Such longitudinal structural members, particularly the members made of Al or Al-alloys, are presently provided by extruding closed shapes having the required cross-sectional configuration and wall thickness in one operation.

However, provision of structural members like bumper beams for vehicles by simply extruding a double chamber closed shape is not offering a low cost, flexible manufacturing method ensuring simultaneously the lowest possible weight of the members. This is due to a number of disadvantages inherent to the extrusion process such as low extruding speed, increased die wear, limited choice of alloys and possibility for provision of thinner walls.

Furthermore, cross-sectional variations along the longitudinal extension of the members, often required due to limited available space or specific performance requests to structural members, are achieved by subsequent re-shaping/deformation of the cross-sectional configuration on pre-determined locations.

It is therefore an object of the present invention to provide a new method of manufacturing longitudinal structural members of closed cross-sectional configuration offering more freedom with regard to the functional configuration of the members and low material consumption.

A further object of the present invention is to provide a cheap manufacturing method and low weight structural members.

These and other objects are achieved in accordance with the present invention by provision of a novel manufacturing method and the resulting members as it appears from the attached patent claims 1 and 5.

Specific features and advantages of the present invention will be apparent from the following detailed descriptions of preferred embodiments of the structural beam with reference to the accompanying drawings, Figs. 1-5, where Fig. 1 is a schematical perspective view of an extruded open member, Fig. 2 illustrates schematically in a perspective view steps of reshaping and removing of excess material from the open member, Fig. 3 is a schematical perspective view of the reshaped member prior to the folding step of interim side walls, Figs. 4a, b show the formed closed shape member with abutted inwardly extending flanges ready for the final joining operation, and Figs. 5a, b illustrate schematically in a perspective and a vertical cross-sectional view, respectively, another embodiment of the member having a constant height along its longitudinal extension.

Referring to Figs. 1-4 illustrating by way of example the individual steps of manufacturing a bumper beam according to the present invention, only a half of the beam divided by a centre line OY-OY is shown in order to facilitate focusing on the preferred embodiment of the beam exhibiting variation in height and provision of a channel recession along the beam to protect the welded seam as a preferred type of joint.

Figure 1 shows in a schematical perspective view an open extruded member 2 comprising a base portion 21 provided with a rib 22 protruding longitudinally along the member.

Inherent advantages of extruding an open member compared to a closed shape configurated member is a higher extruding speed, less wear of the extruding tool (die), larger choice of applicable alloys and possibility of weight-optimizing of the member by provision of thinner walls when and where appropriate.

Figure 2 illustrates schematically provision of lateral flanges 23,24 by means of any suitable method like e. g. pressing, rolling etc., followed optionally by gradual reduction of the flanges'lateral extension in the direction towards the end of the member by removal/cutting of material as depicted by the hatched fields.

Figure 3 illustrates an interim stage of the reshaping process from open to a closed shape configuration of the member 2 comprising provision of interim side walls 25,26 followed by a further reshaping step along the dotted lines, thereby folding the interim side walls 25,26 into a top portion 27 of a closed shape member by abutting the lateral flanges 23,24 on the top of the longitudinally extending rib 22 as shown in Figure 4a, followed by a joining/welding operation.

Figure 4b shows the formed closed shape member 3 in a preferred embodiment suitable for use as a bumper in vehicles exhibiting a channel-like recession 35 accommodating/- protecting a welding seam (joint) 36 joining the abutted flanges 23,24 and the rib 22, thereby providing two separate chambers 31,32.

Advantageously, thanks to the optional provision of flanges 23,24 exhibiting varying width, the channel recession 35 and thus the whole height H of the beam 3 is decreasing from the OY central line towards the end (s) as illustrated in a perspective view in Fig. 4a.

This flexibility with regard to variation of cross-sectional area of the bumper beam is an important advantage resulting from the novel manufacturing method according to the present invention.

Customarily this effect is achieved by deforming (folding) of side walls of readily closed shape extruded members, thus increasing both the weight of the beam, the material con- sumption and thus the cost of the beams.

Although the present invention has been described and illustrated with respect to the preferred features and manner of manufacturing and preferred embodiments, it is to be understood that various changes and modifications may be made to the specifically described and illustrated arrangements without departing from the scope of the present invention.

Thus Figure 5 illustrates schematically in a perspective view and a cross-sectional view, respectively, a simplified embodiment of the structural member 3 exhibiting a constant height along its longitudinal extrusion. The interim side walls 25,26 from Figure 3 are in this case without provision of any lateral flanges directly folded into a top portion of the thereby provided closed shape member 3 being abutted and joined (welded) along the rib 22 providing two separate chambers 31,32.

Apart from the disclosed joining based on welding of the abutted walls of the member (preferentially friction stir welding), any other conventional joining techniques may be applied depending on the actual demand/performance to the structural member.

Consequently, structural members for different purposes having variation in the cross-sectional configuration, or constant cross-section with option for cutting of members having desired length, can be manufactured according to the present invention.