steel dissimilar welds fabricated by friction surfacing assisted friction stir lap welding

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Original Article

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Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction surfacing assisted friction stir lap welding

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Li Zhou a,b,∗ , Mingrun Yu a,b,c , Baiyang Liu b , Zili Zhang b , Shuwei Liu b,1 , Xiaoguo Song a,b , Hongyun Zhao a,b

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a

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b

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c

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Q1

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan

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a r t i c l e

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i n f o

a b s t r a c t

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Article history:

Al/steel dissimilar welds were obtained by friction stir lap welding (FSLW) with an Al inter-

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Received 24 August 2019

layer, which was fabricated by friction surfacing (FS). In present study, the tool pin was

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Accepted 18 October 2019

totally plunged into the Al plate and interlayer without stirring steel, which avoided the

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Available online xxx

tool wear. The Al plate and interlayer were remarkably intermixed as pin length increased.

Keywords:

indicating the enhanced atomic migration and related interfacial bonding. The maximum

A diffusion layer, instead of the intermetallic layer, was found at the Al/steel interface,

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Al/steel dissimilar joining

failure load of the joints of 2.8 kN was reached when the 2.5 mm pin tool was used. The

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Friction surfacing

heterogeneous microstructure, which was caused by the intermixing, was responsible for

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Friction stir welding

the fracture, according to the fracture profiles. Necking and dimples were depicted from the

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Microstructure

fractographies, indicating that the joints were failed by plastic fracture.

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Mechanical properties

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

1.

Introduction

Al/steel hybrid structures are widely used in many industries, especially the Al-steel hybrid body of vehicles, for weight reduction to lower fuel consumption [1–3]. However, perfect Al/steel joints could hardly be obtained by conventional welding processes, and this limits the application of Al/steel structures remarkably. Bad weld appearances, great residual stress, and intermetallic compounds (IMCs) are the key prob-

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lems which troubles Al/steel welding a lot [4–6]. Friction stir welding (FSW) has been hence proposed to joining Al and steel joints in recent years [7,8]. FSW is a revolutionary solid-state welding process, which was invented by The Welding Institution (TWI) in 1991 [8]. Compared to conventional welding processes, FSW generates less heat in dissimilar welding, which reduces the residual stress and IMCs significantly [9–11]. Friction stir lap welding (FSLW), which has been successfully applied to join dissimilar alloys [12,13], is the welding process conducting FSW

∗

Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected] (L. Zhou). 1 This author is now studying in the State Key Laboratory of Nonferrous Metals and Processes, China. https://doi.org/10.1016/j.jmrt.2019.10.046 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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on lap joints. Over the last decade, studies mainly focused on the microstructure evolution and mechanical properties of the FSLW Al/steel joints with different welding parameters [14]. The deformation rate and heat input were reported having significant effects on the microstructure in the FSLW Al/steel joints [15,16]. Patterson et al. [15] and Yazdipour and Heidarzadeh [17] found defects, such as cracks and voids at the interface, were responsible for the decrease of lap shear strength of the Al/steel welds. Hence, it was concluded that the Al/steel welds’ lap shear performance could be improved by eliminating the interfacial defects. Ibrahim et al. [18] reported that the interfacial microstructure was optimized through parameter adjustment, and the weld’s lap shear strength was enhanced due to the elimination of inner defects. On the other hand, IMCs were also considered having determinate influence on the mechanical properties of the Al/steel welds. The IMC layer promoted the mechanical properties of the FSLW Al/steel joints with a proper thickness as investigated by Das et al. [19]. Pourali et al. [20] pointed that the Al-Fe IMCs were mainly formed at the interface, which was responsible for the fracture in the lap shear test. Elrefaey et al. [21] suggested that the formation of IMCs was inhibited and the Al/steel bonding was strengthened, when a small plunge depth was employed. However, it is hard to control the proper plunge depth, as excessive defects or IMCs occurred in the joints once the plunge depth has a little difference from the proper value [22]. Although studies about Al/steel FSLW have been conducted as mentioned above, the tool wear and the formation of IMC are still key problems limiting the application of Al/Steel FSLW. Introducing an interlayer between the Al alloy and steel was found effective to avoid tool wear in Al/steel FSLW [22]. In addition, as reported by Zheng et al. [23], fewer IMCs was observed at the interface in the Zn interlayer assisted Al/steel FSW, resulting better mechanical properties compared with the joints without Zn filler. But the Zn interlayer cannot survive at high temperature due to its low melting point. Al coating is thus widely applied, replacing Zn coating, for high temperature use. In this study, Al coating, which was fabricated on the steel by friction surfacing (FS), was employed to assist Al/steel FSLW in order to avoid tool wear and to reduce IMCs. Defect-free Al/steel joints were obtained by friction surfacing assisted friction stir lap welding (FSaFSLW) with various tool profiles. The microstructure evolution of the Al/steel joints were examined by optical and electronic microscopes to reveal the joining mechanism of Al/steel FSaFSLW. The pin profile was optimized by studying the influence of pin length on the microstructures and mechanical properties. The fracture mechanism was explored following the lap shear tests.

