Resistance element welding of magnesium alloy and austenitic stainless steel in three-sheet configurations

Journal of Materials Processing Tech. 274 (2019) 116292

Contents lists available at ScienceDirect

Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec

Resistance element welding of magnesium alloy and austenitic stainless steel in three-sheet configurations ⁎

S.M. Manladana,b, Y. Zhanga,c, S. Rameshd,e, Y. Caia, S. Aoa, , Z. Luoa,

T

⁎

a

School of Materials Science and Engineering, Tianjin University, Tianjin, 3300350, China Department of Mechanical Engineering, Faculty of Engineering, Bayero University, 3011, Kano, Nigeria c Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China d Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia e Department of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, BE1410, Brunei b

A R T I C LE I N FO

A B S T R A C T

Associate Editor: C.H. Caceres

The Mg alloy and two austenitic stainless steel sheets were joined together by a metallurgical bond across the rivet and two austenitic stainless steel sheets. More heat was generated at the austenitic stainless/austenitic stainless interface than at the rivet/ austenitic stainless steel interface, leading to larger nugget size at the austenitic stainless steel /austenitic stainless steel interface at all welding currents. Thus, the nugget size at the austenitic stainless steel /austenitic stainless steel interface mainly influenced the transition from interfacial to pullout failure modes. The fusion zone microstructure consisted of ferrite and austenite. The microstructure in the edges of the nugget (both in the rivet and ASS) consisted of fine columnar dendritic grains. Owing to variation of temperature gradient and solidification growth rate, the grains morphology changed from columnar dendritic to equiaxed dendritic in the nugget center. The fine grains resulted in high fusion zone hardness. Digital image correlation analysis revealed that the joints could experience joining zone rotation/out-of-plane displacement during lap-shear tests, which reduced the magnitude of strain sustained by the joints in the loading direction. The joint configuration that did not undergo joining zone rotation and failed via pullout failure in the austenitic stainless steel sheet exhibited superior lap-shear performance.

Keywords: Resistance element welding Resistance spot welding Multi-sheet joining Magnesium alloy Austenitic stainless steel

1. Introduction Resistance spot welding (RSW) has been used to join similar and dissimilar materials of equal and unequal thicknesses in three-sheet configurations. Examples of these are uncoated and zinc-coated low carbon steel (LCS) (Harlin et al., 2003); LCS/DP600 steel/TRIP 700 steel (Nielsen et al., 2011); uncoated DQSK steel (Pouranvari and Marashi, 2012a, b); LCS/DP600/DP780 steels (Zhang et al., 2013); galvanized DP1000/bare TRIP980/ TWIP980 steels (Wei et al., 2015); AA5052 Al alloy (Li et al., 2015); galvanized DP780 steels(Wei et al., 2016); zinc-coated DX54/DP600/DP600 steels (Moghadam et al., 2016); austenitic stainless steel (ASS) (Zhang et al., 2016a); 6061-T6 Al alloy (Li et al., 2016); and 22MnB5/ DX54D /22MnB5 steels (Chtourou et al., 2018). The RSW of four steel sheets has also been reported, including 1.3 mm galvanized DP780 steel (Wei et al., 2016) and 0.7/1.2/ 1.2/0.9 mm LCS (Eizadi and Marashi, 2016). These studies have shown that the RSW of multiple sheets is significantly more challenging than that of two sheets due to the additional interfaces. For instance, it is

⁎

difficult to control the heat balance at the sheet/sheet interfaces and to ensure adequate nugget sizes that would result in favorable failure mode. These challenges are compounded when dissimilar materials and/or unequal sheet thicknesses are involved. The successful implementation of multi-sheet joints in vehicles manufacturing would therefore require the development of reliable alternative joining techniques. Recently, Sun et al. (2018) successfully applied flat friction stir spot welding to join three 6061-T6 Al alloy sheets. The process consisted of two steps. In the first step, a back plate having a pre-drilled dent was used to produce a protuberance on the bottom of the joint. In the second step, a smooth back plate and a probeless rotating tool were used to remove the protuberance and keyhole. This process maybe costly and difficult to implement in large scale production. Furthermore, since RSW is fully integrated into vehicles manufacturing lines for decades, techniques based on RSW machine would save equipment investment cost and are therefore more desirable. Resistance element welding (REW) is a joining technique based on

Corresponding authors. E-mail addresses: [email protected] (S. Ao), [email protected] (Z. Luo).

https://doi.org/10.1016/j.jmatprotec.2019.116292 Received 4 March 2019; Received in revised form 18 June 2019; Accepted 1 July 2019 Available online 02 July 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Table 1 Materials chemical composition (wt. %).

