BACKGROUND OF THE INVENTION

The present invention relates to a self-piercing rivet for joining workpieces, in particular made from high tensile steels, having a head which comprises a head diameter, and having a shank which comprises a shank diameter, wherein the shank, on the foot end located opposite the head, comprises an axial recess which has an axial depth, wherein the shank comprises a flat surface portion or a circular cutter on the foot end.

In addition, the present invention relates to a self-piercing riveted joint having at least one upper workpiece and one lower workpiece, of which at least one is realized from a metal, in particular from a high tensile steel, and having a reshaped self-piercing rivet of the type designated above, the head of which abuts against the upper workpiece.

A self-piercing rivet with a flat surface portion at the foot end is disclosed in document DE 10 2013 020 504 A1. During a self-piercing riveting process carried out with the self-piercing rivet, said development of said self-piercing rivet produces a reshaping of the self-piercing rivet which is based less on the rivet shank spreading apart. The achievement of the relatively short axial depth of the recess rather is that the joint is formed as a result of a compressing of the rivet which is produced, in particular, by the counter-pressure of the high tensile steel of the workpiece arrangement. The undercut or the indentation, which is formed as a result, can be relatively small in this case. On account of the high tensile materials, even a relatively small undercut, however, is sufficient to realize the necessary joint tensile strength.

In addition, a self-piercing rivet is disclosed in document EP 1 229 254 A2. Said document proposes providing a self-piercing riveted joint with at least two joining parts produced from a high tensile steel which are joined together by means of a half-hollow self-piercing rivet, which is formed from steel and comprises a rivet head and a rivet shank, which connects thereto, with a rivet foot at the end, the rivet foot being realized in an obtuse manner in the initial state prior to the joining operation. The form of the self-piercing rivet, in this case, is to be identical to the form of a self-piercing rivet as is also known for joining light metal workpieces, from document EP 0 833 063 A1. A favourable reshaping behaviour of the half-hollow self-piercing rivet is to be achieved when joining the high tensile joining parts as a result of the obtuse realization of the rivet foot, the striving of the rivet shank to spread apart being reduced compared to a pointed rivet foot. In this case, the spreading apart is not to occur until the rivet shank penetrates into the lower joining part with the rivet foot. The joining parts are to comprise a tensile strength in excess of 500 N/mm2, up to 1500 N/mm2. The tensile strength of the half-hollow self-piercing rivet used is to be within a range of between 1200 and 1400 N/mm2, but can also reach values of up to 2000 N/mm2.

In order to ensure a suitable spreading-apart behaviour, the quotient from the axial depth of the shank cavity and the rivet foot outside diameter is to be between 0.3 and 0.7. Where an axial depth of the shank cavity is too small, the rivet shank is said not to spread apart sufficiently once the upper steel sheet has been perforated.

A further half-hollow self-piercing rivet is disclosed in document WO 2007/132194 A1. In this case, a central blind hole is to be provided in the shank, wherein the ratio of a difference between an outside and an inside diameter of the shank in the region of the hole is to be within the range of between 0.47 and 0.52.

In addition, document EP 2 314 890 A2 discloses a half-hollow self-piercing rivet for joining high and super high tensile steels, a head diameter generally being smaller than or equal to 1.3 times the shank diameter.

Self-piercing riveted joints of the prior art, by way of which high tensile or super high tensile steels are joined, can, however, still have various problems. On the one hand, the extent of the spreading apart may not be symmetrical with reference to a rivet axis. In addition, the shank can be pressed together and bent. In many cases, it is not even possible to press the rivet into the workpiece arrangement, it even being possible for the self-piercing rivet to fracture.

Self-piercing rivets which are designed to join high tensile or super high tensile steels as a rule comprise a flat surface portion at the foot end, as mentioned above. Other self-piercing rivets such as, for example, C-rivets comprise a circular cutter at the foot end of the shank. A circular cutter is formed, in particular by a cutting line between a radius or a chamfer in the region of an inside surface of the axial recess and a radius or a chamfer toward a radial outside surface of the shank. A circular cutter of this type should be realized for the purpose of severing an upper workpiece of a workpiece arrangement to be joined in a circular manner, the upper workpiece preferably being produced from a steel, but also being able to be produced from another material.

