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Journal of Materials Processing Technology 213 (2013) 1095–1102 Contents lists available at SciVerse ScienceDirect Journal of Materials Processing Technology jou rnal h om epa g e: www.elsevier.com/locate/jmatprotec Application of magnetic pulse welding technique for flexible printed circuit boards (FPCB) lap joints Tomokatsu Aizawa, Keigo Okagawa, Mehrdad Kashani Tokyo Metropolitan College of Industrial Technology, 1-10-40, Higashi-Ohi, Shinagawa-ku, Tokyo 140-0011, Japan a r t i c l e i n f o Article history: Received 6 September 2012 Received in revised form 26 November 2012 Accepted 10 December 2012 Available online xxx Keywords: Magnetic pulse welding (MPW) Solid state bonding Flexible printed circuit board (FPCB) a b s t r a c t The magnetic pulse welding (MPW) is a high speed solid state welding process which has been used successfully to provide metallurgical and electrical bonds between flexible printed circuit board (FPCB) with using: (1) welding with aluminum driver sheet and (2) without driver sheet. The optimum bank energy for reliable bonding was about 1 kJ with 160–180 kA maximum current. The maximum tensile shearing for most welded samples was nearly same as tensile shearing strength of copper layer of FPCB sheet. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Modern electronic devices are required to be thin, lightweight and functionally sophisticated. Therefore, joining thin flexible printed circuit board (FPCB) in different shapes is receiving atten- tion. FPCB and flexible cables constructed from polyester or polyimide film and such films are lightweight, flexible and thin. Varying circuit shapes, dimensions, circuit arrangements and dif- ferent length cable arrangements may be constructed using FPCB. However, a disadvantage to the use of FPCB is the bonding prob- lems. Yoon et al. (2007) studied bonding characteristics of FPCB using solder method and reported high electrical and mechanical properties, while the excessive growth of intermetallic compounds at the joint interface significantly degrades the performance and reliability of the solder joint. Maruo et al. (2004) investigated adhesive-bonding methods using anisotropic conductive adhesive or non-conductive adhesive for FPCB bonding but experimental results show poor electrical property and low mechanical reliability in joint interfaces. Unfortunately, the conventional boding meth- ods for FPCB do not appear to provide an economical and reliable solution to interconnection and construction challenges. The mag- netic pulse welding (MPW) provides an excellent and high speed method for achieving FPCB lap-joint. MPW uses magnetic pres- sure to drive the primary metal against the target metal sweeping away surface contaminants while forcing intimate metal-to-metal contact, thereby producing a solid-state weld. Several technical Corresponding author. Tel.: +81 47 375 7796; fax: +81 47 375 7795. E-mail address: [email protected] (M. Kashani). research papers has been reported about MPW for example Tamaki and Kojima (1988) and Shribman et al. (2002) used conventional MPW method with solenoidal coil for joining tubular parts and investigated its feature. MPW has been theorized and tested for several decades, but equipment limits the total energy stored and this keeps weld lengths to the order of meters or less. Recently, MPW application rapidly growing in industrial application and new development make MPW method well suited for manufacture and assembly in wide range of application. Shribman and Gafri (2001) introduced MPW technique for tube to tube applications. They studied the fundamental equations of MPW process and illustrated some examples of similar and dissimilar weld applications with some interface microstructures. Uhlmann et al. (2005) also studied the applicability and the potentials of MPW for joining of aluminum and magnesium structure which is a new solutions for modern lightweight structures applications. Daehn and Lippold (2009) also proposed a new MPW device and developed it for similar or dissimi- lar thin sheet metal joints application. Recently, several works also were carried out on application of MPW technique in electronics micro-devices. For example, Kashani et al. (2008, 2009) developed a new low energy MPW system which can be used for bonding of wire to terminal plate in electronic devices or making small Copper and Manganin alloys joint as a shunt resistor for using at control circuits. The goal of this work was to introduce new low energy system with modified coil structure for using MPW tech- nique in FPCB lap-joints application which has not been reported before. The present paper examines the detail of the welding pro- cess and welds quality characteristics for FPCB lap-joints in two cases: (1) with aluminum driver sheet and (2) without aluminum driver sheet. 0924-0136/$ see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.12.004

