Materials and Design 165 (2019) 107585

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Dissimilar metal joining of aluminum to steel by ultrasonic plusresistance spot welding - Microstructure and mechanical properties

Ying Lu a, Ellis Mayton a,b, Hyeyun Song a,c, Menachem Kimchi a, Wei Zhang a,⁎a Department of Materials Science and Engineering, The Ohio State University, 1248 Arthur E. Adams Dr, Columbus, OH 43221, USAb Welding Technology Corp., 24775 Crestview Court, Farmington Hills, MI 48335, USAc EWI, 1250 Arthur E. Adams Dr, Columbus, OH 43221, USA

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Developed new dissimilar metal joiningmethod, ultrasonic + resistance spotwelding

• Produced sound joints of 1-mm-thickAA6061-T6 to 0.9-mm-thick low carbonsteel

• Less than 1.5-μm-thick intermetalliclayer formed at aluminum to steel inter-face

• Peak tensile shear strength of 3.2 kNwith button pull-out failure mode

• Temperature in dissimilar joint pre-dicted by multi-physics finite elementmodel

⁎ Corresponding author.E-mail address: zhang.3978@osu.edu (W. Zhang).

https://doi.org/10.1016/j.matdes.2019.1075850264-1275/© 2019 Elsevier Ltd. This is an open access art

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 September 2018Received in revised form 4 January 2019Accepted 4 January 2019Available online 5 January 2019

Anew joiningmethod, i.e. ultrasonic plus resistance spotwelding, is developed for dissimilarmetal joining of alu-minum alloy (Al) to steel. In thismethod, a thin Al insert is first joined to a steel sheet using the solid-state ultra-sonic spot welding (USW). Next, the Al insert side of the steel sheet is welded to an Al sheet by the standardresistance spot welding (RSW). The U + RSW method is used to join 1-mm-thick AA6061-T6 to 0.9-mm-thickAISI 1008 steel with 0.4-mm-thick AA6061-T6 as the insert. The final Al/steel weld shows a brazing featurewith liquid aluminum wetting and spreading on the solid steel surface. At welding current of 16.5 kA, a lessthan 1.5-μm-thick layer of intermetallics is observed at the Al insert/steel interface, corresponding to a highjoint strength of 3.2 kN and a nugget pull-out failure mode. The formation of such a thin layer of intermetallicsis attributed to themetallurgical bond formed at Al to steel interface byUSW,which in turn reduces the electricalresistance and temperature at this interface during subsequent RSW. The effect of USW and RSW parameters onthe interfacial microstructure, nugget size, joint strength and failure mode is further investigated.

icle under the CC BY-NC-ND li

© 2019 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Dissimilar metal weldingAluminum alloySteelMechanical propertiesIntermetallicsAutomotive light-weighting

1. Introduction

To address an increasing demand for vehicle light-weighting, multi-materials designs with steels and light metals, such as aluminum alloys

cense (

(Al), are being extensively pursued [1]. Broadly speaking, there arethree types of processes to join Al to steel. The first type is fusionwelding such as arc or laser welding. Due to the low solubility of ironin aluminum, thick and brittle intermetallic compound (IMC) layercan form at the fusion weld interface, which deteriorates the load bear-ing capacity of the weld. In addition, solidification related welding de-fects, such as shrinkage voids and solidification cracking, can form at

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Fig. 1. Schematic diagram of U + RSW process. VD is the sonotrode's vibration directionfor USW of intermediate joint.

2 Y. Lu et al. / Materials and Design 165 (2019) 107585

the weld interface [2]. The second type of processes for dissimilar metaljoining Al to steel includes mechanical fastening (e.g., self-piercing riv-eting [3]), and solid-state welding (e.g., ultrasonic spot welding(USW) [4], friction stir spot welding (FSSW) [5,6], vaporizing foil actua-tor welding (VFAW) [7], etc.). The third type is resistance spot welding(RSW), which shares many of the aforementioned challenges as fusionwelding of Al to steel. However, as RSW is one of the most widelyused joining methods in automotive industry, developing a soundRSWprocess to join Al to steel is expected to have a large impact on ve-hicle light-weighting.

For direct RSW of Al to steel, Zhang et al. [8–10] observed dual-layerof IMCs including lath-/tongue-like η-Fe2Al5 on steel side and needle-like θ-FeAl3 on aluminum alloy side at the Al to steel interface. At an op-timized welding condition, a high tensile shear strength (TSS) of 3.3 kNwas obtained although the failure mode was the interfacial fracture. Bystudying RSW of Al to galvanized (Zn-coated) steel versus RSW of Al tonon-coated low carbon steel, Arghavani et al. [11] found a beneficial

Fig. 2. Sample geometry for lap-shear te

effect of the zinc coating on reducing IMC thickness and increasingboth TSS and cross-tension strength (CTS) of welds.

One method to increase Al to steel resistance spot weld strength isthrough an improved heat balance by reducing heat loss to water-cooled electrode on the Al side tomaintain a high temperature in the fu-sion zone. This method was used by Satonaka et al. [12] who placed acover plate between the Al sheet and electrode during RSW. Qiu et al.[13–17] also applied a cover plate on the Al side in RSW of aluminumalloy AA5052 to either an austenitic stainless steel or a cold-rolled lowcarbon steel. An interesting finding is that the thickness of IMC layerformed at the interface of Al to austenitic stainless steel (~2 μm) wasthinner than that at the interface of Al to low carbon steel. Correspond-ingly, the peak TSS of 6.5 kN along with a button pull-out failure modewas obtained for the former, whereas the peak TSS of 4.68 kN alongwith an interfacial fracture mode was obtained for the latter.