AZ31 Mg alloy 316 L ASS Q235 steel

Si

Mn

C

Co

Mo

Ni

Cr

Zn

Fe

Al

P

S

Mg

0.1 0.370 0.4

0.2–0.5 1.650 1.0

– 0.029 0.14

– 0.210 –

– 2.050 –

– 10.02 –

– 16.67 –

0.5-1.5 – –

0.005 balance balance

2.5–3.5 0.002 –

0.015 0.034 0.04

0.005 – 0.02

balance – –

RSW that was developed recently by Volkswagen (Innojoin, 2017) to address the challenges of joining Al to advanced high strength steels. As illustrated by Meschut et al. (2014), the technique involves punching a hole in the Al sheet, inserting a rivet (auxiliary element) in the hole, followed by RSW. A metallurgical bond is created between the rivet and the steel, and a force-and-form-locking connection (Innojoin, 2017) is established between the rivet and the Al sheet. The technique has been successfully applied to join Al alloy/22MnB5 steel (Meschut et al., 2014); Al alloy/ Q235 steel (Qiu et al., 2015); Al alloy/ 22MnMoB boron steel (Ling et al., 2016); LITECOR® to 22MnB5 steel (Holtschke and Jüttner, 2016), and Mg alloy to stainless steel (Manladan et al., 2017). Considering the challenges involved in joining multiple sheets by conventional RSW, it is important to explore the possibility of using REW to produce reliable multi-material, multi-sheet joints. The growing interest in Mg alloy and ASS in the transportation industry will invariably require joining them together in three-sheet configurations. This is a very challenging task because of the wide differences in physical and metallurgical properties between them. To the authors knowledge, no work has been reported on joining Mg to steel in three-sheet configurations by any technique. To expand the application of Mg alloy and ASS, in the present work, REW was employed to join 1.5 mm-thick AZ31 Mg alloy to two 0.7mm-thick 316 L ASS sheets using different joint configurations. The microstructures and lap-shear properties of the joints were studied. 3D digital image correlation (DIC) method was used to measure the out-of-plane deformation and strain distribution in the axial direction during the lap-shear test.

2. Materials and methods AZ31 Mg alloy (1.5-mm-thick) and AISI316 L ASS (0.7-mm-thick) were used as the base materials. Q235 steel rivet (5 mm in diameter) was used as the auxiliary element. The chemical compositions of the Mg alloy, ASS, and rivet are shown in Table 1. The dimensions of the welding specimens were 25 × 100 mm in accordance with AWS D17.2 Standard (AWS, 2007). Before welding, alcohol was used to clean the specimens. Abrasive paper was used to further clean the surface oxides on the Mg alloy specimens. The specimens were assembled, as shown in Fig. 1, with an overlap distance of 25 mm. Firstly, the possibility of using conventional RSW to join the three-

Fig. 1. Schematic illustration of the welding process, joint configurations, and tensile-shear sample geometries: (a) conventional RSW, (b) REW.

Fig. 2. Set up of DIC test. 2

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 3. (a) Macrostructure of the RSW joints; (b)-(d) microstructures of the corresponding regions in (a); (e) microstructure of region area E in (c); (f) microstructure of region F in (d).

electrode was used against the ASS side, as illustrated in Fig. 1, to decrease the current density and enhance the cooling rate in the ASS. In the REW process, the truncated cone was used against the rivet side. This was aimed at reducing the current density on the rivet side to avoid excessive melting of the rivet and thus enlargement of the rivet hole. Based on a series of preliminary investigations, for the RSW process, 150 ms welding time, 3.6 kN electrode force, and 6–18 kA welding current (in 2kA increment) were used. A welding time of 180 ms, 3.6 kN electrode force, and of 5–10 welding current (in 1 kA increment) were used for the REW process. For each condition, four samples were produced; one for metallographic examination, three for lap-shear tests. Metallographic samples were cut from the center of the joints. The samples were ground and polished based on standard metallography procedures. The Q235 steel side was etched using 4% nital solution. The

