Method for joining at least two parts to be joined which are arranged so as to overlap at least in a joining zone using a joining element

A method for joining at least two parts to be joined which are arranged so as to overlap at least in a joining zone is disclosed using a joining element which is fed to the joining zone in a joining direction and which is designed so as to be free of undercuts at least in a shaft when seen opposite the joining direction. At least one first part to be joined that interacts first with the joining element in the joining direction, is punched by means of the joining element, and the joining element is pressed into a second part to be joined without punching through the second part. The shaft of the joining element is designed so as to be free of undercuts after being pressed into the second part to be joined and is arranged so as to contact the first and the second part to be joined with a radially applied force in a form-fit-free manner at least when seen opposite the joining direction.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2014/003182, filed Nov. 27, 2014, which designated the United States and has been published as International Publication No. WO 2015/106780 and which claims the priority of German Patent Application, Serial No. 10 2014 000 624.7, filed Jan. 18, 2014, pursuant to 35 U.S.C. 119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a method for joining at least two parts to be joined that are arranged so as to overlap at least in a joining zone using a joining element according to the preamble of claim1.

From DE 103 29 447 A1 a joining method for joining parts that are to be joined and are arranged so as to overlap each other is known which uses a joining element having a shaft region that is free of undercuts. In this disclosed joining method material of a first joining part is urged during the joining process through a punch recess of the second joining part, which recess is generated during the joining method, wherein an undesired punch piece is generated. After urging material of the first joining part through the generated punch opening of the second joining part the material of the first joining part is compressed relative to the second joining part so that a form fit is generated is between the first and second joining part. A disadvantage of this method is that an undesired slug is generated that has to be reliably removed from the joining site. In addition uncoated border regions are generated when urging the first material through the just generated punch opening in the second joining part, which may lead to undesired corrosion.

In DE 10 2011 009 649 A1 the method generally known as solid punch riveting is disclosed. In such a method the used joining element has at least in the shaft region undercuts or grooves and indentations that extend in circumferential direction and into which material is pressed with a die so as to ensure that the solid rivet is form-fittingly seated in the material of the two joining parts.

A disadvantage of such a so-called solid punch riveting is that at least one, or even two slugs of the two joining parts are generated. In particular when the joining parts are made of different materials, two slugs are particularly disadvantageous because after joining with the solid rivet they have to be separated again according to material to be recycled.

From DE 10 2008 005 289 a fastening arrangement of a superstructure component, in particular of a roof element for a vehicle roof on a body of a motor vehicle, is known. In this fastening arrangement joining or fastening is accomplished by the solid punch riveting process, in which two slugs are generated during introduction of the solid rivet into the components to be joined. A disadvantage in this method is that slugs are produced, which involves increased disposal costs.

It is an object of the invention to avoid the disadvantages of the state of the art. It is another object of the invention to provide a joining method which uses a joining element and which minimizes corrosion sensitivity in the region of the joining sites.

A further object of the invention is to provide a joining method in which no free slugs are produced that have to be disposed.

A further object of the invention is to provide a joining method with a simple formed joining element, which has sufficient removal strengths or removal forces.

SUMMARY OF THE INVENTION

It is a further object of the invention to provide a punch rivet joining method which can also be used within a narrow joining zone, in particular in a region of a vehicle body flange with a flange length of 12 mm or less.

These objects are solved with a joining method with the features of claim1. Advantageous embodiments are set forth in the dependent claims.

A method for joining at least two joining parts, which overlap at least in a joining zone, by using a joining element which is supplied to the joining zone in a joining direction F and is configured to be free of undercuts at least in a shaft region viewed in a direction opposite the joining direction F, is refined according to the invention in that by means of the joining element at least one first joining part is punched through, which first interacts with the joining element and the joining element pushes a second joining part into the second joining part without pushing through the second joining part, wherein the shaft region of the joining element after the pushing into the second joining part is arranged so as to contact the first and second joining part so as to be impinged with a force without undercut and at least in joining direction F in the absence of a form fit.

