A6c

Pa

b

a

AA

P8

KHEA

1

wtu

ieoi(

ado

0d

Journal of Materials Processing Technology 211 (2011) 916–924

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

nalysis and comparative study of factors affecting quality in the hemming of016T4AA performed by means of electromagnetic forming and processharacterization

. Jimberta,∗, I. Eguiaa, I. Pereza, M.A. Gutierreza, I. Hurtadob

Labein-TECNALIA, Paque Tecnológico de Bizkaia, Edif. 700, Derio 48160, SpainMondragon Unibertsitatea, Loramendi 4, Arrasate 20500, Spain

r t i c l e i n f o

rticle history:vailable online 27 August 2010

ACS:1.20.Hy

eywords:emminglectromagnetic formingluminum

a b s t r a c t

Hemming is commonly one of the last operations for stamped parts. For this reason it is of critical impor-tance on the performance and perceived quality of assembled vehicles. However, designing the hemmedunion is a complicated task and is deeply influenced by the mechanical properties of the material ofthe bent part. Significant problems can arise in this operation when bending aluminum alloys, becausecracks can appear due to the localized strain during hemming as a result of the low ductility of auto-motive aluminum alloys. This paper presents the development of the electromagnetic forming (EMF)technology for auto body-in-white parts hemming. A relatively simple experimental procedure to per-form a hemming operation based on the principle of EMF is presented in order to compare the variationin the quality parameters of a hemmed joint. The achieved results are compared with the correspondinggeometry hemmed utilizing the conventional process. At the same time, the study is completed with the

development of a new simulation method for the EMF technology. The results obtained during this studyprove the capability of the EMF to obtain quality hem unions simplifying the complicated conventionalhemming operation. In this study a loose coupling EMF hemming simulation method has been developedusing Maxwell® 3D to solve the electromagnetic field computation and Abaqus® to solve the mechanicalcomputation. This simulation method shows good agreement with the physical experiments. Finally,the EMF hemming process is characterized by analyzing the influence of main input parameters on the

s.

quality output parameter

. Introduction and background

The reduction of CO2 emissions by the new legislations world-ide increases the interest of the automotive industry for reducing

he weight of the vehicles. This concern brings new interest in these of aluminum alloys for automotive body materials.

One of the most demanding operations for these types of partss the hemming operation. It consists in a 180◦ folding of thedge of the part with a bending radius of 1–3 times the thicknessf the metal sheet. The conventional hemming process consistsn three operations: bending, pre-hemming and final hemmingFig. 1).

Unfortunately aluminum has low formability compared to steelnd is difficult to hem due to its susceptibility to strain localizationuring the hemming process. This phenomenon produces crackingn the hemmed edge (Lin, 2006). In order to avoid this problem and

∗ Corresponding author at: UPV/EHU, Spain. Tel.: +34 946014308.E-mail address: pello.jimbert@ehu.es (P. Jimbert).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.08.022

© 2010 Elsevier B.V. All rights reserved.

due to the limitations of conventional hemming technologies, thebending radius must be increased and a rope hem used (instead ofthe flat hem used with steels) when working with aluminum alloys.These changes in the design of the hem union reduce the customerperceived quality of the product (Fig. 2).

Moreover dies and tools used for the hemming process aredesigned based on experience and lengthy and costly die tryoutsdue to the required quality of these body parts.

Several studies analyzed a variety of techniques to obtain “flathem” unions with automotive aluminum alloys. Golovashchenko(2005) obtained “flat hem” unions for aluminum alloys by redis-tributing the plastic strain of the bending operation through a largerarea before the hemming. The flanged parts must have a sharpbending radius before the hemming. This new method uses a biggerbending radius to reduce damage on the parts. An additional axial

compression delivers additional material into the bending zoneallowing an additional 10% of pre-strain through the whole formingand assembly operations. The main disadvantage of this solution isthe need of an extra axial compression step which increases processtime and tooling cost.

P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924 917

Fig. 1. Different stages of the conventional mechanical hemming process.