sheets was explored (Fig. 1a). However, the Mg alloy easily separated from the ASS sheet. The materials were joined together using the REW process. To investigate the effect of joint configurations on the lap-shear properties of the REW joints, three different joint configurations (single lap joints, Type I and Type II; and double lap joints, Type III) were produced. The welding process was conducted using a 220 kW, medium-frequency DC RSW machine (2–22 kA). As shown in Fig. 1, in the REW process, prior to welding, a hole (5 mm in diameter) was made at the center of the Mg alloy overlap area, and the solid steel rivet was put in the hole. Electrodes with asymmetrical geometries (made of C18200 Cu alloy, RWMA class II) were used to improve the heat balance. A truncated cone electrode (10 mm tip diameter) and a spherical electrode (sphere radius of 50 mm and face diameter of 20 mm) were used (Manladan et al., 2017). For the RSW process, the truncated cone 3

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 4. Fracture surface morphologies of RSW joints: (a) Mg alloy joining zone, (b) ASS joining zone, (c) ASS BM, and (d) area D in (b).

Table 2 Results of EDS analysis of regions 1 and 2 in Fig. 4d. Composition (wt.%)

Region 1 Region 2

Fe 49.05 5.41

Cr 12.34 1.56

Ni 6.43 1.31

Mg 22.27 83.68

Al 9.91 8.04

ASS side was etched using a solution of 10 g FeCl3, 120 ml H2O, and 30 ml HCl. The Mg side was etched using a 5 g picric acid, 5 ml acetic acid, 10 ml H2O, and 100 ml ethanol solution. The macroscopic morphologies of the joints were observed using Olympus SZX12 stereomicroscope, while the microstructures were observed with an Olympus GX51 microscope. Vickers micro-hardness tester (Huayin HV-1000A) was used to measure the hardness variation across the joint under a load of 200 g for 15 s. Lap-shear tests were done using a CSS-44100 testing system at a speed of 2 mm/min. Three samples were tested for each condition, and the average peak load and energy absorption (area of the

Fig. 6. Nugget diameters as a function of welding current.

Fig. 5. (a) Schematic diagram of the REW joints; (b)-(g) macrostructures of the joints at different welding currents. 4

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 7. (a) Typical macrostructure of the REW joints; (b)-(e) microstructures of the corresponding regions in (a).

Fig. 8. EBSD phase mappings of the FZ.

3. Results and discussion

load–displacement curve up to the peak load) were recorded. Vic 3D-7 system (Correlated Solutions, Inc) was used to conduct the DIC analysis. A pair of CCD cameras was used to capture digital images of the front (Mg/ rivet) side and back (ASS) side of the overlap area of the joints at 5 frames per second during the test. The setup of the DIC test is shown in Fig. 2. Prior to the test, the speckle was applied on the area of interest (AOI), as indicated in Fig. 1b. In line with the work of Comer et al. (2013), the images taken were imported in to the Vic 3D-7 software to measure the out-of-plane displacements and surface strain fields in the axial direction.

3.1. Conventional RSW joints The typical macroscopic morphology and microstructures of the conventional RSW joints are shown in Fig. 3. The Mg alloy was joined to the upper ASS sheet (ASS1) through a welding-brazing mechanism. The Mg alloy melted and spread on the ASS1, leading to nugget formation in the Mg alloy (indicated by black dotted lines in Fig. 3a). The formation of welding-brazing mechanism in joining AZ31 Mg alloy to steel has been attributed to the formation of nano-scale Fe-Al layer on the steel (Wang et al., 2016) as a result of the reaction between Al atoms from the Mg alloy and Fe atoms from the steel. The molten Mg alloy would then spread on this layer and solidify to form the nugget. The ASS1 and ASS2 were joined by a metallurgical bond. As shown in Fig. 3c–f, the fusion zone (FZ) in the ASS sheets is divided into two zones, i.e., the 5

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 9. (a) Fe-C phase diagram; and microstructures (OM/FESEM images) of the: (b)-(c) UCHAZ, (d)-(e) ICHAZ, (f)-(g) BM.

