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DOI: 10.1002/srin.201100270 steel research int. 83 (2012) No. 1

Tailored Profiles Made of Tailor Rolled Str
ips by Roll Forming – Part 2 of 2
Peter Groche*, and Michael Mirtsch

Institute for Production Engineering and Forming Machines, Mechanical Engineering Department, Darmstadt University of Technology,

Darmstadt/Germany

*Corresponding author; e-mail: [email protected]

The main scope of the presented work is to demonstrate the potential of load optimized tubes with a varying thickness distribution in

circumferential direction produced by roll forming. As initial material a so called Tailor Rolled Strip (TRS) sheetmetal coil produced by Strip Profile

Rolling (SPR) method was used instead of plain sheet. The TRS sheet metal is manufactured in a continuously working process by rolling one or

more groves in transverse direction into the sheet metal coil. In this paper, the secondary forming of the TRS sheet metal to TRS tubes is

investigated by means of FE-simulations and roll forming experiments. To simulate the manufacturing process of the TRS tube by FEM, an

integrated consideration of the process is necessary because of the large local strain hardening in the groves of the initial SPR sheet metal. In

experimental roll forming operations welded tubes could be manufactured successfully. The geometrical and material properties of these tubes

are analyzed. The reprocessing of TRS tubes by hydroforming is investigated bymeans of tube bursting tests. It has been found that an additional

annealing process is necessary to achieve deformations in the grooved area during the hydroforming process.

Keywords: lightweight profiles, roll forming, strip profile rolling

Submitted on 2 November 2011, accepted on 2 November 2011

Introduction

Tubes and other profiles made from sheets or strips are ofhigh economic importance. Due to the constant thickness ofconventional sheets and strips, profiles made of these kindsof semi-finished products are disadvantageous for load-optimized light weight structures. Consequently, severalapproaches aim at optimizing the cross section geometries ofprofiles by an adaption of the sheet thickness distribution.Liu et al. investigated the forming of tailor welded tubes

(TWT) by hydroforming [1]. Their investigations showedthat TWT can be used for weight reduction or thicknesshomogenization of products made by hydroforming.Roll forming is a preferred technology for the manufac-

ture of tubes and other kinds of profiles. Different rollforming strategies can be utilized [2]. They influence theprocess design effort as well as the result of a roll formingprocess considerably.Several research activities dealt with combinations of

rolling and roll forming processes. Goertan et al came to theconclusion that a combination of linear flow splitting and rollforming helped to reduce the number of roll forming stepsbecause the band edgewas stiffened by the preceding rollingprocess [3]. A combination of linear bend splitting and rollforming was used in [4] to create profiles with bifurcationsand areas with reduced thickness. Beiter reports about acombination of flexible rolling and roll forming [5]. Hepoints out that suitable roll forming stands have to beequipped with devices that allow a change of the rollspositions during the forming operation.All abovementioned work reached the conclusion that the

usage of sheets or strips with non-uniform thickness enablessubstantial weight reduction in products. At the same time,

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the non-uniform thickness necessitates the consideration ofgeometry and material modifications caused by the preced-ing thickness adjustment during the process and tool designfor subsequent manufacturing steps.The aim of the work presented here is to determine

whether load-optimized tubes, with a varying wall thicknessdistribution in the circumferential direction, can be manu-factured in a continuous manufacturing chain consisting ofStrip Profile Rolling and roll forming operations. Thetargeted so called Tailor Rolled Strip (TRS) tubes could bebeneficially used as light weight parts, especially when theyare loaded by bending moments. In that case, the tubegeometry can be optimized tomaximize the specific bendingstiffness (moment of inertia per unit cross section area).Additionally, for tubes that have to be formed to morecomplex geometries in a subsequent tube hydro-formingoperation, TRS tube could be used to produce parts with anoptimized wall thickness distribution.The manufacturing of the TRS tube was realized by the

two continuous processes: 1.) strip profile rolling (SPR) and2.) roll forming. In the strip profile rolling process, a sheetmetal coil with a varying thickness distribution is created.Afterwards, this TRS coil is formed to a tube by roll forming.These tubes are used as semi-finished parts for subsequenttube hydro-forming operations. The development to man-ufacture TRS with varying wall thickness distribution, bystrip profile rolling has been reported in the first part of thispaper. In the current paper, the secondary processing of TRStube by roll forming and tube hydro forming will bepresented. TheTRS tube formingwas investigated bymeansof finite element (FE) simulations and experimental tube rollforming. Results should show which material propertychanges, during the SPR process, need to be considered for

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Full Paper steel research int. 83 (2012) No. 1

Figure 3. Optimum utilization of material in a TRS tube for bending

load.

an adequate numericalmodelling. The hydroforming ofTRStubes is investigated by means of tube bursting tests.