Anothermethod to improve resistance spotweld strength is throughplacing an insert between theAl and steel sheets to reduce the IMC layerthickness. One insert material is “special” aluminum-clad steel sheet,which was used by Oikawa et al. [18] and later by Sun et al. [19]. Inthe work by Sun et al., a 1.0-mm- or 1.5-mm-thick Al cladded steelsheet was used as the insert placed between an AA5182-O and an AISI1008 mild steel. In cross-tension and coach peel testing, the static anddynamic strengths for resistance spot welds joined with insert werecomparable to those of mechanically joined, self-piercing rivets (SPR).The other insert material, lighter and cheaper than the aluminum-clad steel, is aluminum alloy. Zhang et al. [20] used AA4047(AlSi12) as an insert. The thickness of IMC layer formed at the Al tosteel interface reduced from 1.8 μm to 0.6 μm as the insert thicknessincreased from 100 μm to 400 μm; either wasmuch thinner than thatformed for direct resistance spot welds without an insert. Ibrahimet al. [21] investigated resistance spot welding of AA6061-T6 toAISI 304 austenitic stainless steel with an 80-μm-thick Al-Mg alloy(80 wt% Al and 20 wt% Mg) as the insert. TSS strength of weld withAl-Mg insert was higher than that without the insert. In addition, fa-tigue strength of the joint welded using RSW with insert was higherthan that welded using FSSW. It is noted that all these inserts weredirectly placed between Al and steel sheets without a metallurgicalbond prior to RSW.

In this study, a new joining method, ultrasonic plus resistance spotwelding (abbreviated as U + RSW), was developed for producingsound dissimilarmetal joints of Al to steel. Themethod utilized an Al in-sert, which was first metallurgically bonded to steel sheet using solid-state USW. Next, the Al insert side of the steel sheet was welded to anAl sheet by the standard RSW. Electron microscopy and mechanicaltestingwere used to understand the effect of USWand RSWparameters

nsile testing (dimensions in mm).

Fig. 3. SEM image of the intermediate joint's interfaceproducedbyultrasonic spotweldingof AA6061 insert to 1008 steel. USW parameters were vibration amplitude = 50 μm,normal force = 1.75 kN, and welding energy = 255 J.

3Y. Lu et al. / Materials and Design 165 (2019) 107585

on the interfacial microstructure, nugget size, joint strength and failuremode of dissimilar metal joints of Al to steel.

2. Approaches

2.1. Materials

The base materials used in this study were 1-mm-thick aluminumalloy AA6061-T6, and 0.9-mm-thick cold-rolled AISI 1008 steel. Flatcoupons of 100 mm in length and 38 mm in width were cut from theAl and steel sheets with the coupons' length direction aligned with therolling direction. As a first test, the insert used was 0.4-mm-thickAA6061-T6. Optimization of the insertmaterial and thicknesswill be re-ported in a future work.

2.2. U + RSW

Fig. 1 shows a schematic diagram of U + RSW method. In the firststep, the Al insert was ultrasonic spot welded to the steel sheet to createan intermediate joint. In the second step, the primary joint was createdby resistance spotwelding of theAl sheet to theAl insert side of the steelsheet, where the center of intermediate joint was aligned to the centerof electrodes. Detailed procedure is provided as follows.

Fig. 4.Macrostructure of dissimilarmetal joints of AA6061 to 1008 steel produced by (a) directcurrent of 16.1 kA. For better illustration of the joint dimensions, the Al/insert fusion boundaelectrode force for RSW were kept constant at 0.083 s and 3.56 kN, respectively. In U + RSW,in Fig. 3.

2.2.1. Ultrasonic spot welding for intermediate jointAs shown in Fig. 1, the intermediate joint was created by ultrasonic

spot welding of a 0.4-mm-thick AA6061-T6 insert to a 0.9-mm-thickcold-rolled AISI 1008 steel coupon. An Amtech Ultraweld® 20 2.4-kWlateral driven ultrasonic spot welder, designed to operate at 20 kHzwith a maximum peak-to-peak amplitude of 80 μm, was used forUSW. The dimensions of sonotrode tip were 8 mm × 6 mm, and theanvil tip 12 mm × 12 mm. Both the sonotrode and anvil surfaces hada pyramid knurl pattern to facilitate firm gripping of the samples. Spe-cifically, the spacing between two teeth was about 0.63 mm, and thegroove depth was 0.16 and 0.08 mm for the anvil and sonotrode knurlpatterns, respectively [22]. The samples were grinded with 240 gradesand paper and cleaned by ethanol before welding. During USW, thesonotrode was vibrating along the short dimension of the sample(thus perpendicular to its rolling direction), as illustrated in Fig. 1.

The baseline USW parameters for the intermediate joint, developedbased on those used in a previous study [22], were peak-to-peak vibra-tion amplitude= 50 μm, and normal force= 1.75 kN. The normal forcewas applied over the entire duration of ultrasonic spot welding. TheUSW energy had a strong influence on the joint strength, which wasused as a measure of the extent of metallurgical bond formed at the Alinsert to steel interface. Hence, to study the intermediate joint qualityon the microstructure and mechanical properties of the primary joint,three USW energies were used: 50 J, 150 J, and 255 J. The resultantUSW time increased from 0.05 to 0.23 s as the energy increased from50 to 255 J.