In such a method according to the invention it is particularly advantageous that no undesired slugs are generated. In particular the slug generated during punching through the first joining part by means of the joining element is displaced into a region of the second joining part where it is embedded. The joining element remains substantially undeformed during the joining method according to the invention and, in contrast to the semi solid punch riveting method is not intended to expand. This allows keeping the joining zone particularly small, which enables using punches and dies or down holders with small diameters so that the joining method according to the invention can also be used in the region of small overlap zones, in particular in the region of small vehicle body flanges. It is surprising that during the joining method according to the invention the perforation of the first joining part and the introduction of the joining element into the second joining part along the form direction F results in radial forces acting on the shaft of the joining element so that sufficient removal forces can be achieved, which are for example required for a connection of vehicle body parts by means of riveting. In the method according to the invention a form fit is intentionally avoided and the pressing of the undercut-free joining element results in a pure force fit and/or friction fit between the joining parts and the joining element at the contacting surfaces, wherein the pullout forces generated thereby to pull the joining element out of the joining parts reaches a sufficient level.

In an embodiment of the method according to the invention, a slug that is generated when punching through the first joining part is embedded between a terminal free end of the joining element and the non-punched through second joining part. As a result the slug generated during punching through the first joining part remains in the joining connections between the joining parts to be joined. Thus no waste in the form of a slug that has to be removed and/or disposed from the joining station at which the method according to the invention is performed.

For the method according to the invention, joining elements can be used which have a shaft region and a joining part head, wherein the shaft region is configured undercut-free when viewed in the intended joining direction and is circular, oval, triangular multiangular or polygonal. For example the shaft region of the joining element may taper toward its free end.

The joining element used according to the invention is configured so that it remains unchanged or almost unchanged during the joining process regarding its three dimensional shape. The term “unchanged” or “almost unchanged” means hereby that when the joining element is urged in of course scores or grooves may form on the outer sides of the joining element. The three-dimensional shape of the joining element however remains substantially the same. An example of a situation in which the three-dimensional shape of the joining element does not remain the same is the hollow punch rivet method in which a defined deformation of the semi solid punch rivet is required and intended for forming an undercut in the joining connection. In contrast an appropriate joining element for the joining method according to the invention is a solid body which during the joining retains its three-dimensional shape.

It has been shown that a sufficient strength of the joining site can be achieved when the joining element in the region of the shaft outer surface has a roughness of Ra≤10 μm. Already in such a surface roughness, sufficient frictional forces are generated by the radial forces occurring during the joining on the shaft outer surfaces of the joining element, which radial forces lead to sufficiently high removal forces or removal strengths of the connection site.

Advantageously the joining part is inserted by using a punch and a die. It was shown that a diameter of the shaft region of the joining element smaller than 4 mm, in particular smaller than 3.5 mm, makes it possible to use dies with a diameter of 12 mm or less, in particular less than 10 mm, in the region of the joining zone so that such a die can be used in narrow regions of joining zones, for example in vehicle body flanges.

Advantageously the joining element is made of a material that is harder than the harder one of the two joining parts to be joined.

The method according to the invention is suited for joining two joining parts made of the same or different materials. For example the joining part1and the joining part2can be made of different plastic materials. On the other hand it is also possible that one of the two joining parts is made of plastic and the other one of the two joining parts is made of a metal. The method according to the invention can also be used for joining parts that are made of different metallic materials.

For further improving the joint connection produced with the method according to the invention it is also advantageous to make the joining element from a material that has a higher thermal expansion coefficient α than the greatest thermal expansion coefficient α of the joining parts to be joined.

It can also be advantageous to heat the joining zone, i.e., a partial region of the joining site into which the joining element is to be pushed, prior to performing the joining method according to the invention. Such heating can for example be accomplished with appropriate heating devices in a down holder or a die. When at least one of the joining parts is made of an inductively heatable metal, the joining part made of metal can for example be locally heated by inductive heating prior to pressing the joining part in. For the inductive heating, induction coils can be present in the down holder and/or the die.

As a result the joining parts have a higher temperature than the joining element prior to pressing the joining element, thereby enabling increasing the radial forces acting on the outer surface of the shaft of the joining element after insertion of the joining element. This results in a higher removal strength because the increased radial forces also result in an increased friction force between the joining parts and the joining element, which in turn results in the force fit/friction fit intended according to the invention while avoiding a form fit.

