Abstract: The use of an external magnetic field is proposed to optimize weld seam formation in K-TIG backing welding of medium-thick 9%Ni steel plates. First, the physical characteristics of the excitation coil are considered to select the main circuit topology for the magnetic control power supply(MCPW), and the inductance and capacitance are designed accordingly. Combined with sinusoidal pulse width modulation(SPWM) technology, the principle of generating multi-mode currents is elucidated. A simulation model is developed in Simulink to theoretically demonstrate the feasibility of the approach. Subsequently, using the STM32F103 microcontroller, a digital loop control algorithm is designed to achieve arbitrarily adjustable multi-mode current outputs, including DC, pulse, and AC sine waves. Finally, the MCPW is employed to conduct K-TIG backing welding experiments on medium-thick 9%Ni steel plates. The results indicate that the external alternating longitudinal magnetic field induces arc rotation and oscillation, effectively mitigating defects such as undercut and incomplete fusion caused by heat concentration. As a result, weld seam formation is significantly optimized. Furthermore, a three-dimensional arc numerical model is established to clarify the mechanism by which the external alternating magnetic field influences the K-TIG welding arc.

Key words: 9%Ni steel, K-TIG welding, digital loop control, external magnetic field, multi-mode