As a “quantitative” measure of the extent of metallurgical bondformed in the intermediate joint, the tensile shear strength of the ultra-sonic spot weld was tested. For each ultrasonic energy, three sampleswere welded and then tested in the lap-shear tensile testing configura-tion using a displacement rate of 1 mm/min. A dummy spacer wasinserted between the sample and the grip for proper alignment of thesample.

2.2.2. Resistance spot welding of Al to intermediate jointThe 1-mm-thick AA6061-T6 coupon was resistance spot welded to

the intermediate joint in a single phase 60 Hz alternate current (AC)welder. The Al insert side of the steel sheet faced the Al sheet with thecenter of intermediate joint aligned to the center of electrodes. As illus-trated in Fig. 1, two different types of electrode were used. Facing the Alsheet was a B-type, dome-shaped electrode with 6 mm face diameter,commonly used for RSW of similar metals (e.g., steel to steel or Al toAl). On the other hand, facing the steel sheetwas a C-type, flat electrode

RSW atwelding current of 13.0 kA, (b) U+ RSWat current of 13.3 kA, and (c) U+ RSWatry and the steel HAZ boundary are traced by the dashed lines. The welding time and thethe intermediate joints were ultrasonic spot welded using the same parameters as those

Fig. 7. Thickness distribution of IMCs at the Al/steel interface along the weld widthdirection. Distance 0 corresponds to the weld center. Welding parameters used were thesame as those in Fig. 4(c).

Fig. 5. Effect of welding current on aluminumnugget diameters for U+RSW. An exampledefinition of the two diameters is given in Fig. 4(c). The welding time and the electrodeforce for RSWwere kept constant at 0.083 s (5 cycles) and 3.56 kN, respectively.

4 Y. Lu et al. / Materials and Design 165 (2019) 107585

with surface diameter of 15.875 mm. Such flat electrode was selectedfor the purpose of distributing the current over a large area, thus reduc-ing the current density and consequently the heat build-up in the steelsheet. Minimizing the steel sheet temperature in turn is expected tohelp reduce the IMC growth during RSW.

The electrode force used was 3.56 kN, and a short welding time of5 cycles (0.083 s) was used to minimize the time available for IMCgrowth aswell as to reduce the electrodewear; the latter due to alloyingbetween Al coupon and copper electrode. The welding current variedfrom 12 to 17 kA. For each condition, a set of 4 samples were welded,of which three samples were used for lap-shear tensile testing andone for microstructure characterization. For comparison, the AA6061-T6 to 1018 steel coupons were directly resistance spot welded withoutan insert.

2.3. Bond quality characterization

The samples for interfacial microstructure characterization wereprepared following a common procedure for metallography. Specifi-cally, the welded coupons were cross-sectioned along the weld centerline, and the cross sections were then cold-mounted with epoxy. Thesamples were ground with sand papers, polished with diamond pastes,and finishedwith a vibratory polish using 0.05 μmcolloidal silica for 1 h.For observation of the IMCs at Al to steel interface, un-etched sampleswere analyzed in an FEI Apreo scanning electron microscope (SEM)with energy-dispersive X-ray spectroscopy (EDS). SEM with EDS was

Fig. 6. (a) SEM image showing IMCmorphology at the center of the Al/steel interface, and (b) Eas those in Fig. 4(c).

also used to observe the IMCs on fracture surface of as tensile-sheartested samples. In addition, the cross sections were chemically etchedwith Keller's reagent (2 ml HF (48%) + 3 ml HCl + 5 ml HNO3+ 190ml H2O) for Al alloy and 2% Nital for 1008 steel. The etched sam-ples were observed in optical microscope for measurement of nuggetsize.

The mechanical properties of the dissimilar metal spot welds weretested by lap-shear tensile testing in an MTS 810 testing machine perAmerican Welding Society (AWS) Standard C1.1M [23]. The geometryof a test specimen is shown in Fig. 2. For each welding condition, a setof three repeats were tested under a quasi-static loading conditionwith displacement rate of 1 mm/min. From the load-displacementcurve, the TSS was determined from the peak load, and the fracture en-ergy of the spot weld was calculated by the area under the load-displacement curve till the peak load.

To further compare the joint strength of U + RSWwith the existingliterature data generated on different Al and steelmaterials, the joint ef-ficiencywas calculated. For simplicity, theweld nugget is represented asa cylinder, andwelding defects such as shrinkage voids are ignored [24].For pull-out failure mode, the nominal peak tensile stress of the jointcan be calculated by:

σPF ¼ Peak load2πrt ð1Þ

DS composition profiles along the arrowmarked.Welding parameters usedwere the same

Fig. 8. Comparison between the calculated joint geometry and the experimental macrograph for direct RSW of Al to steel without insert and at welding current of 13 kA, welding time of0.083 s (5 cycles) and electrode force of 3.56 kN. Calculated temperature is given in Celsius. 654 and 1535 °C are the liquidus temperature of Al and steel, respectively.

5Y. Lu et al. / Materials and Design 165 (2019) 107585

where σPF is the nominal peak joint stress for pull-out failure mode, r isthe aluminum nugget radius, and t is the Al sheet thickness. For interfa-cial failure (IF) mode, the nominal peak shear stress (τIF) is calculatedas:

τIF ¼ Peak loadπr2 ð2Þ

The joint efficiency is calculated as the ratio of nominal peak stressover the strength (SF) of Al base metal [25].