Application of Magnetic Pulse Welding Technique for Flexible Printed Circuit

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Page 1: Application of Magnetic Pulse Welding Technique for Flexible Printed Circuit

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Journal of Materials Processing Technology 213 (2013) 1095– 1102

Contents lists available at SciVerse ScienceDirect

Journal of Materials Processing Technology

jou rna l h om epa g e: www.elsev ier .com/ locate / jmatprotec

pplication of magnetic pulse welding technique for flexible printed circuitoards (FPCB) lap joints

omokatsu Aizawa, Keigo Okagawa, Mehrdad Kashani ∗

okyo Metropolitan College of Industrial Technology, 1-10-40, Higashi-Ohi, Shinagawa-ku, Tokyo 140-0011, Japan

r t i c l e i n f o

rticle history:eceived 6 September 2012eceived in revised form6 November 2012

a b s t r a c t

The magnetic pulse welding (MPW) is a high speed solid state welding process which has been usedsuccessfully to provide metallurgical and electrical bonds between flexible printed circuit board (FPCB)with using: (1) welding with aluminum driver sheet and (2) without driver sheet. The optimum bankenergy for reliable bonding was about 1 kJ with 160–180 kA maximum current. The maximum tensile

ccepted 10 December 2012vailable online xxx

eywords:agnetic pulse welding (MPW)

olid state bonding

shearing for most welded samples was nearly same as tensile shearing strength of copper layer of FPCBsheet.

© 2012 Elsevier B.V. All rights reserved.

lexible printed circuit board (FPCB)

. Introduction

Modern electronic devices are required to be thin, lightweightnd functionally sophisticated. Therefore, joining thin flexiblerinted circuit board (FPCB) in different shapes is receiving atten-ion. FPCB and flexible cables constructed from polyester orolyimide film and such films are lightweight, flexible and thin.arying circuit shapes, dimensions, circuit arrangements and dif-

erent length cable arrangements may be constructed using FPCB.owever, a disadvantage to the use of FPCB is the bonding prob-

ems. Yoon et al. (2007) studied bonding characteristics of FPCBsing solder method and reported high electrical and mechanicalroperties, while the excessive growth of intermetallic compoundst the joint interface significantly degrades the performance andeliability of the solder joint. Maruo et al. (2004) investigateddhesive-bonding methods using anisotropic conductive adhesiver non-conductive adhesive for FPCB bonding but experimentalesults show poor electrical property and low mechanical reliabilityn joint interfaces. Unfortunately, the conventional boding meth-ds for FPCB do not appear to provide an economical and reliableolution to interconnection and construction challenges. The mag-etic pulse welding (MPW) provides an excellent and high speedethod for achieving FPCB lap-joint. MPW uses magnetic pres-

ure to drive the primary metal against the target metal sweepingway surface contaminants while forcing intimate metal-to-metalontact, thereby producing a solid-state weld. Several technical

∗ Corresponding author. Tel.: +81 47 375 7796; fax: +81 47 375 7795.E-mail address: [email protected] (M. Kashani).

924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jmatprotec.2012.12.004

research papers has been reported about MPW for example Tamakiand Kojima (1988) and Shribman et al. (2002) used conventionalMPW method with solenoidal coil for joining tubular parts andinvestigated its feature. MPW has been theorized and tested forseveral decades, but equipment limits the total energy stored andthis keeps weld lengths to the order of meters or less. Recently,MPW application rapidly growing in industrial application and newdevelopment make MPW method well suited for manufacture andassembly in wide range of application. Shribman and Gafri (2001)introduced MPW technique for tube to tube applications. Theystudied the fundamental equations of MPW process and illustratedsome examples of similar and dissimilar weld applications withsome interface microstructures. Uhlmann et al. (2005) also studiedthe applicability and the potentials of MPW for joining of aluminumand magnesium structure which is a new solutions for modernlightweight structures applications. Daehn and Lippold (2009) alsoproposed a new MPW device and developed it for similar or dissimi-lar thin sheet metal joints application. Recently, several works alsowere carried out on application of MPW technique in electronicsmicro-devices. For example, Kashani et al. (2008, 2009) developeda new low energy MPW system which can be used for bondingof wire to terminal plate in electronic devices or making smallCopper and Manganin alloys joint as a shunt resistor for using atcontrol circuits. The goal of this work was to introduce new lowenergy system with modified coil structure for using MPW tech-nique in FPCB lap-joints application which has not been reported

