The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-018-1710-x
ORIGINAL ARTICLE
Double-sided friction stir spot welding of steel and aluminum alloy sheets Xiaochong Lyu 1 & Meng Li 1 & Xifeng Li 1 & Jun Chen 1 Received: 12 July 2017 / Accepted: 5 February 2018 # Springer-Verlag London Ltd., part of Springer Nature 2018
Abstract Double-sided friction stir spot welding (FSSW) of dissimilar Al/steel alloy sheets was developed, and the effects of different tool rotational directions and plunge forces on the welding quality were investigated and compared with single-sided FSSW. Experimental results show that the fracture load increases with the increasing of plunge force, and double-sided FSSW can sustain higher fracture load than single-sided FSSW regardless of the plunge force. Two rotational directions result in similar fracture load of the welded part if the plunge force is less than 1.16 kN. If the plunge force is larger than 1.16 kN, opposite rotational direction achieves higher fracture load of the welding point. The formation of intermetallic compounds under different welding conditions at the welding joint was analyzed. Keywords Double-sided friction stir spot welding (FSSW) . Plunge force . Rotational direction . Dissimilar joint
1 Introduction The emerging of new industry and development of modern manufacturing technologies necessitates the combination of dissimilar alloy sheets to provide more specified performance. The demand for producing joints of dissimilar metallic sheets which can provide appropriate tailored mechanical and electrical properties is continuously increasing [1]. For the joining of dissimilar alloys, friction stir spot welding (FSSW) is a typical method widely applied to light metals, such as Al and Mg alloys [2]. Since the required heat input in solid state joining process such as FSSW is much less than resistance spot welding (RSW), nowadays, FSSW has been considered as one of the most promising welding method of steel with aluminum alloys [3]. In dissimilar FSSW of Al and steel, intermetallic compound (IMC) layer is of great interest. Hsieh et al. identified the IMCs in low carbon steel SS400 and AA6061 aluminum alloy as Fe2Al5 and Fe4Al13 [2], and found that IMC thickness increases up to 25 μm along with dwell time while the fracture load increases first and then decreases. Habibnia et al. * Jun Chen
[email protected] 1
Department of Plasticity Technology, Shanghai Jiao Tong University, Shanghai 200030, China
identified the IMCs between AA5050 aluminum alloy and 304 stainless sheets in friction stir welding as Al13Fe4 and Al5Fe2 by XRD spectrum [4]. Tool rotational speed has also influence on the forming of IMC layer and fracture load in AA5083 aluminum alloy and St-12 steel alloy [5]. The tool design is critical in FSSW. According to Piccini et al. [3], tool geometry could influence the bonding area and fracture load in FSSW. Welding tool with a wider end could achieve bigger bonding area and higher fracture load. Previous works mostly used tools with a probe [6, 7]. However, the bonding area is relatively small because the keyhole left as the probe is penetrated into the lower sheet [8]. The keyhole also causes some problems such as welding defect and low fracture load. The pinless FSSW was proposed in 2009 and has received increasing attentions in the recent years [9]. Tozaki et al. proved that a scroll groove on the tool shoulder instead of the probe could achieve better welding properties [10]. Chiou et al. proposed an embedded tool to weld aluminum alloy, which could improve the speed of temperature rise and fracture load of FSSW joints by changing the tool materials [11]. Garg et al. compared the joint strength between the tool with pin and embedded tool [12]. Fracture mode can influence the fracture load and fracture surface in spot welding. In the literatures, fracture modes are usually divided into interfacial fracture and plug fracture [13]. Interfacial fracture is a typical quasi cleavage fracture while plug fracture is a kind of ductile fracture in base metal. Dwell time in FSSW could change the performance of
Int J Adv Manuf Technol Table 1 Element
AA5052 DC05
Chemical composition of AA5052, DC05 and Inconel 601 alloy Chemical composition wt% C
P
– 0.06
– – 0.02 0.02
Inconel 601 0.10 max. –
S
Mg
Si
2.2–2.8 0.25 – 0.02
0.015 max. –
Fe
0.4 0.1 Bal. –
Mn
Ni
Cr
Zn
0.1 0.35
– –
0.15–0.35 0.10 0.15 – – –
0.5 max. Bal. 1.0 max. 1.0 max. 58.0–63.0 21.0–25.0 –
