Joining Aluminium Alloy 5A06 to Stainless Steel 321 by Vaporizing Foil Actuators Welding with an Interlayer

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Article Joining Aluminium Alloy 5A06 to Stainless Steel 321 by Vaporizing Foil Actuators Welding with an Interlayer

Shan Su 1, Shujun Chen 1,*, Yu Mao 2, Jun Xiao 1, Anupam Vivek 2 and Glenn Daehn 2

1 College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, 100 Ping Le Yuan, Beijing 100124, China; [email protected] (S.S.); [email protected] (J.X.) 2 Department of Materials Science and Engineering, The Ohio State University, 2041, College Road, Columbus, OH 43221, USA; [email protected] (Y.M.); [email protected] (A.V.); [email protected] (G.D.) * Correspondence: [email protected]; Tel.: +86-010-67391620

Received: 27 November 2018; Accepted: 29 December 2018; Published: 5 January 2019

Abstract: Direct aluminium–stainless steel joints are difficult to create by the vaporized foil actuator welding (VFAW) method because brittle intermetallic compounds (IMCs) tend to form along the interface. The use of an interlayer as a transition layer between the two materials with vast difference in hardness and ductility was proposed as a solution to reduce the formation of the IMCs. In this work, VFAW was used to successfully weld sheet aluminium alloy 5A06 to stainless steel 321 with a 3003 aluminium alloy interlayer. Input energy levels of 6 kJ, 8 kJ, 10 kJ, and 12 kJ were used and as a trend, higher energy inputs resulted in higher impact velocities, larger weld area, and better mechanical properties. In lap-shear and peel testing, all samples failed at the interface of the interlayer and target. At 10 kJ energy input, flyer velocities up to 935 m/s, lap-shear peak load of 44 kN, and peel load of 2.15 kN were achieved. Microstructure characterization and element distribution were performed, and the results show a wavy pattern created between the flyer and interlayer which have similar properties, and the interface between the interlayer and target was dominated by element diffusion and IMCs identified mainly as FeAl3 and FeAl. The results demonstrate VFAW is a suitable joining method for dissimilar metals such as aluminium alloy and stainless steel, which has a broad and significant application prospect in aerospace and chemical industry.

Keywords: dissimilar materials; interlayer; vaporizing foil actuators welding

1. Introduction Aluminium–stainless steel sheet joints are of great interest for purposes of lightweighting and corrosion protection in industry applications [1]. Aluminium can be joined with stainless steel via adhesive bonding or mechanical fastening [2]. However, in order to weld aluminium to stainless steel, special techniques are required. Dissimilar metals fusion welding processes have many difﬁculties as a result of metallurgical incompatibility [3], the formation of brittle phases, the segregation of high- and low-melting phases due to chemical mismatch, and possibly large residual stresses from the physical mismatch. Fusion-based welding techniques such as resistance spot welding [4] (RSW) and arc and laser welding-brazing [5–8] are most common in aluminium–stainless steel welding. The joints welded through the fusion-based welding process produce brittle intermetallic compound (IMC), which easily leads to fracture due to different physical and chemical properties between dissimilar metals. Solid state welding is a suitable welding method for joining dissimilar metals and can overcome some of the disadvantages associated with the fusion-based welding process [9]. Several solid state welding methods, including explosion welding [10–15] (EXW), ultrasonic welding [16], magnetic

Metals 2019, 9, 43; doi:10.3390/met9010043 www.mdpi.com/journal/metals Metals 2019, 9, 43 2 of 11 pulse welding [17,18] (MPW), and friction stir welding [1,19,20] (FSW), have been used to weld the dissimilar materials such as aluminium alloy and stainless steel. EXW and MPW are variants of impact or collision welding, which has been established as a fast and reliable technology [15]. MPW is applied for the weld length of centimeters, while EXW is more suitable for meters. Corigliano et al. [10,11] investigated the static and fatigue bending tests of explosive welded joints of ASTM A516 low carbon steel and A5086 aluminium alloy with pure aluminium as an interlayer. Han et al. [21] use AA1050 plate as interlayer in EXW of AA5083 aluminium alloy plate and SS41 steel plate. The thin interlayer enhanced the bond strength and suppressed the formation of the brittle interfacial zone and the thickness of the generated interfacial zone increased as the thickness of the interlayer increased from 0.5 mm to 2 mm. Manikandan et al. [22] employed different thickness of interlayer to analyze the energetic conditions of the titanium/304 stainless steel joints welded by explosive welding process. The results show that a thin interlayer leads to successful welding as the use of interlayer splits the kinetic energy deposition between the two interfaces and thereby reduces the possibility of melting or formation of IMCs. Vaporizing foil actuator welding (VFAW) is a novel solid state welding technology making use of the impulse created by vaporization of the metallic foil or wire by the passage of a high current [23]. The high-amplitude impulse accelerates the ﬂyer workpiece toward the target workpiece. The high velocity oblique impact spits out the surface contaminants in the form of a jet and the high pressure metal-to-metal contact leads to formation of metallic bonds [24]. The technology proves to be robust to join several dissimilar metals such as Al–Mg, Al–Cu, Al–Fe, Al–Ti, etc. [25–28]. However, further development and understanding of the process is needed in order to accomplish industrial adoption. There are also certain pairs which cannot be joined directly with this process. In such cases, an interlayer can prove to be useful. In this study, 3003 aluminium alloy was chosen as the interlayer in the 5A06 aluminium alloy and 321 stainless steel VFAW welding process. This research further studied the effects of the interlayer on the microstructure and mechanical properties of the welds. Welded joints were evaluated based on mechanical strength and microstructure. The characteristics of mechanical interlocking and metallurgical bonding areas were studied by advanced microscopy.