before. The present paper examines the detail of the welding pro-cess and welds quality characteristics for FPCB lap-joints in twocases: (1) with aluminum driver sheet and (2) without aluminumdriver sheet.
Page 2: Application of Magnetic Pulse Welding Technique for Flexible Printed Circuit

1096 T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102

2

amg(mita

p

ı

wna(icfbsbsf

b

Fig. 1. Principle of MPW technique.

. Principle of MPW

Magnetic pulse welding (MPW) uses electromagnetic force toccelerate one metal piece (base metal) against another stationaryetal piece (target sheet). When a high magnetic field �B is suddenly

enerated and penetrated into metal sheets, then the eddy currentscurrent density �i) pass through them and as a result, an electro-

agnetic force of �F = �i × �B acts mainly on the base metal sheet andt is accelerated away from the coil and collides rapidly with thearget metal sheet. The eddy current �i and the magnetic pressure pre given as following:

×�i = −�

(∂�B∂t

)(1)

= (B2o − B2

i )

2�=

(B2

o2�

)(1 − e−2x/ı) (2)

=√

2ω��

andk ω = 1√LC

(3)

here �, �, �, Bo and Bi are the electrical conductivity, mag-etic permeability, thickness, the magnetic flux density at lowernd upper surfaces of Al sheet, respectively. The depth of skin effectı) can be obtained by calculation of angular frequency (ω) and its governed by the complete MPW system’s inductivity L and itsapacity C. The skin depth becomes important parameter speciallyor thin sheet metal bonding process. When the thickness of thease sheet metal is the same as the skin depth, then magnetic pres-ure equals 86% of its maximum value and it reaches 98% when thease metal thickness is twice of the skin depth. The appropriate

kin depth and higher magnetic pressure can be adjusted by therequency of the discharge current.

At the moment of collision the colliding surfaces can be cleanedy a large kinetic energy getting before the collision. The velocities

Fig. 2. Principle of welding process in case of using Al driver sheet: (a) before weld-ing and (b) after welding.

attained during this process range from 200 m/s to 500 m/s andthe joining process completed within microsecond. Because ofthe short impact period, the extent of heating might be minimalalong the joints. Therefore, comparing to the traditional fusionwelding process, no significant heat affected zones is produced inMPW joints and it can be noticed as a main advantage (Aizawa andYoshizawa, 2001; Aizawa and Kashani, 2004).

Kakizaki et al. (2010) found that the surface oxide of the metalinterfaces is disrupted due to the jet action and metallurgical bond-ing is achieved between clean surfaces. As shown in Fig. 1, theprinciple of MPW technique can be summarized into three steps:(1) producing high magnetic field, (2) acceleration of base metal,and (3) impaction and bonding.

In the present experiment which has been carried out for FPBClap-joints application, the base and target metals are thin and lightand the skin depth is comparable with thickness of the FPBC copperlayer. Therefore, the impaction is weaker in comparing with thickmetal welding case. In this case, using Al driver sheet can increasethe electromagnetic force and magnetic pressure. The impactionof Aluminum driver sheet with FPCB layers can make a solid statebonding between FPCB sheets. Fig. 2 shows the principle of weldingprocess with using the Al driver sheet. The result of second exper-iment without using Al driver sheet also is reported in this paperfor better comparison.

3. Experimental setup

The block diagram of the discharge system is shown in Fig. 3which is consisted of a capacitor bank (C) and a spark gap switch(G) with a one layer E-shaped flat coil. For optimizing the weldquality, three capacitor banks with different energy storage capac-ity were used in present experimental setup. The capacitor banks(CB) descriptions were summarized in Table 1. The capacitor bank isconnected to the gap-switch and one-turn coil by a low inductancetransmission line.

