Investigation on the Explosive Welding of 1100 AluminumAlloy and AZ31 Magnesium Alloy

Pengwan Chen, Jianrui Feng, Qiang Zhou, Erfeng An, Jingbo Li, Yuan Yuan, and Sanli Ou

(Submitted September 28, 2015; in revised form April 12, 2016; published online June 7, 2016)

The undesirable properties of magnesium alloys include easy embrittlement, low oxidation resistance, anddifficulty in welding with other materials. Their application in industry is, therefore, restricted. In this paper,plates of 1100 aluminum alloy and AZ31 magnesium alloy were successfully welded together using theexplosive welding technique. The influences of the welding parameters on the weld quality were investigated.The surface morphology and microstructure near the weld interface were examined by optical microscopy,scanning electron microscopy (equipped with energy-dispersive x-ray spectroscopy), and transmission elec-tron microscopy. The experimental results demonstrated the typical wavy bonding interface. In addition,elemental diffusion with a thickness of approximately 3 lm occurred near the bonding interface. The twoplates were joined together well at the atomic scale. Nanograins with a size of approximately 5 nm wereobserved in the diffusion layer. The microhardness and shear strength were measured to evaluate themechanical properties, which confirmed that a high quality of bonding was acquired.

Keywords aluminum alloy, explosive welding, magnesium alloy,mechanical properties, microstructure

1. Introduction

Magnesium alloys are promising materials in industry due totheir low density, high specific strength, excellent machinabil-ity, and high damping capacity, but are restricted in applicationby some undesirable properties, such as extreme chemicalreactivity, poor corrosion, and wear resistance. While joining ofmagnesium alloys to other materials, which allow combiningthe beneficial advantages of each component, can be apromising solution to the aforementioned problems, and willalso find wide application in welding technique. Among metalmaterials, aluminum alloys, which are well qualified for theirhigh strength-to-weight ratio, perfect capacity for plasticdeformation and excellent ductility, are used extensively inthe fields such as the aerospace and automobile industries.Additionally, a stable oxide film that covers the surface of thealuminum alloys can work as a good protector from corrosion.Therefore, welded plates of magnesium and aluminum alloyswill be promising structural materials and will be of greatbenefit on reducing energy consumption.

Several techniques have been used in the past to weldaluminum and magnesium alloys. Borrisutthekul et al. (Ref 1)welded aluminum and magnesium alloys using the laserwelding technique. The thickness of the intermetallic layerwas easily controlled using edge-line welding of the lap jointmethod, and a high joining strength was obtained. Firouzdor

et al. (Ref 2) used the friction stir welding method on 6061 Aland AZ31 Mg alloys and investigated the effect of weldingconditions. They demonstrated that welding conditions affectedthe formation of intermetallics and even liquids. Chen et al.(Ref 3) welded the 7075 aluminum and AZ31 magnesium alloysusing vacuum diffusion bonding. They concluded that thequality of the welding joints was affected by bonding temper-ature and holding time. Compared to the above-mentionedwelding techniques, explosive welding is an effective method toreduce the formation of brittle intermetallics (Ref 4, 5). Thus far,some attempts have been made to weld aluminum andmagnesium alloys using the explosive welding method. Ghaderiet al. (Ref 6) analyzed the plates using optical microscopy andscanning electron microscopy and proposed the lower limit ofthe welding parameters based on experimental results. Yan et al.(Ref 7) welded the plates using explosive welding and analyzedtheir microstructure and mechanical properties. They concludedthat explosive welding is an ideal method to weld plates ofaluminum and magnesium alloys together. However, for theexplosive welding of aluminum and magnesium alloys, muchwork, including the influence of welding parameters, themicrostructure variations near the bonding interface, and themechanical properties, is still needed for further investigation.

In this work, we joined the plates of 1100 aluminum andAZ31 magnesium alloys using explosive welding method. Theinfluence of the welding parameters, including the explosivethickness and standoff distance on the bonding interface, wasanalyzed. The surface morphology and microstructure wereexamined using optical microscopy, scanning electron micro-scopy, and transmission electron microscopy. The mechanicalproperties, including microhardness and shear strength, weretested to evaluate the weld quality.

2. Experimental

Commercial plates of 1100 aluminum and AZ31 magnesiumalloys were selected for explosive welding. A sketch of the

Pengwan Chen, Jianrui Feng, Qiang Zhou, Erfeng An, Yuan Yuan,and Sanli Ou, State Key Laboratory of Explosion Science andTechnology, Beijing Institute of Technology, Beijing 100081, China;and Jingbo Li, School of Materials Science and Engineering, BeijingInstitute of Technology, Beijing 100081, China. Contact e-mail:pwchen@bit.edu.cn.

