Patent Application
20050016108
The present invention relates to a lightweight construction element having an inner framework structure of light metal. The invention also relates to a method for producing the lightweight construction element.
The use of light metals is one of the greatest challenges in the construction of locomotion means, especially automobiles, since minimization of weight is one of the most effective methods of reducing fuel consumption.
Against the background of a cost-to-benefit comparison of different light metals, it is clear that the manufacturing costs increase drastically with increasing weight savings by the use of such materials. Thus lightweight construction can only be achieved economically if it becomes possible to compensate for the associated higher material costs by more favorable production processes and in particular by more sparing use of materials.
Lightweight construction elements having an inner framework structure of light metal material have proved advantageous for the purpose of the best possible ratio between weight and load-bearing ability or strength. Such lightweight construction elements can be produced economically by extrusion.
As regards the extrusion process, however, the relative ratio between the size of the section to be pressed and the size of the extrusion press and especially of the chamber diameter is critical for the material flow of a given material. For example, from the Aluminum Textbook published by Aluminium-Verlag of Düsseldorf, 15th Edition 1996, Vol. 2, p. 103, it is known that, in the extrusion of hollow sections made of pure aluminum or of AlMgSi alloys and having uniform wall thickness, a section having a section circle diameter of 450 mm and obtained by extrusion in an 80-MN extrusion press can have a minimum wall thickness of 5 mm, whereas a section having a section circle diameter of 50 mm and obtained by extrusion in a 10-MN extrusion press can have a minimum wall thickness of 1 mm. This shows that large lightweight construction elements can be produced only with relatively thick wall thicknesses by extrusion, meaning higher production costs and, because of the greater component weight, also a negative influence on the fuel consumption of a vehicle containing this component.
Against this background, the object of the present invention is to provide a lightweight construction element having an inner framework structure of light metal, wherein the wall thickness is smaller than in conventional lightweight construction elements produced by extrusion.
This object is achieved according to the invention by the fact that there is taught a lightweight construction element having an inner framework structure of light metal, comprising a plurality of extruded hollow sections joined to one another, the lightweight construction element having a circumscribed circle with a diameter of at least 300 mm and a wall thickness of at most 0.5% of this value. In a particularly preferred embodiment of the invention, the wall thickness is at most 0.35% of the diameter of the circumscribed circle of the lightweight construction element.
According to the invention, therefore, by extruding individual hollow sections and joining the hollow sections in a planar configuration, there can be obtained a lightweight construction element of practically any desired size with a wall thickness, which construction element, because of the technical limitations of the extrusion process, cannot be manufactured in monolithic form or can only be manufactured with much greater linear density or greater wall thickness once its size exceeds a certain value (generally a circumscribed circle with a diameter of larger than 300 mm).
As the Applicant has surprisingly found, the extruded hollow sections of the inventive lightweight construction element can be joined by friction stir welding. Heretofore those skilled in the art have assumed that the friction stir welding technique requires that the workpieces to be welded each have a wall thickness of at least 1.6 mm (most recently stated in a contribution by the Alusuisse Co. at the 2nd Technical Conference on “Advances in Lightweight Automotive Engineering”, Stuttgart, 6 to 7 Nov. 2001). Friction stir welding (FSW) had already been developed almost ten years ago (see European Patent B 0615480). Nevertheless, it is not yet one of the standard joining techniques in the automobile industry, where only resistance welding, inert-gas welding and laser (hybrid) welding have been used heretofore as thermal joining techniques.
It is particularly advantageous in friction stir welding—in contrast to conventional welding techniques—that welding of the two workpieces takes place below the liquidus temperature of the materials to be welded, and so no appreciable risk of development of pores and hot cracks exists. Moreover, even alloys that are difficult or impossible to melt as well as aluminum/magnesium composite elements can be welded with friction stir welding, an accomplishment that is difficult or even impossible with the conventional welding techniques. Thus entirely new possibilities for the production of composite components are created by the friction stir welding technique.
As an alternative to friction stir welding, the individual hollow sections can be joined by adhesive bonding, which has the advantage in particular that the hollow sections to be joined are subjected to only slight thermal stress, whereby development of pores and hot cracks is avoided.
To ensure that the hollow sections composing the lightweight construction element can be joined, they can be provided with appropriate elements in the form of ridges, hooks or grooves, so that the elements in the form of ridges, hooks or grooves of adjacent hollow sections have corresponding shape and can overlap in a planar configuration of the hollow sections, in order to be able, together with the adjacent zones of the sections, to withstand the forces occurring during friction stir welding.
As an alternative to this, it may also be possible to avoid the use of hollow-section elements in the form of ridges, hooks or grooves, in which case the hollow sections are joined only along an abutting edge. To ensure that no deformations of the hollow sections are caused during friction stir welding, the forces occurring at this time must be absorbed by an appropriate fixture, such as an inner mandrel. Avoiding the use of elements in the form of ridges, hooks or grooves can be regarded as advantageous, since this contributes to economy of materials and thus to reduction of costs and weights.
The individual hollow sections may be made of aluminum, magnesium, titanium or alloys thereof. By joining hollow sections of dissimilar materials, it is advantageously possible to produce composite members.
In a particularly advantageous embodiment of the invention, a lightweight construction element comprises a plurality of mutually symmetric, individual hollow sections. Hereby the costs of producing a lightweight construction element can be greatly reduced by a smaller number of tools and simplified logistics.
For production of the inventive lightweight construction element, hollow sections with a wall thickness of at most 0.5% of the diameter of the circumscribed circle of the lightweight construction element manufactured therefrom are produced by extrusion. The extruded hollow sections are then joined in a planar configuration to form a lightweight construction element, in such a way that the lightweight construction element has a circumscribed circle having a diameter of at least 300 mm. Friction stir welding and adhesive bonding are preferably used for joining the hollow sections.
The inventive lightweight construction element produced in this way is preferably used as part of a load-bearing structure, for example in a motor vehicle.
The invention will now be explained in more detail on the basis of practical examples with reference to the attached drawings, wherein
Referring first to
The lightweight construction element illustrated as an example in
By comparison with the manufacture of a corresponding lightweight construction element from two equally large individual hollow sections (circumscribed circle with a diameter of about 250 mm), in which case the wall thickness achievable by the extrusion technique was 2 mm and the individual hollow sections were welded together by laser welding, the weight savings achieved in the inventive lightweight construction element was about 15%.
In a particularly advantageous manner, the endurance limit of the lightweight construction element produced can be greatly increased in the case of hollow sections joined by friction stir welding. For comparison of the endurance limit of lightweight construction elements produced by laser welding and by friction stir welding, appropriately manufactured lightweight construction elements were subjected to a sinusoidally increasing and decreasing tensile stress at various load levels. The result is illustrated in
As is evident from
It is also evident that a fracture of the laser-welded lightweight construction element is generally located in the weld, starting from the upper side of the weld and from hydrogen pores, whereas fractures of the friction-stir-welded lightweight construction element are located in the base metal and start from notches in the section, or in other words extrusion marks or surface irregularities.
In Case II of
In Case III of
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 24 198.8 | May 2002 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP03/01936 | 2/26/2003 | WO |