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Experimentation procedures

In this study, 6061 Al alloy and Q235 steel plates, whose dimensions were 250 mm × 75 mm × 2 mm, were welded by FSaFSLW. An Al interlayer were deposited on the steel using 6061 rods with 20 mm diameter by FS. The microstructures of the base materials are shown in Fig. 1, respectively. The chemical compositions and mechanical properties of base materials are summarized in Table 1.

The Al/steel FSaFSLW are illustrated in Fig. 2. Before FS and FSLW, the surfaces of steel and Al plates were polished by abrasive paper and cleaned by acetone. During FS, the rotating speed and traversing speed were 1500 rpm and 75 mm/min respectively, and the plunging speed was 12 mm/min. After FS, the 6061 plates were lapped to the coated Q235 plates at the retreating side after milling the interlayers to 1 mm thick. FSLW was then performed on the lap joints using the pin tools with different pin length. The welding details are listed in Table 2. Metallographic and mechanical specimens were cut by an electrical discharge machine vertically to the welding direction. Optical microscope (OM, Olympus DSX 510) was used to observe the microstructural characteristics after mechanical polishing. Scanning electron microscope (SEM, Zeiss-MERLIN Compact) equipped with an energy dispersive spectrometer (EDS, EDAX Octane Plus) was further employed to analyze the interfacial microstructure. X-ray diffraction (XRD, D/max2500X) was used to identify the interfacial phases. The microhardness distributions of the Al/Ti joints were measured using the Vickers hardness tester (HMAS-D1000Z) with a load of 100 g and a holding time of 10 s. Three specimens were tested by a universal testing machine (Instron 5967) at a crosshead speed of 6.0 mm/min for each joint.

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Results and discussion

3.1.

Microstructure of Al interlayer

The macrostructure and microstructure of the Al interlayer on Q235 substrate fabricated by FS are shown in Fig. 3. The Al interlayer was partly uncoated at the beginning. This was attributed to the unstable FS processing when the rod started traversing. According to the cross section, the profile of the interlayer was asymmetrical due to the difference between the material flows at advancing and retreating sides, which also results in the occurrence of small tunnels. In addition, flow tracks, including onion rings, could be depicted from the cross section. The grains of Al interlayer were highly refined and equaixed, indicating the dynamic recrystallization during FS. As a result, the average grain size, which was 36.5 ␮m for the rod, declined to 7.6 ␮m after FS. Furthermore, the grains adjacent to the substrate was slightly smaller than the grains at the top of the interlayer, which could be attributed to the difference between the thermo-cycles and material flows. At the interface, the grains of the steel substrate were elongated due to the friction between the Al rod and the substrate.

3.2.

Macrostructure of the Al/steel joints

Fig. 4 shows the appearances of the FSaFSLW Al/steel joints welded by different pin tools. Al/steel joints are obtained by FSaFSLW with no visible defect on the surface. There is no significant difference between the surfaces. The appearance is mainly depended on the welding parameters other than the pin length. In this study, same parameters were selected for the FSLW using different pins, which resulted in little difference between the weld surfaces.

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 1 – Microstructure of base materials: (a) Q235 steel, (b) 6061 rod, (c) 6061 plate.

Table 1 – Chemical compositions and mechanical properties of as-received materials. Elements (wt.%)

6061

Plate Rod

Q235

Al

Cu

Mg

Mn

Bal. Bal. –

0.27 0.30 –

1.61 1.37 –

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Fe

C

Si

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Tensile strength (MPa)

Elongation (%)

247.92 318.24 385.97

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Fig. 2 – Schematic illustration of FSaFSLW: (a) interlayer deposition, (b) FSLW.

Table 2 – Detailed welding parameters of the FSaFSLW of 6061 alloy and Q235 steel.

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No.