2. Experimental Procedure

2.1. Materials and Methods This work follows the VFAW method described by Vivek et al. [23]. The ﬂyer sheet was placed directly against an aluminium foil which was insulated by a 0.12 mm thick polyimide (trade name Kapton) tape and the ends of steel terminals were connected to a capacitor bank. The characteristics of the capacitor bank are shown in Table1.

Table 1. Capacitor bank characteristics.

Capacitance Inductance Resistance Maximum Charging Energy Short Circuit Current Rise Time 426 µF 100 nH 10 mΩ 16 kJ at 8.66 kV 12 µs

As the capacitor bank was discharged, the foil vaporized in 10~15 µs under the current of 100~300 kA. The flyer sheet accelerated to a high velocity (300~1200 m/s) by the forces of the vaporized foil. 0.076 mm thick aluminium foils were used for energy input lower than 10 kJ, and 0.125 mm thick foils were used for 12 kJ energy input. The active area of the foils was 12.7 mm wide and 50.8 mm long. A schematic of the apparatus as well as the actual implementation, the sketch of the aluminium foil, and the schematic of VFAW process are shown in Figure1. Annealed aluminium alloy type 5A06 (1.8 × 70 × 140, in mm) and 3003 (1.02 × 70 × 70, in mm) sheets were chosen as the flyer and interlayer material. 321 stainless steel sheets were cut to 4 mm × 70 mm × 140 mm. The standoff distances between the flyer and interlayer and the interlayer and target were 3 mm and 1.5 mm, respectively, Metals 2019, 9, 43 3 of 11 Metals 2019, 9, x FOR PEER REVIEW 3 of 12 Metals 2019, 9, x FOR PEER REVIEW 3 of 12 mm,inmm, all respectively, experiments.respectively, in Thein all all impacting experiments. experiments. surfaces The The ofimpacting impacting all materials surfaces surfaces were of cleanedof all all material material priors to swere weldingwere cleaned cleaned with prior acetoneprior to to weldingafterwelding being with with ground acetone acetone by after emeryafter being being paper. ground ground by by emery emery paper. paper.

PDVPDV probe probe

(a)(a) (b) (b) 203203 Target sheet Target sheet InterlayerInterlayer sheet sheet 50.850.8 31.5 31.5 RR 25.4 25.4 Standoff Standoff Foil Flyer sheet 12.7 sheet Foil Flyer sheet 12.7 sheet (c) (d) (c) (d) Figure 1. (a) actual implementation of the vaporizing foil actuator welding (VFAW) apparatus; (b) FigureFigure 1. 1. (a()a actual) actual implementation implementation of ofthe the vaporizing vaporizing foil foilactuator actuator welding welding (VFAW) (VFAW) apparatus; apparatus; (b) impact velocity test apparatus; (c) sketch of the aluminium foil; (d) schematic of the VFAW process. impact(b) impact velocity velocity test testapparatus; apparatus; (c) sketch (c) sketch of the of the aluminium aluminium foil; foil; (d) ( dschematic) schematic of ofthe the VFAW VFAW process. process.

