Effects of curvature direction on friction stir welding lap joint of aluminum alloy “S” curved surface

In this paper, the friction stir welding (FSW) of single curvature and double curvature “S” curved surface lap joints were realized using split-type pressing blocks fixture and robotic system with welding parameter 1400 rpm, 150 mm/min. By finite element method (FEM) and axial force analysis, the effects of curvature direction on friction stir welding lap joint of 5083 aluminum alloy “S” curved surface is explored. The results show that Z-axis curvature direction has important influence on joint forming. Z-axis positive curvature makes the actual axial compressive force smaller than the device input, which decreases the interface stress of the lap joint and increases the tendency of upper plate weld edges bending deformation and the probability of interface defects. So the joint mechanical properties are reduced, and the tensile load can be as low as 1.5 kN. Z-axis negative curvature makes the actual axial compressive force greater than the device input, which increases the interface stress of the lap joint. Meanwhile, it decreases the tendency of upper plate weld edges bending deformation and leads to the interface defects disappearing. So the joint mechanical properties are good, and the tensile load can be more than 2.4 kN. For Y-axis curvature direction, its impact is mainly shown in changing the direction of resultant force FR. The inclination of force FR is helpful to eliminate the interface defects of the joint. At the same time, it will cause inconsistent stress between the advancing side (AS) and the retreating side (RS) of the joint interface, which is the reason that the double-curvature lap joint welding process is unstable.


Introduction
Friction stir welding (FSW) is a common technology for manufacturing light alloy plates, which are widely used in aerospace, automobiles, ships and other fields [1,2]. With the expansion of the FSW application range and the demand for complex space structure welding in practical applications, the society needs to develop more flexible FSW equipment and research on the FSW welding process for complex space structures [3,4]. At present, some application of the curved surfaces has been extensively increasing in numerous applications like FSW, 5-axis milling computer numerical controlled (CNC) machines, and elsewhere [5,6]. Three-dimensional curved structure FSW has become a new research hotspot.
In order to realize the FSW of the three-dimensional space surface structure, many scholars have conducted research on this through the welding robot system. Mendes et al. [7] present a complete concept and design of a novel friction stir welding (FSW) robotic platform for welding polymeric materials. Experimental results demonstrate that it is possible to weld plastics with an acceptable level of quality using a robotic FSW platform aided by force/motion control and tuned with appropriate process parameters. Li et al. [8] proposed a 5-axis hybrid robot for FSW. This hybrid robot comprises a 2-SPR-RPS parallel mechanism (with one translational degree of freedom and two rotational degrees of freedom) and two gantries. It can overcome the limitation which traditional FSW machines cannot provide an orientation ability during the welding along a curved surface.
Many scholars have studied the welding deflection compensation and adjustment strategy of robot friction stir welding equipment. Yue et al. [9] present an approach 1 3 for stiffness identification of a 5-DOF hybrid robot named TriMule for friction stir welding. The proposed approach has great potential in deflection prediction and compensation. Backer et al. [10] focus on the robot deflections during FSW by relating process forces to the deviations from the programmed robot path and to the strength of the obtained joint. They concluded that deflections must be compensated for in high-strength alloys, and several strategies can be applied including online sensing or compensation of the deflection in the robot program. Kolegain et al. [11] point out robotic FSW equipment undergo deflection due to their limited stiffness, which leads to position and orientation deviations of the end effector during welding experiments. They studied a feedforward compensation technique based on a deflection model which is coupled with an offline path planning methodology based on Bézier curves. High position and orientation accuracy are achieved, and defect-free weld is performed. Xiao et al. [12] studied a constant plunge depth control based on online trajectory generation for RFSW, which can generate an accurate welding trajectory according to the rough initial reference path and smoothly compensate for the plunge deviation. The proposed method can effectively reduce the vibration caused by compensation during the welding process and reduce flash, which can improve the welding quality. Zhao et al. [13] propose a method of constructing the hybrid stiffness index. Then, based on the soft stiffness index with joint limit constraint, a joint trajectory planning algorithm for the ZK-500 robot and positioner system is proposed. The result of simulations shows that the soft stiffness index with a joint limit constraint can not only ensure the stiffness performance but also improve the smoothness of the joint trajectory in the robot trajectory planning task. Gurdal O et al. [14] presented a robotic finishing system with the scan-and-machine approach. The system combines the workpiece localization, automatic path programming, and process parameter selection features.
