to provide a device for this method.
The first object is achieved by means of the features of the characterizing part of Patent Claim 1.
The second object is achieved by means of the features of the characterizing part of Patent Claim 8.
By this means, a method is provided in which an effective interlock is achieved with the application of a reduced joining force. This applies, in particular, to the materials with a particular tendency to work-hardening. The joining forces which have to be appliedto this extent are reduced such that the tools can withstand this mechanical loading.
According to the invention, it has been established that, in particular in the case of severely work-hardening sheet metal materials, spreading is made more difficult by the fact that the material attempts to move out under pressure stresses, and flows back counter to the direction of action of the punch. By pressing on the parts to be joined during the joining operation, this reverse flow is prevented, and the sheet metal material is forced, by means of the spreading initiated in this way, to form an interlocking joining element in the active range of the die. In the case of materials with a particular tendency to work-hardening, the method therefore also leads to a good interlock which was previously not achieved.
Because the sheet metal parts located around a joint location are pressed onto the die with a force which is so great as to press the material onto a support (the die here) in such a way that the volume of material to be joined can no longer flow back counter to the direction of action of the punch, the situation is specifically achieved in which the upsetting force CA 022~72~1 1998-12-04 -applied is used in an early phase of this upsetting to form an effective interlock. Consequently, a sufficiently strong joint results in an advantageous way, even when, whilst taking care of the tools, the necessary deformation degree at the joint location takes place with the application of a reduced force. As a result, press-joining in particular becomes more economical, since the number of joints which can be carried out with one joining tool is increased considerably.
The pressing force acting on the sheet metal parts around the joint location is a passive force, which must be higher than the reverse-flow force of the material of the parts being joined during the joining operation. According to the invention, it has been established that the pressing force exceeds the reverse-flow force of the material if, preferably, the pressing force is set to be at least so high that, during the joining operation, the sheet metal parts do not gape in the area surrounding the punch.
Furthermore, preferred values for the pressing force, for example for flat, even sheet metal parts, lie at around five times to eight times the forces which are needed merely to strip the joint parts or to prevent them bending up during the displacement phase.
In addition, the pressin~ force is preferably applied to an effective contact area of such a size around the joint location that the pressing force to be applied leaves behind no impression in the metal sheets, and the mould pieces of the die are able to move away sideways during upsetting. The pressing-force active face can therefore be dimensioned to be sufficiently large, so that the resulting pressure loading cannot lead to plastic deformation of the clamped parts to be joined, and to be as small as possible, in order that joints can even be made at poorly accessible locations.
It has been found that metal forming may be improved still further if the sheet metal parts are CA 022~72~1 1998-12-04 joined at a temperature which is increased above the ambient temperature or that of the tools; in this case, a temperature difference of 10~C to 50~C already results in an improvement of the load capacity of the joint, given the same joining force, or in the case of the same load capacity as in the case of joint elements produced at ambient temperature, the joining force may be reduced by up to 15%. One reason for this resides in a reduction in the work-hardening, such as occurs in particular in the case of austenitic sheet metal materials.
Using the press-joining device according to the invention, it is also possible for sheet metal materials with a pronounced tendency to work-hardening to be press-joined, without plastic deformations of punch and anvil or failure of the tool basic body occurring. In this case, the modular construction of the punch and die permits the tool set to be loaded more highly, since cracks are forestalled.
Consequently, the tool set is able to absorb higher joining pressures.
Preferred high-strength materials for the punch and anvil pin are hard metal and high-strength ceramic.
In order to support the punch and anvil pin in a manner suitable for their stress, these may each be supported on a planar supporting plate.
The anvil face preferably has a shape which is suitable for material flow in the form of a circumferential, flat chamfer, with which an improved interlock is achieved with the same sheet metal bottom thickness out of sheet metal. At the same time, such a chamfer leads to the pressure on the edge being relieved, as a result of which the service life is increased.
The active faces of punch and anvil pin may have a star-shaped grinding pattern, as a result of which their service life is further increased and the material flow is improved.