clearly seen in Fig. 3a. For most of the joints, the Mg alloy separated from the ASS sheets after cutting the samples for metallographic examination and while mounting the samples in the tensile test machine. Therefore, the mechanical properties and the effects of joint configuration on the mechanical properties of the RSW joints were not studied. The typical fracture surfaces are shown in Fig. 4. Pores, shrinkage cavities, and cracks are seen in the Mg nugget, indicating poor connection between the Mg alloy and ASS sheets. The joining zone in the ASS sheet (Fig. 4b) has similar characteristics as the ASS BM (Fig. 4c). The grain boundaries are seen clearly, further indicating that the ASS

main FZ (FZ1) and peripheral FZ (FZ2). During solidification, the microstructure of the ASS transforms into a mixture of delta ferrite and austenite, according to the following transformation path (Zhang et al., 2016b): I

II

III

L→ L+ δ → L+ δ+ γ → δ+ γ Since the cooling rate at the periphery of the FZ is higher than at the center, because of the closer contact with the electrodes, the difussion controled reaction (stage III) would have limited time to occur. This leads to higher volume fraction of delta ferrte in the periphery (FZ2). The connection between the Mg alloy and ASS 1 was very poor, as 6

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 10. Hardness distribution in the REW joints (a): hardness indentation paths; (b) vertical hardness profile (path 1); (c) horizontal hardness profiles (paths 2 and 3).

Fig. 11. Lap-shear properties of the REW joints (a) Peak load as a function of welding current; (b) energy absorption as a function of welding current; and (c) comparison of the maximum average peak load and energy absorption.

shown in Fig. 6, the nugget size at the ASS/ASS interface was larger than that at the rivet/ASS interface (asymmetrical nugget) at all welding currents. For both interfaces, the nugget size increased with increasing welding current due to the increase in the volume of molten metal. This is mainly due to the differences in electrical resistivity between the LCS rivet and ASS. The electrical resistivity of ASSs (690–1020 μΩm) is significantly higher than that of LCS (120 μΩm) (AWS, 1982). Therefore, more heat would be generated at the ASS/ASS interface than the ASS/rivet interface. Perhaps, the only concern in the REW of Mg/ASS/ASS sheets observed in this study is the difficulty to

did not melt. However, residual Mg alloy is observed on the surface, as confirmed by the result of EDS analysis (Table 2) of regions 1 and 2 in Fig. 4b. This indicates that some regions of good connection exist on the surface.

3.2. Macro/microstructures of the REW joints Fig. 5a shows a schematic diagram of the REW joints. The macrostructures of the joints at different welding currents are shown in Fig. 5b–g. The nugget is formed across the rivet/ASS1/ASS 2 sheets. As 7

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 12. Schematic illustration of the failure modes in Type I joints: (a) IF and (b) TF.

Fig. 13. Typical load-displacement curves of the REW joints.

the ASS2 (Fig. 7d) are finer than those at the edge of the nugget in the rivet (Fig. 7b). This is likely because of the higher cooling rate experienced in ASS2. The edge of the nugget in the ASS 2 is much closer to the water-cooled electrodes, leading to higher cooling rate. Electron back scattered diffraction (EBSD) phase mapping reveals that the FZ microstructure consisted of ferrite and austenite, as shown in Fig. 8. The Mg alloy in contact with the rivet also melted and re-solidified (region E in Fig. 7a). The microstructure in this region consisted of CDG (Fig. 7e). The microstructure in the heat affected zone (HAZ) of the Q235 steel rivet could be divided into the upper-critical HAZ and inter-critical HAZ, as illustrated in Fig. 5a. According to the Fe-C phase diagram (Fig. 9a), the UCHAZ is heated to a temperature above AC3, transforming the microstructure into austenite. The austenite transformed to martensite (Fig. 9b–c) upon cooling because of the inherent high cooling characteristics of the RSW process (Goodarzi et al., 2009). The maximum temperature reached in the ICHAZ is between AC1 and AC3, and the microstructure transformed into ferrite and austenite. The