2.2.2.2.2.2. Velocity VelocityVelocity Measurement MeasurementMeasurement InputInputInput energies energiesenergies of ofof 6 66 kJ, kJ,kJ, 8 88 kJ, kJ,kJ, 10 1010 kJ, kJ,kJ, and andand 12 1212 kJ kJkJ were werewere used usedused in inin this thisthis study. study.study. Current CurrentCurrent and andand voltage voltagevoltage of ofof the thethe foilfoilfoil vaporized vaporizedvaporized in inin VFAW VFAWVFAW process process were werewere measured measured using usingusing a aa 50 5050 kA:1 kA:1kA:1 V VV Rogowski RogowskiRogowski coil coilcoil and andand 1000:1 1000:11000:1 high highhigh voltagevoltagevoltage probe, probe, respectively.respectively. respectively. The The The velocity velocity velocity of of flyerof flyer flyer and and and interlayer interlayer interlayer impact impact impact to the to to targetthe the target target was recordedwas was recorded recorded using usingphotonicusing photonic photonic Doppler Doppler Doppler velocimetry velocimetry velocimetry (PDV) (PDV) [(PDV)29]. A [29] [29] hole. .A A was hole hole drilled was was drilled indrilled the centerin in the the center of center the backingof of the the backing backing steel fixture steel steel fixtureandfixture the and and interlayer the the interlayer interlayer using transparentusing using transparent transparent acrylic acrylic acrylic as the as replacementas the the replacement replacement of the of targetof the the target target sheet sheet tosheet allow to to allow allow the laser the the laserfocusinglaser focusing focusing probe probe toprobe look to to at look look the at interlayer at the the interlayer interlayer directly. directly. directly. The velocity The The velocity velocity at any distance at at any any distance distance within this within within range this this can range range then canbecan estimatedthen then be be estimated estimated by integration by by integration integration of the resulting of of the the resulting velocity–timeresulting velocity–time velocity–time curve. curve. curve. 2.3. Strength Testing 2.3.2.3. Strength Strength Testing Testing Lap-shear and peel tests were used to analyze the joint strength. A schematic of the strength Lap-shearLap-shear and and peel peel tests tests were were used used to to analyze analyze the the joint joint strength. strength. A A schematic schematic of of the the strength strength testing is shown in Figure2. Three samples from each input energy were tested. For peel testing, testingtesting is is shown shown in in Figure Figure 2. 2. Three Three samples samples from from ea eachch input input energy energy were were tested. tested. For For peel peel testing, testing, the the the flyer sheets were bent to 90◦ with respect to the interlayer and target, and the target sheets were flyerflyer sheets sheets were were bent bent to to 90° 90° with with respect respect to to the the in interlayerterlayer and and target, target, and and the the target target sheets sheets were were fixed fixed fixed to a steel die by a bolt. Testing was carried out using an MTS810 mechanical testing frame at a toto a a steel steel die die by by a a bolt. bolt. Testing Testing was was carried carried out out using using an an MTS810 MTS810 mechanical mechanical testing testing frame frame at at a a constant extension rate of 0.1 mm/s. constantconstant extension extension rate rate of of 0.1 0.1 mm/s. mm/s.

InterlayerInterlayer LoadLoad LoadLoad FlyerFlyer TargetTarget

(a(a) ) (b(b) ) Figure 2. Mechanical testing schematic. (a) Peel test; (b) Lap-shear test. FigureFigure 2. 2. MechanicalMechanical testing testing schematic. schematic. ( (aa)) Peel Peel test; test; ( (bb)) Lap-shear Lap-shear test. test.

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Metals 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 4 of 12 Interfacial morphologies and element content and distribution in the interface of interlayer and Interfacial morphologies and element content and distribution in the interface of interlayer and target were observed by scanning electron microscopy (SEM) using a Zeiss Ultra55 equipped with a target were observed by scanning electron microscopy (SEM) using a Zeiss Ultra55 equipped with a silicon drift detector for energy-dispersive X-ray spectroscopy (EDS). Optical microscopy was used to silicon drift detector for energy-dispersive X-ray spectroscopy (EDS). Optical microscopy was used analyzeto analyze the interfacial the interfacial morphologies morphologies of the of the ﬂyer flyer and and interlayer. interlayer. 3. Results and Discussion 3. Results and Discussion 3.1. Velocity, Current, and Voltage Traces 3.1. Velocity, Current, and Voltage Traces The temporal evolutions of current, voltage, and velocity of the ﬂyer sheet with 8 kJ energy input The temporal evolutions of current, voltage, and velocity of the flyer sheet with 8 kJ energy input in VFAW process are shown in Figure3. in VFAW process are shown in Figure 3.

Burst time 800 Velocity of flyer Velocity (m/s) Velocity Velocity (m/s) Velocity 600 ；；

400 Current 200 Voltage (×10V) Voltage (×10V) 0

； 0 20406080

； 0 20406080 -200 Voltage

-400 Time (μs) Current (×200A) Current (×200A) FigureFigure 3. 3.Voltage, Voltage, current, current, andand velocityvelocity traces with with 8 8 kJ kJ energy energy input. input.

