Based on the arc phase and short-circuit phase and their relationship, the paper considers the changes of the extension length of the welding wire, the arc length, liquid bridge resistance and mass of liquid bridge, combines the improved "mass-spring" model with the loop model of welding power system, puts forward the critical judgment condition of droplet transition, and establishes a more accurate dynamic model for describing the short-circuit transition process. Through the dynamic model, the short-circuit transition frequency, welding current and voltage, contact droplet and residual droplet equivalent radius (including mean and statistical distribution) and the dynamic changes of droplet equivalent radius at different wire feeding speeds were calculated and analyzed, and compared with the experimental results, they were in good agreement. The dynamic changes of droplet displacement, velocity, welding wire extension length and liquid bridge shape are discussed. The results show that the fluctuations of droplet displacement, velocity, and extension length of welding wire is the smallest at the optimal arc point, and the smaller the radius of curvature of the initial liquid bridge is, the better the stability of short-circuit transition is.

K-TIG welding is a highly efficient welding technique that enables full penetration to be achieved with a single pass, without the need for groove preparation or wire filling. Comprehending the distinct arc characteristics of K-TIG welding is the cornerstone for exploring the metallurgical process, heat, and mass transfer phenomena, and serves as a guide for design considerations. In this study, 2D numerical models of arc plasma in K-TIG welding were established. The arc characteristics of the keyhole state and non-penetrated state were compared, while the influence of welding parameters on arc characteristics was further investigated. Finally, the calculated arc pressure was validated by experiments. Results show that the electric potential gradient in the arc column decreases after the keyhole forms, while the temperature increases at the center of the anode surface and decreases on both sides away from the arc center. Higher welding currents and smaller tungsten tip angles will lead to an increase in the temperature and velocity of arc plasma. As the tungsten tip height increases, the arc temperature increases on both sides but decreases in the arc center, and the pressure in the inner wall of the keyhole shrinks.