EW ¼ σWSF ð3Þ

where σW = σPF or τIF depending on the pull-out versus interfacial fail-ure mode, respectively. It is noted that the stress state experienced bythe joint is complex as the joint commonly rotates during lap-shear test-ing. For simplicity, the strength (SF) used to compute the joint efficiencyis determined as the ultimate tensile strength (UTS) of theAl basemetal.

2.4. Finite element simulation of temperature distribution during RSW

As to be discussed in detail in the next section, the nugget and IMCsat the Al/steel interface grew substantially during the resistance spotwelding step of U + RSW. Temperature distribution in the dissimilarmetal joint is essential to understand the growth of nugget and IMCs.In this study, an existing finite element analysis (FEA) model, originallydeveloped for RSWof steel sheets, wasmodified for the dissimilarmetalRSW of Al to steel with and without the metallurgically bonded insert.The existing model was a 3D transient, fully-coupled thermal-electrical-structural model developed based on commercially availableFEA software Abaqus. A detailed description of the existing model isavailable in the literature [26].

Fig. 9. (a) Calculated temperature profiles at Al/steel interface for direct RSW, RSWwith a separsheet, and (b) calculated temperature profiles at three different locations for the U + RSW join

The main modifications to the existing FEA model for dissimilarmetal joint included the new electrode geometry, sheet thickness, andelectrical contact resistance (ECR) at the Al/steel interface. The ECR forAl to steel was defined by multiplying the ECR for copper electrode tosteel [27] by a correction factor that took into account the effect ofoxide layer on the Al sheet, as reported by Wan et al. [28]. This ECR forAl to steel was used in the simulation of the first two cases. For thelast case, the metallurgical bond formed at Al insert/steel interface byUSW was accounted for by setting the ECR at this interface to a half ofthat for copper electrode to steel [27]. In addition, for simplicity, it wasnot considered the effect of prior USW on the resistance spot weldingstep such as the surface indentation.

Three caseswere simulated: (1) direct RSWof Al to steelwithout in-sert, (2) RSWof 3 sheet stack-up comprisingAl sheet, Al insert, and steelsheet, and (3) RSW of Al to steel with metallurgically bonded Al insert.Parts (i.e., electrodes, Al and sheets, and/or Al insert) with the same di-mensions as those in the experiment were meshed using a gradescheme with denser elements placed around the weld center andcoarser elements away from it. The total number of nodes and elementsfor Case 1was 42,015 and 19,982, respectively. Both Cases 2 and 3 had atotal number of 56,561 nodes and 26,040 elements, slightly more thanthose in Case 1 due to the inclusion of Al insert in those two cases.

3. Results and discussion

3.1. Microstructure and strength of intermediate joints welded by USW

At the ultrasonicwelding energy of 50 J, no bond formed at the inter-face of AA6061 insert to cold rolled 1008 steel, and the insertwas peeledoff from the steel sheet easily. As the ultrasonic energy increased to150 J, an interfacial fracture occurred for the intermediate joint with apeak load around 1.2 kN. Upon a further increase of the ultrasonic

ate Al insert, and U+ RSW for which the Al insert wasmetallurgically bonded to the steelt in (a). The resistance spot welding current was 13 kA.

Fig. 10. Effect of welding current on tensile shear strength and fracture energy of welded dissimilar joints of AA6061 to AISI 1008 steel. The welding time and the electrode force for RSWwere kept constant at 0.083 s and 3.56 kN, respectively.

6 Y. Lu et al. / Materials and Design 165 (2019) 107585

energy to 255 J, a nugget pull-out failure mode was obtained with apeak load of approximately 1.6 kN, comparable to that of ultrasonicspot welded 0.4-mm-thick Al/Al joint [22]. Fig. 3 shows the Al/steel in-terface microstructure of the intermediate joint at ultrasonic weldingenergy of 255 J. Due to the low welding energy and short weldingtime in USW, any IMCs formed at the Al/steel interface were too thinto be observed in SEM. As this study is focused on the microstructureand strength of the primary joint, further characterization of the inter-mediate joint's interface using a high spatial resolution technique suchas transmission electron microscope (TEM) is not attempted. In the lit-erature, Xu et al. measured the IMC layer using TEM for ultrasonic spotwelded 1-mm-thick AA6111 to uncoated low alloy DC04 steel atwelding time of 0.3 s [29]. They found only less than 200-nm-thick dis-continuous Fe2Al5 IMC islands existed at the Al/steel interface.

The effect of the intermediate joint quality on the primary jointstrength is briefly discussed in the following. For the intermediate jointswelded at low ultrasonic energy of 50 J, the TSS of primary joints wasabout 2.5 kN. In addition, expulsion or ejection of liquid metal fromthe Al insert/steel interface was observed when the intermediate jointwas resistance spot welded to the Al sheet. As expulsion commonlyleads to high variability in weld quality [28], the ultrasonic energy of50 J was not used. On the other hand, when the USW energy for the in-termediate joint was equal to or above 150 J, expulsion did not occurduring RSW, and the TSS of primary joints increased to 3.2 kN. Sincethe intermediate joint at USW energy of 150 J had an interfacial fracturewhereas that at 255 J had a button-pull failure, the metallurgicalbond of Al insert to steel was likely weaker for the former. Addition-ally, for the latter (high USW energy of 255 J), the IMC layer formedat the Al insert/steel interface was still very thin. Hence, it was cho-sen the intermediate joints welded at USW energy of 255 J for thesubsequent RSW in this study. The microstructure, mechanicalstrength and failure mode of the primary joints are discussed in de-tail in the next sections.