CA 022~72~1 1998-12-04 Further refinements of the invention are to be taken from the following description and the subclaims.
The invention will be explained in more detail below with reference to exemplary embodiments illustrated in the appended figures, in which:
Fig. 1 shows, in schematic form, a partly sectioned side view of a first exemplary embodiment of a press-joining device, Fig. 2 shows a partly sectioned side view of a die with a pressing element according to a further exemplary embodiment of a press-joining device, Fig. 3 shows a plan view of an active face of a punch according to Fig. 2, Fig. 4 shows a side view of an anvil according to a further exemplary embodiment of a press-joining device, Fig. 5 shows a plan view of an active face of an anvil pin according to Fig. 4, Figs. 6, 7 and 8 are enlarged side views of further embodiments of the area marked with a dashed circle in Fig. 4, Fig. 9a shows, in partial section, a press-joining element produced using the device according to Fig. 1, Fig. 9b shows, in partial section, a press-joining element produced using a device without a pressing element, given the same bottom thickness of the joint element as in Fig. 9a, Fig.lOa shows the press-joining element produced according to Fig. 9a, Fig.lOb shows, in partial section, a press-joining element produced using a device without a pressing element, given a bottom thickness of the joint element that is reduced as a result of a greater joining force than in Figs. 9a, 9b and lOa.

Fig. 1 shows a first exemplary embodiment of a device for press-joining, comprising a tool set with a punch 1 CA 022~72~1 1998-12-04 and a die 2, which are each arranged in a press-joining tool holder 3, 4.
As its central area, the die 2 has an anvil 5 which at the top forms an upsetting table 6 (anvil working face) of a press-joining mould cavity 7 in the die 2. The press-joining mould cavity 7 delimits the displacement of a volume of material by means of die elements, projecting vertically with respect to the upsetting table 6, in the form of mould pieces 8, 9, which are able to move out sideways during an upsetting operation. The mould pieces 8, 9 additionally form, with their respective tops 10, 11, a die rest for sheet-metal parts 12, 13 to be joined. In order that, following a respective joining operation, the mould lS pieces 8, 9 which have move out can be put back into position once more and close the die 2, two restoring springs 42, 43 are preferably combined in a spring cap.
The die 2 can be fastened to the tool holder 4 by means of a fastening screw 14.
The punch 1 has a punch pin 15 with a punch active face 16, which penetrates into the press-joining mould cavity 7 with a selectable joining force for a press-joining operation. During this joining operation, a volume of material of the sheet metal parts 12, 13 that is located at a joint location under a punch active face 16 is initially displaced, that is to say displaced out of the plane of the sheet metal parts 12, 13, until a bottom 17 (cf. Figs. 9a and lOa) of the joint being produced rests on the anvil 5, and is then upset with spreading.
The punch 1 is enclosed by a pressing element 18, which permits force to be applied to the sheet metal parts 12, 13 around a joint location, that is to say adjacent to the punch active face 16. For this purpose, the pressing element 18 presses the sheet metal parts 12, 13 lying around the die active face 16 against the die rest. The pressing element 18 is therefore intended to exert a counter-action to a reverse-flow force of a material. The level of the CA 022~72~1 1998-12-04 application of force respectively the pressing force can be controlled. This control may be separate from the application of the joining force or linked with the latter.
Spring elements 19, 20 for applying the pressing-element force are preferably provided. In addition, the pressing element 18 is preferably guided via the spring elements 19, 20 on the tool holder 3 for the punch 1. Such guidance makes it possible in a simple way to increase the application of force by the pressing element 18 during the joining operation. To this end, the pressing element 18 can be provided with an active face 21, against which the punch 1 operates, whilst compressing the spring elements 19, 20 and increasing the pressing force. The pressing element 18 is preferably designed as a pressure plate and can additionally also be used as a stripping element at the end of a joining operation.
In order to apply the pressing-element force, it is also possible for pressure transmitters other than springs to be used to produce the pressure forces.
In addition, it is not necessary for the pressing element 18 to be guided on the punch. It is also possible for the pressing element 18 to carry out its function by means of an external guide and an external drive.
The active face 21 of the pressing element 18 is dimensioned to be sufficiently large when the resulting pressure loading does not lead to plastic deformation of the surface of the sheet metal parts 12, 13 to be joined, and the mould pieces 8, 9 of the die 2 are also able to open during the joining operation.
The punch 1 and the die 2 are preferably modularly constructed. To this end, the punch comprises the punch pin 15, at whose top end the die active face 16 is constructed, and which is pressed into a basic body 23. For this purpose, the basic body 23 preferably has a through hole 24. This modular construction permits the use of different materials for .... .. .