F (left) aG betwe2

adtt

hpdctsvraaaptstltvbotpmcRe

nwlitrnpoem

tE

final hemming. The pre-hemming followed a vertical path with 45angle on the tool face. Blades displacement speed was 1 mm/s forthe pre-hemming and hemming operations.

Samples made by EMF were hemmed in one single operationfrom the 90◦ bending position. Target geometries were compared

Table 1

ig. 2. Different types of hemmed unions. “Flat hemming” union used with steelsAP” identified here is the distance between the tangent break lines, and is smaller005).

Other researchers (Krajewski and Carsley, 2003; Espinosa etl., 2007) have reduced the hem union radius by annealing theeformed area after the bending operation. After the heat treatmenthe material recovers all its formability and can be flat hemmed, buthe integration of the heating step is complex.

In the late eighties a new hemming technology called rollemming appeared (Sawa, 1986). This new process presents a non-lane strain deformation pattern with a component of strain in theirection of the hem line different from the plane strain bendingreated with the conventional hemming. Carsley (2005) theorizedhat the non-plane strain path created during roll hemming enablesharper hemming radius than the plain strain created with the con-entional hemmers. More recently, Le Maoût et al. have studied theoll hemming process for curved pre-strained parts similar to actualutomotive parts. Two different materials were used, a 6000 seriesluminum (Le Maoût et al., 2009a) and a E220BH steel (Le Maoût etl., 2010) both widely used by the automotive industry for externalarts. The goal of these investigations was to gather experimen-al data on hemming of concave and convex edge-curved surfaceamples to validate the numerical simulation of the process ando study the influence of geometry and pre-strain on roll-in andoad. Le Maoût et al. (2009b) have also developed a damage charac-erization experimental model for the 6000 series aluminum alloyalidated with the hemming process simulation. This model haseen used to propose a hemming limit criterion by a critical valuef the void volume fraction for the table top hemming method. Inhe same year Xing et al. (2010) established another fracture limitrediction method using forming limit strain curves from experi-ental data of a 6061-T6 aluminum alloy. In this case the fracture

riterion was validated with a roll hemming process simulation.oll hemming has two main disadvantages: the technology has anlevated process time and an arduous robot programming.

This work presents a completely innovative hemming tech-ology alternative to the conventional ones. The EMF technologyorks especially well with highly electrical conductive materials

ike aluminum alloys (Vohnout, 1998) increasing their formabil-ty limits when free formed. Daehn (2005) attributes this increaseo the inertial effects present in EMF processes. The inertial effecteduces the load generated in the necking area of the part creatingew necking areas and increasing the total elongation. When theart is formed against a die Imbert (2005) attributes this increasef elongation to the inertial ironing effect. The high pressure gen-

rated during the impact delays the creation of microvoids in theaterial raising the formability limits.The high velocity of the flange will lead to a sharper hem union

han the one obtained with the conventional hemming method.MF technology will decrease the set-up time of the hemming

nd “rope hemming” union used with low formability alloys (right). The “apparenten two flat hemming unions than it is between two rope hemming unions (Carsley,

tooling and will eliminate the pre-hemming operation since thehemming is obtained from the 90◦ bent geometry on one singleEMF operation.

2. EMF and conventional hemming experiments

The EMF forming process consists in generating a pulsed mag-netic field by discharging the electrical energy stored in a capacitorbank through a coil. The time varying magnetic field induces eddycurrents in the sheet, which in turn generates an opposing mag-netic field. The interaction of the two fields results in the sheetbeing repelled from the coil at speeds that can reach hundreds ofmeters per second.

A Magneform capacitor bank was used for the EMF hemmingexperiments. Its maximum stored energy is 60 kJ with 1800 �Fcapacitance at 8.3 kV. The total inductance and resistance of thebank are 10.3 nH and 956 ��, respectively.

A 6016T4 aluminum alloy and straight hem geometry were usedin this study. The mechanical properties of this alloy can be seen inTable 1.

The experimental set-up with the straight flat coil and the geo-metrical parameters of the hem union can be seen in Fig. 3.