Some scholars have carried out research on axial force of welding equipment and its control. Mendes et al. [15] performed FSW of a polymer in a robotic system to study the influence of the axial force on weld quality. In a robotic solution, the control of axial force allows to eliminate robot positional errors and guarantee the contact between the FSW tool and the work pieces. Wang et al. [16] focus on the dynamic model identification of axial force in the robotic FSW process. Two dynamic models of axial force are proposed by using the positioning error along the spindle axis and its changing rate. By means of a nonlinear observer and data filtering, experimental studies have been carried out on a robot-based FSW system. The result shows good performance of the identification.
A few scholars have carried out research on influence of weld gap, stirring head offset, and control strategy. Wanjara et al. [17] found that with increasing weld pitch, the occurrence of a "lazy S" defect in the weld nugget of friction stir-welded AA6061 became increasingly pronounced. They found a joint gap value of 0.5 mm may represent a critical limit in regard to the industrial application of the process. Based on these results, a robotic scenario was synthesized and implemented to successfully produce 1-m-long welds. Guillo et al. [18] studied the influence of the offset position of the pin axis relative to the butt weld on the welding quality. Then, they showed a method to compensate the lateral pin deviation in real-time during robotic friction stir welding (RFSW). Shultz et al. [19] presented a shared control strategy that allows a skilled operator to identify irregularities that occur during robotic friction stir welding and assist the robotic system in producing an appropriate response.
At present, most robotic friction stir welding (RFSW) and curved surface structure FSW research is mainly focused on the equipment control, while few research studies have been conducted on the influence of equipment factors on the welding process and welding quality. Therefore, this paper uses the FSW welding robot to explore the "S"-type curved lap friction stir welding process of 5083-H111 aluminum alloy. By combining the method of finite element numerical simulation and experimental analysis, the axial force loading process in FSW and the effects of the lap joint curvature direction on the microstructure and mechanical properties of the weld is explored.

Experimental work
This study utilized the RoboStir FSW robot system (Fig. 1). The system, which is based on the KR1000 robot, has a constant pressure control [20][21][22][23][24][25] and maximum force capacity of 2400 kg. In order to achieve the welding of plates with the curvature on the Z and Y axes, the fixture in Fig. 1a was used, whose pressing blocks are split-type. The single-curvature means the lap joint only has curvature in Z-axis, while double-curvature means the lap joint has curvature both in Z-axis and Y-axis. As shown in Fig. 1d, the bending of the plate toward the Z positive direction is defined as the Z-axis positive curvature, while the opposite is the Z-axis negative curvature. The no bending position of plates is defined as Z-axis no curvature. It also shows that the bending of the welding track toward the Y positive direction is defined as the Y-axis positive curvature, while the opposite is the Y-axis negative curvature. The Z-axis radius of curvature is 200 mm, and Y-axis radius of curvature is 300 mm. The experimental materials were 2-mm-thick 5083-H111 aluminum alloy plates, which were premade as "S" curved shape lap structure, and the specific chemical compositions are shown in Table 1. The mixing tool used concentric circular groove shape shoulder, the diameter of the shoulder was 10 mm, and the length of the stirring pin was 2 mm. During the welding process, the plunge depth was 2.2 mm, the inclination angle of the main shaft changed with constant pressure control at any time, the rotation direction of main shaft was counterclockwise, and the welding parameter was 1400 rpm, 150 mm/min. After welding, the metallographic samples were taken in the direction of the vertical weld by an electric spark cutting machine, and the size was 20 mm × 10 mm. The metallographic samples were polished and etched by Keller reagent (2 ml HF + 3 ml HCL + 5 ml HNO3 + 190 ml H2O) to prepare metallographic specimens. The optical microscope  (OLYMPUS BX41M) was used to observe the interface defect morphology and the microstructure of the lap joints. With reference to the ASTMD-1002-10 standard, a wirecut electric discharge machine is used to intercept the tensile shear samples in the direction of the vertical weld at specific locations, and the sample size is 70 mm × 10 mm × 4mm. The tensile and shear test was carried out on the ZMICK100 electronic universal tensile testing machine, and the tensile rate was 1 mm/min.