JMEPEG (2016) 25:2635–2641 �ASM InternationalDOI: 10.1007/s11665-016-2088-2 1059-9495/$19.00

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initial configuration for explosive welding is illustrated inFig. 1. An AZ31 magnesium alloy plate, with dimensions of200 mm9 150 mm9 10 mm, was selected as the base plate.To avoid the negative boundary effect during the explosivewelding process, an aluminum plate, with dimensions of250 mm9 200 mm9 2 mm (which is larger than the magne-sium plate), was selected as the flyer plate. Four supports oneach corner of the magnesium plate were used to support thealuminum plate. Prior to welding, the magnesium plates wereheat treated at 380 �C for 12 h to homogenize the metallurgicalstructure and improve the impact toughness. To ensure a high-quality weld, the roughness of the bonding surface should beless than 1 lm, which can be obtained by polishing using 600-grit abrasive paper. The rock-expanded ammonium nitrate, witha density of 0.85 g/cm3 and a detonation velocity of approx-imately 3200 m/s, was used in the experiments. An electricdetonator was inserted into the left part of the explosive. Bytriggering the electric detonator, the explosive is detonated anddrives the aluminum plate to impact the magnesium plate.During the high-speed collision process, the aluminum andmagnesium alloy plates are instantaneously welded together.The experimental conditions are shown in Table 1. The impactvelocities were calculated based on the equation proposed byVaidyanathan et al. (Ref 8).

After welding, specimens for metallographic observationswere cut paralleling the explosion direction from the weldedplates. The samples for microstructural observation wereprepared using standard metallographic techniques and etchedfor 30 s in a solution containing 1 mL of HNO3, 1 mL ofCH3COOH, 1 g of C2H2O4, and 150 mL of de-ionized water.The microstructure examinations of the samples were carriedout using optical microscopy, Hitachi S4800 scanning electronmicroscope, and JEM-2010 high-resolution transmission elec-tron microscope. ATEM sample with a thickness of 60 lm wasprepared and thinned using a focused ion beam (FIB)instrument. Elemental analysis of the welded samples wasperformed using the thermo scientific NORAN system 7energy-dispersive spectrometer with a beam spot size of 5 nm.

The Vickers hardness profile was measured across thebonding interface using a HVS-1000Z tester with a load of100 g at different regions. The shear test was carried outaccording to the GB/T6396-1995. Figure 2 shows the shear testspecimen and a schematic of the shear test apparatus. Theshearing area of the sample was 25 mm9 3 mm, and thecompression speed was 1 mm/min. The shear stress s is

s ¼ P=ae;

where a and e are the width and height of test interfaces,respectively.

3. Results and Discussion

Figure 3 presents the welded plates being cut from sample#2 using a wire-electrode cutter. It is observed that the Al andMg alloy plates were joined together tightly, and no obviouscracks were present. This indicates that explosive welding is aneffective method to join 1100 aluminum alloy and AZ31magnesium alloy plates.

3.1 Microstructure of the Weld Interface

The low magnification microstructures of #1, #2, and #6explosive weld samples are shown in Fig. 4(a)-(c) and revealthe wavy nature of the interface. Due to the density differencesand the inclined collision between Al and Mg, the wavyinterface is asymmetric and spikes toward the aluminum plate.The interfacial waves of each sample are not the same due todifferent welding parameters. For sample #1, a regular wavyinterface can be observed, as shown in Fig. 4(a). An increase inthe welding parameters, such as the explosive thickness,expanded the wavy interface and a vortex appeared, as shownin Fig. 4(b). Upon further increases in the welding parameters,both the wavy interface and the vortex were enlarged, and acavity appeared at the center of the vortex, as shown inFig. 4(c).