All the tests were made using the same geometrical inputparameters. Parts were bent with a mechanical sheet metal fold-ing machine using 2.5 mm clearance between blades and 1.2 mmbending radius. The flange is 5 mm height and the thickness of theinner and the outer part is 1.1 mm. The H overlap between coil andflange and the discharged energy from the capacitor bank werethe changing input parameters for the electromagnetic hemmingexperiments. Five different H overlap values were tested: 0%, 20%,40%, 60%, 80% and 100% (expressed on % of the flange height over-lapped with the EMF coil) and five discharge energy values of 1, 2,3, 4 and 5 kilojoules (kJ).

The conventionally hemmed samples were all made using thesame process parameters in two operations: pre-hemming and

◦

Mechanical properties of the AA6016T4 aluminum alloy (Alusuisse, 2007).

Re (MPa) Rm (MPa) Max. strain(%)

Thickness(mm)

Hardness(HV)

AA 6016 T4 123 237 20 1.1 70

918 P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924

Fig. 3. EMF straight hemming experimental set-up (left) and detailed view of the main geometrical parameters (distances in millimetres) (right).

med s

ah

2

tasr

2

rabva

2

e

Fig. 4. Conventionally hemmed sample on the left and EMF hem

fter the experiments (Fig. 4). The target geometry is defined as theem union with a complete hemmed flange as can be seen in Fig. 4.

.1. Results and evaluation

Several quality output parameters were measured in ordero establish the capabilities of the EMF technology for hemmingpplications and were compared to the conventionally hemmedamples. Finally, the simulation method is presented with theesults.

.1.1. Union radiusThe first analysis performed was the measurement of the union

adius. This value can be seen in Fig. 4. Adjusting the H parameternd the discharged energy through the coil, the geometry obtainedy EMF has a smaller union radius than the one obtained con-entionally, meaning EMF has a higher perceived quality due to

reduction of the “apparent GAP” (Fig. 2).

.1.2. Outer surface and crackingAfter hemming, the convex surfaces of the outer parts were

xamined looking for cracks or surface irregularities using a micro-

Fig. 5. Outer surface of the hemmed target geometries for the convent

ample on the right. The dimension of the union radius is shown.

scope (Fig. 5). These observations can quantify the damage sufferedby samples in both cases. Different surface textures were observedwhen analysing the micrographs. Any cracks within one thicknessof the edge of the specimen were not considered according to theASTM standard.

Surface damage was classified according to the ASTM E290-97aBend Test Rating Standard. The conventionally hemmed part gen-erates a surface with a damage of degree 3, which means crackinitiation lines (i.e. lines of localized thinning or crazing) paral-lel to the bend line or cracks visible only with 3× magnification.The surface of the part hemmed by EMF presents a surface with adamage of degree 2 which means no cracking, but heavy orangepeel. The orange peel refers to an effect that arises on the sur-face of metal sheets when they are stretched beyond their elasticlimit.

A cross-section of the union was also analyzed looking for cracks.The conventionally hemmed part presents cracks initiations on the

outer and on the inner surface. The part hemmed by EMF showedno cracks on both surfaces (Fig. 6).

EMF technology applies an inertial energy to the flange gener-ating a tensile stress into the bent area. This way, new material isintroduced into the union edge distributing the total deformation

ionally hemmed part (left) and the part hemmed by EMF (right).

P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924 919

Fig. 6. Section of the unions hemmed conventionally (left) and using the EMF technology (right).

F f a car door by diffractometry where the warp defect can be observed (right, Lange, 2006).

oc

2

foLthatrr

dc

iaw

ig. 7. Schematic of the “warp” defect (left, Livatyali et al., 2004) and visualization o

n a bigger area and reducing the maximum strain and the risk ofracking.

.1.3. WarpThe warp defect was also measured. Warp is a characteristic sur-

ace appearance defect on hemming. This defect is the deflectionf the outer surface at the edge of the union as is shown in Fig. 7.ange (2006) and Lin (2005) attribute the creation of this defect tohe relaxation of the material during the springback after the pre-emming and hemming processes. The warping effect constitutesmajor problem when assembling closure panels, even though

he size of the defect usually ranges in the order of microns. It isesponsible for creating a light deflection on the part when paintededucing the customer perceived final quality.