The Zeiss Supra 55 Sapphire scanning electron microscope was used to observe and analyze the fracture of the tensile-shear specimen to determine the fracture form and the location of the crack source, and combine the metallographic observation to infer the fracture mode.

Axial force loading process simulation model
In this study, a 3D mathematical model of FSW axial force loading process was developed using a commercial FEM software ABAQUS to investigate changing process of the component's stress and deform, which is caused by axial force and lap joint curvature. Before the simulation, some assumptions and simplifications were put forward to the model. In order to focus on the role of axial force in curved lap joint, the model only considered the axial force loading process occurring in the stage of stable welding (the most critical stage of weld forming), ignoring the thermal process, plastic flow, and other mechanical efforts in other stages of welding. This assumption may cause some inaccuracies, but these inaccuracies can be ignored in terms of studying the influence of axial force on joint stress and deformation [26][27][28][29][30].
So the model can simplify the setting of material parameters and boundary conditions. In the stable welding stage, the weld temperature is about 200-300 °C, and the mechanical property parameters of the material can be obtained. The boundary conditions can piecewise load axial force according to different positions of the model.

Geometric model and mesh generation
The computation domain is a 3D mathematical model of lap joint. The X-axis positive direction in Fig. 2 represents the welding direction. The single-curvature model is shown in Fig. 2a. The double-curvature model is shown in Fig. 2b. The red arrow indicates the welding trajectory, and the black arrow indicates the loading position of the tooling pressing block. Mesh generation of the two models is the same, which is shown in Fig. 2c.

Boundary condition
The boundary conditions of two models are as shown in Fig. 3. Fixed constraints are set on the back of the lap structure and the position of pressing block. The axial force loading in single-curvature model is shown in Fig. 3a. The weld trajectory is discretized into multiple segments, and each segment is sequentially loaded with downward pressure which is perpendicular to the weld surface. The axial force loading in double-curvature model is shown in Fig. 3b. The welding trajectory is also divided into several segments, and each segment is sequentially loaded with downward pressure which has an angle with respect to the direction perpendicular to the weld surface.

Material mechanical property setting
The setting of material parameters refers to the theoretical formula and true stress-strain curve in Dr. Xu's research [31][32][33]. According to the true stress-strain curve of 5083 aluminum alloy under the condition of strain rate of 0.013 S −1 and temperature of 200 °C, the mechanical property parameter points are extracted and input into the finite element simulation software, as shown in Fig. 4.
The two models used implicit calculation, and the results of the stress and deformation field of the joint are obtained for analysis.    Fig. 6d, both the edges of advancing and retreating sides of the upper plate weld are bending toward Z positive. The microstructure shows no defects. Position IV is Z-axis no curvature and Y-axis negative curvature position. In Fig. 6e, the lap joint interface shows no bending deformation. The microstructure shows no defects. Position VI is Z-axis negative and Y-axis positive curvature position (including positions V-VII). In Fig. 6f, the lap joint interface shows no bending deformation. The microstructure shows no defects.