3.1. General outlines of apparatus

Fig. 4 shows the general outlines of the magnetic pulse weldingapparatus. The coil was made by Cr–Cu alloy. The main discharge

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T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102 1097

Fig. 3. Block diagram of discharge system

Table 1Electrical characteristics of MPW capacitor bank.

CB1 CB2 CB3

Charging voltage (kV) 13.5 9.1 6.3Capacitor bank (�F) 12 (2 × 6 �F) 24 (4 × 6 �F) 50 (2 × 25 �F)

cpo05dT

Fcc

Total energy (kJ) 1.1 1.0 1.0Residual inductance (nH) 20 15 20

urrent pass through the middle part of coil and the width of thisart was designed to be 2.5 mm and length of 40 mm. The thicknessf E-shaped coil was 2 mm and its inductance was measured about.039 �H. The thickness of Al driver was 0.3 mm with size of

0 mm × 50 mm. The skin depth for a typical discharge in this Alriver sheet was calculated by Eq. (2) and it was about 0.22 mm.his value is enough to prevent of melting of FPCB and apply big

ig. 4. General outlines of apparatus: (a) plan view of coil geometry and dis-harge circuit; CB, capacitor bank; G, gap switch. (b) Cross section view of theoil-containing lap of FPCB sheets with Al driver sheet.

and appearance of discharge circuit.

electromagnetic force to Al driver sheet for rapid collision withFPCB and producing a solid state boding.

3.2. FPCB sample

A typical FPCB samples and its dimensional size which wereused in present experiment are shown in Fig. 5a. Three types ofFPCB sheet (W1, W5 and W10) which almost is used in industrialapplications was chosen in present experiment. The size of FPCBsheet is 40 mm × 40 mm and the width of copper rows are 1 mm,5 mm and 10 mm for W1, W5 and W10, respectively. The cross sec-tion view of all FPCB sheet layers are same and are shown in Fig. 5b.The total thickness of FPCB is about 110 �m and the polyimide filmlayer cover only 30 mm of copper foil and 10 mm of that is uncoated(welding zone).

4. Experimental results and discussion

4.1. Current signal and collision speed

All three different capacitor banks system (CB1, CB2 and CB3)was tested to get the optimum results. The best results wereobtained with CB2 system. The minimum joule heating effect andalso smallest deformation on FPCB sheets was observed by CB2system which had a lower total inductance and faster discharge.Typical current waveform (top signal) and also the signal of col-lision speed circuit (bottom signal) for CB2 system are shown inFig. 6. This current signal was obtained at 1.0 kJ discharge by usinga magnetic probe. The current signal shows that a damping andoscillating current flows through a one-turn coil for the duration ofabout 50 �s and the discharge frequency was about 133 kHz. Themaximum current and maximum magnetic flux density were mea-sured about 160 kA and 23 T at 1.0 kJ discharge in case of using Aldriver sheet. The maximum magnetic pressures were calculated byEq. (2) and it was about 200 MPa for the same discharge.

In order to measure the time of collision and also the collisionspeed of the aluminum driver sheet just before welding, very simplecircuit is prepared to measure the time traveling of the base metal ingap distance which is exist between two FPCB sheets before weld-ing. The detail of this circuit is reported in pervious work (Aizawaet al., 2007). The time between starting discharge and final phaseof welding was approximately about 1.7 �s and the maximum col-lision speed of Al driver sheet just before welding is calculated tobe 380 m/s.

4.2. Effect of gap between FPCB sheets before welding

The collision speed has a relation with the bank energy andthe discharge current and the maximum collision speed can be

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1098 T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102

F opperc

oicsts0icht

Ft

ig. 5. Typical FPCB samples which were used in present experiment:(a) width of cross section of FPCB sheet with fabrication material layer thickness.

btained at the first maximum in the current signal. Therefore, bynserting the appropriate gap distance between sample sheets, theollision time can be nearly same as quarter period of the currentignal at the first maximum current peak. The optimum gap dis-ance has relation with the capacitor bank energy and the dischargeystem inductance. However, our experimental result shows that.4 mm, gap distance between FPCB sheets is necessary for achiev-

ng high weld quality. On other hand, the surfaces of FPCB sheetan be melted because of high Eddy current and increasing of Jouleeating effect in case of samples without inserting gap between

hem (Fig. 7).

