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Xin Wu1

Mem. ASME

Mechanical Engineering,

Wayne State University,

Detroit, MI 48202

e-mail: [email protected]

Jianhui Shang2

Mem. ASME

Hirotec America,

Auburn Hills, MI 48326

e-mail: [email protected]

An Investigation of MagneticPulse Welding of Al/Cuand Interface Characterization

This paper investigated the effect of magnetic pulse welding (MPW) condition (weldingpower, surface scratches, and contamination) on the establishment of welding betweenaluminum and copper tubes, and the associated welding mechanisms. The results showedthat higher applied power and surface scratches in tangential direction were in favor forgood weld, and oil on the surface prevented welding. Direct evidences were obtained onlocal interface melting under a high welding power. CuAl intermetallics with differentatomic ratios were identified by energy dispersion spectrum (EDS) chemical analysis andby microscratching test. The mechanisms of MPW and the process improvement werediscussed. [DOI: 10.1115/1.4027917]

Keywords: magnetic pulse welding, electromagnetic welding, aluminumcopper welding,surface contamination, microstructure characterization

Introduction

A high power magnetic pulse (MP) generated from electromag-netic coils can be used for material processing, such as MPcutting, forming, and welding. The magnetic pulse welding(MPW), also called electromagnetic welding, applies a high pres-sure pulse force on the mating interface and achieve a strongbonding without physical contact between tool (here electromag-netic coil) and workpiece, thus bring various advantages. It is gen-erally recognized that the MPW allows welding dissimilarmaterials, including thermodynamically incompatible materialswhich otherwise is required for fusion welding. It prevents the useof a consumable electrode, and it eliminates heat-affected zoneand improve welding quality and processing repeatability. In addi-tion, it is also considered as an environmental-friendly materialprocess since no postcleaning or finishing is needed as often seenin many other welding processes. In recent years, for energy sav-ing and environment protection in automotive manufacturing,there is a high demand to weld dissimilar materials for lightweightstructures, and to weld electrical power components made of cop-per and aluminum (e.g., power cables, connectors and terminals)in electrical vehicles that is difficult to be welded by fusion weld-ing, due to very high electrical and thermal conductivities. Forthese reasons this welding technique has been recognized as a via-ble approach to join same or dissimilar materials and attract newtheoretical and practical interests [1].

The initial work on electromagnetic force generation techniquesand its application in metal forming and welding were developedin the former Soviet Union, see a comprehensive handbook byBelyy et al. [2]. There are some earlier work published on weldingsystem development, including the MPW study in vacuum withpreheating by Strizhakov [3], the MPW with arc heating by

Yablochnikov [4], the design of apparatus and inductors byYablochnikov [5,6] for welding large diameter thin-walled pipes.In recent years the study on MP forming and welding of light-weight materials has received increased interest, and a series ofpapers were published by Daehns group at Ohio State University,among which Tamhane et al. [7] studied on sample size effect onthe ductility of a ring expansion process, Daehn et al. [8,9]

reported the improved formability in electromagnetic sheet form-

ing, Thomas et al. [10] reported the forming limit testing and anal-ysis on aluminum tube expansion, Fenton and Daehn [11]performed a modeling work on electromagnetic sheet forming,Golowin et al. [12] developed a new type of electromagnetic actu-ator with uniform pressure, and Zhang et al. [13,14] studied on themicrostructures at the vicinity of the welded AlCu interfaceusing electron backscattering diffraction method (EBSD) andshowed that severely deformed and recrystallized fine-grainedmaterials produced near the welded interface.

In terms of the workpiece materials to be joined and the weld-ing process conditions, MPW of Al to Al was studied by Shrib-man et al. [15], who reported that the strength of the weldsreached that of the base metal (A7075-T6). The effect of processparameters on welding aluminum sheets was reported by Koreet al. [16], who showed that for a given discharge energy the shear

strength of the welds reached the maximum value at an optimumcoil standoff distance, and the geometry of the coil also has im-portant effect on the product strength. On welding Al to Cu,Marya et al. [17] and Marya and Marya [18] studied the micro-structures and temperatures at the aluminumcopper interface,and the results showed that a hard copper rich intermetallic phasewith the same composition as the equilibrium c-Cu2Al wasformed. These, along with the observed interfacial voids, wereused as the evidences of Al melting, and a simple analytical modelwas used for estimating the interfacial temperatures that providedfurther support for interface melting. Aluminum to steel weldingwas reported by Aizawa et al. [19], who studied the weldingparameters for several aluminum alloys (A1050, A2017, A3004,A5182, A5052, A6016, and A7075) to weld with a steel. Forwelding Al to Mg, Ben-Artzy et al. [20] reported the formation of

