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Journal of Materials Processing Technology 230 (2016) 131142

Contents lists available at ScienceDirect

Journal ofMaterials Processing Technology

journal homepage: www.elsevier .com/ locate / jmatprotec

Analytical approach for magnetic pulse welding ofsheet connections

Marlon Hahn , Christian Weddeling,Joern Lueg-Althoff, A. Erman TekkayaInstitute of Forming Technology and Lightweight Construction(IUL), TU Dortmund University, Baroper Str. 303, 44227 Dortmund, Germany

a r t i c l e i n f o

Article history:

Received 18 July 2015

Received in revised form

20 November 2015

Accepted 21 November 2015Available online 2 December 2015

Keywords:

Magnetic pulse welding (MPW)

Lightweight structures

Analytical model

Impact velocity

a b s t r a c t

An analytical model to calculate the acting forming pressure in magnetic pulse welding by determining

the magnetic field strength between the flyer sheet and a one-turn coil was presented. By neglecting

plastic deformation ofthe flyer, the model allows to calculate the transient velocity and displacement

behavior, too. The electromagnetic acceleration of5000-series aluminum alloy sheets was investigatedunder various experimental parameters. Utilizing Photon Doppler Velocimetry revealed that the ana-

lytical model appropriately describes the influence ofcurrent amplitude, coil geometry, and, especially,

discharge frequency on the velocity-displacement curve of the flyer and hence on the impact velocity.

The model introduced was applied to compute the impact velocity for the welding oflong lapjoints of

5000-series aluminum alloy sheets and 6000-series aluminum alloy hollow profiles. Through peel tests

it was shown that the weld strength at least complied with the strength ofthe weaker base material as

failure always happened in the flyer sheet. The wavy interface pattern typical for impact welding was

identified with the help ofmetallography.

2015 Elsevier B.V. All rights reserved.

1. Introduction

There is a risingdemand forlightweight structures in transport-related applications with the aim of reducing energyconsumption

to minimize costs as well as environmental pollution so that more

and more light metals are applied in the automotive industry. As a

consequence thereof, manufacturers face the challenge of joining

different grades of aluminum alloys. If welding is the joining pro-

cess of choice, conventional fusion-based techniques often reach

their limitsdue to theoccurrence of microstructural andmechani-

cal changesin theweldbead andheataffectedzone(HAZ)reducing

the strength of the joint and frequently causing hot cracks espe-

cially in welds between 5000- and 6000-series aluminum alloys

(PraveenandYarlagadda,2005). Theseproblemsmaybeavoidedby

utilizing high velocity impact welding processes such as magnetic

pulse welding (MPW). It is a solid-state welding process, which

also allowsto minimize or even eliminate the formationof contin-uous intermetallic phases when joining dissimilar metals (Zhang

et al., 2011). MPW is thereforewell suited for creating strongmet-

allurgical bondsbetween both similar anddissimilarmetalsand its

alloys.

Thegeneralworkingprincipleof impactweldingis illustratedin

Fig. 1. Besides MPW, further impact welding processes are (Zhang

Corresponding author.E-mail address:[email protected](M. Hahn).

et al., 2011): explosivewelding (EXW), laser impact welding (LIW),

andthe latelyby Viveketal.(2013) introducedvaporizing foilactu-

ator welding (VFAW).As outlined by Mori et al. (2013), thetwojoiningpartners, com-

monlynamedflyerand target,collideunder theangleatvelocitiesvimin therange of several hundred m/sproducing impactpressures

of the order of GPa. This process is accompanied by the so-called

jetting effect that leaves behind chemically pure surfaces allowing

a metallic bond to be formed. The atoms of the involved materi-

als are impacted to such an extent that they share and exchange

valence electrons. As a result, a wavy interface morphology is often

observable (see Fig. 1). A common explanation for the evolution

of these waves was given by Ben-Artzy et al. (2010). The authors

stated that reflected shock waves in the joining partners lead to a

KelvinHelmholtz instability.Fora given materialcombination,the

domainof thetwocrucialparameters (impactangle)andvc (colli-

sion velocity)necessary fora successful weld maybeplotted in theform of a weldingwindow, whichoriginates from EXW(Mousavi

and Sartangi, 2009). In contrast to EXW though, both and vc donot remain constant during MPW(Verstraete et al., 2011). A com-

pilation of welding windows as well as different bonding criteria

available in literature so far was presented by Kapil and Sharma

(2015).BymeansofX-raydiffractionanalysisandscanningelectron

microscopy,Koreet al. (2009) found that neither meltedzones nor

intermetallicphasesmaybepresent inmagneticpulsewelds,while

http://dx.doi.org/10.1016/j.jmatprotec.2015.11.021

0924-0136/ 2015 Elsevier B.V. All rightsreserved.
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132 M.Hahn et al./ Journal of Materials Processing Technology 230 (2016) 131142

Nomenclature

Symbol/meaning/unit

a Length of the pressure lead of the tool coil in mmB Magnetic flux density (vector) in GBg Magnetic flux density in the gap between the flyer

and the tool coil in G

C Capacitance of the pulse generator in F

c1, c2 , c3 Constants in theanalytical modelD Flyer displacement in mm

d1, d2 Distances from a two-sided tool coil in mm

Dch Critical flyer displacement in the analytical model

in mm

E0 Initial chargingenergy in J

EL Total magnetic energy in J

f Frequency of the discharge circuit in HzFL Lorentz force (vector) in N/mm3f0, fd,fb Initial (0), Doppler-shifted (d), and beat (b) fre-

quency of the Photon Doppler Velocimeter in Hz

FPeel Test force during peel test (index max for the max-

imum) in N

FUTS Ultimate tensile strength for a specific specimen

geometry in Nh Height of the tool coil in mmH Magnetic field strength (vector) in A/mmh Effective height of the trapezoidal coil in mmHg Magnetic field strength in thegap between the flyer