ig. 6. Typical current signal at 1.0 kJ discharge (top signal) and the signal of collisionime (bottom signal).

rows are 1 mm, 5 mm and 10 mm for W1, W5 and W10 sheet, respectively; and (b)

4.3. Microstructure of joined interface

Fig. 8 shows the lap-joints of three types of FPCB sheets. Thewidth of the weld zone was nearly equal to the middle part ofthe coil (b = 2.5 mm). The welded area has concentrated into twolines with 0.6 mm wideness along of middle part of coil and no heataffected area was observed in polyimide substrate. The lap-jointsof W5-FPCB and W10-FPCB sheets divided to several parts for theoptical microscope observations and also for shearing strength test.

The divided test parts were polished for observing the joinedinterface. Fig. 9 shows the optical microscope images of the weldedarea. These optical images show the welded zone was formed in twoparts with approximately 2 mm apart. The pictures show a wavytransition layer without any significant heat-affected zone or anycrack on copper layer is formed in welded zone. The scanning ionmicroscopic (SIM) image of welding interface also shows a fine-grain microstructure at interface layer.

4.4. Electrical resistances of lap-joints

The electrical resistance of FPCB lap-joint was measured by adigital micro-ohmmeter using KELVIN-TYPE (4-WIRES) method.The measurement setup was shown in Fig. 10. It is necessary toremove polyimide films and adhesive materials from welded sam-ple before resistance measurement. The micro-ohmmeter probesshould be connected near lap-joint on copper sheet. The bond-ing resistance of FPCB lap-joints was measured less than 10 ��for W5 samples and this value is enough low for micro electronicapplications.

4.5. Tensile shear test

Welded samples were investigated on a standard tensile sheartesting machine at test rate of 10 mm/min. The tensile shearingstrength of each welded samples (W1, W5 and W10) were mea-sured and compared with results of no-welded samples (Fig. 11).

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T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102 1099

Fig. 7. Typical FPCB samples (E = 1.0 kJ; I = 200 kA; driver: 0.3 mm Al sheet): (a) without gap and (b) with 0.4 mm gap.

Fig. 8. Lap-joints of W1, W5 and W10-FPCB sheets sample.

Btt

ns

4

dccafAtmutmcacdmta

Table 2Comparison of the typical results and welding process parameters for two cases: (1)with Al driver; (2) without Al driver.

With Al driver sheet Without Al driver sheet

Bank capacitance (�F) 24 (4 × 6 �F) 12 (2 × 6 �F)Bank inductance (�H) 0.018 0.021Coil inductance (�H) 0.041 0.032Total inductance (�H) 0.059 0.053Charging energy (kJ) 1.0 1.1Charging voltage (kV) 9.1 13.5Max. current (kA) 160 180Coil dimension (see Fig. 4) a = 40 mm; b = 2.5 mm;

t = 2 mma = 60 mm; b = 1 mm;t = 10 mm

Discharge currentfrequency (kHz)

133 200

Collision time (�s) 1.7 1.5Maximum collision speed

(m/s)380 330

Max. magnetic flux density(T)

23 30

Max. magnetic pressure(MPa)

200 143

Al driver thickness (mm) 0.3 –Skin depth (mm) 0.22(Al driver sheet) 0.15(Cu layer of FPCB)

ased on the shearing strength test results, it can be obtained thathe maximum tensile shearing of welded sample is little smallerhan tensile shearing strength of no-welded FPCB sheet.

The maximum tensile shearing for most welded samples wasearly same as tensile shearing strength of copper layer of FPCBheet and failure always occurred near of welded line (Fig. 12).

