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Journal of Materials Processing Technology 211 (2011) 944–952

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

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

pplication of high velocity impact welding at varied different length scales

uan Zhanga,∗, Sudarsanam Suresh Babub, Curtis Prothec, Michael Blakelyd,ohn Kwasegroche, Mike LaHae, Glenn S. Daehna

Department of Materials Science and Engineering, Ohio State University, 477 Watts Hall, 2041 College Rd, Columbus, OH 43210, United StatesDepartment of Industrial Welding and Systems Engineering, Ohio State University, Columbus, OH 43221, United StatesDynamic Materials Corporation, Mt. Braddock, PA 15465, United StatesDynamic Materials Corporation, Sugar Land, TX 77478, United StatesContinuum Inc., Santa Clara, CA 95051, United States

r t i c l e i n f o

rticle history:vailable online 11 January 2010

eywords:mpact welding

a b s t r a c t

Three complementary impact welding technologies are described in this paper. They are explosive weld-ing, magnetic pulse welding, and laser impact welding, which have been used to provide metallurgicalbonds between both similar and dissimilar metal pairs. They share the physical principle that generalimpact-driven welding can be carried out by oblique impact but are used at different length scales from

avy interfacexplosive welding (EXW)agnetic pulse welding (MPW)

aser impact welding (LIW)icrohardness

meters to sub-millimeter. The different length scales require different kinds of systems to drive theprocess, and the scales themselves can give different weld morphologies. Metallographic analysis oncross-sections shows a wavy interface morphology which is likely the result of an instability associatedwith jetting, which scours the surfaces clean during impact. The normalized period and amplitude ofthe undulations increase with increasing impact energy density. Microhardness testing results show theimpact welded interface has a much greater hardness than the base metals. This can lead to weldments

l to or

that have strengths equa

. Introduction

There is a growing recognition that optimal lightweight struc-ures for automobiles, aircraft and even bicycles are often createdrom multi-material assemblies. Joining dissimilar high-strengthight alloys has therefore been of significant and growing inter-st. One of the most elegant ways to accomplish dissimilar metalelding is by impact welding. The outstanding advantage of impactelding is that it can minimize the formation of continuous inter-etallic phases while chemically bond dissimilar metals. This

aper presents three types of impact welding technologies. Theyre explosive welding (EXW), magnetic pulse welding (MPW), andaser impact welding (LIW). They all share the same basic principleo join the metals together by impact-driven solid state welding buthese are best applied at different length scales. Fig. 1 schematicallyhows the range of three welds. Explosive welding is well suited forarge planar interfaces, up to meters in extent. As shown in Fig. 1(a),

90-m-long explosive welded refinery column is transited on sev-ral rail cars. Magnetic pulse welding can provide linear or circularelds, but equipment limits the total energy stored and this keepseld lengths to the order of meters or less, with typical widths of

∗ Corresponding author. Tel.: +1 614 313 0358.E-mail address: zhang.561@osu.edu (Y. Zhang).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.01.001

greater than that of the weakest base material.© 2010 Elsevier B.V. All rights reserved.

millimeters to centimeters. Fig. 1(b) shows MPW welded tubes inwhich the outer diameter is about 20 mm. Laser impact welding isa new process, introduced here, that can create spot-like welds onthe order of millimeters in diameter. Fig. 1(c) shows a 3 mm diam-eter spot welding of an aluminum tab on to an aluminum sheet andthis sample was studied in present paper.

All of these three impact welding technologies are based onsolid state impact or collision welding, which has been estab-lished as fast, reliable and cost-effective. Bahrani et al. (1967),Crossland (1982) and Blazynski (1983) studied EXW on a vari-ety of metal combinations at large scales, and their researchwork indicated extensive industrial application. Recently, MPWhas been applied to both similar and dissimilar metals. For exam-ple, Tamaki and Kojima (1988) and Shribman et al. (2002) weldedaluminum tubes together. Hokari et al. (1998) welded aluminumtube to copper tube, Aizawa et al. (2007) welded aluminumalloys to carbon steel sheet, Ben-Artzy et al. (2008) welded alu-minum to magnesium, Hutchinson et al. (2009) welded copperplate to zirconium-based bulk metallic glass plate. MPW hasseen limited, but rapidly growing industrial application (Pul-

sar (http://www.pulsar.co.il/news/?nid=14); Shribman and Gafri,2001; Uhlmann et al., 2005). The commonly welded length scalefor MPW ranges from a few millimeters to several centimeters.Required energies are too great for larger structures and couplingof magnetic energy to the part becomes inefficient at much smaller

Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952 945

Fig. 1. Illustration of explosive welding (EXW), magnetic pulse welding (MPW) andlaser impact welding (LIW) for different length scales. (a) Stainless steel clad to 387atq

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als Corporation. The being welded surfaces were ground to remove

lloyed steel by EXW to make refinery columns. (b) 1018 steel rod impact weldedo AA6063-O by MPW. (c) AA1100 thin plates seam linear welding by LIW. The coinuarter is shown as a length scale reference for LIW.

engths. Recently, Daehn and Lippold (2009) proposed LIW andeveloped for low temperature spot impact welding. It is bestpplied to relatively thin samples (about 200 �m or less, and weldegions of a few millimeters in diameter). One distinctive advan-age of this approach is that it appears applicable to arbitrarily smalloil thicknesses and length scales, and does not rely on the intrin-ic electrical conductivity of the flyer. This makes the method welluited for the manufacture and assembly of micro-devices such asicro-electro-mechanical systems.Bahrani et al. (1967) and Ezra (1973) established the impact

elding principle based on the jetting effect that allows atom-cally clean metal surfaces to join each other. When two platesollide with high velocity at a proper impact angle, a jet forms frometals being deformed. At the collision point, the impact stress

reatly exceeds the yield stress. A jet propagates along the matingnterface, cleans away the surface oxide layer and leaves two atom-cally clean surfaces. Simultaneously, these two virgin surfaces arender a large contact pressure. Bahrani et al. (1967) also pointedut that the impact pressure brings the surface atoms into directontact, allowing a chemical or metallic bond of great strength.otros and Groves (1980) and Salem (1980) concluded that the

et is the essential for impact welding. Palmer et al. (2006) foundhat the collision process often involves severe plastic deformation,

echanical alloying, possibly local melting with rapid solidifica-

ion, and high strain rate induced fluid-like behavior. Due to variednstabilities including Kelvin–Helmholtz instabilities, the weldednterface is often highly heterogeneous. The most common inter-ace morphology is the wavy morphology. Based on the study from

Fig. 2. Schematic diagram of explosive welding process. Jet was generated from thecollision point and propagated along the welded interface.

Bahrani et al. (1967) and Grignon et al. (2004), it is known that thedetailed interface morphology is critically dependent on the impactangle, impact velocity, properties of the materials, and geometry ofthe welded plates. The important parameters that determine theweld quality and interface microstructure are the impact velocityand impact angle (Botros and Groves, 1980). With a fixed impactangle, the impact velocity has a dominant effect. Excessive velocitycan cause significant melting, leading to intermetallic formation,or this can produce brittle damage or spalling in the impactingplates. Insufficient impact velocity may not initiate the jet requiredto remove the surface oxide (Robinson, 1977). Typical estimates ofthe strain rate may be as high as 107 s−1 (Tamaki and Kojima, 1988)produced from an impact velocity which is about 250–400 m/s (Leeet al., 2007).

This paper examines the processes and characteristic of thewelded interfaces for EXW, MPW and LIW joints. The studied mate-rials include aluminum alloy, carbon steel and copper alloy.

2. Experimental methods

2.1. Explosive welding (EXW)

Explosive welding is a solid state impact welding process thatemploys large quantities (usually a few centimeters thick) of highexplosive to accelerate a flyer plate against a target plate. Themethod is well established for wide variety of materials with flatand large surface area. A schematic diagram in Fig. 2 shows the basiccomponents for EXW. The explosive detonation is normally initi-ated at a point or along a line, and the relative velocities, betweenthe explosive detonation front velocity and the flyer plate down-ward velocity, set the contact angle upon impact (Chizari et al.,2009). Changing initial standoff can change the impact velocity andangle. The collision takes place across the entire surface along animpact line that moves from the detonator and produces the plasmajet. The jet atomically cleans the surface as is necessary for weld-ing. The main process parameters for EXW include the surface finishand cleanliness, the placement of the detonator, the standoff dis-tance, and the characteristics of the explosive used, particularly itsdetonation velocity and the pressure produced. These controlledvariables are responsible for the impact velocity and impact anglethat is developed (Palmer et al., 2006).