and the tool coil in A/mm

Hh Magnetic field strength at the sidewall of the tool

coil in A/mm

Hh0, Hy0 Coefficient functions in the analytical model in

A/mm

HS Magnetic field strength due to the skin effect in

A/mm

I Coil current (indexa foramplitudeor peak value) in

A

Ih

Current at the sidewall of the tool coil in A

Ip Current due to the proximity effect in A

IS Current due to the skin effect in A

j Imaginary unitJ Current density (vector) in A/mm2k Complex propagation constant (indices F and T for

flyer and tool coil, respectively) in 1/mm

l Length in mm

L Total inductance of the discharge circuit in H

Li Inner inductance of the pulse generator in H

p Magnetic pressure (index hf for the high-frequency

limit) in MPa

pc Plastic collapse pressure in MPa

R Total resistance of the discharge circuit inRi Inner resistance of thepulse generator in

s Sheet thickness in mm

t Time ins

trise Current rise time in s

v Flyer velocity (index m for measured velocities) in

mm/s

vc Collision velocity in mm/s

vim Impact velocity in mm/s

w Width of the tool coil in mm

w Width of the bottom of the trapezoidal coil in mm Impact angle in

Skin depth in mm Electrical conductivity in 1/

0 Operating wavelength of the Photon DopplerVelocimeter in mm

Magnetic permeability (index 0 for air) in Vs/Amb Density of the flyer material in kg/mm

3

Y Flow stress of the flyer material in MPa

Goebelet al.(2010)similarlyshowedthatthesephenomena cannot

becompletely avoided forsomematerials. InMPWtheelectromag-

netic forming (EMF) technology isused to plastically accelerate theflyer plate.Jablonski and Winkler (1978) stated that the forming

pressure in EMF is generated by penetration of a pulsed magnetic

field into a conductive workpiece to be formed. Themagnetic field

in turn results from a rapid discharge of a capacitor through the

tool coil (see Fig.2a). Materialsof lowelectrical conductivitycan be

formed with the help of thin high-conductivity driver plates (Gies

et al., 2014). Such drivers are positioned between the workpiece

and the coil to provide the forming pressure.

Neglecting the nonlinearity of circuit parameters due

to workpiece deformation, Jablonski and Winkler (1978)

described the coil current I by a simple series RLC (equivalent

resistanceinductancecapacitance) circuit yielding an exponen-

tially damped sine wave with frequency f and initial charging

energy E0:

I(t) =

E02CfL

exp

R

2Lt

sin (2ft) (1)

where

f = 12

1

LC R

2

4L2 . (2)

In order to simplify the analysis, Buehler and Bauer (1968)

approximated the frequency based on the time triseuntil peak cur-

rent Ia as

f= 14trise

. (3)

The transient magnetic field in the vicinity of the workpiece

(flyer plate) induceseddy currents in it that opposethecoil current

implying the appearance of the Lorentz volume forceFL (Lorentz,1895):

FL= J B . (4)J andB are the vectors of current density and magnetic flux

density. FollowingAizawa (2003), thisvolume force can be mathe-

maticallytransformedintoapressurep, also referredto asmagnetic

pressure, acting on both the workpiece and the coil. It can be cal-

culated as

p = B2g

2

1 exp2s

. (5)

Here, s is the flyer thickness and Bg is the magnetic flux den-sity tangential to the flyer surface near the tool coil. The presence

of a transient magnetic field between flyer and coil leads to the

evolution of two related effects, the internally caused skin and

Fig. 1. Schematic of impact welding (Mori et al., 2013).

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M.Hahn et al. / Journal of Materials Processing Technology 230 (2016) 131142 133

Fig. 2. Schematic of MPW: (a) one-sidedaccessibility as illustrated in Weddeling et al. (2014), (b) two-sided accessibility.

the externally caused proximity effect, inducing current crowd-

ing mainly to the surfaces opposite each other in case of reverse

current flow. Leastwise the skin effect can be characterized by an

equivalent conductor thickness, theskindepth (Heaviside, 1951):

=

(f)1/2 . (6)

The parameters f, , and stand for frequency, electri-

cal conductivity, and magnetic permeability. Low inductances

and capacitances facilitate high discharge frequencies, which are

required for attaining an appropriate magnetic field and thus

high forming pressure (Daehn, 2010). Generally, tool coils can be

dividedintothree basiccategoriesafterHarvey andBrower (1958):

compression coils, expansion coils, and flat coils for sheet metal

forming. Certainly hybrids exist, also for welding tasks. Weddeling

etal.(2014), for instance,useda modifiedexpansioncoilintroduced

by Kamal (2005) (called uniform pressure electromagnetic actua-

tor) to manufacture flat lap joints. A tool coil for such weld types

frequently resemblesa single rectangular conductor thepressure

lead having a wider return path away from the weld area (see

Fig. 2a). As can be seen in Fig. 2b, the return path can serve as asecond pressure lead if it is narrow enough and properly placed

below the targetplate so that both joining partners areaccelerated

against one another (Aizawa, 2003). The mathematical description

of the magnetic flux density in Eq. (5) strongly depends on the

coilgeometry, amongother factors.Formulaerelatingthe magnetic

fieldto thedischargecurrentwerereviewedby Psyketal.(2011)for

rotationally symmetric geometries. For a double-sided conductor

configuration with two sheets as shown in Fig. 2b, Aizawa (2003)

provided the followingequation:

Bg= Iw

tan1 w

2d1

+ tan1

w

2d2

. (7)

In this, w is the width of the two pressure leads, d1 and d2 ineach case represent the distance between the coil surface facing

the sheet and the point where magnetic flux density is observed.

