This application is the US national phase of PCT application PCT/DE99/02431 filed Aug. 4, 1999 with a claim to the priorities of German patent applications 19837130.6 and 19837131.4 both filed Aug. 17, 1998.
The invention relates to a method of deforming by high internal pressure at least two hollow bodies each having at least one opening, especially metal tubes or hollow profiles, the hollow body if necessary being produced during an internal high-pressure deformation process.
The hollow bodies in question are normally metal tubes, preferably of steel, aluminum, or the like. These metal tubes can be of circular or noncircular as well for example as of rectangular section. Similarly hollow bodies or blanks are used that are put together from several metal parts. In any case it is necessary that the hollow bodies be open at least at one end as is standard with internal high-pressure deformation processes. It is understood that such an internal high-pressure deformation tool can be made of several parts and can be opened so the finished shaped part can be taken out of it.
Internal high-pressure processes are well known (see German patent 4,320,237). Typically they are used when hollow bodies or metal tubes of complicated shape are needed. To this end a tube is deformed which subsequently is shaped by conventional means. This includes inter alia welding-together shaped metal tubes.
In the prior art the standard method is that the individual hollow body is shaped by the internal high-pressure deformation process according to a predetermined design to have a predetermined shape. Such a hollow body is then connected to another hollow body for example by welding. This second hollow body can also be an internal high-pressure deformed hollow body. Classic parts that are made in this manner are complex frame parts as used for example in motor-vehicle frames and/or as chassis parts.
The normally used connection system often creates distortion of the metal tubes being joined or welded together at the connection region. It is also not impossible that the tube walls are weakened in the critical connection region by the welding procedure which is particularly to be avoided in highly loaded tube joints. This applies also to the case when the above-mentioned tubes must be fastened together to be pressure tight as well as in the exemplary complicated torsion-resisting frame parts as for example needed in a motor-vehicle frame or chassis.
In any case the prior art requires a plurality of steps in order to be able to make such complicated tube or frame constructions. In addition the quality could be improved.
It is therefore the object of the invention to improve on a method of the above-described type such that it allows complicated frame constructions to be made simply and quickly and in addition makes possible a perfect mutual connection of the hollow bodies without distortion.
This object is attained by the invention according to a first alternative in a method of joining two or more hollow bodies each having at least one opening, in particular metal tubes or profiles, by an internal high-pressure deformation process wherein first
in a joint region between a first hollow body and a second hollow body to be connected thereto at least one intended fracture zone is formed in an outer wall of the first hollow body, and thereafter
the outer wall of the first hollow body is expanded outwardly in a closed cavity of a internal high-pressure shaping tool by means of a hydraulic medium at least in the joint region of the two bodies to form a flange, while
the second body to be joined is held in the cavity of the internal high-pressure shaping tool at a predetermined spacing from the flange, and thereafter
the hydraulic medium flows after exceeding the burst pressure for the intended fracture zone into the second hollow body to optionally shape it by internal high-pressure deformation and to join the first hollow body by its flange with the second hollow body.
A second embodiment of the invention is a method of joining two or more hollow bodies each having at least one opening, in particular metal tubes or profiles, by an internal high-pressure deformation process wherein first
in a joint region between a first hollow body and a second hollow body to be connected thereto at least one intended fracture zone is formed in an outer wall of the first hollow body, thereafter
the first hollow body and the second hollow body are engaged together at the joint region, thereafter
the outer wall of the first hollow body is expanded outwardly in a closed cavity of an internal high-pressure shaping tool by means of a hydraulic medium at least in the joint region of the two bodies, and subsequently
the hydraulic medium flows after exceeding the burst pressure for the intended fracture zone into the second hollow body to shape it by internal high-pressure deformation.
Joining according to the invention means solidly connecting and preferably material joining between the hollow bodies.
With the method of the first embodiment of the invention the hollow bodies can be so joined that the first hollow body simultaneously engages with its flange into the second hollow body and thus forms a joint in the joint region.
The first embodiment also provides that the outer wall of the first hollow body is expanded outward to form a flange so that the second hollow body to be joined thereto is held at a spacing from the flange. The expansion of the first hollow body takes place as is standard in a closed cavity of an internal high-pressure deforming tool by means of the described hydraulic pressure medium. According to a preferred embodiment the to-be-joined second hollow body is held in this cavity at a spacing from the flange region.
Normally the mechanical weakening of the outer wall or the formation of the intended fracture zone is achieved in that the intended fracture zone is defined by a blind bore, stamping, recessing, spark-eroded boring, metal-etch boring, or the like. Normally instead of one intended fracture zone a plurality of intended fracture zones are formed in the outer wall of the first hollow body aligned with the inner profile of the second hollow body. This zone or zones are formed before fitting the hollow body into the internal high-pressure deforming tool by means of an external boring and/or stamping device or inside the internal high-pressure deforming tool, preferably by an integrated drill, stamp, or the like.