intermetallic phases with different compositions, and suggestedthat a rapid solidification occurred within a thin melted layer atthe interface, and an energy balance analysis was provided to indi-cate that there was enough energy to melt a thin interfacial layer.For MPW of Ti to Al, Marya and Gerard [21] provided a brief dis-cussed on Ti welding in this overview paper. Many efforts havebeen made on modeling MPW process, see a review article byEl-Azab et al. [22]. Although MP force is more readily to beapplied on conductive metals that allow induced current to gener-ate repulsive magnetic field and force to the applied one from thecoil, the MP force can also be used for consolidating powders andjoining nonmetals, nonconductive materials indirectly throughconductive capitulation, such as that in powder consolidation[23,24]. Another material joining technique with great similarity

1Corresponding author.2Present address: Edison Welding Institute, Columbus, OH 43221.Contributed by the Manufacturing Engineering Division of ASME for publication

in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript receivedFebruary 8, 2013; final manuscript received June 24, 2014; published online August6, 2014. Assoc. Editor: Wei Li.

Journal of Manufacturing Science and Engineering OCTOBER 2014, Vol. 136 / 051002-1CopyrightVC 2014 by ASME

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to the MPW is explosive welding, and the research results in thisarea are also helpful in understanding the MPW process, but willnot be reviewed here. In the above mentioned studies, althoughthe processing conditions of MPW have been studied extensively,the surface topological feature and contamination on the weldingprocess has not been addressed, which should also affect the weld-ing strength.

Significant efforts have been made to understand the mecha-nisms responsible for material joining at a high speed impact, butdue to the highly nonequilibrium condition of the process the pre-

existing knowledge in material process and behaviors, thermody-namics and kinetics under an equilibrium or quasi-equilibriumconditions may not be able to fully explain the current complexphenomena, and controversy speculations on observation interpre-tations often exist. While MPW has been commonly viewed bymajority of the papers as a solid-state welding process, which ismainly based on observations that no heat affected zone exists,several relatively recent papers used more advanced techniqueson analyzing interface microstructure and chemistry, and on ther-mal analysis that suggested the possibility of interface melting,see previously mentioned works [17,18,20]. In addition, Stern andAizenshtein [25] reported the possibility of melting and solidifica-tion near joint interface based on the observation that along thebonding interface either a discontinuous pocket string or a contin-uous transition layer were formed, which was explained either dueto grain refinement from local melting followed by rapid solidifi-

cation, or due to the formation of intermetallic phases (for dissimi-lar materials). On the other hand, the work by Zhang et al. [13] onEBSD study of aluminum MPW showed different findings of arecrystallized/refined microstructure from heavily deformedgrains with high grain boundary angles, implying that the micro-structure evolution was still within the solid state phase and themelting point was not reached. It is notable that both melting/rapid solidification and solid-state deformation/recrystallizationcan produce a fine-grained microstructure with low dislocationdensity and with high-angle grain boundaries; despite the two aretotally different processes at different temperature ranges. How-ever, if a heavily deformed grain shape and high dislocation den-sity were observed, the process must be in solid state, and neitherrecrystallization nor melting would occur, otherwise the deforma-tion microstructure would disappear. It is commonly accepted

[26] that recrystallization occurs after certain critical degree ofcold work followed by heating to 1/32/3 absolute melting tem-peratureTm, and for a pure metal the recrystallization temperaturewould be lower, normally 0.4 Tm. It is not so clear weather it ispossible that under a very high energy input rate and strain rate,within very short time period, a deformed system can remain itsheavily deformed state without recrystallization, or a much highersuperheating temperature is need for melting to occur. Thus, it isinteresting to further investigate the welding conditions and theinvolved three debatable welding processes: (1) melting followedby solidification/crystallization, (2) deformation followed byrecrystallization (without melting), (3) there is no sufficient heatand time of heating for recrystallization or even recovery, so thatthe stored energy from electromagnetic deformation will not bereleased, especially when the time is very short. Another interest-

ing issue in MPW is the formation of wavy interface, which hasbeen widely observed, and commonly considered as a result ofshear instability similar to that observed in fluid dynamics, seeNassiri et al. [27] and Ben-Artzy et al. [28], but the effect of origi-nal surface morphology on the interface waviness and bondingstrength is not reported.