Table 1Comparison of joint efficiency calculated using Eq. (3) for dissimilar metal resistance spot weld

Al Steel Nugget diameter(mm)

Peak load(kN)

Failu

6008-T66 GI coated H220YD 5.8 3.35052 GI coated low carbon steel 9.3 6.55052 Low carbon steel 9.0 4.75052 Stainless steel 304 10.0 6.56008-T66 GI coated H220YD 9.6 6.26061-T6 Low carbon steel 5.4 3.2

3.2. Resistance spot welding with metallurgically bonded insert

3.2.1. Macrostructure of dissimilar metal joint of Al to steelFor direct RSW of Al to steel, it was reported in the literature [30]

that the peak interface temperature was approximately 1000 °C(1273 K), which is higher than the melting temperature of Al butlower than that of steel. A nugget of liquid Al thus formed partly dueto Joule heating generated on the steel side. Liquid Al wet and spreadon the solid steel surface, creating a “brazed” joint [18]. Such brazingfeature of liquid Al over solid steel was observed for the three joints ofAA6061 to 1008 steel obtained in this study, as shown in Fig. 4.

A comparison of Fig. 4(a) and (b) reveals an interesting differencebetween the direct RSW and U+ RSW processes. Both joints were pro-duced with the same welding energy. The direct resistance spot weldexhibited a larger extent of electrode indentation on the Al sheet side,indicating the joint temperature was likely much higher in direct RSWthan U + RSW. In addition, not shown in Fig. 4 is the expulsion, whichtook place at the Al/steel interface in direct resistance spot welding,and did not occur in U + RSW at the same and some higher weldingenergies.

For the two U+ RSW joints shown in Fig. 4(b) and (c), no apparentinterface was observed between the Al sheet and insert in the fusionzone,which indicates a completemelting of the two. Nowelding defectswere observed within the Al nugget or at Al/steel interface at lowwelding currents (e.g., 13.3 kA shown in Fig. 4(b)). On the other hand,at high welding current of 16.1 kA (see Fig. 4(c)), shrinkage voidswith irregular shape formed near the Al/steel interface. It is noted thatsuch shrinkage voids are commonly observed in resistance spot weldsof both Al to Al and steel to steel at high welding currents.

At welding current of 13.3 kA, the nugget penetration into the 1-mm-thick Al sheet was 0.03 mm, and it increased to 0.86 mm whenthe welding current was increased to 16.1 kA, as shown in Fig. 4(b) and (c). The increased nugget penetration was accompanied with

ing of Al to steel. GI standards for pure Zn galvanized.

re mode UTS of Al(MPa)

Nominal peak stress(MPa)

Joint efficiency(%)

Ref.

IF 340 125.8 37 [8]IF 250 95.5 38 [11]IF 250 73.6 29 [15]BP 250 206.9 83 [15]BP 340 205.1 60 [20]BP 310 188.6 61 This study

Fig. 11. Schematic illustration of four different failure modes observed in lap shear tensile testing of Al/steel joints welded by U + RSW.

7Y. Lu et al. / Materials and Design 165 (2019) 107585

an expanded nugget size. Fig. 5 plots the Al nugget diametermeasured atthe Al sheet/Al insert and Al insert/steel interfaces as a function ofwelding current. Due to a limited number of samples available for testing,the extent of deviation for nugget diameter at each welding current wasnot measured. As shown in this figure, the Al nugget diameter increasedquickly as the welding current was increased from 12.5 to 14.5 kA, andthen the increase in nugget diameter was more gradual as the currentwas further increased to 16.5 kA. The minimum nugget size as per AWSstandard D8.1 M is 4

ffiffi

tp

, which is thus 4 mm at the Al/insert interface.Suchminimumnugget size requirement wasmet when thewelding cur-rent was higher than 13.5 kA. It is noted that the nugget pull-out failuremode of U + RSW joints occurred when the aluminum nugget size atAl/insert interface was above 5 mm, which is about 5

ffiffi

tp

.

3.2.2. Microstructure at Al to steel interfaceFig. 6(a) shows the morphology of IMCs at the center of Al insert/

steel interface in the U + RSW joint. A relatively flat interface existedbetween IMCs and steel, whereas needle-like shaped IMCs grew intothe Al insert. This morphology is similar to that observed in direct resis-tance spot welding of Al to steel. The composition profiles across the Al/steel interface are also plotted in Fig. 6(b). The intermixed distribution

Fig. 12. Fracture surface for four types of failure modes at aluminum side (left) and steel side (rout from insert), (c) Type 3 (button pull-out from Al sheet), and (d) Type 4 (interfacial fractur

of Al and Fe elements indicates that the formation of the IMCs wasmainly controlled by inter-diffusion of Al and Fe across the interface ofliquid aluminum alloy and solid steel [9]. From the composition data,the IMC layer was found to likely include Fe2Al5 adjacent to the steelside and FeAl3 next to the Al side.

Fig. 7 plots the thickness of IMC layer along thewidth direction of Al/steel interface. The maximum thickness of IMCs was 1.25 μm, located attheweld center due to the high interface temperature and long interac-tion time at elevated temperature. The thickness of IMCs reduced to-ward the periphery of the weld. For comparison, the maximumthickness in U + RSW with welding current = 16.1 kA and weldingtime = 0.083 s is 1.75 μm thinner than that obtained by direct RSWwith current = 9 kA and welding time of 0.1 s [9]. Actually, this maxi-mum thickness of IMCs in U + RSW is similar to that produced by thesolid-state USW of AA6111 to DC04 steel [29] with a comparable peakload of 3.2 kN; U + RSW joints failed by button pull-out while USWjoints failed by interfacial failure.

