Robotic Friction Stir Welding Research Articles

Active Vibration Avoidance Method for Variable Speed Welding in Robotic Friction Stir Welding Based on Constant Heat Input

Robotic Friction Stir Welding (RFSW) technology integrates the advantages of friction stir welding and industrial robots, finding extensive applications and research in aerospace, shipbuilding, and new energy vehicles. However, the high-speed rotational process of friction stir welding combined with the low stiffness characteristics of serial industrial robots inevitably introduces vibrations during the welding process. This paper investigates the vibration patterns and impacts during the RFSW process and proposes an active vibration avoidance control method for variable speed welding based on constant heat input. This method utilizes a vibration feedback strategy that adjusts the spindle speed actively if the end-effector’s vibration exceeds a threshold, thereby avoiding the modal frequencies of the robot at its current pose. Concurrently, it calculates and adjusts the welding speed of the robot according to the thermal equilibrium equation to maintain constant heat input. A simplified dynamic model of the RFSW robot was established, and the feasibility of this method was validated through simulation experiments. This study fills the gap in vibration analysis of RFSW and provides new insights into control strategies and process optimization for robotic friction stir welding.

Development of a novel nonrigid support friction stir welding repair robot for aluminum alloy train

PurposeThis paper aims to develop a specific type of mobile nonrigid support friction stir welding (FSW) robot, which can adapt to aluminum alloy trucks for rapid online repair.Design/methodology/approachThe friction stir welding robot is designed to complete online repair according to the surface damage of large aluminum alloy trucks. A rotatable telescopic arm unit and a structure for a cutting board in the shape of a petal that was optimized by finite element analysis are designed to give enough top forging force for welding to address the issues of inadequate support and significant deformation in the repair process.FindingsThe experimental results indicate that the welding robot is capable of performing online surface repairs for large aluminum alloy trucks without rigid support on the backside, and the welding joint exhibits satisfactory performance.Practical implicationsCompared with other heavy-duty robotic arms and gantry-type friction stir welding robots, this robot can achieve online welding without disassembling the vehicle body, and it requires less axial force. This lays the foundation for the future promotion of lightweight equipment.Originality/valueThe designed friction stir welding robot is capable of performing online repairs without dismantling the aluminum alloy truck body, even in situations where sufficient upset force is unavailable. It ensures welding quality and exhibits high efficiency. This approach is considered novel in the field of lightweight online welding repairs, both domestically and internationally.

Optimization of Installation Position for Complex Space Curve Weldments in Robotic Friction Stir Welding Based on Dynamic Dual Particle Swarm Optimization

Robotic friction stir welding (RFSW), with its wide application range, ample working space, and task flexibility, has emerged as a vital development in friction stir welding (FSW) technology. However, the low stiffness of serial industrial robots can lead to end-effector deviations and vibrations during FSW tasks, adversely affecting the weld quality. This paper proposes a dynamic dual particle swarm optimization (DDPSO) algorithm through a new comprehensive stability index that considers both the stiffness and vibration stability of the robot to optimize the installation position of complex space curve weldments, thereby enhancing the robot’s stability during the FSW process. The algorithm employs two independent particle swarms for exploration and exploitation tasks and dynamically adjusts task allocation and particle numbers based on current results to fully utilize computational resources and enhance search efficiency. Compared to the standard particle swarm optimization (PSO) algorithm, the DDPSO approach demonstrated superior search capabilities and stability of optimization results. The maximum fitness value improved by 4.2%, the average value increased by 12.74%, and the concentration level of optimization results rose by 72.91% on average. The new optimization method pioneers fresh perspectives for optimizing the stability of RFSW, providing significant grounds for the process optimization and offline programming of complex spatial curve weldments.