A confocal profilometer was used to quantify the value of thiseflection for the geometries obtained electromagnetically andonventionally (Fig. 8).

Fig. 8 displays the measured profiles with the associated max-

mal warp values, are 20 �m for the conventionally hemmed partnd 30 �m for the electromagnetically hemmed part, in otherords, the EMF produced slightly greater warp.

Fig. 8. Warp on mechanical hemming vs. EMF hemming.

Fig. 9. Section of the hemmed union showing the micro-hardness measurementslocations.

2.1.4. Micro-hardness measurementTo finish with the straight hem union characterization some

micro-hardness measurements were made in both samples toevaluate the influence of the high speed deformation. Several mea-surements were made through the thickness direction as shownin Fig. 9. The measurements were made according the ASTME384-08 ae1 Standard Test Method for Microindentation Hard-ness of Materials applying 200 g during 10 s. The exact locationof the measurements are the following (expressed in millimetresfrom the edge of the union): HVa = 0.85 mm, HVb = 0.55 mm and

HVc = 0.25 mm.

Results of the micro-hardness measures are summarized inTable 2.

A slightly lower hardness was obtained for the electromagneti-cally hemmed parts.

Table 2Micro-hardness values after bending 90◦ and after hemming with the differenttechnologies.

Initial hardness = 70 HV HVa HVb HVc

After bending 90◦ 83.5 78.7 85.8After hemming Conventionally 88 82 86

EMF 88 80 84

920 P. Jimbert et al. / Journal of Materials Proces

Fe

ppwc

2

tsttp

patt

-

-

-

ae

ig. 10. Sequence of the coupling between Maxwell® 3D and Abaqus® (Arangurent al., 2008).

The high geometrical quality demanded by these body-in-whitearts is maintained when hemming with the EMF technology com-ared to the conventional hemming method. Similar values ofarp, union radius and micro-hardness were obtained in both

ases.

.2. Simulation

In order to have a better understanding of the deformation pat-ern followed by the flange when hemmed electromagnetically aimulation method was developed. This way the deformation dis-ribution pattern can be analyzed for both technologies as well ashe maximum strain values in order to find an explanation for thehysical results.

Simulating the EMF process implies accounting for multiphysicshenomena. Several physical problems must be taken into accountnd all these problems are tightly related to each other. Currently,here are three different strategies to tackle the numerical simula-ion of the electromagnetic and mechanical coupling:

Direct coupling: In this strategy, all the coupling field effects arecalculated in each element in every time step (all the degrees offreedom are resolved at the same time). The main drawback ofthis strategy is that it takes a great deal of time.Sequential coupling: In each time interval, called macro-time step,the induced Lorentz forces in the workpiece are calculated withthe electromagnetic model, and then automatically transferred asinput loads to the mechanical model. In this model, the workpiecedeformation is calculated and the geometry is updated in orderto transfer it to the electromagnetic model. The Lorentz forcesare again calculated and the iterative process is repeated untilthe forces are negligible. From that moment, only the mechanicalproblem is resolved.Loose coupling: In this case, the Lorentz forces are calculated with-out taking into account the workpiece deformation. These forcesare the input loads of the mechanical simulation, which is carriedout in one step. This strategy neglects the forces evolution duringthe process due to the geometry changes, so in some cases it is aless accurate strategy, but on other hand it takes less computation

time.

At LABEIN-Tecnalia a loose coupling between Maxwell® 3Dnd Abaqus® has been developed (Fig. 10). In this coupling thelectromagnetic problem is solved with Maxwell® 3D, which is a

sing Technology 211 (2011) 916–924

software package that uses FEM analysis to simulate and solve 3Delectromagnetic field problems. Its transient solution type allowsresolving 3D magnetic fields caused by windings supplied by volt-age and/or electrical current sources with arbitrary variation asfunctions of time.