Self-piercing rivets of the above-described form comprise an under-head radius at a transition from the outside surface of the shank toward the head, and the head bottom surface is frequently inclined in relation to a radial plane of the self-piercing rivet and forms a cone portion.

Document US 2016/0084288 A1 has disclosed a half-hollow self-piercing rivet where a projection, which can be melted by means of a weld current in order to be welded with the lower workpiece of a workpiece arrangement, is realized at a foot end of the shank. In the case of said self-piercing rivet, the shank does not spread apart in the radial direction when the setting operation is carried out. In the region of an outside periphery of a head bottom surface, a head of said setting rivet comprises a cutting edge which protrudes axially forward and penetrates into an upper surface of an upper workpiece of the workpiece arrangement and is to generate a press fit as a result.

In the case of self-piercing rivet joints of the prior art, stresses can occur during setting operations in the region of a transition between the shank and the head.

BRIEF SUMMARY OF THE INVENTION

Against said background, it is an object of the invention to provide an improved self-piercing rivet as well as an improved self-piercing riveted joint, both of which are suitable for joining different materials, in particular metals, in particular aluminium or steel and in particular for joining high tensile and super high tensile steels.

Said object is achieved in the case of the self-piercing rivet named in the introduction as a result of an axial circular recess being realized in a region of a transition portion between the head and the shank.

The axial recess makes it possible to reduce the setting forces when setting or carrying out a self-piercing riveting procedure.

The achievement of reducing the setting forces means that damage to the self-piercing rivet and/or of one of the workpieces whilst the self-piercing riveting method is being carried out is less probable or even excluded. For precisely in the case of high tensile and specifically in the case of super high tensile metal sheets, as a rule the setting force increases dramatically as soon as a head bottom surface, which is frequently realized as a slant or with a positive radius, touches an upper surface of the upper workpiece of the workpiece arranged to be joined.

The bottom surface of the head in the region of the head protrusion in relation to the shank is preferably either aligned substantially radially or, however, is chamfered in relation to such a radial plane or is provided with a radius in such a manner that an angle between a shank outside surface and the head protrusion is greater than or equal to 80° and smaller than 140°. Consequently, the head protrusion is preferably not able to penetrate into an upper workpiece surface.

The circular recess makes it possible for the head protrusion to stand out in a circular manner in relation to a radial plane during the setting operation such that the head protrusion assumes an angle of greater than 0° in relation to the radial plane in the set state.

The self-piercing rivet according to the invention, however, is advantageous not only when used in self-piercing riveting procedures where high tensile and super high tensile metal sheets are used. The self-piercing rivet according to the invention is also advantageous in the case of other applications (composite construction, super high tensile steels, standard steel, CFK, Al).

In particular, when carrying out self-piercing riveting procedures using the self-piercing rivet according to the invention, cracks in the self-piercing rivet in the region of the transition portion between the head and the shank are prevented or are less likely to occur.

The circular recess can be realized in an angular manner, however is preferably realized in longitudinal section as a continuous recess with a constant recess development. It is particularly preferred when the recess is realized by a plurality of radii which each continuously connect to one another.

In the case of the self-piercing rivet according to the invention, the ratio of the axial depth of the recess to the shank diameter for applications in conjunction with high tensile steels is preferably smaller than 0.3, in particular smaller than 0.28 and in a particularly preferred manner is smaller than 0.25 or even smaller than 0.2.

The ratio of the axial depth of the recess to the shank diameter is preferably greater than 0.05 and preferably greater than 0.01 and in particular greater than 0.12.