Possible Weight Savings by Usage of TRS Tubes

In order to determine the possible weight savings of anoptimized TRS tube loaded by bending moments, theparametric model shown in Figure 1 is used. The area withreduced thickness was varied to find an optimal ratiobetween bending stiffness (proportional to moment ofinertia, Ix) and material used (proportional to area A).As basic dimensions of the tube an, external diameter

D¼ 60mm, with groove thickness, h¼ 1.5mm and initialwall thickness sþ h¼ 2.5mm were chosen. The angle a

defines thewidth of the areawith the initial wall thickness, inthe circumferential direction of the TRS tube.If the angle a is increased, both the moment of inertia and

the material usage are increased. The change in the momentof inertia, depending on the width of the area with the initialwall thickness, is demonstrated in Figure 2. Here, it can beseen that the material near to the neutral bending axisincreases the weight, but does not contribute to the overallbending stiffness significantly. On the other hand, materialin the area of a¼ 08 or 1808 enhances the stiffnessefficiently.

Figure 1. Geometry definition of the TRS tube.

Figure 2. Resulting moment of inertia of the TRS tube with varying

angle a.

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The best utilization of material, for bending stiffness, isachieved when the ratio of moment of inertia (Ix) and area(A) reaches a maximum. Figure 3 shows this ratiodepending on the angle a. For the given example, the anglea is approximately 458. In this example, an improvement ofapprox. 12% can be achieved by the SPR process, comparedto a tube with uniform thickness.

Mechanical Properties of a TRS

Thickness reduced areas of the TRShave been extensivelyplastically deformed during the strip profile rolling process.Resulting modifications of the material behaviour can bedetected by tensile tests. Therefore, test samples from theTRS have been investigated with and without a recrystal-lization annealing after the rolling process. To visualize thelocal strain in the TRS during the tensile tests, the opticalmeasurement system ARAMIS (GOM) was used. ARAMISis a non-contact and material independent measuringsystem, which enables the analysis of surface strain valuesfor static or dynamic forming operations [6].The original TRS (length 185mm, width 25mm, 65mm

grove width, 1mm grove depth, 2.5mm initial wall thick-ness, material DC01) showed only very little remainingformability and a non-homogeneous material flow duringthe tensile test. As demonstrated in Figure 4 on the left side,the grooved area showed an average elongation of 1.6%.Final failure occurred in themiddle of the test samples. Afterannealing (640 8C, 1h, inert gas: argon), the originalformability was restored and a homogeneous material flowin the grooved area was observed. The received averageuniform elongation was 33%, as seen in Figure 4 on the rightside.Formability during bending was examined by three-point

bending. Additionally, verification of FE-models could beconducted with reduced experimental and simulative effortscompared to investigations of the roll forming process.During the three-point bending operation the TRS is placedon two supporting elements and it is deformed by adownward moving punch. The test stand is illustrated inFigure 5 on the top. Acquisition of the local strains was done

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steel research int. 83 (2012) No. 1 Full Paper

Figure 4. Major strains of the strain hardened (left) and the annealed

TRS (right) during tensile test shortly before fracture.

by the optical measuring system Argus (GOM). With thissystem, the local strains are measured over the surface of thesample, based on the deformation of a deterministic gridpainted on the metal strip. In order to compare the FEpredictions with the experimental results, the local strainsalong a path were plotted in a diagram. The analysed path isshown schematically in Figure 5 on the bottom.

Figure 5. 3-point-bending test: tooling and bent TRS sample (top),

visualization of major strain (bottom).