3.3. Calculated temperature distribution during RSW

Fig. 8 shows the calculated temperature distribution in the direct re-sistant spot weld of Al to steel without an insert, i.e., Case 1 as described

ight): (a) Type 1 (shear fracture at Al/insert interface), (b) Type 2 (partial/full nugget pull-e through insert/steel interface).

Fig. 13. SEM images of fracture surface on steel side for Type 2 (partial/full nugget pull-out from insert): (a) overview, (b) zoomed-in viewof region 1, and (c) zoomed-in viewof region 2.The resistance spot welding current was 14.2 kA.

8 Y. Lu et al. / Materials and Design 165 (2019) 107585

in Section 2.4. The temperature distributionwas taken at time of 0.083 s(i.e., just before the welding current was turn off), and three isothermsare marked in the calculated temperature field to illustrate the pre-dicted joint geometry. The green region, defined by isotherms of 654and 900 °C (927 and 1173 K), represents the melt pool (thus the nug-get) in the Al side. The red region, defined by isotherms of 900 and1535 °C (1173 and 1808 K), corresponds to the heat-affected zone inthe steel side. Superimposed on this figure is the experimentalmacrograph of the joint cross-section. As shown in this figure, the pre-dicted nugget geometry in the Al side, HAZ geometry in the steel side,and electrode indentations are consistent to the respective experimen-tal data. Such good consistence supports the validity of the model incapturing the electrical-mechanical interactions at the electrode/sheetand sheet/sheet interfaces.

In addition to Case 1, the FEA model was used to simulate two othercases; both involved an Al insert. Case 3 captured U + RSW where theAl insert was metallurgically bonded to the steel sheet prior to RSW. Onthe other hand, in Case 2, the Al insert was simply placed between theAl sheet and the steel sheet, creating a 3 sheet stack-up (3T). It is notedthat Case 2 represented the RSW method used by Zhang et al. [20] andIbrahim et al. [21] in the literature. Although it was not evaluated exper-imentally in this study, Case 2was simulated to quantitatively understandthe effect of metallurgical bond on the interface temperature.

Fig. 9(a) compares the calculated temperature profiles at the centerof Al/steel interface for all three cases. The overall behavior is similar for

Fig. 14. SEM images of fracture surface on steel side for Type 3 (nugget pull-out from Al sheet):16.0 kA.

all three cases. Specifically, the interface temperature increased rapidlyin the initial 0.02 s, and the heating rate decreased when the tempera-ture was heated above the solidus temperature of AA6061 (582 °C or855 K) due to latent heat of fusion. When the welding current wasturned off at time = 0.083 s, the temperature dropped rapidly due tohigh thermal conductivity of Al.

The main difference in the three calculated temperature profiles isthe peak temperature reached at the weld center. For direct resistancespot welding, the peak temperature at Al/steel interface was about844 °C (1117 K). When an Al insert was simply placed in between Aland steel sheets without creating a metallurgical bond between insertand steel, the peak temperature at the Al/steel interface was about824 °C (1097 K), just slightly lower than that in direct RSW. On theother hand, when the insert was metallurgically bonded to the steelsheet, the peak temperature at the Al/steel interface was 754 °C(1027 K). As discussed earlier, the IMC growth was mainly controlledby the inter-diffusion of Al and Fe elements. Since such diffusion is athermally activated process, it is thus expected a thinner layer of IMCdue to lower peak temperature and shorter time at elevated tempera-ture in U + RSW compared to either direct RSW or RSWwith an Al in-sert that was not metallurgically bonded to steel. Moreover, a lowerpeak temperature in U + RSW would reduce the propensity for earlyexpulsion at Al/steel interface.

Fig. 9(b) plots the calculated temperature profiles at different loca-tions along the width direction of Al/steel interface for the U + RSW

(a) overview, and (b) zoomed-in view of region 1. The resistance spot welding current was

Fig. 15. SEM images of fracture surface on steel side for Type 4 (interfacial fracture throughAl insert/steel interface): (a) overview, (b) zoomed-in viewof region 1, and (c) zoomed-in viewof region 2. The resistance spot welding current was 16.5 kA.

9Y. Lu et al. / Materials and Design 165 (2019) 107585

joint (i.e., Case 3). The temperature profiles at weld center and 1 mmaway from the center are similar due to high thermal conductivity of liq-uid Al. At a location of 2.4mmaway from theweld center, the peak tem-perature already dropped below the solidus temperature of AA6061. Itis thus expected (1) similar IMC thickness within ±1 mm to the weldcenter due to uniformly high temperature, and (2) minimal IMCs out-side 2.4 mm to the center due to low temperature. Such postulationmade based on the calculated temperature profiles is consistent withthe experimentally-measured IMC thickness distribution shown inFig. 7.

3.4. Mechanical properties of U + RSW joint

Fig. 10(a) shows the tensile shear strength of Al/steel joints weldedby U+ RSW as a function of welding current used in the RSW step. TheTSS of U + RSW joint increased continuously with the increasing

Fig. 16. Effect of welding current on failure mode in lap shear tensile testing of RSW and U+ RSWwelded AA6061 to 1008 steel. The welding time and the electrode force for RSWwere kept constant at 0.083 s and 3.56 kN, respectively.

welding current. At a welding current of 16.5 kA, a peak load of 3.2 kNalong with button pull-out failure mode was obtained; this peak loadis comparable to that of resistance spot welded 1-mm-thick AA6082-T6 [31]. To further compare U + RSW with other resistance spotwelding techniques, the joint efficiency for U+ RSW and relevant liter-ature datawere calculated using Eq. (3) and summarized in Table 1. Theliterature data shows a low joint efficiency of 29 to 37% for interfacialfailure mode. The joint efficiency for U + RSW is comparable to the lit-erature data for button pull-out failure mode. Further optimization of U+ RSW (e.g., insert material and thickness) to improve the joint effi-ciency is planned in a future study.

