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Roll forming of a high strength aluminum tube
Roll forming of a high strength aluminum tube
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Roll forming of a high strength aluminum tube

    Roll forming of a high strength aluminum tube

    The presented paper provides a modelling strategy for roll forming of a high strength aluminum alloy tube. Roll forming allows the cost-effective production of large

quantities of long profiles. Forming of high strength aluminum brings challenges like high springback and poor formability

due to the low Young’s modulus, low ductility and high yield strength. Forming processes with high strength aluminum, such

as the AA7075 alloy, therefore require a detailed process design. Three different forming strategies, one double radius

strategy and two W-forming strategies are discussed in the paper. The paper addresses the question whether common roll

forming strategies are appropriate for the challenge of roll forming of a high strength aluminum micro channel tube. For this purpose, different

forming strategies are investigated numerically regarding buckling, longitudinal strain distribution and final geometry.

While geometry is quite the same for all strategies, buckling and strain distribution differ with every strategy. The result

of the numerical investigation is an open tube that can be welded into a closed tube in a subsequent step. Finally, roll

forming experiments are conducted and compared with the numerical results.Current research in production technology focuses

primarily on increasing resource efficiency and thus follows the approach of fundamental sustainability of processes and

products. High strength aluminum alloys (e.g. AA7075) are commonly used in aerospace applications in spite of their high cost

of about 5 €/kg and poor formability [1]. Due to ambitious legal requirements, such as the CO2 target in automotive

engineering, new lightweight construction concepts are still needed [2]. An excellent basis is offered by the production of

high strength AA7075 thin walled tubes as semi-finished products by roll forming. These can be further processed in

subsequent customized processes such as welding, stamping, cutting or rotary swaging.


    According to DIN 8586, roll forming is a bending technology with rotating tool motion to produce open and closed profiles

[3]. 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 [4]. The incremental nature of the roll forming process also allows forming of high strength

materials, such as ultra high strength steel (UHSS) [5].


    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 [6]. Abeyrathna [5], Park [7] and Bui [8] 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 [9]. Parameters with a large influence on buckling are the stiffness of the sheet and the yield strength of the

material. According to Halmos [10], 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

[6].Groche et al. [11], Park et al. [7], Zou et al. [12] and Lee et al. [13] 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 [14].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 [10, 18].


    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 [18]. 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 [18]. Another approach is described by Jiang et al. [19] 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 [10]. 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.


    FE-Simulation of the roll forming process

    The roll forming tools are designed by numerical simulation of the process. The target geometry is a tube with an outer

diameter of d=54.98mm (ro=27,49mm/ri=25,99mm) and a wall thickness of s0=1.5mm. An AA7075-T6 aluminum alloy is used for the

roll forming process. Table 2 shows the mechanical properties of the alloy.The first forming strategy suggested automatically

by UBECO Profil after defining the target geometry is a double radius-forming strategy and has 27 passes in total. Based on

tube forming sequences in literature [15, 16], the number of passes is reduced to 14 passes by skipping every second pass, in

order to increase process efficiency. After the reduction to 14 passes, the edge strain is still below the critical limit in

every stage of the process according to the PSA. The approach for the first forming strategy is to form the tube in uniform

increments and to keep the longitudinal strain low in the band edge. The further approach is to calculate the stresses of the

formed tube to arrive at the number of passes required. Forming strategy 2R is the first strategy numerically investigated by

the FE-software Marc Mentat.In this paper, roll forming of a high strength extruded aluminum tube is investigated. Due to the difficult determination of the

design parameters, roll forming of high strength aluminum is a challenge. Conventional roll forming strategies quickly reach

their limits when forming aluminum or high strength steels. To form a tube out of high-strength aluminum alloys such as

AA7075, a W-forming strategy is recommended. Another positive influence is the application of a down-hill strategy. The

investigations have shown that an efficient roll forming production line for high strength aluminum tubes can be set up even

with a small number of forming passes. The W-forming strategies showed a good behavior with regard to buckling, compared to

the double radius forming strategy. Forming strategy W2 combines the advantages of few passes with a good final part geometry

thanks to detailed process design. The numerical investigation and the following experiments demonstrated the feasibility of

roll forming a high-strength aluminum tube. It is shown that conventional design methods are also valid for high-strength

materials.A further result of the numerical investigation is that the design of the tools should not be based on longitudinal

strain in the band edge alone. For a first estimation, the elongation of the band edge is a valid factor, but for an exact

process design a numerical simulation should always be performed. In addition, BLS is material dependent, which makes an

analytical calculation even more difficult.


    Regarding the springback angle, the experimental investigations show little deviations from the FE-model. The reasons for

this are the simplified material model, which does not consider combined hardening effects, the influence of the smaller

modulus of elasticity after plastic deformation and compliance of the forming stand. Nevertheless, the simplified FE-model

provides sufficiently accurate results regarding buckling and geometry of the tube.

    Axial crash of thin-walled circular

seamless aluminum tube
is investigated in this study. These kinds of tubes usually are used in automobile and train

structures to absorb the impact energy. An explicit finite element method (FEM) is used to model and analyse the behaviour.

Formulation of the energy absorption and the mean crash force in the range of variables is presented using design of

experiments (DOE) and response surface method (RSM). Comparison with experimental tests has been accomplished in some results

for validation. Also, comparison with the analytical aspect of this problem has been done. Mean crash force has been

considered as a constraint as its value is directly related to the crash severity and occupant injury. The results show that

the triggering causes a decrease in the maximum force level during crash.

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