Digital Twin Virtual Welding Approach of Robotic Friction Stir Welding Based on Co-Simulation of FEA Model and Robotic Model

Robotic friction stir welding has become an important research direction in friction stir welding technology. However, the low stiffness of serial industrial robots leads to substantial, difficult-to-measure end-effector deviations under the welding forces during the friction stir welding process, impacting the welding quality. To more effectively measure the deviations in the end-effector, this study introduces a digital twin model based on the five-dimensional digital twin theory. The model obtains the current data of the robot and six-axis force sensor and calculates the real-time end deviations using the robot model. Based on this, a virtual welding model was realized by integrating the FEA model with the digital twin model using a co-simulation approach. This model achieves pre-process simulation by iteratively cycling through the simulated force from the FEA model and the end displacement from the robot model. The virtual welding model effectively predicts the welding outcomes with a mere 6.9% error in lateral deviation compared to actual welding, demonstrating its potential in optimizing welding parameters and enhancing accuracy and quality. Employing digital twin models to monitor, simulate, and optimize the welding process can reduce risks, save costs, and improve efficiency, providing new perspectives for optimizing robotic friction stir welding processes.

Robotic friction stir welding in lightweight battery assembly of extrusion-cast aluminium alloys

The present study focuses on developing lightweight assembly of two different aluminium alloys extruded and high pressure die cast (HPDC) for battery frame assembly in BEV. The goal is to produce defect-free welds in lap configuration with smooth surface finish. Stationary shoulder friction stir welding (SSFSW) was employed with welding speeds of 3–15 mm/s. EBSD analysis revealed two groups of grains in the stir zone (SZ) due to dynamic recrystallization. Moreover, the grain size of the SZ significantly decreased compared to both alloys. The cast alloy contains large iron particles, and that were broken by the rotating probe, and the stirred material consisted of fine dispersed precipitates. Tensile-shear test found the fracture location at the hook area near to cast, and a model representing fracture behavior is also discussed. With increasing welding speed from 3 to 5 mm/s, the tensile strength found ∼95 and ∼100 MPa, respectively without any significance difference in the fracture behavior and location. Overall, this study provides valuable insights such as materials mixing, grain refinement, and joint strength in dissimilar joining using SSFSW. The findings could be useful in developing optimized welding parameters and improving the overall quality and productivity of the SSFSW process for battery pack assembly in BEV.

A constant plunge depth control strategy for robotic FSW based on online trajectory generation

Robotic friction stir welding (RFSW) usually comes with a huge upsetting force, and the stiffness of the welding system distributes unevenly over the position, which leads to a large deviation of the plunge depth of the tool at the end of the robot. The conventional constant distance tracking control suffers from the problem of unsmooth compensation leading to the vibration of the robot and thus degrading the weld quality. For this problem, a constant plunge depth control based on online trajectory generation for RFSW is studied, which can generate an accurate welding trajectory according to the rough initial reference path and smoothly compensate for the plunge deviation. Initially, three laser-ranging sensors are utilized to measure the pose deviation of the tool in real-time and generate the ideal welding trajectory according to the projection vector method. Then, a deformation compensation model is established to realize the real-time prediction of the correct value. To ensure the smoothness and rapidity of the dynamic tracking process of displacement deviation, we adopt an online trajectory generator as the core of optimization control to meet the process constraints such as speed, acceleration, and jerk during the compensation process. Finally, simulation and experiment are carried out. The results show that the proposed method can effectively reduce the vibration caused by compensation during the welding process and reduce flash, which can improve the welding quality.

Robotic friction stir welding – seam-tracking control, force control and process supervision

Purpose This study aims to enable robotic friction stir welding (FSW) in practice. The use of robots has hitherto been limited, because of the large contact forces necessary for FSW. These forces are detrimental for the position accuracy of the robot. In this context, it is not sufficient to rely on the robot’s internal sensors for positioning. This paper describes and evaluates a new method for overcoming this issue. Design/methodology/approach A closed-loop robot control system for seam-tracking control and force control, running and recording data in real-time operation, was developed. The complete system was experimentally verified. External position measurements were obtained from a laser seam tracker and deviations from the seam were compensated for, using feedback of the measurements to a position controller. Findings The proposed system was shown to be working well in overcoming position error. The system is flexible and reconfigurable for batch and short production runs. The welds were free of defects and had beneficial mechanical properties. Research limitations/implications In the experiments, the laser seam tracker was used both for control feedback and for performance evaluation. For evaluation, it would be better to use yet another external sensor for position measurements, providing ground truth. Practical implications These results imply that robotic FSW is practically realizable, with the accuracy requirements fulfilled. Originality/value The method proposed in this research yields very accurate seam tracking as compared to previous research. This accuracy, in turn, is crucial for the quality of the resulting material.