Two sets of explosive welding experiments were performedon Al/Fe and Cu/Al respectively. For each set, the experimentswere repeated four times and all of the four samples were stud-ied. The explosion welded materials evaluated in this study werea standard explosion clad product produced by Dynamic Materi-

the oxides. One of the samples was taken from an aluminum-steelstructural transition joint material consisting of 3.175 mm thickAA5086, 6.35 mm thick AA1100, and 9.525 mm thick carbon steelSA516-55. This tri-clad material was produced in a single explosion

946 Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952

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ig. 3. Schematic diagram of magnetic pulse welding process. Both the flyer platend the target plate were supported by insulated layers with certain standoff dis-ance between them. The electromagnetic (EM) actuator was connected to capacitorank. The arrows indicate the primary current flow direction.

elding shot. The primary explosive was an ANFO, which was mix-ure of explosive grade ammonium nitrate (AN) prills with aboutwt% Diesel Fuel Oil. The exact composition is proprietary, butommon mixes are discussed in the literature (Turcotte et al., 2003).he flyer plate velocities could be estimated using a modified Gur-ey equation with the applied explosive load (Blazynski, 1983). Theyer plate velocity for aluminum to aluminum welding was esti-ated to be ∼1000 m/s, and for the aluminum to steel welding was900 m/s. The copper-aluminum clad sample was taken from thelectrical transition joint. It consisted of the 3.175 mm thick Cu102s the flyer plate and 9.525 mm thick AA1100 as the target plate.he copper flyer plate velocity was estimated to be ∼450 m/s.

.2. Magnetic pulse welding (MPW)

Since 1969, MPW has been successfully applied for tube toube impulse welding but typically required fairly high electri-

ig. 4. Primary current and magnetic pulse induced flyer velocity for MPW processn Cu/Al joint. The flyer plate material for this specific velocity is Cu110 and thehickness is 3.175 mm. The peak current is 140 kA and rise time is 12 �s. And theeak velocity is about 265 m/s and the time interval from switching to impact isbout 14 �s.

Fig. 5. Schematic diagram of laser impact welding process. The laser pulse ablatesthe substrate. Plasma jet was generated between flyer and target block. For plateto plate welding, the target plate can be attached to the block oblique surface andbeneath the flyer plate with certain angle.

cal energies to be stored in capacitor bank with typical valuesin the range of 20–100 kJ (Khrenov and Chudakov, 1969). Withthe recent development of MPW, the geometry of the weldingworkpiece could be cylindrically symmetrical (Tamaki and Kojima,1988) or asymmetrical (Aizawa et al., 2007; Kore et al., 2007;Lee et al., 2007). Studies by Aizawa’s group (2007) in Tokyo andDate’s group (2007) in Mumbai have developed MPW seam lin-ear welding. These methods tend to use much less energy thanthose for tube welding. For example, the reported energy to weldaluminum alloy plate about 1 mm in thickness to SPCC steel plateis only about 1.4 kJ (Aizawa et al., 2007). Very rapid rise times inthe capacitor discharge circuit are largely responsible for this effi-ciency.

MPW is closely analogous to explosive welding. However, ratherthan explosives, it uses electromagnetic force to accelerate the flyer

plate. Therefore, MPW can be safely and reproducibly used in pro-duction environments controlled by an electric power supply andthe fine adjustment to parameters is straightforward. With properdesign, the energy is fairly efficiently used for accelerating the flyerplate or tube rather than heating or melting the materials. The MPW

Fig. 6. Wavy interface morphology from EXW. (a) Three metal layers were weldedtogether. From top to bottom, the materials are AA5086, AA1100 and carbon steelSA516-55. There were two wavy interfaces and they are Al/Al and Al/Fe interfaces.(b) Welded interface between AA1100 and Cu102.

Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952 947

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Fig. 7. Wavy interface morphology from MPW. (a) Interface between lo

ystem includes a capacitor bank, actuator, and workpieces. Theapacitor bank simply consists of an inductance–capacitance circuitnd an actuator of some impedance. Fig. 3 shows the actuator withhe workpieces. When capacitors are discharged, the high densityurrent flows through the conductive actuator, which is regarded

s primary current. The rise time of the primary current pulse isypically between 5 �s and 30 �s. The discharged primary currentas a dampened sine waveform as shown in Fig. 4. If there is a closedurrent path, the associated electromagnetic field induces a strongecondary current through the nearby metal workpiece. Because

able 1ummary of wave length and amplitude from EXW, MPW, and LIW.

