Attività

p>
According to DIN 8586, roll forming is a bending technology with rotating tool motion to produce open and closed profiles . Several pairs of forming rolls are aligned one behind the other for the forming process. The friction between the rotating forming rolls and the sheet metal causes a forward movement of the sheet. Simultaneously the sheet is formed in and between the stations. For the production of large quantities, roll forming is a cost-effective manufacturing process, compared to tube extrusion or tube drawing. Roll forming can also be competitive for smaller quantities, if the number of forming passes is small enough . The incremental nature of the roll forming process also allows forming of high strength materials, such as ultra high strength steel (UHSS) .

During roll forming there is a limit for the amount of deformation regarding buckling limit strain (BLS), which can be reached in one forming station . Abeyrathna , Park and Bui showed that longitudinal strain has a major impact on product defects, such as bow or buckling. The maximum longitudinal strain occurs in the area of the band edge. Plastic elongation in the roll gap between the forming rolls followed by compression when the sheet leaves the forming rolls leads to buckling. Figure 1 illustrates the elongation, followed by compression when forming a tube. To prevent buckling, the maximum longitudinal strain must be low. Once buckling takes place, welding of the formed tube becomes very difficult or even impossible . Parameters with a large influence on buckling are the stiffness of the sheet and the yield strength of the material. According to Halmos , elongation of the band edge depends on the flange height and inter-station distance ld. High bending angles of a single forming station Θp and a small inter-station distance ld lead to large elongation of the band edge and thus to buckling. For circular sections (e.g. tube), the BLS is 5–10 times higher than the BLS for a U-profile .Groche et al. , Park et al. , Zou et al. and Lee et al. showed that roll forming of high strength materials and especially of high strength aluminum drawn tube brings challenges compared to commonly roll formed steel grades. High strength leads to high springback and thus to less dimensional accuracy in the processed part. Parameters, which have an influence on springback are shown in Table 1. Difficulties regarding aluminum include early fracture due to low ductility, higher springback and redundant deformation. This requires a well-designed forming strategy in order to get the lowest possible springback and buckling in the roll forming process and the best quality of the processed part. In contrast, aluminum shows a good-natured behavior with regard to buckling due to a higher value of BLS compared to steel .The single radius-forming strategy has the advantage to form tubes with different sheet thickness on the same tool. A flower pattern with constant bending radius over the entire cross-section of the sheet is characteristic for the single radius-forming. For high-strength materials, the single radius-forming strategy is not applicable due to high springback caused by the high elastic bending content .

The double radius- and W-forming strategies are appropriate for high strength steels. For both strategies, two radii are combined in each pass, whereby the radius in the edge area is equal to the end radius already in the first pass of the process . In contrast to double radius forming, a negative bending is initially introduced in the middle section in the W-forming process. The main advantage of this strategy is that the final radius can be formed into the band edge area at the first pass of the process . Another approach is described by Jiang et al. with a cage roll forming mill for the production of electric resistance welded pipes.

The height displacement of the profile is called “up-hill” or “down-hill”. During the down-hill strategy, the profile is lowered step by step in each pass. The use of a down-hill forming strategy can reduce plastic elongation in the band edge and thus the number of forming stations . Based on the fundamental differences in roll forming between aluminum and steel, this publication addresses the question if one of the strategies suits for forming a tube of the high-strength aluminum alloy AA7075.