CA 022572~l l998-l2-04 the punch pin 15 and the basic body 23. The punch pin may consist of a first material, specifically a material of relatively high hardness or relatively high strength. Preferred materials for this are hard metal and high-strength ceramic. The basic body 23 may consist of a second material, which is less hard but for this reason, for example, is more ductile than the first material. Thermal treated tool steel is preferred as the second material.
At its end facing away from the working active face 16, the punch pin 15 iS in addition preferably supported on a plate 25, which may consist of the same rnaterial as the punch pin 15 and which, for its part, is supported in the tool holder 3 of a press (not illustrated). The transverse dimensions of the plate 25 are selected such that the load acting on the tool holder 3 can be absorbed by the latter without risk of fracture. The punch 1 can be fastened to the tool holder 3 by means of a fastening screw 26.
The die 2 can be correspondingly modularly constructed, like the punch 1. The anvil 5 is then formed by an anvil pin 22 made of a first material, specifically a material of relatively high hardness or relatively great strength, such as hard metal or high-strength ceramic, for example. The anvil pin 22 iS
pressed into a basic body 27 made of more ductile ~aterial, for which purpose the latter has a through hole 29. The material pairing can be selected as in the case of the punch. The anvil 5 may also be supported on the tool holder 4 via a plate 28. The plate 28 can be designed corresponding to the plate 25 for the punch 1.
Figs. 2 to 5 show a second exemplary embodiment of a press-joining device, which differs from the exemplary embodiment described above only in the differences cited below. Otherwise, the abovementioned designs apply in a corresponding manner, so that the same reference symbols can be used for identical parts.
Fig. 2 shows a punch 1 of modular design having a punch pin 15, on which the die active face 16 iS

CA 022~72~1 1998-12-04 designed, said pin being pressed into a basic body 23 and being supported on a base plate. Part circumferential grooves 30 are used for changing the tool rapidly, and hence for fastening the punch 1 to a tool holder. As in the case of the exemplary embodiment according to Fig. 1, the active end of the punch is surrounded by a pressing element 18, which is supported on the basic body 23 via springs 19, 20. Via this pressing element 18, which encloses the punch 1 as closely as possible, but with a sliding fit, and the springs 19, 20, the pressing forces according to the invention are now applied, and press the pressing element 18 so forcefully onto the die that the sheet metal material is clamped firmly around the punch 1, and the material of the sheet metal on the punch side is thus prevented from flowing upward, that is to say counter to the active direction of the punch 1, during upsetting process. It is possible to press on the sheet metal on the die rest before the start of displacement, so that no bending of the metal sheets during displacing needs to be feared either.
The pressing element 18 according to Fig. 2 differs from that of Fig. 1 in that the active face 21 of the pressing element 18 runs continuously.
Fig. 4 shows the associated die 2 with an anvil 5, which is pressed into a basic body 27 and is supported on a base plate 28. The basic body 27 has, on its side facing the punch 1, oblique surfaces 32, on which die elements (not illustrated) are supported, resting with spring prestress on the anvil 5 and, together with the active face 6 of the anvil, limiting the die cavity.
In order to prevent the risk that the anvil 5 and/or the die pin 15 will break out at the edge under high pressure loading, a circumferential chamfer 31 is provided, whose cross-sectional shape, as illustrated in Figs. 6 to 8, may be variable: rounded (Fig. 6), flat bevelled (Fig. 7) or double bevelled (Fig. 8).
This chamfer also promotes material flow.