TheThe flyer flyer sheet sheet has has a low a low acceleration acceleration (from (from 25 to25 33to µ33s) μ unders) under the the electromagnetic electromagnetic interaction interaction force generatedforce generated by the current by the carryingcurrent carrying foil [30]. foil The [30] voltage. The has voltage a sharp has spike a sharp rise spike and arise sudden and a decrease sudden of current,decrease while of current, the flyer while sheet the accelerated flyer sheet to accelerated 393 m/s instantto 393 m/s at about instant 33.5 at aboutµs. This 33.5 moment μs. This alsomoment called burstalso time called in burst the VFAW time in process. the VFAW This process. voltage This spike voltage rise isspike a typical rise is phenomenon a typical phenomenon in foil vaporized in foil processvaporized due toprocess the resistance due to the increase resistance when increase the foilwhen is the vaporized foil is vaporized and converted and converted into high into pressure high pressure plasma. The flyer and interlayer sheet impacted at the target sheet at a velocity of 788 m/s. plasma.pressure The plasma. flyer and The interlayer flyer and sheet interlayer impacted sheet at impacted the target at sheet the target at a velocity sheet at of a 788velocity m/s. of The 788 velocity m/s. The velocity trace of flyer sheet with other three energies input are shown in Figure 4. The impact traceThe of velocity flyer sheet trace with of flyer other sheet three with energies other inputthree energies are shown input in Figureare shown4. The in impactFigure velocities4. The impact were velocities were 635 m/s, 788 m/, 935 m/s, and 1025 m/s for input energies of 6 kJ, 8 kJ, 10 kJ, and 12 kJ, 635velocities m/s, 788 were m/, 635 935 m/s, m/s, 788 and m/, 1025 935 m/sm/s, and for input1025 m/s energies for input of 6 energies kJ, 8 kJ, of 10 6 kJ, kJ, and8 kJ, 12 10 kJ,kJ, respectively.and 12 kJ, respectively.

1200 6 kJ 8 kJ 1000 1000 10 kJ 12 kJ 800

600

400 Flyer velocity (m/s) velocity Flyer Flyer velocity (m/s) velocity Flyer 200

0 012345 Distance (mm) FigureFigure 4. 4.Flyer Flyer sheet sheet velocity velocity tracestraces atat 66 kJ, 8 kJ, 10 10 kJ, kJ, and and 12 12 kJ kJ input input energies. energies.

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3.2. Mechanical Testing 3.2. Mechanical Testing Mechanical testing results from lap-shear and peel tests are summarized in Table 2. The peak loadMechanical of lap-shear testing and results peel tests from in lap-shearcreased with and energy peel tests input are increased summarized from in 6 Table kJ to2 10. The kJ. peakThe average load of lap-shearlap-shear and peelloads tests were increased 29.9 kN, withand 35 energy kN, 44 input kN, and increased 42.6 kN from for 6the kJ 6 to kJ, 10 8 kJ. kJ, The 10 kJ, average and 12 lap-shear kJ input Metals 2019, 9, x FOR PEER REVIEW 5 of 12 loadsenergies, were 29.9 respectively. kN, and 35 The kN, average 44 kN, peak and 42.6peel kN loads for were the 6 1.62 kJ, 8kN, kJ, 101.65 kJ, kN, and 2.15 12 kJkN,input and energies,2.03 kN, respectively.respectively.3.2. Mechanical The average Testing peak peel loads were 1.62 kN, 1.65 kN, 2.15 kN, and 2.03 kN, respectively. Mechanical testing results from lap-shear and peel tests are summarized in Table 2. The peak load of lap-shear and peel testsTable inTablecreased 2. Lap-shear 2. withLap-shear energy and and input peel peel increased test test results. results. from 6 kJ to 10 kJ. The average Energylap-shear Input loads Lap-Shear were 29.9 Average kN, and 35 TestkN, 44 1 kN, Test and 2 42.6 Test kN for 3 the Peel 6 kJ, Average 8 kJ, 10 kJ, and Test 12 1 kJ input Test 2 Test 3 Energyenergies, Input respectively. Lap-Shear Average The average Test peak 1 peel Test loads 2 were Test 31.62 kN, Peel 1.65 Average kN, 2.15 Test kN, 1 and Test2.03 2kN, Test 3 (kJ) (kN) (kN) (kN) (kN) (kN) (kN) (kN) (kN) (kJ)respectively. (kN) (kN) (kN) (kN) (kN) (kN) (kN) (kN) 6 29.9 28.8 31.9 28.9 1.62 1.59 1.61 1.66 6 29.9 28.8 31.9 28.9 1.62 1.59 1.61 1.66 88 35 35 Table 34.2 2. 34.2 Lap-shear 36.236.2 and peel 34.7 test34.7 results. 1.651.65 1.691.69 1.651.65 1.621.62 1010 44 44 43.5 43.5 45.445.4 43.143.1 2.152.15 2.122.12 2.182.18 2.162.16 Energy Input Lap-Shear Average Test 1 Test 2 Test 3 Peel Average Test 1 Test 2 Test 3 1212 42.6 42.6 42.9 42.9 42.242.2 42.642.6 2.032.03 2.012.01 2.042.04 2.032.03 (kJ) (kN) (kN) (kN) (kN) (kN) (kN) (kN) (kN) 6 29.9 28.8 31.9 28.9 1.62 1.59 1.61 1.66 The load–displacementThe load–displacement8 curves 35 curves of theof 34.2 the lap-shear lap-shear36.2 samples34.7 samp les are are1.65 shown shown in1.69 in Figure Figure1.655 . 5. The The1.62 lap-shear lap-shear testtest fractured surface10 are shown 44 in Figure 43.5 6. The45.4 failure 43.1 zones of all2.15 specimens 2.12 for 2.18lap-shear 2.16 and peel test fractured surface12 are shown 42.6 in Figure6. 42.9 The failure42.2 zones42.6 of all2.03 specimens 2.01 for lap-shear2.04 2.03 and peel test are locatedare located at the at interfacethe interface between between the the interlayer interlayer and and target target plate. plate. As As shown shown inin thethe cross-sectionalcross-sectional morphology of the specimen for tensile test, the good weld area between the flyer plate and interlayer morphology ofThe the load–displacement specimen for curves tensile of test,the lap-shear the good samp weldles are area shown between in Figure the 5. The flyer lap-shear plate andtest interlayer is smallerfractured than surface that betweenare shown the in Figure interlayer 6. The andfailure target zones plate. of all specimens The diameter for lap-shear of the andfailure peel zone test increases is smaller than that between the interlayer and target plate. The diameter of the failure zone increases from are16 locatedmm to at 28.8 the mminterface with between the input the interlayerenergy from and target 6 kJ toplate. 10 kJ,As showtherebyn in increasingthe cross-sectional the peak tensile from 16 mm to 28.8 mm with the input energy from 6 kJ to 10 kJ, thereby increasing the peak tensile force force morphologysustained ofby the the specimen joint. Thefor tensile results test, show the good that weld the area welded between area the flyerof the plate flyer and andinterlayer interlayer was sustainedis by smaller the joint.than that The between results the showinterlayer that and the target welded plate. The area diameter of the of flyer the failure and interlayerzone increases was smaller smaller than that of interlayer and target, and the bonding strength of interlayer and target near the than thatfrom of interlayer 16 mm to 28.8 and mm target, with the and input the energy bonding from strength6 kJ to 10 kJ, of thereby interlayer increasing and targetthe peak near tensile the center is centerforce is smallersustained than by the that joint. of The outside. results Theshow samples that the welded welded area at ofhigher the flyer energies and interlayer had larger was welding smallerinterface thansmaller thatof thaninterlayer of that outside. of interlayerand The flyer, samplesand and target, these welded and correthe bonding atlated higher withstrength energies higher of interlayer bonding had larger and strength.target welding near However,the interface the of interlayerincreasecenter and of flyer, isthe smaller input and than theseenergy that correlated ofslightly outside. reduced The with samples higher the weldedstrength bonding at ofhigher strength.the energiesjoining However, hadbetween larger thethewelding increaseinterlayer of and the inputtarget energyinterface with slightly the of interlayerfailure reduced load and decreasingtheflyer, strength and these from of corre the44lated kN joining towith 42.6 betweenhigher kN. bonding the interlayerstrength. However, and target the with the failure loadincrease decreasing of the input from energy 44 kN slightly to 42.6 reduced kN. the strength of the joining between the interlayer and 50target with the failure load decreasing from 44 kN to 42.650 kN. 5 6 kJ Lap-shear test 50 8 kJ 50 5 6 kJ 10 kJ Lap-shear Peel test test 4 40 8 kJ 40 12 kJ 40 10 kJ 40 Peel test 4 12 kJ 30 30 3 30 30 3