The magnetic energy density is obtained from the electromag-netic calculation in Maxwell® 3D and it is applied as pressure onthe surface of the workpiece that is facing the coil, for the mechan-ical calculation. Three main considerations need to be taken intoaccount for this purpose:

1. The magnetic field lines are perpendicular to the workpiece sur-face, that is to say the workpiece behaves in a similar mannerthan a perfect conductor. This approximation is good enough forhighly conductive materials such as aluminum. In the mechan-ical calculation the pressure is always applied perpendicular toeach element.

2. Maxwell® 3D calculates the magnetic pressure applied to theworkpiece by the magnetic field. The peak time of this pres-sure curve was used to calculate at that moment the maximummagnetic energy density.

3. The maximum pressure distribution over the workpiece isapplied only once, in the mechanical calculation, but it iscorrected in the time scale using the unitary pressure curve cal-culated with Maxwell® 3D. This pressure curve is adjusted usinga high speed camera record of the physical experiments. As theflange separates from the coil the electromagnetic interactiondisappears and the pulsed pressure curve drops to zero.

The physical explanation of the strong relation between distanceand magnetic pressure can be explained as follows: the magneticpressure generated over the part can be calculated using Eq. (1):

P = �0(H21 − H2

2)2

(1)

where �0 is the magnetic permeability of free space (H/m), H1 isthe magnetic field strength on the surface of the part (A/m) and H2is the magnetic field strength on the opposite surface of the part(A/m). For highly electrically conductive materials like aluminumthe whole magnetic field intensity is absorbed by the part so H2can be considered equal to zero and the magnetic pressure can becalculated using Eq. (2):

P = �0(H21)

2(2)

The existing magnetic field strength can be calculated as fol-lows:

H = B

�(3)

where B is the magnetic field and � is the magnetic permeabilityof the material.

According to Biot–Savart law to calculate the magnetic fieldgenerated by a steady current, i.e. a continual flow of charges, forexample through a wire:

B = �0I

4�

∫dlxr̂

r2(4)

where vector dl is the direction of the current, �0 is the magneticconstant, r is the distance between the location of dl and the location

at which the magnetic field is being calculated, and r̂ is a unit vectorin the direction of r.

From Eqs. (3) and (4) can be concluded that the existing Hmagnetic field intensity between the coil and the part decreasesexponentially with the distance among them (r).

P. Jimbert et al. / Journal of Materials Proces

Fc

H

cecrlswtm

amanTtw

b

fltth

ip

s

ig. 11. Unitary pressure curve in time and curve corrected using a high speedamera to be multiplied by the maximum magnetic pressure.

And the magnetic pressure, P, increases by the magnetic field,, squared from Eq. (2).

It is generally accepted, and experimentally validated underonventional forming conditions that the EMF processes are gov-rned by the first part of the cycle of the discharged transienturrent. After the first part of the cycle the part to deform haseacted to the electromagnetic impulse and is already several mil-imetres away from the coil. At this point the electromagnetic forcestop acting over the part and the magnetic repulsion drops to zerohen the distance between the coil and the part is two times the

hickness of the part (Hansen, 1964). From this moment the defor-ation continues due to the inertial energy given to the part.The physical EMF hemming experiments were recorded with

high speed camera at 90.000 frames per second. The preciseoment when the flange separates from the coil was calculated

nalyzing these images. The trigger of the camera was synchro-ised with the electromagnetic discharge of the capacitor bank.his way the generated magnetic pressure and the displacement ofhe flange were calculated at the same time and the pressure curveas corrected according to the displacement of the flange.

In this case, the flange separates completely from the coiletween 10 and 30 ms for the EMF hemming experiments (Fig. 11).

After this moment the magnetic repulsion disappears as theange separates from the coil and the deformation continues dueo the inertial energy. So the pressure curve is corrected and takeno zero at that moment for a more accurate simulation of the EMFemming process.

The electromagnetic pressure impulse is then introduced as annput parameter in the Abaqus® software to solve the mechanicalart of the process.

Prior to the EMF hemming simulation, the part is bent quasi-tatically in order to reproduce the real strains situation (Fig. 12).