All else being equal, Eq. (7) does not consider the variation of the

magnetic field with frequency and workpiece conductivity, mean-

ing it always yields the same magnetic flux density for a given

current value independent of the chosen frequency and conduc-

tivity. Moreover, Eq. (7) is only valid fora symmetric configuration

consisting of two one-turn coils and thus not applicable for the

welding with one-sided accessibility (e.g., welding of sheets onto

larger profiles). Since an analytical approach that overcomes the

disadvantages mentioned above has not yet been found in litera-

ture, an approach which eventually allows to calculate the impact

velocity when one-sidedly using one-turn coils is proposed and

verified in thepresent paper.

Fig. 3. Flyer segment interpreted as fully clamped beam.

Fig. 4. Visualization of magnetic field and current distribution in rectangular coil

and flyer plate with respect to proximity and skin effect.

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2. Analyticalmodel

First, a setup as depicted in Fig. 2 is considered, particularly the

forming or weld area. There, the flyer plate is fixed between two

spacers. It is a reasonable simplification to treat a cross-section of

this flyer segment as a fully clamped beam of density b, length l,andthicknesss under partial, uniformloadingp over thecoil width

w (see Fig. 3).

With flow stress Y, the corresponding static plastic collapse

pressurepcof the beam as known from rigid-plastic theory can be

written as (Jones, 1989)

pc= 2s2Ywl

. (8)

It is assumed that this collapse pressure is much smaller than

the acting magnetic pressure (pcp) so that the influence ofpcon theflyer acceleration is ignored. Here, a one-dimensional rigid-

body motion accordingto Newtons second lawis taken todescribe

the velocity vof the flyer plate and its displacementD :

v= 1bs

pdt , (9)

D = vdt. (10)Once the temporal evolution of the magnetic flux density Bg is

determined, magnetic pressure, flyer velocity, and displacement

may be computed according to Eqs. (5), (9), and (10). For that rea-

son, the electrical part of the model is established in what follows.

If an harmonic magnetic field strengthHas well as good conduc-tors(J= H) areassumed,Maxwellsequations (Maxwell,1865)may be put in the form of the second-order partial differential

equation below:

2 H= k2n H (11)where

kn= 1n +j 1n , n = F, T . (12)

It is noted thatj is the imaginary unit, Fand Trepresent the skindepth in the flyer and the tool coil, respectively. Furthermore, the

total current can be expressed by Ampres circuital law as

I=r

H dr . (13)

Now, solutions of Eq. (11) and boundary conditions that ade-

quately relate to the current distribution indicated in Fig. 4 must

be found. For the determination of the magnetic field strength in

tube compression or expansion, it is a common simplification to

neglect the workpiece movement (Psyk et al., 2011). As the dis-

placements inMPWaregenerally lowin comparison tosheetmetalforming tasks in EMF, thesame simplification is made here as well.

Statements given in the following explicitly refer to Fig. 4, whereH

g =Bg/ applies with0 = 4107 N/A2 in air.The magnetic field strength Hg in the small gap gbetween the

flyer and the tool coil is assumed spatially constant as the flyer

remains in close proximity to the coil until the impact. Regarding

the flyer plate, a one-dimensional field with an exponential decay

from Hg to Hgexps/F

at the side facing the target is already

implicated in Eq. (5). In the flyer, only the proximity effect plays a

rolesinceonlythe inducededdy current Ip emergesthere(no forced

current as in the coil). This differs from the two-dimensional dis-

tributionin thecurrent-carryingcoil, where thetotal current Imay

be split abstractly as follows. On the surface close to the flyer, the

proximityeffect, as an antimirror-image ofIp, aswell as IS, which is

caused by the skineffect, is present. For the bottom side of the coil,

it is supposed that only the skin effect and thus ISremainsbecause

the flyer is too far away to have an influence on the magnetic field

HS there. The magnetic field and the current density in the inner

area of thecoil areassumed to be negligibly small. Locally employ-

ing Ampres law at the bottom of the coil may then simply result

in

IS= |

HS|

w

2

. (14)

Accordingly, the transition from the topto thebottom along the

outer vertical surface of thecoil (heighth) may be expressedby the

residual current Ih as

Ih=h0

Hhdy . (15)

The function Hh is discussed in more detail later in this sec-

tion. Applying Ampres law globally around both the coil and the

corresponding flyer segment, such that Ipcancels out, yields

2 (2IS + Ih) = Hgexp s

Fw + |HS|w + 2

h

0Hhdy , (16)

which, after inserting Eqs. (14) and (15), eventually leads to

|HS| = Hgexp sF

. (17)

For thetwo-dimensional magneticfielddistributionH= [HxHy]in the coil as described above, the following functions, that satisfy

Eq. (11), are proposed here:

Hx(y, t)= Hx1 exp(kTy)+Hx2 exp (kTy) with Hx(0, t)= Hg,Hx(h, t) = HS (18)

Hy (x, y, t) = Hy0 sinhkTc1x

exp

kTc2y

with

1

c21+ 1

c22= 1.

(19)

With a time-dependent function Hh0 and the constant c2, the

real part ofHy at the vertical coil surface (x=w/2) may be written

as

ReHyw

2,y, t

= Hh0exp

yTc2

cos

yTc2

0 for 0 y h (20)

Concerning boundaryconditions, it is assumed that the vertical

magnetic field strengths at the edges of the coil (points P1 and P2)

are specified by:

Re

Hy

w

2, 0, t

= Hg Hh0= Hg , (21)

Re

Hy

w

2,h,t

= |HS| . (22)

As Eq. (22) hasno closed-form solution when solving for c2, the

function Hh with constant c3 is taken to roughly approximate the

regarded real part in the form

Hh= Hgexp yTc3

Re

Hy

w

2,y,t

for 0 y h . (23)

Taking this into account in Eq. (22) ultimately results in

Hh

=Hgexp

sy

Fh . (24)

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It can be shown that this simplification in the end leads to

slightly higher values ofHg compared to using a numerical solu-

tion for c2 in Eq. (22). In this way, Eq. (23) indirectly even takes

account of edge-effects (current concentration at sharp edges of a

conductor). Again utilizing Ampres law, but with an integration

path just around the coil now gives

I= Hgw+ Hgexp sFw + 2h

0

Hhdy , (25)

which, after solving the integral and rearranging, can be rewritten

as

Hg= Iw

1+ exp

s/F

+ 2Fh

1 exp

s/F

/s

. (26)

Eq. (26) is put into Eq. (5) to complete the model providing

p =0I

2 1 exp

2s/F

2w 1+ exps/F+ 2Fh 1 exps/F /s

2 (27)

for the transient pressures based on a measured or calculated coil

current I if its decaying time course is interpreted as a sequence

of harmonic half-waves. Thecharacteristics of the proposed model

are plotted exemplarily in Fig. 5 for an arbitrary point in time.