Here material is removed at the intended fracture zone according to the stretch-to-break of the material of the first hollow body with reference to a predetermined burst pressure. This is of course determined in particular by the material of the first hollow body which is expanded during internal high-pressure deformation to form the flange. In any case there are generally two possibilities. On the one hand the intended fracture zone produces a weakening of the outer wall of the first hollow body and the pressure in the hydraulic medium increases until the zone breaks and the medium flows into the second hollow body. On the other hand the pressure medium can be used at the predetermined maximum pressure so that in this case the intended fracture zone or zones are such that on reaching this maximum pressure in any case the burst pressure is exceeded or the maximum pressure is equal to this burst pressure.
This can be done by appropriate removal of material and/or a weakening of the material in the intended fracture zone. As a result there is a metallurgical (grain) weakening. The invention is based on the recognition that, after exceeding the so-called flow or stretch limit (measured in N/mm2), the material used normally for the metal tubes has a shape change caused by the actual tension that can break it. In any case it is in particular true with stretching tensionxe2x80x94as is particularly obtained with pressure buildupxe2x80x94to create a split at the intended fracture zone. The pressure of the hydraulic medium necessary for this is determined by the characteristics of the material used and the thickness in the region of the intended fracture zone. As a result the thickness of the outer wall can be set in accordance with the material being used and the predetermined internal pressure of the hydraulic medium or its maximum pressure so that on reaching the desired internal pressure or maximum pressure in any case the intended fracture zone will rupture or at least the burst pressure will be reached and in most cases exceeded.
Thus hollow bodies of widely differing wall thickness can be joined in one and the same internal high-pressure deforming tool. Hence a flexible accommodation to wholly different hollow bodies and different workpieces is possible without problems.
According to the second embodiment both hollow bodies are joined at the joint region before the outer wall of the first hollow body is expanded. When this hollow-body connection is made, first an internal high-pressure deformation of the first hollow body is done, namely in the joint region of both bodies. The hydraulic pressure at this location causes, as a result of the intended fracture region there, the hydraulic medium to exceed the burst pressure for the intended fracture zone and flow into the second hollow body. Since this second hollow body is contained in the cavity of the internal high-pressure deforming tool an internal high-pressure deformation (of the second hollow body) is possible. As a result in a simple and fast manner the two hollow bodies that are to be joined are deformed in one internal high-pressure deforming operation so that even complicated tube or frame constructions can be made in a single step. Of course the invention is not limited to the internal high-pressure deformation of two hollow bodies, but any desired number or hollow bodies can first be connected together and then acted upon by a hydraulic medium in the joint region by burst pressure breaking through the fracture zone. Hence even complicated motor-vehicle frames or constructions can be made in a single method step.
Fitting together of the first and second hollow bodies at the fracture zone and the subsequent joining of the hollow bodies inside or outside the internal high-pressure deforming tool can be done with standard material-joining methods such as welding, soldering, shrinking, gluing, or the like. Thus this joining of the two hollow bodies normally takes place outside the internal high-pressure deforming tool. Of course it is also possible to join the hollow bodies inside the internal high-pressure deforming tool, the two hollow bodies for example being united by pressure or a groove/ridge interfit which is subsequently (additionally) glued, riveted, or the like. This necessitates however complicated means inside the internal high-pressure deforming tool which thus must be for example made separable.
On joining the hollow bodies inside the internal high-pressure deforming tool and the provided flange it is possible to unite this formation and the confronting spaced second hollow body with a groove/ridge connection. In this case also complicated means are necessary inside the internal high-pressure deforming tool which must be made separable in the region of the produced flange. Thus it is conceivable that the flange is first formed with an outside diameter which is smaller than an inside diameter of the second body to be connected to it. After this flange is formed, the internal high-pressure deforming tool is opened at this location and the second hollow body is fitted over the flange. Of course up to this time the burst pressure has not been reached. When now the pressure is raised to the burst pressure, the flange engages from inside on the second hollow body like a shrink fit so that further welding can be eliminated.
Finally one normally loads the basic frame of interconnected hollow bodies as a single assembly (after forming the intended fracture zone or zones by grooving) into the internal high-pressure deforming tool and then starts the internal high-pressure deformation, starting with the first hollow body.
Independently of this the invention makes it possible to deform two or more coaxially interfitted hollow bodies in a single internal high-pressure deforming tool. In the simplest case this is done with a double tube where the inner tube (first hollow body) is expanded to form a flange and subsequently this flange is joined with the inner surface of the outer tube (second hollow body). Of course this can be one with two or more hollow bodies which do not have to be aligned coaxially. It is also possible within the scope of the invention to form a flange on the outer tube (second hollow body) with both flanges being connected together. In any case this procedure makes it possible to deform the outer tube after fracturing of the intended fracture zone by internal high pressure. Generally it is thus clear that internal high-pressure deformed frame assemblies can be made with interfitting hollow bodies.
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