Although it was possible to bend the TRS to a 908 anglewithout fracture of the material, a local buckling occurred.FE simulations of the 3-point-bending operation, of TRS,were carried out with the FE software Abaqus (explicitsolver). The element type ‘‘continuum shell elements’’, withan edge length of 3mmand one element in thewall thicknessdirection was used to model the behaviour of a referencestrip with uniformwall thickness.Major strains predicted bythe simulation and obtained from the experiments showed agood agreement, as shown in Figure 6.The FE simulation model, for bending of TRS, included

not only the modification of the geometry, but also the strainhardening of the material due to the thickness reductionduring the SPRprocess. In a first FE simulation, two separateflow curves were used to represent the strain hardenedgrooved material and the material with the initial wallthickness in the TRS. These two flow curves were obtainedby separate tensile tests of the thick (initial wall thickness)and the grooved area of the TRS. The experimentallyobserved buckling of the TRSwas not predicted correctly bythe simulation. The comparison of experimentallymeasuredmajor strain and the predicted major strain by the FEsimulation showed a large deviation of approximately 170%in the centre of the metal strip, as demonstrated in Figure 7.An inhomogeneous strain hardening in the groove of the

TRS caused by the strip profile rolling (SPR) process couldbe identified as reason for the buckling in the bent TRS. Toquantify the local strain-hardening distribution, the Vickershardness values (HV-0.2) were measured along the TRS, asseen in Figure 8. The Vickers hardness distribution shows alocal minimum at the centre of the TRS. In the other areas ofthe groove, nearly constant values of about 200HV-0.2weredetected. At the transition from the groove to the region withthe initial wall thickness, the Vickers hardness does notdecrease abruptly but continuously. This continuous declinecould be explained by the roll-out of the bulge during theflattening pass of the strip profile rolling process. The bulgeflattening mechanism can also be held responsible for thehardness minimum in the center of the sample. Compared to

Figure 6. Comparison of the major strains of sheet metal strip with

uniform thickness.



Figure 8. Vickers hardness distribution of TRS sample.

Figure 10. Comparison of major strains of the TRS using local pre-

strains for the simulation.

Figure 7. Major strains in the TRSusing two separate flowcurves for

the simulation model of the TRS sample.

the other regions of the groove, the influence of the bulgeflattening is negligible in the sample center.Using the functional equation [7]

Rm ¼ 3:38 � HV

and values of Vickers hardness and yield strength fordifferent deeply rolled sheet metal strips, the local strain-hardening in the TRS can be estimated and implementedinto the FE simulation shown in Figure 9.The provision of the pre-strain distribution in the FE

simulation model leads to a significantly improved pre-diction of the local buckling of the TRS. The results of theoptimised simulations are demonstrated in Figure 10.

Figure 9. Local pre strain in the FE model of the TRS.


Roll Forming of a TRS Tube

Roll forming is an economical, continuous bendingprocess in which a sheet metal strip is bent to a profile byseveral pairs of forming rolls (see Figure 11).Tomanufacture a TRS tube with equivalent geometry, the

TRS strip can either be equipped with grooves on the sheetmetal edges or with one groove in the middle of the sheetmetal, as illustrated in Figure 12.Depending on the chosen positions of the grooves, the

welding seam is either located at the thick (groove in middleof the sheet metal) or at the thin band edges of the TRS. Forselecting the arrangement of the groove(s) in the TRS for theroll forming process, several aspects need to be considered.A reduced wall thickness in the range of the band edges

allows an increased process speed, since the speed in highfrequency or laser welding can be increased withoutchanging the maximum power of the equipment. Thiscorrelation is illustrated in Figure 13.The advantage of an increased process speed, in the case of

the arrangement of the grooves at the sheetmetal edges in theTRS, is opposed by a larger risk of band edge waviness, asshown in Figure 14. Band edge waviness can occur becauseduring the roll forming process the band edges travel alonger distance than other areas in the sheet metal. If plasticdeformation occurs in the longitudinal direction of the bandedge during the roll forming process, the unwanted bandedge waviness might appear. So a thicker band edge resultsin a decreased risk of band edge waviness.

Figure 11. Schematic illustration of a roll forming machine.



Figure 12. TRS with a groove in the middle of the sheet metal and

grooves at the edges (top) result in the equivalent tube geometry

(bottom).

Figure 14. Reduced risk of band edge waviness when thicker sheet

metal is used [9].