In this paper, MPW of Cu/Al tube is further studied with thefocus on the effect of initial surface conditions (surface topologyand contamination) on the joint establishment, which is an issuenot being previously reported. The welded interfaces were charac-terized, and the new phase formation was identified and analyzed,and more importantly, some direct evidences on interface meltingwere obtained. The mechanisms of PMW and the means of pro-cess improvement are discussed.

Experimental Condition

Electromagnetic Welding Setup, Specimens, and WeldingConditions.A MP generator used for this study is made byHirotec America (Model Pulsar MP-30I9 Research Edition). Itconsists of a capacitor bank and a high voltage cabinet for charg-ing the capacitors, capable of generating 30 kJ at a charging volt-age of 9 kV. The MP coil system is shown in Figs. 1(a)1(d),which consists of a 5-turn coil in connection with the MP genera-tor and a magnetic field concentrator to intensify the magneticforce applied onto the aluminum tube wall.

The as-received tubes of aluminum alloy AA6063-O and purecopper C110 (abbreviated as Al and Cu thereafter) were machinedby lathe turning on both inner and outer surfaces over the jointlengths. While the Al tube has uniform cross section over lengthof 1.5 mm wall thickness (see Fig.1(e)), the Cu tube has differentwall thicknesses in its end region, following an earlier design byShang [29] for supporting the squeezing force and for axial align-ment, resulting in three AlCu weld-zones, see Fig. 1(f). Beforeand during welding both Al and Cu tubes were firmly clampedfrom outside of the welding zone, but inside the welding zone thetwo tube ends were free from constraints in radial and axialdirections.

Three initial copper tube surface conditions were prepared forinvestigating their effects on welding

Surface condition-A: The as-machined tube surface, pro-duced by lathe turning with the finishing surface containingtangential scratches over its length. See Figs.2(a)and2(b).

Surface condition-B: After the lathe turning (condition-A)additional manual sanding was performed on the Cu outersurfaces with 200-grit coarse sand papers along the axialdirection, which replaced the original tangential lathescratches (Fig.2(b)) with a new set of axial scratches on theCu outer surface (Fig.2(c)), in order to investigate the surfacetopology effect.

Surface condition-C: After the lather turning (condition-A) asilicon-based high-viscosity lubricant oil was applied overthe welding zone surfaces, to produce an artificially contami-nated interface (exaggerated) before welding, in order toinvestigate if the high impact force can break down the con-tamination layer to establish weld.

Three applied voltages of 4.3 kV, 5.2 kV, and 6.0 kV were usedfor charging the battery bank, which is proportional to the energyand power discharged to the coil and is indirectly related to thewelding MP force when other settings are identical. The coil cur-rent and output energy were measured during welding. The actualenergy delivered to the workpieces can be much reduced due tothe loss of magnetic field in the coil-workpiece coupling, and theJoule heat in the coil loop. The energy received by the workpieceis dissipated mainly in the forms of plastic deformation, theinduced current within the workpiece, and the frictional work atthe Cu/Al interface, all of which contribute the heating ofworkpiece.

Welding Quality Evaluation. The welding quality was eval-

uated by peeling test and by interface microstructure examination.For the peeling test two parallel cuts in 5-mm spacing were madeon the Al tube wall along the axial direction. A plier was used toclamp the Al cut strip from its chopped-off end, and manuallytwisted/rolled along the welded strip top. The welding quality wasqualitatively categorized as welded if the peeled surfacesshowed that the fracture mainly occurred within one of the basemetals, or as not-welded if the observed separation mainlyoccurred at the original CuAl contact surfaces, associated withlimited resistance to peel the strip off.

Microstructure examination on the sectioned/polished central-plane surfaces was performed to see if the Al and Cu are tightlyconnected without gap, or if a gap exists between Al and Cu. Byimage processing the percentage of the bonded interface length

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over the total interface length was obtained as another way toevaluate the welding quality.