welding joints. Fracture load by tensile-shear test increases consistently along with the dwell time due to the increased width and height of the hooks [14]. Tool rotational speed has negative influences on the fracture load of AA5052 welding joints despite the increasing of heat input [15]. Plunge speed of the tool shoulder has obvious effect on the hook geometry and mechanical properties of the welding joint [16]. Double-sided FSSW was proposed in 2013 [17]. A rotating anvil was used in FSSW process, which could permit the joining of thicker cross sections, improve the fracture load of the welding joints, and reduce the plunge force. It has been proved that the use of a rotating anvil for FSSW is a viable method to create spot welds with higher quality in Fig. 1 The schematic and real image of the double-sided incremental sheet forming equipment
Cu
Others Al
–
Bal. 0.01 1.0–1.7
thicker weldments. Among most of the researches, the tool plunge is controlled by plunge depth [7], and the plunge force is defined as 10 kN in this method [18]. In plunge force controlling occasion, Msieh et al. achieved FSSW by a constant plunge force at 8 kN [19]. However, no relationship about plunge force and welding properties has been revealed. In the present work, double-sided pinless FFSW was proposed to make dissimilar joints of DC05 steel and AA5052 aluminum alloy sheets. The influences of critical parameters such as rotational direction and plunge force on mechanical properties and microstructure characteristic and fracture mode were investigated with the dwell time and tool rotational speed set as constants to filter their influences.
Int J Adv Manuf Technol Table 2
Fig. 2 Configuration of the FSSW tool
2 Experimental procedures Friction stir spot welds were carried out to make dissimilar lap joints with 1.0-mm thickness AA5052 and 0.8-mm thickness DC05 sheets. The chemical compositions of the experiment materials are shown in Table 1. The Vickers microhardness of the as-received aluminum and steel samples is 70 and 90 HV, respectively, with the sample size as 180 mm × 40 mm for FSSW and tensile-sheer tests. A double-sided incremental sheet forming machine was initially developed and further used for double-sided FSSW process as shown in Fig. 1. The rotation of the left tool is driven by a servo motor while the right one by stepper motor. The compressed air provides plunge pressure on the right tool during the welding process. In the experiments, the steel sheet was placed on the left side and aluminum sheet on the right side. Higher rotational speed enables generating more energy within limited duration, which is helpful in improving the welding quality. Fifteen
Corresponding plunge forces for each plunge pressure
Plunge pressure (MPa)
0.3
0.4
0.5
0.6
0.7
Force (kN)
0.56
0.86
1.16
1.46
1.75
thousand and 2300 RPM which are almost the speed limits of stepper motor and servo motor were set for the two motors to avoid mechanical system vibration. The dwell time was set as a constant of 4 s. A pinless tool was designed for the welding process, and the tool geometry is shown in Fig. 2. The tool is made of Inconel 601 nikel-base alloy with high strength and stiffness at high temperature, whose chemical composition is also shown in Table. 1. The tool rotational direction has two different options as shown in Fig. 3. Plan A means reverse rotation in the left side to the right side, and plan B means the same direction on both side. A single-sided FSSW named as plan C which only the tool at the right side rotates was also conducted for comparison. Before welding, the aluminum alloy and steel surfaces were polished by grinding to remove the surface contaminants. Plunge pressure varied from 0.3 to 0.7 MPa in plan A and B, while varied from 0.4 to 0.7 MPa in plan C based on the try-out knowledge, and the corresponding plunge forces is shown in Table. 2. The tensile-shear tests were carried out at room temperature as a speed of 1 mm/min. Load-displacement curves were recorded, and then, the fracture load and fracture mode were analyzed. The samples were cut by wire EDM along a line through the welding spot center to observe the cross-section morphology by optical microscopy (OM). Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used to observe and analyze the compositions of IMC layer. The microhardness of each welding
Fig. 3 Friction stir welding system: (a) Tool rotation and clamping system. (b) Details of the dimensions. (c) Real experimental system
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sample at different locations was measured and analyzed by a Zwick/Roell Zhμ microhardness tester.