Weldingmethod

Materials(flyer/target)

Flyer platethickness (t)(mm)

Energy density(kJ/m2)

Impact ve(m/s)

EXW Al/Al 3.175 4286 ∼1000Al/Fe 6.35 6944 ∼900Cu/Al 3.175 2861 ∼450

MPW Al/Al 0.50 640 ∼300Al/Fe 0.406 1173 ∼250Cu/Al 0.31 747 ∼265

LIW Al/Al 0.175 53 ∼475Al/Fe 0.050 53 ∼475

bon steel 1008 and AA6061. (b) Interface between Cu110 and AA6061.

the primary current in the actuator and the secondary current inthe flyer plate generally travel in opposite directions, their inter-actions result in a strong repulsive force. Therefore, the flyer plateis repelled from the actuator and accelerates with sufficient veloc-ity to generate impact welding. The primary current, as measured

by Rogowski coils, and the flyer velocity during acceleration andimpact, as measured by Photon Doppler Velocimetry (Johnson etal., 2009), are shown in Fig. 4. Similar to EXW, it is necessary tohave a slight impact angle to form a jet along the mating surface(Lee et al., 2007).

locity Wave length(�) (mm)

Wave amplitude(A) (mm)

�/t A/t �/A

3.60 0.83 1.134 0.261 4.344.00 0.65 0.630 0.102 6.151.26 0.21 0.397 0.066 6.00

0.100 0.010 0.200 0.020 10.000.064 0.009 0.158 0.022 7.110.045 0.004 0.145 0.013 11.25

0.034 0.0020 0.194 0.011 17.000.017 0.0008 0.340 0.016 21.25

948 Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952

by LIW

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Fig. 8. Wavy interface morphology from LIW. (a) Interface morphology made

MPW uses an electromagnetic field and thus the flyer plate muste electrically conductive and plastically deformable, or coupled toconductive driver. It is also suggested that the target plate should

ave a higher yield strength than the flyer plate to avoid extremeeformation of the target (Kistersky, 1996).

In this particular study, AA6061-T6 plate was welded to Cu110late and low carbon steel 1008 plate respectively. For each set,ight samples from the repeated experiments were investigated.

ig. 9. Wave length and wave amplitude map for three different impact weldingrocesses. The map could be divided into two domains, and they are EXW domainnd MPW&LIW domain. In each domain, there are different welded metal pairs. 1-l/Al is EXW for AA5086/AA1100; 1-Al/Fe is EXW on AA1100/carbon steel SA516-55;-Cu/Al is EXW on Cu102/AA1100. 2-Al/Al is MPW on AA6061/AA6061; 2-Al/Fe isPW on AA6061/low carbon steel 1008; 2-Cu/Al is MPW on Cu110/AA6061. 3-Al/Al

s LIW on AA1100/AA1100, 3-Al/Fe is LIW on AA1100/low carbon steel 1010.

on AA1100 welds. (b) Interface between low carbon steel 1010 and AA1100.

A commercial Maxwell Magneform capacitor bank was used, forwhich the maximum stored energy is 16 kJ and maximum workingvoltage is 8.66 kV. There are eight capacitors and each has a capaci-tance of 53.25 �F. The being welded metal plate was in rectangularshape and the length was 100 mm and width was 75 mm. The thick-ness for the flyer plates varies within the range of 0.3–0.5 mm. Thewelding surfaces were lightly sanded and degreased before impact.The sanding and cleaning processes were carried out on a flat steelsurface, and thus the plates were flattened at the same time. In orderto maintain flatness, a thick steel backing is placed behind the tar-get sheet. The standoff distance was 3.15 mm and the overlap widthwas 12 mm. The input energy for successful welding was 4.8 kJ forAl/Al, 5.6 kJ for Cu/Al and 8.8 kJ for Al/Fe. Taking Cu/Al experimentsat 5.6 kJ as an example, the Cu plate was the flyer plate and placednear to the actuator with an insulator layer in between; and theAl plate was the stationary target plate and placed 3.15 mm awayfrom the flyer plate. These two plates had 12 mm wide overlap,which was the possible welding zone. After the energy dischargedfrom the capacitor bank, the primary current flowed through theactuator and reached the peak value of about 140 kA within approx-imately 12 �s, as shown in Fig. 4. Because of the electromagneticforce, the flyer plate was accelerated until it collided against thetarget plate. During this process, the flyer velocity was measured.Fig. 4 also shows the impact velocity was up to 265 m/s at 14 �s.After the collision, the sample vibrated and this was shown by theoscillating of the absolute value for the measured velocity.