Multi-parameter sensing of robotic friction stir welding based on laser circular scanning

Sensing is the premise of process control and is of great importance for improving the weld quality in robotic friction stir welding (RFSW). The kinematic characteristics of the 5-axis robot make existing sensing methods unsuitable for the plunge depth and seam deviation. To address this problem, this paper presented a multi-parameter sensing method for the 5-axis RFSW based on laser circular scanning. The laser displacement sensor rotated around the tool forming a series of measured dots to acquire the three-dimensional profile of the workpiece. The measured dots in a circle could be divided into four groups: plane region, welded region, seam region, and disturbed region. Process parameters were extracted using specific algorithms in a circle. The sensing system simultaneously measured the tool tilt angle, plunge depth, and seam deviation online at the maximum frequency of 10 Hz. The absolute measurement accuracy was 0.04°, 0.01 mm, and 0.05 mm for the tool tilt angle, plunge depth, and seam deviation, respectively. And the maximum standard deviation for ten repeated measurements was 0.005°, 0.005 mm, and 0.009 mm, respectively. The feasibility of the sensing system applied to RFSW had been verified through dynamic measurement experiment. This system greatly reduces the number of sensors and facilitates reliable process sensing. Despite being made for 5-axis robots, the sensing system can be used with 6-axis robots and dedicated machines.

Research and Application of Force-position Mixed-control of Robotic Friction Stir Welding

Research and Application of Force-position Mixed-control of Robotic Friction Stir Welding

Numerical Modeling of Multiphysics Field in Conventional and Stationary Shoulder Friction Stir Welding of Al-Cu Alloy

In robotic friction stir welding, high rotational speed is constantly employed to reduce both robot torque and the welding load. In this paper, a three-dimensional coupled thermo-mechanical model is developed. This model analyzes the multiphysics field in both friction stir welding and stationary shoulder friction stir welding at relatively high rotational speeds. Both experimental and numerical results indicate that the stationary shoulder friction stir welding joint has homogenous microstructure and temperature gradient distribution. Further research finds the peak temperature in the nugget zone of stationary shoulder friction stir welding and friction stir welding is 490° and 530°, respectively. Meanwhile, the effective strain of the stationary shoulder friction stir welding joint is lower when compared to the friction stir welding. However, the strain rate is higher. While being reduced along the thickness direction, the strongest material flow velocity is observed in the top surface of the workpiece. Stationary shoulder friction stir welding is shown as a favorable process for obtaining the homogeneity of the microstructure and reducing joint softening.

Mechanical performance research of friction stir welding robot for aerospace applications

The friction stir welding robot for aerospace applications developed by the research group is subject to the effects of size, working conditions, and other conditions during the operation. The load conditions of the friction stir welding robot are harsh, and the strength and stiffness tests of the whole machine need to be carried out. Five typical working conditions of the friction stir welding robot are analyzed. By analyzing the system composition and configuration of the robot, the loading conditions of the robot stirring head during the welding process are accurately simulated, and this is used as the stiffness and strength check. The boundary conditions of the robot are simulated and analyzed under typical working conditions. The results show that the data of each part of the robot under load are obtained for a given size of the rocket cap welding. After analysis, the maximum normal displacement of the friction stir welding robot reached 0.6424 mm and the maximum stress was 79.21 MPa under the condition of melon flap welding.

Analysis of typical working conditions and experimental research of friction stir welding robot for aerospace applications

In an attempt to address the limitations of single function and the poor process flexibility of existing friction stir welding equipment, a new friction stir welding robot has been developed to address practical problems in three-dimensional surface welding. Based on an analysis of the components of the robotic system, a kinematics model was established, and the forward and backward kinematics solutions were derived for the robot. An iterative nearest point algorithm, iterative closest point based on point cloud matching, was used to plan the welding trajectory for the most complex petal welding conditions, and an experimental study was conducted. The results show that the kinematic and dynamic test results were consistent with the acquired simulation curves for specific sizes of the rocket cap flaps under appropriate welding conditions. The normal error of the weld was less than 85 µm, the average tensile strength was 359 MPa, and the elongation was 7.20%, 78.0%, and 72.0% of 2A14-T6 aluminum alloy. The friction stir welding robot exhibited robust performance, and the proposed trajectory planning method is practical and effective. The appearance, geometric dimension, and mechanical properties of the weld achieved the expected criteria, and high-precision welding of large complex thin-walled surfaces is possible.