3 Results and discussions 3.1 Macrostructure observation FSSW of dissimilar metal sheets have been achieved with the dwell time of 4 s. The macrostructures of several friction stir spot welds on the aluminum side are shown in Fig. 4a–f; it is found that the tool indentation decreases as the plunge pressure declines from 0.7 to 0.3 MPa. Due to the specific transition arc design of the tool, the stir zone (SZ) area increases along with the tool indentation. Figure 4a, d demonstrates that the steel would be penetrated to the aluminum side in plan A and B if the plunge pressure was 0.7 MPa. However, in singlesided FSSW (plan C), no such phenomenon was observed. Under high temperature and pressure in double-sided FSSW, the softened aluminum sheet severely deformed; thus, the steel was penetrated to the aluminum side affected by the friction and the pressure from the left tool. Figure 5 shows the cross-section morphology of the FSSW joints by different welding conditions. It is found that doublesided FSSW as shown in Fig. 5a, c will lead to higher level of indentation than single-sided FSSW as shown in Fig. 5e if the plunge pressure is set as 0.7 MPa. In double-sided FSSW, the
aluminum sheet completely deformed and flowed to the circumference of the SZ; thus, no aluminum was found in SZ. While in single-sided FFSW, the phenomenon was not observed under the same plunge pressure. If the plunge pressure is set as 0.4 MPa, aluminum and steel sheets in plan A (Fig. 5b) will be penetrated into each other more completely than plan B as shown in Fig. 5d. In double-sided FSSW, the interfaces are wave shaped as shown in Fig. 5b, d rather than the straight line in single-sided FSSW as shown in Fig. 5e, which may result in higher fracture load. No obvious hook defects as described by Li et al. [9] are found.
3.2 Mechanical properties The fracture loads of FSSW joints captured by tensile-shear tests are shown in Fig. 6a for three different welding operations, indicating that the fracture load monotonically increases with the increasing plunge pressure. The maximum fracture load is 4.76 kN achieved by plan A with plunge pressure as 0.7 MPa, which is higher than that by single-sided FSSW of similar materials [20, 21]. Double-sided FSSW (plan A and plan B) shows higher fracture load under the same plunge pressure than singl-sided FSSW (plan C). If the plunge pressure is less than 0.6 MPa, the fracture loads in plan A and B are similar. However, if the plunge pressure exceeds 0.5 MPa, plan A will have an obviously higher fracture load than that of plan B.
Fig. 4 Weld surface appearance on aluminum side: (a) plan A, 0.7 MPa; (b) plan A, 0.5 MPa; (c) plan A, 0.3 MPa; (d) plan B, 0.7 MPa; (e) plan B, 0.5 MPa; (f) plan C, 0.7 MPa
Int J Adv Manuf Technol Fig. 5 Cross-section morphology of the FSSW joints: (a) plan A, 0.7 MPa; (b) plan A, 0.4 MPa; (c) plan B, 0.7 MPa; (d) plan B, 0.4 MPa; (e) plan C, 0.7 MPa
The fracture mode of each sample can be seen in Table. 3, where “PF” means plug fracture, “MF” means mixed fracture, and “IF” means interfacial fracture mode. Typical plug fracture, mixed fracture, and interfacial fracture are shown in Fig. 7a–f. In “PF” mode shown in Fig. 7a, b, fracture occurs along the perimeter of bonding area. Aluminum sheet is still stuck on steel sheet because the bonding area is stable and the aluminum sheet is thinned by tool friction. Thus, “PF” usually occurs under high plunge pressure (more than 0.5 MPa) as shown in Table.3. In “IF” mode as shown in Fig. 7e, f, fracture occurs along the interface of two sheets because the bonding area is weak. Thus, “IF” usually occurs under low plunge pressure. Under medium plunge pressure, bonding is stable only for some local areas, so the mixed fracture occurs. Fracture modes could be connected with the tool indentation as shown in Fig. 6b. If the sheet indentation exceeds 0.7 mm, the fracture mode tends to be “PF.” Because the thinning of aluminum sheet is extremely severe and the bonding area is relatively stable as shown in Fig. 5a, c. The initial crack occurs at the edge of the SZ and propagates along the