Fig. 12. Bent geometry before EMF he

sing Technology 211 (2011) 916–924 921

The mechanical part simulations were performed withAbaqus/Explicit, which uses explicit time integration. The blank andthe inner part were modelled with C3D8 elements (3D elementsof 8 nodes) and the base where the blank is placed was modelledwith R3D4 elements (3D rigid elements of 4 nodes). The frictionbetween the base and the inner and outer parts, and also betweenthe deformable parts was represented by a Coulomb friction coef-ficient of 0.1.

The material of the inner and outer parts was an AA6016T4 alu-minum alloy (Table 1). Its mechanical behaviour was consideredisotropic and was modelled by means of stress–strain pairs.

In order to reduce the number of elements in the model, only aslice of 0.5 mm in width was modelled, neglecting the border effectsin the sides of the blank by restricting the transversal movementof the nodes in the extremes of the slice. The nodes of the innerand outer parts placed at the edge of the right side of the modelin Fig. 12 have a longitudinal movement restriction in order toavoid blank displacements in that direction. As the EMF process is ahigh speed forming process no mass scaling was needed. This is animportant difference between the EMF hemming process and theconventional one. In the conventional process time is much higher,so mass scaling is needed in order to simulate in the same times asin the EMF ones.

Simulation results are compared in Fig. 13 against the physicalexperiments. The conventional hemming process was also simu-lated starting from the same bent geometry shown in Fig. 12. Theconventional hemming simulation was made in two steps: pre-hemming and final hemming.

Several conclusions can be drawn from the simulations. Due tothe higher roll-in (roll-in is described in Fig. 15) on the samplesafter the EMF hemming, there is extra material introduced in theunion. This extra material generates a redistribution of the totaldeformation on a larger area and consequently, a reduction of themaximum strain (Fig. 13). This result confirms the conclusions fromthe physical experiments of Section 2.1.

The numerical simulation using a loose coupling strategy andsolid elements for the mechanical part predicted the observedfinal geometry. This simulation also confirms the assumed homo-geneous distribution of the deformation and the reduction of themaximum strain for the electromagnetically hemmed samples. Agood level of agreement can be seen between the physical experi-ments and the simulations in both cases.

3. EMF hemming characterization experiments

After the straight hemming experiments, the EMF process hasshown real capability to produce quality hemmed parts. Continuingwith the development of this new application for the EMF technol-

mming obtained with Abaqus® .

922 P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924

Fig. 13. Hemming simulation samples (up) for the conventional (left) and the EMF process (right). The physical experiment samples are placed below to show the high levelof agreement.

F in inp(

ocs

itae0

oape

i

ig. 14. EMF circular hemming experimental set-up and detailed section of the maright).

gy, more experiments were run. The objective now is the furtherharacterization of the EMF hemming process. For this purpose aecond experimental plan was designed.

Two input parameters were chosen for this study. A geometricalnput parameter, the outer diameter, and a process input parame-er, the H overlapping between coil and flange. These parametersre summarized in Fig. 14. There were three different D outer diam-ters of 30, 60 and 90 mm and five different H overlaps values of%, 20%, 40%, 60% and 80%.

Four output parameters were taken into account to quantify thebtained quality: the roll-in, the bending of the inner part, the warp

nd the crack appearance on the outer surface. These output qualityarameters are summarized in Fig. 15, except for warp, which isxplained in Fig. 7.

The roll-in is an important parameter for the automakers evenf it is not a real defect. The concern of the industry is the control of

Fig. 15. (P) roll-in (left), bending of the inner part (centre)

ut parameters: D outer diameter (left) and H overlapping between coil and flange

this parameter in the different areas of a real part in order to keepthe same distance between different automotive closure panels likecar doors.

Warp is a characteristic defect that appears in the surface of ahemmed area as presented before in Fig. 7.

In the completed hem the inner part cannot be bent because itreduces the quality of the outer part.

Cracking is the fracture of the material due to an excessive strain.It typically occurs on the outer edge of the union and has to beavoided.