It can be seen that the pressure theoretically increases till infin-

ity for an infinitely large current and that it decreases to zero for a

verywidecoil. Themostconspicuouspoint, though, is theexistence

ofa high-frequencylimit,which isphf =p (f) =0.5 (I/w)2 andequivalent to the hypothesis that the current entirely flows on the

coil surface near the flyer plate. Naturally, the pressure becomes

zero for a frequency of zero. The physical explanation for that is

given by the fact that eddy currents are only induced in the flyer

in case of a temporally varying magnetic field. A higher flyer con-

ductivitymathematically equals a higher frequency with regard tomagnetic pressure since both parameters similarly affectp via the

skin depth of the flyer. It is further noted that the assumed mag-

netic field distribution is only valid for flyers situated close to the

pressure lead of the coil. This assertion will be further discussed

later.

Fig. 5. Exemplary analytically calculated pressures for rectangular one-turn coils

and a flyer conductivity of 30.16MS/m according to Eq.(27).

3. Experimental procedure

TheMPWexperiments conducted withinthe scope of this workcan be divided into two major parts: velocity measurements (part

I) and welding experiments (part II). Firstly, data obtained from

part I was used to verify the analytical model introduced above

and, secondly, to identify suitable parametersettings for theactual

welding part. Every experiment was repeated three times for rea-

sons of statistical certainty. The basic setup of both experimental

partsis schematically shown inFig.6. Theproposedmodel assumes

a two-dimensional field distribution in the tool coil, which theo-

retically implies an infinite coil lengthalong the axis of the current

flow. That is why relatively long coils (effective length300mm)were used for the experimental part of the work reported here.

In part I, a flyer plate was accelerated over a distance of

5 mm by a one-turn coil without a real target, but with a hole

drilled into the opposing clamping fixture to allow for recordingthe transient flyer velocity at the central point between the two

5 mm-spacers by means of a Photon Doppler Velocimetry (PDV)

system, which is addressed later in this section. On the basis of

a copper-chrome-zirconiumcoil designed by PoyntingGmbH(coil

type:F-VWB-300-10), twodifferentpressureleadgeometries were

Table 1

Experimental design of part I.

Velocitymeasurements (PDV, 5mm travel)

Each experiment: 3 repetitions Approx. frequency

CMaxwell =504F: 20kHz CSMU= 80F:55 kHz CSMU= 40F: 65kHz

Charging

energy

3.25kJ ,

4.09kJ

5.75kJ

6.68kJ

8.25kJ

Experiments were conducted for both coils (RE, TR) with 1mm thick EN AW 5005A flyers

Energy variation: , frequency variation (with Ia

207kA):

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Fig. 6. Schematic of experimental setup for part I and II ofMPW experiments.

Table 2

Experimental design of part II.

Weldingexperiments

Each experiment:

3 repetitions

Standoffdistance

1mm 2mm

Charging

energy

5.75kJ , ,

8.25kJ , ,

9 kJ

TRcoil onSMUcapacitor bank at approx. 55kHz

Target: EN AW 6060 hollow profile, flyer: 1mm thick EN AW 5005A sheet

Withmandrel in profile: , withoutmandrel:

tested (seeFig.6). Strictlyspeaking,the cross-sectionof thetoolcoil

with the chamfers was a hexagon consisting of a trapezoid and an

adjacentrectangle. To emphasize thegeometryof thepressure lead

in close proximity to the flyer, this coil is called trapezoid (TR) in

Fig.6 andhereafter; thecompletelyrectangular one iscalled REcoil

from now on. In part II, flyer plates were accelerated onto a rect-angular hollow profile to create magnetic pulse welds in the form

of a lap joint havinga predetermined standoff distance,whichwas

ensured by two insulating spacers. In some cases a massive steel

mandrel wasput into the hollow profile to prevent deformation of

the profile upon flyer plate impact. In both experimental parts the

horizontal distance between the two insulating spacers amounted

to50 mm. Theparameters variedarelisted in theensuingtables for

each part.

The experimental design of part I is summarized in Table 1 and

may be further divided into the two subparts frequency variation

and energy variation. When changing the discharge frequency of

the circuit, it is useful to keep the peak current Iaconstant in order

toretaincomparability. Therefore,dependingon thecapacitorbank

configuration, various charging energies needed to be employed

Fig.7. Schematicof thePDVsystemusedfor experimentalpartI (Lueg-Althoff et al.,

2014).

(RLC analysis). Regarding the two coil types, however, the same

charging energies were applied for the frequency variation yield-

ing peak currents that were not perfectly identical but in the same

range (approx. 207 kA, seeTable 1). Two different pulsegenerators

(9kJ Poynting SMU 0612 FS and 32kJ Maxwell Magneform 7000

series)wereused tocovera frequency rangefrom20 kHztill65kHz.

The second subpart, the variation of the charging energy, comes

along with a variation of the peak current; in this case, at a rela-

tivelyconstant frequency of about 55kHz. 1mm thicksheetsmadefrom the aluminum alloy ENAW5005A were chosen asflyer mate-

rial. The rolling direction was always perpendicular to the length

of the pressure lead of the coil. Density and electrical conductiv-

ity of the flyer plates were taken to be 2.70g/cm3 and 30.16MS/m,

respectively (N.N., 2015).