.6. Comparison of results for two cases

The feasibility of MPW technique without using Aluminumriver sheet was investigated and compared with result of Al driverase. In this case, the skin depth is higher than the thickness of theopper layer in FPCB sheet and it is necessary to adjust the appropri-te current discharge frequency and increase the total bank energyor obtaining enough high magnetic pressure comparing with usingl driver case. However, the maximum current should be limited

o prevent from melting of thin copper layer of FPCB sheet. Severalodifications should be considered for reliable welding without

sing Al driver sheet. The width of middle part of coil was modifiedo 1 mm to improve eddy current paths and concentration of the

agnetic pressure in small area of copper layer. The thickness ofoil also was increased to 10 mm for improving the lifetime of coilgainst mechanical shock of discharge pulse. The maximum dis-harge current and the maximum of magnetic flux density at 1.1 kJ

ischarge were measured about 180 kA and 30 T, respectively. Theaximum magnetic pressures were calculated about 143 MPa for

he same discharge. The maximum tensile shearing was obtainedbout 110–120 N for successful welded samples which is a little

Gap distance (mm) 0.4 0.4Bonding resistance (��) 10 10Average tensile strength

(N)130–140 110–120

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1100 T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102

Fig. 9. Optical microscope and SIM images of the welded area in FPCB lap-joint.

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T. Aizawa et al. / Journal of Materials Processing Technology 213 (2013) 1095– 1102 1101

Fig. 10. Bonding resistance measurement for FPCB la

Fig. 11. Tensile strength of welded and no welded samples.

Fig. 12. Typical rupture of FPCB lap-joint sample which made by using Al driversheet.

p-joint using KELVIN-TYPE (4-WIRES) method.

smaller than Al driver case. The wavy transition pattern was not soclear in optical microscope observations in this case. For a bettercomparison of results in both cases, the other typical results andwelding process parameters are summarized in Table 2.

5. Summary and conclusions

In this work, MPW process was investigated for FPCB lap-jointapplication in two cases: (1) with Al driver; (2) without Al driverand the main results are summarized a follows:

• FPCB sheet can be successfully welded in both cases and the opti-mum bank energy for reliable bonding was about 1.0–1.1 kJ.

• The welded area in FPCB lap-joint has concentrated into two lineswith 0.6 mm wideness along of middle part of coil.

• The welded zone has a good joint quality without any damage orheat affect in polyimide substrates of FPCB.

• The deformation of lap-joint is also small for welded samplesusing Al driver sheet.

• The maximum tensile shearing was obtained around 110–140 Nfor successful welded samples for both cases and it was nearlysame as tensile shearing strength of copper layer of FPCB sheet

• The gap distance between FPCB sheets before welding has veryimportant effect on welding quality and its optimum was 0.4 mm.

• The optical microscope and SIM images of the welded area alsoshow that transition layer was formed without any significantheat-affected zone (HAZ). But the wavy transition layer wasformed only for samples which were made by using Al driversheet and this wavy structure was not clear for lap-joint sampleswithout using Al driver sheet.

• Application of MPW process without using driver sheet may berestricted to a particular type of FPCB sheet because of many lim-itations for adjusting eddy current, magnetic pressure and skindepth in very thin copper layer of FPCB sheet. It can be concludethis case is more applicable for FPCB sheets with thicker copperlayer.

Acknowledgments

The authors wish to express thanks to Mr. K. Hanasaki and Mr.Y. Sugiyama of Yazaki Corporation for the observation of joinedinterfaces.

References

Aizawa, T., Kashani, M., 2004. Proc. of IIW International Conference on TechnicalTrends and Future Prospective of Welding Technology for Transportation, Land,Sea, Air and Space , Osaka, p. 378.

Aizawa, T., Yoshizawa, M., 2001. Proc. of 7th Int. Symposium of Japan WeldingSociety , pp. 295–300.

Aizawa, T., Kashani, M., Okagawa, K., 2007. American Welding Journal (AWJ) 86 (5),

119–124.

Daehn, G.S., Lippold, J.C., 2009. Low Temperature Spot Impact Welding Driven With-out Contact. US Patent PCT/US09/36299.

Kakizaki, S., Watanabe, M., Kumai, S., 2010. Proceedings of the 12th InternationalConference of Aluminum Alloys (ICAA12) , pp. 945–949.

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hribman, V., Stern, A., Livshitz, Y., Gafri, O., 2002. Magnetic pulse welding produceshigh-strength aluminum welds. Welding Journal, 33–37.

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