2.3. Laser impact welding (LIW)

Laser impact welding technology has recently been proposedand demonstrated (Daehn and Lippold, 2009). The initial experi-

ments were carried out with a Continuum Powerlite II model 9000Yttrium Aluminum Garnet (YAG) laser at Continuum Inc. in SantaClara, CA on materials, including aluminum alloy 1100 and low car-bon steel 1010. Both the aluminum to aluminum and aluminum tolow carbon steel plates were successfully welded.

Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952 949

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ig. 10. EDS study on EXW welded Al/Fe interface. (a) SEM image, the two whiteas in the pure solid state bond region, position (1); and the other crossed the inte

he pure solid state bond region at position (1). (c) Chemical composition distributi

Laser impact welding is also caused by direct solid state impactelding of a flyer and target, both of which are at nominal ambi-

nt temperature and does not produce gross melting of the metalshich are joined. It is quite different from the laser beam weldinghich has liquid pool along the welding line. As shown in Fig. 5,

ocused and pulsed laser beam shines through a transparent con-nement component adjacent to the flyer. The flyer plate has anptically absorbent surface, in this case a darkened surface from alack permanent marker. The laser ablates the ink and the result-

ng plasma reacts against the confinement. The ablation generatesplasma-based pressure pulse between the confinement and thelate. This accelerates the flyer sheet over a time interval of a fewanoseconds. The plates collide with high velocity. The short dura-ion of the high-pressure pulse causes the surfaces to co-deform,ossibly forming a plasma jet. When the clean metal surfaces meetnder pressure, metallurgical joining is accomplished. Fig. 5 alsohows that the impact angle and the standoff distance can bedjustable by simple geometric variations. The optical output ener-ies for LIW are in the order of a few joules, as opposed to several orens of kilojoules for MPW and megajoules for EXW. Accordingly,he sizes that can be bonded with each technique scale proportion-tely. LIW, it seems, can be tuned to produce impact welding for

lmost arbitrarily small foil thicknesses and length scales.

In the present study AA1100 plates and low carbon steel 1010lates were welded by LIW. The laser beam source was a Contin-um Powerlite II YAG laser, which could output a 3 J pulse in the

nfrared spectrum at 1064 nm wave length. The pulse width was

ndicate the linear EDS scan locations. Both of them were across the interface. Onellic phases, position (2). (b) Chemical composition distribution for linear scan fromlinear scan from the FeAl intermetallics region at position (2).

about 8 ns and the beam was focused to a spot size of a 3 mm diam-eter circle. For similar welding, the material was a low strengthaluminum alloy 1100. The flyer sheet was 3 mm wide and 20 mmlong, with a thickness of 0.175 mm. The target plate was large in alldimensions relative to the flyer plate and had a 15◦ taper as shownin Fig. 5. For dissimilar welding, the flyer plate was AA1100 and was0.05 mm thick, 3 mm wide and 20 mm long. The target plate waslow carbon steel 1010, which was again a large block with a 15◦

tapered flat surface. The real impact region on the flyer plate wasan approximately 3 mm diameter circle. The metal surfaces werelightly ground with sand paper and flattened by steel die prior towelding. For both similar and dissimilar welding, the experimentwas repeated and four samples for each set were investigated inthis paper.