The experimental design of part II is compiled in Table 2. With-

outanticipating results, it canbe seen that only onecapacitorbank

configuration (55 kHz: SMU pulse generator with a capacitance of

80F) and only the TR coil were utilized for the magnetic pulsewelding of the EN AW 5005A flyer plates onto extruded rectangu-

lar EN AW 6060 profiles with a wall thickness of 5mm. Moreover,

chargingenergies ranging from5.75kJ to9 kJas well asstandoffdis-

tances of 1mm and 2mm were deployed with the aim to provide

different impact velocities and angles.

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Fig. 8. IUL peel test setup integrated in Zwick/Roell tensile testing machine.

During every experiment, the coil current was measured with

the help of a Rogowski coil (type: PEM CWT2500B with 500kA

peak current rating in case of the SMU pulse generator and PEMCWT1500R with 300kA peak current rating in case of the Maxwell

pulse generator) placed at the terminals of the capacitor bank. As

mentioned above, a PDV system was used to record velocity-time

graphs. Such an optical measurement systemis illustrated in Fig. 7.

It is based onthe idea thata laser beam of a known initial wave

length 0 (1550 nm here) is reflected from a moving workpiece the flyer plate with a Doppler-shifted frequency so that a com-

binedtime-dependent beatfrequency, which is proportional to the

wanted workpiece velocity, can be detected (see Fig. 7). The func-

tioning of a PDV system is treated in more detail by Strand et al.(2004). Besides theRIO Grande LaserModulewithanoutputpower

of 1W used here, theotherPDV componentsconformedwiththose

described in Daehn et al. (2008). A LeCroy Waverunner 104MXi

oscilloscope having a maximum sampling rate of 10GS/s ensured

the recording of both the PDV data and the coil currents. Further

dataprocessingwas performedwiththeproprietarysoftwareMAT-

LAB.

Fig. 9. Analytical and measured flyer velocities at certain displacements for varied energies: (a)for therectangular coil, (b) forthe trapezoidal coil.

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For the purpose of assessing the static strength of sheet-to-

profile welds (and similar joints), a device for peel tests was

developed and integrated in a Zwick/Roell Z250 tensile testing

machine. With this test rig, which is pictured in Fig. 8, peel tests

can be conducted under different angles.

Theprincipal structure largely resembledthatof a conventional

tensile test, except that the upper clamping assembly (holds pro-

file) additionally featured one degree of freedom in the horizontal

direction, accomplished by a runner on a rail. The runner was con-

nectedto the lower clampingassembly(holdsbent sheet)by a wire

guidedovera pulley so that the vertical force lines of the lower and

upper clamping assembly steadily coincided when peeling off the

sheet fromthe profile. A plotof test force FPeelversus displacement

was made during every experiment.

4. Verification of the analyticalmodel

In case of MPW, the velocity versus displacement graph v(D) of

a flyer is an importanttoolwith regardto determiningstandoffdis-

tances and machine parameters for specific lap joints. Such graphs

constitute the focus of the verification of the analytical approach

developedin Section2. Fortoolcoilshavingageometryonlyslightly

differing from that of a rectangular one (e.g., TR coil in Fig. 6), Eq.

(27) may be modified as

p =0I

21 exp

2s/F

2w +w exp

s/F

+ 2Fh

1 exp

s/F

/s2 (28)

where w is still the width of the surface parallel and close to the

flyer plate(3mm;see Fig.6) whereaswis the widthof theoppositesurface (6mm, also see Fig. 6). In cross-section, h now representsthe lengthof the open polygonal path along the lines connectingw

andw(h = 15.6mm incaseof the TRcoil).Analyticalvelocities (Eq.(9))and displacements(Eq. (10)) generatedfrompressures accord-

ing to Eqs. (27) or (28) were based on measured coil currents in

this work. In the analytical model, dv/dt0 applies, while there isnaturally also a deceleration phase in reality. It is therefore useful

to compare the analytics with experimental results until or at thedisplacement where the measured flyer velocity vm achieved its

maximum, D(vm,max). Such comparisons areshown in Fig. 9a and b

in terms of varying the charging energy for a given capacitor bank

configuration and, thus, a constant discharge frequency (approx.

55kHz).

Maximum velocities ranged from less than 200m/s at a charg-

ing energyof 3.25kJ to 420m/s at 8kJ. The TR coil provided higher

velocities than the RE coil and these higher velocities already

occured at shorter distances compared to the RE coil, which can

be traced back to higher magnetic pressures due to the smaller

pressure lead. Calculated velocities at D(vm,max) were in accept-

able agreement with measured ones for both coil geometries. The

average deviation between model and experiment amounted to

9% for the variation of charging energy, the largest deviation was20% at 8kJ and a comparatively largeflyer displacementof 3.4mm.

In case of the TR coil, the charging energies corresponded to peak

currents ranging from 200kA at 4.8kJ to 303kA at 8kJ. A larger

cross-sectional area comes along with a lower resistance, which

is why higher peak currents were recorded when using the RE coil

(between214kA at3.25kJ and 338kAat8 kJ).Maximum measured

flyervelocitiesandassociatedanalyticalonesresulting fromchang-

ing the frequency while keeping the current amplitude almost

constant (approximately 207 kA) are depicted in Fig. 10a and b for

both coils used.

