**4. Conclusions**

This work mainly investigates the stability of keyhole digging process in VPPA welding. Using comprehensive experimental measurement of weld pool free surface, keyhole boundary and plasma arc pressure, the evolution of keyhole weld pool and the influence mechanism of plasma arc pressure are obtained. The specific conclusions are as follows.


**Author Contributions:** B.X. performed all experiments and wrote this manuscript. S.C. and F.J. provided the basic idea. S.T. and M.T. participated in the discussion on the results and guided the writing of the article.

**Funding:** This work is Supported by National Natural Science Foundation of China (No.51875004), Beijing Natural Science Foundation (3172004), the China Scholarship Council (CSC) for one-year study in University of Alberta for Fan Jiang, the China Scholarship Council (CSC) for one-year study in Osaka University for Bin Xu, International Research Cooperation Seed Fund of Beijing University of Technology (No. 2018B16), the open project of Beijing Engineering Researching Center of Laser Technology (BG0046-2018-09) and start-up funding from Beijing University of Technology.

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