**5. Conclusions**

In this work, the DP-GMAW process based on robot operation for stainless steel 304 was seriously considered; all of the operational procedures could be successfully accomplished, and stable welding processes and weld beads with satisfactory quality could be obtained. After introducing the principle and operational characteristics about this process, orthogonal experimental design based on four key operational parameters and corresponding performance testing, which included the appearance observation and key shape parameters measurement, microstructure analysis, tensile and hardness testing, as well as stability analysis of the electrical signals, were conducted. Then, the FCE method was employed to provide quantitative quality evaluation for the weld bead. According to the combined orthogonal experimental analyses about the appearance, grain size and FCE scores of quality of weld bead, an optimal operational parameter combination for each condition can be obtained. Based on these serious explorations, some important conclusions can be drawn:


This work can provide effective methods for analyzing the influential levels of key operational parameters on one or more performances of the weld, and then obtaining a corresponding optimal parameter combination. The work can improve the process parameter optimization and operational performance in the academic research or actual industrial production. In the future, corresponding works will probably continue to be applied in welding base plates of other materials, or other multi-parameter welding systems, and the influential levels of other operational parameters on the quality of weld bead will be further considered.

**Author Contributions:** There are three authors contributed to this manuscript. P.Y. conducted the experiments and analyzed the experimental results. K.Z. proposed the idea of the work, and originated the experiment, and write the paper. S.H. provided assisted works for the work, and calculated the fuzzy comprehensive evaluation scores and sorted out the experimental data, and conducted the microstructure observation experiments.

**Funding:** This research was funded by National Natural Science Foundation of China, China (Grant No: 51805099, 51605103), Science and Technology Planning Project of Guangdong Province, China (2017B090914005), Science and Technology Program of Guangzhou, China (201805010001), and Beijing Institute of Technology Research Fund Program for Young Scholars, China.

**Conflicts of Interest:** The authors declare no conflict of interest.