Here, the deviationbetween model and experiment also varied

from 0% to not more than 20%, again with an average deviation of

about 9%. It is noticeable that most of the measured and calculated

velocities lay in the same area (ca. 200m/s), only the correspond-

ingdisplacements partlydiffered to a greater extent. So, fora given

peak current and impact velocity, the discharge frequency might

serve as a parameter to adjust the desired standoff distance. Fast

capacitor banks can improve the process efficiency because higher

frequencies allow for achieving the same flyer velocity as with

slowercapacitorsbutwithless energy input(seeFig. 10). Certainly,

this isnota general statementdue to the fact that all circuit param-

eters are of interest when choosing the charging energy. In case of

the 65kHz experiments, for example, a lower capacitance facili-

tated the frequency increase, but, at the same time, an inductance

slightlyhigher than that forthe 55kHz experiments ledto the need

of a higher charging energy to reach the same peak current. The

latter two figures just refer to a specific flyer displacement. Rep-

resentative curves of velocity versus time and displacement are

displayed in Fig. 11a and b for the REcoilas well as in Fig. 12a and

b for the TR coil.

Despite a small difference in the time domain in Fig. 11a, the

related velocity-displacement curve in Fig. 11b shows how accu-

rate the model represented the experiment until the deceleration

phase of the flyer began. The same basically applies to the graphs

in Fig. 12a and b except that there was no differencebetween both

graphs at the beginningof theflyer acceleration, plus a slight over-

estimation at the maximum velocity wasobserved.

Theanalytical model providedsatisfying results until the actualflyer displacement D(vm,max) was reached. As can be seen in

Figs. 11 and 12, the model might also still be helpful in the

early decelerationphase just after vm,max. Moreover, velocity mea-

surements are always necessary to detect D(vm,max). An adequate

domain of definitionshall bedefined independentof specificveloc-

ity measurements. The magnetic field distribution as described in

Section 2 implies that the workpiece movement is ignored and

without specifying a distance that the flyer is located near the

pressure lead of the coil. A characteristic distance Dch between

tool coil and flyer plate, which can be set as maximum admissi-

ble flyer displacement in the analytical model, can be obtained by

considering the total magnetic energy EL :

EL= 12LI2 = 0

2

| H|2dV. (29)For long coils, EL may be assumed to be completely stored in

the gap between the coil and the flyer. To simplify matters, the

high-frequency limitwithHg= I/w is used so thatELcan be written

as

EL=1

2LI 0

2

H2gdV=

02 H2gwaDch=

0aDch2w

I2 (30)

wherea is the lengthof the pressure lead in the directionof current

flow (295mm here). Neglecting the resistance in Eq. (2), the total

inductance L can be expressed in the form

L 1

42f2C . (31)

After substituting Eq. (31) into Eq. (30), solving forDch yields

Dch= w

4a02f2C . (32)

Dch represents a criterion for the maximum flyer displacement at

which the analytical field distribution remains valid. For a given

current, and under the assumptions made above, a higher value of

Dch would lead to a magnetic energy larger than the initial charg-

ingenergy. Nevertheless, theactual maximumflyervelocityvm,maxmightoccurbefore, at,or afterDch. Measuredvelocitiesvm(Dch)and

analytically calculated ones v(Dch) for both the variation of energy

(or peak current Ia) and frequency are collected in Fig. 13a and b

for the respective coil geometry.

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Fig. 10. Analytical andmeasured flyer velocities at certain displacements forvaried frequencies:(a) forthe RE coil, (b) forthe TR coil.

Fig. 11. Example comparison between the analytical model and an experiment performed on the Maxwell pulse generator with the rectangular coil: (a) velocity-time and

current-time graph, (b) velocity-displacement graph.

Fig. 12. Example comparison between the analytical model and an experiment performed on the SMU pulse generator with the trapezoidal coil: (a) velocity-time and

current-time graph, b) velocity-displacement graph.

Thevelocities vm(Dch) usually lay inthesamerangeas the actual

maximum velocities vm,max(compare with Figs. 9 and 10. What is

more, the analytical velocities at Dch were mostly in good agree-

ment with the corresponding experimental velocities (see Fig. 13).

Deviations between them now ranged from perfect agreement to

55%. The largest deviations, though, were outliers in that they per-

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Fig. 13. Experimental and analytically calculated velocities at distanceDch: (a) trapezoidal coil, (b) rectangular coil.

tained to displacements of almost 4mm, which equals four times

the initial thickness of theflyer plate. At such large displacements,

the occurrence of considerable tensional membrane forces make

the assumption of a rigid-body motion (see Eq. (9)) seem inadmis-

sible. Consequently, the model clearly overestimated velocities at

large displacements(several timestheflyer thickness) where plas-

tic work became significant, meaning that the flyer was already in

the decelerationphase fora long time. Another reasonfor the inap-

plicabilityof themodelathighdisplacements is thatthemodel (Eqs.

(27) and(28), respectively)as wellas thecriteriongiventhroughEq.

(32)presumea spatiallyconstant magneticfield in thegapbetween

the flyer plate and the tool coil (see Fig. 4). This simplification con-

notes that the size of the gap and thus the flyer movement do not

influence the magnitude of magnetic pressure. If the gap becomes

too large in reality, though, the electromagnetic coupling and, as a

consequence thereof, the magnetic pressure diminish so that the

field distribution illustrated in Fig. 4 ultimately becomes invalid

at high displacements. It is therefore noted that both the analytical

modelaswell asthe displacementcriterionareonlyfeasibleas long

as a good coupling is ensured. Since maximum standoff distances

are typically only of the order of a very few millimeters or less in

MPW, theformulas introduced herecansupport theprocessdesign

of lap joints without the need of costly velocity measurements.

5. Evaluation ofweld quality

As the previous section showed, theanalytical model proposed

in this article couldbe used to approximate the impactvelocity vim

Fig.14. MPW strengthevaluation forpeel tests:maximum testforceversus impact velocity fordifferent standoffdistances(withand without putting a massive steel mandrel

in theEN AW 6060 hollowprofileduring magnetic pulse welding).

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Fig. 15. Exemplary photograph and micrograph of MPW sheet-to-profile lap joint.

(first impact onto the profile in this case) for the welding experi-

ments mentionedin Section3 (experimentalpartII). Themaximum

test force Fmax recorded during the peel test also explained in Sec-

tion 3 was taken as a representative value for the weld strength.