Due to the results of the tensile and the three-point bendingtests, it could be expected that the tube roll forming of TRSdiffers from the tube roll forming of a sheet metal with auniform wall thickness.Before the FE-simulation model of TRS roll forming was

set up, simulations to optimize the roll forming of aconventional tube with uniform wall thickness wereconducted. These simulations of reference tubes wereoptimized in such a way that the accumulated plastic strain

Figure 13. Correlation of the wall thickness of sheet metal and the

achievable welding speed in high frequency welding [8].


was minimized by varying the arrangement of the tools. Thearrangement of the tools, of the optimized simulation, wasthen used to simulate the roll forming process of TRS.Due tosymmetry, only half of the TRS was simulated. The front ofthe TRSwas equippedwith a sharpened tip to allow an easierfeed into the gap of the roll pairs. The length, of themodelledTRS, was approximately four times the forming standdistance. The geometry of the tools was provided by JansenAG. This tooling was also used for later experimental rollforming of TRS. The simulated forming process included 17forming stands. Figure 15 shows the principal arrangementof the rolls and the sharpened tip of the sheet metal.Since the exact position of the side rolls, in the horizontal

direction, was not predefined, they have been set upaccording to the flower pattern. The flower pattern showsthe predicted sheet geometry, in each forming stand, fromthe flat sheet to the finished tube, as seen in Figure 16.The accumulated plastic strains (PEEQ in Abaqus), along

half of the circumference of the reference tube, are plottedfor each of the forming stands in Figure 17 top. From thevery large plastic deformation of the sheet edge in the secondforming stand (side roll), it could be deduced that thearrangement of these side rolls was not optimal. Preferably,the accumulated plastic strains should increase more evenlywith each successive forming stand. Through variations ofthe side rolls arrangement, the increase of accumulatedplastic strain was much more homogeneous and themaximum value was reduced: The results are demonstratedin Figure 17 bottom.

Figure 15. FE-model of ‘‘sharpened’’ sheet metal (left) and arrange-

ment of the roll forming tools (right).



Figure 16. Flower pattern and approximately aligned side roll.

Figure 18. Contour of reference tube (strip with constant thickness).

The resulting good roundness of the simulated tube isdemonstrated through comparison of the tube cross section(only half along the circumference) and the reference circlein Figure 18.To adapt the optimized roll forming simulation of the

reference tube to the roll forming simulation of the TRS

Figure 17. Change in the accumulated plastic strain at the individual

forming stands along half of the tube circumference before (top) and

after (bottom) optimization.


tube, the changed geometry (groove) and the pre-hardeningboth the SPR is implemented. If the roll geometries are thesame for the reference tube and TRS, a local gap with nocontact between the grooved area of the TRS and the upperrolls would occur. This gap could allow for uncontrolled orunwanted forming. Therefore, an additional simulationinvestigated, whether a local support of the grooved area,by geometrically modified tools, could enhance the TRStube roll forming process. In Figure 19, the cross sections ofthe TRS tubes (only half of the circumference), simulatedwith andwithout the adapted tools for the local support of the

Figure 19. Modified tool for forming TRS (top) and contour of TRS

tubes using ‘‘modified tool’’ and ‘‘reference tool’’ (bottom).



Figure 20. Reduced deviation of geometry due to modification of

tools 7th stand for forming TRS.

Figure 21. Bandedgewavinessof TRS tube (grooveson theedges);

top: with strain hardening, bottom: without strain hardening.

grooves, are plotted. The adapted tools result in a smallimprovement of the roundness of the finished tube,compared to the geometry achievable with the original tools.The largest deviations of roundness, between the SPR

tubes manufactured with the reference and modified tools,occur in the pre-hardened area with unchanged wallthickness close to the groove. This deviation was almostcompletely eliminated by a modification of the toolgeometry of the 7th forming stand as shown in Figure 20.The FE simulations presented so far dealt with the roll

forming of TRS with a groove in the center of the sheet. Asdescribed above, a geometrically adequate tube can bemanufactured by roll forming a TRS with grooves on thelongitudinal sheet edges. This, however, results in anincreased risk of unwanted band edge waviness, due to thereduced wall thickness at the edges. With reduced wallthickness, it is more likely that the material wouldexperience unwanted plastic elongations in the longitudinaldirection. On the other hand, the pre-hardening of thematerial in the TRS groovesmay prevent the development ofband edge waviness. The yield stress increases from the pre-hardening, so that plastic deformation could be reduced.Figure 21 shows a comparison of the resulting band edge

waviness of the TRS tubes, during the last two formingstands. The tubes are plotted in a sectional view (cut inlengthwise direction). The band edge waviness can beobserved in the top view. The band edges of the TRS tube‘‘grooves in band edges’’ (pre-hardening) show slightwaviness (areas 1a. and 2a.). The band edge waviness ofthe SPR with the original material properties is much larger(areas 1b. and 2b.) The pre-hardening of the SPRmaterial inthe grooves shows a positive effect on the tube roll forming.The experimental tube forming and welding was con-

ducted on an industrial roll forming machine of Jansen AG.Existing rolling tools for roll forming of conventional tubes(uniform wall thickness distribution) were used.The roll forming line included 17 forming stands already

described with the FE simulation model, a welding station


(high frequency welding) to close the tube and 8 calibrationstands. The latter consisted of alternating undriven verticalside rolls and driven horizontal rolls.Within this calibration,the outside diameter of the welded tube was reduced from60.4mm to 60mm by compression, to increase the dimen-sional accuracy of the tubes. For the experimental rollforming process, different kinds of sheet metal were used:
� S
hei