The remainder of this paper provides a comparative study ofthe interface structures produced by EXW, MPW and LIW accord-ing to the conditions described above. For each welding method,a series of samples were prepared and the cross-sections weretaken through the middle part of the welded flyer–target inter-face, because the joint in the vicinity of the edge was usually weak.The samples were sectioned and polished through standard met-allurgical procedures. The final polish was conducted on a polish

cloth with 1 �m diamond paste. These sections were then exam-ined by an optical microscope (OM), scanning electron microscope(SEM) and energy dispersive X-ray spectroscopy (EDS). The micro-hardness was measured on the exposed surfaces with 10 g, 50 g and200 g load at 20 s dwell time. The wave feather and the microhard-

950 Y. Zhang et al. / Journal of Materials Processing Technology 211 (2011) 944–952

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ig. 11. EDS study on EXW welded Cu/Al interface. (a) SEM image, the two whiteas in the pure solid state bond region, position (1); and the other crossed the inte

he pure solid state bond region at position (1). (c) Chemical composition distributi

ess data were the average results collected from all of the samplesor each method.

. Results and discussion

.1. Wavy characterization

The welded interfaces from these three welding processes indi-ate wavy morphology as shown in Figs. 6(a and b), 7(a and b),and 8(a and b). For EXW and MPW, the wavy structures show bothon-symmetric and symmetric waves and it is similar for all of theepeated samples; for LIW, the interface is nearly flat and the wavesre more symmetric. The wave lengths and amplitudes increaseith both increasing input energy and sample thicknesses. In EXW,

he wave loses symmetry and forms crests shape interface. LIWurface demonstrates periodic sine waves with quite small waveengths and amplitudes. In all cases, the two materials do genuinelydhere to one another except the edge, and detailed mechanicalesting is in progress.

For each cross-section sample, the wave length and wave ampli-ude were measured. All of the samples in every welding methodhowed the similar wavy features, but at widely different ampli-udes. The average results from all of the examined samples areummarized in Table 1. The wave length generally correlates withmpact velocity. Since it has been suggested that the flyer plate

hickness (t) will influence the wave shape, wave length (�) andhe amplitude (A) (Ben-Artzy et al., 2006), in this study, the waveength and amplitude were normalized as �/t and A/t.

Fig. 9 is the map of the normalized wave length (�/t) and wavemplitude (A/t) for the three different welding methods. It indi-

ndicate the linear EDS scan locations. Both of them were across the interface. Onellic phases, position (2). (b) Chemical composition distribution for linear scan fromlinear scan from the CuAl intermetallics region at position (2).

cates that, high impact energies result in long wave length andincreased amplitude, and amplitude and wave length tend to scalewith each other. The lower energy intensities for MPW and LIWresult in smaller waves. With the large excess energy of EXW, theinterface still has periodic wave fashion, but the wave shape losesthe symmetry and has strong vortices. With lower impact velocityin LIW, the interface has smoother wave with reduced amplitude.

The wavy morphology increases the intimate contact area, andcan aid interlocking between two metal surfaces for strong bond.Most researchers think of the waviness as being caused by aKelvin–Helmholtz instability (Bahrani et al., 1967; Akbari Mousaviand Al-Hassani, 2005; Ben-Artzy et al., 2006). The oblique impactprocess generates intense shear stress and the shear stress is spreadout from the collision point. When the shear stress exceeds thewelding plate yield stress, the plates can be regarded as an inviscidfluids (Salem, 1980) and the high values of plastic strain occur nearthe impact. From the collision point to the entire mating interface,the velocity distribution, the strain and strain rate distribution havelarge variations. Interacting with the disturbances from the fluid-like welding plates, the variation is believed to result in the periodicwavy interface. Despite sample to sample differences, it appearsthat the same basic mechanism works over a range of length andenergy scales. Varied impact velocities and angles, which were notstudied presently, are also expected to cause variations.

3.2. Interfacial intermetallic phase formation

It is also interesting to point out that there is extensive inter-metallic phase formation in the corner of wave vortex for EXW andMPW as shown in Figs. 6(a and b) and 7(b), while the interface for

Y. Zhang et al. / Journal of Materials Process

Fig. 12. Summary of microhardness testing results for three welding methods. (a)Microhardness changes across EXW welded Al/Fe and Cu/Al interface. The testingload is 200 g and the dwell time is 20 s. (b) Microhardness changes across the inter-fl

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Acknowledgement

aces from MPW welded AA6061-T6 pairs and LIW welded AA1100 pairs. The testingoad for MPW interface is 50 g and for LIW interface is 10 g, the dwell time is 20 s.

