Fixing element for a friction welding joint

The invention relates to a fixing element having a concentric end face forming a frictional surface for friction welding joining to a flat part by rotational force and axial force applied by the fixing element to the part, and having multiple radially oriented ridges on the end face. The ridges are limited in the direction of rotation by a front groove introduced into the friction surface, the ridges rising only slightly over the friction surface in the axial direction and being provided with a scraping surface that pushes the material scraped during the friction welding process into the front groove.

The invention relates to a fixing element having a concentric end face forming a frictional surface for friction welding joining to a flat part by rotational force and axial force applied by the fixing element to the part, and having multiple radially oriented ridges on the end face.

In a fixing element designed as a light-metal bolt according to DE 201 09 359 U1, the concentric end face of the bolt is already provided with radially oriented ridges for the purpose of friction welding, the ridges acting as star-shaped cutting edges and, due to this design, being able to mechanically cut through an oxide layer formed on a metal sheet so that a secure friction welding joint may be produced between a light-metal bolt and an aluminum sheet. According to this prior art, the ridges, which are designed as milling cutter-like cutting edges, transport the oxide layer present on the sheet, together with material that was plasticized by the friction welding process, outward and away from the joining zone in an annular ridge. The oxide, which represents a contaminant, is therefore mixed with plasticized bolt material and transported outward to the annular ridge, where a bolt material permeated with contaminants lends stability to the bolt welding joint in the form of the annular ridge, while the actual joining zone no longer has any bolt material at its disposal for the bolt welding joint in the area of the bolt end face.

The object of the invention is to design the fixing element described above, which is provided with multiple radially oriented ridges on its end face, in such a way that, on the one hand, contaminants are safely eliminated during friction welding and, on the other hand, however, the plasticized material of the fixing element and of the part fixed thereto is optimally utilized for producing the friction welding joint. According to the invention, this object is achieved by the fact that the ridges are limited by a front groove introduced into the friction surface, the ridges rising only slightly above the friction surface in the axial direction and being provided with a scraping surface that pushes the scraped material into the front groove during the friction welding process.

Forming a front groove in front of each ridge on the end face of the fixing element in such a manner that the front groove is introduced into the friction surface and the ridges rise only slightly above the friction surface in the axial direction causes the scraping surface to first rub away contaminants, in particular oxide, which is pushed by the ridges into the front groove, during friction welding, the ridges, however, being quickly worn away during welding of the bolt, due to their slightly raised height over the friction surface, so that the friction surface of the fixing element very quickly comes into direct contact with the surface of the part, very quickly resulting in frictional action and thus plasticization of the material of both the fixing element and the part, practically all of this material then being also available for forming the friction welding joint over the entire end surface of the fixing element. This results in a particularly secure joint between the fixing element and the part, which may also be particularly quickly produced in an automatic manufacturing process, due to the given dimensions. In designing the friction surface, it is also possible for each of the ridges to be limited on both sides in the direction of rotation by a front groove and a back groove, the residual material of the relevant ridge stripped away by the friction surface being pushed into the back groove by plastic deformation. The provision of a back groove in addition to the front groove opens up the possibility of the back groove accommodating the residual material of the relevant ridge scraped away during friction welding, so that this residual material is not pushed into the friction welding joint, where it could cause problems. The design of the fixing element having a front groove and a back groove on the friction surface also makes it possible to rotate the fixing element in any direction for the purpose of friction welding.

The design of the ridges may be suitably selected as a function of the material of the flat part. If the flat part is a steel part, providing the scraping surface with a sharp edge on either one side or both sides is favorable for removing oxide. The sharp edge on both sides is advantageous, in particular, due to the production engineering of the relevant fixing element. If, however, a relatively soft material, such as aluminum, is used for the flat part, it is sufficient to have a rounded scraping surface that merges with the ridge.

In order to accommodate the abraded material produced by the friction welding process, which also travels outward in the radial direction, a concentrically circumferential annular grove is suitably provided in the outer region of the end face. This annular groove then easily accommodates the material pushed radially in front of the ridges. The annular groove is sealed radially to the outside after the fixing element has been fully welded on, so that the material pushed into the annular groove is securely enclosed and is not able to result in any type of contamination.

The fixing element itself may be designed, in particular, as a bolt; however, it is also possible to use a nut as a fixing element for the purposes of this invention.

The friction surface of the fixing element may be shaped as a flat cone having a cone angle α=160° to 178°. If a convex cone is used, this has an advantage in that the friction surface initially comes into contact with the part by the tip of its cone during the friction welding process and, by applying a particularly high pressure, the cone flows outward when the two parts to be joined melt and thereby gradually covers the entire friction surface. However, it is also possible to design a concave friction surface as a flat cone. In this case, the outer edge of the friction surface comes into contact first when the fixing element is pressed against the part, the friction surface then entrapping the plasticized material in its interior during the friction welding process and does not allow it to escape to the outside.

