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Article

Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel

by
Tianhan Hu
1,
Bolong Li
1,
Zhen Li
1,
Kai Ding
1,
Tianhai Wu
2,3,
Hua Pan
2,3,* and
Yulai Gao
1,*
1
State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
2
State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel Group), Shanghai 201900, China
3
Automobile Steel Research Institute, R&D Center, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, China
*
Authors to whom correspondence should be addressed.
Metals 2024, 14(11), 1209; https://doi.org/10.3390/met14111209
Submission received: 23 September 2024 / Revised: 14 October 2024 / Accepted: 18 October 2024 / Published: 23 October 2024
(This article belongs to the Special Issue Advances in Welding Processes of Metallic Materials)
Browse Figures
Versions Notes

Abstract

:
Considering energy conservation and emission reductions, lightweight automobiles have become a research focus in the automotive industry. Steel/aluminum joining is regarded as an ideal lightweight structure, which can not only reduce the energy consumption but also ensure safety and is already attracting extensive attention. In this study, aluminum alloy 6061 and B410LA steel sheets were successfully joined by refill friction stir spot welding. The tensile properties, microhardness distribution and interfacial microstructure characteristics of the steel/Al welded joints were investigated. The maximum tensile load of the steel/Al joint was 4.3 kN. The mechanical properties of the steel/Al refill friction stir spot welded joint were largely determined by the bonding quality of the sleeve-plunging zone. With the stirring of the sleeve and the pin during the refill friction stir spot welding, work hardening occurred in the stir zone (SZ). The microhardness of the SZ was significantly higher than that of the steel base metal (BM) and could be detected on the steel side. The Fe-Al intermetallic compound (IMC) layer was continuously distributed at the interface of the sleeve-plunging zone, revealing good uniformity in the thickness. In particular, a hook-and-vortex-like structure formed during the refill friction stir spot welding process in the sleeve-plunging zone, producing a mechanical interlock effect at the interface. The ideal mechanical properties of the welded joint could be attributed to the good quality of the metallurgical and mechanical bonding at the interface, especially the mechanical interlock effect, thereby depending on the hook-and-vortex-like structure.