For further increasing the strength or for achieving a tight flange connection it can be advantageous to generate or at least prepare an adhesive connection between the joining parts, in particular in the region of the joining flanges, prior to performing the joining method according to the invention. The term preparing an adhesive connection for example means to apply an adhesive that is cured in a later step.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1shows an exemplary joining part arrangement, which is suitable for the joining method according to the invention. The joining part arrangement has a first joining part1, a second joining part2and optionally a third joining part3. The joining parts1,2,3for example form a first joining flange4and a second joining flange5. In the region of the joining flanges4and5, joining elements are inserted with the joining method according to the invention for example in joining directions F. the joining directions F are hereby preferably perpendicular to the joining flanges4,5. In the shown example according toFIG. 1, the second and the third joining parts2,3are for example made of a metal material, wherein the second and the third joining part2,3in the region of the joining flanges4,5are connected with each other by means of spot welding. With the method according to the invention the first joining part1is to be connected in the region of the joining flanges4,5with the already joined second and third joining part2,3. The first joining part1can hereby also be made of a steel material or a different metal material for example aluminum or plastic.

In the following the method according to the invention is exemplary explained by way ofFIG. 2, whereinFIG. 2shows a joining flange4,5into which a joining element6is inserted. The joining element6was pressed into the joining flange4,5by means of a punch7along the joining direction F. During the pressing in of the joining element6along the joining direction F the joining element6first comes into contact with the first joining part1. Further movement of the punch7causes the first joining part1to be punched through so that a slug8is generated. The slug8together with a terminal free end9of the joining element6is pressed onto the second joining part2in joining direction F, wherein—importantly—the second joining part2is not completely punched through but is only partially deformed in the joining direction F. As a result the slug8is embedded between the joining element6, in particular the free end9, and the non-punched through remainder of the second joining part2. When a third joining part3is involved in the formation of the joining flange4,5, the third joining part is locally deformed optionally in a recess region10of the die11by urging in the joining element6in the joining direction F. Surprisingly, urging in the joining element6in joining direction F results in radial forces R that are sufficient to interact with an outer surface12of a shaft13of the joining element6. Due to these radial forces friction forces R are generated that act in opposition to pulling the joining element6out of the joining parts1,2against the joining direction F. These friction forces are sufficient to achieve a sufficient removal force at which the joining parts would be released from each other. Advantageously a down holder14can be arranged which surrounds the die7, wherein of course after insertion of the joining element6into the joining flange4,5the die7as well as the down holder14and also the die11are removed again. In case of a three-layered construction of the joining flange4,5as shown inFIG. 2it may be advantageous to connect the joining parts2and3to each other already prior to the insertion of the joining element6. For example spot welding may be used for this purpose, which is schematically indicated by a spot welding lens15. The down holder14and the die11each have contact surfaces20or21which are in contact with the joining flanges4,5during insertion of the joining element6in the region of a joining zone22. Via these contact surfaces20,21heat may be introduced into the joining zone if desired. For this purpose it may be advantages to heat the down holder14and the die11in the region of the contact surfaces20,21by means of heating devices (not shown).

As a result the joining element6sits after performance of the joining method according to the invention undercut-free and at least viewed in joining direction F in the absence of a form fit with its shaft13in the joining parts1,2.

An appropriate joining element6for the method according to the invention is shown inFIGS. 3ato 3c. Such a joining element6has a shaft13and a head23. During the joining process the outside24the head23interacts with the punch7. Relative to the outside24the head of the joining element6tapers relative to the outside2,4preferably with an angle α>120°, and transitions into the shaft13via a radius region25. At the face of its free end9the shaft13has a flat indentation26which when viewed in longitudinal extent L of the joining element6extends away from the free end9by a small degree, preferably by less than ⅕, preferably by less than 1/10 of the longitudinal extent L. As an alternative also an even end face can be used.

The joining element6is in the example according to theFIGS. 3ato 3cconfigured as rotationally symmetric component wherein the indentation26is configured tapered. The indentation26extends in radial direction viewed from a center axis M of the joining element6up until almost to the outer surface12of the shaft13so that a circumferential centering edge27is formed. The centering edge27causes a decrease of the tilting tendency during placement of the joining element6onto the joining part1, before the joining element6is pressed in and supports the through punching of the first joining part1by means of the joining element6. The outer ≤surface12of the shaft13has preferably a surface roughness Raof 10 μm or less. With the exception of the indentation26the joining element6is configured as a solid part (seeFIG. 3c) so that a change of the three-dimensional shape of the joining element6during joining, in particular with the exception except of a minimal change of the outer surface12of the shaft13(i.e., scorings or grooves) does not occur.