Measurement of Strain Distribution and Plastic Work. Thecross-sectional images were also used for dimension and strainmeasurement. The binary images were input into a user-developedMATLABprogram, and the axial and radial coordinates (x, y) of Aland Cu boundaries were identified and obtained. From the outerdiameterdand thicknesstof Al and Cu at each x-coordinate (pixel)the average strain components in hoop, thickness and axial

directions, as well as von Mises effective strain over the tube axial

positionx were calculated based on the following formulations:

ehx ln dx tx

d0 t0

(1)

etx ln tx

t0

(2)

exx ln A0

Ax

ln

d0 t0dx tx

t0

tx

eh et (3)

ee 2=3e2

x e2he

2t

0:5(4)

where the subscripts x, h, t stand for axial, hoop, and thicknessdirections, respectively, and d(x), t(x), and A(x) are the outerdiameter, thickness, and the cross-sectional area at sectionx; the

subscript 0 stands for their initial values. The work done pervolume,w(x), is the product of the effective strain and stress; herethe mean of yield strength and ultimate strength Yfrom referenceswas used as the ideal work without considering strain hardeningand strain rate effect on the flow stress. The total plastic workWof entire deformation zone was obtained by further integrating theunit energy per volume w over weld zone volume V over the xrange [a,b]

wx

ee0

redee Yeex (5)

W

ba

wx Axdx

ba

pdx tx tx Y eexdx (6)

Fig. 2 As-machined Al and Cu tube samples before welding (a)and the produced surface scratches on the Cu tubes in hoop

direction (b). The further sanded surface is shown in (c).

Fig. 1 The electromagnetic system (a) and its assembly schematics (b), consisting of an elec-trical coil (c), a field concentrator made of copper (d), the tubular workpieces Al (e), and Cuwith special end geometry (f)

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This work is one part of the total energy dissipation in the MPW,to be used and discussed for later estimation of the heatgeneration.

Microstructure Characterization. The welding conditionswere repeated for producing samples not damaged by peeling test,for microstructure examination. The welded samples were slowlycut along the central axis with a bench saw under water cooling,to avoid possible microstructural change. Samples were ground tothe finishing step with 4000-mesh metallurgical sand paper under

water cooling, followed by cloth polishing to 0.05 lm Al2O3pow-der abrasives water suspension in the finishing pass. For each sam-ple the entire weld zone was photographed with a stereomicroscope (for low magnification image) and with a metallurgi-cal microscope (by Olympus), both allows mounting a digitalcamera with image software (OptixCam OCView).

Additional examinations on microstructures and fracturedsurfaces were performed under a scanning electron microscopy(SEM, model Hitachi-2400). The chemical composition distribu-tion across the interfaces was analyzed with an EDS probe, andthe obtained chemical compositions were compared with the equi-librium CuAl binary phase diagram. In addition, in order to char-acterize the local mechanical properties near the interface region,microscratch test was performed under a constant load and with amicroindenter that slided across the interface on a sectioned andpolished surface, and the scratched line width was measured toreflect the strength variation near the interface region relative tothe two base metals.

Results

General Features of the MPW and the Effect on AppliedWelding Energy. A typical recorded coil current vs. time curve,performed at 5.2 kV charging voltage, is shown in Fig.3(a). Thecurrent curve shows a sinusoidal shape with rapid decay in the

amplitude, and the maximum current of 178 kA was reached at2 105 s at the first peak, and the measured total welding outputenergy was 11.2 kJ. Varying the charging voltage has resulted indifferent peak current and output energy, but the curve shaperemained the same. The peeled and also central-sectioned sampleat 5.2 kV, see in Fig. 3(b), shows significant diameter reductionwithin the welding zone, for both Al and Cu, and the magnitudeof the diameter reduction increases toward Al tube end (from leftto right). The Al tube wall is almost in full contact with the Cutube; except for the Cu corner transition zone-II. Figure 3(c)

shows that the peeled/fractured Al surface is very rough, and cer-tain amount of aluminum materials left over on the Cu surface.Due to the strong bonding significant force and torque wereneeded to peel the Al strip apart from the welded interface. Thiswelded result applies to all surface conditions A and B. How-ever, the oil-contaminated sample (surface condition C) was notwelded in the sense that during sample sectioning one half of thespecimen was broken from the Al and Cu weld surface. Anothernonbroken half had less than 10% of bonded length, and was eas-ily separated. The percentage of the welded length over total inter-face length on the sectioned surface is given in Table1.

Weld Zone Deformation and Bonded Length. For surfacecondition A Fig.4 shows the selected axial cross-sectional imagesunder different welding voltage that were used for the measure-

ment of strains, and Figs.5(a)5(c) show the measured distribu-tions of true strain components of Cu and Al at 5.2 kV as anexample. It can be seen that toward the Al end the strain increaseswith x, but in the transition zone II the high strain is due to theover calculation of its wall thickness under the current thicknessmeasurement/calculation method. Figure 5(d) shows the effectivestrain distribution for all surface conditions and the applied charg-ing voltages. By integration of the plastic work for all casesFig.5(e)shows that the total plastic works of Al, Cu and their sumincrease with increasing the charging voltage. Figure5(f)shows thepercentage welded length increases with voltage, for surface condi-tion A. For surface condition B the welded length is slightly lowerthan A, and for the contaminated surface the welded length is muchlower than condition A, based on limited samples available.