3.1. EMF hemming characterization results

A specific configuration of a hemmed flange was establishedas the reference geometry, called “target geometry” (Fig. 16) tocompare the different quality output aspects when changing the

and cracks on the outer surface of the union (right).

P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924 923

imtgiewv

o

3

t

qthooooif

tferptw

sh

tp

Fig. 17. Hemmability window for the AA6016T4 (thickness = 1.1 mm, bending Rradius = 1.2 mm and flange height = 5 mm) is represented by the area between thetwo lines.

Fig. 16. EMF hemming target geometry.

nput parameters. This target geometry fixes the quality require-ents and is defined as the hemmed union before the flange hits

he inner part. The discharged energy is increased until the targeteometry is obtained for every input parameter value. Then, thenput parameter value is changed for the next experiment and thenergy adjusted again until the target geometry is obtained. Thisay, target geometries obtained using different input parameter

alues can be compared in terms of the output quality parameters.All the results of this study are presented assessing the influence

f each input parameter on the studied output parameters:

.1.1. H overlapThe H overlap parameter determines the exact location where

he electromagnetic impulse is applied to the flange.This study revealed the importance of this parameter on the final

uality of the hemmed parts. High H overlap values do not enablehe achievement of the target geometry presented in Fig. 16. Theigh overlap gives a hemmed union with a bent inner part. Thepposite, the use of a low value of H can cause damage on the edgef the union generating cracks or a heavy orange peel. The bendingf the inner part and the cracking limit values change as a functionf the outer diameter of the part. By plotting these cracking andnner part bending limits on a graph the EMF hemmability windowor the AA6016T4 aluminum alloy is generated (Fig. 17).

The hemmability window is defined as the area between thewo lines of the graph in Fig. 17. This area represents the safe rangeor the H overlap values to obtain a quality hem union for the differ-nt outer diameters. The main EMF hemming process parameter isepresented on one axis (the H overlap) while the main geometricalarameter is represented on the other (the contour of the part orhe outer diameter). Therefore, hemmability window is very usefulhen designing the EMF hemming process for a new part.

In this case the hemmability window is very narrow due to the

mall bending radius of 1.2 mm used in these experiments. A wideremmability window will be obtained by increasing this value.

The roll-in was also measured for the different outer diame-ers and the results plotted as a function of the H overlap inputarameter in Fig. 18.

Fig. 19. Warp for diffe

Fig. 18. Roll-in vs. H overlap for the different outer diameters.

The roll-in increases linearly with an increasing H overlap, butis independent of the outer diameter.

The maximum warp values for the different H overlaps are plot-ted in Fig. 19.

A slight increase of the warp occurs when increasing the H over-lap.

3.1.2. Outer diameterThis is the main geometrical parameter for the automotive

hemmed parts. Actual part contours are basically a combination ofdifferent outer diameter areas and straight areas. After the experi-ments the output quality parameters where analyzed for the 3 outerdiameters.

A slight increase in the obtained warp was observed for thebigger outer diameter values (Fig. 20).

When the target geometry is obtained, adjusting the EMF dis-charged energy and the H overlap; the roll-in stays constant for

different outer diameters (Fig. 21).

This is useful when designing EMF hemming process for actualautomotive parts because the roll-in will be constant for the targetgeometry in every area of the part.

rent H overlaps.

924 P. Jimbert et al. / Journal of Materials Processing Technology 211 (2011) 916–924

Fig. 20. Warp for the 3

a

chroan

4

tEmphfathEpad

Fig. 21. Roll-in for the 3 outer diameters.

The main EMF hemming process parameters have been analyzednd the hemmability window has been established.

H overlap has shown to be the most important parameter toontrol in order to obtain a quality hemmed union. Inside theemmability window its value has to be kept to the minimum toeduce warp. This strategy can bring problems of cracking on theuter surface when using a small bending radius, and can bringreduction of the EMF coil life due to an increase of the energy

eeded.