Advantageously the joining element6has at least in the region of the shaft13a coating, for example a mixture of zinc, aluminum or nickel. When the joining element has a coating the coating has, at least in the region of the shaft13, a surface roughness Raof ≤10 μm.

A further function of the indention26is to at least partially receive the generated slug8and guiding the slug8during entering the second form part2.

In the exemplary embodiment3ato3cthe shaft13of the joining element6is cylindrical, in any case undercut-free. The material for the joining element6is preferably selected so that for the joining element6a material is used that is harder than the hardest of the two joining parts1,2to be joined. Advantageously the thermal expansion coefficient α of the joining element6is greater than the greatest thermal expansion coefficient α of the parts1,2, to be joined.

The term “undercut-free” in the context of the invention also includes three-dimensional shapes of the shaft13, which taper undercut-free toward the free end9. Thus the shaft13can for example be configured as a truncated cone, wherein center lines28of the outer surface12intersect at an acute angle β of a few degrees, in particular less than 3 to 5°, with the center line M (shown with dashed lines inFIG. 3c). The choice of the angle b is mostly based on a friction coefficient between the outer surface12and the joining parts1,2and is selected by a person with skill in the art so that sufficient removal forces can be achieved in the joint generated by the method according to the invention.

For flange connections that are common in the construction of vehicle bodies a diameter of the shaft13of the joining element6of less than 4 mm, in particular less than 3.5 mm, has proven advantageous. In the case of a shaft13that tapers toward the free end9, the greatest diameter of the shaft13is designated DS, which prevails directly at the transition to the radius region25.

A diameter DK of the head23of the joining element is preferably 1.5 to 2 times the diameter DS of the shaft13.

The joining element6is in particular made of high-strength tempered steel or of a material, which has a hardness of at least 520 HV, preferably 540 HV to 650 HV. The shaft13may in the region of the free end9be configured to have a stronger taper for example with a chamfer or a transition radius.

The above stated hardness values of the joining element6preferable apply to the joining of high-strength vehicle body panels. When for example a first and a second joining part1,2are connected with the method according to the invention which are made of a soft material, for example plastic or aluminum, of course joining elements6with a lower hardness may be used. In particular for joining materials other than steel it may be useful to use a non-coated joining element6, for example made of a so-called stainless steel.

For increasing the surface of the outside24of the shaft13that takes part in forming the force fit and/or friction fit, groove like indentations29and/or rib like elevations30can be provided. The groove like indentions29and/or rib like elevations30extend hereby parallel to the longitudinal extent, i.e., parallel to the center axis M, of the joining element6which is synonymous with a parallel direction relative to the joining direction F.

The method according to the invention advantageously achieves generating a simply producible and especially with regard to preventing entrance of humidity optimized joining connection. The reason herefore is that at least one outer joining part is not punched through so that this joining part, in the example ofFIG. 2the joining part2, is not perforated and thus remains protected against moisture. Further the method according to the invention can be performed cost-effectively because a pre punching of the joining part1in the region of the joining zone22is not required. On the other hand the slug generated during the punching through the joining part1by means of the joining element6is embedded in the joining part2so that disposal of the slug8and separation of materials of the slug form other slug8is not required.

A further particular advantages is that the method according to the invention enables using particularly small joining elements6for example with a shaft diameter of only maximally 4 mm which allows keeping an outer diameter of the joining auxiliary tools (die11and down holder14) small. This makes it possible that the method according to the invention can also be used for particularly short joining flanges4,5with flange lengths of less than 15 mm, in particular 12 mm or less.

The method according to the invention is particularly suited for connecting sheet metal joining parts1to be punched through which have a thickness of 0.5 mm to 1.5 mm, in particular 0.8 mm to 1.2 mm joining part thickness. Furthermore sheet metal joining parts2into which the slug8are embedded, can have a thickness of 1.5 mm to 6 mm. hereby the thicknesses of the joining parts1,2to be joined can be the same or different. Preferably the thicker one of the two joining parts (in the exemplary embodiment the joining part2) is the one, which serves for receiving the slug8and which is not punched through.