In above true strain calculation using Eqs.(5) and (6), the yield

stresses were from available data [30,31], and an average value ofthe yield stress and tensile strength was taken, for copper (C110)at 144.5 MPa and for aluminum (AA6063-O) at 82.7 MPa. Theerror in the strain measurement, based on the transition zone-IIstrain measurement using current coordinate for tilted tube dimen-sions for actual wall thickness strain, is in the range of615%, andthe average treatment for the actual nonuniform deformation (thatneglects the redundant work and underestimates the actual plasticwork). This first-order approximation provides a means to esti-mate the total energy received by the workpiece. It is well knowthat during plastic deformation more than 90% of the receivedexternal work is dissipated in the form of heat, so that this mea-surement provide a very conservative estimation of the heatingenergy in the MP welding using this simple and straightforwardmethod of geometrical measurement, which can be useful for

Fig. 3 The measured current wave for welding at 5.2kV (a),and the resulting specimen cross section (b) showing diameterreduction of Al and Cu, and the peeled fracture surface (c)showing very rough aluminum inner surface, indicating strongbonding and ductile fracture within Al

Table 1 Peeling test results and bonded percentage lengthsfor different welding conditions. The percentage is the bondedlength over the total interface length estimated from micro-graphs of sectioned surfaces.

Cu surface condition 4.6 kV 5.2 kV 6.0 kV

A Scratches inhoop direction

Welded (55%) Welded (63%) Welded (73%)

B Scratches inaxial direction

N/A Welded (59%) N/A

C Oil-contaminatedover A

N/A N/A Not welded(10%)



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further interface temperature analysis (not provided here). Otherheat sources not considered include Joule heat from induction cur-rent and interfacial frictional heat at the interface. A direct multi-physics energy calculation is very complex involving energyconversion and transformation from the capacitor to the primarycoil, the secondary coil, the field concentrator of unknown effi-

ciency, and finally to the induced field at the workpiece that can-not be conveniently calculated and directly measured.

The Effect of Surface Contamination.For the oil-contaminated sample (surface condition C) very weak bondingstrength was obtained even under the highest applied voltage(6.0 kV). For majority areas the interface bonding was not estab-lished. The observation of the separated Al and Cu surfaces showsthat

On the Al outer surface, Figs. 6(a)and 6(b), the lath turningscratches are clearly seen (since no solid contact on it), butthe spacing of the scratches is increasing toward Al tube end(right side), consistent with the axial strain distribution, seeFig.5, indicating the contacting sequence was from Cu zone-

I (with a large OD and small clearance), then passed the tran-sition zone-II and developed increased shear displacementsin zone-III.

On the Al inner surface, Figs.6(c)and 6(d), the majority ofthe welding zone turned to black, apparently the originallythick and transparent viscous oil turned to dry powders (car-bon black) after MPW, suggesting that the temperature washigh enough to decompose the oil. In addition, the areas withmost black powders were within the middle 1/3 of the weld-ing zone (zone-III), suggesting that the temperature and pres-sure of the middle zone was the highest over the entireprocessing zone. The lathe turning scratches were clearlyseen on both Al and Cu mating surfaces, but with many dam-ages and deformations.

Comparing samples with and without oil contamination underthe same applied voltage (6.0 kV), the contaminated surface didnot establish welded interface in majority contact area, under sim-ilar condition or actually has slightly higher effective strain, likelydue to a reduced interface friction from oil lubrication, so that theAl tube can move in axial direction more freely.

This indicates the oil contaminated surface cannot be easilybroken by MP pressure, even with significant plastic deformation.This is different from the oxidation layer that can be broken byMP force as reported by previous study [32]. Thus, for MPW a

clean surface is still required to establish a strong bonding, but aquantitative description of the cleanness requirement needs to befurther studied.

The Effect of Surface Topology and Scratch Orientation

Scratches in Hoop Direction. For the sample with lathe turningscratches in hoop direction and welded at 6.0kV the opticalmicrographs are shown in Fig. 7, with the weld zone shown inthree consecutive segments, labeled as L1, L2, and L3, respec-tively. It shows that wavy interfaces were formed, and both theamplitude and wavelength of the waves increase toward the free-end of Al tube (to right). The measured average wavelength wasabout 190lm, which is close to lathe machining scratch pitch dis-tance shown on the outer surface machine scratches; thus, it is

necessary to further check if this correlation is just a coincidence,and if so the sample with initial scratches in axial direction shouldhave the similar wavelength.