20 2 Load (kN) Load 20 20 2 Load (kN) Load 20 Peel test peak load testPeel (kN) peak Peel test peak load (kN) load testPeel peak

10 10 10 1 1 10 Lap-shear test peak load (kN) Lap-shear test peak load(kN)

0 0 0 012340 6 8 10 12 0 0 6 8 10 12 01234Displacement (mm) Energy input (kJ) Energy input (kJ) Displacement (mm)

(a) (b) (a) (b) Figure 5.FigureLap-shear 5. Lap-shear and and peel peel test test results results atat 6 kJ, kJ, 8 8kJ, kJ, 10 10kJ, kJ,and and12 kJ 12energies kJ energies input. (a) input. Load– (a) Load– displacement curves of lap-shear test; (b) Lap-shear and peel average load. displacementdisplacement curves curves of lap-shear of lap-shear test; test; (b) ( Lap-shearb) Lap-shear and and peel peel average average load. load.

6 kJ 8 kJ

Figure 6. Cont. Metals 2019, 9, 43 6 of 11 Metals 2019, 9, x FOR PEER REVIEW 6 of 12

Metals 2019, 9, x FOR PEER REVIEW 6 of 12

6 kJ 8 kJ

10 kJ 12 kJ

FigureFigure 6. Lap-shear 6. Lap-shear tested10 kJ tested samples samples failed failed area area at 6at kJ, 6 kJ, 8 kJ, 8 kJ, 1210 10kJ kJ, kJ, and and 12 12 kJ kJ input input energies. energies.

3.3. Interfacial3.3. InterfacialFigure Morphologies Morphologies 6. Lap-shear tested samples failed area at 6 kJ, 8 kJ, 10 kJ, and 12 kJ input energies.