. Discussion and conclusions

This experimental and computational work presents a solutiono the problems encountered when hemming aluminum alloys.MF technology expands hemming possibilities for automotive alu-inum alloys by reducing the final damage on the part. This is

ossible due to the high speed forming promoted by EMF. Theigh speed induced in the flange makes a wider and more uni-

orm distribution of the deformation through all the hemmedrea. Conventional hemming techniques restrict the deformationo the bending radius area by applying the deformation in the pre-

emming and the final hemming operations quasi-statically. TheMF launches the flange from the 90◦ bending position avoiding there-hemming operation, distributing the deformation on a biggerrea due to the acquired inertial energy and reducing the maximumegree of damage and the risk of cracking.

outer diameters.

Further work will be needed to study the interaction betweendifferent input parameters on a more complex part. The devel-opment of a 3D fully coupled simulation method will help in theaccomplishment of this task.

References

Alusuisse, 2007. Product specification.Aranguren, I., Pérez, I., González, B., Eguia, I., 2008. A new multiphysics simulation

approach for electromagnetic forming. In: ECCOMAS 2008, Venice, Italy.Carsley, J.E., 2005. Microstructural evolution during bending: conventional vs. roller

hemming. In: 6th Global Innovations Proceedings, TMS Annual Meeting, SanFrancisco, California, USA, February 13–17, pp. 169–174.

Daehn, G.S., 2005. High velocity metal forming. In: Daehn, G.S. (Ed.), ASM Handbook,vol. 14. Forming and Forging.

Espinosa, R.H., Das, S., Weaver, J., 2007. Simulation of warm forming assisted hem-ming to study the effects of process parameters on product quality. In: SAE WorldCongress, Detroit, USA.

Golovashchenko, S., 2005. Sharp flanging and flat hemming of aluminum exteriorbody panels. Journal of Materials Engineering and Performance 14, 508–515.

Hansen, H.J., 1964. The Principle of Electromagnetic Metal Forming, MagneformDocument, San Diego.

Imbert, J., 2005. Increased Formability and the Effects of the Tool/Sheet Interactionin Electromagnetic Forming of Aluminum Alloy Sheet. Dissertation, Waterloo,Ontario, Canada.

Krajewski, P.E., Carsley, J.E., 2003. In: Das, S.K. (Ed.), Heat Treatment Effects onBending in AA6111, Aluminum, TMS, pp. 25–35.

Lange, C., 2006. Etude physique et modélisation numérique du procédé de sertissagede pièces de carrosserie, C. Lange, Tesis, Ecole Nationale Supérieure des Minesde Paris.

Le Maoût, N., Thuillier, S., Manach, P.Y., 2009a. Classical and roll-hemming processesof pre-strained metallic sheets. Experimental Mechanics, doi:10.1007/s11340-009-9297-7.

Le Maoût, N., Thuillier, S., Manach, P.Y., 2009b. Aluminum alloy damage evolution fordifferent strain paths – application to hemming process. Engineering FractureMechanics 76 (9), 1202–1214.

Le Maoût, N., Thuillier, S., Manach, P.Y., 2010. Drawing, flanging and hemming ofmetallic thin sheets: a multi-step process. Materials & Design 31 (6), 2725–2736.

Lin, G., 2005. A computational design-of-experiments study of hemming process forautomotive aluminum alloys. In: Proc. IMechE, vol. 219, Part B.

Lin, G., 2006. Quality and Formability of Automotive Aluminum Alloys. Dissertation,The University of Michigan.

Livatyali, H., Laxhuber, T., Altan, T., 2004. Experimental investigation of formingdefects inflat surface–convex edge hemming. Journal of Materials ProcessingTechnology 146, 20–27.

Sawa, M., Torai Eng, K.K., 1986. Roller Type Hemming Device and Its Method, JP

61262432 (A).

Vohnout, V.J., 1998. A Hybrid Quasi-static/Dynamic Process for Forming Large SheetMetal Parts From Aluminum Alloys. Ph.D. Dissertation, The Ohio State Univer-sity, Columbus, OH.

Xing, H., Lin, Z.Q., Li, S.H., Zhao, Y.X., 2010. Fracture limit prediction for roller hem-ming of aluminum alloy sheet. Materials & Design 31 (3), 1410–1416.