Mechanical properties in different positions
Mechanical properties of different positions of single-curvature "S"-shaped lap joint are shown in Fig. 7a. At position III (Z-axis positive curvature position), the tensile load of the joint is minimum, which is 1.5 kN. In Fig. 6a, there are defects on advancing side of upper plate and interface on the Z-axis positive curvature position (positions I-III). Besides, there is bending deformation on edge of upper plate weld. These phenomena cause the decrease of tensile load. In the Z-axis negative curvature position (positions V-VII), as shown in Fig. 6c, there are less defects and the tensile loads are great, whose value can increase to more than 2.4 kN. Figures 8a-c show the fracture morphology of different positions of single-curvature "S"-shaped weld. In positions III and IV, the fracture shows cleavage river pattern and tearing ridge, which is dissociative rupture; but in position VI, Mechanical properties of different positions of doublecurvature "S"-shaped lap joint are shown in Fig. 7b. At position II (Z-axis positive and Y-axis positive curvature position), the tensile load of the joint is minimum, which is 1.7 kN. From positions II to VII, the tensile load of the joint increases gradually. In Fig. 6d, there is bending deformation on edge of upper plate weld at the positive II, which may be the reason of the minimum tensile load. The tensile loads of different positions of the weld are different, indicating that the welding process is unstable. The tensile load can get a maximum to 3.0 kN. Figures 8d-f show the fracture morphology of different positions of double-curvature "S"-shaped weld. In positions II, IV, and VI, the fracture all shows some dimples and tearing ridge, which is quasidissociation rupture.
Comprehensively analyzing the above joints morphology and mechanical properties, it shows that curvature of the Z-axis and Y-axis has a great influence on the mechanical properties of the weld joint. In single-curvature "S"-shaped weld, the edge of the upper plate weld of the joint tends to bend toward Z positive in the Z-axis positive curvature position, resulting in reduced tensile load of the lap joint. This is related to the stress state of the lap joint to be welded when the axial force is loading. In double-curvature "S"-shaped  weld, the tensile loads of different positions of the weld are different. This is affected by the combined action of Z-axis and Y-axis curvatures. Because the Y-axis curvature is realized by the change of axial force motion state during welding, it is necessary to further study the axial force loading process of FSW equipment and its influence on the lap joint.

Axial force loading process analysis
Figures 9a-c show simulated stress field of single-curvature lap joint during axial force loading process. When the axial force is loaded, the shape of the stress concentration zone near weld area of three selected positions has little difference, but the peak value of the stress shows a gradually increasing trend from position III to position VI. The peak stress is 353.5, 360.6, and 490 MPa, respectively. Figure  Figures 9d-f shows simulated stress field of double-curvature lap joint during axial force loading process. When the axial force is loaded, the shape of the stress concentration zone near weld area of three selected positions is different. In Y-axis positive curvature position (positions II and VI), the stress on retreating side of the weld is higher than advancing side. The peak stress is 522.7 and 598.6 MPa, respectively. On the contrary, the stress on advancing side of the weld is higher in Y-axis negative curvature position (position IV). The peak stress is 596.9 MPa. Figure 10b shows the cross-section of simulated stress field. The white dotted line indicates the stress concentration distribution of the final nugget zone (NZ). As shown by the black arrow position in the figure, the interface stress concentration first appears on retreating side then changes into advancing side, and finally appears on retreating side. Its value shows an increasing tendency from about 225 MPa (position II) increasing to about 275 MPa (position IV) and then decreasing to about 300 MPa (position VI). Figure 11a shows simulated deformation field of singlecurvature lap joint during axial force loading process. The white dotted line indicates the shape of the NZ. It can be seen from the figure that the blue area in the weld center gradually increases from position III to position VI, indicating that the pressing depth of the weld position gradually increases when the axial force is loaded. In the interface position, as shown in the black dotted boxes 1 to 3, its color Fig. 9 Simulated stress field of "S"-shaped lap joint during axial force loading process: a single curvature and b double curvature changes from orange to yellow and then to green, indicating that the deformation along the Z positive direction gradually decreases. The same phenomenon can be observed in Fig. 6, from position III to position VI (As shown