Scratches in Axial Direction. The longitudinally scratchedspecimen (surface condition B), after welded and sectioned alongthe axial direction, is shown in Fig. 8. In this case only the Cuinner surface were sanded in longitudinal direction, while the Alinner surface was still with lathe scratches in tangential direction,but Cu has much higher strength than Al. The results show that

With the pre-existing surface markers of Cu and Al in theperpendicular cross-over arrangement, the produced interfacewaves were relatively random, and no obvious periodic fea-ture was recognized.

For the fractured interface region, the shape profiles of thefractured two edges in Cu and Al sides do not match witheach other, though with some similarity. This suggests thatthe plastic deformation occurred after separation, so that theedge profile and location alignment have been changed andshifted.

The original handmade sanding scratches in axial direction onCu surface and in tangential direction on Al did not show a clearperiodic feature. From the view of interface bonding, if the twomating surfaces have the same surface scratch orientations a largerinterface area can be generated that is in favor of all three possiblebonding mechanisms, i.e., the mechanical mixing/interlocking,physical bonding, and chemical reaction. In contrast, for thescratches in perpendicular directions between the two weldingsurfaces, the effective surface contact area and associated weldingeffect was reduced.

The wavy surfaces have been widely reported in other MPWstudies. Based on Ben-Artzy et al. [28], the wavy interface forma-tion in MPW was explained by KelvinHelmholtz instabilitymechanism: under the MP force the reflected shock waves interactwith the welding collision point at the interface, where interfer-ences are the source for the wave initiation. Based on this mecha-nism, the initial surface feature (here the lathe turning scratcheson both mating surfaces) served as the initiator of formed wavesand affected the welded wavy wavelength within certain range,similar to that in unstable vibration under a periodic externalforce; but in the case of the two mating surfaces scratched in per-pendicular directions, and especially when the stronger scratchsets were not coincide with the MP shear direction the more ran-dom wavelengths were produced.

Fig. 4 Selected images of sectioned samples under threeapplied voltages, and the binary images of Cu and Al used forstrain calculation



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Interface Reaction and Evidence of Melting. The new phaseformation requires a rapid mass transportation by diffusion overcertain time period, but the first MP peak is only in 105 s, so thepeak temperature and interface deformation become criticallyimportant to provide a high diffusion rate for the interface reac-tion, and the combined thermomechanical condition determinesthe type and amount of the reaction product and its morphology/

microstructure.Mechanical mixing is one way to reduce diffusion path for newphase formation, as seen in Fig.9, where the wavy interface con-tained a 1040 lm thickness of the new phase with distinct color/contrast from the parent Al and Cu, and some Cu base metalshave been completely wrapped within the new phase pocket.Mechanical mixing is a necessary condition in forming such a dis-continuous new phase across the interface (because atomic diffu-sion path must produce a continuous profile). In addition, in Fig.9(b)some microcracks can be seen within the new phase region,which are stopped at the boundaries of the more ductile base met-als (this is different from the polishing scratches as the latter aremore straight and continuous crossing different phases), indicatingthe new phase is more brittle than the base metals.

Interface Melting.The direct evidence of interface melting wasobtained for a sample welded at 6.0 kV, see Fig.10, where a roundpore was found inside the new phase region, and by refocusing tothe bottom of the pore it shows the smooth inner surface of thepore with cracks. This pore cannot be formed from pull-out orintrude-in of an inclusion during welding or sample sectioning/polishing (otherwise a sharp edge and surface would be pro-

duced). In addition, from its round shape, smooth edge, and glassypore surface, it cannot be formed by high-pressure impact in solidstate (otherwise the pore will be crashed to a collapsed shaperather than a round shape). Thus, it is more reasonable to believeit was formed from liquid phase. The process involved a thermallyassisted diffusion process at a high temperature for minimizingthe surface energy. Two more evidences of melted sites werefound on the same sample from SEM observation; see Fig.11, inwhich some pores from trapped gas and a glassy pore surfaceappearance can be clearly seen. This morphology can only be pro-duced from melting. In addition, several cracks exist on the poresurface, indicating a high thermal stress during rapid solidifica-tion. All the three pores shown have a size in the range of2030lm in diameter. Within the current welding operation

Fig. 5 The distributions of the true strain components along the axial distance from Cu headend (at x50) to Al tube free end (at x520 mm), for Al (a) and Cu (b) at 5.2 kV. Also shown arethe effective strains for Al and Cu for this specimen (c), and for the specimens welded at differ-ent voltages and surface preparation conditions (d). The plastic works (e) and the bondedlength fraction (f) are plotted for different welding voltages.