In addition, the above object is achieved by a self-piercing riveted joint having an upper workpiece and having a lower workpiece, of which at least one is realized from a metal, in particular a high tensile steel, and having a reshaped self-piercing rivet, the head of which abuts against the upper workpiece, the self-piercing rivet being in particular a self-piercing rivet according to the invention.

Finally, the above object is achieved by a method for producing a self-piercing riveted joint, in particular a self-piercing riveted joint of the type designated above, having the steps of providing a workpiece arrangement, which comprises at least one upper and one lower workpiece, and driving a self-piercing rivet of the type according to the invention into the workpiece arrangement at a self-piercing force.

Where there is a flat surface portion at the foot end, during the self-piercing riveting procedure there is a reshaping of the self-piercing rivet which is based less on a spreading apart of the rivet shank. The achievement of a relatively short axial depth of the recess is that the joint is formed as a result of a compressing of the rivet which is brought about, in particular, by the counter pressure of the high tensile steel of the workpiece arrangement. The undercut or the indentation which is formed as a result can be relatively small in this case. On account of high tensile materials, even a relatively small undercut, however, is sufficient to realize the necessary joint tensile strength.

In addition, a further achievement of the preferred relatively small axial depth of the recess is that the self-piercing rivet maintains a clearly higher level of stability which makes it possible to punch through even high tensile and super high tensile steels.

The flat surface portion at the foot end forms a contribution to the novel production of self-piercing riveted joints with high tensile steels. In other words, it is preferred when the end face of the foot end, also designated as the cutting edge and being as a rule circular, is realized at least in portions in a flat manner, preferably aligned perpendicular to a longitudinal axis of the self-piercing rivet.

The upper workpiece of the workpiece arrangement is preferably produced from steel for this case and has a tensile strength which is preferably greater than 800 N/mm2, in particular greater than 1000 N/mm2. The tensile strength of at least the upper workpiece can be up to 1500 N/mm2and beyond.

The tensile strength of the bottom workpiece is—without heating—preferably limited to approximately 600 N/mm2.

In other words, forming steels, as are known under the designation “Usibor®”, where the micro-structure, prior to a heat treatment, consists, in particular, of a ferrite-perlite framework, are also able to be joined with the necessary joint strength using the self-piercing rivet according to the invention.

It is preferred that the strength or hardness of the self-piercing rivet is adapted in a corresponding manner. In addition, it is preferred that the self-piercing rivet is a half-hollow self-piercing rivet which is produced in particular so as to be rotationally symmetrical and/or in one piece from steel.

The minimum rivet length is preferably the thickness of the upper workpiece plus a length which is preferably greater than 2 mm and in particular equal to 3 mm. The maximum rivet length is preferably within the range of the thickness of the workpiece arrangement.

Whilst above-preferred developments of a self-piercing rivet are described for the application with high tensile and super high tensile steels, where the self-piercing rivet comprises a flat surface portion at the foot end and preferably a relatively small axial depth of a central recess, other application examples of the self-piercing rivet according to the invention can also be realized for applications with normal steels or other materials (for example aluminium).

In said case, the ratio of the axial depth of the recess to the shank diameter is preferably clearly greater than 0.5, in particular greater than 0.75 and can even reach values of 1 or above. In addition, in this case a circular cutter is preferably realized at the foot end of the shank, as has been disclosed in general, for example, for so-called C-rivets.

The object is consequently fully achieved.

It is particularly preferred in one variant when the head comprises a head bottom surface which is realized substantially perpendicular to a longitudinal axis of the self-piercing rivet.

In other words, it is preferred when a transition region between the shank and the head is not formed by a radius or an inclined cone surface. For this would result in said transition region, even at a relatively early time, contacting an upper surface of the uppermost workpiece when a self-piercing riveting procedure is carried out, which leads to high setting forces.

In other words, the development of the head, in such a manner that the head bottom surface thereof is realized substantially perpendicular to a longitudinal axis of the self-piercing rivet, can contribute to a reduction in setting forces.