Performance analysis and optimization of a rigid-flexible parallel manipulator

In order to meet the design requirements of friction stir welding equipment, a three-degree-of-freedom rigid-flexible 3UPS-UP-3Cable-Driven (3UPS-UP-3CD) parallel manipulator was applied to the main feed mechanism of the friction stir welding robot. The kinematics analysis of the new parallel manipulator was carried out, and the complete homogeneous dimensional Jacobian matrix of the 3UPS-UP-3CD manipulator was solved. Through kinematic performance analysis, the carrying capacity, dexterity and stiffness of 3UPS-UP-3CD manipulator are obviously better than 3UPS-UP mechanism, and the impact of cables on the workspace can also be eliminated by adjusting the dimension parameter. The composite quality workspace map of the manipulator was drawn, and the composite kinematic performance index was proposed. Multiobjective optimization was completed for 3UPS-UP-3CD manipulator, and its performance was improved. The dynamic analysis of the manipulator was carried out, and the driving forces of two mechanisms were calculated under two trajectories. The results showed that the maximum driving force of 3UPS-UP-3CD manipulator was lower than 3UPS-UP mechanism, which proved that 3UPS-UP-3CD manipulator was more suitable for heavy-duty conditions.

Joint Trajectory Time Optimization of Cobot Based on Particle Swarm Optimization

In order to improve the working efficiency of the robot and make the welding quality of the part of skin group improved greatly. We Researched on the end trajectory of the Cobot. In the application scenario of Friction Stir Welding (FSW) test bench, an algorithm of joint space trajectory is proposed according to the kinematics characteristics of the robot and the practical application needs.Based on particle swarm optimization (PSO), the motion trajectory of robot joint is fitted by high order polynomial interpolation. While ensuring the end trajectory, the stability of the joint motion is guaranteed. The simulation experiment is carried out by using MATLAB software. The joint space optimal motion planning is realized. The comparison algorithm is verified by experiments. According to the experimental results, the particle swarm optimization algorithm improves the efficiency of the cooperative robot significantly. It is helpful to the end stability of friction stir welding robot.

Kinematics and dynamics analysis of a new-type friction stir welding robot and its simulation

The mechanical configuration, structural composition, and five typical working conditions of a newly developed friction stir welding robot are introduced. The kinematics model of the friction stir welding robot is established and the forward kinematics equations, inverse kinematics equations, and the Jacobian matrix are solved. In addition, the dynamics model of the friction stir welding robot is also built by using the Lagrange method. The centroid position coordinate and inertia matrix of each part are obtained. Finally, the dynamic equation of friction stir welding robot is determined. According to the kinematics and dynamics model of robots, simulation analysis for friction stir welding robot based on virtual prototyping technology was carried out. The trajectory equation of the weld joint under the condition of melon petal welding is established, the spline trajectory is fitted by many discrete points measured by the contact probe, and the trajectory planning of each joint and the changing laws of motion parameters under the friction stir welding robot melon petal welding condition are obtained. The movement laws and the loading conditions of each joint can be better controlled by designers, and provide solid theoretical support for the static and dynamic characteristics analysis and structural optimization of the friction stir welding robot.

Theoretical Calculation and Simulation Analysis of Axial Static Stiffness of Double-Nut Ball Screw with Heavy Load and High Precision

Double-nut ball screws bear the action of bidirectional pretightening force, leading to the deformation of the contact area between the ball and the raceway. Under this condition, it is important to analyze and calculate the static stiffness of the ball screw. However, the conventional calculation method is inaccurate. Hence, a new method for the static stiffness analysis of a double-nut ball screw is proposed. Through the structural analysis of the ball screw and internal load distribution, a load deformation model was established based on the Hertzian contact theory. Through the load analysis of the ball screw, a static stiffness model of the ball screw was established and applied to a case study and a finite element simulation. The rigidity of THK double-nut ball screws used in the X-axis feed system of a high-stiffness heavy-duty friction stir welding robot (developed by the research group) was calculated. When the workload was lower than 1.1 × 104 N, the slope of the double-nut static stiffness curve increased significantly with the increase in the workload, and when the workload was greater than 1.1 × 104 N, its upward slope tended to stabilize. The simulated and experimental stiffness curves were in good agreement; when the external axial load was greater than 2.8 × 104 N, the stiffness value calculated using the finite element method gradually converged to the theoretical value; and when the axial load reached 3.0 × 104 N, the simulation and test curves matched well. The analysis method of the double-nut ball screw was found to be concise and accurate, and the stiffness curves calculated using the two methods were consistent. The simulation analysis of the static stiffness presented herein is expected to aid the design of double-nut ball screws of high-rigidity heavy-duty equipment.