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window only three melting sites were found, all under the highestwelding voltage (6.0 kV).

Characterization of New Phase Formed on the WeldingInterface.SEM micrographs of two welded interface regions areshown in Fig.12, where a transition zone between two base met-als can be clearly seen. SEMEDM analysis was performed acrossthe interface for the two samples under a spot operation mode inorder to collect more signals over longer time period for betteraccuracy, and the results are shown in Fig. 13, given the newphase a chemical composition in the range of 6080 at. % Cu andbalanced Al.

The equilibrium AlCu phase diagram (shown in the Appendix)may be a helpful reference. When heated to above 800 K andcooled down to room temperature, intermetallic compounds Alm:-Cuncan form with various Stoichiometric atomic ratios, includingAlCu, Al9Cu11, and Al2Cu3, as labeled at the bottom of the phase

diagram. Under current nonequilibrium condition involving a highpressure and very short time the actual phase formation may notexactly follow this diagram but with some offset of phase region.From the SEMEDS results, the measured Al:Cu ratio of the newphase in the interface region was within the range of 1:1 to 2:3range. From the above observation and analysis we can concludethat there existed a chemical reaction and AlCu intermetallicformation.

Microscratching test was performed for a sample welded at5.2 kV. We did not use microharness test with the considerationthat it often has data scattering and more repeats are required fordata averaging, so it is not suitable for small area measurementcontaining a high hardness gradient. In contrast, a microscratchingtest can be much simpler and informative for the current purposeto compare the relative properties of the new phase and base met-als at the small area across the interface. Vickers micro-indenterwas used with one of the diagonal axis aligned along the scratch-ing direction running across the interface, under a 120 gf load anda constant speed of 2lm/s on the polished surface, see Fig.14.The produced scratch was much narrower (4lm) in width withinthe new phase region than that in Cu base metal (1214lm) andin Al base metal (14 lm), as marked in the figure, indicating thatthe intermetallic phase is much harder/stronger than the two basemetals. Also shown are the multiple cracks in the new phaseregion, again indicating this new compound is brittle.

Discussions

On the Interface Reaction Process. For the current materialsystem and MPW conditions, the observed phenomena may leadto the following understanding of welding process

Upon application of MP force, large amount of energy, ata very high energy density and energy input rate, wasadded into the aluminum tube and resulted in Al wallimpact with the outer surface of Cu tube. The kineticenergy converted to surface deformation energy and adia-batic heating at a high rate, with no sufficient time for heattransfer, causing rapid temperature rising in the weldingzonewithin a thin layer of aluminum and copper contact-ing surfaces.

Mechanical mixing of two metals at the interface occurredas the result of severe plastic deformation. Surface topologyhas important effect on the mechanical mixing and wavyinterface formation.

Fig. 6 The sample with oil contamination (not welded): (a) and(b) Al outer surface; (c) and (d) Al inner surface; and (e) and (f)

Cu outer surface. The right side shows the local enlargement ofthe boxed area.

Fig. 7 A sectioned sample welded at 6.0 kV (top), and themicrostructures over the three consecutive segments L1, L2,and L3 from left to right

Fig. 8 Interfaces of the sample with the original copper surfacesanded in axial direction. No correlation of the failed interfaceedge and the original lathe-turned pitch can be identified.



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Interface temperature increases significantly, and dependingon the magnitude of applied energy interface melting mayoccur.

New intermetallic compounds formed, which involved masstransportation and interdiffusion of Cu and Al atoms in solidphase or liquid phase. This is a thermally activated processrequiring certain time to start and finish. The thickness ofthe intermetallic phase was in 05 lm, depending on theapplied energy and the surface conditions.

Due to the sequential process along the welding length, theearlier formed brittle intermetallic phase may subject to fur-ther shear deformation from later impact deformation in

front of the welding zone, which may cause microstructuraldamage.

Continued heat transfer and cooling to RT may result inthermal stress development, and the final welded interfaceoften contains certain separation zone, with some localcracks within the brittle intermetallic zone.