IW is relatively flat and the intermetallic phases are largely absent.he pockets of intermetallics are likely formed by intense localeating, melting, mixing and rapid solidification in wave vortices.he interface forms discontinuous pockets, but the most weldedegion is free from intermetallic phases in all cases. The intermetal-ic phase along the EXW welded interface was taken as an examplen this paper because it was more obvious by larger length scale. Thehemical composition changes cross the interface were studied byDS from different locations as shown in Figs. 10(a) and 11(a). Andhe EDS results are shown in Figs. 10(b and c) and 11(b and c). In theegion of an apparent pure solid state bond, the chemical compo-ition profile has sharp interface composition change, as shown inigs. 10(b) and 11(b), whereas for the interface with intermetallics,

he composition gradually changes from one phase to other phaseith several steps. And these steps in Figs. 10(c) and 11(c) indicate

aried Al–Fe and Al–Cu compounds respectively.

ing Technology 211 (2011) 944–952 951

The structure and distribution of the intermetallic regions thatform in impact welding are quite important. Fusion welding ofdissimilar metals is essentially impossible, because wide regionsof intermetallic form and these are almost universally very brit-tle. Thus, fusion welded interfaces will have very poor reliabilityand poor mechanical strength. While intermetallics can form onimpact welding, these regions are discontinuous and the result-ing overall structure is expected to have excellent toughness in allcases, because none of these collision-welding processes lead to theformation of continuous intermetallic phases.

3.3. Interfacial microhardness

The average vickers microhardness (HV) values taken from theinterface region are shown in Fig. 12(a and b). For each of the threeimpact welding processes, the interfacial hardness is higher thanthat in the base metals. Local plastic deformation and impact shockharden the local regions. For EXW interface, within 250 �m widerange on either side, the hardness increases obviously, as show inFig. 12(a). Beyond the strengthened region, the hardness is same asthe base metals. For MPW and LIW, the hardened interface regionon either side is smaller and the width is about 50 �m and 20 �mrespectively, as shown in Fig. 12(b). The size of these hardenedzones largely scales with the energy input and wave lengths seen ineach case. This is significant because in fusion welding, local heat-ing generates heat affected zones (HAZ) that are almost universallysofter than the base metals, which reduces joint efficiency. Thus,impact welding holds the opportunity to produce welds with muchgreater joint efficiencies.

4. Conclusion

The structures that result from three types of solid state impactwelding processes are studied and compared, and the three weld-ing processes are explosive welding, magnetic pulse welding andlaser impact welding. The study shows that high velocity obliqueimpact welding is feasible for all length scales with proper impactangle and velocity. And all of them can be applied to join simi-lar and dissimilar materials. The wavy interface is observed at alllength scales, and it is presumed that cleaning of the interface byjetting is also important in all cases. The jetting removes surfaceoxide layer and the oblique impact brings the two clean surfacesjoint together. In each case a wavy interface was generated and thewave length and amplitude are related to both flyer plate thicknessand flyer impact velocity. High energy density results in long wavelength and great wave amplitude. EXW has wavy interface withvortex; MPW has wavy interface near sinusoidal profile; LIW hasa relatively gently curved surface. Interfacial intermetallic phasewas often found to exist in the wave vortex of explosively weldedsamples, but there was no continuous intermetallic layer alongthe welded interface. Intermetallics were possible but isolated inMPW, and were not observed in LIW. It was also found that allof the three welding processes result in the improved microhard-ness along the interface after high velocity impact welding. Whileexplosive welding has earned a reputation for excellent weld qual-ity between widely dissimilar metals, the magnetic pulse and laserimpact techniques produce similar structures. These techniquesare significant because they can be easily applied in typical manu-facturing environments, and therefore offer new opportunities fordissimilar metals welding.

The financial support from American Welding Society (AWS) fora student fellowship to Yuan Zhang is highly appreciated.