According to an alternative development, the head comprises a head bottom surface which is aligned at an angle of between 5° and 45°, in particular between 5° and 20° or within a range of between 15° and 45° with reference to a radial plane.

In the case of said variant, a head bottom surface of the head is chamfered overall radially outside the axial circular recess or is realized in a substantially conical manner.

The setting forces are able to be clearly reduced in the case of said development too. As a result of the chamfer, in many embodiments after a setting operation and after bending off the head protrusion in the substantially axial direction, the inclined head bottom surface is able to rest in a substantially flat manner on an upper surface of the upper workpiece, for example for the purposes of providing a seal. This applies in particular when the self-piercing rivet is driven relatively far into the workpiece arrangement. An upper surface of the head of the self-piercing rivet is preferably aligned with an upper surface of the upper workpiece, or even lies somewhat below the upper surface of the upper workpiece such that the self-piercing rivet is quasi countersunk into the workpiece arrangement.

According to a further preferred embodiment, the axial circular recess is realized on a head bottom surface of the head which protrudes radially in relation to the shank.

In this connection, the circular recess preferably does not extend radially into the region of the shank.

In the case of said embodiment, high forces can still be transmitted in the axial direction by means of the shank, it being possible, where applicable, for the head protrusion to be bent off.

In addition, it is advantageous when the axial circular recess, in a first radial recess portion which faces the shank, comprises a greater axial depth than in a second radial recess portion.

This also contributes to the head protrusion being able to be bent slightly in relation to the radial plane during the setting operation. A virtual bending “axis”, which is realized in this case in a circular manner concentrically with respect to a longitudinal axis of the shank, is preferably located in this case in the first radial recess portion.

The first radial recess portion is preferably directly adjacent the shank. The second recess portion is preferably at a spacing radially from a head outside periphery.

According to a further preferred embodiment, a ratio of the radial length of the axial circular recess to a radial head protrusion length is greater than 0.2. It is particularly preferred when said ratio is greater than 0.3, in particular greater than 0.4 or even 0.5. The ratio of the radial length of the axial circular recess to a radial head protrusion length is preferably smaller than 0.9, in particular smaller than 0.8 and preferably smaller than 0.7.

As a result, the setting force is able to be further reduced as a longer part of the head protrusion is bent off in relation to the longitudinal axis of the shank when a setting operation is carried out.

In addition, it is advantageous when a ratio of an axial depth of the axial circular recess to an axial head height is greater than 0.15. The ratio is preferably greater than 0.25, in particular greater than 0.3. The ratio is in particular smaller than 0.6, preferably smaller than 0.5 and in particular smaller than 0.4.

The maximum axial depth of the axial circular recess, which departs in the axial direction preferably from an axial position of the self-piercing rivet, which is defined by a bottommost position of the head bottom surface which is located radially outside the circular recess, is preferably assumed for calculating said ratio.

The above-named ratio of the axial depth of the circular recess to the axial head height also contributes to a reduction in the setting forces.

According to a further preferred embodiment, a head bottom surface of the head is chamfered in a portion radially outside the axial circular recess.

The chamfered portion, in the case of said embodiment, is either the entire portion of the head bottom surface located radially outside the axial circular recess. The chamfered portion of the head bottom surface, however, can also be at a spacing from the axial circular recess in the radial direction in such a manner that a substantially radially aligned head bottom surface portion is realized between the axial circular recess and the chamfered portion.

The chamfer is preferably such that said chamfered head bottom surface portion in the set state rests in a substantially flat manner on an upper surface of a joined workpiece.

A sealing tightness of the set self-piercing riveted joint can be increased as a result.

The alignment of the head bottom surface portion or of the cone angle thereof is preferably within a range greater than 0° and smaller than 45°. Said cone angle of the head bottom surface located radially on the outside preferably corresponds to an intended bending of the head protrusion when carrying out the setting operation in relation to a radial plane.