Superimposed in Fig. 10(a) is the TSS data for direct resistance spotwelding without insert, which is limited to low welding currents sincesevere expulsion occurred at welding current above 13.6 kA. Moreover,a mild expulsion at Al/steel interface was already observed at lowwelding current of 12.3 kA in direct resistance spot welding, whichwas not observed until the welding current was above 16.5 kA in U+ RSW. At welding current of 13.6 kA, the TSS of U + RSW joint is0.76 kN (or 54%) higher than that of direct resistance spot weld. More-over, the direct resistance spot weld failed by interfacial fracture with alow fracture energy less than 0.15 J, as shown in Fig. 10(b). On the otherhand, for dissimilar metal joints welded by U + RSW, the fracture en-ergy increased with welding current with a maximum value of approx-imately 1 J at welding current of 16.5 kA. The high peak load andfracture energy for U + RSW are attributed to the thin layer of IMCs(b1.5 μm) formed at Al/steel interface without expulsion. In summary,Fig. 10 shows that both TSS and fracture energy are significantly im-proved for U + RSW welded AA6061 to 1008 steel compared to directresistance spot welding.

3.5. Failure modes of dissimilar metal joint welded by U + RSW

Four major failure modes were observed in Al to steel joints weldedbyU+RSW, as schematically illustrated in Fig. 11 [18]. Thesemodes areshear fracture at Al/insert interface (Type 1), partial/full nugget pull-outfrom insert (Type 2), button pull-out from Al sheet (Type 3), and inter-facial fracture through insert/steel interface (Type 4). Type 1 and 3 fail-ure modes are similar to the interfacial and button pull-out failuremodes respectively, commonly observed in RSW of aluminum alloys[32] as well as direct RSW of Al to steel without insert [33]. Type 2

10 Y. Lu et al. / Materials and Design 165 (2019) 107585

and 4 failuremodes are unique to Al/steel joints weldedwith insert. Theoptical macrographs of fracture surfaces for the four types of failuremodes are shown in Fig. 12.

The fracture surfaces for different failure modes were further ob-served in SEM. The fracture surface for Type 1 (shear fracture at Al/in-sert interface) is similar to that in resistance spot welded AA6061 withinterfacial failure mode [32]; for brevity, the SEM images of the fracturesurface for Type I are not further shown.

For Type 2 (partial/full nugget pull-out from insert), the SEM imagesof the fracture surface are shown in Fig. 13. It can be observed mixedfracture features with dimples at region 1 and brittle cleavage at region2. EDS results showed that region 1 was aluminum, and region 2 wasFeAl3. In the literature, Zhang et al. [9] explained that themixed fracturefeatures could be caused by aluminum nugget bulging into steel sheet.However, there was no obvious nugget bulging at Al/steel interface ob-served in this study. Instead, a possible reason for mixed failure featuresis the thin and discontinuous nature of the IMC layer formed at the Al/steel interface. In other words, near the weld center where the IMClayer was thickest (still b1.5 μm) and uniform, the crack likely propa-gated through IMCs, resulting in brittle cleavage feature (see region2). On the other hand, outside the weld center where the IMC layerwas thin and discontinuous, some Al was metallurgically bonded tosteel. As the crack propagated through Al near the interface, the ductilefeature formed with Al dimples left on the fracture surface at steel side.

Fig. 14 shows the fracture surface of Type 3 button pull-out failuremode. Deep dimples can be observed at region 1, indicating a ductilefailure. Fig. 15 shows the fracture surface of Type 4 failure modewhere the crack initiated and propagated through Al insert/steel inter-face. This interfacial failure mode is commonly observed in direct resis-tance spot welding of Al to steel [34]. Similar to Type 2 failure mode,mixed features were observed with dimples from aluminum insertstrongly bonded to steel at region 1, and brittle cleavage due to crackingthrough IMCs at region 2.

Failure modes for joints made by U + RSW were significantly af-fected by welding current as shown in Fig. 16. As the welding currentwas less than 13.6 kA, the shear fracture occurred at the Al/insert inter-face (Type 1) with low TSS and fracture energy, which is due to thesmall nugget size and limited nugget penetration into Al sheet.Superimposed in this figure is the data for direct resistance spotwelds, which also failed in the Type 1 mode. But unlike U + RSW, theshear fracture in direct RSW occurred due to the thick layer of IMCsformed at the Al/steel interface.

As thewelding current was between 13.6 kA and 15.2 kA, the failuremode changed to partial/full nugget pull-out from insert (Type 2). Thisis because the nugget size at Al/insert interface increased with increas-ingwelding current.Within this current range, the nugget diameterwasabout 4 to 5 mm (see Fig. 5). It is postulated that the nugget size waslarge enough so that the crack, initiated in the insert, propagatedthrough the thickness direction of insert, and led to a final failurethrough the IMCs at the Al insert/steel interface.