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eferences

izawa, T., Kashani, M., Okagawa, K., 2007. Application of magnetic pulse weldingfor aluminum alloys and SPCC steel sheet joints. Welding Journal 86, 119s–124s.

kbari Mousavi, A.A., Al-Hassani, S.T.S., 2005. Numerical and experimental studiesof the mechanism of the way interface formations in explosive/impact welding.Journal of Mechanics and Physics of Solids 53, 2501–2528.

ahrani, A.S., Black, T.J., Crossland, B., 1967. The mechanics of wave formation inexplosive welding. Proceedings of the Royal Society of London. Series A: Math-ematical and Physical Sciences 296 (1445), 123–136.

en-Artzy, A., Stern, A., Frage, N., Shribman, V., 2008. Interface phenomena inaluminium magnesium magnetic pulse welding. Science and Technology ofWelding and Joining 13 (4), 402–408.

en-Artzy, A., Stern A., Frage, N., Shribman, V., Sadot, O., 2006. Wave formationmechanism in magnetic pulse welding. Personal communication.

lazynski, T.Z., 1983. Explosive Welding, Forming, and Compaction. Applied SciencePublishers, London, New York.

otros, K.K., Groves, T.K., 1980. Characteristics of the wavy interface and the mecha-nism of its formation in high-velocity impact welding. Journal of Applied Physics51 (7), 3715–3721.

hizari, M., Al-Hassani, S.T.S., Barrett, L.M., 2009. Effect of flyer shape on the bondingcriteria in impact welding of plates. Journal of Materials Processing Technology209 (1), 445–454.

rossland, B., 1982. Explosive Welding of Metals and its Application. ClarendonPress, Oxford University Press, Oxford, New York.

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

zra, A.A., 1973. Principles and Practice of Explosive Metalworking. Garden CityPress, London.

rignon, F., Benson, D., Vecchio, K.S., Meyers, M.A., 2004. Explosive welding of alu-minum to aluminum: analysis, computations and experiments. InternationalJournal of Impact Engineering 30, 1333–1351.

okari, H., Sato, T., Kawauchi, K., Muto, A., 1998. Magnetic impulse welding ofaluminium tube and copper tube with various core materials. Welding Inter-national 12 (8), 619–626.

ing Technology 211 (2011) 944–952

Hutchinson, N., Zhang, Y., Daehn, G., Flores, K., 2009. Solid State Joining of Zr-BasedBulk Metallic Glass. TMS, San Francisco, CA.

Johnson, J.R., Taber, G., Vivek, A., Zhang, Y., Golowin, S., Banik, K., Fenton, G.K., Daehn,G., 2009. Coupling experiment and simulation in electromagnetic forming usingphoton Doppler velocimetry. Metal Forming 80 (5), 359–365.

Khrenov, K.K., Chudakov, V.A., 1969. Magnetic pulse welding of butt joints betweentubes. Automatic Welding USSR 22, 75.

Kistersky, L., 1996. Welding process turns out tubular joints fast. MetalworkingDistributor 140 (4), 41–42.

Kore, S.D., Date, P.P., Kulkarni, S.V., 2007. Effect of process parameters on electro-magnetic impact welding of aluminum sheets. International Journal of ImpactEngineering 34, 1327–1341.

Lee, K.-J., Shinji, K., Takashi, A., Aizawa, T., 2007. Interfacial microstructure andstrength of steel/aluminum alloy lap joint fabricated by magnetic pressure seamwelding. Materials Science and Engineering A 471, 95–101.

Palmer, T.A., Elmer, J.W., Brasher, D., Butler, D., Riddle, R., 2006. Development of anexplosive welding process for producing high-strength welds between niobiumand 6061-T651 aluminum. Welding Journal, 252–263.

Robinson, J.L., 1977. Fluid mechanics of copper: viscous energy dissipation in impactwelding. Journal of Applied Physics 48 (6), 2202–2207.

Salem, S.A.L., 1980. Some aspects of liquid/solid interaction at high speeds. Ph.D.Thesis. UMIST.

Shribman, V., Gafri, O., 2001. Magnetic Pulse Welding and Joining—A New Tool forthe Automotive Industry.

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

Tamaki, K., Kojima, M., 1988. Factors affecting the result of electromagnetic weld-ing of aluminum tube. Transactions of the Japan Welding Society 19 (1),53–59.

Turcotte, R., Lightfoot, P.D., Fouchard, R., Jones, D.E.G., 2003. Thermal hazard assess-ment of AN and AN-based explosives. Journal of Hazardous Materials 101 (1),1–27.

Uhlmann, E., Hahn, R., Jurgasch, D., 2005. Pulsed-magnetic hot joining-new solu-tions for modern lightweight structures. Steel Research International 76 (2),245–250.