Rotating speed (r/min)

Traversing speed (mm/min)

Plunge depth (mm)

Tilting angle (◦ )

Pin length (mm)

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100

0.1

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Fig. 5 shows the cross-sections of the Al/steel joints. The area of the weld was enlarged as the pin length increased. Hook structure, which was commonly formed beside the interlayer in FSLW joint, was not observed though the interlayer has been significantly stirred. According to the microstructure evolution, the joint was composed of stir zone (SZ), thermomechanic affected zone (TMAZ), and heat affected zone (HAZ). The pin length mainly affected the SZs of the joints. It is suggested that the SZ is enlarged while the pin length increasing. It should be noticed that, when the 2.0 mm pin was used, the interlayer was deformed slightly, and the interface between the Al plate and interlayer is clear and straight comparing with the other two joints. As the pin length increased to 2.5 mm, the Al interlayer was remarkably suppressed at the advancing

2.5 2.9

side of the SZ. A small amount of the intermixing of Al plate and interlayer could be depicted from the cross section. When the pin further elongated to 2.9 mm, the plasticized Al plate was stirred into the interlayer, meanwhile, the stirred interlayer was also extruded into the Al plate. It was indicated that the weld material flowed downward at the advancing side and upward at the retreating side, as illustrated in the overviews.

3.3.

Microstructure of the Al/steel joints

Fig. 6 shows the microstructure in different zones of the FSaFSLW Al/steel joint welded by 2.5 mm pin tool. The SZ consisted of refined and equaixed grains, which was attributed to the dynamic recrystallization powered by the deformation and

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 5 – Cross-section of the welds with different pin length: (a) 2.0 mm, (b) 2.5 mm, (c) 2.9 mm.

Fig. 3 – Macro- and microstructure of Al interlayer: (a) the appearance of Al interlayer, (b) cross-section of Al interlayer, (c) micrograph of area A, (d) micrograph of area B.

The SEM micrographs and EDS linear results of the Al/steel interfaces are shown in Fig. 7, which were obtained in order to explore the bonding mechanism of the Al/steel FSaFSLW. The Al/steel interfaces were clear and straight, and no obvious Al/steel intermixing was depicted from the micrographs. Tiny fragments of the steel could be observed in the interlayer according to the EDS results. Furthermore, the Al and Fe atoms were diffused to the opposite side according to the EDS linear results. However, no proper Al–Fe IMC could be found according to such a low Fe content (∼10 at.%) from the Al–Fe binary phase diagram. Therefore, the FSaFSLW is considered to join mainly by atomic diffusion mainly. A little amount of Al–Fe IMCs might precipitate in the diffusion layer, as the Fe content was much higher than the solubility of Fe in Al (0.02 at.%) [24,25].

3.4. Fig. 4 – Appearance of the welds with different pin length: (a) 2.0 mm, (b) 2.5 mm, (c) 2.9 mm.

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heat input during welding. In addition, the interlayer was partly stirred into the Al plate at the advancing side of the interface, resulting in the intermixing which was related to the material flow closely. Furthermore, the dissolution of the precipitates in SZ was more significantly in the Al plate than the interlayer. The microstructure of the TMAZ was composed of the elongated and the equaixed grains in both Al plate and interlayer. It was suggested that the microstructure in the TMAZ partially experienced the dynamic recrystallization during FSLW. As for the HAZ, the microstructure was only affected by welding heat. Therefore, the grain structure was coarser than that before welding.

Microhardness

Fig. 8 shows the microhardness distribution of the joint. The microhardness remarkably varied at the middle of Al plate. The hardness distributions of the Al plates are similar and W-shaped when different pin tools are used, indicating that the pin length has little influence on the hardness profile of the Al plate. At the center of Al plate, the hardness in SZ is slightly lower than that of the BM, which could be attributed to the refined grains and the precipitate dissolution. As the precipitates dissolved and the grains partly recrystallized, the hardness further declined at TMAZ due to the larger grain size mainly. In HAZ, the hardness further decreased to 68 HV, and gradually rose to the hardness of the BM at the boundaries. Fig. 8b shows the microhardness distribution at the middle of the interlayer. The interlayer consists of highly refined and equaixed grains. The hardness of the interlayer decreased to 66 HV from 98 HV for the Al rod. After welding, the hardness of the interlayer slightly varied with a narrower width due to that

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 6 – Microstructure of typical joint (2.5 mm): (a) top of SZ, (b) middle of SZ, (c) bottom of SZ, (d) TMAZ in Al plate, (e) TMAZ in interlayer, (f) HAZ in Al plate.

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the welding processes at the interlayer are less affected by the shoulder. It is indicated that the deformation and heat in the interlayer are less than those in the Al plate during welding. As a result, the hardness slightly decreased in the SZ of the interlayer, which could be attributed to the dissolution of the precipitates. The hardness then gradually rose at the TMAZ due to the working hardening caused by the deformation. No

obvious variation of the hardness was measured at the HAZ due to less heat generated during welding. The hardness distribution along the center line perpendicular to the interface is shown in Fig. 10c. It could be seen that the hardness of the steel slightly increased to 168 HV beside the interface, which was attributed to the deformed grains mentioned before. The hardness sharply declined to about 60 HV at the SZ.