As welding current was higher than 15.2 kA, it was observed somevariability in the failure mode. The following three failure modes oc-curred: partial/full nugget pull-out from insert (Type 2), nugget pull-out from Al sheet (Type 3), and interfacial fracture across Al/steel inter-face (Type 4), although Type 4 fracture occurred much less frequently.Several competing factors likely contributed to the variability in the fail-ure mode. Specifically, at high welding current, the resulting large nug-get size and high electrode indentation on Al sheet (see Fig. 4(c))promoted the nugget pull-out from Al sheet (Type 3). On the otherhand, the thick layer of IMCs at andwelding defects near the Al/steel in-terface might lead to the partial/full nugget pull-out of insert from steel(Type 2). In the worst case, those IMCs and defects were sufficiently se-vere to cause the interfacial fracture across Al/steel interface (Type 4).As mentioned previously, further optimization of U + RSW(e.g., insert material and thickness) to ensure a consistency in failuremode (e.g., Type 3) is planned in future study.

4. Summary and conclusions

In summary, a new joining method, U + RSW, was developed fordissimilar metal spot joining of 1-mm-thick AA6061-T6 to 0.9-mm-thick cold-rolled 1008 steel. In this method, a thin Al insert was firstjoined to the steel sheet using solid-state USW. The Al insert side ofthe steel sheet was then welded to the Al sheet by standard RSW. Theeffect of welding parameters on the interfacial microstructure, nuggetsize, joint strength and failuremodewas investigated. Themain conclu-sions are as follows:

(1) The TSS of the intermediate joint created by USWwas around 1.2to 1.6 kN when the USW energy was equal to or above 150 J, in-dicating a strongmetallurgical bond formed at the Al insert/steelinterface. The thickness of IMC layer at this interfacewas too thinto clearly observe in SEM. From the relevant literature data, theIMC layer thickness was likely around 200 nm (0.2 μm).

(2) When RSWof the intermediate joint to the Al sheet, no expulsionwas observed at the Al insert/steel interface until a high weldingcurrent of 16.5 kA. For comparison, severe expulsion occurred indirect RSW of Al to steel without insert at a low welding currentof 13 kA. The dissimilar metal joint of Al to steel created by U+ RSW showed a brazing feature with liquid aluminumwettingand spreading on solid steel surface. In other words, a nuggetformed on the Al side, while a heat affected zone formed onsteel side. The maximum thickness of IMC layer, observed atthe weld center, was less than 1.5 μm.

(3) The predicted nugget geometry in the Al side, HAZ geometry inthe steel side, and electrode indentations by the FEA model ofdissimilar metal RSW were consistent to the respective experi-mental data. The calculated temperature profiles showed that U+ RSW had a lower peak temperature and shorter time at ele-vated temperature when compared to direct RSW.

(4) Both TSS and fracture energy of joints created by U + RSW in-creased with increasing welding current. At welding current of16.5 kA, a TSS of 3.2 kN was obtained, which is comparable tothat of resistance spot welded 1-mm-thick AA6068. The joint ef-ficiency for U + RSWwas 61%, comparable to the relevant liter-ature data of dissimilar resistance spot welded Al to steel.

(5) Four different types of failure modes were observed, and theywere significantly affected by the welding current. As thewelding current was less than 13.6 kA, a shear fracture occurredat Al/insert interface (Type 1 failure mode). With welding cur-rent between 13.6 kA and 15.2 kA, the failure mode changed topartial/full nugget pull-out from insert (Type 2). As the weldingcurrent was above 15.2 kA, a variability in failure modes was ob-served including Type 2, nugget pull-out from aluminum sheet(Type 3), and interfacial fracture through Al insert/steel interface(Type 4). Such variability was likely caused by competing factorssuch as nugget size, electrode indentation, IMC layer thickness,and welding defects near the Al/steel interface. For both Types2 and 4, the fracture surface exhibited mixed features with dim-ples from aluminum strongly bonded to steel, and brittle cleav-age due to cracking through IMCs.

CRediT authorship contribution statement

Ying Lu: Formal analysis, Investigation, Writing - original draft. EllisMayton: Investigation.Hyeyun Song: Investigation.MenachemKimchi:Supervision. Wei Zhang: Conceptualization, Supervision, Writing - re-view & editing.

Acknowledgements

This works is funded in part by a gift fund fromOhio State University(OSU) Simulation Innovation and Modeling Center (SIMCenter)

11Y. Lu et al. / Materials and Design 165 (2019) 107585

through support fromHonda R&DAmericas, Inc. The authorswould liketo thank Colleen Hilla of OSUWelding Engineering for helpingwith tak-ing SEM images of ultrasonic spot joint andMitchell Matheny of EWI forhelpful discussion on ultrasonic spot welding.

Data availability statement

The raw/processed data required to reproduce these findings cannotbe shared at this time due to technical or time limitations.

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Dissimilar metal joining of aluminum to steel by ultrasonic plus resistance spot welding - Microstructure and mechanical p...1. Introduction2. Approaches2.1. Materials2.2. U+RSW2.2.1. Ultrasonic spot welding for intermediate joint2.2.2. Resistance spot welding of Al to intermediate joint

2.3. Bond quality characterization2.4. Finite element simulation of temperature distribution during RSW

3. Results and discussion3.1. Microstructure and strength of intermediate joints welded by USW3.2. Resistance spot welding with metallurgically bonded insert3.2.1. Macrostructure of dissimilar metal joint of Al to steel3.2.2. Microstructure at Al to steel interface

3.3. Calculated temperature distribution during RSW3.4. Mechanical properties of U+RSW joint3.5. Failure modes of dissimilar metal joint welded by U+RSW

4. Summary and conclusionsAcknowledgementsData availability statementReferences