According to a further preferred embodiment, which provides an independent invention in conjunction with the preamble of claim1, the central recess, in longitudinal section, is in the form of a truncated cone, wherein a recess bottom of the recess is realized in the form of a cone, wherein a ratio of a cone angle of the recess bottom to a cone angle of the recess is smaller than 0.4.

The ratio of said angle is preferably smaller than 0.3, in particular smaller than 0.25. Said ratio is preferably greater than 0.05 and in particular greater than 0.1, preferably greater than 0.2.

The cone-shaped recess bottom, which preferably forms a very flat cone, makes it possible, in particular, to simplify the producibility of the self-piercing rivet and/or to realize an optimized receiving of a slug and/or an improved radial widening for the purposes of forming an undercut when carrying a setting operation.

When the recess, in longitudinal section, is in the form of a truncated cone, the diameter of the recess in the region of the foot end is preferably greater than that in the region of a bottom of the recess. In the case of said embodiment, the bottom of the recess can preferably be flat, but can also be curved in a concave or convex manner, or realized in a cone-shaped manner, as above.

According to a further preferred embodiment, the central recess is arcuate in longitudinal section.

The arcuate form, in this case, can be produced by one single radius such that the recess, in longitudinal section, is in the form of a circular arc.

It is particularly advantageous, however, when the recess, in longitudinal section, comprises the form of a pointed arch or a gothic arch.

Such an arch form is produced by two arches constructed from circles with an apex.

In this case, it is preferred when the apex is rounded in a suitable manner by means of a radius.

In addition, it is preferred in the case of the pointed arch shape when the centre points of the respective arches are in each case located on different sides—when viewed in longitudinal section—of a longitudinal centre axis of the self-piercing rivet to the respectively associated arch.

In the case of the two above-mentioned embodiments—in the form of truncated cone or in the form of an arch—it is advantageous for the self-piercing forces acting from the head end to be introduced suitably into the foot end.

All in all, it is additionally preferred when the central recess does not comprise a cylindrical portion.

A cylindrical portion in the recess can lead to instability and, where applicable, fractures in the case of very high punching pressures.

The stability of the self-piercing rivet overall can be increased by dispensing with a cylindrical portion inside the recess.

According to a further embodiment, the central recess has a recess volume, wherein a ratio of the recess volume to the volume of the shank is smaller than 0.25, in particular smaller than 0.18 and/or greater than 0.05, in particular greater than 0.1.

The recess volume is calculated in this connection proceeding from the foot end of the self-piercing rivet. The volume of the shank is the volume of the shank where the shank comprises a standard outside diameter, that is exclusively a possible transition portion toward a head of the self-piercing rivet, but including the recess volume which is consequently also contained in the volume of the shank.

The relatively small recess volume on the one hand produces a high level of stability for the self-piercing rivet. On the other hand, a slug detached from the upper workpiece is not received by the recess but rather during the self-piercing operation is pushed by the rivet in front of itself. The advantageous achievement here is that material is reshaped to a greater extent inside a die of a self-piercing riveting tool instead of being deformed inside the recess.

According to a further preferred embodiment, the flat surface portion is realized as a circular surface portion and comprises, in cross section, a radial width, wherein the ratio of the radial width of the circular surface portion to the shank diameter is greater than 0.05 and/or smaller than 0.25.

The self-piercing rivet is preferably produced from a steel with a hardness of at least 500 HV10 (1630 MPa), in particular with a hardness of at least 650 HV10, in particular with at least 700 HV10. As a rule, the hardness is less than 800 HV10.

In the case of the self-piercing riveted joint according to the invention, it is preferred when the axial thickness of the upper workpiece is greater than or equal to the axial depth of the central recess in the non-deformed state.

In addition, it is advantageous in the case of the self-piercing riveted joint according to the invention when a slug is separated from the upper workpiece and when less than 50% of the volume of the slug is situated inside the deformed central recess, in particular less than 30%, preferably less than 25% and particularly preferred less than 20%.

