## Materials and Methods
### Welding Experiments
#### 3.4.3 Tele-Welding Experiment  
A 3 \(\times\) 1 within-participants experiment was designed to validate whether the designed MRVF scheme facilitated novice welder control of a robotic tele-welding platform to achieve quality welding results and to assess the user experience. The null hypothesis (H0) of the repeated-measures ANOVA was that the baseline, MRnoVF, and MRVF tele-welding paradigms are equally effective in welding quality and welder experience for novices, in terms of effectiveness, intuitiveness, and learnability, using the VCMM imitation-based motion mapping approach as the basis for teleoperation. In this work, three visualization modules in the tele-welding HRI platform, shown in Figure 4, were tested to validate the efficacy of the proposed MRVF tele-welding paradigm. In particular, to show the differences between the 2D baseline, MR, and MRVF settings for remote tele-welding. Specifically, the three modules were as follows:  
* Baseline: Perform the tele-welding operation with a non-immersive display using monoscopic streams (Figure 4a). The display screen was a standard 27-inch PC monitor. The 2D visualization was transmitted from the monoscopic camera mounted on the welding robot. The welder manipulated the master haptic robot for the welding robot control without haptic effects. The non-immersive 2D display was used as the baseline condition, as it is commonly used for visual feedback in typical remote-controlled welding systems. * MRnoVF: Conduct the tele-welding task with immersive MR-HMD with overlaid monocular images on the top of the virtual workpiece (Figure 4c). The MRnoVF scheme is a limited version of the proposed MRVF module because it does not provide the participants with haptic cues to support hand maneuvering. The haptic device was deployed to command the UR5 arm for welding but provided no force feedback to the operator. * MRVF: MRVF incorporates combined planar prevention and conical guidance haptic cues in the immersive MR workspace (Figure 4d). The user maneuvered the haptic device within the constraints provided by guidance and prevention VFs while welding with the remotely placed robot. The user inspected the real-time pose of the physical welding robot via the scaled virtual replica in the scene. The participants ran through all three experimental setups distinguished by increasing levels of visual and haptic HRI modalities. First, each participant read the instructions and completed a pre-task questionnaire recording age, gender, and familiarity with welding, robotics, and MR experience. The objective of each trial was then explained. Each subject was given the same introduction that demonstrated the proposed intuitive tele-welding platform with the visual/haptic feedback modules they were going to use before testing, ensuring standardized, consistent training for all subjects. After a demonstration, the participants were given 2 min to experience the MR-enhanced telerobotic welding system to familiarize themselves with the haptic robot, mixed reality imagery, and robotic welding platform. The MR-HMD and haptic stylus were fitted on each participant at the user site. When the subject sent verbal confirmation, the MR welding workspace appeared as intended, and they started completing the teleoperated robotic welding tasks as required. Each participant completed the typical horizontal flat welding task under each experimental condition (2D baseline, MRnoVF, MRVF). The condition sequence was randomized to mitigate learning and fatigue effects. After completing each experimental task using one control-feedback condition, the participants filled out a questionnaire about the HRI module to directly compare the three conditions. The participants were given unbounded time to complete the welding tasks but were instructed to navigate the torch from a given pose to the desired welding starting pose as effectively as they could.