from Figs. 6a to 6c), the edges bending deformation forwards Z positive direction of upper plate weld gradually decreases. Figure 11b shows simulated deformation field of double-curvature lap joint during axial force loading process. The white dotted line indicates the shape of the NZ. From position II to position VI, the pressing depth of the weld position also gradually increases. In the interface position, as shown by the black arrow position, its color changes from green (position II, the value is positive) to cyan (positions IV and VI, the value is negative), indicating that the deformation along the Z direction changed from positive to negative, which also explains the phenomenon in Figs. 6d-f, the bending deformation of interface showing in position II but disappearing in position IV and VI.  This phenomenon means the axial force F N of same value has different effects because of the lap joint curvature. The force state of single-curvature joint in axial force loading process is analyzed in Fig. 12a. In positions I-III with Z-axis positive curvature, the thermal action in the welding process causes the expansion of the lap joint, which produces the force F 1 and F 2 with the restraint of the pressing blocks 1 and 2. This will lead to the generation of the upward force F 1 which is opposite to the axial force F N . The resultant force F R (F R = F N − F 1 ′), which is the actual axial compressive force, will be smaller than F N . Thus, the interface stress of the lap joint will be smaller than other positions. This eventually leads to the edges of the upper plate weld bending toward Z positive (as shown in Fig. 6a). In position IV with Z-axis no curvature, the force F 1 and F 2 are in opposite direction and offset their effects to lap joint, which means no other force is generated. The resultant force F R (F R = F N ) is the actual axial compressive force. Thus, the interface stress of the lap joint will be a little bigger than positions I-III. This eventually leads to the edges of the upper plate weld bending toward Z positive but the bending deformation getting a little smaller (as shown in Fig. 6b). In positions V-VI with Z-axis negative curvature, the force F 1 and F 2 with the restraint will lead to the generation of the downward force F 2 ′, which has same direction as the axial force F N . The resultant force F R (F R = F N + F 2 ′), which is the actual axial compressive force, will be greater than F N . Thus, the interface stress of the lap joint will be bigger. This eventually leads to the edges of the upper plate weld bending deformation disappearing (as shown in Fig. 6c).
The force state of double-curvature joint axial force loading process is analyzed in Fig. 12b. In positions I-III with Z-axis positive and Y-axis positive curvature, the force F′, which points to the Z positive direction, occurs because of Fig. 12 The force state of different positions of weld during "S"-shaped lap joint FSW process: a single curvature and b double curvature (two viewing angles) material thermal expansion, restraint, and Z-axis positive curvature. Due to Y-axis positive curvature, the axial force F N deflects a certain angle toward the Y positive direction during welding. This means an included angle between the axial force F N and F′ will be greater than 90°. So the direction of the actual resultant force F R will lie between Y positive direction and Z negative direction, and its value will be smaller than F N . Thus, the stress on retreating side of the weld will be higher than advancing side. This eventually leads to the edges of the upper plate weld bending toward Z positive and causes the edge bending deformation of retreating side bigger than advancing side (as shown in Fig. 6d). In position IV with Z-axis no curvature and Y-axis negative curvature, no other force was generated because Z-axis has no curvature. Due to Y-axis negative curvature, the axial force F N deflects a certain angle toward the Y negative direction during welding. The direction of the actual resultant force F R (F R = F N ) will lie between Y negative direction and Z negative direction. Thus, the stress on advancing side of the weld will be higher than retreating side. This eventually leads to the interface bending deformation disappearing (as shown in Fig. 6e). In positions V-VI with Z-axis negative and Y-axis positive curvature, the force F′, which points to the Z negative direction, occurs because of material thermal expansion, restraint, and Z-axis negative curvature. Due to Y-axis positive curvature, the axial force F N deflects a certain angle toward the Y positive direction during welding. This means an included angle between the axial force F N and F′ will be smaller than 90°. So the direction of the actual resultant force F R will lie between Y positive direction and Z negative direction, and its value will be greater than F N . Thus, the stress on retreating side of the weld will be higher than advancing side. This eventually leads to the interface bending deformation disappearing (as shown in Fig. 6f).