PRmaterial with a groove in the center (68mmwide and1mm deep),

� s
heetmetalwith ageometrically similarmilledgroove and � r eference material without a groove.
The additional tubes with the milled groove were ofinterest for the subsequent tube burst tests. The test materialswere welded to each other creating a long band which couldbe continuously roll formed, welded and cut to tube sectionsof 6 meters. The roll formed tubes with the milled grooveshowed a strong saber (curved) shape. Since the TRS tubeswith the similar geometry did not result in a saber shape, itcan be concluded that its pre-hardening has a positiveinfluence on the roll forming process.

Properties of TRS Tubes

Roundness measurements were carried out for theproduced tubeswith a 3-D coordinatemeasurementmachineas seen in Figure 22. The tested tubes were arranged onthe machine in such a way, that the welding seam waspointing upwards. The roundness measurement of one tubeincluded 40 coordinates along the circumference of the tube.The maximum deviation of the tube roundness compared tothe target circle (diameter: 60mm) and the form deviationswere obtained from the test. The resulting form deviationswere calculated by summation of the absolute values of themaximum positive and the maximum negative deviation. InFigure 22 (upper right) the contours of the measured tubesare plotted in a scaling factor (20), for better visibility. Thedashed inner and outer circles represent the maximumnegative and the maximum positive deviation, respectively.The form deviation of the TRS tube (0.31mm) is

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Figure 22. Results of roundness measurements for reference and

TRS tube.

Figure 23. Schematic of the tube-bursting test with fixed ends.

Figure 24. Internal pressure and expansion of tubes in the bursting

test.

approximately twice as large as the form deviation of thereference tube with uniform wall thickness (0.14mm).In a subsequent forming operation, the quality of TRS

tubes was investigated by a tube hydro-forming test. Duringa tube hydro-forming operation, the tube is expanded andformed by an internal pressure. This pressure is generallyachieved by a liquid which is pressurizing the inner wall ofthe tube and forming it into the cavity of a surrounding upperand lower die. By an additional application of axialmechanical pressure on the tube ends, the resultingformability of the tube can be increased by pushing extramaterial into the cavities [10].To study the formability of themanufactured TRS tubes in

tube hydro forming operations, tube-bursting tests wereconducted. The tube-bursting test is a test method tocharacterize tube quality for tube hydro forming operations[11]. Tube-bursting tests can be carried out with differentboundary conditions on the tube ends. The tube ends caneither be fixed, free or they can be pushed or pulled.Depending on the process variant, different strain statesoccur in the tube during the forming process. In this study,the tube ends were fixed during the test. Due to thisconstraint, only a small increase of the tube diameter can beachieved before the tube bursts. However, by a directcomparison of the test results obtained from the TRS tube,the reference tube (uniformwall thickness) and the tubewiththe milled groove, conclusions about the changed form-ability could be made. A state of plain strain occurs in themiddle of a tube with uniform thickness distribution duringthis test, if a sufficient free expanding length is taken intoaccount [11]. The major strain is therefore always pointingin the circumferential direction. A schematic drawing of thetube-bursting test with fixed ends is illustrated in Figure 23.The top half of the figure schematically shows the tubeinserted into the tool before the ends are fixed and before thepressure is applied. In the lower part of the figure, the tube


ends are fixed and sealed by conical punches, so that theinternal pressure pi causes the radial expansion of the tube.The radial expansion of the tube was measured in the middleof the tube by two oppositely arranged distance sensors.In Figure 24, the results of the tube-bursting tests are

shown for the different tube types. In the diagram, the radialexpansions (separate for left and right sensor) and theforming pressures are plotted. The welding seam wasarranged, for all the tubes, at the position of the rightdistance sensor. So for the tubes with grooves, the groovewas placed at the left distance sensor. The TRS tubes showedonly very small radial expansions before bursting. Thebursting always occurred in the grooved area of the TRStubes. The reason for the little formability was the largestrain hardening caused by the strip profile rolling process.By optical measurements with the systemArgus (GOM), thelocal strainwasmade visible and it could be observed that, inthe grooved area of the SPR-tube, only very small strainoccurred, as demonstrated in Figure 25. After an annealingprocess, a TRS tube could obtain significantly larger and



Figure 25. Major strains of a TRS tube test after the tube bursting.

more uniform strains in the bursting test. The observed radialexpansions were quantitatively comparable to the tubeswitha milled groove.