Expressing the strength in terms of stress would not be feasible

here due to the fact that the area truly welded was not accessible

nondestructively. Hence,adiagramwhereFmax isplottedversus vimfor various experimental configurations is shown in Fig. 14. Even

though the impact angles were not known here (not reliably mea-

surable in situ), it canbe statedthat higher standoff distanceswere

accompanied by higher impact angles.

Data points lying around Fmax =0 symbolize that no weld was

achieved in those cases. From vim =400m/s on, it seems that the

maximum load the joint was able to bear was reached (approx.

3kN), independent of impact angle (or standoff distance) and

impact velocity. Certainly, this maximum force indicated a min-

imum peel strength of the joint because all welded specimens

failed in the base metal of the flyer plate near the weld seam while

the weld seam itself remained free of failure (see photograph in

Fig. 14). It can also be seen from Fig. 14 that the force FUTS, whichcorresponds to the ultimate tensile strength of the flyer material,

was a little higher than Fmax of the joints. This observation may

be explained by the stress state of a flyer segment in the region

wherefailure occured duringpeeling (see sketch in Fig. 14): On the

one hand, the test load FPeel acted as a tensional membrane force

within the flyer. On the other hand, the test force also caused a

bending moment in a flyer cross-section close to the weld seam.

This moment was characterized by tensional stresses near the tar-

get (profile here) and compressive stresses near theopposing flyer

surface. Consequently, the superposition of tensional stresses gen-

erated by FPeel and its associated bending moment led to a lower

maximum test force than in pure tension. Furthermore, it can be

concludedfrom Fig. 14that the usageof a mandrel topreventdefor-

mationof theprofile didnot affectthe weld strength. Naturally, the

experimentalsetup is less complex andmoreflexible ifa mandrel is

not required. When no mandrel was used, the deflection resulting

from theflyer impactreduced the innerheight of thehollow profile

from 50mm to approximately 49mm (1/5 of the wall thickness).

With mandrel, no deflection of the profile could be detected. Yet,

Psyk et al. (2014) showed that the target deformation can signifi-

cantlyinfluence thejointquality if the flyer and thetarget aremore

similar in thickness than in the present study.

Finally, in Fig. 15, a welded specimen, representative of thesuc-

cessful welding experiments, was regarded on the macro as well

as on the micro scale to further evaluate the quality of the MPW

joints. In the etched microsection, it can be seen that there were

twosmall symmetric regions, where the flyer wasactually welded

to theprofile (bigger grains in the profiledue to extrusionprocess),

while noweld could be created in the center. Since a small fraction

of the flyer surface was parallel to the targetsurfaceat theveryfirst

impact, the impactanglewas too low for the formation of a weld in

this central region. Within theweldedregion, however, the typical

wavy interface could be observed. Microscopically, neither inter-layers nor local melt zones are visible (see Fig. 15). Raoelison et al.

(2013) found waves of about the same amplitude (ca. 20m) intubular MPWjoints of aluminum alloy 6060 and claimed that such

continuous interfacial waves without voids imply a good and per-

manentbonding, whichagainendorses thepeel test results shown

in Fig. 14.

6. Conclusions

Forthemagneticpulsewelding(MPW)offlat sheetsusinga one-

turn coil, the following conclusions can be drawn from the work

presented:

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A simplified analytical model that allows to compute the mag-neticpressureaswellas thevelocity-timeanddisplacement-time

history of the flyer plate until the first impact onto the target

was introduced. It takes into account the geometry, the current

amplitude, and the discharge frequency. The model was verified experimentally by utilizing Photon

Doppler Velocimetry (PDV) to record the transient flyer veloci-

ties for various charging energies (3.258.25kJ) and frequencies

(approx. 20kHz to approx. 70kHz). Average deviations between

the model and the experiments amounted to 9%. Further insight into the impact welding process was gained with

the help of the modelbyshowing that an impactvelocity ofabout

400m/s isnecessary forthemagneticpulseweldingof 1mmthick

EN AW 5005A sheet onto an EN AW 6060 hollow profile. Etchedmicrosections made clear that a wavy interfacemorphol-

ogy is present in the welded regions in which no interlayers,

voids, or melt zones could be found.

Acknowledgements

This paper is based on investigations of the Collaborative

Research Center SFB/TR 10, subproject A10 Joining by forming,

which is kindly supported by the German Research Foundation

(DFG). The peel test used for this work has been developed within

the scope of subproject A1 of thepriority program SPP1640 (join-

ingby plastic deformation) also funded by theDFG.

References

Aizawa, T., 2003. Magnetic pressure seam welding methodfor aluminium sheets.Weld. Int. 17 (12), 929933, http://dx.doi.org/10.1533/wint.2003.3199.

Ben-Artzy, A., Stern,A., Frage, N., Shribman,V., Sadot, O., 2010. Wave formationmechanism in magnetic pulse welding. Int. J. Impact Eng. 37 (4), 397404,http://dx.doi.org/10.1016/j.ijimpeng.2009.07.008.

Buehler, H., Bauer,D., 1968. Ein Beitrag zur Magnetumformung rohrfrmigerWerkstcke. Werkstatt und Betrieb 110 (9), 513516.

Daehn,G.S., Zhang, Y., Golowin, S., Banik, K., Vivek, A., Johnson, J.R., Taber, G.,Fenton, G.K., Henchi, I.,Leplattenier, P., 2008. Coupling experimentandsimulation in electromagnetic forming usingPhoton Doppler Velocimetry.Proceedings of the3rd International Conference on High SpeedFormingICHSF, 3544 https://eldorado.tu-dortmund.de/handle/2003/27105.

Daehn, G.S., 2010. Energy field methods and electromagnetic sheet metal forming.In: Zhang, W. (Ed.), Intelligent Energy Field Manufacturing: InterdisciplinaryProcess Innovations.CRC Press,pp. pp. 471504, ISBN 978-1420071016.