1. Introduction

With increasing environmental issues and energy crises, energy conservation and emission reductions have become common challenges faced globally. In the past several years, lightweight automobiles have become the research focus in automobile manufacturing [1]. Several studies [2,3] have shown that lightweight automobiles could significantly cut down on fuel consumption and exhaust emissions during vehicle driving. This not only helps relieve energy pressure but also makes positive contributions to environmental protection. In particular, steel/Al joining was considered as an ideal lightweight structure [4]. High-strength steel has the characteristics of high strength, good formability and excellent corrosion resistance, while Al alloys possess the advantages of low density, high specific strength, good corrosion resistance and easy recycling. The reasonable joining of steel and Al alloy in the vehicle body could achieve the purpose of weight reduction while ensuring safety. Therefore, steel/Al hybrid structures have increasingly been applied in the design and manufacturing of frame components used in automobile bodies [5,6].
The main welding techniques used in steel/Al joining include resistance spot welding (RSW) [7], laser welding [8], cold metal transfer (CMT) [9], magnetic pulse welding (MPW) [10], friction stir welding (FSW) [11], etc. However, the variance in the expansion coefficient, thermal conductivity and elastic modulus between Fe and Al could lead to the uneven heating of the steel/Al, which are dissimilar metals, during the welding process [12,13]. These conditions could cause inhomogeneous thermal deformation, resulting in large residual stress [14]. In addition, there exists a significant difference in the density and melting temperature between Fe and Al. An uneven weld metal (WM) composition could be produced due to the different densities and melting points during the welding process, thereby increasing the risk of WM fracture. In particular, several kinds of intermetallic compounds (IMCs) were easily formed according to various element ratios between Fe and Al, such as Fe2Al5 (an orthorhombic structure), Fe4Al13 (a monoclinic structure), FeAl3 (a monoclinic structure) [12,15,16,17,18], etc. The formation of binary Fe-Al IMCs with a high brittleness could seriously deteriorate the properties of steel/Al joints. The IMCs generated at the interface zone could induce crack initiation, deteriorating the joint bonding quality [19]. Thus, controlling the interfacial microstructure, especially the IMC layer, is particularly important for welded joint properties.
With the advantages of a low heat input and the control of the IMC layer, the solid-phase welding technique is widely used in the field of steel/Al welding [20,21,22,23]. Among these welding techniques, the FSW technique has become one of the primary choices for achieving high-quality steel/Al joining. At present, the spot welding joint is the most widely used joint form in the automobile industry, with the manufacturing of a car requiring about 2000 welding spots [24]. The FSW technique also evolved the form of spot welding to facilitate this application. Refill friction stir spot welding [10] can improve the joint bonding quality because of the absence of the keyhole [10,25] compared with traditional friction stir spot welding (FSSW) [26]. Furthermore, refill friction stir spot welding can effectively control the formation of brittle intermetallic compounds (IMCs), improving the bonding quality of the welded joint [24,27].
Piccini et al. [28] investigated the influence of the length of the stirring pin on the mechanical properties of the refill friction stir spot joint of the AA6063 Al alloy and low-carbon steel. The results indicated that the thermomechanical effect of the joint interface was enhanced as the length of the stirring pin decreased. The maximum tensile load of the joint also increased. At the same time, the fracture mode transferred from the interface fracture (IF) to the button fracture (BF). In addition, the impact of the shape of the stirring pin on the bonding quality of the steel/Al welded joint was also studied [24]. The bonding quality of steel/Al joints prepared using the different stirring pin shapes was compared in the experiment. It was found that the bonding quality of the joint was the best when the stirring pin was conical, with a height of 0.4 mm and a diameter of 9.6 mm. Ding et al. [29] also prepared a steel/Al welded joint. In order to reduce the consumption of the pin, the pin did not enter into the steel matrix, which also produced a good bonding quality in the welded joint. Li et al. [30] discovered that the mechanical bonding induced by the plunging of the sleeve could effectively enhance the connection quality of steel/Al welded joints. The amorphous layer or the IMC layer was formed at the interface zone of the steel/Al refill friction stir spot welded joint under the action of stirring heat and mechanical force. Ogura et al. [31] observed the amorphous phase in the welded joint of 3003 Al alloy and SUS304 stainless steel. Rest et al. [32] found a double-IMC-layer structure in the steel/Al refill friction stir spot welded joint, which included an Fe2Al5 phase near the steel side and an FeAl3 phase near the Al side. Tanaka et al. [33] optimized the rotation speed of the pin to limit the IMC growth. They deemed that the bonding quality of the steel/Al welded joint was inversely proportional to the IMC thickness. Chen et al. [34] held that the growth of the IMC layer was affected by the plunging depths of the pin. When the plunging position of the pin was close to the steel base metal (BM), discontinuous IMCs were formed at the interface zone of the steel/Al refill friction stir spot welded joint. The IMC layers that formed at the interface were continuous when the pin was plunged into the steel BM. Bozzi et al. [35] joined 6061 aluminum alloy and IF steel by using the friction stir spot welding technique. The IMC layer was observed at the interface of the steel/Al welded joint. The thickness of the IMC layer was quantified as a function of the rotational speed of the stirring pin and the plunging depth. The entanglement of oval IMCs was detected by transmission electron microscopy (TEM). In addition, they deemed that an IMC layer with appropriate thickness was necessary to achieve good metallurgical bonding of steel/Al welded joints. Meanwhile, some typical interface morphology could be observed in the steel/Al refill friction stir spot welded joint [36]. In order to obtain better mechanical properties, it is often required that the steel/Al refill friction stir spot welded joint possesses good metallurgical and mechanical bonding. Nevertheless, little information is available for the effect of enhanced mechanical bonding on the mechanical properties of the joint.
In this study, the welded joints were prepared employing the refill friction stir spot welding technique. The mechanical properties of the joint were revealed by the tensile and microhardness tests. The microstructure of the joint was observed. In addition, the element distribution at the interface was detected to elucidate the role of the mechanical and metallurgical bonding in the mechanical properties.