Summary

By the two continuous processes strip profile rolling androll forming, load adapted TRS tubes were manufactured. ATRS is produced by the strip profiling process. During thisprocess, wide grooves can be rolled into sheet metals. The socalled TRS were formed to tubes by roll forming. TRS tubescan be beneficially used for light weight products which areloaded with bending moment, as shown by analyticalcalculations.Also, TRS tubes could be beneficially used as a semi-

finished part for a tube hydro forming process. In that case,the thicker wall thickness of the TRS tube could be allocatedat areas with large deformations. Tube bursting tests showedthat an additional annealing process is necessary to restorethe formability of the TRS tube, if deformation of the rolledarea of the tube is required. To simulate the manufacturingprocess of the TRS tube byFEM, an integrated considerationof the process is necessary, because the local strainhardening of the material has a significant influence on theforming results. This influence could be demonstrated andanalyzed by 3-point bending tests of TRS. For the rollforming process, the strain hardening of the TRS shows a


positive effect: the TRS tube with a groove in the middle ofthe sheet metal showed, after the experimental roll forming,no unwanted saber shape, opposite to a geometrically similartube with a milled groove (quasi annealed material).Furthermore, FE-simulations predicted that during the rollforming of a TRS with grooves on the band edges the risk ofband edge waviness is reduced due to the strain hardening.

Acknowledgements

The presented work was carried out in a joint projectfunded by FOSTA (Research Association for SteelApplication), with the title ‘‘Development of a productionprocess for tubes with non-constant wall thickness distribu-tion by roll forming of tailor rolled strips’’. This project wascarried out in cooperation between the Institute forProduction Engineering and Forming Machines (TUDarmstadt) and the Institute of Metal Forming (RWTHAachen), along with several industrial partners. The authorswish to thank the FOSTA for funding and the projectpartners for the fruitful cooperation.

References

[1] G. Liu, S. Yuan, G. Chu: FEA on deformation behavior of tailor-welded tube in hydroforming, Journal of Materials ProcessingTechnology 187–188 (2007) 287–291.

[2] P. Groche G. von Breitenbach: Roll forming strategies for weldedtubes, Steel Research International 79/1 (2008), 40–46.

[3] M.O. Gortan, D. Vucic, P. Groche, H. Livatyali: Roll forming ofbranched profiles, Journal of Materials Processing Technology209 (2009) 5837–5844.

[4] P. Groche, J. Ringler, T. Abu Shreehah: Bending–rolling combi-nations for strips with optimized cross-section geometries; CIRPAnnals - Manufacturing Technology 58 (2009) 263–266.

[5] P. Beiter: Leichtbau im Automobil- und Transportsektor durch Roll-profile aus flexibel gewalzten hoher- und hochstfesten Mehrphasen-stahlen, 7 Fachtagung Walzprofilieren, Darmstadt, 2010.

[6] Homepage of the GOM mbH, www.gom.com.[7] H. Bargel, G. Schulze: Werkstoffkunde, 2008, Springer Verlag.[8] U. Dilthey: Schweißtechnische Fertigungs-verfahren 1, 2006,

Springer Verlag.[9] G. T. Halmos: Roll Forming Handbook, 2006, Verlag: Taylor &

Francis Ltd.[10] P. Groche, R. Steinheimer, D. Schmoeckel: Process stability in the

tube hydroforming process In: 53. Mitgliederversammlung des CIRP:Montreal, Canada, August 24-30, 2003 / International Institution forProduction Engineering Research.- Uetendorf: Ed. Colibri, 2003.-482 S.- (CIRP annals; 52,1).- ISBN 3-905277-39-5 Ed. Colibri,Uetendorf.

[11] G. v. Breitenbach: Methode zur Analyse, Bewertung und Optimier-ung der Prozesskette Profilieren langsnahtgeschweiter Rohre fur dasInnenhoch:druck-Umformen, 2007, Shaker Aachen.


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