Design optimization of the ram structure of friction stir welding robot

This paper proposes a method for designing the ram structure of friction stir welding (FSW) robots using finite element analysis. Under the given working condition, force analysis for the ram structure is performed. By analyzing the boundary and load cases on the ram structure, we optimize the topology and size. Through ram-structure optimization considering the static and dynamic characteristics, we can achieve lightweight design of the ram structure and effectively improve the FSW robot’s welding precision. The optimization flow and method can be applied to heavy-load robots and high-stiffness structure.

Off-line path programming for three-dimensional robotic friction stir welding based on Bézier curves

Purpose Robotic friction stir welding (RFSW) is an innovative process which enables solid-state welding of aluminum parts using robots. A major drawback of this process is that the robot joints undergo elastic deformation during the welding, because of the high forces induced by the process. This leads to tool deviation and incorrect orientation. There is currently no computer-aided manufacturing/computer-aided design (CAD) software for generating off-line paths which integrates robot deflections, and the main purpose of this study is to propose an off-line methodology to plan a path for RFSW with the integration of the deflections. Design/methodology/approach The approach is divided into two steps. The first step consists of extracting position and orientation data from CAD models of the workpieces and adding the deflections calculated with a deflection model to generate a suitable path for performing RFSW. The second step consists of the smooth fitting of the suitable path using Bézier curves. Findings The method is experimentally validated by welding a curved workpiece using a Kuka KR500-2MT robot. A suitable tool position and orientation were calculated to perform this welding, an experimental procedure was set up, a defect-free weld was performed and a high accuracy was achieved in terms of position and orientation. Practical implications This method can help manufacturers to easily perform RFSW for three-dimensional workpieces regardless of the lateral tool deviation, loss of the right orientation and control force stability. Originality/value The originality of this method lies in compensating for robot deflections without using expensive sensors, which is the most commonly used method for compensating for robot deflection. This off-line method can lead to a reduction in programming time in comparison with teach programming method and leads to reduced investment costs in comparison with commercial off-line programming packages.

A feedforward deflection compensation scheme coupled with an offline path planning for robotic friction stir welding

Robotic friction stir welding is an innovative process which allows the welding of aluminum workpieces with robots. With their six-axis, serial industrial robots allow the welding of three-dimensional complex geometries but, under high load induced by the process, they undergo deflection due to their limited stiffness. Position and orientation deviations of the end effector are observed during welding experiments leading to weld defects. In this paper, a feedforward compensation technique based on a deflection model is implemented to compensate the deviations. This technique is coupled with an offline path planning methodology based on Bézier curves. The methodology is validated by performing an arc of circular helix welding path. High position and orientation accuracy are achieved and defect-free weld is performed.

High Rotation Speed Friction Stir Welding for 2014 Aluminum Alloy Thin Sheets

In this study, 2014 aluminum alloy sheets with 1 mm thickness are welded successfully by friction stir welding (FSW) robot under the condition of high rotation speed. When the high rotation speed of 10,000-16,500 rpm is applied, the lower axial pressure (less than 200 N) is obtained, which reduces stiffness requirements for equipment. Welding deformation is inevitable because high rotation speed can easily result in rapid heating rate and uneven heat input. The welding distortion caused by two cooling methods is measured, respectively, by laser range finder. The experimental results show that the welding distortion is smaller under the condition of water cooling. When the rotation speed is up to 15,000 rpm and welding speed 50-170 mm/min, the whole welding process is controllable. Under the higher rotation speed condition, the welding defects disappear gradually and more stable mechanical properties can be obtained up to 75% of base metal (ω = 16,000 rpm, ν = 110 mm/min). The results of different welding parameters demonstrate that the high rotation speed can increase material mixing and reduce the axial force (z force), and it can benefit lightweight sheet welding by using FSW robot.