2. Materials and Methods

Aluminum alloy 6061-T6 and B410LA steel were selected as the BMs for the welding. The chemical compositions of the welding materials are listed in Table 1. Al alloy 6061 and B410LA steel sheets were overlapped by refill friction stir spot welding. The thicknesses of the plates were 1 mm (Al) and 0.8 mm (steel). The refill friction stir spot welding machine (FFSW-04-1D) was employed to join the steel and Al-based alloy. Figure 1 displays a schematic of the refill friction stir spot welding equipment and process. The welding tool was composed of three parts: the clamping ring, sleeve and stirring pin (see Figure 1a). The clamping ring could play a role in fixing the plates and preventing the welding material from overflowing. As the main components for stirring materials, the outer walls of the sleeve and pin generally had threads [37]. Figure 1b–e illustrate the welding process. The entire welding process was divided into four stages. Figure 1b shows the first stage of the welding process. The stirring pin and the sleeve rotated together, with the same rotational speed of 1500 rpm. The second stage is presented in Figure 1c. The sleeve was moved downward by 1.01 mm at a moving speed of 50 mm/min. The stirring pin was moved upward by 2.00 mm at a moving speed of 100 mm/min. Subsequently, the sleeve was retracted by 1.01 mm at a retracting speed of 50 mm/min. Meanwhile, the stirring pin was plunged by 2.00 mm at a plunging speed of 100 mm/min (see Figure 1d). Eventually, the entire welding tool was raised and separated from the steel/Al welded joint (see Figure 1e). The overall time required for preparing one welding spot was about 3.4 s. In particular, the preheating stage required 1 s. The time for the plunging stage was 1.2 s. The refilling stage lasted for 1.2 s.
The mechanical properties of the welded joint were revealed by the tensile test and microhardness test. The specimens (30 mm × 170 mm) with welding spot of a diameter of 9 mm were prepared, and we performed the tensile tests on the electronic universal testing machine (Instron 5581, Instron, Norwood, MA, USA). To observe the microstructure of the specimens, sandpapers of 80, 400, 800, 1200, 1500, and 2000 mesh were used to grind the welded joints. The grinding direction was perpendicular to the surface of the welding spot, and 4% HNO3 + C2H5OH etchant with a volume ratio of 5:1 was used to corrode B410LA steel, with an etching time of 5 s. Keller’s reagent (HF + HCl + HNO3 + H2O with the volume ratio of 1:1:1:6) was used to corrode 6061 aluminum alloy, with an etching time of 120 s. The microstructure details of the steel BM (B410LA) and Al alloy BM (6061) were observed by optical microscopy (OM, Zeiss Imager A2m, Zeiss, Oberkochen, Germany). The interfacial microstructure characteristics of the steel/Al refill friction stir spot welded joint were detected by using the scanning electron microscope (SEM, Phenom Pro XL, Phenom-World, Eindhoven, The Netherland). SEM (VEGA 3 SBH, TESCAN, Brno, Czech Republic) and ultra-depth-of-field microscope (Zeiss Smartzoom 5, Zeiss, Oberkochen, Germany) were employed to observe the fracture morphology of the steel/Al joints. The element distribution at the interface was analyzed by the energy-dispersive X-ray spectroscopy (EDS, Bruker Xflash, Bruker, Billerica, MA, USA) and electro-probe microanalyzer (EPMA, EPMA-8050G, Shimadzu Corporation, Kyoto, Japan). Electron backscatter diffraction (EBSD, Zeiss Sigma 300, Zeiss, Oberkochen, Germany) was used to investigate the grain size and misorientation angle in the sleeve-plunging zone. The microhardness distribution at the steel side and Al alloy side of the welded joint was analyzed by employing the digital microhardness tester (MH-5L, Hengyi Precision Instrument Co., Ltd., Shanghai, China). The load for measuring the microhardness of the Al alloy side was set at 0.49 N, while the load for measuring the microhardness of the steel side was set at 1.96 N.