On Welding Mechanisms. The experimental observations sug-gest that multiple joining mechanisms co-exist and operate simulta-neously. These mechanisms include mechanical interlocking (atmacron and micron scales), physical bonding (atomic adhesion fromfresh new surface contacts), and chemical bonding/reaction (newcompound formation). The contribution of each mechanism dependson the material pairs to be welded, the applied energy level and thesurface topological condition. Chemical bonding from intermetallic

formation has two opposite effects on bonding strength: on the oneside, it greatly increases the bonding strength; on the other hand, ittends to form microcracks under the condition of a high temperaturegradient, a high thermal stress, and a large plastic deformation, espe-cially when the intermetallic layer is too thick.

The observations indicate that (1) the formation of intermetallicphase at the interface can occur by solid-state reaction withoutmelting, which plays important role in bonding strength; (2) inter-face local melting is possible, but is not widely observed and isnot a necessary condition for intermetallic formation. Thus, thedominating MPW mechanism is still a solid-state process.

The surface roughness/scratch orientation not only affects me-chanical interlocking and physical bonding but also it affects thelocal contact stress as well, which changes the chemical reaction

Fig. 9 New compound phase formed at the welding interface region that mixes with basematerials (a) and (b), and contains microcracks (b), observed under optical microscope;incomplete intermetallic phase formation at a wavy pocket is seen in (c), with the Al wave frontconverted to intermetallic but the tail still remained as base Al

Fig. 10 The optical micrograph of a sample welded at 6.0 kV,and a round pore with a smooth surface was observed (a). Byrefocusing at the pore bottom (b) some surface cracks wererevealed.



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driving force and kinetics. Thus, further study can be helpful onoptimization of workpiece surface topology as one of the processdesign element. As for the surface contamination, it plays as a bar-rier for all three joining mechanisms, and give a sensitive but neg-ative impact to the weld establishment, so that a clean surface or

coated surface with favorable chemical composition can be oneway to enhance the weldability; on the other hand, this bondingprohibiting layer can provide a useful means for special weldingdesign of selective patterns containing combined welding andnonwelding features.

Fig. 11 The sample welded under 6.0kW power (a), with three local melting zones shown in

(b) and (c) as evident by the smooth glassy pore surfaces with cracks; (d) the local enlarge-ment of one pore shown in (b).

Fig. 12 SEM micrographs of two interface regions and their local enlargements. The roughCu surface was from chemical etching.



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Conclusion

(1) AlCu tubes have been successfully welded by applyingelectromagnetic pulse force over Al tube outer surfaces,under different electromagnetic power input levels and sur-face conditions. Welding was not established on oil conta-minated surfaces.

(2) The processing parameters that affect welding strength

include not only the chemical property and compatibility oftwo joining materials, the power used in the electromag-netic welding, but also the surface topological conditions(the surface roughness, scratch orientations, and contamina-tion). A correlation between the wavelength of lathe turningscratches and that of welded interface waves wereobserved.

(3) Intermetallic compounds formed at the welded interface,and the microstructure, chemical composition, and mechan-ical properties were examined, identified, and tested.

(4) Evidences on liquid phase formation have been obtainedunder a high power welding condition, which producedsmooth and glassy pore surface on the trapped air bubbles,and with surface cracks.

(5) The strain and deformation energy by MP force were meas-ured from geometry of welded specimens as a means oflow bond estimation of total heat generation, but local heatat the interface needs to be further analyzed.

(6) Welding mechanisms responsible for MPW have been fur-ther discussed based on the experimental observations. Thedirection of process improvement is provided in terms ofpower application and interface preparation.

Acknowledgment

The authors like to express their sincere thanks to ProfessorGlenn Daehn, who offered a lightening discussion and lab tour atthe early stage of this study.

Funding support of this study was from Hirotec America and

Wayne State University internal fund.

Nomenclature

d(x),t(x),A(x) deformed tube outer diameter, thickness, andcross-sectional area atx

d0,t0,A0 initial tube outer diameter, thickness, and cross-sectional area

w(x) plastic work per volume atxW total plastic work over welding length

ee(x) true effective (von Mises) strain atxet(x) true strain component in thickness direction atxex(x) true strain component in axial direction atxeh(x) true strain component in hoop direction atx

Appendix: The Equilibrium AlCu Phase Diagram

The equilibrium AlCu phase diagram is shown in Fig.15, gen-erated using commercial software COMPUTHERM [33] and pub-lished binary phase diagram database.

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Fig. 15 The AlCu binary phase diagram, computed with com-mercial software and with available thermodynamics database


http://dx.doi.org/10.1108/01445150010321742http://dx.doi.org/10.1108/01445150010321742http://dx.doi.org/10.1108/01445150010321742

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