circumference of the SZ until the fracture appears. The highest level of indentation has been achieved by highest plunge pressure as 0.7 MPa in plan A. If the plunge pressure is less than 0.6 MPa, the indentations of plan A and B are similar, which means the heat input of plan A and B are similar. According to Bozzi et al., higher indentation contributes to the formation of IMC layers [22]. The welding joint could be strengthened by IMC layer if the layer is less than 7 μm. Figure 6c shows the relationship between nugget diameter of the joint and the plunge pressure, indicating that among all of the three plans, nugget diameter increases with the increasing of the plunge pressure. Nugget diameter in double-sided FSSW is always higher than that in single-sided FSSW. If the plunge pressure is lower than 0.5 MPa, the nugget diameters in plan A and B are similar. However, if the plunge pressure exceeds 0.5 MPa, the nugget diameter in plan A is obviously higher than that in plan B. The evolution of fracture load in Fig. 6a is consistent with the increase of nugget diameter, meaning that higher nugget diameter could lead to higher fracture load. In “IF” mode, higher nugget diameter means higher bonding area, since the bonding strength in unit area
Int J Adv Manuf Technol
Fig. 6 Plunge pressure vs (a) fracture load, (b) tool indentation, (c) nugget diameter, (d) fracture load per unit area
is similar, so that higher bonding area could lead to higher fracture lode. In “PF” mode, fracture occurs along the perimeter of nugget area on the aluminum sheet. The tearing area increases with the increasing of the perimeter, leading to higher fracture load. The average fracture load per unit area (AFLPUA) is shown in Fig. 6d. AFLPUAs in plan A and B increase first then decline sharply with plunge pressure as 0.5 MPa. Although the strength of the welding area is higher if the plunge pressure is higher than 0.5 MPa, the thinning of the sheet metal is severe, which turns the sheet metal fracture mode to “PF.” While in single-sided FSSW, fracture load per
Table 3
Fracture mode evolution in different welding parameters
Pressure/force Plan A Plan B Plan C
0.7 PF PF IF
0.6 PF PF IF
0.5 MF MF IF
0.4 IF IF IF
0.3 IF IF –
unit area remains relatively stable despite of the increased plunge pressure, which attributes to the interfacial fracture in all cases.
3.3 Microstructure observation The SEM microstructures from the sectional view of the FSSW sheets and the EDS results were analyzed. For the three welding plans, the interfaces at the center of the welding spots under different plunge pressure are shown in Fig. 8a, c, e, g and i, while the EDS line scan results are shown in Fig. 8b, d, f, h and j. If the plunge pressure is 0.7 MPa, the average IMC thicknesses in plan A, B, and C are 6.8, 4.9, and 3.0 μm, respectively, indicating that compared with plan B, plan A could promote the formation of IMC layers under high pressure, which results in higher fracture load. On the other hand, if the plunge pressure decreases down to 0.4 MPa, the IMC thicknesses of plan A and B are 3.3 and 4.0 μm, respectively. If the plunge pressure is 0.7 MPa in plan A and B, Fe of steel and Al of aluminum alloy are mixed more completely; thus,
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Fig. 7 Typical fracture modes: (a) and (b) plug fracture; (c) and (d) mixed fracture; (e) and (f) interfacial fracture
IMC is also formed inside aluminum alloy sheet as shown in Fig. 8a, e, which can improve the joint strength. If the EDS scans through the IMC inside Al, the amount of Al drops down and the amount of Fe gets increasing. This phenomenon turns more pronouncedly if the IMC is bigger. This shows that in double-sided FSSW, IMC layers are obviously thicker than that of single-sided FSSW, contributing to higher fracture load as shown in Fig. 6a. Among all of the three plans, IMC thickness increases along with plunge pressure and indentation, which is consistent with the results by Bozzi et al. using similar materials [22]. Fracture surfaces of typical PF (zone A in Fig. 7b) and IF (zone B in Fig. 7f) were observed by SEM, the images of the steel side are shown in Fig. 9. Dimples were observed in Fig. 8a, indicating that the ductile fracture occurred in “PF” mode on the base material. In Fig. 9b, no dimple was observed, which means that fracture occurs in IMC layer. EDS results of points A to C in Table 4 demonstrate that point B and C are mainly IMC while point A is aluminum alloy. In “PF” mode, IMC layer is stable, so the fracture tends to occur at inner aluminum alloy. On the contrary, in “IF” mode, no IMC was formed or the IMC is too weak; fracture tends to occur on the interface.
3.4 Vickers microhardness The microhardness evolutions along the transverse direction and vertical direction of the welding spot are shown in Fig.