3. Results and Discussion

Figure 2 presents the appearances of three typical steel/Al welded joints before the tensile test. The dimension of the specimens is illustrated in Figure 2a. The length of both the 6061Al alloy and B410LA steel plates was 100 mm, with 30 mm width. The length of the overlapping zone of the two plates was 30 mm. As shown in Figure 2b–d, the weld spot diameter was 9 mm.
The appearances of the steel/Al refill friction stir spot welded joints after the tensile test and experimental data are shown in Figure 3. The tensile test results illustrated that all three specimens failed in the Al alloy plate, showing a button fracture (BF) mode (see Figure 3b–d). The maximum tensile load of the steel/Al joint was 4.3 kN. These data indicated that B410LA steel and 6061Al alloy plates were effectively bonded, exhibiting excellent tensile properties. Moreover, Lin et al. [38] conducted the tensile-shear tests on the steel/Al cold metal transfer (CMT) brazed lap joints. The results demonstrated that the maximum tensile-shear load of the joint was 2.1 kN. Huang et al. [39] employed the laser welding-brazing technique to join 6061 Al alloy and 22MnB5 steel. The maximum tensile-shear load of the obtained steel/Al joint was 2.8 kN. According to the results by Li et al. [30], the maximum fracture load of the ST16 steel (Angang Steel Company Limited, Anshan, China)-Aleris Superlite 200 ST Al alloy (Aleris, Beachwood, OH, USA) refill friction stir spot welded joint was 3.7 kN. Therefore, compared with steel/Al joints fabricated with other welding methods and BMs, the refill friction stir spot welded joint of B410LA steel and 6061 Al alloy prepared in this study revealed better mechanical properties.
Figure 4 shows the fracture morphology of the steel/Al refill friction stir spot welded joint. The fracture location of the joint was in the BM of Al alloy. Al alloy had good toughness. Therefore, dimples could be observed in different fracture zones.
In general, the characteristic zones of the steel/Al refill friction stir spot welded joint can be divided into the stir zone (SZ), heat-affected zone (HAZ), thermos-mechanically affected zone (TMAZ), and BMs [40]. Owing to the difference in plunging position, the SZ could be further divided into the sleeve- and pin-plunging zones. The microhardness distribution in various welding characteristic zones of the steel/Al refill friction stir spot welded joint is presented in Figure 5. The microhardness at the steel and Al alloy sides of the welded joint were obtained. At the Al alloy side, the microhardness of the stir zone was equivalent to that of the BM of the Al alloy. An obvious decrease in microhardness could be observed on both sides of the stir zone, so the zone was determined as the HAZ. Meanwhile, the microhardness of the stir zone significantly higher than that of the BM of steel could be detected in the steel side. With the stirring of the sleeve and the pin during the refill friction stir spot welding, work hardening occurred in the stir zone. During the refilling stage, the hot compression behavior could induce dynamic recrystallization in the stir zone, thereby promoting the refinement of grain size. Therefore, this process probably also led to the increase in the microhardness of the stir zone [41]. The microhardness of the sleeve-plunging zone was higher than that of the pin-plunging zone, indicating that more drastic stirring was produced in this zone. That is to say, the effect of the metallurgical and mechanical bonding in the sleeve-plunging zone was more prominent.
The mechanical properties of 6061 Al alloy and B410LA steel are listed in Table 2. The tensile strength of the steel/Al refill friction stir spot welded joint was comparable to that of 6061 Al alloy. At the Al alloy side, the microhardness of the stir zone was similar to that of the Al alloy BM, while, at the steel side, it could be detected that the microhardness of the stir zone was higher than that of the steel BM.
The microstructure of the BM of B410LA steel and the 6061Al alloy is presented in Figure 6. The microstructure of the steel BM was the granular pearlite distributed in equiaxed ferrite grains, as can be seen in Figure 6a,b. The microstructure of Al alloy BM was mainly α-Al matrix (see Figure 6c,d).
