23-08-2021, 06:53 AM
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 [url=http://www.aluminium-tube-
pipe.com/aluminum-alloy-tube/]aluminum alloy tube[/url]. 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 [url=http://www.aluminium-tube-
pipe.com/aluminum-alloy-tube/aluminum-micro-channel-tube/]aluminum micro channel tube[/url]. 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 [url=http://www.aluminium-tube-pipe.com/aluminum-
alloy-tube/extruded-aluminum-tube/]extruded aluminum tube[/url] 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.
Roll forming of a high strength aluminum tube
The presented paper provides a modelling strategy for roll forming of a high strength [url=http://www.aluminium-tube-
pipe.com/aluminum-alloy-tube/]aluminum alloy tube[/url]. 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 [url=http://www.aluminium-tube-
pipe.com/aluminum-alloy-tube/aluminum-micro-channel-tube/]aluminum micro channel tube[/url]. 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 [url=http://www.aluminium-tube-pipe.com/aluminum-
alloy-tube/extruded-aluminum-tube/]extruded aluminum tube[/url] 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.