FIG. 1ashows fixing element1designed as a bolt, including shank2and bolt head3, which in this case is designed as a hexagon for clamping into a driving element. On the side facing away from shank2, head3has an end face that is designed as friction surface4. Three ridges5,6and7extending outward in the radial direction rise from the friction surface, a front groove8and9(10is not visible) being provided in front of each ridge with regard to a direction of rotation according to the sketched rotating arrow, i.e., in the clockwise direction, these grooves limiting ridges5,6and7in the direction of rotation. Ridges5,6and7rise only slightly above friction surface4and each have a scraping surface (see ridge5inFIG. 1a) on their side facing away from friction surface4. Head3includes a truncated cone3T having a small diameter end3Tsd which forms an outer perimeter of the friction surface4. The truncated cone3T has a surface which slopes outwardly and away from the friction surface4. Head has flat sides which extend in an axial direction away from a large diameter end3Tld of the truncated cone3T.

The scraping surface being shown more clearly inFIG. 1b, which is discussed in greater detail below. The raised height of ridges5,6and7or that of scraping surfaces11limiting them in the axial direction is slightly less than 1.0 mm. Ridges5,6and7are advantageously hardened.

Scraping surfaces11may lie together on a plane; however, they may also be provided in the form of coaxial cones (which is discussed in greater detailed below in connection withFIGS. 4 and 5), as shown below in connection withFIGS. 4,5and6. Fixing element1is pressed against a flat part for the purpose of friction welding and placed in rotary motion, which first causes the surface of the part to be scraped due to the contact between scraping surfaces11and the flat part, thereby cleaning this surface for the later actual friction welding process. The material scraped off the surface, e.g., contaminants, oxides or the like, is pushed by ridges5,6and7in the direction of rotation and thus reaches adjacent front groove8,9or10, where this removed material does not interfere with the following friction welding process. By correspondingly rotating and pressing structural member1against a part, the surfaces rubbing together are heated in the known manner by friction during friction welding, the materials of the parts being pressed together ultimately softening and thereby completing the friction welding process. The material originating in ridges5,6and7is also largely pushed into front groove8,9and10, so that it is unable to interfere with the friction welding joint.

The fixing element shown inFIG. 1ahas in its axial center concentric bore28, into which ridges5,6and7and front groove8,9and10run out.

FIG. 1bshows a perspective view of an extract fromFIG. 1a, the cross-section of ridge5being clearly shown, which is limited axially to the outside by scraping surface11and has front groove8in front of ridge5in the direction of rotation, the front groove being used to accommodate abraded material and residual material of ridge5.

FIG. 2shows a modification of the design according toFIG. 1, a fixing element1again being shown which is designed as a bolt having a similar design to that inFIG. 1a. However, fixing element1in this case has a front groove15,16and17as well as a back groove18,19and20in addition to three ridges12,13and14in friction surface4, which increases the material accommodation capacity over that of only ridges12,13and14and, moreover, also makes it possible to mount fixing element1in the clockwise direction as well as the counterclockwise direction. Reference is also hereby made to the explanations regardingFIGS. 1aand1b.

FIG. 3again shows a fixing element1designed as a bolt, which largely corresponds to the one according toFIG. 1a, but has concentrically circumferential annular groove22in the region of its friction surface21. This annular groove is used to accommodate any material that may be pushed radially by ridges23,24and25.

FIG. 4shows a cross-sectional view along line IV-IV fromFIG. 1a, which demonstrates that in this case both friction surface26and scraping surface27are shaped like a coaxial flat cone, namely a cone having an angle of α=175°, it being noted that this angle may lie in a range between 160° and 178°. Both friction surface26and scraping surface27are inclined by the same angle. As a result, the middle region of the end face of the fixing element comes into contact with a part when fixing element1is pressed against the part, which applies to both the scraping surface and subsequently also to the friction surface. When using this specific embodiment, an abrasion and friction welding action occurs in the center of head3, after which the scraping and friction welding actions extend radially to the outside and over the entire end surface of head3.

FIG. 5shows a variant of the illustration according toFIG. 4, in which scraping surface29and friction surface30are positioned at right angles to the longitudinal axis of part1. In this specific embodiment, scraping surfaces29therefore press against a part over their entire length when brought into contact with the part, after which friction surface30follows, which also comes into contact with a part over its entire surface.

FIG. 6shows a further variant of the illustration according toFIG. 4, in which scraping surface31is flat all over, while friction surface32is shaped like a flat cone, as illustrated inFIG. 3.

With regard to the fixing elements according toFIGS. 4,5and6, it should be further noted that the bolts illustrated here each are provided with a thread on their shanks2, this being intended to merely indicate that, after friction welding, the relevant bolts may naturally be provided with a screw connection for the purpose of screwing on any additional part or for other purposes.

FIG. 7shows a perspective view of a fixing element33, which is designed as a hexagon nut and has friction surface34on its one end face. Like in the design according toFIG. 1a, three ridges35,36and37are provided in friction surface34, these ridges together with the front groove running in front of them accommodating scraped-away or otherwise removed material in the same manner as those according toFIG. 1a.

FIG. 8shows fixing element33in a cross-sectional view along line VIII-VIII, which shows that in this case friction surface34is shaped like a concave cone, which causes the outer edge of friction surface34to first come into contact with the surface of a part when this fixing element is set in place and thereby causes the friction welding process to progress from the outside to the inside.