Abstract: A double-inverter variable polarity plasma arc(VPPA) welding system with high-low frequency modulation, controlled by single chip microcomputer is introduced and its characteristics of output signal with different conditions of adding high-low frequency pulse are tested and analyzed. Its welding test is performed and the results show the system runs well. It is analyzed from the time and frequency domain of the characteristic signals that compare with 1 kHz, when pulse frequency increases to 2 kHz the slope of pulse reduces 1.2 times and higher harmonic increases 1.4 times. The welding experiment results show that it can get better welding gap than typical VPPA welding and obtain uniform ripple welding line when process parameters including high frequency pulse, positive and negative polarity currents etc. are chosen appropriately. After examination of the weld microstructure, it is found that the pulse VPPA could obtain finer weld grains than typical VPPA. Because of improving quality of welding, the welding procedure is significant for high-intensity Al-alloy welding in aerospace.

Key words: pulse;variable polarity plasma arc;signal analysis