The3.3. Interfacial interfaceThe interface Morphologies between between 3003 3003 and and 5A06 5A06 is dominated is dominated by by a wave-likea wave-like bond, bond, whereas whereas AA3003 AA3003 andand SS321SS321 are bonded are bonded with with IMC IMC possibly possibly caused caused by elementby element diffusion diffusion and and mixing. mixing. The The wavy-bonding wavy-bonding The interface between 3003 and 5A06 is dominated by a wave-like bond, whereas AA3003 and regions with the input energy of 6, 8, 10, and 12 kJ are shown in the red frame of Figure 7. A regionsSS321 with are bonded the input with energy IMC possibly of 6, 8,caused 10, andby element 12 kJ diffusion are shown and inmi thexing. red The framewavy-bonding of Figure 7. symmetrically distributed wavy-bonding region is formed at the Al–Al interface area at A symmetricallyregions with the distributed input energy wavy-bonding of 6, 8, 10, and region 12 kJ are is shown formed in atthe thered Al–Alframe of interface Figure 7. areaA at approximatelysymmetricallyapproximately 5 mm distributed 5 from mm the from centralwavy-bonding the area.central The area. region metals The ofis metals theformed interlayer of theat theinterlayer and Al–Al the ﬂyer andinterface plate the areflyerarea embedded plateat are withapproximately eachembedded other, with the 5 mm cresteach from other, height the the graduallycentral crest heightarea. increases The graduall metals fromy increasesof thethe centerinterlayer from to the theand center sides, the toflyer andthe plate sides, the distanceare and the betweenembeddeddistance the wavy between with bondingeach the other, wavy region the bonding crest and height the region center graduall and decreases they increases center with decreases from increasing the center with energy.increasingto the sides, When energy. and the the inputWhen the input energy increases, the tendency of the crest tilting towards the center increases. energydistance increases, between the the tendency wavy bonding of the crest region tilting and the towards center thedecreases center with increases. increasing energy. When the input energy increases, the tendency of the crest tilting towards the center increases.

6 kJ 6 kJ

8 kJ8 kJ

10 kJ10 kJ

12 kJ 12 kJ Figure 7. Interface morphologies of ﬂyer and interlayer at 6 kJ, 8 kJ, 10 kJ, and 12 kJ input energies. Figure 7. Interface morphologies of flyer and interlayer at 6 kJ, 8 kJ, 10 kJ, and 12 kJ input energies.

Metals 2019, 9, x FOR PEER REVIEW 7 of 12 Metals 2019, 9, 43 7 of 11 Figure 7. Interface morphologies of flyer and interlayer at 6 kJ, 8 kJ, 10 kJ, and 12 kJ input energies.

TheMetals distance2019, 9, x FOR between PEER REVIEW thecrest height, λ, and the distance, L, between two waves are7 of given 12 in The distance between the crest height, λ, and the distance, L, between two waves are given in Figure8. The crest height and inter-wave interval increase with the input energy increase. The crest Figure 8. TheFigure crest 7. Interface height morphologies and inter-wave of flyer interval and interlayer increase at 6 kJ,with 8 kJ, the 10 kJ, input and 12 energy kJ input increase. energies. The crest heights are 96, 116, 120, and 124 µm, whereas the inter-wave intervals are 332, 358, 471, and 493 µm. heights are 96, 116, 120, and 124 μm, whereas the inter-wave intervals are 332, 358, 471, and 493 μm. The distance between the crest height, λ, and the distance, L, between two waves are given in Figure 8. The crest height and inter-wave interval increase with the input energy increase. The crest heights are 96, 116, 120, and 124 μm, whereas the inter-wave intervals are 332, 358, 471, and 493 μm.

Figure 8. InterfaceInterface morphologies morphologies of of wave wave bonding area with 6 kJ energy input. Figure 8. Interface morphologies of wave bonding area with 6 kJ energy input. Figure 99 showsshows thethe distributiondistribution patternpattern atat thethe interfaceinterface betweenbetween aluminiumaluminium alloyalloy 30033003 andand stainless steelFigure 321 9 shows when thethe distribution input energy pattern is 8 atkJ. the The interface graph between shows that aluminium the bonding alloy 3003region and at the interfacestainless between steel 321aluminium when the alloy input 3003 energy and and is stainless stainless 8 kJ. The steel steel graph 321 321 shows consists consists that of ofthe the the bonding mechanical-clinch mechanical-clinch region at the area, area, IMC interfacebonding bonding between area, area, and and aluminium an an unwelded unwelded alloy 3003 area. area. and TheThe stainless unwelded unwelded steel 321area area consists is located is located of the at mechanical-clinch the at thecenter center of the of area, joint, the joint, the mechanical-clinchthe mechanical-clinchIMC bonding area, zone and zoneby an element byunwelded element diffusion area. diffusion The is outsideunwelded is outside the area IMC is the located bonding IMC at bonding thezone, center and zone, of the the IMC andjoint, thebonding the IMC zonebonding mechanical-clinchis flat zone near is the ﬂat aluminium near zone the by aluminiumelement alloy diffusionand alloy irregularly is and outside irregularly undulated the IMC undulatedbonding near the zone, nearstainless and the the stainlesssteel. IMC bonding steel. zone is flat near the aluminium alloy and irregularly undulated near the stainless steel.