.

CA 022~72~1 1998-12-04 The mutually facing active faces 16, 6 of punch 1 and/or anvil 5 are provided with a surface finish which promotes the flow of the material to be joined, as can be seen on the star-shaped grinding pattern in Figs. 3 and 5. The active faces 16 and 6 in the case of the device according to Fig. 1 may also be designed correspondingly.
A press-joining method for connecting sheet metal parts 12, 13 lying one upon another comprises a joining operation in which, by means of a punch 1 and a die 2, at a joint location a volume of material of the sheet metal parts 12, 13 is displaced locally and, as a result of upsetting perpendicular to the plane of the sheet metal, is joined as the material flows. During this joining operation, the sheet metal parts 12, 13 surrounding the joint location are pressed onto the die 2 with a pressing force which is sufficient to prevent any flow of material counter to the direction of action of the punch 1.
The pressing force can be kept constant during the entire joining operation. Alternatively, the pressing force can be increased during the joining operation, in order that, at least during upsetting, a pressing force which exceeds the reverse-flow force acts in the area surrounding the joint location, whereas the said force can be selected to be lower before this. It is also possible for a first pressing force to be applied around the active face of the punch before or shortly before the joining operation.
The sheet metal parts 12, 13 are pressed in an essentially stationary manner on the die 2, to be specific with such a pressing force that the reverse flow of material is prevented. This applies, in particular, for materials with a particular tendency to work-hardening, or pre-hardened materials.
The pressing force is in particular selected such that, during the joining operation, the sheet metal parts 12, 13 do not gape in the area surrounding _ _ CA 022~72~1 1998-12-04 the punch 1, that is to say the sheet metal parts 12, 13 lie flat one upon another.
In addition, the pressing force for flat, even sheet metal parts can be selected such that it is five times to eight times the forces which are needed merely to prevent the joint parts bending up during the displacement phase. A guide value, mentioned by way of example, for the force applied in the case of a joint element diameter of 5 mm is 3000 N, if, for example, austenitic stainless steel sheets are being joined.
In this case, the pressing force is preferably applied to an effective contact area of such a size around the joint location that the pressing force to be used leaves behind no impression in the sheet metal parts, and the die can open sideways.
Furthermore, the temperature of the sheet metal parts 12, 13 can be increased above the ambient temperature. Preference is given to temperatures that are 10~C to 50~C higher than the ambient temperature.
2 0 Fig. 9a shows, by way of example, a joint produced using the above-described press-joining devices, the pressing force exerted by the pressing element 18 during the joining operation being indicated by the arrows and the letter F. Sheet metal parts 12, 13 made of a particularly work-hardening material and having an initial sheet metal thickness of about 1 mm in each case have been connected with interlock by a joint element whose bottom 17 has been upset between the active face 16 of the punch and the working face 6 of the anvil in the mould cavity 7, whilst reducing the overall initial thickness. A spreading 33 has been formed as a result of material flow, for which purpose the mould pieces 8, 9 have moved outwards. The result is a joint with a good interlock. At the end of the joining operation, the sheet metal parts 12, 13 do not gape in the area surrounding the punch, so that these sheet metal parts lie flat one upon another and without any gap areas.

CA 022~72~1 1998-12-04 By comparison with this, Fig. 9b shows a joint with a punch 1 and a die 2 with a punch assembly but without a pressing element. In the case of achieving the same bottom thickness, that is to say approximately equal joining forces, an effective interlock is still not achieved.
Figs. lOa and lOb serve to clarify the inventive method further. The joint according to Fig. lOa corresponds to that illustrated in Fig. 9a.
The joint of Fig. lOb differs from that of Fig. 9b in that increased joining forces have been used, which have led to a higher upsetting degree of the bottom 17 of the joint. However, the spreading 34 achieved in this way still does not exhibit an adequate interlock.
Figs. 9a, 9b and lOa, lOb make it clear that the inventive method permits the use of lower joining forces, since the effective interlock is already achieved with lower upsetting rate.