The microstructure of the steel/Al refill friction stir spot welded joint is displayed in Figure 7. No obvious defects such as holes could be found in the joint (see Figure 7a). There were two obvious groups of hook structure at the interface. They were caused by the plunging of the sleeve. During the refill friction stir spot welding process, the metal at the edge of the sleeve was softened by the friction and entered the BM of Al alloy under the pressure effect, thereby forming the hook structure. The pin-plunging zone presented the flat interface, and the quality of the interface bonding was good (see Figure 7c,e).
In order to further investigate the relationship between the mechanical properties and the interface morphology of the steel/Al joints, the microstructure of various interface zones of the steel/Al welded joint was detected by SEM, and the line scanning results were obtained by EDS. Figure 8 shows the corresponding results. Compared with the sleeve-plunging zone, the pin-plunging zone displayed a more flat-straight interface. The dispersed steel particles could be observed (see Figure 8b,e). During the welding process, the friction of the sleeve and the pin could crush the steel at the surface, and the broken steel particles would be embedded in the softer Al alloy matrix. In addition, according to the line scanning results (see Figure 8c,f), it could be found that Fe and Al elements diffused mutually at the interface during the welding process.
Figure 9 presents the microstructure details of the sleeve-plunging zone of the steel/Al joint. Figure 9a,b present the hook structure formed by the plunging of the sleeve. The mechanical interlock effect produced by this structure was beneficial to enhancing the strength of the steel/Al joint. It could also be found that the flat and continuous IMC layer was generated at the interface, revealing that the effective metallurgical bonding was formed in the sleeve-plunging zone. In addition, the vortex-like structure mainly composed of IMC was detected in the BM of steel (see Figure 9c,d). The IMC was distributed in the BMs on both sides, producing the mechanical interlock effect and making the interface bonding much stronger [36]. The appearance of the hook and vortex-like structures indicated that the sleeve-plunging zone produced qualified mechanical bonding. Therefore, the bonding condition in the sleeve-plunging zone crucially determined the mechanical properties of the steel/Al refill friction stir spot welded joint.
To shed much light on the effect of the interfacial morphology on the mechanical properties of the steel/Al refill friction stir spot welded joint, the interfacial morphology, element distribution, grain size and misorientation angle of the sleeve-plunging zone of the steel/Al refill friction stir spot welded joint were detected. The corresponding results are presented in Figure 10. No obvious material loss and welding defects were detected in the sleeve-plunging zone. The uniform and continuous Fe-Al IMC layer could be found at the interface according to the result of element analysis (see Figure 10c,d). The uniform IMC layer indicated that effective metallurgical bonding was formed at the sleeve-plunging zone. In addition, the hook-and-vortex-like structure can be observed in Figure 10a,b, successfully producing a mechanical interlock effect. The vortex-like structure was composed of both Fe and Al. The grains of the steel BM were fine in the sleeve-plunging zone, with an average grain size of 0.58 μm. The high-angle grain boundaries (HAGBs) accounted for a relatively large proportion (64.1%).
Figure 11 shows a schematic of the formation of the interlock morphology during the refill friction stir spot welding process. As the sleeve plunged, more welding materials were stirred, and, thus, the Fe atoms at the edge of the welding spot moved to one side, as shown in Figure 11a. The macroscopic performance was that the steel matrix entered the softer Al alloy matrix via the stirring effect, forming the typical hook-like morphology at the interface between the Al alloy and steel. Meanwhile, the Al and Fe atoms at the intermediate part of the sleeve-plunging zone were simultaneously mixed together by stirring, which eventually led to the vortex-like structure. According to the results of Geyer et al. [42], the hook structure could easily initiate the micro-cracks, which could deteriorate the mechanical properties of the joint. However, this situation could be different when the hook structure bonds effectively with the matrix. Firstly, the hook structure could not easily initiate cracks when gapless metallurgical bonding with the Al alloy matrix was formed. Secondly, the interlock morphology enhanced by the hook structure could effectively improve the shear strength of the steel/Al refill friction stir spot welded joint.