10a, b. In transverse direction, the microhardness was measured as 80 μm from the welding interface, while in vertical direction, microhardness was measured at the welding spot center. Figure 10a shows that the hardness of DC05 and AA5052 in the thermo mechanically affected zone (TMAZ) is generally higher than that in the heat-affected zone (HAZ) and the base material. The enhanced hardness in TMAZ of steel is due to the plastic deformation and strain hardening effect by the rotating tool [20]. In TMAZ, the hardness of DC05 of double-sided FSSW is obviously higher than that in single-sided FSSW. Since the tool only rotates at aluminum alloy side in plan C, steel only undergoes high-pressure and high-temperature procedure; the microhardness is similar to HAZ. In the HAZ, material has experienced a thermal cycle which could modify the microstructure and mechanical properties. The microstructure of aluminum alloy in HAZ has fully recrystallized [3]. The main reason for the microhardness reduction in HAZ is due to the dissolution of fine precipitation into aluminum matrix and coarsening of second phase [20]. Figure 10b shows that DC05 demonstrates higher hardness near the sheet surface in double-sided FSSW because the plastic deformation near the sheet surface is more severe. The left tool is kept still in singlesided FSSW, which causes the stable hardness of the steel along vertical direction. The hardness of AA5052 generally retains due to the combination of the effect of strain hardening and recrystallization.
Int J Adv Manuf Technol Fig. 8 SEM images and EDS line scanning results of FSSW joints: (a) and (b) plan A, 0.7 MPa; (c) and (d) plan A, 0.4 MPa; (e) and (f) plan B, 0.7 MPa; (g) and (h) plan B, 0.4 MPa; (i) and (j) plan C, 0.7 MPa
2500
Counts
Al drops
Mg Al Fe
2000
1500
1000
IMC
Fe rises
500
0
(a)
0
(b)
2
4
6
8
10
12
14
16
18
20
Distance (μm)
3000
Mg Al Fe
2500
Counts
2000 1500 1000 500 0 0
(c)
1
(d)
2
3
4
5
6
7
8
9
10
Distance (μm)
Al drops
4500
Mg Al Fe
4000 3500
Counts
3000 2500 2000 1500
IMC
1000
Fe rises
500 0 0
(e)
2
4
6
8
10
Distance (μm)
(f)
12
14
16
Al drops
3500
Mg Al Fe
3000 2500
Counts
2000 1500
IMC
1000
Fe rises
500 0 0
(g)
1
2
3
4
5
6
7
8
9 10 11 12 13 14
Distance (μm)
(h)
Al drops
3000
Mg Al Fe
2500
Counts
2000 1500 1000
IMC
500
Fe rises
0
(i)
0
(j)
1
2
3
4
5
6
7
8
Distance (μm)
9
10 11 12
Int J Adv Manuf Technol Fig. 9 SEM images of fracture surfaces: (a) Zone A, PF. (b) Zone B, IF
Table 4 EDS results of point A to C (wt%)
Point
Fe
Al
Mg
A
1.5
96.3
2.2
B C
87.1 41.8
12.9 56.5
– 1.6
Fig. 10 Hardness evolution of welded sheets: (a) along transverse direction; (b) along vertical direction
4 Conclusions In the present work, an innovative double-sided FSSW approach of DC05 and AA5052 sheets has been developed. Comparative experimental studies with single-sided FSSW have been made to evaluate the welding ability of the approach. Some conclusive remarks are drawn as follows: (1) For both rotational directions, double-sided FSSW achieves higher fracture load than single-sided one regardless of the change of plunge pressure, which proves the superiority of the proposed double-sided FSSW. A maximum of 4.76 kN fracture load has been achieved by double-sided FSSW. (2) In all three plans, the fracture load increases along with the plunge pressure because higher plunge pressure
could promote the increase of welding area and indentation and IMCs. However, higher plunge pressure causes severe sheet thinning. Fracture mode turned to “PF” if the tool indentation exceeds 0.7 mm. (3) Under high plunge pressure (more than 0.5 MPa), plan A has higher indentation, welding area, and IMC thickness than plan B, which contributes to the higher fracture load achieved by plan A. While under low plunge pressure (less than 0.5 MPa), plan B has lower welding area but thicker IMC layers; thus, the fracture load is similar to plan A, which proves that if the IMC thickness is below 7 μm; IMC layers can strengthen the welding joints in double-sided FSSW. (4) On steel side, microhardness increases remarkably due to the rotation of the left tool, especially near the sheet surface (from 90 to 122 HV). However, the effect of
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single-sided FSSW on the steel hardness can be negligible. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China through grant no. 51675332.
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