**Bin Xu 1,2, Shinichi Tashiro 2, Fan Jiang 1,3,\*, Shujun Chen <sup>1</sup> and Manabu Tanaka <sup>2</sup>**


Received: 11 March 2019; Accepted: 29 March 2019; Published: 1 April 2019

**Abstract:** The keyhole digging process associated with variable polarity plasma arc (VPPA) welding remains unclear, resulting in poor control of welding stability. The VPPA pressure directly determines the dynamics of the keyhole and weld pool in the digging process. Here, through a high speed camera, high frequency pulsed diode laser light source and X-ray transmission imaging system, we reveal the potential physical phenomenon of a keyhole weld pool. The keyhole depth changes periodically corresponding to the polarity conversion period if the current is same in the electrode negative (EN) phase and electrode positive (EP) phase. There exist three distinct regimes of keyhole and weld pool behavior in the whole digging process, due to the arc pressure attenuation and energy accumulation effect. The pressure in the EP phase is smaller than that of the EN phase, causing the fluctuation of the weld pool free surface. Based on the influence mechanism of energy and momentum transaction, the arc pressure output is balanced by separately adjusting the current in each polarity. Finally, the keyhole fluctuation during the digging process is successfully reduced and welding stability is well controlled.

**Keywords:** VPPA welding; keyhole; digging process; plasma arc pressure; electrode energy balance; X-ray transmission system