form the nugget exactly across the rivet/ASS interface, as shown in Fig. 5b–g. This shifting of the nugget at the rivet/ASS interface did not affect the lap-shear properties of the joints. The joints failed mainly in the ASS sheets or Mg alloy, as discussed in section 3.4. At 10kA, due to the high heat input, pores are formed in the nugget center, and the rivet hole widened due to excessive melting of the Mg alloy, as shown in Fig. 5g. The microstructures in the edges of the nugget, both in the rivet (Fig. 7b) and ASS 2 (Fig. 7d), consisted of columnar dendritic grains (CDG). Equiaxed dendritic grains (EDG) are observed in the nugget center (Fig. 7c), indicating that columnar-to-equiaxed transtion (CET) has occurred. According to the solidification theory of Kou (1987), CET is governed by the temperature gradient (G) to solidification growth rate (R) ratio. Lower G/R ratio leads to higher degree of constitutional supercooling and thus promotes CET. The formation of equiaxed grains in the nugget center could be attributed to lower value of the G/R ratio in the nugget center. The columnar grains at the edge of the nugget in 8

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 14. Out-of-plane displacements and axial surface strains at different stages of the lap-shear test for Type I (TF): (a1-e1) front side; (a2-e2) back side.

phase boundaries in the FZ. Alenius et al. (2006) also observed high hardness value (about 400 HV) in the FZ of two-sheet RSW joints between LCS and ASS. The UCHAZ of the Q235 steel rivet exhibited the highest hardness (average value of approximately 393.6 HV), which is due to the martensitic microstructure (Fig. 9b–c). The average hardness of the ICHAZ (approximately 241.67 HV) is lower than that of the UCHAZ due to the presence of ferrite in the ICHAZ. The Mg alloy side exhibited little hardness variation, with an average hardness of approximately 63.5 HV.

ferrite remained upon cooling while the austenite transformed to martensite. Thus, the microstructure consisted of ferrite and martensite (Fig. 9d–e). As shown in Fig. 9f–g, the microstructure of the as-received rivet consisted mainly of ferrite and some pearlite (Manladan et al., 2017). 3.3. Hardness distribution across the REW joints The vertical (path 1 in Fig. 10a) and horizontal hardness profiles (paths 2 and 3 in Fig. 10b) across the joints are shown Fig. 10. The hardness variation is consistent with the microstructural gradient in the HAZ and FZ. The average hardness of the FZ is approximately 357.9 HV which is significantly higher than those of the ASS BM (approximately 170 HV) and the Q235 steel BM (approximately 194.12 HV). FZ hardness depends on the hardness of the individual constituents and the strengthening effect of the grain and phase boundaries (Pouranvari et al., 2016). As discussed in the preceding section, the FZ consisted mainly of fine columnar grains of ferrite and austenite. Therefore, the high hardness of the FZ could be related to the high area fraction of

3.4. Lap-shear properties of the joints Referring to Fig. 11a and b, the welding current strongly influenced the peak load and energy absorption ability of the joints. Generally, for all the joints configurations (Type I, Type II, and Type III), the peak load and energy absorption increased with increasing welding current. This is mainly due to the increase in nugget diameter at both interfaces (Fig. 6). For Type I joint, compared to those at a welding current of 9 kA, a slight decrease in peak load and energy absorption is observed at 9

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 15. Out-of-plane displacements and axial surface strain at different stages of the lap-shear test for Type II (PO): (a1-e1) front side; (a2-e2) back side.

Fig. 16. Illustration of the loading conditions and failure location in Type III joints.

largely because of the differences in secondary bending/joining zone rotation tendencies and failure mechanism/location.

welding current of 10 kA, despite the increase in nugget size. The high heat resulted in slight increase in the size of the rivet hole, as shown in Fig. 5g. Fig. 11 also shows that the joint configuration strongly influences the lap-shear properties of the joints. Under the same welding conditions, Type III joints exhibited the most superior lap-shear performance, followed by Type II, and then Type I joints. These differences in lapshear performance between the three types of joint configurations is

3.5. Failure mode of the REW joints 3.5.1. Failure mechanism of Type I joints As indicated in Fig. 11, Type I joints exhibited two types of failure modes. At welding currents of 5 and 6 kA, the joints failed via 10

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 17. Out-of-plane displacements and axial surface strain at different stages of the lap-shear test for Type III (PO): (a1-e1) front side; (a2-e2) back side.