.

Claims

claims

1. Press-joining method for connecting sheet metal parts lying one upon another by means of a joining operation in which, by means of a punch and a die, at a joint location a volume of material of the sheet metal parts is displaceded locally and, as a result of upsetting perpendicular to the plane of the sheet metal, is joined as the material flows, characterized in that, during the joining operation, sheet metal parts surrounding the joint location are pressed onto the die with a pressing force which is sufficient to prevent any flow of material counter to the direction of action of the punch.

2. Press-joining method according to Claim 1, characterized in that the pressing force is selected such that, during the joining operation, the sheet metal parts do not gape in the area surrounding the punch.

3. Press-joining method according to Claim 1 or 2, characterized in that the pressing force for flat, even sheet metal parts is selected such that it is five times to eight times the forces which are used merely to strip the joint parts or to prevent them bending up during the displacement phase.

4. Press-joining method according to one of Claims 1 to 3, characterized in that the pressing force is applied to an effective contact area of such a size around the joint location that the pressing force to be used leaves behind no impression in the sheet metal parts, and the die can open sideways.

5. Press-joining method according to one of Claims 1 to 4, characterized in that a pressing force is applied to the surrounding sheet metal parts before the joining operation is started.

6. Press-joining method according to one of Claims 1 to 5, characterized in that the temperature of the sheet metal parts is increased above the ambient temperature.

7. Press-joining method according to Claim 6, characterized in that the sheet metal parts are heated to a temperature 10°C to 50°C higher than the ambient temperature.

8. Press-joining device for connecting sheet metal parts lying one upon another using a die and a punch that can penetrate into a hollow mould in the die, the die having an anvil and mould pieces which project vertically from the said anvil, can pivot out sideways and are intended to limit the hollow mould, in which displacing takes place locally, characterized in that the punch (1) is enclosed by a pressing element (18) for applying a force to the sheet metal parts (12, 13), and the anvil (5) and the punch (1) each have a pin (15, 22) which has an active face (16, 6) and is made of a high-strength material, and which is pressed into a basic body (23, 27) made of another material.

9. Press-joining device according to Claim 8, characterized in that the basic bodies of the punch and anvil (23, 27) have through holes which accommodate the respective pins (15, 22), located inside them.

10. Press-joining device according to Claim 8 or 9, characterized in that the pins (15, 22) are each supported at the end opposite the respective active face (16, 6) on a planar supporting plate (25, 28), via which pressure loading can be introduced, at the anvil (5) and the punch (1), into a tool holder (3, 4).

11. Press-joining device according to Claim 10, characterized in that the supporting plates (25, 28) consist of a high-strength material.

12. Press-joining device according to one of Claims 8 to 11, characterized in that the high-strength material used can be hard metal or high-strength ceramic.

13. Press-joining device according to one of Claims 8 to 12, characterized in that the active faces (6, 16) of the anvil (5) and/or the punch (1) are provided with a circumferential chamfer (31).

14. Press-joining device according to Claim 13, characterized in that the circumferential chamfer (31) is rounded.

15. Press-joining device according to Claim 13, characterized in that the circumferential chamfer (31) has a triangular cross section.

16. Press-joining device according to Claim 13, characterized in that the circumferential chamfer (31) has a broken cross-sectional shape that approximates rounding.

17. Press-joining device according to one of Claims 8 to 16, characterized in that the active faces (6, 16) of anvil (5) and/or punch (1) are provided with a surface finish which promotes material flow.

18. Press-joining device according to one of Claims 8 to 17, characterized in that the pressing element (18) is a pressure plate that is guided on the punch (1) and is supported by one or more prestressed springs (19, 20).

19. Press-joining device according to one of Claims 8 to 18, characterized in that the pressing element (18) is guided on the basic body (23) of the punch (1).