This results from the self-piercing rivet being realized such that it is compressed in a substantial manner, as a result of which the volume of the recess is reduced such that the slug is pushed substantially in front of the rivet during the self-piercing riveting operation.

As a result, material of the lower workpiece can be displaced in a suitable manner inside the die by means of the slug such that it flows behind an undercut of the shank of the self-piercing rivet.

All in all, it is additionally advantageous when the shank of the reshaped self-piercing rivet forms an undercut in relation to forces in the direction of the head, wherein the ratio of undercut to shank diameter is smaller than 0.1 and/or greater than 0.01.

This results in the extent of the undercut being comparatively small. When joining high tensile steels, a small undercut of this type, however, is sufficient to realize the necessary joint strength.

According to a further preferred embodiment of the self-piercing riveted joint, the ratio of the axial length of the self-piercing rivet after the reshaping and of the axial length of the self-piercing rivet prior to the reshaping is greater than 0.8 and/or smaller than 0.95.

This results in the self-piercing rivet only being compressed in a comparatively small manner on account of its predetermined hardness, which also results in a relatively small undercut in the radial direction.

This also results in the minimum length of the self-piercing rivet preferably being produced from the thickness of the upper workpiece plus a value of preferably 3 or 3.5 mm, whereas the maximum length of the self-piercing rivet is preferably calculated by the overall thickness of the workpiece arrangement plus 1 mm or is equal to the overall thickness of the workpiece arrangement.

Generally speaking, it is also possible to carry out a self-piercing riveting procedure with the self-piercing rivet according to the invention where a self-piercing riveted joint is produced, in particular a self-piercing riveted joint of the type described above, having the steps of providing a workpiece arrangement, which comprises at least one upper and one lower workpiece, and of driving a self-piercing rivet of the type according to the invention into the workpiece arrangement at a self-piercing force.

In the case of said method, it is advantageous when the workpiece arrangement is supported on a die with a die volume, into which at least the lower workpiece is driven, the ratio of die volume to a volume of the self-piercing rivet preferably being greater than or equal to 1.0 and/or smaller than or equal to 1.5.

The die volume is the volume into which material at least of the lower workpiece flows during the self-piercing riveting operation, the upper edge of the die recess, provided for this purpose, being substantially flush with a support surface. The die recess, in this case, is preferably in the form of a truncated cone, with a relatively large diameter in the region of the contact surface and a smaller diameter in the region of a bottom of the die volume.

All in all, the following can be additionally noted in a supplementary manner. In the case of conventional self-piercing riveting, the forming of the undercut is a feature that is relevant to the quality of the joint strength. When the rivet according to the invention has more strength, this feature is no longer exclusively applicable. The rivet then requires a relatively firm upper workpiece, the slug of which the compresses the rivet and, at the same time, presses it somewhat apart. The undercut in the joint is not generated by a classic spreading apart in contrast to rivets of the prior art, but rather as a result of a compressing of the rivet which is brought about as a result of the counter pressure of the high tensile steel. In the case of such rivets, the application range begins as a rule initially at 800 N/mm2, in particular at 1000 m N/mm2tensile strength of the upper workpiece. Steels of said strength category have been used in vehicle construction as a result of the reinforced use of lightweight structures produced from high tensile metal sheets. The scope of use of said self-piercing rivet downward is preferably limited by a minimum self-piercing force of 8 kN—the force to perforate/punch through the high tensile workpiece arrangement. A sufficient compressing (not primarily spreading) of the self-piercing rivet starts from said force, and the necessary extent of compression, which is preferably at least 0.15 mm, is obtained. To evaluate the joint quality, the compression extent must be looked at along with the formation of the undercut. The compression extent is calculated from the axial length of the self-piercing rivet prior to the reshaping minus the axial length of the self-piercing rivet after the reshaping, i.e. in the set state.

It is obvious that the features named above and those yet to be explained below are usable not only in the combination provided in each case, but also in other combinations or standing on their own without departing from the framework of the present invention.