4. Conclusions

The B410LA steel and 6061 Al alloy sheets were successfully joined by the refill friction stir spot welding technique. The relationship between the mechanical properties and microstructure of the steel/Al welded joint was investigated. Meanwhile, the roles of the mechanical and metallurgical bonding in the mechanical properties were elucidated. The main conclusions are listed as follows:
(1)
The steel/Al refill friction stir spot welded joints exhibited excellent tensile-shear properties. All the specimens were fractured at the base metal (BM) of the Al alloy, exhibiting a button fracture (BF) mode. The maximum tensile-shear load of the steel/Al refill friction stir spot welded joint was as high as 4.3 kN. A significantly higher microhardness of the stir zone (SZ) than that of the steel BM could be detected in the steel side. The microhardness of the sleeve-plunging zone was higher than that of the pin-plunging zone, indicating that more drastic stirring was produced in this zone.
(2)
The steel/Al refill friction stir spot welded joints were successfully bonded with the interface, without any obvious welding defects. The flat and continuous intermetallic compound (IMC) layer could be observed at the interface, and hook-and-vortex-like structures could also be detected.
(3)
The bonding condition in the sleeve-plunging zone crucially determined the mechanical properties of the steel/Al refill friction stir spot welded joint. A uniform Fe-Al IMC layer was found in the sleeve-plunging zone, indicating the formation of effective metallurgical bonding in this zone. In particular, hook-and-vortex-like structures formed by the plunging of the sleeve could also be observed. These characteristic structures produced the mechanical interlock effect, leading to the qualified mechanical bonding in the zone of sleeve plunging.