indicating an increase in the local bearing stress. As the severity of the secondary bending increased, due to the reduction in stiffness and limited ductility of the Mg alloy, failure occurred abruptly through the hole center. The secondary bending and strain distribution on the ASS side of the overlap area are consistent with the phenomenon on the Mg side. The compressive strains in the ASS sheet perpendicular to the loading direction increased with increased secondary bending of the sheets.

interfacial failure (IF) mode at the rivet/ASS interface, as illustrated in Fig. 12a. At higher welding currents and consequently larger nugget diameter, the resistance of the joints to IF mode increased. The failure occurred in the Mg alloy, through the hole center, perpendicular to the loading direction, as illustrated in Fig. 12b. This type of failure mode is mostly observed in riveted and bolted joints, and it is referred to as tension failure (TF) (Wang et al., 2017). Fig. 13a compares the typical load-displacement curves of Type I joints that failed in IF and TF modes. Although the TF is accompanied with relatively higher peak load and energy absorption, for both failure modes, the load dropped suddenly upon reaching the peak load. Since the TF sustained higher load and is more complex, it is characterized further. During lap-shear tests, due to the eccentricities in the load path, out-of-plane deformations (referred to as secondary bending (Skorupa et al., 2015) occurs. As shown in Fig. 14, as the test progressed, the Mg alloy sheet increasingly experienced secondary bending. A compressed region occurred on the primary bearing surface of the Mg alloy under the rivet. The size of this region increased as the test progressed,

3.5.2. Failure mechanism of Type II joints In Type II joints, the load was sustained mainly by the nugget at the ASS/ASS interface. Irrespective of the welding current, the joints failed in pull out (PO) mode in which the pulled ASS sheet withdrew from the nugget. The typical load-displacement curve for the joint is shown in Fig. 13b. Unlike in Type I joints, upon reaching the peak load, the failure occurred gradually. The out-of-plane displacements and strain distributions in Type II (PO) are shown in Fig. 15. It can be seen that the joining zone underwent severe rotation. The ASS side experienced large 11

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

Fig. 18. Fracture surface morphologies: (a) IF mode (rivet side), (b) IF mode (ASS side), (c) TF in the Mg alloy, and (d) PO in the ASS sheet.

mode at the rivet/ASS interface exhibit fish-scale like dimples (Fig. 18a and b), further indicating that the IF is governed by shear stress at the sheet/sheet interface. The surface appearance is also consistent with the low energy absorption of the IF mode joints. The PO failure surface in the ASS (Fig. 18d) exhibits circular, further indicating higher energy absorption. The circular dimples also indicate that, although the global load is shear, the fracture at the materials level is tensile. The fracture surface morphology of the Type I (TF) (Fig. 18c) is also consistent with the low energy absorption of the joints on account of the limited ductility of Mg alloy, especially when compared to ASS

amount of deformation and the strain is localized in the periphery of the nugget, eventually leading to PO failure. The strains on the front side are significantly less than that in Type I (TF) joints. The strain on the ASS side are significantly higher than that of the Type I (TF) joints. 3.5.3. Failure mechanism of Type III joints At 5kA welding current, IF failure first occurred at the rivet/ASS interface (due to the small nugget size at this interface) and then PO occurred in the ASS sheet. At higher welding current, PO occurred in the pulled ASS sheet. As shown in Fig. 13c, the characteristics of the load-displacement curve for Type III PO failure mode is similar to that of Type II up to the peak load. The Type III joints sustained higher peak load and absorbed higher energy despite the fact that both joints possessed the same nugget size and failed in the pulled ASS sheet. This can be attributed to differences in joining zone rotation tendency. Mechanics-based analyses of spot welds in ductile materials (Chao, 2003) have shown that the nugget rotation results in a combined tensile/shear stress field at the loading bearing area. The higher the degree of rotation, the lower the proportion of normal strain/stress component at the periphery of the joining. As shown in the photo of the failed samples in Fig. 13c and schematically illustrated in Fig. 16, because of the absence of eccentricities in the load path, the joining zone of Type III (PO) did not undergo rotation. The results of the DIC test (Fig. 17) also indicate that the joining zone did not bend/rotate, although the free edges of the Mg alloy and ASS sheets were slightly displaced out of the plane. Regions of compression also appeared in the bearing surfaces of the rivet hole in the Mg side. However, these are smaller compared to those in Type I (TF). The strain distribution in the surfaces of the Mg alloy and ASS sheets of the Type III (PO) joints are more uniform. The pulled ASS was found to under more uniform and extensive elongation than that in Type II PO. These could account for the superior tensile-shear performance of this joint.