Table 1 lists the results of the interfacial waves with respectto different welding parameters. The formation of the wavyinterface was caused by a jet that penetrated the bondinginterface. As mentioned by Crossland (Ref 4), during the high-speed collision, high temperature and pressure are generated atthe collision point, causing the jet to eject in the front of thecollision point with a velocity greater than 5000 m/s. Due to theinclined collision and resistance of the bonding surface, the jetchanges direction to penetrate the bonding surfaces periodi-cally, thereby forming the wavy interface. The interfacial wavesof Al-Mg were much larger than those of other welded plates,such as Cu-steel and W-Cu, as described by Bina et al. (Ref 9)

Fig. 1 Schematic view of the explosive welding of Al-Mg alloyplates

Table 1 Experimental conditions and some welding results

No.Explosive

thickness, mmStandoff

distance, mmImpact velocity,

m/sCollisionangle, �

Impactenergy, KJ

Wave length,lm

Wave amplitude,lm

Shear strength,MPa

#1 9 1 680 12.20 62.42 330 70 62#2 9 2 822 14.76 91.22 451 94 63#3 13.5 1 807 14.49 87.92 440 95 64#4 13.5 2 976 17.54 128.60 1046 281 68#5 18 1 911 16.37 112.04 985 285 64#6 18 2 1103 19.85 164.24 1129 258 71

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and Manikandan et al. (Ref 10). This is because of the lowerhardness of Al-Mg, in which case the jet will have sufficientenergy to penetrate the bonding surfaces to form the largerinterfacial waves. Therefore, due to the increase in the weldingparameters, the jet will have much more energy to penetrate thebonding surface to form larger interfacial waves. The formationof the vortex at the back of the interfacial waves is also due tojet penetration. Due to surface resistance, the jet will be dividedinto two parts when penetrating the aluminum plate. Most ofthe jet will travel forward; however, a small part will swirl back

Fig. 2 Shear strength tests: (a) A shear test sample and (b) theschematic illustration of the shear test apparatus

Fig. 3 Welded sample of the 1100 aluminum alloy and AZ31 mag-nesium alloy plates

Fig. 4 Microstructures of the interfacial waves using the opticalmicroscope: (a) #1, (b) #2, and (c) #6

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in a counterclockwise direction continually penetrating thealuminum to form the vortex. The cavity formation is caused bythe high welding parameters. In this situation, part of the air inthe front of the collision zone will be captured by the jet andswirl back with the jet to penetrate the aluminum, therebycausing the formation of cavities at the center of vortex.

To observe the microstructure more clearly, the enlargedmicrographs from sample #2 are shown in Fig. 5 using SEM.Figure 5(a) shows the fine-grain crystals near the bondinginterface on the magnesium side. The size of these grains is

approximately 0.5-1 lm and is much smaller than the originalsize, which is 10-30 lm. The formation of these fine grains iscaused by dynamic recrystallization during the high-strain-rateimpact process. Adiabatic shear bands (ASBs) can be observedon the magnesium side in Fig. 5(b), which originate from thefine-grain crystals near the bonding interface and disappear inthe region far away from the interface. This phenomenon is dueto drastic plastic deformation under a high strain rate at thecollision points, which results in rapidly rising temperature forthe formation of adiabatic shear bands. Away from the adiabaticshear bands, a deformation twin structure appears, as shown inFig. 5(c). The reason for this occurrence is that away from thecollision point, the temperature falls significantly and is nothigh enough to form ASBs, but the high strain can beaccommodated by twinning, leading to the formation of thetwin structure.

Transmission electron microscopy (TEM) was used toanalyze bonding interfaces at the nanoscale. Fine-grain crystals,with an approximate size of 100 nm, were generated at thealuminum and magnesium sides near the bonding interface, asshown in Fig. 6(a) and (b). These fine-grain crystals are muchsmaller and closer to the bonding interface compared to thecrystals observed using the SEM mentioned earlier. Theformation of these grains is also caused by dynamic recrystal-lization during the high-strain-rate impact process. A transitionlayer was observed at the bonding interface, and the interface ofMg-transition layer is shown in Fig. 6(c).

The atomic-scale Al-transition layer and Mg-transition layerbonding interfaces are shown in Fig. 6(d) and (e), respectively.The results indicate that these materials are bonded together atthe atomic scale. Nanograins with a size of approximately 5 nmwere observed in the transition layer, as shown in Fig. 6(f). It isrecognized that during a high-speed collision, high temperatureis generated, causing the bonding interface to melt. The fastcooling rate results in the gradual generation of nanograins. Theformation of nanocrystals at the bonding interface of Cu-Al wasalso observed using TEM by Zhou et al. (Ref 11).