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 7 – Interfacial microstructure: a) SEM micrograph (b) EDS linear results of the interface welded by 2.0 mm pin tool, (c) SEM micrograph (d) EDS linear results of the interface welded by 2.5 mm pin tool; (e) SEM micrograph (f) EDS linear results of the interface welded by 2.9 mm pin tool.

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3.5.

Tensile shear test

Fig. 9 shows the failure load and elongation of the FSaFSLW Al/steel joints. The failure load of the joints reached the maximum of 2.8 kN with the 2.5 mm pin tool, which was improved by ∼30% of the joint without FS interlayer. The elongation reached the maximum of 2.8% with the 2.9 pin tool. When the 2.0 mm pin tool was used, the failure load and elongation both reached the minimum, which were 1.9 kN and 1.2% respectively. As the pin length increased to 2.5 mm, higher failure

load and elongation of 2.8 kN and 2.0% were obtained. When the pin length further increased to 2.9 mm, the Al plate was significantly stirred into the interlayer, resulting in the uneven microstructure of the interlayer. The intermixing of Al plate and interlayer reduced the cross-section area of the interlayer, which was negative on the strength improvement. As a result, the joint welded by 2.9 mm pin tool has a lower failure load of 2.2 kN and a higher elongation of 2.6%. Fig. 10 shows the cross sections of the fractured tensile specimens of the FSaFSLW Al/steel joints. The joint, which

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 8 – Microhardness distribution of the welds: (a) Al plate, (b) interlayer, (c) welds center.

Fig. 9 – Tensile shear test results of the welded joints.

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was welded by 2.0 mm pin tool, fractured along the Al/steel interface. The fracture between Al plate and interlayer could be attributed to the low bonding strength caused by the insufficient stirring and friction due to the short pin. Meanwhile, the interlayer was torn off the steel under the influence of the load during testing. When the pin length increased to 2.5 mm, the joint fractured along the interlayer, due to the promoted bonding between the Al plate and the interlayer. As

2.9 mm pin tool was employed, the fracture developed along the boundaries between the Al plate and the interlayer in the intermixing region, and also fractured at the interlayer finally. This suggested that the uneven microstructure distribution, which was caused by materials flow during welding, should be responsible for the failure of the joint welded by 2.9 mm pin tool.

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 10 – Cross-sections of the fractured joints of different pin length: (a) 2.0 mm, (b) 2.5 mm, (c) 2.9 mm.

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SEM was employed to observe the fracture surface of the specimen welded by 2.9 mm pin tool in order to reveal the fracture mechanism as shown in Fig. 11. According to the variation of microstructure, the fracture surface was divided into three regions as shown in Fig. 11a. The joints were torn along the interface between the interlayer and Al plate, resulting in the shear dimples in Fig. 11b. Equiaxed dimples were observed at regions B and C as shown in Fig. 11c and d. The average size of the dimples in region B was larger than that in region C. It was indicated that the cracks

started from the center and developed towards the interface. The XRD spectrums of the fracture surfaces are given in Fig. 12. As the interlayer and the steel were tore along the interface, the XRD spectrums are used to identify the interfacial phases. The peaks of IMC were hardly depicted from the XRD spectrums, which indicated that little IMC was formed in the joint. It was further concluded that the interlayer and the substrate are bonded by atomic diffusion mainly, which further proofs the discussion above.

Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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Fig. 11 – Morpholgies of fracured suface: (a) overview o, (b) region A, (c) region B, (d) region C.

Fig. 12 – XRD spectrums obtained from fracture surfaces: (a) Al side, (b) steel side.

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Conclusion

FSaFSLW was employed to join the AA6061 alloy and Q235 steel in this study. The FS interlayer was composed of highly refined and equiaxed grains. The defect-free Al/steel welds were obtained without stirring steel by FSaFSLW. The Al plate and interlayer were intermixed significantly after welding. A diffusion layer, whose Fe content was ∼10 at.%, were formed at the Al/steel interface replacing the IMCs in the conventional welds, and the thickness of diffusion layer was increased as pin length increased. The ultimate lap shear strength of the FSaFSLW Al/steel weld was reached 2.8 kN with a 2.5 mm pin tool, which was ∼30% improved comparing to the conventional Al/steel weld. The fracture was propagated by interface

department and void coalescence, which resulted in the necking and dimples in the fractographies.

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Conflict of interest The authors declare no conflicts of interest.

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Acknowledgements This studied was kindly supported by National Natural Science Foundation of China (Grant No. 51974100). Mingrun Yu acknowledges the support from the China Scholarship Council for the one-year study at the Joining and Welding Research Institute, Osaka University.

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Please cite this article in press as: Zhou L, et al. Microstructure and mechanical properties of Al/steel dissimilar welds fabricated by friction JMRTEC 1023 1–10 surfacing assisted friction stir lap welding. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.10.046

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