4. Conclusions The following conclusions were drawn from this study.

• Conventional resistance spot welding is not suitable for joining Mg alloy/ASSs in multi-sheet configurations • Resistance element welding has the potentials of being a reliable •

• •

technique for joining Mg alloy and ASS sheets in three-sheet configuration The resistance element welding joints were produced by a metallurgical bonding across the rivet/austenitic stainless steel/austenitic stainless steel. The microstructure at the edges of the joints in the rivet and austenitic stainless steel consist of fine columnar grains. The microstructure in the nugget center consists of equiaxed grains The nugget size at the rivet/ austenitic stainless steel interface was lower than that at the austenitic stainless steel/austenitic stainless steel interface, and therefore influenced the failure mode transition. The joints configuration affects the mechanical performance of the joints. At optimum conditions, joints that did not undergo joining zone rotation and failed in the austenitic stainless steel exhibited superior lap-shear performance.

Acknowledgements

3.6. Fracture surface morphologies of the REW joints The fracture surface morphologies of the joints that failed in IF

This work was supported by National Key R&D Program of China 12

Journal of Materials Processing Tech. 274 (2019) 116292

S.M. Manladan, et al.

(No.2018YFB1107900), Natural Science Foundation of Tianjin City (No. 18JCQNJC04100), and the National Natural Science Foundation of China (No. 51575383). S.M. Manladan acknowledges China Postdoctoral Science Foundation for Postdoctoral International Exchange Program.

aluminum alloy to uncoated 22MnMoB boron steel. Mater. Manuf. Process. 31, 2174–2180. Manladan, S.M., Yusof, F., Ramesh, S., Zhang, Y., Luo, Z., Ling, Z., 2017. Microstructure and mechanical properties of resistance spot welded in welding-brazing mode and resistance element welded magnesium alloy/austenitic stainless steel joints. J. Mater. Process. Technol. 250, 45–54. Meschut, G., Hahn, O., Janzen, V., Olfermann, T., 2014. Innovative joining technologies for multi-material structures. Weld. World 58, 65–75. Moghadam, M., Tiedje, N.S., Choobi, M.S., Nielsen, C.V., Pantleon, K., Bay, N.O., 2016. Bonding mechanisms in spot welded three layer combinations. 9th International Seminar Conference on Advances in Resistance Welding. Nielsen, C.V., Friis, K.S., Zhang, W., Bay, N., 2011. Three-sheet spot welding of advanced high-strength steels. Weld. J. 90. Pouranvari, M., Khorramifar, M., Marashi, P., 2016. Ferritic–austenitic stainless steels dissimilar resistance spot welds: metallurgical and failure characteristics. Sci. Technol. Weld. Join. Pouranvari, M., Marashi, S., 2012a. Failure behavior of three-steel sheets resistance spot welds: effect of joint design. J. Mater. Eng. Perform. 21, 1669–1675. Pouranvari, M., Marashi, S., 2012b. Weld nugget formation and mechanical properties of three-sheet resistance spot welded low carbon steel. Can. Metall. Q. 51, 105–110. Qiu, R., Wang, N., Shi, H., Cui, L., Hou, L., Zhang, K., 2015. Joining steel to aluminum alloy by resistance spot welding with a rivet. Int. J. Mater. Res. 106, 60–65. Skorupa, M., Korbel, A., Skorupa, A., Machniewicz, T., 2015. Observations and analyses of secondary bending for riveted lap joints. Int. J. Fatigue 72, 1–10. Sun, Y., Fujii, H., Zhu, S., Guan, S., 2018. Flat friction stir spot welding of three 6061-T6 aluminum sheets. J. Mater. Process. Technol. Wang, W.-M., Khan, H.A., Li, J., Miller, S.F., Trimble, A.Z., 2017. Classification of failure modes in friction stir blind riveted lap-shear joints with dissimilar materials. J. Manuf. Sci. Eng. 139 021005. Wang, X., Sun, D., Sun, Y., 2016. Influence of Cu-interlayer thickness on microstructures and mechanical properties of MIG-welded Mg-steel joints. J. Mater. Eng. Perform. 25, 910–920. Wei, S., Liu, R., Lv, D., Lin, L., Lu, X., Ding, S., 2016. Effect of joint configuration on resistance spot weldability of galvanised DP780 steel sheets. Sci. Technol. Weld. Join. 21, 178–185. Wei, S., Liu, R., Lv, D., Lin, L., Xu, R., Guo, J., Wang, K., Lu, X., 2015. Weldability and mechanical properties of similar and dissimilar resistance spot welds of three-layer advanced high strength steels. Sci. Technol. Weld. Join. 20, 20–26. Zhang, Y., Li, Y., Luo, Z., Feng, Y., Zhou, J., 2016a. Effect of joint design on the failure behaviour of three-stack-up austenitic stainless steel resistance spot welds. Sci. Technol. Weld. Join. 21, 484–492. Zhang, Y., Li, Y., Luo, Z., Feng, Y., Zhou, J., 2016b. Effect of joint design on the failure behaviour of three-stack-up austenitic stainless steel resistance spot welds. Sci. Technol. Weld. Join. 1–9. Zhang, Y., Shen, J., Zhao, Y., Wang, P.-C., Carlson, B., 2013. Effect of adhesive characteristics on the weld quality in weld bonding multiple steel sheets. Weld. J. 92.