3.2 Elemental Analysis

Elemental line scan analysis across the bonding interface[wave crest and wave trough, labeled as 1 and 2 in Fig. 7(a)]was conducted using SEM equipped with EDS to investigatethe elemental variation. The results are shown in Fig. 7(b) and(c), revealing that elemental diffusion, with a thickness ofapproximately 3 lm, occurred at both the wave crest and thewave trough, and no any trace of intermetallics is detected inthe bonding interface. During the drastic collision, hightemperature (>1000 K) and pressure (>1010 Pa) (Ref 12,13) are generated at the collision point, causing the elementaldiffusion. What is more, due to the fast welding characteristic(approximately 10�6 s) of explosive welding, there is notenough time to form intermetallic compound at the bondinginterface. Yan et al. (Ref 7) reported similar result of elementaldiffusion on the Al/Mg interface after explosive welding.However, intermetallic compounds may easily form whenusing other welding methods, such as laser welding (Ref 1),friction stir welding (Ref 2), and diffusion welding (Ref 3),which can be attributed to their long heating time. Therefore, itcan be concluded that explosive welding is an effective methodto avoid the formation of intermetallic compounds at thebonding interface. However, for the explosive welding of othermaterials, such as Al/Cu (Ref 14), Ti/steel (Ref 15), and Al/

Fig. 5 Microstructure at Mg alloy side: (a) fine grains near thebonding interface, (b) ASBs structure, and (c) twin structure

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steel (Ref 16), intermetallics are detected at the bondinginterface, which may be attributed to their higher weldingparameters.

Due to the fast welding process, it is not easy to formintermetallics at the bonding interface. However, many litera-ture studies have reported the formation of intermetallics at thevortex (Ref 17, 18). As mentioned above, the vortex formation

is due to the high-speed jet (>5000 m/s) penetrating thebonding plate, in which the generated heat is not able to rapidlydissipate. In this paper, the elemental composition of the vortexis measured using the EDS. An Al-rich phase with aconcentration of approximately 73.1 at.% Al and 26.9 at.%Mg is observed. The result indicates that a possiblemetastable phase of Al2Mg is formed at the vortex.

Fig. 6 Microstructure of the weld interface using TEM: (a) fine grains of Al alloy near the bonding interface, (b) fine grains of Mg alloy nearthe bonding interface, (c) interface of the Mg-transition layer, (d) interface of the Al-transition layer at the atomic scale, (e) interface of the Mg-transition layer at the atomic scale, and (f) nanograins at the transition layer

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3.3 Mechanical Properties

Microhardness measurements were performed across thebonding interface of sample #2. The original microhardnessesof the 1100 aluminum and AZ31 magnesium alloys weremeasured to be approximately 42 and 92 HV, respectively. Thehardnesses for both aluminum and magnesium sides away from

the bonding interface were almost equal to the original value.However, the hardness near the bonding interface tends toincrease, as shown in Fig. 8. In the vicinity of the interface, thehardness of aluminum exceeded 50 HV and the hardness ofAZ31 was greater than 110 HV, which is due to the dynamichardening caused by drastic plastic deformation. The resultsobtained from the vortex exhibits significantly different hard-ness values. The average measured microhardness in the vortexis approximately 142 HV, which is much stiffer than thebonding interface.

Shear strength is a significant factor for evaluating thebonding interface and general welding quality. The measuredshear strengths for each welding sample are listed in Table 1.During the shear strength test, failure occurred at the 1100aluminum alloy side, but no crack was seen at the bondinginterface, which indicates that the shear strength of the bondinginterface is higher than the measured values of 60-70 MPa. Theshear strength is independent of the bonding interface but tendsto increase slightly with an increase in the welding parameters.This explains the hardening of the aluminum and magnesiumalloys near the bonding interface. As the welding parametersincreased, a higher impact velocity was generated at thecollision point, causing a higher strain rate loading. Therefore,more hardening occurred at both the aluminum and magnesiumalloy sides near the bonding interface.

4. Conclusions

In this paper, plates of 1100 aluminum and AZ31 magne-sium alloys were successfully welded together using theexplosive welding technique. A wavy interface was generatedat the bonding interface, and the size of the interfacial wavestended to increase with increasing welding parameters. Finegrains, with a size of approximately 100 nm, were generated inthe region near the bonding interface. Nanograins, with a sizeof approximately 5 nm, were generated in the bondinginterface. An elemental diffusion layer with a thickness ofapproximately 3 lm was observed between the plates. Thehardness near the bonding interface tended to increase. Finally,shear strength tests confirmed that a high-quality weld wasacquired through explosive welding.

Fig. 7 (a) Elemental analysis using SEM and EDS, (b) elementalline scan across the wave crest, and (c) elemental line scan acrossthe wave trough

Fig. 8 Hardness profile across the welded interface of sample #2

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Acknowledgments

The authors would like to express their thanks for the financialsupport of the National Natural Science Foundation of China underGrant Nos. 11521062 and 11472054.

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