## Materials and Methods
### MRVF Visual/Haptic Workspace  
Digital twin technology was used to capture the physical UR5 robot pose during operation and allowed the welders to view the rotation status of each joint [41]. The  
Figure 3: The imitation-based and robot-assisted teleoperation of the GMAW process. (**a**) Telerobotic welding platform including the UR5 manipulator, welding torch, and the attached vision system; (**b**) a novice welder conducting immersive and intuitive robot-assisted welding with haptic guidance; (**c**) a professional welder performing remote-controlled robotic GMAW operation for comparison. Figure 2: The communication scheme of the MRVF tele-welding hardware apparatus. combination of the virtual twin and onsite video streams in MR provided comprehensive real-time monitoring of the robot's operating status. It also provided assistance in accurately and efficiently amending the welding motion based on data from the robot model. The scale ratio for the virtual UR5 robot was 1:5 so the digital twin data and motions fit the user's view in the MR welding workspace. Virtual fixtures (VFs) can be divided into guidance fixtures and prevention fixtures. The proposed MRVF presented uses a combination of both to guide the users to efficiently navigate to the initial welding point and effectively prevent the torch tip from colliding with the workpiece. During the welding process, the electrode needs to contact the molten weld pool to ensure the filler metal can be transferred from the electrode to the work. However, contact between the torch tip and the workpiece must be prevented to avoid damage. In the MRVF tele-welding workspace (Figure 4d), a transparent prevention VF panel remains overlaid on the virtual workpiece with a 2D display of the actual welding process to minimize collisions of the torch tip manipulated by the user and the workpiece. The welding experiments revealed it is relatively difficult to move the torch to the exact weld starting point for novice users, and this torch alignment process is often time-consuming and increases overall task completion time. In the MR workspace, a conical guidance fixture is installed with the tip aligned to the welding start point, as shown in  
Figure 4: The visualization of tele-welding interaction modules used in the tele-welding experiments. (**a**) Typical 2D visual feedback for remote-controlled robotic welding where the user uses a monitor to observe the welding process, without the immersive HMD usage; (**b**) the virtual replica of the physical welding workpiece; (**c**) tele-welding MR platform without haptic effects including welding virtual workpiece, overlaid RGB stream, virtual welding torch, and the scaled digital twin of UR5; (**d**) the MRVF module involving hybrid guidance and prevention VFs. Figure 4d. The user simply moves the torch head to the wide end and then quickly moves the virtual torch tip to the cone tip position by following the resistance of the inner wall of the conical shape, and the actual torch is simultaneously driven to the intended welding start point. Interaction between the haptic robot and the MR environment occurs at the haptic interface point (HIP), representing the corresponding position of the physical haptic probe of the master haptic robot [42; 43]. The force exerted on the haptic stylus is calculated by simulating a spring between the proxy and the HIP. The resistance force exerted by the haptic stylus to the user's hand is proportional to the distance between the proxy point and the HIP. Figure 5 illustrates how haptic rendering and robot control are implemented using a master-controlled HIP and proxy-controlled robot (MHPR) architecture [44]. Considering the proxy point never violates the constraints imposed by the virtual fixtures, the welding robot will not collide with the workpiece, even though the operator overcomes the resistance force.