Gies, S., Weddeling, C., Tekkaya, A.E., 2014. Proceedings of the6th InternationalConference on High SpeedFormingICHSF, 315324 https://eldorado.tu-dortmund.de/handle/2003/33476.

Goebel, G.,Kaspar, J., Herrmannsdrfer, T., Brenner, B., Beyer, E.,2010. Insights intointermetallic phases on pulsewelded dissimilar metal joints. Proceedings ofthe 4th International Conferenceon High Speed FormingICHSF,127136https://eldorado.tu-dortmund.de/handle/2003/27191.

Harvey, G.W., Brower, D.F., 1958. Metal Forming Deviceand Method. US-PatentNo.2976907.

Heaviside,O., 1951. Electromagnetic Theory. The Complete & Unabridged Edition.E. & F.N. Spon.

Jablonski, J., Winkler, R., 1978. Analysis of the electromagnetic forming process. Int.J. Mech. Sci. 20, 315325, http://dx.doi.org/10.1016/0020-7403(78) 90093-0.

Jones, N., 1989. Structural Impact, 1st ed. Cambridge University Press, Cambridge,ISBN978-0521628907.

Kamal, M., 2005. A uniform pressure electromagnetic actuator forforming flatsheets. In:Ph.D. Thesis. The Ohio State University http://rave.ohiolink.edu/etdc/view?acc num=osu1127230699.

Kapil, A., Sharma, A., 2015. Magnetic pulsewelding: an efficient andenvironmentally friendly multi-material joining technique. J. Cleaner Prod.100, 3558, http://dx.doi.org/10.1016/j.jclepro.2015.03.042.

Kore, S.D., Imbert, J., Worswick, M.J., Zhou, Y., 2009. Electromagnetic impact

welding of Mg to Al sheets. Sci. Technol. Weld. Join. 14 (6), 549553, http://dx.doi.org/10.1179/136217109X449201.

Lorentz, H.A., 1895. Versuch einer Theorie der elektrischen und optischenErscheinungen in bewegten Krpern. E. J. Brill, Leiden(in German).

Lueg-Althoff, J., Lorenz, A., Gies, S., Weddeling, C., Goebel, G., Tekkaya, A.E., Beyer,E., 2014. Magnetic pulse welding by electromagnetic compression:determination of theimpactvelocity.Adv. Mater.Res. 966967, 489499,http://dx.doi.org/10.4028/www.scientific.net/amr.966-967.489.

Maxwell, J.K., 1865. A dynamical theoryof theelectromagnetic field. Phil. Trans.R.Soc. Lond. 155, 459512.

Mori, K.,Bay, N., Fratini, L., Micari, F.,Tekkaya, A.E., 2013. Joining by plasticdeformation. CIRP Ann.-Manuf. Technol. 62 (2), 673694, http://dx.doi.org/10.1016/j.cirp.2013.05.004.

Mousavi, A.S.AAkbari, Sartangi, P.F., 2009. Experimental investigation of explosivewelding of cp-titanium/AISI 304 stainlesssteel. Mater. Des. 30 (3), 459468,http://dx.doi.org/10.1016/j.matdes.2008.06.016.

N.N., 2015. aluSELECT database. European Aluminium Association (accessed18.06.15.).

Praveen, P., Yarlagadda,P.K.D.V., 2005. Meeting challengesin welding of aluminum

alloysthrough pulse gas metal arcwelding. J. Mater. Process. Technol.164165, 11061112, http://dx.doi.org/10.1016/j.jmatprotec.2005.02.224.

Psyk, V.,Risch, D., Kinsey, B.L., Tekkaya, A.E., Kleiner, M., 2011. Electromagneticforminga review. J. Mater.Process.Technol.211 (5), 787829, http://dx.doi.org/10.1016/j.jmatprotec.2010.12.012.

Psyk, V.,Lieber, T., Kurka, P.,Drossel, W.-G.,2014. Electromagnetic joining ofhybrid tubes forhydroforming. Procedia CIRP 23, 16, http://dx.doi.org/10.1016/j.procir.2014.10.063.

Raoelison, R.N., Buiron, N., Rachik, M., Haye, D., Franz, G., Habak, M., 2013. Study oftheelaborationof a practical weldability windowin magnetic pulse welding. J.Mater. Process. Technol. 213, 13481354, http://dx.doi.org/10.1016/j.jmatprotec.2013.03.004.

Strand, O.T., Berzins,L.V., Goosman, D.R., Kuhlow, W.W., Sargis, P.D., Whitworth,T.L., 2004. Velocimetry usingheterodyne techniques. Proc. SPIE5580, 26thInternational Congress on High-Speed Photography and Photonics, http://dx.doi.org/10.1117/12.567579.

Verstraete, J., De Waele, W., Faes, K., 2011. Magnetic pulse welding: lessons to belearned from explosive welding. Sustainable Constr. Des. 2 (3), 458464, ISSN

2032-7471.Vivek, A., Hansen, S.R., Liu, B.C., Daehn, G.S., 2013. Vaporizing foil actuator: a toolforcollision welding. J. Mater. Process. Technol. 213 (12), 23042311, http://dx.doi.org/10.1016/j.jmatprotec.2013.07.006.

Weddeling, C., Hahn, M., Daehn,G.S., Tekkaya, A.E., 2014. Uniform pressureelectromagnetic actuatoran innovative tool for magnetic pulse welding.Proceedings of the International Conference on Manufactureof LightweightComponentsManuLight2014 vol. 18, 156161, http://dx.doi.org/10.1016/j.procir.2014.06.124, Procedia CIRP.

Zhang, Y., Babu, S.S., Prothe, C., Blakely, M., Kwasegroch, J., LaHa, M., Daehn,G.S.,2011. Application of high velocity impact welding at varied differentlengthscales. J. Mater. Process. Technol. 211, 944952, http://dx.doi.org/10.1016/j.jmatprotec.2010.01.001.
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