References Alenius, M., Pohjanne, P., Somervuori, M., Hanninen, H., 2006. Exploring the mechanical properties of spot welded dissimilar joints for stainless and galvanized steels. Weld. J. 85, 305. AWS, 1982. Metals and their weldability. In: Kearns, W.H. (Ed.), Welding Handbook, 7th ed. American Welding Society, Miami, FL. AWS, 2007. Specification for Resistance Welding for Aerospace Applications, D17.2. American Welding Society. Chao, Y., 2003. Failure mode of spot welds: interfacial versus pullout. Sci. Technol. Weld. Join. 8, 133–137. Chtourou, R., Chaari, F., Haugou, G., Leconte, N., Markiewicz, E., Zouari, B., 2018. Experimental study of the mechanical strength and the failure of multi-sheet, multimaterial spot-welded assemblies under pure and combined loading conditions. EPJ Web of Conferences p. 02009. Comer, A., Katnam, K., Stanley, W., Young, T., 2013. Characterising the behaviour of composite single lap bonded joints using digital image correlation. Int. J. Adhes. Adhes. 40, 215–223. Eizadi, H., Marashi, S., 2016. On the resistance spot welding of four-sheet stack of unequal sheet thickness. Sci. Technol. Weld. Join. 1–6. Goodarzi, M., Marashi, S., Pouranvari, M., 2009. Dependence of overload performance on weld attributes for resistance spot welded galvanized low carbon steel. J. Mater. Process. Technol. 209, 4379–4384. Harlin, N., Jones, T., Parker, J., 2003. Weld growth mechanism of resistance spot welds in zinc coated steel. J. Mater. Process. Technol. 143, 448–453. Holtschke, N., Jüttner, S., 2016. Joining lightweight components by short-time resistance spot welding. Weld. World 1–9. Innojoin, 2017. Development and Evaluation of Advanced Welding Technologies for Multi-material Design With Dissimilar Sheet Metals. http://www.innovatienetwerk. be/attachments/get/4818/InnoJoin_-_Summary_-_part_1.pdf. Kou, S., 1987. Welding Metallurgy. New York. . Li, Y., Yan, F., Luo, Z., Chao, Y., Ao, S., Cui, X., 2015. Weld growth mechanisms and failure behavior of three-sheet resistance spot welds made of 5052 aluminum alloy. J. Mater. Eng. Perform. 24, 2546–2555. Li, Y., Zhang, Y., Luo, Z., Shan, H., Feng, Y., Ling, Z., 2016. Failure mode transition of triple-thin-sheet aluminum alloy resistance spot welds under tensile-shear loads. Weld. J. 95, 479s–490s. Ling, Z., Li, Y., Luo, Z., Feng, Y., Wang, Z., 2016. Resistance element welding of 6061

13