Author Contributions

Resources, T.W. and H.P.; conceptualization, T.H., B.L. and Z.L.; methodology, T.H. and K.D.; software, T.H., B.L. and Z.L.; validation, Z.L. and K.D.; data curation, Z.L. and T.H.; formal analysis, T.H. and B.L.; investigation, T.H. and B.L.; writing—original draft, T.H. and B.L.; writing—review and editing, K.D. and Y.G.; supervision, T.W., H.P. and Y.G; project administration, T.W., H.P. and Y.G.; funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant no. 52101042).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

Authors Tianhai Wu, Hua Pan were employed by the company Automobile Steel ResearchInstitute, R&D Center, Baoshan Iron & Steel Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic of the refill friction stir spot welding equipment and process: (a) welding equipment, (b–e) welding process. The direction of the arrow in this figure indicates the moving direction of the welding tool.
Figure 1. Schematic of the refill friction stir spot welding equipment and process: (a) welding equipment, (b–e) welding process. The direction of the arrow in this figure indicates the moving direction of the welding tool.
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Figure 2. Appearances of steel/Al refill friction stir spot welded joints before tensile test: (a) the appearances of the specimens, and (b–d) the appearances of the characteristic welding zones.
Figure 2. Appearances of steel/Al refill friction stir spot welded joints before tensile test: (a) the appearances of the specimens, and (b–d) the appearances of the characteristic welding zones.
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Figure 3. Appearances of steel/Al refill friction stir spot welded joints after tensile test and tensile test results: (a) the appearances of tested specimens, (b–d) the magnified images of fractured zone, and (e) fracture location and tensile load of the tested joints.
Figure 3. Appearances of steel/Al refill friction stir spot welded joints after tensile test and tensile test results: (a) the appearances of tested specimens, (b–d) the magnified images of fractured zone, and (e) fracture location and tensile load of the tested joints.
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Figure 4. Fracture morphology of steel/Al refill friction stir spot welded joint: (a) macrostructure of fracture, and (b–e) microstructure of different zones of fracture.
Figure 4. Fracture morphology of steel/Al refill friction stir spot welded joint: (a) macrostructure of fracture, and (b–e) microstructure of different zones of fracture.
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Figure 5. The microhardness distribution in different welding characteristic zones of steel/Al refill friction stir spot welded joint: (a) metallographic image of the whole joint, (b,c) the microstructure of the sleeve-plunging zone, and (d) the microhardness for the zone marked with the lines in (a).
Figure 5. The microhardness distribution in different welding characteristic zones of steel/Al refill friction stir spot welded joint: (a) metallographic image of the whole joint, (b,c) the microstructure of the sleeve-plunging zone, and (d) the microhardness for the zone marked with the lines in (a).
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Figure 6. The metallographic microstructure of steel BM and Al alloy BM: (a,b) the microstructure of steel BM (B410LA), and (c,d) the microstructure of Al alloy BM (6061).
Figure 6. The metallographic microstructure of steel BM and Al alloy BM: (a,b) the microstructure of steel BM (B410LA), and (c,d) the microstructure of Al alloy BM (6061).
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Figure 7. The metallographic microstructure of the steel/Al refill friction stir spot welded joint: (a) the whole joint, (b,c) the metallographic microstructure of the sleeve-plunging zone, (d) the metallographic microstructure of the pin-plunging zone, and (e) the magnified image of zone C arrowed in (a).
Figure 7. The metallographic microstructure of the steel/Al refill friction stir spot welded joint: (a) the whole joint, (b,c) the metallographic microstructure of the sleeve-plunging zone, (d) the metallographic microstructure of the pin-plunging zone, and (e) the magnified image of zone C arrowed in (a).
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Figure 8. The microstructure of different interface zones of steel/Al refill friction stir spot welded joint: (a,b) the sleeve-plunging zone, (d,e) the pin-plunging zone, (c,f) the line scanning results of zones A and B, respectively.
Figure 8. The microstructure of different interface zones of steel/Al refill friction stir spot welded joint: (a,b) the sleeve-plunging zone, (d,e) the pin-plunging zone, (c,f) the line scanning results of zones A and B, respectively.
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Figure 9. The microstructure details of the sleeve-plunging zone of the steel/Al refill friction stir spot welded joint: (a,b) the hook structure, and (c,d) the vortex-like structure.
Figure 9. The microstructure details of the sleeve-plunging zone of the steel/Al refill friction stir spot welded joint: (a,b) the hook structure, and (c,d) the vortex-like structure.
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Figure 10. The element distribution, grain size and misorientation angle of the sleeve-plunging zone of steel/Al refill friction stir spot welded joint: (a,b) the microstructure details at the interface, (c–f) the distribution of Al, Fe, Si, C elements, (g) inverse pole figure (IPF) map of the steel BM, (h) grain size, and (i) misorientation angle. The red line in (h) represents the fitting curve.
Figure 10. The element distribution, grain size and misorientation angle of the sleeve-plunging zone of steel/Al refill friction stir spot welded joint: (a,b) the microstructure details at the interface, (c–f) the distribution of Al, Fe, Si, C elements, (g) inverse pole figure (IPF) map of the steel BM, (h) grain size, and (i) misorientation angle. The red line in (h) represents the fitting curve.
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Figure 11. Schematic of the formation of the interlock morphology in refill friction stir spot welding process: (a) the Al and Fe atom diffusion during the refill friction stir spot welding process, (b) the interlock morphology in the steel/Al refill friction stir spot welded joint. The yellow arrows in (a) indicate that the steel matrix entered the softer Al alloy matrix via the stirring effect.
Figure 11. Schematic of the formation of the interlock morphology in refill friction stir spot welding process: (a) the Al and Fe atom diffusion during the refill friction stir spot welding process, (b) the interlock morphology in the steel/Al refill friction stir spot welded joint. The yellow arrows in (a) indicate that the steel matrix entered the softer Al alloy matrix via the stirring effect.
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Table 1. The chemical composition of Al alloy 6061 and B410LA steel (wt. %).
Table 1. The chemical composition of Al alloy 6061 and B410LA steel (wt. %).
Base MetalsMgSiCuMnCrTiZnCAlFe
6061-T61.00.60.2<0.150.1<0.150.4-Bal.<0.7
B410LA-0.15-1.60.030.02-0.140.04Bal.
Table 2. Mechanical properties of 6061 Al alloy and B410LA steel.
Table 2. Mechanical properties of 6061 Al alloy and B410LA steel.
MaterialsTensile Strength
(MPa)		Yield Strength
(MPa)		Elongation
(%)		Microhardness
(HV)
60613102761375
B410LA≥590410–560≥17180
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MDPI and ACS Style

Hu, T.; Li, B.; Li, Z.; Ding, K.; Wu, T.; Pan, H.; Gao, Y. Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel. Metals 2024, 14, 1209. https://doi.org/10.3390/met14111209

AMA Style

Hu T, Li B, Li Z, Ding K, Wu T, Pan H, Gao Y. Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel. Metals. 2024; 14(11):1209. https://doi.org/10.3390/met14111209

Chicago/Turabian Style

Hu, Tianhan, Bolong Li, Zhen Li, Kai Ding, Tianhai Wu, Hua Pan, and Yulai Gao. 2024. "Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel" Metals 14, no. 11: 1209. https://doi.org/10.3390/met14111209

APA Style

Hu, T., Li, B., Li, Z., Ding, K., Wu, T., Pan, H., & Gao, Y. (2024). Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel. Metals, 14(11), 1209. https://doi.org/10.3390/met14111209

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