(a)

(b) (b)

(c)

Figure 9. Interface morphologies of 3003 aluminium and 321 stainless steel at 8 kJ energy input. (c) (a) unwelded area. (b) IMC bonding area. (c) Mechanical-clinch area.

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Figure 9. Interface morphologies of 3003 aluminium and 321 stainless steel at 8 kJ energy input. (a) Metalsunwelded2019, 9, 43 area. (b) IMC bonding area. (c) Mechanical-clinch area. 8 of 11

The morphologies of the IMC bonding area at energies input 6, 10, and 12 kJ are shown in Figure The morphologies of the IMC bonding area at energies input 6, 10, and 12 kJ are shown in Figure 10. 10. The length and thickness of the IMC bonding zone at the interface increases and decreases with The length and thickness of the IMC bonding zone at the interface increases and decreases with the input the input energy, respectively. When the input energy is 6 kJ, the largest thickness of IMC at 50 μm energy, respectively. When the input energy is 6 kJ, the largest thickness of IMC at 50 µm is achieved, is achieved, whereas its length is approximately 300 μm. When the input energy increases to 10 kJ, whereas its length is approximately 300 µm. When the input energy increases to 10 kJ, the thickness of the thickness of the interface is reduced to 30 μm, and the IMC is discontinuously distributed with the interface is reduced to 30 µm, and the IMC is discontinuously distributed with approximately 2 mm approximately 2 mm in length. The thickness of IMC reduced to 26 μm with energy input increased in length. The thickness of IMC reduced to 26 µm with energy input increased to 12 kJ. to 12 kJ.

6 kJ

10 kJ

12 kJ

Figure 10. Interface morphologies with 6 kJ, 10 kJ, and 12 kJ input energies.

The interface morphologies morphologies of of the the center center area area with with 12 12 kJkJ energy energy input input is shown is shown in Figure in Figure 11. 11At. Athigh high impact impact velocities velocities and and low low impact impact angles, angles, over overheatedheated regions regions were were formed formed at at the the center center of the interface between the interlayer and target at the input energy levellevel ofof 1212 kJkJ causingcausing damagedamage on the interlayer, andand therefore therefore the the bonding bonding strength strength was was impaired, impaired, with with the the tensile tensile load load decreased decreased to 42.6 to 42.6 kN. kN.

Metals 2019, 9, 43 9 of 11 Metals 2019, 9, x FOR PEER REVIEW 9 of 12

Metals 2019, 9, x FOR PEER REVIEW 9 of 11

Figure 11. Interface morphologies of center area with 1212 kJkJ energyenergy input.input.

As shown from the failure areas of the specimens afterafter the lap-shear and peel tests, the weld area Figure 11. Interface morphologies of center area with 12 kJ energy input. at the interface between aluminium alloy 3003 and stainlessstainless steel 321 is significantlysignificantly larger than that between aluminiumAs shown from alloy the 30033003failure and areas aluminium of the specimens alloyalloy 5A06.5A06. after the The lap failure-shear and zone peel of tests, each the joint weld is located area at the centralcentralat the interface area ofof thebetweenthe interfaceinterface aluminium betweenbetween alloy aluminiumaluminiu 3003 andm stainless alloy 3003 steel and 321 isstainless significantly steel larger 321. Inthan In the that VFAW process,between whichwhich aluminium isis driven alloy by the 3003 energy and aluminium of aluminium alloy 5A06 foil vaporization,. The failure zone the of flyerflyer each plate joint initiallyis located collides at withwith thethe central interlayer. interlayer. area Theof Thethe flyer interface flyer plate plate between and interlayerand aluminium interlayer then alloy impactthen 3003 impact the and target stainless the plate target steel together, 321.plate In thusthetogether, VFAW producing thus aproducing highprocess, collision a whichhigh angle iscollision driven between by angle the the energy flyerbetween plate of aluminium the and flyer external foil plate vaporization, side and of theexternal interlayer.the flyer side plate of This theinitially collision interlayer. collides prevents This with the interlayer. The flyer plate and interlayer then impact the target plate together, thus thecollision formation prevents of a the reliable formation weld. of The a reliable collision weld. angle The between collision the angle interlayer between and centralthe interlayer area of and the producing a high collision angle between the flyer plate and external side of the interlayer. This targetcentral plate area increasesof the target externally, plate in thuscreases rendering externally, the bondingthus rendering strength the of bonding the region strength smaller of than the thatregion of collision prevents the formation of a reliable weld. The collision angle between the interlayer and smaller than that of the external side. the externalcentral area side. of the target plate increases externally, thus rendering the bonding strength of the region The element line scanning results at the interface between aluminium alloy 3003 and stainless Thesmaller element than that line ofscanning the external results side. at the interface between aluminium alloy 3003 and stainless steel 321 areThe givenelement in line Figure scanning 1212.. The results welded at the areas interface at the between interface aluminium between alloy aluminium 3003 and alloy stainless 3003 and stainlesssteel steel321 are 321 given and in at Figure the central 12. The area welded close areas to the at the joint interface are mainly between characterized aluminium alloy by IMC. 3003 Withinand a certainstainless distance steel from 321 and the centerat the central of the area joint, close the to collisionco thellision joint angle are mainly between characterized the interlayer by IMC. and Within target a plate increases.increases.certain At distance the interface, from the a cente 3 μµmr of thick the joint,bonding the collision region angle is formed. between The the intensity interlayer is andsignificantly significantly target plate larger than increases. that in the At central the interface, area. Thea 3 μm EDS thick scan bonding results region of Points is formed. 1–7 are The shown intensity in Tableis significantly3 3.. TheThe IMCsIMCs larger alongalong than that in the central area. The EDS scan results of Points 1–7 are shown in Table 3. The IMCs along the interfaceinterface betweenbetween aluminiumaluminium alloyalloy 30033003 andand stainlessstainless steelsteel 321321 areare FeAlFeAl andand FeAlFeAl33.. the interface between aluminium alloy 3003 and stainless steel 321 are FeAl and FeAl3. 12000 12000 Cr 10000 10000 Ni Al Cr Fe 8000 Fe 8000 Ni 6000 Al 6000 CPS CPS 4000 4000