It can be seen from the above that Z-axis curvature direction has important influence on joint forming. By split-type pressing block fixture, the material thermal expansion causes upward force F 1 ′ in Z-axis positive curvature position but causes downward force F 2 ′ in Z-axis negative curvature position. So the actual axial compressive force F R is smaller than the device input F N in Z-axis positive curvature position. It will cause the interface stress of the lap joint decreasing. Then, it will cause the tendency of upper plate weld edges bending deformation and the probability of interface defects increasing, while F R is greater than F N in Z-axis negative curvature position, which will cause the interface stress of the lap joint to increase. Then it will cause the tendency of upper plate weld edge bending deformation to decrease and the interface defects to disappear.
For Y-axis curvature direction, its impact is mainly shown in changing the direction of resultant force F R . In Y-axis positive curvature position, the force F R lies between Y positive direction and Z negative direction. This inclination of force F R is helpful to eliminate the interface defects of the joint. At the same time, it will cause the interface stress of retreating side bigger than advancing side. When cooperating with Z-axis positive curvature, the value of force F R is less than F N , resulting in the edge bending deformation of retreating side bigger than advancing side. When cooperating with Z-axis negative curvature, the value of force F R is greater than F N , resulting in the edge bending deformation disappearing. In Y-axis negative curvature position, the force F R lies between Y negative direction and Z negative direction. This inclination of force F R is also helpful to eliminate the interface defects of the joint. It will cause the interface stress of advancing side bigger than retreating side. When cooperating with Z-axis no curvature, the value of force F R is equal to F N , resulting in the edge bending deformation disappearing. The changing direction of F R causes inconsistent stress between the advancing side and the retreating side of the joint interface, which is the reason why the doublecurvature lap joint welding process is unstable.

Conclusions
The friction stir welding of single-curvature and doublecurvature "S" curved surface lap joints was realized using split-type pressing block fixture and robotic system with welding parameters 1400 rpm and 150 mm/min in this study. With the FEM model and axial force analysis, the effects of curvature direction friction stir welding lap joint of 5083 aluminum alloy "S" curved surface are explored. The conclusions are as follow: (1) In the axial force loading process of Z-axis singlecurvature FSWed lap joints, Z-axis positive curvature makes the actual axial compressive force F R smaller than the device input F N while Z-axis negative curvature makes the actual axial compressive force F R greater than the device input F N . (2) Due to F R smaller than F N , it means that Z-axis positive curvature decreases the interface stress of the lap joint and increases the tendency of upper plate weld edges bending deformation and the probability of interface defects. Finally, the joint mechanical properties are reduced, and the tensile load can be as low as 1.5 kN. Because of F R greater than F N , it means that Z-axis negative curvature increases the interface stress of the lap joint. Meanwhile, it decreases the tendency of upper plate weld edge bending deformation and leads to the interface defects disappearing. So the joint mechanical properties are good, and the tensile load can be more than 2.4 kN. (3) In the axial force loading process of Z-axis and Y-axis double-curvature FSWed lap joints, Y-axis positive curvature makes actual axial compressive force F R lie between Y positive direction and Z negative direction. This inclination of force F R is helpful to eliminate the interface defects of the joint. At the same time, it will cause the interface stress of retreating side to be bigger than advancing side. (4) When Y-axis positive curvature is cooperating with Z-axis positive curvature, the value of force F R is less than the device input F N , resulting in the edge bending deformation of retreating side being bigger than advancing side. So, the minimum tensile load of joints, 1.7 kN, shows in the Z-axis positive curvature and Y-axis positive curvature position. When Y-axis positive curvature is cooperating with Z-axis negative curvature, the value of force F R is greater than the device input F N , resulting in the edge bending deformation disappearing. Finally, the maximum tensile load of joints, 3.0 kN, shows in the Z-axis negative curvature and Y-axis negative curvature position. (5) Y-axis negative curvature makes actual axial compressive force F R lie between Y negative direction and Z negative direction. This inclination of force F R is also helpful to eliminate the interface defects of the joint. It will cause the interface stress of advancing side bigger than retreating side. When cooperating with Z-axis no curvature, the value of force F R is equal to the device input F N , resulting in the edge bending deformation disappearing. The maximum tensile load of joints is between maximum and minimum.