2000 2000

0 0 0 5 10 15 20 25 30 35 0 1020304050 Distance(a) (μm) (b)Distance (μm)

FigureFigure 12. Element 12. Element distributions distributions of of the the 3003–321 3003–321 interface interface at different areas. areas. (a ()a EDS) EDS line line scan scan of line of line 1; 1; (b) EDS line scan of line 2. (b) EDS line scan of line(a) 2. (b)

FigureTable Table12. 3. ElementPoint 3. Point EDS distributionsEDS compositional compositional of the result result 3003–321 at at thethe interfaceinterface of ofat the thedifferent interlayer interlayer areas. and and target(a) target EDS sheet line sheet (at. scan %). (at. of %). line 1; (b) EDS line scan of lineEDS 2. EDS EDS EDS EDS EDS EDS EDS EDS EDS EDS EDS EDS EDS Spot 1 Spot 2 Spot 3 Spot 4 Spot 5 Spot 6 Spot7 Spot 1 Spot 2 Spot 3 Spot 4 Spot 5 Spot 6 Spot7 Table 3. Point EDSAl compositional42.3 0.77 result 42.49 at the interface62.99 of67.16 the interlayer66.79 and67.17 target sheet (at. %). AlFe 42.56 42.3 69.79 0.77 42.47 42.49 21.61 62.99 22.84 67.16 22.54 66.79 22.25 67.17 FeEDS 42.56 EDS 69.79 EDS 42.47 ED 21.61S EDS 22.84 22.54EDS EDS 22.25

Spot 1 Spot 2 Spot 3 Spot 4 Spot 5 Spot 6 Spot7 Al 42.3 0.77 42.49 62.99 67.16 66.79 67.17 Fe 42.56 69.79 42.47 21.61 22.84 22.54 22.25

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4. Conclusions 5A06 aluminium alloy and 321 stainless steel were welded by VFAW using an AA3003 interlayer and joints with good structure and properties were obtained. With input energies of 6 kJ, 8 kJ, and 10 kJ, the shear and tensile strengths are increased due to increased welded area. The joints with the best mechanical properties were obtained at the input energy of 10 kJ, which had a failure load of 44 kN in lap shear mode and 2.15 kN in peel mode of loading. Microstructure characterization revealed a wavy pattern between the flyer and interlayer which have similar properties. The interface between the interlayer and target showed the presence of IMCs, identified as Al3Fe and FeAl, which were continuously or intermittently distributed along the interface depending on the input energy. The results show that the use of an interlayer is a viable solution for impact welding of materials which are otherwise difficult to weld.

Author Contributions: Data curation, S.S.; Funding acquisition, S.C. and G.D.; Investigation, S.S. and A.V.; Methodology, A.V.; Project administration, S.C.; Software, J.X.; Supervision, G.D.; Writing—original draft, S.S.; Writing—review & editing, Y.M. Funding: This research was funded by National Nature Science Foundation of China (Grant Number 51575012). The OSU work was funded by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technology Office under award number DE-0007813. The author gratefully acknowledges financial support from China Scholarship Council. Conflicts of Interest: The authors declare no conflict of interest. Disclaimer: Neither the U. S. Government, nor any of its employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy or completeness of any information, product, or process disclosed, or represents that its manufacture or use would not infringe privately owned rights. Any reference to a commercial product, process, or service does not constitute an endorsement or favoring by the U. S. Government. The views and opinions of the authors stated do not necessarily reflect those of the U. S. Government.

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